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Versions: 00 01 02 03 draft-ietf-babel-source-specific

Network Working Group                                         M. Boutier
Internet-Draft                                             J. Chroboczek
Updates: 6126bis                       IRIF, University of Paris-Diderot
(if approved)                                              June 15, 2017
Intended status: Standards Track
Expires: December 17, 2017


                    Source-Specific Routing in Babel
                 draft-boutier-babel-source-specific-02

Abstract

   This document describes an extension to the Babel routing protocol to
   support source-specific routing.

Status of this Memo

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   This Internet-Draft will expire on December 17, 2017.

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Table of Contents

   1.  TODOs  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction and background  . . . . . . . . . . . . . . . . .  3
   3.  Data Structures  . . . . . . . . . . . . . . . . . . . . . . .  3
     3.1.  The Source Table . . . . . . . . . . . . . . . . . . . . .  3
     3.2.  The Route Table  . . . . . . . . . . . . . . . . . . . . .  3
     3.3.  The Table of Pending Requests  . . . . . . . . . . . . . .  4
   4.  Data Forwarding  . . . . . . . . . . . . . . . . . . . . . . .  4
   5.  Protocol Operation . . . . . . . . . . . . . . . . . . . . . .  5
     5.1.  Source-specific messages . . . . . . . . . . . . . . . . .  5
     5.2.  Route Acquisition  . . . . . . . . . . . . . . . . . . . .  5
     5.3.  Wildcard requests  . . . . . . . . . . . . . . . . . . . .  6
   6.  Backwards compatibility  . . . . . . . . . . . . . . . . . . .  7
     6.1.  Loop-avoidance . . . . . . . . . . . . . . . . . . . . . .  8
     6.2.  Starvation and Blackholes  . . . . . . . . . . . . . . . .  8
   7.  Protocol Encoding  . . . . . . . . . . . . . . . . . . . . . .  9
     7.1.  Source Prefix sub-TLV  . . . . . . . . . . . . . . . . . .  9
     7.2.  Source-specific Update . . . . . . . . . . . . . . . . . .  9
     7.3.  Source-specific (Route) Request  . . . . . . . . . . . . . 10
     7.4.  Source-Specific Seqno Request  . . . . . . . . . . . . . . 10
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   9.  Security considerations  . . . . . . . . . . . . . . . . . . . 10
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 10
     10.2. Informative References . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
























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1.  TODOs

   o  Source Prefix sub-TLV type: TBD
   o  check references (Section) for BABEL in 6126bis


2.  Introduction and background

   Source-specific routing (other known as Source Address Dependant
   Routing, SAD Routing or SADR) is an extension to traditional next-hop
   routing where packets are routed according to both their destination
   and their source address.  This document describes the source-
   specific routing extension to the Babel routing protocol as defined
   in 6126bis [BABEL].  It notably requires the sub-TLV mandatory bit.

   Background information about source-specific routing is provided in
   [SS-ROUTING].


3.  Data Structures

   This extension adds some data to the data structures maintained by a
   Babel node.

3.1.  The Source Table

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

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

   If a source table entry has a zero length source prefix (splen equals
   to 0), then the entry is a non-source-specific entry, and is treated
   just like a source table entry defined by the original Babel
   protocol.

   With this extension the route entry contains a source which itself
   contains a source prefix.  Notwithstanding the accidental similarity
   in their names, these are two very different concepts, and should not
   be confused.

3.2.  The Route Table

   Every Babel node maintains a route table, as described in [BABEL],
   Section 3.2.6.  With this extension, this table is indexed by the
   5-tuple (prefix, plen, source prefix, source plen, router-id)



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   obtained from the associated source table entry.

   If a route table entry has a zero length source prefix, then the
   entry is a non-source-specific entry, and is treated just like a
   route table entry defined by the original Babel protocol.

3.3.  The Table of Pending Requests

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

   o  the source prefix being requested.


4.  Data Forwarding

   In next-hop routing, if two routing table entries overlap, then one
   is necessarily more specific than the other; the "longest prefix
   rule" specifies that the most specific applicable routing table entry
   is chosen.

   With source-specific routing, there might no longer be a most
   specific applicable prefix: two routing table entries might match a
   given packet without one necessarily being more specific than the
   other.  Consider for example the following fragment of a routing
   table:

      (2001:DB8:0:1::/64, ::/0, A)
      (::/0, 2001:DB8:0:2::/64, B)

   This specifies that all packets with destination in 2001:DB8:0:1::/64
   are to be routed through A, while packets with a source in 2001:DB8:
   0:2::/64 are to be routed through B. A packet with source 2001:DB8:0:
   2::42 and destination 2001:DB8:0:1::57 matches both rules, although
   neither is more specific than the other.  A choice is necessary, and
   unless the choice being made is the same on all routers in a routing
   domain, persistent routing loops may occur.

   A Babel implementation MUST choose routing table entries by using the
   so-called destination-first ordering, where a routing table entry R1
   is preferred to a 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 more
   formal terms, routing table entries are compared using the
   lexicographic product of the destination prefix ordering by the
   source prefix ordering.)




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

   o  If the lower layers implement the destination-first ordering, then
      the Babel implementation MAY use them directly;
   o  If the lower layers can hold source-specific routes, but not with
      the right semantics, then the Babel implementation MUST
      disambiguate the routing table by using a suitable disambiguation
      algorithm (see [SS-ROUTING] for such an algorithm);
   o  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.  We only described the fundamental differences
   between the original protocol and the extension in this section.  The
   other mechanisms described in [BABEL] (Section 3) may be infered by
   using pairs of (destination, source) prefixes instead of just
   (destination) prefixes.

5.1.  Source-specific messages

   A route of this extension with a zero-length source prefix is the
   same than a route without source prefix (a route of the classical
   Babel).  In both of the cases, packets are accepted independantly of
   their source address.  Thus, a route is said source-specific only if
   its source prefix has a non-zero length.

   Three messages are used to communicate informations on routes:
   Updates, Route Requests and Seqno Requests.  With this extension,
   these messages carry an additionnal source prefix if (and only if)
   the corresponding route is source-specific.  More formally, an
   Update, a Route Request and a Seqno Request MUST carry a source
   prefix if they concern a source-specific route (non-zero length
   source prefix) and MUST NOT carry a source prefix otherwise (zero
   length source prefix).  A message which carry a source prefix is said
   source-specific.

5.2.  Route Acquisition

   When a non-source-specific Babel node receives a source-specific
   update, it just ignores it.

   On receipt of a source-specific update (id, prefix, source prefix,



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   seqno, metric), a source-specific Babel node behaves as described in
   [BABEL] Section 3.5.4 though indexing entries by (neigh, id, prefix,
   source prefix).  When a source-specific Babel node receives a non-
   source-specific update, it MUST consider this update as carrying a
   zero length source prefix.

5.3.  Wildcard requests

   TODO: behaviour to be defined.

5.3.1.  Proposal 1

   The original Babel protocol states that when a node receives a
   wildcard route request, it SHOULD send a full routing table dump.
   This extension does not change this statement: a source-specific node
   SHOULD send a full routing table dump when receiving a wildcard
   request.

   Source-specific wildcard requests does not exist: a wildcard request
   SHOULD NOT carry a source prefix.

5.3.2.  Proposal 2

   We assume that a mandatory sub-TLV has a corresponding non-mandatory
   sub-TLV.  This proposal is like Proposal 3 but instead of having
   multiple wildcard request TLVs, one for each kind of routes
   understood, we use one wildcard request with sub-TLVs corresponding
   to the extension.  To have a full routing table dump, a node sends a
   wildcard requests with a non-mandatory Source sub-TLV.

   A source-specific node SHOULD always attach a non-mandatory Source
   sub-TLV to its wildcard requests.

   This proposal has been rejected because it implies to share the space
   of non-mandatory and mandatory sub-TLVs.

5.3.3.  Proposal 3

   The Babel protocol provides the ability to request a full routing
   table dump by sending a "wildcard request", a route request with the
   AE field set to 0.  As the original protocol has no source-specific
   routes, such a request may only concern non-source-specific routes.
   This extension does not modify the semantics of wildcard requests in
   that sense: a wildcard request prompts the receiver to send its non-
   source-specific routes only, and a Babel node SHOULD NOT send any
   source-specific updates in reply to a wildcard request.

   To obtain a dump of the source-specific routes, a source-specific



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   wildcard request MUST be used.  A source-specific wildcard request is
   a wildcard request carrying a zero length source prefix.

   When a node receives a source-specific wildcard request, it SHOULD
   send a dump of its routes which are source-specific "only".  It
   SHOULD NOT send any non-source-specific routes in reply to a source-
   specific wildcard request.  It SHOULD NOT send any source-specific
   routes which are under the effect of a future extension.  Such
   extension should detail how to handle the possible combinations.

   In consequence, a node requiring a full routing table dump must send
   both a non-source-specific wildcard request and a source-specific
   wildcard request.

5.3.4.  Proposal 4

   Wildcard requests are deprecated.  Either deprecate it in 6126bis, or
   say the following.

   A node receiving a wildcard request SHOULD ignore it.

   This proposal has been rejected because wildcard requests speeds up
   the convergence of the network on boot.  This is considered
   important.

5.3.5.  Note on Overhead between (1) and (3)

   Sending one wildcard request (1) instead of a few something-specific
   wildcard requests (3) in a negligible gain.

   Non-source-specific nodes sending requests to source-specific nodes
   may reduce the global overhead with (3).  But, if the network has no
   source-specific route, there is no overhead to reduce; if there is
   only a few source-specific routes (like in a home network), the
   overhead would be negligible.  Thus, the interesting case is when
   there is a lot of source-specific routes.

   We can imagine a network with a source-specific backbone announcing a
   default route and catching all trafic.  Good old routers not
   supporting this extensions would be put at some backbone leafs.  Is
   sbabeld part of that use case ?

   Couldn't we just send a Route Request for *default* ?


6.  Backwards compatibility

   The protocol extension defined in this document is, to a great



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   extent, interoperable with the base protocol defined in [BABEL] (and
   all its known 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.

   TODO: Should we put a warning to say it's not the case with the
   Experimental Track Babel ?

6.1.  Loop-avoidance

   The extension defined in this protocol uses a new Mandatory sub-TLV
   to carry the source prefix information.  As discussed in Section 4.4
   of [BABEL], this encoding ensures that non-source-specific routers
   will silently ignore the whole TLV, which is necessary to avoid
   persistent routing loops in hybrid networks.

   Consider two nodes A and B, with A source-specific announcing a route
   to (D, S).  Suppose that B ignores the source prefix information when
   it receives the update, and reannounces it as D. This is reannounced
   to A, which treats it as (D, ::/0).  Packets destined to D but not
   sourced in S will be forwarded by A to B, and by B to A, causing a
   persistent routing loop:

       (D,S)                 (D)
        <--                 <--
     ------ A ----------------- B
              -->
             (D,::/0)

6.2.  Starvation and Blackholes

   In general, discarding of source-specific routes by non-source-
   specific routers will cause routing 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.

   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.  However, introducing such a default route in the
   network may in the same time introduce a blackhole.  This tradeoff is
   let to the administrator.







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7.  Protocol Encoding

   This extension defines a new sub-TLV used to carry a source prefix by
   the three following existing messages: Update, Route Request and
   Seqno Request.

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 = TBD   |    Length     |  Source Plen  | Source Prefix...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Fields:
   Type      Set to TBD to indicate a Source Prefix sub-TLV.
   Length    The length of the body, exclusive of the Type and Length
             fields.
   Source Plen  The length of the advertised source prefix.  This MUST
             NOT be 0.
   Source Prefix  The source prefix being advertised.  This field's size
             is (Source Plen)/8 rounded upwards.

   The Source Prefix field's encoding is the same than the Prefix's.  It
   is defined by the AE field of the corresponding TLV.

   Remark that this sub-TLV is a Mandatory sub-TLV.  The whole TLV MUST
   be ignored if that TLV is not recognized.  Otherwise, routing loops
   may occur.

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 than routes with non-source-specific Updates (see [BABEL])
   except for wildcard updates.

   Wildcard updates MUST NOT carry any source prefix.  Wildcard updates
   (in fact, wildcard retraction) are used when a Babel node stops: a
   receiver retracts all routes announced by the announcing node.  There
   is no use case for source-specific wildcard updates.  A source-
   specific Babel node receiving a (legacy) wildcard update MUST
   retracts all routes it learns from this node (including source-
   specific ones).

   Contrary to the destination prefix, this extension does not compress
   the source prefix attached to Updates.  The destination prefix uses
   compression as defined in [BABEL] for Updates with Mandatory



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   extensions.

   However, as defined in [BABEL] (Section 4.5), the compression is
   allowed for the destination prefix of source-specific routes.  Legacy
   implementation will correctly update their parser state, while
   ignoring the whole TLV afterwards.

7.3.  Source-specific (Route) Request

   TODO: A source-specific Route Request prompts the receiver to send an
   update for a given pair of destination and source prefixes.  It MUST
   NOT be used to request a full routing table dump.  The Source Prefix
   sub-TLV of a wildcard source-specific Route Request (Request with AE
   equals to 0 and a Source Prefix sub-TLV) MIGHT be ignored: a receiver
   MIGHT reply by a full routing table dump.

7.4.  Source-Specific Seqno Request

   A source-specific Seqno Request is just like a Seqno Request for a
   source-specific route.  It uses the same mechanisms described in
   [BABEL].


8.  IANA Considerations

   IANA is instructed to add the following entry to the "Babel sub-TLV
   Types" registry:

                +------+---------------+-----------------+
                | Type | Name          | Reference       |
                +------+---------------+-----------------+
                | TBD  | Source Prefix | (this document) |
                +------+---------------+-----------------+


9.  Security considerations

   The extension defined in this document adds a new sub-TLV to three
   TLVs already present in the original Babel protocol.  It does not by
   itself change the security properties of the protocol.


10.  References

10.1.  Normative References

   [BABEL]    Chroboczek, J., "The Babel Routing Protocol", Internet
              Draft draft-ietf-babel-rfc6126bis-02, May 2017.



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10.2.  Informative References

   [SS-ROUTING]
              Boutier, M. and J. Chroboczek, "Source-Specific Routing",
              August 2014.

              In Proc.  IFIP Networking 2015.  A slightly earlier
              version is available online from
              http://arxiv.org/pdf/1403.0445.


Authors' Addresses

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

   Email: boutier@irif.fr


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

   Email: jch@irif.fr






















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