Generalized SRv6 Network Programming for SRv6 Compression

Segment routing (SR) is a source routing paradigm that explicitly indicates the forwarding path for packets at the ingress node by inserting an ordered list of instructions, called segments. When segment routing is deployed on the IPv6 data plane, it is called SRv6 . For support of SR, a new routing header called Segment Routing Header (SRH), which contains a list of SIDs and other information, has been defined in . In use cases like Traffic Engineering, an ordered SID List with multiple SIDs is inserted into the SRH to steer packets along an explicit path. However, the size of SIDs (16 bytes per SID) in SRH proposes challenges for packet processing and payload efficiency . In order to solve this problem, this document proposes Generalized Segment Routing over IPv6 (G-SRv6) Networking Programming for SRv6 compression. G-SRv6 supports to encode multiple types of Segments in an SRH, called Generalized SRH (G-SRH). In SRv6 Compression, the G-SRH can carry multiple SRv6 SID and G-SID(Generalized Segment Identifier) containers in the SID list. A G-SID container may include an SRv6 SID or multiple G-SIDs and optional padding. A G-SID can be a 32-bits value of the original SRv6 SID, which contains the node ID and function ID. By carrying G-SIDs instead of 128 bits SRv6 SID, the problem of SRv6 header size can be solved, and the solution is compatible with SRv6.

This document makes use of the terms defined in , and , and the reader is assumed to be familiar with that terminology. This document introduces the following terms: Compressible SRv6 SID: It is the 128-bit SRv6 SID whose format can be compressed. It is composed by Common Prefix and Generalized Segment Identifier (G-SID) and optional arguments and padding. Common Prefix: It is the same prefix shared by multiple SIDs. G-SRv6: Generalized SRv6 Network Programming G-SRH: Generalized Segment Routing Header. It keeps the same format and code point with original SRH, which can carry multiple G-SIDs and original SIDs. G-SID: Generalized Segment Identifier.It is a Compressed SID(C-SID) . G-SID Container: Generalized Segment Identifier Container.It is a C-SID container .

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.

This section describes the concepts of G-SRv6.

In an SRv6 domain, the SIDs are allocated from an address block, called SID space. Therefore, the SIDs allocated from the same SID space share the common prefix. Also, if the length of the SID is less than 128 bits, then padding is required. In an SID List, the common prefix and padding are redundant. Reducing the redundant information can reduce the overhead of SRv6. This document defines a Generalized SID (G-SID) to carry the different part of the original SRv6 SID in the SRH to reduce the size of the SRH. The G-SID can be a 32-bits value following the common prefix in the original SRv6 SID. An SRv6 SID with this format is called compressible SRv6 SID. The format of a compressible SRv6 SID with 32-bits G-SID is shown in Figure 1.

| Figure 1. 32 bits G-SID in SRv6 SID ]]> In order to indicate the format of the SRv6 SID is compressible, control plane extension may be considered. This is out of scope of this document, and can be described in other documents.

In order to align with 128 bits, a 128 bit G-SID Container is defined. A G-SID Container is a 128 bits value, and it may contain different type of SIDs: an SRv6 SID: A G-SID Container contains a single SRv6 SID. A Micro SID Carrier: A G-SID Container contains a Micro SID carrier . Multiple G-SIDs: A G-SID Container contains multiple G-SIDs and optional padding. When G-SID is a 32-bits value, a G-SID Container can consist of 4 G-SIDs. If the length of G-SIDs in a G-SID Container is less than 128 bits, then padding is required.

In order to locate the G-SID within the G-SID Container, this section defines Generalized SID Index (SI) to indicate the location of the G-SID within the current G-SID Container. SI is a location argument of the G-SID, which is the least bits in the argument part. When G-SID is a 32 bits value, the SI is the least 2 bits in Argument.

In order to indicate the SRv6 compression processing, updating the next 32-bits G-SID to the IPv6 DA, this section defines COC(Continue of Compression) Flavor. When a node receives an SID with COC Flavor, it indicates to update the G-SID part in IPv6 DA with the next 32 bits G-SID. When a node receives an SID without COC Flavor, the node processes the packet as a normal SRv6 packet , for example, update the IPv6 DA with the next 128 bits SID if SL >0. Therefore, if the behavior of the last G-SID in the G-SID list has no COC Flavor, then the next 128 bits SID will be updated to the DA, so it indicates the end of the compression sub-path. When COC Flavor applies to END, END.X and END.T, the SIDs can be advertised via the IS-IS , and the SRv6 SID Structure Sub-Sub-TLV MUST be carried to indicate the format of the SRv6 SID. The Locator.Block length indicates the length of the common prefix, and the G-SID is the following 32-bits value after the Block, which contains the Node ID and Function ID.

G-SRH supports to encode different types of segment in a single SRH without modifying the encapsulation format of SRH. When an SRv6 path travels normal SRv6 nodes and compressed SRv6 nodes, the SRv6 SID and G-SIDs can be encoded in a single G-SRH. For easier understanding, this document assumes that the Compressible SRv6 SID consists of 64 bits common prefix and 32 bits G-SID. The encoding can be shown as follows.

Where: Common Prefix: the common prefix shared by the Compressible SRv6 SIDs in the current compression sub-path. Usually, it is the prefix of the SID space, called Locator Block in control plane . Operators are free to configure the length and the value of the common prefix based on the address planning of their networking. G-SID: 32-bits Generalized SID. Padding: Must be zero. When the length of G-SIDs within the G-SID Container is less than 128 bits, then padding is needed.

This section describes the pseudo code of COC Flavor, and it replaces the S13 and S14 of End, End.X, and End.T's pseudo code . The pseudo code is shown below. When N receives a packet whose IPv6 DA is S and S is a local SID with COC Flavor, N does:

Ref1: an SID with COC flavor indicates the SRv6 compression processing that the node needs to update the next 32 bits G-SID to the IPv6 DA. When the SI is greater than 0, the next G-SID is the next G-SID in the current G-SID Container. Otherwise, the next G-SID is the first G-SID in the next G-SID Container. Ref2: B is the length of the Locator Block . An SID without COC Flavor will be processed following the SRv6 processing. The node will update the next 128 bit SID to the IPv6 DA if the SL > 0.

This section describes a simple example of G-SRv6 for compression. The reference topology is shown below.

Nodes 0 - 10 are G-SRv6 enabled nodes within the SRv6 domain, and node 0 is the ingress node of the G-SRv6 path while the node 10 is the egress node. Nodes CE1 and CE2 are tenants of VPN 10, and they are outside of the SRv6 domain. In order to ease the reading of the example, this section introduces a simplified SID allocation schema. 2001:db8::/64 is dedicated to the internal SRv6 SID space, which is the common prefix for the SIDs as well. Node k has 2001:db8:0:0:k::/80 for its local SID space. Its SIDs will be explicitly allocated from that block. 2001:db8:0:0:k:1:: represents the End.X SID with COC allocated by node K, and it is associated with interface N of node K. For instance, 2001:db8:0:0:1:1:: represents the End.X with COC flavor allocated by node 1. 2001:db8:0:0:k:2:: represents the End.X SID without COC allocated by node K, and it is associated with interface N of node K. For instance, 2001:db8:0:0:1:2:: represents the End.X without COC flavor allocated by node 1. 2001:db8:0:0:10:10:: is an END.DT4 SID initiated by node 10, which is associated with the VRF10. Therefore, the SID 2001:db8:0:0:1:1::, 2001:db8:0:0:2:1::, 2001:db8:0:0:3:1::, 2001:db8:0:0:4:1::, 2001:db8:0:0:5:1::, 2001:db8:0:0:6:1::, 2001:db8:0:0:7:1::, 2001:db8:0:0:8:1:: are SRv6 End.X SIDs with COC Flavor, and 2001:db8:0:0:9:2:: is a Compressible SRv6 End.X SID. The SID list [2001:db8:0:0:1:1::, 2001:db8:0:0:2:1::, 2001:db8:0:0:3:1::, 2001:db8:0:0:4:1::, 2001:db8:0:0:5:1::, 2001:db8:0:0:6:1::, 2001:db8:0:0:7:1::, 2001:db8:0:0:8:1::, 2001:db8:0:0:9:2::, 2001:db8:0:0:10:10::] is calculated for a strict TE path from Node 1 to Node 10 for the VPN traffic of tenant 10. In G-SRv6, the SID list can be encoded as [2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:2, 2001:db8:0:0:10:10::] in reduced mode. The G-SID Container encoding is shown below.

The packets forwarding procedures: Node 0 sends the SRv6 packet with G-SRH to the node 1.The SL is 3. The Active SID in IPv6 DA is 2001:db8:0:0:1:1::. When node 1 receives the packet, the IPv6 DA is 2001:db8:0:0:1:1::, which is a Local End.X with COC Flavor SID. The SRH.SL is 3, and DA.SI is 0. The node processes the packet: SL = SL-1,DA.SI =3, pointing to the next G-SID 2:1, and updates SRH[SL=2][DA.SI=3] to the IPv6 DA[CP:CP+31], where CP is the length of the common prefix. The packet is forwarded with the new IPv6 DA 2001:db8:0:0:2:1:C::, to the node 2. When node 2 receives the packet, the IPv6 DA is 2001:db8:0:0:2:1:C::, which is a Local End.X with COC Flavor SID. The SRH.SL is 2, and DA.SI is 3. The node processes the packet: DA.SI --, pointing to the next G-SID 3:1, and updates SRH[SL=2][DA.SI=2] to the IPv6 DA[CP:CP+31]. The packet is forwarded with the new IPv6 DA 2001:db8:0:0:3:1:8::, to the node 3. Similar to node 1 and 2, the node 3,4,5,6,7,8 process the packet and forward with the new IPv6 DA. When node 9 receives the packet, the IPv6 DA is 2001:db8:0:0:9:2::, which is a Local End.X SID. The SRH.SL is 1. The node updates the next SID 2001:db8:0:0:10:10:: to the IPv6 DA and forwards the packet to the node 10. Node 10 receives the packet, and the IPv6 DA is an VPN SID allocated by itself, the node processes the SRv6 VPN SID. This illustration shows that 70 % overhead of SID list is removed in G-SRv6(10 x 16 Bytes to 3 x 16 Bytes), also, it shows the capabilities of encoding G-SIDs and SRv6 SIDs in a single G-SRH.

G-SRv6 is fully compatible with SRv6 No SRH encapsulation modification. No new address consumption: Compressible SRv6 SIDs can be allocated from the Locator allocated to the node. No new route advertisements: Compressible SRv6 SIDs can share the same locator with the normal SRv6 SID. No security policy modification: when reusing the Locator with SRv6 SIDs, no security policy need to be updated. No control plane modification: Controller can install the SR policy with 128-bits G-SID Containers, and the ingress treats the G-SID Container as an opaque 128-bits SID without understanding the structure of it. G-SRv6 capable nodes understand the COC flavor behaviors, while Compression disable SRv6 nodes are unaware of Compression. G-SRv6 reduces the SRv6 encapsulation size. 128 bits to 32 bits, up to 75 % overhead is reduced. More overhead is reduced when the G-SID is a 16-bits value. G-SRv6 has efficient address consumption and easy to deploy Operators are free to allocate an SID space from their address space. No affect of networking(i.e. routes and ACL security policies) by using the existing Locator to allocate compressible SRv6 SIDs. G-SRv6 is hardware friendly Same SRv6 processing flow with a new IPv6 DA update method Leverages the mature hardware capabilities (DA update, DA longest match) Avoids extra lookup in indexed mapping table G-SRv6 supports incremental deployments, which can be deployed on demand.

The G-SRv6 mechanism has been implemented on the following 10+ hardware devices, software implementations and SDN controllers. They had also successfully participated in the series of joint interoperability testing events hosted by China Mobile from June 2020 to November 2020. The following hardware devices and software implementations had successfully passed the series of G-SRv6 dataplane interoperability testing (in alphabetical order). Chipsets Broadcom Jericho 2 BCM88690 Centec CTC7132 Intel Barefoot Tofino BFN-T10 Marvell Falcon 98CX8580 Devices Cisco ASR 9000 Cisco IOS XRv9000 Huawei NE40E Huawei NE5000E H3C CR16010H-FA H3C CR19000-8 Ruijie F9300 Switch ZTE M6000-8S Plus ZTE M6000-3S Test Equipment IXIA XGS12 Spirent TestCenter N4U The following hardware devices and software implementations had successfully passed the series of G-SRv6 with control plane interoperability test (in alphabetical order). China Unitechs Unified Controller Huawei NE40E and NE5000E H3C CR16010H-FA and CR19000-8 Spirent TestCenter N4U ZTE M6000-8S Plus and M6000-3S Regarding open-source implementations, G-SRv6 has been implemented on Linux Kernel.

In addition, China Mobile had come up with China Unitechs, Huawei, ZTE and H3C to successfully deploy trial of G-SRv6 (with control plane) in their three province branch networks in November 2020, respectively. The details are listed below (in alphabetical order). Huawei devices with a China Unitechs Unified Controller, Guangdong Province. L3VPN over G-SRv6 BGP TE policy. H3C devices with a China Unitechs Unified Controller, Zhejiang Province. L3VPN over G-SRv6 BGP TE policy. ZTE devices with a China Unitechs Unified Controller, Henan Province. L3VPN over G-SRv6 BGP TE policy. More information of G-SRv6 interop-test and deployment status will be updated as the work progresses.

This section describes the protocol extension requirements.

REQ1-01: An SRv6 compression path can be represented as a G-SID Container list consists of a compressible SRv6 SID and G-SID Containers. REQ1-02: A G-SID Container consists of at most 4 (32-bits) G-SIDs, if the number of G-SID is less than 4, then padding is required to align with 128 bits. REQ1-03: If the first Compressible SRv6 SID is copied to the IPv6 DA, then following G-SIDs should be updated to the IPv6 DA by the nodes along the SRv6 compression sub-path accordingly. REQ1-04: The last G-SID in the G-SID Container for the SRv6 compression sub-path is the a G-SID without COC flavor. REQ1-05: When process the G-SID with COC flavor in the IPv6 DA, the next G-SID is updated to the IPv6 DA.

REQ1-11: ISIS/OSPF/BGP-LS/PCEP extensions for advertising the capabilities of supporting G-SRv6 for SRv6 compression. REQ1-12: ISIS/OSPF/BGP-LS/BGP extensions for advertising Compressible SRv6 SIDs. REQ1-13: ISIS/OSPF/BGP-LS/BGP extensions for advertising the Continue-of-compression(COC) flavor SID. REQ1-21: BGP SR Policy extensions for programming a G-SRv6 path combining with Compressible SRv6 SIDs and SRv6 SIDs. REQ1-31: PCEP SR Policy extensions for programming a G-SRv6 path combining with G-SIDs and SRv6 SIDs. REQ1-32: PCEP extensions for programming a G-SRv6 path combining with G-SIDs and SRv6 SIDs.

This document requests IANA to allocate the following codepoints for COC flavor behaviors within the "SRv6 Endpoint Behaviors" sub-registry under the top-level "Segment Routing Parameters" registry.

The security considerations described in , and are applicable to this specification. No additional security measure is required.

TBD