RFC9117: Revised Validation Procedure for BGP Flow Specifications
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Related keywords:
(BGP flowspec)
Internet Engineering Task Force (IETF) J. Uttaro
Request for Comments: 9117 AT&T
Updates: 8955 J. Alcaide
Category: Standards Track C. Filsfils
ISSN: 2070-1721 D. Smith
Cisco
P. Mohapatra
Sproute Networks
August 2021
Revised Validation Procedure for BGP Flow Specifications
Abstract
This document describes a modification to the validation procedure
defined for the dissemination of BGP Flow Specifications. The
dissemination of BGP Flow Specifications as specified in RFC 8955
requires that the originator of the Flow Specification match the
originator of the best-match unicast route for the destination prefix
embedded in the Flow Specification. For an Internal Border Gateway
Protocol (iBGP) received route, the originator is typically a border
router within the same autonomous system (AS). The objective is to
allow only BGP speakers within the data forwarding path to originate
BGP Flow Specifications. Sometimes it is desirable to originate the
BGP Flow Specification from any place within the autonomous system
itself, for example, from a centralized BGP route controller.
However, the validation procedure described in RFC 8955 will fail in
this scenario. The modification proposed herein relaxes the
validation rule to enable Flow Specifications to be originated within
the same autonomous system as the BGP speaker performing the
validation. Additionally, this document revises the AS_PATH
validation rules so Flow Specifications received from an External
Border Gateway Protocol (eBGP) peer can be validated when such a peer
is a BGP route server.
This document updates the validation procedure in RFC 8955.
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 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9117.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction
2. Definitions of Terms Used in This Memo
3. Motivation
4. Revised Validation Procedure
4.1. Revision of Route Feasibility
4.2. Revision of AS_PATH Validation
5. Topology Considerations
6. IANA Considerations
7. Security Considerations
8. References
8.1. Normative References
8.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
[RFC8955] defines BGP Network Layer Reachability Information (NLRI)
[RFC4760] that can be used to distribute traffic Flow Specifications
amongst BGP speakers in support of traffic filtering. The primary
intention of [RFC8955] is to enable downstream autonomous systems to
signal traffic filtering policies to upstream autonomous systems. In
this way, traffic is filtered closer to the source and the upstream
autonomous systems avoid carrying the traffic to the downstream
autonomous systems only to be discarded. [RFC8955] also enables more
granular traffic filtering based upon upper-layer protocol
information (e.g., protocol or port numbers) as opposed to coarse IP
destination prefix-based filtering. Flow Specification NLRIs
received from a BGP peer is subject to validity checks before being
considered feasible and subsequently installed within the respective
Adj-RIB-In.
The validation procedure defined within [RFC8955] requires that the
originator of the Flow Specification NLRI match the originator of the
best-match unicast route for the destination prefix embedded in the
Flow Specification. The aim is to make sure that only speakers on
the forwarding path can originate the Flow Specification. Let's
consider the particular case where the Flow Specification is
originated in any location within the same Local Domain as the
speaker performing the validation (for example, by a centralized BGP
route controller), and the best-match unicast route is originated in
another Local Domain. In order for the validation to succeed for a
Flow Specification received from an iBGP peer, it would be necessary
to disseminate such Flow Specification NLRI directly from the
specific border router (within the Local Domain) that is advertising
the corresponding best-match unicast route to the Local Domain.
Those border routers would be acting as de facto route controllers.
This approach would be, however, operationally cumbersome in a Local
Domain with numerous border routers having complex BGP policies.
Figure 1 illustrates this principle. R1 (the upstream router) and RR
(a route reflector) need to validate the Flow Specification whose
embedded destination prefix has a best-match unicast route (dest-
route) originated by ASBR2. ASBR2 could originate the Flow
Specification, and it would be validated when received by RR and R1
(from their point of view, the originator of both the Flow
Specification and the best-match unicast route will be ASBR1).
Sometimes the Flow Specification needs to be originated within AS1.
ASBR1 could originate it, and the Flow Specification would still be
validated. In both cases, the Flow Specification is originated by a
router in the same forwarding path as the dest-route. For the case
where AS1 has thousands of ASBRs, it becomes impractical to originate
different Flow Specification rules on each ASBR in AS1 based on which
ASBR each dest-route is learned from. To make the situation more
tenable, the objective is to advertise all the Flow Specifications
from the same route controller.
R1(AS1) --- RR(AS1) --- ASBR1(AS1) --- ASBR2(AS2)
|
route controller(AS1)
Figure 1
This document describes a modification to the validation procedure
described in [RFC8955], by allowing Flow Specification NLRIs to be
originated from a centralized BGP route controller located within the
Local Domain and not necessarily in the data-forwarding path. While
the proposed modification cannot be used for inter-domain
coordination of traffic filtering, it greatly simplifies distribution
of intra-domain traffic filtering policies within a Local Domain that
has numerous border routers having complex BGP policies. By relaxing
the validation procedure for iBGP, the proposed modification allows
Flow Specifications to be distributed in a standard and scalable
manner throughout the Local Domain.
Throughout this document, some references are made to
AS_CONFED_SEQUENCE segments; see Sections 4.1 and 5. If
AS_CONFED_SET segments are also present in the AS_PATH, the same
considerations apply to them. Note, however, that the use of
AS_CONFED_SET segments is not recommended [RFC6472]. Refer to
[CONFED-SET] as well.
2. Definitions of Terms Used in This Memo
Local Domain: the local AS or the local confederation of ASes
[RFC5065].
eBGP: BGP peering to a router not within the Local Domain.
iBGP: Both classic iBGP and any form of eBGP peering with a router
within the same confederation (i.e., iBGP peering is a peering
that is not eBGP as defined above).
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Motivation
Step (b) of the validation procedure in Section 6 of [RFC8955] is
defined with the underlying assumption that the Flow Specification
NLRI traverses the same path, in the inter-domain and intra-domain
route distribution graph, as that of the longest-match unicast route
for the destination prefix embedded in the Flow Specification.
In the case of inter-domain traffic filtering, the Flow Specification
originator at the egress border routers of an AS (e.g., RTR-D and
RTR-E of AS1 in Figure 2) matches the eBGP neighbor that advertised
the longest match destination prefix (see RTR-F and RTR-G,
respectively, in Figure 2).
Similarly, at the upstream routers of an AS (see RTR-A and RTR-B of
AS1 in Figure 2), the Flow Specification originator matches the
egress iBGP border routers that had advertised the unicast route for
the best-match destination prefix (see RTR-D and RTR-E, respectively,
in Figure 2). This is true even when upstream routers select paths
from different egress border routers as the best route based upon IGP
distance. For example, in Figure 2:
RTR-A chooses RTR-D as the best route
RTR-B chooses RTR-E as the best route
/ - - - - - - - - - - - - - -
| AS1 |
+-------+ +-------+
| | | | | |
| RTR-A | | RTR-B |
| | | | | |
+-------+ +-------+
| \ / |
iBGP \ / iBGP
| \ / |
+-------+
| | | |
| RTR-C |
| | RC | |
+-------+
| / \ |
/ \
| iBGP / \ iBGP |
+-------+ +-------+
| | RTR-D | | RTR-E | |
| | | |
| | | | | |
+-------+ +-------+
| | | |
- - -|- - - - - - - - -|- - -/
| eBGP eBGP |
- - -|- - - - - - - - -|- - -/
| | | |
+-------+ +-------+
| | | | | |
| RTR-F | | RTR-G |
| | | | | |
+-------+ +-------+
| AS2 |
/ - - - - - - - - - - - - - -
Figure 2
It is highly desirable that mechanisms exist to protect each AS
independently from network security attacks using the BGP Flow
Specification NLRI for intra-AS purposes only. Network operators
often deploy a dedicated Security Operations Center (SOC) within
their AS to monitor and detect such security attacks. To mitigate
attacks within an AS, operators require the ability to originate
intra-AS Flow Specification NLRIs from a central BGP route controller
that is not within the data forwarding plane. In this way, operators
can direct border routers within their AS with specific attack-
mitigation actions (drop the traffic, forward to a pipe-cleaning
location, etc.).
In addition, an operator may extend the requirements above for a
group of ASes via policy. This is described in Section 4.1 (b.2.3)
of the validation procedure.
A central BGP route controller that originates Flow Specification
NLRI should be able to avoid the complexity of having to determine
the egress border router whose path was chosen as the best for each
of its neighbors. When a central BGP route controller originates
Flow Specification NLRI, the rest of the speakers within the AS will
see the BGP route controller as the originator of the Flow
Specification in terms of the validation procedure rules. Thus, it
is necessary to modify step (b) of the validation procedure described
in [RFC8955] such that an iBGP peer that is not within the data
forwarding plane may originate Flow Specification NLRIs.
4. Revised Validation Procedure
4.1. Revision of Route Feasibility
Step (b) of the validation procedure specified in Section 6 of
[RFC8955] is redefined as follows:
| b) One of the following conditions MUST hold true:
|
| 1. The originator of the Flow Specification matches the
| originator of the best-match unicast route for the
| destination prefix embedded in the Flow Specification (this
| is the unicast route with the longest possible prefix
| length covering the destination prefix embedded in the Flow
| Specification).
|
| 2. The AS_PATH attribute of the Flow Specification is empty or
| contains only an AS_CONFED_SEQUENCE segment [RFC5065].
|
| 1. This condition SHOULD be enabled by default.
|
| 2. This condition MAY be disabled by explicit
| configuration on a BGP speaker.
|
| 3. As an extension to this rule, a given non-empty AS_PATH
| (besides AS_CONFED_SEQUENCE segments) MAY be permitted
| by policy.
Explanation:
Receiving either an empty AS_PATH or one with only an
AS_CONFED_SEQUENCE segment indicates that the Flow Specification
was originated inside the Local Domain.
With the above modification to the [RFC8955] validation procedure,
a BGP peer within the Local Domain that is not within the data-
forwarding path can originate a Flow Specification.
Disabling the new condition above (see step b.2.2 in Section 4.1)
could be a good practice if the operator knew with certainty that
a Flow Specification would not be originated inside the Local
Domain. An additional case would be if it was known for a fact
that only the right egress border routers (i.e., those that were
also egress border routers for the best routes) were originating
Flow Specification NLRI.
Also, policy may be useful to permit a specific set of non-empty
AS_PATHs (see step b.2.3 in Section 4.1). For example, it could
validate a Flow Specification whose AS_PATH contained only an
AS_SEQUENCE segment with ASes that were all known to belong to the
same administrative domain.
4.2. Revision of AS_PATH Validation
Section 6 of [RFC8955] states:
| BGP implementations MUST also enforce that the AS_PATH
| attribute of a route received via the External Border Gateway
| Protocol (eBGP) contains the neighboring AS in the left-most
| position of the AS_PATH attribute. While this rule is optional
| in the BGP specification, it becomes necessary to enforce it
| here for security reasons.
This rule prevents the exchange of BGP Flow Specification NLRIs at
Internet exchanges with BGP route servers, which by design don't
insert their own AS number into the AS_PATH (Section 2.2.2.1 of
[RFC7947]). Therefore, this document also redefines the [RFC8955]
AS_PATH validation procedure referenced above as follows:
| BGP Flow Specification implementations MUST enforce that the AS
| in the left-most position of the AS_PATH attribute of a Flow
| Specification route received via the External Border Gateway
| Protocol (eBGP) matches the AS in the left-most position of the
| AS_PATH attribute of the best-match unicast route for the
| destination prefix embedded in the Flow Specification NLRI.
Explanation:
For clarity, the AS in the left-most position of the AS_PATH means
the AS that was last added to an AS_SEQUENCE.
This proposed modification enables the exchange of BGP Flow
Specification NLRIs at Internet exchanges with BGP route servers
while at the same time, for security reasons, prevents an eBGP
peer from advertising an inter-domain Flow Specification for a
destination prefix that it does not provide reachability
information for.
Comparing only the left-most AS in the AS-PATH for eBGP-learned
Flow Specification NLRIs is roughly equivalent to checking the
neighboring AS. If the peer is a route server, security is
necessarily weakened for the Flow Specification NLRI, as it is for
any unicast route advertised from a route server. An example is
discussed in the Security Considerations section.
Redefinition of this AS_PATH validation rule for a Flow
Specification does not mean that the original rule in [RFC8955]
cannot be enforced as well. Its enforcement remains optional per
Section 6.3 of [RFC4271]. That is, a BGP speaker can enforce the
first AS in the AS_PATH to be the same as the neighbor AS for a
route belonging to any Address Family (including Flow
Specification Address Family). If the BGP speaker peer is not a
route server, when enforcing this optional rule, the security
characteristics are exactly equivalent to those specified in
[RFC8955].
Alternatively, enforcing this optional rule for unicast routes
(even if not enforced on Flow Specification NLRIs) achieves
exactly the same security characteristics. The reason is that,
after all validations, the neighboring AS will be the same as the
left-most AS in the AS-PATH for the unicast route, and the left-
most AS in the AS_PATH for the unicast route will be the same as
the left-most AS in the AS_PATH for the Flow Specification NLRI.
Therefore, the neighboring AS will be the same as the left-most AS
in the AS_PATH for the Flow Specification NLRI (as the original
AS_PATH validation rule in [RFC8955] states).
Note, however, that not checking the full AS_PATH allows any rogue
or misconfigured AS the ability to originate undesired Flow
Specifications. This is a BGP security threat, already present in
[RFC8955], but out of the scope of this document.
Using the new rule to validate a Flow Specification route received
from a peer belonging to the same Local Domain is out of the scope
of this document. Note that although it's possible, its utility
is dubious. Although it is conceivable that a router in the same
Local Domain could send a rogue update, only eBGP risk is
considered within this document (in the same spirit as the
aforementioned AS_PATH validation in [RFC4271]).
5. Topology Considerations
[RFC8955] indicates that the originator may refer to the originator
path attribute (ORIGINATOR_ID) or (if the attribute is not present)
the transport address of the peer from which the BGP speaker received
the update. If the latter applies, a network should be designed so
it has a congruent topology amongst unicast routes and Flow
Specification routes. By congruent topology, it is understood that
the two routes (i.e., the Flow Specification route and its best-match
unicast route) are learned from the same peer across the AS. That
would likely not be true, for instance, if some peers only negotiated
one Address Family or if each Address Family peering had a different
set of policies. Failing to have a congruent topology would result
in step (b.1) of the validation procedure to fail.
With the additional second condition (b.2) in the validation
procedure, non-congruent topologies are supported within the Local
Domain if the Flow Specification is originated within the Local
Domain.
Explanation:
Consider the following scenarios of a non-congruent topology
without the second condition (b.2) being added to the validation
procedure:
1. Consider a topology with two BGP speakers with two iBGP
peering sessions between them, one for unicast and one for
Flow Specification. This is a non-congruent topology. Let's
assume that the ORIGINATOR_ID attribute was not received
(e.g., a route reflector receiving routes from its clients).
In this case, the Flow Specification validation procedure will
fail because of the first condition (b.1).
2. Consider a confederation of ASes with local AS X and local AS
Y (both belonging to the same Local Domain), and a given BGP
speaker X1 inside local AS X. The ORIGINATOR_ID attribute is
not advertised when propagating routes across local ASes.
Let's assume the Flow Specification route is received from
peer Y1 and the best-match unicast route is received from peer
Y2. Both peers belong to local AS Y. The Flow Specification
validation procedure will also fail because of the first
condition (b.1).
Consider now that the second condition (b.2) is added to the
validation procedure. In the scenarios above, if Flow
Specifications are originated in the same Local Domain, the
AS_PATH will be empty or contain only an AS_CONFED_SEQUENCE
segment. Condition (b.2) will evaluate to true. Therefore, using
the second condition (b.2), as defined by this document,
guarantees that the overall validation procedure will pass. Thus,
non-congruent topologies are supported if the Flow Specification
is originated in the same Local Domain.
Flow Specifications originated in a different Local Domain sill
need a congruent topology. The reason is that in a non-congruent
topology, the second condition (b.2) evaluates to false and only
the first condition (b.1) is evaluated.
6. IANA Considerations
This document has no IANA actions.
7. Security Considerations
This document updates the route feasibility validation procedures for
Flow Specifications learned from iBGP peers and through route
servers. This change is in line with the procedures described in
[RFC8955] and, thus, security characteristics remain essentially
equivalent to the existing security properties of BGP unicast
routing, except as detailed below.
The security considerations discussed in [RFC8955] apply to this
specification as well.
This document makes the original AS_PATH validation rule (Section 6.3
of [RFC4271]) again OPTIONAL (Section 4.2) for Flow Specification
Address Family (the rule is no longer mandatory as had been specified
by [RFC8955]). If that original rule is not enforced for Flow
Specification, it may introduce some new security risks. A speaker
in AS X peering with a route server could advertise a rogue Flow
Specification route whose first AS in AS_PATH was Y. Assume Y is the
first AS in the AS_PATH of the best-match unicast route. When the
route server advertises the Flow Specification to a speaker in AS Z,
it will be validated by that speaker. This risk is impossible to
prevent if the Flow Specification route is received from a route
server peer. If configuration (or other means beyond the scope of
this document) indicates that the peer is not a route server, that
optional rule SHOULD be enforced for unicast and/or for Flow
Specification routes (as discussed in the Revision of AS_PATH
Validation section, just enforcing it in one of those Address
Families is enough). If the indication is that the peer is not a
route server or there is no conclusive indication, that optional rule
SHOULD NOT be enforced.
A route server itself may be in a good position to enforce the
AS_PATH validation rule described in the previous paragraph. If it
is known that a route server is not peering with any other route
server, it can enforce the AS_PATH validation rule across all its
peers.
BGP updates learned from iBGP peers are considered trusted, so the
Traffic Flow Specifications contained in BGP updates are also
considered trusted. Therefore, it is not required to validate that
the originator of an intra-domain Traffic Flow Specification matches
the originator of the best-match unicast route for the destination
prefix embedded in that Flow Specification. Note that this
trustworthiness consideration is not absolute and the new possibility
that an iBGP speaker could send a rogue Flow Specification is
introduced.
The changes in Section 4.1 don't affect the validation procedures for
eBGP-learned routes.
It's worth mentioning that allowing (or making operationally
feasible) Flow Specifications to originate within the Local Domain
makes the network overall more secure. Flow Specifications can be
originated more readily during attacks and improve the stability and
security of the network.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://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,
<https://www.rfc-editor.org/info/rfc4271>.
[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
DOI 10.17487/RFC4760, January 2007,
<https://www.rfc-editor.org/info/rfc4760>.
[RFC5065] Traina, P., McPherson, D., and J. Scudder, "Autonomous
System Confederations for BGP", RFC 5065,
DOI 10.17487/RFC5065, August 2007,
<https://www.rfc-editor.org/info/rfc5065>.
[RFC7947] Jasinska, E., Hilliard, N., Raszuk, R., and N. Bakker,
"Internet Exchange BGP Route Server", RFC 7947,
DOI 10.17487/RFC7947, September 2016,
<https://www.rfc-editor.org/info/rfc7947>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8955] Loibl, C., Hares, S., Raszuk, R., McPherson, D., and M.
Bacher, "Dissemination of Flow Specification Rules",
RFC 8955, DOI 10.17487/RFC8955, December 2020,
<https://www.rfc-editor.org/info/rfc8955>.
8.2. Informative References
[CONFED-SET]
Kumari, W., Sriram, K., Hannachi, L., and J. Haas,
"Deprecation of AS_SET and AS_CONFED_SET in BGP", Work in
Progress, Internet-Draft, draft-ietf-idr-deprecate-as-set-
confed-set-05, 12 March 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-
deprecate-as-set-confed-set-05>.
[RFC6472] Kumari, W. and K. Sriram, "Recommendation for Not Using
AS_SET and AS_CONFED_SET in BGP", BCP 172, RFC 6472,
DOI 10.17487/RFC6472, December 2011,
<https://www.rfc-editor.org/info/rfc6472>.
Acknowledgements
The authors would like to thank Han Nguyen for his direction on this
work as well as Waqas Alam, Keyur Patel, Robert Raszuk, Eric Rosen,
Shyam Sethuram, Susan Hares, Alvaro Retana, and John Scudder for
their review and comments.
Authors' Addresses
James Uttaro
AT&T
200 S. Laurel Ave
Middletown, NJ 07748
United States of America
Email: ju1738@att.com
Juan Alcaide
Cisco
Research Triangle Park
7100 Kit Creek Road
Morrisville, NC 27709
United States of America
Email: jalcaide@cisco.com
Clarence Filsfils
Cisco
Email: cf@cisco.com
David Smith
Cisco
111 Wood Ave South
Iselin, NJ 08830
United States of America
Email: djsmith@cisco.com
Pradosh Mohapatra
Sproute Networks
Email: mpradosh@yahoo.com