Path Segment for SRv6 (Segment Routing
in IPv6)Huawei Technologiesc.l@huawei.comChina Mobilechengweiqiang@chinamobile.comHuawei Technologiesmach.chen@huawei.comHuawei TechnologiesDivyashree Techno Park, WhitefieldBangaloreKarnataka560066Indiadhruv.ietf@gmail.comCisco Systems, Inc.Canadargandhi@cisco.comChina TelecomGuangzhouzhuyq8@chinatelecom.cnSPRING Working GroupSegment Routing (SR) allows for a flexible definition of end-to-end
paths by encoding an ordered list of instructions, called "segments".
The SR architecture can be implemented over an MPLS data plane as well
as an IPv6 data plane.Currently, Path Segment has been defined to identify an SR path in
SR-MPLS networks, and is used for various use-cases such as end-to-end
SR Path Protection and Performance Measurement (PM) of an SR path. This
document defines the Path Segment to identify an SRv6 path in an IPv6
network.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 an MPLS data plane, called
SR-MPLS , a segment identifier (SID) is present
as an MPLS label. When segment routing is deployed on an IPv6 data
plane, a SID is presented as a 128-bit value, and it can be an IPv6
address of a local interface but it does not have to be. To support SR
in an IPv6 network, a Segment Routing Header (SRH) is used.In an SR-MPLS network, when a packet is transmitted along an SR path,
the labels in the MPLS label stack will be swapped or popped, so no
label or only the last label may be left in the MPLS label stack when
the packet reaches the egress node. Thus, the egress node can not
determine from which ingress node or SR path the packet came from.
Therefore, to identify an SR-MPLS path, a Path Segment is defined in
.Likewise, a path needs to be identified in an SRv6 network for
several use cases such as binding bidirectional paths and end-to-end performance
measurement .An SRv6 path MAY be identified by the content of a segment list.
However, the segment list may not be a good key, since the length of a
segment list is flexible according to the number of required SIDs. Also,
the length of a segment list may be too long to be a key when it
contains many SIDs. For instance, if packet A uses an SRH with 3 SIDs
while Packet B uses an SRH with 10 SIDs, the key to identify these two
paths will be a 384-bits value and a 1280-bits value, respectively.
Further, an SRv6 path cannot be identified by the information carried by
the SRH in reduced mode as the first SID is not
present.Furthermore, different SRv6 policies may use the same segment list
for different candidate paths, so the traffic of different SRv6 policies
are merged, resulting in the inability to measure the performance of the
specific path.To solve the above issues, this document defines a new SRv6 segment
called "SRv6 Path Segment", which is a 128-bits value, to identify an
SRv6 path.When the SRv6 Path Segment is used in reduced mode SRH , the entire path information is indicated by the Path
Segment, and the performance will be better than using the entire
segment list as the path identifier, while the overhead is equivalent to
the SRH in normal mode. Furthermore, with SRv6 Path Segment, each SRv6
candidate path can be identified and measured, even when they use the
same segment list.An SRv6 Path Segment MUST NOT be copied to the IPv6 destination
address, so it is not routable.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.MPLS: Multiprotocol Label Switching.PM: Performance Measurement.SID: Segment ID.SR: Segment Routing.SR-MPLS: Segment Routing with MPLS data plane.SRH: Segment Routing Header.PSID: Path Segment Identifier.PSP: Penultimate Segment Popping.Further, this document makes use of the terms defined in and .Similar to SR-MPLS Path Segment , SRv6 Path Segment may also
be used to identify an SRv6 Path in some use cases:Performance Measurement: For Passive measurement , path identification at the measuring points is
the pre-requisite . SRv6 Path segment can
be used by the measuring points (e.g., the ingress/egress nodes of
an SRv6 path) or a centralized controller to correlate the packets
counts/timestamps, then packet loss/delay can be calculated.Bi-directional SRv6 Path Association: In some scenarios, such as
mobile backhaul transport networks, there are requirements to
support bidirectional paths. Like SR-MPLS , to support
bidirectional SRv6 paths, a straightforward way is to bind two
unidirectional SRv6 paths to a single bidirectional path. SRv6 Path
segments can be used to correlate the two unidirectional SRv6 paths
at both ends of the path. defines how to use PCEP and
Path Segment to initiate a bidirectional SR path.End-to-end Path Protection: For end-to-end 1+1 path protection
(i.e., Live-Live case), the egress node of an SRv6 path needs to
know the set of paths that constitute the primary and the
secondary(s), to select the primary packet for onward transmission,
and to discard the packets from the secondary(s), so each SRv6 path
needs a unique path identifier at the egress node, which can be an
SRv6 Path Segment.As defined in , an SRv6 segment is a 128-bit
value.To identify an SRv6 path, this document defines a new segment called
SRv6 Path Segment.Depending on the use case, an SRv6 Path Segment identifies:an SRv6 path within an SRv6 domainan SRv6 Policya Candidate-path or a SID-List in a SRv6 Policy Note that, based on the use-case, a SRv6 Path Segment can be
used for different SID-Lists within an SR Policy.This document proposes two types of SRv6 Path Segment format.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 network
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 a unique local ID to identify an SRv6 Path and its
endpoint behavior.The Function Type of an SRv6 Path Segment is END.PSID (End
Function with Path Segment Identifier).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.This section describes the SRv6 Path Segment encoding in SRH.The SRv6 Path Segment MUST appear only once in a segment list, and it
MUST appear as the last entry in the segment list. To indicate the existence of a Path Segment in the SRH, this
document defines a P-flag in the SRH flag field. The encapsulation of
SRv6 Path Segment is shown below.P-bit: set when SRv6 Path Segment is inserted. It MUST be
ignored when a node does not support SRv6 Path Segment
processing.SRH.P-bit processing can be enabled or disabled by configuration on
devices, it can be done by CLI, NETCONF YANG or other ways, and this
is out of the scope of this document.The pseudo code of SRH.P-bit processing is described as below.Ref1: The SRv6 Path Segment processing is accosiated with the
specific application, such as SRv6 Path Segment based Performance
measurement, so this is out of the scope of this document.In some use cases, only the egress need to process the SRv6 Path
Segment, therefore, the P-bit processing can be done at the egress
node only while the intermediate nodes do not need to process it. This
feature can be enabled by configuration like CLI , NETCONF YANG or
other ways. In this case, the pseudo code is described as below.A Path Segment is a local segment allocated by an egress node. A Path
Segment can be allocated through several ways, such as CLI, BGP , PCEP or other ways. The mechanisms
through which a Path Segment is allocated are out of scope of this
document.When a Path Segment is allocated by the egress, it MUST be
distributed to the ingress node of the path that identified by the path
segment. In this case, only the egress will process the Path Segment,
and other nodes specified by SIDs in the segment list do not know how to
process the Path Segment.Depending on the use case, a Path Segment may be distributed to the
SRv6 nodes along the SRv6 path. In this case, the SRv6 nodes that
learned the Path Segment may process the 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 segment list,
and it MUST appear as the last entry. Only the one that appears as
the last entry in the SID list will be processed. An SRv6 Path
Segment that appears at any 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 support 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.This I-D requests the IANA to allocate, within the "SRv6 Endpoint
Behaviors" sub-registry belonging to the top-level "Segment-routing with
IPv6 data plane (SRv6) Parameters" registry, the following
allocations:This document also requests IANA to allocate bit position TBA within
the "Segment Routing Header Flags" registry defined in .This document does not introduce additional security requirements and
mechanisms other than the ones described in .The authors would like to thank Stefano Previdi and Zafar Ali for
their valuable comments and suggestions.