MPLS Working Group K. Kompella
Internet-Draft R. Balaji
Intended status: Standards Track R. Thomas
Expires: 18 July 2021 Juniper Networks
14 January 2021
Label Distribution Using ARP
draft-kompella-mpls-larp-09
Abstract
This document describes extensions to the Address Resolution Protocol
to distribute MPLS labels for IPv4 and IPv6 host addresses.
Distribution of labels via ARP enables simple plug-and-play operation
of MPLS, which is key to deploying MPLS in data centers and
enterprises.
Status of This Memo
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This Internet-Draft will expire on 18 July 2021.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Approach . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Overview of Ethernet ARP . . . . . . . . . . . . . . . . . . 3
3. L-ARP Protocol Operation . . . . . . . . . . . . . . . . . . 4
3.1. Setup . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Egress Operation . . . . . . . . . . . . . . . . . . . . 5
3.3. Ingress Operation . . . . . . . . . . . . . . . . . . . . 5
3.4. Data Plane . . . . . . . . . . . . . . . . . . . . . . . 6
4. Attributes . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Secondary Attributes . . . . . . . . . . . . . . . . . . 7
5. L-ARP Message Format . . . . . . . . . . . . . . . . . . . . 7
5.1. Hardware Address Format . . . . . . . . . . . . . . . . . 9
5.2. CT TLV . . . . . . . . . . . . . . . . . . . . . . . . . 10
6. L-ARP Client Server Synchronisation . . . . . . . . . . . . . 10
6.1. L-ARP NAK . . . . . . . . . . . . . . . . . . . . . . . . 10
6.2. Bulk withdrawal . . . . . . . . . . . . . . . . . . . . . 11
6.3. Garbage Collection Requirements . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1. Normative References . . . . . . . . . . . . . . . . . . 12
10.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
This document describes extensions to the Address Resolution Protocol
(ARP) [RFC0826] to advertise label bindings for IP host addresses.
While there are well-established protocols, such as LDP, RSVP and
BGP, that provide robust mechanisms for label distribution, these
protocols tend to be relatively complex, and often require detailed
configuration for proper operation. There are situations where a
simpler protocol may be more suitable from an operational standpoint.
An example is the case where an MPLS Fabric is the underlay
technology in a Data Center; here, MPLS tunnels originate from host
machines. The host thus needs a mechanism to acquire label bindings
to participate in the MPLS Fabric, but in a simple, plug-and-play
manner. Existing signaling/routing protocols do not always meet this
need. Labeled ARP (L-ARP) is a proposal to fill that gap.
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1.1. Requirements Language
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].
The term "server" will be used in this document to refer to an ARP/
L-ARP server; the term "host" will be used to refer to a compute
server or other device acting as an ARP/L-ARP client.
1.2. Approach
ARP is a nearly ubiquitous protocol; every device with an Ethernet
interface, from hand-helds to hosts, have an implementation of ARP.
ARP is plug-and-play; ARP clients do not need configuration to use
ARP. That suggests that ARP may be a good fit for devices that want
to source and sink MPLS tunnels, but do so in a zero-config, plug-
and-play manner, with minimal impact to their code.
The approach taken here is to create a minor variant of the ARP
protocol, labeled ARP (L-ARP), which is distinguished by a new
hardware type, MPLS-over-Ethernet. Regular (Ethernet) ARP (E-ARP)
and L-ARP can coexist; a device, as an ARP client, can choose to send
out an E-ARP or an L-ARP request, depending on whether it needs
Ethernet or MPLS connectivity. Another device may choose to function
as an E-ARP server and/or an L-ARP server, depending on its ability
to provide an IP-to-Ethernet and/or IP-to-MPLS mapping.
2. Overview of Ethernet ARP
In the most straightforward mode of operation [RFC0826], ARP queries
are sent to resolve "directly connected" IP addresses. The ARP
request is broadcast, with the Target Protocol Address field (see
Section 5 for a description of the fields in an ARP message) carrying
the IP address of another node in the same subnet. All the nodes in
the LAN receive this ARP request. All the nodes, except the node
that owns the IP address, ignore the ARP request. The IP address
owner learns the MAC address of the sender from the Source Hardware
Address field in the ARP request, and unicasts an ARP reply to the
sender. The ARP reply carries the replying node's MAC address in the
Source Hardware Address field, thus enabling two-way communication
between the two nodes.
A variation of this scheme, known as "proxy ARP" [RFC2002], allows a
node to respond to an ARP request with its own MAC address, even when
the responding node does not own the requested IP address.
Generally, the proxy ARP response is generated by routers to attract
traffic for prefixes they can forward packets to. This scheme
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requires the host to send ARP queries for the IP address the host is
trying to reach, rather than the IP address of the router. When
there is more than one router connected to a network, proxy ARP
enables a host to automatically select an exit router without running
any routing protocol to determine IP reachability. Unlike regular
ARP, a proxy ARP request can elicit multiple responses, e.g., when
more than one router has connectivity to the address being resolved.
The sender must be prepared to select one of the responding routers.
Yet another variation of the ARP protocol, called 'Gratuitous ARP'
[RFC2002], allows a node to update the ARP cache of other nodes in an
unsolicited fashion. Gratuitous ARP is sent as either an ARP request
or an ARP reply. In either case, the Source Protocol Address and
Target Protocol Address contain the sender's address, and the Source
Hardware Address is set to the sender's hardware address. In case of
a gratuitous ARP reply, the Target Hardware Address is also set to
the sender's address.
3. L-ARP Protocol Operation
The L-ARP protocol builds on the proxy ARP model, and also leverages
gratuitous ARP model for asynchronous updates.
In this memo, we will refer to L-ARP clients (that make L-ARP
requests) and L-ARP servers (that send L-ARP responses). In
Figure 1, H1, H2 and H3 are L-ARP clients, and T1, T2 and T3 are
L-ARP servers. T4 is a member of the MPLS Fabric that may not be an
L-ARP server. Within the MPLS Fabric, the usual MPLS protocols (IGP,
LDP, RSVP-TE) are run. Say H1, H2 and H3 want to establish MPLS
tunnels to each other (for example, they are using BGP MPLS VPNs as
the overlay virtual network technology). H1 might also want to talk
to a member of the MPLS Fabric, say T. Also, the "protocol"
addresses in L-ARP requests are either IPv4 or IPv6 addresses; note
that while it is common to use Neighbor Discovery (ND) [RFC4861] for
"regular" ARP requests when dealing with IPv6 (i.e., to obtain
Ethernet MAC addresses corresponding to an IPv6 address), ND is not
used when the ARP request is for an MPLS label.
. . . . . . .
. .
H1 --- T1 T4
\ . MPLS .
\ . .
\ . Fabric .
H2 --- T2 T3 --- H3
. .
. . . . . . .
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Figure 1: MPLS Fabric
3.1. Setup
In Figure 1, the nodes T1-T4, and those in between making up the
"MPLS Fabric" are assumed to be running some protocol whereby they
can signal MPLS reachability to themselves and to other nodes (like
hosts H1-H3). T1-T3 are L-ARP servers; T4 need not be, since it
doesn't have an attached L-ARP client. H1-H3 are L-ARP clients.
3.2. Egress Operation
A node (say T3) that wants an attached node (say H3) to have MPLS
reachability allocates a label L3 to reach H3 and advertises this
label into the MPLS Fabric. This can be triggered by configuration
on T3, or when T3 first receives an L-ARP request from H3 (indicating
that H3 wants MPLS connectivity), or via some other protocol. T3
then advertises (H3, L3) to its peers in the MPLS Fabric so that all
members of the Fabric have connectivity to H3. This advertisement
can be one of the following:
* a "proxy" LDP message (sent on behalf of H3) with prefix H3 and
label L3; or
* a node SID advertised on behalf of H3; or
* a labeled BGP advertisement, with prefix H3, label L3 and next hop
self.
On receiving a packet with label L3, T3 pops the label and send
the packet to H3. (In the case of labeled BGP, there would be a
two-label stack, with outer label to reach T3 and inner label of
L3.) This is the usual operation of an MPLS Fabric, with the
addition of advertising labels for nodes outside the fabric.
3.3. Ingress Operation
A node (say H1, an L-ARP client) that needs an MPLS tunnel to another
node (say H3) identified by a host address (either IPv4 or IPv6)
broadcasts over all its interfaces an L-ARP request with the Target
Protocol Address set to H3 and Hardware Type set to "MPLS-over-
Ethernet". A node receiving the L-ARP request (say T1, an L-ARP
server) does the following:
1. checks if it has reachability to H3. If not, it ignores the
L-ARP request.
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2. if it does, T1 allocates a label TL3 to reach H3 (if it doesn't
already have such a label) and installs an L-FIB entry to swap L1
with the label (stack) to reach H3.
3. sends a (proxy) L-ARP reply to H1 with the Source Hardware
Address (SHA) set to (L, M), where M is T1's metric to H3. T1
may also set some attribute bits in the SHA.
3.4. Data Plane
To send a packet to H3 over an MPLS tunnel, H1 pushes L1 onto the
packet, sets the destination MAC address to M1 and sends it to T1.
On receiving this packet, T1 swaps the top label with the label(s)
for its MPLS tunnel to H3. If T1's reachability to H3 is via a
SPRING label stack, the label L1 acts as an implicit binding SID.
If H1 and H3 have an overlay connection (say an IPVPN [RFC4364] VPN-
foo) whereby VM1 on H1 wishes to talk to VM3 on H3 over VPN-foo, H1
does the following:
1. H1 learns information about VPN-foo via BGP (or an SDN
controller), including the VPN label VL3 to use to talk to VM3;
2. H1 installs a VRF for VPN-foo, with prefix VM3, label VL3 and
next hop H3;
3. H1 binds the local "veth" interface to VM1 to this VRF.
4. When VM1 sends a packet to dest IP address VM3 over its veth
interface, H1 looks up VM3 in the corresponding VRF, gets label
VL3. It then sends an L-ARP request for next hop H3, and gets
TL3.
5. Finally, H1 pushes the label pair (TL3, VL3) onto the packet from
VM1 and sends this to T1. This packet will then end up at VM3 on
H3.
Note that H1 broadcasts its L-ARP request over its attached
interfaces. H1 may receive several L-ARP replies; in that case, H1
can select any subset of these to send MPLS packets destined to H3.
As described later, the L-ARP response may contain certain parameters
that enable the client to make an informed choice. If the target H3
belongs to one of the subnets that H1 participates in, and H3 is
capable of sending L-ARP replies, H1 can use H3's response to send
MPLS packets to H3.
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4. Attributes
In addition to carrying a label stack to be used in the data plane,
an L-ARP reply carries some attributes that are typically used in the
control plane. One of these is a metric. The metric is the distance
from the L-ARP server to the destination. This allows an L-ARP
client that receives multiple responses to decide which ones to use,
and whether to load-balance across some of them. The metric
typically will be the IGP shortest path distance from server to the
destination; this makes comparing metrics from different servers
meaningful.
Another attribute is Entropy Label (EL) Capability. This attribute
says whether the destination is EL capable (ELC). In Figure 1, if T3
advertises a label to reach H3 and T3 is ELC, T3 can include in its
signaling to T1 that it is ELC. In that case, T1's L-ARP reply to H1
can have ELC bit set. This tells H1 that it may include (below the
outermost label) an Entropy Label Indicator followed by an Entropy
Label. This will help improve load balancing across the MPLS Fabric,
and possibly on the last hop to H3.
4.1. Secondary Attributes
Beyond the basic attributes that are carried with every L-ARP
request, there more optional attributes, for example, to ask for
certain characteristics of the path traffic takes to the destination.
These attributes are carried in TLVs that are carried in L-ARP
requests and replies.
One such TLV is the "CT" TLV. This TLV allows the L-ARP client to
request a label to a destination over a tunnel in the Transport Class
given by CT [I-D.kaliraj-idr-bgp-classful-transport-planes]. To
satisfy this request, the L-ARP server creates (or finds) a tunnel to
the destination that is routed over the CT Transport Plane, allocates
a label L, inserts an entry in the LFIB to swap L to the tunnel, and
sends L to the L-ARP client in its reply.
5. L-ARP Message Format
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ar$hrd | ar$pro |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ar$hln | ar$pln | ar$op |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// ar$sha (ar$hln octets) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// ar$spa (ar$pln octets) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// ar$tha (ar$hln octets) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// ar$tpa (ar$pln octets) //
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Value //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Value //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: L-ARP Packet Format
ar$hrd: Hardware Type: MPLS-over-Ethernet. The value of the field
used here is HTYPE-MPLS. To start with, we will use the
experimental value HW_EXP2 (256).
ar$pro: Protocol Type: IPv4/IPv6. The value of the field used here
is 0x0800 to resolve an IPv4 address and 0x86DD to resolve an IPv6
address.
ar$hln: Hardware Address Length: 6
ar$pln: Protocol Address Length: for an IPv4 address, the length is
4 octets; for an IPv6 address, it is 16.
ar$op: Operation Code: set to 1 for request, 2 for reply, and 10 for
ARP-NAK. Other op codes may be used as needed.
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ar$sha: Source Hardware Address: In an L-ARP request, this is
usually all zeros. In an L-ARP reply, Source Hardware Address is
the label to reach ar$spa, as specified in Figure 3 below.
ar$spa: Source Protocol Address: In an L-ARP request, this field
carries the sender's IP address. In an L-ARP reply, this field
carries the requested IP address (which may not be the sender's IP
address).
ar$tha: Target Hardware Address: In an L-ARP message, this is all
zeros.
ar$tpa: Target Protocol Address: In an L-ARP request, this field
carries the IP address for which the client is seeking an MPLS
label.
Type: a 2-octet field defining the Type of the TVL
Length: a 2-octet field defining the Length L of the TVL
Value: an L-octet field with the Value of the TLV
5.1. Hardware Address Format
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MPLS Label (20 bits) |E|Z|Z|Z| Metric ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... (3 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Label Format in L-ARP
MPLS Label:
The 20-bit label
E-bit:
Entropy Label Capable: this flag indicates whether the
corresponding label in the label stack can be followd by an
Entropy Label. If this flag is set, the client has the option of
inserting ELI and EL as specified in [RFC6790]. The client can
choose not to insert ELI/EL pair. If this flag is clear, the
client must not insert ELI/EL after the corresponding label.
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Z: These bits are not used, and SHOULD be set to zero on sending
and
ignored on receipt.
Metric:
The IGP metric to ar$tha from the point of view of the L-ARP
replier.
5.2. CT TLV
The CT TLV has Type (TBD) and Length 4 octets; the Value field
consists of the CT attribute.
6. L-ARP Client Server Synchronisation
The information that is communicated in the L-ARP reply can change
and hence it is necessary to have both the client and server
synchronised with each other. Loss of synchronisation between client
and server can have undesirable side effects such as packet drops or
packets getting diverted to wrong or suboptimal path. To keep the
client cache synchronised with server L-ARP protocol employs two
methods
1. Periodic refresh from L-ARP client side: To prevent stale
information remaining in the cache indefinitely, L-ARP client
should periodically send unicast L-ARP requests and refresh its
cache. In the absence of any replies for a configured no of
times, L-ARP client should purge the entry from its cache. The
server SHOULD send an explicit L-ARP NAK message in reply for
such unicast L-ARP requests received if it has no mapping for the
requested IP or when the entry is withdrawn.
2. Explicit notifications from server side: On advertised parameter
changes, the L-ARP server should broadcast an unsolicited L-ARP
reply with the updated parameters. L-ARP client on receipt of
the unsolicited reply should update the cache if the entry
already exists in its cache. If the entry does not exist client
should drop the unsolicited L-ARP packet without any processing.
6.1. L-ARP NAK
On events like label withdrawal the L-ARP server SHOULD notify the
clients to invalidate the cache entry by broadcasting an L-ARP NAK
message for that label. On receiving the NAK message a node SHOULD
delete the cache entry associated with the corresponding label.
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6.2. Bulk withdrawal
In cases where the server doesn't store advertised label bindings in
a persistent storage, it would be necessary to withdraw all the
advertised labels in case of server events like reboot. To handle
such scenarios L-ARP provides a bulk withdrawal request of its
advertised labels. Bulk withdrawal L-ARP request is made by
broadcasting an L-ARP NAK packet with all-zero address in the source
protocol address field.
6.3. Garbage Collection Requirements
To limit the storage needed on both server and client side for the
L-ARP caches, a node may need to periodically garbage-collect old
entries. The client can follow a LRU-based policy to reclaim the
entries. On server side the liveness of the entry can be determined
by the periodic refreshes from the client and in the absence of any
refreshes for a configurable time interval the labels advertised can
be reclaimed.
7. Security Considerations
There are many possible attacks on ARP: ARP spoofing, ARP cache
poisoning and ARP poison routing, to name a few. These attacks use
gratuitous ARP as the underlying mechanism, a mechanism used by
L-ARP. Thus, these types of attacks are applicable to L-ARP.
Furthermore, ARP does not have built-in security mechanisms; defenses
rely on means external to the protocol.
It is well outside the scope of this document to present a general
solution to the ARP security problem. One simple answer is to add a
TLV that contains a digital signature of the contents of the ARP
message. This TLV would be defined for use only in L-ARP messages,
although in principle, other ARP messages could use it as well. Such
an approach would, of course, need a review and approval by the
Security Directorate. If approved, the type of this TLV and its
procedures would be defined in this document. If some other
technique is suggested, the authors would be happy to include the
relevant text in this document, and refer to some other document for
the full solution.
8. IANA Considerations
IANA is requested to allocate a new ARP hardware type (from registry
hrd) for HTYPE-MPLS.
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9. Acknowledgments
Many thanks to Shane Amante for his detailed comments and
suggestions. Many thanks to the team in Juniper prototyping this
work for their suggestions on making this variant workable in the
context of existing ARP implementations. Thanks too to Luyuan Fang,
Alex Semenyaka and Dmitry Afanasiev for their comments and
encouragement.
10. References
10.1. Normative References
[RFC0826] Plummer, D., "An Ethernet Address Resolution Protocol: Or
Converting Network Protocol Addresses to 48.bit Ethernet
Address for Transmission on Ethernet Hardware", STD 37,
RFC 826, DOI 10.17487/RFC0826, November 1982,
<https://www.rfc-editor.org/info/rfc826>.
[RFC2002] Perkins, C., Ed., "IP Mobility Support", RFC 2002,
DOI 10.17487/RFC2002, October 1996,
<https://www.rfc-editor.org/info/rfc2002>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and
L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
RFC 6790, DOI 10.17487/RFC6790, November 2012,
<https://www.rfc-editor.org/info/rfc6790>.
10.2. Informative References
[I-D.kaliraj-idr-bgp-classful-transport-planes]
Vairavakkalai, K., Venkataraman, N., and B. Rajagopalan,
"BGP Classful Transport Planes", Work in Progress,
Internet-Draft, draft-kaliraj-idr-bgp-classful-transport-
planes-00, 8 May 2020, <http://www.ietf.org/internet-
drafts/draft-kaliraj-idr-bgp-classful-transport-planes-
00.txt>.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
2006, <https://www.rfc-editor.org/info/rfc4364>.
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[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,
<https://www.rfc-editor.org/info/rfc4861>.
Authors' Addresses
Kireeti Kompella
Juniper Networks
1133 Innovation Way
Sunnyvale 94089
USA
Phone: +1-408-745-2000
Email: kireeti.kompella@gmail.com
R. Balaji
Juniper Networks
Survey No.111/1 to 115/4, Wing A & B
Bangalore 560103
India
Email: balajir@juniper.net
Reji Thomas
Juniper Networks
Survey No.111/1 to 115/4, Wing A & B
Bangalore 560103
India
Email: rejithomas@juniper.net
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