Network Working Group J. Iyengar
Internet-Draft I. Swett
Intended status: Informational Google
Expires: January 9, 2017 July 8, 2016

QUIC Congestion Control And Loss Recovery
draft-iyengar-quic-loss-recovery-00

Abstract

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 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.

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1. Introduction

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 [draft-hamilton-quic-transport-protocol].

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 parts of the QUIC transmission machinery that are necessary to describe the congestion control and loss recovery mechanisms. The document then describes QUIC's default congestion control and default loss recovery, followed by a list of the various TCP mechanisms that QUIC implements (in spirit) in its loss recovery mechanisms.

2. Design of the QUIC Transmission Machinery

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 can contain several frames; we outline the frames that are important to the loss detection and congestion control machinery below.

2.1. Relevant Differences Between QUIC and TCP

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.

2.1.1. Monotonically Increasing Sequence Numbers

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 transmission 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 sequence number is strictly increasing, and directly encodes transmission order. A higher QUIC sequence number signifies that the packet was sent later, and a lower QUIC sequence number signifies that the packet was sent earlier.

This design point significantly simplifies loss detection mechanisms for QUIC. Most TCP mechanisms implicitly attempt to infer transmission ordering based on the TCP sequence numbers; a non-trivial task, especially when TCP timestamps are not available.

QUIC resends lost packets with new packet sequence numbers when retransmission is necessary, 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 sequence number.

2.1.2. No Reneging

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.

2.1.3. More ACK Ranges

QUIC supports up to 255 ACK ranges, opposed to TCP’s 3 SACK ranges. In high loss environments, this speeds recovery.

2.1.4. Explicit Correction For Delayed Acks

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.

3. An Overview of QUIC Loss Recovery

We briefly describe QUIC's actions on packet transmission, ack reception, and timer expiration events.

3.1. On Sending a Packet

A retransmission timer may be set based on the mode:

3.2. On Receiving an ACK

The following steps are performed when an ACK is received:

3.3. On Timer Expiration

QUIC uses one loss recovery timer, which when set, can be in one of several states. When the timer expires, the state determines the action to be performed. (TODO: describe when the timers are set)

4. Congestion Control

(describe NewReno-style congestion control for QUIC.)

5. 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.

5.1. 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.

5.2. 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 sequence number to measure the FACK reordering threshold. Currently QUIC does not implement an adaptive threshold as many TCP implementations(ie: the Linux kernel) do.

5.3. 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.

5.4. 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.

5.5. 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 ¼ SRTT after the final outstanding packet is acked.

5.6. 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.

5.7. RFC 6937 (Proportional Rate Reduction)

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

5.8. 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.

5.9. 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 ⅛ of the connection min_rtt.

6. References

6.1. Normative References

[RFC2119] Bradner, S., "Key Words for use in RFCs to Indicate Requirement Levels", March 1997.

6.2. Informative References

[draft-hamilton-quic-transport-protocol] Hamilton, R., Iyengar, J., Swett, I. and A. Wilk, "QUIC: A UDP-Based Multiplexed and Secure Transport", July 2016.

Authors' Addresses

Janardhan Iyengar Google EMail: jri@google.com
Ian Swett Google EMail: ianswett@google.com

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