]> ICMP Extension Header Length Field HPE
USA rbonica@juniper.net
China Telecom
China hexm4@chinatelecom.cn
ZTE Corporation
China xiao.min2@zte.com.cn
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. This document updates RFC 4884.
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. New implementations SHOULD always include the length field, even though it is not needed when the ICMP message ends with an ICMP Extension Structure. This document updates .
Motivation enhances the PROBE utility and provides motivation for this document. The enhancement adds a new field to the ICMP Extended Echo and ICMP Extended Echo Reply messages. To maintain compatibility with legacy PROBE implementations, the new field is added after the ICMP Extension Structure. New PROBE implementations infer ICMP Extension Structure length from the ICMP message type. acknowledges that this is not a desirable behavior and advises against its use in the future. This document provides an alternative to inferring ICMP Extension Structure length from other message contents.
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 as updated by this document.
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 as per .
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 .
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.
Backwards Compatibility Legacy implementations set the length field to 0 as per section 7 of . 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.
Legacy implementation do not recognize ICMP messages that rely on the ICMP Extension Header length field. This is because when the document was published, the IETF had not yet standardized any messages that rely on ICMP Extension Header length field. Assume that, at some time in the future, the IETF standardizes a new ICMP message and that message relies on the ICMP Extension Header length field. Legacy implementations will not recognize the new message because of its type and will not examine its content. An ICMP implementation MUST be capable of processing the ICMP Extension Header length field before recognizing any message that relies on it.
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, Erik Vynke and Michael Welzl for their review and helpful suggestions.
References 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. Informative References PROBE: A Utility for Probing Interfaces Arista Networks Juniper Networks Arista Networks Google Verizon Orange This document describes a network diagnostic tool called PROBE. PROBE is similar to PING in that it can be used to query the status of a probed interface, but it differs from PING in that it does not require bidirectional connectivity between the probing and probed interfaces. Instead, PROBE requires bidirectional connectivity between the probing interface and a proxy interface. The proxy interface can reside on the same node as the probed interface, or it can reside on a node to which the probed interface is directly connected. This document updates RFC 4884 and obsoletes RFC 8335.