Destination-IP-Community Filter for BGP Flow Specification

BGP Flow Specification, originally defined in , 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 large-scale networks, configuring individual Flowspec rules based on explicit destination IP prefix matches can lead to severe TCAM and control-plane entry scale bottlenecks. This document specifies a new FSv2 component type under the Routing Filter Family to support community-level filtering. The match field targets the BGP Community attribute of the destination IP prefix. This functionality is designed to operate within a single administrative domain.

FSv2: Flow Specification Version 2 Destination-IP-Community: The BGP Standard Community associated with the reachability route of the destination IP address.

This document defines a new FSv2 component encoded within the Routing Filter Family (Type 1400). Unlike legacy FSv1 raw byte streams, FSv2 utilizes explicit TLV formatting. This document defines a new Flow Specification component type encoded within the BGP Flowspec NLRI. Filter Family: 1400 (Routing Filter Family) Component Type: 300 (DIP COM) Encoding format for the DIP COM component:

| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ]]> Where: Flags (4 bits): Defined in FSv2 specifications. Type (12 bits): Set to 300 for DIP COM. Length (16 bits): Length of the value field in octets. <[numeric_op, Destination-IP-Community]+>: Contains a list of {numeric_op, Destination-IP-Community} pairs that are used to match the Destination-IP-Community. numeric_op: This component uses the Numeric Operator (numeric_op) described in Section 4.1.1 of , Type 300 component values SHOULD be encoded as 4 octets (numeric_op len=10). Destination-IP-Community (32 bits): BGP Community value to match.

The implementation of Destination-IP-Community filtering relies on a cross-coupling mechanism between the Unicast BGP Forwarding Table (FIB) and the Flowspec Packet Inspection Engine. The detailed processing workflow is defined in the following three steps: Control-Plane Community Dissemination and FIB Tagging: When a BGP speaker receives unicast BGP routes carrying BGP Communities (e.g., Standard Communities), and those routes are selected as best paths and installed into the Forwarding Information Base (FIB), the local system MUST preserve the associated BGP Community attributes (or a local mapped metadata tag representing the Community) within the corresponding FIB entries. For example: FIB Entry for Destination Prefix A: Tagged with Community 64597:1 FIB Entry for Destination Prefix B: Tagged with Community 64597:2 Flowspec Rule Installation: When a BGP Flowspec rule containing the Destination-IP-Community component is received, the system programs a filtering rule into the forwarding hardware. The match parameters include both standard RFC 8955/8956 n-tuples (e.g., Destination IP, Source IP, Protocol, Port) and the target Community value specified in the Destination-IP-Community component (denoted as COMM_Rule, e.g., 64597:1). Datapath Two-Stage Packet Matching: Upon receiving a data packet: The device evaluates the packet against installed Flowspec rules. If a candidate rule contains a Destination-IP-Community match condition, the forwarding engine performs a FIB lookup using the packet's Destination IP address to extract the associated FIB entry's Community attribute (denoted as COMM_FIB). Evaluation: If COMM_FIB matches COMM_Rule (e.g., both are 64597:1), the Destination-IP-Community match condition is satisfied, and the configured Flowspec action (e.g., redirect, drop, rate-limit) is executed. If COMM_FIB does not match COMM_Rule (e.g., COMM_FIB is 64597:2 while COMM_Rule is 64597:1), the match condition fails, and packet processing continues to subsequent rules or default forwarding.

This section illustrates a representative operational scenario for community-level traffic steering. Consider the topology in .

In AS64597, ISP operator policy requires redirecting all traffic from IP Prefix 61 bound for AS64598 and AS64599 to traverse R3 first. Using traditional BGP Flowspec rules, router R1 requires multiple "Destination Prefix + Source Prefix" filter entries:

Using the Destination-IP-Community filter defined in this document, the operator configures a single "Destination Community + Source Prefix" rule on R1:

This mechanism significantly reduces control-plane propagation state and hardware TCAM resource usage on network processors.

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

This specification extends BGP Flow Specification Version 2 (FSv2) matching capabilities. All security considerations defined in apply. Unauthorized injection or modification of Destination-IP-Community Flowspec rules could lead to traffic misdirection, unexpected dropping, or Denial-of-Service (DoS) conditions. To mitigate these risks: This feature MUST be restricted to a single administrative domain or strictly trusted peer boundaries. BGP speakers MUST filter or strip this Flowspec component type at eBGP domain boundaries unless explicitly allowed by bilateral policy. Standard BGP Flowspec validation procedures MUST be enforced to prevent unauthorized prefix rule propagation.

The following people made significant contributions to this document:

TBD.