Internet DRAFT - draft-ietf-spring-srv6-path-segment
draft-ietf-spring-srv6-path-segment
SPRING Working Group C. Li
Internet-Draft Huawei Technologies
Intended status: Standards Track W. Cheng
Expires: 21 April 2024 China Mobile
M. Chen
D. Dhody
Huawei Technologies
Y. Zhu
China Telecom
19 October 2023
Path Segment for SRv6 (Segment Routing in IPv6)
draft-ietf-spring-srv6-path-segment-07
Abstract
Segment 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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 21 April 2024.
Li, et al. Expires 21 April 2024 [Page 1]
Internet-Draft SRv6 Path Segment October 2023
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Use Cases for SRv6 Path Segment . . . . . . . . . . . . . . . 4
3. SRv6 Path Segment . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Format of an SRv6 Path Segment . . . . . . . . . . . . . 6
3.1.1. SRv6 Path Segment: Locator and Local ID . . . . . . . 6
3.1.2. SRv6 Path Segment: Global ID . . . . . . . . . . . . 6
4. Encoding of an SRv6 Path Segment . . . . . . . . . . . . . . 7
4.1. SRH.P-flag . . . . . . . . . . . . . . . . . . . . . . . 7
5. SRv6 Path Segment Allocation . . . . . . . . . . . . . . . . 9
6. Processing of SRv6 Path Segment . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 10
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
11.1. Normative References . . . . . . . . . . . . . . . . . . 11
11.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
Segment routing (SR) [RFC8402] 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.
Li, et al. Expires 21 April 2024 [Page 2]
Internet-Draft SRv6 Path Segment October 2023
When segment routing is deployed on an MPLS data plane, called SR-
MPLS [RFC8660], 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) [RFC8754] is used.
In SR, a path needs to be identified for several use cases such as
binding bidirectional paths [I-D.ietf-pce-sr-bidir-path] and end-to-
end performance measurement [I-D.gandhi-spring-udp-pm].
Additionally, 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. To identify an SR-MPLS path, a Path Segment is defined
in [I-D.ietf-spring-mpls-path-segment].
An SRv6 path could be identified by the content of a segment list.
However, the segment list is not be a good key identifier, 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 [RFC8754] 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 the "SRv6 Path Segment", which in total is an 128-bits value,
to identify an SRv6 path.
When the SRv6 Path Segment is used in reduced mode SRH [RFC8754], 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.
Li, et al. Expires 21 April 2024 [Page 3]
Internet-Draft SRv6 Path Segment October 2023
1.1. Requirements Language
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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
1.2. Terminology
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 [RFC8402]
and [RFC8986].
2. Use Cases for SRv6 Path Segment
Similar to SR-MPLS Path Segment [I-D.ietf-spring-mpls-path-segment],
SRv6 Path Segment may also be used to identify an SRv6 Path in some
use cases:
* Performance Measurement: For Passive measurement [RFC7799], path
identification at the measuring points is the pre-requisite
[I-D.ietf-spring-mpls-path-segment]. 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.
Li, et al. Expires 21 April 2024 [Page 4]
Internet-Draft SRv6 Path Segment October 2023
* Bi-directional SRv6 Path Association: In some scenarios, such as
mobile backhaul transport networks, there are requirements to
support bidirectional paths. Like SR-MPLS
[I-D.ietf-spring-mpls-path-segment], 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. [I-D.ietf-pce-sr-bidir-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.
3. SRv6 Path Segment
As defined in [RFC8986], an SRv6 segment is a 128-bit value.
To identify an SRv6 path, this document defines a new segment called
SRv6 Path Segment. An SRv6 Path Segment will not be used for routing
so it should not be copied to the IPv6 destination address.
[RFC8754] states that the SR segment endpoint node creates Forwarding
Information Base (FIB) entries for its local SIDs (without
constraining the details of implementation). In order to provide a
new independent 128-bit ID space for Path Segment, the Path Segment
is required to be stored seperate from the other SIDs (for example in
a different table from the FIB).
Depending on the use case, an SRv6 Path Segment identifies:
* an SRv6 path within an SRv6 domain
* an SRv6 Policy
* a Candidate-path or a SID-List in a SRv6 Policy [RFC9256]
Note that, based on the use-case, a SRv6 Path Segment can be used for
different SID-Lists within an SR Policy.
Li, et al. Expires 21 April 2024 [Page 5]
Internet-Draft SRv6 Path Segment October 2023
3.1. Format of an SRv6 Path Segment
This document defines two formats of the SRv6 Path Segment. A future
document MAY add further new formats for the SRv6 Path Segment,
provided the SRv6 PSID value remains unique irrespective of the
format.
3.1.1. SRv6 Path Segment: Locator and Local ID
As per [RFC8986], an SRv6 SID consists 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 remaining
bits of the SID MUST be zero.
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, which is END.PSID (End Function with Path Segment
Identifier). The Argument part is optional according to the use
cases.
+--------------------------------------------------------------+
| Locator | Function ID |Arg(Opt) |
+--------------------------------------------------------------+
|<-------------------------128 bits--------------------------->|
Figure 1. PSID Format following LOC:FUNCT:ARG
3.1.2. SRv6 Path Segment: Global ID
An SRv6 Path Segment ID can be a Global ID, and its format depends on
the use case.
+--------------------------------------------------------------+
| Global PSID |
+--------------------------------------------------------------+
|<-------------------------128 bits--------------------------->|
Figure 2. 128-bit Global PSID
Li, et al. Expires 21 April 2024 [Page 6]
Internet-Draft SRv6 Path Segment October 2023
4. Encoding of an 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.
4.1. SRH.P-flag
To indicate the existence of a Path Segment in the SRH, this document
defines a P-flag in the SRH flag field, and it is to be allocated
(The following P-flag is for illustration only and will be modified
to the right bit once the P-flag is allocated). The encapsulation of
SRv6 Path Segment is shown below.
Li, et al. Expires 21 April 2024 [Page 7]
Internet-Draft SRv6 Path Segment October 2023
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len | Routing Type | Segments Left |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Last Entry | Flags |P| Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Segment List[0] (128 bits IPv6 address) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
...
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Segment List[n-1] (128 bits IPv6 address) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| SRv6 Path Segment (Segment List[n],128 bits IPv6 value) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// //
// Optional Type Length Value objects (variable) //
// //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3. SRv6 Path Segment in SID List
* P-flag: set when SRv6 Path Segment is inserted. A node that does
not understand the P-flag will ignore it as described in
[RFC8754]. A node that understands the P-flag but does not
support SRv6 Path Segment processing MUST ignore the P-flag. If
the P-flag is unset or the P-flag is ignored when processing, the
SRv6 Path Segment processing is skipped or ignored.
SRH.P-flag 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-flag processing is described as below.
Li, et al. Expires 21 April 2024 [Page 8]
Internet-Draft SRv6 Path Segment October 2023
S01. if SRH.P-flag processing is enabled:
S02. if SRH.P-flag is set:
S03. SRv6 Path Segment processing ;;ref1
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-flag 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.
S01. if SRH.P-flag processing is enabled:
S02. if intermediate node processing is disabled:
S03. if SRH.P-flag is set and SRH.SL == 0:
S03. SRv6 Path Segment processing
S04 else:
S05. if SRH.P-flag is set:
S06. SRv6 Path Segment processing
5. SRv6 Path Segment Allocation
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
[I-D.ietf-idr-sr-policy-path-segment], PCEP
[I-D.ietf-pce-sr-path-segment] 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. This is out of the scope of this document, and may be
studied in the future if needed.
6. Processing of SRv6 Path Segment
When the SRv6 Path Segment is used, the following rules apply:
Li, et al. Expires 21 April 2024 [Page 9]
Internet-Draft SRv6 Path Segment October 2023
* The SRv6 Path Segment MUST appear only once in a segment list, and
it MUST appear as the last entry. Placing an SRv6 Path Segment at
any other location in the SID list will result in unpredictable
forwarding behavior. Only the one that appears as the last entry
in the SID list will be processed.
* 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 [RFC8986]) MUST be
disabled.
* The ingress needs to set the P-flag 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-flag is set. When the P-flag 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.
7. IANA Considerations
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:
Value Description Reference
--------------------------------------------------------------
TBA1 End.PSID - SRv6 Path Segment [This.ID]
8. Security Considerations
This document does not introduce additional security requirements and
mechanisms other than the ones described in [RFC8402].
9. Contributors
Li, et al. Expires 21 April 2024 [Page 10]
Internet-Draft SRv6 Path Segment October 2023
Zhenbin Li
Huawei Technologies
Huawei Campus, No. 156 Beiqing Rd.
Beijing 100095
China
Email: lizhenbin@huawei.com
Jie Dong
Huawei Technologies
Huawei Campus, No. 156 Beiqing Rd.
Beijing 100095
China
Email: jie.dong@huawei.com
10. Acknowledgements
The authors would like to thank Adrian Farrel, Stefano Previdi, and
Zafar Ali for their valuable comments and suggestions.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
(SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
<https://www.rfc-editor.org/info/rfc8754>.
Li, et al. Expires 21 April 2024 [Page 11]
Internet-Draft SRv6 Path Segment October 2023
[RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
(SRv6) Network Programming", RFC 8986,
DOI 10.17487/RFC8986, February 2021,
<https://www.rfc-editor.org/info/rfc8986>.
11.2. Informative References
[I-D.gandhi-spring-udp-pm]
Gandhi, R., Filsfils, C., Voyer, D., Salsano, S., Ventre,
P. L., and M. Chen, "UDP Path for In-band Performance
Measurement for Segment Routing Networks", Work in
Progress, Internet-Draft, draft-gandhi-spring-udp-pm-02,
14 September 2018, <https://datatracker.ietf.org/doc/html/
draft-gandhi-spring-udp-pm-02>.
[I-D.ietf-idr-sr-policy-path-segment]
Li, C., Li, Z., Yin, Y., Cheng, W., and K. Talaulikar, "SR
Policy Extensions for Path Segment and Bidirectional
Path", Work in Progress, Internet-Draft, draft-ietf-idr-
sr-policy-path-segment-08, 16 August 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-sr-
policy-path-segment-08>.
[I-D.ietf-pce-sr-bidir-path]
Li, C., Chen, M., Cheng, W., Gandhi, R., and Q. Xiong,
"Path Computation Element Communication Protocol (PCEP)
Extensions for Associated Bidirectional Segment Routing
(SR) Paths", Work in Progress, Internet-Draft, draft-ietf-
pce-sr-bidir-path-12, 9 September 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-pce-sr-
bidir-path-12>.
[I-D.ietf-pce-sr-path-segment]
Li, C., Chen, M., Cheng, W., Gandhi, R., and Q. Xiong,
"Path Computation Element Communication Protocol (PCEP)
Extension for Path Segment in Segment Routing (SR)", Work
in Progress, Internet-Draft, draft-ietf-pce-sr-path-
segment-08, 24 August 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-pce-sr-
path-segment-08>.
Li, et al. Expires 21 April 2024 [Page 12]
Internet-Draft SRv6 Path Segment October 2023
[I-D.ietf-spring-mpls-path-segment]
Cheng, W., Li, H., Li, C., Gandhi, R., and R. Zigler,
"Path Segment in MPLS Based Segment Routing Network", Work
in Progress, Internet-Draft, draft-ietf-spring-mpls-path-
segment-16, 12 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-spring-
mpls-path-segment-16>.
[RFC7799] Morton, A., "Active and Passive Metrics and Methods (with
Hybrid Types In-Between)", RFC 7799, DOI 10.17487/RFC7799,
May 2016, <https://www.rfc-editor.org/info/rfc7799>.
[RFC8660] Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing with the MPLS Data Plane", RFC 8660,
DOI 10.17487/RFC8660, December 2019,
<https://www.rfc-editor.org/info/rfc8660>.
[RFC9256] Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov,
A., and P. Mattes, "Segment Routing Policy Architecture",
RFC 9256, DOI 10.17487/RFC9256, July 2022,
<https://www.rfc-editor.org/info/rfc9256>.
Authors' Addresses
Cheng Li
Huawei Technologies
Email: c.l@huawei.com
Weiqiang Cheng
China Mobile
Email: chengweiqiang@chinamobile.com
Mach(Guoyi) Chen
Huawei Technologies
Email: mach.chen@huawei.com
Dhruv Dhody
Huawei Technologies
Divyashree Techno Park, Whitefield
Bangalore 560066
Karnataka
India
Email: dhruv.ietf@gmail.com
Li, et al. Expires 21 April 2024 [Page 13]
Internet-Draft SRv6 Path Segment October 2023
Yongqing Zhu
China Telecom
Guangzhou
Email: zhuyq8@chinatelecom.cn
Li, et al. Expires 21 April 2024 [Page 14]
