Embedding LOOPS in Geneve

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Table of Contents

1. Introduction

LOOPS (Local Optimizations on Path Segments) aims to provide local in-network loss recovery. The LOOPS problems and opportunities draft [I-D.li-tsvwg-loops-problem-opportunities] illustrates some typical scenarios where LOOPS are applicable. One way to use LOOPS is to map it onto a tunnel protocol. The path segment on which LOOPS is applied then is a tunnel, which can be an existing one or created on purpose.¶

LOOPS allows the packet loss recovery to be performed over specific segments instead of end-to-end, enabling faster and more reliable data delivery. [I-D.welzl-loops-gen-info] defines the information to be carried between LOOPS ingress and egress nodes in a generic way, giving a guideline on defining the common elements to embed LOOPS functions in various tunnel protocols.¶

Geneve [I-D.ietf-nvo3-geneve] is an encapsulation protocol that can be used to create overlay tunnels. It defines an extensible TLV structure to carry so-called "tunnel options". The present document employs this flexibility, specifying how to embed LOOPS in Geneve. This specification covers the format and Geneve-specific procedures only: the actual LOOPS function and procedures are defined in [I-D.welzl-loops-gen-info].¶

LOOPS has two modes of loss recovery, retransmission and forward error correction (FEC). The current version of the present document covers retransmission only.¶

2. Terminology

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.¶

This document makes use of the terminology defined in [I-D.welzl-loops-gen-info].¶

3. Geneve LOOPS Frame Format

Figure 1 shows the format of the Geneve Header and a single Geneve Option, as defined in [I-D.ietf-nvo3-geneve]. Geneve LOOPS defines a new Option class called LOOPS to carry LOOPS forward and backward information.¶

Geneve Header and Option:¶

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ver|  Opt Len  |O|C|    Rsvd.  |          Protocol Type        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Virtual Network Identifier (VNI)       |    Reserved   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Variable Length Options                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          Option Class         |      Type     |R|R|R| Length  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Variable Option Data                     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

In the Geneve Option structure, a Geneve LOOPS option uses the following values:¶

• Option Class: TBD1 for LOOPS (see Section 5).¶
• Type: Based on the substructure already defined in Geneve, which uses bit 0 (the most significant bit) to indicate a critical option (see see Figure 2), LOOPS defines two type numbers: 0 for LOOPS retransmission mode, and 64 for FEC mode. The present document only addresses messages with LType=0.¶

TBD: Additional type numbers could be defined, possibly obviating the need for some of the flags in the current option structure.¶

 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|C|    LType    |
+-+-+-+-+-+-+-+-+

• C: Critical bit as defined in [I-D.ietf-nvo3-geneve].¶

• LType: LOOPS Mode.¶

  • 0: Retransmission mode. In this mode, the LOOPS option format and operations follow this document.¶
  • 64: FEC mode¶
  • Further mode values can be assigned in an IANA registry (see Section 5.2).¶
• Length: Length of Variable Option Data field, expressed in four byte multiples excluding the option header, ranging from 0 to 31. As the option header is another four bytes, the total length of the option in bytes is therefore 4 * (1 + Length), yielding a maximum total length of 128 bytes.¶

• Variable option data: consists of two parts, Flags and Flag Based Data, as shown in Figure 3.¶

  • Flags: 16 bits, as described in next subsection. Some of the flags indicate the presence of additional data in the field of Flag Based Data.¶
  • Flag Based Data: This field consists of one or multiple optional data blocks whose presence is indicated by the corresponding flag bits. Any remaining bytes needed to reach a multiple of four bytes are filled with zeroes.¶

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Flags               |                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
 |                                                               |
 ~                         Flag Based Data                       ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |I|R|D|S|T|E|A|R|             |B|
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4. Security Considerations

The security considerations of [I-D.welzl-loops-gen-info] and [I-D.ietf-nvo3-geneve] apply.¶

5. IANA Considerations

Acknowledgements

Sami Boutros provided some advice on the use of Geneve in this protocol binding.¶

Author's Address