Extension Mechanism for the Babel Routing Protocol
draft-chroboczek-babel-extension-mechanism-04

Network Working Group                                      J. Chroboczek
Internet-Draft                          PPS, University of Paris-Diderot
Updates: 6126 (if approved)                               March 27, 2015
Intended status: Experimental
Expires: September 28, 2015

           Extension Mechanism for the Babel Routing Protocol
             draft-chroboczek-babel-extension-mechanism-04

Abstract

   This document defines the encoding of extensions to the Babel routing
   protocol [RFC6126].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on September 28, 2015.

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Table of Contents

   1.  Extending the Babel routing protocol  . . . . . . . . . . . .   2
   2.  Mechanisms for extending the Babel protocol . . . . . . . . .   3
     2.1.  New versions of the Babel protocol  . . . . . . . . . . .   3
     2.2.  New TLVs  . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.3.  Sub-TLVs  . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.4.  The Flags field . . . . . . . . . . . . . . . . . . . . .   4
     2.5.  Packet trailer  . . . . . . . . . . . . . . . . . . . . .   5
   3.  Format of sub-TLVs  . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Sub-TLVs specified in this document . . . . . . . . . . .   5
     3.2.  Unknown sub-TLVs  . . . . . . . . . . . . . . . . . . . .   6
   4.  Choosing between extension mechanisms . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   9
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Extending the Babel routing protocol

   A Babel packet [RFC6126] contains a header followed by a sequence of
   TLVs, each of which is a sequence of octets having an explicit type
   and length.  The original Babel protocol has the following provisions
   for including extension data:

   o  a Babel packet with a version number different from 2 MUST be
      silently ignored ([RFC6126], Section 4.2);

   o  an unknown TLV MUST be silently ignored ([RFC6126], Section 4.3);

   o  except for Pad1 and PadN, all TLVs are self-terminating, and any
      extra data included in a TLV MUST be silently ignored ([RFC6126],
      Section 4.2);

   o  the Flags field of the Update TLV contains 6 undefined bits that
      MUST be silently ignored ([RFC6126], Section 4.4.9);

   o  any data following the last TLV of a Babel packet MUST be silently
      ignored ([RFC6126], Section 4.2).

   Each of these provisions provides a place to store data needed by
   extensions of the Babel protocol.  However, in the absence of any
   further conventions, independently developed extensions to the Babel
   protocol might make conflicting uses of the available space, and
   therefore lead to implementations that would fail to interoperate.
   This document formalises a set of rules for extending the Babel

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   protocol that are designed to ensure that no such incompatibilities
   arise, and that are currently respected by a number of deployed
   extensions.

   In the rest of this document, we call "original protocol" the
   protocol defined in [RFC6126], and "extended protocol" any extension
   of the Babel protocol that follows the rules set out in this
   document.

2.  Mechanisms for extending the Babel protocol

   This section describes each of the mechanisms available for extending
   the Babel protocol.

2.1.  New versions of the Babel protocol

   The header of a Babel packet contains an eight-bit protocol version.
   The current version of the Babel protocol is version 2; any packets
   containing a version number different from 2 MUST be silently
   ignored.

   Versions 0 and 1 were earlier experimental versions of the Babel
   protocol that have seen some modest deployment; these version numbers
   SHOULD NOT be reused by future versions of the Babel protocol.
   Version numbers larger than 2 might be used by a future incompatible
   protocol.

2.2.  New TLVs

   An extension may carry its data in a new TLV type.  Such new TLVs
   will be silently ignored by implementations of the original Babel
   protocol, as well as by other extended implementations of the Babel
   protocol, as long as the TLV types do not collide.

   All new TLVs MUST have the format defined in [RFC6126], Section 4.3.
   New TLVs SHOULD be self-terminating, in the sense defined in the next
   section, and any data found after the main data section of the TLV
   SHOULD be treated as a series of sub-TLVs.

   TLV types 224 through 254 are reserved for Experimental Use
   [RFC3692].  TLV type 255 is reserved for expansion of the TLV type
   space, in the unlikely event that eight bits turn out not to be
   enough.

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2.3.  Sub-TLVs

   With the exception of the Pad1 TLV, all Babel TLVs carry an explicit
   length.  With the exception of Pad1 and PadN, all TLVs defined by the
   original protocol are self-terminating, in the sense that the length
   of the meaningful data that they contain (the "natural length") can
   be determined without reference to the explicitly encoded length.  In
   some cases, the natural length is trivial to determine: for example,
   a HELLO TLV always has a natural length of 2 (4 including the Type
   and Length fields).  In other cases, determining the natural length
   is not that easy, but needs to be done in any case by an
   implementation that interprets the given TLV: for example, the
   natural length of an Update TLV depends on both the prefix length and
   the amount of prefix compression being performed.

   If the explicit length of a TLV defined by the original protocol is
   larger than its natural length, the extra space present in the TLV is
   silently ignored by an implementation of the original protocol;
   extended implementations MAY use it to store arbitrary data, and
   SHOULD structure the additional data as a sequence of sub-TLVs.
   Unlike TLVs, the sub-TLVs themselves need not be self-terminating.

   An extension MAY be assigned one or more sub-TLV types.  Sub-TLV
   types are assigned independently from TLV types: the same numeric
   type can be assigned to a TLV and a sub-TLV.  Sub-TLV types are
   assigned globally: once an extension is assigned a given sub-TLV
   number, it MAY use this number within any TLV; however, the
   interpretation of a given sub-TLV type can depend on which particular
   TLV it is embedded within.

   Sub-TLV types 224 through 254 are reserved for Experimental Use
   [RFC3692].  TLV type 255 is reserved for expansion of the sub-TLV
   type space, in the unlikely event that eight bits turn out not to be
   enough.  The format of sub-TLVs is defined in Section 3 below.

2.4.  The Flags field

   The Flags field is an eight-bit field in the Update TLV.  Bits 0 and
   1 (the bits with values 80 and 40 hexadecimal) are defined by the
   original protocol, and MUST be recognised and used by every
   implementation.  The remaining six bits are not currently used, and
   are silently ignored by implementations of the original protocol.

   Due to the small size of the Flags field, it is NOT RECOMMENDED that
   one or more bits be assigned to an extension; a sub-TLV SHOULD be
   assigned instead.  An implementation MUST ignore any bits in the
   Flags field that it does not know about, and MUST send them as zero.

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2.5.  Packet trailer

   A Babel packet carries an explicit length in its header.  A Babel
   packet is carried by a UDP datagram, which in turn contains an
   explicit length in its header.  It is possible for a UDP datagram
   carrying a Babel packet to be larger than the size of the Babel
   packet.  In that case, the extra space after the Babel packet, known
   as the packet trailer, is silently ignored by an implementation of
   the original protocol.

   The packet trailer was originally intended to be used as a
   cryptographic trailer.  However, the authentication extension to
   Babel [RFC7298] ended up using a pair of new TLVs, and no currently
   deployed extension of Babel uses the packet trailer.  The format and
   purpose of the packet trailer is therefore currently left undefined.

3.  Format of sub-TLVs

   A sub-TLV has exactly the same structure as a TLV.  Except for Pad1
   (Section 3.1.1), all sub-TLVs have the following structure:

    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     |     Body...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Fields :

   Type      The type of the sub-TLV.

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

   Body      The sub-TLV body, the interpretation of which depends on
             both the type of the sub-TLV and the type of the TLV within
             which it is embedded.

3.1.  Sub-TLVs specified in this document

   This document defines two types of sub-TLVs, Pad1 and PadN.  These
   two sub-TLVs MUST be correctly parsed and ignored by any extended
   implementation of the Babel protocol that uses sub-TLVs.

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3.1.1.  Pad1

    0
    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |   Type = 0    |
   +-+-+-+-+-+-+-+-+

   Fields :

   Type      Set to 0 to indicate a Pad1 sub-TLV.

   This sub-TLV is silently ignored on reception.

3.1.2.  PadN

    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 = 1   |    Length     |      MBZ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Fields :

   Type      Set to 1 to indicate a PadN sub-TLV.

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

   MBZ       Set to 0 on transmission.

   This sub-TLV is silently ignored on reception.

3.2.  Unknown sub-TLVs

   Any unknown sub-TLV MUST be silently ignored by an extended
   implementation that uses sub-TLVs.

4.  Choosing between extension mechanisms

   New versions of the Babel protocol should only be defined if the new
   version is not backwards compatible with the original protocol.

   In many cases, an extension could be implemented either by defining a
   new TLV, or by adding a new sub-TLV to an existing TLV.  For example,
   an extension whose purpose is to attach additional data to route
   updates can be implemented either by creating a new "enriched" Update
   TLV, or by adding a sub-TLV to the Update TLV.

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   The two encodings are treated differently by implementations that do
   not understand the extension.  In the case of a new TLV, the whole
   unknown TLV is ignored by an implementation of the original protocol,
   while in the case of a new sub-TLV, the TLV is parsed and acted upon,
   and the unknown sub-TLV is silently ignored.  Therefore, a sub-TLV
   should be used by extensions that extend the Update in a compatible
   manner (the extension data may be silently ignored), while a new TLV
   must be used by extensions that make incompatible extensions to the
   meaning of the TLV (the whole TLV must be thrown away if the
   extension data is not understood).

   Using a new bit in the Flags field is equivalent to defining a new
   sub-TLV while using less space in the Babel packet.  Due to the
   limited Flags space, and the doubtful space savings, we do not
   recommend the use of bits in the Flags field -- a new sub-TLVs should
   be used instead.

   We refrain from making any recommendations about the usage of the
   packet trailer due to the lack of implementation experience.

5.  IANA Considerations

   IANA is to create three new registries, called "Babel TLV types",
   "Babel sub-TLV types", and "Babel Flags values".  The allocation
   policy for all three of these registries is Expert Review with
   Specification Required [RFC5226].

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     The initial value of the Babel TLV types registry is as follows:

   +---------+---------------------------------------+-----------------+
   | Type    | Name                                  | Reference       |
   +---------+---------------------------------------+-----------------+
   | 0       | Pad1                                  | [RFC6126]       |
   |         |                                       |                 |
   | 1       | PadN                                  | [RFC6126]       |
   |         |                                       |                 |
   | 2       | Acknowledgment Request                | [RFC6126]       |
   |         |                                       |                 |
   | 3       | Acknowledgment                        | [RFC6126]       |
   |         |                                       |                 |
   | 4       | Hello                                 | [RFC6126]       |
   |         |                                       |                 |
   | 5       | IHU                                   | [RFC6126]       |
   |         |                                       |                 |
   | 6       | Router-Id                             | [RFC6126]       |
   |         |                                       |                 |
   | 7       | Next Hop                              | [RFC6126]       |
   |         |                                       |                 |
   | 8       | Update                                | [RFC6126]       |
   |         |                                       |                 |
   | 9       | Route Request                         | [RFC6126]       |
   |         |                                       |                 |
   | 10      | Seqno Request                         | [RFC6126]       |
   |         |                                       |                 |
   | 11      | TS/PC                                 | [RFC7298]       |
   |         |                                       |                 |
   | 12      | HMAC                                  | [RFC7298]       |
   |         |                                       |                 |
   | 13      | Source-specific Update                | [BABEL-SS]      |
   |         |                                       |                 |
   | 14      | Source-specific Request               | [BABEL-SS]      |
   |         |                                       |                 |
   | 15      | Source-specific Seqno Request         | [BABEL-SS]      |
   |         |                                       |                 |
   | 224-254 | Reserved for Experimental Use         | (this document) |
   |         |                                       |                 |
   | 255     | Reserved for expansion of the type    | (this document) |
   |         | space                                 |                 |
   +---------+---------------------------------------+-----------------+

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   The initial value of the Babel sub-TLV types registry is as follows:

   +---------+-------------------------------------+-------------------+
   | Type    | Name                                | Reference         |
   +---------+-------------------------------------+-------------------+
   | 0       | Pad1                                | (this document)   |
   |         |                                     |                   |
   | 1       | PadN                                | (this document)   |
   |         |                                     |                   |
   | 2       | Diversity                           | [BABEL-DIVERSITY] |
   |         |                                     |                   |
   | 3       | Timestamp                           | [BABEL-RTT]       |
   |         |                                     |                   |
   | 224-254 | Reserved for Experimental Use       | (this document)   |
   |         |                                     |                   |
   | 255     | Reserved for expansion of the type  | (this document)   |
   |         | space                               |                   |
   +---------+-------------------------------------+-------------------+

    The initial value of the Babel Flags values registry is as follows:

                  +-----+-------------------+-----------+
                  | Bit | Name              | Reference |
                  +-----+-------------------+-----------+
                  | 0   | Default prefix    | [RFC6126] |
                  |     |                   |           |
                  | 1   | Default router-id | [RFC6126] |
                  +-----+-------------------+-----------+

6.  Acknowledgments

   I am grateful to Denis Ovsienko and Gabriel Kerneis for their
   feedback on previous versions of this document.

7.  References

7.1.  Normative References

   [RFC3692]  Narten, T., "Assigning Experimental and Testing Numbers
              Considered Useful", BCP 82, RFC 3692, January 2004.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC6126]  Chroboczek, J., "The Babel Routing Protocol", RFC 6126,
              February 2011.

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7.2.  Informative References

   [BABEL-DIVERSITY]
              Chroboczek, J., "Diversity Routing for the Babel Routing
              Protocol", draft-chroboczek-babel-diversity-routing-00
              (work in progress), July 2014.

   [BABEL-RTT]
              Jonglez, B. and J. Chroboczek, "Delay-based Metric
              Extension for the Babel Routing Protocol", draft-jonglez-
              babel-rtt-extension-00 (work in progress), July 2014.

   [BABEL-SS]
              Boutier, M. and J. Chroboczek, "Source-Specific Routing in
              Babel", draft-boutier-babel-source-specific-00 (work in
              progress), November 2014.

   [RFC7298]  Ovsienko, D., "Babel Hashed Message Authentication Code
              (HMAC) Cryptographic Authentication", RFC 7298, July 2014.

Author's Address

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

   Email: jch@pps.univ-paris-diderot.fr

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