SRv6 Network Programming

Segment Routing leverages the source routing paradigm. An ingress node steers a packet through an ordered list of instructions, called segments. Each one of these instructions represents a function to be called at a specific location in the network. A function is locally defined on the node where it is executed and may range from simply moving forward in the Segment List to any complex user-defined behavior. Network programming combines segment routing functions, both simple and complex, to achieve a networking objective that goes beyond mere packet routing. This document defines the SRv6 Network Programming concept and specifies the main segment routing behaviors to enable the creation of interoperable overlays with underlay optimization (Service Level Agreement). The companion document illustrates the concepts defined in this document. Familiarity with the Segment Routing Header is expected.

The following terms used within this document are defined in : Segment Routing, SR Domain, Segment ID (SID), SRv6, SRv6 SID, SR Policy, Prefix SID and Adjacency SID. The following terms used within this document are defined in : SRH, SR Source Node, Transit Node, SR Segment Endpoint Node and Reduced SRH. NH: Next-header field of the IPv6 header . NH=SRH means that the next-header of the IPv6 header is Routing Header for IPv6(43) with the Type field set to 4. SL: The Segments Left field of the SRH FIB: Forwarding Information Base. A FIB lookup is a lookup in the forwarding table. SA: Source Address DA: Destination Address 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 of this document. SRv6 Segment Endpoint behavior: A packet processing behavior executed at an SRv6 Segment Endpoint Node. of this document defines SRv6 Segment 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 is SA, Destination Address is DA, and next-header is 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. SRH[n]: A shorter representation of Segment List[n], as defined in .

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 when, and only when, they appear in all capitals, as shown here.

RFC8402 defines an SRv6 Segment Identifier as an IPv6 address explicitly associated with the segment. When an SRv6 SID is in the Destination Address field of an IPv6 header of a packet, it is routed through an IPv6 network as an IPv6 address. Its processing is defined in section 4.3 and reproduced here as a reminder. Without constraining the details of an implementation, the SR Segment Endpoint Node creates Forwarding Information Base (FIB) entries for its local SIDs. When an SRv6-capable node receives an IPv6 packet, it performs a longest-prefix-match lookup on the packets destination address. This lookup can return any of the following: A FIB entry that represents a locally instantiated SRv6 SID A FIB entry that represents a local interface, not locally instantiated as an SRv6 SID A FIB entry that represents a non-local route No Match This document formally defines behaviors and parameters for SRv6 SIDs.

This document defines an SRv6 SID as consisting of LOC:FUNCT:ARG, where a locator (LOC) is encoded in the L most significant bits of the SID, followed by F bits of function (FUNCT) and A bits of arguments (ARG). L, the locator length, is flexible, and an operator is free to use the locator length of their choice. F and A may be any value as long as L+F+A <= 128. When L+F+A is less than 128 then the remainder of the SID MUST be zero. A locator may be represented as B:N where B is the SRv6 SID block (IPv6 subnet allocated for SRv6 SIDs by the operator) and N is the identifier of the parent node instantiating the SID. When the LOC part of the SRv6 SIDs is routable, it leads to the node which instantiates the SID. The FUNCT is an opaque identification of a local behavior bound to the SID. The term "function" refers to the bit-string in the SRv6 SID. The term "behavior" identifies the behavior bound to the SID. The behaviors are defined in Section 4 of this document. An SRv6 endpoint behavior MAY require additional information for its processing (e.g. related to the flow or service). This information may be encoded in the ARG bits of the SID. In such a case, the semantics and format of the ARG bits are defined as part of the SRv6 endpoint behavior specification. The ARG value of a routed SID SHOULD remain constant among packets in a given flow. Varying ARG values among packets in a flow may result in different ECMP hashing and cause re-ordering.

Locators are assigned consistent with IPv6 infrastructure allocation. For example, an network operator may: Assign block B::/48 to the SR domain Assign a unique B:N::/64 block to each SRv6-enabled node in the domain As an example, one mobile service provider has commercially deployed SRv6 across more than 1000 commercial routers and 1800 whitebox routers. All these devices are enabled for SRv6 and advertise SRv6 SID's. The provider historically deployed IPv6 and assigned infrastructure address from a portion of the fc00::/7 prefix. They further subdivided the prefix into three /48 prefixes (Country X, Country Y, Country Z) to support their SRv6 infrastructure. From those /48 prefixes each router is assigned a /64 prefix from which all SIDs of that router are allocated. In another example, a large mobile and fixed line service provider has commercially deployed SRv6 in their country-wide network. This provider is assigned a /20 prefix by a RIR. They sub-allocated a few /48 prefixes to their infrastructure to deploy SRv6. Each router is assigned a /64 prefix from which all SIDs of that router are allocated. IPv6 address consumption in both these examples is minimum, representing one billionth and one millionth of the assigned address space respectively. A service provider receiving the current minimum allocation of a /32 from a RIR may assign a /48 prefix to their infrastructure deploying SRv6, and subsequently allocate /64 prefixes for SIDs at each SRv6 node. The /48 assignment is one sixty five thousandth (1/2^16) of the usable IPv6 address space available for assignment by the provider. When an operator instantiates a SID at a node, they specify a SID value B:N:FUNCT and the behavior bound to the SID using one of the IANA codepoints of the registry of SRv6 Endpoint Behaviors defined in this document. The node advertises the SID, B:N:FUNCT, in the control-plane (see ) together with the IANA Endpoint Behavior codepoint (see ) identifying the behavior of the SID. A remote node uses the IANA behavior codepoint to map the received SID (B:N:FUNCT) to a behavior. A remote node selects a desired behavior at an advertising node by selecting the SID (B:N:FUNCT) advertised with the desired behavior. A remote node cannot infer the behavior by examination of the FUNCT value of a SID. Therefore the IANA Endpoint Behavior codepoint is advertised along with the SID in the control plane. As an example, a network operator may: Assign an SRv6 SID block 2001:db8:bbbb::/48 from their in-house operation block for their SRv6 infrastructure Assign an SRv6 Locator 2001:db8:bbbb:3::/64 to the Router 3 in their SR Domain At Router 3, within the locator 2001:db8:bbbb:3::/64, network operator or the router performs dynamic assignment for: Function 11 associated with the behavior End.X (Endpoint with cross-connect) between router 3 and its connected neighbor router 4. This SID is advertised in the control plane as 2001:db8:bbbb:3:11:: with IANA Behavior value of 5. Function 12 associated with the behavior End.X (Endpoint with cross-connect) between router 3 and its connected neighbor router 2. This SID is advertised in the control plane as 2001:db8:bbbb:3:12:: with IANA Behavior value of 5.

Most often, the node N would advertise IPv6 prefix(es) matching the LOC parts covering its SIDs or shorter-mask prefix. The distribution of these advertisements and calculation of their reachability are routing protocol specific aspects that are outside the scope of this document. An SRv6 SID is said to be routed if its SID belongs to an IPv6 prefix advertised via a routing protocol. An SRv6 SID that does not fulfill this condition is non-routed. Let's provide a classic illustration: Node N is configured explicitly with two SIDs: 2001:DB8:B:1:100:: and 2001:DB8:B:2:101::. The network learns about a path to 2001:DB8:B:1::/64 via the IGP and hence a packet destined to 2001:DB8:B:1:100:: would be routed up to N. The network does not learn about a path to 2001:DB8:B:2::/64 via the IGP and hence a packet destined to 2001:DB8:B:2:101:: would not be routed up to N. A packet could be steered to a non-routed SID 2001:DB8:B:2:101:: by using a SID list <...,2001:DB8:B:1:100::,2001:DB8:B:2:101::,...> where the non-routed SID is preceded by a routed SID to the same node. Routed and non-routed SRv6 SIDs are the SRv6 instantiation of global and local segments, respectively [RFC8402].

Each FIB entry indicates the behavior associated with a SID instance and its parameters. This document defines a new set of behaviors in addition to that defined in RFC8754 Section 4.3.1. Following is a set of well-known behaviors that can be associated with a SID.

The list is not exhaustive. In practice, any function can be attached to a local SID: e.g. a node N can bind a SID to a local VM or container which can apply any complex processing on the packet. The following sub-sections detail the behaviors, introduced in this document, that a node (N) binds to a SID (S). Section 4.16 defines flavors of some of these behaviors.

The Endpoint behavior ("End" for short) is the most basic behavior. It is the instantiation of a Prefix-SID . When N receives a packet whose IPv6 DA is S and S is a local End SID, N does:

max_LE) or (Segments Left > Last Entry+1)) { S10. Send an ICMP Parameter Problem to the Source Address, Code 0 (Erroneous header field encountered), Pointer set to the Segments Left field. Interrupt packet processing and discard the packet. S11. } S12. Decrement Hop Limit by 1 S13. Decrement Segments Left by 1 S14. Update IPv6 DA with Segment List[Segments Left] S15. Submit the packet to the egress IPv6 FIB lookup and transmission to the new destination S16. } ]]> Notes: The End behavior operates on the same FIB table (i.e. VRF, L3 relay id) associated to the packet. Hence the FIB lookup on line S15 is done in the same FIB table as the ingress interface.

When processing the Upper-layer Header of a packet matching a FIB entry locally instantiated as an SRv6 End SID, if Upper-layer Header processing is allowed by local configuration (e.g. ICMPv6), then process the upper-layer header. Otherwise, send an ICMP parameter problem message to the Source Address and discard the packet. Error code 4 (SR Upper-layer Header Error) and Pointer set to the offset of the upper-layer header.

The "Endpoint with cross-connect to an array of layer-3 adjacencies" behavior (End.X for short) is a variant of the End behavior. It is the SRv6 instantiation of an Adjacency-SID and it is required to express any traffic-engineering policy. Any SID instance of this behavior is associated with a set, J, of one or more Layer-3 adjacencies. When N receives a packet destined to S and S is a local End.X SID, the line S15 from the End processing is replaced by the following:

Notes: S15. If the set J contains several L3 adjacencies, then one element of the set is selected based on a hash of the packet's header . If a node N has 30 outgoing interfaces to 30 neighbors, usually the operator would explicitly instantiate 30 End.X SIDs at N: one per layer-3 adjacency to a neighbor. Potentially, more End.X could be explicitly defined (groups of layer-3 adjacencies to the same neighbor or to different neighbors). Note that if N has an outgoing interface bundle I to a neighbor Q made of 10 member links, N may allocate up to 11 End.X local SIDs: one for the bundle(LAG) itself and then up to one for each Layer-2 member link. When the End.X behavior is associated with a BGP Next-Hop, it is the SRv6 instantiation of the BGP Peering Segments .

The "Endpoint with specific IPv6 table lookup" behavior (End.T for short) is a variant of the End behavior. The End.T behavior is used for multi-table operation in the core. For this reason, an instance of the End.T behavior is associated with an IPv6 FIB table T. When N receives a packet destined to S and S is a local End.T SID, the line S15 from the End processing is replaced by the following:

The "Endpoint with decapsulation and cross-connect to an array of IPv6 adjacencies" behavior (End.DX6 for short) is a variant of the End.X behavior. One of the applications of the End.DX6 behavior is the L3VPNv6 use-case where a FIB lookup in a specific tenant table at the egress PE is not required. This is equivalent to the per-CE VPN label in MPLS . The End.DX6 SID MUST be the last segment in a SR Policy, and it is associated with one or more L3 IPv6 adjacencies J. When N receives a packet destined to S and S is a local End.DX6 SID, N does the following processing:

When processing the Upper-layer header of a packet matching a FIB entry locally instantiated as an SRv6 End.DX6 SID, the following is done:

Notes: S01. 41 refers to IPv6 encapsulation as defined by IANA allocation for Internet Protocol Numbers. S05. If the End.DX6 SID is bound to an array of L3 adjacencies, then one entry of the array is selected based on the hash of the packet's header .

The "Endpoint with decapsulation and cross-connect to an array of IPv4 adjacencies" behavior (End.DX4 for short) is a variant of the End.X behavior. One of the applications of the End.DX4 behavior is the L3VPNv4 use-case where a FIB lookup in a specific tenant table at the egress PE is not required. This is equivalent to the per-CE VPN label in MPLS . The End.DX4 SID MUST be the last segment in a SR Policy, and it is associated with one or more L3 IPv4 adjacencies J. When N receives a packet destined to S and S is a local End.DX4 SID, N does the following processing:

When processing the Upper-layer header of a packet matching a FIB entry locally instantiated as an SRv6 End.DX4 SID, the following is done:

Notes: S01. 4 refers to IPv4 encapsulation as defined by IANA allocation for Internet Protocol Numbers S05. If the End.DX4 SID is bound to an array of L3 adjacencies, then one entry of the array is selected based on the hash of the packet's header .

The "Endpoint with decapsulation and specific IPv6 table lookup" behavior (End.DT6 for short) is a variant of the End.T behavior. One of the applications of the End.DT6 behavior is the L3VPNv6 use-case where a FIB lookup in a specific tenant table at the egress PE is required. This is equivalent to the per-VRF VPN label in MPLS . Note that an End.DT6 may be defined for the main IPv6 table in which case and End.DT6 supports the equivalent of an IPv6inIPv6 decapsulation (without VPN/tenant implication). The End.DT6 SID MUST be the last segment in a SR Policy, and a SID instance is associated with an IPv6 FIB table T. When N receives a packet destined to S and S is a local End.DT6 SID, N does the following processing:

When processing the Upper-layer header of a packet matching a FIB entry locally instantiated as an SRv6 End.DT6 SID, N does the following:

The "Endpoint with decapsulation and specific IPv4 table lookup" behavior (End.DT4 for short) is a variant of the End behavior. One of the applications of the End.DT4 behavior is the L3VPNv4 use-case where a FIB lookup in a specific tenant table at the egress PE is required. This is equivalent to the per-VRF VPN label in MPLS . Note that an End.DT4 may be defined for the main IPv4 table in which case an End.DT4 supports the equivalent of an IPv4inIPv6 decapsulation (without VPN/tenant implication). The End.DT4 SID MUST be the last segment in a SR Policy, and a SID instance is associated with an IPv4 FIB table T. When N receives a packet destined to S and S is a local End.DT4 SID, N does the following processing:

When processing the Upper-layer header of a packet matching a FIB entry locally instantiated as an SRv6 End.DT4 SID, N does the following:

The "Endpoint with decapsulation and specific IP table lookup" behavior (End.DT46 for short) is a variant of the End.DT4 and End.DT6 behavior. One of the applications of the End.DT46 behavior is the L3VPN use-case where a FIB lookup in a specific IP tenant table at the egress PE is required. This is equivalent to single per-VRF VPN label (for IPv4 and IPv6) in MPLS. Note that an End.DT46 may be defined for the main IP table in which case an End.DT46 supports the equivalent of an IPinIPv6 decapsulation(without VPN/tenant implication). The End.DT46 SID MUST be the last segment in a SR Policy, and a SID instance is associated with an IPv4 FIB table T4 and an IPv6 FIB table T6. When N receives a packet destined to S and S is a local End.DT46 SID, N does the following processing:

When processing the Upper-layer header of a packet matching a FIB entry locally instantiated as an SRv6 End.DT46 SID, N does the following:

The "Endpoint with decapsulation and Layer-2 cross-connect to an outgoing L2 interface (OIF)" (End.DX2 for short) is a variant of the endpoint behavior. One of the applications of the End.DX2 behavior is the L2VPN/EVPN VPWS use-case. The End.DX2 SID MUST be the last segment in a SR Policy, and it is associated with one outgoing interface I. When N receives a packet destined to S and S is a local End.DX2 SID, N does:

When processing the Upper-layer header of a packet matching a FIB entry locally instantiated as an SRv6 End.DX2 SID, the following is done:

Notes: S04. An End.DX2 behavior could be customized to expect a specific IEEE header (e.g. VLAN tag) and rewrite the egress IEEE header before forwarding on the outgoing interface.

The "Endpoint with decapsulation and specific VLAN table lookup" behavior (End.DX2V for short) is a variant of the End.DX2 behavior. One of the applications of the End.DX2V behavior is the EVPN Flexible cross-connect use-case. The End.DX2V behavior is used to perform a lookup of the Ethernet frame VLANs in a particular L2 table. Any SID instance of this behavior is associated with an L2 Table T. When N receives a packet whose IPv6 DA is S and S is a local End.DX2 SID, the processing is identical to the End.DX2 behavior except for the Upper-layer header processing which is modified as follows:

Notes: An End.DX2V behavior could be customized to expect a specific VLAN format and rewrite the egress VLAN header before forwarding on the outgoing interface.

The "Endpoint with decapsulation and specific unicast MAC L2 table lookup" behavior (End.DT2U for short) is a variant of the End behavior. One of the applications of the End.DT2U behavior is the EVPN Bridging unicast. Any SID instance of the End.DT2U behavior is associated with an L2 Table T. When N receives a packet whose IPv6 DA is S and S is a local End.DT2U SID, the processing is identical to the End.DX2 behavior except for the Upper-layer header processing which is as follows:

Notes: S05. In EVPN, the learning of the exposed MAC Source Address is done via the control plane.

The "Endpoint with decapsulation and specific L2 table flooding" behavior (End.DT2M for short) is a variant of the End.DT2U behavior. Two of the applications of the End.DT2M behavior are the EVPN Bridging BUM with ESI filtering and the EVPN ETREE use-cases. Any SID instance of this behavior is associated with a L2 table T. Additionally the behavior MAY take an argument: "Arg.FE2". It is an argument specific to EVPN ESI filtering and EVPN-ETREE used to exclude specific OIF (or set of OIFs) from L2 table T flooding. When N receives a packet whose IPv6 DA is S and S is a local End.DT2M SID, the processing is identical to the End.DX2 behavior except for the Upper-layer header processing which is as follows:

Notes: S05. In EVPN, the learning of the exposed MAC Source Address is done via control plane Arg.FE2 is encoded in the SID as an (k*x)-bit value. These bits represent a list of up to k OIFs, each identified with an x-bit value. Values k and x are defined on a per End.DT2M SID basis. The interface identifier 0 indicates an empty entry in the interface list.

This is a variation of the End behavior. One of its applications is to express scalable traffic-engineering policies across multiple domains. it is one of the SRv6 instantiations of a Binding SID . Any SID instance of this behavior is associated with an SR Policy B and a source address A. When N receives a packet whose IPv6 DA is S and S is a local End.B6.Encaps SID, does:

max_LE) or (Segments Left > (Last Entry+1)) { S10. Send an ICMP Parameter Problem to the Source Address, Code 0 (Erroneous header field encountered), Pointer set to the Segments Left field. Interrupt packet processing and discard the packet. S11. } S12. Decrement Hop Limit by 1 S13. Decrement Segments Left by 1 S14. Update IPv6 DA with Segment List[Segments Left] S15. Push a new IPv6 header with its own SRH containing B S16. Set the outer IPv6 SA to A S17. Set the outer IPv6 DA to the first SID of B S18. Set the outer PayloadLength, Traffic Class, FlowLabel and Next-Header fields S19. Submit the packet to the egress IPv6 FIB lookup and transmission to the new destination S20. } ]]> Notes: S14. The SRH MAY be omitted when the SRv6 Policy B only contains one SID and there is no need to use any flag, tag or TLV. S17. The Payload Length, Traffic Class and Next-Header fields are set as per . The Flow Label is computed as per . When processing the Upper-layer header of a packet matching a FIB entry locally instantiated as an SRv6 End.B6.Encaps SID, process the packet as per .

This is an optimization of the End.B6.Encaps behavior. End.B6.Encaps.Red reduces the size of the SRH by one SID by excluding the first SID in the SRH of the new IPv6 header. Thus the first segment is only placed in the IPv6 Destination Address of the new IPv6 header and the packet is forwarded according to it. The SRH Last Entry field is set as defined in Section 4.1.1 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 "Endpoint bound to an SR-MPLS Policy" is a variant of the End behavior. The End.BM behavior is required to express scalable traffic-engineering policies across multiple domains where some domains support the MPLS instantiation of Segment Routing. This is an SRv6 instantiation of an SR-MPLS Binding SID . Any SID instance of this behavior is associated with an SR-MPLS Policy B. When N receives a packet whose IPv6 DA is S and S is a local End.BM SID, does:

The PSP, USP and USD flavors are variants of the End, End.X and End.T behaviors. For each of these behaviors these flavors MAY be supported for a SID either individually or in combinations.

SR Segment Endpoint Nodes advertise the SIDs instantiated on them via control plane protocols as described in . Different behavior ids are allocated for flavored and unflavored SIDs (see ). An SR Segment Endpoint Node that offers both PSP and non-PSP flavored behavior advertises them as two different SIDs. The SR Segment Endpoint Node only advertises the PSP flavor if the operator enables this capability at the node. The PSP operation is deterministically controlled by the SR Source Node. A PSP-flavored SID is used by the Source SR Node when it needs to instruct the penultimate SR Segment Endpoint Node listed in the SRH to remove the SRH from the IPv6 header.

SR Segment Endpoint Nodes receive the IPv6 packet with the Destination Address field of the IPv6 Header equal to its SID address. A penultimate SR Segment Endpoint Node is one that, as part of the SID processing, copies the last SID from the SRH into the IPv6 Destination Address and decrements Segments Left value from one to zero. The PSP operation only takes place at a penultimate SR Segment Endpoint Node and does not happen at any Transit Node. When a SID of PSP-flavor is processed at a non-penultimate SR Segment Endpoint Node, the PSP behavior is not performed as described in the pseudocode below since Segments Left would not be zero. The SRH processing of the End, End.X and End.T behaviors are modified: after the instruction "S14. Update IPv6 DA with Segment List[Segments Left]" is executed, the following instructions must be executed as well:

The usage of PSP does not increase the MTU of the IPv6 packet and hence does not have any impact on the PMTU discovery mechanism. As a reminder, defines in section 5 the SR Deployment Model within the SR Domain . Within this framework, the Authentication Header (AH) is not used to secure the SRH as described in Section 7.5 of . This behavior does not contravene Section 4 of because the current destination address of the incoming packet is the address of the node executing the PSP behavior.

One use-case for the PSP functionality is streamlining the operation of an egress border router.

+R2+-------->+R3+-------->+R4+-------->+ePE| | R1| +--+ +--+ +--+ |R5 | +-+-+ +-----+ +-----+ +-----+ +-----+ +-+-+ | |IPv6 | |IPv6 | |IPv6 | |IPv6 | | | |DA=R3| |DA=R3| |DA=R5| |DA=R5| | | +-----+ +-----+ +-----+ +-----+ | | | SRH | | SRH | | IP | | IP | | | |SL=1 | |SL=1 | +-----+ +-----+ | | | R5 | | R5 | | | +-----+ +-----+ | | | IP | | IP | | | +-----+ +-----+ | | | +----------------------------------------------------+ ]]> In the above illustration, for a packet sent from iPE to ePE, node R3 is an intermediate traffic engineering waypoint and is the penultimate segment endpoint router; the node that copies the last segment from the SRH into the IPv6 Destination Address and decrements segments left to 0. The SDN controller knows that no-other node after R3 needs to inspect the SRH, and it instructs R3 to remove the exhausted SRH from the packet by using a PSP-flavored SID. The benefits for the egress PE are straightforward: as part of the decapsulation process the egress PE is required to terminates less bytes from the packet. if a lookup on an upper-layer IP header is required (e.g. per-VRF VPN), the header is more likely to be within the memory accessible to the lookup engine in the forwarding ASIC.

The SRH processing of the End, End.X and End.T behaviors are modified: the instructions S02-S04 are substituted by the following ones:

The Upper-layer header processing of the End, End.X and End.T behaviors are modified as follows:

An implementation that supports the USD flavor in conjunction with the USP flavor MAY optimize the packet processing by first looking whether the conditions for the USD flavor are met, in which case it can proceed with USD processing else do USP processing.

This section describes a set of SR Policy Headend behaviors.

This list can be expanded in case any new functionality requires it.

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. N steers the transit packets P1 and P2 into an SR Policy with a Source Address T and a Segment list <S1, S2, S3>. The H.Encaps encapsulation behavior is defined as follows:

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 TTL or Hop Limit that is decremented as described in ). The H.Encaps behavior is valid for any kind of Layer-3 traffic. This behavior is commonly used for L3VPN with IPv4 and IPv6 deployments. It may be also used for 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. S03: As described in (IPv6 Flow Label Specification)

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.

The H.Encaps.L2 behavior encapsulates a received Ethernet 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 or frame check sequence (FCS) from the Ethernet frame upon encapsulation and the decapsulating node MUST regenerate the preamble or FCS before forwarding Ethernet frame.

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 teh SRH of the pushed IPv6 header. The first SID is only places 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.

A node supporting this document SHOULD implement a combined traffic counter (packets and bytes) per local SID entry, for traffic that matched that SID and was processed correctly.

When a flow-based selection within a set needs to be performed, the source address, the destination address and the flow label MUST be included in the flow-based hash. This occurs when a FIB lookup is performed and multiple ECMP paths exist to the updated destination address. This occurs when End.X, End.DX4, or End.DX6 are bound to an array of adjacencies. This occurs when the packet is steered in an SR policy whose selected path has multiple SID lists. Additionally, any transit router in an SRv6 domain includes the outer flow label in its ECMP load-balancing hash .

The security considerations for Segment Routing are discussed in . More specifically for SRv6 the security considerations and the mechanisms for securing an SR domain are discussed in . Together, they describe the required security mechanisms that allow establishment of an SR domain of trust to operate SRv6-based services for internal traffic while preventing any external traffic from accessing or exploiting the SRv6-based services. This document introduces SRv6 Endpoint and SR Policy Headend behaviors for implementation on SRv6 capable nodes in the network. As such, this document does not introduce any new security considerations.

In an SDN environment, one expects the controller to explicitly provision the SIDs and/or discover them as part of a service discovery function. Applications residing on top of the controller could then discover the required SIDs and combine them to form a distributed network program. The concept of "SRv6 network programming" refers to the capability for an application to encode any complex program as a set of individual functions distributed through the network. Some functions relate to underlay SLA, others to overlay/tenant, others to complex applications residing in VM and containers. This section provides a high level overview of the control-plane protocols involved with SRv6 and their specification.

The End, End.T and End.X SIDs express topological behaviors and hence are expected to be signaled in the IGP together with the flavors PSP, USP and USD. The IGP should also advertise the maximum SRv6 SID depth (MSD) capability of the node for each type of SRv6 operation. In particular, the SR source (e.g., H.Encaps), intermediate endpoint (e.g., End, End.X) and final endpoint (e.g., End.DX4, End.DT6) behaviors. These capabilities are factored in by an SR Source Node (or a controller) during the SR Policy computation. The presence of SIDs in the IGP do not imply any routing semantics to the addresses represented by these SIDs. The routing reachability to an IPv6 address is solely governed by the, non-SID-related, IGP prefix reachability information that includes locators. Routing is not governed neither influenced in any way by a SID advertisement in the IGP. These SIDs provide important topological behaviors for the IGP to build TI-LFA based FRR solutions and for TE processes relying on IGP topology database to build SR policies.

BGP-LS provides the functionality for topology discovery that includes the SRv6 capabilities of the nodes, their locators and locally instantiated SIDs . This enables controllers or applications to build an inter-domain topology that can be used for computation of SR Policies using the SRv6 SIDs.

The End.DX4, End.DX6, End.DT4, End.DT6, End.DT46, End.DX2, End.DX2V, End.DT2U and End.DT2M SIDs can be signaled in BGP .

The following table summarizes behaviors for SIDs that can be signaled in which each respective control plane protocol. IGP BGP-LS BGP IP/VPN/EVPN End (PSP, USP, USD) X X End.X (PSP, USP, USD) X X End.T (PSP, USP, USD) X X End.DX6 X X X End.DX4 X X X End.DT6 X X X End.DT4 X X X End.DT46 X X X End.DX2 X X End.DX2V X X End.DT2U X X End.DT2M X X End.B6.Encaps X End.B6.Encaps.Red X End.B6.BM X The following table summarizes which SR Policy Headend capabilities are signaled in which signaling protocol. IGP BGP-LS BGP IP/VPN/EVPN H.Encaps X X H.Encaps.Red X X H.Encaps.L2 X H.Encaps.L2.Red X The previous table describes generic capabilities. It does not describe specific instantiated SR policies. For example, a BGP-LS advertisement of H.Encaps behavior would describe the capability of node N to perform a H.Encaps behavior, specifically it would describe how many SIDs could be pushed by N without significant performance degradation. As a reminder, an SR policy is always assigned a Binding SID . BSIDs are also advertised in BGP-LS as shown in . Hence, the only focuses on the generic capabilities related to H.Encaps.

This document requests IANA to allocate, in the "Protocol Numbers" registry (https://www.iana.org/assignments/protocol-numbers/protocol-numbers.xhtml), a new value for "Ethernet" with the following definition: The value 143 in the Next Header field of an IPv6 header or any extension header indicates that the payload is an Ethernet . IANA has done a temporary allocation of Protocol Number 143.

This document requests IANA to create a new top-level registry called "Segment Routing Parameters". This registry is being defined to serve as a top-level registry for keeping all other Segment Routing sub-registries. Additionally, a new sub-registry "SRv6 Endpoint Behaviors" is to be created under top-level "Segment Routing Parameters" registry. This sub-registry maintains 16-bit identifiers for the SRv6 Endpoint behaviors. This registry is established to provide consistency for control plane protocols which need to refer to these behaviors. These values are not encoded in the function bits within a SID. The range of the registry is 0-65535 (0x0000 - 0xFFFF) and has the following registration rules and allocation policies: Range Hex Registration procedure Notes 0 0x0000 Reserved Not to be allocated 1-32767 0x0001-0x7FFF FCFS 32768-65534 0x8000-0xFFFE Reserved 65535 0xFFFF Reserved Opaque Requests for allocation from within the FCFS range must include a point of contact and preferably also a brief description of how the value will be used. This information may be provided with a reference to an Internet Draft or an RFC or in some other documentation that is permanently and readily available.

The initial registrations for the sub-registry are as follows: Value Hex Endpoint behavior Reference 0 0x0000 Reserved Not to be allocated 1 0x0001 End (no PSP, no USP) [This.ID] 2 0x0002 End with PSP [This.ID] 3 0x0003 End with USP [This.ID] 4 0x0004 End with PSP&USP [This.ID] 5 0x0005 End.X (no PSP, no USP) [This.ID] 6 0x0006 End.X with PSP [This.ID] 7 0x0007 End.X with USP [This.ID] 8 0x0008 End.X with PSP&USP [This.ID] 9 0x0009 End.T (no PSP, no USP) [This.ID] 10 0x000A End.T with PSP [This.ID] 11 0x000B End.T with USP [This.ID] 12 0x000C End.T with PSP&USP [This.ID] 14 0x000E End.B6.Encaps [This.ID] 15 0x000F End.BM [This.ID] 16 0x0010 End.DX6 [This.ID] 17 0x0011 End.DX4 [This.ID] 18 0x0012 End.DT6 [This.ID] 19 0x0013 End.DT4 [This.ID] 20 0x0014 End.DT46 [This.ID] 21 0x0015 End.DX2 [This.ID] 22 0x0016 End.DX2V [This.ID] 23 0x0017 End.DT2U [This.ID] 24 0x0018 End.DT2M [This.ID] 25 0x0019 Reserved [This.ID] 27 0x001B End.B6.Encaps.Red [This.ID] 28 0x001C End with USD [This.ID] 29 0x001D End with PSP&USD [This.ID] 30 0x001E End with USP&USD [This.ID] 31 0x001F End with PSP, USP & USD [This.ID] 32 0x0020 End.X with USD [This.ID] 33 0x0021 End.X with PSP&USD [This.ID] 34 0x0022 End.X with USP&USD [This.ID] 35 0x0023 End.X with PSP, USP & USD [This.ID] 36 0x0024 End.T with USD [This.ID] 37 0x0025 End.T with PSP&USD [This.ID] 38 0x0026 End.T with USP&USD [This.ID] 39 0x0027 End.T with PSP, USP & USD [This.ID] 40-32766 Unassigned 32767 0x7FFF The SID defined in RFC8754 [This.ID] 32768-65534 Reserved 65535 0xFFFF Opaque [This.ID]

The authors would like to acknowledge Stefano Previdi, Dave Barach, Mark Townsley, Peter Psenak, Thierry Couture, Kris Michielsen, Paul Wells, Robert Hanzl, Dan Ye, Gaurav Dawra, Faisal Iqbal, Jaganbabu Rajamanickam, David Toscano, Asif Islam, Jianda Liu, Yunpeng Zhang, Jiaoming Li, Narendra A.K, Mike Mc Gourty, Bhupendra Yadav, Sherif Toulan, Satish Damodaran, John Bettink, Kishore Nandyala Veera Venk, Jisu Bhattacharya, Saleem Hafeez and Brian Carpenter.

Daniel Bernier Bell Canada Canada Email: daniel.bernier@bell.ca Dirk Steinberg Lapishills Consulting Limited Cyprus Email: dirk@lapishills.com Robert Raszuk Bloomberg LP United States of America Email: robert@raszuk.net Bruno Decraene Orange France Email: bruno.decraene@orange.com Bart Peirens Proximus Belgium Email: bart.peirens@proximus.com Hani Elmalky Google United States of America Email: helmalky@google.com Prem Jonnalagadda Barefoot Networks United States of America Email: prem@barefootnetworks.com Milad Sharif SambaNova Systems United States of America Email: milad.sharif@sambanova.ai David Lebrun Google Belgium Email: dlebrun@google.com Stefano Salsano Universita di Roma "Tor Vergata" Italy Email: stefano.salsano@uniroma2.it Ahmed AbdelSalam Gran Sasso Science Institute Italy Email: ahmed.abdelsalam@gssi.it Gaurav Naik Drexel University United States of America Email: gn@drexel.edu Arthi Ayyangar Arrcus, Inc United States of America Email: arthi@arrcus.com Satish Mynam Arrcus, Inc United States of America Email: satishm@arrcus.com Wim Henderickx Nokia Belgium Email: wim.henderickx@nokia.com Shaowen Ma Juniper Singapore Email: mashao@juniper.net Ahmed Bashandy Individual United States of America Email: abashandy.ietf@gmail.com Francois Clad Cisco Systems, Inc. France Email: fclad@cisco.com Kamran Raza Cisco Systems, Inc. Canada Email: skraza@cisco.com Darren Dukes Cisco Systems, Inc. Canada Email: ddukes@cisco.com Patrice Brissete Cisco Systems, Inc. Canada Email: pbrisset@cisco.com Zafar Ali Cisco Systems, Inc. United States of America Email: zali@cisco.com Ketan Talaulikar Cisco Systems, Inc. India Email: ketant@cisco.com