Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!CS.PURDUE.EDU!narten From: narten@CS.PURDUE.EDU (Thomas Narten) Newsgroups: comp.protocols.tcp-ip Subject: Re: sendmail hanging followup Message-ID: <8905222032.AA14712@percival.cs.purdue.edu> Date: 22 May 89 20:32:14 GMT References: Sender: daemon@ucbvax.BERKELEY.EDU Organization: The Internet Lines: 22 > There is a gateway which is clobering packets greater than about 1k. > This would cause sendmail to hang when attempting to deliver messages > > 1k because the packets with the message couldn't get through. I'm > not sure which hosts this affects, but the problem is being worked on. Without suggesting that this unnamed gateway exhibits reasonable behavior, let me point out that any TCP implementation that blindly sends out packets larger than 576 bytes in size is asking for trouble because no host or gateway is required to be able to *accept* datagrams larger than 576 bytes in size. The TCP layer should divide data generated by the application into reasonably-sized TCP segments. (Thus, large sendmail writes aren't your problem.) Current wisdom suggests that 576 bytes is an upper limit on the size of IP datagrams that a host should send in the absense of explicit information that larger packets won't cause problems. (See Mogul and Kent's 1987 SIGCOMM paper "Fragmentation Considered Harmful" for more details on the evils of large packets.) So, if your sending TCP is generating 1000 byte packets to send across the Internet, your TCP/IP implementation needs work. Thomas Narten