IPv4 routes with an IPv6 next hop in the Babel routing protocol
draft-ietf-babel-v4viav6-06
| Document | Type | Active Internet-Draft (babel WG) | |
|---|---|---|---|
| Author | Juliusz Chroboczek | ||
| Last updated | 2021-07-02 (latest revision 2021-06-18) | ||
| Replaces | draft-bastian-babel-v4ov6 | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
| Formats | pdf htmlized bibtex | ||
| Stream | WG state | Submitted to IESG for Publication (wg milestone: Jul 2021 - IESG submission of I... ) | |
| Document shepherd | Donald Eastlake | ||
| Shepherd write-up | Show (last changed 2021-06-18) | ||
| IESG | IESG state | Publication Requested | |
| Action Holders |
(None)
|
||
| Consensus Boilerplate | Yes | ||
| Telechat date | |||
| Responsible AD | Martin Vigoureux | ||
| Send notices to | d3e3e3@gmail.com | ||
Network Working Group J. Chroboczek
Internet-Draft IRIF, University of Paris
Updates: 8966 (if approved) 18 June 2021
Intended status: Standards Track
Expires: 20 December 2021
IPv4 routes with an IPv6 next hop in the Babel routing protocol
draft-ietf-babel-v4viav6-06
Abstract
This document defines an extension to the Babel routing protocol that
allows annoncing routes to an IPv4 prefix with an IPv6 next-hop,
which makes it possible for IPv4 traffic to flow through interfaces
that have not been assigned an IPv4 address. This document updates
RFC 8966.
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 https://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 20 December 2021.
Copyright Notice
Copyright (c) 2021 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 (https://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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Specification of Requirements . . . . . . . . . . . . . . 3
2. Protocol operation . . . . . . . . . . . . . . . . . . . . . 3
2.1. Announcing v4-via-v6 routes . . . . . . . . . . . . . . . 4
2.2. Receiving v4-via-v6 routes . . . . . . . . . . . . . . . 4
2.3. Prefix and seqno requests . . . . . . . . . . . . . . . . 5
2.4. Other TLVs . . . . . . . . . . . . . . . . . . . . . . . 5
3. ICMPv4 and PMTU discovery . . . . . . . . . . . . . . . . . . 5
4. Protocol encoding . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Prefix encoding . . . . . . . . . . . . . . . . . . . . . 6
4.2. Changes to existing TLVs . . . . . . . . . . . . . . . . 7
5. Backwards compatibility . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
The role of a routing protocol is to build a routing table, a data
structure that maps network prefixes in a given family (IPv4 or IPv6)
to next hops, pairs of an outgoing interface and a neighbour's
network address, for example:
destination next hop
2001:db8:0:1::/64 eth0, fe80::1234:5678
203.0.113.0/24 eth0, 192.0.2.1
When a packet is routed according to a given routing table entry, the
forwarding plane typically uses a neighbour discovery protocol (the
Neighbour Discovery protocol (ND) [RFC4861] in the case of IPv6, the
Address Resolution Protocol (ARP) [RFC0826] in the case of IPv4) to
map the next-hop address to a link-layer address (a "MAC address"),
which is then used to construct the link-layer frames that
encapsulate forwarded packets.
It is apparent from the description above that there is no
fundamental reason why the destination prefix and the next-hop
address should be in the same address family: there is nothing
preventing an IPv6 packet from being routed through a next hop with
an IPv4 address (in which case the next hop's MAC address will be
obtained using ARP), or, conversely, an IPv4 packet from being routed
through a next hop with an IPv6 address. (In fact, it is even
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possible to store link-layer addresses directly in the next-hop entry
of the routing table, which is commonly done in networks using the
OSI protocol suite).
The case of routing IPv4 packets through an IPv6 next hop is
particularly interesting, since it makes it possible to build
networks that have no IPv4 addresses except at the edges and still
provide IPv4 connectivity to edge hosts. In addition, since an IPv6
next hop can use a link-local address that is autonomously
configured, the use of such routes enables a mode of operation where
the network core has no statically assigned IP addresses of either
family, which significantly reduces the amount of manual
configuration required.
We call a route towards an IPv4 prefix that uses an IPv6 next hop a
"v4-via-v6" route. This document describes an extension that allows
the Babel routing protocol [RFC8966] to announce v4-via-v6 routes
across interfaces that have no IPv4 addresses assigned. Section 3
describes procedures that ensure that all routers can originate
ICMPv4 packets, even if they have not been assigned any IPv4
addresses.
The extension described in this document is inspired by a previously
defined extension to the BGP protocol [RFC5549]. This document
updates [RFC8966].
1.1. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Protocol operation
The Babel protocol fully supports dual-stack operation: all data that
represent a neighbour address or a network prefix are tagged by an
Address Encoding (AE), a small integer that identifies the address
family (IPv4 or IPv6) of the address of prefix, and describes how it
is encoded. This extension defines a new AE, called v4-via-v6, which
has the same format as the existing AE for IPv4 addresses. This new
AE is only allowed in TLVs that carry network prefixes: TLVs that
carry a neighbour address use one of the normal encodings for IPv6
addresses.
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2.1. Announcing v4-via-v6 routes
A Babel node that needs to announce an IPv4 route over an interface
that has no assigned IPv4 address MAY make a v4-via-v6 announcement.
In order to do so, it first establishes an IPv6 next-hop address in
the usual manner (either by sending the Babel packet over IPv6, or by
including a Next Hop TLV containing an IPv6 address and using AE 2 or
3); it then sends an Update, with AE equal to 4 (v4-via-v6)
containing the IPv4 prefix being announced.
If the outgoing interface has been assigned an IPv4 address, then, in
the interest of maximising compatibility with existing routers, the
sender SHOULD prefer an ordinary IPv4 announcement; even in that
case, however, it MAY send a v4-via-v6 announcement. A node SHOULD
NOT send both ordinary IPv4 and v4-via-v6 announcements for the same
prefix over a single interface (if the update is sent to a multicast
address) or to a single neighbour (if sent to a unicast address),
since doing that provides no benefit while doubling the amount of
routing traffic.
Updates with infinite metric are retractions: they indicate that a
previously announced route is no longer available. Retractions do
not require a next hop, and there is therefore no difference between
v4-via-v6 retractions and ordinary retractions. A node MAY send IPv4
retractions only, or it MAY send v4-via-v6 retractions on interfaces
that have not been assigned an IPv4 address.
2.2. Receiving v4-via-v6 routes
Upon reception of an Update TLV with AE equal to 4 (v4-via-v6) and
finite metric, a Babel node computes the IPv6 next hop, as described
in Section 4.6.9 of [RFC8966]. If no IPv6 next hop exists, then the
Update MUST be silently ignored. If an IPv6 next hop exists, then
the node MAY acquire the route being announced, as described in
Section 3.5.3 of [RFC8966]; the parameters of the route are as
follows:
* the prefix, plen, router-id, seqno, metric MUST be computed as for
an IPv4 route, as described in Section 4.6.9 of [RFC8966];
* the next hop MUST be computed as for an IPv6 route, as described
in Section 4.6.9 of [RFC8966]: it is taken from the last preceding
Next Hop TLV with an AE field equal to 2 or 3; if no such entry
exists, and if the Update TLV has been sent in a Babel packet
carried over IPv6, then the next hop is the network-layer source
address of the packet.
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An Update TLV with a v4-via-v6 AE and metric equal to infinity is a
retraction: it announces that a previously available route is being
retracted. In that case, no next hop is necessary, and the
retraction is treated as described in Section 4.6.9 of [RFC8966].
As usual, a node MAY ignore the update, e.g., due to filtering
(Appendix C of [RFC8966]). If a node cannot install v4-via-v6
routes, e.g., due to hardware or software limitations, then routes to
an IPv4 prefix with an IPv6 next hop MUST NOT be selected, as
described in Section 3.5.3 of [RFC8966].
2.3. Prefix and seqno requests
Prefix and seqno requests are used to request an update for a given
prefix. Since they are not related to a specific next hop, there is
no semantic difference between IPv4 and v4-via-v6 requests.
Therefore, a node SHOULD NOT send requests of either kind with the AE
field being set to 4 (v4-via-v6); instead, it SHOULD request IPv4
updates by sending requests with the AE field being set to 1 (IPv4).
When receiving requests, AEs 1 (IPv4) and 4 (v4-via-v6) MUST be
treated in the same manner: the receiver processes the request as
described in Section 3.8 of [RFC8966]. If an Update is sent, then it
MAY be sent with AE 1 or 4, as described in Section 2.1 above,
irrespective of which AE was used in the request.
When receiving a request with AE 0 (wildcard), the receiver SHOULD
send a full route dump, as described in Section 3.8.1.1 of [RFC8966].
Any IPv4 routes contained in the route dump MAY use either AE 1
(IPv4) or AE 4 (v4-via-v6), as described in Section 2.1 above.
2.4. Other TLVs
The only other TLVs defined by [RFC8966] that carry an AE field are
Next Hop and TLV. Next Hop and IHU TLVs MUST NOT carry the AE 4 (v4-
via-v6).
3. ICMPv4 and PMTU discovery
The Internet Control Message Protocol (ICMPv4, or simply ICMP)
[RFC0792] is a protocol related to IPv4 that is primarily used to
carry diagnostic and debugging information. ICMPv4 packets may be
originated by end hosts (e.g., the "destination unreachable, port
unreachable" ICMPv4 packet), but they may also be originated by
intermediate routers (e.g., most other kinds of "destination
unreachable" packets).
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Some protocols deployed in the Internet rely on ICMPv4 packets sent
by intermediate routers. Most notably, path MTU Discovery (PMTUd)
[RFC1191] is an algorithm executed by end hosts to discover the
maximum packet size that a route is able to carry. While there exist
variants of PMTUd that are purely end-to-end [RFC4821], the variant
most commonly deployed in the Internet has a hard dependency on
ICMPv4 packets originated by intermediate routers: if intermediate
routers are unable to send ICMPv4 packets, PMTUd may lead to
persistent blackholing of IPv4 traffic.
Due to this kind of dependency, every Babel router that is able to
forward IPv4 traffic MUST be able originate ICMPv4 traffic. Since
the extension described in this document enables routers to forward
IPv4 traffic received over an interface that has not been assigned an
IPv4 address, a router implementing this extension MUST be able to
originate ICMPv4 packets even when the outgoing interface has not
been assigned an IPv4 address.
There are various ways to meet this requirement, and choosing between
them is left to the implementation. For example, if a router has an
interface that has been assigned an IPv4 address, or if an IPv4
address has been assigned to the router itself (to the "loopback
interface"), then that IPv4 address may be "borrowed" to serve as the
source of originated ICMPv4 packets. If no IPv4 address is
available, a router may choose a source address from a prefix known
to be unused, for example from a suitably chosen private address
range [RFC1918]. If no more suitable address is available, then a
router MAY use the IPv4 dummy address 192.0.0.8 as the source address
of the IMCPv4 packets that it sends. Note however that using the
same address on multiple routers may hamper debugging and fault
isolation, e.g., when using the "traceroute" utility.
4. Protocol encoding
This extension defines the v4-via-v6 AE, whose value is 4. This AE
is solely used to tag network prefixes, and MUST NOT be used to tag
neighbour addresses, e.g. in Next Hop or IHU TLVs.
This extension defines no new TLVs or sub-TLVs.
4.1. Prefix encoding
Network prefixes tagged with AE 4 (v4-via-v6) MUST be encoded and
decoded just like prefixes tagged with AE 1 (IPv4), as described in
Section 4.3.1 of [RFC8966].
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A new compression state for AE 4 (v4-via-v6) distinct from that of AE
1 (IPv4) is introduced, and MUST be used for address compression of
prefixes tagged with AE 4, as described in Section 4.6.9 of [RFC8966]
4.2. Changes to existing TLVs
The following TLVs MAY be tagged with AE 4 (v4-via-v6):
* Update (Type = 8)
* Route Request (Type = 9)
* Seqno Request (Type = 10)
As AE 4 (v4-via-v6) is suitable only for network prefixes, IHU
(Type = 5) and Next-Hop (Type = 7) TLVs MUST NOT be tagged with AE 4.
Such (incorrect) TLVs MUST be ignored upon reception.
4.2.1. Update
An Update (Type = 8) TLV with AE 4 is constructed as described in
Section 4.6.9 of [RFC8966] for AE 1 (IPv4), with the following
specificities:
* Prefix. The Prefix field is constructed according to Section 4.1
above.
* Next Hop. The next hop is determined as described in Section 2.2
above.
4.2.2. Other TLVs
When tagged with the AE 4, Route Request and Seqno Request updates
MUST be constructed and decoded as described in Section 4.6 of
[RFC8966], and the network prefixes contained within them decoded as
described in Section 4.1 above.
5. Backwards compatibility
This protocol extension adds no new TLVs or sub-TLVs.
This protocol extension uses a new AE. As discussed in Appendix D of
[RFC8966] and specified in the same document, implementations that do
not understand the present extension will silently ignore the various
TLVs that use this new AE. As a result, incompatible versions will
ignore v4-via-v6 routes. They will also ignore requests with AE 4,
which, as stated in Section 2.3, are NOT RECOMMENDED.
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Using a new AE introduces a new compression state, used to parse the
network prefixes. As this compression state is separate from other
AEs' states, it will not interfere with the compression state of
unextended nodes.
This extension reuses the next-hop state from AEs 2 and 3 (IPv6), but
makes no changes to the way in which it is updated, and therefore
causes no compatibility issues.
As mentioned in Section 2.1, ordinary IPv4 announcements are
preferred to v4-via-v6 announcements when the outgoing interface has
an assigned IPv4 address; doing otherwise would prevent routers that
do not implement this extension from learning the route being
announced.
6. IANA Considerations
IANA has allocated value 4 in the "Babel Address Encodings" registry
as follows:
+====+===========+=================+
| AE | Name | Reference |
+====+===========+=================+
| 4 | v4-via-v6 | (this document) |
+----+-----------+-----------------+
Table 1
7. Security Considerations
The extension defined in this document does not fundamentally change
the security properties of the Babel protocol. However, by allowing
IPv4 routes to be propagated across routers that have not been
assigned IPv4 addresses, it might invalidate the assumptions made by
network administrators, which could conceivably lead to security
issues.
For example, if an island of IPv4-only hosts is separated from the
IPv4 Internet by routers that have not been assigned IPv4 addresses,
a network administrator might reasonably assume that the IPv4-only
hosts are unreachable from the IPv4 Internet. This assumption is
broken if the intermediary routers implement the extension described
in this document, which might expose the IPv4-only hosts to traffic
from the IPv4 Internet. If this is undesirable, the flow of IPv4
traffic must be restricted by the use of suitable filtering rules
(Appendix C of [RFC8966]) together with matching packet filters in
the data plane.
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8. Acknowledgments
This protocol extension was originally designed, described and
implemented in collaboration with Theophile Bastian. Margaret Cullen
pointed out the issues with ICMP and helped coin the phrase "v4-via-
v6". The author is also indebted to Donald Eastlake, Toke Hoiland-
Jorgensen, and David Schinazi.
9. References
9.1. Normative References
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, DOI 10.17487/RFC0792, September 1981,
<https://www.rfc-editor.org/info/rfc792>.
[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/rfc/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC8966] Chroboczek, J. and D. Schinazi, "The Babel Routing
Protocol", RFC 8966, DOI 10.17487/RFC8966, January 2021,
<https://www.rfc-editor.org/info/rfc8966>.
9.2. Informative 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/rfc/rfc826>.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/info/rfc1191>.
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
J., and E. Lear, "Address Allocation for Private
Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
February 1996, <https://www.rfc-editor.org/info/rfc1918>.
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[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
<https://www.rfc-editor.org/info/rfc4821>.
[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/rfc/rfc4861>.
[RFC5549] Le Faucheur, F. and E. Rosen, "Advertising IPv4 Network
Layer Reachability Information with an IPv6 Next Hop",
RFC 5549, DOI 10.17487/RFC5549, May 2009,
<https://www.rfc-editor.org/rfc/rfc5549>.
Author's Address
Juliusz Chroboczek
IRIF, University of Paris
Case 7014
75205 Paris Cedex 13
France
Email: jch@irif.fr
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