Re: last-hop routers, raising the MTU

mogul (Jeffrey Mogul) Tue, 12 December 1989 02:27 UTC

Received: by (5.54.5/4.7.34) id AA06895; Mon, 11 Dec 89 18:27:55 PST
From: mogul (Jeffrey Mogul)
Message-Id: <>
Date: 11 Dec 1989 1827-PST (Monday)
To: jnc@PTT.LCS.MIT.EDU (Noel Chiappa)
Cc: mtudwg
Subject: Re: last-hop routers, raising the MTU
In-Reply-To: jnc@PTT.LCS.MIT.EDU (Noel Chiappa) / Sun, 10 Dec 89 22:55:41 EST. <8912110355.AA25661@PTT.LCS.MIT.EDU>

	    Whoa! I don't think your assertion that the last hop router will
    know the host's MTU is correct. Routers have to be able to receive and
    send the MTU on any interface, but they aren't required to know what any
    host is using (and have no way of finding that info out anyway). The rule
    is that 'a host must not send a larger datagram [than 576] without explicit
    knowledge or prior arrangement with the destination host'. This puts the
    onus on the end hosts, and frees the routers from getting involved in the
    game. It is perfectly possible to send a packet to a host that's bigger than
    the host is ready to receive....

Noel, you're confusing MTU and MSS.  The assertion is correct, ignoring
the possibility of translucent bridges, or else how could fragmentation
possibly work?!