Bicone Source Address Validation

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Table of Contents

1. Introduction

Source address spoofing is one of the most serious security threats to today's Internet. It serves as a main attack vector for Distributed Denial of Service (DDoS) attacks and is commonly used in reflective DDoS attacks. To mitigate source address spoofing, many source address validation (SAV) solutions (e.g., BCP38 [RFC2827] and BCP84 [RFC3704] [RFC8704]) have been proposed. The primary design goal of SAV solutions is avoiding improper block (i.e., blocking legitimate traffic) while maintaining directionality, especially in partial deployment scenarios (see [I-D.ietf-savnet-inter-domain-problem-statement] and [RFC8704]). However, previous SAV solutions cannot meet the design goal due to technical limitations (see [I-D.ietf-savnet-intra-domain-problem-statement] and [I-D.ietf-savnet-inter-domain-problem-statement]).¶

Existing advanced SAV solutions (e.g., EFP-uRPF [RFC8704]) typically generate ingress SAV allowlist filters by using information related to customer cone. Specifically, they aim to generate an allowlist on an interface facing a customer or lateral peer AS by identifying prefixes in the customer's or lateral peer AS's customer cone.¶

However, solely using an allowlist may cause legitimate traffic to be blocked if the allowlist fails to cover all prefixes in a customer cone. This document analyzes potential improper block problems of solely using allowlist filters, and proposes Bicone SAV to minimize improper block. Bicone SAV focuses on generating robust ingress SAV filters for an interface facing a customer or lateral peer AS. In addition to applying an allowlist like existing advanced SAV solutions, Bicone SAV applies a blocklist generated by identifying prefixes in the provider cone.¶

The reader is encouraged to be familiar with [RFC8704], [I-D.ietf-sidrops-aspa-profile], [RFC6482], and [I-D.ietf-sidrops-aspa-verification].¶

2. Terminology

Provider cone: the set of ASes an AS can reach by using only customer-to-provider links.¶

3. Improper Block Problems of Solely Using An Allowlist

The basic idea of existing advanced SAV solutions is generating an allowlist by using information related to a customer's (or lateral peer's) customer cone. Specifically, they identify prefixes in a customer's (or lateral peer's) customer cone and only accepts data packets with source addresses belonging to these prefixes on the interface facing that customer (or lateral peer). This is because data packets received from a customer or lateral peer should use source addresses belonging to prefixes in the customer's or lateral peer's customer cone unless there is a route leak [RFC7908].¶

EFP-uRPF (BCP84 [RFC8704]) generates allowlists by using BGP UPDATE messages related to the customer cone. However, if a multi-homed AS in the customer cone limits the propagation of its prefixes, EFP-uRPF may miss these prefixes in the allowlist, resulting in improper block. Section 3.3 of [RFC8704] has given such an example of limited propagation of prefixes where a multi-homed customer AS attaches NO_EXPORT to all prefixes announced to one transit provider. Figure 1 illustrates a similar scenario of limited propagation of prefixes in customer cone. Since AS1 attaches NO_EXPORT to routes for P1 and P2 announced to AS2, routes for P1 and P2 are not propagated on the AS2-AS4 interface. Because AS4 never receives routes for P1 and P2 from its customer AS2, both EFP-uRPF Algorithm A and Algorithm B at AS4 will block legitimate data packets received on AS4-AS2 interface with source addresses in P1 or P2.¶

                           P1[AS5 AS3 AS1]
                           P2[AS5 AS3 AS1]
               +---------+ (P2P/P2C) +---------+
               |   AS4   |<----------+   AS5   |
               +---------+           +---------+
                    /\                 /\
      P1 and P2 not /                  / P1[AS3 AS1]
       propagated  /                  /  P2[AS3 AS1]
            (C2P) /                  /   (C2P)
           +---------+       +---------+
           |   AS2   |       |   AS3   |
           +---------+       +---------+
                 /\           /\
P1[AS1] NO_EXPORT \           / P1[AS1]
P2[AS1] NO_EXPORT  \         /  P2[AS1]
            (C2P)   \       /   (C2P)
                   +---------+
                   |   AS1   |
                   +---------+
                      P1, P2 (prefixes originated)

Some more recent SAV solutions (e.g., BAR-SAV [I-D.ietf-sidrops-bar-sav]) additionally use Autonomous System Provider Authorization (ASPA) [I-D.ietf-sidrops-aspa-profile] and Route Origin Authorization (ROA) [RFC6482] to generate a more robust allowlist. However, since registering ASPAs and ROAs is optional for network operators, ASPAs and ROAs will be partially deployed for a long time. When some ASPAs and ROAs are missing, the SAV solution will still fail to discover all prefixes in a customer's or lateral peer's customer cone, leading to improper block.¶

Overall, considering the complexity of inter-domain routing, existing SAV solutions which solely use an allowlist may fail to identify all prefixes in a customer's (or lateral peer's) customer cone. In this case, the generated allowlist may result in improper block. To minimize improper block, Bicone SAV additionally generates a blocklist on an interface facing a customer or lateral peer by using BGP UPDATE messages, ASPAs, and ROAs that are related to the provider cone.¶

In the following, Section 4 describes how to generate the blocklist and Section 5 describes how to perform more robust ingress SAV filtering by using both allowlist and blocklist filters.¶

4. Generating A Blocklist Using Information Related to Provider Cone

Bicone SAV enhances the robustness of SAV through using a blocklist on an interface facing a customer or lateral peer. The provider cone of an AS is defined as the set of ASes an AS can reach by using only customer-to-provider links. Considering prefixes associated with ASes in the provider cone should not be used as source addresses in data packets received from any customer or lateral peer AS unless there is a route leak [RFC7908]. The blocklist should contain prefixes belonging to the provider cone.¶

To generate a blocklist, an AS first identifies ASes in its provider cone by using ASPAs and AS-PATH in BGP UPDATE messages. Then, it discovers prefixes belonging to these ASes in provider cone and includes those prefixes in the blocklist. Data packets received from customers or lateral peers with source addresses in the blocklist should be blocked.¶

Note that if an AS cannot identify all ASes in the provider cone when ASPAs are partially deployed, the generated blocklist will not include all prefixes in the provider cone. In this case, the generated blocklist can still block spoofing traffic received from customers and lateral peers with source addresses in the blocklist. Therefore, Bicone SAV provides immediate incremental benefits to early adopters.¶

A detailed description of blocklist generation procedure is as follows:¶

1. Create the set of all directly connected Provider ASNs. Call it AS-set Z(1).¶

2. Create the set of all unique AS_PATHs in Adj-RIBs-In of all interfaces facing Providers.¶

3. For each unique AS_PATH with N (N>1) ASNs, i.e., [ASN_{1}, ASN_{2}, ..., ASN_{i}, ASN_{i+1}, ..., ASN_{N}] where ASN_{i} is the ith ASN in AS_PATH and the first ASN (i.e., ASN_{1}) is a directly connected Provider ASN. If all unique AS_PATHs have been processed, go to Step 8.¶

4. Let i = N¶

5. Decrement i to i-1.¶

6. If ASN_{i} authorizes ASN_{i+1} as a Provider in ASN_{i}'s ASPA, ASNs from ASN_{1} to ASN_{i+1} (i.e., ASN_{1}, ASN_{2}, ..., ASN_{i}, and ASN_{i+1}) are included in AS-set Z(1) and go to Step 3.¶

7. If i == 1, go to Step 3. Else, go to Step 5.¶

8. Let k = 1.¶

9. Increment k to k+1.¶

10. Create AS-set Z(k) of ASNs that are not in AS-set Z(k-1) but are authorized as Providers in ASPAs of any ASN in AS-set Z(k-1).¶

11. If AS-set Z(k) is null, then set k_max = k-1 and go to Step 12. Else, form the union of AS-set Z(k) and AS-set Z(k-1) as AS-set Z(k) and go to Step 9.¶

12. Select all ROAs in which the authorized origin ASN is in AS-set Z(k_max). Form the union of the sets of prefixes in the selected ROAs. Call it Prefix-set P1.¶

13. Using the routes in Adj-RIBs-In of all interfaces facing Providers, create a set of prefixes originated by any ASN in AS-set Z(k_max). Call it Prefix-set P2.¶

14. Form the union of Prefix-set P1 and Prefix-set P2. Apply this union set as a blocklist on every interface facing a Customer or Lateral Peer.¶

5. Using Both Allowlist and Blocklist Filters

Although using a blocklist helps reduce improper block when an allowlist does not include all prefixes in a customer's or lateral peer's customer cone, it may also block legitimate traffic in the CDN and DSR scenario as existing SAV solutions do (see [I-D.ietf-savnet-inter-domain-problem-statement] and [I-D.ietf-sidrops-bar-sav]). To minimize improper block, it is recommended to use both allowlist and blocklist filters. The allowlist is generated by discovering as many prefixes in the customer's or lateral peer's customer cone as possible (see [RFC8704] and [I-D.ietf-sidrops-bar-sav]) and the blocklist is generated by discovering as many prefixes in the provider cone as possible (see Section 4).¶

A detailed description of SAV procedure is as follows:¶

1. Check if source addresses of data packets received from a customer or lateral peer are included in the corresponding allowlist. If so, these data packets are accepted. If not, go to Step 2.¶

2. Check if source addresses of data packets received from a customer or lateral peer are included in the corresponding blocklist. If so, these data packets are blocked. If not, these data packets should be accepted to avoid improper block.¶

If an AS can make sure the generated allowlist covers all prefixes in a customer's or lateral peer's customer cone, it does not need to use a blocklist on that corresponding interface and can only accept data packets received on that interface with source addresses in the allowlist.¶

6. Security Considerations

The security considerations described in [RFC8704], [I-D.ietf-sidrops-bar-sav], [I-D.ietf-sidrops-aspa-profile], [RFC6482], and [I-D.ietf-sidrops-aspa-verification] also applies to this document.¶

7. IANA Considerations

This document has no IANA requirements.¶

Authors' Addresses