Looks good in theory. Will check some more. Have you double checked
against RFC4342 as that overrides parts of RFC 3448 (see section 5 in
particular)?
Ian
On 11/22/06, Gerrit Renker <gerrit@xxxxxxxxxxxxxx> wrote:
[DCCP]: Fix bug in calculation of first t_nom and first t_ipi
Problem:
--------
Currently the inter-packet-interval for the first packet is set to 500msec instead
of the 1 second specified in RFC 3448 (detailed derivation below).
Furthermore, the code is complicated, since t_ipi is, in fact, added twice to t_0 = t_now.
Solution:
---------
This patch
* sets the initial value of t_ipi to 1 second, as per RFC 3448
* simplifies the initial setting of t_ipi and t_delta through a case analysis
Detailed Derivation:
--------------------
When the first CCID 3 packet is sent in ccid3_hc_tx_send_packet, t_ipi is set to
hctx->ccid3hctx_t_ipi = TFRC_INITIAL_IPI;
which is USEC_PER_SEC / 4 = 250 msec. This is then added twice to t_0, due to:
case TFRC_SSTATE_NO_SENT:
// ...
/* Set nominal send time for initial packet */
hctx->ccid3hctx_t_nom = now;
timeval_add_usecs(&hctx->ccid3hctx_t_nom, hctx->ccid3hctx_t_ipi);
rc = 0;
break;
// ...
/* Can we send? if so add options and add to packet history */
if (rc == 0) {
// ...
timeval_add_usecs(&hctx->ccid3hctx_t_nom, hctx->ccid3hctx_t_ipi);
}
As a result, t_1 = t_0 + t_ipi = t_now + t_ipi
= t_0 + 500msec
This contradicts RFC 3448, due to the following requirements:
a) Section 4.2, "Sender Initialization"
"the value of X[/s] is set to 1 packet/second [...] The
initial values for R (RTT) and t_RTO are undefined"
b) Section 4.5, "Preventing Oscillations
R_sqmean is undefined since R is undefined
Therefore, X_init = X
c) Section 4.6, "Scheduling of Packet Transmissions"
t_ipi = X_init / s = 1 second
A further simplification is possible for the initial `delta' value, which is
defined in [RFC 3448, section 4.6] as
t_delta = min(t_ipi/2, t_gran/2)
t_gran/2 here is TFRC_OPSYS_HALF_TIME_GRAN = USEC_PER_SEC / (2 * HZ).
Since Linux always schedules with a higher granularity than 1 second,
the initial value of t_delta is thus simply
t_delta = min(USEC_PER_SEC / 2 , USEC_PER_SEC / (2 * HZ))
= USEC_PER_SEC / (2 * HZ)
Lastly, TFRC_INITIAL_IPI is not referenced anywhere else and can thus be removed.
Signed-off-by: Gerrit Renker <gerrit@xxxxxxxxxxxxxx>
--
Ian McDonald
Web: http://wand.net.nz/~iam4
Blog: http://imcdnzl.blogspot.com
WAND Network Research Group
Department of Computer Science
University of Waikato
New Zealand
-
To unsubscribe from this list: send the line "unsubscribe dccp" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
