wdiff draft-ietf-idr-optional-transitive-03.txt draft-ietf-idr-optional-transitive-04.txt

Internet Engineering Task Force (IETF) J. Scudder
Internet-Draft
Internet Draft Juniper Networks
Update: 1997, 4271, 4360 (if approved) E. Chen
Intended status: Status: Standards Track E. Chen P. Mohapatra
Expires: April 1, 2011 26, 2012 K. Patel
Cisco Systems
September 28, 2010
October 25, 2011

Revised Error Handling for Optional Transitive BGP Attributes
draft-ietf-idr-optional-transitive-03.txt 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 in as a session reset would impact not only
routes with the case of optional transitive
attributes. offending attribute, but also other valid routes
exchanged over the session. This document partially revises BGP's error-handling rules the
error handling for
optional transitive attributes, UPDATE messages, 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

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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 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 whose Partial flag is set; 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 attribute attributes -- in
effect the attributes 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 BGP's error-handling rules the error handling for optional transitive
attributes, UPDATE
messages, and provides guidelines for the authors of documents
defining new optional transitive attributes. It also Finally, it 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. Specification of 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].

2. Neighbor-Complete Flag Bit

It is desirable Revision to know whether a neighbor recognizes, or does not
recognize, a given optional transitive attribute. Base Specification

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] first paragraph of Section 4.3. The fifth high-order bit (bit 4) 6.3 of the Attribute Flags octet [RFC4271] is revised as
follows:

Old Text:

All errors detected while processing the Neighbor-Complete bit. It
indicates whether the neighbor that sent the UPDATE 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
indicated by 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, NOTIFICATION message with one instead of zero
indicating that a neighbor does recognize the attribute. Error Code
UPDATE Message Error. The reason
for this difference is that during error subcode elaborates on the period while this
specification is being adopted, some BGP speakers will recognize specific
nature of the
Neighbor-Complete flag and some will not. Since error.

New text:

An error detected while processing the previous
definition [RFC4271] of bit 4 required it to UPDATE message for which a
session reset is specified MUST be sent as zero, indicated by sending the use
of one to mean "attribute recognized" allows
NOTIFICATION message with the recipient Error Code UPDATE Message Error.
The error subcode elaborates on the specific nature of such a
flag to unequivocally determine that a neighbor does recognize the
given attribute.

Use error.

The error handling of the flag on receipt is discussed following case described 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 remains unchanged:

If the BGP session. Instead, when
such an attribute Withdrawn Routes Length or Total Attribute Length
is determined to be malformed, the UPDATE message
containing that attribute SHOULD be treated as though all contained
routes had been withdrawn just as too large (i.e., if they had been listed in the
WITHDRAWN ROUTES field of the UPDATE message, thus causing them to be
removed from Withdrawn Routes Length + Total Attribute
Length + 23 exceeds the Adj-RIB-In according to message Length), then the procedures Error Subcode
MUST be set to Malformed Attribute List.

The error handling of [RFC4271].
In the following case described in Section 6.3 of an optional transitive
[RFC4271] is revised

If any recognized attribute which has no effect
on route selection or installation, Attribute Flags that conflict with
the malformed attribute MAY
instead be discarded and Attribute Type Code, then the UPDATE message continue to Error Subcode MUST 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 set to
Attribute Flags Error. The Data field MUST
provide specifics regarding what constitutes an error for that contain the erroneous
attribute (type, length, and how that error is to be handled.

Note value).

as follows:

If any attribute has Attribute Flags that conflict with the
Attribute Type Code, then the revised error handling only applies when an individual
optional transitive attribute is received with its Partial flag set
and Neighbor-Complete flag clear SHOULD be logged, and deemed to the
Attribute Flags MUST be erroneous. In reset to the
event that an correct value. The UPDATE
message is deemed MUST continue to be malformed in any processed.

The error handling of all other
way then the procedures cases described in Section 6.3 of
[RFC4271] MUST be applied. This that specify a session reset is
likewise the case if an optional transitive revised as follows.

When a path attribute in an UPDATE message 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 determined to be treated according to
the procedures of [RFC4271] include the cases where the Total
Attribute Length is inconsistent with
malformed, the UPDATE message length, or where
there is more than one containing that attribute with a given type code. Also,
implicit MUST be
treated as though all contained routes had been withdrawn just as if
they had been listed in the foregoing paragraph is the fact that if due to an
error, including those WITHDRAWN ROUTES field (or in an optional transitive attribute, the other
attributes
MP_UNREACH_NLRI attribute [RFC4760bis] if appropriate) of the UPDATE message cannot be correctly parsed, then the
procedures of [RFC4271] continue to apply. Examples of errors that
would continue
message, thus causing them to be treated removed from the Adj-RIB-In
according to the procedures of [RFC4271]
include [RFC4271]. In the cases where case of an
attribute which has no effect on route selection or installation, the Total Attribute Length is inconsistent
with
malformed attribute MAY instead be discarded and the UPDATE message length, where the Attribute Length is inconsistent
with
continue to be processed. For the Total Attribute Length, or where there is more than one
attribute with a given type code. Also, implicit in sake of brevity, the foregoing
paragraph former
approach is termed "treat-as-withdraw", and the fact that if due to an error, including those in an
optional transitive attribute, the other attributes latter as "attribute
discard".

The approach of the UPDATE
message cannot "treat-as-withdraw" MUST be correctly parsed, then used for the procedures error
handling of [RFC4271]
continue to apply.

In the specific case cases described in Section 6.3 of incorrect path attribute flags -- i.e., a
path attribute [RFC4271] that is known by its type code to be Optional
specify a session reset and
Transitive but whose flags are not set accordingly -- involve any of the behavior
specified by [RFC4271] SHALL following attributes:
ORIGIN, AS_PATH, NEXT_HOP, MULTI_EXIT_DISC, and LOCAL_PREF.

The approach of "attribute discard" MUST be followed. (Consider that in used for the case error
handling of such an error, the "tunneling" argument given above does not
apply, by definition.)

Finally, we observe that cases described in order to treat an UPDATE as though all
contained routes had been withdrawn as discussed above, the NLRI
field and/or MP_REACH Section 6.3 of [RFC4271] that
specify a session reset and MP_UNREACH [RFC4760] attributes need to be
successfully parsed. If this were not possible, involve any of the UPDATE would
necessarily be following attributes:
ATOMIC_AGGREGATE and AGGREGATOR.

When multiple malformed attributes exist in some way beyond an UPDATE message, if the
same approach (either "treat-as-withdraw" or "attribute discard") is
specified for the scope handling of this 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 therefore, the procedures of [RFC4271] would continue how that
error is to
apply.

4. Parsing of NLRI Fields

We be handled.

Finally, we observe that in order to treat an UPDATE as though all contained
routes had been withdrawn as discussed above, use the approach of "treat-as-
withdraw", the entire NLRI field and/or MP_REACH and MP_UNREACH [RFC4760]
[RFC4760bis] attributes need to be successfully parsed. If this were is
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 apply.
Alternatively the error handling procedures specified in [RFC4760bis]
for disabling a particular AFI/SAFI MAY be followed.

3. Parsing of NLRI Fields

To facilitate the determination of the NLRI field in an UPDATE with a
malformed attributes, we strongly RECOMMEND that attribute, the MP_REACH or MP_UNREACH attribute (if
present) SHOULD be encoded as the very first path attribute in an UPDATE.

Traditionally
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 are is carried immediately following all the
attributes in an UPDATE
[RFC4271]. 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.

Furthermore, we observe that the NLRI for the IPv4 unicat address
family can equally well be carried in the MP_REACH attribute of an
UPDATE when the IPV4 unicast address family capability is shared
(i.e., both advertised and received) over a BGP session. For the
same sake of better debugging and fault handling, we also RECOMMEND
that the MP_REACH attribute be used and be placed as the very first
path attribute in an UPDATE in this case.

4. Operational Considerations

Although the "treat as withdraw" "treat-as-withdraw" error-handling behavior defined in
Section 3 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. 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" "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" "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 handled by the "attribute
discard" instead of the
containing UPDATE being treated as a withdraw as discussed in
Section 3, "treat-as-withdraw" approach, 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 a malformed optional
transitive attributes attribute
to be diagnosed. At a minimum, such facilities
SHOULD MUST include logging
an error listing the NLRI involved, and containing the entire
malformed UPDATE message when such an attribute is detected.

6. The
malformed UPDATE message SHOULD be analyzed, and the root cause
SHOULD be investigated.

5. Error Handling Procedures for Existing Optional Transitive Attributes

6.1.

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 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), using the procedures approach of [RFC4271] MUST be followed with
respect to an Optional Attribute Error.

6.2. "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 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), using the procedures approach of [RFC4271] MUST be followed with respect to an Optional Attribute
Error.

6.3. "treat-as-withdraw".

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), using the procedures approach of [RFC4271] MUST be followed with respect to an Optional Attribute
Error. "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. Acknowledgements 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. The Neighbor-Complete
flag was introduced as the result of helpful discussion with Jie Dong
and Mach Chen.

9. IANA Considerations

IANA is requested to establish topic, and maintain a registry of BGP Path
Attribute Flags. Flags one through four are defined in [RFC4271].
Flag five is defined in Section 2 review of this document. Future
allocations are to be made according to the IETF Standards Action
policy [RFC5226].

10. References

10.1.

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.

10.2. Informative References

[RFC4760]

[RFC4760bis]
Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
January 2007.
draft-ietf-idr-rfc4760bis-03.txt, work in progress,
August 2011.

Appendix A. Why not discard UPDATES? 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

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
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

Email: Systems, Inc.

EMail: enkechen@cisco.com

Pradosh Mohapatra
Cisco Systems, Inc.

EMail: pmohapat@cisco.com
Keyur Patel
Cisco Systems, Inc.

EMail: keyupate@cisco.com