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2. Terminology

2. Terminology​

The following terms used within this document are defined in [RFC8402]: Segment Routing (SR), SR Domain, Segment ID (SID), SRv6, SRv6 SID, SR Policy, Prefix-SID, and Adj-SID.

The following terms used within this document are defined in [RFC8754]: Segment Routing Header (SRH), SR source node, transit node, SR Segment Endpoint Node, Reduced SRH, Segments Left, and Last Entry.

The following terms are used in this document as defined below:

FIB: Forwarding Information Base. A FIB lookup is a lookup in the forwarding table.

SA: Source Address

DA: Destination Address

L3: Layer 3

L2: Layer 2

MAC: Media Access Control

EVPN: Ethernet VPN

ESI: Ethernet Segment Identifier

Per-CE VPN label: A single label for each attachment circuit that is shared by all routes with the same "outgoing attachment circuit" (Section 4.3.2 of [RFC4364])

Per-VRF VPN label: A single label for the entire VPN Routing and Forwarding (VRF) table that is shared by all routes from that VRF (Section 4.3.2 of [RFC4364])

SL: The Segments Left field of the SRH

SRv6 SID function: The function part of the SID is an opaque identification of a local behavior bound to the SID. It is formally defined in Section 3.1 of this document.

SRv6 Endpoint behavior: A packet processing behavior executed at an SRv6 Segment Endpoint Node. Section 4 of this document defines SRv6 Endpoint behaviors related to traffic-engineering and overlay use cases. Other behaviors (e.g., service programming) are outside the scope of this document.

An SR Policy is resolved to a SID list. A SID list is represented as <S1, S2, S3> where S1 is the first SID to visit, S2 is the second SID to visit, and S3 is the last SID to visit along the SR path.

(SA,DA) (S3, S2, S1; SL) represents an IPv6 packet with:

  • Source Address (SA), Destination Address (DA), and next header (SRH).

  • SRH with SID list <S1, S2, S3> with Segments Left = SL.

    Note the difference between the <> and () symbols: <S1, S2, S3> represents a SID list where S1 is the first SID and S3 is the last SID to traverse. (S3, S2, S1; SL) represents the same SID list but encoded in the SRH format where the rightmost SID in the SRH is the first SID and the leftmost SID in the SRH is the last SID. When referring to an SR Policy in a high-level use case, it is simpler to use the <S1, S2, S3> notation. When referring to an illustration of the detailed packet behavior, the (S3, S2, S1; SL) notation is more convenient.

  • The payload of the packet is omitted.

2.1. Requirements Language​

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

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