Revised Error Handling for BGP UPDATE Messages
RFC 7606
| Document | Type | RFC - Proposed Standard (August 2015) Errata | |
|---|---|---|---|
| Authors | Enke Chen , John Scudder , Prodosh Mohapatra , Keyur Patel | ||
| Last updated | 2019-12-19 | ||
| Replaces | draft-ietf-idr-optional-transitive, draft-chen-ebgp-error-handling | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
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| Additional resources | Mailing list discussion | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Rob Shakir | ||
| Shepherd write-up | Show Last changed 2014-11-24 | ||
| IESG | IESG state | RFC 7606 (Proposed Standard) | |
| Action Holders |
(None)
|
||
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | Alvaro Retana | ||
| Send notices to | (None) | ||
| IANA | IANA review state | Version Changed - Review Needed | |
| IANA action state | No IANA Actions |
RFC 7606
Internet Engineering Task Force (IETF) E. Chen, Ed.
Request for Comments: 7606 Cisco Systems, Inc.
Updates: 1997, 4271, 4360, 4456, 4760, J. Scudder, Ed.
5543, 5701, 6368 Juniper Networks
Category: Standards Track P. Mohapatra
ISSN: 2070-1721 Sproute Networks
K. Patel
Cisco Systems, Inc.
August 2015
Revised Error Handling for BGP UPDATE Messages
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 because 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, 5543, 5701, and 6368.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7606.
Chen, et al. Standards Track [Page 1]
RFC 7606 Revised Error Handling for BGP August 2015
Copyright Notice
Copyright (c) 2015 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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
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material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Chen, et al. Standards Track [Page 2]
RFC 7606 Revised Error Handling for BGP August 2015
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Error-Handling Approaches . . . . . . . . . . . . . . . . . . 5
3. Revision to BGP UPDATE Message Error Handling . . . . . . . . 5
4. Attribute Length Fields . . . . . . . . . . . . . . . . . . . 7
5. Parsing of Network Layer Reachability Information (NLRI)
Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Encoding NLRI . . . . . . . . . . . . . . . . . . . . . . 8
5.2. Missing NLRI . . . . . . . . . . . . . . . . . . . . . . 8
5.3. Syntactic Correctness of NLRI Fields . . . . . . . . . . 9
5.4. Typed NLRI . . . . . . . . . . . . . . . . . . . . . . . 9
6. Operational Considerations . . . . . . . . . . . . . . . . . 10
7. Error-Handling Procedures for Existing Attributes . . . . . . 11
7.1. ORIGIN . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.2. AS_PATH . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.3. NEXT_HOP . . . . . . . . . . . . . . . . . . . . . . . . 12
7.4. MULTI_EXIT_DISC . . . . . . . . . . . . . . . . . . . . . 12
7.5. LOCAL_PREF . . . . . . . . . . . . . . . . . . . . . . . 12
7.6. ATOMIC_AGGREGATE . . . . . . . . . . . . . . . . . . . . 12
7.7. AGGREGATOR . . . . . . . . . . . . . . . . . . . . . . . 13
7.8. Community . . . . . . . . . . . . . . . . . . . . . . . . 13
7.9. ORIGINATOR_ID . . . . . . . . . . . . . . . . . . . . . . 13
7.10. CLUSTER_LIST . . . . . . . . . . . . . . . . . . . . . . 13
7.11. MP_REACH_NLRI . . . . . . . . . . . . . . . . . . . . . . 14
7.12. MP_UNREACH_NLRI . . . . . . . . . . . . . . . . . . . . . 14
7.13. Traffic Engineering Path Attribute . . . . . . . . . . . 14
7.14. Extended Community . . . . . . . . . . . . . . . . . . . 14
7.15. IPv6 Address Specific BGP Extended Community Attribute . 15
7.16. ATTR_SET . . . . . . . . . . . . . . . . . . . . . . . . 15
8. Guidance for Authors of BGP Specifications . . . . . . . . . 15
9. Security Considerations . . . . . . . . . . . . . . . . . . . 16
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
10.1. Normative References . . . . . . . . . . . . . . . . . . 17
10.2. Informative References . . . . . . . . . . . . . . . . . 18
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
Chen, et al. Standards Track [Page 3]
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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 because a session reset
impacts 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. This is because
attributes may have been propagated without being checked by
intermediate routers that don't recognize the attributes. In effect,
the attributes may have been tunneled; when they reach a router that
recognizes and checks the attributes, the session that is reset may
not be associated with the router that is at fault. To make matters
worse, in such cases, although the problematic attributes may have
originated with a single update transmitted by a single BGP speaker,
by the time they encounter a router that checks them they may have
been replicated many times and thus may cause the reset of many
peering sessions. Thus, the damage inflicted may be multiplied
manyfold.
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 message 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], [RFC5543], [RFC5701], and [RFC6368] 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].
Chen, et al. Standards Track [Page 4]
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2. Error-Handling Approaches
In this document, we refer to four different approaches to handle
errors found in a BGP UPDATE message. They are as follows (listed in
order, from the one with the "strongest" action to the one with the
"weakest" action):
o Session reset: This is the approach used throughout the base BGP
specification [RFC4271], where a NOTIFICATION is sent and the
session terminated.
o AFI/SAFI disable: Section 7 of [RFC4760] allows a BGP speaker that
detects an error in a message for a given AFI/SAFI to optionally
"ignore all the subsequent routes with that AFI/SAFI received over
that session". We refer to this as "disabling a particular AFI/
SAFI" or "AFI/SAFI disable".
o Treat-as-withdraw: In this approach, the UPDATE message containing
the path attribute in question 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].
o Attribute discard: In this approach, the malformed attribute MUST
be discarded and the UPDATE message continues to be processed.
This approach MUST NOT be used except in the case of an attribute
that has no effect on route selection or installation.
3. Revision to BGP UPDATE Message Error Handling
This specification amends Section 6.3 of [RFC4271] in a number of
ways. See Section 7 for treatment of specific path attributes.
a. The first paragraph 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.
Chen, et al. Standards Track [Page 5]
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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.
b. Error handling for the following case 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.
c. Attribute Flag error handling is revised as follows:
Old Text:
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).
New Text:
If the value of either the Optional or Transitive bits in
the Attribute Flags is in conflict with their specified
values, then the attribute MUST be treated as malformed and
the "treat-as-withdraw" approach used, unless the
specification for the attribute mandates different handling
for incorrect Attribute Flags.
d. If any of the well-known mandatory attributes are not present in
an UPDATE message, then "treat-as-withdraw" MUST be used. (Note
that [RFC4760] reclassifies NEXT_HOP as what is effectively
discretionary.)
e. "Treat-as-withdraw" MUST be used for the cases that specify a
session reset and involve any of the attributes ORIGIN, AS_PATH,
NEXT_HOP, MULTI_EXIT_DISC, or LOCAL_PREF.
f. "Attribute discard" MUST be used for any of the cases that
specify a session reset and involve ATOMIC_AGGREGATE or
AGGREGATOR.
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g. If the MP_REACH_NLRI attribute or the MP_UNREACH_NLRI [RFC4760]
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 (whether
recognized or unrecognized) 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 will continue
to be processed.
h. When multiple attribute errors exist in an UPDATE message, if the
same approach (as described in Section 2) 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.
i. The Withdrawn Routes field MUST be checked for syntactic
correctness in the same manner as the NLRI field. This is
discussed further below and in Section 5.3.
j. 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 -- what
this entails is discussed in more detail in Section 5. If this
is not possible, the procedures of [RFC4271] and/or [RFC4760]
continue to apply, meaning that the "session reset" approach (or
the "AFI/SAFI disable" approach) MUST be followed.
4. 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:
o In the first case, the length of the last encountered path
attribute would cause the Total Attribute Length to be exceeded
when parsing the enclosed path attributes.
o In the second case, fewer than three octets remain (or fewer than
four octets, if the Attribute Flags field has the Extended Length
bit set) when beginning to parse the attribute. That is, this
case exists if there remains unconsumed data in the path
attributes but yet insufficient data to encode a single minimum-
sized path attribute.
In either of these 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
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Attribute Length MUST be relied upon to enable the beginning of the
NLRI field to be located.
For all path attributes other than those specified as having an
attribute length that may be zero, it SHALL be considered a syntax
error for the attribute to have a length of zero. Of the path
attributes considered in this specification, only AS_PATH and
ATOMIC_AGGREGATE may validly have an attribute length of zero.
5. Parsing of Network Layer Reachability Information (NLRI) Fields
5.1. Encoding NLRI
To facilitate the determination of the NLRI field in an UPDATE
message with a malformed attribute:
o The MP_REACH_NLRI or MP_UNREACH_NLRI attribute (if present) SHALL
be encoded as the very first path attribute in an UPDATE message.
o An UPDATE message MUST NOT contain more than one of the following:
non-empty Withdrawn Routes field, non-empty Network Layer
Reachability Information field, MP_REACH_NLRI attribute, and
MP_UNREACH_NLRI attribute.
Since older BGP speakers may not implement these restrictions, an
implementation MUST still be prepared to receive these fields in any
position or combination.
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 message. When such an UPDATE message 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.
5.2. Missing NLRI
[RFC4724] specifies an End-of-RIB message (EoR) that can be encoded
as an UPDATE message that contains only a MP_UNREACH_NLRI attribute
that encodes no NLRI (it can also be a completely empty UPDATE
message in the case of the "legacy" encoding). In all other well-
specified cases, an UPDATE message either carries only withdrawn
routes (either in the Withdrawn Routes field or the MP_UNREACH_NLRI
attribute) or it advertises reachable routes (either in the Network
Layer Reachability Information field or the MP_REACH_NLRI attribute).
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Thus, if an UPDATE message is encountered that does contain path
attributes other than MP_UNREACH_NLRI and doesn't encode any
reachable NLRI, we cannot be confident that the NLRI have been
successfully parsed as Section 3 (j) requires. For this reason, if
any path attribute errors are encountered in such an UPDATE message
and if any encountered error specifies an error-handling approach
other than "attribute discard", then the "session reset" approach
MUST be used.
5.3. 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_NLRI or MP_UNREACH_NLRI 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.
o The attribute flags of the attribute are inconsistent with those
specified in [RFC4760].
o The length of the MP_UNREACH_NLRI attribute is less than 3, or the
length of the MP_REACH_NLRI attribute is less than 5.
5.4. Typed NLRI
Certain address families, for example, MCAST-VPN [RFC6514], MCAST-
VPLS [RFC7117], and EVPN [RFC7432] have NLRI that are typed. Since
supported type values within the address family are not expressed in
the Multiprotocol BGP (MP-BGP) capability [RFC4760], it is possible
for a BGP speaker to advertise support for the given address family
and subaddress family while still not supporting a particular type of
NLRI within that AFI/SAFI.
Chen, et al. Standards Track [Page 9]
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A BGP speaker advertising support for such a typed address family
MUST handle routes with unrecognized NLRI types within that address
family by discarding them, unless the relevant specification for that
address family specifies otherwise.
6. 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 message received on an Internal BGP (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 suboptimal
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 an
incremental update and could cause invalid routes to be kept.
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.
Section 8 mentions that "attribute discard" should not be used in
cases where "the attribute in question has or may have an effect on
route selection." Although all cases that specify "attribute
discard" in this document do not affect route selection by default,
in principle, routing policies could be written that affect selection
based on such an attribute. Operators should take care when writing
Chen, et al. Standards Track [Page 10]
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such policies to consider the possible consequences of an attribute
discard. In general, as long as such policies are only applied to
external BGP sessions, correctness issues are not expected to arise.
7. Error-Handling Procedures for Existing Attributes
In the following subsections, we elaborate on the conditions for
error-checking various path attributes and specify what approach(es)
should be used to handle malformations. It is possible that
implementations may apply other error checks not contemplated here.
If so, the error handling approach given here should generally be
applied.
This section addresses all path attributes that are defined at the
time of this writing that were not defined with error handling
consistent with Section 8 and that are not marked as "deprecated" in
the "BGP Path Attributes" registry [IANA-BGP-ATTRS]. Attributes 17
(AS4_PATH), 18 (AS4_AGGREGATOR), 22 (PMSI_TUNNEL), 23 (Tunnel
Encapsulation Attribute), 26 (AIGP), 27 (PE Distinguisher Labels),
and 29 (BGP-LS Attribute) do have error handling consistent with
Section 8 and thus are not further discussed herein. Attributes 11
(DPA), 12 (ADVERTISER), 13 (RCID_PATH / CLUSTER_ID), 19 (SAFI
Specific Attribute), 20 (Connector Attribute), 21 (AS_PATHLIMIT), and
28 (BGP Entropy Label Capability Attribute) are deprecated and thus
are not further discussed herein.
7.1. ORIGIN
The attribute is considered malformed if its length is not 1 or if it
has an undefined value [RFC4271].
An UPDATE message with a malformed ORIGIN attribute SHALL be handled
using the approach of "treat-as-withdraw".
7.2. AS_PATH
An AS_PATH is considered malformed if an unrecognized segment type is
encountered or if it contains a malformed segment. A segment is
considered malformed if any of the following are true:
o There is an overrun where the Path Segment Length field of the
last segment encountered would cause the Attribute Length to be
exceeded.
o There is an underrun where after the last successfully parsed
segment there is only a single octet remaining (that is, there is
not enough unconsumed data to provide even an empty segment
header).
Chen, et al. Standards Track [Page 11]
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o It has a Path Segment Length field of zero.
An UPDATE message with a malformed AS_PATH attribute SHALL be handled
using the approach of "treat-as-withdraw".
[RFC4271] also says that an implementation optionally "MAY check
whether the leftmost ... AS in the AS_PATH attribute is equal to the
autonomous system number of the peer that sent the message". A BGP
implementation SHOULD also handle routes that violate this check
using "treat-as-withdraw" but MAY follow the "session reset" behavior
if configured to do so.
7.3. NEXT_HOP
The attribute is considered malformed if its length is not 4
[RFC4271].
An UPDATE message with a malformed NEXT_HOP attribute SHALL be
handled using the approach of "treat-as-withdraw".
7.4. MULTI_EXIT_DISC
The attribute is considered malformed if its length is not 4
[RFC4271].
An UPDATE message with a malformed MULTI_EXIT_DISC attribute SHALL be
handled using the approach of "treat-as-withdraw".
7.5. LOCAL_PREF
The error handling of [RFC4271] is revised as follows:
o if the LOCAL_PREF 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 message SHALL be handled using the approach of "treat-as-
withdraw".
7.6. 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".
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7.7. AGGREGATOR
The error conditions specified in [RFC4271] for the attribute are
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 [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".
7.8. Community
The error handling of [RFC1997] is revised as follows:
o The Community attribute SHALL be considered malformed if its
length is not a non-zero multiple of 4.
o An UPDATE message with a malformed Community attribute SHALL be
handled using the approach of "treat-as-withdraw".
7.9. 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 message SHALL be handled using the approach of "treat-as-
withdraw".
7.10. 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
Chen, et al. Standards Track [Page 13]
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o if received from an internal neighbor, it SHALL be considered
malformed if its length is not a non-zero multiple of 4. If
malformed, the UPDATE message SHALL be handled using the approach
of "treat-as-withdraw".
7.11. MP_REACH_NLRI
If the Length of Next Hop Network Address field of the MP_REACH
attribute is inconsistent with that which was expected, the attribute
is considered malformed. Since the next hop precedes the NLRI field
in the attribute, in this case it will not be possible to reliably
locate the NLRI; thus, the "session reset" or "AFI/SAFI disable"
approach MUST be used.
"That which was expected", while somewhat vague, is intended to
encompass the next hop specified for the Address Family Identifier
and Subsequent Address Family Identifier fields and is potentially
modified by any extensions in use. For example, if [RFC5549] is in
use, then the next hop would have to have a length of 4 or 16.
Sections 3 and 5 provide further discussion of the handling of this
attribute.
7.12. MP_UNREACH_NLRI
Sections 3 and 5 discuss the handling of this attribute.
7.13. Traffic Engineering Path Attribute
We note that [RFC5543] does not detail what constitutes
"malformation" for the Traffic Engineering path attribute. A future
update to that specification may provide more guidance. In the
interim, an implementation that determines (for whatever reason) that
an UPDATE message contains a malformed Traffic Engineering path
attribute MUST handle it using the approach of "treat-as-withdraw".
7.14. Extended Community
The error handling of [RFC4360] is revised as follows:
o The Extended Community attribute SHALL be considered malformed if
its length is not a non-zero multiple of 8.
o 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.
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7.15. IPv6 Address Specific BGP Extended Community Attribute
The error handling of [RFC5701] is revised as follows:
o The IPv6 Address Specific Extended Community attribute SHALL be
considered malformed if its length is not a non-zero multiple of
20.
o An UPDATE message with a malformed IPv6 Address Specific Extended
Community attribute SHALL be handled using the approach of "treat-
as-withdraw".
Note that a BGP speaker MUST NOT treat an unrecognized IPv6 Address
Specific Extended Community Type or Sub-Type as an error.
7.16. ATTR_SET
The final paragraph of Section 5 of [RFC6368] is revised as follows:
Old Text:
An UPDATE message with a malformed ATTR_SET attribute SHALL be
handled as follows. If its Partial flag is set and its
Neighbor-Complete flag is clear, the UPDATE message is treated
as a route withdraw as discussed in [OPT-TRANS-BGP]. Otherwise
(i.e., Partial flag is clear or Neighbor-Complete is set), the
procedures of the BGP-4 base specification [RFC4271] MUST be
followed with respect to an Optional Attribute Error.
New Text:
An UPDATE message with a malformed ATTR_SET attribute SHALL be
handled using the approach of "treat as withdraw".
Furthermore, the normative reference to [OPT-TRANS-BGP] in [RFC6368]
is removed.
8. Guidance for Authors of BGP Specifications
A document that specifies a new BGP attribute MUST provide specifics
regarding what constitutes an error for that attribute and how that
error is to be handled. Allowable error-handling approaches are
detailed in Section 2. The "treat-as-withdraw" approach is generally
preferred and the "session reset" approach is discouraged. Authors
of BGP documents are also reminded to review the discussion of
optional transitive attributes in the first paragraph of the
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Introduction of this document. The document SHOULD also provide
consideration of what debugging facilities may be required to permit
issues caused by a malformed attribute to be diagnosed.
For any malformed attribute that 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.
Authors can refer to Section 7 for examples.
9. 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. Since the intervening routers do not recognize the
attribute, they propagate it without checking it. When the malformed
attribute arrives at a router that does recognize the given attribute
type, that router resets the session over which it arrived. Since
significant fan-out can occur between the attacker and the routers
that do recognize the attribute type, this attack could potentially
be particularly harmful.
The improved error handling of this specification could in theory
interact badly with some now-known weaker cryptographic mechanisms
should such be used in future to secure BGP. For example, if a
(fictional) mechanism that did not supply data integrity was used, an
attacker could manipulate ciphertext in any attempt to change or
observe how the receiver reacts. Absent this specification, the BGP
session would have been terminated; with this specification, the
attacker could make potentially many attempts. While such a
confidentiality-only mechanism would not be defined today, we have in
the past seen mechanism definitions that result in similar, though
not as obviously exploitable, vulnerabilities [RFC7366]. The
approach recommended today to avoid such issues is to prefer use of
Authenticated Encryption with Additional Data (AEAD) ciphers
[RFC5116] and thus to discard messages that don't verify.
In other respects, this specification does not change BGP's security
characteristics.
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10. References
10.1. Normative References
[IANA-BGP-ATTRS]
IANA, "BGP Path Attributes",
<http://www.iana.org/assignments/bgp-parameters>.
[RFC1997] Chandra, R., Traina, P., and T. Li, "BGP Communities
Attribute", RFC 1997, DOI 10.17487/RFC1997, August 1996,
<http://www.rfc-editor.org/info/rfc1997>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<http://www.rfc-editor.org/info/rfc4271>.
[RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
Communities Attribute", RFC 4360, DOI 10.17487/RFC4360,
February 2006, <http://www.rfc-editor.org/info/rfc4360>.
[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
<http://www.rfc-editor.org/info/rfc4456>.
[RFC4724] Sangli, S., Chen, E., Fernando, R., Scudder, J., and Y.
Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724,
DOI 10.17487/RFC4724, January 2007,
<http://www.rfc-editor.org/info/rfc4724>.
[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
DOI 10.17487/RFC4760, January 2007,
<http://www.rfc-editor.org/info/rfc4760>.
[RFC5543] Ould-Brahim, H., Fedyk, D., and Y. Rekhter, "BGP Traffic
Engineering Attribute", RFC 5543, DOI 10.17487/RFC5543,
May 2009, <http://www.rfc-editor.org/info/rfc5543>.
[RFC5701] Rekhter, Y., "IPv6 Address Specific BGP Extended Community
Attribute", RFC 5701, DOI 10.17487/RFC5701, November 2009,
<http://www.rfc-editor.org/info/rfc5701>.
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[RFC6368] Marques, P., Raszuk, R., Patel, K., Kumaki, K., and T.
Yamagata, "Internal BGP as the Provider/Customer Edge
Protocol for BGP/MPLS IP Virtual Private Networks (VPNs)",
RFC 6368, DOI 10.17487/RFC6368, September 2011,
<http://www.rfc-editor.org/info/rfc6368>.
[RFC6793] Vohra, Q. and E. Chen, "BGP Support for Four-Octet
Autonomous System (AS) Number Space", RFC 6793,
DOI 10.17487/RFC6793, December 2012,
<http://www.rfc-editor.org/info/rfc6793>.
10.2. Informative References
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<http://www.rfc-editor.org/info/rfc5116>.
[RFC5549] Le Faucheur, F. and E. Rosen, "Advertising IPv4 Network
Layer Reachability Information with an IPv6 Next Hop",
RFC 5549, DOI 10.17487/RFC5549, May 2009,
<http://www.rfc-editor.org/info/rfc5549>.
[RFC6514] Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter, "BGP
Encodings and Procedures for Multicast in MPLS/BGP IP
VPNs", RFC 6514, DOI 10.17487/RFC6514, February 2012,
<http://www.rfc-editor.org/info/rfc6514>.
[RFC7117] Aggarwal, R., Ed., Kamite, Y., Fang, L., Rekhter, Y., and
C. Kodeboniya, "Multicast in Virtual Private LAN Service
(VPLS)", RFC 7117, DOI 10.17487/RFC7117, February 2014,
<http://www.rfc-editor.org/info/rfc7117>.
[RFC7366] Gutmann, P., "Encrypt-then-MAC for Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", RFC 7366, DOI 10.17487/RFC7366, September 2014,
<http://www.rfc-editor.org/info/rfc7366>.
[RFC7432] Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
2015, <http://www.rfc-editor.org/info/rfc7432>.
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Acknowledgements
The authors wish to thank Juan Alcaide, Deniz Bahadir, Ron Bonica,
Mach Chen, Andy Davidson, Bruno Decraene, Stephen Farrell, Rex
Fernando, Jeff Haas, Chris Hall, Joel Halpern, Dong Jie, Akira Kato,
Miya Kohno, Warren Kumari, Tony Li, Alton Lo, Shin Miyakawa, Tamas
Mondal, Jonathan Oddy, Tony Przygienda, Robert Raszuk, Yakov Rekhter,
Eric Rosen, Shyam Sethuram, Rob Shakir, Naiming Shen, Adam Simpson,
Ananth Suryanarayana, Kaliraj Vairavakkalai, Lili Wang, and Ondrej
Zajicek for their observations and discussion of this topic and
review of this document.
Authors' Addresses
Enke Chen (editor)
Cisco Systems, Inc.
Email: enkechen@cisco.com
John G. Scudder (editor)
Juniper Networks
Email: jgs@juniper.net
Pradosh Mohapatra
Sproute Networks
Email: mpradosh@yahoo.com
Keyur Patel
Cisco Systems, Inc.
Email: keyupate@cisco.com
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