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<P><FONT size=2>Minor note - MTU is technically Layer 3 (as opposed to link
layer or layer 2). So it is quite correct to describe the MTU as the link
layer payload size. So, as noted, 1518 bytes is the frame size at layer
2.<BR><BR>However, it is very important to keep in mind that MTU and path MTU
discovery operate at Layer 3. For example, boundaries between differing
MTUs should be handled by Layer 3 devices (not switches) to avoid end-to-end
issues that can arise.<BR><BR>Loki</FONT></P>
<P><FONT size=2>----<BR><BR>"Joe Wrote:"</FONT></P>
<P><FONT size=2></FONT><FONT size=2><BR>Date: Thu, 21 Apr 2005 09:28:28
-0700<BR>From: Joe Touch <touch@ISI.EDU><BR>Subject: Re: [e2e] Question on
MTU<BR>To: Arjuna Sathiaseelan <arjuna.sathiaseelan@gmail.com><BR>Cc:
end2end-interest@postel.org<BR>Message-ID:
<4267D4AC.8090503@isi.edu><BR>Content-Type: text/plain;
charset=ISO-8859-1<BR><BR><BR>MTU usually refers to a link layer, and denotes
the maximum link ayboad<BR>size, excluding link header/trailer info. For
Ethernet, such<BR>header/trailers
include:<BR><BR> - 14 byte
header<BR> - 4 byte 802.1q (VLAN)
tag<BR> - 4 byte CRC<BR><BR>Standard
ethernet has 1518 byte frames, but 802.1q ethernet has 1522<BR>byte frames. From
the link frame size, subtract the link header/trailer<BR>to get the MTU.
Standard ethernet has an MTU of 1500 bytes, but there<BR>are jumbograms of 9,000
bytes in the extended ethernet spec.<BR><BR>MSS usually refers to a transport
protocol, e.g., TCP, and denotes the<BR>max payload size there too. It is also
relative to the network (IPv4,<BR>IPv6) protocol _and_ link layer
used.<BR><BR>And just as link layer overhead sizes vary, so do network layer
overhead<BR>sizes (minimums of 20 for IPv4, 40 for IPv6 - larger if options
are<BR>included, e.g., 48 for IPv6 with jumbogram
option).<BR><BR></P></FONT></BODY></HTML>