Internet DRAFT - draft-ietf-homenet-babel-profile
draft-ietf-homenet-babel-profile
Network Working Group J. Chroboczek
Internet-Draft IRIF, University of Paris-Diderot
Intended status: Standards Track July 18, 2018
Expires: January 19, 2019
Homenet profile of the Babel routing protocol
draft-ietf-homenet-babel-profile-07
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
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This Internet-Draft will expire on January 19, 2019.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirement Language . . . . . . . . . . . . . . . . . . 2
1.2. Background . . . . . . . . . . . . . . . . . . . . . . . 2
2. The Homenet profile of Babel . . . . . . . . . . . . . . . . 3
2.1. Requirements . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Optional features . . . . . . . . . . . . . . . . . . . . 5
3. Interactions between HNCP and Babel . . . . . . . . . . . . . 5
3.1. Requirements . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Optional features . . . . . . . . . . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
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 [RFC6126bis]. 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. Requirement 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:
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o RFC 6126bis [RFC6126bis] 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 RFC 6126bis Babel 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.
o The informative Appendix A of RFC 6126bis 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.
o While RFC 6126bis does not provide an algorithm for route
selection, its Section 3.6 suggests selecting the route with
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].
o Five RFCs and Internet-Drafts define optional extensions to Babel:
HMAC-based authentication [RFC7298], source-specific routing
[BABEL-SS], delay-based routing [BABEL-RTT] and ToS-specific
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.
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REQ2: a Homenet implementation of Babel MUST implement the IPv6
subset of the protocol defined in the body of RFC 6126bis.
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 6126bis. 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 draft-ietf-babel-source-
specific [BABEL-SS].
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 non-specific
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
RFC 6126bis. 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 sub-optimal 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 RFC 6126bis 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
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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 RFC 6126bis and described in detail in Section III.E of
[BABEL-RTT]. 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, 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
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communication channel between HNCP and Babel, in existing
implementations HNCP installs a route in the operating system's
kernel which 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 non-trivial 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 (non-specific) 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 non-specific 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 non-specific default route, the former will
override the latter if all nodes implement source-specific
routing. Announcing an additional non-specific route is allowed,
since doing that causes no harm and might simplify operations in
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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.
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 requires no actions from IANA.
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6. 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.
7. References
7.1. Normative References
[BABEL-SS]
Boutier, M. and J. Chroboczek, "Source-Specific Routing in
Babel", draft-ietf-babel-source-specific-03 (work in
progress), August 2018.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997.
[RFC6126bis]
Chroboczek, J. and D. Schinazi, "The Babel Routing
Protocol", Internet Draft draft-ietf-babel-rfc6126bis-04,
October 2017.
[RFC7788] Stenberg, M., Barth, S., and P. Pfister, "Home Networking
Control Protocol", RFC 7788, DOI 10.17487/RFC7788, April
2016.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017.
7.2. Informative References
[BABEL-RTT]
Jonglez, B. and J. Chroboczek, "Delay-based Metric
Extension for the Babel Routing Protocol", draft-jonglez-
babel-rtt-extension-01 (work in progress), May 2015.
[BABEL-Z] Chroboczek, J., "Diversity Routing for the Babel Routing
Protocol", draft-chroboczek-babel-diversity-routing-01
(work in progress), February 2016.
[DELAY-BASED]
Jonglez, B. and J. Chroboczek, "A delay-based routing
metric", March 2014.
Available online from http://arxiv.org/abs/1403.3488
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[RFC7298] Ovsienko, D., "Babel Hashed Message Authentication Code
(HMAC) Cryptographic Authentication", RFC 7298, July 2014.
[ToS-SPECIFIC]
Chouasne, G., "https://tools.ietf.org/id/
draft-chouasne-babel-tos-specific-00.xml", draft-chouasne-
babel-tos-specific-00 (work in progress), July 2017.
Author's Address
Juliusz Chroboczek
IRIF, University of Paris-Diderot
Case 7014
75205 Paris Cedex 13
France
Email: jch@irif.fr
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