No code change, cosmetical changes only:
* whitespace cleanup via scripts/cleanfile,
* remove self-references to filename at top of files,
* fix coding style (extraneous brackets),
* fix documentation style (kernel-doc-nano-HOWTO).
Thanks are due to Ivo Augusto Calado who raised these issues by
submitting good-quality patches.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>
String literals are constant, and usually, we can also tag the array
of pointers const too, moving it to the .rodata section.
Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: David S. Miller <davem@davemloft.net>
Thanks to Wei and Arnaldo for pointing out the correct
new reference for CCID-3.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>
Removed the __exit annotation of tfrc_lib_exit(), in order to suppress the following section mismatch messages:
WARNING: net/dccp/dccp.o(.text+0xd9): Section mismatch in reference from the function ccid_cleanup_builtins() to the function .exit.text:tfrc_lib_exit()
The function ccid_cleanup_builtins() references a function in an exit section.
Often the function tfrc_lib_exit() has valid usage outside the exit section
and the fix is to remove the __exit annotation of tfrc_lib_exit.
WARNING: net/dccp/dccp.o(.init.text+0x48): Section mismatch in reference from the function ccid_initialize_builtins() to the function .exit.text:tfrc_lib_exit()
The function __init ccid_initialize_builtins() references
a function __exit tfrc_lib_exit().
This is often seen when error handling in the init function
uses functionality in the exit path.
The fix is often to remove the __exit annotation of
tfrc_lib_exit() so it may be used outside an exit section.
Signed-off-by: Leonardo Potenza <lpotenza@inwind.it>
Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>
This patch integrates the TFRC library, which is a dependency of CCID-3 (and
CCID-4), with the new use of CCIDs in the DCCP module.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>
Based on Arnaldo's earlier patch, this patch integrates the standardised
CCID congestion control plugins (CCID-2 and CCID-3) of DCCP with dccp.ko:
* enables a faster connection path by eliminating the need to always go
through the CCID registration lock;
* updates the implementation to use only a single array whose size equals
the number of configured CCIDs instead of the maximum (256);
* since the CCIDs are now fixed array elements, synchronization is no
longer needed, simplifying use and implementation.
CCID-2 is suggested as minimum for a basic DCCP implementation (RFC 4340, 10);
CCID-3 is a standards-track CCID supported by RFC 4342 and RFC 5348.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>
Two registration routines, for SP and NN features, are provided by this patch,
replacing a previous routine which was used for both feature types.
These are internal-only routines and therefore start with `__feat_register'.
It further exports the known limits of Sequence Window and Ack Ratio as symbolic
constants.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Acked-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: David S. Miller <davem@davemloft.net>
This implements [RFC 3448, 4.5], which performs congestion avoidance behaviour
by reducing the transmit rate as the queueing delay (measured in terms of
long-term RTT) increases.
Oscillation can be turned on/off via a module option (do_osc_prev) and via sysfs
(using mode 0644), the default is off.
Overflow analysis:
------------------
* oscillation prevention is done after update_x(), so that t_ipi <= 64000;
* hence the multiplication "t_ipi * sqrt(R_sample)" needs 64 bits;
* done using u64 for sqrt_sample and explicit typecast of t_ipi;
* the divisor, R_sqmean, is non-zero because oscillation prevention is first
called when receiving the second feedback packet, and tfrc_scaled_rtt() > 0.
A detailed discussion of the algorithm (with plots) is on
http://www.erg.abdn.ac.uk/users/gerrit/dccp/notes/ccid3/sender_notes/oscillation_prevention/
The algorithm has negative side effects:
* when allowing to decrease t_ipi (leads to a large RTT) and
* when using it during slow-start;
both uses are therefore disabled.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
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>
rfc3448bis allows three different ways of tracking the packet size `s':
1. using the MSS/MPS (at initialisation, 4.2, and in 4.1 (1));
2. using the average of `s' (in 4.1);
3. using the maximum of `s' (in 4.2).
Instead of hard-coding a single interpretation of rfc3448bis, this implements
a choice of all three alternatives and suggests the first as default, since it
is the option which is most consistent with other parts of the specification.
The patch further deprecates the update of t_ipi whenever `s' changes. The
gains of doing this are only small since a change of s takes effect at the
next instant X is updated:
* when the next feedback comes in (within one RTT or less);
* when the nofeedback timer expires (within at most 4 RTTs).
Further, there are complications caused by updating t_ipi whenever s changes:
* if t_ipi had previously been updated to effect oscillation prevention (4.5),
then it is impossible to make the same adjustment to t_ipi again, thus
counter-acting the algorithm;
* s may be updated any time and a modification of t_ipi depends on the current
state (e.g. no oscillation prevention is done in the absence of feedback);
* in rev-06 of rfc3448bis, there are more possible cases, depending on whether
the sender is in slow-start (t_ipi <= R/W_init), or in congestion-avoidance,
limited by X_recv or the throughput equation (t_ipi <= t_mbi).
Thus there are side effects of always updating t_ipi as s changes. These may not
be desirable. The only case I can think of where such an update makes sense is
to recompute X_calc when p > 0 and when s changes (not done by this patch).
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
The per-CCID menu has several dependencies on EXPERIMENTAL. These are redundant,
since net/dccp/ccids/Kconfig is sourced by net/dccp/Kconfig and since the
latter menu in turn asserts a dependency on EXPERIMENTAL.
The patch removes the redundant dependencies as well as the repeated reference
within the sub-menu.
Further changes:
----------------
Two single dependencies on CCID-3 are replaced with a single enclosing `if'.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
The patch updates CCID-3 with regard to the latest rfc3448bis-06:
* in the first revisions of the draft, MSS was used for the RFC 3390 window;
* then (from revision #1 to revision #2), it used the packet size `s';
* now, in this revision (and apparently final), the value is back to MSS.
This change has an implication for the case when no RTT sample is available,
at the time of sending the first packet:
* with RTT sample, 2*MSS/RTT <= initial_rate <= 4*MSS/RTT;
* without RTT sample, the initial rate is one packet (s bytes) per second
(sec. 4.2), but using s instead of MSS here creates an imbalance, since
this would further reduce the initial sending rate.
Hence the patch uses MSS (called MPS in RFC 4340) in all places.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
This patch is a requirement for enabling ECN support later on. With that change
in mind, the following preparations are done:
* renamed handle_loss() into congestion_event() since it returns true when a
congestion event happens (it will eventually also take care of ECN packets);
* lets tfrc_rx_congestion_event() always update the RX history records, since
this routine needs to be called for each non-duplicate packet anyway;
* made all involved boolean-type functions to have return type `bool';
Updating the RX history records is now only necessary for the packets received
up to sending the first feedback. The receiver code becomes again simpler.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
This updates the computation of X_recv with regard to Errata 610/611 for
RFC 4342 and draft rfc3448bis-06, ensuring that at least an interval of 1
RTT is used to compute X_recv. The change is wrapped into a new function
ccid3_hc_rx_x_recv().
Further changes:
----------------
* feedback is not sent when no data packets arrived (bytes_recv == 0), as per
rfc3448bis-06, 6.2;
* take the timestamp for the feedback /after/ dccp_send_ack() returns, to avoid
taking the transmission time into account (in case layer-2 is busy);
* clearer handling of failure in ccid3_first_li().
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
This improves the receiver RTT sampling algorithm so that it tries harder to get
as many RTT samples as possible.
The algorithm is based the concepts presented in RFC 4340, 8.1, using timestamps
and the CCVal window counter. There exist 4 cases for the CCVal difference:
* == 0: less than RTT/4 passed since last packet -- unusable;
* > 4: (much) more than 1 RTT has passed since last packet -- also unusable;
* == 4: perfect sample (exactly one RTT has passed since last packet);
* 1..3: sub-optimal sample (between RTT/4 and 3*RTT/4 has passed).
In the last case the algorithm tried to optimise by storing away the candidate
and then re-trying next time. The problem is that
* a large number of samples is needed to smooth out the inaccuracies of the
algorithm;
* the sender may not be sending enough packets to warrant a "next time";
* hence it is better to use suboptimal samples whenever possible.
The algorithm now stores away the current sample only if the difference is 0.
Applicability and background
----------------------------
A realistic example is MP3 streaming where packets are sent at a rate of less
than one packet per RTT, which means that suitable samples are absent for a
very long time.
The effectiveness of using suboptimal samples (with a delta between 1 and 4) was
confirmed by instrumenting the algorithm with counters. The results of two 20
second test runs were:
* With the old algorithm and a total of 38442 function calls, only 394 of these
calls resulted in usable RTT samples (about 1%), and 378 out of these were
"perfect" samples and 28013 (unused) samples had a delta of 1..3.
* With the new algorithm and a total of 37057 function calls, 1702 usable RTT
samples were retrieved (about 4.6%), 5 out of these were "perfect" samples.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
This updates the CCID-3 receiver in part with regard to errata 610 and 611
(http://www.rfc-editor.org/errata_list.php), which change RFC 4342 to use the
Receive Rate as specified in rfc3448bis, requiring to constantly sample the
RTT (or use a sender RTT).
Doing this requires reusing the RX history structure after dealing with a loss.
The patch does not resolve how to compute X_recv if the interval is less
than 1 RTT. A FIXME has been added (and is resolved in subsequent patch).
Furthermore, since this is all TFRC-based functionality, the RTT estimation
is now also performed by the dccp_tfrc_lib module. This further simplifies
the CCID-3 code.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
The only state information that the CCID-3 receiver keeps is whether initial
feedback has been sent or not. Further, this overlaps with use of feedback:
* state == TFRC_RSTATE_NO_DATA as long as no feedback has been sent;
* state == TFRC_RSTATE_DATA as soon as the first feedback has been sent.
This patch reduces the duplication, by memorising the type of the last feedback.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
This migrates more TFRC-related code into the dccp_tfrc_lib:
* sampling of the packet size `s' (which is only needed until the first
loss interval is computed (ccid3_first_li));
* updating the byte-counter `bytes_recvd' in between sending feedbacks.
The result is a better separation of CCID-3 specific and TFRC specific
code, which aids future integration with ECN and e.g. CCID-4.
Further changes:
----------------
* replaced magic number of 536 with equivalent constant TCP_MIN_RCVMSS;
(this constant is also used when no estimate for `s' is available).
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
This changes the return type of tfrc_lh_update_i_mean() to void, since that
function returns always `false'. This is due to
len = dccp_delta_seqno(cur->li_seqno, DCCP_SKB_CB(skb)->dccpd_seq) + 1;
if (len - (s64)cur->li_length <= 0) /* duplicate or reordered */
return 0;
which means that update_i_mean can only increase the length of the open loss
interval I_0, and hence the value of I_tot0 (RFC 3448, 5.4). Consequently the
test `i_mean < old_i_mean' at the end of the function always evaluates to false.
There is no known way by which a loss interval can suddenly become shorter,
therefore the return type of the function is changed to void. (That is, under
the given circumstances step (3) in RFC 3448, 6.1 will not occur.)
Further changes:
----------------
* the function is now called from tfrc_rx_handle_loss, which is equivalent
to the previous way of calling from rx_packet_recv (it was called whenever
there was no new or pending loss, now it is also updated when there is
a pending loss - this increases the accuracy a bit);
* added a FIXME to possibly consider NDP counting as per RFC 4342 (this is
not implemented yet).
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
This enables the TFRC code to begin loss detection (as soon as the module
is loaded), using the latest updates from rfc3448bis-06, 6.3.1:
* when the first data packet(s) are lost or marked, set
* X_target = s/(2*R) => f(p) = s/(R * X_target) = 2,
* corresponding to a loss rate of ~ 20.64%.
The handle_loss() function is now called right at the begin of rx_packet_recv()
and thus no longer protected against duplicates: hence a call to rx_duplicate()
has been added. Such a call makes sense now, as the previous patch initialises
the first entry with a sequence number of GSR.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
This patch
1) separates history allocation and initialisation, to facilitate early
loss detection (implemented by a subsequent patch);
2) removes duplication by using the existing tfrc_rx_hist_purge() if the
allocation fails. This is now possible, since the initialisation routine
3) zeroes out the entire history before using it.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
In the congestion-avoidance phase a decay of p towards 0 is natural once fewer
losses are encountered. Hence the warning message "p is below resolution" is
not necessary, and thus turned into a debug message by this patch.
The TFRC_SMALLEST_P is needed since in theory p never actually reaches 0. When
no further losses are encountered, the loss interval I_0 grows in length,
causing p to decrease towards 0, causing X_calc = s/(RTT * f(p)) to increase.
With the given minimum-resolution this congestion avoidance phase stops at some
fixed value, an approximation formula has been added to the documentation.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Since CCIDs are only used during the established phase of a connection,
they have very little internal state; this specifically reduces to:
* "no packet sent" if and only if s == 0, for the TX packet size s;
* when the first packet has been sent (i.e. `s' > 0), the question is whether
or not feedback has been received:
- if a feedback packet is received, "feedback = yes" is set,
- if the nofeedback timer expires, "feedback = no" is set.
Thus the CCID only needs to remember state about whether or not feedback
has been received. This is now implemented using a boolean flag, which is
toggled when a feedback packet arrives or the nofeedback timer expires.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
The DCCP base time resolution is 10 microseconds (RFC 4340, 13.1 ... 13.3).
Using a timer with a lower resolution was found to trigger the following
bug warnings/problems on high-speed networks (e.g. local loopback):
* RTT samples are rounded down to 0 if below resolution;
* in some cases, negative RTT samples were observed;
* the CCID-3 feedback timer complains that the feedback interval is 0,
since the feedback interval is in the order of 1 RTT or less and RTT
measurement rounded this down to 0;
On an Intel computer this will for instance happen when using a
boot-time parameter of "clocksource=jiffies".
The following system log messages were observed:
11:24:00 kernel: BUG: delta (0) <= 0 at ccid3_hc_rx_send_feedback()
11:26:12 kernel: BUG: delta (0) <= 0 at ccid3_hc_rx_send_feedback()
11:26:30 kernel: dccp_sample_rtt: unusable RTT sample 0, using min
11:26:30 last message repeated 5 times
This patch defines a global constant for the time resolution, adds this in
timer.c, and checks the available clock resolution at CCID-3 module load time.
When the resolution is worse than 10 microseconds, module loading exits with
a message "socket type not supported".
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
