CCID-2's cwnd increases like TCP during slow-start, which has implications for
* the local Sequence Window value (should be > cwnd),
* the Ack Ratio value.
Hence an exponential growth, if it does not reflect the actual network
conditions, can quickly lead to instability.
This patch adds congestion-window validation (RFC2861) to CCID-2:
* cwnd is constrained if the sender is application limited;
* cwnd is reduced after a long idle period, as suggested in the '90 paper
by Van Jacobson, in RFC 2581 (sec. 4.1);
* cwnd is never reduced below the RFC 3390 initial window.
As marked in the comments, the code is actually almost a direct copy of the
TCP congestion-window-validation algorithms. By continuing this work, it may
in future be possible to use the TCP code (not possible at the moment).
The mechanism can be turned off using a module parameter. Sampling of the
currently-used window (moving-maximum) is however done constantly; this is
used to determine the expected window, which can be exploited to regulate
DCCP's Sequence Window value.
This patch also sets slow-start-after-idle (RFC 4341, 5.1), i.e. it behaves like
TCP when net.ipv4.tcp_slow_start_after_idle = 1.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
This replaces a switch statement with a test, using the equivalent
function dccp_data_packet(skb). It also doubles the range of the field
`rx_num_data_pkts' by changing the type from `int' to `u32', avoiding
signed/unsigned comparison with the u16 field `dccps_r_ack_ratio'.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
This moves CCID-2's initial window function into the header file, since several
parts throughout the CCID-2 code need to call it (CCID-2 still uses RFC 3390).
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Acked-by: Leandro Melo de Sales <leandro@ic.ufal.br>
This patch replaces an almost identical replication of code: large parts
of dccp_parse_options() re-appeared as ccid2_ackvector() in ccid2.c.
Apart from the duplication, this caused two more problems:
1. CCIDs should not need to be concerned with parsing header options;
2. one can not assume that Ack Vectors appear as a contiguous area within an
skb, it is legal to insert other options and/or padding in between. The
current code would throw an error and stop reading in such a case.
Since Ack Vectors provide CCID-specific information, they are now processed
by the CCID directly, separating this functionality from the main DCCP code.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
This patch brings the Ack Vector interface up to date. Its main purpose is
to lay the basis for the subsequent patches of this set, which will use the
new data structure fields and routines.
There are no real algorithmic changes, rather an adaptation:
(1) Replaced the static Ack Vector size (2) with a #define so that it can
be adapted (with low loss / Ack Ratio, a value of 1 works, so 2 seems
to be sufficient for the moment) and added a solution so that computing
the ECN nonce will continue to work - even with larger Ack Vectors.
(2) Replaced the #defines for Ack Vector states with a complete enum.
(3) Replaced #defines to compute Ack Vector length and state with general
purpose routines (inlines), and updated code to use these.
(4) Added a `tail' field (conversion to circular buffer in subsequent patch).
(5) Updated the (outdated) documentation for Ack Vector struct.
(6) All sequence number containers now trimmed to 48 bits.
(7) Removal of unused bits:
* removed dccpav_ack_nonce from struct dccp_ackvec, since this is already
redundantly stored in the `dccpavr_ack_nonce' (of Ack Vector record);
* removed Elapsed Time for Ack Vectors (it was nowhere used);
* replaced semantics of dccpavr_sent_len with dccpavr_ack_runlen, since
the code needs to be able to remember the old run length;
* reduced the de-/allocation routines (redundant / duplicate tests).
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
This updates CCID-2 to use the CCID dequeuing mechanism, converting from
previous continuous-polling to a now event-driven mechanism.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>
This patch reorganises the return value convention of the CCID TX sending
function, to permit more flexible schemes, as required by subsequent patches.
Currently the convention is
* values < 0 mean error,
* a value == 0 means "send now", and
* a value x > 0 means "send in x milliseconds".
The patch provides symbolic constants and a function to interpret return values.
In addition, it caps the maximum positive return value to 0xFFFF milliseconds,
corresponding to 65.535 seconds. This is possible since in CCID-3/4 the
maximum possible inter-packet gap is fixed at t_mbi = 64 sec.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>
This removes the argument `more' from ccid_hc_tx_packet_sent, since it was
nowhere used in the entire code.
(Btw, this argument was not even used in the original KAME code where the
function initially came from; compare the variable moreToSend in the
freebsd61-dccp-kame-28.08.2006.patch kept by Emmanuel Lochin.)
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Change "return (EXPR);" to "return EXPR;"
return is not a function, parentheses are not required.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
The `options_received' struct is redundant, since it re-duplicates the existing
`p' and `x_recv' fields. This patch removes the sub-struct and migrates the
format conversion operations to ccid3_hc_tx_parse_options().
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
This adds a function to take care of the following, separate cases occurring in
the computation of the Loss Rate p:
* 1/(2^32-1) is mapped into 0% as per RFC 4342, 8.5;
* 1/0 is mapped into 100%, the maximum;
* to avoid that p = 1/x is rounded down to 0 when x is very large, since this
means accidentally re-entering slow-start indicated by p == 0, the minimum
resolution value of p is now returned instead;
* a bug in ccid3_hc_rx_getsockopt is fixed: 1/0 was mapped into ~0U.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
This patch is thanks to an investigation by Leandro Sales de Melo and his
colleagues. They worked out two state diagrams which highlight the fact that
the xxx_TERM states in CCID-3/4 are in fact not necessary.
And this can be confirmed by in turn looking at the code: the xxx_TERM states
are only ever set in ccid3_hc_{rx,tx}_exit(): when CCID-3 sets the state
to xxx_TERM, it is at a time where no more processing should be going on,
hence it is not necessary to introduce a dedicated exit state - this is already
implied by unloading the CCID.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
This
1. adds packet type information to ccid_hc_{rx,tx}_parse_options(). This is
necessary, since table 3 in RFC 4340, 5.8 leaves it to the CCIDs to state
which options may (not) appear on what packet type.
2. adds such a check for CCID-3's {Loss Event, Receive} Rate as specified in
RFC 4340 8.3 ("Receive Rate options MUST NOT be sent on DCCP-Data packets")
and 8.5 ("Loss Event Rate options MUST NOT be sent on DCCP-Data packets").
3. removes an unused argument `idx' from ccid_hc_{rx,tx}_parse_options(). This
is also no longer necessary, since the CCID-specific option-parsing routines
are passed every single parameter of the type-length-value option encoding.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
This simplifies and consolidates the TX option-parsing code:
1. The Loss Intervals option is not currently used, so dead code related to
this option is removed. I am aware of no plans to support the option, but
if someone wants to implement it (e.g. for inter-op tests), it is better
to start afresh than having to also update currently unused code.
2. The Loss Event and Receive Rate options have a lot of code in common (both
are 32 bit, both have same length etc.), so this is consolidated.
3. The test against GSR is not necessary, because
- on first loading CCID3, ccid_new() zeroes out all fields in the socket;
- ccid3_hc_tx_packet_recv() treats 0 and ~0U equivalently, due to
pinv = opt_recv->ccid3or_loss_event_rate;
if (pinv == ~0U || pinv == 0)
hctx->p = 0;
- as a result, the sequence number field is removed from opt_recv.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
This removes the RTT-sampling function tfrc_tx_hist_rtt(), since
1. it suffered from complex passing of return values (the return value both
indicated successful lookup while the value doubled as RTT sample);
2. when for some odd reason the sample value equalled 0, this triggered a bug
warning about "bogus Ack", due to the ambiguity of the return value;
3. on a passive host which has not sent anything the TX history is empty and
thus will lead to unwanted "bogus Ack" warnings such as
ccid3_hc_tx_packet_recv: server(e7b7d518): DATAACK with bogus ACK-28197148
ccid3_hc_tx_packet_recv: server(e7b7d518): DATAACK with bogus ACK-26641606.
The fix is to replace the implicit encoding by performing the steps manually.
Furthermore, the "bogus Ack" warning has been removed, since it can actually be
triggered due to several reasons (network reordering, old packet, (3) above),
hence it is not very useful.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
This fixes a subtle bug in the calculation of the inter-packet gap and shows
that t_delta, as it is currently used, is not needed.
The algorithm from RFC 5348, 8.3 below continually computes a send time t_nom,
which is initialised with the current time t_now; t_gran = 1E6 / HZ specifies
the scheduling granularity, s the packet size, and X the sending rate:
t_distance = t_nom - t_now; // in microseconds
t_delta = min(t_ipi, t_gran) / 2; // `delta' parameter in microseconds
if (t_distance >= t_delta) {
reschedule after (t_distance / 1000) milliseconds;
} else {
t_ipi = s / X; // inter-packet interval in usec
t_nom += t_ipi; // compute the next send time
send packet now;
}
Problem:
--------
Rescheduling requires a conversion into milliseconds (sk_reset_timer()). The
highest jiffy resolution with HZ=1000 is 1 millisecond, so using a higher
granularity does not make much sense here.
As a consequence, values of t_distance < 1000 are truncated to 0. This issue
has so far been resolved by using instead
if (t_distance >= t_delta + 1000)
reschedule after (t_distance / 1000) milliseconds;
This is unnecessarily large, a lower bound is t_delta' = max(t_delta, 1000).
And it implies a further simplification:
a) when HZ >= 500, then t_delta <= t_gran/2 = 10^6/(2*HZ) <= 1000, so that
t_delta' = MAX(1000, t_delta) = 1000 (constant value);
b) when HZ < 500, then t_delta = 1/2*MIN(rtt, t_ipi, t_gran) <= t_gran/2,
so that 1000 <= t_delta' <= t_gran/2.
The maximum error of using a constant t_delta in (b) is less than half a jiffy.
Fix:
----
The patch replaces t_delta with a constant, whose value depends on CONFIG_HZ,
changing the above algorithm to:
if (t_distance >= t_delta')
reschedule after (t_distance / 1000) milliseconds;
where t_delta' = 10^6/(2*HZ) if HZ < 500, and t_delta' = 1000 otherwise.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
This makes RTAX_RTO_MIN also available to CCID-3, replacing the compile-time
RTO lower bound with a per-route tunable value.
The original Kconfig option solved the problem that a very low RTT (in the
order of HZ) can trigger too frequent and unnecessary reductions of the
sending rate.
This tunable does not affect the initial RTO value of 2 seconds specified in
RFC 5348, section 4.2 and Appendix B. But like the hardcoded Kconfig value,
it allows to adapt to network conditions.
The same effect as the original Kconfig option of 100ms is now achieved by
> ip route replace to unicast 192.168.0.0/24 rto_min 100j dev eth0
(assuming HZ=1000).
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>
Using a fixed RTO_MIN of 0.2 seconds was found to cause problems for CCID-2
over 802.11g: at least once per session there was a spurious timeout. It
helped to then increase the the value of RTO_MIN over this link.
Since the problem is the same as in TCP, this patch makes the solution from
commit "05bb1fad1cde025a864a90cfeb98dcbefe78a44a"
"[TCP]: Allow minimum RTO to be configurable via routing metrics."
available to DCCP.
This avoids reinventing the wheel, so that e.g. the following works in the
expected way now also for CCID-2:
> ip route change 10.0.0.2 rto_min 800 dev ath0
Luckily this useful rto_min function was recently moved to net/tcp.h,
which simplifies sharing code originating from TCP.
Documentation also updated (plus minor whitespace fixes).
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>
This patch consolidates initial-window code common to TCP and CCID-2:
* TCP uses RFC 3390 in a packet-oriented manner (tcp_input.c) and
* CCID-2 uses RFC 3390 in packet-oriented manner (RFC 4341).
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>
This removes the wrappers around the sk timer functions, since not much is
gained from using them: the BUG_ON in start_rto_timer will never trigger
since that function is called only if:
* the RTO timer expires (rto_expire, and then timer_pending() is false);
* in tx_packet_sent only if !timer_pending() (BUG_ON is redundant here);
* previously in new_ack, after stopping the timer (timer_pending() false).
Removing the wrappers also clears the way for eventually replacing the
RTO timer with the icsk-retransmission-timer, as it is already part of the
DCCP socket.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>
Since CCID-2 is de facto a mini implementation of TCP, it makes sense to share
as much code as possible.
Hence this patch aligns CCID-2 timestamping with TCP timestamping.
This also halves the space consumption (on 64-bit systems).
The necessary include file <net/tcp.h> is already included by way of
net/dccp.h. Redundant includes have been removed.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>
The current CCID-2 RTT estimator code is in parts broken and lags behind the
suggestions in RFC2988 of using scaled variants for SRTT/RTTVAR.
That code is replaced by the present patch, which reuses the Linux TCP RTT
estimator code.
Further details:
----------------
1. The minimum RTO of previously one second has been replaced with TCP's, since
RFC4341, sec. 5 says that the minimum of 1 sec. (suggested in RFC2988, 2.4)
is not necessary. Instead, the TCP_RTO_MIN is used, which agrees with DCCP's
concept of a default RTT (RFC 4340, 3.4).
2. The maximum RTO has been set to DCCP_RTO_MAX (64 sec), which agrees with
RFC2988, (2.5).
3. De-inlined the function ccid2_new_ack().
4. Added a FIXME: the RTT is sampled several times per Ack Vector, which will
give the wrong estimate. It should be replaced with one sample per Ack.
However, at the moment this can not be resolved easily, since
- it depends on TX history code (which also needs some work),
- the cleanest solution is not to use the `sent' time at all (saves 4 bytes
per entry) and use DCCP timestamps / elapsed time to estimated the RTT,
which however is non-trivial to get right (but needs to be done).
Reasons for reusing the Linux TCP estimator algorithm:
------------------------------------------------------
Some time was spent to find a better alternative, using basic RFC2988 as a first
step. Further analysis and experimentation showed that the Linux TCP RTO
estimator is superior to a basic RFC2988 implementation. A summary is on
http://www.erg.abdn.ac.uk/users/gerrit/dccp/notes/ccid2/rto_estimator/
In addition, this estimator fared well in a recent empirical evaluation:
Rewaskar, Sushant, Jasleen Kaur and F. Donelson Smith.
A Performance Study of Loss Detection/Recovery in Real-world TCP
Implementations. Proceedings of 15th IEEE International
Conference on Network Protocols (ICNP-07), 2007.
Thus there is significant benefit in reusing the existing TCP code.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>
This removes the dec_pipe function and improves the way the RTO timer is rearmed
when a new acknowledgment comes in.
Details and justification for removal:
--------------------------------------
1) The BUG_ON in dec_pipe is never triggered: pipe is only decremented for TX
history entries between tail and head, for which it had previously been
incremented in tx_packet_sent; and it is not decremented twice for the same
entry, since it is
- either decremented when a corresponding Ack Vector cell in state 0 or 1
was received (and then ccid2s_acked==1),
- or it is decremented when ccid2s_acked==0, as part of the loss detection
in tx_packet_recv (and hence it can not have been decremented earlier).
2) Restarting the RTO timer happens for every single entry in each Ack Vector
parsed by tx_packet_recv (according to RFC 4340, 11.4 this can happen up to
16192 times per Ack Vector).
3) The RTO timer should not be restarted when all outstanding data has been
acknowledged. This is currently done similar to (2), in dec_pipe, when
pipe has reached 0.
The patch onsolidates the code which rearms the RTO timer, combining the
segments from new_ack and dec_pipe. As a result, the code becomes clearer
(compare with tcp_rearm_rto()).
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>
This removes the ccid2_hc_tx_check_sanity function: it is redundant.
Details:
The tx_check_sanity function performs three tests:
1) it checks that the circular TX list is sorted
- in ascending order of sequence number (ccid2s_seq)
- and time (ccid2s_sent),
- in the direction from `tail' (hctx_seqt) to `head' (hctx_seqh);
2) it ensures that the entire list has the length seqbufc * CCID2_SEQBUF_LEN;
3) it ensures that pipe equals the number of packets that were not
marked `acked' (ccid2s_acked) between `tail' and `head'.
The following argues that each of these tests is redundant, this can be verified
by going through the code.
(1) is not necessary, since both time and GSS increase from one packet to the
next, so that subsequent insertions in tx_packet_sent (which advance the `head'
pointer) will be in ascending order of time and sequence number.
In (2), the length of the list is always equal to seqbufc times CCID2_SEQBUF_LEN
(set to 1024) unless allocation caused an earlier failure, because:
* at initialisation (tx_init), there is one chunk of size 1024 and seqbufc=1;
* subsequent calls to tx_alloc_seq take place whenever head->next == tail in
tx_packet_sent; then a new chunk of size 1024 is inserted between head and
tail, and seqbufc is incremented by one.
To show that (3) is redundant requires looking at two cases.
The `pipe' variable of the TX socket is incremented only in tx_packet_sent, and
decremented in tx_packet_recv. When head == tail (TX history empty) then pipe
should be 0, which is the case directly after initialisation and after a
retransmission timeout has occurred (ccid2_hc_tx_rto_expire).
The first case involves parsing Ack Vectors for packets recorded in the live
portion of the buffer, between tail and head. For each packet marked by the
receiver as received (state 0) or ECN-marked (state 1), pipe is decremented by
one, so for all such packets the BUG_ON in tx_check_sanity will not trigger.
The second case is the loss detection in the second half of tx_packet_recv,
below the comment "Check for NUMDUPACK".
The first while-loop here ensures that the sequence number of `seqp' is either
above or equal to `high_ack', or otherwise equal to the highest sequence number
sent so far (of the entry head->prev, as head points to the next unsent entry).
The next while-loop ("while (1)") counts the number of acked packets starting
from that position of seqp, going backwards in the direction from head->prev to
tail. If NUMDUPACK=3 such packets were counted within this loop, `seqp' points
to the last acknowledged packet of these, and the "if (done == NUMDUPACK)" block
is entered next.
The while-loop contained within that block in turn traverses the list backwards,
from head to tail; the position of `seqp' is saved in the variable `last_acked'.
For each packet not marked as `acked', a congestion event is triggered within
the loop, and pipe is decremented. The loop terminates when `seqp' has reached
`tail', whereupon tail is set to the position previously stored in `last_acked'.
Thus, between `last_acked' and the previous position of `tail',
- pipe has been decremented earlier if the packet was marked as state 0 or 1;
- pipe was decremented if the packet was not marked as acked.
That is, pipe has been decremented by the number of packets between `last_acked'
and the previous position of `tail'. As a consequence, pipe now again reflects
the number of packets which have not (yet) been acked between the new position
of tail (at `last_acked') and head->prev, or 0 if head==tail. The result is that
the BUG_ON condition in check_sanity will also not be triggered, hence the test
(3) is also redundant.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>
Gerrit Renker
David S. Miller
Eric Dumazet