Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!rpi!crdgw1!cs.albany.edu!percival.albany.edu!narten From: narten@cs.albany.edu (Thomas Narten) Newsgroups: comp.protocols.tcp-ip Subject: Re: ICMP redirects Message-ID: Date: 25 Sep 90 01:09:11 GMT References: <6592@ncrcae.Columbia.NCR.COM> <1990Sep24.235052.16616@julius.cs.uiuc.edu> Sender: narten@cs.albany.edu Organization: Computer Science Department, SUNY at Albany, Albany, NY 12222 Lines: 28 In-reply-to: zweig@cs.uiuc.edu's message of 24 Sep 90 23:50:52 GMT >>I have a question about routing and ICMP. Can ICMP redirects be used to update >>routing tables or does it just convey info about a route not being appropriate? Yes and yes. A router sends ICMP redirects to a host saying, "the datagram you just sent me destined to Y should be sent to Z [possibly, but not necessarily a router], which you can reach directly". Naturally, a host that received such a message should change its routing table to reflect the more direct path. Note that redirects generally only work for hosts and not routers, because a router determines when to send a redirect by checking 1) the source address in the forwarded datagram, and 2) noting the address Z of machine the datagram is being forwarded to. If the two addresses are on the same net, a redirect is in order. Note: To get a redirect, you must be sending the misdirected datagram to a router in the first place, so there is still a need for finding a viable router. The IETF Router discovery group is working on that problem. >ICMP redirects are from routers and say "Hey! You're sending IP packets for >host A to me when they should really go directly to A!" Replace "go directly to A" with "go directly to B" and I'll agree. Thomas Narten -- Thomas Narten narten@cs.albany.edu