[e2e] MTU - IP layer

Loki Jorgenson ljorgenson at apparentnetworks.com
Thu Apr 21 12:27:42 PDT 2005

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.

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.



"Joe Wrote:"

Date: Thu, 21 Apr 2005 09:28:28 -0700
From: Joe Touch <touch at ISI.EDU>
Subject: Re: [e2e] Question on MTU
To: Arjuna Sathiaseelan <arjuna.sathiaseelan at gmail.com>
Cc: end2end-interest at postel.org
Message-ID: <4267D4AC.8090503 at isi.edu>
Content-Type: text/plain; charset=ISO-8859-1

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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