| IP option 11, MTU Probe |
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Description | Glossary | RFCs | Publications | Obsolete RFCs |
| Protocol suite: | TCP/IP. |
| Protocol type: | Connectionless network layer protocol. |
| Option length: | 4 bytes. |
| Host implementation: | Obsolete. |
| Router implementation: | Obsolete. |
| Links: | IANA: IP option numbers. |
The IP MTU Probe option can be used to find the lowest MTU out of the network paths that a nonfragmented datagram has traveled.
The MTU value will be returned in the MTU Reply option.
This option is now obsolete.
RFC 1063, pg 2:
This option always contains the lowest MTU of all the networks that have been traversed so far by the datagram.
A host that sends this option must initialize the value field to be the MTU of the directly-connected network. If the host is multi-homed, this should be for the first-hop network.
Each gateway that receives a datagram containing this option must compare the MTU field with the MTUs of the inbound and outbound links for the datagram. If either MTU is lower than the value in the MTU field of the option, the option value should be set to the lower MTU. (Note that gateways conforming to RFC-1009 may not know either the inbound interface or the outbound interface at the time that IP options are processed. Accordingly, support for this option may require major gateway software changes).
Any host receiving a datagram containing this option should confirm that value of the MTU field of the option is less than or equal to that of the inbound link, and if necessary, reduce the MTU field value, before processing the option.
If the receiving host is not able to accept datagrams as large as specified by the value of the MTU field of the option, then it should reduce the MTU field to the size of the largest datagram it can accept.
| MAC header | IP header | IP option 11 | Data |
IP Option 11:
| 00 | 01 | 02 | 03 | 04 | 05 | 06 | 07 | 08 | 09 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Type | Length | MTU Value | |||||||||||||||||||||||||||||
Type. 8 bits. Set to 11.
| 00 | 01 | 02 | 03 | 04 | 05 | 06 | 07 |
|---|---|---|---|---|---|---|---|
| C | Class | Option | |||||
C, Copy flag. 1 bit. Cleared to 0.
Indicates that this option is not to be copied into all fragments.Class. 2 bits. Cleared to 0.
The option is a control option.Option. 5 bits. Set to 11.
The IP option number.
Length. 8 bits. Set to 4.
Total length of the option.
MTU Value.
16 bits.
Contains the lowest MTU of all the networks that have been traversed so far by the datagram.
RFCs:
[RFC 1063] IP MTU Discovery Options.
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Description | Glossary | RFCs | Publications | Obsolete RFCs |