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Real-time data over IP networks using LLQ.

 
 
Jaco Versfeld
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      05-03-2004
Hi,

When supporting real-time data over IP networks, scheduling should be
implemented, like WFQ and its variants or LLQ (Low Latency Queueing).

Does LLQ have any disadvantages at this stage? Does any packet loss
occur in the high priority classes when using LLQ?

Thanks,
Jaco
 
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Ben
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      05-03-2004
Cisco will tell you LLQ has no disadvantages. It combines all the advantages
of other types of queueing.

Packet loss can occur in the priority queue, but only as you define it. For
the priority queue you must specify a maximum bandwidth which is policed
(i.e. exceed action is drop) to ensure the other queues are not starved.

So packet loss will only occur if you have not configured enough bandwidth
for the priority queue.




"Jaco Versfeld" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) om...
> Hi,
>
> When supporting real-time data over IP networks, scheduling should be
> implemented, like WFQ and its variants or LLQ (Low Latency Queueing).
>
> Does LLQ have any disadvantages at this stage? Does any packet loss
> occur in the high priority classes when using LLQ?
>
> Thanks,
> Jaco



 
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Jaco Versfeld
Guest
Posts: n/a
 
      05-03-2004
Thanks for the reply.

Will high-priority data be dropped or delayed excessively when a lot
of low priority data is received first at the classifier?

Thank you very much,
Jaco


"Ben" <(E-Mail Removed)> wrote in message news:<4096175f$(E-Mail Removed)>...
> Cisco will tell you LLQ has no disadvantages. It combines all the advantages
> of other types of queueing.
>
> Packet loss can occur in the priority queue, but only as you define it. For
> the priority queue you must specify a maximum bandwidth which is policed
> (i.e. exceed action is drop) to ensure the other queues are not starved.
>
> So packet loss will only occur if you have not configured enough bandwidth
> for the priority queue.
>
>
>
>
> "Jaco Versfeld" <(E-Mail Removed)> wrote in message
> news:(E-Mail Removed) om...
> > Hi,
> >
> > When supporting real-time data over IP networks, scheduling should be
> > implemented, like WFQ and its variants or LLQ (Low Latency Queueing).
> >
> > Does LLQ have any disadvantages at this stage? Does any packet loss
> > occur in the high priority classes when using LLQ?
> >
> > Thanks,
> > Jaco

 
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Ivan Ostres
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Posts: n/a
 
      05-03-2004
In article <(E-Mail Removed) >,
http://www.velocityreviews.com/forums/(E-Mail Removed) says...
> Thanks for the reply.
>
> Will high-priority data be dropped or delayed excessively when a lot
> of low priority data is received first at the classifier?
>
>
>


IMHO, no. If I understood it correctly (from IOS inside book), high
priority traffic will be delayed just for the time needed to empty TX
ring, and that shouldn't be too long for today's interface speeds. The
data will be dropped if high priority data will fill its buffer before
TX buffer becomes empty of low priority data.

I hope I got this info right. If I'm wrong, I hope someone will note so,
and explain to me too .

--
Ivan
 
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Ben
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Posts: n/a
 
      05-04-2004
Hmmm, I am not sure I understand the question...

Do you mean is there a problem with inbound traffic getting dropped before
it is classified?

This is unlikely. Firstly if there were congestion into an interface the
egress router would have already dealt with it (with it's own QoS policy or
queueing mechanism). If not, then it should be configured to do so. Or, the
service provider would simply be dropping excess traffic in the case of a
WAN link (again an issue for the egress router to sort out).

Secondly, if a received packet arrives at the interface buffers and finds it
full, it is simply copied to the shared memory buffers instead (no
performance hit). Packets can then be switched, classified, whatever from
there. To overload the router's internal memory buffers is more of a
configuration or hardware issue than a congestion issue. For classification
to fail once the packet is buffered is more of a CPU utilisation issue.

The main issue now occurs after classification and a packet has been
delivered to the appropriate queue. What happens if that queue is full?
There queues too are finite configurable space in shared memory. WRED is the
mechanism that can deal with 'tail-drop' - indiscriminite dropping of
packets when a queue is full.
It use the same class information as LLQ and basically works by pro-actively
dropping packets as a queue approaches being full (to avoid tcp slowstart)
and in a class weighted fashion - not just 'last in first dropped'.

Let me know if I have totally misunderstood what you were asking.


"Jaco Versfeld" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) om...
> Thanks for the reply.
>
> Will high-priority data be dropped or delayed excessively when a lot
> of low priority data is received first at the classifier?
>
> Thank you very much,
> Jaco
>
>
> "Ben" <(E-Mail Removed)> wrote in message

news:<4096175f$(E-Mail Removed)>...
> > Cisco will tell you LLQ has no disadvantages. It combines all the

advantages
> > of other types of queueing.
> >
> > Packet loss can occur in the priority queue, but only as you define it.

For
> > the priority queue you must specify a maximum bandwidth which is policed
> > (i.e. exceed action is drop) to ensure the other queues are not starved.
> >
> > So packet loss will only occur if you have not configured enough

bandwidth
> > for the priority queue.
> >
> >
> >
> >
> > "Jaco Versfeld" <(E-Mail Removed)> wrote in message
> > news:(E-Mail Removed) om...
> > > Hi,
> > >
> > > When supporting real-time data over IP networks, scheduling should be
> > > implemented, like WFQ and its variants or LLQ (Low Latency Queueing).
> > >
> > > Does LLQ have any disadvantages at this stage? Does any packet loss
> > > occur in the high priority classes when using LLQ?
> > >
> > > Thanks,
> > > Jaco



 
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Jaco Versfeld
Guest
Posts: n/a
 
      05-05-2004
Hi,

Thanks a lot for the answer. You understood me 100% correct.

Actually, I am a postgrad student researching forward error correction
(FEC) techniques to compliment congestion avoidance techniques (like
LLQ).

(Long story, but I have a bursary at a large Telco company and have to
finish my degree in the next few months. When I started two and a
half years ago, I researched FEC techniques to alleviate the effect of
packet loss. I spent most of my time researching and designing such
codes. I only came to know of LLQ quite recently, and it was already
too late to change my research subject.)

What I want to do now, is to see whether FEC can compliment LLQ. My
hope was that the classifier at the LLQ Router could be modelled as a
finite length queue, like the Best Effort Routers of the Internet.
Thus, if the classifier is "overwhelmed" by a burst of packets
containing low priority and high priority data, the probability of
packet loss of high priority packets in the burst would be high, which
can be recovered using FEC.

Are there any other places and/or scenarios in modern networks where
packet loss of high priority data can occur?

Thank you very much for the comments thus far, it helped me a lot,
Jaco



"Ben" <(E-Mail Removed)> wrote in message news:<4097c2b1$(E-Mail Removed)>...
> Hmmm, I am not sure I understand the question...
>
> Do you mean is there a problem with inbound traffic getting dropped before
> it is classified?
>
> This is unlikely. Firstly if there were congestion into an interface the
> egress router would have already dealt with it (with it's own QoS policy or
> queueing mechanism). If not, then it should be configured to do so. Or, the
> service provider would simply be dropping excess traffic in the case of a
> WAN link (again an issue for the egress router to sort out).
>
> Secondly, if a received packet arrives at the interface buffers and finds it
> full, it is simply copied to the shared memory buffers instead (no
> performance hit). Packets can then be switched, classified, whatever from
> there. To overload the router's internal memory buffers is more of a
> configuration or hardware issue than a congestion issue. For classification
> to fail once the packet is buffered is more of a CPU utilisation issue.
>
> The main issue now occurs after classification and a packet has been
> delivered to the appropriate queue. What happens if that queue is full?
> There queues too are finite configurable space in shared memory. WRED is the
> mechanism that can deal with 'tail-drop' - indiscriminite dropping of
> packets when a queue is full.
> It use the same class information as LLQ and basically works by pro-actively
> dropping packets as a queue approaches being full (to avoid tcp slowstart)
> and in a class weighted fashion - not just 'last in first dropped'.
>
> Let me know if I have totally misunderstood what you were asking.

 
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Ben
Guest
Posts: n/a
 
      05-06-2004
I would tend to think they are different functionality but I don't know a
lot about FEC.

Is FEC really just an advanced form of CRC check that can not only determine
errors, but repair them?

In that case, where would it be implemented? Between two routers on WAN link
that traverse a service-providers switched network?
Or within the service-provider's network at each switch?

What I am getting at is that LLQ is dealing specifically with queueing...a
method for assigning traffic to a software queue if and only if the
interface's hardware queue is full (indicating congestion).

Quality of Service is a general term and incorporates queueing but also
classifying, marking, policing and shaping traffic. From that point of view
FEC could be considered a complementary QoS feature - another method of
guaranteeing quality. But I am not really sure how it explicitly relates to
queueing as such...
Unless you were to relate FEC to classification (really a separate process
to queueing that LLQ "notices"). For example you might only employ FEC on
high priority data.

However, I guess there are two types of data loss, intentional (for want of
a better word) and unintentional. Intentional occurs because of congestion,
unintentional occurs due to some form of corruption en route or from a
malfunctioning interface. Only one discriminates between high and low
priority data. Congestion usually occurs at points of aggregration,
corruption occurs due to malfunctioning hardware or cable interference.

Would FEC deal with the latter, or both? If dealing with corruption only,
there would no way to discriminate - the packet markings themselves could be
corrupted.


"Jaco Versfeld" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) om...
> Hi,
>
> Thanks a lot for the answer. You understood me 100% correct.
>
> Actually, I am a postgrad student researching forward error correction
> (FEC) techniques to compliment congestion avoidance techniques (like
> LLQ).
>
> (Long story, but I have a bursary at a large Telco company and have to
> finish my degree in the next few months. When I started two and a
> half years ago, I researched FEC techniques to alleviate the effect of
> packet loss. I spent most of my time researching and designing such
> codes. I only came to know of LLQ quite recently, and it was already
> too late to change my research subject.)
>
> What I want to do now, is to see whether FEC can compliment LLQ. My
> hope was that the classifier at the LLQ Router could be modelled as a
> finite length queue, like the Best Effort Routers of the Internet.
> Thus, if the classifier is "overwhelmed" by a burst of packets
> containing low priority and high priority data, the probability of
> packet loss of high priority packets in the burst would be high, which
> can be recovered using FEC.
>
> Are there any other places and/or scenarios in modern networks where
> packet loss of high priority data can occur?
>
> Thank you very much for the comments thus far, it helped me a lot,
> Jaco
>
>
>
> "Ben" <(E-Mail Removed)> wrote in message

news:<4097c2b1$(E-Mail Removed)>...
> > Hmmm, I am not sure I understand the question...
> >
> > Do you mean is there a problem with inbound traffic getting dropped

before
> > it is classified?
> >
> > This is unlikely. Firstly if there were congestion into an interface the
> > egress router would have already dealt with it (with it's own QoS policy

or
> > queueing mechanism). If not, then it should be configured to do so. Or,

the
> > service provider would simply be dropping excess traffic in the case of

a
> > WAN link (again an issue for the egress router to sort out).
> >
> > Secondly, if a received packet arrives at the interface buffers and

finds it
> > full, it is simply copied to the shared memory buffers instead (no
> > performance hit). Packets can then be switched, classified, whatever

from
> > there. To overload the router's internal memory buffers is more of a
> > configuration or hardware issue than a congestion issue. For

classification
> > to fail once the packet is buffered is more of a CPU utilisation issue.
> >
> > The main issue now occurs after classification and a packet has been
> > delivered to the appropriate queue. What happens if that queue is full?
> > There queues too are finite configurable space in shared memory. WRED is

the
> > mechanism that can deal with 'tail-drop' - indiscriminite dropping of
> > packets when a queue is full.
> > It use the same class information as LLQ and basically works by

pro-actively
> > dropping packets as a queue approaches being full (to avoid tcp

slowstart)
> > and in a class weighted fashion - not just 'last in first dropped'.
> >
> > Let me know if I have totally misunderstood what you were asking.



 
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Jaco Versfeld
Guest
Posts: n/a
 
      05-06-2004
Hi,

Cyclic Redundancy Checks (CRC) is a subset of Forward error correction
codes. CRCs can detect errors, but with CRCs you can actually correct
errors as well. (If I am correct, CRCs are actually called BCH codes.
Normally if you can detect 2t errors with a Code, the Code will be
able to correct t errors, given that t or less errors occured)

I have looked at Packet Erasure Codes. Any FEC code can be used as a
Packet Erasure Code. The basic idea is that you take k data packets,
use a "mathematical recipe" and calculate h extra packets. You then
transmit the n = h + k packets over the network. Any h or less
packets can be dropped, and from the received packets you can
reconstruct the original k data packets. The major advantage of
packet erasure codes is that the recovery usually is a lot faster than
retransmission techniques, like ARQ. (For normal file transfer, have a
look at www.digitalfountain.com )


The Technology experts at the Telco (who also gave me the bursary) say
that errors on modern networks are very low (with probability
10^(-6)), so that it is not advisable to consider correcting these
errors.

It would be great to find a scenario where Packet erasure codes could
be used to alleviate the effect of congestion errors, i.e., packets
dropped due to buffer overflow, especially of high priority data. The
major disadvantage of packet erasure codes is that extra packets need
to be transmitted and in an already congested environment, this can
sometimes cause more trouble than helping.

However, when protecting only high priority data, this can help to
recover packets that might be dropped, due to congestion or network
errors.

A possible setup suggested by many authors was to encode the packets
of a source at the application layer, send the data over the network,
and decode the packets at the sink at the application layer. Some
also suggested that a PEC (packet erasure code) can be implemented at
the lower layers. The best performance "theoretically" would be
gained when the encoding and decoding is separated by the maximum
amount of network hops. A lot of research has gone into PEC codes
before LLQ. The researchers thought that this could be an answer to
packet loss in Best Effort networks.


Is there some chance that the classifier of an LLQ router can drop
high priority data "unintentionally" when the outgoing rate of this
class is still below the allocated bandwidth, due to a burst of data
received? (I.e., can the classifier be "overwhelmed" by a big burst
of incoming data?)

Thank you very much for the help thus far, I have learned a lot
Jaco


"Ben" <(E-Mail Removed)> wrote in message news:<YMimc.22667$(E-Mail Removed)>...
> I would tend to think they are different functionality but I don't know a
> lot about FEC.
>
> Is FEC really just an advanced form of CRC check that can not only determine
> errors, but repair them?
>
> In that case, where would it be implemented? Between two routers on WAN link
> that traverse a service-providers switched network?
> Or within the service-provider's network at each switch?
>
> What I am getting at is that LLQ is dealing specifically with queueing...a
> method for assigning traffic to a software queue if and only if the
> interface's hardware queue is full (indicating congestion).
>
> Quality of Service is a general term and incorporates queueing but also
> classifying, marking, policing and shaping traffic. From that point of view
> FEC could be considered a complementary QoS feature - another method of
> guaranteeing quality. But I am not really sure how it explicitly relates to
> queueing as such...
> Unless you were to relate FEC to classification (really a separate process
> to queueing that LLQ "notices"). For example you might only employ FEC on
> high priority data.
>
> However, I guess there are two types of data loss, intentional (for want of
> a better word) and unintentional. Intentional occurs because of congestion,
> unintentional occurs due to some form of corruption en route or from a
> malfunctioning interface. Only one discriminates between high and low
> priority data. Congestion usually occurs at points of aggregration,
> corruption occurs due to malfunctioning hardware or cable interference.
>
> Would FEC deal with the latter, or both? If dealing with corruption only,
> there would no way to discriminate - the packet markings themselves could be
> corrupted.

 
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