Network Working Group W. Cheng
Internet-Draft H. Li
Intended status: Standards Track China Mobile
Expires: March 19, 2020 M. Chen
Huawei
R. Gandhi
Cisco Systems, Inc.
R. Zigler
Broadcom
September 16, 2019
Path Segment in MPLS Based Segment Routing Network
draft-ietf-spring-mpls-path-segment-01
Abstract
A Segment Routing (SR) path is identified by an SR segment list.
Only the full segment list identifies the full end-to-end path, and
one segment or a partial set of segments from the list cannot
identify the SR path and distinguish it from other SR paths that may
be partially congruent. But path identification is a pre-requisite
for various use-cases such as performance measurement (PM) of an SR
path.
In SR for MPLS (SR-MPLS) the segment identifiers are stripped from
the packet through label popping as the packet transits the network.
This means that when a packet reaches the egress of the SR path, it
is not possible to determine on which SR path it traversed the
network.
This document defines a new type of segment that is referred to as
Path Segment, which is used to identify an SR path in an SR-MPLS
network. When used, it is inserted at the ingress node of the SR
path and immediately follows the last segment identifier in the
segment list of the SR path. The Path Segment will not be popped off
until it reaches the egress node of the SR path.
A Path Segment can be used by the egress node to implement path
identification hence to support various use-cases including SR path
PM, end-to-end 1+1 SR path protection, and bidirectional SR paths
correlation.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 3
2. Path Segment . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Nesting of Path Segments . . . . . . . . . . . . . . . . . . 5
4. Path Segment Allocation . . . . . . . . . . . . . . . . . . . 6
5. Path Segment for PM . . . . . . . . . . . . . . . . . . . . . 7
6. Path Segment for Bi-directional SR Path . . . . . . . . . . . 7
7. Path Segment for End-to-end Path Protection . . . . . . . . . 7
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. Security Considerations . . . . . . . . . . . . . . . . . . . 8
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 8
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
12.1. Normative References . . . . . . . . . . . . . . . . . . 9
12.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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1. Introduction
Segment Routing (SR) [RFC8402] is a source routed forwarding method
that allows to directly encode forwarding instructions (called
segments) in each packet, hence it enables steering traffic through a
network without the per-flow states maintained on the transit nodes.
Segment Routing can be instantiated on an MPLS data plane or an IPv6
data plane. The former is called SR-MPLS
[I-D.ietf-spring-segment-routing-mpls], the latter is called SRv6
[RFC8402]. SR-MPLS leverages the MPLS label stack to construct SR
paths.
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
that 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
cannot determine along which SR path the packet came.
However, to support use cases like end-to-end 1+1 path protection
(Live-Live case), bidirectional path correlation or performance
measurement (PM), the ability to implement path identification is a
pre-requisite. More detail can be found in Section 5, 6, and 7.
Therefore, this document introduces a new segment that is referred to
as the Path Segment. A Path Segment is defined to uniquely identify
an SR path in an SR-MPLS network in the context of the egress node.
It is normally used by egress nodes for path identification or
correlation.
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
BCP14 [RFC2119][RFC8174] when, and only when, they appear in all
capitals, as shown here.
1.2. Abbreviations
DM: Delay Measurement.
LM: Loss Measurement.
MPLS: Multiprotocol Label Switching.
PM: Performance Measurement.
PSID: Path Segment ID.
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SID: Segment ID.
SL: Segment List.
SR: Segment Routing.
SR-MPLS: Segment Routing instantiated on MPLS data plane.
SRv6: Segment Routing instantiated on IPv6 data plane
2. Path Segment
A Path Segment is a single label that is assigned from the Segment
Routing Local Block (SRLB) or Segment Routing Global Block (SRGB) of
the egress node of an SR path. It means that the Path Segment is
unique in the context of the egress node of the SR path. When a Path
Segment is used, the Path Segment MUST be inserted at the ingress
node and MUST immediately follow the last label of the SR path. The
Path Segment may be used to identify an SR-MPLS Policy, its
Candidate-Path (CP), or a SID List (SL)
[I-D.ietf-spring-segment-routing-policy] terminating on an egress
node depending on the use-case.
The value of the TTL field in the MPLS label stack entry containing
the Path Segment MUST be set to the same value as the TTL of the last
label stack entry for the last segment in the SR path. If the Path
Segment is the bottom label, the S bit MUST be set.
Normally, the intermediate nodes will not see the Path Segment label
and do not know how to process it. A Path Segment presenting to an
intermediate node is an error condition.
The egress node MUST pop the Path Segment. The egress node MAY use
the Path Segment for further processing. For example, when
performance measurement is enabled on the SR path, it can trigger
packet counting or timestamping.
The label stack with Path Segment is as below (Figure 1):
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+--------------------+
| ... |
+--------------------+
| Label 1 |
+--------------------+
| Label 2 |
+--------------------+
| ... |
+--------------------+
| Label n |
+--------------------+
| Path Segment |
+--------------------+
| ... |
+--------------------+
Figure 1: Label Stack with Path Segment
Where:
o The Labels 1 to n are the segment label stack used to direct how
to steer the packets along the SR path.
o The Path Segment identifies the SR path in the context of the
egress node of the SR path.
3. Nesting of Path Segments
Binding SID (BSID) [RFC8402] can be used for SID list compression.
With BSID, an end-to-end SR path can be split into several sub-paths,
each sub-path is identified by a BSID. Then an end-to-end SR path
can be identified by a list of BSIDs, therefore, it can provide
better scalability.
BSID and Path SID (PSID) can be combined to achieve both sub-path and
end-to-end path monitoring. A reference model for such a combination
in (Figure 2) shows an end-to-end path (A->D) that spans three
domains (Access, Aggregation and Core domain) and consists of three
sub-paths, one in each sub-domain (sub-path (A->B), sub-path (B->C)
and sub-path (C->D)). Each sub-path is allocated a BSID. For
nesting the sub-paths, each sub-path is allocated a PSID. Then, the
SID list of the end-to-end path can be expressed as <BSID1, BSID2,
..., BSIDn, e-PSID>, where the e-PSID is the PSID of the end-to-end
path. The SID list of a sub-path can be expressed as <SID1, SID2,
...SIDn, s-PSID>, where the s-PSID is the PSID of the sub-path.
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Figure 2 shows the details of the label stacks when PSID and BSID are
used to support both sub-path and end-to-end path monitoring in a
multi-domain scenario.
/--------\ /--------\ /--------\
/ \ / \ / \
A{ Access }B{ Aggregation }C{ Core }D
\ / \ / \ /
\--------/ \--------/ \--------/
Sub-path(A->B) Sub-path(B->C) Sub-path(C->D)
|<--------------->|<-------------->|<-------------->|
E2E Path(A->D)
|<------------------------------------------------->|
+------------+
~A->B SubPath~
+------------+ +------------+
|s-PSID(A->B)| ~B->C SubPath~
+------------+ +------------+
| BSID(B->C) | |s-PSID(B->C)|
+------------+ +------------+ +------------+
| BSID(C->D) | | BSID(C->D) | ~C->D SubPath~
+------------+ +------------+ +------------+ +------------+
|e-PSID(A->D)| |e-PSID(A->D)| |e-PSID(A->D)| |e-PSID(A->D)|
+------------+ +------------+ +------------+ +------------+
Figure 2: Nesting of Path Segments
4. Path Segment Allocation
Several ways can be used to allocate the Path Segment.
One way is to set up a communication channel (e.g., MPLS Generic
Associated Channel (G-ACh)) between the ingress node and the egress
node, and the ingress node of the SR path can directly send a request
to the egress node to ask for a Path Segment.
Another way is to leverage a centralized controller (e.g., PCE, SDN
controller) to assign the Path Segment. PCEP based Path Segment
allocation is defined in [I-D.li-pce-sr-path-segment], and SR-policy
based path segment allocation is defined in
[I-D.li-idr-sr-policy-path-segment-distribution].
A Path Segment can be used in the case of PHP, where some labels MAY
be popped off at the penultimate hop of an SR path, but the Path
Segment MUST NOT be popped off until it reaches at the egress LSR.
In the case of a Path Segment that is assigned by a centralized
controller, the controller must make sure (e.g., by some capability
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discovery mechanisms) that the egress LSR knows and can process the
Path Segment.
5. Path Segment for PM
As defined in [RFC7799], performance measurement can be classified
into Active, Passive and Hybrid measurement. For Passive
measurement, path identification at the measuring points is the pre-
requisite. Path segment can be used by the measuring points (e.g.,
the ingress/egress nodes of an SR path) or a centralized controller
to correlate the packets counts/timestamps that are from the ingress
and egress nodes to a specific SR path, then packet loss/delay can be
calculated.
Performance Delay Measurement (DM) and Loss Measurements (LM) in SR
networks with MPLS data plane can be found in
[I-D.gandhi-spring-sr-mpls-pm] and [I-D.gandhi-spring-udp-pm].
6. Path Segment for Bi-directional SR Path
With the current SR architecture, an SR path is a unidirectional
path. In some scenarios, for example, mobile backhaul transport
network, there are requirements to support bidirectional paths, and
the path is normally treated as a single entity and both directions
of the path have the same fate, for example, failure in one direction
will result in switching at both directions.
MPLS supports this by introducing the concepts of co-routed
bidirectional LSP and associated bidirectional LSP. With SR, to
support bidirectional paths, a straightforward way is to bind two
unidirectional SR paths to a single bidirectional path. Path
Segments can be used to correlate the two unidirectional SR paths at
both ends of the paths.
[I-D.li-pce-sr-bidir-path] defines how to use PCEP and Path segment
to initiate a bidirectional SR path, and
[I-D.li-idr-sr-policy-path-segment-distribution] defines how to use
SR policy and Path segment to initiate a bidirectional SR path.
7. Path Segment for End-to-end Path Protection
For end-to-end 1+1 path protection (i.e., Live-Live case), the egress
node of an SR path needs to know the set of paths that constitute the
primary and the secondaries, in order to select the primary packet
for onward transmission, and to discard the packets from the
secondaries.
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To do this, each path needs a path identifier that is unique at the
egress node. Depending on the design, this is a single unique path
segment label chosen by the egress PE.
There then needs to be a method of binding this path identifiers into
equivalence groups such that the egress PE can determine the set of
packets that represent a single path and its secondary.
It is obvious that this group can be instantiated in the network by
an SDN controller and that the Path Segment achieves the desired
function.
8. IANA Considerations
This document does not require any IANA actions.
9. Security Considerations
This document does not introduce additional security requirements and
mechanisms other than the ones described in [RFC8402].
10. Contributors
The following individuals also contribute to this document.
o Cheng Li, Huawei
o Lei Wang, China Mobile
o Shuangping Zhan, ZTE
o Greg
11. Acknowledgements
The authors would like to thank Adrian Farrel, Stewart Bryant,
Alexander Vainshtein, Andrew G. Malis and Loa Andersson for their
review, suggestions and comments to this document.
The authors would like to acknowledge the contribution from Alexander
Vainshtein on "Nesting of Path Segments".
12. References
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12.1. Normative References
[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.
[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>.
12.2. Informative References
[I-D.gandhi-spring-sr-mpls-pm]
Gandhi, R., Filsfils, C., daniel.voyer@bell.ca, d.,
Salsano, S., Ventre, P., and M. Chen, "Performance
Measurement in Segment Routing Networks with MPLS Data
Plane", draft-gandhi-spring-sr-mpls-pm-03 (work in
progress), September 2018.
[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.
[]
Filsfils, C., Dukes, D., Previdi, S., Leddy, J.,
Matsushima, S., and d. daniel.voyer@bell.ca, "IPv6 Segment
Routing Header (SRH)", draft-ietf-6man-segment-routing-
header-22 (work in progress), August 2019.
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[I-D.ietf-spring-segment-routing-policy]
Filsfils, C., Sivabalan, S., daniel.voyer@bell.ca, d.,
bogdanov@google.com, b., and P. Mattes, "Segment Routing
Policy Architecture", draft-ietf-spring-segment-routing-
policy-03 (work in progress), May 2019.
[I-D.li-idr-sr-policy-path-segment-distribution]
Li, C., Chen, M., Dong, J., and Z. Li, "Segment Routing
Policies for Path Segment and Bidirectional Path", draft-
li-idr-sr-policy-path-segment-distribution-01 (work in
progress), October 2018.
[I-D.li-pce-sr-bidir-path]
Li, C., Chen, M., Cheng, W., Li, Z., Dong, J., Gandhi, R.,
and Q. Xiong, "PCEP Extensions for Associated
Bidirectional Segment Routing (SR) Paths", draft-li-pce-
sr-bidir-path-06 (work in progress), August 2019.
[I-D.li-pce-sr-path-segment]
Li, C., Chen, M., Cheng, W., Dong, J., Li, Z., Gandhi, R.,
and Q. Xiong, "Path Computation Element Communication
Protocol (PCEP) Extension for Path Segment in Segment
Routing (SR)", draft-li-pce-sr-path-segment-08 (work in
progress), August 2019.
[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
Weiqiang Cheng
China Mobile
Email: chengweiqiang@chinamobile.com
Han Li
China Mobile
Email: lihan@chinamobile.com
Mach(Guoyi) Chen
Huawei
Email: mach.chen@huawei.com
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Rakesh Gandhi
Cisco Systems, Inc.
Canada
Email: rgandhi@cisco.com
Royi Zigler
Broadcom
Email: royi.zigler@broadcom.com
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