Extension Mechanism for the Babel Routing Protocol
A formal scheme to add new features to the Babel routing protocol without breaking old implementations.
Chroboczek, J., "Extension Mechanism for the Babel Routing Protocol", RFC 7557, DOI 10.17487/RFC7557, May 2015, <https://www.rfc-editor.org/info/rfc7557>.
@misc{rfc7557,
series = {Request for Comments},
number = 7557,
howpublished = {RFC 7557},
publisher = {RFC Editor},
doi = {10.17487/RFC7557},
url = {https://www.rfc-editor.org/info/rfc7557},
author = {J. Chroboczek},
title = {{Extension Mechanism for the Babel Routing Protocol}},
pagetotal = 11,
year = 2015,
month = may,
} RFC text
Contents
Independent Submission J. Chroboczek
Request for Comments: 7557 PPS, University of Paris-Diderot
Updates: 6126 May 2015
Category: Experimental
ISSN: 2070-1721
Extension Mechanism for the Babel Routing Protocol
Abstract
This document defines the encoding of extensions to the Babel routing
protocol, as specified in RFC 6126.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This is a contribution to the RFC Series, independently
of any other RFC stream. The RFC Editor has chosen to publish this
document at its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7557.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 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. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . 10
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
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
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 use the term "original protocol" for
the protocol defined in [RFC6126], and "extended protocol" for any
extension of the Babel protocol that follows the rules set out in
this document.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in [RFC2119].
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.
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 this needs to be done anyway 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.
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.
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.
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-TLV 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 has created three new registries, called "Babel TLV Types",
"Babel Sub-TLV Types", and "Babel Flags Values". The allocation
policy for each of these registries is Specification Required
[RFC5226].
The initial values in the "Babel TLV Types" registry are 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 | |
+---------+-----------------------------------------+---------------+
The initial values in the "Babel Sub-TLV Types" registry are as
follows:
+---------+-----------------------------------------+---------------+
| Type | Name | Reference |
+---------+-----------------------------------------+---------------+
| 0 | Pad1 | this document |
| | | |
| 1 | PadN | this document |
| | | |
| 2 | Diversity | [BABEL-DIV] |
| | | |
| 3 | Timestamp | [BABEL-RTT] |
| | | |
| 224-254 | Reserved for Experimental Use | this document |
| | | |
| 255 | Reserved for expansion of the type | this document |
| | space | |
+---------+-----------------------------------------+---------------+
The initial values in the "Babel Flags Values" registry are as
follows:
+-----+-------------------+-----------+
| Bit | Name | Reference |
+-----+-------------------+-----------+
| 0 | Default prefix | [RFC6126] |
| | | |
| 1 | Default router-id | [RFC6126] |
| | | |
| 2-7 | Unassigned | |
+-----+-------------------+-----------+
6. Security Considerations
This document specifies the structure of fields that are already
present in the original Babel protocol and does not, by itself, raise
any new security considerations. Specific extensions may change the
security properties of the protocol, for example, by adding security
mechanisms [RFC7298] or by enabling new kinds of attack.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3692] Narten, T., "Assigning Experimental and Testing Numbers
Considered Useful", BCP 82, RFC 3692,
DOI 10.17487/RFC3692, January 2004,
<http://www.rfc-editor.org/info/rfc3692>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.
[RFC6126] Chroboczek, J., "The Babel Routing Protocol", RFC 6126,
DOI 10.17487/RFC6126, April 2011,
<http://www.rfc-editor.org/info/rfc6126>.
7.2. Informative References
[BABEL-DIV] Chroboczek, J., "Diversity Routing for the Babel Routing
Protocol", Work in Progress, draft-chroboczek-babel-
diversity-routing-00, July 2014.
[BABEL-RTT] Jonglez, B. and J. Chroboczek, "Delay-based Metric
Extension for the Babel Routing Protocol", Work in
Progress, draft-jonglez-babel-rtt-extension-01, May 2015.
[BABEL-SS] Boutier, M. and J. Chroboczek, "Source-Specific Routing
in Babel", Work in Progress, draft-boutier-babel-
source-specific-01, May 2015.
[RFC7298] Ovsienko, D., "Babel Hashed Message Authentication Code
(HMAC) Cryptographic Authentication", RFC 7298,
DOI 10.17487/RFC7298, July 2014,
<http://www.rfc-editor.org/info/rfc7298>.
Acknowledgments
I am grateful to Denis Ovsienko and Gabriel Kerneis for their
feedback on previous draft versions of this document.
Author's Address
Juliusz Chroboczek
PPS, University of Paris-Diderot
Case 7014
75205 Paris Cedex 13
France
EMail: jch@pps.univ-paris-diderot.fr
AI ELI5
On this page
§1Extending the Babel Routing Protocol
Babel is a routing protocol defined in RFC 6126 that routers use to exchange network route information. Like any protocol, Babel needs to evolve to support new features, but there is a challenge: implementations must not break when they receive data they do not understand.
RFC 6126 already includes provisions for this: packets with unknown version numbers, unknown message types (TLVs), and extra data in existing messages are silently ignored by the original protocol. The problem is that without formal rules, independently developed extensions might conflict, using the same extension space in incompatible ways. This RFC formalizes five extension mechanisms with rules designed to ensure extensions remain compatible with old implementations.
Babel needs to grow, but old routers must still work even when they see new features they don't know about.
§2Five Extension Mechanisms
The RFC defines five ways to extend Babel, each with specific rules.
Protocol versions: The Babel packet header includes an eight-bit version field. The current version is 2. A future incompatible protocol could use version 3 or higher; implementations receiving an unknown version MUST silently ignore the packet. Versions 0 and 1 were earlier experimental versions and SHOULD NOT be reused.
New TLVs: An extension can define a new TLV (Type-Length-Value) type to carry its data. Unknown TLVs are silently ignored, so this works safely. New TLVs SHOULD be self-terminating (their meaningful length can be determined without knowing their explicit length field) and any extra data in them SHOULD be structured as sub-TLVs. TLV types 224 through 254 are reserved for experimental use; type 255 is reserved for future expansion.
Sub-TLVs: Many Babel TLVs are self-terminating, meaning their meaningful length is smaller than their explicit length. Extensions can store data in the extra space as sub-TLVs, which need not be self-terminating. Sub-TLVs have their own type and length fields and are interpreted based on which TLV contains them. Sub-TLV types 224–254 are for experimental use; type 255 is reserved for expansion.
Flags field: The Update TLV contains an eight-bit Flags field. Bits 0 and 1 are defined by the original protocol; the remaining six bits are undefined and silently ignored. Due to limited space, using flags is NOT RECOMMENDED; a sub-TLV is preferred.
Packet trailer: A Babel packet fits inside a UDP datagram, which may have extra space. The extra data at the end (the packet trailer) is silently ignored. Originally intended for cryptographic trailers, no currently deployed extension uses it, so its format remains undefined.
Extensions can add a new version number, new message types, extra data within messages, new flag bits, or trailing data—each with backward-compatible rules.
§3Sub-TLV Format and Structure
Sub-TLVs use the same structure as TLVs: a Type byte, a Length byte, and a Body. Except for the special Pad1 type (which is just a single zero byte), all sub-TLVs have these three fields.
Two sub-TLVs are defined by this document for padding: Pad1 (type 0) and PadN (type 1). Pad1 is a single zero byte used to fill one byte; PadN includes a length and a body of zero bytes, allowing padding of any length. Both are silently ignored on reception. Unknown sub-TLVs MUST also be silently ignored by implementations that use sub-TLVs.
The interpretation of a sub-TLV type can depend on which TLV contains it, allowing the same sub-TLV type number to mean different things in different contexts.
Sub-TLVs are tiny messages within messages, with type and length fields. Some are for padding, and unknown ones are always ignored.
§4Choosing Among Extension Mechanisms
The RFC provides guidance on which mechanism to use for a given extension.
New versions should only be used when the new version is not backwards compatible with the original protocol, since defining a new version breaks compatibility with old implementations.
New TLVs vs. sub-TLVs: Both can carry extension data. When an old implementation sees an unknown TLV, it ignores the entire TLV and cannot act on it. When it sees a known TLV with unknown sub-TLVs, it parses and acts on the known TLV and silently ignores the unknown sub-TLVs. A sub-TLV should be used when the extension is compatible with the meaning of the TLV it is attached to (the extension data is optional). A new TLV must be used when the extension fundamentally changes the TLV's meaning and should not be acted upon by old implementations that do not understand it.
Flags field: Using a bit in the Flags field saves space compared to a sub-TLV, but the available space is tiny (only 6 bits). Due to space constraints and small savings, a sub-TLV is recommended instead.
Packet trailer: Due to lack of implementation experience, no recommendations are made about its use.
Use new versions rarely, new TLVs when meaning changes fundamentally, sub-TLVs for compatible add-ons, and generally avoid flags.
§5IANA Registries and Interoperability
The RFC establishes three IANA registries to coordinate extension assignments: one for TLV types, one for sub-TLV types, and one for Flags values. The allocation policy is Specification Required, meaning any extension type must be assigned by IANA after publication of a specification.
For each registry, the RFC provides initial values. The TLV types include the original types from RFC 6126 (Pad1, Hello, Update, etc.) plus entries from related extensions like source-specific routing and HMAC authentication. Types 224–254 are reserved for Experimental Use, and type 255 is reserved for future expansion of the type space.
By centralizing these registries, the RFC ensures that no two independently developed extensions accidentally use the same type numbers and conflict with each other, maintaining interoperability across implementations.
IANA keeps official lists of type numbers for extensions, preventing conflicts and ensuring different Babel implementations can work together.