ICMPV6 RFC 4443 PDF

Gupta, Ed. Please refer to the current edition of the "Internet Official Protocol Standards" STD 1 for the standardization state and status of this protocol. Distribution of this memo is unlimited. Introduction 1.

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Tappan Consultant C. Pignataro Cisco Systems, Inc. Please refer to the current edition of the "Internet Official Protocol Standards" STD 1 for the standardization state and status of this protocol.

Distribution of this memo is unlimited. 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. Bonica, et al. The impact of these changes on compliant implementations is discussed, and new requirements for future implementations are presented.

Table of Contents 1. Conventions Used in This Document ICMP Extensibility Backwards Compatibility Interaction with Network Address Translation Security Considerations IANA Considerations Normative References Informative References The extension structure supports multi-part ICMP operation. Protocol designers can make an ICMP message carry additional information by encoding that information in the extension structure.

This document also addresses a fundamental problem in ICMP extensibility. All of the ICMP messages addressed by this memo include an "original datagram" field. Although the "original datagram" field is of variable length, the ICMP message does not include a field that specifies its length. Application software infers the length of the "original datagram" field from the total length of the ICMP message.

If an extension structure were appended to the message without adding a length attribute for the "original datagram" field, the message would become unparsable. Specifically, application software would not be able to determine where the "original datagram" field ends and where the extension structure begins.

Therefore, this document proposes a length attribute as well as an extension structure that is appended to the ICMP message. The current memo also addresses backwards compatibility with existing ICMP implementations that either do not implement the extensions defined herein or implement them without adding the required length attributes. In particular, this document addresses backwards compatibility with certain, widely deployed, MPLS-aware ICMPv4 implementations that send the extensions defined herein without adding the required length attribute.

The current memo does not define any ICMP extension objects. It defines only the extension header and a common header that all extension objects share. The above mentioned memos share a common characteristic. In this case, as in many others, appending information to the existing ICMP Time Expired Message is preferable to defining a new message and emitting two messages whenever a packet is dropped due to TTL expiration.

The above mentioned messages include an "original datagram" field, and the message formats are updated to specify a length attribute for the "original datagram" field.

It is recommended that all new messages support extensions. Protocol designers can extend ICMP messages by simply appending fields or data structures to them. In RFC , the "original datagram" field includes the IP header plus the next eight octets of the original datagram. Unfortunately, the "original datagram" field lacks a length attribute.

Application software infers the length of this field from the total length of the ICMP message. In order to solve this problem, this memo introduces an 8-bit length attribute to the following ICMPv4 messages. The length attribute represents the length of the "original datagram" field. Space for the length attribute is claimed from reserved octets, whose value was previously required to be zero. For ICMPv4 messages, the length attribute represents bit words.

When the length attribute is specified, the "original datagram" field MUST be zero padded to the nearest bit boundary. Because the Bonica, et al. For ICMPv6 messages, the length attribute represents bit words.

Because the fifth octet of each of the impacted ICMPv6 messages was reserved for future use, this octet was selected as the location of the length attribute in ICMPv6.

If the original datagram did not contain octets, the "original datagram" field MUST be zero padded to octets. See Section 5. The following sub-sections depict length attribute as it has been introduced to selected ICMP messages. However, a length attribute is added to the second word. The length attribute represents length of the padded "original datagram" field, measured in bit words. It is depicted only to demonstrate that those bits are not available for assignment in this memo.

Some ICMP implementations, produced between and the time of this publication, may send a non-compliant version of ICMP extensions described in this memo. When they do this, they send exactly octets representing the original datagram, zero padding if required. They also calculate checksums as described in this document. However, they do not specify a length attribute to be associated with the "original datagram" field.

They are insensitive to the length attribute that is associated with the "original datagram" field. Non-compliant implementations parse the extensions defined in this memo, but only in conjunction with the Time Expired and Destination Unreachable messages. They require the "original datagram" field to contain exactly octets and are insensitive to the length attribute that is associated with the "original datagram" field. Non-compliant applications were produced between and the time of publication of this memo.

Compliant applications comply fully with the specifications of this document. In order to demonstrate backwards compatibility, Table 1 describes how members of each application category would parse each category of ICMP message. Fortunately, the extensions are guaranteed to begin at least octets beyond the beginning of the "original datagram" field.

So, only those ICMP applications that process the th octet of the "original datagram" field will be adversely effected. To date, only two applications falling into this category have been identified, and the degree to which they are effected is minimal. The impact of this issue is considered to be minimal because many ICMP messages are discarded for other reasons e. Another theoretically possible, but highly improbably scenario occurs when ICMP extensions overwrite the portion of the original datagram field that represents the TCP header, causing the TCP stack to operate upon the wrong TCP connection.

This scenario is highly unlikely because it occurs only when the TCP header appears at or beyond the th octet of the original datagram field and then only when the extensions approximate a valid TCP header. If the total ICMPv4 message length is less than the length of its IP header plus octets, the application correctly determines that the message does not contain any extensions. All of these octets would be required if extensions were present.

In order to represent a valid Extension Header, it must contain a valid version number and checksum. If it does not contain a valid version number and checksum, the application correctly determines that the message does not contain any extensions. Non-compliant applications assume that the ICMPv4 Extension Structure begins on the th octet of the Time Exceeded message, after a octet field representing the padded "original datagram" message.

It is possible that a non-compliant application will parse an ICMPv4 message incorrectly under the following conditions: - the message does not contain extensions - the original datagram field contains octets or more - selected octets of the original datagram field represent the correct values for an extension header version number and checksum Although this is possible, it is very unlikely.

A similar analysis can be performed for ICMPv6. However, the numeric constants would change as appropriate. Non-Compliant Application Receives ICMP Message with Compliant Extensions When a non-compliant application receives a message that contains compliant ICMP extensions, it will parse those extensions correctly only if the "original datagram" field contains exactly octets. This is because non-compliant applications are insensitive to the length attribute that is associated with the "original datagram" field.

They assume its value to be However, each implementation will decide how many octets to include. If the length attribute is zero, the compliant application MUST determine that the message contains no extensions. In this case, that determination is technically correct, but not backwards compatible with the non-compliant implementation that originated the ICMP message. Specifically, when a TRACEROUTE application operating in non- compliant mode receives a sufficiently long ICMP message that does not specify a length attribute, it will parse for a valid extension header at a fixed location, assuming a octet "original datagram" field.

If the application detects a valid version and checksum, it will treat the octets that follow as an extension structure. These fields include the "original datagram" field mentioned above.

However, if a NAT device modifies the "original datagram" field, it should modify only the leading octets of that field, which represent the outermost IP header. Because the outermost IP header is guaranteed to be contained by the first octets of the "original datagram" field, ICMP extensions and NAT will not interfere with one another.

It is conceivable that a NAT implementation might overstep the restrictions of RFC and overwrite the length attribute specified by this memo. If a NAT implementation were to overwrite the length attribute with zeros, the resulting packet will be indistinguishable from a packet that was generated by a non-compliant ICMP implementation. The Extension Structure contains exactly one Extension Header followed by one or more objects.

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Internet Control Message Protocol for IPv6

Tappan Consultant C. Pignataro Cisco Systems, Inc. Please refer to the current edition of the "Internet Official Protocol Standards" STD 1 for the standardization state and status of this protocol. Distribution of this memo is unlimited. 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.

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RFC Errata

Gupta, Ed. Please refer to the current edition of the "Internet Official Protocol Standards" STD 1 for the standardization state and status of this protocol. Distribution of this memo is unlimited. Conta, et al. Message General Format Message Source Address Determination Message Checksum Calculation

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