Re: network unreachable


Thomas Narten (narten@purdue.edu)
12 Jun 88 19:31:41 GMT


In article <2512@mandrill.CWRU.Edu> > edguer@mandrill (Aydin Edguer) writes:
>I am trying to understand why certain sites are unreachable from my machine.
>Background: [...recently joined NSFnet; prior connection via CSNET..]

>Question(s):
> Why would sites which were reachable become unreachable like this?
> Is this due (perhaps) to the dismantling of the ARPAnet which I understand
> is taking place?

No. Both Purdue and decwrl are still reachable via the ARPANET.

> What can I do to find out why they have become unreachable?

The problem is broken routing tables, and I suspect NSFnet (or its
regionals).

We have been unable to reach net 128.146 (ohio-state) for a week, and
the same is apparently the case for 129.22. Both of these nets are
reachable from NSFnet sites, but not from us. Moreover, our butterfly
core gateway has routes for these nets that point to 10.4.0.14
(psc.psc.edu). Since we can ping psc.psc.edu, I conclude that we are
correctly advertising routes to the core. In other words, we (Purdue
and ARPANET) are managing routes correctly.

By implication, NSFnet is not correctly propagating all of the routes
it learns from the ARPANET. (Note that for the most part, we can reach
NSFnet sites; only a handful cause trouble.)

> What can I do to try to reach them now that my router has given up?

Contact your regional NOC (if there is one), and if that doesn't help,
call the NSFnet NOC.

The following information would be of help:

1) Do you have a route to get to Purdue (either explicit or default)?
For that matter, do you have routes to other sites that are on the
ARPANET? Can you ping 10.0.0.37 (purdue's arpanet connection)?

What about 10.4.0.14? If both of us can ping a common gateway, but
not each other.......

2) Do you get ICMP unreachable messages when attempting to reach the
networks in question? The best check for this is a 4.3 system with a
modified ping that prints the name of the gateway that generated the
ICMP message.

Alternatively, one can open TCP connection to the host in question.
For instance, "ftp 128.10.2.1" will abort with a "network unreachable"
error if ICMP unreachables are returned; "connection timed out" on the
other hand, indicates that packets are going into a black hole. This
works on 4.3 BSD (running the latest tcp/ip code) and Sun OS systems.

--
Thomas Narten
narten@cs.purdue.edu or	{ucbvax,decvax,ihnp4}!purdue!narten



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