Subject: [Diffserv] Comments on VW EF draf

From: Shahram Davari <[email-masked]>
Date: Sun, 26 Mar 2000 14:20:52 -0800

Hi,

I have a few questions/comments on the VW-BA draft:

1) In section 2.6 the voice capacity of IP network is compared to that
of a TDM Telco trunk, and it was concluded that there is a 25% BW
penalty for IP. However, it is not clear why for IP a 200 byte per 20 msec
(equivalent to 80 kbit/sec rate) is assumed, while for the TDM a 64Kbit/sec
rate. Is this overhead due to RTP/UDP/IP header? if so why header
compression is not considered for the comparison?

2) Section 6.1.1.1 says that:

"in order to get the worst jitter a packet from each ingress must arrive at
each of the M border routers at the same time and must be transmitted to the
interior router queue for the bottleneck link B at the same time"

Since this section is about dumpbell bottle neck, why is it necessary for
the packets to arrive at the ingress routers at the same time? to achieve
the worst jitter case we only need the packets to arrive at the 
bottleneck at the same time, don't we?

3) Section 6.1.1.1 says "bottleneck B" while the figure shows the
bottleneck link as "L". Which one is correct?

4) Section 6.1.1.1 says: "The links between the border routers and the
bottleneck router must be enough larger than B that the packets are
still sitting in B's queue when our (A,Z) packet arrives at the end of
a burst of N, that is L>NxB"

- What does B's queue mean does it mean the queue for the bottleneck L?

- instead of the sentence "must be enough larger" isn't it better
 to say "must be just large enough".

- How is the L>NB equation derived? It doesn't seem correct to me because,
if L is infinitely large, then any packet arriving at bottleneck won't see
any other packets in front of it.

According to my calculations if you have M links arriving at the
bottleneck point and each carrying a burst of N packets, then in order
for the last packet in a burst to experience jitter due to the other
packets in front of it, the time to transmit one packet over bottleneck
link "L" must not exceed the time that it takes to receive a burst of
N packets. That is:

S/L < NS/B  =>  L<B/N

4) Last paragraph of section 6.1.1 says " the amount of time to send an
EF packet on each link can be written as fT/(total no. of EF-marked flow
crossing the link). Is this always true. Don't you think that this holds
only when the full f% capacity of the link is used by EF.

5) Section 6.1.1.2 finds the worst case jitter as fT. But the implicit
assumption is that an EF flow won't encounter another EF flow more than
once. However, in multipath routing an EF-flow may encounter another
EF flow many times. For example consider the following scenario:

          X                    X
     /--------->\          /-------->\
    /            \        /           \
-->A              B----> C             D------>
    \            /        \           /
     \--------->/          \-------->/
         Y                      Y

Flow X and Y encounter each other more than once.

I appreciate the authors reply.

Regards,

-Shahram

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From: Kathleen Nichols <[email-masked]>
Date: Wed, 29 Mar 2000 03:02:06 -0800

Shahram Davari wrote:
....
> - What does B's queue mean does it mean the queue for the bottleneck L?

Yes, the terminology is clearly messed up here. I was attempting
to merge a couple of different examples and use consistent
terminology and ended up with that messed up section. I'll
fix it in the next rev.

> link "L" must not exceed the time that it takes to receive a burst of

I'll look through this later and change it if it is wrong.

> only when the full f% capacity of the link is used by EF.

I think the words clearly say we make the worst case assumptions.
Using the full "f" is to make a worst case.

> EF flow many times. For example consider the following scenario:

I think Van answered this in his remarks and I hope he plans to
put this into the revision. I've been through this with
other people before and if you work out what has to happen
here for some specific example, it's usually enlightening.

Thanks for the close read,

	Kathie

---

From: Anna Charny <[email-masked]>
Date: Wed, 29 Mar 2000 17:33:53 -0800

Kathie,

>Shahram Davari wrote:

<snip>

>here for some specific example, it's usually enlightening.

Would it be possible to give a brief summary of how this was answered by 
Van for those
of us who were not present at the meeting?

Thanks in advance,
Anna