Asymmetric Extended Route Optimization (AERO)
draft-templin-aerolink-73
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
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|
|---|---|---|---|
| Author | Fred Templin | ||
| Last updated | 2016-10-13 | ||
| Replaced by | draft-templin-intarea-6706bis, draft-templin-intarea-6706bis | ||
| RFC stream | (None) | ||
| Formats | |||
| Additional resources | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
| Telechat date | (None) | ||
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| Send notices to | (None) |
draft-templin-aerolink-73
Network Working Group F. Templin, Ed.
Internet-Draft Boeing Research & Technology
Obsoletes: rfc5320, rfc5558, rfc5720, October 13, 2016
rfc6179, rfc6706 (if
approved)
Intended status: Standards Track
Expires: April 16, 2017
Asymmetric Extended Route Optimization (AERO)
draft-templin-aerolink-73.txt
Abstract
This document specifies the operation of IP over tunnel virtual links
using Asymmetric Extended Route Optimization (AERO). Nodes attached
to AERO links can exchange packets via trusted intermediate routers
that provide forwarding services to reach off-link destinations and
redirection services for route optimization. AERO provides an IPv6
link-local address format that supports operation of the IPv6
Neighbor Discovery (ND) protocol and links IPv6 ND to IP forwarding.
Admission control and address/prefix provisioning are supported by
the Dynamic Host Configuration Protocol for IPv6 (DHCPv6), while
mobility management and route optimization are naturally supported
through dynamic neighbor cache updates. Although DHCPv6 and IPv6 ND
messaging are used in the control plane, both IPv4 and IPv6 are
supported in the data plane. AERO is a widely-applicable tunneling
solution especially well suited to mobile Virtual Private Networks
(VPNs) and other applications as described in this document.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 16, 2017.
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Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Asymmetric Extended Route Optimization (AERO) . . . . . . . . 7
3.1. AERO Link Reference Model . . . . . . . . . . . . . . . . 7
3.2. AERO Node Types . . . . . . . . . . . . . . . . . . . . . 8
3.3. AERO Routing System . . . . . . . . . . . . . . . . . . . 9
3.4. AERO Interface Link-local Addresses . . . . . . . . . . . 10
3.5. AERO Interface Characteristics . . . . . . . . . . . . . 12
3.6. AERO Interface Initialization . . . . . . . . . . . . . . 14
3.6.1. AERO Relay Behavior . . . . . . . . . . . . . . . . . 14
3.6.2. AERO Server Behavior . . . . . . . . . . . . . . . . 15
3.6.3. AERO Client Behavior . . . . . . . . . . . . . . . . 15
3.7. AERO Interface Neighbor Cache Maintenace . . . . . . . . 15
3.8. AERO Interface Forwarding Algorithm . . . . . . . . . . . 17
3.8.1. Client Fowarding Algorithm . . . . . . . . . . . . . 18
3.8.2. Server Fowarding Algorithm . . . . . . . . . . . . . 18
3.8.3. Relay Fowarding Algorithm . . . . . . . . . . . . . . 19
3.9. AERO Interface Encapsulation and Re-encapsulation . . . . 19
3.10. AERO Interface Decapsulation . . . . . . . . . . . . . . 20
3.11. AERO Interface Data Origin Authentication . . . . . . . . 20
3.12. AERO Interface Packet Size Issues . . . . . . . . . . . . 21
3.13. AERO Interface Error Handling . . . . . . . . . . . . . . 23
3.14. AERO Router Discovery, Prefix Delegation and
Autoconfiguration . . . . . . . . . . . . . . . . . . . . 26
3.14.1. AERO DHCPv6 and IPv6 ND Service Model . . . . . . . 26
3.14.2. AERO Client Behavior . . . . . . . . . . . . . . . . 27
3.14.3. AERO Server Behavior . . . . . . . . . . . . . . . . 29
3.15. AERO Interface Route Optimization . . . . . . . . . . . . 31
3.15.1. Reference Operational Scenario . . . . . . . . . . . 31
3.15.2. Concept of Operations . . . . . . . . . . . . . . . 33
3.15.3. Message Format . . . . . . . . . . . . . . . . . . . 33
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3.15.4. Sending Predirects . . . . . . . . . . . . . . . . . 34
3.15.5. Re-encapsulating and Relaying Predirects . . . . . . 35
3.15.6. Processing Predirects and Sending Redirects . . . . 36
3.15.7. Re-encapsulating and Relaying Redirects . . . . . . 38
3.15.8. Processing Redirects . . . . . . . . . . . . . . . . 38
3.15.9. Server-to-Client and Client-to-Server Redirection . 39
3.15.10. Server-to-Server Redirection . . . . . . . . . . . . 40
3.16. Neighbor Unreachability Detection (NUD) . . . . . . . . . 40
3.17. Mobility Management . . . . . . . . . . . . . . . . . . . 41
3.17.1. Announcing Link-Layer Address Changes . . . . . . . 42
3.17.2. Bringing New Links Into Service . . . . . . . . . . 42
3.17.3. Removing Existing Links from Service . . . . . . . . 42
3.17.4. Implicit Mobility Management . . . . . . . . . . . . 42
3.17.5. Moving to a New Server . . . . . . . . . . . . . . . 43
3.17.6. Packet Queueing for Mobility . . . . . . . . . . . . 44
3.17.7. Alternate Mobility Security Model . . . . . . . . . 44
3.18. Multicast Considerations . . . . . . . . . . . . . . . . 44
4. AERO Variations . . . . . . . . . . . . . . . . . . . . . . . 45
4.1. Operation on Host-Only IPv6 AERO Links . . . . . . . . . 45
4.2. Operation on AERO Links Without DHCPv6 Services . . . . . 46
4.3. Operation on Server-less AERO Links . . . . . . . . . . . 46
4.4. Operation on Client-less AERO Links . . . . . . . . . . . 46
4.5. Manually-Configured AERO Tunnels . . . . . . . . . . . . 47
4.6. Encapsulation Avoidance on Relay-Server Dedicated Links . 47
4.7. Encapsulation Protocol Version Considerations . . . . . . 47
5. Implementation Status . . . . . . . . . . . . . . . . . . . . 47
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 47
7. Security Considerations . . . . . . . . . . . . . . . . . . . 48
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 49
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 50
9.1. Normative References . . . . . . . . . . . . . . . . . . 50
9.2. Informative References . . . . . . . . . . . . . . . . . 52
Appendix A. AERO Alternate Encapsulations . . . . . . . . . . . 59
Appendix B. When to Insert an Encapsulation Fragment Header . . 60
Appendix C. Autoconfiguration for Constrained Platforms . . . . 61
Appendix D. Extending AERO Links Through Security Gateways . . . 62
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 63
1. Introduction
This document specifies the operation of IP over tunnel virtual links
using Asymmetric Extended Route Optimization (AERO). The AERO link
can be used for tunneling to neighboring nodes over either IPv6 or
IPv4 networks, i.e., AERO views the IPv6 and IPv4 networks as
equivalent links for tunneling. Nodes attached to AERO links can
exchange packets via trusted intermediate routers that provide
forwarding services to reach off-link destinations and redirection
services for route optimization [RFC5522].
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AERO provides an IPv6 link-local address format that supports
operation of the IPv6 Neighbor Discovery (ND) [RFC4861] protocol and
links IPv6 ND to IP forwarding. Admission control and address/prefix
provisioning are supported by the Dynamic Host Configuration Protocol
for IPv6 (DHCPv6) [RFC3315], while mobility management and route
optimization are naturally supported through dynamic neighbor cache
updates. Although DHCPv6 and IPv6 ND messaging are used in the
control plane, both IPv4 and IPv6 can be used in the data plane.
A node's AERO interface can be configured over multiple underlying
interfaces. From the standpoint of IPv6 ND, AERO interface neighbors
therefore may appear to have multiple link-layer addresses. Each
link-layer address is subject to change due to mobility, and link-
layer address changes are signaled by IPv6 ND messaging the same as
for any IPv6 link.
AERO is applicable to a wide variety of use cases. For example, it
can be used to coordinate the Virtual Private Network (VPN) links of
mobile nodes (e.g., cellphones, tablets, laptop computers, etc.) that
connect into a home enterprise network via public access networks
using services such as OpenVPN [OVPN]. AERO is also applicable to
aviation applications for both manned and unmanned aircraft where the
aircraft is treated as a mobile node that can connect an Internet of
Things (IoT). Numerous other use cases are also in scope.
The AERO mobile VPN capability and Border Gateway Protocol (BGP)-
based core routing system can further be employed either in
conjunction or separately according to the specific use case (see
Section 4). This allows for correct fitting of the (modular) AERO
components to match the specific application. The remainder of this
document presents the AERO specification.
2. Terminology
The terminology in the normative references applies; the following
terms are defined within the scope of this document:
AERO link
a Non-Broadcast, Multiple Access (NBMA) tunnel virtual overlay
configured over a node's attached IPv6 and/or IPv4 networks. All
nodes on the AERO link appear as single-hop neighbors from the
perspective of the virtual overlay even though they may be
separated by many underlying network hops. The AERO mechanisms
can also operate over native link types (e.g., Ethernet, WiFi
etc.) when a tunnel virtual overlay is not needed.
AERO interface
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a node's attachment to an AERO link. Since the addresses assigned
to an AERO interface are managed for uniqueness, AERO interfaces
do not require Duplicate Address Detection (DAD) and therefore set
the administrative variable DupAddrDetectTransmits to zero
[RFC4862].
AERO address
an IPv6 link-local address constructed as specified in
Section 3.4.
AERO node
a node that is connected to an AERO link.
AERO Client ("Client")
a node that issues DHCPv6 messages to receive IP Prefix
Delegations (PDs) from one or more AERO Servers. Following PD,
the Client assigns an AERO address to the AERO interface for use
in IPv6 ND exchanges with other AERO nodes. A node that acts as
an AERO Client on one AERO interface can also act as an AERO
Server on a different AERO interface.
AERO Server ("Server")
a node that configures an AERO interface to provide default
forwarding services for AERO Clients. The Server assigns an
administratively provisioned IPv6 link-local unicast address to
the AERO interface to support the operation of DHCPv6 and the IPv6
ND protocol. An AERO Server can also act as an AERO Relay.
AERO Relay ("Relay")
a node that configures an AERO interface to relay IP packets
between nodes on the same AERO link and/or forward IP packets
between the AERO link and the native Internetwork. The Relay
assigns an administratively provisioned IPv6 link-local unicast
address to the AERO interface the same as for a Server. An AERO
Relay can also act as an AERO Server.
ingress tunnel endpoint (ITE)
an AERO interface endpoint that injects encapsulated packets into
an AERO link.
egress tunnel endpoint (ETE)
an AERO interface endpoint that receives encapsulated packets from
an AERO link.
underlying network
a connected IPv6 or IPv4 network routing region over which the
tunnel virtual overlay is configured.
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underlying interface
an AERO node's interface point of attachment to an underlying
network.
link-layer address
an IP address assigned to an AERO node's underlying interface.
When UDP encapsulation is used, the UDP port number is also
considered as part of the link-layer address. Link-layer
addresses are used as the encapsulation header source and
destination addresses.
network layer address
the source or destination address of the encapsulated IP packet.
end user network (EUN)
an internal virtual or external edge IP network that an AERO
Client connects to the rest of the network via the AERO interface.
The Client sees each EUN as a "downstream" network and sees the
AERO interface as its point of attachment to the "upstream"
network.
AERO Service Prefix (ASP)
an IP prefix associated with the AERO link and from which more-
specific AERO Client Prefixes (ACPs) are derived.
AERO Client Prefix (ACP)
an IP prefix derived from an ASP and delegated to a Client, where
the ACP prefix length must be no shorter than the ASP prefix
length and must be no longer than 64 for IPv6 or 32 for IPv4.
base AERO address
the lowest-numbered AERO address from the first ACP delegated to
the Client (see Section 3.4).
Throughout the document, the simple terms "Client", "Server" and
"Relay" refer to "AERO Client", "AERO Server" and "AERO Relay",
respectively. Capitalization is used to distinguish these terms from
DHCPv6 client/server/relay [RFC3315].
The terminology of DHCPv6 [RFC3315] and IPv6 ND [RFC4861] (including
the names of node variables and protocol constants) applies to this
document. Also throughout the document, the term "IP" is used to
generically refer to either Internet Protocol version (i.e., IPv4 or
IPv6).
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. Lower case
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uses of these words are not to be interpreted as carrying RFC2119
significance.
3. Asymmetric Extended Route Optimization (AERO)
The following sections specify the operation of IP over Asymmetric
Extended Route Optimization (AERO) links:
3.1. AERO Link Reference Model
.-(::::::::)
.-(:::: IP ::::)-.
(:: Internetwork ::)
`-(::::::::::::)-'
`-(::::::)-'
|
+--------------+ +--------+-------+ +--------------+
|AERO Server S1| | AERO Relay R1 | |AERO Server S2|
| Nbr: C1; R1 | | Nbr: S1; S2 | | Nbr: C2; R1 |
| default->R1 | |(P1->S1; P2->S2)| | default->R1 |
| P1->C1 | | ASP A1 | | P2->C2 |
+-------+------+ +--------+-------+ +------+-------+
| | |
X---+---+-------------------+------------------+---+---X
| AERO Link |
+-----+--------+ +--------+-----+
|AERO Client C1| |AERO Client C2|
| Nbr: S1 | | Nbr: S2 |
| default->S1 | | default->S2 |
| ACP P1 | | ACP P2 |
+------+-------+ +------+-------+
| |
.-. .-.
,-( _)-. ,-( _)-.
.-(_ IP )-. +-------+ +-------+ .-(_ IP )-.
(__ EUN )--|Host H1| |Host H2|--(__ EUN )
`-(______)-' +-------+ +-------+ `-(______)-'
Figure 1: AERO Link Reference Model
Figure 1 presents the AERO link reference model. In this model:
o AERO Relay R1 aggregates AERO Service Prefix (ASP) A1, acts as a
default router for its associated Servers S1 and S2, and connects
the AERO link to the rest of the IP Internetwork.
o AERO Servers S1 and S2 associate with Relay R1 and also act as
default routers for their associated Clients C1 and C2.
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o AERO Clients C1 and C2 associate with Servers S1 and S2,
respectively. They receive AERO Client Prefix (ACP) delegations
P1 and P2, and also act as default routers for their associated
physical or internal virtual EUNs. (Alternatively, Clients can
act as multi-addressed hosts without serving any EUNs).
o Simple hosts H1 and H2 attach to the EUNs served by Clients C1 and
C2, respectively.
Each node on the AERO link maintains an AERO interface neighbor cache
and an IP forwarding table the same as for any link. In common
operational practice, there may be many additional Relays, Servers
and Clients.
3.2. AERO Node Types
AERO Relays provide default forwarding services to AERO Servers.
Each Relay also peers with each Server in a dynamic routing protocol
instance to discover the Server's list of associated ACPs (see
Section 3.3). Relays forward packets between neighbors connected to
the same AERO link and also forward packets between the AERO link and
the native IP Internetwork. Relays present the AERO link to the
native Internetwork as a set of one or more AERO Service Prefixes
(ASPs) and serve as a gateway between the AERO link and the
Internetwork. Relays maintain an AERO interface neighbor cache entry
for each AERO Server, and maintain an IP forwarding table entry for
each AERO Client Prefix (ACP). AERO Relays can also be configured to
act as AERO Servers.
AERO Servers provide default forwarding services to AERO Clients.
Each Server also peers with each Relay in a dynamic routing protocol
instance to advertise its list of associated ACPs (see Section 3.3).
Servers configure a DHCPv6 server function and act as delegating
routers to facilitate Prefix Delegation (PD) exchanges with Clients.
Each delegated prefix becomes an ACP taken from an ASP. Servers
forward packets between AERO interface neighbors, and maintain an
AERO interface neighbor cache entry for each Relay. They also
maintain both neighbor cache entries and IP forwarding table entries
for each of their associated Clients. AERO Servers can also be
configured to act as AERO Relays.
AERO Clients act as requesting routers to receive ACPs through DHCPv6
PD exchanges with AERO Servers over the AERO link. Each Client can
associate with a single Server or with multiple Servers, e.g., for
fault tolerance, load balancing, etc. Each IPv6 Client receives at
least a /64 IPv6 ACP, and may receive even shorter prefixes.
Similarly, each IPv4 Client receives at least a /32 IPv4 ACP (i.e., a
singleton IPv4 address), and may receive even shorter prefixes.
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Clients maintain an AERO interface neighbor cache entry for each of
their associated Servers as well as for each of their correspondent
Clients.
3.3. AERO Routing System
The AERO routing system comprises a private instance of the Border
Gateway Protocol (BGP) [RFC4271] that is coordinated between Relays
and Servers and does not interact with either the public Internet BGP
routing system or the native IP Internetwork interior routing system.
Relays advertise only a small and unchanging set of ASPs to the
native routing system instead of the full dynamically changing set of
ACPs.
In a reference deployment, each AERO Server is configured as an
Autonomous System Border Router (ASBR) for a stub Autonomous System
(AS) using an AS Number (ASN) that is unique within the BGP instance,
and each Server further peers with each Relay but does not peer with
other Servers. Similarly, Relays do not peer with each other, since
they will reliably receive all updates from all Servers and all
Relays will therefore have a consistent view of the AERO link ACP
delegations.
Each Server maintains a working set of associated ACPs, and
dynamically announces new ACPs and withdraws departed ACPs in its BGP
updates to Relays. Clients are expected to remain associated with
their current Servers for extended timeframes, however Servers SHOULD
selectively suppress BGP updates for impatient Clients that
repeatedly associate and disassociate with them in order to dampen
routing churn.
Each Relay configures a black-hole route for each of its ASPs. By
black-holing the ASPs, the Relay will maintain forwarding table
entries only for the ACPs that are currently active, and packets
destined to all other ACPs will correctly incur Destination
Unreachable messages due to the black hole route. Relays do not send
BGP updates for ACPs to Servers, but instead originate a default
route. In this way, Servers have only partial topology knowledge
(i.e., they know only about the ACPs of their directly associated
Clients) and they forward all other packets to Relays which have full
topology knowledge.
Scaling properties of the AERO routing system are limited by the
number of BGP routes that can be carried by Relays. At the time of
this writing, the global public Internet BGP routing system manages
well in excess of 500K routes with linear growth and no signs of
router resource exhaustion [BGP]. Therefore, assuming each BGP
router can eventually carry 1M or more routes, this means that at
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least 1M Clients can be serviced by a single set of Relays. A means
of increasing scaling would be to assign a different set of Relays
for each set of ASPs. In that case, each Server still peers with
each Relay, but the Server institutes route filters so that it only
sends BGP updates to the specific set of Relays that aggregate the
ASP. For example, if the ASP for the AERO link is 2001:db8::/32, a
first set of Relays could service the ASP segment 2001:db8::/40, a
second set of Relays could service 2001:db8:0100::/40, a third set
could service 2001:db8:0200::/40, etc.
Assuming up to 1K sets of Relays, the AERO routing system can then
accommodate 1B ACPs with no additional overhead for Servers and
Relays (for example, it should be possible to service 1B /64 ACPs
taken from a /34 ASP and even more for shorter ASPs). In this way,
each set of Relays services a specific set of ASPs that they
advertise to the native routing system, and each Server configures
ASP-specific routes that list the correct set of Relays as next hops.
This arrangement also allows for natural incremental deployment, and
can support small scale initial deployments followed by dynamic
deployment of additional Clients, Servers and Relays without
disturbing the already-deployed base.
Note that in an alternate routing arrangement each set of Relays
could advertise an aggregated ASP for the link into the native
routing system even though each Relay services only smaller segments
of the ASP. In that case, a Relay upon receiving a packet with a
destination address covered by the ASP segment of another Relay can
simply tunnel the packet to the correct Relay. The tradeoff then is
the penalty for Relay-to-Relay tunneling compared with reduced
routing information in the native routing system.
Finally, Relays may have multiple Routing Information Base (RIB)
entries for a single ACP advertised by multiple Servers, but will
place only one entry in the Forwarding Information Base (FIB).
Servers can assign a BGP weight to their peering configurations so
that Relays can determine preferences for ACPs learned from multiple
Servers. In this way, Relays can choose the Server with the highest
weight and insert the corresponding RIB route into the FIB. The
Relay can then fail over to a Server with lower weight in case of ACP
withdrawal or Server failure.
3.4. AERO Interface Link-local Addresses
AERO interface link-local address types include administratively-
provisioned addresses and AERO addresses.
Administratively-provisioned addresses are allocated from the range
fe80::/96 and assigned to a Server or Relay's AERO interface.
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Administratively-provisioned addresses MUST be managed for uniqueness
by the administrative authority for the AERO link. (Note that fe80::
is the IPv6 link-local subnet router anycast address, and
fe80::ffff:ffff is the address used by Clients to bootstrap AERO
address autoconfiguration. These special addresses are therefore not
available for administrative provisioning.)
An AERO address is an IPv6 link-local address with an embedded prefix
based on an ACP and associated with a Client's AERO interface. AERO
addresses remain stable as the Client moves between topological
locations, i.e., even if its link-layer addresses change.
For IPv6, AERO addresses begin with the prefix fe80::/64 and include
in the interface identifier (i.e., the lower 64 bits) a 64-bit prefix
taken from one of the Client's IPv6 ACPs. For example, if the AERO
Client receives the IPv6 ACP:
2001:db8:1000:2000::/56
it constructs its corresponding AERO addresses as:
fe80::2001:db8:1000:2000
fe80::2001:db8:1000:2001
fe80::2001:db8:1000:2002
... etc. ...
fe80::2001:db8:1000:20ff
For IPv4, AERO addresses are based on an IPv4-mapped IPv6 address
[RFC4291] formed from an IPv4 ACP and with a Prefix Length of 96 plus
the ACP prefix length. For example, for the IPv4 ACP 192.0.2.32/28
the IPv4-mapped IPv6 ACP is:
0:0:0:0:0:FFFF:192.0.2.16/124
The Client then constructs its AERO addresses with the prefix
fe80::/64 and with the lower 64 bits of the IPv4-mapped IPv6 address
in the interface identifier as:
fe80::FFFF:192.0.2.16
fe80::FFFF:192.0.2.17
fe80::FFFF:192.0.2.18
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... etc. ...
fe80:FFFF:192.0.2.31
When the Server delegates ACPs to the Client, both the Server and
Client use the lowest-numbered AERO address from the first ACP
delegation as the "base" AERO address. (For example, for the ACP
2001:db8:1000:2000::/56 the base address is 2001:db8:1000:2000.) The
Client then assigns the base AERO address to the AERO interface and
uses it for the purpose of maintaining the neighbor cache entry. If
the Client has multiple AERO addresses (i.e., when there are multiple
ACPs and/or ACPs with short prefix lengths), the Client originates
IPv6 ND messages using the base AERO address as the source address
and accepts and responds to IPv6 ND messages destined to any of its
AERO addresses as equivalent to the base AERO address. In this way,
the Client maintains a single neighbor cache entry that may include
multiple AERO addresses.
3.5. AERO Interface Characteristics
AERO interfaces use encapsulation (see: Section 3.9) to exchange
packets with neighbors attached to the AERO link.
AERO interfaces maintain a neighbor cache, and use both DHCPv6 and
IPv6 ND control messaging to manage the creation, modification and
deletion of neighbor cache entries. AERO interfaces use standard
DHCPv6 messaging for prefix delegation, admission control and
neighbor cache entry management. AERO interfaces use unicast IPv6 ND
Neighbor Solicitation (NS), Neighbor Advertisement (NA), Router
Solicitation (RS) and Router Advertisement (RA) messages for neighbor
cache management the same as for any IPv6 link. AERO interfaces use
two IPv6 ND redirection message types -- the first known as a
Predirect message and the second being the standard Redirect message
(see Section 3.15).
AERO interface ND messages include one or more Source/Target Link-
Layer Address Options (S/TLLAOs) formatted as shown in Figure 2:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length = 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface ID | UDP Port Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ IP Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P00|P01|P02|P03|P04|P05|P06|P07|P08|P09|P10|P11|P12|P13|P14|P15|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P16|P17|P18|P19|P20|P21|P22|P23|P24|P25|P26|P27|P28|P29|P30|P31|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P32|P33|P34|P35|P36|P37|P38|P39|P40|P41|P42|P43|P44|P45|P46|P47|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P48|P49|P50|P51|P52|P53|P54|P55|P56|P57|P58|P59|P60|P61|P62|P63|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: AERO Source/Target Link-Layer Address Option (S/TLLAO)
Format
In this format:
o Type is set to '1' for SLLAO or '2' for TLLAO the same as for IPv6
ND.
o Length is set to the constant value '5' (i.e., 5 units of 8
octets).
o Reserved is set to the value '0' on transmission and ignored on
receipt.
o Interface ID is set to an integer value between 0 and 65535
corresponding to an underlying interface of the AERO node.
o UDP Port Number and IP Address are set to the addresses used by
the AERO node when it sends encapsulated packets over the
underlying interface. When UDP is not used as part of the
encapsulation, UDP Port Number is set to the value '0'. When the
encapsulation IP address family is IPv4, IP Address is formed as
an IPv4-mapped IPv6 address as specified in Section 3.4.
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o P[i] is a set of 64 Preference values that correspond to the 64
Differentiated Service Code Point (DSCP) values [RFC2474]. Each
P(i) is set to the value '0' ("disabled"), '1' ("low"), '2'
("medium") or '3' ("high") to indicate a preference level for
packet forwarding purposes.
AERO interfaces may be configured over multiple underlying
interfaces. For example, common mobile handheld devices have both
wireless local area network ("WLAN") and cellular wireless links.
These links are typically used "one at a time" with low-cost WLAN
preferred and highly-available cellular wireless as a standby. In a
more complex example, aircraft frequently have many wireless data
link types (e.g. satellite-based, cellular, terrestrial, air-to-air
directional, etc.) with diverse performance and cost properties.
If a Client's multiple underlying interfaces are used "one at a time"
(i.e., all other interfaces are in standby mode while one interface
is active), then IPv6 ND messages include only a single S/TLLAO with
Interface ID set to a constant value. In that case, the Client would
appear to have a single underlying interface but with a dynamically
changing link-layer address.
If the Client has multiple active underlying interfaces, then from
the perspective of IPv6 ND it would appear to have multiple link-
layer addresses. In that case, IPv6 ND messages MAY include multiple
S/TLLAOs -- each with an Interface ID that corresponds to a specific
underlying interface of the AERO node.
3.6. AERO Interface Initialization
3.6.1. AERO Relay Behavior
When a Relay enables an AERO interface, it first assigns an
administratively-provisioned link-local address fe80::ID to the
interface. Each fe80::ID address MUST be unique among all AERO nodes
on the link, and is taken from the range fe80::/96 but excluding the
special addresses fe80:: and fe80::ffff:ffff. The Relay then engages
in a dynamic routing protocol session with all Servers on the link
(see: Section 3.3), and advertises its assigned ASPs into the native
IP Internetwork.
Each Relay subsequently maintains an IP forwarding table entry for
each active ACP covered by its ASP(s), and maintains a neighbor cache
entry for each Server on the link. Relays exchange NS/NA messages
with AERO link neighbors the same as for any AERO node, however they
typically do not perform explicit Neighbor Unreachability Detection
(NUD) (see: Section 3.16) since the dynamic routing protocol already
provides reachability confirmation.
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3.6.2. AERO Server Behavior
When a Server enables an AERO interface, it assigns an
administratively-provisioned link-local address fe80::ID the same as
for Relays. The Server further configures a DHCPv6 server function
to facilitate DHCPv6 PD exchanges with AERO Clients. The Server
maintains a neighbor cache entry for each Relay on the link, and
manages per-Client neighbor cache entries and IP forwarding table
entries based on control message exchanges. Each Server also engages
in a dynamic routing protocol with each Relay on the link (see:
Section 3.3).
When the Server receives an NS/RS message from a Client on the AERO
interface it returns an NA/RA message. The Server further provides a
simple link-layer conduit between AERO interface neighbors. In
particular, when a packet sent by a source Client arrives on the
Server's AERO interface and is destined to another AERO node, the
Server forwards the packet at the link layer without ever disturbing
the network layer and without ever leaving the AERO interface.
3.6.3. AERO Client Behavior
When a Client enables an AERO interface, it uses the special
administratively-provisioned link-local address fe80::ffff:ffff as
the source network-layer address in DHCPv6 PD messages to obtain one
or more ACPs from an AERO Server. Next, the Client assigns the base
AERO address to the AERO interface and sends an RS to the Server to
receive an RA. In this way, the DHCPv6 PD exchange securely
bootstraps autoconfiguration of unique link-local address(es) while
the RS/RA exchange establishes link-layer addresses and
autoconfigures AERO link parameters. The Client maintains a neighbor
cache entry for each of its Servers and each of its active
correspondent Clients. When the Client receives IPv6 ND messages on
the AERO interface it updates or creates neighbor cache entries,
including link-layer address information.
3.7. AERO Interface Neighbor Cache Maintenace
Each AERO interface maintains a conceptual neighbor cache that
includes an entry for each neighbor it communicates with on the AERO
link, the same as for any IPv6 interface [RFC4861]. AERO interface
neighbor cache entires are said to be one of "permanent", "static" or
"dynamic".
Permanent neighbor cache entries are created through explicit
administrative action; they have no timeout values and remain in
place until explicitly deleted. AERO Relays maintain a permanent
neighbor cache entry for each Server on the link, and AERO Servers
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maintain a permanent neighbor cache entry for each Relay. Each entry
maintains the mapping between the neighbor's fe80::ID network-layer
address and corresponding link-layer address.
Static neighbor cache entries are created and maintained through
DHCPv6 PD and IPv6 ND exchanges as specified in Section 3.14, and
remain in place for durations bounded by prefix delegation lifetimes.
AERO Servers maintain static neighbor cache entries for the ACPs of
each of their associated Clients, and AERO Clients maintain a static
neighbor cache entry for each of their associated Servers. When an
AERO Server delegates prefixes via DHCPv6 PD, it creates a static
neighbor cache entry for the Client using the Client's base AERO
address as the network-layer address and associates all of the
Client's other AERO addresses with the neighbor cache entry. When
the Client receives the prefix delegation, it creates a static
neighbor cache entry for the Server based on the DHCPv6 Reply message
link-local source address as the network-layer address and the
encapsulation IP source address and UDP source port number as the
link-layer address. The Client then sends an RS message to inform
the Server of its link-layer addresses and to solicit an RA. When
the Server returns an RA message, the Client uses the
autoconfiguration information in the RA message to configure AERO
interface parameters.
Dynamic neighbor cache entries are created or updated based on
receipt of Predirect/Redirect messages as specified in Section 3.15,
and are garbage-collected when keepalive timers expire. AERO Clients
maintain dynamic neighbor cache entries for each of their active
correspondent Clients with lifetimes based on IPv6 ND messaging
constants.
When an AERO Client receives a valid Predirect message it creates or
updates a dynamic neighbor cache entry for the Predirect target
network-layer and link-layer addresses. The node then sets an
"AcceptTime" variable in the neighbor cache entry to ACCEPT_TIME
seconds and uses this value to determine whether packets received
from the correspondent can be accepted. The node resets AcceptTime
when it receives a new Predirect, and otherwise decrements AcceptTime
while no Predirects have been received. It is RECOMMENDED that
ACCEPT_TIME be set to the default constant value 40 seconds to allow
a 10 second window so that the AERO redirection procedure can
converge before AcceptTime decrements below FORWARD_TIME (see below).
When an AERO Client receives a valid Redirect message it creates or
updates a dynamic neighbor cache entry for the Redirect target
network-layer and link-layer addresses. The Client then sets a
"ForwardTime" variable in the neighbor cache entry to FORWARD_TIME
seconds and uses this value to determine whether packets can be sent
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directly to the correspondent. The node resets ForwardTime when it
receives a new Redirect, and otherwise decrements ForwardTime while
no Redirects have been received. It is RECOMMENDED that FORWARD_TIME
be set to the default constant value 30 seconds to match the default
REACHABLE_TIME value specified for IPv6 ND [RFC4861].
The Client also sets a "MaxRetry" variable to MAX_RETRY to limit the
number of keepalives sent when a correspondent may have gone
unreachable. It is RECOMMENDED that MAX_RETRY be set to 3 the same
as described for IPv6 ND address resolution in Section 7.3.3 of
[RFC4861].
Different values for ACCEPT_TIME, FORWARD_TIME and MAX_RETRY MAY be
administratively set, if necessary, to better match the AERO link's
performance characteristics; however, if different values are chosen,
all nodes on the link MUST consistently configure the same values.
Most importantly, ACCEPT_TIME SHOULD be set to a value that is
sufficiently longer than FORWARD_TIME to allow the AERO redirection
procedure to converge.
When there may be a Network Address Translator (NAT) between the
Client and the Server, or if the path from the Client to the Server
should be tested for reachability, the Client can send periodic RS
messages to the Server to receive RA replies. The RS/RA messaging
will keep NAT state alive and test Server reachability without
disturbing the DHCPv6 server.
3.8. AERO Interface Forwarding Algorithm
IP packets enter a node's AERO interface either from the network
layer (i.e., from a local application or the IP forwarding system) or
from the link layer (i.e., from the AERO tunnel virtual link).
Packets that enter the AERO interface from the network layer are
encapsulated and forwarded into the AERO link, i.e., they are
tunnelled to an AERO interface neighbor. Packets that enter the AERO
interface from the link layer are either re-admitted into the AERO
link or forwarded to the network layer where they are subject to
either local delivery or IP forwarding. In all cases, the AERO
interface itself MUST NOT decrement the network layer TTL/Hop-count
since its forwarding actions occur below the network layer.
AERO interfaces may have multiple underlying interfaces and/or
neighbor cache entries for neighbors with multiple Interface ID
registrations (see Section 3.5). The AERO node uses each packet's
DSCP value to select an outgoing underlying interface based on the
node's own preference values, and also to select a destination link-
layer address based on the neighbor's underlying interface with the
highest preference value. If multiple outgoing interfaces and/or
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neighbor interfaces have a preference of "high", the AERO node sends
one copy of the packet via each of the (outgoing / neighbor)
interface pairs; otherwise, the node sends a single copy of the
packet.
The following sections discuss the AERO interface forwarding
algorithms for Clients, Servers and Relays. In the following
discussion, a packet's destination address is said to "match" if it
is a non-link-local address with a prefix covered by an ASP/ACP, or
if it is an AERO address that embeds an ACP, or if it is the same as
an administratively-provisioned link-local address.
3.8.1. Client Fowarding Algorithm
When an IP packet enters a Client's AERO interface from the network
layer the Client searches for a neighbor cache entry that matches the
destination. If there is a match, the Client uses one or more link-
layer addresses in the entry as the link-layer addresses for
encapsulation and admits the packet into the AERO link. Otherwise,
the Client uses the link-layer address in a static neighbor cache
entry for a Server as the encapsulation address.
When an IP packet enters a Client's AERO interface from the link-
layer, if the destination matches one of the Client's ACPs or link-
local addresses the Client decapsulates the packet and delivers it to
the network layer. Otherwise, the Client drops the packet silently.
3.8.2. Server Fowarding Algorithm
When an IP packet enters a Server's AERO interface from the network
layer, the Server searches for a static or dynamic neighbor cache
entry that matches the destination. If there is a match, the Server
uses one or more link-layer addresses in the entry as the link-layer
addresses for encapsulation and admits the packet into the AERO link.
Otherwise, the Server uses the link-layer address in a permanent
neighbor cache entry for a Relay (selected through longest-prefix
match) as the link-layer address for encapsulation.
When an IP packet enters a Server's AERO interface from the link
layer, the Server processes the packet as follows:
o if the destination matches one of the Server's own addresses the
Server decapsulates the packet and forwards it to the network
layer for local delivery.
o else, if the destination matches a static or dynamic neighbor
cache entry the Server first determines whether the neighbor is
the same as the one it received the packet from. If so, the
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Server MUST drop the packet silently to avoid looping; otherwise,
the Server uses the neighbor's link-layer address(es) as the
destination for encapsulation and re-admits the packet into the
AERO link.
o else, the Server uses the link-layer address in a permanent
neighbor cache entry for a Relay (selected through longest-prefix
match) as the link-layer address for encapsulation.
3.8.3. Relay Fowarding Algorithm
When an IP packet enters a Relay's AERO interface from the network
layer, the Relay searches its IP forwarding table for an ACP entry
that matches the destination and otherwise searches for a neighbor
cache entry that matches the destination. If there is a match, the
Relay uses the link-layer address in the corresponding neighbor cache
entry as the link-layer address for encapsulation and forwards the
packet into the AERO link. Otherwise, the Relay drops the packet and
(for non-link-local addresses) returns an ICMP Destination
Unreachable message subject to rate limiting (see: Section 3.13).
When an IP packet enters a Relay's AERO interface from the link-
layer, the Relay processes the packet as follows:
o if the destination does not match an ASP, or if the destination
matches one of the Relay's own addresses, the Relay decapsulates
the packet and forwards it to the network layer where it will be
subject to either local delivery or IP forwarding.
o else, if the destination matches an ACP entry in the IP forwarding
table, or if the destination matches the link-local address in a
permanent neighbor cache entry, the Relay first determines whether
the neighbor is the same as the one it received the packet from.
If so the Relay MUST drop the packet silently to avoid looping;
otherwise, the Relay uses the neighbor's link-layer address as the
destination for encapsulation and re-admits the packet into the
AERO link.
o else, the Relay drops the packet and (for non-link-local
addresses) returns an ICMP Destination Unreachable message subject
to rate limiting (see: Section 3.13).
3.9. AERO Interface Encapsulation and Re-encapsulation
AERO interfaces encapsulate IP packets according to whether they are
entering the AERO interface from the network layer or if they are
being re-admitted into the same AERO link they arrived on. This
latter form of encapsulation is known as "re-encapsulation".
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The AERO interface encapsulates packets per the Generic UDP
Encapsulation (GUE) procedures in
[I-D.ietf-nvo3-gue][I-D.herbert-gue-fragmentation], or through an
alternate encapsulation format (see: Appendix A). For packets
entering the AERO interface from the network layer, the AERO
interface copies the "TTL/Hop Limit", "Type of Service/Traffic Class"
[RFC2983], "Flow Label"[RFC6438].(for IPv6) and "Congestion
Experienced" [RFC3168] values in the packet's IP header into the
corresponding fields in the encapsulation IP header. For packets
undergoing re-encapsulation, the AERO interface instead copies these
values from the original encapsulation IP header into the new
encapsulation header, i.e., the values are transferred between
encapsulation headers and *not* copied from the encapsulated packet's
network-layer header. (Note especially that by copying the TTL/Hop
Limit between encapsulation headers the value will eventually
decrement to 0 if there is a (temporary) routing loop.) For IPv4
encapsulation/re-encapsulation, the AERO interface sets the DF bit as
discussed in Section 3.12.
When GUE encapsulation is used, the AERO interface next sets the UDP
source port to a constant value that it will use in each successive
packet it sends, and sets the UDP length field to the length of the
encapsulated packet plus 8 bytes for the UDP header itself plus the
length of the GUE header (or 0 if GUE direct IP encapsulation is
used). For packets sent to a Server or Relay, the AERO interface
sets the UDP destination port to 8060, i.e., the IANA-registered port
number for AERO. For packets sent to a Client, the AERO interface
sets the UDP destination port to the port value stored in the
neighbor cache entry for this Client. The AERO interface then either
includes or omits the UDP checksum according to the GUE
specification.
3.10. AERO Interface Decapsulation
AERO interfaces decapsulate packets destined either to the AERO node
itself or to a destination reached via an interface other than the
AERO interface the packet was received on. Decapsulation is per the
procedures specified for the appropriate encapsulation format.
3.11. AERO Interface Data Origin Authentication
AERO nodes employ simple data origin authentication procedures for
encapsulated packets they receive from other nodes on the AERO link.
In particular:
o AERO Servers and Relays accept encapsulated packets with a link-
layer source address that matches a permanent neighbor cache
entry.
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o AERO Servers accept authentic encapsulated DHCPv6 and IPv6 ND
messages from Clients, and create or update a static neighbor
cache entry for the Client based on the specific message type.
o AERO Clients and Servers accept encapsulated packets if there is a
static neighbor cache entry with a link-layer address that matches
the packet's link-layer source address.
o AERO Clients and Servers accept encapsulated packets if there is a
dynamic neighbor cache entry with an AERO address that matches the
packet's network-layer source address, with a link-layer address
that matches the packet's link-layer source address, and with a
non-zero AcceptTime.
Note that this simple data origin authentication is effective in
environments in which link-layer addresses cannot be spoofed. In
other environments, each AERO message must include a signature that
the recipient can use to authenticate the message origin, e.g., as
for common VPN systems such as OpenVPN [OVPN]. In environments where
end systems use end-to-end security, however, it may be sufficient to
require signatures only for AERO DHCPv6, IPv6 ND and ICMP control
plane messages and omit signatures for data plane messages.
3.12. AERO Interface Packet Size Issues
The AERO interface is the node's attachment to the AERO link. The
AERO interface acts as a tunnel ingress when it sends a packet to an
AERO link neighbor and as a tunnel egress when it receives a packet
from an AERO link neighbor. AERO interfaces observe the packet
sizing considerations for tunnels discussed in
[I-D.ietf-intarea-tunnels] and as specified below.
The Internet Protocol expects that IP packets will either be
delivered to the destination or a suitable Packet Too Big (PTB)
message returned to support the process known as IP Path MTU
Discovery (PMTUD) [RFC1191][RFC1981]. However, PTB messages may be
crafted for malicious purposes such as denial of service, or lost in
the network [RFC2923]. This can be especially problematic for
tunnels, where a condition known as a PMTUD "black hole" can result.
For these reasons, AERO interfaces employ operational procedures that
avoid interactions with PMTUD, including the use of fragmentation
when necessary.
AERO interfaces observe two different types of fragmentation. Source
fragmentation occurs when the AERO interface (acting as a tunnel
ingress) fragments the encapsulated packet into multiple fragments
before admitting each fragment into the tunnel. Network
fragmentation occurs when an encapsulated packet admitted into the
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tunnel by the ingress is fragmented by an IPv4 router on the path to
the egress. Note that a packet that incurs source fragmentation may
also incur network fragmentation.
IPv6 specifies a minimum link Maximum Transmission Unit (MTU) of 1280
bytes [RFC2460]. Although IPv4 specifies a smaller minimum link MTU
of 68 bytes [RFC0791], AERO interfaces also observe the IPv6 minimum
for IPv4 even if encapsulated packets may incur network
fragmentation.
IPv6 specifies a minimum Maximum Reassembly Unit (MRU) of 1500 bytes
[RFC2460], while the minimum MRU for IPv4 is only 576 bytes [RFC1122]
(note that common IPv6 over IPv4 tunnels already assume a larger MRU
than the IPv4 minimum).
AERO interfaces therefore configure an MTU that MUST NOT be smaller
than 1280 bytes, MUST NOT be larger than the minimum MRU among all
nodes on the AERO link minus the encapsulation overhead ("ENCAPS"),
and SHOULD NOT be smaller than 1500 bytes. AERO interfaces also
configure a Maximum Segment Unit (MSU) as the maximum-sized
encapsulated packet that the ingress can inject into the tunnel
without source fragmentation. The MSU value MUST NOT be larger than
(MTU+ENCAPS) and MUST NOT be larger than 1280 bytes unless there is
operational assurance that a larger size can traverse the link along
all paths.
All AERO nodes MUST configure the same MTU/MSU values for reasons
cited in [RFC3819][RFC4861]; in particular, multicast support
requires a common MTU value among all nodes on the link. All AERO
nodes MUST configure an MRU large enough to reassemble packets up to
(MTU+ENCAPS) bytes in length; nodes that cannot configure a large-
enough MRU MUST NOT enable an AERO interface.
The network layer proceeds as follow when it presents an IP packet to
the AERO interface. For each IPv4 packet that is larger than the
AERO interface MTU and with the DF bit set to 0, the network layer
uses IPv4 fragmentation to break the packet into a minimum number of
non-overlapping fragments where the first fragment is no larger than
the MTU and the remaining fragments are no larger than the first.
For all other IP packets, if the packet is larger than the AERO
interface MTU, the network layer drops the packet and returns a PTB
message to the original source. Otherwise, the network layer admits
each IP packet or fragment into the AERO interface.
For each IP packet admitted into the AERO interface, the interface
(acting as a tunnel ingress) encapsulates the packet. If the
encapsulated packet is larger than the AERO interface MSU the ingress
source-fragments the encapsulated packet into a minimum number of
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non-overlapping fragments where the first fragment is no larger than
the MSU and the remaining fragments are no larger than the first.
The ingress then admits each encapsulated packet or fragment into the
tunnel, and for IPv4 sets the DF bit to 0 in the IP encapsulation
header in case any network fragmentation is necessary. The
encapsulated packets will be delivered to the egress, which
reassembles them into a whole packet if necessary.
Several factors must be considered when fragmentation is needed. For
AERO links over IPv4, the IP ID field is only 16 bits in length,
meaning that fragmentation at high data rates could result in data
corruption due to reassembly misassociations [RFC6864][RFC4963]. For
AERO links over both IPv4 and IPv6, studies have also shown that IP
fragments are dropped unconditionally over some network paths [I-
D.taylor-v6ops-fragdrop]. In environments where IP fragmentation
issues could result in operational problems, the ingress SHOULD
employ intermediate-layer source fragmentation (see: [RFC2764] and
[I-D.herbert-gue-fragmentation]) before appending the outer
encapsulation headers to each fragment. Since the encapsulation
fragment header reduces the room available for packet data, but the
original source has no way to control its insertion, the ingress MUST
include the fragment header length in the ENCAPS length even for
packets in which the header is absent.
3.13. AERO Interface Error Handling
When an AERO node admits encapsulated packets into the AERO
interface, it may receive link-layer or network-layer error
indications.
A link-layer error indication is an ICMP error message generated by a
router on the path to the neighbor or by the neighbor itself. The
message includes an IP header with the address of the node that
generated the error as the source address and with the link-layer
address of the AERO node as the destination address.
The IP header is followed by an ICMP header that includes an error
Type, Code and Checksum. Valid type values include "Destination
Unreachable", "Time Exceeded" and "Parameter Problem"
[RFC0792][RFC4443]. (AERO interfaces ignore all link-layer IPv4
"Fragmentation Needed" and IPv6 "Packet Too Big" messages since they
only emit packets that are guaranteed to be no larger than the IP
minimum link MTU as discussed in Section 3.12.)
The ICMP header is followed by the leading portion of the packet that
generated the error, also known as the "packet-in-error". For
ICMPv6, [RFC4443] specifies that the packet-in-error includes: "As
much of invoking packet as possible without the ICMPv6 packet
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exceeding the minimum IPv6 MTU" (i.e., no more than 1280 bytes). For
ICMPv4, [RFC0792] specifies that the packet-in-error includes:
"Internet Header + 64 bits of Original Data Datagram", however
[RFC1812] Section 4.3.2.3 updates this specification by stating: "the
ICMP datagram SHOULD contain as much of the original datagram as
possible without the length of the ICMP datagram exceeding 576
bytes".
The link-layer error message format is shown in Figure 3 (where, "L2"
and "L3" refer to link-layer and network-layer, respectively):
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
| L2 IP Header of |
| error message |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L2 ICMP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
~ ~ P
| IP and other encapsulation | a
| headers of original L3 packet | c
~ ~ k
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ e
~ ~ t
| IP header of |
| original L3 packet | i
~ ~ n
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~ e
| Upper layer headers and | r
| leading portion of body | r
| of the original L3 packet | o
~ ~ r
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
Figure 3: AERO Interface Link-Layer Error Message Format
The AERO node rules for processing these link-layer error messages
are as follows:
o When an AERO node receives a link-layer Parameter Problem message,
it processes the message the same as described as for ordinary
ICMP errors in the normative references [RFC0792][RFC4443].
o When an AERO node receives persistent link-layer Time Exceeded
messages, the IP ID field may be wrapping before earlier fragments
awaiting reassembly have been processed. In that case, the node
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SHOULD begin including integrity checks and/or institute rate
limits for subsequent packets.
o When an AERO node receives persistent link-layer Destination
Unreachable messages in response to encapsulated packets that it
sends to one of its dynamic neighbor correspondents, the node
SHOULD test the path to the correspondent using Neighbor
Unreachability Detection (NUD) (see Section 3.16). If NUD fails,
the node SHOULD set ForwardTime for the corresponding dynamic
neighbor cache entry to 0 and allow future packets destined to the
correspondent to flow through a default route.
o When an AERO Client receives persistent link-layer Destination
Unreachable messages in response to encapsulated packets that it
sends to one of its static neighbor Servers, the Client SHOULD
test the path to the Server using NUD. If NUD fails, the Client
SHOULD associate with a new Server and send a DHCPv6 Release
message to the old Server as specified in Section 3.17.5.
o When an AERO Server receives persistent link-layer Destination
Unreachable messages in response to encapsulated packets that it
sends to one of its static neighbor Clients, the Server SHOULD
test the path to the Client using NUD. If NUD fails, the Server
SHOULD cancel the DHCPv6 PD for the Client's ACP, withdraw its
route for the ACP from the AERO routing system and delete the
neighbor cache entry (see Section 3.16 and Section 3.17).
o When an AERO Relay or Server receives link-layer Destination
Unreachable messages in response to an encapsulated packet that it
sends to one of its permanent neighbors, it treats the messages as
an indication that the path to the neighbor may be failing.
However, neighbor reachability will be determined by the dynamic
routing protocol.
When an AERO Relay receives a packet for which the network-layer
destination address is covered by an ASP, if there is no more-
specific routing information for the destination the Relay drops the
packet and returns a network-layer Destination Unreachable message
subject to rate limiting. The Relay first writes the network-layer
source address of the original packet as the destination address of
the message and determines the next hop to the destination. If the
next hop is reached via the AERO interface, the Relay uses the IPv6
address "::" or the IPv4 address "0.0.0.0" as the source address of
the message, then encapsulates the message and forwards it to the
next hop within the AERO interface. Otherwise, the Relay uses one of
its non link-local addresses as the source address of the message and
forwards it via a link outside the AERO interface.
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When an AERO node receives an encapsulated packet for which the
reassembly buffer it too small, it drops the packet and returns an
network-layer Packet Too Big (PTB) message. The node first writes
the MRU value into the PTB message MTU field, writes the network-
layer source address of the original packet as the destination
address of the message and determines the next hop to the
destination. If the next hop is reached via the AERO interface, the
node uses the IPv6 address "::" or the IPv4 address "0.0.0.0" as the
source address of the message, then encapsulates the message and
forwards it to the next hop within the AERO interface. Otherwise,
the node uses one of its non link-local addresses as the source
address of the message and forwards it via a link outside the AERO
interface.
When an AERO node receives any network-layer error message via the
AERO interface, it examines the network-layer destination address.
If the next hop toward the destination is via the AERO interface, the
node re-encapsulates and forwards the message to the next hop within
the AERO interface. Otherwise, if the network-layer source address
is the IPv6 address "::" or the IPv4 address "0.0.0.0", the node
writes one of its non link-local addresses as the source address,
recalculates the IP and/or ICMP checksums then forwards the message
via a link outside the AERO interface.
3.14. AERO Router Discovery, Prefix Delegation and Autoconfiguration
AERO Router Discovery, Prefix Delegation and Autoconfiguration are
coordinated by the DHCPv6 and IPv6 ND control messaging protocols as
discussed in the following Sections.
3.14.1. AERO DHCPv6 and IPv6 ND Service Model
Each AERO Server configures a DHCPv6 server function to facilitate PD
requests from Clients. Each Server is provisioned with a database of
ACP-to-Client ID mappings for all Clients enrolled in the AERO
system, as well as any information necessary to authenticate each
Client. The Client database is maintained by a central
administrative authority for the AERO link and securely distributed
to all Servers, e.g., via the Lightweight Directory Access Protocol
(LDAP) [RFC4511], via static configuration, etc.
Therefore, no Server-to-Server DHCPv6 PD state synchronization is
necessary, and Clients can optionally hold separate PDs for the same
ACPs from multiple Servers. In this way, Clients can associate with
multiple Servers, and can receive new PDs from new Servers before
deprecating PDs received from existing Servers. This provides the
Client with a natural fault-tolerance and/or load balancing profile.
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AERO Clients and Servers use unicast IPv6 ND messages to maintain
neighbor cache entries the same as for any link. AERO Servers act as
default routers for AERO Clients, and therefore send unicast RA
messages with configuration information in response to a Client's RS
message.
The following sections specify the Client and Server behavior.
3.14.2. AERO Client Behavior
AERO Clients discover the link-layer addresses of AERO Servers via
static configuration (e.g., from a flat-file map of Server addresses
and locations), or through an automated means such as DNS name
resolution. In the absence of other information, the Client resolves
the FQDN "linkupnetworks.[domainname]" where "linkupnetworks" is a
constant text string and "[domainname]" is a DNS suffix for the
Client's underlying network (e.g., "example.com"). After discovering
the link-layer addresses, the Client associates with one or more of
the corresponding Servers.
To associate with a Server, the Client acts as a requesting router to
request ACPs through a DHCPv6 PD exchange [RFC3315][RFC3633]. The
Client's DHCPv6 Solicit message includes fe80::ffff:ffff as the IPv6
source address, 'All_DHCP_Relay_Agents_and_Servers' as the IPv6
destination address, the address of the Client's underlying interface
as the link-layer source address and the link-layer address of the
Server as the link-layer destination address. The Client also
includes a Client Identifier option with the Client's DUID, and an
Identity Association for Prefix Delegation (IA_PD) option. If the
Client is pre-provisioned with ACPs associated with the AERO service,
it MAY also include the ACPs in the IA_PD to indicate its preferences
to the DHCPv6 server. The Client finally includes any additional
DHCPv6 options (including any necessary authentication options to
identify itself to the DHCPv6 server), and sends the encapsulated
Solicit message via any available underlying interface.
When the Client attempts to perform a DHCPv6 PD exchange with a
Server that is too busy to service the request, the Client may
receive an error status code such as "NoPrefixAvail" in the Server's
Reply [RFC3633] or no Reply at all. In that case, the Client SHOULD
discontinue DHCPv6 PD attempts through this Server and try another
Server. When the Client receives a Reply from the AERO Server it
creates a static neighbor cache entry with the Server's link-local
address as the network-layer address and the Server's encapsulation
address as the link-layer address. Next, the Client autoconfigures
AERO addresses for each of the delegated ACPs and assigns the base
AERO address to the AERO interface.
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The Client then prepares a unicast RS message to send to the Server
in order to obtain a solicited RA. The Client includes its base AERO
address as the network-layer source address, the Server's link-local
address as the network-layer destination address, the Client's link-
layer address as the link-layer source address, and Server's link-
layer address as the link-layer destination address. The Client also
includes one or more SLLAOs formatted as described in Section 3.5 to
register its link-layer address(es) with the Server.
The first SLLAO MUST correspond to the underlying interface over
which the Client will send the RS. The Client MAY include additional
SLLAOs specific to other underlying interfaces, but if so it MUST
have assurance that there will be no NATs on the paths to the Server
via those interfaces (otherwise, the Client can register additional
link-layer addresses with the Server by sending subsequent
unsolicited NA messages after the initial RS/RA exchange). The
Server will use the S/TLLAOs to populate its link-layer address
information for the Client.
When the Client receives an RA from the AERO Server (see
Section 3.14.3), it configures a default route with the Server as the
next hop via the AERO interface. The Client next examines the Code
value in the RA message; if Code was 1 the Client can assume there
was a NAT on the path to the Server. The Client also caches any ASPs
included in Prefix Information Options (PIOs) as ASPs to associate
with the AERO link, and assigns the MTU/MSU values in the MTU options
to its AERO interface while configuring an appropriate MRU. This
configuration information applies to the AERO link as a whole, and
all AERO nodes will use the same values.
Following autoconfiguration, the Client sub-delegates the ACPs to its
attached EUNs and/or the Client's own internal virtual interfaces.
In the former case, the Client acts as a router for nodes on its
attached EUNs. In the latter case, the Client acts as a host and can
configure as many addresses as it wants from /64 prefixes taken from
the ACPs and assign them to either an internal virtual interface
("weak end-system") or to the AERO interface itself ("strong end-
system") [RFC1122] while black-holing the remaining portions of the
/64s. The Client subsequently renews its ACP delegations through
each of its Servers by sending DHCPv6 Renew messages.
After the Client registers its Interface IDs and their associated
'P(i)' values, it may wish to change one or more Interface ID
registrations, e.g., if an underlying interface becomes unavailable,
if cost profiles change, etc. To do so, the Client prepares an
unsolicited NA message to send over any available underlying
interface. The NA MUST include a S/TLLAO specific to the selected
available underlying interface as the first S/TLLAO and MAY include
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any additional S/TLLAOs specific to other underlying interfaces. The
Client includes fresh 'P(i)' values in each S/TLLAO to update the
Server's neighbor cache entry. If the Client wishes to disable some
or all DSCPs for an underlying interface, it includes an S/TLLAO with
'P(i)' values set to 0 ("disabled").
If the Client wishes to discontinue use of a Server it issues a
DHCPv6 Release message to both delete the Server's neighbor cache
entry and release the DHCPv6 PD.
3.14.3. AERO Server Behavior
AERO Servers configure a DHCPv6 server function on their AERO links.
AERO Servers arrange to add their encapsulation layer IP addresses
(i.e., their link-layer addresses) to a static map of Server
addresses for the link and/or the DNS resource records for the FQDN
"linkupnetworks.[domainname]" before entering service.
When an AERO Server receives a prospective Client's Solicit on its
AERO interface, and the Server is too busy to service the message, it
SHOULD return a Reply with status code "NoPrefixAvail" per [RFC3633].
Otherwise, the Server authenticates the message. If authentication
succeeds, the Server determines the correct ACPs to delegate to the
Client by searching the Client database.
Next, the Server prepares a Reply message to send to the Client while
using fe80::ID as the network-layer source address, the link-local
address taken from the Client's Solicit as the network-layer
destination address, the Server's link-layer address as the source
link-layer address, and the Client's link-layer address as the
destination link-layer address. The Server also includes an IA_PD
option with the delegated ACPs. For IPv4 ACPs, the prefix included
in the IA_PD option is in IPv4-mapped IPv6 address format and with
prefix length set as specified in Section 3.4.
When the Server sends the Reply message, it creates a static neighbor
cache entry for the Client using the base AERO address as the
network-layer address and with lifetime set to no more than the
smallest PD lifetime. The Client will subsequently issue an RS
message with one or more SLLAO options and with the Client's base
AERO address as the source address.
When the Server receives the RS message, it first verifies that a
neighbor cache entry for the Client exists (otherwise, it discards
the RS). The Server then updates the neighbor cache entry link-layer
address(es) by recording the information in each SLLAO option indexed
by the Interface ID and including the UDP port number, IP address and
P(i) values. For the first SLLAO in the list, however, the Server
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records the actual encapsulation source UDP and IP addresses instead
of those that appear in the SLLAO in case there was a NAT in the
path.
The Server then prepares a unicast RA message to send back to the
Client using fe80::ID as the network-layer source address, the
Client's base AERO address as the network-layer destination address,
the Server's link-layer address as the source link-layer address, and
the source link-layer address of the RS message as the destination
link-layer address. In the RA message, if the actual encapsulation
addresses in the RS message were the same as those that appeared in
the first SLLAO (see above), the Server sets the Code field to 0;
otherwise it sets Code to 1. The Server then includes one or more
PIOs that encode the ASPs for the AERO link, and with flags A=0; L=1.
The Server also includes two MTU options - the first MTU option
includes the MTU for the link and the second MTU option includes the
MSU for the link (see Section 3.12).
When the Server delegates the ACPs, it also creates an IP forwarding
table entry for each ACP so that the AERO BGP-based routing system
will propagate the ACPs to all Relays that aggregate the
corresponding ASP (see: Section 3.3).
After the initial DHCPv6 PD Solicit/Reply and IPv6 ND RS/RA
exchanges, the AERO Server maintains the neighbor cache entry for the
Client until the PD lifetimes expire. If the Client issues a Renew,
the Server extends the PD lifetimes. If the Client issues a Release,
or if the Client does not issue a Renew before the lifetime expires,
the Server deletes the neighbor cache entry for the Client and
withdraws the IP routes from the AERO routing system.
3.14.3.1. Lightweight DHCPv6 Relay Agent (LDRA)
AERO Clients and Servers are always on the same link (i.e., the AERO
link) from the perspective of DHCPv6. However, in some
implementations the DHCPv6 server and AERO interface driver may be
located in separate modules. In that case, the Server's AERO
interface driver module can act as a Lightweight DHCPv6 Relay Agent
(LDRA)[RFC6221] to relay DHCPv6 messages to and from the DHCPv6
server module.
When the LDRA receives a DHCPv6 message from a Client addressed to
either 'All_DHCP_Relay_Agents_and_Servers' or the Server's fe80::ID
unicast address, it wraps the message in a Relay-Forward message
header and includes an Interface-ID option that includes enough
information to allow the LDRA to forward the resulting Reply message
back to the Client (this information may include the Client's UDP and
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IP addresses, a security association identifier, etc). The LDRA then
forwards the message to the DHCPv6 server.
When the DHCPv6 server prepares a Reply message, it wraps the message
in a Relay-Reply message and echoes the Interface-ID option. The
DHCPv6 server then delivers the Relay-Reply message to the LDRA,
which discards the Relay-Reply wrapper and delivers the DHCPv6
message to the Client based on the information in the Interface ID
option.
3.15. AERO Interface Route Optimization
When a source Client forwards packets to a prospective correspondent
Client within the same AERO link domain (i.e., one for which the
packet's destination address is covered by an ASP), the source Client
MAY initiate an AERO link route optimization procedure. The
procedure is based on an exchange of IPv6 ND messages using a chain
of AERO Servers and Relays as a trust basis.
Although the Client is responsible for initiating route optimization,
the Server is the policy enforcement point that determines whether
route optimization is permitted. For example, on some AERO links
route optimization would allow traffic to circumvent critical
network-based traffic interception points. In those cases, the
Server can simply discard any route optimization messages instead of
forwarding them.
The following sections specify the AERO link route optimization
procedure.
3.15.1. Reference Operational Scenario
Figure 4 depicts the AERO link route optimization reference
operational scenario, using IPv6 addressing as the example (while not
shown, a corresponding example for IPv4 addressing can be easily
constructed). The figure shows an AERO Relay ('R1'), two AERO
Servers ('S1', 'S2'), two AERO Clients ('C1', 'C2') and two ordinary
IPv6 hosts ('H1', 'H2'):
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+--------------+ +--------------+ +--------------+
| Server S1 | | Relay R1 | | Server S2 |
+--------------+ +--------------+ +--------------+
fe80::2 fe80::1 fe80::3
L2(S1) L2(R1) L2(S2)
| | |
X-----+-----+------------------+-----------------+----+----X
| AERO Link |
L2(C1) L2(C2)
fe80::2001:db8:0:0 fe80::2001:db8:1:0
+--------------+ +--------------+
|AERO Client C1| |AERO Client C2|
+--------------+ +--------------+
2001:DB8:0::/48 2001:DB8:1::/48
| |
.-. .-.
,-( _)-. 2001:db8:0::1 2001:db8:1::1 ,-( _)-.
.-(_ IP )-. +-------+ +-------+ .-(_ IP )-.
(__ EUN )--|Host H1| |Host H2|--(__ EUN )
`-(______)-' +-------+ +-------+ `-(______)-'
Figure 4: AERO Reference Operational Scenario
In Figure 4, Relay ('R1') assigns the administratively-provisioned
link-local address fe80::1 to its AERO interface with link-layer
address L2(R1), Server ('S1') assigns the address fe80::2 with link-
layer address L2(S1),and Server ('S2') assigns the address fe80::3
with link-layer address L2(S2). Servers ('S1') and ('S2') next
arrange to add their link-layer addresses to a published list of
valid Servers for the AERO link.
AERO Client ('C1') receives the ACP 2001:db8:0::/48 in a DHCPv6 PD
exchange via AERO Server ('S1') then assigns the address
fe80::2001:db8:0:0 to its AERO interface with link-layer address
L2(C1). Client ('C1') configures a default route and neighbor cache
entry via the AERO interface with next-hop address fe80::2 and link-
layer address L2(S1), then sub-delegates the ACP to its attached
EUNs. IPv6 host ('H1') connects to the EUN, and configures the
address 2001:db8:0::1.
AERO Client ('C2') receives the ACP 2001:db8:1::/48 in a DHCPv6 PD
exchange via AERO Server ('S2') then assigns the address
fe80::2001:db8:1:0 to its AERO interface with link-layer address
L2(C2). Client ('C2') configures a default route and neighbor cache
entry via the AERO interface with next-hop address fe80::3 and link-
layer address L2(S2), then sub-delegates the ACP to its attached
EUNs. IPv6 host ('H2') connects to the EUN, and configures the
address 2001:db8:1::1.
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3.15.2. Concept of Operations
Again, with reference to Figure 4, when source host ('H1') sends a
packet to destination host ('H2'), the packet is first forwarded over
the source host's attached EUN to Client ('C1'). Client ('C1') then
forwards the packet via its AERO interface to Server ('S1') and also
sends a Predirect message toward Client ('C2') via Server ('S1').
Server ('S1') then re-encapsulates and forwards both the packet and
the Predirect message out the same AERO interface toward Client
('C2') via Relay ('R1').
When Relay ('R1') receives the packet and Predirect message, it
consults its forwarding table to discover Server ('S2') as the next
hop toward Client ('C2'). Relay ('R1') then forwards both the packet
and the Predirect message to Server ('S2'), which then forwards them
to Client ('C2').
After Client ('C2') receives the Predirect message, it process the
message and returns a Redirect message toward Client ('C1') via
Server ('S2'). During the process, Client ('C2') also creates or
updates a dynamic neighbor cache entry for Client ('C1').
When Server ('S2') receives the Redirect message, it re-encapsulates
the message and forwards it on to Relay ('R1'), which forwards the
message on to Server ('S1') which forwards the message on to Client
('C1'). After Client ('C1') receives the Redirect message, it
processes the message and creates or updates a dynamic neighbor cache
entry for Client ('C2').
Following the above Predirect/Redirect message exchange, forwarding
of packets from Client ('C1') to Client ('C2') without involving any
intermediate nodes is enabled. The mechanisms that support this
exchange are specified in the following sections.
3.15.3. Message Format
AERO Redirect/Predirect messages use the same format as for IPv6 ND
Redirect messages depicted in Section 4.5 of [RFC4861]. AERO
Redirect/Predirect messages formats are identical except that
Redirect messages use Code=0, while Predirect messages use Code=1.
The Redirect/Predirect message format is shown in Figure 5:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (=137) | Code (=0/1) | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Target Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Destination Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
Figure 5: AERO Redirect/Predirect Message Format
3.15.4. Sending Predirects
When a Client forwards a packet with a source address from one of its
ACPs toward a destination address covered by an ASP (i.e., toward
another AERO Client connected to the same AERO link), the source
Client MAY send a Predirect message forward toward the destination
Client via the Server.
In the reference operational scenario, when Client ('C1') forwards a
packet toward Client ('C2'), it MAY also send a Predirect message
forward toward Client ('C2'), subject to rate limiting (see
Section 8.2 of [RFC4861]). Client ('C1') prepares the Predirect
message as follows:
o the link-layer source address is set to 'L2(C1)' (i.e., the link-
layer address of Client ('C1')).
o the link-layer destination address is set to 'L2(S1)' (i.e., the
link-layer address of Server ('S1')).
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o the network-layer source address is set to fe80::2001:db8:0:0
(i.e., the base AERO address of Client ('C1')).
o the network-layer destination address is set to the AERO address
corresponding to the destination address of Client ('C2').
o the Type is set to 137.
o the Code is set to 1 to indicate "Predirect".
o the Target Address is set to fe80::2001:db8:0:0 (i.e., the base
AERO address of Client ('C1')).
o the Destination Address is set to the source address of the
originating packet that triggered the Predirection event. (If the
originating packet is an IPv4 packet, the address is constructed
in IPv4-mapped IPv6 address format).
o the message includes one or more TLLAOs set to appropriate values
for Client ('C1')'s underlying interfaces.
o the message includes one or more Route Information Options (RIOs)
[RFC4191] that include Client ('C1')'s ACPs.
o the message SHOULD include a Timestamp option and a Nonce option.
o the message includes a Redirected Header Option (RHO) that
contains the originating packet truncated if necessary to ensure
that at least the network-layer header is included but the size of
the message does not exceed 1280 bytes.
Note that the act of sending Predirect messages is cited as "MAY",
since Client ('C1') may have advanced knowledge that the direct path
to Client ('C2') would be unusable or otherwise undesirable. If the
direct path later becomes unusable after the initial route
optimization, Client ('C1') simply allows packets to again flow
through Server ('S1').
3.15.5. Re-encapsulating and Relaying Predirects
When Server ('S1') receives a Predirect message from Client ('C1'),
it first verifies that the TLLAOs in the Predirect are a proper
subset of the Interface IDs in Client ('C1')'s neighbor cache entry.
If the Client's TLLAOs are not acceptable, Server ('S1') discards the
message. Otherwise, Server ('S1') validates the message according to
the Redirect message validation rules in Section 8.1 of [RFC4861],
except that the Predirect has Code=1. Server ('S1') also verifies
that Client ('C1') is authorized to use the ACPs encoded in the RIOs
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of the Predirect. If validation fails, Server ('S1') discards the
Predirect; otherwise, it copies the correct UDP Port number and IP
Address for Client ('C1')'s underlying link into the first TLLAO in
case the addresses have been subject to NAT.
Server ('S1') then examines the network-layer destination address of
the Predirect to determine the next hop toward Client ('C2') by
searching for the AERO address in the neighbor cache. Since Client
('C2') is not one of its neighbors, Server ('S1') re-encapsulates the
Predirect and relays it via Relay ('R1') by changing the link-layer
source address of the message to 'L2(S1)' and changing the link-layer
destination address to 'L2(R1)'. Server ('S1') finally forwards the
re-encapsulated message to Relay ('R1') without decrementing the
network-layer TTL/Hop Limit field.
When Relay ('R1') receives the Predirect message from Server ('S1')
it determines that Server ('S2') is the next hop toward Client ('C2')
by consulting its forwarding table. Relay ('R1') then re-
encapsulates the Predirect while changing the link-layer source
address to 'L2(R1)' and changing the link-layer destination address
to 'L2(S2)'. Relay ('R1') then relays the Predirect via Server
('S2').
When Server ('S2') receives the Predirect message from Relay ('R1')
it determines that Client ('C2') is a neighbor by consulting its
neighbor cache. Server ('S2') then re-encapsulates the Predirect
while changing the link-layer source address to 'L2(S2)' and changing
the link-layer destination address to 'L2(C2)'. Server ('S2') then
forwards the message to Client ('C2').
3.15.6. Processing Predirects and Sending Redirects
When Client ('C2') receives the Predirect message, it accepts the
Predirect only if the message has a link-layer source address of one
of its Servers (e.g., L2(S2)). Client ('C2') further accepts the
message only if it is willing to serve as a redirection target.
Next, Client ('C2') validates the message according to the Redirect
message validation rules in Section 8.1 of [RFC4861], except that it
accepts the message even though Code=1 and even though the network-
layer source address is not that of it's current first-hop router.
In the reference operational scenario, when Client ('C2') receives a
valid Predirect message, it either creates or updates a dynamic
neighbor cache entry that stores the Target Address of the message as
the network-layer address of Client ('C1') , stores the link-layer
addresses found in the TLLAOs as the link-layer addresses of Client
('C1'), and stores the ACPs encoded in the RIOs of the Predirect as
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the ACPs for Client ('C1'). Client ('C2') then sets AcceptTime for
the neighbor cache entry to ACCEPT_TIME.
After processing the message, Client ('C2') prepares a Redirect
message response as follows:
o the link-layer source address is set to 'L2(C2)' (i.e., the link-
layer address of Client ('C2')).
o the link-layer destination address is set to 'L2(S2)' (i.e., the
link-layer address of Server ('S2')).
o the network-layer source address is set to fe80::2001:db8:1:0
(i.e., the base AERO address of Client ('C2')).
o the network-layer destination address is set to fe80::2001:db8:0:0
(i.e., the base AERO address of Client ('C1')).
o the Type is set to 137.
o the Code is set to 0 to indicate "Redirect".
o the Target Address is set to fe80::2001:db8:1:0 (i.e., the base
AERO address of Client ('C2')).
o the Destination Address is set to the destination address of the
originating packet that triggered the Redirection event. (If the
originating packet is an IPv4 packet, the address is constructed
in IPv4-mapped IPv6 address format).
o the message includes one or more TLLAOs set to appropriate values
for Client ('C2')'s underlying interfaces.
o the message includes one or more Route Information Options (RIOs)
that include Client ('C2')'s ACPs.
o the message SHOULD include a Timestamp option and MUST echo the
Nonce option received in the Predirect (i.e., if a Nonce option is
included).
o the message includes as much of the RHO copied from the
corresponding Predirect message as possible such that at least the
network-layer header is included but the size of the message does
not exceed 1280 bytes.
After Client ('C2') prepares the Redirect message, it sends the
message to Server ('S2').
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3.15.7. Re-encapsulating and Relaying Redirects
When Server ('S2') receives a Redirect message from Client ('C2'), it
first verifies that the TLLAOs in the Redirect are a proper subset of
the Interface IDs in Client ('C2')'s neighbor cache entry. If the
Client's TLLAOs are not acceptable, Server ('S2') discards the
message. Otherwise, Server ('S2') validates the message according to
the Redirect message validation rules in Section 8.1 of [RFC4861].
Server ('S2') also verifies that Client ('C2') is authorized to use
the ACPs encoded in the RIOs of the Redirect message. If validation
fails, Server ('S2') discards the Redirect; otherwise, it copies the
correct UDP Port number and IP Address for Client ('C2')'s underlying
link into the first TLLAO in case the addresses have been subject to
NAT.
Server ('S2') then examines the network-layer destination address of
the Redirect to determine the next hop toward Client ('C1') by
searching for the AERO address in the neighbor cache. Since Client
('C1') is not a neighbor, Server ('S2') re-encapsulates the Redirect
and relays it via Relay ('R1') by changing the link-layer source
address of the message to 'L2(S2)' and changing the link-layer
destination address to 'L2(R1)'. Server ('S2') finally forwards the
re-encapsulated message to Relay ('R1') without decrementing the
network-layer TTL/Hop Limit field.
When Relay ('R1') receives the Redirect message from Server ('S2') it
determines that Server ('S1') is the next hop toward Client ('C1') by
consulting its forwarding table. Relay ('R1') then re-encapsulates
the Redirect while changing the link-layer source address to 'L2(R1)'
and changing the link-layer destination address to 'L2(S1)'. Relay
('R1') then relays the Redirect via Server ('S1').
When Server ('S1') receives the Redirect message from Relay ('R1') it
determines that Client ('C1') is a neighbor by consulting its
neighbor cache. Server ('S1') then re-encapsulates the Redirect
while changing the link-layer source address to 'L2(S1)' and changing
the link-layer destination address to 'L2(C1)'. Server ('S1') then
forwards the message to Client ('C1').
3.15.8. Processing Redirects
When Client ('C1') receives the Redirect message, it accepts the
message only if it has a link-layer source address of one of its
Servers (e.g., 'L2(S1)'). Next, Client ('C1') validates the message
according to the Redirect message validation rules in Section 8.1 of
[RFC4861], except that it accepts the message even though the
network-layer source address is not that of it's current first-hop
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router. Following validation, Client ('C1') then processes the
message as follows.
In the reference operational scenario, when Client ('C1') receives
the Redirect message, it either creates or updates a dynamic neighbor
cache entry that stores the Target Address of the message as the
network-layer address of Client ('C2'), stores the link-layer
addresses found in the TLLAOs as the link-layer addresses of Client
('C2') and stores the ACPs encoded in the RIOs of the Redirect as the
ACPs for Client ('C2').. Client ('C1') then sets ForwardTime for the
neighbor cache entry to FORWARD_TIME.
Now, Client ('C1') has a neighbor cache entry with a valid
ForwardTime value, while Client ('C2') has a neighbor cache entry
with a valid AcceptTime value. Thereafter, Client ('C1') may forward
ordinary network-layer data packets directly to Client ('C2') without
involving any intermediate nodes, and Client ('C2') can verify that
the packets came from an acceptable source. (In order for Client
('C2') to forward packets to Client ('C1'), a corresponding
Predirect/Redirect message exchange is required in the reverse
direction; hence, the mechanism is asymmetric.)
3.15.9. Server-to-Client and Client-to-Server Redirection
In some environments, the Server nearest the target Client may need
to serve as a proxy redirection target, e.g., if direct Client-to-
Client communications are not possible. In that case, when the
source Client sends a Predirect message the target Server prepares a
corresponding Redirect the same as if it were the target Client (see:
Section 3.15.6), except that it writes its own link-layer address in
the TLLAO option. The Server must then maintain a dynamic neighbor
cache entry for the redirected source Client.
Similarly, when the source Client must send all packets via its own
Server and cannot act on a route optimization request, the source
Server can send a Predirect message toward the target Client. The
target Client then prepares a corresponding Redirect message the same
as for Client-to-Client route optimization and sends the Redirect
message back to the source Server.
Thereafter, if a Client moves to a new Server, the old Server sends
ICMP "Destination Unreachable" messages subject to rate limiting in
response to data packets received from a correspondent node to report
that the route optimization ForwardTime should be set to 0. The
correspondent Client (or Server) then allows future packets destined
to the departed Client to again flow through its own Server (or
Relay). Note however that the old Server retains the neighbor cache
entry and does not set AcceptTime to 0 since there may be many
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packets in flight. When the old Server receives these packets, it
forwards them to a Relay which will forward them to the departed
Client's new Server. AcceptTime will then eventually decrement to 0
once the correspondent node processes and acts on the Destination
Unreachables.
In any case, a Server MUST NOT send a BGP update to its Relays for
Clients discovered through dynamic route optimization redirection.
BGP updates are only to be sent for the Server's working set of
statically-associated Clients.
3.15.10. Server-to-Server Redirection
If neither the source nor target Clients are capable of sending
packets other than via their own Servers, a Server-to-Server route
optimization can still be employed. In that case, the source
Client's Server can send a Predirect message via a Relay to the AERO
address of the target Client, and the Relay will forward the message
to the target Client's Server. The target Server prepares the
Redirect message the same as if it were the target Client, except
that it writes its own link-layer address in the TLLAO option then
sends a Redirect message back to the source Server via the Relay.
Both Servers must then maintain a dynamic neighbor cache entry for
the redirected Clients.
Thereafter, if a Client moves to a new Server, the old Server sends
ICMP "Destination Unreachable" messages subject to rate limiting in
response to data packets forwarded by the correspondent Server to
report that the route optimization ForwardTime should be set to 0.
The correspondent Server then allows future packets destined to the
departed Client to again flow through its own Relay. Note however
that the old Server retains the neighbor cache entry and does not set
AcceptTime to 0 since there may be many packets in flight. When the
old Server receives these packets, it forwards them to a Relay which
will forward them to the departed Client's new Server. AcceptTime
will then eventually decrement to 0 once the correspondent node
processes and acts on the Destination Unreachables.
In any case, a Server MUST NOT send a BGP update to its Relays for
Clients discovered through dynamic route optimization redirection.
BGP updates are only to be sent for the Server's working set of
statically-associated Clients..
3.16. Neighbor Unreachability Detection (NUD)
AERO nodes perform Neighbor Unreachability Detection (NUD) by sending
unicast NS messages with SLLAOs to elicit solicited NA messages from
neighbors the same as described in [RFC4861]. NUD is performed
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either reactively in response to persistent L2 errors (see
Section 3.13) or proactively to update neighbor cache entry timers
and/or link-layer address information.
When an AERO node sends an NS/NA message, it MUST use one of its
link-local addresses as the IPv6 source address and a link-local
address of the neighbor as the IPv6 destination address. When an
AERO node receives an NS message or a solicited NA message, it
accepts the message if it has a neighbor cache entry for the
neighbor; otherwise, it ignores the message.
When a source AERO node is redirected to a target AERO node it SHOULD
proactively test the direct path by sending an initial NS message to
elicit a solicited NA response. While testing the path, the source
node can optionally continue sending packets via its Server (or
Relay), maintain a small queue of packets until target reachability
is confirmed, or (optimistically) allow packets to flow directly to
the target.
While data packets are still flowing, the source node thereafter
periodically tests the direct path to the target node (see
Section 7.3 of [RFC4861]) in order to keep dynamic neighbor cache
entries alive. When the target node receives a valid NS message, it
resets AcceptTime to ACCEPT_TIME and updates its cached link-layer
addresses (if necessary). When the source node receives a solicited
NA message, it resets ForwardTime to FORWARD_TIME and updates its
cached link-layer addresses (if necessary). If the source node is
unable to elicit a solicited NA response from the target node after
MaxRetry attempts, it SHOULD set ForwardTime to 0. Otherwise, the
source node considers the path usable and SHOULD thereafter process
any link-layer errors as a hint that the direct path to the target
node has either failed or has become intermittent.
When ForwardTime for a dynamic neighbor cache entry expires, the
source node resumes sending any subsequent packets via a Server (or
Relay) and may (eventually) attempt to re-initiate the AERO
redirection process. When AcceptTime for a dynamic neighbor cache
entry expires, the target node discards any subsequent packets
received directly from the source node. When both ForwardTime and
AcceptTime for a dynamic neighbor cache entry expire, the node
deletes the neighbor cache entry.
3.17. Mobility Management
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3.17.1. Announcing Link-Layer Address Changes
When a Client needs to change its link-layer addresses, e.g., due to
a mobility event, it sends unsolicited NAs to its neighbors using the
new link-layer address as the source address and with TLLAOs that
include the updated Client link-layer information.
The Client MAY send up to MaxRetry unsolicited NA messages in
parallel with sending actual data packets in case one or more NAs are
lost. If all NAs are lost, the Client will eventually invoke NUD by
sending NS messages that include SLLAOs.
3.17.2. Bringing New Links Into Service
When a Client needs to bring new underlying interfaces into service
(e.g., when it activates a new data link), it sends unsolicited NAs
to its neighbors using the new link-layer address as the source
address and with TLLAOs that include the new Client link-layer
information.
3.17.3. Removing Existing Links from Service
When a Client needs to remove existing underlying interfaces from
service (e.g., when it de-activates an existing data link), it sends
unsolicited NAs to its neighbors with TLLAOs that include P(i) values
set to "disabled".
If the Client needs to send the unsolicited NAs over a link other
than the one being removed from service, it MUST include a TLLAO for
the sending link as the first TLLAO and include the TLLAO for the
link being removed from service as an additional TLLAO.
3.17.4. Implicit Mobility Management
AERO interface neighbors MAY include a configuration knob that allows
them to perform implicit mobility management in which no IPv6 ND
messaging is used. In that case, the Client only transmits packets
over a single interface at a time, and the neighbor always observes
packets arriving from the Client from the same link-layer source
address.
If the Client's underlying interface address changes (either due to a
readdressing of the original interface or switching to a new
interface) the neighbor immediately updates the neighbor cache entry
for the Client and begins accepting and sending packets to the
Client's new link-layer address. This implicit mobility method
applies to use cases such as cellphones with both WiFi and Cellular
interfaces where only one of the interfaces is active at a given
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time, and the Client automatically switches over to the backup
interface if the primary interface fails.
3.17.5. Moving to a New Server
When a Client associates with a new Server, it performs the Client
procedures specified in Section 3.14.2.
When a Client disassociates with an existing Server, it sends a
DHCPv6 Release message via a new Server with its base AERO address as
the network-layer source address and the unicast link-local address
of the old Server as the network-layer destination address. The new
Server then encapsulates the Release message in a DHCPv6 Relay-
Forward message header, writes the Client's source address in the
'peer-address' field, and writes its own link-local address in the IP
source address (i.e., the new Server acts as a DHCPv6 relay agent).
The new Server then forwards the message to an Relay, which forwards
the message to the old Server based on the network-layer destination
address.
When the old Server receives the Release, it first authenticates the
message then releases the DHCPv6 PDs and deletes the Client's ACP
routes. The old Server then deletes the Client's neighbor cache
entry so that any in-flight packets will be forwarded via a Relay to
the new Server, which will forward them to the Client. The old
Server finally returns a DHCPv6 Relay-Reply message via an Relay to
the new Server, which will decapsulate the DHCPv6 Reply message and
forward it to the Client.
When the new Server forwards the Reply message, the Client can delete
both the default route and the neighbor cache entry for the old
Server. (Note that since Release/Reply messages may be lost in the
network the Client SHOULD retry until it gets a Reply indicating that
the Release was successful. If the Client does not receive a Reply
after MaxRetry attempts, the old Server may have failed and the
Client should discontinue its Release attempts.)
Note that this DHCPv6 relay-chaining approach is necessary to avoid
failures, e.g., due to temporary routing fluctuations. In
particular, the Client should always be able to forward messages via
its new Server but may not always be able to send messages directly
to an old Server. But, the new Server and Old Server should always
be able to exchange messages with one another.
Finally, Clients SHOULD NOT move rapidly between Servers in order to
avoid causing excessive oscillations in the AERO routing system.
Such oscillations could result in intermittent reachability for the
Client itself, while causing little harm to the network. Examples of
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when a Client might wish to change to a different Server include a
Server that has gone unreachable, topological movements of
significant distance, etc.
3.17.6. Packet Queueing for Mobility
AERO Clients and Servers should maintain a small queue of packets
they have recently sent to an AERO neighbor, e.g., a 1 second window.
If the AERO neighbor moves, the AERO node MAY retransmit the queued
packets to ensure that they are delivered to the AERO neighbor's new
location.
Note that this may have performance implications for asymmetric
paths. For example, if the AERO neighbor moves from a 50Mbps link to
a 128Kbps link, retransmitting a 1 second window could cause
significant congestion. However, any retransmission bursts will
subside after the 1 second window.
3.17.7. Alternate Mobility Security Model
In some environments, an AERO node may have no way of authenticating
any unsolicited NA messages it receives. In that case, the target
AERO node SHOULD ignore any unsolicited NA messages it receives, and
the source AERO node SHOULD inform the target of its new link-layer
addresses by sending a fresh Predirect message via its Server (or
Relay). The target AERO node can then accept the Predirect message
and update its link-layer addresses based on the Predirect TLLAOs.
3.18. Multicast Considerations
When the underlying network does not support multicast, AERO Clients
map link-scoped multicast addresses to the link-layer address of a
Server, which acts as a multicast forwarding agent. The AERO Client
also serves as an IGMP/MLD Proxy for its EUNs and/or hosted
applications per [RFC4605] while using the link-layer address of the
Server as the link-layer address for all multicast packets.
When the underlying network supports multicast, AERO nodes use the
multicast address mapping specification found in [RFC2529] for IPv4
underlying networks and use a TBD site-scoped multicast mapping for
IPv6 underlying networks. In that case, border routers must ensure
that the encapsulated site-scoped multicast packets do not leak
outside of the site spanned by the AERO link.
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4. AERO Variations
AERO can be used in many different variations based on the specific
use case. The following sections discuss variations that adhere to
the AERO principles while allowing selective application of AERO
components.
4.1. Operation on Host-Only IPv6 AERO Links
IPv6 AERO links typically have ASPs that cover many candidate ACPs of
length /64 or shorter. However, in some cases it may be desirable to
use AERO over links that have only a /64 ASP. This can be
accommodated by treating all Clients on the AERO link as simple hosts
that receive /128 prefix delegations.
In that case, each Client configures an administratively-provisioned
link-local address instead of an AERO address, i.e., the same as for
Servers and Relays. The Client discovers its link-local address by
including an IA_NA option in its DHCPv6 Solicit message to the
Server. The Server responds by returning the Client's
administratively-provisioned link-local address in the IA_NA option
plus any IPv6 addresses for the Client in IA_PD options with prefix
length /128.
For example, if the ASP for the host-only IPv6 AERO link is
2001:db8:1000:2000::/64, each Client will receive one or more /128
IPv6 prefix delegations such as 2001:db8:1000:2000::1/128,
2001:db8:1000:2000::2/128, etc. The Client then assigns the /128s to
the AERO interface as IPv6 addresses, and the Client's applications
treat the AERO interface as an ordinary host interface.
In this arrangement, the Client conducts route optimization in the
same sense as discussed in Section 3.15, except that the Predirect
message network-layer source address is the Client's
administratively-assigned link-local address and the network-layer
destination address is the same as the destination address of the
packet that triggered the redirection. All other aspects of AERO
operation are the same as described in earlier sections.
This has applicability for nodes that act as a Client on an
"upstream" AERO link, but also act as a Server on "downstream" AERO
links. More specifically, if the node acts as a Client to receive a
/64 prefix from the upstream AERO link it can then act as a Server to
provision /128s to Clients on downstream AERO links.
Note that, due to the nature of the AERO address format, valid IPv6
ACP lengths are either /64 or shorter, or exactly /128 (i.e., prefix
lengths between /65 and /127 cannot be accommodated).
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4.2. Operation on AERO Links Without DHCPv6 Services
When Servers on the AERO link do not provide DHCPv6 services,
operation can still be accommodated through administrative
configuration of ACPs on AERO Clients. In that case, administrative
configurations of AERO interface neighbor cache entries on both the
Server and Client are also necessary. However, this may interfere
with the ability for Clients to dynamically change to new Servers,
and can expose the AERO link to misconfigurations unless the
administrative configurations are carefully coordinated.
4.3. Operation on Server-less AERO Links
In some AERO link scenarios, there may be no Servers on the link and/
or no need for Clients to use a Server as an intermediary trust
anchor. In that case, each Client acts as a Server unto itself to
establish neighbor cache entries by performing direct Client-to-
Client IPv6 ND message exchanges, and some other form of trust basis
must be applied so that each Client can verify that the prospective
neighbor is authorized to use its claimed ACP.
When there is no Server on the link, Clients must arrange to receive
ACPs and publish them via a secure alternate PD authority through
some means outside the scope of this document.
4.4. Operation on Client-less AERO Links
In some environments, the AERO service may be useful for mobile nodes
that do not implement the AERO Client function and do not perform
encapsulation. For example, if the mobile node has a way of
injecting its ACP into the access subnetwork routing system an AERO
Server connected to the same access network can accept the ACP prefix
injection as an indication that a new mobile node has come onto the
subnetwork. The Server can then inject the ACP into the BGP routing
system the same as if an AERO Client/Server DHCPv6 PD exchange had
occurred. If the mobile node subsequently withdraws the ACP from the
access network routing system, the Server can then withdraw the ACP
from the BGP routing system.
In this arrangement, AERO Servers and Relays are used in exactly the
same ways as for environments where DHCPv6 Client/Server exchanges
are supported. However, the access subnetwork routing systems must
be capable of accommodating rapid ACP injections and withdrawals from
mobile nodes with the understanding that the information must be
propagated to all routers in the system. Operational experience has
shown that this kind of routing system "churn" can lead to overall
instability and routing system inconsistency.
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4.5. Manually-Configured AERO Tunnels
In addition to the dynamic neighbor discovery procedures for AERO
link neighbors described above, AERO encapsulation can be applied to
manually-configured tunnels. In that case, the tunnel endpoints use
an administratively-provisioned link-local address and exchange NS/NA
messages the same as for dynamically-established tunnels.
4.6. Encapsulation Avoidance on Relay-Server Dedicated Links
In some environments, AERO Servers and Relays may be connected by
dedicated point-to-point links, e.g., high speed fiberoptic leased
lines. In that case, the Servers and Relays can participate in the
AERO link the same as specified above but can avoid encapsulation
over the dedicated links. In that case, however, the links would be
dedicated for AERO and could not be multiplexed for both AERO and
non-AERO communications.
4.7. Encapsulation Protocol Version Considerations
A source Client may connect only to an IPvX underlying network, while
the target Client connects only to an IPvY underlying network. In
that case, the target and source Clients have no means for reaching
each other directly (since they connect to underlying networks of
different IP protocol versions) and so must ignore any redirection
messages and continue to send packets via their Servers.
5. Implementation Status
Production user-level and kernel-level AERO implementations have been
developed and are undergoing internal testing within Boeing.
An initial public release of the AERO proof-of-concept source code
was announced on the intarea mailing list on August 21, 2015, and a
pointer to the code is available in the list archives.
6. IANA Considerations
The IANA has assigned a 4-octet Private Enterprise Number "45282" for
AERO in the "enterprise-numbers" registry.
The IANA has assigned the UDP port number "8060" for an earlier
experimental version of AERO [RFC6706]. This document obsoletes
[RFC6706] and claims the UDP port number "8060" for all future use.
No further IANA actions are required.
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7. Security Considerations
AERO link security considerations are the same as for standard IPv6
Neighbor Discovery [RFC4861] except that AERO improves on some
aspects. In particular, AERO uses a trust basis between Clients and
Servers, where the Clients only engage in the AERO mechanism when it
is facilitated by a trust anchor.
Redirect, Predirect and unsolicited NA messages SHOULD include a
Timestamp option (see Section 5.3 of [RFC3971]) that other AERO nodes
can use to verify the message time of origin. Predirect, NS and RS
messages SHOULD include a Nonce option (see Section 5.3 of [RFC3971])
that recipients echo back in corresponding responses. In cases where
spoofing cannot be mitigated through other means, however, all AERO
IPv6 ND messages should employ Secure Neighbor Discovery (SeND)
[RFC3971].
AERO links must be protected against link-layer address spoofing
attacks in which an attacker on the link pretends to be a trusted
neighbor. Links that provide link-layer securing mechanisms (e.g.,
IEEE 802.1X WLANs) and links that provide physical security (e.g.,
enterprise network wired LANs) provide a first line of defense,
however AERO nodes SHOULD also use DHCPv6 securing services (e.g.,
Secure DHCPv6 [I-D.ietf-dhc-sedhcpv6], etc.) for Client
authentication and network admission control. Following
authenticated DHCPv6 PD procedures, AERO nodes MUST ensure that the
source of data packets corresponds to the node to which the prefixes
were delegated.
AERO Clients MUST ensure that their connectivity is not used by
unauthorized nodes on their EUNs to gain access to a protected
network, i.e., AERO Clients that act as routers MUST NOT provide
routing services for unauthorized nodes. (This concern is no
different than for ordinary hosts that receive an IP address
delegation but then "share" the address with other nodes via some
form of Internet connection sharing.)
AERO Clients, Servers and Relays on the open Internet are susceptible
to the same attack profiles as for any Internet nodes. For this
reason, IP security SHOULD be used when AERO is employed over
unmanaged/unsecured links using securing mechanisms such as IPsec
[RFC4301], IKE [RFC5996] and/or TLS [RFC5246]. In some environments,
however, the use of end-to-end security from Clients to correspondent
nodes (i.e., other Clients and/or Internet nodes) could obviate the
need for IP security between AERO Clients, Servers and Relays.
AERO Servers and Relays present targets for traffic amplification DoS
attacks. This concern is no different than for widely-deployed VPN
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security gateways in the Internet, where attackers could send spoofed
packets to the gateways at high data rates. This can be mitigated by
connecting Relays and Servers over dedicated links with no
connections to the Internet and/or when connections to the Internet
are only permitted through well-managed firewalls.
Traffic amplification DoS attacks can also target an AERO Client's
low data rate links. This is a concern not only for Clients located
on the open Internet but also for Clients in protected enclaves.
AERO Servers can institute rate limits that protect Clients from
receiving packet floods that could DoS low data rate links.
8. Acknowledgements
Discussions both on IETF lists and in private exchanges helped shape
some of the concepts in this work. Individuals who contributed
insights include Mikael Abrahamsson, Mark Andrews, Fred Baker, Bob
Braden, Stewart Bryant, Brian Carpenter, Wojciech Dec, Ralph Droms,
Adrian Farrel, Sri Gundavelli, Brian Haberman, Joel Halpern, Tom
Herbert, Sascha Hlusiak, Lee Howard, Andre Kostur, Ted Lemon, Andy
Malis, Satoru Matsushima, Tomek Mrugalski, Alexandru Petrescu, Behcet
Saikaya, Joe Touch, Bernie Volz, Ryuji Wakikawa and Lloyd Wood.
Members of the IESG also provided valuable input during their review
process that greatly improved the document. Discussions on the v6ops
list in the December 2015 through January 2016 timeframe further
helped clarify AERO multi-addressing capabilities. Special thanks go
to Stewart Bryant, Joel Halpern and Brian Haberman for their
shepherding guidance during the publication of the AERO first
edition.
This work has further been encouraged and supported by Boeing
colleagues including M. Wayne Benson, Dave Bernhardt, Cam Brodie,
Balaguruna Chidambaram, Irene Chin, Bruce Cornish, Claudiu Danilov,
Wen Fang, Anthony Gregory, Jeff Holland, Ed King, Gene MacLean III,
Rob Muszkiewicz, Sean O'Sullivan, Kent Shuey, Brian Skeen, Mike
Slane, Carrie Spiker, Brendan Williams, Julie Wulff, Yueli Yang, and
other members of the BR&T and BIT mobile networking teams. Wayne
Benson is especially acknowledged for his outstanding work in
converting the AERO proof-of-concept implementation into production-
ready code.
Earlier works on NBMA tunneling approaches are found in
[RFC2529][RFC5214][RFC5569].
Many of the constructs presented in this second edition of AERO are
based on the author's earlier works, including:
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o The Internet Routing Overlay Network (IRON)
[RFC6179][I-D.templin-ironbis]
o Virtual Enterprise Traversal (VET)
[RFC5558][I-D.templin-intarea-vet]
o The Subnetwork Encapsulation and Adaptation Layer (SEAL)
[RFC5320][I-D.templin-intarea-seal]
o AERO, First Edition [RFC6706]
Note that these works cite numerous earlier efforts that are not also
cited here due to space limitations. The authors of those earlier
works are acknowledged for their insights.
This work is aligned with the NASA Safe Autonomous Systems Operation
(SASO) program under NASA contract number NNA16BD84C.
This work is aligned with the FAA as per the SE2025 contract number
DTFAWA-15-D-00030.
This work is aligned with the Boeing Information Technology (BIT)
MobileNet program.
This work is aligned with the Boeing Research and Technology (BR&T)
autonomous systems networking program.
9. References
9.1. Normative References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<http://www.rfc-editor.org/info/rfc768>.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<http://www.rfc-editor.org/info/rfc791>.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, DOI 10.17487/RFC0792, September 1981,
<http://www.rfc-editor.org/info/rfc792>.
[RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
DOI 10.17487/RFC2003, October 1996,
<http://www.rfc-editor.org/info/rfc2003>.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <http://www.rfc-editor.org/info/rfc2460>.
[RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in
IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473,
December 1998, <http://www.rfc-editor.org/info/rfc2473>.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
DOI 10.17487/RFC2474, December 1998,
<http://www.rfc-editor.org/info/rfc2474>.
[RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
C., and M. Carney, "Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
2003, <http://www.rfc-editor.org/info/rfc3315>.
[RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
Host Configuration Protocol (DHCP) version 6", RFC 3633,
DOI 10.17487/RFC3633, December 2003,
<http://www.rfc-editor.org/info/rfc3633>.
[RFC3971] Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
"SEcure Neighbor Discovery (SEND)", RFC 3971,
DOI 10.17487/RFC3971, March 2005,
<http://www.rfc-editor.org/info/rfc3971>.
[RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and
More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191,
November 2005, <http://www.rfc-editor.org/info/rfc4191>.
[RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
for IPv6 Hosts and Routers", RFC 4213,
DOI 10.17487/RFC4213, October 2005,
<http://www.rfc-editor.org/info/rfc4213>.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<http://www.rfc-editor.org/info/rfc4861>.
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[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862,
DOI 10.17487/RFC4862, September 2007,
<http://www.rfc-editor.org/info/rfc4862>.
[RFC6434] Jankiewicz, E., Loughney, J., and T. Narten, "IPv6 Node
Requirements", RFC 6434, DOI 10.17487/RFC6434, December
2011, <http://www.rfc-editor.org/info/rfc6434>.
9.2. Informative References
[BGP] Huston, G., "BGP in 2015, http://potaroo.net", January
2016.
[I-D.herbert-gue-fragmentation]
Herbert, T. and F. Templin, "Fragmentation option for
Generic UDP Encapsulation", draft-herbert-gue-
fragmentation-02 (work in progress), October 2015.
[I-D.ietf-dhc-sedhcpv6]
Jiang, S., Li, L., Cui, Y., Jinmei, T., Lemon, T., and D.
Zhang, "Secure DHCPv6", draft-ietf-dhc-sedhcpv6-14 (work
in progress), October 2016.
[I-D.ietf-intarea-tunnels]
Touch, J. and W. Townsley, "IP Tunnels in the Internet
Architecture", draft-ietf-intarea-tunnels-03 (work in
progress), July 2016.
[I-D.ietf-nvo3-gue]
Herbert, T., Yong, L., and O. Zia, "Generic UDP
Encapsulation", draft-ietf-nvo3-gue-04 (work in progress),
July 2016.
[I-D.templin-intarea-grefrag]
Templin, F., "GRE Tunnel Level Fragmentation", draft-
templin-intarea-grefrag-04 (work in progress), July 2016.
[I-D.templin-intarea-seal]
Templin, F., "The Subnetwork Encapsulation and Adaptation
Layer (SEAL)", draft-templin-intarea-seal-68 (work in
progress), January 2014.
[I-D.templin-intarea-vet]
Templin, F., "Virtual Enterprise Traversal (VET)", draft-
templin-intarea-vet-40 (work in progress), May 2013.
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[I-D.templin-ironbis]
Templin, F., "The Interior Routing Overlay Network
(IRON)", draft-templin-ironbis-16 (work in progress),
March 2014.
[OVPN] OpenVPN, O., "http://openvpn.net", October 2016.
[RFC0879] Postel, J., "The TCP Maximum Segment Size and Related
Topics", RFC 879, DOI 10.17487/RFC0879, November 1983,
<http://www.rfc-editor.org/info/rfc879>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<http://www.rfc-editor.org/info/rfc1122>.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<http://www.rfc-editor.org/info/rfc1191>.
[RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers",
RFC 1812, DOI 10.17487/RFC1812, June 1995,
<http://www.rfc-editor.org/info/rfc1812>.
[RFC1930] Hawkinson, J. and T. Bates, "Guidelines for creation,
selection, and registration of an Autonomous System (AS)",
BCP 6, RFC 1930, DOI 10.17487/RFC1930, March 1996,
<http://www.rfc-editor.org/info/rfc1930>.
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, DOI 10.17487/RFC1981, August
1996, <http://www.rfc-editor.org/info/rfc1981>.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC2131, March 1997,
<http://www.rfc-editor.org/info/rfc2131>.
[RFC2529] Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4
Domains without Explicit Tunnels", RFC 2529,
DOI 10.17487/RFC2529, March 1999,
<http://www.rfc-editor.org/info/rfc2529>.
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[RFC2675] Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms",
RFC 2675, DOI 10.17487/RFC2675, August 1999,
<http://www.rfc-editor.org/info/rfc2675>.
[RFC2764] Gleeson, B., Lin, A., Heinanen, J., Armitage, G., and A.
Malis, "A Framework for IP Based Virtual Private
Networks", RFC 2764, DOI 10.17487/RFC2764, February 2000,
<http://www.rfc-editor.org/info/rfc2764>.
[RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
DOI 10.17487/RFC2784, March 2000,
<http://www.rfc-editor.org/info/rfc2784>.
[RFC2890] Dommety, G., "Key and Sequence Number Extensions to GRE",
RFC 2890, DOI 10.17487/RFC2890, September 2000,
<http://www.rfc-editor.org/info/rfc2890>.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery",
RFC 2923, DOI 10.17487/RFC2923, September 2000,
<http://www.rfc-editor.org/info/rfc2923>.
[RFC2983] Black, D., "Differentiated Services and Tunnels",
RFC 2983, DOI 10.17487/RFC2983, October 2000,
<http://www.rfc-editor.org/info/rfc2983>.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<http://www.rfc-editor.org/info/rfc3168>.
[RFC3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
"DNS Extensions to Support IP Version 6", RFC 3596,
DOI 10.17487/RFC3596, October 2003,
<http://www.rfc-editor.org/info/rfc3596>.
[RFC3819] Karn, P., Ed., Bormann, C., Fairhurst, G., Grossman, D.,
Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
Wood, "Advice for Internet Subnetwork Designers", BCP 89,
RFC 3819, DOI 10.17487/RFC3819, July 2004,
<http://www.rfc-editor.org/info/rfc3819>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<http://www.rfc-editor.org/info/rfc4271>.
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[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <http://www.rfc-editor.org/info/rfc4291>.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
December 2005, <http://www.rfc-editor.org/info/rfc4301>.
[RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", RFC 4443,
DOI 10.17487/RFC4443, March 2006,
<http://www.rfc-editor.org/info/rfc4443>.
[RFC4459] Savola, P., "MTU and Fragmentation Issues with In-the-
Network Tunneling", RFC 4459, DOI 10.17487/RFC4459, April
2006, <http://www.rfc-editor.org/info/rfc4459>.
[RFC4511] Sermersheim, J., Ed., "Lightweight Directory Access
Protocol (LDAP): The Protocol", RFC 4511,
DOI 10.17487/RFC4511, June 2006,
<http://www.rfc-editor.org/info/rfc4511>.
[RFC4555] Eronen, P., "IKEv2 Mobility and Multihoming Protocol
(MOBIKE)", RFC 4555, DOI 10.17487/RFC4555, June 2006,
<http://www.rfc-editor.org/info/rfc4555>.
[RFC4592] Lewis, E., "The Role of Wildcards in the Domain Name
System", RFC 4592, DOI 10.17487/RFC4592, July 2006,
<http://www.rfc-editor.org/info/rfc4592>.
[RFC4605] Fenner, B., He, H., Haberman, B., and H. Sandick,
"Internet Group Management Protocol (IGMP) / Multicast
Listener Discovery (MLD)-Based Multicast Forwarding
("IGMP/MLD Proxying")", RFC 4605, DOI 10.17487/RFC4605,
August 2006, <http://www.rfc-editor.org/info/rfc4605>.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
<http://www.rfc-editor.org/info/rfc4821>.
[RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
Errors at High Data Rates", RFC 4963,
DOI 10.17487/RFC4963, July 2007,
<http://www.rfc-editor.org/info/rfc4963>.
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[RFC4994] Zeng, S., Volz, B., Kinnear, K., and J. Brzozowski,
"DHCPv6 Relay Agent Echo Request Option", RFC 4994,
DOI 10.17487/RFC4994, September 2007,
<http://www.rfc-editor.org/info/rfc4994>.
[RFC5213] Gundavelli, S., Ed., Leung, K., Devarapalli, V.,
Chowdhury, K., and B. Patil, "Proxy Mobile IPv6",
RFC 5213, DOI 10.17487/RFC5213, August 2008,
<http://www.rfc-editor.org/info/rfc5213>.
[RFC5214] Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
DOI 10.17487/RFC5214, March 2008,
<http://www.rfc-editor.org/info/rfc5214>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[RFC5320] Templin, F., Ed., "The Subnetwork Encapsulation and
Adaptation Layer (SEAL)", RFC 5320, DOI 10.17487/RFC5320,
February 2010, <http://www.rfc-editor.org/info/rfc5320>.
[RFC5494] Arkko, J. and C. Pignataro, "IANA Allocation Guidelines
for the Address Resolution Protocol (ARP)", RFC 5494,
DOI 10.17487/RFC5494, April 2009,
<http://www.rfc-editor.org/info/rfc5494>.
[RFC5522] Eddy, W., Ivancic, W., and T. Davis, "Network Mobility
Route Optimization Requirements for Operational Use in
Aeronautics and Space Exploration Mobile Networks",
RFC 5522, DOI 10.17487/RFC5522, October 2009,
<http://www.rfc-editor.org/info/rfc5522>.
[RFC5558] Templin, F., Ed., "Virtual Enterprise Traversal (VET)",
RFC 5558, DOI 10.17487/RFC5558, February 2010,
<http://www.rfc-editor.org/info/rfc5558>.
[RFC5569] Despres, R., "IPv6 Rapid Deployment on IPv4
Infrastructures (6rd)", RFC 5569, DOI 10.17487/RFC5569,
January 2010, <http://www.rfc-editor.org/info/rfc5569>.
[RFC5720] Templin, F., "Routing and Addressing in Networks with
Global Enterprise Recursion (RANGER)", RFC 5720,
DOI 10.17487/RFC5720, February 2010,
<http://www.rfc-editor.org/info/rfc5720>.
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[RFC5844] Wakikawa, R. and S. Gundavelli, "IPv4 Support for Proxy
Mobile IPv6", RFC 5844, DOI 10.17487/RFC5844, May 2010,
<http://www.rfc-editor.org/info/rfc5844>.
[RFC5949] Yokota, H., Chowdhury, K., Koodli, R., Patil, B., and F.
Xia, "Fast Handovers for Proxy Mobile IPv6", RFC 5949,
DOI 10.17487/RFC5949, September 2010,
<http://www.rfc-editor.org/info/rfc5949>.
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)",
RFC 5996, DOI 10.17487/RFC5996, September 2010,
<http://www.rfc-editor.org/info/rfc5996>.
[RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
April 2011, <http://www.rfc-editor.org/info/rfc6146>.
[RFC6179] Templin, F., Ed., "The Internet Routing Overlay Network
(IRON)", RFC 6179, DOI 10.17487/RFC6179, March 2011,
<http://www.rfc-editor.org/info/rfc6179>.
[RFC6204] Singh, H., Beebee, W., Donley, C., Stark, B., and O.
Troan, Ed., "Basic Requirements for IPv6 Customer Edge
Routers", RFC 6204, DOI 10.17487/RFC6204, April 2011,
<http://www.rfc-editor.org/info/rfc6204>.
[RFC6221] Miles, D., Ed., Ooghe, S., Dec, W., Krishnan, S., and A.
Kavanagh, "Lightweight DHCPv6 Relay Agent", RFC 6221,
DOI 10.17487/RFC6221, May 2011,
<http://www.rfc-editor.org/info/rfc6221>.
[RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "Network Configuration Protocol
(NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
<http://www.rfc-editor.org/info/rfc6241>.
[RFC6275] Perkins, C., Ed., Johnson, D., and J. Arkko, "Mobility
Support in IPv6", RFC 6275, DOI 10.17487/RFC6275, July
2011, <http://www.rfc-editor.org/info/rfc6275>.
[RFC6355] Narten, T. and J. Johnson, "Definition of the UUID-Based
DHCPv6 Unique Identifier (DUID-UUID)", RFC 6355,
DOI 10.17487/RFC6355, August 2011,
<http://www.rfc-editor.org/info/rfc6355>.
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[RFC6422] Lemon, T. and Q. Wu, "Relay-Supplied DHCP Options",
RFC 6422, DOI 10.17487/RFC6422, December 2011,
<http://www.rfc-editor.org/info/rfc6422>.
[RFC6438] Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
for Equal Cost Multipath Routing and Link Aggregation in
Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011,
<http://www.rfc-editor.org/info/rfc6438>.
[RFC6691] Borman, D., "TCP Options and Maximum Segment Size (MSS)",
RFC 6691, DOI 10.17487/RFC6691, July 2012,
<http://www.rfc-editor.org/info/rfc6691>.
[RFC6706] Templin, F., Ed., "Asymmetric Extended Route Optimization
(AERO)", RFC 6706, DOI 10.17487/RFC6706, August 2012,
<http://www.rfc-editor.org/info/rfc6706>.
[RFC6864] Touch, J., "Updated Specification of the IPv4 ID Field",
RFC 6864, DOI 10.17487/RFC6864, February 2013,
<http://www.rfc-editor.org/info/rfc6864>.
[RFC6935] Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
UDP Checksums for Tunneled Packets", RFC 6935,
DOI 10.17487/RFC6935, April 2013,
<http://www.rfc-editor.org/info/rfc6935>.
[RFC6936] Fairhurst, G. and M. Westerlund, "Applicability Statement
for the Use of IPv6 UDP Datagrams with Zero Checksums",
RFC 6936, DOI 10.17487/RFC6936, April 2013,
<http://www.rfc-editor.org/info/rfc6936>.
[RFC6939] Halwasia, G., Bhandari, S., and W. Dec, "Client Link-Layer
Address Option in DHCPv6", RFC 6939, DOI 10.17487/RFC6939,
May 2013, <http://www.rfc-editor.org/info/rfc6939>.
[RFC6980] Gont, F., "Security Implications of IPv6 Fragmentation
with IPv6 Neighbor Discovery", RFC 6980,
DOI 10.17487/RFC6980, August 2013,
<http://www.rfc-editor.org/info/rfc6980>.
[RFC7078] Matsumoto, A., Fujisaki, T., and T. Chown, "Distributing
Address Selection Policy Using DHCPv6", RFC 7078,
DOI 10.17487/RFC7078, January 2014,
<http://www.rfc-editor.org/info/rfc7078>.
[TUNTAP] Wikipedia, W., "http://en.wikipedia.org/wiki/TUN/TAP",
October 2014.
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Appendix A. AERO Alternate Encapsulations
When GUE encapsulation is not needed, AERO can use common
encapsulations such as IP-in-IP [RFC2003][RFC2473][RFC4213], Generic
Routing Encapsulation (GRE) [RFC2784][RFC2890] and others. The
encapsulation is therefore only differentiated from non-AERO tunnels
through the application of AERO control messaging and not through,
e.g., a well-known UDP port number.
As for GUE encapsulation, alternate AERO encapsulation formats may
require encapsulation layer fragmentation. For simple IP-in-IP
encapsulation, an IPv6 fragment header is inserted directly between
the inner and outer IP headers when needed, i.e., even if the outer
header is IPv4. The IPv6 Fragment Header is identified to the outer
IP layer by its IP protocol number, and the Next Header field in the
IPv6 Fragment Header identifies the inner IP header version. For GRE
encapsulation, a GRE fragment header is inserted within the GRE
header [I-D.templin-intarea-grefrag].
Figure 6 shows the AERO IP-in-IP encapsulation format before any
fragmentation is applied:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Outer IPv4 Header | | Outer IPv6 Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|IPv6 Frag Header (optional)| |IPv6 Frag Header (optional)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Inner IP Header | | Inner IP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | |
~ ~ ~ ~
~ Inner Packet Body ~ ~ Inner Packet Body ~
~ ~ ~ ~
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Minimal Encapsulation in IPv4 Minimal Encapsulation in IPv6
Figure 6: Minimal Encapsulation Format using IP-in-IP
Figure 7 shows the AERO GRE encapsulation format before any
fragmentation is applied:
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Outer IP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| GRE Header |
| (with checksum, key, etc..) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| GRE Fragment Header (optional)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Inner IP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ ~
~ Inner Packet Body ~
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Minimal Encapsulation Using GRE
Alternate encapsulation may be preferred in environments where GUE
encapsulation would add unnecessary overhead. For example, certain
low-bandwidth wireless data links may benefit from a reduced
encapsulation overhead.
GUE encapsulation can traverse network paths that are inaccessible to
non-UDP encapsulations, e.g., for crossing Network Address
Translators (NATs). More and more, network middleboxes are also
being configured to discard packets that include anything other than
a well-known IP protocol such as UDP and TCP. It may therefore be
necessary to determine the potential for middlebox filtering before
enabling alternate encapsulation in a given environment.
In addition to IP-in-IP, GRE and GUE, AERO can also use security
encapsulations such as IPsec and SSL/TLS. In that case, AERO control
messaging and route determination occur before security encapsulation
is applied for outgoing packets and after security decapsulation is
applied for incoming packets.
AERO is especially well suited for use with VPN system encapsulations
such as OpenVPN [OVPN].
Appendix B. When to Insert an Encapsulation Fragment Header
An encapsulation fragment header is inserted when the AERO tunnel
ingress needs to apply fragmentation to accommodate packets that must
be delivered without loss due to a size restriction. Fragmentation
is performed on the inner packet while encapsulating each inner
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packet fragment in outer IP and encapsulation layer headers that
differ only in the fragment header fields.
The fragment header can also be inserted in order to include a
coherent Identification value with each packet, e.g., to aid in
Duplicate Packet Detection (DPD). In this way, network nodes can
cache the Identification values of recently-seen packets and use the
cached values to determine whether a newly-arrived packet is in fact
a duplicate. The Identification value within each packet could
further provide a rough indicator of packet reordering, e.g., in
cases when the tunnel egress wishes to discard packets that are
grossly out of order.
In some use cases, there may be operational assurance that no
fragmentation of any kind will be necessary, or that only occasional
large control messages will require fragmentation. In that case, the
encapsulation fragment header can be omitted and ordinary
fragmentation of the outer IP protocol version can be applied when
necessary.
Appendix C. Autoconfiguration for Constrained Platforms
On some platforms (e.g., popular cell phone operating systems), the
act of assigning a default IPv6 route and/or assigning an address to
an interface may not be permitted from a user application due to
security policy. Typically, those platforms include a TUN/TAP
interface [TUNTAP] that acts as a point-to-point conduit between user
applications and the AERO interface. In that case, the Client can
instead generate a "synthesized RA" message. The message conforms to
[RFC4861] and is prepared as follows:
o the IPv6 source address is the Client's AERO address
o the IPv6 destination address is all-nodes multicast
o the Router Lifetime is set to a time that is no longer than the
ACP DHCPv6 lifetime
o the message does not include a Source Link Layer Address Option
(SLLAO)
o the message includes a Prefix Information Option (PIO) with a /64
prefix taken from the ACP as the prefix for autoconfiguration
The Client then sends the synthesized RA message via the TUN/TAP
interface, where the operating system kernel will interpret it as
though it were generated by an actual router. The operating system
will then install a default route and use StateLess Address
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AutoConfiguration (SLAAC) to configure an IPv6 address on the TUN/TAP
interface. Methods for similarly installing an IPv4 default route
and IPv4 address on the TUN/TAP interface are based on synthesized
DHCPv4 messages [RFC2131].
Appendix D. Extending AERO Links Through Security Gateways
When an enterprise mobile node moves from a campus LAN connection to
a public Internet link, it must re-enter the enterprise via a
security gateway that has both a physical interface connection to the
Internet and a physical interface connection to the enterprise
internetwork. This most often entails the establishment of a Virtual
Private Network (VPN) link over the public Internet from the mobile
node to the security gateway. During this process, the mobile node
supplies the security gateway with its public Internet address as the
link-layer address for the VPN. The mobile node then acts as an AERO
Client to negotiate with the security gateway to obtain its ACP.
In order to satisfy this need, the security gateway also operates as
an AERO Server with support for AERO Client proxying. In particular,
when a mobile node (i.e., the Client) connects via the security
gateway (i.e., the Server), the Server provides the Client with an
ACP in a DHCPv6 PD exchange the same as if it were attached to an
enterprise campus access link. The Server then replaces the Client's
link-layer source address with the Server's enterprise-facing link-
layer address in all AERO messages the Client sends toward neighbors
on the AERO link. The AERO messages are then delivered to other
nodes on the AERO link as if they were originated by the security
gateway instead of by the AERO Client. In the reverse direction, the
AERO messages sourced by nodes within the enterprise network can be
forwarded to the security gateway, which then replaces the link-layer
destination address with the Client's link-layer address and replaces
the link-layer source address with its own (Internet-facing) link-
layer address.
After receiving the ACP, the Client can send IP packets that use an
address taken from the ACP as the network layer source address, the
Client's link-layer address as the link-layer source address, and the
Server's Internet-facing link-layer address as the link-layer
destination address. The Server will then rewrite the link-layer
source address with the Server's own enterprise-facing link-layer
address and rewrite the link-layer destination address with the
target AERO node's link-layer address, and the packets will enter the
enterprise network as though they were sourced from a node located
within the enterprise. In the reverse direction, when a packet
sourced by a node within the enterprise network uses a destination
address from the Client's ACP, the packet will be delivered to the
security gateway which then rewrites the link-layer destination
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address to the Client's link-layer address and rewrites the link-
layer source address to the Server's Internet-facing link-layer
address. The Server then delivers the packet across the VPN to the
AERO Client. In this way, the AERO virtual link is essentially
extended *through* the security gateway to the point at which the VPN
link and AERO link are effectively grafted together by the link-layer
address rewriting performed by the security gateway. All AERO
messaging services (including route optimization and mobility
signaling) are therefore extended to the Client.
In order to support this virtual link grafting, the security gateway
(acting as an AERO Server) must keep static neighbor cache entries
for all of its associated Clients located on the public Internet.
The neighbor cache entry is keyed by the AERO Client's AERO address
the same as if the Client were located within the enterprise
internetwork. The neighbor cache is then managed in all ways as
though the Client were an ordinary AERO Client. This includes the
AERO IPv6 ND messaging signaling for Route Optimization and Neighbor
Unreachability Detection.
Note that the main difference between a security gateway acting as an
AERO Server and an enterprise-internal AERO Server is that the
security gateway has at least one enterprise-internal physical
interface and at least one public Internet physical interface.
Conversely, the enterprise-internal AERO Server has only enterprise-
internal physical interfaces. For this reason security gateway
proxying is needed to ensure that the public Internet link-layer
addressing space is kept separate from the enterprise-internal link-
layer addressing space. This is afforded through a natural extension
of the security association caching already performed for each VPN
client by the security gateway.
Author's Address
Fred L. Templin (editor)
Boeing Research & Technology
P.O. Box 3707
Seattle, WA 98124
USA
Email: fltemplin@acm.org
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