Path: utzoo!utgpu!watserv1!watmath!att!pacbell.com!ucsd!swrinde!zaphod.mps.ohio-state.edu!think.com!barmar
From: barmar@think.com (Barry Margolin)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: problem with ftp
Keywords: ftp tcp
Message-ID: <1991Feb9.190400.6078@Think.COM>
Date: 9 Feb 91 19:04:00 GMT
References: <1991Feb6.184300@sicsun.epfl.ch>
Sender: news@Think.COM
Organization: Thinking Machines Corporation, Cambridge MA, USA
Lines: 30

In article <1991Feb6.184300@sicsun.epfl.ch> goud@sicsun.epfl.ch (Mireille Goud) writes:
>        - If the 2 hosts are on the same network (same B class
>address) the size of the packets is negociated between the 2 hosts.
>        - If the 2 hosts are not on the same network (they don't have
>the same class address) the size of the packets is 512 bytes.
>1. Is this a ftp problem or a tcp problem ?

It's a problem/feature of many TCP implementations.  The reason it is done
is that the goal is to prevent fragmentation of datagrams.  Hosts on the
same network can agree to base the max segment size (MSS) on the maximum
packet size (called the MTU -- Max Transmission Unit) of the network.  If
they are on different networks, though, they don't know what the smallest
MTU is of all the intervening networks, except that all networks in a
TCP/IP internet are required to support at least 512-byte packets
(actually, I think the number is something like 568, to allow 512-byte TCP
segments plus IP and TCP headers).

>2. Is there a solution to have big packets (4K bytes on fddi) even if
>the 2 hosts are not on the same network ?

There is work going on to develop "path MTU discovery" mechanisms.  Also,
some TCP implementations use a larger MSS for subnet that are part of the
same subnetted network than for outside networks (on the assumption that
local area media are faster than wide area media, so the consequences of
greater retransmission due to loss of a fragment are less severe).
--
Barry Margolin, Thinking Machines Corp.

barmar@think.com
{uunet,harvard}!think!barmar