Internet DRAFT - draft-ietf-idr-optional-transitive
draft-ietf-idr-optional-transitive
Internet Engineering Task Force (IETF) J. Scudder
Internet Draft Juniper Networks
Update: 1997, 4271, 4360 (if approved) E. Chen
Intended Status: Standards Track P. Mohapatra
Expires: April 26, 2012 K. Patel
Cisco Systems
October 25, 2011
Revised Error Handling for BGP UPDATE Messages
draft-ietf-idr-optional-transitive-04.txt
Abstract
According to the base BGP specification, a BGP speaker that receives
an UPDATE message containing a malformed attribute is required to
reset the session over which the offending attribute was received.
This behavior is undesirable as a session reset would impact not only
routes with the offending attribute, but also other valid routes
exchanged over the session. This document partially revises the
error handling for UPDATE messages, and provides guidelines for the
authors of documents defining new optional attributes. Finally, it
revises the error handling procedures for several existing
attributes.
Status of this Memo
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This Internet-Draft will expire on April 26, 2012.
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Copyright Notice
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This document is subject to BCP 78 and the IETF Trust's Legal
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described in the Simplified BSD License.
1. Introduction
According to the base BGP specification [RFC4271], a BGP speaker that
receives an UPDATE message containing a malformed attribute is
required to reset the session over which the offending attribute was
received. This behavior is undesirable as a session reset would
impact not only routes with the offending attribute, but also other
valid routes exchanged over the session. In the case of optional
transitive attributes, the behavior is especially troublesome and may
present a potential security vulnerability. The reason is that such
attributes may have been propagated without being checked by
intermediate routers that do not recognize the attributes -- in
effect the attribute may have been tunneled, and when they do reach a
router that recognizes and checks them, the session that is reset may
not be associated with the router that is at fault.
The goal for revising the error handling for UPDATE messages is to
minimize the impact on routing by a malformed UPDATE message, while
maintaining protocol correctness to the extent possible. This can be
achieved largely by maintaining the established session and keeping
the valid routes exchanged, but removing the routes carried in the
malformed UPDATE from the routing system.
This document partially revises the error handling for UPDATE
messages, and provides guidelines for the authors of documents
defining new optional attributes. Finally, it revises the error
handling procedures for several existing attributes. Specifically,
the error handling procedures of [RFC4271], [RFC1997], and [RFC4360]
are revised.
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1.1. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Revision to Base Specification
The first paragraph of Section 6.3 of [RFC4271] is revised as
follows:
Old Text:
All errors detected while processing the UPDATE message MUST be
indicated by sending the NOTIFICATION message with the Error Code
UPDATE Message Error. The error subcode elaborates on the specific
nature of the error.
New text:
An error detected while processing the UPDATE message for which a
session reset is specified MUST be indicated by sending the
NOTIFICATION message with the Error Code UPDATE Message Error.
The error subcode elaborates on the specific nature of the error.
The error handling of the following case described in Section 6.3 of
[RFC4271] remains unchanged:
If the Withdrawn Routes Length or Total Attribute Length
is too large (i.e., if Withdrawn Routes Length + Total Attribute
Length + 23 exceeds the message Length), then the Error Subcode
MUST be set to Malformed Attribute List.
The error handling of the following case described in Section 6.3 of
[RFC4271] is revised
If any recognized attribute has Attribute Flags that conflict with
the Attribute Type Code, then the Error Subcode MUST be set to
Attribute Flags Error. The Data field MUST contain the erroneous
attribute (type, length, and value).
as follows:
If any attribute has Attribute Flags that conflict with the
Attribute Type Code, then the error SHOULD be logged, and the
Attribute Flags MUST be reset to the correct value. The UPDATE
message MUST continue to be processed.
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The error handling of all other cases described in Section 6.3 of
[RFC4271] that specify a session reset is revised as follows.
When a path attribute in an UPDATE message is determined to be
malformed, the UPDATE message containing that attribute MUST be
treated as though all contained routes had been withdrawn just as if
they had been listed in the WITHDRAWN ROUTES field (or in the
MP_UNREACH_NLRI attribute [RFC4760bis] if appropriate) of the UPDATE
message, thus causing them to be removed from the Adj-RIB-In
according to the procedures of [RFC4271]. In the case of an
attribute which has no effect on route selection or installation, the
malformed attribute MAY instead be discarded and the UPDATE message
continue to be processed. For the sake of brevity, the former
approach is termed "treat-as-withdraw", and the latter as "attribute
discard".
The approach of "treat-as-withdraw" MUST be used for the error
handling of the cases described in Section 6.3 of [RFC4271] that
specify a session reset and involve any of the following attributes:
ORIGIN, AS_PATH, NEXT_HOP, MULTI_EXIT_DISC, and LOCAL_PREF.
The approach of "attribute discard" MUST be used for the error
handling of the cases described in Section 6.3 of [RFC4271] that
specify a session reset and involve any of the following attributes:
ATOMIC_AGGREGATE and AGGREGATOR.
When multiple malformed attributes exist in an UPDATE message, if the
same approach (either "treat-as-withdraw" or "attribute discard") is
specified for the handling of these malformed attributes, then the
specified approach MUST be used. Otherwise "treat-as-withdraw" MUST
be used.
A document which specifies a new attribute MUST provide specifics
regarding what constitutes an error for that attribute and how that
error is to be handled.
Finally, we observe that in order to use the approach of "treat-as-
withdraw", the entire NLRI field and/or MP_REACH and MP_UNREACH
[RFC4760bis] attributes need to be successfully parsed. If this is
not possible, the procedures of [RFC4271] continue to apply.
Alternatively the error handling procedures specified in [RFC4760bis]
for disabling a particular AFI/SAFI MAY be followed.
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3. Parsing of NLRI Fields
To facilitate the determination of the NLRI field in an UPDATE with a
malformed attribute, the MP_REACH or MP_UNREACH attribute (if
present) SHOULD be encoded as the very first path attribute in an
UPDATE as recommended by [RFC4760bis]. An implementation, however,
MUST still be prepared to receive these fields in any position.
If the encoding of [RFC4271] is used, the NLRI field for the IPv4
unicast address family is carried immediately following all the
attributes in an UPDATE. When such an UPDATE is received, we observe
that the NLRI field can be determined using the "Message Length",
"Withdrawn Route Length" and "Total Attribute Length" (when they are
consistent) carried in the message instead of relying on the length
of individual attributes in the message.
4. Operational Considerations
Although the "treat-as-withdraw" error-handling behavior defined in
Section 2 makes every effort to preserve BGP's correctness, we note
that if an UPDATE received on an IBGP session is subjected to this
treatment, inconsistent routing within the affected Autonomous System
may result. The consequences of inconsistent routing can include
long-lived forwarding loops and black holes. While lamentable, this
issue is expected to be rare in practice, and more importantly is
seen as less problematic than the session-reset behavior it replaces.
When a malformed attribute is indeed detected over an IBGP session,
we recommend that routes with the malformed attribute be identified
and traced back to the ingress router in the network where the routes
were sourced or received externally, and then a filter be applied on
the ingress router to prevent the routes from being sourced or
received. This will help maintain routing consistency in the
network.
Even if inconsistent routing does not arise, the "treat-as-withdraw"
behavior can cause either complete unreachability or sub-optimal
routing for the destinations whose routes are carried in the affected
UPDATE message.
Note that "treat-as-withdraw" is different from discarding an UPDATE
message. The latter violates the basic BGP principle of incremental
update, and could cause invalid routes to be kept. (See also
Appendix A.)
For any malformed attribute which is handled by the "attribute
discard" instead of the "treat-as-withdraw" approach, it is critical
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to consider the potential impact of doing so. In particular, if the
attribute in question has or may have an effect on route selection or
installation, the presumption is that discarding it is unsafe, unless
careful analysis proves otherwise. The analysis should take into
account the tradeoff between preserving connectivity and potential
side effects.
Because of these potential issues, a BGP speaker MUST provide
debugging facilities to permit issues caused by a malformed attribute
to be diagnosed. At a minimum, such facilities MUST include logging
an error listing the NLRI involved, and containing the entire
malformed UPDATE message when such an attribute is detected. The
malformed UPDATE message SHOULD be analyzed, and the root cause
SHOULD be investigated.
5. Error Handling Procedures for Existing Optional Attributes
5.1. AGGREGATOR
The error handling of [RFC4271] is revised as follows:
The AGGREGATOR attribute SHALL be considered malformed if any of the
following applies:
o Its length is not 6 (when the "4-octet AS number capability" is
not advertised to, or not received from the peer [RFC4893]).
o Its length is not 8 (when the "4-octet AS number capability" is
both advertised to, and received from the peer).
An UPDATE message with a malformed AGGREGATOR attribute SHALL be
handled using the approach of "attribute discard".
5.2. Community
The error handling of [RFC1997] is revised as follows:
The Community attribute SHALL be considered malformed if its length
is not a nonzero multiple of 4.
An UPDATE message with a malformed Community attribute SHALL be
handled using the approach of "treat-as-withdraw".
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5.3. Extended Community
The error handling of [RFC4360] is revised as follows:
The Extended Community attribute SHALL be considered malformed if its
length is not a nonzero multiple of 8.
An UPDATE message with a malformed Extended Community attribute SHALL
be handled using the approach of "treat-as-withdraw".
Note that a BGP speaker MUST NOT treat an unrecognized Extended
Community Type or Sub-Type as an error.
6. IANA Considerations
This document makes no request of IANA.
7. Security Considerations
This specification addresses the vulnerability of a BGP speaker to a
potential attack whereby a distant attacker can generate a malformed
optional transitive attribute that is not recognized by intervening
routers (which thus propagate the attribute unchecked) but that
causes session resets when it reaches routers that do recognize the
given attribute type.
In other respects, this specification does not change BGP's security
characteristics.
8. Acknowledgments
The authors wish to thank Ron Bonica, Mach Chen, Andy Davidson, Dong
Jie, Rex Fernando, Joel Halpern, Akira Kato, Miya Kohno, Tony Li,
Alton Lo, Shin Miyakawa, Tamas Mondal, Jonathan Oddy, Robert Raszuk,
Yakov Rekhter, Rob Shakir, Naiming Shen, Shyam Sethuram, Ananth
Suryanarayana, and Kaliraj Vairavakkalai for their observations and
discussion of this topic, and review of this document.
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9. Normative References
[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.
[RFC4893] Vohra, Q. and E. Chen, "BGP Support for Four-octet AS
Number Space", RFC 4893, May 2007.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC4760bis]
Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4",
draft-ietf-idr-rfc4760bis-03.txt, work in progress,
August 2011.
Appendix A. Why not discard UPDATE messages?
A commonly asked question is "why not simply discard the UPDATE
message instead of treating it like a withdraw? Isn't that safer and
easier?" The answer is that it might be easier, but it would
compromise BGP's correctness so is unsafe. Consider the following
example of what might happen if UPDATE messages carrying bad
attributes were simply discarded:
AS1 ---- AS2
\ /
\ /
\ /
AS3
o AS1 prefers to reach AS3 directly, and advertises its route to
AS2.
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o AS2 prefers to reach AS3 directly, and advertises its route to
AS1.
o Connections AS3-AS1 and AS3-AS2 fail simultaneously.
o AS1 switches to prefer AS2's route, and sends an update message
which includes a withdraw of its previous announcement. The
withdraw is bundled with some advertisements. It includes a bad
attribute. As a result, AS2 ignores the message.
o AS2 switches to prefer AS1's route, and sends an update message
which includes a withdraw of its previous announcement. The
withdraw is bundled with some advertisements. It includes a bad
attribute. As a result, AS1 ignores the message.
The end result is that AS1 forwards traffic for AS3 towards AS2, and
AS2 forwards traffic for AS3 towards AS1. This is a permanent (until
corrected) forwarding loop.
Although the example above discusses route withdraws, we observe that
in BGP the announcement of a route also withdraws the route
previously advertised. The implicit withdraw can be converted into a
real withdraw in a number of ways; for example, the previously-
announced route might have been accepted by policy, but the new
announcement might be rejected by policy. For this reason, the same
concerns apply even if explicit withdraws are removed from
consideration.
10. Authors' Addresses
John G. Scudder
Juniper Networks
Email: jgs@juniper.net
Enke Chen
Cisco Systems, Inc.
EMail: enkechen@cisco.com
Pradosh Mohapatra
Cisco Systems, Inc.
EMail: pmohapat@cisco.com
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Keyur Patel
Cisco Systems, Inc.
EMail: keyupate@cisco.com
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