The Use of Path Segment in SR Inter-domain Scenarios
draft-xiong-spring-path-segment-sr-inter-domain-00
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| Authors | Quan Xiong , Greg Mirsky , Weiqiang Cheng | ||
| Last updated | 2019-07-03 | ||
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draft-xiong-spring-path-segment-sr-inter-domain-00
SPRING Quan Xiong
Internet-Draft Greg Mirsky
Intended status: Standards Track ZTE Corporation
Expires: January 4, 2020 Weiqiang Cheng
China Mobile
July 3, 2019
The Use of Path Segment in SR Inter-domain Scenarios
draft-xiong-spring-path-segment-sr-inter-domain-00
Abstract
This document discusses the inter-domain scenarios for SR-MPLS
network and proposes the solution with the use of Path Segment.
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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 . . . . . . . . . . . . . . . . . . 3
3. Path Segment for SR-MPLS Inter-domain . . . . . . . . . . . . 3
3.1. Inter-domain Path Segment . . . . . . . . . . . . . . . . 4
3.2. End-to-end Path Segment . . . . . . . . . . . . . . . . . 4
4. SR-MPLS Inter-domain Scenarios . . . . . . . . . . . . . . . 5
4.1. Stitching Inter-domain with i-Path . . . . . . . . . . . 5
4.2. Nesting Inter-domain with e-Path . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. 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". A segment can represent any
instruction, topological or service based. A segment can have a
semantic local to an SR node or global within an SR domain. SR
supports 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.
[I-D.ietf-spring-mpls-path-segment] defines a Path Segment identifier
to support bidirectional path correlation for transport network. In
multi-domain scenarios, the SR bidirectional end-to-end tunnel MAY be
established with the use of Path Segments. The SR-MPLS inter-domain
models include the stitching and nesting inter-domain models. Path
Segment MAY be used to indicate the inter-domain path or the end-to-
end path and correlate the inter-domain paths or end-to-end
unidirectional paths.
This document discusses the inter-domain scenarios for SR-MPLS
networks and proposes the solution with the use of Path Segment for
end-to-end bidirectional SR path.
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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).
A->B SID list: The SID List from SR node A to SR node B.
AS: Autonomous System.
ASBR: Autonomous System Border Router. Router used to connect
together ASes of the same or different service providers via one or
more inter-AS links.
Domains:Autonomous System (AS) or IGP Area. An Autonomous System is
composed by one or more IGP area.
e-Path: End-to-end Path segment.
s-Path: Sub-path Path Segment.
Inter-Area: Two IGP areas interconnects with an ABR in a AS.
Inter-AS: Two ASes interconnects with an ASBR.
IGP: Interior Gateway Protocol.
i-Path/i-PSID: Inter-domain Path Segment.
SR: Segment Routing.
SR-MPLS: Segment Routing with MPLS data plane.
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. Path Segment for SR-MPLS Inter-domain
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3.1. Inter-domain Path Segment
In the stitching inter-domain model, the end-to-end SR path being
split into multiple segments section 4.2. And each segment can be
identified by an inter-domain path segment (i-Path or i-PSID). The
inter-domain path segment is valid in the corresponding domain and
the border nodes maintain the forwarding entries of that i-Path
segment mapping to the next i-Path. In the headend node, the i-Path
can be mapped to the inter-domain path of reverse direction and
correlates the two unidirectional paths. The border nodes should
install the following MPLS data entries for Path segments:
incoming label: i-Path
outgoing label: the SID list of the next domain or link + next i-Path
Taking Figure 1 as an example, the border node X installs the MPLS
data entries:
incoming label: i-Path(A->X)
outgoing label: X->Y SID list + i-Path(X->Y)
The i-Path can be a locally unique label and assigned from the
Segment Routing Local Block (SRLB). It is required that the
controller(e.g., PCE) assigns the label to ensure the ingress and the
egress node can recognize it and it also can be assigned from egress
node of each domain. PCEP based i-Path allocation and procedure is
defined in [I-D.xiong-pce-stateful-pce-sr-inter-domain].
3.2. End-to-end Path Segment
The nesting inter-domain model is 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 stitching
label to correlate the two domains. The use of the binding SID
[RFC8402] is also recommended to reduce the size of lable stack
section 4.2.
The e-Path can be a globally unique or local label. If the e-Path is
globally unique, it MUST be assigned from the SRGB block of each
domain. If the e-Path is a local label, it is required that the
controller(e.g., PCE) or a super controller (e.g., hierarchical PCE)
assigns the label to ensure the ingress(A) and the egress node(Z) can
recognize it and there is no SID collision in the ingress and egress
domains.
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4. SR-MPLS Inter-domain Scenarios
The domains of the networks may be IGP Areas or ASes and the inter-
domain scenario may be inter-Area or inter-AS. The multiple SR-MPLS
domains may be interconnect with a ABR within areas or inter-link
between ASes. This document takes IGP Areas domains for example.
SR-MPLS domains can be deployed as Figure 1 shown.
+ + +
+ + + + + +
+ + + + + +
+ + + + + +
A SR-MPLS X SR-MPLS Y SR-MPLS Z
+ IGP 1 + + IGP 2 + + IGP 3 +
+ + + + + +
+ + + + + +
+ + +
Figure 1: SR-MPLS and MPLS-TP interworking Scenario
Two SR-MPLS inter-domain models are discussed in this document
including the stitching and nesting inter-domain model which are
described in Section 4.1 and Section 4.2 respectively.
4.1. Stitching Inter-domain with i-Path
The Figure 1 displays the border node inter-domain scenario. SR node
X and SR node Y are the border nodes of two different domains. The
i-Paths from A->X, X->Y, and Y->Z are used for the inter-domain path
segment. The ingress SR node A encapsulates the data packet with
i-Path (A->X) and A->X SID list. The data packet is forwarded to SR
node X according to the A->X SID list. Node X pushes the i-Path
(X->Y) and X->Y SID list based on the above mentioned forwarding
entry. The data packet is forwarded to node Y and then to the SR
node Z based on the same forwarding procedure. In node Z, the i-Path
(Y->Z) can be mapped to the path from Z to Y of reverse direction and
correlates the two unidirectional paths. The packet transmission of
the reverse direction is the same with the forwarding direction with
different i-Paths.
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.................. ................. ....................
. . . . . .
+-----+ +-----+ +-----+ +-----+
| A | | X | | Y | | Z |
+-----+ +-----+ +-----+ +-----+
. SR Domain 1 . . SR Domain 2 . . SR Domain 3 .
.................. ................. ....................
|<------------------>|<------------------>|<--------------->|
i-Path(A->X) i-Path(X->Y) i-Path(Y->Z)
Node A Node X Node Y Node Z
+-------------+ +-------------+ +-------------+
|A->X SID list| |X->Y SID list| |Y->Z SID list|
+-------------+ +-------------+ +-------------+ +--------------+
|i-Path(A->X) |---->|i-Path(X->Y) |---->|i-Path(Y->Z) |--->| Payload |
+-------------+ +-------------+ +-------------+ +--------------+
| Payload | | Payload | | Payload |
+-------------+ +-------------+ +-------------+
Figure 2: Stitching Border Node Inter-Domain Scenario
4.2. Nesting Inter-domain with e-Path
Figure 3 shows the SR-MPLS nesting inter-domain scenario. The
e-Path(A->Z) is used to indicate the end-to-end path. The s-Path is
used to identify the domain's sub-path. The e-Path, s-Path and SR
list are pushed by the ingress node. The e-Path is used to correlate
the two unidirectional SR paths to an SR bidirectional path. The
s-Path can be used as stitching label to correlate the two inter-
domain sub-paths.
The use of the binding SID [RFC8402] is also recommended to replace
the SR list of each domain. As shown in Figure 3, the B-SID(X->Y) is
used to replace the X->Y SID list. Ingress node A pushes
e-Path(A->Z), B-SID(Y->Z), B-SID(X-Y), s-Path(A->X) and A->X SID list
in turn. When the packet is received at node X, the s-Path(A-X) and
X->Y SID list are popped, and the new s-Path(X->Y) is pushed. Also,
X->Y SID list replaces B-SID(X->Y) to indicate that packet to be
forwarded from node X to node Y. The data packet reaches the SR node
Z according to the same forwarding procedure. In SR node Z, the
e-Path (A->Z) is used to correlate the two unidirectional end-to-end
paths.
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.................. ................. ....................
. . . . . .
+-----+ +-----+ +-----+ +-----+
| A | | X | | Y | | Z |
+-----+ +-----+ +-----+ +-----+
. SR Domain 1 . . SR Domain 2 . . SR Domain 3 .
.................. ................. ....................
|<------------------>|<------------------>|<--------------->|
s-Path(A->X) s-Path(X->Y) s-Path(Y->Z)
|<--------------------------------------------------------->|
e-Path(A->Z)
Node A
+-------------+
|A->X SID list| Node X
+-------------+ +-------------+
|s-Path(A->X) | |X->Y SID list| Node Y
+-------------+ +-------------+ +-------------+
|B-SID(X->Y) | --> |s-Path(X->Y) | |Y->Z SID list|
+-------------+ +-------------+ +-------------+
|B-SID(Y->Z) | |B-SID(Y->Z) | --> |s-Path(Y->Z) | Node Z
+-------------+ +-------------+ +-------------+ +-------------+
|e-Path(A->Z) | |e-Path(A->Z) | |e-Path(A->Z) | --> |e-Path(A->Z) |
+-------------+ +-------------+ +-------------+ +-------------+
| Payload | | Payload | | Payload | | Payload |
+-------------+ +-------------+ +-------------+ +-------------+
Figure 3: Nesting Inter-Domain Scenario
5. Security Considerations
TBA
6. Acknowledgements
TBA
7. IANA Considerations
TBA
8. Normative References
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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-00 (work in progress),
March 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.
[I-D.xiong-pce-stateful-pce-sr-inter-domain]
Xiong, Q., hu, f., Mirsky, G., and W. Cheng, "Stateful PCE
for SR-MPLS-TP Inter-domain", draft-xiong-pce-stateful-
pce-sr-inter-domain-00 (work in progress), December 2018.
[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
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Weiqiang Cheng
China Mobile
Beijing
China
Email: chengweiqiang@chinamobile.com
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