Subject: [NDProxy#16] Loop prevention, revisited

From: "Dave Thaler" <[email-masked]>
Date: Tue, 15 Feb 2005 19:47:20 -0800

Summary (from Erik Nordmark):
> I still think the document should say "MUST prevent loops; 
> SHOULD run IEEE 802.1D to prevent loops" and then talk about 
> cases when the protocol is not needed. The one example we have
> is the case of PPP (e.g. to a GGSN) where the probability of a 
> loop would be very small, so in that particular case there is no
> need for 802.1D (and there might be other examples, but the 802.11
> scenario in the document isn't one of them).

Discussion so far: 
http://www.icir.org/dthaler/NDProxyIssues.htm#Issue 16 

The problematic text in section 6 is:
> Loop prevention can be done done by having the proxy implement the
> Spanning Tree Algorithm and Protocol as defined in [BRIDGE] on all
> proxy interfaces. Loop prevention is OPTIONAL, and is useful only
> if the proxy can be deployed in an environment where physical loops
> are possible. For example, in a cell phone which proxies between a
> PPP dialup link and a local Ethernet interface, it is typically safe
> to assume that physical loops are not possible and hence there is
> no need to support the Spanning Tree Protocol (STP).
>
> If loop prevention is implemented, it is done as follows.

Proposed replacement text:
> An implementation MUST have some means of preventing loops.
> Loop prevention SHOULD be done by having the proxy implement the
> Spanning Tree Algorithm and Protocol as defined in [BRIDGE] on all
> proxy interfaces, as described below. This mechanism is required
> only if the proxy can be deployed in an environment where physical 
> loops are possible. For example, in a cell phone which proxies 
> between a PPP dialup link and a local Ethernet interface, it is 
> typically safe to assume that physical loops are not possible and
> hence there is no need to support the Spanning Tree Protocol (STP).
>
> Loop prevention using STP is done as follows.

-Dave

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From: Pekka Savola <[email-masked]>
Date: Wed, 16 Feb 2005 07:36:17 +0200 (EET)

On Tue, 15 Feb 2005, Dave Thaler wrote:
>> proxy interfaces, as described below.

Is this strong enough for interoperability across ND-proxies from 
different vendors?  Loop prevention won't do much good unless every 
vendor implements an interoperable mechanism..

The text is also a bit contradictory: "An implementation MUST have 
some means.." and "The mechanism is required only if...".  Granted, 
there are some cases where it may be physically impossible (or 
extremely difficult) to use the implementation to form a loop, but in 
the general case, the implementor really cannot know how the devices 
will end up being used.  Maybe the first sentence needs to be reworded 
slightly to include an escape clause there as well.

-- 
Pekka Savola                 "You each name yourselves king, yet the
Netcore Oy                    kingdom bleeds."
Systems. Networks. Security. -- George R.R. Martin: A Clash of Kings