Source Address Validation Using BGP UPDATEs, ASPA, and ROA (BAR-SAV)

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Table of Contents

2. Same Procedure Applies to Customers and Lateral Peers

The same procedure applies for the construction of a permissible ingress SAV filter for a customer or lateral peer interface. Customers and lateral peers should only transmit data packets with source addresses belonging to only the prefixes that are authorized to be used by the ASes in their respective customer cones (CC). The CC includes the AS belonging to the customer or lateral peer.¶

3. SAV Using ASPA and ROA (Procedure X)

The method/procedure (called Procedure X) described in this section is for future scenarios when ASPA and ROA adoption is ubiquitous. In that scenario, robust SAV filters can be generated from the RPKI information (ASPA and ROA data) alone. The procedure is applicable for ingress SAV filter design for customer and lateral peer interfaces. An ISP may use Procedure X on customer interfaces if it requires all its customers to register ROAs and ASPAs.¶

A description of Procedure X (one that makes use of only ASPA and ROA data):¶

• Step A: Compute the set of ASNs in the Customer's or Lateral Peer's customer cone using ASPA data.¶
• Step B: Compute from ROA data the set of unique prefixes authorized to be announced by the ASNs found in Step A. Keep only the unique prefixes. This set is the permissible prefix list for SAV for the interface in consideration.¶

A detailed description of Procedure X is as follows:¶

1. Let the Customer or Lateral Peer ASN be denoted as AS-k.¶
2. Let i = 1. Initialize: AS-set S(1) = {AS-k}.¶
3. Increment i to i+1.¶
4. Create AS-set S(i) of all ASNs whose ASPA data declares at least one ASN in AS-set S(i-1) as a Provider.¶
5. If AS-set S(i) is null, then set i_max = i - 1 and go to Step 6. Else, go to Step 3.¶
6. Form the union of the sets, S(i), i = 1, 2, ..., i_max, and name this union as AS-set A.¶
7. Select all ROAs in which the authorized origin ASN is equal to any ASN in AS-set A. Form the union of the sets of prefixes listed in the selected ROAs. Name this union set of prefixes as P-set.¶
8. Apply P-set as the list of permissible prefixes for SAV.¶

4. SAV using BGP UPDATE Messages, ASPA, and ROA (BAR-SAV)

SAV using BGP UPDATE Messages, ASPA, and ROA (BAR-SAV) is described in this section and is meant for the period when there is a partial deployment of ROAs and ASPAs. To compensate for incomplete RPKI information, BAR-SAV augments ASPA data with BGP UPDATE AS_PATH data for discovering CC ASes, and it augments ROA data with BGP UPDATE data for discovering all prefixes associated with ASes in the CC. The details of this procedure are described below.¶

BAR-SAV additionally incorporates a refined version of Algorithm A of EFP-uRPF (Section 3.1 of [RFC8704]). Algorithm A in [RFC8704] picked only the originating ASes from AS_PATHs received on the customer or lateral peer interface in consideration and included them for SAV filter computation. The variant of Algorithm A in [RFC8704] used here includes all ASes in the AS_PATHs for the SAV filter computation. Unless there is a route leak [RFC7908], each AS is a customer of the AS added next in AS_PATHs of BGP UPDATE messages received from a customer or lateral peer. Further customer-provider AS relations within the CC are discovered by examining all unique ASes in the AS_PATHs in BGP UPDATEs received on all interfaces (from transit providers, customers, lateral peers, and IBGP peers). This is described in the step-by-step procedure later in this section.¶

Note that if a multi-homed AS is present in an above-mentioned AS_PATH and did not originate any prefix in the CC in consideration but originated a prefix into an overlapping neighboring CC, then the AS and prefix will still be detected and included in the design of the SAV filter. This improves the accuracy of the SAV filter in the BAR-SAV method in comparison to Algorithm A in [RFC8704].¶

One should not compute a customer cone by separately processing ASPA data and AS_PATH data and then merging the two sets of ASes at the end. Doing so is likely to miss ASes from the customer cone. Instead, both ASPAs and AS_PATHs should be used to iteratively expand the discovered customer cone. When new ASes are discovered, both ASPA and AS_PATH data should be used to discover customers of those ASes. This process is repeated for newly discovered customer ASes until there are no new ASes to be found.¶

If a transit provider-to-customer relationship, e.g., from AS X to AS Y, is deduced from AS_PATH data but the ASPA data contradicts it (i.e., AS Y has ASPA and it does not include AS X as a transit provider), then the ASPA data prevails, and AS Y must not be considered to be a customer of X. This design principle is reflected in Step 5 of the procedure described below. (Please see discussion about route leaks in Section 7.)¶

A detailed description of the BAR-SAV procedure is as follows:¶

1. Let the Customer or Lateral Peer ASN be denoted as AS-k.¶
2. Let i = 1. Initialize: AS-set Z(1) = {AS-k}.¶
3. Increment i to i+1.¶
4. Create AS-set A(i) of all ASNs whose ASPA data declares at least one ASN in AS-set Z(i-1) as a Provider.¶
5. Create AS-set B(i) of all "non-ASPA" customer ASNs each of which is a customer of at least one ASN in AS-set Z(i-1) according to unique AS_PATHs in Adj-RIBs-In [RFC4271] of all interfaces at the BGP speaker computing the SAV filter. "Non-ASPA" ASN are ASNs that declare no provider in ASPA data.¶
6. Form the union of AS-sets A(i) and B(i) and call it AS-set C. From AS-set C, remove any ASNs that are present in Z(j), for j=1 to j=(i-1). Call the resulting set Z(i).¶
7. If AS-set Z(i) is null, then set i_max = i - 1 and go to Step 8. Else, go to Step 3.¶
8. Form the union of the AS-sets, Z(i), i = 1, 2, ..., i_max, and name this union as AS-set D.¶
9. Select all ROAs in which the authorized origin ASN is in AS-set D. Form the union of the sets of prefixes listed in the selected ROAs. Name this union set of prefixes as Prefix-set P1.¶
10. Using the routes in Adj-RIBs-In of all interfaces, create a list of all prefixes originated by any ASN in AS-set D. Name this set of prefixes as Prefix-set P2.¶
11. Form the union of Prefix-sets P1 and P2. Apply this union set as the list of permissible prefixes for SAV.¶

5. Operational Recommendations

Network operators SHOULD implement the BAR-SAV method (Section 4) for computing the permissible ingress prefix list for SAV on interfaces facing customers and lateral peers. BAR-SAV offers immediate incremental benefits to early adopters.¶

The operational recommendations provided in Section 3.2 of [RFC8704] are applicable and helpful for BAR-SAV (Section 4). Since Procedure X (Section 3) and the BAR-SAV procedure (Section 4) benefit from the registration of ROAs, network operators are RECOMMENDED to register ROAs and enable ROV in their ASes. When ASPA becomes available, network operators are also RECOMMENDED to register ASPAs at that time.¶

The registration of ROAs and ASPAs helps with the detection and inclusion of otherwise hidden prefixes in the permissible list for SAV. As mentioned earlier, prefixes hidden in other techniques often arise from the use of multi-homing in conjunction with limited propagation of prefixes in a given CC (for example, by attaching NO_EXPORT to all prefixes announced from a customer AS to a transit provider AS). In these situations, the registration of ASPAs helps improve the accuracy of SAV.¶

                 +----------+   P3[AS5 AS3]  +------------+
                 |    AS4   |<---------------|     AS5    |
                 +----------+      (P2P)     +------------+
                     /\   /\                        /\
                     /     \                         \
             P1[AS1]/       \P2[AS2]                  \P3[AS3]
              (C2P)/         \(C2P)                    \(C2P)
                  /           \                         \
           +----------+    +----------+           +----------+
           |  AS1 (P1)|    | AS2 (P2) |           | AS3 (P3) |
           +-----+----+    +----+-----+           +-----+----+
                 +              +                       +
               User       Edge Server (DSR)      Anycast Server

           Consider AS4 generating its SAV list
           CDN's ROAs: {P3 AS3}, {P3, AS2}, {P2, AS2}
           AS2 should not/does not announce P3
           With the SAV methods in this document,
             AS4 correctly includes P2 and P3 in its SAV list

6. IANA Considerations

This document includes no request to IANA.¶

7. Security Considerations

The security considerations described in [RFC8704], [RFC6811], and [I-D.ietf-sidrops-aspa-profile] also apply to this document.¶

The security and robustness of BAR-SAV are strengthened by supporting mechanisms for detecting and dropping routes that are misoriginations or leaks. It is advised that the BGP UPDATEs received at BGP speakers are vetted using ROV (using ROAs and/or trusted IRR route objects) and prefix filtering (see [RFC6811] [RFC7454] [NIST-800-189]). It is also advised that one or more of the available methods to prevent, detect, and mitigate route leaks are also deployed (e.g., [RFC9234] [I-D.ietf-grow-route-leak-detection-mitigation] [I-D.ietf-sidrops-aspa-verification] [sriram1] [sriram2]).¶

Acknowledgements

The authors would like to thank Oliver Borchert for comments and discussion.¶

Authors' Addresses