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ianswett merged 2 commits into from
Feb 7, 2017
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Move references to informational references #190

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Move references to informational references
ianswett Jan 23, 2017
7024518
Fix informative references (#218)
MikeBishop Jan 24, 2017
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104 changes: 21 additions & 83 deletions draft-ietf-quic-recovery.md
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Expand Up @@ -59,6 +59,13 @@ normative:

informative:

RFC3782:
RFC6582:
RFC5827:
RFC5682:
RFC6937:
I-D.dukkipati-tcpm-tcp-loss-probe:

--- abstract

QUIC is a new multiplexed and secure transport atop UDP. QUIC builds on decades
Expand Down Expand Up @@ -234,9 +241,9 @@ mechanisms described in this section.
* rto_count: The number of times an rto has been sent without receiving an ack.

* smoothed_rtt: The smoothed RTT of the connection, computed as described in
{{!RFC6298}}
{{?RFC6298}}

* rttvar: The RTT variance.
* rttvar: The RTT variance, computed as described in {{?RFC6298}}

* reordering_threshold: The largest delta between the largest acked
retransmittable packet and a packet containing retransmittable frames before
Expand Down Expand Up @@ -283,7 +290,7 @@ Pseudocode for SetLossDetectionAlarm follows:
alarm_duration = max(1.5 * smoothed_rtt, kMinTLPTimeout) << handshake_count;
handshake_count++;
else if (largest sent packet is acked):
// Early retransmit alarm.
// Early retransmit {{!RFC 5827}} with an alarm to reduce spurious retransmits.
alarm_duration = 0.25 x smoothed_rtt;
else if (tlp_count < kMaxTLPs):
// Tail Loss Probe alarm.
Expand Down Expand Up @@ -318,13 +325,16 @@ are as follows:
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.

* sent_bytes: The number of bytes sent in the packet.

Pseudocode for OnPacketSent follows:

~~~
OnPacketSent(packet_number, is_retransmittable):
OnPacketSent(packet_number, is_retransmittable, sent_bytes):
# TODO: Clarify the data in sent_packets.
sent_packets[packet_number] = {now}
sent_packets[packet_number].time = now
if is_retransmittable:
sent_packets[packet_number].bytes = sent_bytes
SetLossDetectionAlarm()
~~~

Expand All @@ -338,7 +348,7 @@ Pseudocode for OnAckReceived and UpdateRtt follow:
OnAckReceived(ack):
// If the largest acked is newly acked, update the RTT.
if (sent_packets[ack.largest_acked]):
rtt_sample = now - sent_packets[ack.largest_acked]
rtt_sample = now - sent_packets[ack.largest_acked].time
if (rtt_sample > ack.ack_delay):
rtt_sample -= ack.delay;
UpdateRtt(rtt_sample)
Expand All @@ -351,6 +361,7 @@ Pseudocode for OnAckReceived and UpdateRtt follow:


UpdateRtt(rtt_sample):
// Based on {{?RFC6298}}.
if (smoothed_rtt == 0):
smoothed_rtt = rtt_sample
rttvar = rtt_sample / 2
Expand Down Expand Up @@ -425,84 +436,11 @@ Pseudocode for DetectLostPackets follows:
# Congestion Control

(describe NewReno-style congestion control for QUIC.)
(describe appropriate byte counting.)
(define recovery based on packet numbers.)
(describe min_rtt based hystart.)
(describe how QUIC's F-RTO delays reducing CWND until an ack is received.)

# TCP mechanisms in QUIC

QUIC implements the spirit of a variety of RFCs, Internet drafts, and other
well-known TCP loss recovery mechanisms, though the implementation details
differ from the TCP implementations.


## RFC 6298 (RTO computation)

QUIC calculates SRTT and RTTVAR according to the standard formulas. An RTT
sample is only taken if the delayed ack correction is smaller than the measured
RTT (otherwise a negative RTT would result), and the ack's contains a new,
larger largest observed packet number. min_rtt is only based on the observed
RTT, but SRTT uses the delayed ack correction delta.

As described above, QUIC implements RTO with the standard timeout and CWND
reduction. However, QUIC retransmits the earliest outstanding packets rather
than the latest, because QUIC doesn't have retransmission ambiguity. QUIC uses
the commonly accepted min RTO of 200ms instead of the 1s the RFC specifies.

## FACK Loss Recovery (paper)

QUIC implements the algorithm for early loss recovery described in the FACK
paper (and implemented in the Linux kernel.) QUIC uses the packet number to
measure the FACK reordering threshold. Currently QUIC does not implement an
adaptive threshold as many TCP implementations (i.e., the Linux kernel) do.

## RFC 3782, RFC 6582 (NewReno Fast Recovery)

QUIC only reduces its CWND once per congestion window, in keeping with the
NewReno RFC. It tracks the largest outstanding packet at the time the loss is
declared and any losses which occur before that packet number are considered
part of the same loss event. It's worth noting that some TCP implementations
may do this on a sequence number basis, and hence consider multiple losses of
the same packet a single loss event.

## TLP (draft)

QUIC always sends two tail loss probes before RTO is triggered. QUIC invokes
tail loss probe even when a loss is outstanding, which is different than some
TCP implementations.

## RFC 5827 (Early Retransmit) with Delay Timer

QUIC implements early retransmit with a timer in order to minimize spurious
retransmits. The timer is set to 1/4 SRTT after the final outstanding packet is
acked.

## RFC 5827 (F-RTO)

QUIC implements F-RTO by not reducing the CWND and SSThresh until a subsequent
ack is received and it's sure the RTO was not spurious. Conceptually this is
similar, but it makes for a much cleaner implementation with fewer edge cases.

## RFC 6937 (Proportional Rate Reduction)

PRR-SSRB is implemented by QUIC in the epoch when recovering from a loss.

## TCP Cubic (draft) with optional RFC 5681 (Reno)

TCP Cubic is the default congestion control algorithm in QUIC. Reno is also an
easily available option which may be requested via connection options and is
fully implemented.

## Hybrid Slow Start (paper)

QUIC implements hybrid slow start, but disables ack train detection, because it
has shown to falsely trigger when coupled with packet pacing, which is also on
by default in QUIC. Currently the minimum delay increase is 4ms, the maximum is
16ms, and within that range QUIC exits slow start if the min_rtt within a round
increases by more than one eighth of the connection mi

## RACK (draft)

QUIC's loss detection is by it's time-ordered nature, very similar to RACK.
Though QUIC defaults to loss detection based on reordering threshold in packets,
it could just as easily be based on fractions of an rtt, as RACK does.

# IANA Considerations

Expand Down