Inserting, Processing And Deleting IPv6 Extension Headers

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Table of Contents

1. Introduction

In IPv6 [RFC8200] optional internet-layer information is encoded in extension headers. As specified by [RFC8200], "extension headers (except for the Hop-by-Hop Options header) are not processed, inserted, or deleted by any node along a packet's delivery path, until the packet reaches the node (or each of the set of nodes, in the case of multicast) identified in the Destination Address field of the IPv6 header".¶

The statement quoted above identifies nodes upon which extension headers are not processed, inserted, or deleted. It does not imply that extension headers can be processed, inserted, or deleted on any other node along a packet's delivery path.¶

This document provides guidance regarding the processing, insertion, and deletion of IPv6 extension headers. It clarifies the statement quoted above and updates [RFC8200].¶

2. Terminology

The following terms are used in this document:¶

• Source node - An IPv6 source node accepts data from an upper-layer protocol, prepends an IPv6 header, and sends the resulting IPv6 packet to a destination node.¶
• Final destination node - An IPv6 final destination node receives an IPv6 packet and delivers its payload to an upper-layer protocol.¶
• Delivery path - A packet's delivery path is a series of nodes that a packet traverses on route to its final destination. The delivery path includes the final destination node.¶
• Segment - A segment is a series of links and nodes in a packet's delivery path. An IPv6 Routing header steers packets from segment to segment along the delivery path. If a packet contains a Routing header, its delivery path can contain multiple segments. If a packet does not contain a Routing header, its delivery path contains only one segment.¶
• Segment egress node - A segment egress node terminates a segment. When a packet arrives at a segment egress node, its IPv6 Destination Address identifies an interface that belongs to the node. All final destination nodes are also segment egress nodes.¶
• Extension header processing - Each IPv6 extension header is associated with a procedure. For example, the Fragment header is associated with fragmentation and reassembly procedures. Extension header processing is the reception of an extension header and the execution of its associated procedure.¶

4. Motivation

The following are reasons why extension headers are not inserted by nodes along a packet's delivery path:¶

• Nodes that execute Path MTU Discovery (PMTUD) [RFC8201] procedures can send packets that are nearly as large as the Path MTU. Adding an extension header to such a packet can cause MTU black holing.¶
• IPv6 Authentication Header [RFC4302] processing relies on the immutability of the Payload Length field in the IPv6 header. When a node along a packet's delivery path inserts an extension header, it must also update the Payload Length field in the IPv6 header. Therefore, it causes IPv6 Authentication Header processing to fail on the final destination node.¶
• When a source node sends a packet to a final destination node, and a node along the packet's delivery path inserts an extension header, the final destination node will mistakenly attribute the extension header to the source node. Attackers can leverage this mistaken attribution.¶

The following are reasons why extension headers, except for the Routing header, are not deleted by any node along a packet's delivery path:¶

• IPv6 Authentication Header processing relies on the immutability of the Payload Length field in the IPv6 header. When a node along a packet's delivery path inserts an extension header, it must also update the Payload Length field in the IPv6 header. Therefore, it causes IPv6 Authentication Header processing to fail on the final destination node.¶
• When a source node sends a packet to a final destination node, and a node along the packet's delivery path removes an extension header, the resulting packet may not elicit the behavior intended by the source node. For example, if a Destination Options header is removed, none of the options that it contains will be delivered to the final destination node.¶

The following are reasons why Routing headers can be deleted by any segment egress node when the Segments Left field is equal to zero and the packet does not contain an authentication header:¶

• Because every segment that the routing header contains has already been processed.¶
• Because [RFC8986] has set a precedent for deletion in this case.¶

7. Acknowledgements

Thanks to Bob Hinden, Brian Carpenter, Tom Herbert and Fernando Gont for their comments and review.¶

Authors' Addresses