Subject: [tcpm] TCP friendly formulation in CUBIC-draft ?

From: Karen Elisabeth Egede Nielsen <[email-masked]>
Date: Tue, 15 Mar 2016 09:19:54 +0100

Hi Lisong, Neal, All,

I understand the basic principle of the TCP friendliness function and I
have no problem

with having the window follow TCP New Reno when the CUBIC algorithm would
return a smaller value.

I.e., following section 1 of the draft:

When the cubic

   function grows slower than the window of Standard TCP, CUBIC simply

   follows the window size of Standard TCP to ensure fairness to

   Standard TCP in a small BDP network.

With the disclaimer that I have not studied reference [FHP00] I have,
however, a problem understanding how this is achieved

with formula (Eq. 4) of section 3.2 of draft-ietf-tcpm-cubic-01.

>From whatever in [FDP00] one get that the average window of AIMD TCP CC is

       AVG_W_aimd = [ alpha_aimd * (1+beta_aimd) /

                      (2*(1-beta_aimd)*p) ]^0.5          (eq. 3)

which with alpha = 1 and beta = 0,5 indeed yields AVG_W_Newreno =
(1.5/p)^0.5. (Good otherwise we were in trouble).

One can further observe from (3), of course,  that generally for AIMD
CC with  alpha equal to

   3*(1-beta)/(1+beta) then one get the same average window as of NewReno
TCP.  But how does it follow that

W_aimd(t) = W_max*beta_aimd +

                   [3*(1-beta_aimd)/(1+beta_aimd)] * (t/RTT) (Eq. 4)

is a reasonable estimate for the window of standard TCP CC as quoted as
being the intention in section 1 ?

QUESTION:

Is (Eq. 4) the best implementation choice for comparison with TCP New Reno
that were manageable from an implementation perspective or is (Eq 4) the
rule that a CUBIC implementation

should aim to follow ? I.e., that the Linux TCP implementation aims to
follow ?

In the latter case, then I think one should probably use the following
formulation in Section 1:

When the cubic

   function grows slower than the window of standard AIMD CC, CUBIC simply

   follows the window size of standard AIMD CC to ensure fairness to

   Standard TCP in a small BDP network.

And then probably relate to that the fairness comes (?) from calibration
towards same average window - - -

Thanks !

BR, Karen

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*Karen Egede Nielsen*

Software Architect, Ph.D.

*Tieto Denmark A/S*

R&D, Telecom & Media

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Direct Phone / Mobile +45 25134336

E-mail: [email-masked]

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