Subject: Re: Your reply to my suggestion/proposal...

From: Choon Lee <[email-masked]>
Date: Mon, 16 Aug 1993 14:13:05 -0700

Scott,

Good start on our journey towards the goal.

I have some more questions on your proposal:

>> > >> 
>> > >> 
>> > >> The other scenario I've thought about is where two or more DLSw's are
>> > >> connected to the same LAN or bridged LAN:
>> > >> 
>> > >>                      +-------+              +-------+
>> > >>                      |  S 1  |              |  S 2  |
>> > >>                      +---+---+              +---+---+
>> > >>                          |                      |
>> > >>              N1 -------+-+----     N2  -------+-+----
>> > >>                        |                      |
>> > >>                 +------+-------+      +-------+------+
>> > >>                 |   DLSw A     |      |   DLSw B     |
>> > >>                 +---+---+---+--+      +---+---+---+--+
>> > >>                    /|\ /|\ /|\           /|\ /|\ /|\
>> > >>                     |   |   +-------------+   |   |
>> > >>                     |   +-------+ +-----------+   |
>> > >>                     |            X              +-+
>> > >>                     |   +-------+ +---------+   |
>> > >>             |      \|/ \|/                 \|/ \|/        |
>> > >>             |     +-+---+--+             +--+---+-+       |
>> > >> +-----+     +-----+ DLSw C |             | DLSw D +-------+   +-----+
>> > >> | S 3 +-----+     +--------+             +--------+       +---+ S 4 |
>> > >> +-----+     |                                             |   +-----+
>> > >>             |                                             |
>> > >>             |     +--------+             +--------+       |
>> > >>             +-----+ Brdg A +<----------->+ Brdg B +-------+
>> > >>             |     +--------+             +--------+       |
>> > >>             |                                             |
>> > >>            N3                                            N3'
>> > >> 
>> > >> 

>> > >> 
>> > In the case of parallel DLSw (in diagram above, imagine LANs N1 and N2 are
>> > bridged, DLSw A connected only to C, and B connected only to D), C thinks that 
>> > A is its only partner and D thinks that B is its only partner. How do they
>> > figure out that there's a loop?
>> > 
>> 
>> Good question.  In that situation my suggested solution doesn't cut
>> it.  But there is a way that it can be extended to fix the problem,
>> though.  TWIK frames broadcast on the LAN's contain the full IP
>> address of partner DLSw (DLSw A send a TWIK containting C's IP Addr, B
>> sends a TWIK with D's IP Addr, C send a TWIK w/ A's Addr and D send a
>> TWIK with B`s addr).  If receiving DLSw not only compare these partner
>> IP Addrs with their own partners, but also compare the IP Network
>> portion of these addrs (add the partner's subnet mask to the TWIK
>> packet, in addition to the IP addr), then each could detect that
>> another DLSw on the LAN has a partner on the same net as one of his
>> partners.  Then apply the same exclusion principle, but extended
>> slightly; ie: that only the DLSw with the lowest IP address can talk
>> with a partner DLSw on a given network.  In the new scenario you
>> describe, A and B would recognize that even though they don't share a
>> partner the partners they have are on the same network.  In that case
>> the DLSw with the lowest IP address would include its partner in its
>> partner group (say A has the lowest addr, then it would put C's addr
>> in its group).  B, recognizing that its partner is on the same net as
>> A's partner, and that its IP address is larger than A's, would not
>> include D in its partner goup (in fact, its group would be empty, thus
>> it wouldn't be able to talk to anyone).
>>
I believe that the IP network can be set up quite differently for routers (which
DLSw nodes are). In a IP router, the network for every interface is different.
For example, in the parallel DLSw case (described above), the IP network of DLSw 
node A at the LAN side, call it IP.A.LAN is different from that on the WAN side, IP.A.DLSW. Also, the IP.A.DLSW can also be different from IP.C.DLSW (although they 
can be the same). Furthermore, IP.A.LAN can be differnt from IP.B.LAN (although
they are physically on the same LAN, they can be logically on different IP networks).

The IP network that C knows about its partner would be IP.A.DLSW 
(does it even know that, or does it know only about the *address* rather
than the network?), since that is the one used in the TCP/IP connection. 
Similarly, D knows about its parther's IP network as IP.B.DLSW. More likely than 
not, IP.A.DLSW is different from IP.B.DLSW. In this case, C would not know
that its partner and D's partner are on the same LAN.

Regards,
Choon Lee
3Com Corp.

---

From: [email-masked] (Scott Adams)
Date: Mon, 16 Aug 1993 18:06:33 -0400 (EDT)

Choon Lee,

Thanks for the fast responses.

> that its partner and D's partner are on the same LAN.

How embarrassing!  That's what I get for dashing off a quick reply
without spending enough time thinking about the problem in more depth.
I'm going to slink away and think about this some more...

> 3Com Corp.

Humbly,

Scott