Network Working Group D. Meyer

Network Working Group D. Meyer
Request for Comments: 4384 February 2006
BCP: 114
Category: Best Current Practice

BGP Communities for Data Collection

Status of This Memo

This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2006).

Abstract

BGP communities (RFC 1997) are used by service providers for many
purposes, including tagging of customer, peer, and geographically
originated routes. Such tagging is typically used to control the
scope of redistribution of routes within a provider's network and to
its peers and customers. With the advent of large-scale BGP data
collection (and associated research), it has become clear that the
information carried in such communities is essential for a deeper
understanding of the global routing system. This memo defines
standard (outbound) communities and their encodings for export to BGP
route collectors.

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Table of Contents

1. Introduction ....................................................2
2. Definitions .....................................................3
2.1. Peers and Peering ..........................................3
2.2. Customer Routes ............................................3
2.3. Peer Routes ................................................3
2.4. Internal Routes ............................................4
2.5. Internal More Specific Routes ..............................4
2.6. Special Purpose Routes .....................................4
2.7. Upstream Routes ............................................4
2.8. National Routes ............................................4
2.9. Regional Routes ............................................4
3. RFC 1997 Community Encoding and Values ..........................5
4. Community Values for BGP Data Collection ........................5
4.1. Extended Communities .......................................7
4.2. Four-Octet AS Specific Extended Communities ................9
5. Note on BGP UPDATE Packing ......................................9
6. Acknowledgements ................................................9
7. Security Considerations ........................................10
7.1. Total Path Attribute Length ...............................10
8. IANA Considerations ............................................10
9. References .....................................................11
9.1. Normative References ......................................11
9.2. Informative References ....................................11

1. Introduction

BGP communities [RFC1997] are used by service providers for many
purposes, including tagging of customer, peer, and geographically
originated routes. Such tagging is typically used to control the
scope of redistribution of routes within a provider's network and to
its customers and peers. Communities are also used for a wide
variety of other applications, such as allowing customers to set
attributes such as LOCAL_PREF [RFC1771] by sending appropriate
communities to their service provider. Other applications include
signaling various types of Virtual Private Networks (VPNs) (e.g.,
Virtual Private LAN Service (VPLS) [VPLS]), and carrying link
bandwidth for traffic engineering applications [RFC4360].

With the advent of large-scale BGP data collection [RV] [RIS] (and
associated research), it has become clear that the geographical and
topological information, as well as the relationship the provider has
to the source of a route (e.g., transit, peer, or customer), carried
in such communities is essential for a deeper understanding of the
global routing system. This memo defines standard communities for
export to BGP route collectors. These communities represent a
significant part of information carried by service providers as of

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this writing, and as such could be useful for internal use by service
providers. However, such use is beyond the scope of this memo.
Finally, those involved in BGP data analysis are encouraged to verify
with their data sources as to which peers implement this scheme (as
there is a large amount of existing data as well as many legacy
peerings).

The remainder of this memo is organized as follows. Section 2
provides the definition of terms used as well as the semantics of the
communities used for BGP data collection, and Section 3 defines the
corresponding encodings for RFC 1997 [RFC1997] communities. Finally,
Section 4 defines the encodings for use with extended communities
[RFC4360].

2. Definitions

In this section, we define the terms used and the categories of
routes that may be tagged with communities. This tagging is often
referred to as coloring, and we refer to a route's "color" as its
community value. The categories defined here are loosely modeled on
those described in [WANG] and [HUSTON].

2.1. Peers and Peering

Consider two network service providers, A and B. Service providers A
and B are defined to be peers when (i) A and B exchange routes via
BGP, and (ii) traffic exchange between A and B is settlement-free.
This arrangement is also typically known as "peering". Peers
typically exchange only their respective customer routes (see
"Customer Routes" below), and hence exchange only their respective
customer traffic. See [HUSTON] for a more in-depth discussion of the
business models surrounding peers and peering.

2.2. Customer Routes

Customer routes are those routes that are heard from a customer via
BGP and are propagated to peers and other customers. Note that a
customer can be an enterprise or another network service provider.
These routes are sometimes called client routes [HUSTON].

2.3. Peer Routes

Peer routes are those routes heard from peers via BGP, and not
propagated to other peers. In particular, these routes are only
propagated to the service provider's customers.

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2.4. Internal Routes

Internal routes are those routes that a service provider originates
and passes to its peers and customers. These routes are frequently
taken out of the address space allocated to a provider.

2.5. Internal More Specific Routes

Internal more specific routes are those routes that are frequently
used for circuit load balancing purposes and Interior Gateway
Protocol (IGP) route reduction. They also may correspond to customer
services that are not visible outside the service provider's network.
Internal more specific routes are not exported to any external peer.

2.6. Special Purpose Routes

Special purpose routes are those routes that do not fall into any of
the other classes described here. In those cases in which such
routes need to be distinguished, a service provider may color such
routes with a unique value. Examples of special purpose routes
include anycast routes and routes for overlay networks.

2.7. Upstream Routes

Upstream routes are typically learned from an upstream service
provider as part of a transit service contract executed with the
upstream provider.

2.8. National Routes

These are route sets that are sourced from and/or received within a
particular country.

2.9. Regional Routes

Several global backbones implement regional policy based on their
deployed footprint and on strategic and business imperatives.
Service providers often have settlement-free interconnections with an
Autonomous System (AS) in one region, and that same AS is a customer
in another region. This mandates use of regional routing, including
community attributes set by the network in question to allow easy
discrimination among regional routes. For example, service providers
may treat a route set received from another service provider in
Europe differently than the same route set received in North America,
as it is common practice to sell transit in one region while peering
in the other.

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3. RFC 1997 Community Encoding and Values

In this section, we provide RFC 1997 [RFC1997] community values for
the categories described above. RFC 1997 communities are encoded as
BGP Type Code 8, and are treated as 32-bit values ranging from
0x0000000 through 0xFFFFFFF. The values 0x0000000 through 0x0000FFFF
and 0xFFFF0000 through 0xFFFFFFFF are reserved.

The best current practice among service providers is to use the
high-order two octets to represent the provider's AS number, and the
low-order two octets to represent the classification of the route, as
depicted below:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| <AS> | <Value> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

where <AS> is the 16-bit AS number. For example, the encoding
0x2A7C029A would represent the AS 10876 with value 666.

4. Community Values for BGP Data Collection

In this section, we define the RFC 1997 community encoding for the
route types described above for use in BGP data collection. It is
anticipated that a service provider's internal community values will
be converted to these standard values for output to a route
collector.

This memo follows the best current practice of using the basic format
<AS>:<Value>. The values for the route categories are described in
the following table:

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Category Value
===============================================================
Reserved <AS>:0000000000000000
Customer Routes <AS>:0000000000000001
Peer Routes <AS>:0000000000000010
Internal Routes <AS>:0000000000000011
Internal More Specific Routes <AS>:0000000000000100
Special Purpose Routes <AS>:0000000000000101
Upstream Routes <AS>:0000000000000110
Reserved <AS>:0000000000000111-
<AS>:0000011111111111
National and Regional Routes <AS>:0000100000000000-
<AS>:1111111111111111
Encoded as <AS>:<R><X><CC>
Reserved National and Regional values <AS>:0100000000000000-
<AS>:1111111111111111

Where

<AS> is the 16-bit AS
<R> is the 5-bit Region Identifier
<X> is the 1-bit satellite link indication
X = 1 for satellite links, 0 otherwise
<CC> is the 10-bit ISO-3166-2 country code [ISO3166]

and <R> takes the values:

Africa (AF) 00001
Oceania (OC) 00010
Asia (AS) 00011
Antarctica (AQ) 00100
Europe (EU) 00101
Latin America/Caribbean Islands (LAC) 00110
North America (NA) 00111
Reserved 01000-11111

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That is:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| <AS> | <R> |X| <CC> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

For example, the encoding for a national route over a terrestrial
link in AS 10876 from the Fiji Islands would be:

<AS> = 10876 = 0x2A7C
<R> = 00010
<X> = 0
<CC> = Fiji Islands Country Code = 242 = 0011110010

In this case, the low-order 16 bits are 0001000011110010 = 0x10F2.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x2A7C | 0x10F2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Note that a configuration language might allow the specification of
this community as 10876:4338 (0x10F2 == 4338 decimal).

Finally, note that these categories are not intended to be mutually
exclusive, and multiple communities can be attached where
appropriate.

4.1. Extended Communities

In some cases, the values and their encoding described in Section 4
may clash with a service provider's existing community assignments.
Extended communities [RFC4360] provide a convenient mechanism that
can be used to avoid such clashes.

The Extended Communities attribute is a transitive optional BGP
attribute with the Type Code 16 and consists of a set of extended
communities of the following format:

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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 high | Type low(*) | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Value |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

For purposes of BGP data collection, we encode the communities
described in Section 4 using the two-octet AS specific extended
community type, which has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x00 | Sub-Type | Global Administrator |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Local Administrator |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The two-octet AS specific extended community attribute encodes the
service provider's two-octet Autonomous System number (as assigned by
a Regional Internet Registry, or RIR) in the Global Administrator
field, and the Local Administrator field may encode any information.

This memo assigns Sub-Type 0x0008 for BGP data collection, and
specifies that the <Value> field, as defined in Section 3.1, is
carried in the low-order octets of the Local Administrator field.
The two high-order octets of the Local Administrator field are
reserved, and are set to 0x00 when sending and ignored upon receipt.

For example, the extended community encoding for 10876:4338
(representing a terrestrial national route in AS 10876 from the Fiji
Islands) would be:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x00 | 0x0008 | 0x2A7C |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x00 | 0x00 | 0x10F2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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4.2. Four-Octet AS Specific Extended Communities

The four-octet AS specific extended community is encoded as follows:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x02 | 0x0008 | Global Administrator |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Global Administrator (cont.) | 0x10F2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

In this case, the four-octet Global Administrator sub-field contains
a four-octet Autonomous System number assigned by the IANA.

5. Note on BGP UPDATE Packing

Note that data collection communities have the potential of making
the attribute set of a specific route more unique than it would be
otherwise (since each route collects data that is specific to its
path inside one or more ASes). This, in turn, can affect whether
multiple routes can be grouped in the same BGP update message, and it
may lead to increased use of bandwidth, router CPU cycles, and
memory.

6. Acknowledgements

The community encoding described in this memo germinated from an
interesting suggestion from Akira Kato at WIDE. In particular, the
idea would be to use the collection community values to select paths
that would result in (hopefully) more efficient access to various
services. For example, in the case of RFC 3258 [RFC3258] based DNS
anycast service, BGP routers may see multiple paths to the same
prefix, and others might be coming from the same origin with
different paths, but others might be from different region/country
(with the same origin AS).

Joe Abley, Randy Bush, Sean Donelan, Xenofontas Dimitropoulos, Vijay
Gill, John Heasley, Geoff Huston, Steve Huter, Michael Patton,
Olivier Marce, Ryan McDowell, Rob Rockell, Rob Thomas, Pekka Savola,
Patrick Verkaik, and Alex Zinin all made many insightful comments on
early versions of this document. Henk Uijterwaal suggested the use
of the ISO-3166-2 country codes.

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7. Security Considerations

While this memo introduces no additional security considerations into
the BGP protocol, the information contained in the communities
defined in this memo may in some cases reveal network structure that
was not previously visible outside the provider's network. As a
result, care should be taken when exporting such communities to route
collectors. Finally, routes exported to a route collector should
also be tagged with the NO_EXPORT community (0xFFFFFF01).

7.1. Total Path Attribute Length

The communities described in this memo are intended for use on egress
to a route collector. Hence an operator may choose to overwrite its
internal communities with the values specified in this memo when
exporting routes to a route collector. However, operators should in
general ensure that the behavior of their BGP implementation is
well-defined when the addition of an attribute causes a PDU to exceed
4096 octets. For example, since it is common practice to use
community attributes to implement policy (among other functionality
such as allowing customers to set attributes such as LOCAL_PREF), the
behavior of an implementation when the attribute space overflows is
crucial. Among other behaviors, an implementation might usurp the
intended attribute data or otherwise cause indeterminate failures.
These behaviors can result in unanticipated community attribute sets,
and hence result in unintended policy implications.

8. IANA Considerations

This memo assigns a new Sub-Type for the AS specific extended
community type in the First Come First Served extended transitive
category. The IANA has assigned Sub-Type 0x0008 as defined in
Section 4.1.

In addition, the IANA has created two registries for BGP Data
Collection Communities, one for standard communities and one for
extended communities. Both of these registries will initially be
populated by the values described in Section 4. IETF Consensus, as
described in [RFC2434], usually through the Global Routing Operations
Working Group (grow), is required for the assignment of new values in
these registries (in particular, for <Value> or <R> in the table of
values for the route categories in Section 4).

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9. References

9.1. Normative References

[ISO3166] "ISO 3166 Maintenance agency (ISO 3166/MA)", Web
Page: http://www.iso.org/iso/en/prods-services/
iso3166ma/index.html, 2004.

[RFC1771] Rekhter, Y. and T. Li (Editors), "A Border Gateway
Protocol (BGP-4)", RFC 1771, March 1995.

[RFC1997] Chandra, R. and P. Traina, "BGP Communities
Attribute", RFC 1997, August 1996.

[RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
Communities Attribute", RFC 4360, January 2006.

9.2. Informative References

[HUSTON] Huston, G., "Interconnection, Peering, and
Settlements",
http://www.isoc.org/inet99/proceedings/1e/1e_1.htm

[RFC2434] Narten, T., and H. Alvestrand, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP
26, RFC 2434, October 1998.

[RFC3258] Hardie, T., "Distributing Authoritative Name Servers
via Shared Unicast Addresses", RFC 3258, April 2002.

[RIS] "The RIPE Routing Information Service", Web Page:
http://www.ripe.net/ris, 2004.

[RV] Meyer, D., "The Routeviews Project", Web Page:
http://www.routeviews.org, 2002.

[VPLS] Kompella, K., et al., "Virtual Private LAN Service",
Work in Progress, April 2005.

[WANG] Wang, F. and L. Gao, "Inferring and Characterizing
Internet Routing Policies", ACM SIGCOMM Internet
Measurement Conference 2003.

Author's Address

David Meyer

EMail: [email protected]

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