2.1. SAV Using Access Control List

Ingress/egress ACLs are maintained to list acceptable (or alternatively, unacceptable) prefixes for the source addresses in the incoming/outgoing Internet Protocol (IP) packets. Any packet with a source address that fails the filtering criteria is dropped. The ACLs for the ingress/egress filters need to be maintained to keep them up to date. Updating the ACLs is an operator-driven manual process; hence, it is operationally difficult or infeasible.¶

Typically, the egress ACLs in access aggregation devices (e.g., CMTS, PDN-GW) permit source addresses only from the address spaces (prefixes) that are associated with the interface on which the customer network is connected. Ingress ACLs are typically deployed on border routers and drop ingress packets when the source address is spoofed (e.g., belongs to obviously disallowed prefix blocks, IANA special-purpose prefixes [SPAR-v4][SPAR-v6], provider's own prefixes, etc.).¶

2.2. SAV Using Strict Unicast Reverse Path Forwarding

Note: In the figures (scenarios) in this section and the subsequent sections, the following terminology is used:¶

• "fails" means drops packets with legitimate source addresses.¶
• "works (but not desirable)" means passes all packets with legitimate source addresses but is oblivious to directionality.¶
• "works best" means passes all packets with legitimate source addresses with no (or minimal) compromise of directionality.¶
• The notation Pi[ASn ASm ...] denotes a BGP update with prefix Pi and an AS_PATH as shown in the square brackets.¶

In the strict uRPF method, an ingress packet at a border router is accepted only if the Forwarding Information Base (FIB) contains a prefix that encompasses the source address and forwarding information for that prefix points back to the interface over which the packet was received. In other words, the reverse path for routing to the source address (if it were used as a destination address) should use the same interface over which the packet was received. It is well known that this method has limitations when networks are multihomed, routes are not symmetrically announced to all transit providers, and there is asymmetric routing of data packets. Asymmetric routing occurs (see Figure 1) when a customer AS announces one prefix (P1) to one transit provider (ISP-a) and a different prefix (P2) to another transit provider (ISP-b) but routes data packets with source addresses in the second prefix (P2) to the first transit provider (ISP-a) or vice versa. Then, data packets with a source address in prefix P2 that are received at AS2 directly from AS1 will get dropped. Further, data packets with a source address in prefix P1 that originate from AS1 and traverse via AS3 to AS2 will also get dropped at AS2.¶

           +------------+ ---- P1[AS2 AS1] ---> +------------+
           | AS2(ISP-a) | <----P2[AS3 AS1] ---- | AS3(ISP-b) |
           +------------+                       +------------+
                    /\                             /\
                     \                             /
                      \                           /
                       \                         /
                 P1[AS1]\                       /P2[AS1]
                         \                     /
                        +-----------------------+
                        |      AS1(customer)    |
                        +-----------------------+
                          P1, P2 (prefixes originated)

          Consider data packets received at AS2
          (1) from AS1 with a source address (SA) in P2, or
          (2) from AS3 that originated from AS1 with a SA in P1:
                    * Strict uRPF fails
                    * Feasible-path uRPF fails
                    * Loose uRPF works (but not desirable)
                    * Enhanced feasible-path uRPF works best

2.3. SAV Using Feasible-Path Unicast Reverse Path Forwarding

The feasible-path uRPF technique helps partially overcome the problem identified with the strict uRPF in the multihoming case. The feasible-path uRPF is similar to the strict uRPF, but in addition to inserting the best-path prefix, additional prefixes from alternative announced routes are also included in the RPF list. This method relies on either (a) announcements for the same prefixes (albeit some may be prepended to effect lower preference) propagating to all transit providers performing feasible-path uRPF checks or (b) announcement of an aggregate less-specific prefix to all transit providers while announcing more-specific prefixes (covered by the less-specific prefix) to different transit providers as needed for traffic engineering.¶

As an example, in the multihoming scenario (see Scenario 2 in Figure 2), if the customer AS announces routes for both prefixes (P1, P2) to both transit providers (with suitable prepends if needed for traffic engineering), then the feasible-path uRPF method works. It should be mentioned that the feasible-path uRPF works in this scenario only if customer routes are preferred at AS2 and AS3 over a shorter non-customer route. However, the feasible-path uRPF method has limitations as well. One form of limitation naturally occurs when the recommendation (a) or (b) mentioned above regarding propagation of prefixes is not followed.¶

Another form of limitation can be described as follows. In Scenario 2 (described here, illustrated in Figure 2), it is possible that the second transit provider (ISP-b or AS3) does not propagate the prepended route for prefix P1 to the first transit provider (ISP-a or AS2). This is because AS3's decision policy permits giving priority to a shorter route to prefix P1 via a lateral peer (AS2) over a longer route learned directly from the customer (AS1). In such a scenario, AS3 would not send any route announcement for prefix P1 to AS2 (over the P2P link). Then, a data packet with a source address in prefix P1 that originates from AS1 and traverses via AS3 to AS2 will get dropped at AS2.¶

          +------------+  routes for P1, P2   +------------+
          | AS2(ISP-a) |<-------------------->| AS3(ISP-b) |
          +------------+        (P2P)         +------------+
                    /\                            /\
                     \                            /
               P1[AS1]\                          /P2[AS1]
                       \                        /
         P2[AS1 AS1 AS1]\                      /P1[AS1 AS1 AS1]
                         \                    /
                        +-----------------------+
                        |      AS1(customer)    |
                        +-----------------------+
                          P1, P2 (prefixes originated)

        Consider data packets received at AS2 via AS3
        that originated from AS1 and have a source address in P1:
        * Feasible-path uRPF works (if the customer route to P1
          is preferred at AS3 over the shorter path)
        * Feasible-path uRPF fails (if the shorter path to P1
          is preferred at AS3 over the customer route)
        * Loose uRPF works (but not desirable)
        * Enhanced feasible-path uRPF works best

2.4. SAV Using Loose Unicast Reverse Path Forwarding

In the loose uRPF method, an ingress packet at the border router is accepted only if the FIB has one or more prefixes that encompass the source address. That is, a packet is dropped if no route exists in the FIB for the source address. Loose uRPF sacrifices directionality. It only drops packets if the source address is unreachable in the current FIB (e.g., IANA special-purpose prefixes [SPAR-v4][SPAR-v6], unallocated, allocated but currently not routed).¶

2.5. SAV Using VRF Table

The Virtual Routing and Forwarding (VRF) technology [RFC4364] [Juniper] allows a router to maintain multiple routing table instances separate from the global Routing Information Base (RIB). External BGP (eBGP) peering sessions send specific routes to be stored in a dedicated VRF table. The uRPF process queries the VRF table (instead of the FIB) for source address validation. A VRF table can be dedicated per eBGP peer and used for uRPF for only that peer, resulting in strict mode operation. For implementing loose uRPF on an interface, the corresponding VRF table would be global, i.e., contains the same routes as in the FIB.¶

3.1. Description of the Method

The enhanced feasible-path uRPF (EFP-uRPF) method adds greater operational robustness and efficacy to existing uRPF methods discussed in Section 2. That is because it avoids dropping legitimate data packets and compromising directionality. The method is based on the principle that if BGP updates for multiple prefixes with the same origin AS were received on different interfaces (at border routers), then incoming data packets with source addresses in any of those prefixes should be accepted on any of those interfaces. The EFP-uRPF method can be best explained with an example, as follows:¶

Let us say, in its Adj-RIBs-In [RFC4271], a border router of ISP-A has the set of prefixes {Q1, Q2, Q3}, each of which has AS-x as its origin and AS-x is in ISP-A's customer cone. In this set, the border router received the route for prefix Q1 over a customer-facing interface while it learned the routes for prefixes Q2 and Q3 from a lateral peer and an upstream transit provider, respectively. In this example scenario, the enhanced feasible-path uRPF method requires Q1, Q2, and Q3 be included in the RPF list for the customer interface under consideration.¶

Thus, the EFP-uRPF method gathers feasible paths for customer interfaces in a more precise way (as compared to the feasible-path uRPF) so that all legitimate packets are accepted while the directionality property is not compromised.¶

The above-described EFP-uRPF method is recommended to be applied on customer interfaces. It can also be extended to create the RPF lists for lateral peer interfaces. That is, the EFP-uRPF method can be applied (and loose uRPF avoided) on lateral peer interfaces. That will help to avoid compromising directionality for lateral peer interfaces (which is inevitable with loose uRPF; see Section 2.4).¶

Looking back at Scenarios 1 and 2 (Figures 1 and 2), the EFP-uRPF method works better than the other uRPF methods. Scenario 3 (Figure 3) further illustrates the enhanced feasible-path uRPF method with a more concrete example. In this scenario, the focus is on operation of the EFP-uRPF at ISP4 (AS4). ISP4 learns a route for prefix P1 via a C2P interface from customer ISP2 (AS2). This route for P1 has origin AS1. ISP4 also learns a route for P2 via another C2P interface from customer ISP3 (AS3). Additionally, AS4 learns a route for P3 via a lateral P2P interface from ISP5 (AS5). Routes for all three prefixes have the same origin AS (i.e., AS1). Using the enhanced feasible-path uRPF scheme and given the commonality of the origin AS across the routes for P1, P2, and P3, AS4 includes all of these prefixes in the RPF list for the customer interfaces (from AS2 and AS3).¶

                 +----------+   P3[AS5 AS1]  +------------+
                 | AS4(ISP4)|<---------------|  AS5(ISP5) |
                 +----------+      (P2P)     +------------+
                     /\   /\                        /\
                     /     \                        /
         P1[AS2 AS1]/       \P2[AS3 AS1]           /
              (C2P)/         \(C2P)               /
                  /           \                  /
           +----------+    +----------+         /
           | AS2(ISP2)|    | AS3(ISP3)|        /
           +----------+    +----------+       /
                    /\           /\          /
                     \           /          /
               P1[AS1]\         /P2[AS1]   /P3[AS1]
                  (C2P)\       /(C2P)     /(C2P)
                        \     /          /
                     +----------------+ /
                     |  AS1(customer) |/
                     +----------------+
                          P1, P2, P3 (prefixes originated)

         Consider that data packets (sourced from AS1)
         may be received at AS4 with a source address
         in P1, P2, or P3 via any of the neighbors (AS2, AS3, AS5):
         * Feasible-path uRPF fails
         * Loose uRPF works (but not desirable)
         * Enhanced feasible-path uRPF works best

3.2. Operational Recommendations

The following operational recommendations will make the operation of the enhanced feasible-path uRPF robust:¶

For multihomed stub AS:¶

• A multihomed stub AS should announce at least one of the prefixes it originates to each of its transit provider ASes. (It is understood that a single-homed stub AS would announce all prefixes it originates to its sole transit provider AS.)¶

For non-stub AS:¶

• A non-stub AS should also announce at least one of the prefixes it originates to each of its transit provider ASes.¶
• Additionally, from the routes it has learned from customers, a non-stub AS SHOULD announce at least one route per origin AS to each of its transit provider ASes.¶

3.3. A Challenging Scenario

It should be observed that in the absence of ASes adhering to above recommendations, the following example scenario, which poses a challenge for the enhanced feasible-path uRPF (as well as for traditional feasible-path uRPF), may be constructed. In the scenario illustrated in Figure 4, since routes for neither P1 nor P2 are propagated on the AS2-AS4 interface (due to the presence of NO_EXPORT Community), the enhanced feasible-path uRPF at AS4 will reject data packets received on that interface with source addresses in P1 or P2. (For a little more complex example scenario, see slide #10 in [Sriram-URPF].)¶

                 +----------+
                 | AS4(ISP4)|
                 +----------+
                     /\   /\
                     /     \  P1[AS3 AS1]
      P1 and P2 not /       \ P2[AS3 AS1]
        propagated /         \ (C2P)
          (C2P)   /           \
           +----------+    +----------+
           | AS2(ISP2)|    | AS3(ISP3)|
           +----------+    +----------+
                    /\           /\
                     \           / P1[AS1]
    P1[AS1] NO_EXPORT \         / P2[AS1]
    P2[AS1] NO_EXPORT  \       / (C2P)
                 (C2P)  \     /
                     +----------------+
                     |  AS1(customer) |
                     +----------------+
                          P1, P2 (prefixes originated)

       Consider that data packets (sourced from AS1)
       may be received at AS4 with a source address
       in P1 or P2 via AS2:
       * Feasible-path uRPF fails
       * Loose uRPF works (but not desirable)
       * Enhanced feasible-path uRPF with Algorithm A fails
       * Enhanced feasible-path uRPF with Algorithm B works best

3.4. Algorithm B: Enhanced Feasible-Path uRPF with Additional Flexibility across Customer Cone

Adding further flexibility to the enhanced feasible-path uRPF method can help address the potential limitation identified above using the scenario in Figure 4 (Section 3.3). In the following, "route" refers to a route currently existing in the Adj-RIBs-In. Including the additional degree of flexibility, the modified algorithm called Algorithm B (implemented in a transit AS) can be described as follows:¶

1. Create the set of all directly connected customer interfaces. Call it Set I = {I1, I2, ..., Ik}.¶
2. Create the set of all unique prefixes for which routes exist in Adj-RIBs-In for the interfaces in Set I. Call it Set P = {P1, P2, ..., Pm}.¶
3. Create the set of all unique origin ASes seen in the routes that exist in Adj-RIBs-In for the interfaces in Set I. Call it Set A = {AS1, AS2, ..., ASn}.¶
4. Create the set of all unique prefixes for which routes exist in Adj-RIBs-In of all lateral peer and transit provider interfaces such that each of the routes has its origin AS belonging in Set A. Call it Set Q = {Q1, Q2, ..., Qj}.¶
5. Then, Set Z = Union(P,Q) is the RPF list that is applied for every customer interface in Set I.¶

When Algorithm B (which is more flexible than Algorithm A) is employed on customer interfaces, the type of limitation identified in Figure 4 (Section 3.3) is overcome and the method works. The directionality property is minimally compromised, but the proposed EFP-uRPF method with Algorithm B is still a much better choice (for the scenario under consideration) than applying the loose uRPF method, which is oblivious to directionality.¶

So, applying the EFP-uRPF method with Algorithm B is recommended on customer interfaces for the challenging scenarios, such as those described in Section 3.3.¶

3.5. Augmenting RPF Lists with ROA and IRR Data

It is worth emphasizing that an indirect part of the proposal in this document is that RPF filters may be augmented from secondary sources. Hence, the construction of RPF lists using a method proposed in this document (Algorithm A or B) can be augmented with data from Route Origin Authorization (ROA) [RFC6482], as well as Internet Routing Registry (IRR) data. Special care should be exercised when using IRR data because it is not always accurate or trusted. In the EFP-uRPF method with Algorithm A (see Section 3.1.1), if a ROA includes prefix Pi and ASj, then augment the RPF list of each customer interface on which at least one route with origin ASj was received with prefix Pi. In the EFP-uRPF method with Algorithm B, if ASj belongs in Set A (see Step #3 Section 3.4) and if a ROA includes prefix Pi and ASj, then augment the RPF list Z in Step 5 of Algorithm B with prefix Pi. Similar procedures can be followed with reliable IRR data as well. This will help make the RPF lists more robust about source addresses that may be legitimately used by customers of the ISP.¶

3.6. Implementation and Operations Considerations

3.7. Summary of Recommendations

Depending on the scenario, an ISP or enterprise AS operator should follow one of the following recommendations concerning uRPF/SAV:¶

1. For directly connected networks, i.e., subnets directly connected to the AS, the AS under consideration SHOULD perform ACL-based SAV.¶
2. For a directly connected single-homed stub AS (customer), the AS under consideration SHOULD perform SAV based on the strict uRPF method.¶

3. For all other scenarios:¶

   • The EFP-uRPF method with Algorithm B (see Section 3.4) SHOULD be applied on customer interfaces.¶
   • The loose uRPF method SHOULD be applied on lateral peer and transit provider interfaces.¶

It is also recommended that prefixes from registered ROAs and IRR route objects that include ASes in an ISP's customer cone SHOULD be used to augment the pertaining RPF lists (see Section 3.5 for details).¶