At 08:50 AM 9/16/01, Circusnuts wrote:
>I believe the correct way to answer this question is, the MTU effects any
>interface to interface communication where a TCP handshake takes place.
>That would mean incoming or outgoing. The window of information must match
>what I expect to receive.
>
>Have I come close ???
Nope. Sorry. ;-) TCP is end-to-end, so "interface-to-interface" has no
meaning at the TCP layer. You may be thinking about the TCP segment size.
Each side of a TCP session specifies the size of the largest TCP segment
that it can handle receiving. This info is carried in the TCP Options field
in a SYN packet during the 3-way handshake. The two sides do not need to
agree. It is not a negotiated value.
The segment size is the size of each message. This is not the same as the
window size which is much larger. The window size is how much data the host
is ready to receive before the other side should stop and wait for an ACK.
Regardless, the original question is down a layer and not specific to TCP.
When IP goes to send a datagram, if the datagram is larger than the MTU of
the output data-link-layer interface, IP fragments the datagram. The end
recipient reassembles it.
How does the TCP segment size relate to MTU? It usually defaults to
something that matches the local interface. For example on a PC that is on
Ethernet, it defaults to 1460 (1500 minus the 20-byte IP header and 20-byte
TCP header).
Cisco lets you set both the interface MTU and an IP MTU. They can actually
differ, but there's generally no need for them to be different.
As far as incoming frames, I doubt you could affect this by setting the
interface MTU. My guess is that checking the size of any incoming frame is
done at the chip level. An Ethernet chip would trash a frame bigger than
1522 (counting header, CRC, and any tagging) and report a giant.
Priscilla
>Phil
>
>----- Original Message -----
>From: "Lists Wizard"
>To:
>Sent: Sunday, September 16, 2001 2:19 AM
>Subject: MTU Question [7:20096]
>
>
> > Hi Groups,
> >
> > I am a little confused about how the MTU size
> > configured on an interface affects the transmission of
> > packets through that interface. My question is does it
> > affects packets received on the interface or packets
> > transmitted out of the interface?
> >
> >
> > Thanks
> >
> > Lw
> >
> > __________________________________________________
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________________________
Priscilla Oppenheimer
http://www.priscilla.com
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