Internet Exchange BGP Route Server
draft-ietf-idr-ix-bgp-route-server-12

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 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 among multiple Internet routers.

Status of This Memo

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Copyright Notice

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Table of Contents

• 1. Introduction to Multilateral Interconnection
• 1.1. Notational Conventions
• 2. Technical Considerations for Route Server Implementations
• 2.1. Client UPDATE Messages
• 2.2. Attribute Transparency
• 2.2.1. NEXT_HOP Attribute
• 2.2.2. AS_PATH Attribute
• 2.2.2.1. Route Server AS_PATH management
• 2.2.2.2. Route Server client AS_PATH management
• 2.2.3. MULTI_EXIT_DISC Attribute
• 2.2.4. Communities Attributes
• 2.3. Per-Client Policy Control in Multilateral Interconnection
• 2.3.1. Path Hiding on a Route Server
• 2.3.2. Mitigation of Path Hiding
• 2.3.2.1. Multiple Route Server RIBs
• 2.3.2.2. Advertising Multiple Paths
• 2.3.3. Implementation Suggestions
• 3. Security Considerations
• 4. IANA Considerations
• 5. Acknowledgments
• 6. References
• 6.1. Normative References
• 6.2. Informative References
• Authors' Addresses

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 external BGP 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 Internet exchange points.

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 external BGP. 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; these are outlined in Section 2 of this document. Route server functionality is not mandatory in BGP implementations.

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. 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].

2. Technical Considerations for Route Server Implementations

A route server uses the [RFC4271] Border Gateway Protocol to broker network reachability information amongst its clients. There are some differences between the behaviour of a BGP route server and a BGP implementation which 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 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 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 which 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.

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 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 behaviour 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 which 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 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, except as defined by local 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, but are included for information purposes only.

2.3.1. Path Hiding on a Route Server

     ___      ___
    /   \    /   \
 ..| AS1 |..| AS2 |..
:   \___/    \___/   :
:       \    / |     :
:        \  /  |     :
: IXP     \/   |     :
:         /\   |     :
:        /  \  |     :
:    ___/____\_|_    :
:   /   \    /   \   :
 ..| AS3 |..| AS4 |..
    \___/    \___/