A question about 802.1d based CoS

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Hi,
I hava a switch Support 802.d layer two based CoS (can set each port
for 0-7 priorty. in fact it map to four queues e.g 0-2 ->q1, 3-4
-->q2, 5-->q3, and 6-7->q4 and it is weight round robin 10:5:2:1
->q4:q3:q2:q1)The question is that suppose i have two port (p1 and
p2)using line speed send diff traffic go to p3 and set p1 to 7 and p2
to 5. and all ports are same line speed without flow control.

Does p3 drop total half packets and 1/3 from p1 and 2/3 from p2
(10:5)?

if p3 does not congestion, does this CoS still work ?


if it connect another switch via trunk, does this CoS still work?

Does incoming tranffic add tag when it going to source port in the
switch and take off this tag when it going out of dest. port?


What is diff. between QoS and CoS?

Thanks,

LL

 

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In article <dc998cfd.0410192104.4d384174@posting.google.com>,
wld <aaabbb16@hotmail.com> wrote:
:I hava a switch Support 802.d layer two based CoS

:if it connect another switch via trunk, does this CoS still work?

Yes, but the other switch might have different ideas about what
the priorities mean.


oes incoming tranffic add tag when it going to source port in the
:switch and take off this tag when it going out of dest. port?

Yes.

:What is diff. between QoS and CoS?

http://www.btglobalservices.com/business/global/en/business_zone/business_innovations/glossary.html

CoS and QoS are ways of managing data traffic on a network. Quality
of Service (QoS) refers to the end-to-end delivery of information -
for example, data, voice and video - in real-time, reliably,
consistently and securely across the network, meeting agreed
service level assurances. QoS can vary by the type of traffic being
transported. For example, speech quality is sensitive to even
slight delays, while data - e-mails, for example - is more
tolerant. Class of Service (CoS) is a way of categorising traffic,
enabling prioritisation by type. Each CoS can be offered different
bandwidth or other parameters appropriate to the Qos required for
the traffic type.


In other words, CoS is about figuring out what the priority of
particular traffic should be, and QoS is about getting traffic
to the other end with priority appropriate for the classification.
--
Come to think of it, there are already a million monkeys on a million
typewriters, and Usenet is NOTHING like Shakespeare. -- Blair Houghton.

 

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In article <dc998cfd.0410192104.4d384174@posting.google.com>,
aaabbb16@hotmail.com (wld) wrote:

> Hi,
> I hava a switch Support 802.d layer two based CoS (can set each port
> for 0-7 priorty.

Do you mean that you are assigning *frame* priority based on the arrival
port? Remember, "priority" in 802.1D means the priority assigned to a
*frame*. Assigning frame priority based on arrival port seems odd to me.
It implies that *all* frames arriving on some port are "more important"
than *all* frames arriving on some other port. If the intent is to give
particular *users* higher priority, the assignment should rather be
based on MAC address (not port), so that the priority assignment "moves"
with the station.

Nonetheless, let's continue with your hypothetical situation (perhaps it
is a school assignment?).

> in fact it map to four queues e.g 0-2 ->q1, 3-4
> -->q2, 5-->q3, and 6-7->q4 and it is weight round robin 10:5:2:1
> ->q4:q3:q2:q1)

You have four classes of service on each output port. This is fairly
common.

Is the weighting being performed on a *frame* basis (i.e., 10 frames, 5
frames, 2 frames, 1 frame) or on a *byte* basis (e.g., 100K bytes, 50K
bytes, 20K bytes, 10K bytes)? Either is possible, and the resulting
traffic will vary. For simplicity, I will assume that the weighting is
on a byte basis; if all frames are assumed to be the same length (bad
assumption in practice), there is no difference between the two schemes.

By the way be *very careful* pronouncing "four queue system" in English!
:^)

> The question is that suppose i have two port (p1 and
> p2)using line speed send diff traffic

What do you mean by "diff traffic"? Differentiated service? Different?

> go to p3 and set p1 to 7 and p2
> to 5. and all ports are same line speed without flow control.
>

So your scenario has two steady-state, wire-speed frame streams, one at
priority 7 (highest class of service) and one at priority 5 (next
highest class of service) targeted to a single output port.

> Does p3 drop total half packets and 1/3 from p1 and 2/3 from p2
> (10:5)?
>

Using the weightings you provided, frames arriving from port 1 will
consume 10/15 (two-thirds) of the capacity of port 3, and frames
arriving from port 2 will consume 5/15 (one-third) of the capacity.
Assuming that this situation lasts "forever" (i.e., steady-state), and
the switch has finite buffers, 1/3 of the frames arriving on port 1, and
2/3 of the frames arriving on port 2 will ultimately be discarded.

> if p3 does not congestion, does this CoS still work ?
>

"Work" for what purpose? Obviously, if the purpose is to forward all of
the frames, no class-of-service scheme "works" if the offered load
exceeds the switch or port capacity in the steady-state. I discuss this
problem at length in Chapter 13 of "The Switch Book". Priority provides
a means to ride-through transient overload conditions without adversely
impacting the performance of time-sensitive applications; it cannot
resolve a situation where the steady-state demands placed on either the
switch or a link exceeds the capacity of the switch or link.

>
> if it connect another switch via trunk, does this CoS still work?
>

Same answer as above.

> Does incoming tranffic add tag when it going to source port in the
> switch and take off this tag when it going out of dest. port?
>

I don't understand this question. A tag-aware switch should tag untagged
frames for transmission on a "tagged" output port; similarly it should
strip the tag from received frames when transmitting them onto an
"untagged" output port.

>
> What is diff. between QoS and CoS?
>

That all depends on how you define QoS and CoS; many people use
different (and conflicting) definitions, so you get different answers.
This is also discussed at length in Chapter 13.


--
Rich Seifert Networks and Communications Consulting
21885 Bear Creek Way
(408) 395-5700 Los Gatos, CA 95033
(408) 228-0803 FAX

Send replies to: usenet at richseifert dot com

 

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Rich Seifert wrote:

>> What is diff. between QoS and CoS?
>>
>
> That all depends on how you define QoS and CoS; many people use
> different (and conflicting) definitions, so you get different answers.
> This is also discussed at length in Chapter 13.

I think ATM introduced some level of rigor in the definition of CoS and
QoS, but I agree that these terms are not typically used with any
standard meaning.

CoS or "service category" in ATM would be things like constant bit rate,
variable bit rate (real time), variable bit rate (non real time),
available bit rate, unspecified bit rate, and guaranteed frame rate. So
it's a more or less qualitative description of the type of virtual
circuit the calling party wants to set up. At the two extremes would be
constant bit rate to emulate a dedicated leased line, and uspecified bit
rate to emulate a best effort packet-switched network.

Traffic descriptors, such as peak cell rate, sustained cell rate, and
minimum cell rate, quantify the type of connection the calling party
wants to set up. For some classes, a traffic descriptor might not be
applicable. For example, in constant bit rate connections, peak and
sustained cell rates would be the same.

QoS is a quantitative specification of link quality, applied to the
specific connection the calling party is trying to set up. The basic
parameters would be end to end latency, jitter, and cell loss rate. For
example, a constant bit rate connection might ask for very low jitter
and low latency, to better emulate a dedicated line, but might not be
too concerned with cell loss rate. If this setup is for a voice link, an
occasional glitch in a voice connection might be acceptable, if this
level of loss allows the network to provide a constant bit rate link. An
unspecified bit rate connection would not be too concerned about QoS
parameters, because it is only trying to emulate best effort service.

In IP, Differentiated Services (RFC 2475) works to introduce some of
these different types of services in IP sessions. I don't think there's
any such rigor or standard definitions implied in the use of the
priority levels with IEEE 802.1Q, however.

Bert