Source Address Validation in Inter-domain Networks (Inter-domain SAVNET) Gap Analysis, Problem Statement, and Requirements

Source address validation in inter-domain networks (Inter-domain SAVNET) is vital to mitigate source address spoofing between Autonomous Systems (ASes). Inter-domain SAV is essential to the Internet security . Many efforts have been taken on the tasks of inter-domain SAV. Ingress filtering is a typical method of inter-domain SAV. Strict uRPF reversely looks up the FIB table and requires that the valid incoming interface must be the same interface which would be used to forward traffic to the source address in the FIB table. Feasible-path uRPF (FP-uRPF) , taking a looser SAV than strict uRPF, is designed to add more alternative valid incoming interfaces for the source address. To be more flexible about directionality, BCP 84 recommends that i) the loose uRPF method which loses directionality completely SHOULD be applied on lateral peer and transit provider interfaces, and that ii) the Enhanced FP-uRPF (EFP-uRPF) method with Algorithm B, looser than strict uRPF, FP-uRPF, and EFP-uRPF with Algorithm A, SHOULD be applied on customer interfaces. Routers deploying EFP-uRPF accept a data packet from customer interfaces only when the source address of the packet is contained in that of the customer cone. Despite the diversity of inter-domain SAV mechanisms, there are still some points that are under considered but important for enhancing Internet security. Moreover, in the currently focused SAV work scope, these mechanisms may lead to improper permit or improper block problems in some scenarios. This document does an analysis of the existing inter-domain SAV mechanisms and answers: i) what are the technical gaps, ii) what are the major problems needing to be solved, and iii) what are the potential directions for further enhancing inter-domain SAV.

SAV: Source Address Validation, i.e., validating the authenticity of a packet's source IP address. SAV rule: The rule generated by intra-domain SAV mechanisms that determines valid incoming interfaces for a specific source prefix. SAV table: The data structure that stores SAV rules on the data plane. The router queries its local SAV table to validate the authenticity of source addresses. Improper block: The packets with legitimate source IP addresses are blocked improperly due to inaccurate SAV rules. Improper permit: The packets with spoofed source IP addresses are permitted improperly due to inaccurate SAV rules.

BCP 84 recommends loose uRPF at provider/peer interfaces and EFP-uRPF at customer interfaces. The followings are the gap analysis of these SAV mechanisms.

Existing SAV mechanisms may have improper permit problems that the packets with spoofed source addresses are considered as legal. Here are two cases where improper permit will appear.

The first case is at provider or peer interfaces where loose uRPF is deployed. Loose uRPF almost accepts any source address, which fails to protect ASes in the customer cone from externally injected attacks.

Figure 1 shows a reflection attack scenario. AS 3 is the provider of AS 4. AS 4 is the provider of AS 1 and AS 2. EFP-uRPF is deployed at AS 4's customer interfaces, and loose uRPF is implemented at AS 4's provider interface. Assume a reflection attacker is attached to AS 3. It sends packets spoofing source addresses of P1 to the server located in AS 2 for attacking the victim in AS 1. However, this attack cannot be successfully blocked though AS 4 has deployed inter-domain SAV.

The second case is at customer interfaces where EFP-uRPF with algorithm B is deployed. It allows packets with source addresses of the customer cone to enter from any customer interfaces to avoid potential improper block problems that EFP-uRPF with algorithm A may have. However, vulnerability is imported. Although EFP-uRPF with algorithm B can prevent ASes inside the customer cone from using source addresses of ASes outside the customer cone, it sacrifices the directionality of traffic from different customers, which will lead to improper permit problems.

In Figure 2, assume AS 4 implements EFP-uRPF with algorithm B at customer interfaces. Under EFP-uRPF with algorithm B, AS 4 will generate SAV rules with legitimate P1 and P2 at both customer interfaces. When the attacker in AS 2 spoofs source address of AS 1, AS 4 will improperly permit these packets with spoofed source addresses of prefix P1. The same also applies when the attacker in AS 1 forges source prefix P2. That is to say, EFP-uRPF algorithm B cannot prevent source address spoofing between ASes of the customer cone.

In some cases, existing SAV mechanisms may improperly block the packets with legitimate source addresses. Here are two cases where improper permit will appear.

Figure 3 presents a NO_EXPORT scenario. AS 1 is the common customer of AS 2 and AS 3. AS 4 is the provider of AS 2. The relationship between AS 3 and AS 4 is customer-to-provider (C2P) or peer-to-peer (P2P). All arrows in Figure 2 represent BGP advertisements. AS 2 owns prefix P2 and advertises it to AS 4 through BGP. AS 3 also advertises its own prefix P3 to AS 4. AS 1 has prefix P1 and advertises the prefix to the providers, i.e., AS 2 and AS 3. After receiving the route for prefix P1 from AS 1, AS 3 propagates this route to AS 4. Differently, AS 2 does not propagate the route for prefix P1 to AS 4, since AS 1 adds the NO_EXPORT community attribute in the BGP advertisement destined to AS 2. In the end, AS 4 only learns the route for prefix P1 from AS 3. Assume that AS 3 is the customer of AS 4. If AS 4 runs EFP-uRPF with algorithm A at customer interfaces, the packets with source addresses of P1 are required to arrive only from AS 3. When AS 1 sends the packets with legitimate source addresses of prefix P1 to AS 4 through AS 2, AS 4 will improperly block these packets. EFP-uRPF with algorithm B works well in this case. Assume that AS 3 is the peer of AS 4. AS 4 will never learn the route of P1 from its customer interfaces. So, no matter EFP-uRPF with algorithm A or that with algorithm B are used by AS 4, there will be improper block problems. Besides the NO_EXPORT case above, there are also many route filtering policies that can result in interruption of BGP advertisement. Improper block may be induced by existing inter-domain SAV mechanisms in such cases, and it is hard to prevent networks from taking these configurations.

Anycast is a network addressing and routing methodology. An anycast IP address is shared by devices in multiple locations, and incoming requests are routed to the location closest to the sender. Therefore, anycast is widely used in Content Delivery Network (CDN) to improve the quality of service by bringing the content to the user as soon as possible. In practice, anycast IP addresses are usually announced only from some locations with a lot of connectivity. Upon receiving incoming requests from users, requests are then tunneled to the edge locations where the content is. Subsequently, the edge locations do direct server return (DSR), i.e., directly sending the content to the users. To ensure that DSR works, servers in edge locations must send response packets with anycast IP address as the source address. However, since edge locations never advertise the anycast prefixes through BGP, an intermediate AS with EFP-uRPF may improperly block these response packets.

Figure 4 shows a specific DSR scenario. The anycast IP prefix (i.e., prefix P3) is only advertised from AS 3 through BGP. Assume AS 3 is the provider of AS 4. AS 4 is the provider of AS 1 and AS 2. When users in AS 1 send requests to the anycast destination IP, the forwarding path from users to anycast servers is AS 1 -> AS 4 -> AS 3. Anycast servers in AS 3 receive the requests and then tunnel them to the edge servers in AS 2. Finally, the edge servers send the content to the users with source addresses of prefix P3. The reverse forwarding path is AS 2 -> AS 4 -> AS 1. Since AS 4 never receives routing information for prefix P3 from AS 2, EFP-uRPF algorithm A/EFP-uRPF algorithm B or other existing uRPF-like mechanisms at AS 4 will improperly block the response packets from AS 2.

Existing SAV mechanisms validate upstream (i.e., the packets from customers to providers) strictly but take a loose validation for downstream (i.e., the packets from providers/peers to customers). Even an AS as well as its provider deploy BCP 84, the AS may still suffer source address spoofing attacks. In particular, the victim network in a reflection attack cannot gain additional protection by deploying existing SAV mechanisms. For example, in the reflection attack scenario in Figure 1, even if the victim network (i.e. AS1) and its provider (i.e. AS4) deploy SAV, AS1 is still vulnerable to the reflection attack from AS3, because it cannot help AS4 accurately identify and reject the forged request from AS3. The scenario in Figure 2 is another example. AS 4 deploying EFP-uRPF with algorithm B cannot prevent source address spoofing within the same customer cone. Technical limitations induce inaccurate SAV (especially improper permit) and further degrade the incentive of deployers. A detailed discussion about misaligned incentive can be found in .

Existing inter-domain SAV mechanisms have accuracy gaps in some scenarios like routing asymmetry induced by BGP route policies or configurations. Particularly, EFP-uRPF takes the RPF list in data-plane, which means the packets from customer interfaces with unknown source prefixes (not appear in the RPF list) will be discarded directly. Improper block issues will arise when legitimate source prefixes are not accurately learned by EFP-uRPF. The root cause is that these mechanisms leverage local RIB table of routers to learn the source addresses and determine the valid incoming interface, which may not match the real data-plane forwarding path from the source. It may mistakenly consider a valid incoming interface as invalid, resulting in improper block problems; or consider an invalid incoming interface as valid, resulting in improper permit problems. Essentially, it is impossible to generate an accurate SAV table solely based on the router's local information due to the existence of asymmetric routes.

Existing inter-domain SAV mechanisms pay more attention to upstream (traffic from customer to provider/peer), resulting in weak source address checking of downstream (traffic from provider/peer to customer). It only prevents customer cone from originating spoofed traffic, but does not protect customer cone from receiving spoofed traffic from outside customer cone. Therefore, the "strict upstream but weak downstream checking" makes the benefits of deploying SAV mainly flow to ASes outside the customer cone. Moreover, the AS who deploys SAV is still vulnerable to reflection attack, because the victim with SAV deployment does not participate in protecting its source addresses from being forged.

Inter-domain SAVNET focuses on narrowing the technical gaps of existing inter-domain SAV mechanisms. The new inter-domain SAV mechanism MUST satisfy the following requirements.

The new inter-domain SAV mechanism MUST ensure accurate SAV, i.e., avoiding improper block problems while trying best to reduce improper permit problems. Generating SAV rules solely depending on local routing information (e.g., RIB) results in inaccurate SAV in the case of asymmetric routing. Accurate SAV requires that SAV rules MUST match real data-plane paths. Therefore, to ensure the accuracy of SAV, extra information out of local routing information is required as a supplement.

The new inter-domain SAV mechanism MUST provide direct incentives. It would be attractive if the network who deploys SAV can protect itself from source address spoofing attacks (espacially reflection attacks) instead of only providing protection to others. In particular, the mechanism MUST work for both upstream and downstream traffic, and downstream SHOULD be under the same SAV criteria as upstream. It would be easy to achieve perfect all-round protection supposing SAV is fully deployed. But when some ASes do not deploy SAV, efforts are needed to narrow the gaps as much as possible.

The new inter-domain SAV mechanism MUST provide protection for source addresses even the mechanism is partially deployed. It is impractical to assume that all the ASes or most of the ASes enable SAV simultaneously. Partial deployment or incremental deployment have to be considered during the work of SAV.

The new inter-domain SAV mechanism MUST not induce much overhead. Any improvement designs for SAV mechanisms MUST not overload control plane. Besides, data plane MUST not modify packets for simplifying the process of data plane, which keeps same as existing SAV mechanisms.

This document focus on the same scope corresponding to existing inter-domain SAV mechanisms. Generally, it includes all IP-encapsulated scenarios: Native IP forwarding: including both global routing table forwarding and CE site forwarding of VPN; IP-encapsulated Tunnel (IPsec, GRE, SRv6, etc.): focusing on the validation of the outer layer IP address; Both IPv4 and IPv6 addresses Scope does not include: Non-IP packets: including MPLS label-based forwarding and other non-IP-based forwarding.

SAV rules can be generated based on route information (FIB/RIB) or non-route information. If the information is poisoned by attackers, the SAV rules will be false. Lots of legal packets may be dropped improperly or malicious traffic with spoofed source addresses may be permitted improperly. Route security should be considered by routing protocols. Non-route information should also be protected by corresponding mechanisms or infrastructure. If SAV mechanisms or protocols require information exchange, there should be some considerations on the avoidance of message alteration or message injection. The SAV procedure referred in this document modifies no field of packets. So, security considerations on data-plane is not in the scope of this document.

This document does not request any IANA allocations.