Re: MSS vs MTU

TCP and IPv4 headers are 20 bytes long each, and IPv6 header is 40 bytes
long, so the MSS is equal to MTU minus 40 when using IPv4, and MTU minus 60
when using IPv6 (in most cases, MTU for Ethernet is 1500 bytes).
MSS and MTU are almost the same, yet not so much. MSS, which is the largest
segment (layer 4, yet not including the layer 4 header) that can fit on the
current physical medium. MTU, which is the largest packet (layer 3,
including the layer 3 header) that can be transmitted.
The MSS is used during the 3-way handshake of TCP to let each side know that
maximum segment size they can transmitt in a single frame. It's purpose is
to minimize IP fragmentation. However, that is only each side. Who only
knows what is in the middle. That is why each entity in between must know
what its MTU size is in case fragmentation is required somewhere along the
path.
Hopefully that helps.
Thanks kindly.

On Thu, Oct 29, 2009 at 3:13 AM, Mark Stephanus Chandra <
mark.chandra_at_gmail.com> wrote:

> Can Anyone tell me what is the difference between MSS and MTU ?
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> I have look in Cisco DOC explanation, it mention about the PPPOE technology
> that need adjusment in MSS, but still the explanation make no sense to me.
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> Regards
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> Mark Stephanus Chandra - CCIE#23887
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-- 
Mark Jackson, CCIE #4736
Senior Network, Security and Voice Architect
858.705.1861
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