Extension Mechanism for the Babel Routing ProtocolPPS, University of Paris-DiderotCase 701475205 Paris Cedex 13Francejch@pps.univ-paris-diderot.frThis document defines the encoding of extensions to the
Babel routing protocol .A Babel packet 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:a Babel packet with a version number different from 2 MUST be
silently ignored (, Section 4.2);any unknown TLV MUST be silently ignored (,
Section 4.3);all TLVs are self-terminating, and any extra data included in a TLV
MUST be silently ignored (, Section 4.2);the Flags field of the Update TLV contains 6 undefined bits that
MUST be silently ignored (, Section 4.4.9);any data following the last TLV of a Babel packet MUST be silently
ignored (, 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.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.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.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
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.Except for Pad1 (see below), a Sub-TLV has exactly the same
structure as a Babel TLV:Fields :
The type of the sub-TLV.The length of the body, exclusive of the Type and
Length fields.The TLV body, the interpretation of which
depends on the type.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.Fields :
Set to 0 to indicate a Pad1 TLV.This sub-TLV is silently ignored on reception.Fields :
Set to 1 to indicate a PadN TLV.The length of the body, exclusive of the Type and
Length fields.Set to 0 on transmission.This sub-TLV is silently ignored on reception.Any unknown sub-TLV MUST be silently ignored by an extended
implementation that uses sub-TLVs.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
SHOULD use a sub-TLV in preference to a new flag. No registry of flag
assignments is currently defined.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 ended up using a pair of new TLVs, and no
currently deployed extension of Babel uses the packet trailer.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.Babel HMAC Cryptographic AuthenticationThe Babel Routing Protocol