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Re: MTU of the Internet?

  • From: Patrick W. Gilmore
  • Date: Sat Feb 07 13:06:29 1998

At 10:17 AM 2/6/98 -0600, Stephen Sprunk wrote:

>Does anyone have data to show if any terminal servers or client stacks
>will honor TOS bits and/or known interactive port numbers when
>ordering packets for transmission across slow links?

This is relatively trivial with Cisco terminal servers, but it does have to
be done manually.  I'd be really surprised if no other vendor can
specifically order packets in the queue by port number or something.

Cisco does automatically take specific steps to allow smaller packets
(regardless of TOS or port number) through the box faster when coming from
many slow interfaces to one fast interface.  The idea is, why should a
bunch of (completed) telnet packets wait for the ethernet/T1 while a
gigantic 1500 byte FTP packet is still serializing from a 28.8 modem
connection?  This should help "perceived" response times on interactive
(e.g. telnet w/ 64 byte packets) stuff while not actually decreasing the
throughput of large file transfers on other peoples' links.  Supposedly,
some other vendors place a packet in the outgoing queue when the first byte
arrives.  If you have a bunch of small packets come in (and complete) just
after the big packet starts, you could create delays and waste bandwidth on
the uplink.

Of course, I've never actually measured the effects this, nor do I have
first hand knowledge of how other vendors handle the process.  I'm just
taking Cisco's word for it (which you are more than welcome to disprove
with real data - if possible ;).

>Stephen Sprunk          "Oops."                 Email: [email protected]


Patrick W. Gilmore                      voice: +1-650-482-2840
Director of Operations, CCIE #2983        fax: +1-650-482-2844
                "Tomorrow's Performance Today"