Source-Specific Routing in the Babel Routing Protocol
RFC 9079

Document Type RFC - Proposed Standard (August 2021; No errata)
Authors Matthieu Boutier  , Juliusz Chroboczek 
Last updated 2021-08-26
Replaces draft-boutier-babel-source-specific
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Internet Engineering Task Force (IETF)                        M. Boutier
Request for Comments: 9079                                 J. Chroboczek
Category: Standards Track                      IRIF, University of Paris
ISSN: 2070-1721                                              August 2021

         Source-Specific Routing in the Babel Routing Protocol

Abstract

   Source-specific routing, also known as Source Address Dependent
   Routing (SADR), is an extension to traditional next-hop routing where
   packets are forwarded according to both their destination address and
   their source address.  This document describes an extension for
   source-specific routing to the Babel routing protocol.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9079.

Copyright Notice

   Copyright (c) 2021 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
   (https://trustee.ietf.org/license-info) 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

   1.  Introduction and Background
     1.1.  Application to Multihoming
     1.2.  Other Applications
     1.3.  Specificity of Prefix Pairs
   2.  Specification of Requirements
   3.  Data Structures
     3.1.  The Source Table
     3.2.  The Route Table
     3.3.  The Table of Pending Seqno Requests
   4.  Data Forwarding
   5.  Protocol Operation
     5.1.  Protocol Messages
     5.2.  Wildcard Messages
   6.  Compatibility with the Base Protocol
     6.1.  Starvation and Blackholes
   7.  Protocol Encoding
     7.1.  Source Prefix Sub-TLV
     7.2.  Source-Specific Update
     7.3.  Source-Specific Route Request
     7.4.  Source-Specific Seqno Request
   8.  IANA Considerations
   9.  Security Considerations
   10. References
     10.1.  Normative References
     10.2.  Informative References
   Acknowledgments
   Authors' Addresses

1.  Introduction and Background

   The Babel routing protocol [RFC8966] is a distance vector routing
   protocol for next-hop routing.  In next-hop routing, each node
   maintains a forwarding table that maps destination prefixes to next
   hops.  The forwarding decision is a per-packet operation that depends
   on the destination address of the packets and on the entries of the
   forwarding table.  When a packet is about to be routed, its
   destination address is compared to the prefixes of the routing table:
   the entry with the most specific prefix containing the destination
   address of the packet is chosen, and the packet is forwarded to the
   associated next hop.  Next-hop routing is a simple, well-understood
   paradigm that works satisfactorily in a large number of cases.

   The use of next-hop routing limits the flexibility of the routing
   system in two ways.  First, since the routing decision is local to
   each router, a router A can only select a route ABC...Z if its
   neighbouring router B has selected the route BC...Z.  Second, the
   only criterion used by a router to choose a route is the destination
   address: two packets with the same destination follow the same route.
   Yet, there are other data in the IP header that could conceivably be
   used to guide the routing decision -- the Type of Service (ToS) octet
   and, of course, the source address.

   Source-specific routing [SS-ROUTING], or Source Address Dependent
   Routing (SADR), is a modest extension to next-hop routing where the
   forwarding decision depends not only on the destination address but
   also on the source address of the packet being routed, which makes it
   possible for two packets with the same destination but different
   source addresses to be routed following different paths.

   This document describes a source-specific routing extension for the
   Babel routing protocol [RFC8966].  This involves minor changes to the
   data structures, which must include a source prefix in addition to
   the destination prefix already present, and some changes to the
   Update, Route Request, and Seqno Request TLVs, which are extended
   with a source prefix.  The source prefix is encoded using a mandatory
   sub-TLV ([RFC8966], Section 4.4).

1.1.  Application to Multihoming

   Multihoming is the practice of connecting a single network to two or
   more transit networks.  The main application of source-specific
   routing is a form of multihoming known as "multihoming with multiple
   addresses".

   Classical multihoming consists of assigning a provider-independent
   range of addresses to the multihomed network and announcing it to all
   transit providers.  While classical multihoming works well for large
   networks, the cost of obtaining a provider-independent address range
   and announcing it globally in the Internet is prohibitive for small
   networks.  Unfortunately, it is not possible to implement classical
   multihoming with ordinary provider-dependent addresses: in a network
   connected to two providers A and B, a packet with a source address
   allocated by A needs to be routed through the edge router connected
   to A.  If it is routed through the edge router connected to B, it
   will most likely be filtered (dropped), in accordance with [BCP84].

   In multihoming with multiple addresses, every host in the multihomed
   network is assigned multiple addresses, one for each transit
   provider.  Additional mechanisms are needed in order (i) to choose,
   for each packet, a source address that is associated with a provider
   that is currently up, and (ii) to route each packet towards the
   router connected to the provider associated with its source address.
   One might argue that multihoming with multiple addresses splits the
   difficult problem of multihoming into two simpler sub-problems.

   The issue of choosing a suitable source address is a decision local
   to the sending host and is an area of active research.  The simplest
   solution is to use a traditional transport-layer protocol, such as
   TCP, and to probe all available source addresses at connection time,
   analogously to what is already done with destination addresses,
   either sequentially [RFC6724] or in parallel [RFC8305].  Since the
   transport-layer protocol is not aware of the multiple available
   addresses, flows are interrupted when the selected provider goes down
   (from the point of view of the user, all TCP connections are dropped
   when the network environment changes).  A better user experience can
   be provided by making all of the potential source and destination
   addresses available to higher-layer protocols, either at the
   transport layer [RFC8684] [RFC4960] or at the application layer
   [RFC8445].

   Source-specific routing solves the problem of routing a packet to the
   edge router indicated by its source address.  Every edge router
   announces into the routing domain a default route specific to the
   prefix associated with the provider it is connected to.  This route
   is propagated all the way to the routers on the access link, which
   are therefore able to route every packet to the correct router.
   Hosts simply send packets to their default router -- no host changes
   are necessary at the network layer.

1.2.  Other Applications

   In addition to multihoming with multiple addresses, we are aware of
   two applications of source-specific routing.  Tunnels and VPNs are
   packet encapsulation techniques that are commonly used in the
   Internet to establish a network-layer topology that is different from
   the physical topology.  In some deployments, the default route points
   at the tunnel; this causes the network stack to attempt to send
   encapsulated packets through the tunnel, which causes it to break.
   Various solutions to this problem are possible, the most common of
   which is to point a host route at the tunnel endpoint.

   When source-specific routing is available, it becomes possible to
   announce through the tunnel a default route that is specific to the
   prefix served by the tunnel.  Since the encapsulated packets have a
   source address that is not within that prefix, they are not routed
   through the tunnel.

   The third application of source-specific routing is controlled
   anycast.  Anycast is a technique in which a single destination
   address is used to represent multiple network endpoints, collectively
   called an "anycast group".  A packet destined to the anycast group is
   routed to an arbitrary member of the group, typically the one that is
   nearest according to the routing protocol.

   In many applications of anycast, such as DNS root servers, the
   nondeterminism of anycast is acceptable; some applications, however,
   require finer control.  For example, in some Content Distribution
   Networks (CDNs), every endpoint is expected to handle a well-defined
   subset of the client population.  With source-specific routing, it is
   possible for each member of the anycast group to announce a route
   specific to its client population, a technique that is both simpler
   and more robust than manually tweaking the routing protocol's metric
   ("prepending" in BGP).

1.3.  Specificity of Prefix Pairs

   In ordinary next-hop routing, when multiple routing table entries
   match the destination of a packet, the "longest prefix rule" mandates
   that the most specific entry applies.  The reason why this rule makes
   sense is that the set of prefixes has the following "tree property":

      For any prefixes P and P', either P and P' are disjoint, or one is
      more specific than the other.

   It would be a natural proposition to order pairs of prefixes
   pointwise: to define that (D,S) is more specific than (D',S') when D
   is more specific than D and S is more specific than S'.
   Unfortunately, the set of pairs of prefixes with the pointwise
   ordering doesn't satisfy the tree property.  Indeed, consider the
   following two pairs:

      (2001:db8:0:1::/64, ::/0) and (::/0, 2001:db8:0:2::/64)

   These two pairs are not disjoint (a packet with destination
   2001:db8:0:1::1 and source 2001:db8:0:2::1 is matched by both), but
   neither is more specific than the other.  The effect is that there is
   no natural, unambiguous way to interpret a routing table such as the
   following:

             destination                source     next-hop
       2001:db8:0:1::/64                  ::/0            A
                    ::/0     2001:db8:0:2::/64            B

   A finer ordering of pairs of prefixes is required in order to avoid
   all ambiguities.  There are two natural choices: destination-first
   ordering, where (D,S) is more specific than (D',S') when

   *  D is strictly more specific than D', or

   *  D = D', and S is more specific than S'

   and, symmetrically, source-first ordering, in which sources are
   compared first and destinations second.

   Expedient as it would be to leave the choice to the implementation,
   this is not possible: all routers in a routing domain must use the
   same ordering lest persistent routing loops occur.  Indeed, consider
   the following topology:

      A --- B --- C --- D

   Suppose that A announces a route for (::/0, 2001:db8:0:2::/64), while
   D announces a route for (2001:db8:0:1::/64, ::/0).  Suppose further
   that B uses destination-first ordering while C uses source-first
   ordering.  Then a packet that matches both routes, say, with
   destination 2001:db8:0:1::1 and source 2001:db8:0:2::1, would be sent
   by B towards D and by C towards A and would therefore loop
   indefinitely between B and C.

   This document mandates (Section 4) that all routers use destination-
   first ordering, which is generally believed to be more useful than
   source-first ordering.  Consider the following topology, where A is
   an edge router connected to the Internet and B is an internal router
   connected to an access network N:

     (::/0, S)             (D, ::/0)
      Internet --- A --- B --- N

   A announces a source-specific default route with source S (::/0, S),
   while B announces a nonspecific route to prefix D.  Consider what
   happens to a packet with a destination in D and a source in S.  With
   destination-first ordering, the packet is routed towards the network
   N, which is the only way it can possibly reach its destination.  With
   source-first ordering, on the other hand, the packet is sent towards
   the Internet, with no hope of ever reaching its destination in N.

2.  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.

3.  Data Structures

   A number of the conceptual data structures described in Section 3.2
   of [RFC8966] contain a destination prefix.  This specification
   extends these data structures with a source prefix.  Data from the
   original protocol, which do not specify a source prefix, are stored
   with a zero-length source prefix, which matches the exact same set of
   packets as the original, non-source-specific data.

3.1.  The Source Table

   Every Babel node maintains a source table, as described in [RFC8966],
   Section 3.2.5.  A source-specific Babel node extends this table with
   the following field:

   *  The source prefix (sprefix, splen) specifying the source address
      of packets to which this entry applies.

   The source table is now indexed by 5-tuples of the form (prefix,
   plen, sprefix, splen, router-id).

   Note that the route entry contains a source (see Sections 2 and 3.2.5
   of [RFC8966]) that itself contains both destination and source
   prefixes.  These are two different concepts and must not be confused.

3.2.  The Route Table

   Every Babel node maintains a route table, as described in [RFC8966],
   Section 3.2.6.  Each route table entry contains, among other data, a
   source, which this specification extends with a source prefix as
   described above.  The route table is now indexed by 5-tuples of the
   form (prefix, plen, sprefix, splen, neighbour), where the first four
   components are obtained from the source.

3.3.  The Table of Pending Seqno Requests

   Every Babel node maintains a table of pending seqno requests, as
   described in [RFC8966], Section 3.2.7.  A source-specific Babel node
   extends this table with the following entry:

   *  The source prefix (sprefix, splen) being requested.

   The table of pending seqno requests is now indexed by 5-tuples of the
   form (prefix, plen, sprefix, splen, router-id).

4.  Data Forwarding

   As noted in Section 1.3, source-specific tables can, in general, be
   ambiguous, and all routers in a routing domain must use the same
   algorithm for choosing applicable routes.  An implementation of the
   extension described in this document MUST choose routing table
   entries by using destination-first ordering, where routing table
   entry R1 is preferred to routing table entry R2 when either R1's
   destination prefix is more specific than R2's or the destination
   prefixes are equal and R1's source prefix is more specific than R2's.

   In practice, this means that a source-specific Babel implementation
   must take care that any lower layer that performs packet forwarding
   obey these semantics.  More precisely:

   *  if the lower layers implement destination-first ordering, then the
      Babel implementation SHOULD use them directly;

   *  if the lower layers can hold source-specific routes but not with
      the right semantics, then the Babel implementation MUST either
      silently ignore any source-specific routes or disambiguate the
      routing table by using a suitable disambiguation algorithm (see
      Section V.B of [SS-ROUTING] for such an algorithm);

   *  if the lower layers cannot hold source-specific routes, then a
      Babel implementation MUST silently ignore any source-specific
      routes.

5.  Protocol Operation

   This extension does not fundamentally change the operation of the
   Babel protocol, and we therefore only describe differences between
   the original protocol and the extended protocol.

   In the original protocol, three TLVs carry a destination prefix:
   Update, Route Request, and Seqno Request TLVs.  This specification
   extends these messages so that they may carry a Source Prefix sub-
   TLV, as described in Section 7.  The sub-TLV is marked as mandatory
   so that an unextended implementation will silently ignore the whole
   enclosing TLV.  A node obeying this specification MUST NOT send a TLV
   with a zero-length source prefix; instead, it sends a TLV with no
   Source Prefix sub-TLV.  Conversely, an extended implementation MUST
   interpret an unextended TLV as carrying a source prefix of zero
   length.  Taken together, these properties ensure interoperability
   between the original and extended protocols (see Section 6).

5.1.  Protocol Messages

   This extension allows three TLVs of the original Babel protocol to
   carry a source prefix: Update TLVs, Route Request TLVs, and Seqno
   Request TLVs.

   In order to advertise a route with a non-zero length source prefix, a
   node sends a source-specific update, i.e., an update with a Source
   Prefix sub-TLV.  When a node receives a source-specific update
   (prefix, source prefix, router-id, seqno, metric) from a neighbour
   neigh, it behaves as described in [RFC8966], Section 3.5.3, except
   that the entry under consideration is indexed by (prefix, plen,
   sprefix, splen, neigh) rather than just (prefix, plen, neigh).

   Similarly, when a node needs to send a request of either kind that
   applies to a route with a non-zero length source prefix, it sends a
   source-specific request, i.e., a request with a Source Prefix sub-
   TLV.  When a node receives a source-specific request, it behaves as
   described in Section 3.8 of [RFC8966], except that the request
   applies to the route table entry carrying the source prefix indicated
   by the Source Prefix sub-TLV.

5.2.  Wildcard Messages

   In the original protocol, the address encoding (AE) value 0 is used
   for wildcard messages: messages that apply to all routes of any
   address family and with any destination prefix.  Wildcard messages
   are allowed in two places in the protocol: wildcard retractions are
   used to retract all of the routes previously advertised by a node on
   a given interface, and wildcard route requests are used to request a
   full dump of the route table from a given node.  Wildcard messages
   are intended to apply to all routes, including routes decorated with
   additional data and AE values to be defined by future extensions;
   hence, this specification extends wildcard operations to apply to all
   routes, whatever the value of the source prefix.

   More precisely, a node receiving an update with the AE field set to 0
   and the Metric field set to infinity (a wildcard retraction) MUST
   apply the route acquisition procedure described in Section 3.5.3 of
   [RFC8966] to all of the routes that it has learned from the sending
   node, whatever the value of the source prefix.  A node MUST NOT send
   a wildcard retraction with an attached source prefix, and a node that
   receives a wildcard retraction with a source prefix MUST ignore the
   retraction.

   Similarly, a node that receives a route request with the AE field set
   to 0 (a wildcard route request) SHOULD send a full routing table
   dump, including routes with a non-zero length source prefix.  A node
   MUST NOT send a wildcard request that carries a source prefix, and a
   node receiving a wildcard request with a source prefix MUST ignore
   the request.

6.  Compatibility with the Base Protocol

   The protocol extension defined in this document is, to a great
   extent, interoperable with the base protocol defined in [RFC8966]
   (and all previously standardised extensions).  More precisely, if
   non-source-specific routers and source-specific routers are mixed in
   a single routing domain, Babel's loop-avoidance properties are
   preserved, and, in particular, no persistent routing loops will
   occur.

   However, this extension is encoded using mandatory sub-TLVs,
   introduced in [RFC8966], and therefore is not compatible with the
   older version of the Babel routing protocol [RFC6126], which does not
   support mandatory sub-TLVs.  Consequently, this extension MUST NOT be
   used in a routing domain in which some routers implement [RFC6126];
   otherwise, persistent routing loops may occur.

6.1.  Starvation and Blackholes

   In general, the discarding of source-specific routes by non-source-
   specific routers will cause route starvation.  Intuitively, unless
   there are enough non-source-specific routes in the network, non-
   source-specific routers will suffer starvation and discard packets
   for destinations that are only announced by source-specific routers.

   In the common case where all source-specific routes are originated at
   one of a small set of edge routers, a simple yet sufficient condition
   for avoiding starvation is to build a connected source-specific
   backbone that includes all of the edge routers and announce a non-
   source-specific default route towards the backbone.

7.  Protocol Encoding

   This extension defines a new sub-TLV used to carry a source prefix:
   the Source Prefix sub-TLV.  It can be used within an Update, Route
   Request, or Seqno Request TLV to match a source-specific entry of the
   route table in conjunction with the destination prefix natively
   carried by these TLVs.

   Since a source-specific routing entry is characterised by a single
   destination prefix and a single source prefix, a source-specific
   message contains exactly one Source Prefix sub-TLV.  A node MUST NOT
   send more than one Source Prefix sub-TLV in a TLV, and a node
   receiving more than one Source Prefix sub-TLV in a single TLV MUST
   ignore the TLV.  It MAY ignore the whole packet.

7.1.  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 = 128  |    Length     |  Source Plen  | Source Prefix...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Fields:

   Type           Set to 128 to indicate a Source Prefix sub-TLV.

   Length         The length of the body, in octets, exclusive of the
                  Type and Length fields.

   Source Plen    The length of the advertised source prefix, in bits.
                  This MUST NOT be 0.

   Source Prefix  The source prefix being advertised.  This field's size
                  is (Source Plen)/8 octets rounded upwards.

   The length of the body TLV is normally of size 1+(Source Plen)/8
   rounded upwards.  If the Length field indicates a length smaller than
   that, then the sub-TLV is corrupt, and the whole enclosing TLV must
   be ignored; if the Length field indicates a length that is larger,
   then the extra octets contained in the sub-TLV MUST be silently
   ignored.

   The contents of the Source Prefix sub-TLV are interpreted according
   to the AE of the enclosing TLV.  If a TLV with AE equal to 0 contains
   a Source Prefix sub-TLV, then the whole enclosing TLV MUST be
   ignored.  If a TLV contains multiple Source Prefix sub-TLVs, then the
   whole TLV MUST be ignored.

   Note that this sub-TLV is a mandatory sub-TLV.  Therefore, as
   described in Section 4.4 of [RFC8966], the whole TLV MUST be ignored
   if that sub-TLV is not understood (or malformed).

7.2.  Source-Specific Update

   The source-specific update is an Update TLV with a Source Prefix sub-
   TLV.  It advertises or retracts source-specific routes in the same
   manner as routes with non-source-specific updates (see [RFC8966]).  A
   wildcard retraction (update with AE equal to 0) MUST NOT carry a
   Source Prefix sub-TLV.

   Babel uses a stateful compression scheme to reduce the size taken by
   destination prefixes in Update TLVs (see Section 4.5 of [RFC8966]).
   The source prefix defined by this extension is not compressed.  On
   the other hand, compression is allowed for the destination prefixes
   carried by source-specific updates.  As described in Section 4.5 of
   [RFC8966], unextended implementations will correctly update their
   parser state while otherwise ignoring the whole TLV.

7.3.  Source-Specific Route Request

   A source-specific route request is a Route Request TLV with a Source
   Prefix sub-TLV.  It prompts the receiver to send an update for a
   given pair of destination and source prefixes, as described in
   Section 3.8.1.1 of [RFC8966].  A wildcard request (route request with
   AE equal to 0) MUST NOT carry a Source Prefix sub-TLV; if a wildcard
   request with a Source Prefix sub-TLV is received, then the request
   MUST be ignored.

7.4.  Source-Specific Seqno Request

   A source-specific seqno request is a Seqno Request TLV with a Source
   Prefix sub-TLV.  It requests that the receiving node perform the
   procedure described in Section 3.8.1.2 of [RFC8966] but applied to a
   pair consisting of a destination and source prefix.

8.  IANA Considerations

   IANA has allocated sub-TLV number 128 for the Source Prefix sub-TLV
   in the "Babel Sub-TLV Types" registry.

9.  Security Considerations

   The extension defined in this document adds a new sub-TLV to three
   sub-TLVs already present in the original Babel protocol and does not
   change the security properties of the protocol itself.  However, the
   additional flexibility provided by source-specific routing might
   invalidate the assumptions made by some network administrators, which
   could conceivably lead to security issues.

   For example, a network administrator might be tempted to abuse route
   filtering (Appendix C of [RFC8966]) as a security mechanism.  Unless
   the filtering rules are designed to take source-specific routing into
   account, they might be bypassed by a source-specific route, which
   might cause traffic to reach a portion of a network that was thought
   to be protected.  A network administrator might also assume that no
   route is more specific than a host route and use a host route in
   order to direct traffic for a given destination through a security
   device (e.g., a firewall); source-specific routing invalidates this
   assumption, and, in some topologies, announcing a source-specific
   route might conceivably be used to bypass the security device.

10.  References

10.1.  Normative References

   [BCP84]    Baker, F. and P. Savola, "Ingress Filtering for Multihomed
              Networks", BCP 84, RFC 3704, March 2004.

              Sriram, K., Montgomery, D., and J. Haas, "Enhanced
              Feasible-Path Unicast Reverse Path Forwarding", BCP 84,
              RFC 8704, February 2020.

              <https://www.rfc-editor.org/info/bcp84>

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8966]  Chroboczek, J. and D. Schinazi, "The Babel Routing
              Protocol", RFC 8966, DOI 10.17487/RFC8966, January 2021,
              <https://www.rfc-editor.org/info/rfc8966>.

10.2.  Informative References

   [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
              RFC 4960, DOI 10.17487/RFC4960, September 2007,
              <https://www.rfc-editor.org/info/rfc4960>.

   [RFC6126]  Chroboczek, J., "The Babel Routing Protocol", RFC 6126,
              DOI 10.17487/RFC6126, April 2011,
              <https://www.rfc-editor.org/info/rfc6126>.

   [RFC6724]  Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
              <https://www.rfc-editor.org/info/rfc6724>.

   [RFC8305]  Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
              Better Connectivity Using Concurrency", RFC 8305,
              DOI 10.17487/RFC8305, December 2017,
              <https://www.rfc-editor.org/info/rfc8305>.

   [RFC8445]  Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
              Connectivity Establishment (ICE): A Protocol for Network
              Address Translator (NAT) Traversal", RFC 8445,
              DOI 10.17487/RFC8445, July 2018,
              <https://www.rfc-editor.org/info/rfc8445>.

   [RFC8684]  Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C.
              Paasch, "TCP Extensions for Multipath Operation with
              Multiple Addresses", RFC 8684, DOI 10.17487/RFC8684, March
              2020, <https://www.rfc-editor.org/info/rfc8684>.

   [SS-ROUTING]
              Boutier, M. and J. Chroboczek, "Source-Specific Routing",
              IFIP Networking Conference,
              DOI 10.1109/IFIPNetworking.2015.7145305, May 2015,
              <http://arxiv.org/pdf/1403.0445>.

Acknowledgments

   The authors are indebted to Donald Eastlake, Joel Halpern, and Toke
   Hoiland-Jorgensen for their help with this document.

Authors' Addresses

   Matthieu Boutier
   IRIF, University of Paris
   Case 7014
   75205 Paris Cedex 13
   France

   Email: boutier@irif.fr

   Juliusz Chroboczek
   IRIF, University of Paris
   Case 7014
   75205 Paris Cedex 13
   France

   Email: jch@irif.fr