Subject: Re: [ippm] I-D ACTION:draft-ietf-ippm-bw-capacity-01.txt

From: Joseph Ishac <[email-masked]>
Date: Wed, 5 Apr 2006 19:11:55 -0400

Resurrecting this thread in order to recapture Patrik's comments.

After IETF week, I put some more thought into the two comments below and
the changes made to the draft, and would like to solicit some input
from the list.  There were two major comments:

1) Nominal Physical Link Capacity (NPLC) "does not vary with time"

I agree, and have a proposed change to the text below.  However, I had a
question for the list.  Take the following example:

 (a) 802.11b switching speeds from 11 Mbps to 5 Mbps
 (b) A channel bonded 802.11g going from 108 Mbps to 54 Mbps (single channel)

I can see (b) easily argued as a changing NPLC, however one could
potentially argue case (a) either way [speed was decreased in order to
accommodate signal properties and thus technically the NPLC is still 11
Mbps]

So the question is, is it important to distinguish between such cases or
is it good enough to say that NomCap (t,t+I) > C(t,t+I), for all values
of t and I.  (Especially given that the NPLC is not used to define other
values in the document)

The proposed modifications to the text currently reads as:
=====
Nominal Physical Link Capacity:

Or NomCap(L) is the theoretical maximum amount of data that the link can
support.  For example, an OC-3 link would be capable of roughly 155
Mbps.  We stress that this is a measurement at the physical layer and
not the network IP layer, which we will define separately.  While
NomCap(L) is typically constant over time, there are links whose
characteristics may allow otherwise, such as the dynamic activation of
additional transponders for a satellite link.  However, the nominal
physical link capacity will always be greater than the information
carrying capacity of the link.
=====

2) "Link" (Current) vs. "Hop", and concept of VLANs/multiple physical
links between routers.

After some more thought, I believe the document is accurate with the use
of "link".  Our definitions are consistent with the framework in that a
link connects two "hosts" or "nodes" and does not necessarily make them
routers.  IP packets are still transported over each link, and that's
the property we're looking for.  A hop in your terms would be treated as
a "cloud subpath" (again from the framework) and would be equivalent to
calculating the capacity over that path.

So, does text need to be added to further clarify this or can it be
leaved as is?

Thanks,

-Joseph

On Mon, Nov 21, 2005 at 01:52:09PM +0100, Patrik Arlos wrote:
> Hi Joseph, Philip: 
> 
> 
> In your definition of Nominal Physical Link Capacity (NPLC), you state that
> it 'does not vary with time'. I do not think that it holds for wireless
> channels. The argument being that the signal interference can change quite
> rapidly, and at some point the sending speed has to be altered to compensate
> for the interference. Cf. the different speeds of 802.11 or UMTS. However,
> from the following discussion, the time varying behaviour of the NPLC is not
> interesting. Since it only acts as the upper limit of the capacity, i.e.,
> C(t,t+I)<NomCap (t,t+I). Furthermore, if the desire is to define capacity at
> the IP layer the behaviour of the underlying technologies should not matter.
> 
> 
>  
> 
> How about using 'hop' instead of 'link' in the IP layer terminology? A hop
> involves a sender, a receiver and a set of link layer links. I do think that
> is would resolve some other problems as well, for instance when virtual LAN
> (VLAN) is used a 'hop' might cover multiple physical links, without any IP
> layer treatment. I.e., from the IP layer's viewpoint there is a direct
> connection to the destination.
> 
>  
> 
> For instance, let S and D denote the IP address of the sending and receiving
> entities, respectively. The path between S and D is described as sequence of
> hops. A hop covers all communication entities involved in the transfer of an
> IP packet between two logically adjacent entities. Via the number of hops
> (hop count) we can identify the number of networks that has to be traversed
> in order to reach D from S. The number of hops is only dependent on the IP
> layer configuration, not the underlying configurations. For example, if S
> and D are located on the same IP network, then hop count is zero. A hop
> count of one would mean that the IP traffic passes one router to reach the
> destination. 
> 
> /Patrik
> 
> Patrik Arlos
> 
> SE/BTH/Sweden
> 
> +46 (0)455-385654

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---

From: "Patrik Arlos" <[email-masked]>
Date: Tue, 18 Apr 2006 10:10:28 +0200

Hi, 

1) Would it not be possibly to say that a speed reduction is due to packet
loss/signal degradation. What I'm getting at is that we can view the NPLC as
fixed, it's just subject to more disturbances, thus making the perceived
capacity (as viewed from layer +1) smaller than the NPLC. 

In short, I agree that it is sufficient to say that "NomCap (t,t+I) >
C(t,t+I), for all values of t and I."

2) I think that you should add one line that highlights how link-layer links
are supposed to be treated, just for clarification. 

Can you send a copy/url to the current working document?

Regards,
Patrik

Patrik Arlos
Ph.D Telecommunications Systems
SE/BTH/Sweden
+46 (0)455-385444

> -----Original Message-----
> From: Joseph Ishac [mailto:[email-masked]]
> Sent: den 6 april 2006 01:12
> To: [email-masked]
> Subject: Re: [ippm] I-D ACTION:draft-ietf-ippm-bw-capacity-01.txt
>

---

From: Joseph Ishac <[email-masked]>
Date: Mon, 24 Apr 2006 18:25:06 -0400

> C(t,t+I), for all values of t and I."

Ok, cool.

> are supposed to be treated, just for clarification. 

I added a two sentence blurb that attempts to do that.  See...

> Can you send a copy/url to the current working document?

http://roland.grc.nasa.gov/~jishac/share/draft-ietf-ippm-bw-capacity-02.txt

The changes fall in Section 2, page 5.

Thanks!

-Joseph