]> ICMP Extension Structure Length Field HPE
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Internet INTAREA Group ICMP The ICMP Extension Structure (RFC4884) does not have a length field. Therefore, unless the length of the Extension Structure can be inferred from other data in the ICMP message, the Extension Structure must be the last item in the ICMP message. This document updates RFC 4884 to define a length field for the ICMP Extension Structure. When length information is provided, receivers can use it to parse ICMP messages. Specifically, receivers can use length information to determine the offset at which the item after the ICMP Extension Structure begins.
Introduction The ICMP Extension Structure has variable length. However, it does not include a field that reflects its length. Therefore, implementations can parse the ICMP Extension Structure only when it appears at the end of an ICMP message. It is good practice for a variable-length data structure to include a field that reflects its length. This allows implementations to parse the structure even when it does not appear at the end of a message. The ICMP Extension Structure includes reserved bits that are available for this purpose. This document adds a length field to the ICMP Extension Header. It does not define data items that might follow the ICMP Extension Structure. The specifications of this document apply to all ICMP Extension Structures, regardless of whether they appear in ICMPv4 or ICMPv6 messages. This document UPDATES .
Conventions and Definitions 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 BCP14 when, and only when, they appear in all capitals, as shown here.
The ICMP Extension Structure An ICMP Extension Structure contains exactly one Extension Header followed by one or more objects. The Extension Header format is defined in Section 7 of . This document modifies the Extension Header format by allocating the lower 8 bits of the reserved field for a new length field. depicts the updated Extension Header format.
ICMP Extension Header As Updated By This Document 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 Version Rsvd Length Checksum
Version: 4 bits.
  • ICMP Extension Header version number. This is version 2 as per .
Reserved (Rsvd): 4 bits
  • MUST be set to 0 by the sender and MUST be ignored by the receiver.
Length: 8 bits
  • This field represents the length of the ICMP Extension Structure, including all options and optional padding, but excluding the ICMP Extension Header. The length is measured in 4-byte words. Legacy implementations set this field to 0 as per section 7 of . Therefore, implementation SHOULD NOT drop packets if this field is set to 0.
Checksum: 16 bits
  • As per , the checksum is the one's complement of the one's complement sum of the data structure, with the checksum field replaced by zero for the purpose of computing the checksum. An all-zero value means that no checksum was transmitted. See Section 5.2 of for a description of how this field is used.
The ICMP Extension Structure MUST be zero-padded so that it ends on a 4-byte boundary. If it does not end on a 4-byte boundary, the receiving node will parse the ICMP message incorrectly and may discard it. The receiver MUST silently discard an ICMP message in the following cases:
  • The length field in the ICMP Extension Header indicates that the ICMP Extension Structure is too large to fit in the ICMP message.
  • The length field in the final ICMP Extension Object indicates that the final ICMP Extension Object is too large to fit in the ICMP Extension Structure.
  • The final three bytes of the ICMP Extension Structure are neither padding (i.e., zeros) nor part of a well-formed ICMP Extension Object.
Backwards Compatibility Legacy implementations that do not support the mechanism defined in this document set the length field to zero when sending a packet and ignore the length field in received ICMP messages. Such implementations require one of the following:
  • The ICMP Extension Structure is final item in the ICMP packet.
  • The length of the ICMP Extension Structure can be inferred from other fields in the packet.
Currently, no mechanisms rely on the ICMP extension structure length field. Should such mechanisms be defined in the future, backward compatibility with legacy implementations should be discussed for each case.
UPDATES to RFC 4884
Length Field In Section 7 of , an ICMP Extension Header contains a 12-bit reserved field. of this document allocates the lower 8 bits of that field for a new length field. provides a diagram of the updated ICMP Extension header.
Malformed Extension Headers offered no advice regarding the processing of malformed ICMP Extension Headers. of this document offers the following advice: The receiver MUST silently discard an ICMP message in the following cases:
  • The length field in the ICMP Extension Header indicates that the ICMP Extension Structure is too large to fit in the ICMP message.
  • The length field in the ICMP Extension Structure is less than to the total length of ICMP Extension Objects.
  • The final three bytes of the ICMP Extension Structure are neither padding (i.e., zeros) nor part of a well-formed ICMP Extension Object.
Padding Requirement In , the ICMP Extension Structure was not required to end on a 4-byte boundary. of this document adds the following requirement: The ICMP Extension Structure MUST be zero-padded so that it ends on a 4-byte boundary. If it does not end on a 4-byte boundary, the receiving node will parse the ICMP message incorrectly and may discard it.
Ignore Reserved Field describes the reserved field of the ICMP Extension Header as follows: Must be set to 0. of this document describes the reserved field as follows: MUST be set to 0 by the sender and MUST be ignored by the receiver.
IANA Considerations This document requires no IANA actions.
Security Considerations This document introduces no security vulnerabilities. However, it does inherit security considerations from .
Acknowledgements Thanks to Tom Herbert, Jen Linkova, Erik Vynke and Michael Welzl for their review and helpful suggestions.
Normative References Internet Control Message Protocol Extended ICMP to Support Multi-Part Messages This document redefines selected ICMP messages to support multi-part operation. A multi-part ICMP message carries all of the information that ICMP messages carried previously, as well as additional information that applications may require. Multi-part messages are supported by an ICMP extension structure. The extension structure is situated at the end of the ICMP message. It includes an extension header followed by one or more extension objects. Each extension object contains an object header and object payload. All object headers share a common format. This document further redefines the above mentioned ICMP messages by specifying a length attribute. All of the currently defined ICMP messages to which an extension structure can be appended include an "original datagram" field. The "original datagram" field contains the initial octets of the datagram that elicited the ICMP error message. Although the original datagram field is of variable length, the ICMP message does not include a field that specifies its length. Therefore, in order to facilitate message parsing, this document allocates eight previously reserved bits to reflect the length of the "original datagram" field. The proposed modifications change the requirements for ICMP compliance. The impact of these changes on compliant implementations is discussed, and new requirements for future implementations are presented. This memo updates RFC 792 and RFC 4443. [STANDARDS-TRACK] Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification This document describes the format of a set of control messages used in ICMPv6 (Internet Control Message Protocol). ICMPv6 is the Internet Control Message Protocol for Internet Protocol version 6 (IPv6). [STANDARDS-TRACK] Key words for use in RFCs to Indicate Requirement Levels In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.