[e2e] MTU - IP layer

Loki Jorgenson ljorgenson at apparentnetworks.com
Thu Apr 21 13:55:29 PDT 2005

Hmmmmmm - that's an interesting reading of RFC 791 - and the distinction
of fragments over datagrams could be made in that way.

My observation remains that MTU is conceptually defined and implemented
at Layer 3.  Making pains to define it in Layer 2 terms in order to
ensure its scope includes all valid cases makes sense - and yet I find
it challenged.  Promoting the subtle distinction of "Frame payload" over
"packet/datagram" doesn't seem beneficial.

Prehaps I'm favouring the pragmatic over the precise....


-----Original Message-----
From: Joe Touch [mailto:touch at ISI.EDU] 
Sent: Thursday, April 21, 2005 1:30 PM
To: Loki Jorgenson
Cc: end2end-interest at postel.org
Subject: Re: [e2e] MTU - IP layer

Hash: SHA1

L3 packet size isn't referred to as MTU, esp. in IP (rfc791); it is
datagram length (or total length).

Fragments in IP must be less than or equal to the MTU, which there (791)
refers to the max payload of the L2.

path MTU discovery is equivalent to path "max link payload" discovery,
rather than path "max network payload" discovery.

IMO, therefore, MTU really refers to the L2 payload size, which is not
the same as the L3 'frame' size (size of the total IP packet), but is
related to the size of an L3 fragment.


Loki Jorgenson wrote:
> 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
> 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
> MTUs should be handled by Layer 3 devices (not switches) to avoid
> end-to-end issues that can arise.
> Loki
> ----
> "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
> 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
> 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
> 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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