[e2e] MTU - IP layer

[Joe Touch]

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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.

Joe

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 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.
> 
> 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 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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