Subject: [ippm] Question 3 - draft-ietf-ippm-reordering-07.txt

From: Nischal Piratla <[email-masked]>
Date: Sun, 17 Oct 2004 21:04:42 -0600

Here is the definition for Type-P-Reordered from the draft. To explain this concept, the draft presents two terms namely, s and NextEXp.
It is also stated that NextExp value cannot decrease. And, if Type-P-Reordered is FALSE, then the NextExp value will increase.

The corresponding piece of text from the document is pasted below:

"3.3 Definition:

   If a packet is found to be reordered by comparison with the Next
   Expected value, its Type-P-Reordered = TRUE; otherwise Type-P-
   Reordered = FALSE, as defined below:

   Morton, et al.      Standards Track exp. Feb 2005               Page 6
   Packet Reordering Metric for IPPM                          August 2004 

   The value of Type-P-Reordered is defined as TRUE if s < NextExp (the
   packet is reordered). In this case, NextExp value does not change.

   The value of Type-P-Reordered is defined as FALSE if s >= NextExp
   (the packet is in-order). In this case, NextExp is set to s+1.

   Since the Next Expected value cannot decrease, it provides a non-
   reversing order criterion to identify reordered packets.

   This definition can also be specified in pseudo-code.

   On successful arrival of a packet with sequence number s:
        if s >= NextExp then /* s is in-order */
                NextExp = s + 1;
                Type-P-Reordered = False;
        else     /* when s < NextExp */
                Type-P-Reordered = True"

The question is about the way the Type-P-Reordered is defined. 

Let us consider the example of two packet reordering from the draft.

                   \/ \/
   s = 1, 2, 3, 4, 7, 5, 6, 8, 9, 10
   i = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
                      /\ /\

In this example, will the arrival of Type-P-Reordered be considered TRUE or FALSE, when packet with sequence number '6' arrives?
If it is TRUE then the definition seems to stand good for Reordering-Extent metric where packet '6' is assigned an extent. 
But n-reordering metric doesn't seem to fit the group of Type-P-Reordered metrics, as it does not account for this reordering.
On the other hand, if the Type-P-Reordered is FALSE, then Reordering-Extent doesn't strictly belong to this class as it computes an extent for a packet that is not reordered!

In other words, does Type-P-Reordered mean that the above definition is strictly followed in treating a packet as in-order or out-of-order?
May be I missed something here. Can someone please clarify the justification on having both the metrics as Type-P-Reordered metrics?

Thanks,
Nischal Piratla

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---

From: Al Morton <[email-masked]>
Date: Mon, 18 Oct 2004 00:25:31 -0400

At 11:04 PM 10/17/2004, Nischal Piratla wrote:
>...In other words, does Type-P-Reordered mean that the above definition is 
>strictly followed in treating a packet as in-order or out-of-order?
>May be I missed something here. Can someone please clarify the 
>justification on having both the metrics as Type-P-Reordered metrics?
>
>Thanks,
>Nischal Piratla
Type-P-Reordered is strictly followed, and packet 6 is TRUE = Reordered.

n-reordering exists as a separate metric, with a different name:

5. Metrics Focused on Receiver Assessment: A TCP-Relevant Metric

5.1 Metric Name:

    Type-P-packet-n-Reordering-Stream
    ...
The fact that n-reordered packets are a subset of Type-P-Reordered =TRUE
is stated several times in the draft. For example, at the end of
section 5.4:
...
    A packet's n-reordering is sometimes equal to its reordering extent,
    e. n-reordering is different in the following ways:

    1. n is a count of early packets with consecutive arrival positions
    at the receiver.

    2. Reordered packets may not be n-reordered, but will have an
    extent, e (see the examples).

Al

---

From: Nischal Piratla <[email-masked]>
Date: Mon, 18 Oct 2004 01:22:17 -0600

Al Morton wrote:

>> ...In other words, does Type-P-Reordered mean that the above 
>> definition is strictly followed in treating a packet as in-order or 
>> out-of-order?

Al,
If n-reordered packets are a subset of Type-P-Reordered=TRUE then does 
it mean that n-reordering considers some Type-P-Reordered =TRUE as 
reordered and others as not reordered. However, the definition of 
Type-P-Reordered clearly states when a packet is considered reordered 
and when it is not. With this subset assumption, the difference between 
out-of-order and in-order packets is not clear as far as n-reordering is 
concerned..
Type-P-Reordered is either TRUE or FALSE and if n-reordered packets are 
a subset of TYPE-P-Reordered then it implies that the metric is not 
considering all out-of-order packets as reordered. Is n-reordering 
metric not able to capture reordering completely, as far as the 
definition of Type-P-Reordered goes?

In other words, Type-P-Reordered =TRUE corresponds to reordered packets. 
What does it mean to say that a subset of these packets are considered 
by a reorder metric to measure reordering? Are there two different 
definitions to classify a packet as reordered in this draft?

Comments are appreciated.
Thanks,
Nischal

Voice: +1 970-491-7067/7974

---

From: Al Morton <[email-masked]>
Date: Mon, 18 Oct 2004 08:18:33 -0400

At 03:22 AM 10/18/2004, Nischal Piratla wrote:
>Type-P-Reordered is either TRUE or FALSE and if n-reordered packets are a 
>subset of TYPE-P-Reordered then it implies that the metric is not 
>considering all out-of-order packets as reordered. Is n-reordering metric 
>not able to capture reordering completely, as far as the definition of 
>Type-P-Reordered goes?

The definition of n-reordering does not assign a value of n
for all Type-P-Reordered=TRUE packets. So, the short answer is, yes.

>What does it mean to say that a subset of these packets are considered by 
>a reorder metric to measure reordering? Are there two different 

n-reordering is a special designation of a reordered packet's position
w.r.t. in-order packets, the "n" in n-reordering.
n-reordering's relevance to TCP NewReno was considered important enough
to include this metric. Several folks have found it useful.

hth,
Al