Regarding MSS and MTU

Hi all dear friends,

As usual this is always greatful website for me. Recently here I found article on TCP ( I know I am very late to read this article ).

This Article is very much in depth.. I don't know who wrote this article..btw whoever done it... really really done wonderful job. I salute him ( or entire firewall team ) for this fantastic article.

I read that article in depth, everything is properly ( acutally in depth ) explained.. But I didnot understand following thing which is really made me more confused...


***TCP Analysis - Section 6: TCP Options****
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It would also benefit us to recognise the correct terminology that corresponds to each level of the OSI Model: The TCP Header and Data is called a Segment (Layer 4), while the IP Header and the Segment is called an IP Datagram (Layer 3).

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TCP usually computes the Maximum Segment Size (MSS) that results in IP Datagrams that match the network MTU. In practice, this means the MSS will have such a value that if we add the IP Header as well, the IP Datagram (IP Header+TCP Header+DATA) would be equal to the network MTU.

If the MSS option is omitted by one or both ends of the connection, then the value of 536 bytes will be used. The MSS value of 536 bytes is defined by RFC 1122 and is calculated by taking the default value of an IP Datagram, 576 bytes, minus the standard length of the IP and TCP Header (40 bytes), which gives us 536 bytes.

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My question is that If you said like

MSS = TCP header + Data = Segment

MTU = IP header + TCP header + Data = IP Datagram

Then on what basis and how can you say that

The MSS value of 536 bytes is defined by RFC 1122 and is calculated by taking the default value of an IP Datagram, 576 bytes, minus the standard length of the IP and TCP Header (40 bytes), which gives us 536 bytes.

whereas Ip datagram = IP header + TCP header + Data

if you minus both the IP and TCP header then surely you will get 536 bytes.. but then how come it will be IP datagram as you already removed IP header from it.

Please advice where am I wrong.

I really applogize for asking silly question.

Awaiting your kind reply.

Thanks.

cscrtr

I think you raised a good point. MSS = MTU - 40, thats for sure. Then MSS will certainly not include the TCP header, only the data. Check here:

technet.microsoft.com/en-us/library/cc940069.aspx
msdn.microsoft.com/en-us/library/aa916746.aspx
www.windowsecurity.com/whitepapers/MS_Wi...ntation_Details.html

Some diagrams shown above prove that MSS does not include the TCP header.

So I think that there might be a slight error in our TCP options section. Specially that headers/format diagram. I will inform Chris about that to let us know if it needs correction or correct me if I'm wrong.

cscrtr,

You are correct. MSS actually refers to the data segment size and does not include the TCP Header!

The article has been corrected and thank you letting us know !

If you happen to find any other errors, please feel free to notify us.

Cheers mate,

cscrtr,

You are correct. MSS actually refers to the data segment size and does not include the TCP Header!

The article has been corrected and thank you letting us know !

If you happen to find any other errors, please feel free to notify us.

Cheers mate,

Hi Chris,

Thanks for correcting the diagram.

But still in discription ( very first line after the diagram ) it is mentioned same thing. i.e.

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You can see the Maximum Segment Size consists of the TCP Header and Data, while the Maximum Transfer Unit includes the MSS plus the IP Header.

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Thanks again.

cscrtr