Revised Validation Procedure for BGP Flow Specifications
draft-ietf-idr-bgp-flowspec-oid-11

Abstract

This document describes a modification to the validation procedure defined in [RFC5575bis] for the dissemination of BGP Flow Specifications. [RFC5575bis] 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. This allows only BGP speakers within the data forwarding path (such as autonomous system border routers) to originate BGP Flow Specifications. Though it is possible to disseminate such Flow Specifications directly from border routers, it may be operationally cumbersome in an autonomous system with a large number of border routers having complex BGP policies. The modification proposed herein enables Flow Specifications to be originated from a centralized BGP route controller.

Status of This Memo

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Table of Contents

• 1. Requirements Language
• 2. Introduction
• 3. Motivation
• 4. Revised Validation Procedure
• 4.1. Revision of Route Feasibility
• 4.2. Revision of AS_PATH Validation
• 5. Other RFC5575bis Considerations
• 6. Topology Considerations
• 7. IANA Considerations
• 8. Security Considerations
• 9. Acknowledgements
• 10. Normative References
• Authors' Addresses

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.

2. Introduction

[RFC5575bis] defined a new BGP [RFC4271] capability that can be used to distribute traffic Flow Specifications amongst BGP speakers in support of traffic filtering. The primary intention of [RFC5575bis] 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. [RFC5575bis] 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 [RFC5575bis] 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. This allows only BGP speakers within the data forwarding path (such as autonomous system border routers) to originate BGP Flow Specification NLRIs. Though it is possible to disseminate such Flow Specification NLRIs directly from border routers, it may be operationally cumbersome in an autonomous system with a large number of border routers having complex BGP policies.

This document describes a modification to the [RFC5575bis] 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.

3. Motivation

Step (b) of the validation procedure in [RFC5575bis], 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 ASN1 in figure 1) matches the eBGP neighbor that advertised the longest match destination prefix (see RTR-F and RTR-G respectively in figure 1). Similarly, at the ingress border routers of ASN (see RTR-A and RTR-B of ASN1 in figure 1), 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 1). This is true even when ingress border routers select paths from different egress border routers as best path based upon IGP distance. For example, in figure 1:

• RTR-A chooses RTR-D's path as best
• RTR-B chooses RTR-E as the best path

                  / - - - - - - - - - - - - -  -
                  |           ASN1              |
                    +-------+        +-------+
                  | |       |        |       |  |
                    | RTR-A |        | RTR-B |
                  | |       |        |       |  |
                    +-------+        +-------+
                  |       \           /         |
                     iBGP  \         / iBGP
                  |         \       /           |
                            +-------+
                  |         |       |           |
                            | RTR-C |
                  |         |  RC   |           |
                            +-------+
                  |           /   \             |
                             /     \
                  |   iBGP  /       \ iBGP      |
                    +-------+        +-------+
                  | | RTR-D |        | RTR-E |  |
                    |       |        |       |
                  | |       |        |       |  |
                    +-------+        +-------+
                  |     |                 |     |
                   - - -|- - - - - - - - -|- - -/
                        | eBGP       eBGP |
                   - - -|- - - - - - - - -|- - -/
                  |     |                 |     |
                    +-------+        +-------+
                  | |       |        |       |  |
                    | RTR-F |        | RTR-G |
                  | |       |        |       |  |
                    +-------+        +-------+
                  |           ASN2              |
                  / - - - - - - - - - - - - -  -
	 

Figure 1

It is highly desirable that the mechanisms exist to protect each ASN independently from network security attacks using the BGP Flow Specification NLRI for intra-domain purposes only. Network operators often deploy a dedicated Security Operations Center (SOC) within their ASN to monitor and detect such security attacks. To mitigate attacks within a domain (AS or group of ASes), operators require the ability to originate intra-domain 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 ASN with specific attack mitigation actions (drop the traffic, forward to a clean-pipe center, etc.).

To originate a Flow Specification NLRI, a central BGP route controller must set itself as the originator in the Flow Specification NLRI. This is necessary given the route controller is originating the Flow Specification rather than reflecting it, and to avoid the complexity of having to determine the egress border router whose path was chosen as the best in each of the ingress border routers. Thus, it is necessary to modify step (b) of the [RFC5575bis] validation procedure 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 [RFC5575bis], section 6 is redefined as follows:

1. 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 does not contain AS_SET and/or AS_SEQUENCE segments.
      1. This condition SHOULD be enabled by default. This default behavior should validate an empty AS_PATH.
      2. This condition MAY be disabled by configuration on a BGP speaker.
      3. As an exception to this rule, a given AS_PATH with AS_SET and/or AS_SEQUENCE segments MAY be validated by policy.

Explanation:

• In this context, an empty AS_PATH means that it does not have AS_SET and/or AS_SEQUENCE segments, and local domain means the local AS [RFC4271] or the local confederation of ASes (in the case that the local AS belongs to a confederation of ASes [RFC5065]). Thus, receiving a Flow Specification with an empty AS_PATH indicates that the Flow Specification was originated inside the local domain.
• With the above modification to the [RFC5575bis] 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 (b.2.2) may be a good practice when the operator knows with certainty that there is not a Flow Specification originated inside the local domain.
• Also, policy may be useful to validate a specific set of non-empty AS_PATHs (b.2.3). For example, it could validate a Flow Specification whose AS_PATH contains only an AS_SEQUENCE with ASes that are all known to belong to the same administrative domain.

4.2. Revision of AS_PATH Validation

[RFC5575bis] 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. Therefore, this document also redefines the [RFC5575bis] 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 the 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 last ASes added is sufficient for eBGP learned Flow Specification NLRIs. Requiring a full AS_PATH match would limit origination of inter-domain Flow Specifications to the origin AS of the best-match unicast route for the destination prefix embedded in the Flow Specification only. As such, a full AS_PATH validity check may prevent transit ASes from originating inter-domain Flow Specifications, which is not desirable.
• Redefinition of this AS_PATH validation rule for a Flow Specification does not mean that the original rule in [RFC5575bis] cannot be enforced as well. Its enforcement remains optional per [RFC4271] section 6.3. That is, we can enforce the first AS in the AS_PATH to be the same as the neighbor AS for any address-family route (including a Flow Specification).
• Using the new rule to validate a Flow Specification received from an Internal Border Gateway Protocol (iBGP) peer is out of the scope of this document. Note that in most scenarios such validation would be redundant.
• Using the new rule to validate a Flow Specification route received from an External Border Gateway Protocol (eBGP) peer belonging to the same local domain (in the case that the local AS belongs to a confederation of ASes) is out of the scope of this document. Note that although it's possible, its utility is dubious.

5. Other RFC5575bis Considerations

This section clarifies some of the terminology and rules referenced in [RFC5575bis]. Namely:

• In the context of this document and [RFC5575bis], AS_PATH attribute is defined as the reconstructed AS path information (by combining AS_PATH and AS4_PATH attributes, if the BGP speaker is a NEW speaker and receives the route from an OLD speaker), according to section 4.2.3 of [RFC6793].

• Support for two-octet AS only implementations is out of the scope of this document (i.e. it's assumed that the BGP speaker supports [RFC6793]).

6. Topology Considerations

[RFC5575bis] 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 we received the update. If the latter applies, a network should be designed so it has a congruent topology.

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 without the second condition (b.2) being added to the validation procedure:
  1. Consider a topology with two BGP speakers with two 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 topology with a BGP speaker within a confederation of ASes, inside local AS X. ORIGINATOR_ID attribute is not advertised within the local domain. Let's assume the Flow Specification route is received from peer A and the best-match unicast route is received from peer B. Both peers belong in local AS Y. Both AS X and AS Y belong to the same local domain. The Flow Specification validation procedure will also fail because of the first condition (b.1).

• In the examples above, if Flow Specifications are originated in the same local domain, AS_PATH will not contain AS_SET and/or AS_SEQUENCE segments. When the second condition (b.2) in the validation procedure is used, the validation procedure will pass. Thus, non-congruent topologies are supported if the Flow Specification is originated in the same local domain.
• Even when the second condition (b.2) is used in the validation procedure, a Flow Specification originated in a different local domain needs a congruent topology. AS_SEQUENCE is not empty and the first condition (b.1) in the validation procedure needs to be evaluated. Because transport addresses for Flow Specification and unicast routes are different, the validation procedure will fail. This is true both across domains and within domains. Consider both cases:
  • Consider the first example. If the Flow Specification route is originated in another AS, the validation procedure will fail because the topology is non-congruent within the domain.
  • Consider the second example and modify it so AS X and AS Y belong to different local domains (no confederation of ASes exists). The validation procedure will fail because the topology is non-congruent across domains.

7. IANA Considerations

This memo includes no request to IANA.

8. Security Considerations

No new security issues are introduced by relaxing the validation procedure for IBGP learned Flow Specifications. With this proposal, the security characteristics of BGP Flow Specifications remain equivalent to the existing security properties of BGP unicast routing.

BGP updates learned from iBGP peers are trusted so the Traffic Flow Specifications contained in BGP updates are 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. This proposal continues to enforce the validation Procedure for eBGP learned Traffic Flow Specifications, as per [RFC5575bis] rules. In this way, the security properties of [RFC5575bis] are maintained such that an EBGP peer cannot cause a denial-of-service attack by advertising an inter-domain Flow Specification for a destination prefix that it does not provide reachability information for.

9. 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 and Shyam Sethuram for their review comments.

10. Normative References

Authors' Addresses