



Network Working Group                                            H. Chen
Internet-Draft                                                    J. Liu
Intended status: Standards Track                                   W. Li
Expires: 1 August 2026                                           K. Zhao
                                                      Nanjing University
                                                         28 January 2026


       SRv6 Segment Identifier Compression for Satelite Networks
                  draft-chen-spring-satellite-csid-00

Abstract

   This document defines a compression method for Segment Identifiers
   (SIDs) in Segment Routing over IPv6 (SRv6) specifically designed for
   satellite networks.  By leveraging the topological location
   information of satellite nodes, a 128-bit SID is compressed into a
   32-bit Compressed SID (C-SID).  The method introduces a "SID
   Container" structure to support the co-existence of 128-bit SIDs and
   32-bit C-SIDs within a single Segment List without modifying the SRv6
   control plane or the fixed SRH header format defined in RFC 8754.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 1 August 2026.

Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights



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   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Design Goals  . . . . . . . . . . . . . . . . . . . . . . . .   3
   5.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   4
   6.  Protocol Specification  . . . . . . . . . . . . . . . . . . .   4
     6.1.  C-SID Structure and Encoding  . . . . . . . . . . . . . .   4
     6.2.  SID Container and Multi-scale Orchestration . . . . . . .   5
     6.3.  Forwarding and Decoding Logic . . . . . . . . . . . . . .   5
       6.3.1.  Identification and Reading  . . . . . . . . . . . . .   5
       6.3.2.  Decoding to IPv6 Address  . . . . . . . . . . . . . .   5
       6.3.3.  Operational Advantages  . . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
     7.1.  Privacy and Topology Exposure . . . . . . . . . . . . . .   6
     7.2.  Address Scanning  . . . . . . . . . . . . . . . . . . . .   6
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   10. Normative References  . . . . . . . . . . . . . . . . . . . .   6
   11. Informative References  . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   Satelite networks face unique constraints such as limited onboard
   computing power and storage resources.  When applying SRv6 to
   satellite constellations, long service paths or complex policies lead
   to significant Segment Routing Header (SRH) [RFC8754] overhead, which
   reduces effective payload and increases the risk of MTU
   fragmentation.  Existing compression schemes often rely on complex
   endpoint behaviors or rigid block planning, which are difficult to
   implement and maintain in dynamic satellite environments.  This
   document [CHEN-SAT-CSID] proposes a lightweight compression method
   that maps satellite topological semantics (layer, orbit, and node
   index) directly into a 32-bit C-SID.  This approach enhances network
   observability and efficiency while maintaining full compatibility
   with standard SRv6 data plane processing [RFC8986].








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2.  Conventions Used in This Document

   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.  Terminology

   SID Container:  A 128-bit basic unit in the SRH Segment List used to
      carry either one 128-bit SID or multiple 32-bit C-SIDs.

   C-SID:  Compressed Segment Identifier, 32 bits in length.

   CSI (Compressed SID Index):  A field used to indicate the position of
      a C-SID within a SID Container.

   C-Flag (Compression Flag):  A flag used to distinguish between C-SIDs
      and standard SIDs.

4.  Design Goals

   This document targets satellite constellation environments where SRv6
   policies may traverse long paths and frequently change.  The
   mechanism is designed to achieve the following goals:

   *  Compatibility: Preserve the fixed SRH header format and standard
      SRv6 processing model.

   *  No control-plane extension: Avoid modifications to SRv6 control
      plane signaling; only the segment list representation is
      optimized.

   *  Topology awareness: Encode satellite topological semantics (layer,
      orbit, and node index) directly into the identifier.

   *  Multi-scale orchestration: Allow interleaving of compressed 32-bit
      C-SIDs and regular 128-bit SIDs within a single Segment List.

   *  Implementation simplicity: Use lightweight packing/decoding logic
      suitable for resource-constrained onboard platforms.









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5.  Protocol Overview

   The Segment List in an SRH [RFC8754] is reorganized into 128-bit
   units called SID Containers.  A container operates in either Standard
   Mode (one 128-bit SID) or Compressed Mode (1 to 4 C-SIDs).  A 32-bit
   C-SID carries satellite topological semantics and SRv6 behavior
   information.  During forwarding, a node reads the active container
   indicated by Segment Left (SL) and decides whether to process a full
   SID or a C-SID based on a container flag bit.  When a C-SID is
   encountered, it can be decoded to a 128-bit IPv6 address for
   forwarding, without changing the SRH base format.

6.  Protocol Specification

6.1.  C-SID Structure and Encoding

   A C-SID MUST be defined as a 32-bit field.  The structure is as
   follows:

        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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           C-Locator           | C-Func  |     C-Args     |CSI|F|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Figure 1: C-SID Format (CSI is 2 bits; C-Arguments is 8 bits)

   Bit numbering in Figure 1 follows the convention used in IETF bit
   diagrams where the leftmost bit is bit 0.  The fields MUST be
   assigned as: C-Locator bits 0-15, C-Func bits 16-20, C-Args bits
   21-28, CSI bits 29-30, and F bit 31.

   C-Locator (16 bits):  Carries satellite topological information.  It
      is divided into three sub-fields representing the layer index,
      orbit index, and in-orbit index of the node.

   C-Function (5 bits):  Represents the segment behavior instructions
      mapped from the original SID [RFC8986].

   C-Arguments (8 bits):  Carries behavior extension parameters.

   CSI (2 bits):  Indicates the slot index within the SID Container.

   *  11: Slot 1 (bits 0-31 of the container)

   *  10: Slot 2

   *  01: Slot 3



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   *  00: Slot 4 (bits 96-127)

   C-Flag (1 bit): MUST be set to 1 if the unit is a C-SID; it MUST be
   set to 0 if it is a standard SID or padding.

6.2.  SID Container and Multi-scale Orchestration

   The SRH Segment List is reorganized into 128-bit SID Containers.
   Each container MUST function in one of two modes:

   1.  Standard Mode: Carries a single 128-bit uncompressed SID.  The
       lowest bit (C-Flag) of the container MUST be set to 0.

   2.  Compressed Mode: Carries 1 to 4 C-SIDs.  The lowest bit (C-Flag)
       of the container MUST be set to 1.

   This design allows for multi-scale identifier orchestration where
   C-SIDs and standard SIDs are interleaved within the same Segment
   List.  If a container is not fully filled with 4 C-SIDs, the
   remaining bits SHOULD be set to zero as padding, with their C-Flags
   marked as 0.

6.3.  Forwarding and Decoding Logic

6.3.1.  Identification and Reading

   When a satellite node processes an SRH, it SHOULD locate the current
   SID Container via the Segment Left (SL) pointer.  The type is judged
   by the lowest bit (C-Flag):

   *  If C-Flag == 0: Read as a standard SID.  After processing,
      decrement SL by 1.

   *  If C-Flag == 1: Read the C-SID.

      -  If CSI != 0: Process the C-SID and offset the internal reading
         pointer by 32 bits within the container.

      -  If CSI == 0: After processing, the container is exhausted.
         Decrement SL by 1 to point to the next container.

6.3.2.  Decoding to IPv6 Address

   To facilitate forwarding, a 32-bit C-SID MUST be decoded into a
   128-bit IPv6 destination address:

   *  Prefix (48 bits): Taken from the common prefix of the satellite
      constellation (derived from the Source IP address).



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   *  C-Locator (16 bits): Appended to the prefix to form a 64-bit
      addressing prefix.

   *  Behavioral Mapping (16 bits): C-SID bits C[15:0] MUST be mapped to
      bits 65-80 of the IPv6 address.

   *  Final Flags: The C-Flag MUST be mapped to the lowest bit of the
      IPv6 address, with remaining bits set to zero.

6.3.3.  Operational Advantages

   *  No Protocol Extension: Works with existing SRH formats [RFC8754]
      and SRv6 control plane protocols.

   *  Topological Awareness: Directly correlates SIDs with satellite
      coordinates (layer/orbit/node).

   *  High Efficiency: Eliminates the need to carry 128-bit Block
      prefixes in the first segment of compressed lists.

7.  Security Considerations

   As satellite networks are vulnerable to link exposure, the integrity
   of the SID Container SHOULD be protected.  Standard SRH security
   mechanisms SHOULD be applied [RFC8754].

7.1.  Privacy and Topology Exposure

   Because C-SIDs embed topological semantics, deployments SHOULD
   consider the risk of topology disclosure on untrusted links.

7.2.  Address Scanning

   Implementations SHOULD consider rate limiting and filtering to reduce
   scanning and probing risks in exposed environments.

8.  IANA Considerations

   This document requests no new allocations from IANA.

9.  Acknowledgements

   The authors thank the IETF SPRING working group and the satellite
   networking research community for discussions that motivated this
   work.

10.  Normative References




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   [RFC2119]  RFC Editor, "Key words for use in RFCs to Indicate
              Requirement Levels", March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC8174]  RFC Editor, "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", May 2017,
              <https://www.rfc-editor.org/rfc/rfc8174>.

   [RFC8754]  RFC Editor, "IPv6 Segment Routing Header (SRH)", March
              2020, <https://www.rfc-editor.org/rfc/rfc8754>.

   [RFC8986]  RFC Editor, "Segment Routing over IPv6 (SRv6) Network
              Programming", February 2021,
              <https://www.rfc-editor.org/rfc/rfc8986>.

11.  Informative References

   [CHEN-SAT-CSID]
              Chen, H., "SRv6 Segment Identifier Compression for
              Satelite Networks", 2026.  Unpublished work provided by
              the authors.

Authors' Addresses

   Hongyan Chen
   Nanjing University
   163 Xianlin Avenue, Qixia District
   Nanjing
   Jiangsu, 210023
   China
   Email: chenhongyan6176@163.com


   Jun Liu
   Nanjing University
   Nanjing
   Jiangsu,
   China
   Email: Johnson.liu@nju.edu.cn


   Wenfeng Li
   Nanjing University
   Nanjing
   Jiangsu,
   China
   Email: wenfeng@nju.edu.cn




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   Kanglian Zhao
   Nanjing University
   Nanjing
   Jiangsu,
   China
   Email: zhaokanglian@nju.edu.cn













































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