Internet DRAFT - draft-tsvwg-quic-loss-recovery
draft-tsvwg-quic-loss-recovery
Network Working Group J. Iyengar
Internet-Draft I. Swett
Intended status: Informational Google
Expires: June 20, 2016 December 18, 2015
QUIC Loss Recovery And Congestion Control
draft-tsvwg-quic-loss-recovery-01
Abstract
QUIC (Quick UDP Internet Connection) is a new multiplexed and secure
transport atop UDP, designed from the ground up and optimized for
HTTP/2 semantics. While built with HTTP/2 as the primary application
protocol, 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 loss recovery, and
where applicable, attributes the TCP equivalent in RFCs, Internet-
drafts, academic papers, and/or TCP implementations.
Status of This Memo
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This Internet-Draft will expire on June 20, 2016.
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1. Introduction
QUIC (Quick UDP Internet Connection) is a new multiplexed and secure
transport atop UDP, designed from the ground up and optimized for
HTTP/2 semantics. While built with HTTP/2 as the primary application
protocol, 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-tsvwg-quic-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 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 loss recovery
mechanisms. The document then describes QUIC's loss recovery,
followed by a list of the various TCP mechanisms that QUIC re-
interprets.
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.
o STREAM frames contain application data. Crypto handshake data is
also sent as STREAM data, and uses the reliability machinery of
QUIC underneath.
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o ACK frames contain acknowledgment information. QUIC uses a NACK-
based scheme, where the largest_observed packet number is
reported, and packets with sequence numbers lesser than the
largest_observed not yet seen are reported as NACK ranges. The
ACK frame also includes a receive timestamp for each packet newly
acked.
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 "sequence 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 SACK Reneging
QUIC ACKs contain information that is equivalent to TCP SACK, but
QUIC does not allow any acked packet to be reneged, greatly
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simplifying implementations on both sides and reducing memory
pressure on the sender.
2.1.3. More NACK Ranges
QUIC supports up to 255 NACK 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:
o If the handshake has not completed, start a handshake timer.
* 1.5x the SRTT, with exponential backoff.
o If there are outstanding packets which have been NACKed, possibly
set the loss timer
* Depends on the loss detection implementation, default is
0.25RTT in the case of Early Retransmit.
o If fewer than 2 TLPs have been sent, compute and restart TLP
timer.
* Timer is set for max(10ms, 2*SRTT) if there are multiple
packets in flight
* Timer is set to max(1.5*SRTT + delayed ack timer, 2*SRTT) if
there is only one packet in flight.
o If 2 TLPs have been sent, set the RTO timer.
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* Timer is set to max(200ms, SRTT+4*RTTVAR) with exponential
backoff after the first RTO.
3.2. On Receiving an ACK
The following steps are performed when an ACK is received:
o Validate the ack, including ignoring any out of order acks.
o Update RTT measurements.
o Sender marks unacked packets lower than the largest_observed and
not NACKed in this ACK frame as ACKED.
o Packets with packet number lesser than the largest_observed that
are NACKed have missing_reports incremented based on
FACK.(largest_observed - missing packet number)
o Threshold is set to 3 by default.
o Packets with missing_reports > threshold are marked for
retransmission. This logic implements Fast Retransmission and
FACK-based retransmission together.
o If nacked packets are outstanding and the largest observed is the
largest sent packet, the retransmission timer will be set to
0.25SRTT, implementing Early Retransmit with timer.
o Stop timers if no packets are outstanding.
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)
o Handshake state:
* Retransmit any outstanding handshake packets.
o Loss timer state:
* Lose the outstanding packets which have been NACKed so far.
* Report the loss to the congestion controller.
* Retransmit as many as the congestion controller allows.
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o TLP state:
* Retransmit the smallest outstanding packet which is
retransmittable.
* Do not mark any packets as lost until an ACK arrives.
* Restart timer for a TLP or RTO.
o RTO state:
* Retransmit the two smallest outstanding packets which are
retransmittable.
* Do not collapse the congestion window (ie: set to 1 packet)
until an ack arrives and confirms that the RTO was not
spurious. Note that this step obviates the need to implement
FRTO.
* Restart the timer for next RTO (with exponential backoff.)
4. 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.
4.1. 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.
4.2. 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.
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Currently QUIC does not implement an adaptive threshold as many TCP
implementations(ie: the Linux kernel) do.
4.3. 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.
4.4. 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.
4.5. 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 1/4 SRTT after the final
outstanding packet is acked.
4.6. 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.
4.7. RFC 6937 (Proportional Rate Reduction)
PRR-SSRB is implemented by QUIC in the epoch when recovering from a
loss.
4.8. 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.
4.9. 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
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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.
5. References
5.1. Normative References
[RFC2119] Bradner, S., "Key Words for use in RFCs to Indicate
Requirement Levels", March 1997.
5.2. Informative References
[draft-tsvwg-quic-protocol]
Hamilton, R., Iyengar, J., Swett, I., and A. Wilk, "QUIC:
A UDP-Based Secure and Reliable Transport For HTTP/2",
July 2015.
Authors' Addresses
Janardhan Iyengar
Google
Email: jri@google.com
Ian Swett
Google
Email: ianswett@google.com
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