Generalized SRv6 Network Programming for
SRv6 CompressionChina MobileNo.32 Xuanwumen west streetBeijing100053Chinachengweiqiang@chinamobile.comHuawei TechnologiesHuawei Campus, No. 156 Beiqing Rd.Beijing100095Chinalizhenbin@huawei.comHuawei TechnologiesHuawei Campus, No. 156 Beiqing Rd.Beijing100095Chinac.l@huawei.comCisco Systems, IncFrancefclad@cisco.comZTE CorporationShenzhenChinaliu.aihua@zte.com.cnChina TelecomTechnology Innovation park, Changping DistrictBeijingChinaxiechf@chinatelecom.cnChina MobileNo.32 Xuanwumen west streetBeijingliuyisong@chinamobile.comBroadcomIsraelshay.zadok@broadcom.com
Routing Area
SPRING Working GroupThis document proposes Generalized Segment Routing over IPv6 (G-SRv6)
Networking Programming for SRv6 compression.G-SRv6 can reduce the overhead of SRv6 by encoding the Generalized
SIDs(G-SID) in SID list, and it also supports to program SRv6 SIDs and
G-SIDs in a single SRH to support incremental deployment and smooth
upgrade.G-SRv6 is fully compatible with SRv6 with no modification of SRH, no
new address consumption, no new route creation, and even no modification
of control plane.G-SRv6 for Compression is designed based on the Compressed SRv6
Segment List Encoding in SRH framework.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 ProgrammingG-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.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 SRv6No 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 deployOperators 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 friendlySame SRv6 processing flow with a new IPv6 DA update
methodLeverages the mature hardware capabilities (DA update, DA
longest match)Avoids extra lookup in indexed mapping tableG-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).ChipsetsBroadcom Jericho 2 BCM88690Centec CTC7132Intel Barefoot Tofino BFN-T10Marvell Falcon 98CX8580Devices Cisco ASR 9000Cisco IOS XRv9000Huawei NE40EHuawei NE5000EH3C CR16010H-FAH3C CR19000-8Ruijie F9300 SwitchZTE M6000-8S PlusZTE M6000-3STest EquipmentIXIA XGS12Spirent TestCenter N4UThe 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 ControllerHuawei NE40E and NE5000EH3C CR16010H-FA and CR19000-8Spirent TestCenter N4UZTE M6000-8S Plus and M6000-3SRegarding 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.TBDTBD