The Use of Path Segment in SR Inter-domain Scenarios
draft-xiong-spring-path-segment-sr-inter-domain-02

Abstract

This document illustrates the inter-domain scenarios for SR-MPLS networks to support end-to-end bidirectional tunnel across multiple domains with the use of Path Segments.

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Table of Contents

• 1. Introduction
• 2. Conventions used in this document
• 2.1. Terminology
• 2.2. Requirements Language
• 3. End-to-end Path Segment
• 3.1. S-PSID
• 3.2. N-PSID
• 4. SR-MPLS Inter-domain Scenarios
• 4.1. Stitching of Path Segments
• 4.2. Nesting of Path Segments
• 5. Security Considerations
• 6. Acknowledgements
• 7. IANA Considerations
• 8. Normative References
• Authors' Addresses

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.

As defined in [RFC8402], the headend of an SR Policy binds a Binding Segment ID (B-SID) to its policy. The B-SID could be bound to a SID List or selected path and used to stitch the SR list and the SR Label Switched Paths (LSP) across multiple domains. In some scenarios, for example, a mobile backhaul transport network, it is required to provide end-to-end bidirectional path across SR networks. [I-D.ietf-spring-mpls-path-segment] defines a path segment identifier to support bidirectional path correlation for transport network. In the multi-domain scenarios, the SR bidirectional end-to-end path MAY be established with the use of path segments. Path segment MAY be used to indicate the end-to-end bidirectional path to achieve the path monitoring including nesting of Path Segments or Path SID (N-PSID) and stitching of Path Segments or Path SID (S-PSID).

This document illustrates the inter-domain scenarios for SR-MPLS networks to support end-to-end bidirectional tunnel across multiple domains with the use of Path Segments.

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. 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.

B-SID: Binding Segment ID.

Domains:Autonomous System (AS) or IGP Area. An Autonomous System is composed by one or more IGP areas.

e-Path: End-to-end Path Segment.

Inter-Area: Two IGP areas interconnects with an ABR in an AS.

Inter-AS: Two ASes interconnects with an ASBR.

IGP: Interior Gateway Protocol.

N-PSID: Nesting of Path Segments.

S-PSID: Stitching of Path Segments.

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. End-to-end Path Segment

3.1. S-PSID

As described in [I-D.ietf-spring-mpls-path-segment], an end-to-end Path Segment, also referred to as e-Path. In the inter-domain scenario, the end-to-end SR path is split into multiple segments. And each segment can be identified by S-PSIDs in stitching model. The correlation of path segments can stitch the inter-domain paths and bind unidirectional paths. The S-PSIDs are valid in the corresponding domain and the border nodes maintain the forwarding entries of that S-PSID. At the headend node, the S-PSID can correlate the inter-domain path of reverse direction and bind the two unidirectional paths.

The S-PSID 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 S-PSID allocation and procedure is defined in [I-D.xiong-pce-stateful-pce-sr-inter-domain].

3.2. N-PSID

As described in [I-D.ietf-spring-mpls-path-segment], an end-to-end Path Segment, also referred to as e-Path. In nesting model, the e-Path is also referred to as N-PSID which 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 N-PSID. The use of the B-SID is also recommended to reduce t he size of label stack section 4.2 and stitch the SR list and the SR LSP. The N-PSID can be used to indicate the end-to-end path and achieve the bidirectional path monitoring.

The N-PSID can be a globally unique or local label. If the N-PSID is globally unique, it MUST be assigned from the SRGB block of each domain. If the N-PSID 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.

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 interconnected with a ABR within areas or inter-link between ASes. This document takes IGP Areas domains for example. The border link scenarios are in future discussion. 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 inter-domain Scenario

Two SR-MPLS inter-domain models are discussed in this document including using the stitching and nesting of Path Segments which are described in Section 4.1 and Section 4.2 respectively.

4.1. Stitching of Path Segments

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 S-PSIDs 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 S-PSID (A->X), B-SID(Y->Z), B-SID(X->Y) 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 S-PSID (X->Y), B-SID(Y->Z) 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 b ased on the same forwarding procedure. In node Z, the S-PSID (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 S-PSID. The stitching of path segments can achieve the inter-domain path monitoring.

 
      ..................   .................   ....................
      .                .   .               .   .                  .
  +-----+             +-----+             +-----+              +-----+
  |  A  |             |  X  |             |  Y  |              |  Z  |
  +-----+             +-----+             +-----+              +-----+
      .  SR Domain 1   .   .  SR Domain 2  .   .   SR Domain 3    .
      ..................   .................   ....................

Service Layer:	 
    |<----------------------End-to-end Service--------------->| 
Path Segment:
    |<-----S-PSID----->o<------S-PSID----->o<-----S-PSID----->| 
LSP/Tunnel:
    |<------SR-LSP---->|<-----SR-LSP------>|<-----SR-LSP----->|
Node:
    |<----SID List---->|<-----SID List---->|<----SID List---->|

    Node A               Node X             Node Y             Node Z      
+-------------+           
|A->X SID list|            
+-------------+     +-------------+     
| B-SID(X->Y) | --->|X->Y SID list|
+-------------+     +-------------+     +-------------+    
| B-SID(Y->Z) |     | B-SID(Y->Z) | --->|Y->Z SID list|
+-------------+     +-------------+     +-------------+    +--------------+
|S-PSID(A->X) |     |S-PSID(X->Y) |     |S-PSID(Y->Z) | -->|   Payload    |
+-------------+     +-------------+     +-------------+    +--------------+
|  Payload    |     |   Payload   |     |  Payload    |     
+-------------+     +-------------+     +-------------+    
	   

Figure 2: Stitching of Path Segments in Inter-Domain Scenario

4.2. Nesting of Path Segments

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 N-PSID, B-SID and SR list are pushed by the ingress node. The N-PSID is used to correlate the two unidirectional SR paths to an SR bidirectional path.

The use of the B-SID 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 N-PSID(A->Z), B-SID(Y->Z), B-SID(X->Y), and A->X SID list in turn. When the packet is received at node X, the X->Y SID list are popped. 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 N-PSID (A->Z) is used to correlate the two unidirectional end-to-end paths.

      ..................   .................   ....................
      .                .   .               .   .                  .
  +-----+             +-----+             +-----+              +-----+
  |  A  |             |  X  |             |  Y  |              |  Z  |
  +-----+             +-----+             +-----+              +-----+
      .  SR Domain 1   .   .  SR Domain 2  .   .   SR Domain 3    .
      ..................   .................   .................... 
						   
Service Layer:
    |<----------------------End-to-end Service--------------->|
Path Segment:
    |<------------------------N-PSID------------------------->|
LSP/Tunnel:
    |<------SR-LSP---->o<-------SR-LSP----->o<-----SR-LSP---->|
Node:
    |<----SID List---->|<-----SID List----->|<----SID List--->|
                                
     Node A            Node X                Node Y            Node Z             
+-------------+                        
|A->X SID list|              
+-------------+     +-------------+     
|B-SID(X->Y)  | --> |X->Y SID list|     
+-------------+     +-------------+     +-------------+
|B-SID(Y->Z)  |     |B-SID(Y->Z)  | --> |Y->Z SID list|        
+-------------+     +-------------+     +-------------+     +-------------+
|N-PSID(A->Z) |     |N-PSID(A->Z) |     |N-PSID(A->Z) | --> |  Payload    |
+-------------+     +-------------+     +-------------+     +-------------+
|  Payload    |     |   Payload   |     |  Payload    |     
+-------------+     +-------------+     +-------------+     

   

Figure 3: Nesting of Path Segments in Inter-Domain Scenario

5. Security Considerations

TBA

6. Acknowledgements

TBA

7. IANA Considerations

TBA

8. Normative References

Authors' Addresses