Subject: QoS in Shared Media

From: "Naidu, Venkata" <[email-masked]>
Date: Fri, 4 May 2001 10:58:44 -0400

Hi All

  I have a doubt regarding "QoS in Shared Media".  I don't 
  know how much this is applicable to this discussion list :-)

  But anyway, I am hearing about 40/10(OC768) Gig Ethernet 
  evolution. If not at the core, these technologies may be
  at the enterprise/access networks(?).

  Moreover, The QoS issue is especially interesting over
  wireless access networks as they are likely to be bottlenecks 
  (compared to wireline links) and they are often shared
  media (e.g., wire less LANs). Providing service 
  differentiation on shared media  is  difficult  due  to
  the risk of collisions between packets of different priority.

  * So, first, Is MPLS useful/applicable to these technologies?

  * If so, how the present singling protocols (RSVP/CR-LDP) or
    diffserv TE extended protocols will take care of this?
    As far as I know, classical RSVP is designed for such goals
    and ofcourse, present IP routing protocols also runs
    on such shared media, P2P and NBMA environments.

  * In a simple environment Ethernet -> ATM/NBMA -> Ethernet
    How resource reservation takes place and is there something
    more the present signaling/routing protocols need to do
    for QoS guaranties.
 
  I didn't find anything useful in TE-WG drafts!
  Is that I am missing something?

Thanks & Regards
--Venkata Naidu

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From: David Charlap <[email-masked]>
Date: Fri, 04 May 2001 13:17:29 -0400

>   at the enterprise/access networks(?).

GigE and 10GigE are not shared-media networks.  Despite the fact that
they're based on Ethernet, every implementation I've ever seen uses them
as point-to-point links between switches/routers/hosts.

>   the risk of collisions between packets of different priority.

QoS guarantees are impossible over shared media.  There is always the
possibility that somebody on the bus (perhaps a box running malicious
code) will saturate the shared bus with garbage, forcing everybody else
to wait or have collisions.

If you can assume that everybody plays by the rules, then boxes on a
shared bus can coordinate their access to provide some level of
guaranteed service.  Token Ring is one such example of this.  TDM- based
technologies (such as channelized SONET) are another.

>   * So, first, Is MPLS useful/applicable to these technologies?

MPLS is useful even without QoS.  It allows for the design of faster
routers, or alternatively the use of less expensive forwarding chips at
slower speeds.

The explicit-route aspects of traffic engineering work over shared-media
networks as well, even though QoS still can't be guaranteed, which may
also be of use.

>     on such shared media, P2P and NBMA environments.

Classical RSVP's ability to work over non-RSVP links is only a detection
algorithm - it will detect non-RSVP links and indicate this to the
egress node.  The egress node then has to deal with the knowledge that
any requested QoS may not actually be acheivable, due to the presence of
those links along the path.

>     more the present signaling/routing protocols need to do

If the Ethernet is point-to-point, there is no problem.  If it is a
shared bus, then the operator should realize that QoS can't be
guaranteed.  Labels can still be allocated and used.  Resources can
still be reserved on the routers.  But the line itself may cause delays,
queue backups and lost data.

This shouldn't be a problem, however.  If an network admin chooses to
set up a shared-media network, then he should be aware of these
factors.  If these factors are unacceptible, then he should restrict the
links to point-to-point links.

-- David

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From: "Jay Wang" <[email-masked]>
Date: Fri, 4 May 2001 11:15:49 -0700

>technologies (such as channelized SONET) are another.

In fact if the transmission infrastructure of the shared media has the
intelligence or capability built in to coordinate the admission of traffic
into the shared media, it makes the life of QoS at higher level easier. The
RPR(IEEE 802.17) is apparently marching toward this direction.  At least,
Cisco's DPT/SRP-fa (cisco's version of RPR) ensures global bandwidth
fairness
for all nodes on the same ring from what I know.

- jay

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From: "HANSEN CHAN" <[email-masked]>
Date: Fri, 04 May 2001 14:21:01 -0400

> for all nodes on the same ring from what I know.

I really would like to understand how RPR or DPT can ensure global bandwidth
fairness. Any link or pointer?

Thanks,
Hansen

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From: "Jay Wang" <[email-masked]>
Date: Fri, 4 May 2001 11:26:34 -0700

Hansen,

This may be a good place to get an introduction on the topic:

http://www.cisco.com/warp/public/cc/techno/wnty/dpty/tech/dptm_wp.htm

- jay

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From: Vinay Bannai <[email-masked]>
Date: Fri, 04 May 2001 11:50:08 -0700

> fairness. Any link or pointer?

The discussions in IEEE 802.17 are still continuing and there are vaired
solutions that have been proposed. The fundamental premise is at each node in
the RPR ring, based on some bits in the header the add and pass traffic are
scheduled based on the QoS bits set in the header. Also within a RPR capable box
or a line card, the traffic is scheduled on to the ring based on QoS.

Vinay