MTU vs MSS? When to change MTU and when to change MSS?

Question

Here is the question.

When and Why do we decrease MSS instead of MTU.

In the router that we can configure can configure both of them separately.

Suppose I have kept MSS as 1260.I have not made any changes to the MTU. Now if the TCP and the IP header do not have any options then they will be using 40 bytes( 20 TCP + 20 IP).

So does this mean that the MTU in this case is 1300.

Now the second scenario.

I decreased the MTU to 1300. I did not make any change to the MSS. Again there are no options in the TCP and IP options.

So does this mean that the MSS in this case is 1260.

If both the scenarios have same outcome then please help me understand the following.

1. In which case should we decrease the MTU and leave MSS untouched ?
2. In which case should we decrease the MSS and leave MTU untouched ?

Thank You

Answer 1

MTU is typically a reference to the physical medium of the directly connected interface. As a general rule, you want to leave this as the maximum the medium can handle. There are occasional cases where you need to enable jumbo frames or adjust MTU due to overlay networks or fancy encapsulation schemes.

MSS is mostly a reference to the end-to-end path. This isn't commonly manually set, and is instead left to functions such as Path MTU discovery to dynamically set on a connection by connection basis. The most common case is TCP, which can better 'tune' a connection for optimal end-to-end bandwidth usage if it has full control over the segments being sent, as opposed to having the segments being fragmented unknowingly along the path.

Another way of looking at it...

MTU should be set solely based upon the physical medium and/or fringe encapsulation or jumbo frame needs. MTU affects every frame sent out an interface.

MSS is dynamically set based upon the collective path MTU, typically on a connection by connection basis. Dynamic adjustment of the MSS is tuned for each connection, but doesn't affect other connections, or other 'non-connection-oriented' communications.

Answer 2

To add to the previous comment, there are two MTU settings in the network itself that needs to be set. There is the Ethernet MTU , which you set on the switches. Again, set this to the maximum permissive value.

Then there is the IP MTU you set on routers. Typically, if there are no options set (like the DF bit), then this just means the router will fragment when a packet exceeds this size. If you have the DF bit (do not fragment) set , then the router will drop a packet that is above its permitted IP MTU.

Coming to your question - the IP MTU is not of concern, since you arent using any option bits, it will just fragment at the set size. But the Ethernet MTU has to be greater than your MSS. Another way of saying this is that the MSS should be smaller than the ethernet MTU size. Generally, you wont set this MSS manually, but rather use path-mtu discovery, which finds the least MTU between two end points, and adjusts the MSS to be lower than that value.

Answer 3

You may want to decrease MSS without decreasing MTU since you may want to put a limit on the size of the TCP segment. You have to remember that not everything transmitted is TCP. You may, and probably do, have other layer-4 protocols used with IP, and you may not want to decrease their sizes.