Internet Engineering Task Force E. Chen, Ed.
Internet-Draft Cisco Systems, Inc.
Updates: 1997, 4271, 4360, 4456, 4760, 5701 (if approved)J. Scudder, Ed.
Intended status: Standards Track Juniper Networks
Expires: August 18, 2014 P. Mohapatra
Cumulus Networks, Inc.
K. Patel
Cisco Systems, Inc.
February 14, 2014
Revised Error Handling for BGP UPDATE Messages
draft-ietf-idr-error-handling-06
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 attributes. Finally, it revises
the error handling procedures for a number of existing attributes.
This document updates error handling for RFCs 1997, 4271, 4360, 4456,
4760, and 5701.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 18, 2014.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Revision to Base Specification . . . . . . . . . . . . . . . 4
3. Parsing of NLRI Fields . . . . . . . . . . . . . . . . . . . 6
3.1. Inconsistency of Attribute Length Fields . . . . . . . . 6
3.2. Syntactic Correctness of NLRI Fields . . . . . . . . . . 7
4. Operational Considerations . . . . . . . . . . . . . . . . . 7
5. Error Handling Procedures for Existing Attributes . . . . . . 8
5.1. ORIGIN . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.2. AS_PATH . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.3. NEXT_HOP . . . . . . . . . . . . . . . . . . . . . . . . 9
5.4. MULTI_EXIT_DESC . . . . . . . . . . . . . . . . . . . . . 9
5.5. LOCAL_PREF . . . . . . . . . . . . . . . . . . . . . . . 9
5.6. ATOMIC_AGGREGATE . . . . . . . . . . . . . . . . . . . . 9
5.7. AGGREGATOR . . . . . . . . . . . . . . . . . . . . . . . 9
5.8. Community . . . . . . . . . . . . . . . . . . . . . . . . 10
5.9. Extended Community . . . . . . . . . . . . . . . . . . . 10
5.10. IPv6 Address Specific BGP Extended Community Attribute . 10
5.11. ORIGINATOR_ID . . . . . . . . . . . . . . . . . . . . . . 11
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5.12. CLUSTER_LIST . . . . . . . . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
9. Normative References . . . . . . . . . . . . . . . . . . . . 12
Appendix A. Why not discard UPDATE messages? . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
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 attributes. Finally, it revises the error handling
procedures for a number of existing attributes. Specifically, the
error handling procedures of, [RFC1997], [RFC4271], [RFC4360],
[RFC4456], [RFC4760] and [RFC5701] are revised.
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. 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 recognized attribute has Attribute Flags that conflict with
the Attribute Type Code, then the attribute MUST be treated as
malformed and the treat-as-withdraw approach (see below) used,
unless the specification for the attribute mandates different
handling for incorrect Attribute Flags.
The error handling of all other cases involving path attributes as
described in Section 6.3 of [RFC4271] that specify a session reset is
revised as follows.
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When a path attribute (other than the MP_REACH_NLRI attribute
[RFC4760] or the MP_UNREACH_NLRI attribute [RFC4760]) 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 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".
If any of the well-known mandatory attributes are not present in an
UPDATE message, then the approach of "treat-as-withdraw" MUST be used
for the error handling.
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.
If the MP_REACH_NLRI attribute or the MP_UNREACH_NLRI attribute
appears more than once in the UPDATE message, then a NOTIFICATION
message MUST be sent with the Error Subcode "Malformed Attribute
List". If any other attribute appears more than once in an UPDATE
message, then all the occurrences of the attribute other than the
first one SHALL be discarded and the UPDATE message continue to be
processed.
When multiple attribute errors exist in an UPDATE message, if the
same approach (either "session reset", or "treat-as-withdraw" or
"attribute discard") is specified for the handling of these malformed
attributes, then the specified approach MUST be used. Otherwise the
approach with the strongest action MUST be used following the order
of "session reset", "treat-as-withdraw" and "attribute discard" from
the strongest to the weakest.
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.
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Finally, we observe that in order to use the approach of "treat-as-
withdraw", the entire NLRI field and/or the MP_REACH_NLRI and
MP_UNREACH_NLRI 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 [RFC4760]
for disabling a particular AFI/SAFI MAY be followed. One notable
case where it would be not possible to successfully parse the NLRI is
if the NLRI field is found to be "syntactically incorrect" (see
Section 3.2). It can be seen that therefore, this part of [RFC4271]
Section 6.3 necessarily continues to apply:
The NLRI field in the UPDATE message is checked for syntactic
validity. If the field is syntactically incorrect, then the Error
Subcode MUST be set to Invalid Network Field.
Furthermore, this document extends RFC 4271 by mandating that the
Withdrawn Routes field SHALL be checked for syntactic correctness in
the same manner as the NLRI field.
3. Parsing of NLRI Fields
To facilitate the determination of the NLRI field in an UPDATE with a
malformed attribute, the MP_REACH_NLRI or MP_UNREACH_NLRI attribute
(if present) SHALL be encoded as the very first path attribute in an
UPDATE. 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.
3.1. Inconsistency of Attribute Length Fields
There are two error cases in which the Total Attribute Length value
can be in conflict with the enclosed path attributes, which
themselves carry length values. In the "overrun" case, as the
enclosed path attributes are parsed, the length of the last
encountered path attribute would cause the Total Attribute Length to
be exceeded. In the "underrun" case, as the enclosed path attributes
are parsed, after the last successfully-parsed attribute, fewer than
three bytes remain, or fewer than four bytes, if the Attribute Flags
field has the Extended Length bit set -- that is, there remains
unconsumed data in the path attributes but yet insufficient data to
encode a single minimum-sized path attribute. In either of these
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cases an error condition exists and the treat-as-withdraw approach
MUST be used (unless some other, more severe error is encountered
dictating a stronger approach), and the Total Attribute Length MUST
be relied upon to enable the beginning of the NLRI field to be
located.
3.2. Syntactic Correctness of NLRI Fields
The NLRI field or Withdrawn Routes field SHALL be considered
"syntactically incorrect" if either of the following are true:
o The length of any of the included NLRI is greater than 32,
o When parsing NLRI contained in the field, the length of the last
NLRI found exceeds the amount of unconsumed data remaining in the
field.
Similarly, the MP_REACH or MP_UNREACH attribute of an update SHALL be
considered to be incorrect if any of the following are true:
o The length of any of the included NLRI is inconsistent with the
given AFI/SAFI (for example, if an IPv4 NLRI has a length greater
than 32 or an IPv6 NLRI has a length greater than 128),
o When parsing NLRI contained in the attribute, the length of the
last NLRI found exceeds the amount of unconsumed data remaining in
the attribute.
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.
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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
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 Attributes
5.1. ORIGIN
The attribute is considered malformed if its length is not 1, or it
has an undefined value [RFC4271].
An UPDATE message with a malformed ORIGIN attribute SHALL be handled
using the approach of "treat-as-withdraw".
5.2. AS_PATH
The error conditions for the attribute have been defined in
[RFC4271].
An UPDATE message with a malformed AS_PATH attribute SHALL be handled
using the approach of "treat-as-withdraw".
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5.3. NEXT_HOP
The error conditions for the NEXT_HOP attribute have been defined in
[RFC4271].
An UPDATE message with a malformed NEXT_HOP attribute SHALL be
handled using the approach of "treat-as-withdraw".
5.4. MULTI_EXIT_DESC
The attribute is considered malformed if its length is not 4
[RFC4271].
An UPDATE message with a malformed MULTI_EXIT_DESC attribute SHALL be
handled using the approach of "treat-as-withdraw".
5.5. LOCAL_PREF
The attribute is considered malformed if its length is not 4
[RFC4271].
An UPDATE message with a malformed LOCAL_PREF attribute SHALL be
handled as follows:
o using the approach of "attribute discard" if the UPDATE message is
received from an external neighbor, or
o using the approach of "treat-as-withdraw" if the UPDATE message is
received from an internal neighbor.
In addition, if the attribute is present in an UPDATE message from an
external neighbor, the approach of "attribute discard" SHALL be used
to handle the unexpected attribute in the message.
5.6. ATOMIC_AGGREGATE
The attribute SHALL be considered malformed if its length is not 0
[RFC4271].
An UPDATE message with a malformed ATOMIC_AGGREGATE attribute SHALL
be handled using the approach of "attribute discard".
5.7. AGGREGATOR
The error conditions specified in [RFC4271] for the attribute are
revised as follows:
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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 [RFC6793]).
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".
5.8. Community
The error handling of [RFC1997] is revised as follows:
The Community attribute SHALL be considered malformed if its length
is nonzero and is not a multiple of 4.
An UPDATE message with a malformed Community attribute SHALL be
handled using the approach of "treat-as-withdraw".
5.9. Extended Community
The error handling of [RFC4360] is revised as follows:
The Extended Community attribute SHALL be considered malformed if its
length is nonzero and is not a 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.
5.10. IPv6 Address Specific BGP Extended Community Attribute
The error handling of [RFC5701] is revised as follows:
The IPv6 Address Specific Extended Community attribute SHALL be
considered malformed if its length is nonzero and is not a multiple
of 20.
An UPDATE message with a malformed IPv6 Address Specific Extended
Community attribute SHALL be handled using the approach of "treat-as-
withdraw".
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Note that a BGP speaker MUST NOT treat an unrecognized IPv6 Address
Specific Extended Community Type or Sub-Type as an error.
5.11. ORIGINATOR_ID
The error handling of [RFC4456] is revised as follows.
o If the ORIGINATOR_ID attribute is received from an external
neighbor, it SHALL be discarded using the approach of "attribute
discard", or
o if received from an internal neighbor, it SHALL be considered
malformed if its length is not equal to 4. If malformed, the
UPDATE SHALL be handled using the approach of "treat-as-withdraw".
5.12. CLUSTER_LIST
The error handling of [RFC4456] is revised as follows.
o If the CLUSTER_LIST attribute is received from an external
neighbor, it SHALL be discarded using the approach of "attribute
discard", or
o if received from an internal neighbor, it SHALL be considered
malformed if its length is not a multiple 4. If malformed, the
UPDATE SHALL be handled using the approach of "treat-as-withdraw".
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
The authors wish to thank Juan Alcaide, Ron Bonica, Mach Chen, Andy
Davidson, Bruno Decraene, Dong Jie, Rex Fernando, Joel Halpern, Akira
Kato, Miya Kohno, Tony Li, Alton Lo, Shin Miyakawa, Tamas Mondal,
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Jonathan Oddy, Robert Raszuk, Yakov Rekhter, Eric Rosen, Rob Shakir,
Naiming Shen, Shyam Sethuram, Ananth Suryanarayana, Kaliraj
Vairavakkalai and Lili Wang for their observations and discussion of
this topic, and review of this document.
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.
[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 4456, April 2006.
[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760, January
2007.
[RFC5701] Rekhter, Y., "IPv6 Address Specific BGP Extended Community
Attribute", RFC 5701, November 2009.
[RFC6793] Vohra, Q. and E. Chen, "BGP Support for Four-Octet
Autonomous System (AS) Number Space", RFC 6793, December
2012.
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:
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AS1 ---- AS2
\ /
\ /
\ /
AS3
Figure 1
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.
Authors' Addresses
Enke Chen (editor)
Cisco Systems, Inc.
Email: enkechen@cisco.com
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John G. Scudder (editor)
Juniper Networks
Email: jgs@juniper.net
Pradosh Mohapatra
Cumulus Networks, Inc.
Email: pmohapat@cumulusnetworks.com
Keyur Patel
Cisco Systems, Inc.
Email: keyupate@cisco.com
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