| Internet-Draft | Revised Flowspec Validation Procedure | April 2021 |
| Uttaro, et al. | Expires 14 October 2021 | [Page] |
This document describes a modification to the validation procedure defined for the dissemination of BGP Flow Specifications. The dissemination of BGP Flow Specifications requires that the originator of the Flow Specification matches the originator of the best-match unicast route for the destination prefix embedded in the Flow Specification. For an iBGP received route, the originator is typically a border router within the same autonomous system. 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 any place within the autonomous system itself, for example, from a centralized BGP route controller. However, the validation procedure 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 AS_PATH validation rules so Flow Specifications received from an eBGP peer can be validated when such peer is a BGP route server.¶
This document updates the validation procedure in RFC8955.¶
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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.¶
[RFC8955] defines a BGP NLRI [RFC4271] 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 system(s) avoid carrying the traffic to the downstream autonomous system only to be discarded. [RFC8955] also enables more granular traffic filtering based upon upper layer protocol information (e.g., protocol port numbers) as opposed to coarse IP destination prefix-based filtering. Flow specification NLRIs received from a BGP peer are 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 matches the originator of the best-match unicast route for the destination prefix embedded in the Flow Specification. The aim is making 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 autonomous system than the speaker performing the validation (for example by a centralized BGP route controller), and the best-match unicast route is originated in another autonomous system. In order for validation to succeed for a Flow Specification received from an iBGP peer, it could be possible to disseminate such Flow Specification NLRIs directly from the specific border router (within the local autonomous system) that is advertising the corresponding best-match unicast route to the local autonomous system. This approach would be, however, operationally cumbersome in an autonomous system with a large number of border routers having complex BGP policies.¶
Figure 1 illustrates this principle. R1 (the upstream router) and RR 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. Sometimes the Flow Specification needs to be originated on AS1. ASBR1 could originate it, and 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 rules on each ASBR in AS1 based on which ASBR each dest- route is learned from. 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 [RFC8955] validation procedure allowing Flow Specification NLRIs to be originated from a centralized BGP route controller within the local autonomous system that is not 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 an autonomous system which has a large number of 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 an autonomous system.¶
Step (b) of the validation procedure in [RFC8955], section 6 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 best route based upon IGP distance. For example, in Figure 2:¶
/ - - - - - - - - - - - - - -
| 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 clean-pipe center, etc.).¶
In addition, an operator MAY extend the requirements above for a group of ASes via policy. This is described in section (b.2.3) of the validation procedure.¶
A central BGP route controller that originates a 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 a Flow Specification NLRI, the rest of the speakers within the AS will see the BGP 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 [RFC8955] validation procedure such that an iBGP peer that is not within the data forwarding plane may originate Flow Specification NLRIs.¶
Step (b) of the validation procedure specified in [RFC8955], section 6 is redefined as follows:¶
One of the following conditions MUST hold true:¶
The AS_PATH attribute of the Flow Specification is empty or contains only AS_CONFED_SEQUENCE and/or AS_CONFED_SET segments [RFC5065].¶
Explanation:¶
[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.¶
This rule prevents the exchange of BGP Flow Specification NLRIs at Internet exchanges with BGP route servers [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:¶
[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 ipv4 unicast routes and Flow Specification routes. By congruent topology, it is understood that for the two equivalent routes (i.e. the Flow Specification route and its best-match unicast route) are learned from the same peer accross the AS. That would likely not be true, for instance, if some peers only negotiated one type of address-family or if each address-family had a different set of policies.¶
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 validation procedure preceding this document. The second condition (b.2) does not exist. The two following scenarios have a non-congruent topology:¶
This memo includes no request to IANA.¶
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 in [rfc8955] and thus maintain security characteristics equivalent to the existing security properties of BGP unicast routing.¶
The security considerations discussed in [RFC8955] apply to this specification as well.¶
The original AS_PATH validation rule ([RFC4271] section 6.3) becomes hereby optional (section 4.2). If that original rule is actually not enforced it may introduce some security risks. A peer (or a client of a route server peer) in AS X 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). This risk is impossible to prevent if the Flow Specification route is received from a route server peer. If that peer is known for a fact not to be a route server, that optional rule SHOULD be enforced for Flow Specification routes.¶
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 flow destination prefix. Note that this trustworthy consideration is not absolute and the new possibility than an iBGP speaker could send a rouge Flow Specification is introduced.¶
The changes in Section 4.1 don't affect the validation procedures for eBGP-learned routes.¶
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 and Shyam Sethuram for their review comments.¶