TCP/IP details Volume 1: protocol note-MTU discovery

The path MTU is the minimum MTU on any network in the path between the current two hosts. The path MTU is found to inherit from the IP header and set not to split (DF) bits,

To find whether the router on the current path needs to shard the IP datagram being sent. If the DF bit is set for an IP datagram to be forwarded and its length is

If MTU is exceeded, the router returns an error of ICMP inaccessibility.

TCP path MTU discovery is performed as follows: when a connection is established, TCP uses the minimum MTU in the output interface or the MSS declared by the peer interface as the starting packet.

Segment Size. The MTU path does not allow TCP to exceed the MSS declared by the peer. If no MSS is specified for the peer end, the default value is 536.

Once the starting packet segment size is selected, DF bits are set for all IP datagram sent by TCP over the connection. If an intermediate router needs

When a data packet with the DF flag configured is sharded, It discards the data packet and generates an ICMP fault.

If this ICMP error is received, TCP reduces the segment size and retransmits the packet. If the router generates a new ICMP error, the packet segment size is

Set MTU to the next hop minus the header length of IP and TCP. If an earlier ICMP error occurs, you must try the next possible minimum MTU. When this

When a retransmission occurs due to an ICMP error, the congestion window does not need to change, but it must start slowly. Because the route can be dynamically changed, the path MTU is reduced at the last time.

After a period of time, you can use a large value.

For non-local destinations, the default MSS is usually 536 bytes. the MTU path can be used to avoid sharding when the MTU is less than 576 of the intermediate link. For

The local target host can also avoid partitioning when the MTU of the intermediate link is smaller than the endpoint network. However, in order to make the path MTU more efficient and make full use of MTU greater

576 of the WAN, a realization must stop using for non-local purposes of the preset 536MTU default value. A better choice for MSS is the MTU of the output interface.