Re: [PCN] Proposed Mod to draft-pcn-marking-behaviour

[<[e-mail]>]

Btw, although this looks like a lot of text below, it is only slight
wording changes to improve the clarity - but I thought it easiest to
include paragraphs rather than isolated sentences.

The only substantive issue is the one explained at the start. 

 

-----Original Message-----
From: Eardley,PL,Philip,CXR9 R 
Sent: 24 June 2009 09:23
To: [e-mail]
Subject: Proposed Mod to draft-pcn-marking-behaviour

 

I've been editing in the various comments on
http://tools.ietf.org/html/draft-ietf-pcn-marking-behaviour-03 

In Section 2.4, both Michael and Ingemar said they found talk of MTU
obscure, so have been working on phrasing this better.

However I also realised another issue - S2.4 says:-

 

   A PCN-node MUST implement an excess-traffic-meter that has behaviour
   functionally equivalent to the following.

[description of 'base' excess-traffic-meter]

   In addition to the above, ... the meter SHOULD [do] ... packet size
independent marking

 

What this says, now I read it properly, is that you MUST implement
'base' marking behaviour AND also you SHOULD implement "packet size
independent marking".

 

Whereas what I think we actually wanted to say was: you MUST implement
some kind of excess-traffic-marking behaviour, either the 'base' marking
behaviour or "packet size independent marking" - the latter is suggested
ie you SHOULD implement "packet size independent marking" and if you do,
then you don't have to implement the 'base' marking behaviour as well. I
checked with michael and toby and this is their understanding as well.

 

If this isn't your understanding, then please shout now! I'm trying to
get a new version out COP tomorrow Thursday, taking account of all the
comments on -03, including the ietf last call ones. Thanks.

 

 

 

Assuming this is ok, I'm proposing the following wording, which also
gets rid of the obscure talk about MTU.

 

S2.4 now reads:-

...   A PCN-node MUST implement an excess-traffic-meter. The
excess-traffic-meter SHOULD indicate excess-traffic-marking independent
of packet size ("packet size independent marking") but otherwise MUST
use the "baseline" metering behaviour.
               
The "baseline" excess-traffic-meter has behaviour functionally
equivalent to the following.
 
   The meter acts like a token bucket, which is sized in bits and has a
configured reference rate.  The amount of tokens in the token bucket is
termed Tetm.  Tokens are added at the reference rate (PCN-excess-rate),
to a maximum value BSetm.  Tokens are removed equal to the size in bits
of the metered-packet, to a minimum Tetm=0.  If the token bucket is
empty (Tetm = 0), then the meter indicates to the marking function that
the packet is to be excess-traffic-marked. (Explanation of
abbreviations: T is short for Tokens, BS for bucket size, and etm for
excess-traffic-meter.)
 
"Packet size independent marking" means that the size of the packet does
not influence the decision about whether the meter indicates to the
marking function that the packet is to be excess-traffic-marked.
 

 

I also think it best to update Appendix A.2 so that it uses "packet size
independent marking":-

   The following steps are performed when a PCN-packet arrives on a
   link:
 
   o  Tetm = min(BSetm, Tetm + (now - lastUpdate) * PCN-excess-rate); //
      add tokens to token bucket
 
   o  if (packet_mark != excess-traffic-marked) then // do not meter
      packets that are already excess-traffic-marked
 
   o
 
      *  if (Tetm < 0) then packet_mark = excess-traffic-marked; // do
         excess-traffic-marking.  The algorithm ensures this is
         independent of packet size
 
      *  else Tetm = Tetm - packet_size; // remove tokens from token
         bucket if don't mark packet
 
   o  lastUpdate = now // Note: 'now' has the same value as in step 1

 

 

Also tweaked Appendix B.6:

   "Packet size independent marking" - excess-traffic-marking that is
independent of packet size - is specified as a SHOULD in Section 2.4.
With the "baseline" excess-traffic-meter behaviour, large packets are
more likely to be excess-traffic-marked than small packets, because
packets are marked if the number of tokens in the packet is smaller than
the packet size. This means that, with some edge behaviours, flows with
large packets are more likely to be terminated than flows with small
packets [I-D.briscoe-tsvwg-byte-pkt-mark] [Menth09]. It can be achieved
by a small modification of the "baseline" excess-traffic-meter: the
number of tokens in the bucket can become negative; if this number is
negative at a packet's arrival, the packet is marked; otherwise, the
amount of tokens equal to the packet size is removed from the bucket. 

 

"Packet size independent marking" is a 'SHOULD', rather than a 'MUST',
because it may be slightly harder for some equipment to implement, and
the impact of not doing it is undesirable but moderate (sufficient
traffic is terminated, but flows with large packets are more likely to
be terminated).  
 
Thanks, 
phil