TCP MSS vs MTU

T

to0kenZA

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Hi guys,

Could someone please explain to me exactly what the difference is between Maximum Segment Size and Maximum Transmission Unit?

As far as I understand, the MSS on router interfaces can be changed to limit the size of packets transitioning between the routers to prevent packet fragmentation. Is that not the same as MTU?

Also, If a router's MTU is set to say 1300 does that mean that it may drop packets that are larger?


How does it work between networks where the devices are not in your control?


Any insight would be greatly appreciated, thanks in advance!
 
MTU is determined by Layer 2
MSS is the maximum size of the data that TCP can send in a single segment.

MSS is usually matched to MTU or even PMTU for obvious reasons..

If you are set at 1300, and you get larger, you get fragmentation, not dropped packets.

when devices are not in control, they are usually set to standards, but can also negotiate the sizes..

/edit, apologies for the quick, somewhat disjointed reply, will correct it nicely a little later!
 
tau1z said:
MTU is determined by Layer 2
MSS is the maximum size of the data that TCP can send in a single segment.

MSS is usually matched to MTU or even PMTU for obvious reasons..

If you are set at 1300, and you get larger, you get fragmentation, not dropped packets.

when devices are not in control, they are usually set to standards, but can also negotiate the sizes..

/edit, apologies for the quick, somewhat disjointed reply, will correct it nicely a little later!
Click to expand...

Cool, thanks. It does shed a lot more light on the subject for me, thanks again!
 
tau1z said:
MTU is determined by Layer 2
MSS is the maximum size of the data that TCP can send in a single segment.

MSS is usually matched to MTU or even PMTU for obvious reasons..

If you are set at 1300, and you get larger, you get fragmentation, not dropped packets.

when devices are not in control, they are usually set to standards, but can also negotiate the sizes..

/edit, apologies for the quick, somewhat disjointed reply, will correct it nicely a little later!
Click to expand...

Should MMS not have a lower value than MTU/PMTU to cater for TCP/IP overheads?

MTU to be derived from PMTU (lowest value in path) minus the TCP/IP overhead?

http://www.cisco.com/en/US/tech/tk827/tk369/technologies_white_paper09186a00800d6979.shtml
 
tau1z said:
Agreed, apologies, by matched i didnt mean equal exactly....
But you are 100% correct!
Click to expand...

Please don't apologise, it's farking hard for technical people to get something across in a simple form :D
 
ponder said:
Please don't apologise, it's farking hard for technical people to get something across in a simple form :D
Click to expand...

I'm sure we can all talk technical here oaks, lol.

Tau1z's first post was good enough for me, I know you should not match the MSS and the MTU exactly. Shot for the input okes, I couldn't really find any coherent explanations on international forums. :D
 
to0kenZA said:
I'm sure we can all talk technical here oaks, lol.

Tau1z's first post was good enough for me, I know you should not match the MSS and the MTU exactly. Shot for the input okes, I couldn't really find any coherent explanations on international forums. :D
Click to expand...

local is ...lekker......apparently ;)
 
Roman4604 said:
MSS should always be MTU - 40 (20 for IP header + 20 for TCP header).
Click to expand...

Packet size, often referred to as MTU (Maximum Transmission Unit) is the greatest amount of data that can be transferred in one physical frame on the network. For Ethernet, the MTU is 1500 bytes, for PPPoE 1492, dial-up connections often use 576.

Each transmission unit consists of header and actual data. The actual data is referred to as MSS (Maximum Segment Size), which defines the largest segment of TCP data that can be transmitted. Essentially,
MTU=MSS + TCP & IP headers.

MSS=MTU-40 <-- a standard 40 byte header (20 byte IP and 20 byte TCP)
 
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