Subject: Re: MTU, MRU, and MSS

From: Vernon Schryver <[email-masked]>
Date: Wed, 4 Nov 1998 12:26:51 -0700 (MST)

> From: [email-masked]

> However, I am not aware of any consideration of the possibility
> that MTU and MRU (which I assume means Maximum Receive Unit?) might
> be different.  Is this really a problem?

Please excuse me for trying again.
I think I've finally understood the point and figured out a good response.

Yes, MTU's and MRU's do sometimes differ, but in essentially all cases,
the MTU is no larger than the MRU.  You can control the sizes of your
transmitted packets, but not the sizes of the packets you receive.
Reasonable systems are prepared for the worst ('be generous ...')
and so able to swallow the largest possible packet size, even when
configured or built to send something smaller ('be conservative ...').
If you tell the TCP peer to limit its segment size, you'll naturally
compute a value based on your configured transmit limit instead of your
worst case nightmare receive size.

Vernon Schryver    [email-masked]

---

From: "Eric A. Hall" <[email-masked]>
Date: Wed, 04 Nov 1998 11:37:36 -0800

> compute a value based on your configured transmit limit instead of
> your worst case nightmare receive size.

This makes sense, but it is it always efficient, particularly when the
MTU is considerably smaller than the MRU?

Sometimes you want a very small MTU (fill faster) but a very large MRU
(receive larger). Forcing MSS to the (much smaller) MTU prevents this
from happening.

-- 
Eric A. Hall                                            [email-masked]
+1-650-685-0557                                    http://www.ehsco.com