wingo@ncrcae.Columbia.NCR.COM (Dave Wingo) (09/25/90)
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? Example would be to have an X terminal on a network 90. talking to a host A also on net 90. Then I introduce yet another host B which has two(2) network interface cards that bridge 90 to say net 89. I then put host A on net 89 and provide it with a new 89 internet number. Question then is can the X terminal find the gateway to host A via ICMP only or will rip or gated need to be provided on the X terminal to update route tables???? Thanks in advance.... David Wingo wingo@ncrcae.Columbia.NCR.COM
zweig@cs.uiuc.edu (Johnny Zweig) (09/25/90)
wingo@ncrcae.Columbia.NCR.COM (Dave Wingo) writes: > >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? >Example would be to have an X terminal on a network 90. talking to a host A >also on net 90. Then I introduce yet another host B which has two(2) network >interface cards that bridge 90 to say net 89. I then put host A on net 89 and >provide it with a new 89 internet number. Question then is can the X terminal >find the gateway to host A via ICMP only or will rip or gated need to be >provided on the X terminal to update route tables???? > >Thanks in advance.... David Wingo wingo@ncrcae.Columbia.NCR.COM Finding the gateway (the X terminal sounds like it talks IP) from a given host (the X terminal is a host in the sense of being an endpoint-machine for IP traffic) is a problem whose solution is still in the works. Some fine day everyone will run Router Discovery Protocol so a host can use ARP to see if another is on its local net, then punt everything else to the gateway it selected from RDP. Right now, there are a number of icky ways the X terminal might go about finding out that B (in your scenario) is a router. 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!" if the X terminal uses ARP to find A in the first place, it will find that A is not around anymore and have to find the gateway -- the ICMP redirect might occur in the case where A gets put onto the X terminal's net when it didn't used to be and somehow the silly thing just had the route stuck in its head. The netlanders will correct me I'm sure, but I think ICMP redirects are pretty uncommon on nets that use ARP+whatever ad hoc method to locate gateways (since nets with more than one gateway are pretty uncommon). -Johnny ICMP
narten@cs.albany.edu (Thomas Narten) (09/25/90)
>>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
dc@graphon.com (Darren Croke) (09/25/90)
In article <NARTEN.90Sep24210911@percival.albany.edu>, narten@cs.albany.edu (Thomas Narten) writes: > > 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. > It's currently common for X terminals to have a setup field to allow a default gateway to be specified. Darren Croke dc@graphon.com