Network Working Group F.J. Baker
Internet-Draft Cisco Systems
Intended status: Standards Track August 28, 2013
Expires: March 01, 2014
IPv6 Source/Destination Routing using IS-IS
draft-baker-ipv6-isis-dst-src-routing-01
Abstract
This note describes the changes necessary for IS-IS to route IPv6
traffic from a specified prefix to a specified prefix.
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Table of Contents
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1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
2. Theory of Routing . . . . . . . . . . . . . . . . . . . . . . 2
2.1. Notation . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Dealing with ambiguity . . . . . . . . . . . . . . . . . 3
2.3. Interactions with other constraints . . . . . . . . . . . 4
3. Extensions necessary for IPv6 Source/Destination Routing in
IS-IS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Source Prefix sub-TLV . . . . . . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 6
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
This specification builds on IS-IS for IPv6 [RFC5308] and its
extensible TLV. This note defines the sub-TLV for an IPv6 [RFC2460]
Source Prefix, to define routes from a source prefix to a destination
prefix.
This implies not simply routing "to a destination", but routing "to
that destination AND from a specified source". It may be combined
with other qualifying attributes, such as "traffic going to that
destination AND using a specified flow label AND from a specified
source prefix". The obvious application is egress routing, as
required for a multihomed entity with a provider-allocated prefix
from each of several upstream networks. Traffic within the network
could be source/destination routed as well, or could be implicitly or
explicitly routed from "any prefix", ::/0. Other use cases are
described in [I-D.baker-rtgwg-src-dst-routing-use-cases]. If a FIB
contains a route to a given destination from one or more prefixes not
including ::/0, and a given packet destined there that has a source
address that is in none of them, the packet in effect has no route,
just as if the destination itself were not in the route table.
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].
2. Theory of Routing
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Both IS-IS and OSPF perform their calculations by building a lattice
of routers and links from the router performing the calculation to
each router, and then use routes (sequences in the lattice) to get to
destinations that those routes advertise connectivity to. Following
the SPF algorithm, calculation starts by selecting a starting point
(typically the router doing the calculation), and successively adding
{link, router} pairs until one has calculated a route to every router
in the network. As each router is added, including the original
router, destinations that it is directly connected to are turned into
routes in the route table: "to get to 2001:db8::/32, route traffic to
{interface, list of next hop routers}". For immediate neighbors to
the originating router, of course, there is no next hop router;
traffic is handled locally.
In this context, the route is qualified by a source prefix; It is
installed into the FIB with the destination prefix, and the FIB
applies the route if and only if the IPv6 source address also matches
the advertised prefix. Of course, there may be multiple LSPs in the
RIB with the same destination and differing source prefixes; these
may also have the same or differing next hop lists. The intended
forwarding action is to forward matching traffic to one of the next
hop routers associated with this destination and source prefix, or to
discard non-matching traffic as "destination unreachable".
LSAs that lack a source prefix sub-TLV match any source address
(i.e., the source prefix TLV defaults to ::/0), by definition.
2.1. Notation
For the purposes of this document, a route from the prefix A to the
prefix B (in other words, whose source prefix is A and whose
destination prefix is B) is expressed as A->B. A packet with the
source address A and the destination address B is similarly described
as A->B.
2.2. Dealing with ambiguity
In any routing protocol, there is the possibility of ambiguity. For
example, one router might advertise a fairly general prefix - a
default route, a discard prefix (which consumes all traffic that is
not directed to an instantiated subnet), or simply an aggregated
prefix while another router advertises a more specific one. In
source/destination routing, potentially ambiguous cases include cases
in which the link state database contains two routes A->B' and A'->B,
in which A' is a more specific prefix within the prefix A and B' is a
more specific prefix within the prefix B. Traditionally, we have
dealt with ambiguous destination routes using a "longest match first"
rule. If the same datagram matches more than one destination prefix
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advertised within an area, we follow the route with the longest
matching prefix.
With source/destination routes, as noted in
[I-D.baker-rtgwg-src-dst-routing-use-cases], we follow a similar but
slightly different rule; the FIB lookup MUST yield the route with the
longest matching destination prefix that also matches the source
prefix constraint. In the event of a tie on the destination prefix,
it MUST also match the longest matching source prefix among those
options.
An example of the issue is this. Suppose we have two routes:
1. 2001:db8:1::/48 -> 2001:db8:3:3::/64
2. 2001:db8:2::/48 -> 2001:db8:3::/48
and a packet
2001:db8:2::1 -> 2001:db8:3:3::1
If we require the algorithm to follow the longest destination match
without regard to the source, the destination address matches
2001:db8:3:3::/64 (the first route), and the source address doesn't
match the constraint of the first route; we therefore have no route.
The FIB algorithm, in this example, must therefore match the second
route, even though it is not the longest destination match, because
it also matches the source address.
2.3. Interactions with other constraints
In the event that there are other constraints on routing, such as
proposed in [I-D.baker-ipv6-isis-dst-flowlabel-routing], the effect
is a logical AND. The FIB lookup must yield the route with the
longest matching destination prefix that also matches each of the
constraints.
3. Extensions necessary for IPv6 Source/Destination Routing in IS-IS
Section 2 of [RFC5308] defines the "IPv6 Reachability TLV", and
carries in it destination prefix advertisements. It has the
capability of extension, using sub-TLVs.
We define the Source Prefix Sub-TLV as in Section 3.1. As noted in
Section 2, any IPv6 Reachability TLV that does not specify a source
prefix is understood to as specifying ::/0 as the source prefix.
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3.1. Source Prefix sub-TLV
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 |Prefix Length | Prefix
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Source Prefix Sub-TLV
Source Prefix Type: assigned by IANA
TLV Length: Length of the sub-TLV in octets
Prefix Length: Length of the prefix in bits
Prefix: (source prefix length+7)/8 octets of prefix
4. IANA Considerations
The source prefix type mentioned in Section 3 must be defined.
5. Security Considerations
While source/destination routing could be used as part of a security
solution, it is not really intended for the purpose. The approach
limits routing, in the sense that it routes traffic to an appropriate
egress, or gives a way to prevent communication between systems not
included in a source/destination route, and in that sense could be
considered similar to an access list that is managed by and scales
with routing.
6. Acknowledgements
7. References
7.1. Normative References
[ISO.10589.1992]
International Organization for Standardization,
"Intermediate system to intermediate system intra-domain-
routing routine information exchange protocol for use in
conjunction with the protocol for providing the
connectionless-mode Network Service (ISO 8473)", ISO
Standard 10589, 1992.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC2460] Deering, S.E. and R.M. Hinden, "Internet Protocol, Version
6 (IPv6) Specification", RFC 2460, December 1998.
[RFC5308] Hopps, C., "Routing IPv6 with IS-IS", RFC 5308, October
2008.
7.2. Informative References
[I-D.baker-ipv6-isis-dst-flowlabel-routing]
Baker, F., "Using IS-IS with Role-Based Access Control",
draft-baker-ipv6-isis-dst-flowlabel-routing-00 (work in
progress), February 2013.
[I-D.baker-rtgwg-src-dst-routing-use-cases]
Baker, F., "Requirements and Use Cases for Source/
Destination Routing", draft-baker-rtgwg-src-dst-routing-
use-cases-00 (work in progress), August 2013.
Appendix A. Change Log
Initial Version: February 2013
updated Version: August 2013
Author's Address
Fred Baker
Cisco Systems
Santa Barbara, California 93117
USA
Email: fred@cisco.com
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