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Add OnPacketSentCC #780

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Sep 19, 2017
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Add OnPacketSentCC #780

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548342b
Add OnPacketSentCC
ianswett Sep 19, 2017
f17d3dc
Fix formatting and minor clarifications
ianswett Sep 19, 2017
a249176
Add more text on Recovery Period
ianswett Sep 19, 2017
463d2ff
Martin's comments from Aug 24th
ianswett Sep 19, 2017
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99 changes: 54 additions & 45 deletions draft-ietf-quic-recovery.md
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Original file line number Diff line number Diff line change
Expand Up @@ -326,23 +326,23 @@ are as follows:

* packet_number: The packet number of the sent packet.

* is_retransmittable: A boolean that indicates whether the packet contains at
least one frame requiring reliable deliver. The retransmittability of various
QUIC frames is described in {{QUIC-TRANSPORT}}. If false, it is still
acceptable for an ack to be received for this packet. However, a caller MUST
NOT set is_retransmittable to true if an ack is not expected.
* is_ack_only: A boolean that indicates whether a packet only contains an
ACK frame. If true, it is still expected an ack will be received for
this packet, but it is not congestion controlled.

* sent_bytes: The number of bytes sent in the packet.
* sent_bytes: The number of bytes sent in the packet, not including UDP or IP
overhead, but including QUIC framing overhead.

Pseudocode for OnPacketSent follows:

~~~
OnPacketSent(packet_number, is_retransmittable, sent_bytes):
OnPacketSent(packet_number, is_ack_only, sent_bytes):
time_of_last_sent_packet = now
largest_sent_packet = packet_number
sent_packets[packet_number].packet_number = packet_number
sent_packets[packet_number].time = now
if is_retransmittable:
if !is_ack_only:
OnPacketSentCC(sent_bytes)
sent_packets[packet_number].bytes = sent_bytes
SetLossDetectionAlarm()
~~~
Expand Down Expand Up @@ -601,17 +601,21 @@ number of acknowledged bytes when each ack is processed.
Slow start exits to congestion avoidance. Congestion avoidance in NewReno
uses an additive increase multiplicative decrease (AIMD) approach that
increases the congestion window by one MSS of bytes per congestion window
acknowledged. When a loss is detected, NewReno decreases the congestion
window by the loss reduction factor and sets the slow start threshold
to the new congestion window.
acknowledged. When a loss is detected, NewReno halves the congestion
window and sets the slow start threshold to the new congestion window.

## Recovery Period

Recovery is a period of time beginning with detection of a lost packet.
It ends when all packets outstanding at the time recovery began have been
acknowledged or lost. During recovery, the congestion window is not
increased or decreased. As such, multiple lost packets only decrease the
congestion window once as long as they're lost before exiting recovery.
Because QUIC retransmits frames, not packets, it defines the end of
recovery as all packets outstanding at the start of recovery being
acknowledged or lost. This is slightly different from TCP's definition of
recovery ending when the lost packet that started recovery is acknowledged.
During recovery, the congestion window is not increased or decreased.
As such, multiple lost packets only decrease the congestion window once as
long as they're lost before exiting recovery. This causes QUIC to decrease
the congestion window multiple times if retransmisions are lost, but limits
the reduction to once per round trip.

## Tail Loss Probe

Expand All @@ -636,7 +640,9 @@ The pacing rate is a function of the mode, the congestion window, and
the smoothed rtt. Specifically, the pacing rate is 2 times the
congestion window divided by the smoothed RTT during slow start
and 1.25 times the congestion window divided by the smoothed RTT during
slow start.
slow start. In order to fairly compete with flows that are not pacing,
it is recommended to not pace the first 10 sent packets when exiting
quiescence.
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I wonder if this entire section should be removed and left to implementers. But this is fine for now, let's discuss that separately.


## Pseudocode

Expand Down Expand Up @@ -669,22 +675,25 @@ bytes_in_flight:
: The sum of the size in bytes of all sent packets that contain at least
one retransmittable or PADDING frame, and have not been acked or
declared lost. The size does not include IP or UDP overhead.
Ack only frames do not count towards byte_in_flight.
Packets only containing ack frames do not count towards byte_in_flight
to ensure congestion control does not impede congestion feedback.

congestion_window:
: Maximum number of bytes in flight that may be sent.

end_of_recovery:
: The packet number after which QUIC will no longer be in recovery.
: The largest packet number sent when QUIC detects a loss. When a larger
packet is acknowledged, QUIC exits recovery.

ssthresh
: Slow start threshold in bytes. When the congestion window is below
ssthresh, it grows by the number of bytes acknowledged for each ack.
ssthresh, the mode is slow start and the window grows by the number of
bytes acknowledged.

### Initialization

At the beginning of the connection, initialize the loss detection variables as
follows:
At the beginning of the connection, initialize the congestion control
variables as follows:

~~~
congestion_window = kInitialWindow
Expand All @@ -693,22 +702,35 @@ follows:
ssthresh = infinite
~~~

### On Packet Sent

Whenever a packet is sent, and is contains non-ACK frames,
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Instead of and is contains non-ACK frames, I assume you mean and it contains non-ACK frames,?

the packet increases bytes_in_flight.

~~~
OnPacketSentCC(bytes_sent):
bytes_in_flight += bytes_sent
~~~

### On Packet Acknowledgement

Invoked from loss detection's OnPacketAcked and is supplied with
acked_packet from sent_packets.

Pseudocode for OnPacketAckedCC follows:

~~~
OnPacketAckedCC(acked_packet):
// Remove from bytes_in_flight.
bytes_in_flight -= acked_packet.bytes
if (acked_packet.packet_number < end_of_recovery):
// Do not increase congestion window in recovery period.
return
if (congestion_window < ssthresh):
// Slow start.
congestion_window += acked_packets.bytes
else:
// Congestion avoidance.
congestion_window +=
acked_packets.bytes / congestion_window
kDefaultMss * acked_packets.bytes / congestion_window
~~~

### On Packets Lost
Expand All @@ -718,6 +740,9 @@ are detected lost.

~~~
OnPacketsLost(lost_packets):
// Remove lost packets from bytes_in_flight.
for (lost_packet : lost_packets):
bytes_in_flight -= lost_packet.bytes
largest_lost_packet = lost_packets.last()
// Start a new recovery epoch if the lost packet is larger
// than the end of the previous recovery epoch.
Expand All @@ -731,33 +756,13 @@ are detected lost.
### On Retransmission Timeout Verified

QUIC decreases the congestion window to the minimum value once the
retransmission timeout has been confirmed to not be spurious when
the first post-RTO acknowledgement is processed.
retransmission timeout has been verified.

~~~
OnRetransmissionTimeoutVerified()
congestion_window = kMinimumWindow
~~~

### Pacing Packets

QUIC sends a packet if there is available congestion window and
sending the packet does not exceed the pacing rate.

TimeToSend returns infinite if the congestion controller is
congestion window limited, a time in the past if the packet can be
sent immediately, and a time in the future if sending is pacing
limited.

~~~
TimeToSend(packet_size):
if (bytes_in_flight + packet_size > congestion_window)
return infinite
return time_of_last_sent_packet +
(packet_size * smoothed_rtt) / congestion_window
~~~


# IANA Considerations

This document has no IANA actions. Yet.
Expand All @@ -773,6 +778,10 @@ This document has no IANA actions. Yet.
> **RFC Editor's Note:** Please remove this section prior to
> publication of a final version of this document.

## Since draft-ietf-quic-recovery-05

- Add more congestion control text (#776)

## Since draft-ietf-quic-recovery-04

No significant changes.
Expand Down