Extended Message Support for BGP
Arrcus & IIJ
5147 Crystal Springs
Bainbridge Island
WA
98110
United States of America
randy@psg.com
Arrcus, Inc.
keyur@arrcus.com
Cisco Systems
170 W. Tasman Drive
San Jose
CA
95134
United States of America
dward@cisco.com
The BGP specification (RFC 4271) mandates a maximum BGP message size of 4,096
octets. As BGP is extended to support new Address Family Identifiers
(AFIs), Subsequent AFIs (SAFIs), and other
features, there is a need to extend the maximum message size beyond
4,096 octets. This document updates the BGP specification by
extending the maximum message size from 4,096 octets to 65,535 octets
for all messages except for OPEN and KEEPALIVE messages.
Introduction
The BGP specification mandates a maximum
BGP message size of 4,096 octets. As BGP is extended to support
new AFIs, SAFIs, and other capabilities (e.g., BGPsec and BGP - Link
State (BGP-LS) ), there is a
need to extend the maximum message size beyond 4,096 octets. This
document provides an extension to BGP to extend the message size limit
from 4,096 octets to 65,535 octets for all messages except for OPEN and
KEEPALIVE messages.
Requirements Language
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 BCP 14
when, and only when, they appear in all capitals, as shown here.
BGP Extended Message
A BGP message over 4,096 octets in length is a BGP Extended
Message.
BGP Extended Messages have a maximum message size of 65,535
octets. The smallest message that may be sent is a BGP
KEEPALIVE, which consists of 19 octets.
BGP Extended Message Capability
The BGP Extended Message Capability is a new BGP capability defined with Capability Code 6 and
Capability Length 0.
To advertise the BGP Extended Message Capability to a peer, a BGP
speaker uses BGP Capabilities Advertisement . By advertising the BGP Extended Message Capability
to a peer, a BGP speaker conveys that it is able to receive and properly
handle BGP Extended Messages (see ).
Peers that wish to use the BGP Extended Message Capability MUST
support error handling for BGP UPDATE messages per .
Operation
The BGP Extended Message Capability applies to all messages except
for OPEN and KEEPALIVE messages. These exceptions
reduce the complexity of providing backward compatibility.
A BGP speaker that is capable of receiving BGP
Extended Messages SHOULD advertise the BGP Extended Message
Capability to its peers using BGP Capabilities Advertisement . A BGP speaker MAY
send BGP Extended Messages to a
peer only if the BGP Extended Message Capability was received from that
peer.
An implementation that advertises the BGP Extended Message
Capability MUST be capable of receiving a message with a length up
to and including 65,535 octets.
Applications generating information that might be encapsulated
within BGP messages MUST limit the size of their payload to take the
maximum message size into account.
If a BGP message with a length greater than 4,096 octets is
received by a BGP listener who has not advertised the BGP Extended
Message Capability, the listener will generate a NOTIFICATION with
the Error Subcode set to Bad Message Length ().
A BGP UPDATE will (if allowed by policy, best path, etc.) typically
propagate throughout the BGP-speaking Internet and hence to BGP
speakers that may not support BGP Extended Messages. Therefore, an
announcement in a BGP Extended Message where the size of the attribute
set plus the NLRI is larger than 4,096 octets may cause lack of
reachability.
A BGP speaker that has advertised the BGP Extended Message
Capability to its peers may receive an UPDATE from one of its peers
that produces an ongoing announcement that is larger than 4,096
octets. When propagating that UPDATE onward to a neighbor that has
not advertised the BGP Extended Message Capability, the speaker
SHOULD try to reduce the outgoing message size by removing
attributes eligible under the "attribute discard" approach of . If the message is still too big, then it must
not be sent to the neighbor ().
Additionally, if the NLRI was previously advertised to that peer, it
must be withdrawn from service ().
If an Autonomous System (AS) has multiple internal BGP speakers
and also has multiple external BGP neighbors, care must be taken to ensure a consistent view within the
AS in order to present a consistent
external view. In the context of BGP Extended Messages, a
consistent view can only be guaranteed if all the Internal BGP (iBGP) speakers
advertise the BGP Extended Message Capability. If that is not the
case, then the operator should consider whether or not the BGP Extended
Message Capability should be advertised to external peers.
During the incremental deployment of BGP Extended Messages and
use of the "attribute discard" approach of in an iBGP mesh or with
External BGP (eBGP) peers, the
operator should monitor any routes dropped and any discarded
attributes.
Error Handling
A BGP speaker that has the ability to use BGP Extended Messages but
has not advertised the BGP Extended Message Capability, presumably
due to configuration, MUST NOT accept a BGP Extended Message. A
speaker MUST NOT implement a more liberal policy accepting BGP
Extended Messages.
A BGP speaker that does not advertise the BGP Extended Message
Capability might also genuinely not support BGP Extended Messages. Such
a speaker will follow the error-handling procedures of if it receives a BGP Extended Message. Similarly,
any speaker that treats an improper BGP Extended Message as a fatal
error MUST follow the error-handling procedures of .
Error handling for UPDATE messages, as specified in
, is unchanged. However, if a
NOTIFICATION is to be sent to a BGP speaker that has not advertised
the BGP Extended Message Capability, the size of the message MUST
NOT exceed 4,096 octets.
It is RECOMMENDED that BGP protocol developers and implementers
are conservative in their application and use of BGP Extended Messages.
Future protocol specifications MUST describe how to handle peers
that can only accommodate 4,096 octet messages.
Changes to RFC 4271
states "The value of the Length field
MUST always be at least 19 and no greater than 4096." This document
changes the latter number to 65,535 for all messages except for OPEN and
KEEPALIVE messages.
specifies
raising an error if the length of a message is over 4,096 octets. For
all messages except for OPEN and KEEPALIVE messages, if the receiver has advertised the
BGP Extended Message Capability, this document raises that limit to
65,535.
IANA Considerations
IANA has made the following allocation in the "Capability Codes"
registry:
Addition to "Capability Codes" Registry
Value |
Description |
Reference |
6 |
BGP Extended Message |
RFC 8654 |
Security Considerations
This extension to BGP does not change BGP's underlying security
issues .
Due to increased memory requirements for buffering, there may be
increased exposure to resource exhaustion, intentional or
unintentional.
If a remote speaker is able to craft a large BGP Extended Message
to send on a path where one or more peers do not support BGP
Extended Messages, peers that support BGP Extended Messages may:
- act to reduce the outgoing message (see ) and, in doing so, cause an attack by discarding
attributes one or more of its peers may be expecting. The attributes eligible under the
"attribute discard" approach must have no effect on route selection or
installation .
- act to reduce the outgoing message (see ) and, in doing so, allow a downgrade attack. This
would only affect the attacker's message, where 'downgrade' has
questionable meaning.
- incur resource load (processing, message resizing, etc.)
when reformatting the large messages.
References
Normative References
Informative References
Acknowledgments
The authors thank Alvaro Retana for an amazing review; Enke Chen,
Susan Hares, John Scudder, John Levine, and Job Snijders for their
input; and Oliver Borchert and Kyehwan Lee for their implementations
and testing.