Internet Engineering Task Force (IETF) E. Jasinska

Internet Engineering Task Force (IETF) E. Jasinska
Request for Comments: 7947 BigWave IT
Category: Standards Track N. Hilliard
ISSN: 2070-1721 INEX
R. Raszuk
Bloomberg LP
N. Bakker
Akamai Technologies B.V.
September 2016

Internet Exchange BGP Route Server

Abstract

This document outlines a specification for multilateral
interconnections at Internet Exchange Points (IXPs). Multilateral
interconnection is a method of exchanging routing information among
three or more External BGP (EBGP) 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
IXPs, to facilitate simplified interconnection among multiple
Internet routers.

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
http://www.rfc-editor.org/info/rfc7947.

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Copyright Notice

Copyright (c) 2016 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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction to Multilateral Interconnection . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . . 5
2.2.2.1. Route Server AS_PATH Management . . . . . . . . . 5
2.2.2.2. Route Server client AS_PATH Management . . . . . 5
2.2.3. MULTI_EXIT_DISC Attribute . . . . . . . . . . . . . . 5
2.2.4. Communities Attributes . . . . . . . . . . . . . . . 5
2.3. Per-Client Policy Control in Multilateral Interconnection 6
2.3.1. Path Hiding on a Route Server . . . . . . . . . . . . 6
2.3.2. Mitigation of Path Hiding . . . . . . . . . . . . . . 7
2.3.2.1. Multiple Route Server RIBs . . . . . . . . . . . 7
2.3.2.2. Advertising Multiple Paths . . . . . . . . . . . 8
2.3.3. Implementation Suggestions . . . . . . . . . . . . . 9
3. Security Considerations . . . . . . . . . . . . . . . . . . . 9
4. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.1. Normative References . . . . . . . . . . . . . . . . . . 10
4.2. Informative References . . . . . . . . . . . . . . . . . 10
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12

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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 (inter-AS) routing protocol, is
commonly used to facilitate exchange of network reachability
information over such media.

While bilateral EBGP sessions between exchange participants were
previously the most common means of exchanging reachability
information, the overhead associated with dense interconnection can
cause substantial operational scaling problems for participants of
larger IXPs.

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 multilateral interconnection participant (usually
referred to as a "route server client") announces network
reachability information to the route server using EBGP. The route
server, in turn, forwards this information to each 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 a route
reflector [RFC4456], except that it operates using EBGP instead of
Internal BGP (IBGP). Certain adaptions to [RFC4271] are required to
enable an EBGP router to operate as a route server; these are
outlined in Section 2 of this document. Route server functionality
is not mandatory in BGP implementations.

The term "route server" is often used in a different context 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. Notational Conventions

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

A route server uses BGP [RFC4271] to broker network reachability
information amongst its clients. There are some differences between
the behavior of a BGP route server and a BGP implementation that is
strictly compliant with [RFC4271]. These differences are described
as follows.

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 be omitted from the route server's Loc-RIB or Loc-RIBs, due to
filters configured for the purpose 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 Network Layer
Reachability Information (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
from its Loc-RIB or Loc-RIBs to its clients as determined by local
policy.

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 Decision 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 IXP operator configuration)
and SHOULD be passed on to other route server clients.

2.2.1. NEXT_HOP Attribute

The NEXT_HOP is a well-known mandatory BGP attribute that defines the
IP address of the router used as the next hop to the destinations
listed in the NLRI 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].

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2.2.2. AS_PATH Attribute

AS_PATH is a well-known mandatory attribute that identifies the ASes
through which routing information carried in the UPDATE message has
passed.

2.2.2.1. Route Server AS_PATH Management

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 the route server client BGP Decision Process,
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. This
differs from the behavior specified in Section 5.1.2 of [RFC4271],
which requires that the BGP speaker prepends its own AS number as the
last element of the AS_PATH segment. This is a recommendation rather
than a requirement solely to provide backwards compatibility with
legacy route server client implementations that do not yet support
the requirements specified in Section 2.2.2.2.

2.2.2.2. Route Server client AS_PATH Management

In contrast to what is recommended in Section 6.3 of [RFC4271], route
server clients need to be able to accept UPDATE messages where the
leftmost AS in the AS_PATH attribute is not equal to the AS number of
the route server that sent the UPDATE message. If the route server
client BGP system has implemented a check for this, the BGP
implementation MUST allow this check to be disabled and SHOULD allow
the check to be disabled on a per-peer basis.

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. Contrary to
Section 5.1.4 of [RFC4271], if applied to an NLRI UPDATE sent to a
route server, this attribute SHOULD be propagated to other route
server clients, and the route server SHOULD NOT modify its value.

2.2.4. Communities Attributes

The BGP Communities [RFC1997] and Extended Communities [RFC4360]
attributes are 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, except as defined by local

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policy. If a Communities attribute is intended for processing by the
route server itself, as determined by local policy, 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 to ensure 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".

Neither Section 2.3 nor its subsections form part of the normative
specification of this document; they are included for information
purposes only.

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. The lines between AS1, AS2, AS3, and AS4 represent
interconnection relationships, whether via bilateral or multilateral
connections.

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 by implementing outbound filtering on the BGP session towards that

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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 the BGP Decision Process [RFC4271] is used, when the same
prefix is advertised to a route server from multiple route server
clients, the route server will select a single 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 the BGP Decision Process on the route server, but AS2's policy
prevented the route server from sending the path to AS1, 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 BGP Decision Process is performed only once
on the route server for all clients.

Path hiding will only occur on route servers that employ per-client
policy control; if an IXP operator deploys a route server without
implementing a per-client routing policy control system, 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. Mitigation of Path Hiding

There are several approaches that can be taken to mitigate against
path hiding.

2.3.2.1. Multiple Route Server RIBs

The most portable method to allow for per-client policy control
without the occurrence of path hiding is to use a route server BGP
implementation that 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 can optimize this by maintaining paths not subject to
filtering policies in a global Loc-RIB, with per-client Loc-RIBs
stored as deltas.

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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 path to a particular prefix;
consequently, the path-hiding problem described in Section 2.3.1
would disappear.

We present two methods that 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 [RFC6774] is a simple way
to distribute 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 that may be distributed to a client is
constrained by the number of BGP sessions that 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

[RFC7911] 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 that already exists in its Adj-RIB-In,
this approach requires explicit support for the feature both on the
route server and on its clients.

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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 a send-only
mode with the route server clients, which would result in negotiating
a receive-only mode from the client to the route server.

2.3.3. Implementation Suggestions

Authors of route server implementations may wish to consider one of
the methods described in Section 2.3.2 to allow per-client route
server policy control without path hiding.

Recommendations for route server operations are described separately
in [RFC7948].

3. Security Considerations

The path-hiding problem outlined in Section 2.3.1 can be used in
certain circumstances to proactively block third-party path
announcements from other route server clients. Route server
operators should be aware that security issues may arise unless steps
are taken to mitigate against path hiding.

The AS_PATH check described in Section 2.2.2 is normally enabled in
order to check for malformed AS paths. If this check is disabled,
the route server client loses the ability to check incoming UPDATE
messages for certain categories of problems. This could potentially
cause corrupted BGP UPDATE messages to be propagated where they might
not be propagated if the check were enabled. Regardless of any
problems relating to malformed UPDATE messages, this check is also
used to detect BGP loops; removing the check could potentially cause
routing loops to be formed. Consequently, this check SHOULD NOT be
disabled by IXP participants unless it is needed to establish BGP
sessions with a route server and, if possible, should only be
disabled for peers that are route servers.

Route server operators should carefully consider the security
practices discussed in "BGP Operations and Security" [RFC7454].

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4. References

4.1. Normative References

[RFC1997] Chandra, R., Traina, P., and T. Li, "BGP Communities
Attribute", RFC 1997, DOI 10.17487/RFC1997, August 1996,
<http://www.rfc-editor.org/info/rfc1997>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<http://www.rfc-editor.org/info/rfc4271>.

[RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
Communities Attribute", RFC 4360, DOI 10.17487/RFC4360,
February 2006, <http://www.rfc-editor.org/info/rfc4360>.

4.2. Informative References

[RFC1863] Haskin, D., "A BGP/IDRP Route Server alternative to a full
mesh routing", RFC 1863, DOI 10.17487/RFC1863, October
1995, <http://www.rfc-editor.org/info/rfc1863>.

[RFC4223] Savola, P., "Reclassification of RFC 1863 to Historic",
RFC 4223, DOI 10.17487/RFC4223, October 2005,
<http://www.rfc-editor.org/info/rfc4223>.

[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
<http://www.rfc-editor.org/info/rfc4456>.

[RFC6774] Raszuk, R., Ed., Fernando, R., Patel, K., McPherson, D.,
and K. Kumaki, "Distribution of Diverse BGP Paths",
RFC 6774, DOI 10.17487/RFC6774, November 2012,
<http://www.rfc-editor.org/info/rfc6774>.

[RFC7454] Durand, J., Pepelnjak, I., and G. Doering, "BGP Operations
and Security", BCP 194, RFC 7454, DOI 10.17487/RFC7454,
February 2015, <http://www.rfc-editor.org/info/rfc7454>.

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RFC 7947 IXP BGP Route Server September 2016

[RFC7911] Walton, D., Retana, A., Chen, E., and J. Scudder,
"Advertisement of Multiple Paths in BGP", RFC 7911,
DOI 10.17487/RFC7911, July 2016,
<http://www.rfc-editor.org/info/rfc7911>.

[RFC7948] Hilliard, N., Jasinska, E., Raszuk, R., and N. Bakker,
"Internet Exchange BGP Route Server Operations", RFC 7948,
DOI 10.17487/RFC7948, September 2016,
<http://www.rfc-editor.org/info/rfc7948>.

Acknowledgments

The authors would like to thank Ryan Bickhart, Steven Bakker, Martin
Pels, Chris Hall, Aleksi Suhonen, Bruno Decraene, Pierre Francois,
and Eduardo Ascenco Reis for their valuable input.

In addition, the authors would like to acknowledge the developers of
BIRD, OpenBGPD, Quagga, and IOS whose BGP implementations include
route server capabilities that are compliant with this document.

Route server functionality was described in 1995 in [RFC1863], and
modern route server implementations are based on concepts developed
in the 1990s by the Routing Arbiter Project and the Route Server Next
Generation (RSNG) Project, managed by ISI and Merit. Although the
original RSNG code is no longer in use at any IXPs, the IXP community
owes a debt of gratitude to the many people who were involved in
route server development in the 1990s. Note that [RFC1863] was made
historical by [RFC4223].

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Authors' Addresses

Elisa Jasinska
BigWave IT
ul. Skawinska 27/7
Krakow, MP 31-066
Poland

Email: [email protected]

Nick Hilliard
INEX
4027 Kingswood Road
Dublin 24
Ireland

Email: [email protected]

Robert Raszuk
Bloomberg LP
731 Lexington Ave
New York City, NY 10022
United States of America

Email: [email protected]

Niels Bakker
Akamai Technologies B.V.
Kingsfordweg 151
Amsterdam 1043 GR
Netherlands

Email: [email protected]

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