]> ICMP Extension Header Length Field Juniper Networks
USA rbonica@juniper.net
China Telecom
China hexm4@chinatelecom.cn
ZTE Corporation
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Huawei
Israel tal.mizrahi.phd@gmail.com
Internet INTAREA Group ICMP The ICMP Extension Structure 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 defines 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 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 defines 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.
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 depicts the ICMP Extension Header.
ICMP Extension Header
Version: 4 bits
  • Defined in
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.
Reserved (Rsvd): 4 bits
  • Defined in
Checksum: 16 bits
  • Defined in
The ICMP Extension Structure MUST be zero-padded so that it ends on a 4-byte boundary.
Backwards Compatibility Legacy implementations set the length field to 0. When the length field is set to 0, it conveys no information and cannot be used to parse the ICMP packet. In these cases, one of the following statements MUST be true:
  • The ICMP Extension Structure is the final item in the ICMP packet.
  • The length of the ICMP Extension Structure can be inferred from other fields in the packet.
IANA Considerations This document requires no IANA actions.
Security Considerations This document introduces no security vulnerabilities.
Normative References 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] 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.