Internet DRAFT - draft-boutier-babel-source-specific
draft-boutier-babel-source-specific
Network Working Group M. Boutier
Internet-Draft J. Chroboczek
Updates: 6126bis (if approved) IRIF, University of Paris-Diderot
Intended status: Standards Track July 03, 2017
Expires: January 4, 2018
Source-Specific Routing in Babel
draft-boutier-babel-source-specific-03
Abstract
This document describes an extension to the Babel routing protocol to
support source-specific routing.
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Table of Contents
1. TODOs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Introduction and background . . . . . . . . . . . . . . . . . 2
3. Data Structures . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. The Source Table . . . . . . . . . . . . . . . . . . . . 3
3.2. The Route Table . . . . . . . . . . . . . . . . . . . . . 3
3.3. The Table of Pending Requests . . . . . . . . . . . . . . 3
4. Data Forwarding . . . . . . . . . . . . . . . . . . . . . . . 3
5. Protocol Operation . . . . . . . . . . . . . . . . . . . . . 5
5.1. Source-specific messages . . . . . . . . . . . . . . . . 5
5.2. Route Acquisition . . . . . . . . . . . . . . . . . . . . 5
5.3. Wildcard retractions (update) . . . . . . . . . . . . . . 5
5.4. Wildcard requests . . . . . . . . . . . . . . . . . . . . 6
6. Compatibility with the base protocol . . . . . . . . . . . . 8
6.1. Loop-avoidance . . . . . . . . . . . . . . . . . . . . . 8
6.2. Starvation and Blackholes . . . . . . . . . . . . . . . . 8
7. Protocol Encoding . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Source Prefix sub-TLV . . . . . . . . . . . . . . . . . . 9
7.2. Source-specific Update . . . . . . . . . . . . . . . . . 9
7.3. Source-specific (Route) Request . . . . . . . . . . . . . 10
7.4. Source-Specific Seqno Request . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. Security considerations . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative References . . . . . . . . . . . . . . . . . . 10
10.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. TODOs
o Source Prefix sub-TLV type: TBD
o check references (Section) for BABEL in 6126bis
o define wildcard Requests behaviour
2. Introduction and background
Source-specific routing (also known as Source Address Dependant
Routing, SAD Routing or SADR) is an extension to traditional next-hop
routing where packets are routed according to both their destination
and their source address. This document describes the source-
specific routing extension to the Babel routing protocol as defined
in 6126bis [BABEL].
Background information about source-specific routing is provided in
[SS-ROUTING].
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3. Data Structures
This extension adds some data to the data structures maintained by a
Babel node.
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:
o the source prefix (sprefix, splen) specifying the source address
of packets to which this entry applies.
If a source table entry has a zero length source prefix (splen equals
to 0), then the entry is a non-source-specific entry, and is treated
just like a source table entry defined by the original Babel
protocol.
With this extension the route entry contains a source which itself
contains a source prefix. These are two very different concepts, and
should not be confused.
3.2. The Route Table
Every Babel node maintains a route table, as described in [BABEL],
Section 3.2.6. With this extension, this table is indexed by the
5-tuple (prefix, plen, source prefix, source plen, router-id)
obtained from the associated source table entry.
If a route table entry has a zero length source prefix, then the
entry is a non-source-specific entry, and is treated just like a
route table entry defined by the original Babel protocol.
3.3. The Table of Pending Requests
Every Babel node maintains a table of pending 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.
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.
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With source-specific routing, there might no longer be a most
specific applicable prefix: two routing table entries might match a
given packet without one necessarily being more specific than the
other. Consider for example the following fragment of a routing
table:
(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 packets with a source in
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 informations are available 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.)
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.
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5. Protocol Operation
This extension does not fundamentally change the operation of the
Babel protocol. We only describe the fundamental differences between
the original protocol and the extension in this section. The other
mechanisms described in [BABEL] (Section 3) are extended to pairs of
(destination, source) prefixes instead of just (destination)
prefixes.
5.1. Source-specific messages
Three messages are used to communicate informations on routes:
Updates, Route Requests and Seqno Requests. With this extension,
these messages carry an additionnal source prefix if (and only if)
the corresponding route is source-specific. More formally, an
Update, a Route Request and a Seqno Request MUST carry a source
prefix if they concern a source-specific route (non-zero length
source prefix) and MUST NOT carry a source prefix otherwise (zero
length source prefix). A message which carries a source prefix is
said to be source-specific.
5.2. Route Acquisition
When a non-source-specific Babel node receives a source-specific
update, it silently ignores it.
TODO{On receipt of a source-specific update (id, prefix, source
prefix, seqno, metric), a source-specific Babel node behaves as
described in [BABEL] Section 3.5.4 though indexing entries by (neigh,
id, prefix, source prefix).} When a source-specific Babel node
receives a non-source-specific update, it MUST treat this update as
carrying a zero length source prefix.
5.3. Wildcard retractions (update)
The original protocol defines a wildcard update with AE equals to 0
as being a wildcard retraction. A node receiving a wildcard
retraction on an interface must consider that the sending node
retracts all the routes it advertised on this interface.
Wildcard retractions are used when a node is about to leave the
network. Thus, this extension does not define source-specific
wildcard retraction, but extends wildcard retraction to apply also to
source-specific routes. More formally, a wildcard update MUST NOT
carry a source prefix, and a source-specific Babel node receiving a
(legacy) wildcard update MUST retracts all routes it learns from this
node (including source-specific ones).
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5.4. Wildcard requests
TODO: behaviour to be defined.
5.4.1. Proposal 1
The original Babel protocol states that when a node receives a
wildcard route request, it SHOULD send a full routing table dump.
This extension does not change this statement: a source-specific node
SHOULD send a full routing table dump when receiving a wildcard
request.
Source-specific wildcard requests does not exist: a wildcard request
SHOULD NOT carry a source prefix.
5.4.2. Proposal 2
We assume that a mandatory sub-TLV has a corresponding non-mandatory
sub-TLV. This proposal is like Proposal 3 but instead of having
multiple wildcard request TLVs, one for each kind of route
understood, we use one wildcard request with sub-TLVs corresponding
to the extension. To have a full routing table dump, a node sends a
wildcard requests with a non-mandatory Source sub-TLV.
A source-specific node SHOULD always attach a non-mandatory Source
sub-TLV to its wildcard requests.
This proposal has been rejected because it implies to share the space
of non-mandatory and mandatory sub-TLVs.
5.4.3. Proposal 3 (mentionned by Juliusz)
The Babel protocol provides the ability to request a full routing
table dump by sending a "wildcard request", a route request with the
AE field set to 0. As the original protocol has no source-specific
routes, such a request may only concern non-source-specific routes.
This extension does not modify the semantics of wildcard requests in
that sense: a wildcard request prompts the receiver to send its non-
source-specific routes only, and a Babel node SHOULD NOT send any
source-specific updates in reply to a wildcard request.
To obtain a dump of the source-specific routes, a source-specific
wildcard request MUST be used. A source-specific wildcard request is
a wildcard request carrying a zero length source prefix.
When a node receives a source-specific wildcard request, it SHOULD
send a dump of its routes which are source-specific "only". It
SHOULD NOT send any non-source-specific routes in reply to a source-
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specific wildcard request. It SHOULD NOT send any source-specific
routes which are under the effect of a future extension. Such
extension should detail how to handle the possible combinations.
In consequence, a node requiring a full routing table dump must send
both a non-source-specific wildcard request and a source-specific
wildcard request.
5.4.4. Proposal 4 (mentionned by Juliusz)
Wildcard requests are deprecated. Either deprecate it in 6126bis, or
say the following.
A node receiving a wildcard request SHOULD ignore it.
This proposal has been rejected because wildcard requests speeds up
the convergence of the network on boot. This is considered
important.
5.4.5. Proposal 5 (mentionned by David)
By default, a vanilla wildcard request triggers a dump of all non-
specific routes. We define a new non-mandatory sub-TLV on Route
Requests called "Requested Route Types" that contains an array of all
the types of routes this request is requesting.
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 = TBD | Length | RR Type 1 | RR Type 2...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
We also create a registry of Requested Route (RR) types, for example:
0 = Regular
1 = Source-Specific
2 = TOS-specific
etc.
A node receiving a Requested Route Types sub-TLV in a wildcard
request SHOULD sends back a dump of all its routes corresponding to
the requested types or to a combination of these types.
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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 its known 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 not compatible with the Experimental
Track's Babel Routing Protocol (RFC 6126). It requires the mandatory
sub-TLV introduced in [BABEL]. 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 merely ignores the source prefix
information when it receives the update rather than ignoring the sub-
TLV, and reannounces the route as D. This reannouncement 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)
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
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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 by
the three following existing messages: Update, Route Request and
Seqno Request.
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 = TBD | Length | Source Plen | Source Prefix...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Fields:
Type Set to TBD 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.
The Source Prefix field's encoding (AE) is the same as the Prefix's.
It is defined by the AE field of the corresponding TLV.
Note that this sub-TLV is a Mandatory sub-TLV. The whole TLV MUST be
ignored if that TLV is not recognized as described in Section 4.4.
Otherwise, routing loops may occur.
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]).
This TLV MUST NOT be attached to wildcard updates.
Contrary to the destination prefix, this extension does not compress
the source prefix attached to Updates. The destination prefix uses
compression as defined in [BABEL] for Updates with Mandatory
extensions.
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However, as defined in [BABEL] (Section 4.5), the compression is
allowed for the destination prefix of source-specific routes. Legacy
implementation will correctly update their parser state, while
ignoring the whole TLV afterwards.
7.3. Source-specific (Route) Request
TODO: A source-specific Route Request prompts the receiver to send an
update for a given pair of destination and source prefixes. It MUST
NOT be used to request a full routing table dump. The Source Prefix
sub-TLV of a wildcard source-specific Route Request (Request with AE
equals to 0 and a Source Prefix sub-TLV) MIGHT be ignored: a receiver
MIGHT reply by a full routing table dump.
7.4. Source-Specific Seqno Request
A source-specific Seqno Request is just like a Seqno Request for a
source-specific route. It uses the same mechanisms described in
[BABEL].
8. IANA Considerations
IANA is instructed to add the following entry to the "Babel sub-TLV
Types" registry:
+------+---------------+-----------------+
| Type | Name | Reference |
+------+---------------+-----------------+
| TBD | Source Prefix | (this document) |
+------+---------------+-----------------+
9. Security considerations
The extension defined in this document adds a new sub-TLV to three
TLVs already present in the original Babel protocol. It does not by
itself change the security properties of the protocol.
10. References
10.1. Normative References
[BABEL] Chroboczek, J., "The Babel Routing Protocol", Internet
Draft draft-ietf-babel-rfc6126bis-02, May 2017.
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10.2. Informative References
[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
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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