Network Working Group                                         F.J. Baker
Internet-Draft                                             Cisco Systems
Intended status: Standards Track                            D. Lamparter
Expires: April 21, 2016                                           NetDEF
                                                        October 19, 2015

              IPv6 Source/Destination Routing using IS-IS


   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.

   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

   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 April 21, 2016.

Copyright Notice

   Copyright (c) 2015 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
   ( 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.

Table of Contents

Baker & Lamparter        Expires April 21, 2016                 [Page 1]

Internet-Draft      IS-IS Source/Destination Routing        October 2015

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
   2.  Theory of Routing . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Notation  . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Dealing with ambiguity  . . . . . . . . . . . . . . . . .   4
     2.3.  Multi-topology Routing  . . . . . . . . . . . . . . . . .   5
   3.  Protocol encoding for IPv6 Source Prefix information  . . . .   6
     3.1.  Source Prefix sub-TLV . . . . . . . . . . . . . . . . . .   6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   7
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Appendix A.  Correctness considerations . . . . . . . . . . . . .   8
   Appendix B.  Change Log . . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   This specification defines how to exchange destination/source routing
   [I-D.ietf-rtgwg-dst-src-routing] information in IS-IS for IPv6
   [RFC5308] routing environments.  To this extent, a new sub-TLV for an
   IPv6 [RFC2460] Source Prefix is added, and Multi Topology Routing
   [RFC5120] is employed to address compatibility and isolation

   The router MUST implement the Destination/Source Routing mechanism
   described in [I-D.ietf-rtgwg-dst-src-routing].  This implies not
   simply routing "to a destination", but routing "to that destination
   AND from a specified source".  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

Baker & Lamparter        Expires April 21, 2016                 [Page 2]

Internet-Draft      IS-IS Source/Destination Routing        October 2015

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Theory of Routing

   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".

   TLVs that lack a source prefix sub-TLV match any source address
   (i.e., the source prefix TLV defaults to ::/0), by definition.

   To ensure that routers without support for Destination/Source routing
   are excluded from path calculation for routes with a non-default
   source prefix, a separate MTID is used to carry Destination/Source
   routes.  A router MUST NOT participate in a topology with such an
   MTID unless it implements Destination/Source routing.

   There is a distinct Destination/Source Routing MTID for each of the
   underlying base MT topologies the information applies to.  The set of
   routes propagated towards the forwarding plane is the union of the
   information in the base topology and the D/S Routing MTID.  Incoming
   connectivity information with a default or non-present source prefix
   is advertised in the base topology, routes with non-default source
   prefix are advertised in the D/S Routing MTID.

Baker & Lamparter        Expires April 21, 2016                 [Page 3]

Internet-Draft      IS-IS Source/Destination Routing        October 2015

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
   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

   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

Baker & Lamparter        Expires April 21, 2016                 [Page 4]

Internet-Draft      IS-IS Source/Destination Routing        October 2015

   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.  Multi-topology Routing

   As outlined in Section 2, this document specifies the use of separate
   topologies for Multi Topology Routing [RFC5120] to carry Destination/
   Source routing information.  These topologies form pairs with a base
   topology each as follows:

   base               base    D/S
   designated usage   MTID    MTID
   default topology   0       TBD-MT0
   IPv4 management    1       n/a
   IPv6 default       2       TBD-MT2
   IPv4 multicast     3       n/a
   IPv6 multicast     4       n/a
   IPv6 management    5       TBD-MT5

                     Destination/Source Routing MTIDs

   The rationale for in-/excluding base MTIDs to provide a D/S MTID for
   is as follows:

   MTID 0:  The base (non-MTR) topology in some installations carries
      all routing information, including IPv6 reachabilities.  In such a
      setup, the topology with MTID TBD-MT0 is used to carry associated
      D/S reachabilities.

   MTIDs 1 and 3:  Topologies with MTID 1 and 3 carry exclusively IPv4
      reachabilities.  Thus, no IPv6 D/S topology is created to
      associate with them.

   MTID 2:  The topology with MTID 2 carries IPv6 reachabilities in
      common M-ISIS setups.  (MTID 0 in such cases carries exclusively
      IPv4 reachability information.)  Associated IPv6 D/S
      reachabilities MUST be carried in MTID TBD-MT2.

   MTID 4:  MTID 4, while carrying IPv6 connectivity information, is
      used for multicast RPF lookups.  Since Destination/Source routing
      is not compatible with multicast RPF lookups, no associated D/S
      MTID is defined for IS-IS.

Baker & Lamparter        Expires April 21, 2016                 [Page 5]

Internet-Draft      IS-IS Source/Destination Routing        October 2015

   MTID 5:  An alternate management/administration topology may carry
      its routing information in MTID 5.  Destination/Source routing is
      applicable to this and MUST use MTID TBD-MT5 to carry associated
      reachability TLVs.

   Note that the different topology ID is the sole and only mechanism of
   both capability detection and backwards compatibility.  D/S routing
   will operate correctly if D/S routing information is put in the same
   topology as non-D/S information, but adding an IS that does not
   support D/S routing will then -undetectably- lead to incorrect
   routing decisions, possibly including loops.

   As this compatibility mechanism is not considered optional, M-ISIS
   MUST therefore be implemented for supporting the protocol outlined in
   this document.  Even installations that previously used only MTID 0
   (i.e.  no M-ISIS) would need to start using MTID TBD-MT0.

   Systems that use topology IDs different than the values reserved by
   IANA should apply the considerations from this section analogously.

3.  Protocol encoding for IPv6 Source Prefix information

   Destination/Source reachabilities are originated using TLV 237, using
   an additional sub-TLV to carry the source prefix as follows.

   As noted in Section 2, any IPv6 Reachability TLV that does not
   specify a source prefix is functionally identical to specifying ::/0
   as the source prefix.  Such routes SHOULD NOT be originated into the
   D/S MTID, but rather into the base MTID.

3.1.  Source Prefix sub-TLV

   The following Sub-TLV is defined for TLV 237:

    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:  TBD-TLV (assigned by IANA)

   TLV Length:  Length of the sub-TLV in octets

   Prefix Length:  Length of the prefix in bits

Baker & Lamparter        Expires April 21, 2016                 [Page 6]

Internet-Draft      IS-IS Source/Destination Routing        October 2015

   Prefix:  (source prefix length+7)/8 octets of prefix

4.  IANA Considerations

   IANA is requested to allocate Values from the "IS-IS Multi-Topology
   ID Values" registry as follows:

   TBD-MT0:  IPv6 Dest/Source routing corresponding to topology 0

   TBD-MT2:  Reserved for IPv6 Dest/Source routing corresponding to
      topology 2

   TBD-MT5:  Reserved for IPv6 Dest/Source routing corresponding to
      topology 5

   Additionally, IANA is requested to allocate an IS-IS codepoint from
   the "Sub-TLVs for TLVs 135, 235, 236, and 237" registry:

   Type:  TBD-TLV

   Description:  IPv6 SADR Source Prefix

   Applicable to TLV 237:  Yes

   Applicable to TLVs 135, 235, 236:  No

5.  Security Considerations

   The same injection and resource exhaustion attack scenarios as with
   all routing protocols apply.

   Security considerations from [I-D.ietf-rtgwg-dst-src-routing] are
   particularly relevant to this document, in particular the possibility
   to inject (more) specific routes to hijack traffic.

6.  Privacy Considerations

   No privacy considerations apply to this document, as it only
   specifies routing control plane information.

7.  Acknowledgements

   Thanks to Les Ginsberg, Chris Hopps and Acee Lindem for valuable
   feedback on this document.  (TODO: incomplete.)

8.  References

Baker & Lamparter        Expires April 21, 2016                 [Page 7]

Internet-Draft      IS-IS Source/Destination Routing        October 2015

8.1.  Normative References

              Lamparter, D., "Destination/Source Routing", draft-ietf-
              rtgwg-dst-src-routing-00 (work in progress), October 2015.

   [IS-IS]    ISO/IEC, "Intermediate System to Intermediate System
              Intra-Domain Routing Exchange Protocol for use in
              Conjunction with the Protocol for Providing the
              Connectionless-mode Network Service (ISO 8473)", ISO/IEC
              10589:2002, Second Edition, 2002.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2460]  Deering, S.E. and R.M. Hinden, "Internet Protocol, Version
              6 (IPv6) Specification", RFC 2460, December 1998.

   [RFC5120]  Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
              Topology (MT) Routing in Intermediate System to
              Intermediate Systems (IS-ISs)", RFC 5120, February 2008.

   [RFC5308]  Hopps, C., "Routing IPv6 with IS-IS", RFC 5308, October

8.2.  Informative References

              Baker, F., "Using IS-IS with Token-based Access Control",
              draft-baker-ipv6-isis-dst-flowlabel-routing-01 (work in
              progress), August 2013.

              Baker, F., "Requirements and Use Cases for Source/
              Destination Routing", draft-baker-rtgwg-src-dst-routing-
              use-cases-01 (work in progress), October 2014.

Appendix A.  Correctness considerations

   While Multi-Topology routing in general can be assumed to work
   correctly when used on its own, this may not apply to a scenario
   mixing route calculation results as suggested in this document.
   However, this specific application is easily understandable as

      Systems that do not implement D/S routing will not participate in
      the D/S topology.  They will calculate SPF in the base topology.
      Packets routed by such system will either (a) cross only non-D/S

Baker & Lamparter        Expires April 21, 2016                 [Page 8]

Internet-Draft      IS-IS Source/Destination Routing        October 2015

      routers and reach the last hop as intended, or (b) cross a D/S
      router at some point.

      For case (b), the D/S router may (b1) or may not (b2) have a more
      specific D/S route.  In case (b2), packets will be routed based on
      the same decisions that a non-D/S system would apply, so they will
      reach their last hop without any differences.

      For case (b1), a break in forwarding behaviour happens for packets
      as they hit the first D/S-capable router, possibly after
      traversing some non-D/S systems.  That router will apply D/S
      routing - which, since the path calculation is performed in the D/
      S topology, means that the packet is from there on routed on a
      path that only contains D/S capable systems.  It will thus reach
      the D/S last hop as intended.

      Packets starting out on a D/S-capable router fall into cases (b1)
      or (b2) as if a non-D/S router routed them first.

      If, for case (b1), the system knows of the existence of a more
      specific D/S route, but cannot calculate a valid path, it may
      either apply non-D/S routing (i.e.  not install any route) or
      discard the packet (i.e.  install a discard route).  The next hop
      will either be a non-D/S system, or a D/S system with the same
      link-state information (and thus again unable to calculate a valid
      path -- or, more specifically, won't calculate a path that
      includes the previous router).

   The compatibility mechanics thus rest on 2 pillars:

      D/S routes will match as more specific if applicable

      Packets will transit into D/S routing but not out of it

Appendix B.  Change Log

   (to be removed)

   Initial Version:  February 2013

   updated Version:  August 2013

   Added MTR:  August 2014

   Split into 4 drafts:  October 2014

   Dropped 'Critical Sub-TLV' drafts  June 2015

Baker & Lamparter        Expires April 21, 2016                 [Page 9]

Internet-Draft      IS-IS Source/Destination Routing        October 2015

   MT clarifications  October 2015

Authors' Addresses

   Fred Baker
   Cisco Systems
   Santa Barbara, California  93117


   David Lamparter
   Leipzig  04103


Baker & Lamparter        Expires April 21, 2016                [Page 10]