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From: smb@ulysses.att.com (Steven Bellovin)
Newsgroups: comp.protocols.tcp-ip,comp.dcom.lans
Subject: Re: IP down scaling (linear or what) ?
Keywords: ip scaling performance
Message-ID: <14106@ulysses.att.com>
Date: 11 Dec 90 16:03:20 GMT
References: <58@nixeid.UUCP> <1990Dec7.104135.12@ux1.cso.uiuc.edu> <1990Dec10.181110@envy.bellcore.com>
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In article <1990Dec10.181110@envy.bellcore.com>, karn@envy.bellcore.com (Phil R. Karn) writes:
> In article <1990Dec7.104135.12@ux1.cso.uiuc.edu>, krol@ux1.cso.uiuc.edu
> (Ed Krol) writes:
> |> Just remember that as the link speed decreases the IP (and
> presumably)
> |> TCP headers take up a greater percentage of the available bandwidth.
> |> Leaving less room for real data.  (At a link speed of about 500 baud
> |> you have just enough bandwidth to send TCP IP headers and do HDLC
> |> bitstuffing - sorry no room for data)
> 
> Nonsense. The overhead of a TCP/IP link is entirely determined by the
> amount of data in each packet vs the size of the headers. The link
> speeds don't enter into it at all.

However, there are problems with large packets on slow links.  For
example, on a 9.6Kbps link, a 1K byte packet takes just under a second
to transmit.  If you have a smart router that tries to give priority to
short packets (i.e., telnet keystrokes and echoes), it will be helpless --
it can't pre-empt the packet that's already going.  (Well, it could,
I suppose, but that wouldn't be a very good idea...)

There's a second problem if you're going several hops at low speeds.
For each packet, a router can only start sending it once the whole
packet has been received.  Thus, if you have to go 4 hops at 9600 bps,
the total time to clock the bits onto the wire is about 4 seconds.  If
the same data were broken up into 256 byte chunks (or fragments), you
can get considerable overlap, as you can have several routers transmitting
simultaneously..