Extension Mechanism for the Babel Routing Protocol
draft-chroboczek-babel-extension-mechanism-00

Network Working Group                                      J. Chroboczek
Internet-Draft                          PPS, University of Paris-Diderot
Intended status: Experimental                              June 30, 2013
Expires: January 1, 2014

           Extension Mechanism for the Babel Routing Protocol
             draft-chroboczek-babel-extension-mechanism-00

Abstract

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

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Table of Contents

   1.  Extending the Babel routing protocol . . . . . . . . . . . . .  3
   2.  Mechanisms for extending the Babel protocol  . . . . . . . . .  4
     2.1.  New versions of the Babel protocol . . . . . . . . . . . .  4
     2.2.  New TLVs . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.3.  Sub-TLVs . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.4.  The Flags field  . . . . . . . . . . . . . . . . . . . . .  6
     2.5.  Packet trailer . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Choosing between extension mechanisms  . . . . . . . . . . . .  8
   4.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10

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1.  Extending the Babel routing protocol

   A Babel packet [BABEL] consists of a sequence of TLVs, each of which
   is a sequence of octets having an explicit type and length.  The base
   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 ([BABEL], Section 4.2);

   o  any unknown TLV MUST be silently ignored ([BABEL], Section 4.3);

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

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

   o  any data following the last TLV of a Babel packet MUST be silently
      ignored ([BABEL], 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 rules, different extensions of the Babel protocol might make
   conflicting uses of the available space, and therefore lead to
   implementations that might fail to interoperate.  The following
   paragraphs set up a set of rules for using the available extension
   space that are designed to ensure that no such incompatibilities
   arise.

   In the rest of this document, we call "base protocol" the protocol
   defined in RFC 6126, and "extended protocol" any extension of the
   Babel protocol that follows the rules set out in this document.

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2.  Mechanisms 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 currently deployed version of Babel is version 2; any packets
   containing a version number different from 2 MUST be silently
   ignored.

   Versions 0 and 1 were 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 future versions should it be
   found necessary to define a non-backwards compatible version of the
   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 base 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 RFC 6126, Section 4.3.
   Additionally, they 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.

   An extension may be assigned one or more extension TLV types.  A
   registry of known TLV types is being maintained by Juliusz
   Chroboczek.

2.3.  Sub-TLVs

   All Babel TLVs carry an explicit length.  In addition, all Babel
   TLVs, whether defined by the base protocol or by extensions, are
   self-terminating, in the sense that their actual length (the "base
   length") can be determined without reference to the explicit length.
   In some cases, the base length is trivial to determine: for example,
   a HELLO TLV always has a base length of 2 (4 including the Type and
   Length fields).  In other cases, determining the base length is not
   that easy, but is done in any case by any implementation that knows
   the given TLV: for example, the base length of an Update TLV depends
   on both the prefix length and the amount of compression being
   performed.

   If the explicit length of a TLV is larger than its base length, the
   extra space present in the TLV is silently ignored by an

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   implementation of the base protocol, and is used by extended
   implementations to store 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 used by different
   extensions.  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 meaning of a given sub-TLV type may depend on the
   TLV it is embedded within.

   A registry of known sub-TLV types is being maintained by Juliusz
   Chroboczek.

2.3.1.  Format of sub-TLVs

   Except for Pad1 (see below), a Sub-TLV has exactly the same structure
   as a Babel 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      |    Length     |     Body...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Fields :

   Type      The type of the sub-TLV.

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

   Body      The TLV body, the interpretation of which depends on the
             type.

2.3.2.  Standard sub-TLVs

   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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2.3.2.1.  Pad1

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

   Fields :

   Type      Set to 0 to indicate a Pad1 TLV.

   This sub-TLV is silently ignored on reception.

2.3.2.2.  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 TLV.

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

   MBZ       Set to 0 on transmission.

   This sub-TLV is silently ignored on reception.

2.3.3.  Unknown sub-TLVs

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

2.4.  The Flags field

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

   Extensions to the Babel protocol MAY use the six unused bits of the
   Flags field.  However, due to the small size of the Flags field, they

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   SHOULD use a sub-TLV in preference to a new flag.  No registry of
   flag assignments is currently defined.

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 needed to contain the packet.  In that case, the extra
   space after the Babel packet is known as the packet trailer, and is
   silently ignored by an implementation of the base protocol.

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

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3.  Choosing between extension mechanisms

   New versions of the Babel protocol should only be defined if the new
   version is not backwards compatible with the current base 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.  In
   particular, the most common purpose of extensions is to attach
   additional data to routing updates; such extensions are either
   implemented by creating a new "enriched" Update TLV, or by adding a
   sub-TLV to the Update TLV.

   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 a base implementation, 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 preferred 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 piece of data 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, but uses less space in the Babel packet.  Due to the high
   risk of collision in the limited Flags space, and the doubtful space
   savings, we do not recommend the use of the Flags field in future
   extensions.

   This document refrains from making any recommendations about the
   usage of the packet trailer due to the lack of implementation
   experience.

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4.  References

   [AUTH]   Ovsienko, D., "Babel HMAC Cryptographic Authentication",
            Internet Draft draft-ovsienko-babel-hmac-authentication-03,
            April 2013.

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

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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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