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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Internet Layer (OSI Network Layer) Protocols
           9  Internet Control Message Protocol (ICMP/ICMPv4 and ICMPv6)
                9  ICMP Message Types and Formats
                     9  ICMP Version 6 (ICMPv6) Error Message Types and Formats

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ICMPv6 Destination Unreachable Messages
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ICMPv6 Time Exceeded Messages
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ICMPv6 Packet Too Big Messages
(Page 2 of 3)

ICMPv6 Packet Too Big Message Format

Table 104 and Figure 153 show the format for ICMPv6 Packet Too Big messages:


Table 104: ICMPv6 Packet Too Big Message Format

Field Name

Size (bytes)

Description

Type

1

Type: Identifies the ICMPv6 message type; for Packet Too Big messages this is set to 2.

Code

1

Code: Not used for this message type; set to 0.

Checksum

2

Checksum: 16-bit checksum field for the ICMP header, as described in the topic on the ICMP common message format.

MTU

4

MTU: The maximum transmission unit (MTU) size, in bytes, of the physical link over which the router wanted to send the datagram, but was not able to do so due to the datagram's size. Including this value in the Packet Too Big message tells the source device the size it needs to use for its next transmission to this destination to avoid this problem in the future (at least for this particular link.)

Original Datagram Portion

Variable

Original Datagram Portion: As much of the IPv6 datagram as will fit without causing the size of the ICMPv6 message (including its own IP header) to exceed the minimum IPv6 MTU of 1280 bytes.



Figure 153: ICMPv6 Packet Too Big Message Format

 


Key Concept: In IPv6, routers are not allowed to fragment datagrams that are too large to send over a physical link to which they are connected. An oversized datagram is dropped, and an ICMPv6 Packet Too Big message sent back to the datagram’s originator to inform it of this occurrence.



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