Subject: tunnels over asymetrical links?

From: [email-masked] (Andrew Patrick)
Date: Fri, 5 Jul 96 10:42:38 EDT

Is it true that tunnels will not work if the unicast link between the
two endpoints does not follow the same path inbound and outbound?

I am working with an ISP who has two connections to the MCI backbone
managed by BGP.  As it is currently operating, outbound packets are
going directly to MCI, while inbound packets travel through an
intermediate ISP:

Outbound: local tunnel host --> ISP  --> MCI tunnel host
Inbound: MCI tunnel host --> ISP 2 --> ISP 1 --> local tunnel host

Will a DVMRP tunnel work between the "local tunnel host" and the "MCI
tunnel host" in this situation?

-- 
           Andrew Patrick, Ph.D. <[email-masked]>
               Networks Services & Interfaces Laboratory
                     Communications Research Centre
                       http://debra.dgbt.doc.ca/

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From: Bill Fenner <[email-masked]>
Date: Fri, 5 Jul 1996 09:23:00 PDT

>two endpoints does not follow the same path inbound and outbound?

No.  Tunnels don't care about the underlying unicast path at all.

  Bill

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From: Stephen Casner <[email-masked]>
Date: Sat, 6 Jul 1996 00:34:21 -0700 (PDT)

> No.  Tunnels don't care about the underlying unicast path at all.

Yes, but if the difference in unicast routing causes the incoming
interface to be different than the outgoing one (if the tunnel machine
has multiple interfaces), the RPF check will fail, won't it?

							-- Steve

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From: Bill Fenner <[email-masked]>
Date: Sat, 6 Jul 1996 01:16:03 PDT

In message <[email-masked]> y
>has multiple interfaces), the RPF check will fail, won't it?

No.  The RPF check is performed on the tunnel virtual interface, upon which
the packet "arrives" after it has been decapsulated.  The decapsulator only
needs to have the source and destination IP addresses of the packet agree
with the configured tunnel.

The biggest worry with a multi-homed host is that the tunnel configuration
might not agree with the unicast routing table.  Due to an implementation
detail in the UNIX kernel, the tunnel packets can end up with the wrong
IP source address when this happens.  The solution is to install a static
host route for the tunnel endpoint.

  Bill

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From: Walid Dabbous <[email-masked]>
Date: Sat, 06 Jul 1996 13:40:38 +0200

>has multiple interfaces), the RPF check will fail, won't it?

yes this check will fail. We considered this problem in the
Unidirectional Link Routing BoF in Montreal. Basically we proposed
modifications to DVMRP in order to allow such packets to be forwarded.

The udlr directory:
ftp://zenon.inria.fr/rodeo/udlr/
contains some documents describing the general problem of integrating
unidirectional links in Internet routing, and proposed
modifications for RIP, OSPF and DVMRP. However, these documents will
be updated very soon to take into account the remarks/comments/suggestions
during the BoF. 

Walid