5. SR Policy Headend Behaviors

This section describes a set of SRv6 Policy Headend [RFC8402] behaviors.

Behavior	Description
H.Encaps	SR Headend with Encapsulation in an SR Policy
H.Encaps.Red	H.Encaps with Reduced Encapsulation
H.Encaps.L2	H.Encaps Applied to Received L2 Frames
H.Encaps.L2.Red	H.Encaps.Red Applied to Received L2 Frames

Table 2: SR Policy Headend Behaviors

This list is not exhaustive, and future documents may define additional behaviors.

5.1. H.Encaps: SR Headend with Encapsulation in an SR Policy

Node N receives two packets P1=(A, B2) and P2=(A,B2)(B3, B2, B1; SL=1). B2 is neither a local address nor SID of N.

Node N is configured with an IPv6 address T (e.g., assigned to its loopback).

N steers the transit packets P1 and P2 into an SRv6 Policy with a Source Address T and a segment list <S1, S2, S3>.

The H.Encaps encapsulation behavior is defined as follows:

S01.   Push an IPv6 header with its own SRH
S02.   Set outer IPv6 SA = T and outer IPv6 DA to the first SID
          in the segment list
S03.   Set outer Payload Length, Traffic Class, Hop Limit, and
          Flow Label fields
S04.   Set the outer Next Header value
S05.   Decrement inner IPv6 Hop Limit or IPv4 TTL
S06.   Submit the packet to the IPv6 module for transmission to S1

Note:

S03: As described in [RFC2473] and [RFC6437].

After the H.Encaps behavior, P1' and P2' respectively look like:

(T, S1) (S3, S2, S1; SL=2) (A, B2)
(T, S1) (S3, S2, S1; SL=2) (A, B2) (B3, B2, B1; SL=1)

The received packet is encapsulated unmodified (with the exception of the IPv4 TTL or IPv6 Hop Limit that is decremented as described in [RFC2473]).

The H.Encaps behavior is valid for any kind of L3 traffic. This behavior is commonly used for L3VPN with IPv4 and IPv6 deployments. It may be also used for TI-LFA [SR-TI-LFA] at the Point of Local Repair.

The push of the SRH MAY be omitted when the SRv6 Policy only contains one segment and there is no need to use any flag, tag, or TLV.

5.2. H.Encaps.Red: H.Encaps with Reduced Encapsulation

The H.Encaps.Red behavior is an optimization of the H.Encaps behavior.

H.Encaps.Red reduces the length of the SRH by excluding the first SID in the SRH of the pushed IPv6 header. The first SID is only placed in the Destination Address field of the pushed IPv6 header.

After the H.Encaps.Red behavior, P1' and P2' respectively look like:

(T, S1) (S3, S2; SL=2) (A, B2)
(T, S1) (S3, S2; SL=2) (A, B2) (B3, B2, B1; SL=1)

The push of the SRH MAY be omitted when the SRv6 Policy only contains one segment and there is no need to use any flag, tag, or TLV.

5.3. H.Encaps.L2: H.Encaps Applied to Received L2 Frames

The H.Encaps.L2 behavior encapsulates a received Ethernet [IEEE.802.3_2018] frame and its attached VLAN header, if present, in an IPv6 packet with an SRH. The Ethernet frame becomes the payload of the new IPv6 packet.

The Next Header field of the SRH MUST be set to 143.

The push of the SRH MAY be omitted when the SRv6 Policy only contains one segment and there is no need to use any flag, tag, or TLV.

The encapsulating node MUST remove the preamble (if any) and frame check sequence (FCS) from the Ethernet frame upon encapsulation, and the decapsulating node MUST regenerate, as required, the preamble and FCS before forwarding the Ethernet frame.

5.4. H.Encaps.L2.Red: H.Encaps.Red Applied to Received L2 Frames

The H.Encaps.L2.Red behavior is an optimization of the H.Encaps.L2 behavior.

H.Encaps.L2.Red reduces the length of the SRH by excluding the first SID in the SRH of the pushed IPv6 header. The first SID is only placed in the Destination Address field of the pushed IPv6 header.

The push of the SRH MAY be omitted when the SRv6 Policy only contains one segment and there is no need to use any flag, tag, or TLV.