GRE Tunnel Fragmentation
draft-templin-intarea-grefrag-02.txt

Abstract

GRE tunnels use IPv4 or IPv6 fragmentation of the delivery packet when the delivery packet exceeds the tunnel MTU, or when otherwise necessary. This can cause problems when unmitigated IPv4 fragemntation ensues, or when middleboxes drop IPv6 fragments unconditionally. This document introduces GRE tunnel fragmentation which avoids these pitfalls..

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Table of Contents

• 1. Introduction
• 2. GRE Fragmentation Header
• 3. GRE Tunnel Fragmentation Procedures
• 4. IANA Considerations
• 5. Security Considerations
• 6. Acknowledgements
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Author's Address

1. Introduction

GRE is specified in the following RFCs: [RFC2784][RFC2890][RFC7676]. [RFC7588] further discusses GRE fragmentation considerations. In its current manifestation, GRE allows for fragmentation of the payload packet only if it is an IPv4 packet with the Don't Fragment (DF) bit set to 0. GRE also allows for fragmentation of the delivery packet, but this can cause problems in some applications. A third option (introduced here) is for the GRE tunnel to perform tunnel fragmentation and reassembly on the payload packet.

In this way, the ingress can fragment the payload packet (while treating the payload packet's headers as ordinary data) and encapsulate each fragment in a separate delivery header. The GRE header requires a new fragment header field to support this.

This tunnel fragmentation method was first suggested in Section 3.1.7 of [RFC2764], and also appears in more recent works [I-D.templin-aerolink] [I-D.herbert-gue-fragmentation].

2. GRE Fragmentation Header

Figure 1 shows the GRE header as specified in [RFC2784][RFC2890] but with a new optional "Fragment Header" and a new control bit "F":

    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| |K|S|F|   Reserved0   | Ver |         Protocol Type         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Checksum (optional)      |       Reserved1 (Optional)    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Key (optional)                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Sequence Number (Optional)                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Fragment Header (Optional)                    |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+