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Versions: 00 01 draft-ietf-rtgwg-dst-src-routing

rtgwg                                                       D. Lamparter
Internet-Draft                                                    NetDEF
Intended status: Standards Track                           June 27, 2015
Expires: December 29, 2015


                       Destination/Source Routing
                draft-lamparter-rtgwg-dst-src-routing-01

Abstract

   This note specifies using packets' source addresses in route lookups
   as additional qualifier to be used in route lookup.  This applies to
   IPv6 [RFC2460] in general with specific considerations for routing
   protocol left for separate documents.

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
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   This Internet-Draft will expire on December 29, 2015.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
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   described in the Simplified BSD License.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
   2.  Principle of operation  . . . . . . . . . . . . . . . . . . .   3
     2.1.  Lookup ordering and disambiguation  . . . . . . . . . . .   3
     2.2.  Ordering Rationale  . . . . . . . . . . . . . . . . . . .   4
   3.  Applicability To Specific Situations  . . . . . . . . . . . .   4
     3.1.  Recursive Route Lookups . . . . . . . . . . . . . . . . .   4
     3.2.  Unicast Reverse Path Filtering  . . . . . . . . . . . . .   5
     3.3.  Multicast Reverse Path Forwarding . . . . . . . . . . . .   5
   4.  Interoperability  . . . . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   7
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   9.  Change Log  . . . . . . . . . . . . . . . . . . . . . . . . .   7
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     10.2.  Informative References . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   Since connectivity providers generally secure their ingress along the
   lines of BCP 38 [RFC2827], small multihomed networks have a need to
   ensure their traffic leaves their network with a correct combination
   of source address and exit taken.  This applies to networks of a
   particular pattern where the provider's default (dynamic) address
   provisioning methods are used and no fixed IP space is allocated,
   e.g.  home networks, small business users and mobile ad-hoc setups.

   While IPv4 networks would conventionally use NAT or policy routing to
   produce correct behaviour, this not desirable to carry over to IPv6.
   Instead, assigning addresses from multiple prefixes in parallel
   shifts the choice of uplink to the host.  However, now for finding
   the proper exit the source address of packets must be taken into
   account.

   For a general introduction and aspects of interfacing routers to
   hosts, refer to [I-D.sarikaya-6man-sadr-overview].

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].




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2.  Principle of operation

   The mechanism in this document is such that a source prefix is added
   to all route entries.  This document assumes all entries have a
   source prefix, with ::/0 as default value for entries installed
   without a specified source prefix.  This need not be implemented in
   this particular way, however the system MUST behave exactly as if it
   were.  In particular, a difference in behaviour between routes with a
   source prefix of ::/0 and routes without source prefix MUST NOT be
   visible.

   For uniqueness considerations, the source prefix factors MUST be
   taken into account for comparisons.  Two routes with identical
   information except the source prefix MAY exist and MUST be installed
   and matched.

2.1.  Lookup ordering and disambiguation

   Adding further criteria to be looked up when forwarding packets on a
   hop-by-hop basis has the very fundamental requirement that all
   routers behave the same way in choosing the most specific route when
   there are multiple eligible routes.

   For longest-match lookups, the source prefix is matched after the
   destination prefix.  This is to say, first the longest matching
   destination prefix is found, then the table is searched for the route
   with the longest source prefix match, while only considering routes
   with exactly the destination prefix previously found.  If and only if
   no such route exists (because none of the source prefixes match), the
   lookup moves to the next less specific destination prefix.

   A router MUST continue to a less specific destination prefix if no
   route matches on the source prefix.  It MUST NOT terminate lookup on
   such an event.

   Using A < B to mean "A is more specific than B", this is represented
   as:

   A < B :=    Adst <  Bdst
           || (Adst == Bdst && Asrc < Bsrc)











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2.2.  Ordering Rationale

   The ordering described by this document (destination before source)
   could as well be reversed, which would lead to semantically different
   behavior.

   Choosing destination to be evaluated first caters to the assumption
   that local networks should have full, contiguous connectivity to each
   other.  This implies that those specific local routes always match
   first based on destination, and use a zero ("all sources") source
   prefix.

   If the source prefix were to be matched first, this would result in a
   less specific (e.g.  default) route with a source prefix to match
   before those local routes.  In other terms, this would essentially
   divide local connectivity into zones based on source prefix, which is
   not the intention of this document.

   Hence, this document describes destination-first lookup.

3.  Applicability To Specific Situations

3.1.  Recursive Route Lookups

   TBD, multiple possible approaches:

      variant 1: ignore dst-src routes, only use routes with src ::/0

      variant 2: exact-match src prefixes from resolvee to resolvent
      (will not work for a lot of cases)

      variant 3: longer-match src prefixes from resolvee to resolvent
      (nexthop src may be superset of looked-up route)

      variant 4: create multiple instances of the route whose nexthop is
      resolved, with different source prefixes

   (Variant 4:)

   When doing recursive nexthop resolution, the route that is being
   resolved is installed in potentially multiple copies, inheriting all
   possible more-specific routes that match the nexthop as destination.
   The algorithm to do this is:

   1.  form the set of attributes for lookup by using the (unresolved,
       recursive) nexthop as destination (with full host prefix length,
       i.e.  /128), copy all other attributes from the original route




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   2.  find all routes that overlap with this set of attributes
       (including both more-specific and less-specific routes)

   3.  order the result from most to less specific

   4.  for each route, install a route using the original route's
       destination and the "logical and" overlap of each extra match
       attribute with same attribute from the set.  Copy nexthop data
       from the route under iteration.  Then, reduce the set of extra
       attributes by what was covered by the route just installed
       ("logical AND NOT").

   Example recursive route resolution

   route to be resolved:
   2001:db8:1234::/48, source 2001:db8:3456::/48,
                       recursive nexthop via 2001:db8:abcd::1

   routes considered for recursive nexthop:
   ::/0,                                              via fe80::1
   2001:db8:abcd::/48,                                via fe80::2
   2001:db8:abcd::/48,   source 2001:db8:3456:3::/64, via fe80::3
   2001:db8:abcd::1/128, source 2001:db8:3456:4::/64, via fe80::4

   recursive resolution result:
   2001:db8:1234::/48,   source 2001:db8:3456::/48,   via fe80::2
   2001:db8:1234::/48,   source 2001:db8:3456:3::/64, via fe80::3
   2001:db8:1234::/48,   source 2001:db8:3456:4::/64, via fe80::4


3.2.  Unicast Reverse Path Filtering

   Unicast reverse path filtering MUST use dst-src routes analog to its
   usage of destination-only routes.  However, the system MAY match
   either only incoming source against routes' destinations, or it MAY
   match source and destination against routes' destination and source.
   It MUST NOT ignore dst-src routes on uRPF checks.

3.3.  Multicast Reverse Path Forwarding

   Multicast Reverse Path Lookups are used to find paths towards the
   (known) sender of multicast packets.  Since the destination of these
   packets is the multicast group, it cannot be matched against the
   source part of a dst-src route.  Therefore, dst-src routes MUST be
   ignored for Multicast RPF lookups.

4.  Interoperability




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   Since a router implementing source/destination routing can have
   additional, more specific routes than one that doesn't implement
   source/destination routing, persistent loops can form between these
   systems.  To prevent this from happening, a simple rule must be
   followed:

   The set of qualifiers used to route a particular packet MUST be a
   subset of the qualifiers supported by the next hop.

   This means in particular that a router using the source address as
   extra qualifier MUST NOT route packets based on a source/destination
   route to a system that doesn't support source/destination routes (and
   hence doesn't understand the route).

   There are 3 possible approaches to avoid such a condition:

   1.  discard the packet (treat as destination unreachable)

   2.  calculate an alternate topology including only routers that
       support qualifier A

   3.  if the lookup returns the same nexthop without using qualifier A,
       use that result (i.e., the nexthop is known to correctly route
       the packet)

   Above considerations require under all circumstances a knowledge of
   the next router's capabilities.  For routing protocols based on hop-
   by-hop flooding (RIP [RFC2080], BGP [RFC4271]), knowing the peer's
   capabilities - or simply relying on systems to only flood what they
   understand - is sufficient.  Protocols building a link-state database
   (OSPF [RFC5340], IS-IS [RFC5308]) have the additional opportunity to
   calculate alternate paths based on knowledge of the entire domain,
   but cannot rely on routers flooding only link state they support
   themselves.

5.  IANA Considerations

   This document makes no requests to IANA.

6.  Security Considerations

   Systems operating under the principles of this document can have
   routes that are more specific than the previously most specific, i.e.
   host routes.  This can be a security concern if an operator was
   relying on the impossibility of hijacking such a route.

   While source/destination routing could be used as part of a security
   solution, it is not really intended for the purpose.  The approach



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   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.

7.  Privacy Considerations

   If a host's addresses are known, injecting a dst-src route allows
   isolation of traffic from that host, which may compromise privacy.
   However, this requires access to the routing system.  As with similar
   problems with the destination only, defending against it is left to
   general mechanisms protecting the routing infrastructure.

8.  Acknowledgements

   The base underlying this document was first outlaid by Ole Troan and
   Lorenzo Colitti in [I-D.troan-homenet-sadr] for application in the
   homenet area.

   This document is largely the result of discussions with Fred Baker
   and derives from [I-D.baker-ipv6-isis-dst-src-routing].

9.  Change Log

   Initial Version:  April 2015: merged routing-extra-qualifiers draft,
      new ordering rationale section

   Initial Version:  October 2014

10.  References

10.1.  Normative References

   [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.

10.2.  Informative References

   [I-D.baker-ipv6-isis-dst-src-routing]
              Baker, F., "IPv6 Source/Destination Routing using IS-IS",
              draft-baker-ipv6-isis-dst-src-routing-01 (work in
              progress), August 2013.

   [I-D.sarikaya-6man-sadr-overview]



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              Sarikaya, B., "Overview of Source Address Dependent
              Routing", draft-sarikaya-6man-sadr-overview-01 (work in
              progress), September 2014.

   [I-D.troan-homenet-sadr]
              Troan, O. and L. Colitti, "IPv6 Multihoming with Source
              Address Dependent Routing (SADR)", draft-troan-homenet-
              sadr-01 (work in progress), September 2013.

   [RFC2080]  Malkin, G. and R. Minnear, "RIPng for IPv6", RFC 2080,
              January 1997.

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, May 2000.

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [RFC5308]  Hopps, C., "Routing IPv6 with IS-IS", RFC 5308, October
              2008.

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, July 2008.

Author's Address

   David Lamparter
   NetDEF
   Leipzig  04103
   Germany

   Email: david@opensourcerouting.org

















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