MPLS Quan Xiong
Internet-Draft Greg Mirsky
Intended status: Standards Track ZTE Corporation
Expires: April 19, 2020 Weiqiang Cheng
China Mobile
October 17, 2019
The Use of Path Segment in SR-MPLS and MPLS Interworking
draft-xiong-mpls-path-segment-sr-mpls-interworking-01
Abstract
This document discusses the SR-MPLS and MPLS interworking scenarios
and proposes the solution with the use of path segments.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions used in this document . . . . . . . . . . . . . . 3
2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Requirements Language . . . . . . . . . . . . . . . . . . 4
3. SR-MPLS Interworking with MPLS . . . . . . . . . . . . . . . 4
3.1. Stitching Model with Path Segment . . . . . . . . . . . . 5
3.2. Nesting Model with Path Segment . . . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Normative References . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
Segment Routing (SR) leverages the source routing paradigm. A node
steers a packet through an SR Policy instantiated as an ordered list
of instructions called "segments". SR supports a per-flow explicit
routing while maintaining per-flow state only at the ingress nodes of
the SR domain. Segment Routing can be instantiated on MPLS data
plane which is referred to as SR-MPLS
[I-D.ietf-spring-segment-routing-mpls]. SR-MPLS leverages the MPLS
label stack to construct the SR path.
In some scenarios, for example, a mobile backhaul transport network,
it is required to provide end-to-end bidirectional tunnel across
multiple domains. IP/MPLS technology can be deployed in these
domains, which may serve as an access, aggregation, or core network.
Further, using SR architecture, the IP/MPLS network may be upgraded
to support the SR-MPLS technology. As such transformation is
performed incrementally, by one domain at the time, operators are
faced with a requirement to support the interworking between MPLS and
SR-MPLS networks at the boundaries to provide the end-to-end
bidirectional service. [I-D.ietf-spring-mpls-path-segment] defined a
path segment identifier to support SR path PM, end-to-end 1+1 SR path
protection and bidirectional SR paths correlation.
As defined in [RFC8402], the headend of an SR Policy binds a Bingding
Segment ID (BSID) to a policy. The BSID could be bound to a SID List
or selected path and used to stitch the service across multiple
domains. For example, as discussed in Section 3
[I-D.ietf-spring-mpls-path-segment], the BSID can be used to identify
a sub-path and stitched them into an end-to-end SR path in the
nesting model. The BSID and path segment can be combined to achieve
the inter-domain path monitoring. But the solution is not
appropriate for the stitching model. The policy MUST be instantiated
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before the end-to-end service and it can not deploy domains
incrementally. Moreover, all of the BSIDs MUST be pushed onto the
label stack at the headend but not all of them are popped at an edge
node. The edge node pops one BSID that is bound to a SID List. That
cannot meet the independence requirement in the stitching model
especially when the domains belong to different operators.
This document discusses the interworking scenarios between SR-MPLS
and MPLS networks and proposes the solution with the use of path
segments. The stitching and correlation of Path Segments are
proposed to realize the inter-domain stitching and path monitoring.
2. Conventions used in this document
2.1. Terminology
ABR: Area Border Routers. Routers used to connect two IGP areas
(areas in OSPF or levels in IS-IS).
AS: Autonomous System. An Autonomous System is composed by one or
more IGP areas.
ASBR: Autonomous System Border Router. A router used to connect
together ASes of the same or different service providers via one or
more inter-AS links.
Border Node: An ABR that interconnects two or more IGP areas.
Border Link: Two ASes are interconnected with ASBRs.
BSID: Binding Segment ID.
Domains: Autonomous System (AS) or IGP Area. An Autonomous System is
composed of one or more IGP areas.
e-Path: End-to-end Path segment.
IGP: Interior Gateway Protocol.
i-Path/i-PSID: Inter-domain Path Segment.
PM: Performance Measurement.
SR: Segment Routing.
SR-MPLS: Segment Routing with MPLS data plane.
s-Path: Sub-path Path Segment.
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VPN: Virtual Private Network.
2.2. 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.
3. SR-MPLS Interworking with MPLS
It is required to establish the end-to-end VPN service across the
access network, aggregation network, and core network. For example,
SR-MPLS may be deployed in access and core network, and MPLS may be
deployed in the aggregation network. The network interworking should
be taken into account in deployment are the following:
o Border Node or Border Link
o Stitching Model or Nesting Model
o End-to-End OAM or Per-domain OAM
The domains of the networks may be IGP Areas or ASes. The SR-MPLS
and MPLS networks can be interconnected with a border node between
IGP areas or border links between ASes. This document takes IGP
Areas domains for example. MPLS domain can be deployed between two
SR-MPLS domains, as Figure 1 shows. The packets being transmitted
along the SR path in SR-MPLS domains by using the SID list at the
ingress node. And the path in MPLS domains can be pre-configuration
either via NMS or via the MPLS control plane signaling.
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B E X
+ + . . + +
+ + . . + +
+ + . . + +
A SR-MPLS C MPLS G SR-MPLS Z
+ IGP 1 + . IGP 2 . + IGP 3 +
+ + . . + +
+ + . . + +
D F Y
|<---Access Network--->|<-Aggregation Network->|<----Core Network---->|
Figure 1: SR-MPLS and MPLS interworking Scenario
The VPN service across the SR-MPLS and MPLS domains is an end-to-end
bidirectional path. In the SR-MPLS network, a Path Segment uniquely
identifies an SR path and can be used for the bidirectional path.
This document proposed the solution with path segment used in the
interworking scenario including the stitching and nesting models.
3.1. Stitching Model with Path Segment
It is a common requirement that SR-MPLS needs to interwork with MPLS
when SR is incrementally deployed in the MPLS domain. Figure 2 shows
the stitching model of SR-MPLS inter-working with MPLS.
The end-to-end bidirectional path across the SR-MPLS and MPLS network
is split into multiple segments. And each segment can be identified
by an inter-domain path segment (i-Path or i-PSID) as defined in
[I-D.xiong-spring-path-segment-sr-inter-domain]. The correlation of
path segments can stitch the inter-domain paths and bind
unidirectional paths. The i-Paths are valid in the corresponding
domain and the border nodes maintain the forwarding entries of that
i-Path segment that maps to the next i-Path and the related labels,
e.g, SID list or MPLS labels. In the headend node, the i-Path can
correlate the inter-domain path of reverse direction and bind the two
unidirectional paths. The interworking with path segments can
support the inter-domain stitching and path monitoring.
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+-----------------+ ................ +-----------------+
| +---+ | . +---+ . | +---+ |
| | B | | . | E | . | | X | |
| +---+ | . +---+ . | +---+ |
| / \ | . / \ . | / \ |
| +---+ SR-MPLS +-----+ MPLS +-----+ SR-MPLS +---+ |
| | A | domain1 | C | domain2 | G | domain3 | Z | |
| +---+ +-----+ +-----+ +---+ |
| \ / | . \ / . | \ / |
| +---+ | . +---+ . | +---+ |
| | D | | . | F | . | | Y | |
| +---+ | . +---+ . | +---+ |
+-----------------+ ................ +-----------------+
|<----SID List---->|<--- MPLS Label--->|<----SID List---->|
|<-----i-Path----->|<------i-Path----->|<-----i-Path----->|
|<----------------------VPN Service---------------------->|
Figure 2: Stitching Model of SR-MPLS and MPLS interworking
3.2. Nesting Model with Path Segment
Figure 3 displays the nesting model of SR-MPLS and MPLS interworking.
Comparing with the stitching model, the path segment presents end-to-
end encapsulation in the packet from an SR-MPLS domain to an MPLS
domain. As described in [I-D.ietf-spring-mpls-path-segment], an end-
to-end path segment, also referred to as e-Path, is used to indicate
the end-to-end path, and an s-Path is used to indicate the intra-
domain path. The e-Path is encapsulated at the ingress nodes and
decapsulated at the egress nodes. The transit nodes, even the border
nodes of domains, are not aware of the e-Path segment. The s-Path
can be used as a stitching label to correlate the two domains. The
use of the binding SID [RFC8402] is also recommended to reduce the
size of the label stack and stitch the inter-domain paths.
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+-----------------+ ................ +-----------------+
| +---+ | . +---+ . | +---+ |
| | B | | . | E | . | | X | |
| +---+ | . +---+ . | +---+ |
| / \ | . / \ . | / \ |
| +---+ SR-MPLS +-----+ MPLS +-----+ SR-MPLS +---+ |
| | A | domain1 | C | domain2 | G | domain3 | Z | |
| +---+ +-----+ +-----+ +---+ |
| \ / | . \ / . | \ / |
| +---+ | . +---+ . | +---+ |
| | D | | . | F | . | | Y | |
| +---+ | . +---+ . | +---+ |
+-----------------+ ................ +-----------------+
|<----SID List---->|<-- MPLS Label-->|<----SID List---->|
|<-----s-Path----->|<------s-Path----->|<-----s-Path----->|
|<------------------------e-Path------------------------->|
|<----------------------VPN Service---------------------->|
Figure 3: Nesting Model of SR-MPLS and MPLS interworking
4. Security Considerations
TBA
5. Acknowledgements
TBA
6. IANA Considerations
TBA
7. Normative References
[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-01 (work in progress),
September 2019.
[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.
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[I-D.xiong-spring-path-segment-sr-inter-domain]
Xiong, Q., Mirsky, G., and W. Cheng, "The Use of Path
Segment in SR Inter-domain Scenarios", draft-xiong-spring-
path-segment-sr-inter-domain-00 (work in progress), July
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>.
Authors' Addresses
Quan Xiong
ZTE Corporation
No.6 Huashi Park Rd
Wuhan, Hubei 430223
China
Phone: +86 27 83531060
Email: xiong.quan@zte.com.cn
Greg Mirsky
ZTE Corporation
USA
Email: gregimirsky@gmail.com
Weiqiang Cheng
China Mobile
Beijing
China
Email: chengweiqiang@chinamobile.com
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