Subject: Re: Ethernet Circuit Types

From: Giles Heron <[email-masked]>
Date: 25 Jul 2002 11:35:31 +0000

On Wed, 2002-07-24 at 19:11, Eric Rosen wrote:
> 
> Giles> Though you  could argue  that the  only reason you  would ever  see a
> Giles> service delimiting  tag that needs to  be stripped is  that the other
> Giles> end is  incapable of stripping  it.  In which  case the other  end is
> Giles> highly unlikely to be able to add it on egress - so you would need to
> Giles> add  the same  service delimiting  tag  when sending  in the  reverse
> Giles> direction?
> 
> For the sake of argument, let's  consider this scenario, and look at the two
> ends.  
> 
> Ex hypothesi, one end is  incapable of stripping/adding tags.  The other end
> has to be  configured with the service delimiting tags to  add, and it knows
> by virtue of this configuration that it has to add/strip tags. 
> 
> So what's the point of signaling two different circuit types?  Each endpoint
> does what it is configured to do (or is capable of doing), the signaling has
> nothing to do with it.

agreed - you don't necessarily need to signal anything if you have it
configured.

I think one argument was that signalling helps you to catch
configuration errors.

I suppose an analogue for this is the MTU parameter in draft-martini. 
You could argue that there is no need for this - both ends should just
configure the correct MTU size and be done with it.  However as an SP
it's nice to know that a link which is "up" will work.

On this basis you could argue that if we retain the VLAN mode then each
end should advertise the VLAN tag it will send and the VLAN tag it
expects to receive (with a value of -1 for "don't care" allowed for the
latter...)

Giles

-- 
=================================================================
Giles Heron    Principal Network Architect    PacketExchange Ltd.
ph: +44 7880 506185              "if you build it they will yawn"
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From: Luca Martini <[email-masked]>
Date: Thu, 25 Jul 2002 14:33:32 -0600

> Ex hypothesi, one end is  incapable of stripping/adding tags.  The other end

that "end" should be redesigned. >:-)

> does what it is configured to do (or is capable of doing), the signaling has

it is 2 different encapsulations. same as ATM with VPI/VCI or ATM without VPI/VCI.
I do not see this as a signaling requirement. the packet comes in with a tag , 
it should be transmitted  as is. ( + encapsulation )
The remote end then can either have an NSP function ( or switch function ), or 
just transmit the decapsulated packet out to the CE.

Ethernet, unlike ATM, has a special case where tags can be absent from the packet.
I think that this is a special encapsulation , not just another "tag number".
A pure Ethernet ( no tag ) mode is necessary for those cases where tag are not 
desirable, such as VPLS.

Luca