[PCN] PCN admission marking algorithm

I think we have consensus on the admission marking behaviour. This is a
first attempt to capture that in Stds language, is it any use?

If an interface of the PCN-domain performs PCN Admission Marking then it
performs three functions, which are outlined in
draft-ietf-pcn-architecture as follows:

   o  Packet classify - decide whether an incoming packet is a PCN-
      packet or not.  Another PCN WG document will specify encoding,
      using the DSCP and/or ECN fields.

   o  PCN-meter - measure the 'amount of PCN-traffic'.  The measurement
      is made as an aggregate of all PCN-packets, and not per flow.

   o  PCN-mark - algorithms determine whether to PCN-mark PCN-packets
      and what packet encoding is used (as specified in another PCN WG
      document).

These functions are now described in more detail.

Note: The PCN-node MAY implement these three functions on either its
ingoing or outgoing interfaces. The same choice MUST be made for every
PCN-node in one PCN-domain. 

Classify function:

If a packet's ECN/DSCP fields have the value PCN, as defined in
[RFC-encoding], then it MUST be classified as a PCN-packet.

If a packet's ECN/DSCP fields do not have the value PCN, as defined in
[RFC-encoding], then it MUST NOT be classified as a PCN-packet.

Meter function:

Note: The meter is described as a 'token bucket with threshold', however
the implementation is not standardised. For example, it could equally
well be implemented as a virtual queue (which operates with negative
tokens).

A packet classified as a PCN-packet MUST be metered as follows. 

The interface MUST have:

[1] a configured bit rate, termed PCN-lower-rate

[2] a meter for PCN-packets, which MUST have the following behaviour:

a token bucket, which 

1A tokens are added at the PCN-lower-rate, to a maximum TB.size

1B tokens are removed equal to the size of the PCN-packet, to a minimum
TB.size=0

2 if TB.fill < TB.threshold, then the meter indicates "admission mark"
to the Mark function

Note: Step 1A can be triggered by a packet and so can be done at the
same time as Step 1B.

Note: Step 2 can be performed before or after step 1

If TB.fill < size of the PCN-packet, then the meter MAY:

a. leave TB.fill unaltered and indicate "admission mark" to the Mark
function

b. set TB.fill = 0 and indicate "admission mark" to the Mark function

c. set TB.fill = 0 and check if TB.fill < TB.threshold and if it is,
then indicate "admission mark" to the Mark function        

Note: Option a is expected to be the simplest to implement. 

Mark function:

If the meter indicates to "admission mark" a PCN-packet, then:

[1] this step is REQUIRED, unless [RFC-encoding] standardises an
encoding that allows a PCN-packet to be simultaneously "admission
marked" and "termination marked":

If the PCN-packet's codepoint is already "termination mark" then it MUST
NOT be altered, ie the following step is not performed.

[2] the PCN-packet's ECN/DSCP fields MUST be set to the codepoint
"admission marked", as defined in [RFC-encoding]

we could also add an example algorithm, such as that in
http://www.ietf.org/internet-drafts/draft-charny-pcn-comparison-00.txt
Fig 10.1
<http://www.ietf.org/internet-drafts/draft-charny-pcn-comparison-00.txt%
20Fig%2010.1>  on page 21.

Comments?

phil

[PCN] PCN admission marking algorithm - consensus?