Encapsulation of Path Segment in
SRv6Huawei Technologiesc.l@huawei.comChina Mobilechengweiqiang@chinamobile.comHuawei TechnologiesHuawei Campus, No. 156 Beiqing Rd.Beijing100095Chinalizhenbin@huawei.comHuawei TechnologiesDivyashree Techno Park, WhitefieldBangalore560066Indiadhruv.ietf@gmail.com6man Working GroupSegment Routing (SR) allows for a flexible definition of end-to-end
paths by encoding paths as sequences of sub-paths, called "segments".
Segment routing architecture can be implemented over IPv6 data plane,
called SRv6. In some use-cases such as end-to-end SR Path Protection and
Performance Measurement (PM), SRv6 path need to be identified. This
document defines the encoding and processing of Path Segment in SRv6
networks.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 IPv6 dataplane, it is called SRv6
, and it uses the a new IPv6 Extension Header (EH) called the IPv6 Segment Routing
Header (SRH) to construct SRv6 path. As per
, an SRv6
segment is a 128-bit value, which can be represented as LOC:FUNCT, where
LOC is the L most significant bits and FUNCT is the 128-L least
significant bits. Most often the LOC part of the SID is routable and
leads to the node which instantiates that SID. The FUNCT part of the SID
is an opaque identification of a local function bound to the SID.In several use cases, such as binding bidirectional path and end-to-end performance
measurement , the ability
to implement path identification is a pre-requisite. In SRv6, an SRv6
path can be identified by the content of the segment list. However, the
segment list may not be a good key to identify an SRv6 path, since the
length of segment list is too long and flexible according to the number
of SIDs. Therefore,
defines SRv6 Path Segment in order to identify an SRv6 path.This document defines the encoding and processing of SRv6 Path
Segment in SRv6 networks.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.PM: Performance Measurement.SID: Segment ID.SL: Segment List.SR: Segment Routing.SRH: Segment Routing Header.PSID: Path Segment Identifier.PSP: Penultimate Segment Popping.Further, this document makes use of the terms defined in and .This section will describe the encoding of SRv6 Path Segment in SRH. As per , an SRv6 Path Segment is a
128-bits value, which identifies an SRv6 path. Depending on the use
case, an SRv6 Path Segment can identify:an SRv6 path within an SRv6 domainan SRv6 Policya Candidate-paths or a SID-List in a SRv6 Policy .The SRv6 Path Segment MUST appear only once in a SID list, and it
MUST appear at the last entry, so the SRv6 Path Segment MUST NOT be
copied to the IPv6 destination address. The format of the SRv6 Path
Segment follows the format described in section 2.2.In order to indicate the existence of Path Segment in the SRH, this
document defines a P-bit in SRH flag field. The encapsulation of SRv6
Path Segment is shown below.P-bit: set when SRv6 Path Segment is inserted. It SHOULD be
ignored when a node does not support SRv6 Path Segment
processing.This document proposes two types of SRv6 Path Segment format.Editor's Note: Authors would like to request comments of these
encoding mechanisms of SRv6 Path Segment. The appropriate encoding
will be maintained while the rest will be deleted in the future
version of this document.As per ,
an SRv6 segment is a 128-bit value, which can be represented as
LOC:FUNCT, where LOC is the L most significant bits and FUNCT is the
128-L least significant bits. L is called the locator length and is
flexible. Each operator is free to use the locator length it
chooses. Most often the LOC part of the SID is routable and leads to
the node which instantiates that SID. The FUNCT part of the SID is
an opaque identification of a local function bound to the SID. The
FUNCT value zero is invalid.SRv6 Path Segment can follow the format, where the LOC part
identifies the egress node that allocates the Path Segment, and the
FUNCT part is an unique local ID to identify an SRv6 Path towards to
the egress on the egress.The Function Type of SRv6 Path Segment is END.PSID (End Function
with Path Segment Identifier, to be allocated by IANA).The proposed P bit can be used to identify that the last SID is
an SRv6 Path Segment.An SRv6 Path Segment ID can be a Global ID, and its format
depends on the use case.The SRv6 Path Segment will not be copied to the IPv6 Destination
Address, so the SRv6 Path Segment ID can be allocated from an
independent 128-bits ID Space. In this case, a new table should be
maintained at the node for SRv6 Path Segment. The proposed P bit can
be used to identify that the last SID is an SRv6 Path Segment and
need to be looked up in the SRv6 Path Segment table.As per , an SRv6 Path
Segment is a local segment allocated by an egress node. An SRv6 Path
Segment can be allocated through several ways, such as CLI, BGP , PCEP or other means. The mechanisms
through which an SRv6 Path Segment is allocated is out of scope of this
document.When the SRv6 Path Segment is allocated by the egress, it MUST be
distributed to the ingress node. In this case, only the egress will
process the SRv6 Path Segment, and other nodes specified by SIDs in the
SID list do not know how to process the SRv6 Path Segment.An SRv6 Path Segment may be distributed to the SRv6 nodes along the
SRv6 path. In this case, the SRv6 nodes that learn SRv6 Path Segment may
process the SRv6 Path Segment depending on the use case.When the SRv6 Path Segment is used, the following rules apply:The SRv6 Path Segment MUST appear only once in a SID list, and it
MUST appear at the last entry. Only the one that appears at the last
entry in the SID list will be processed. SRv6 Path Segment appears
at other location in the SID list will be treated as an error.When an SRv6 Path Segment is inserted, the SL MUST be initiated
to be less than the value of Last Entry, and will not point to SRv6
Path Segment. For instance, when the Last entry is 4, the SID
List[4] is the SRv6 Path Segment, so the SL MUST be set to 3 or
other numbers less than Last entry.The SRv6 Path Segment MUST NOT be copied to the IPv6 destination
address.Penultimate Segment Popping (PSP, as defined in ) MUST be
disabled.The ingress needs to set the P-bit when an SRv6 Path Segment is
inserted in the SID List. Nodes that supporting SRv6 Path Segment
processing will inspect the last entry to process SRv6 Path Segment
when the P-bit is set. When the P-bit is unset, the nodes will not
inspect the last entry.The specific SRv6 Path Segment processing depends on use cases,
and it is out of scope of this document.TBATBATBA