IDR Working Group E. Jasinska
Internet-Draft Limelight Networks
Intended status: Standards Track N. Hilliard
Expires: September 27, 2012 INEX
R. Raszuk
NTT MCL Inc.
N. Bakker
AMS-IX B.V.
March 26, 2012
Internet Exchange Route Server
draft-ietf-idr-ix-bgp-route-server-00
Abstract
This document outlines a specification for multilateral
interconnections at Internet exchange points (IXPs). Multilateral
interconnection is a method of exchanging routing information between
three or more exterior BGP speakers using a single intermediate
broker system, referred to as a route server. Route servers are
typically used on shared access media networks, such as Internet
exchange points (IXPs), to facilitate simplified interconnection
between multiple Internet routers.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 27, 2012.
Copyright Notice
Copyright (c) 2012 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
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Provisions Relating to IETF Documents
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Table of Contents
1. Introduction to Multilateral Interconnection . . . . . . . . . 3
1.1. Specification of Requirements . . . . . . . . . . . . . . 3
2. Technical Considerations for Route Server Implementations . . 4
2.1. Client UPDATE Messages . . . . . . . . . . . . . . . . . . 4
2.2. Attribute Transparency . . . . . . . . . . . . . . . . . . 4
2.2.1. NEXT_HOP Attribute . . . . . . . . . . . . . . . . . . 4
2.2.2. AS_PATH Attribute . . . . . . . . . . . . . . . . . . 4
2.2.3. MULTI_EXIT_DISC Attribute . . . . . . . . . . . . . . 5
2.2.4. Communities Attributes . . . . . . . . . . . . . . . . 5
2.3. Per-Client Policy Control in Multilateral
Interconnection . . . . . . . . . . . . . . . . . . . . . 5
2.3.1. Path Hiding on a Route Server . . . . . . . . . . . . 6
2.3.2. Implementing Per-Client Policy Control . . . . . . . . 7
2.3.2.1. Multiple Route Server RIBs . . . . . . . . . . . . 7
2.3.2.2. Advertising Multiple Paths . . . . . . . . . . . . 7
3. Security Considerations . . . . . . . . . . . . . . . . . . . 8
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1. Normative References . . . . . . . . . . . . . . . . . . . 9
6.2. Informative References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
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1. Introduction to Multilateral Interconnection
Internet exchange points (IXPs) provide IP data interconnection
facilities for their participants, typically using shared Layer-2
networking media such as Ethernet. The Border Gateway Protocol (BGP)
[RFC4271], an inter-Autonomous System routing protocol, is commonly
used to facilitate exchange of network reachability information over
such media.
While bilateral exterior BGP sessions between exchange participants
were previously the most common means of exchanging reachability
information, the overhead associated with dense interconnection has
caused substantial operational scaling problems for Internet exchange
point participants.
Multilateral interconnection is a method of interconnecting BGP
speaking routers using a third party brokering system, commonly
referred to as a route server and typically managed by the IXP
operator. Each of the multilateral interconnection participants
(usually referred to as route server clients) announces network
reachability information to the route server using exterior BGP, and
the route server in turn forwards this information to each other
route server client connected to it, according to its configuration.
Although a route server uses BGP to exchange reachability information
with each of its clients, it does not forward traffic itself and is
therefore not a router.
A route server can be viewed as similar in function to an [RFC4456]
route reflector, except that it operates using EBGP instead of iBGP.
Certain adaptions to [RFC4271] are required to enable an EBGP router
to operate as a route server, which are outlined in Section 2 of this
document.
The term "route server" is often in a different context used to
describe a BGP node whose purpose is to accept BGP feeds from
multiple clients for the purpose of operational analysis and
troubleshooting. A system of this form may alternatively be known as
a "route collector" or a "route-views server". This document uses
the term "route server" exclusively to describe multilateral peering
brokerage systems.
1.1. Specification of Requirements
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
[RFC2119].
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2. Technical Considerations for Route Server Implementations
2.1. Client UPDATE Messages
A route server MUST accept all UPDATE messages received from each of
its clients for inclusion in its Adj-RIB-In. These UPDATE messages
MAY by omitted from the route server's Loc-RIB or Loc-RIBs, due to
filters configured for the purposes of implementing routing policy.
The route server SHOULD perform one or more BGP Decision Processes to
select routes for subsequent advertisement to its clients, taking
into account possible configuration to provide multiple NLRI paths to
a particular client as described in Section 2.3.2.2 or multiple Loc-
RIBs as described in Section 2.3.2.1. The route server SHOULD
forward UPDATE messages where appropriate from its Loc-RIB or Loc-
RIBs to its clients.
2.2. Attribute Transparency
As a route server primarily performs a brokering service,
modification of attributes could cause route server clients to alter
their BGP best path selection process for received prefix
reachability information, thereby changing the intended routing
policies of exchange participants. Therefore, contrary to what is
specified in section 5. of [RFC4271], route servers SHOULD NOT by
default (unless explicitly configured) update well-known BGP
attributes received from route server clients before redistributing
them to their other route server clients. Optional recognized and
unrecognized BGP attributes, whether transitive or non-transitive,
SHOULD NOT be updated by the route server (unless enforced by local
IX operator configuration) and SHOULD be passed on to other route
server clients.
2.2.1. NEXT_HOP Attribute
The NEXT_HOP, a well-known mandatory BGP attribute, defines the IP
address of the router used as the next hop to the destinations listed
in the Network Layer Reachability Information field of the UPDATE
message. As the route server does not participate in the actual
routing of traffic, the NEXT_HOP attribute MUST be passed unmodified
to the route server clients, similar to the "third party" next hop
feature described in section 5.1.3. of [RFC4271].
2.2.2. AS_PATH Attribute
AS_PATH is a well-known mandatory attribute which identifies the
autonomous systems through which routing information carried in the
UPDATE message has passed.
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As a route server does not participate in the process of forwarding
data between client routers, and because modification of the AS_PATH
attribute could affect route server client best path calculations,
the route server SHOULD NOT prepend its own AS number to the AS_PATH
segment nor modify the AS_PATH segment in any other way.
2.2.3. MULTI_EXIT_DISC Attribute
MULTI_EXIT_DISC is an optional non-transitive attribute intended to
be used on external (inter-AS) links to discriminate among multiple
exit or entry points to the same neighboring AS. If applied to an
NLRI UPDATE sent to a route server, the attribute (contrary to
section 5.1.4 of [RFC4271]) SHOULD be propagated to other route
server clients and the route server SHOULD NOT modify the value of
this attribute.
2.2.4. Communities Attributes
The BGP COMMUNITIES ([RFC1997]) and Extended Communities ([RFC4360])
attributes are attributes intended for labeling information carried
in BGP UPDATE messages. Transitive as well as non-transitive
Communities attributes applied to an NLRI UPDATE sent to a route
server SHOULD NOT be modified, processed or removed. However, if
such an attribute is intended for processing by the route server
itself, it MAY be modified or removed.
2.3. Per-Client Policy Control in Multilateral Interconnection
While IXP participants often use route servers with the intention of
interconnecting with as many other route server participants as
possible, there are circumstances where control of path distribution
on a per-client basis is important for ensuring that desired
interconnection policies are met.
The control of path distribution on a per-client basis can lead to a
path being hidden from the route server client. We refer to this as
"path hiding", which is described in Section 2.3.1.
Route server implementations SHOULD implement one of the methods
described in Section 2.3.2, for the operator to be able to allow the
control of path distribution on a per-client basis without the
occurrence of "path hiding".
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2.3.1. Path Hiding on a Route Server
___ ___
/ \ / \
..| AS1 |..| AS2 |..
: \___/ \___/ :
: \ / | :
: \ / | :
: IXP \/ | :
: /\ | :
: / \ | :
: ___/____\_|_ :
: / \ / \ :
..| AS3 |..| AS4 |..
\___/ \___/
Figure 1: Per-Client Policy Controlled Interconnection at an IXP
Using the example in Figure 1, AS1 does not directly exchange prefix
information with either AS2 or AS3 at the IXP, but only interconnects
with AS4.
In the traditional bilateral interconnection model, per-client policy
control to a third party exchange participant is accomplished either
by not engaging in a bilateral interconnection with that participant
or else by implementing outbound filtering on the BGP session towards
that participant. However, in a multilateral interconnection
environment, only the route server can perform outbound filtering in
the direction of the route server client; route server clients depend
on the route server to perform their outbound filtering for them.
Assuming a traditional best path selection, when the same prefix is
advertised to a route server from multiple route server clients, the
route server will select a single best path for propagation to all
connected clients. If, however, the route server has been configured
to filter the calculated best path from reaching a particular route
server client, then that client will not receive a path for that
prefix, although alternate paths received by the route server might
have been policy compliant for that client. This phenomenon is
referred to as "path hiding".
For example, in Figure 1, if the same prefix were sent to the route
server via AS2 and AS4, and the route via AS2 was preferred according
to BGP's traditional best path selection, but AS1's policy prevents
AS2's path from being accepted, then AS1 would never receive a path
to this prefix, even though the route server had previously received
a valid alternative path via AS4. This happens because the best path
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selection is performed only once on the route server for all clients.
Path hiding will only occur on route servers which employ per-client
policy control; if an IXP operator deploys a route server without the
possibility for policy control, then path hiding does not occur, as
all paths are considered equally valid from the point of view of the
route server.
2.3.2. Implementing Per-Client Policy Control
In this section, we describe the alternatives to provide per-client
policy control while preventing the occurrence of path hiding.
2.3.2.1. Multiple Route Server RIBs
The most portable means to allow for per-client policy control
without the occurrence of path hiding, is by using a route server BGP
implementation which performs the per-client best path calculation
for each set of paths to a prefix, which results after the route
server's client policies have been taken into consideration. This
can be implemented by using per-client Loc-RIBs, with path filtering
implemented between the Adj-RIB-In and the per-client Loc-RIB.
Implementations MAY optimize this by maintaining paths not subject to
filtering policies in a global Loc-RIB, with per-client Loc-RIBs
stored as deltas.
This implementation is highly portable, as it makes no assumptions
about the feature capabilities of the route server clients.
2.3.2.2. Advertising Multiple Paths
The path distribution model described above assumes standard BGP
session encoding where the route server sends a single path to its
client for any given prefix. This path is selected using the BGP
path selection decision process described in [RFC4271]. If, however,
it were possible for the route server to send more than a single path
to a route server client, then route server clients would no longer
depend on receiving a single best path to a particular prefix;
consequently, the path hiding problem described in Section 2.3.1
would disappear.
We present two methods which describe how such increased path
diversity could be implemented.
2.3.2.2.1. Diverse BGP Path Approach
The Diverse BGP Path proposal as defined in
[I-D.ietf-grow-diverse-bgp-path-dist] is a simple way to distribute
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multiple prefix paths from a route server to a route server client by
using a separate BGP session from the route server to a client for
each different path.
The number of paths which may be distributed to a client is
constrained by the number of BGP sessions which the server and the
client are willing to establish with each other. The distributed
paths may be established from the global BGP Loc-RIB on the route
server in addition to any per-client Loc-RIB. As there may be more
potential paths to a given prefix than configured BGP sessions, this
method is not guaranteed to eliminate the path hiding problem in all
situations. Furthermore, this method may significantly increase the
number of BGP sessions handled by the route server, which may
negatively impact its performance.
2.3.2.2.2. BGP ADD-PATH Approach
The [I-D.ietf-idr-add-paths] Internet draft proposes a different
approach to multiple path propagation, by allowing a BGP speaker to
forward multiple paths for the same prefix on a single BGP session.
As [RFC4271] specifies that a BGP listener must implement an implicit
withdraw when it receives an UPDATE message for a prefix which
already exists in its Adj-RIB-In, this approach requires explicit
support for the feature both on the route server and on its clients.
If the ADD-PATH capability is negotiated bidirectionally between the
route server and a route server client, and the route server client
propagates multiple paths for the same prefix to the route server,
then this could potentially cause the propagation of inactive,
invalid or suboptimal paths to the route server, thereby causing loss
of reachability to other route server clients. For this reason, ADD-
PATH implementations on a route server SHOULD enforce send-only mode
with the route server clients, which would result in negotiating
receive-only mode from the client to the route server.
3. Security Considerations
The path hiding problem outlined in section Section 2.3.1 can be used
in certain circumstances to proactively block third party path
announcements from other route server clients.
4. IANA Considerations
The new set of mechanism for route servers does not require any new
allocations from IANA.
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5. Acknowledgments
The authors would like to thank Ryan Bickhart, Steven Bakker, Chris
Hall, Bruno Decraene and Pierre Francois for their valuable input.
In addition, the authors would like to acknowledge the developers of
BIRD, OpenBGPD and Quagga, whose open source BGP implementations
include route server capabilities which are compliant with this
document.
6. References
6.1. Normative References
[I-D.ietf-grow-diverse-bgp-path-dist]
Raszuk, R., Fernando, R., Patel, K., McPherson, D., and K.
Kumaki, "Distribution of diverse BGP paths.",
draft-ietf-grow-diverse-bgp-path-dist-06 (work in
progress), November 2011.
[I-D.ietf-idr-add-paths]
Walton, D., Chen, E., Retana, A., and J. Scudder,
"Advertisement of Multiple Paths in BGP",
draft-ietf-idr-add-paths-06 (work in progress),
September 2011.
[RFC1997] Chandrasekeran, R., Traina, P., and T. Li, "BGP
Communities Attribute", RFC 1997, August 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
Communities Attribute", RFC 4360, February 2006.
[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 4456, April 2006.
6.2. Informative References
[RFC1863] Haskin, D., "A BGP/IDRP Route Server alternative to a full
mesh routing", RFC 1863, October 1995.
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[RFC4223] Savola, P., "Reclassification of RFC 1863 to Historic",
RFC 4223, October 2005.
[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
January 2007.
[RFC5065] Traina, P., McPherson, D., and J. Scudder, "Autonomous
System Confederations for BGP", RFC 5065, August 2007.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
Authors' Addresses
Elisa Jasinska
Limelight Networks
2220 W 14th St
Tempe, AZ 85281
US
Email: elisa@llnw.com
Nick Hilliard
INEX
4027 Kingswood Road
Dublin 24
IE
Email: nick@inex.ie
Robert Raszuk
NTT MCL Inc.
101 S Ellsworth Avenue Suite 350
San Mateo, CA 94401
US
Email: robert@raszuk.net
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Niels Bakker
AMS-IX B.V.
Westeinde 12
Amsterdam, NH 1017 ZN
NL
Email: niels.bakker@ams-ix.net
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