Internet Engineering Task Force J. Scudder
Internet-Draft Juniper Networks
Intended status: Standards Track E. Chen
Expires: September 27, 2010 Cisco Systems
March 26, 2010
Error Handling for Optional Transitive BGP Attributes
draft-ietf-idr-optional-transitive-02.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 in the case of optional transitive
attributes. This document revises BGP's error-handling rules for
optional transitive attributes, and provides guidelines for the
authors of documents defining new optional transitive attributes. It
also introduces a new Path Attribute flag, Neighbor-Complete, to
allow more accurate fault-finding. Finally, it revises the error
handling procedures for several existing optional transitive
attributes.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 27, 2010.
Copyright Notice
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Copyright (c) 2010 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
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described in the 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 in the case of optional
transitive attributes whose Partial flag is set; the reason is that
such attributes may have been propagated without being checked by
intermediate routers that do not recognize the attribute -- in effect
the attributes 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. This document
revises BGP's error-handling rules for optional transitive
attributes, and provides guidelines for the authors of documents
defining new optional transitive attributes. It also revises the
error handling procedures for several existing optional transitive
attributes. Specifically, the error handling procedures of
[RFC4271], [RFC1997], and [RFC4360] are revised.
Error handling procedures are not revised if the error can be imputed
to the direct neighbor. A new flag, Neighbor-Complete, is introduced
which, when used, allows the direct neighbor's involvement to be
determined unequivocally. Imputation of "blame" to the direct
neighbor is achieved by checking the Partial flag and the Neighbor-
Complete flag. If the Partial flag is clear, or the Neighbor-
Complete flag is set, the original error handling procedures remain
in force.
1.1. Requirements Language
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].
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2. Neighbor-Complete Flag Bit
It is desirable to know whether a neighbor recognizes, or does not
recognize, a given optional transitive attribute. The Partial Path
Attribute flag does not provide exactly this information -- it only
enables the determination that a given neighbor did understand such
an attribute, if the flag is set to zero. However, if the flag is
set to one all that can be concluded is that some BGP speaker in the
path did not understand the attribute, it cannot be determined
whether the speaker in question was the neighbor or some other
speaker.
To remedy this, we introduce a new Path Attribute Flag to those
defined in [RFC4271] Section 4.3. The fifth high-order bit (bit 4)
of the Attribute Flags octet is the Neighbor-Complete bit. It
indicates whether the neighbor that sent the message recognizes the
attribute (if set to one) or does not recognize it (if set to zero).
The Neighbor-Complete flag only applies to optional transitive
attributes. For other types of attributes the flag MUST be sent as
zero and ignored when received.
A BGP speaker MUST set the Neighbor-Complete flag to one when sending
a recognized, or zero when sending an unrecognized, optional
transitive path attribute to its neighbor.
The Neighbor-Complete flag is the equivalent of the Partial flag,
with two differences. First, it is reset on a hop-by-hop basis.
Second, its "polarity" is reversed, with one instead of zero
indicating that a neighbor does recognize the attribute. The reason
for this difference is that during the period while this
specification is being adopted, some BGP speakers will recognize the
Neighbor-Complete flag and some will not. Since the previous
definition [RFC4271] of bit 4 required it to be sent as zero, the use
of one to mean "attribute recognized" allows the recipient of such a
flag to unequivocally determine that a neighbor does recognize the
given attribute.
Use of the flag on receipt is discussed in Section 3.
3. Revision to Base Specification
Section 6.3 of [RFC4271] is revised as follows. The paragraphs
related to "any recognized attribute" and "an optional attribute" do
not apply to optional transitive attributes received with their
Partial flag set and Neighbor-Complete flag clear -- an error limited
to such an attribute SHALL NOT be responded to by sending a
NOTIFICATION message or resetting the BGP session. Instead, when
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such an attribute is determined to be malformed, the UPDATE message
containing that attribute SHOULD be treated as though all contained
routes had been withdrawn just as if they had been listed in the
WITHDRAWN ROUTES field 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 optional transitive 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.
An example of an attribute which has no effect on route selection or
installation is the AGGREGATOR attribute.
A document which specifies an optional transitive attribute MUST
provide specifics regarding what constitutes an error for that
attribute and how that error is to be handled.
Note that the revised error handling only applies when an individual
optional transitive attribute is received with its Partial flag set
and Neighbor-Complete flag clear and deemed to be erroneous. In the
event that an UPDATE message is deemed to be malformed in any other
way then the procedures of [RFC4271] MUST be applied. This is
likewise the case if an optional transitive attribute is received
whose Partial flag is not set or whose Neighbor-Complete flag is set
-- this is because the detected error can be imputed to the direct
peer.
Examples of errors which would continue to be treated according to
the procedures of [RFC4271] include the cases where the Total
Attribute Length is inconsistent with the message length, or where
there is more than one attribute with a given type code. Also,
implicit in the foregoing paragraph is the fact that if due to an
error, including those in an optional transitive attribute, the other
attributes of the UPDATE message cannot be correctly parsed, then the
procedures of [RFC4271] continue to apply.
In the specific case of incorrect path attribute flags -- i.e., a
path attribute that is known by its type code to be Optional and
Transitive but whose flags are not set accordingly -- the behavior
specified by [RFC4271] SHALL be followed. (Consider that in the case
of such an error, the "tunneling" argument given above does not
apply, by definition.)
Finally, we observe that in order to treat an UPDATE as though all
contained routes had been withdrawn as discussed above, the NLRI
field and/or MP_REACH and MP_UNREACH [RFC4760] attributes need to be
successfully parsed. If this were not possible, the UPDATE would
necessarily be malformed in some way beyond the scope of this
document and therefore, the procedures of [RFC4271] would continue to
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apply.
4. Operational Considerations
Although the "treat as withdraw" error-handling behavior defined in
Section 3 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.
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 discarded instead of the
containing UPDATE being treated as a withdraw as discussed in
Section 3, it is critical 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 malformed optional
transitive attributes to be diagnosed. At a minimum, such facilities
SHOULD include logging an error when such an attribute is detected.
5. Error Handling Procedures for Existing Optional Transitive
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:
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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 as follows. If its Partial flag is set and its Neighbor-
Complete flag is clear, either the attribute MUST be discarded or the
UPDATE containing it treated as a withdraw as discussed in Section 3.
Otherwise (i.e. if its Partial flag is clear or its Neighbor-Complete
flag is set), the procedures of [RFC4271] MUST be followed with
respect to an Optional Attribute Error.
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 as follows. If its Partial flag is set and its Neighbor-
Complete flag is clear, the update containing it MUST be treated as a
withdraw as discussed in Section 3. Otherwise (i.e. if its Partial
flag is clear or its Neighbor-Complete flag is set), the procedures
of [RFC4271] MUST be followed with respect to an Optional Attribute
Error.
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 as follows. If its Partial flag is set and its Neighbor-
Complete flag is clear, the update containing it MUST be treated as a
withdraw as discussed in Section 3. Otherwise (i.e. if its Partial
flag is clear or its Neighbor-Complete flag is set), the procedures
of [RFC4271] MUST be followed with respect to an Optional Attribute
Error.
Note that a BGP speaker MUST NOT treat an unrecognized Extended
Community Type or Sub-Type as an error.
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6. 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.
7. Acknowledgements
The authors wish to thank Ron Bonica, Andy Davidson, Dong Jie, Rex
Fernando, Joel Halpern, Akira Kato, Miya Kohno, Alton Lo, Shin
Miyakawa, Jonathan Oddy, Robert Raszuk, Yakov Rekhter, Rob Shakir,
Ananth Suryanarayana, and Kaliraj Vairavakkalai for their
observations and discussion of this topic. The Neighbor-Complete
flag was introduced as the result of helpful discussion with Jie Dong
and Mach Chen.
8. IANA Considerations
IANA is requested to establish and maintain a registry of BGP Path
Attribute Flags. Flags one through four are defined in [RFC4271].
Flag five is defined in Section 2 of this document. Future
allocations are to be made according to the IETF Standards Action
policy [RFC5226].
9. References
9.1. 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.
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[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.
9.2. Informative References
[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
January 2007.
Appendix A. Why not discard UPDATES?
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.
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
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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.
Authors' Addresses
John G. Scudder
Juniper Networks
Email: jgs@juniper.net
Enke Chen
Cisco Systems
Email: enkechen@cisco.com
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