Gerrit Renker 53ac9570c8 dccp ccid-3: Simplify computing and range-checking of t_ipi ... This patch simplifies the computation of t_ipi, avoiding expensive computations to enforce the minimum sending rate. Both RFC 3448 and rfc3448bis (revision #06), as well as RFC 4342 sec 5., require at various stages that at least one packet must be sent per t_mbi = 64 seconds. This requires frequent divisions of the type X_min = s/t_mbi, which are later converted back into an inter-packet-interval t_ipi_max = s/X_min = t_mbi. The patch removes the expensive indirection; in the unlikely case of having a sending rate less than one packet per 64 seconds, it also re-adjusts X. The following cases document conformance with RFC 3448 / rfc3448bis-06: 1) Time until receiving the first feedback packet: * if the sender has no initial RTT sample then X = s/1 Bps > s/t_mbi; * if the sender has an initial RTT sample or when the first feedback packet is received, X = W_init/R > s/t_mbi. 2) Slow-start (p == 0 and feedback packets come in): * RFC 3448 (current code) enforces a minimum of s/R > s/t_mbi; * rfc3448bis (future code) enforces an even higher minimum of W_init/R. 3) Congestion avoidance with no absence of feedback (p > 0): * when X_calc or X_recv/2 are too low, the minimum of X_min = s/t_mbi is enforced in update_x() when calling update_send_interval(); * update_send_interval() is, as before, only called when X changes (i.e. either when increasing or decreasing, not when in equilibrium). 4) Reduction of X without prior feedback or during slow-start (p==0): * both RFC 3448 and rfc3448bis here halve X directly; * the associated constraint X >= s/t_mbi is nforced here by send_interval(). 5) Reduction of X when p > 0: * X is modified indirectly via X_recv (RFC 3448) or X_recv_set (rfc3448bis); * in both cases, control goes back to section 4.3 (in both documents); * since p > 0, both documents use X = max(min(...), s/t_mbi), which is enforced in this patch by calling send_interval() from update_x(). I think that this analysis is exhaustive. Should I have forgotten a case, the worst-case consideration arises when X sinks below s/t_mbi, and is then increased back up to this minimum value. Even under this assumption, the behaviour is correct, since all lower limits of X in RFC 3448 / rfc3448bis are either equal to or greater than s/t_mbi. Note on the condition X >= s/t_mbi <==> t_ipi = s/X <= t_mbi: since X is scaled by 64, and all time units are in microseconds, the coded condition is: t_ipi = s * 64 * 10^6 usec / X <= 64 * 10^6 usec This simplifies to s / X <= 1 second <==> X * 1 second >= s > 0. (A zero `s' is not allowed by the CCID-3 code). Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>		2008-09-04 07:45:43 +02:00
..
lib	dccp ccid-3: Update the RX history records in one place	2008-09-04 07:45:42 +02:00
ccid2.c	tcp/dccp: Consolidate common code for RFC 3390 conversion	2008-09-04 07:45:39 +02:00
ccid2.h	dccp ccid-2: Replace broken RTT estimator with better algorithm	2008-09-04 07:45:39 +02:00
ccid3.c	dccp ccid-3: Simplify computing and range-checking of t_ipi	2008-09-04 07:45:43 +02:00
ccid3.h	dccp ccid-3: Always perform receiver RTT sampling	2008-09-04 07:45:41 +02:00
Kconfig	dccp ccid-3: Measuring the packet size s with regard to rfc3448bis-06	2008-09-04 07:45:42 +02:00
Makefile	[DCCP] CCID2: Initial CCID2 (TCP-Like) implementation	2006-03-20 17:41:47 -08:00