Network Working Group T. Bastian
Internet-Draft Ecole Normale Superieure, Paris
Updates: 6126bis (if approved) J. Chroboczek
Intended status: Experimental IRIF, University of Paris-Diderot
Expires: April 23, 2021 October 20, 2020
IPv4 routes with an IPv6 next-hop in the Babel routing protocol
draft-ietf-babel-v4viav6-00
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.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Specification of Requirements . . . . . . . . . . . . . . 3
2. Protocol operation . . . . . . . . . . . . . . . . . . . . . 3
2.1. Announcing v4-over-v6 routes . . . . . . . . . . . . . . 3
2.2. Receiving v4-over-v6 routes . . . . . . . . . . . . . . . 3
2.3. Prefix and seqno requests . . . . . . . . . . . . . . . . 4
2.4. Other TLVs . . . . . . . . . . . . . . . . . . . . . . . 4
3. Backwards compatibility . . . . . . . . . . . . . . . . . . . 4
4. Protocol encoding . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Prefix encoding . . . . . . . . . . . . . . . . . . . . . 5
4.2. Changes for existing TLVs . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
Traditionally, a routing table maps a network prefix of a given
address family to a next-hop address in the same address family. The
sole purpose of this next-hop address is to serve as an input to a
protocol that will map it to a link-layer address, Neighbour
Discovery (ND) [RFC4861] in the case of IPv6, Address Resolution
(ARP) [RFC0826] in the case of IPv4. Therefore, there is no reason
why the address family of the next hop address should match that of
the prefix being announced: an IPv6 next-hop yields a link-layer
address that is suitable for forwarding both IPv6 or IPv4 traffic.
We call a route towards an IPv4 prefix that uses an IPv6 next hop a
"v4-over-v6" route. Since an IPv6 next-hop can use a link-local
address that is autonomously configured, the use of v4-over-v6 routes
enables a mode of operation where the network core has no statically
assigned IP addresses of either family, thus significantly reducing
the amount of manual configuration.
This document describes an extension that allows the Babel routing
protocol [RFC6126bis] to announce routes towards IPv6 prefixes with
IPv4 next hops. The extension is inspired by a previously defined
extension to the BGP protocol [RFC5549].
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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 double-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-over-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 the normal encodings for IPv6
addresses.
2.1. Announcing v4-over-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-over-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); it then sends
an Update with AE equal to TBD 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 use a v4-over-v6 announcement. A node SHOULD
NOT send both ordinary IPv4 and v4-over-v6 annoucements 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 doubles the amount of routing traffic while
providing no benefit.
2.2. Receiving v4-over-v6 routes
Upon reception of an Update TLV with a v4-over-v6 AE, a Babel node
computes the IPv6 next-hop, as described in Section 4.6.9 of
[RFC6126bis]. 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
[RFC6126bis]; the parameters of the route are as follows:
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o the prefix, plen, router-id, seqno, metric MUST be computed as for
an IPv4 route, as described in Section 4.6.9 of [RFC6126bis];
o the next-hop MUST be computed as for an IPv6 route, as described
in Section 4.6.9 of [RFC6126bis]: 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.
As usual, a node MAY ignore the update, e.g., due to filtering
(Appendix C of [RFC6126bis]). If a node cannot install v4-over-v6
routes, eg., 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 [RFC6126bis].
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 ordinary IPv4 and v4-over-v6 requests.
A node SHOULD NOT send requests of either kind with the AE field
being set to TBD (v4-over-v6); instead, it SHOULD request IPv4
updates using requests with the AE field being set to 1 (IPv4).
When receiving requests, AEs 1 (IPv4) and TBD (v4-over-v6) MUST be
treated in the same manner: the receiver processes the request as
described in Section 3.8 of [RFC6126bis]. If an Update is sent, then
it MAY be sent with AE 1 or TBD, 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
[RFC6126bis]. Any IPv4 routes contained in the route dump MAY use
either AE 1 or AE TBD, as described in Section 2.1 above.
2.4. Other TLVs
The only other TLV defined by [RFC6126bis] that carries an AE field
is the IHU TLV. IHU TLVs MUST NOT carry the AE TBD (v4-over-v6).
3. 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
[RFC6126bis] and specified in the same document, implementations that
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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-over-v6 routes. They will also ignore
requests with AE TBD, which, as stated in Section 2.3, are NOT
RECOMMENDED.
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 it is updated, and therefore causes no
compatibility issues.
As mentioned in Section 2.1, ordinary IPv4 announcements are
preferred to v4-over-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.
4. Protocol encoding
This extension defines the v4-over-v6 AE, whose value is TBD. This
AE is solely used to tag network prefixes, and MUST NOT be used to
tag peers' addresses, eg. in Next-Hop or IHU TLVs.
This extension defines no new TLVs or sub-TLVs.
4.1. Prefix encoding
Network prefixes tagged with AE TBD MUST be encoded and decoded as
prefixes tagged with AE 1 (IPv4), as described in Section 4.3.1 of
[RFC6126bis].
A new compression state for AE TBD (v4-over-v6) distinct from that of
AE 1 (IPv4) is introduced, and MUST be used for address compression
of prefixes tagged with AE TBD, as described in Section 4.6.9 of
[RFC6126bis]
4.2. Changes for existing TLVs
The following TLVs MAY be tagged with AE TBD:
o Update (Type = 8)
o Route Request (Type = 9)
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o Seqno Request (Type = 10)
As AE TBD is suitable only to tag network prefixes, IHU (Type = 5)
and Next-Hop (Type = 7) TLVs MUST NOT be tagged with AE TBD. Such
TLVs MUST be silently ignored.
4.2.1. Update
An Update (Type = 8) TLV with AE = TBD is constructed as described in
Section 4.6.9 of [RFC6126bis] for AE 1 (IPv4), with the following
specificities:
o Prefix. The Prefix field is constructed according to the
Section 4.1 above.
o Next hop. The next hop is determined as described in Section 2.2
above.
4.2.2. Other valid TLVs tagged with AE = TBD
Any other valid TLV tagged with AE = TBD MUST be constructed and
decoded as described in Section 4.6 of [RFC6126bis]. Network
prefixes within MUST be constructed and decoded as described in
Section 4.1 above.
5. IANA Considerations
IANA is requested to allocate a value (4 suggested) in the "Babel
Address Encodings" registry as follows:
+-----+------------+-----------------+
| AE | Name | Reference |
+-----+------------+-----------------+
| TBD | v4-over-v6 | (this document) |
+-----+------------+-----------------+
6. Security Considerations
This extension does not fundamentally change the security properties
of the Babel protocol: as described in Section 6 of [RFC6126bis],
Babel must be protected by a suitable cryptographic mechanism in
order to be made secure.
However, enabling this extension will allow IPv4 traffic to flow
through sections of a network that have not been assigned IPv4
addresses, which, in turn, might allow IPv4 traffic to reach areas of
the network that were previously inaccessible to such traffic. If
this is undesirable, the flow of IPv4 traffic must be restricted by
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the use of suitable filtering rules (Appendix C of [RFC6126bis])
together with matching access control rules in the data plane.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997.
[RFC6126bis]
Chroboczek, J. and D. Schinazi, "The Babel Routing
Protocol", draft-ietf-babel-rfc6126bis-17 (work in
progress), February 2020.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017.
7.2. 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.
[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.
[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.
Authors' Addresses
Theophile Bastian
Ecole Normale Superieure, Paris
France
Email: contact@tobast.fr
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Juliusz Chroboczek
IRIF, University of Paris-Diderot
Case 7014
75205 Paris Cedex 13
France
Email: jch@irif.fr
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