[e2e] Question on MTU

Joe Touch touch at ISI.EDU
Thu Apr 21 09:28:28 PDT 2005

Hash: SHA1

Arjuna Sathiaseelan wrote:
> Dear All,
>   I would be very much obliged if you could let me know the following:
> As MTU is the maximum amount of information per packet that can be
> sent on the wire, does it include the MSS + TCP header + IP header +
> DL header (with error correction codes) or is it just the MSS + TCP
> header + IP header?
> Because for the Ethernet - which has a MTU of 1500 bytes - we usually
> have 1460 bytes as MSS + 20 bytes TCP header + 20 bytes IP header.
> What about the header that would be added in link layer?
> Please do clarify me.
> Regds,
> Arjuna

MSS and MTU both omit headers, i.e., they are payload sizes.

MTU usually refers to a link layer, and denotes the maximum link ayboad
size, excluding link header/trailer info. For Ethernet, such
header/trailers include:

	- 14 byte header
	- 4 byte 802.1q (VLAN) tag
	- 4 byte CRC

Standard ethernet has 1518 byte frames, but 802.1q ethernet has 1522
byte frames. From the link frame size, subtract the link header/trailer
to get the MTU. Standard ethernet has an MTU of 1500 bytes, but there
are jumbograms of 9,000 bytes in the extended ethernet spec.

MSS usually refers to a transport protocol, e.g., TCP, and denotes the
max payload size there too. It is also relative to the network (IPv4,
IPv6) protocol _and_ link layer used.

And just as link layer overhead sizes vary, so do network layer overhead
sizes (minimums of 20 for IPv4, 40 for IPv6 - larger if options are
included, e.g., 48 for IPv6 with jumbogram option).


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