Subject: [Diffserv] Definition of a BA

From: Dan Grossman <[email-masked]>
Date: Tue, 11 Apr 2000 11:31:56 -0400

The recent discussion of BAs raised a possible problem with the definition in 
RFC 2475.

DS behavior aggregate: a collection of packets with the same DS codepoint 
crossing a link in a particular direction.

The problem is that AF (and experimental or future PHB groups) in fact use 
more than one DSCP.  Presumably, all packets which are in the PHB group are 
part of the BA.

The obvious fix would be to change the definition to "a collection of packets 
which are members of the same PHB group crossing a link in a particular 
direction."  The problem with this is that there might be multiple BAs which 
are of the same PHB group crossing a link, each with its own set of DSCPs.

The best fix I can think of is to invent a new term: 

DSCP Group: A set of DSCPs which select the members of a PHB group.   

Thus, 

DS behavior aggregate: a collection of packets having DSCPs which are members 
of the same DSCP group, crossing a link in a particular direction.

A less clear way of doing this without a new definition:

DS behavior aggregate: a collection of packets with the same DS codepoint (or 
related DSCPs that select the same PHB group) crossing a link in a particular 
direction.

Comments?  

I will update the terminology draft when there is WG consensus.

_______________________________________________
diffserv mailing list
[email-masked]
http://www1.ietf.org/mailman/listinfo/diffserv
Archive: http://www-nrg.ee.lbl.gov/diff-serv-arch/

---

From: Juha Heinanen <[email-masked]>
Date: Tue, 11 Apr 2000 22:11:48 +0300 (EEST)

i thought that we already agreed that af consists of four phb groups and
that the text will be fixed when a new version of the rfc is spinned
out.  diffserv terminology is already now way beyond comprehension and
adding a new term (dscp group) is more more nail to the coffin.

-- juha

---

From: Lloyd Wood <[email-masked]>
Date: Tue, 11 Apr 2000 20:32:55 +0100 (BST)

On Tue, 11 Apr 2000, Juha Heinanen wrote:

> diffserv terminology is already now way beyond comprehension 

hear, hear.

L.

look, it's not the vocabulary that's supposed to scale.

<[email-masked]>PGP<http://www.ee.surrey.ac.uk/Personal/L.Wood/>

---

From: Dan Grossman <[email-masked]>
Date: Tue, 11 Apr 2000 15:50:14 -0400

> out. 
Yes.  But the problem is with the definition of BA, not the definition of PHB 
group.
 
> diffserv terminology is already now way beyond comprehension and
True.  Can you suggest terms to take away?

> adding a new term (dscp group) is more more nail to the coffin.
I take it you prefer the second solution?
> -- juha
Dan

---

From: Keith McCloghrie <[email-masked]>
Date: Tue, 11 Apr 2000 15:03:43 -0700 (PDT)

> related DSCPs that select the same PHB group) crossing a link in a particular 
 
How about:

  DS behavior aggregate: a collection of packets with the same DS
  codepoint(s) crossing a link in a particular direction.

Keith.

---

From: Grenville Armitage <[email-masked]>
Date: Tue, 11 Apr 2000 20:27:50 -0700

> more than one DSCP.  Presumably, all packets which are in the PHB group are

Hmmm... perhaps my head is still flushing the opposite way after the
IETF, but can you remind me why must we presume that packets in a
PHB Group are part of the same BA?  (Or is this tied up with the
impending re-definition of BA?)

cheers,
gja
________________________________________________________________________
Grenville Armitage                    http://members.home.net/garmitage/
Bell Labs Research Silicon Valley

---

From: Juha Heinanen <[email-masked]>
Date: Wed, 12 Apr 2000 08:04:18 +0300 (EEST)

> impending re-definition of BA?)

why do we need to differentiate BAs and PHB groups?  why can't we make
them to be the same and get rid of the other term?

-- juha

---

From: Brian E Carpenter <[email-masked]>
Date: Wed, 12 Apr 2000 08:48:20 -0500

Juha,

> them to be the same and get rid of the other term?

Because a BA is at some level a mathematical model that describes the 
statistical behaviour of a set of traffic flows. That seems to me to 
be a very different animal from a PHB Group, which simply describes
a mechanism. 

But I take your general point about acronym minimisation. We mustn't add
teriminology for its own sake.

   Brian

---

From: Dan Grossman <[email-masked]>
Date: Wed, 12 Apr 2000 09:57:25 -0400

> impending re-definition of BA?)
Good question.  Yes,  it's partly tied up with the impending redefinition of BA, since now BAs are becoming important to the diffserv architecture in a way they weren't before.  The question really is, are all the packets belonging to an AF group _instance_ and crossing a link in a particular direction part of the same BA?  Does remarking cause packets to shift from one BA to another?

---

From: Juha Heinanen <[email-masked]>
Date: Wed, 12 Apr 2000 16:53:26 +0300 (EEST)

> be a very different animal from a PHB Group, which simply describes

why don't you then talk about edge-to-edge behavior of bhp groups rather
than invent a new term?

-- juha

---

From: Dan Grossman <[email-masked]>
Date: Wed, 12 Apr 2000 09:51:12 -0400

>   codepoint(s) crossing a link in a particular direction.
Nice thought, but too inclusive.  It probably was not intended that completely 
unrelated DSCPs be part of the same BA.

Dan

---

From: Brian E Carpenter <[email-masked]>
Date: Wed, 12 Apr 2000 08:55:05 -0500

Grenville,

Grenville Armitage wrote:
...
> impending re-definition of BA?)

A BA classifier only looks at the DSCP, so a BA classifier cannot
discriminate between hypothetically different "BAs" whose packets carry 
the same DSCP. Since we want interior routers to be able to use BA
classifiers, it follows that at a given interior router interface, 
all packets belonging to a given PHB Group are in the same BA.

   Brian

---

From: Brian E Carpenter <[email-masked]>
Date: Wed, 12 Apr 2000 10:03:44 -0500

> why don't you then talk about edge-to-edge behavior of bhp groups rather

1) because BA is shorter to write
2) because BA isn't a new term anyway

   Brian

---

From: "Brijesh Kumar" <[email-masked]>
Date: Wed, 12 Apr 2000 11:50:47 -0400

> Brian Carpenter wrote:

> all packets belonging to a given PHB Group are in the same BA.

I think it should be other way around. All packets belonging to a given
BA (i.e., having the same codepoint) would be in the same PHB group
at a given router interface. But the reverse  cannot be true since one can
map more than one BAs (i.e., packets having different codepoints) to the
same PHB group within a router. Is that not correct?

Secondly, I have a question. It is not clear from any draft of the DiffServe
documents
what is the relationship between PHBs selected by the class selector
codepoints with
regard to EF and AF PHB? RFC 2474 only says that a PHB selected by a bigger
numerical value of a class selector codepoint will have a preferential
treatment
over a PHB selected by a smaller numerical value of a class selector
codepoint
(default PHB selected by the codepoint 000000 getting the least preferential
treatment). Does it mean that every implementation must choose its own
preference
for defined PHBs?

Cheers,

--brijesh