Internet Engineering Task Force (IETF) J. Chroboczek

Internet Engineering Task Force (IETF) J. Chroboczek
Request for Comments: 9080 IRIF, University of Paris-Diderot
Category: Standards Track August 2021
ISSN: 2070-1721

Homenet Profile of the Babel Routing Protocol

Abstract

This document defines the exact subset of the Babel routing protocol
and its extensions that is required by an implementation of the
Homenet protocol suite, as well as the interactions between the Home
Networking Control Protocol (HNCP) and Babel.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9080.

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Table of Contents

1. Introduction
1.1. Requirements Language
1.2. Background
2. The Homenet Profile of Babel
2.1. Requirements
2.2. Optional Features
3. Interactions between HNCP and Babel
3.1. Requirements
3.2. Optional Features
4. Security Considerations
5. IANA Considerations
6. References
6.1. Normative References
6.2. Informative References
Acknowledgments
Author's Address

1. Introduction

The core of the Homenet protocol suite consists of the Home
Networking Control Protocol (HNCP) [RFC7788], a protocol used for
flooding configuration information and assigning prefixes to links,
combined with the Babel routing protocol [RFC8966]. Babel is an
extensible, flexible, and modular protocol: minimal implementations
of Babel have been demonstrated that consist of a few hundred lines
of code, while the "large" implementation includes support for a
number of extensions and consists of over ten thousand lines of C
code.

This document consists of two parts. The first specifies the exact
subset of the Babel protocol and its extensions that is required by
an implementation of the Homenet protocol suite. The second
specifies how HNCP interacts with Babel.

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

1.2. Background

The Babel routing protocol and its extensions are defined in a number
of documents:

* RFC 8966 [RFC8966] defines the Babel routing protocol. It allows
Babel's control data to be carried either over link-local IPv6 or
over IPv4 and in either case allows announcing both IPv4 and IPv6
routes. It leaves link cost estimation, metric computation, and
route selection to the implementation. Distinct implementations
of Babel [RFC8966] will interoperate, in the sense that they will
maintain a set of loop-free forwarding paths. However, if they
implement conflicting options, they might not be able to exchange
a full set of routes. In the worst case, an implementation that
only implements the IPv6 subset of the protocol and an
implementation that only implements the IPv4 subset of the
protocol will not exchange any routes. In addition, if
implementations use conflicting route selection policies,
persistent oscillations might occur.

* The informative Appendix A of [RFC8966] suggests a simple and
easy-to-implement algorithm for cost and metric computation that
has been found to work satisfactorily in a wide range of
topologies.

* While RFC 8966 does not provide an algorithm for route selection,
its Section 3.6 suggests selecting the route with the smallest
metric with some hysteresis applied. An algorithm that has been
found to work well in practice is described in Section III.E of
[DELAY-BASED].

* Four documents define optional extensions to Babel: authentication
based on Hashed Message Authentication Code (HMAC) [RFC8967],
source-specific routing [RFC9079], delay-based routing
[BABEL-RTT], and ToS-specific (Type of Service) routing
[ToS-SPECIFIC]. All of these extensions interoperate with the
core protocol as well as with each other.

2. The Homenet Profile of Babel

2.1. Requirements

REQ1: A Homenet implementation of Babel MUST encapsulate Babel
control traffic in IPv6 packets sent to the IANA-assigned
port 6696 and either the IANA-assigned multicast group
ff02::1:6 or to a link-local unicast address.

Rationale: Since Babel is able to carry both IPv4 and IPv6
routes over either IPv4 or IPv6, choosing the protocol
used for carrying control traffic is a matter of
preference. Since IPv6 has some features that make
implementations somewhat simpler and more reliable
(notably properly scoped and reasonably stable link-local
addresses), we require carrying control data over IPv6.

REQ2: A Homenet implementation of Babel MUST implement the IPv6
subset of the protocol defined in the body of RFC 8966.

Rationale: Support for IPv6 routing is an essential
component of the Homenet architecture.

REQ3: A Homenet implementation of Babel SHOULD implement the IPv4
subset of the protocol defined in the body of RFC 8966. Use
of other techniques for acquiring IPv4 connectivity (such as
multiple layers of NAT) is strongly discouraged.

Rationale: Support for IPv4 will likely remain necessary
for years to come, and even in pure IPv6 deployments,
including code for supporting IPv4 has very little cost.
Since HNCP makes it easy to assign distinct IPv4 prefixes
to the links in a network, it is not necessary to resort
to multiple layers of NAT, with all of its problems.

REQ4: A Homenet implementation of Babel MUST implement source-
specific routing for IPv6, as defined in RFC 9079 [RFC9079].

Rationale: Source-specific routing is an essential
component of the Homenet architecture. Source-specific
routing for IPv4 is not required, since HNCP arranges
things so that a single nonspecific IPv4 default route is
announced (Section 6.5 of [RFC7788]).

REQ5: A Homenet implementation of Babel must use metrics that are
of a similar magnitude to the values suggested in Appendix A
of [RFC8966]. In particular, it SHOULD assign costs that are
no less than 256 to wireless links and SHOULD assign costs
between 32 and 196 to lossless wired links.

Rationale: If two implementations of Babel choose very
different values for link costs, combining routers from
different vendors will cause suboptimal routing.

REQ6: A Homenet implementation of Babel SHOULD distinguish between
wired and wireless links; if it is unable to determine
whether a link is wired or wireless, it SHOULD make the
worst-case hypothesis that the link is wireless. It SHOULD
dynamically probe the quality of wireless links and derive a
suitable metric from its quality estimation. Appendix A of
[RFC8966] gives an example of a suitable algorithm.

Rationale: Support for wireless transit links is a
distinguishing feature of Homenet, and one that is
requested by our users. In the absence of dynamically
computed metrics, the routing protocol attempts to
minimise the number of links crossed by a route and
therefore prefers long, lossy links to shorter, lossless
ones. In wireless networks, "hop-count routing is worst-
path routing".

While it would be desirable to perform link-quality
probing on some wired link technologies, notably power-
line networks, these kinds of links tend to be difficult
or impossible to detect automatically, and we are not
aware of any published link-quality algorithms for them.
Hence, we do not require link-quality estimation for wired
links of any kind.

2.2. Optional Features

OPT1: A Homenet implementation of Babel MAY perform route selection
by applying hysteresis to route metrics, as suggested in
Section 3.6 of [RFC8966] and described in detail in
Section III.E of [DELAY-BASED]. However, hysteresis is not
required, and the implementation may simply pick the route
with the smallest metric.

Rationale: Hysteresis is only useful in congested and
highly dynamic networks. In a typical home network, which
is stable and uncongested, the feedback loop that
hysteresis compensates for does not occur.

OPT2: A Homenet implementation of Babel may include support for
other extensions to the protocol, as long as they are known
to interoperate with both the core protocol and source-
specific routing.

Rationale: A number of extensions to the Babel routing
protocol have been defined over the years; however, they
are useful in fairly specific situations, such as routing
over global-scale overlay networks [BABEL-RTT] or multi-
hop wireless networks with multiple radio frequencies
[BABEL-Z]. Hence, with the exception of source-specific
routing, no extensions are required for Homenet.

3. Interactions between HNCP and Babel

The Homenet architecture cleanly separates configuration, which is
done by HNCP, from routing, which is done by Babel. While the
coupling between the two protocols is deliberately kept to a minimum,
some interactions are unavoidable.

All the interactions between HNCP and Babel consist of HNCP causing
Babel to perform an announcement on its behalf (under no
circumstances does Babel cause HNCP to perform an action). How this
is realised is an implementation detail that is outside the scope of
this document; while it could conceivably be done using a private
communication channel between HNCP and Babel, in existing
implementations, HNCP installs a route in the operating system's
kernel that is later picked up by Babel using the existing
redistribution mechanisms.

3.1. Requirements

REQ7: If an HNCP node receives a DHCPv6 prefix delegation for
prefix P and publishes an External-Connection TLV containing
a Delegated-Prefix TLV with prefix P and no Prefix-Policy
TLV, then it MUST announce a source-specific default route
with source prefix P over Babel.

Rationale: Source-specific routes are the main tool that
Homenet uses to enable optimal routing in the presence of
multiple IPv6 prefixes. External connections with
nontrivial prefix policies are explicitly excluded from
this requirement, since their exact behaviour is
application specific.

REQ8: If an HNCP node receives a DHCPv4 lease with an IPv4 address
and wins the election for NAT gateway, then it MUST act as a
NAT gateway and MUST announce a (nonspecific) IPv4 default
route over Babel.

Rationale: The Homenet stack does not use source-specific
routing for IPv4; instead, HNCP elects a single NAT
gateway and publishes a single default route towards that
gateway ([RFC7788], Section 6.5).

REQ9: If an HNCP node assigns a prefix P to an attached link and
announces P in an Assigned-Prefix TLV, then it MUST announce
a route towards P over Babel.

Rationale: Prefixes assigned to links must be routable
within the Homenet.

3.2. Optional Features

OPT3: An HNCP node that receives a DHCPv6 prefix delegation MAY
announce a nonspecific IPv6 default route over Babel in
addition to the source-specific default route mandated by
requirement REQ7.

Rationale: Since the source-specific default route is more
specific than the nonspecific default route, the former
will override the latter if all nodes implement source-
specific routing. Announcing an additional nonspecific
route is allowed, since doing that causes no harm and
might simplify operations in some circumstances, e.g.,
when interoperating with a routing protocol that does not
support source-specific routing.

OPT4: An HNCP node that receives a DHCPv4 lease with an IPv4
address and wins the election for NAT gateway SHOULD NOT
announce a source-specific IPv4 default route.

Rationale: Homenet does not require support for IPv4
source-specific routing. Announcing IPv4 source-specific
routes will not cause routing pathologies (blackholes or
routing loops), but it might cause packets sourced in
different parts of the Homenet to follow different paths,
with all the confusion that this entails.

4. Security Considerations

Both HNCP and Babel carry their control data in IPv6 packets with a
link-local source address, and implementations are required to drop
packets sent from a global address. Hence, they are only susceptible
to attacks from a directly connected link on which the HNCP and Babel
implementations are listening.

The security of a Homenet network relies on having a set of
"Internal", "Ad Hoc", and "Hybrid" interfaces (Section 5.1 of
[RFC7788]) that are assumed to be connected to links that are secured
at a lower layer. HNCP and Babel packets are only accepted when they
originate on these trusted links. "External" and "Guest" interfaces
are connected to links that are not trusted, and any HNCP or Babel
packets that are received on such interfaces are ignored. ("Leaf"
interfaces are a special case since they are connected to trusted
links, but HNCP and Babel traffic received on such interfaces is
ignored.) This implies that the security of a Homenet network
depends on the reliability of the border discovery procedure
described in Section 5.3 of [RFC7788].

If untrusted links are used for transit, which is NOT RECOMMENDED,
then any HNCP and Babel traffic that is carried over such links MUST
be secured using an upper-layer security protocol. While both HNCP
and Babel support cryptographic authentication, at the time of
writing, no protocol for autonomous configuration of HNCP and Babel
security has been defined.

5. IANA Considerations

This document has no IANA actions.

6. References

6.1. 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,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC7788] Stenberg, M., Barth, S., and P. Pfister, "Home Networking
Control Protocol", RFC 7788, DOI 10.17487/RFC7788, April
2016, <https://www.rfc-editor.org/info/rfc7788>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8966] Chroboczek, J. and D. Schinazi, "The Babel Routing
Protocol", RFC 8966, DOI 10.17487/RFC8966, January 2021,
<https://www.rfc-editor.org/info/rfc8966>.

[RFC9079] Boutier, M. and J. Chroboczek, "Source-Specific Routing in
the Babel Routing Protocol", RFC 9079,
DOI 10.17487/RFC9079, August 2021,
<https://www.rfc-editor.org/rfc/rfc9079>.

6.2. Informative References

[BABEL-RTT]
Jonglez, B. and J. Chroboczek, "Delay-based Metric
Extension for the Babel Routing Protocol", Work in
Progress, Internet-Draft, draft-ietf-babel-rtt-extension-
00, 26 April 2019, <https://datatracker.ietf.org/doc/html/
draft-ietf-babel-rtt-extension-00>.

[BABEL-Z] Chroboczek, J., "Diversity Routing for the Babel Routing
Protocol", Work in Progress, Internet-Draft, draft-
chroboczek-babel-diversity-routing-01, 15 February 2016,
<https://datatracker.ietf.org/doc/html/draft-chroboczek-
babel-diversity-routing-01>.

[DELAY-BASED]
Jonglez, B., Boutier, M., and J. Chroboczek, "A delay-
based routing metric", March 2014,
<http://arxiv.org/abs/1403.3488>.

[RFC8967] Dô, C., Kolodziejak, W., and J. Chroboczek, "MAC
Authentication for the Babel Routing Protocol", RFC 8967,
DOI 10.17487/RFC8967, January 2021,
<https://www.rfc-editor.org/info/rfc8967>.

[ToS-SPECIFIC]
Chouasne, G. and J. Chroboczek, "TOS-Specific Routing in
Babel", Work in Progress, Internet-Draft, draft-chouasne-
babel-tos-specific-00, 3 July 2017,
<https://datatracker.ietf.org/doc/html/draft-chouasne-
babel-tos-specific-00>.

Acknowledgments

A number of people have helped with defining the requirements listed
in this document. I am especially indebted to Barbara Stark and
Markus Stenberg.

Author's Address

Juliusz Chroboczek
IRIF, University of Paris-Diderot
Case 7014
75205 Paris CEDEX 13
France

Email: [email protected]