| Internet-Draft | icmp-eh-len | November 2024 |
| Bonica, et al. | Expires 25 May 2025 | [Page] |
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.¶
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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 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.¶
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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
Figure 1 depicts the ICMP Extension Header.¶
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¶
Reserved (Rsvd): 4 bits¶
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.¶
Checksum: 16 bits¶
The ICMP Extension Structure MUST be zero-padded so that it ends on a 4-byte boundary.¶
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:¶
This document requires no IANA actions.¶
This document introduces no security vulnerabilities.¶
Thanks to Tom Herbert for his review and helpful suggestion.¶