Subject: Re: MTU, MRU, and MSS

From: Vernon Schryver <[email-masked]>
Date: Wed, 4 Nov 1998 10:45:16 -0700 (MST)

> From: [email-masked] (Alan Cox)
> To: [email-masked] (Eric A. Hall)
> Cc: [email-masked]

>                                                         ...   SGI's 
> appear to advertise a fixed max of 48K on HIPPI. 
> ...

The last time I looked at SGI's source, that's not at all how it works.
Instead, a slightly modifed version of the familiar BSD computes a nice
value for the MSS that is based on the link MTU, other things such as
socket option, the global configuration switch, the MMU page size and the
sizes of various kinds of buffers.

Vernon Schryver    [email-masked]

---

From: "Eric A. Hall" <[email-masked]>
Date: Wed, 04 Nov 1998 10:06:17 -0800

> value for the MSS that is based on the link MTU

Well, that's the question. Shouldn't this be calculated according to
MRU, particularly in those situations where MTU and MRU differ (PPP
circuits with different MTU and MRU sizes, TR nets with fixed MTU but
variable MRUs, etc)? 

In such cases, the MSS should be defined according to the MRU of the
local network, according to RFC 793 anyway. What value is there in
deriving MSS from MTU instead?