BGP Flow Specification for SRv6

defines the BGP Flow Specification (BGP-FS) Network Layer Reachability Information (NLRI) to distribute traffic filtering rules, which consist of an n-tuple of matching criteria applied to IP packets. extends this framework to IPv6 data packets. Additionally, extends Flowspec rules for Layer 2 Ethernet frames, and specifies BGP Flow Specification Version 2. Segment Routing over IPv6 (SRv6) leverages the IPv6 data plane using a Segment Routing Header (SRH) to instantiate Segment Routing policy paths. The SRv6 Network Programming architecture defines the structure of an SRv6 Segment Identifier (SID), typically formatted as LOC:FUNCT:ARG::, where: LOC (Locator): A routable prefix assigned to an SRv6 node that instantiates the SID. Operators may choose variable locator lengths. FUNCT (Function): An opaque identifier bound to a local function executed on the node (e.g., End, End.X, End.DT4). ARG (Argument): Optional arguments appended immediately following the FUNCT field for specialized local processing. Existing IPv6 BGP-FS components allow matching against the entire 128-bit IPv6 Destination Address (DA). However, SRv6 SIDs embed structured sub-fields (Locator, Function, and Argument) within the IPv6 DA. Filtering traffic based solely on a full 128-bit mask lacks the flexibility needed to apply policy rules to specific SRv6 functions or argument ranges across different locators. This document defines a new BGP Flow Specification component type to support sub-field matching for SRv6 SIDs. The matching field is evaluated against the IPv6 Destination Address (or active SID within an SRH when present, as shown in ). To support this feature, a BGP-FS NLRI with AFI = 2 (IPv6) MUST be used.

Loc | Func | Arg | | +-------------+ | +-------------------+ | | Segment[1] | | | +-------------+ | | | ... | | SR Header| +-------------+ | (RFC 8754)| | Segment[n] | | | +-------------+ | | +-------------+ | | ~ Option TLV ~ | | +-------------+ | +-----------------------------+]]>

BGP FS: BGP Flow Specification FS: Flow Specification SR: Segment Routing SRH: SR Header. SRv6: IPv6 Segment Routing, SRv6 is a method of forwarding IPv6 packets on the network based on the concept of source routing. SID: Segment Identifier BSID: Binding SID

A Flow Specification NLRI consists of zero or more matching components. Components Type 1 through 13 are defined in and . This document defines a new component type for SRv6 SID sub-field filtering.

An SRv6 SID consists of Locator, Function, and Argument fields as defined in . In operational scenarios, operators may need to filter traffic matching specific locators, functions, or arguments, or combinations thereof. The new component type TBD1 (SRv6 SID Structure Match) allows fine-grained matching on these individual fields or aggregated field combinations. Encoding: <type, LOC-Len, FUNCT-Len, ARG-Len, [op, value]+> Indicates the new component type (TBD1, to be assigned by IANA). Length of the Locator field in bits. Length of the Function field in bits. Length of the Argument field in bits. A sequence of one or more {operator, value} pairs specifying the matching logic applied to the SID sub-fields. Validation Constraint: The sum of LOC-Len, FUNCT-Len, and ARG-Len MUST NOT exceed 128 bits. If (LOC-Len + FUNCT-Len + ARG-Len) > 128, the NLRI is considered malformed, and BGP Error Handling MUST be applied according to and . The Operator byte (op) is encoded as follows:

The operator semantics follow the Numeric Operator definition in : e - end-of-list bit. Set in the last {op, value} pair in the sequence. a - AND bit. If unset, the previous term is logically ORed with the current one. If set, the operation is a logical AND. It should be unset in the first operator byte of a sequence. The AND operator has higher priority than OR for the purposes of evaluating logical expressions. field type: SID's LOC SID's FUNCT SID's ARG SID's LOC:FUNCT SID's FUNCT:ARG SID's LOC:FUNCT:ARG lt, gt, eq - Less-than, Greater-than, and Equal-to comparison operators, evaluated bitwise or numerically between the extracted SID sub-field and the provided value. The length of the Value field following the operator byte is determined by the selected field type, rounded up to the nearest byte boundary (i.e., ceil(bits / 8)).

Consider a rule targeting all SRv6 traffic directed to Locator 2001:db8:3::/48 with a Function ID within the range [0x0100, 0x0300]. Given: LOC-Len = 48 bits (6 octets) FUNCT-Len = 16 bits (2 octets) ARG-Len = 64 bits (8 octets) Hexadecimal Encoding Sequence:

=100 FUN<=300 0x11 TBD1 30 10 40 01 2001 0db8 0003 4b 0100 8d 0300 ^ ^ ^ | | | Length of LOC FUN ARG]]> Detailed Field Breakdown:

= 0x0100 value FUNCT's value = 0100 0x8d op end-of-list, "AND", FUNCT <= 0x0300 value FUNCT's value = 0300 ]]>

This document introduces no new security vulnerabilities beyond those already documented in and . Because Flowspec rules can result in packet dropping or rate-limiting, implementations MUST verify the authorization of BGP peers sending SRv6 Flowspec rules to prevent potential Denial-of-Service (DoS) attacks or traffic hijacking.

Under "Flow Spec Component Types" registry, IANA is requested to assign the following values:

The authors would like to thank Joel Halpern, Jeffrey Haas, Ketan Talaulikar, Aijun Wang, Dhruv Dhody, Ming Shen and Rainsword Wang for their valuable suggestions and comments.