Unified Identifier in IPv6 Segment Routing Networks
draft-mirsky-6man-unified-id-sr-03

Abstract

Segment Routing architecture leverages the paradigm of source routing. It can be realized in a network data plane by prepending the packet with a list of instructions, a.k.a. segments. A segment can be encoded as a Multi-Protocol Label Switching (MPLS) label, IPv4 address, or IPv6 address. Segment Routing can be applied in MPLS data plane by encoding segments in MPLS label stack. It also can be applied to IPv6 data plane by encoding a list of segment identifiers in IPv6 Segment Routing Extension Header (SRH). This document extends the use of the SRH to unified identifiers encoded as MPLS label and IPv4 address, to support interworking between SR-MPLS and SRv6.

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
• 1.1. Conventions used in this document
• 1.1.1. Terminology
• 1.1.2. Requirements Language
• 2. Segment Routing Extension Header: Benefits and Challenges
• 3. Support of Multiple SID Lengths in IPv6 Segment Routing Extension Header
• 4. The Use Case of Unified Segment Identifier
• 4.1. Interworking Scenario Between SR-MPLS and SRv6
• 5. Operations with Unified Segment Identifier
• 5.1. Egress SR Tunnel Node
• 6. IANA Considerations
• 7. Security Considerations
• 8. Acknowledgements
• 9. Normative References
• Authors' Addresses

1. Introduction

Segment Routing architecture [RFC8402] leverages the paradigm of source routing. It can be realized in a network data plane by prepending the packet with a list of instructions, a.k.a. segment identifiers (SIDs). A segment can be encoded as a Multi-Protocol Label Switching (MPLS) label, IPv4 address, or IPv6 address. Segment Routing can be applied in MPLS data plane by encoding 20-bits SIDs in MPLS label stack [I-D.ietf-spring-segment-routing-mpls]. It also can be applied to IPv6 data plane by encoding a list of 128-bits SIDs in IPv6 Segment Routing Extension Header (SRH) [I-D.ietf-6man-segment-routing-header]. Applicability of 32-bits SID that may represent an IPv4 address has not been defined.

SR extensions to Interior Gateway Protocols (IGP), IS-IS [I-D.ietf-isis-segment-routing-extensions], OSPF [I-D.ietf-ospf-segment-routing-extensions], and OSPFv3 [I-D.ietf-ospf-ospfv3-segment-routing-extensions], defined how 20-bits and 32-bits SIDs advertised and bound to SR objects and/or instructions. Extensions to BGP link-state address family [I-D.ietf-idr-bgp-ls-segment-routing-ext] enabled propagation of segment information of variable length via BGP.

This document extends the use of the SRH [I-D.ietf-6man-segment-routing-header] to SIDs for unified identifier encoded as MPLS label and IPv4 address to support interworking between SR-MPLS and SRv6.

1.1. Conventions used in this document

1.1.1. Terminology

SR: Segment Routing

SRH: Segment Routing Extension Header

MPLS: Multiprotocol Label Switching

SR-MPLS: Segment Routing using MPLS dataplane

SID: Segment Identifier

IGP: Interior Gateway Protocol

OAM: Operation, Administration and Maintenance

TE: Traffic Engineering

SRv6: Segment Routing in IPv6

U-SID: Unified Segment Identifier

1.1.2. 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.

2. Segment Routing Extension Header: Benefits and Challenges

Many functions related to Operation, Administration and Maintenance (OAM) require identification of the SR tunnel ingress and the path, constructed by segments, between the ingress and the egress SR nodes. Combination of IPv6 encapsulation [RFC8200] and SRH [I-D.ietf-6man-segment-routing-header], referred to as SRv6, comply with these requirements while it is challenging when applying SR in MPLS networks, also referred to as SR-MPLS.

On the other hand, the size of IPv6 SID presents a scaling challenge to use topological instructions that define strict explicit traffic engineered (TE) path in combination with service-based instructions. At the same time, that is where SR-MPLS approach provides better results due to smaller SID length.

SR-MPLS is currently larger-scale used to metro networks than SRv6. With the gradual deployment of SRv6 in the core networks, it becomes necessary to support interworking between SR-MPLS and SRv6. It's operationally more efficient and straightforward if SRv6 can use the same size SIDs as in SR-MPLS. The SRH can be extended to define the same as in SR-MPLS SID length to support the unified segment identifier (U-SID). As a result, end-to-end SR tunnel may use U-SIDs across SR-MPLS and SRv6 domains.

3. Support of Multiple SID Lengths in IPv6 Segment Routing Extension Header

    
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Next Header   |  Hdr Ext Len  | Routing Type  | Segments Left |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Last Entry   |     Flags     |              Tag              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |            Segment List[0] (128 bits IPv6 address)            |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                                                               |
                                  ...
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |            Segment List[n] (128 bits IPv6 address)            |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //                                                             //
    //         Optional Type Length Value objects (variable)       //
    //                                                             //
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1: SRH format

In section 3 of [I-D.ietf-6man-segment-routing-header] SRH format has been defined as presented in Figure 1

This document defines the new field Size in the SRH Flags field as a two-bits field with the following values:

• 0b00 - 128-bits SID;
• 0b01 - 20-bits SID;
• 0b10 - 32-bits SID
• 0b11 - reserved for future use.

0b01 and 0b10 can be used as an indication for the U-SID length. When the value of the S field is 0b01 the 20-bit SID may be encoded as 20-bit label-only or in four octets then occupies the 20 rightmost bits.

Entries of the segment list in the SRH MUST be of the same length.

4. The Use Case of Unified Segment Identifier

The U-SID can be used for interworking between SR-MPLS and SRv6. The SR-MPLS is large-scale used to the metro network, such as, for example, the backhaul metro network of CMCC. When the core network uses SRv6, such as, for example, the core network of CMCC, it can use the U-SID to interworking with SR-MPLS in the metro network for end-to-end tunnel.

4.1. Interworking Scenario Between SR-MPLS and SRv6

SR-MPLS uses SR SIDs as MPLS label in MPLS stack over IPv4 or IPv6, and the SIDs are 20-bits long. SRv6 uses SR SIDs as IPv6 extension header in SRH over IPv6, and the SIDs are 128-bits long.

    
        +---------+          +------------------------------------+
        |         |          |           IPv6 header              |
        |IPv4/IPv6|          +------------------------------------+
        |         |          |           SRH                      |
        +---------+          +------------------------------------+
        |  USID1  |          | USID1 | USID2 | ...  | USID6 | Res |
        +---------+          +------------------------------------+
        |  USID2  |          | USID7 |...    | USIDn| Null  | Res |
        +---------+          +------------------------------------+
        | ...     |          +           Payload                  |
        +---------+          +------------------------------------+
        |  USIDn  |
        +---------+
        | Payload |
        +---------+

Figure 2: 20-bits long U-SIDs Encapsulation

    
        +---------+          +----------------------------------+
        |         |          |           IPv6 header            |
        |IPv4/IPv6|          +----------------------------------+
        |         |          |           SRH                    |
        +---------+          +----------------------------------+
        |  USID1  |          | USID1  | USID2  | ...   | USID4  |
        +---------+          +----------------------------------+
        |  USID2  |          | USID5  |...     | USIDn | Null   |
        +---------+          +----------------------------------+
        | ...     |          +           Payload                |
        +---------+          +----------------------------------+
        |  USIDn  |
        +---------+
        | Payload |
        +---------+

Figure 3: 32-bits long U-SIDs Encapsulation

The U-SID uses the same length SIDs in MPLS stack and SRH, and the SID is either 20-bits or 32-bits long. There can be put six 20-bit USIDs and left 8-bit reserved or four 32-bits long USIDs in the space of the single 128-bits long header. The encapsulation is illustrated in Figure 2 and Figure 3.

    
        +-----+       +-----+       +-----+       +-----+
        |  A  +-------+  B  +-------+  E  +-------+  F  |
        +-----+       +--+--+       +--+--+       +--+--+
           |   SR-MPLS   |             |    SRv6     |
           |             |             |             |
        +-----+       +--+--+       +--+--+       +--+--+
        |  C  |-------|  D  +-------+  G  +-------+  H  |
        +-----+       +-----+       +-----+       +-----+

Figure 4: SR-MPLS and SRv6 interworking

The SR-MPLS and SRv6 interworking is illustrated in Figure 4. An end-to-end SR tunnel from A to F crosses the SR-MPLS and SRv6 domains. The SRv6 border nodes (E/G) receive SR-MPLS packets and forward them into the SRv6 domain using U-SIDs.

    
        +--------+
        |IP(A->F)|
        +--------+
        | USID(A)|
        +--------+      +--------+
        | USID(B)|      |IP(E->F)|
        +--------+      +--------+      +----------------+
        |USID(E1)|      |USID(E1)|      |IP(E->F)        |
        +--------+      +--------+      +----------------+
        |USID(E2)|      |USID(E2)|      |  SRH           |
        +--------+      +--------+      +----------------+
        | USID(F)|      | USID(F)|      | SID(E2)| SID(F)|
        +--------+      +--------+      +----------------+
        | Payload| ---> | Payload| ---> | payload        |
        +--------+      +--------+      +----------------+

Figure 5: An Example of Packe Forwarding Based on U-SID

The SRv6 edge node is assigned two U-SIDs (e.g., E1 and E2), one for SR- MPLS and the other for SRv6. Figure 5 demonstrates an example of the packet forwarding.

5. Operations with Unified Segment Identifier

When the SRH is used to include 20-bits or 32-bits U-SIDs the ingress and transit nodes of an SR tunnel act as described in Section 5.1 and Section 5.2 of [I-D.ietf-6man-segment-routing-header] respectively.

5.1. Egress SR Tunnel Node

TBD

6. IANA Considerations

IANA is requested to allocate from the Segment Routing Header Flags registry the two-bits long field referred to as Size.

7. Security Considerations

This specification inherits all security considerations of [RFC8402] and [I-D.ietf-6man-segment-routing-header].

8. Acknowledgements

TBD

9. Normative References

Authors' Addresses