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Fix time loss detection and early retransmit #394

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Mar 13, 2017
Merged

Fix time loss detection and early retransmit #394

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876df8f
Fix time loss detection and early retransmit
ianswett Mar 13, 2017
26bd685
Fix trailing whitespace
ianswett Mar 13, 2017
55d791c
Fix inconsistencies and nits
ianswett Mar 13, 2017
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89 changes: 52 additions & 37 deletions draft-ietf-quic-recovery.md
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Original file line number Diff line number Diff line change
Expand Up @@ -184,7 +184,7 @@ kReorderingThreshold (default 3):
: Maximum reordering in packet number space before FACK style loss detection
considers a packet lost.

kTimeReorderingThreshold (default 1/8):
kTimeReorderingFraction (default 1/8):
: Maximum reordering in time sapce before time based loss detection considers
a packet lost. In fraction of an RTT.

Expand All @@ -208,11 +208,6 @@ mechanisms described in this section.
loss_detection_alarm:
: Multi-modal alarm used for loss detection.

alarm_mode:
: QUIC maintains a single loss detection alarm, which switches
between various modes. This mode is used to determine the duration of the
alarm.

handshake_count:
: The number of times the handshake packets have been
retransmitted without receiving an ack.
Expand All @@ -239,14 +234,19 @@ reordering_threshold:
retransmittable packet and a packet containing retransmittable frames before
it's declared lost.

use_time_loss:
: When true, loss detection operates solely based on reordering
threshold in time, rather than in packet number gaps.
time_reordering_fraction:
: The reordering window as a fraction of max(smoothed_rtt, latest_rtt).

loss_time:
: The time at which the next packet will be considered lost based on early
transmit or exceeding the reordering window in time.

sent_packets:
: An association of packet numbers to information about them, including a time
field indicating the time a packet was sent and a bytes field indicating the
packet's size.
: An association of packet numbers to information about them, including a number
field indicating the packet number, a time field indicating the time a packet
was sent, and a bytes field indicating the packet's size. sent_packets is
ordered by packet number, and packets remain in sent_packets until
acknowledged or lost.

## Initialization

Expand All @@ -258,8 +258,13 @@ follows:
handshake_count = 0
tlp_count = 0
rto_count = 0
reordering_threshold = kReorderingThreshold
use_time_loss = false
if (UsingTimeLossDetection())
reordering_threshold = infinite
time_reordering_fraction = kTimeReorderingFraction
else:
reordering_threshold = kReorderingThreshold
time_reordering_fraction = infinite
loss_time = 0
smoothed_rtt = 0
rttvar = 0
initial_rtt = kDefaultInitialRtt
Expand All @@ -285,6 +290,7 @@ Pseudocode for OnPacketSent follows:

~~~
OnPacketSent(packet_number, is_retransmittable, sent_bytes):
sent_packets[packet_number].packet_number = packet_number
sent_packets[packet_number].time = now
if is_retransmittable:
sent_packets[packet_number].bytes = sent_bytes
Expand All @@ -306,8 +312,8 @@ Pseudocode for OnAckReceived and UpdateRtt follow:
rtt_sample -= ack.delay
UpdateRtt(rtt_sample)
// Find all newly acked packets.
for acked_packet in DetermineNewlyAckedPackets():
OnPacketAcked(acked_packet)
for acked_packet_number in DetermineNewlyAckedPackets():
OnPacketAcked(acked_packet_number)

DetectLostPackets(ack.largest_acked_packet)
SetLossDetectionAlarm()
Expand Down Expand Up @@ -336,13 +342,11 @@ as lost.
Pseudocode for OnPacketAcked follows:

~~~
OnPacketAcked(acked_packet):
OnPacketAcked(acked_packet_number):
handshake_count = 0
tlp_count = 0
rto_count = 0
# TODO: Don't remove packets immediately, since they can be
# used for detecting spurous retransmits.
sent_packets.remove(acked_packet)
sent_packets.remove(acked_packet_number)
~~~

## Setting the Loss Detection Alarm
Expand Down Expand Up @@ -406,10 +410,9 @@ Pseudocode for SetLossDetectionAlarm follows:
alarm_duration = 2 * smoothed_rtt
alarm_duration = max(alarm_duration, kMinTLPTimeout)
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I think this comment can fit in a line

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It was removed.

alarm_duration = alarm_duration << handshake_count
else if (largest sent packet is acked):
// Early retransmit
// with an alarm to reduce spurious retransmits.
alarm_duration = 0.25 * smoothed_rtt
else if (loss_time != 0):
// Early retransmit timer or time loss detection.
alarm_duration = loss_time - now
else if (tlp_count < kMaxTLPs):
// Tail Loss Probe
if (retransmittable_packets_outstanding = 1):
Expand Down Expand Up @@ -442,9 +445,9 @@ Pseudocode for OnLossDetectionAlarm follows:
RetransmitAllHandshakePackets();
handshake_count++;
// TODO: Clarify early retransmit and time loss.
else if ():
else if (loss_time != 0):
// Early retransmit or Time Loss Detection
DetectLostPackets(acked_packet)
DetectLostPackets(largest_acked_packet)
else if (tlp_count < kMaxTLPs):
// Tail Loss Probe.
if (HasNewDataToSend()):
Expand All @@ -463,9 +466,9 @@ Pseudocode for OnLossDetectionAlarm follows:
## Detecting Lost Packets

Packets in QUIC are only considered lost once a larger packet number is
acknowledged. DetectLostPackets is called every time there is a new largest
packet or if the loss detection alarm fires the previous largest acked packet is
supplied.
acknowledged. DetectLostPackets is called every time an ack is received.
If the loss detection alarm fires and the loss_time is set, the previous
largest acked packet is supplied.

### Handshake Packets

Expand All @@ -481,19 +484,31 @@ DetectLostPackets takes one parameter, acked, which is the largest acked packet.
Pseudocode for DetectLostPackets follows:

~~~
DetectLostPackets(acked):
DetectLostPackets(largest_acked):
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nit: "largest_acked" --> "largest_acked_number" ?

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It's a packet, not a number, because I use fields of the packet below, such as the time it was sent.

loss_time = 0
lost_packets = {}
foreach (unacked less than acked):
time_delta = acked.time_sent - unacked.time_sent
packet_delta = acked.packet_number - unacked.packet_number
if (time_delta > kTimeReorderThreshold * smoothed_rtt):
delay_until_lost = infinite;
if (time_reordering_fraction != infinite):
delay_until_lost =
(1 + time_reordering_fraction) * max(latest_rtt, smoothed_rtt)
else if (largest_acked.packet_number == largest_sent_packet):
// Early retransmit alarm.
delay_until_lost = 9/8 * max(latest_rtt, smoothed_rtt)
foreach (unacked less than largest_acked.packet_number):
time_since_sent = now() - unacked.time_sent
packet_delta = largest_acked.packet_number - unacked.packet_number
if (time_since_sent > delay_until_lost):
lost_packets.insert(unacked)
else if (packet_delta > reordering_threshold)
lost_packets.insert(unacked)

// Inform the congestion controller of lost packets.
else if (loss_time == 0 && delay_until_lost != infinite):
loss_time = delay_until_lost - time_since_sent

// Inform the congestion controller of lost packets and
// lets it decide whether to retransmit immediately.
OnPacketsLost(lost_packets)
MaybeRetransmit(lost_packets)
foreach (packet in lost_packets)
sent_packets.remove(packet.packet_number)
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does it make sense, for editorializing, to move these packets to a "lost_packets" structure? doesn't need to be done here, but thinking aloud

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Possibly, though I still need a way to inform the congestion control of newly lost packets. I was thinking of creating a pending_retransmissions data structure in congestion control to store any lost packets that couldn't be sent immediately?

~~~

# Congestion Control
Expand Down