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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.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on March 01, 2014.

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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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