| QUIC | J. Iyengar, Ed. |
| Internet-Draft | I. Swett, Ed. |
| Intended status: Standards Track | |
| Expires: September 14, 2017 | March 13, 2017 |
QUIC Loss Detection and Congestion Control
draft-ietf-quic-recovery-02
QUIC is a new multiplexed and secure transport atop UDP. QUIC builds on decades of transport and security experience, and implements mechanisms that make it attractive as a modern general-purpose transport. QUIC implements the spirit of known TCP loss detection mechanisms, described in RFCs, various Internet-drafts, and also those prevalent in the Linux TCP implementation. This document describes QUIC loss detection and congestion control, and attributes the TCP equivalent in RFCs, Internet-drafts, academic papers, and TCP implementations.
Discussion of this draft takes place on the QUIC working group mailing list (quic@ietf.org), which is archived at https://mailarchive.ietf.org/arch/search/?email_list=quic.
Working Group information can be found at https://github.com/quicwg; source code and issues list for this draft can be found at https://github.com/quicwg/base-drafts/labels/recovery.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 14, 2017.
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QUIC is a new multiplexed and secure transport atop UDP. QUIC builds on decades of transport and security experience, and implements mechanisms that make it attractive as a modern general-purpose transport. The QUIC protocol is described in [QUIC-TRANSPORT].
QUIC implements the spirit of known TCP loss recovery mechanisms, described in RFCs, various Internet-drafts, and also those prevalent in the Linux TCP implementation. This document describes QUIC congestion control and loss recovery, and where applicable, attributes the TCP equivalent in RFCs, Internet-drafts, academic papers, and/or TCP implementations.
This document first describes pre-requisite parts of the QUIC transmission machinery, then discusses QUIC’s default congestion control and loss detection mechanisms, and finally lists the various TCP mechanisms that QUIC loss detection implements (in spirit.)
The words “MUST”, “MUST NOT”, “SHOULD”, and “MAY” are used in this document. It’s not shouting; when they are capitalized, they have the special meaning defined in [RFC2119].
All transmissions in QUIC are sent with a packet-level header, which includes a packet sequence number (referred to below as a packet number). These packet numbers never repeat in the lifetime of a connection, and are monotonically increasing, which makes duplicate detection trivial. This fundamental design decision obviates the need for disambiguating between transmissions and retransmissions and eliminates significant complexity from QUIC’s interpretation of TCP loss detection mechanisms.
Every packet may contain several frames. We outline the frames that are important to the loss detection and congestion control machinery below.
There are some notable differences between QUIC and TCP which are important for reasoning about the differences between the loss recovery mechanisms employed by the two protocols. We briefly describe these differences below.
TCP conflates transmission sequence number at the sender with delivery sequence number at the receiver, which results in retransmissions of the same data carrying the same sequence number, and consequently to problems caused by “retransmission ambiguity”. QUIC separates the two: QUIC uses a packet sequence number (referred to as the “packet number”) for transmissions, and any data that is to be delivered to the receiving application(s) is sent in one or more streams, with stream offsets encoded within STREAM frames inside of packets that determine delivery order.
QUIC’s packet number is strictly increasing, and directly encodes transmission order. A higher QUIC packet number signifies that the packet was sent later, and a lower QUIC packet number signifies that the packet was sent earlier. When a packet containing frames is deemed lost, QUIC rebundles necessary frames in a new packet with a new packet number, removing ambiguity about which packet is acknowledged when an ACK is received. Consequently, more accurate RTT measurements can be made, spurious retransmissions are trivially detected, and mechanisms such as Fast Retransmit can be applied universally, based only on packet number.
This design point significantly simplifies loss detection mechanisms for QUIC. Most TCP mechanisms implicitly attempt to infer transmission ordering based on TCP sequence numbers - a non-trivial task, especially when TCP timestamps are not available.
QUIC ACKs contain information that is equivalent to TCP SACK, but QUIC does not allow any acked packet to be reneged, greatly simplifying implementations on both sides and reducing memory pressure on the sender.
QUIC supports up to 256 ACK ranges, opposed to TCP’s 3 SACK ranges. In high loss environments, this speeds recovery.
QUIC ACKs explicitly encode the delay incurred at the receiver between when a packet is received and when the corresponding ACK is sent. This allows the receiver of the ACK to adjust for receiver delays, specifically the delayed ack timer, when estimating the path RTT. This mechanism also allows a receiver to measure and report the delay from when a packet was received by the OS kernel, which is useful in receivers which may incur delays such as context-switch latency before a userspace QUIC receiver processes a received packet.
We now describe QUIC’s loss detection as functions that should be called on packet transmission, when a packet is acked, and timer expiration events.
Constants used in loss recovery and congestion control are based on a combination of RFCs, papers, and common practice. Some may need to be changed or negotiated in order to better suit a variety of environments.
We first describe the variables required to implement the loss detection mechanisms described in this section.
At the beginning of the connection, initialize the loss detection variables as follows:
loss_detection_alarm.reset()
handshake_count = 0
tlp_count = 0
rto_count = 0
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
After any packet is sent, be it a new transmission or a rebundled transmission, the following OnPacketSent function is called. The parameters to OnPacketSent are as follows:
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
SetLossDetectionAlarm()
When an ack is received, it may acknowledge 0 or more packets.
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].time
if (rtt_sample > ack.ack_delay):
rtt_sample -= ack.delay
UpdateRtt(rtt_sample)
// Find all newly acked packets.
for acked_packet_number in DetermineNewlyAckedPackets():
OnPacketAcked(acked_packet_number)
DetectLostPackets(ack.largest_acked_packet)
SetLossDetectionAlarm()
UpdateRtt(rtt_sample):
// Based on {{RFC6298}}.
if (smoothed_rtt == 0):
smoothed_rtt = rtt_sample
rttvar = rtt_sample / 2
else:
rttvar = 3/4 * rttvar + 1/4 * (smoothed_rtt - rtt_sample)
smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * rtt_sample
When a packet is acked for the first time, the following OnPacketAcked function is called. Note that a single ACK frame may newly acknowledge several packets. OnPacketAcked must be called once for each of these newly acked packets.
OnPacketAcked takes one parameter, acked_packet, which is the packet number of the newly acked packet, and returns a list of packet numbers that are detected as lost.
Pseudocode for OnPacketAcked follows:
OnPacketAcked(acked_packet_number):
handshake_count = 0
tlp_count = 0
rto_count = 0
sent_packets.remove(acked_packet_number)
QUIC loss detection uses a single alarm for all timer-based loss detection. The duration of the alarm is based on the alarm’s mode, which is set in the packet and timer events further below. The function SetLossDetectionAlarm defined below shows how the single timer is set based on the alarm mode.
The initial flight has no prior RTT sample. A client SHOULD remember the previous RTT it observed when resumption is attempted and use that for an initial RTT value. If no previous RTT is available, the initial RTT defaults to 200ms. Once an RTT measurement is taken, it MUST replace initial_rtt.
Endpoints MUST retransmit handshake frames if not acknowledged within a time limit. This time limit will start as the largest of twice the rtt value and MinTLPTimeout. Each consecutive handshake retransmission doubles the time limit, until an acknowledgement is received.
Handshake frames may be cancelled by handshake state transitions. In particular, all non-protected frames SHOULD be no longer be transmitted once packet protection is available.
When stateless rejects are in use, the connection is considered immediately closed once a reject is sent, so no timer is set to retransmit the reject.
Version negotiation packets are always stateless, and MUST be sent once per per handshake packet that uses an unsupported QUIC version, and MAY be sent in response to 0RTT packets.
Tail loss probes [I-D.dukkipati-tcpm-tcp-loss-probe] and retransmission timeouts[RFC6298] are an alarm based mechanism to recover from cases when there are outstanding retransmittable packets, but an acknowledgement has not been received in a timely manner.
Early retransmit [RFC5827] is implemented with a 1/4 RTT timer. It is part of QUIC’s time based loss detection, but is always enabled, even when only packet reordering loss detection is enabled.
Pseudocode for SetLossDetectionAlarm follows:
SetLossDetectionAlarm():
if (retransmittable packets are not outstanding):
loss_detection_alarm.cancel();
return
if (handshake packets are outstanding):
// Handshake retransmission alarm.
if (smoothed_rtt == 0):
alarm_duration = 2 * initial_rtt
else:
alarm_duration = 2 * smoothed_rtt
alarm_duration = max(alarm_duration, kMinTLPTimeout)
alarm_duration = alarm_duration << handshake_count
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):
alarm_duration = 1.5 * smoothed_rtt + kDelayedAckTimeout
else:
alarm_duration = kMinTLPTimeout
alarm_duration = max(alarm_duration, 2 * smoothed_rtt)
else:
// RTO alarm
if (rto_count = 0):
alarm_duration = smoothed_rtt + 4 * rttvar
alarm_duration = max(alarm_duration, kMinRTOTimeout)
else:
alarm_duration = loss_detection_alarm.get_delay() << 1
loss_detection_alarm.set(now + alarm_duration)
QUIC uses one loss recovery alarm, which when set, can be in one of several modes. When the alarm fires, the mode determines the action to be performed.
Pseudocode for OnLossDetectionAlarm follows:
OnLossDetectionAlarm():
if (handshake packets are outstanding):
// Handshake retransmission alarm.
RetransmitAllHandshakePackets();
handshake_count++;
// TODO: Clarify early retransmit and time loss.
else if (loss_time != 0):
// Early retransmit or Time Loss Detection
DetectLostPackets(largest_acked_packet)
else if (tlp_count < kMaxTLPs):
// Tail Loss Probe.
if (HasNewDataToSend()):
SendOnePacketOfNewData()
else:
RetransmitOldestPacket()
tlp_count++
else:
// RTO.
RetransmitOldestTwoPackets()
rto_count++
SetLossDetectionAlarm()
Packets in QUIC are only considered lost once a larger packet number is 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.
The receiver MUST ignore unprotected packets that ack protected packets. The receiver MUST trust protected acks for unprotected packets, however. Aside from this, loss detection for handshake packets when an ack is processed is identical to other packets.
DetectLostPackets takes one parameter, acked, which is the largest acked packet.
Pseudocode for DetectLostPackets follows:
DetectLostPackets(largest_acked):
loss_time = 0
lost_packets = {}
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)
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)
foreach (packet in lost_packets)
sent_packets.remove(packet.packet_number)
(describe NewReno-style congestion control [RFC6582] for QUIC.) (describe appropriate byte counting.) (define recovery based on packet numbers.) (describe min_rtt based hystart.) (describe how QUIC’s F-RTO [RFC5682] delays reducing CWND.) (describe PRR [RFC6937])
This document has no IANA actions. Yet.
| [QUIC-TRANSPORT] | Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed and Secure Transport" |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |