SPRING Working Group C. Li
Internet-Draft Huawei Technologies
Intended status: Standards Track W. Cheng
Expires: May 4, 2021 China Mobile
M. Chen
D. Dhody
Huawei Technologies
R. Gandhi
Cisco Systems, Inc.
October 31, 2020
Path Segment for SRv6 (Segment Routing in IPv6)
draft-li-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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 4, 2021.
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Copyright Notice
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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 . . . . . . . . . . . . . 5
3.1.1. SRv6 Path Segment: Locator and Local ID . . . . . . . 5
3.1.2. SRv6 Path Segment: Global ID . . . . . . . . . . . . 6
4. SRv6 Path Segment Allocation . . . . . . . . . . . . . . . . 6
5. Processing of SRv6 Path Segment . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 8
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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.
When segment routing is deployed on an MPLS data plane, called SR-
MPLS [I-D.ietf-spring-segment-routing-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
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be. To support SR in an IPv6 network, a Segment Routing Header (SRH)
[RFC8754] 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
[I-D.ietf-spring-mpls-path-segment].
Likewise, a path needs to be identified in an SRv6 network 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].
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 [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 "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 [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.
An SRv6 Path Segment MUST NOT be copied to the IPv6 destination
address, so it is not routable.
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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
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 [RFC8402]
and [I-D.ietf-spring-srv6-network-programming].
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:
o 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.
o 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
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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.
o 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 [I-D.ietf-spring-srv6-network-programming], an SRv6
segment is a 128-bit value.
To identify an SRv6 path, this document defines a new segment called
SRv6 Path Segment.
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
SRv6 Path Segment, an SRH.P-flag is defined in
[I-D.li-6man-srv6-path-segment-encap].
Depending on the use case, an SRv6 Path Segment identifies:
o an SRv6 path within an SRv6 domain
o an SRv6 Policy
o a Candidate-path or a SID-List in a SRv6 Policy
[I-D.ietf-spring-segment-routing-policy]
Note that, based on the use-case, a SRv6 Path Segment can be used for
different SID-Lists within an SR Policy.
3.1. Format of an SRv6 Path Segment
This document proposes two types of SRv6 Path Segment format.
3.1.1. SRv6 Path Segment: Locator and Local ID
As per [I-D.ietf-spring-srv6-network-programming], 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
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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).
+--------------------------------------------------------------+
| Locator | Function ID |
+--------------------------------------------------------------+
|<-------------------------128 bits--------------------------->|
Figure 2. PSID in Format LOC:FUNCT
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.
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.
+--------------------------------------------------------------+
| Global ID/PSID |
+--------------------------------------------------------------+
|<-------------------------128 bits--------------------------->|
Figure 3. A Global ID as an PSID
4. 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.
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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.
5. Processing of SRv6 Path Segment
When the SRv6 Path Segment is used, the following rules apply:
o 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.
o 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.
o The SRv6 Path Segment MUST NOT be copied to the IPv6 destination
address.
o Penultimate Segment Popping (PSP, as defined in
[I-D.ietf-spring-srv6-network-programming]) MUST be disabled.
o 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.
o The specific SRv6 Path Segment processing depends on use cases,
and it is out of scope of this document.
6. 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:
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Value Description Reference
--------------------------------------------------------------
TBA1 End.PSID - SRv6 Path Segment [This.ID]
7. Security Considerations
This document does not introduce additional security requirements and
mechanisms other than the ones described in [RFC8402].
8. Contributors
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
9. Acknowledgements
The authors would like to thank Stefano Previdi and Zafar Ali for
their valuable comments and suggestions.
10. References
10.1. Normative References
[I-D.ietf-spring-srv6-network-programming]
Filsfils, C., Camarillo, P., Leddy, J., Voyer, D.,
Matsushima, S., and Z. Li, "SRv6 Network Programming",
draft-ietf-spring-srv6-network-programming-24 (work in
progress), October 2020.
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[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>.
10.2. Informative References
[I-D.gandhi-spring-udp-pm]
Gandhi, R., Filsfils, C., daniel.voyer@bell.ca, d.,
Salsano, S., Ventre, P., and M. Chen, "UDP Path for In-
band Performance Measurement for Segment Routing
Networks", draft-gandhi-spring-udp-pm-02 (work in
progress), September 2018.
[I-D.ietf-idr-sr-policy-path-segment]
Li, C., Li, Z., Telecom, C., Cheng, W., and K. Talaulikar,
"SR Policy Extensions for Path Segment and Bidirectional
Path", draft-ietf-idr-sr-policy-path-segment-01 (work in
progress), August 2020.
[I-D.ietf-pce-sr-bidir-path]
Li, C., Chen, M., Cheng, W., Gandhi, R., and Q. Xiong,
"PCEP Extensions for Associated Bidirectional Segment
Routing (SR) Paths", draft-ietf-pce-sr-bidir-path-03 (work
in progress), September 2020.
[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)",
draft-ietf-pce-sr-path-segment-01 (work in progress), May
2020.
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[I-D.ietf-spring-mpls-path-segment]
Cheng, W., Li, H., Chen, M., Gandhi, R., and R. Zigler,
"Path Segment in MPLS Based Segment Routing Network",
draft-ietf-spring-mpls-path-segment-03 (work in progress),
September 2020.
[I-D.ietf-spring-segment-routing-mpls]
Bashandy, A., Filsfils, C., Previdi, S., Decraene, B.,
Litkowski, S., and R. Shakir, "Segment Routing with MPLS
data plane", draft-ietf-spring-segment-routing-mpls-22
(work in progress), May 2019.
[I-D.ietf-spring-segment-routing-policy]
Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
P. Mattes, "Segment Routing Policy Architecture", draft-
ietf-spring-segment-routing-policy-08 (work in progress),
July 2020.
[I-D.li-6man-srv6-path-segment-encap]
Li, C., Cheng, W., Li, Z., and D. Dhody, "Encapsulation of
Path Segment in SRv6", draft-li-6man-srv6-path-segment-
encap-03 (work in progress), September 2020.
[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>.
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
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Dhruv Dhody
Huawei Technologies
Divyashree Techno Park, Whitefield
Bangalore, Karnataka 560066
India
Email: dhruv.ietf@gmail.com
Rakesh Gandhi
Cisco Systems, Inc.
Canada
Email: rgandhi@cisco.com
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