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RE: [rbridge] Last Call: draft-ietf-trill-routing-reqs (TRILL Routing Requirements in Support of RBridges) to Informational RFC

2007-03-21 03:55:31

        Thank you for your comments.  Please see below...


Eric Gray
Principal Engineer

-----Original Message-----
From: rbridge-bounces(_at_)postel(_dot_)org 
[mailto:rbridge-bounces(_at_)postel(_dot_)org] On Behalf Of Dinesh G Dutt
Sent: Tuesday, March 20, 2007 10:33 PM
To: ietf(_at_)ietf(_dot_)org
Cc: rbridge(_at_)postel(_dot_)org; IETF-Announce
Subject: Re: [rbridge] Last Call: 
draft-ietf-trill-routing-reqs (TRILL Routing Requirements in 
Support of RBridges) to Informational RFC

I have a few comments on the document.

- Section 1, Bridging Limitations: The first two paragraphs are 
structured around the logic: because ethernet header doesn't have 
a TTL or a hop count, the only choice was to use spanning tree. 
IEEE 802.1 has defined several headers such as 802.1Q header that 
carries the VLAN. If they wanted to add a TTL, they could have. 
They picked spanning tree and said that therefore they didn't need 
a TTL.  To the extent this represents history, I think it is 
inaccurate. To the extent it attempts to explain the rationale for 
RBridges, it seems unnecessary. A sufficient replacement maybe 
along the lines of: 

"Spanning Tree Protocol and its variants are the control protocol 
deployed in current 802.1D Ethernet bridges.  This protocol 
constructs a single tree out of a mesh of network connections. 
This tree eliminates usage of equal cost multipaths and results in 
non-optimal pair-wise forwarding."

This is a reasonable proposal for replacement text.  If there
are no objections from the working group, or the IESG, I would
be happy to make this change.


- Section 1, Bridging Limitations: More specific comments:
  - "Because of the potential for severe impact from looping traffic, 
    many (if not most) current bridge implementations stop forwarding
    of traffic frames following a topology change event and restart 
    only after STP/RSTP is complete" is incorrect. All 802.1D bridges 
    allow (R/M)STP to complete before moving a port to forwarding 
    state. I'd remove the phrase in parentheses.

Good suggestion.  Assuming the same acceptance, I can make this
change as well.

  - "Inefficient inter-bridge connection usage". What do you 
    mean by this phrase?

I assume this is a reasonably well understood aspect of using
a spanning tree as opposed to using shortest path routing.

It is not difficult to come up with a reasonable scenario in
which shortest path forwarding results in 80% of the total
link-by-link forwarding load that is generated by the same
amount of traffic in a corresponding spanning tree scenario.

The reason why this happens is that a spanning tree may be
constructed in which the path for some destinations will
traverse at least one additional link, when arriving from
some sources.  Practically speaking, unless a spanning tree 
is constructed per-source bridge, it's easy to show that 
this will be true for at least some source and destination 

Assuming the simplistic (VLAN-free) scenario that is basic
to the "configuration-free" capability that is part of the
chartered goals of TRILL, there would only be one spanning
tree in a bridged network.  Hence, in this scenario, there
would be many cases in which traffic traverses at least one
additional link.

If traffic is demonstrably required to traverse more links
than some theoretical minimum, than link utilization is -
by definition - less efficient than it theoretically can

- Section 1.2, Backward Compatibility and section 4.1: "...they 
terminate a bridged spanning tree, (i.e. - they do not forward 

I thought that we have not concluded the discussion on preventing 
loops for interconnected Bridges and RBridges based on the email 
thread that started a while back. Putting a decision in this 
section on the solution seems a little unnecessary. 

I am not sure that this text - or something like it - is unnecessary 
from a compatibility perspective, and it may be the case that this 
change would require a new working group last call.  However, if it 
is acceptable to the IESG either that the change does not require a 
new last call, or a second working group last call is needed, then I 
would be happy to make this change as well.

What is proposed in the current solution is to run a spanning tree 
protocol instance per port which maybe not scalable. 

I think something like "It's strongly desirable to minimize the
interaction between the bridges and Rbridges and constrain a 
spanning tree" is more appropriate.

Yet it is difficult to imagine how this would translate to a 
requirement that would make sense to someone evaluating the 
acceptability of a routing protocol for the TRILL problem-space.
Perhaps it would be simpler to omit the offending text?

- Ethernet and 802 is used interchangeably. Isn't Ethernet 
802.3 only ? 
Look at: or

I don't see anything on what I consider to be another 
important goal: to 
have a single protocol to compute unicast, multicast and broadcast 
routes. This reduces operational overhead by having to understand and 
debug a single protocol.

The IESG wrote:
The IESG has received a request from the Transparent 
Interconnection of 
Lots of Links WG (trill) to consider the following document:

- 'TRILL Routing Requirements in Support of RBridges '
   <draft-ietf-trill-routing-reqs-02.txt> as an Informational RFC

The IESG plans to make a decision in the next few weeks, 
and solicits
final comments on this action.  Please send substantive 
comments to the
ietf(_at_)ietf(_dot_)org mailing lists by 2007-03-30. Exceptionally, 
comments may be sent to iesg(_at_)ietf(_dot_)org instead. In either 
case, please 
retain the beginning of the Subject line to allow automated sorting.

The file can be obtained via

IESG discussion can be tracked via

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