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 standard structure of an SRv6 Segment Identifier (SID), formatted as LOC:FUNCT:ARG. Furthermore, defines Compressed SRv6 Segment Identifiers (CSIDs) to optimize encoding efficiency, supporting two primary flavors: NEXT-CSID and REPLACE-CSID. In CSID container architecture, multiple CSIDs are packed within a 128-bit IPv6 Destination Address (DA) container. As packets traverse CSID nodes, the active CSID position within the container dynamically changes or shifts. Existing IPv6 BGP-FS components allow matching against the entire 128-bit IPv6 Destination Address (DA). However, both standard SRv6 SIDs and CSID containers embed structured sub-fields within the IPv6 DA. Filtering traffic based solely on a static 128-bit mask lacks the flexibility needed to apply policy rules to specific SRv6 function IDs or active Compressed SIDs across dynamically shifting DA containers. This document defines two new BGP Flow Specification component types: Component Type TBD1: Standard SRv6 SID Structure Match . Component Type TBD2: SRv6 CSID Dynamic Structure Match . 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 CSID: Compressed Segment Identifier NEXT-CSID: NEXT-CSID Flavor defined in REPLACE-CSID: REPLACE-CSID Flavor defined in In this document, the length parameters defining the constituent parts of a SID or CSID structure are defined as follows: LOC-Len: Locator Length in bits for a standard 128-bit SRv6 SID (Component Type TBD1). LB-Len: Locator-Block Length in bits for a CSID structure (Component Type TBD2). LN-Len: Locator-Node Length in bits for a CSID structure. F-Len: Function Length in bits for a SID or CSID structure. ARG-Len: Argument Length in bits for a standard 128-bit SRv6 SID. In addition, the sum of LN-Len and F-Len (LN-Len + F-Len) represents the total length of a single CSID (also referred to as the "CSID length").

A Flow Specification NLRI consists of zero or more matching components. Components Type 1 through 13 are defined in and . This document defines two new component types for SRv6 SID and CSID sub-field filtering. Component Type TBD1: SRv6 Standard SID Structure Match Component Type TBD2: SRv6 CSID Dynamic Structure Match

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 (Standard 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)).

defines Compressed SRv6 Segment Identifiers (C-SIDs) to reduce header overhead, supporting two primary flavors: NEXT-CSID and REPLACE-CSID. Per Section 3 and 4 of , both flavors position the currently active CSID immediately following the Locator-Block (LB) in the IPv6 Destination Address (DA):

Filtering rules MAY target either the Active CSID (applicable to both NEXT-CSID and REPLACE-CSID flavors) or the Next CSID (applicable specifically to the NEXT-CSID flavor).

]]> Indicates component type TBD2 (to be assigned by IANA). Locator-Block Length in bits (e.g., 32 bits). Locator-Node Length in bits (e.g., 16 bits). Function Length in bits (e.g., 16 bits). A sequence of one or more {operator, value} pairs. Validation Constraint: (LB-Len + LN-Len + F-Len) MUST NOT exceed 128 bits. LB-Len, LN-Len, and F-Len MUST NOT be 0. If validation fails, BGP Error Handling MUST be applied according to and . If any field length in bits is not a multiple of 8, the corresponding value field MUST be right-aligned with leading zero padding to the nearest octet boundary. The Operator byte (op) for TBD2 is encoded as:

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: Locator-Block (LB) Active CSID Node ID (LN) Active CSID Function ID (F) Active CSID (LN:F) Next CSID (LN:F) Full CSID (LB:LN:F) Next CSID Node ID (LN) Next CSID Function (F) Field Type 101 represents the Full CSID structure combining the Locator-Block (LB) and the Active CSID (LN:F) into a single matching value. Dynamic Match Processing Logic: When evaluating Component Type TBD2 against an incoming IPv6 packet: Locator-Block: Extracted starting at bit offset 0 with length LB-Len. Active CSID: The Active CSID (LN:F) is extracted starting at bit offset LB-Len with length (LN-Len + F-Len). In the context of the evaluating node, this represents the currently active CSID positioned immediately after the Locator-Block in the IPv6 DA. Next CSID: For NEXT-CSID flavor: The subsequent CSID is extracted immediately following the Active CSID at bit offset (LB-Len + LN-Len + F-Len) with length (LN-Len + F-Len). If the extracted Next CSID is all zeros (indicating no further CSID exists in the container or a padded container), the Next CSID evaluation MUST result in no match (False). For REPLACE-CSID flavor: Since the REPLACE-CSID structure does not support concatenated subsequent C-SIDs within the IPv6 DA container, any rule evaluating Next CSID (field type 100) against a REPLACE-CSID packet MUST result in no match (False).

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 ]]>

Consider an SRv6 network using a CSID container structure (applicable to either NEXT-CSID or REPLACE-CSID flavors) with Locator-Block 2001:db8::/32 (LB-Len=32), Node ID length LN-Len=16, and Function ID length F-Len=16. The rule targets traffic where the Active CSID Node ID equals 0x001A. Given: LB-Len = 32 bits (4 octets) LN-Len = 16 bits (2 octets) F-Len = 16 bits (2 octets) Hexadecimal Sequence:

Detailed Field Breakdown:

In a multi-hop SRv6 domain using the NEXT-CSID flavor, policy rules may require matching both the node currently processing the packet (Active CSID) and the subsequent downstream segment (Next CSID) to enforce strict path-aware traffic filtering. (Note: This dual-matching capability is specific to the NEXT-CSID flavor). Consider a NEXT-CSID network structured with: Locator-Block Length (LB-Len) = 32 bits (4 octets) Node ID Length (LN-Len) = 16 bits (2 octets) Function ID Length (F-Len) = 16 bits (2 octets) Matching Criterion: Filter traffic where: 1. Active CSID (LN:F) equals 0x0001000A 2. AND Next CSID (LN:F) equals 0x0002000B Hexadecimal Sequence:

Detailed Field Breakdown:

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 the "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.