Source-IP-Origin-AS Filter for BGP Flow Specification

BGP Flow Specification, originally defined in and , has evolved to Flow Specification Version 2 (FSv2) as specified in . FSv2 introduces an extensible, TLV-based filter family architecture with explicit rule ordering and modular components. In certain DDoS mitigation scenarios, an operator may need to apply rate-limiting, redirection, or filtering to traffic sourced from a particular network (Autonomous System), even when the specific source IP prefixes within that AS are numerous or rapidly changing. Manually updating hundreds of prefix-based FlowSpec rules is inefficient and consumes excessive Hardware Forwarding Information Base (FIB) / TCAM resources. This document introduces a new FSv2 component under the Routing Filter Family that allows operators to use the Source Origin AS as a matching criterion.

FSv2: Flow Specification Version 2 Source-IP-Origin-AS: The origin AS number associated with the source IP address of a transit packet, determined from the rightmost AS in the AS_PATH attribute of the active BGP route matching that source IP.

This document proposes a new FSv2 component encoded within the Routing Filter Family (Type 1400). Unlike legacy FSv1 raw byte streams, FSv2 utilizes explicit TLV formatting. Filter Family: 1400 (Routing Filter Family) Component Type: 200 (SIP-AS) Encoding format for the SIP-AS component:

| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ]]> Where: Flags (4 bits): Defined in FSv2 specifications. Type (12 bits): Set to 200 for SIP-AS. Length (16 bits): Length of the value field in octets. <[numeric_op, Source-IP-Origin-AS]+>: Contains a list of {numeric_op, Source-IP-Origin-AS} pairs that are used to match the Source-IP-Origin-AS. numeric_op: This component uses the Numeric Operator (numeric_op) described in Section 4.1.1 of , Type 200 component values SHOULD be encoded as 4 octets (numeric_op len=10). Source-IP-Origin-AS (32 bits): A 4-octet Autonomous System Number encoded in network byte order.

As defined in , rules and components follow strict ordering principles. The SIP-AS component (Type 200) within the Routing Filter Family (1400) MUST be evaluated deterministically according to FSv2 component sequencing rules.

When a BGP speaker receives and installs an FSv2 route containing the Routing Filter Family with the SIP-AS component (Type 200), the matching process for a transit data packet MUST proceed as follows: The router extracts the packet's Source IP address. The router performs a longest-prefix match (LPM) lookup for the Source IP in its local BGP Loc-RIB / FIB to retrieve the associated active BGP route. The router evaluates the AS_PATH attribute of the retrieved BGP route: The Origin AS is defined as the rightmost AS number in the AS_SEQUENCE segment of the AS_PATH. If the AS_PATH ends with an AS_SET, standard evaluation or local policy applies. If the evaluated Origin AS matches the criterion in the FSv2 rule, the packet matches this component. In multi-homing scenarios, the match succeeds if ANY valid active path for the source prefix matches the Origin AS defined in the rule.

This section describes how to use this function in a simple scenario. Consider the topology shown in . Router R2 in AS64597 receives traffic originating from AS64598 destined for IP Prefix 61. The operator wants to redirect all traffic originating from AS64598 towards Router R3 before forwarding it to IP Prefix 61.

Using the traditional method, ISP AS64597 must inject multiple "Destination Prefix + Source Prefix" FlowSpec rules into Router R2:

Using the method defined in this draft, ISP AS64597 needs to inject only a single "Destination Prefix + Source-IP-Origin-AS" FlowSpec rule:

This method saves significant TCAM space on network elements, reduces BGP UPDATE messaging load on the control plane, and simplifies operational management when a source AS advertises numerous dynamic prefixes.

In addition to the security considerations in , operators MUST be aware of routing inconsistency risks, BGP origin validation via RPKI , and FSv2 route validation procedures.

IANA is requested to make the following allocations under the BGP Flow Specification Version 2 (FSv2) registries defined in :

The following people made significant contributions to this document:

The authors would like to thank Susan Hares, Ming Shen, Nan Geng, Ziqing Cao for their valuable comments and suggestions.