Network Working Group M. Boutier
Internet-Draft J. Chroboczek
Intended status: Standards Track IRIF, University of Paris-Diderot
Expires: August 4, 2018 January 31, 2018
Source-Specific Routing in Babel
draft-ietf-babel-source-specific-03
Abstract
Source-specific routing (also known as Source-Address Dependent
Routing, SADR) is an extension to traditional next-hop routing where
packets are forwarded according to both their destination and their
source address. This document describes an extension for source-
specific routing to the Babel routing protocol.
Status of This Memo
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Table of Contents
1. Introduction and background . . . . . . . . . . . . . . . . . 2
2. Specification of Requirements . . . . . . . . . . . . . . . . 3
3. Data Structures . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. The Source Table . . . . . . . . . . . . . . . . . . . . 3
3.2. The Route Table . . . . . . . . . . . . . . . . . . . . . 4
3.3. The Table of Pending Seqno Requests . . . . . . . . . . . 4
4. Data Forwarding . . . . . . . . . . . . . . . . . . . . . . . 4
5. Protocol Operation . . . . . . . . . . . . . . . . . . . . . 5
5.1. Protocol Messages . . . . . . . . . . . . . . . . . . . . 6
5.2. Wildcard Messages . . . . . . . . . . . . . . . . . . . . 6
6. Compatibility with the base protocol . . . . . . . . . . . . 7
6.1. Loop-avoidance . . . . . . . . . . . . . . . . . . . . . 7
6.2. Starvation and Blackholes . . . . . . . . . . . . . . . . 8
7. Protocol Encoding . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Source Prefix sub-TLV . . . . . . . . . . . . . . . . . . 8
7.2. Source-specific Update . . . . . . . . . . . . . . . . . 9
7.3. Source-specific (Route) Request . . . . . . . . . . . . . 9
7.4. Source-Specific Seqno Request . . . . . . . . . . . . . . 9
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
9. Security considerations . . . . . . . . . . . . . . . . . . . 9
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1. Normative References . . . . . . . . . . . . . . . . . . 10
11.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction and background
The Babel routing protocol [BABEL] is a distance vector routing
protocol for next-hop routing. In next-hop routing, each node
maintains a forwarding table which maps destination prefixes to next
hops. The forwarding decision is a per-packet operation which
depends on the destination address of the packets and on the entries
of the forwarding table. When a packet is about to be routed, its
destination address is compared to the prefixes of the routing table:
the entry with the most specific prefix containing the destination
address of the packet is chosen, and the packet is forwarded to the
associated next-hop. Next-hop routing is a simple, well understood
paradigm that works satisfactorily in a large number of cases.
Source-specific routing [SS-ROUTING], or Source Address Dependent
Routing (SADR) [DSR], is a modest extension to next-hop routing where
the forwarding decision depends not only on the destination address
but also on the source address of the packet being routed, which
makes it possible for two packets with the same destination but
different source addresses to be routed following different paths.
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The forwarding tables are extended to map pairs of prefixes
(destination, source) to next hops. When multiple entries match a
given packet, the one with the most specific destination prefix is
chosen, and, in case of equality, the one with the most specific
source prefix.
The main application of source-specific routing is a form of
multihoming known as multihoming with multiple addresses. When using
this technique in a network connected to multiple providers, every
host is assigned multiple addresses, one per provider. When a host
sources a packet, it picks one of its addresses as the source
address, and source-specific routing is used to route the packet to
an edge router that is connected to the corresponding provider, which
is compatible with [BCP84]. Unlike classical multihoming, this
technique is applicable to small networks, as it does not require the
use of provider-independent addresses, or cause excessive growth of
the global routing table. More details are given in [SS-ROUTING] and
[DSR].
This document describes a source-specific routing extension for the
Babel routing protocol [BABEL]. This involves minor changes to the
data structures, which must include a source prefix in addition to
the destination prefix already present, and some changes to the
Update, Route Request and Seqno Request TLVs, which are extended with
a source prefix. The source prefix is encoded using a mandatory sub-
TLV ([BABEL] Section 4.4).
2. Specification of Requirements
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].
3. Data Structures
A number of the conceptual data structures described in Section 3.2
of [BABEL] contain a destination prefix. This specification extends
these data structures with a source prefix. Data from the original
protocol, which do not specify a source prefix, are stored with a
zero length source prefix, which matches exactly the same set of
packets as the original, non-source-specific data.
3.1. The Source Table
Every Babel node maintains a source table, as described in [BABEL]
Section 3.2.5. A source-specific Babel node extends this table with
the following field:
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o The source prefix specifying the source address of packets to
which this entry applies.
The source table is now indexed by triples of the form (prefix,
source prefix, router-id).
Note that the route entry contains a source which itself contains a
source prefix. These are two very different concepts that should not
be confused.
3.2. The Route Table
Every Babel node maintains a route table, as described in [BABEL]
Section 3.2.6. Each route table entry contains, among other data, a
source, which this specification extends with a source prefix as
described above. The route table is now indexed by triples of the
form (prefix, source prefix, neighbour), where the prefix and source
prefix are obtained from the source.
3.3. The Table of Pending Seqno Requests
Every Babel node maintains a table of pending seqno requests, as
described in [BABEL], Section 3.2.7. A source-specific Babel node
extends this table with the following entry:
o The source prefix being requested.
The table of pending seqno requests is now indexed by triples of the
form (prefix, source prefix, router-id).
4. Data Forwarding
In next-hop routing, if two routing table entries overlap, then one
is necessarily more specific than the other; the "longest prefix
rule" specifies that the most specific applicable routing table entry
is chosen.
With source-specific routing, there might no longer be a most
specific applicable entry: two routing table entries might match a
given packet without one necessarily being more specific than the
other. Consider for example the following routing table:
destination source next-hop
2001:DB8:0:1::/64 ::/0 A
::/0 2001:DB8:0:2::/64 B
This specifies that all packets with destination in 2001:DB8:0:1::/64
are to be routed through A, while all packets with source in
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2001:DB8:0:2::/64 are to be routed through B. A packet with source
2001:DB8:0:2::42 and destination 2001:DB8:0:1::57 matches both rules,
although neither is more specific than the other. A choice is
necessary, and unless the choice being made is the same on all
routers in a routing domain, persistent routing loops may occur.
More details are given in [SS-ROUTING] Section IV.C.
A Babel implementation MUST choose routing table entries by using the
so-called destination-first ordering, where a routing table entry R1
is preferred to a routing table entry R2 when either R1's destination
prefix is more specific than R2's, or the destination prefixes are
equal and R1's source prefix is more specific than R2's. (In more
formal terms, routing table entries are compared using the
lexicographic product of the destination prefix ordering by the
source prefix ordering.) This is consistent with the behaviour
described in Section 3.3 of [DSR].
In practice, this means that a source-specific Babel implementation
must take care that any lower layer that performs packet forwarding
obey this semantics. In particular:
o If the lower layers implement the destination-first ordering, then
the Babel implementation MAY use them directly;
o If the lower layers can hold source-specific routes, but not with
the right semantics, then the Babel implementation MUST
disambiguate the routing table by using a suitable disambiguation
algorithm (see [SS-ROUTING] Section V.B for such an algorithm);
o If the lower layers cannot hold source-specific routes, then a
Babel implementation MUST silently ignore (drop) any source-
specific routes.
5. Protocol Operation
This extension does not fundamentally change the operation of the
Babel protocol, and we therefore only describe differences between
the original protocol and the extended protocol.
In the original protocol, three TLVs carry a destination prefix:
Updates, Route Requests and Seqno Requests. This specification
extends these messages to optionally carry a source prefix sub-TLV,
as described in Section 7 below. The sub-TLV is marked as mandatory,
so that an unextended implementation will silently ignore the whole
enclising TLV. A node obeying this specification MUST NOT send a TLV
with a zero-length source prefix: instead, it sends a TLV with no
source prefix sub-TLV. Conversely, an extended implementation MUST
interpret an unextended TLV as carrying a source prefix of zero
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length. Taken together, these properties ensure interoperability
between the original and extended protocols (see Section 6 below).
5.1. Protocol Messages
This extension allows three TLVs of the original Babel protocol to
carry a source prefix: Update TLVs, Route Request TLVs and Seqno
Request TLVs.
In order to advertise a route with a non-zero-length source prefix, a
node sends a source-specific Update, i.e., an Update with a source
prefix sub-TLV. When a node receives a source-specific Update
(prefix, source prefix, router-id, seqno, metric) from a neighbour
neigh, it behaves as described in [BABEL] Section 3.5.4, except that
the entry under consideration is indexed by (prefix, source prefix,
neigh) rather than just (prefix, neigh).
Similarly, when a node needs to send a Request of either kind that
applies to a route with a non-zero length source prefix, it sends a
source-specific Request, i.e., a Request with a source prefix sub-
TLV. When a node receives a source-specific Request, it behaves as
described in Section 3.8 of [BABEL], except that the request applies
to the Route Table entry carrying the source prefix indicated by the
sub-TLV.
5.2. Wildcard Messages
In the original protocol, the Address Encoding value 0 is used for
wildcard messages: messages that apply to all routes, of any address
family and with any destination prefix. Wildcard messages are
allowed in two places in the protocol: wildcard retractions are used
to retract all of the routes previously advertised by a node on a
given interface, and wildcard Route Requests are used to request a
full dump of the Route Table from a given node. Wildcard messages
are intended to apply to all routes, including routes decorated with
additional data and AE values to be defined by future extensions, and
hence this specification extends wildcard operations to apply to all
routes, whatever the value of the source prefix.
More precisely, a node receiving an Update with the AE field set to 0
and the Metric field set to infinity (a wildcard retraction) MUST
apply the route acquisition procedure described in Section 3.5.4 of
[BABEL] to all of the routes that is has learned from the sending
node, whatever the value of the source prefix. A node MUST NOT send
a wildcard retraction with an attached source prefix, and a node that
receives a wildcard retraction with a source prefix MUST silently
ignore it.
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Similarly, a node that receives a route request with the AE field set
to 0 (a wildcard route request) SHOULD send a full routing table
dump, including routes with a non-zero-length source prefix. A node
MUST NOT send a wildcard request that carries a source prefix, and a
node receiving a wildcard request with a with a source prefix MUST
silently ignore it.
6. Compatibility with the base protocol
The protocol extension defined in this document is, to a great
extent, interoperable with the base protocol defined in [BABEL] (and
all of its extensions). More precisely, if non-source-specific
routers and source-specific routers are mixed in a single routing
domain, Babel's loop-avoidance properties are preserved, and, in
particular, no persistent routing loops will occur.
However, this extension is encoded using mandatory sub-TLVs,
introduced in [BABEL], and therefore is not compatible with the older
version of the Babel Routing Protocol [RFC6126]. Consequently, this
extension MUST NOT be used with routers implementing RFC 6126,
otherwise persistent routing loops may occur.
6.1. Loop-avoidance
The extension defined in this protocol uses a new Mandatory sub-TLV
to carry the source prefix information. As discussed in Section 4.4
of [BABEL], this encoding ensures that non-source-specific routers
will silently ignore the whole TLV, which is necessary to avoid
persistent routing loops in hybrid networks.
Consider two nodes A and B, with A source-specific announcing a route
to (D, S). Suppose that B (non source-specific) merely ignores the
source prefix information when it receives the update rather than
ignoring the whole TLV, and re-announces the route as D. This re-
announcement reaches A, which treats it as (D, ::/0). Packets
destined to D but not sourced in S will be forwarded by A to B, and
by B to A, causing a persistent routing loop:
(D,S) (D)
<-- <--
------ A ----------------- B
-->
(D,::/0)
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6.2. Starvation and Blackholes
In general, discarding source-specific routes by non-source-specific
routers will cause route starvation. Intuitively, unless there are
enough non-source-specific routes in the network, non-source-specific
routers will suffer starvation, and discard packets for destinations
that are only announced by source-specific routers.
A simple yet sufficient condition for avoiding starvation is to build
a connected source-specific backbone that includes all of the edge
routers, and announce a (non-source-specific) default route towards
the backbone.
7. Protocol Encoding
This extension defines a new sub-TLV used to carry a source prefix:
the Source Prefix sub-TLV. It can be used within an Update, a Route
Request or a Seqno Request TLV to match a source-specific entry of
the Route Table, in conjunction with the destination prefix natively
carried by these TLVs.
Since a source-specific routing entry is characterized by a single
destination prefix and a single source prefix, a source-specific
message contains exactly one Source Prefix sub-TLV. A node MUST NOT
send more that one Source Prefix sub-TLV in a TLV, and a node
receiving more than one Source Prefix sub-TLV in a single TLV SHOULD
ignore this TLV. It MAY ignore the whole packet.
7.1. Source Prefix sub-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 = 128 | Length | Source Plen | Source Prefix...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Fields:
Type Set to 128 to indicate a Source Prefix sub-TLV.
Length The length of the body, exclusive of the Type and Length
fields.
Source Plen The length of the advertised source prefix. This MUST
NOT be 0.
Source Prefix The source prefix being advertised. This field's size
is (Source Plen)/8 rounded upwards.
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The contents of the source prefix sub-TLV are interpreted according
to the AE of the enclosing TLV.
Note that this sub-TLV is a mandatory sub-TLV. Threfore, as
described in Section 4.4 of [BABEL], the whole TLV MUST be ignored if
that sub-TLV is not understood (or malformed). Otherwise, routing
loops may occur (see Section 6.1).
7.2. Source-specific Update
The source-specific Update is an Update TLV with a Source Prefix sub-
TLV. It advertises or retracts source-specific routes in the same
manner than routes with non-source-specific Updates (see [BABEL]). A
wildcard retraction (Update with AE equals to 0) MUST NOT carry a
Source Prefix sub-TLV.
Contrary to the destination prefix, this extension does not compress
the source prefix attached to Updates. However, compression is
allowed for the destination prefix of source-specific routes. As
described in Section 4.5 of [BABEL], unextended implementations will
correctly update their parser state while otherwise ignoring the
whole TLV.
7.3. Source-specific (Route) Request
A source-specific Route Request is a Route Request TLV with a Source
Prefix sub-TLV. It prompts the receiver to send an update for a
given pair of destination and source prefixes, as described in
Section 3.8.1.1 of [BABEL]. A wildcard request (Route Request with
AE equals to 0) MUST NOT carry a Source Prefix sub-TLV.
7.4. Source-Specific Seqno Request
A source-specific Seqno Request is a Seqno Request TLV with a Source
Prefix sub-TLV. It requests the receiving node to perform the
procedure described in Section 3.8.1.2 of [BABEL], but applied to a
pair of a destination and source prefix.
8. IANA Considerations
IANA has allocated sub-TLV number 128 for the Source Prefix sub-TLV
in the Babel Sub-TLV Numbers registry.
9. Security considerations
The extension defined in this document adds a new sub-TLV to three
TLVs already present in the original Babel protocol, and does not in
itself change the security properties of the protocol. However,
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source-specific routing gives more control over routing to the
sending hosts, which might have security implications (see Section 8
of [DSR]).
10. Acknowledgments
The authors are grateful to Joel Halpern for his help with this
document.
11. References
11.1. Normative References
[BABEL] Chroboczek, J., "The Babel Routing Protocol", Internet
Draft draft-ietf-babel-rfc6126bis-04, May 2017.
[BCP84] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, March 2004.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997.
11.2. Informative References
[DSR] Lamparter, D. and A. Smirnov, "Destination/Source
Routing", Internet Draft draft-ietf-rtgwg-dst-src-routing-
06, May 2018.
[RFC6126] Chroboczek, J., "The Babel Routing Protocol
(Experimental)", RFC 6126, February 2011.
[SS-ROUTING]
Boutier, M. and J. Chroboczek, "Source-Specific Routing",
August 2014.
In Proc. IFIP Networking 2015. A slightly earlier
version is available online from http://arxiv.org/
pdf/1403.0445.
Authors' Addresses
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Matthieu Boutier
IRIF, University of Paris-Diderot
Case 7014
75205 Paris Cedex 13
France
Email: boutier@irif.fr
Juliusz Chroboczek
IRIF, University of Paris-Diderot
Case 7014
75205 Paris Cedex 13
France
Email: jch@irif.fr
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