Revised Validation Procedure for BGP Flow SpecificationsAT&T200 S. Laurel AveMiddletownNJ07748United States of Americaju1738@att.comCisco7100 Kit Creek RoadResearch Triangle ParkMorrisvilleNC27709United States of Americajalcaide@cisco.comCiscocf@cisco.comCisco111 Wood Ave SouthIselinNJ08830United States of Americadjsmith@cisco.comSproute Networksmpradosh@yahoo.com
Routing
IDRBGP flowspec
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
.
Copyright Notice
Copyright (c) 2021 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
() in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with
respect to this document. Code Components extracted from this
document must 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.
Table of Contents
. Introduction
. Definitions of Terms Used in This Memo
. Motivation
. Revised Validation Procedure
. Revision of Route Feasibility
. Revision of AS_PATH Validation
. Topology Considerations
. IANA Considerations
. Security Considerations
. References
. Normative References
. Informative References
Acknowledgements
Authors' Addresses
Introduction defines BGP Network Layer
Reachability Information (NLRI) that can be used to distribute traffic Flow
Specifications amongst BGP speakers in support of traffic
filtering. The primary intention of 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. 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 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.
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)
This document describes a modification to the validation procedure described in , 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 and . 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 . Refer to as well.
Definitions of Terms Used in This Memo
Local Domain:
the local AS or the local confederation of ASes .
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
when, and only when, they appear in all capitals, as shown here.
MotivationStep (b) of the validation procedure in 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 ) matches the eBGP neighbor that
advertised the longest match destination prefix (see RTR-F and RTR-G,
respectively, in ).
Similarly, at the upstream routers of an AS (see RTR-A and RTR-B of
AS1 in ), 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 ). 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 :
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 |
/ - - - - - - - - - - - - - -
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 (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
such that an iBGP peer that is not within the data forwarding plane
may originate Flow Specification NLRIs.
Revised Validation ProcedureRevision of Route FeasibilityStep (b) of the validation procedure specified in is
redefined as follows:
b)
One of the following conditions MUST hold true:
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).
The AS_PATH attribute of the Flow Specification is empty or
contains only an AS_CONFED_SEQUENCE segment .
This condition SHOULD be
enabled by default.
This condition MAY be disabled by
explicit configuration on a BGP speaker.
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 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 ) 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 ). 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.
Revision of AS_PATH Validation
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 (). Therefore, this document also
redefines the 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
cannot be enforced as well. Its enforcement remains optional per . 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 .
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 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 , 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 ).
Topology Considerations 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:
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).
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.
IANA ConsiderationsThis document has no IANA actions.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 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 apply to this
specification as well.
This document makes the original AS_PATH validation rule () again
OPTIONAL () for Flow Specification Address Family (the rule is no longer
mandatory as had been specified by ). 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 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.
ReferencesNormative ReferencesKey words for use in RFCs to Indicate Requirement LevelsIn many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.A Border Gateway Protocol 4 (BGP-4)This document discusses the Border Gateway Protocol (BGP), which is an inter-Autonomous System routing protocol.The primary function of a BGP speaking system is to exchange network reachability information with other BGP systems. This network reachability information includes information on the list of Autonomous Systems (ASes) that reachability information traverses. This information is sufficient for constructing a graph of AS connectivity for this reachability from which routing loops may be pruned, and, at the AS level, some policy decisions may be enforced.BGP-4 provides a set of mechanisms for supporting Classless Inter-Domain Routing (CIDR). These mechanisms include support for advertising a set of destinations as an IP prefix, and eliminating the concept of network "class" within BGP. BGP-4 also introduces mechanisms that allow aggregation of routes, including aggregation of AS paths.This document obsoletes RFC 1771. [STANDARDS-TRACK]Multiprotocol Extensions for BGP-4This document defines extensions to BGP-4 to enable it to carry routing information for multiple Network Layer protocols (e.g., IPv6, IPX, L3VPN, etc.). The extensions are backward compatible - a router that supports the extensions can interoperate with a router that doesn't support the extensions. [STANDARDS-TRACK]Autonomous System Confederations for BGPThe Border Gateway Protocol (BGP) is an inter-autonomous system routing protocol designed for Transmission Control Protocol/Internet Protocol (TCP/IP) networks. BGP requires that all BGP speakers within a single autonomous system (AS) must be fully meshed. This represents a serious scaling problem that has been well documented in a number of proposals.This document describes an extension to BGP that may be used to create a confederation of autonomous systems that is represented as a single autonomous system to BGP peers external to the confederation, thereby removing the "full mesh" requirement. The intention of this extension is to aid in policy administration and reduce the management complexity of maintaining a large autonomous system.This document obsoletes RFC 3065. [STANDARDS-TRACK]Internet Exchange BGP Route ServerThis document outlines a specification for multilateral interconnections at Internet Exchange Points (IXPs). Multilateral interconnection is a method of exchanging routing information among three or more External BGP (EBGP) speakers using a single intermediate broker system, referred to as a route server. Route servers are typically used on shared access media networks, such as IXPs, to facilitate simplified interconnection among multiple Internet routers.Ambiguity of Uppercase vs Lowercase in RFC 2119 Key WordsRFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.Dissemination of Flow Specification RulesThis document defines a Border Gateway Protocol Network Layer Reachability Information (BGP NLRI) encoding format that can be used to distribute (intra-domain and inter-domain) traffic Flow Specifications for IPv4 unicast and IPv4 BGP/MPLS VPN services. This allows the routing system to propagate information regarding more specific components of the traffic aggregate defined by an IP destination prefix. It also specifies BGP Extended Community encoding formats, which can be used to propagate Traffic Filtering Actions along with the Flow Specification NLRI. Those Traffic Filtering Actions encode actions a routing system can take if the packet matches the Flow Specification. This document obsoletes both RFC 5575 and RFC 7674.Informative ReferencesDeprecation of AS_SET and AS_CONFED_SET in BGPGoogle, Inc.USA NISTUSA NISTJuniper Networks, Inc. BCP 172 (i.e., RFC 6472) recommends not using AS_SET and
AS_CONFED_SET in the Border Gateway Protocol. This document advances
this recommendation to a standards requirement in BGP; it proscribes
the use of the AS_SET and AS_CONFED_SET types of path segments in the
AS_PATH. This is done to simplify the design and implementation of
BGP and to make the semantics of the originator of a route clearer.
This will also simplify the design, implementation, and deployment of
various BGP security mechanisms. This document (if approved) updates
RFC 4271 and RFC 5065 by eliminating AS_SET and AS_CONFED_SET types,
and obsoletes RFC 6472.
Work in ProgressRecommendation for Not Using AS_SET and AS_CONFED_SET in BGPThis document recommends against the use of the AS_SET and AS_CONFED_SET types of the AS_PATH in BGPv4. This is done to simplify the design and implementation of BGP and to make the semantics of the originator of a route more clear. This will also simplify the design, implementation, and deployment of ongoing work in the Secure Inter-Domain Routing Working Group. This memo documents an Internet Best Current Practice.AcknowledgementsThe authors would like to thank for
his direction on this work as well as ,
, ,
, ,
, ,
and for their review and comments.
Authors' AddressesAT&T200 S. Laurel AveMiddletownNJ07748United States of Americaju1738@att.comCisco7100 Kit Creek RoadResearch Triangle ParkMorrisvilleNC27709United States of Americajalcaide@cisco.comCiscocf@cisco.comCisco111 Wood Ave SouthIselinNJ08830United States of Americadjsmith@cisco.comSproute Networksmpradosh@yahoo.com