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Versions: 00

Network Working Group                                        G. Chouasne
Internet-Draft                                             J. Chroboczek
Updates: 6126 (if approved)             PPS, University of Paris-Diderot
Intended status: Experimental                               July 3, 2017
Expires: January 4, 2018


                     TOS-Specific Routing in Babel
                  draft-chouasne-babel-tos-specific-00

Abstract

   This document describes an extension to the Babel routing protocol to
   support TOS-specific routing.  This version is using mandatory sub-
   TLVs.

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 January 4, 2018.

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   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.




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Table of Contents

   1.  Introduction and background . . . . . . . . . . . . . . . . .   2
   2.  Protocol Operation  . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Conceptual Description  . . . . . . . . . . . . . . . . .   3
     2.2.  Data Structures . . . . . . . . . . . . . . . . . . . . .   3
     2.3.  ToS-specific sub-TLV  . . . . . . . . . . . . . . . . . .   4
     2.4.  ToS-specific Messages . . . . . . . . . . . . . . . . . .   5
   3.  Interaction . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  ToS interpretation  . . . . . . . . . . . . . . . . . . .   6
     3.2.  IP version  . . . . . . . . . . . . . . . . . . . . . . .   6
     3.3.  Interaction with non-Tos-specific Babel . . . . . . . . .   6
     3.4.  Interaction with the Source-specific routing extension  .   7
     3.5.  Forwarding Behavior . . . . . . . . . . . . . . . . . . .   7
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   5.  Security considerations . . . . . . . . . . . . . . . . . . .   8
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction and background

   The Type of Service (ToS) or Differentiated Services Code Point
   (DSCP) is a field of the IPv4 and IPv6 headers that can be used to
   request different per-hop behaviour when forwarding IP packets with
   identical source and destination.  The most common application of the
   ToS field is to request different queueing policies (priority, drop
   probability, etc.) from an AQM.

   A slightly less common application of the ToS field is to take it
   into account in addition to the destination address when performing a
   routing decision.  For example, a router that has a low-latency
   default route with high monetary cost might announce it with a "low-
   latency" ToS, and thus avoid carrying ordinary best-effort traffic
   over the expensive route.  A router that performs ToS-specific
   routing maintains a routing table which instead of being merely
   indexed by destination prefixes is indexed by pairs of a prefix and a
   ToS value.  In order to be routed according to a given entry in the
   routing table, a packet must match not only the destination prefix
   but also the ToS value.  ToS-specific forwarding is described in more
   detail in Section 3.5.

   Just like ordinary routes, ToS-specific routes can be installed
   manually but are more commonly installed by a dynamic routing
   protocol.  This document specifies an extension to the Babel routing
   protocol [BABEL] that allows it to carry ToS-specific routes.  This
   extension considers the ToS field as an opaque value that is only



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   compared for equality, but ignores the two bits that have been
   reserved for Explicit Congestion Notification (ECN) [RFC3168], and is
   therefore compatible with the defintion of the ToS field used by DSCP
   [DSCP].

   The extended protocol remains interoperable with the unextended Babel
   protocol in a sense made precise in Section 3.3.

2.  Protocol Operation

2.1.  Conceptual Description

   ToS-specific routes are routes that are defined by the same
   informations as classic routes, plus a ToS.  This extension adds ToS-
   specific routes to the non-ToS-specific routes handled by the
   original Babel protocol.

   This extension doesn't change the processing of non-ToS-specific
   routes.  A node implementing this extension behaves exactly like a
   node implementing the original protocol as far as non-ToS-specific
   routes are concerned.

   ToS-specific routes are treated analogously to non-ToS-specific
   routes, except for an additional ToS field in a number of data
   structures (Section 2.2) and in the encoding of Update and Request
   TLVs, which are augmented with a sub-TLV carrying the ToS.  This sub-
   TLV has the mandatory bit set ([BABEL] Section 4.4), and hence,
   Updates and Requests for ToS-specific routes will be ignored by nodes
   implementing only the original protocol.  Therefore, ToS-specific
   routes can only consist of a sequence of nodes implementing this
   extension.

2.2.  Data Structures

   An implementation of Babel that implements this extension needs to
   add a ToS field to a number of data structures it maintains.

2.2.1.  The Source Table

   The source table maintained by any Babel node, as described in
   [BABEL], Section 3.2.4, is extended with the following field:

   o  the ToS specifying the ToS of packets to which this entry applies.








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2.2.2.  The Table of Pending Requests

   The table of pending requests, maintained by any Babel node, as
   described in [BABEL], Section 3.2.4, is extended with the following
   field:

   o  the ToS being requested.

   The table is now indexed by (prefix, ToS) pairs.

2.3.  ToS-specific sub-TLV

   This extension defines a new sub-TLV that adds a ToS field to a Babel
   packet.  It turns regular Updates, Route Requests and Seqno Requests
   into ToS-specific Updates, Route Requests and Seqno Requests.  Other
   TLVs MUST NOT be sent with this sub-TLV attached.  If any are
   received, they will be silently ignored, as described in Section 4.4
   of [BABEL].

   A node MUST send ToS-specific Update, Route Request and Seqno Request
   if the route they advertise is ToS-specific.  Otherwise, a node MUST
   send non-ToS-specific Update, Route Request and Seqno Request.

   The ToS sub-TLV is defined as follow:

   0                   1                   2
   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Type  = TBD  |    Length     |      ToS      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields :

   Type      Set to TBD to indicate a ToS-specific mandatory sub-TLV.

   Length    The length of the body, exclusive of the Type and Length
             fields.  It MUST be set to 1.

   ToS       The ToS value of the ToS-specific route.  This MUST be a
             multiple of 4 (i.e. the two least-significant bits MUST be
             0).

   The value TBD has the mandatory bit set to 1 and this sub-TLV must
   therefore be handled in accordance with Section 4.4 of [BABEL].







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2.4.  ToS-specific Messages

   This section describes the handling of ToS-specific messages.

2.4.1.  Updates

   Whenever a ToS-specific Update is sent by a node implementing this
   extension, the source table MUST be updated as follows : if an entry
   indexed by (prefix, plen, router-id, ToS), exists, it MUST be
   modified as described in section 3.7.3 of [BABEL].  Otherwise, a new
   entry is created with value (prefix, plen, router-id, ToS, seqno,
   metric).

2.4.2.  Requests

   Whenever a node implementing this extension sends a ToS-specific
   Request, it MUST add its content as an entry to the Pending Requests
   Table, as described in section 3.2.7 of [BABEL], with the suitable
   ToS.

2.4.2.1.  Route Requests

   When a node implementing this extension receives a ToS-specific Route
   Request for a prefix (prefix, plen) and a ToS t, it checks its route
   table for a selected route with this prefix, plen, and with ToS t in
   the corresponding source.  If such a route doesn't exist, it MUST
   send a ToS-specific retraction for that prefix.

   When a node implementing this extension receives a wildcard Route
   Request, it SHOULD send a full routing table dump, as described in
   Section 3.8.1.1 of [BABEL].  Therefore, it SHOULD also dump its ToS-
   specific routes.

2.4.2.2.  Seqno Requests

   When a node receives a Seqno Request for a prefix (prefix, plen) with
   a ToS-specific sub-TLV with ToS t, it MUST send a ToS-specific update
   with ToS t for a selected route specified by the Plen, Prefix and
   Source ToS field, with either a router-id different from what is
   specified by the Router-Id field, or a Seqno no less than what is
   specified by the Seqno field.  If there is no such route in the Route
   Table, it MUST forward the seqno request according to the rules
   defined in Section 3.8.1.2 of [BABEL].








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3.  Interaction

   This section describes the interaction of this extension with other
   protocols and other versions of Babel.

3.1.  ToS interpretation

   Several interpretations have been defined for the ToS field.

   This extension ignores the last two bits of the field, while the
   first six bits of the ToS field are opaque to this extension.  Hence,
   it is compatible with all extensions that don't use the last two bits
   of the field.  This is the case of all interpretations known to us.
   In particular, it is compatible with the Differentiated Service Field
   interpretation (DSCP), defined in RFC 2474, the original
   interpretation described in RFC 791, and the ECN protocol, described
   in RFC 3168.

   In the case of the DSCP interpretation, one must note that a packet
   with a DSCP value will follow the route with the ToS being four times
   this value.

   Details on how packets are forwarded are provided in Section 3.5.

3.2.  IP version

   This protocol extension is compatible with IPV4 and IPV6.  AE fields
   MUST be filled accordingly, as described in Section 4.1.3 of [BABEL].

3.3.  Interaction with non-Tos-specific Babel

   The encoding of ToS-specific Updates and Requests is using a reserved
   sub-TLV.  This means that, as defined in section 4.4 of [BABEL], they
   will be ignored by nodes that don't implement this extension, which
   means that non-ToS-specific nodes will not treat ToS-specific routes.

   In a topology of routers implementing ToS-specific Babel and non-ToS-
   specific Babel, all packets will reach their destination if it is
   reachable.  Non-ToS-specific packets will follow the same routes as
   if ToS-specific routers were non-ToS-specific routers.  ToS-specific
   packets may switch to a non-ToS-specific route if and only if there
   is no route for the required ToS.

   This extension uses a mandatory bit on each sub-TLV.  It is necessary
   that this bit is set to 1 for all ToS-specific sub-TLV to avoid
   loops, as shown in the following example.





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   Consider two neighboring routers A and B, A implementing the ToS-
   specific Babel extension, and B implementing just the base Babel
   protocol.  Suppose that A has a ToS-specific route for a prefix P and
   ToS t.

   B will receive an update from A with this ToS-specific route.
   Suppose that B reads the update TLV but drops the ToS-specific sub-
   TLV.  It will now forwards packets for P through A.

   B will then send an update for the route to (P) to A.  A will take B
   as next-hop for the matching packets.

   When a packet with no ToS arrives to A or B, it will travel between A
   and B indefinitely, as shown in the figure below.

                (P)            (P)
                -->            <--
   ----------- A ----------------- B -----------
           <--
         (P, t)

3.4.  Interaction with the Source-specific routing extension

   The ToS-specific routing extension is compatible with the source-
   specific extension.  A node implementing ToS-specific routing and
   source-specific routing handles ToS-specific routes and source-
   specific routes.  To achieve this, data structures are extended with
   a ToS field and a source field.

   In principle, a node could handle routes that are both ToS- and
   source-specific.  In that case, Updates and Requests advertising
   source-and-ToS-specific routes would need to be sent with both the
   source prefix sub-TLV and the ToS sub-TLV, and a route preference
   ordering for source-and-ToS-specific packets would need to be
   defined.  Until such an ordering is defined, updates with both the
   source prefix and ToS sub-TLVs MUST NOT be sent, and, if received,
   MUST be silently dropped.

3.5.  Forwarding Behavior

   When a packet for an address A arrives to a node, it may match
   several routes.  The node MUST choose the route with the destination-
   first ordering.

   This ordering only considers routes that have either no ToS or the
   Tos of the packet (ignoring the last two bits).  It forwards a packet
   through the route with the most specific prefix.  If there are two
   routes with the same prefix, then one of them has no ToS and the



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   other has the ToS of the packet (ignoring the last two bits).  The
   packet is following the latter.  If there is no route with a matching
   prefix, the packet is dropped.

   When a ToS-specific route is retracted while the router has another
   non-ToS-specific route, packets should fall back to this non-ToS-
   specific route as fast as possible, to avoid more delay.  Hence, it
   is RECOMMENDED to use the algorithm described in Section 3.5.5 of
   [BABEL]

   A Babel implementation of this extension must ensure that these
   semantics are observed.  If they aren't, the implementation MUST
   disambiguate the routing tables by using a suitable algorithm (for
   example the algorithm of Boutier [SS-ROUTING]).

   It may be the case that the forwarding plane cannot handle some ToS
   values.  In that case, routes with these values as ToS MUST NOT be
   selected and therefore MUST NOT be announced.

4.  IANA Considerations

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

               +------+------------------+-----------------+
               | Type | Name             | Reference       |
               +------+------------------+-----------------+
               | TBD  | ToS-specific TLV | (this document) |
               +------+------------------+-----------------+

5.  Security considerations

   The extension defined in this protocol defines three new sub-TLVs for
   defined TLVs.  This extension doesn't alterate any of the security
   properties of the base protocol.

6.  References

6.1.  Normative References

   [BABEL]    Chroboczek, J., "The Babel Routing Protocol", draft-
              chroboczek-babel-rfc6126bis-03 (work in progress), July
              2016.








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6.2.  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

   Gwendoline Chouasne
   PPS, University of Paris-Diderot
   Case 7014
   75205 Paris Cedex 13
   France

   Email: gwendoline.chouasne-guillon@ens.fr


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

   Email: jch@irif.fr























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