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What are MTU, RWIN, TTL, PMTU?

(the following copied from and other misc locations)

What is MTU?

MTU is Maximum Transmission Unit. Different data protocols have different package sizes that cannot be exceeded. Knowledge of the MTU is required by TCP so that the package sizes can be optimised to prevent fragmentation during transmission across the internet. The Windows 95 default is 1500 which is the Ethernet standard MTU. The internet "standard" for MTU is 576 but many ISPs use MTU's greater than this (my ISP uses 1500). However, if you are connecting to a site on the other side of the world (or country) then your data may have to go through a router which has an MTU of 576. This would cause delays and retransmissions in your requested data if your MTU was greater than 576. The minimum permitted value is 68. See here for a step by step guide to calculating your MTU.

What is MSS?

The beginning of each TCP transfer includes an announcement by the TCP requester of the Maximum (data) Segment Size it will accept (along with the address to which the segments should be sent). The TCP sender may use any segment size it prefers that does not exceed this number. If this TCP segment size is small enough to fit (along with the TCP header and the IP header) into the smallest MTU encountered enroute, it will pass unfragmented.

What is RWIN?

RWIN, or Default Receive Window, is the amount of unacknowledged data than can be outstanding on a TCP connection.

What is TTL?

TTL is Time to Live and details the number of hops a packet can take before it expires. The default is 32 (128 with DUN 1.2 installed). Setting this value too low may mean your packets time-out before reaching their destination. However, setting this value too high may increase the delay before dead packets are retransmitted. Setting 32 or 64 should be fine for most people..

What is RTOmax?

It is the maximum time a TCP sender will wait for an ACK before re-transmitting the segment. Consequently, your RTOmax will not affect the speed of your TCP data reception.

What is PMTU Discovery? What's PMTU?

PMTU (Path Maximum Transmission Unit) Discovery is detailed in RFC 1191. It is a method for allowing two hosts to negotiate the size of the MTU by exchanging their Maximum Segment Size (MSS) values. MTU = MSS +40. PMTUBlackHoleDetect is an option for modifying the PMTUDiscovery algorithm to detect routers that do not send notification when they have dropped packets that could not be fragmented.

Another good source is: While the information is meant for use on Solaris, much of it can be adapted for use on other platforms.

Last updated on 2001-07-30 14:00:00 -0700, by Shalom Craimer

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