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tcp: Fix delayed ack

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When doing tcp rx testing, I saw a lot of retransmission because of the
delayed ACK.  Our current delayed ACK algorithm does not comply with
what RFC 1122 suggests.

As described in RFC 1122, a host may delay sending an ACK response by up
to 500 ms. Additionally, with a stream of full-sized incoming segments,
ACK responses must be sent for every second segment.

=== Before ===
[asias@hjpc pingpong]$ go run client-rxrx.go
Bytes Sent(MiB):  100
Total Time(Secs):  322.620879376
Bandwidth(MiB/Sec):  0.30996133974160595

78 2.412385 192.168.66.100 -> 192.168.66.123 TCP 32174 37672 > 10000
   [ACK] Seq=2149425323 Ack=1000001 Win=229 Len=32120
79 2.612985 192.168.66.100 -> 192.168.66.123 TCP 1514 [TCP Retransmission]
   37672 > 10000 [ACK] Seq=2149425323 Ack=1000001 Win=229 Len=1460
80 2.613131 192.168.66.123 -> 192.168.66.100 TCP 54 10000 > 37672
   [ACK] Seq=1000001 Ack=2149457443 Win=29200 Len=0

=== After ===
[asias@hjpc pingpong]$ go run client-rxrx.go
Bytes Sent(MiB):  100
Total Time(Secs):  0.244951095
Bandwidth(MiB/Sec):  408.2447559583271

No retransmission is seen.
This commit is contained in:
Asias He
2014-11-12 18:09:22 +08:00
committed by Avi Kivity
parent 8e47ed8b06
commit ee023f4f84
1 changed files with 28 additions and 7 deletions
Download Patch File Download Diff File Expand all files Collapse all files

35
net/tcp.hh
View File

@@ -207,6 +207,7 @@ private:
std::chrono::milliseconds _retransmit_timeout{3000};
static constexpr uint16_t _max_nr_retransmit{5};
timer _retransmit;
uint16_t _nr_full_seg_received = 0;
public:
tcb(tcp& t, connid id);
void input(tcp_hdr* th, packet p);
@@ -226,7 +227,7 @@ private:
void merge_out_of_order();
void insert_out_of_order(tcp_seq seq, packet p);
void trim_receive_data_after_window();
bool should_send_ack();
bool should_send_ack(uint16_t seg_len);
void clear_delayed_ack();
packet get_transmit_packet();
void start_retransmit_timer() { _retransmit.rearm(clock_type::now() + _retransmit_timeout); };
@@ -390,7 +391,7 @@ tcp<InetTraits>::tcb::tcb(tcp& t, connid id)
, _foreign_ip(id.foreign_ip)
, _local_port(id.local_port)
, _foreign_port(id.foreign_port) {
_delayed_ack.set_callback([this] { output(); });
_delayed_ack.set_callback([this] { _nr_full_seg_received = 0; output(); });
_retransmit.set_callback([this] { retransmit(); });
}
@@ -508,7 +509,7 @@ void tcp<InetTraits>::tcb::input(tcp_hdr* th, packet p) {
}
}
// Send <ACK> to ack data only when data is present in this packet
do_output = should_send_ack();
do_output = should_send_ack(seg_len);
}
}
if (th->f_fin) {
@@ -616,6 +617,8 @@ void tcp<InetTraits>::tcb::input(tcp_hdr* th, packet p) {
}
// send some stuff
if (do_output || (do_output_data && can_send())) {
// Since we will do output, we can canncel scheduled delayed ACK.
clear_delayed_ack();
output();
}
}
@@ -795,13 +798,31 @@ void tcp<InetTraits>::tcb::close() {
}
template <typename InetTraits>
bool tcp<InetTraits>::tcb::should_send_ack() {
if (_delayed_ack.cancel()) {
bool tcp<InetTraits>::tcb::should_send_ack(uint16_t seg_len) {
// We've received a TSO packet, do ack immediately
if (seg_len > _rcv.mss) {
_nr_full_seg_received = 0;
_delayed_ack.cancel();
return true;
} else {
_delayed_ack.arm(400ms);
}
// We've received a full sized segment, ack for every second full sized segment
if (seg_len == _rcv.mss) {
if (_nr_full_seg_received++ >= 1) {
_nr_full_seg_received = 0;
_delayed_ack.cancel();
return true;
}
}
// If the timer is armed and its callback hasn't been run.
if (_delayed_ack.armed()) {
return false;
}
// If the timer is not armed, schedule a delayed ACK.
_delayed_ack.arm(500ms);
return false;
}
template <typename InetTraits>