9.3 ICMP Host and Network Unreachable Errors

9.3 ICMP Host and Network Unreachable Errors

The ICMP "host unreachable" error message is sent by a router when it receives an IP datagram that it cannot deliver or forward. (Figure 6.10 shows the format of the ICMP unreachable messages.) We can see this easily on our network by taking down the dialup SLIP link on the router sun, and trying to send a packet through the SLIP link from any of the other hosts that specify sun as the default router.

Older implementations of the BSD TCP/IP software generated either a host unreachable, or a network unreachable, depending on whether the destination was on a local subnet or not. 4.4BSD generates only the host unreachable.

Recall from the netstat output for the router sun shown in the previous section that the routing table entries that use the SLIP link are added when the SLIP link is brought up, and deleted when the SLIP link is brought down. This means that when the SLIP link is down, there is no default route on sun. But we don't try to change all the other host's routing tables on our small network, having them also remove their default route. Instead we count on the ICMP host unreachable generated by sun for any packets that it gets that it cannot forward.

We can see this by running ping on svr4, for a host on the other side of the dialup SLIP link (which is down):

 svr4 %  ping gemini  ICMP Host Unreachable from gateway sun (140.252.13.33)     ICMP Host Unreachable from gateway sun (140.252.13.33)  ^?   type interrupt key to stop  

Figure 9.2 shows the tcpdump output for this example, run on the host bsdi.

Figure 9.2. ICMP host unreachable in response to ping.

When the router sun finds no route to the host gemini, it responds to the echo request with a host unreachable.

If we bring the SLIP link to the Internet up, and try to ping an IP address that is not connected to the Internet, we expect an error. What is interesting is to see how far the packet gets into the Internet, before the error is returned:

 sun %  ping 192.82.148.1   this IP address is not connected to the Internet  PING 192.82.148.1: 56 data bytes    ICMP Host Unreachable from gateway enssl42.UT.westnet.net (192.31.39.21)     for icmp from sun (140.252.1.29) to 192.82.148.1 

Looking at Figure 8.5 we see that the packet made it through six routers before detecting that the IP address was invalid. Only when it got to the border of the NSFNET backbone was the error detected . This implies that the six routers that forwarded the packet were doing so because of default entries, and only when it reached the NSFNET backbone did a router have complete knowledge of every network connected to the Internet. This illustrates that many routers can operate with just partial knowledge of the big picture.

[Ford, Rekhter, and Braun 1993] define a top-level routing domain as one that maintains routing information to most Internet sites and does not use default routes. They note that five of these top-level routing domains exist on the Internet: the NSFNET backbone, the Commercial Internet Exchange (CIX), the NASA Science Internet (NSI), SprintLink, and the European IP Backbone (EBONE).