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Transport QUIC This document describes loss detection and congestion control mechanisms for QUIC. Note to Readers Discussion of this draft takes place on the QUIC working group mailing list (quic@ietf.org), which is archived at . Working Group information can be found at ; source code and issues list for this draft can be found at .

Introduction QUIC is a new multiplexed and secure transport protocol atop UDP, specified in . This document describes congestion control and loss recovery for QUIC. Mechanisms described in this document follow the spirit of existing TCP congestion control and loss recovery mechanisms, described in RFCs, various Internet-drafts, or academic papers, and also those prevalent in TCP implementations.

Conventions and Definitions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. Definitions of terms that are used in this document:

Ack-eliciting frames:

All frames other than ACK, PADDING, and CONNECTION_CLOSE are considered ack-eliciting.

Ack-eliciting packets:

Packets that contain ack-eliciting frames elicit an ACK from the receiver within the maximum acknowledgement delay and are called ack-eliciting packets.

In-flight:

Packets are considered in-flight when they are ack-eliciting or contain a PADDING frame, and they have been sent but are not acknowledged, declared lost, or discarded along with old keys.

Design of the QUIC Transmission Machinery All transmissions in QUIC are sent with a packet-level header, which indicates the encryption level and includes a packet sequence number (referred to below as a packet number). The encryption level indicates the packet number space, as described in . Packet numbers never repeat within a packet number space for the lifetime of a connection. Packet numbers are sent in monotonically increasing order within a space, preventing ambiguity. This design obviates the need for disambiguating between transmissions and retransmissions; this eliminates significant complexity from QUIC's interpretation of TCP loss detection mechanisms. QUIC packets can contain multiple frames of different types. The recovery mechanisms ensure that data and frames that need reliable delivery are acknowledged or declared lost and sent in new packets as necessary. The types of frames contained in a packet affect recovery and congestion control logic:

• All packets are acknowledged, though packets that contain no ack-eliciting frames are only acknowledged along with ack-eliciting packets.
• Long header packets that contain CRYPTO frames are critical to the performance of the QUIC handshake and use shorter timers for acknowledgement.
• Packets containing frames besides ACK or CONNECTION_CLOSE frames count toward congestion control limits and are considered in-flight.
• PADDING frames cause packets to contribute toward bytes in flight without directly causing an acknowledgment to be sent.

Relevant Differences Between QUIC and TCP Readers familiar with TCP's loss detection and congestion control will find algorithms here that parallel well-known TCP ones. However, protocol differences between QUIC and TCP contribute to algorithmic differences. These protocol differences are briefly described below.

Separate Packet Number Spaces QUIC uses separate packet number spaces for each encryption level, except 0-RTT and all generations of 1-RTT keys use the same packet number space. Separate packet number spaces ensures acknowledgement of packets sent with one level of encryption will not cause spurious retransmission of packets sent with a different encryption level. Congestion control and round-trip time (RTT) measurement are unified across packet number spaces.

Monotonically Increasing Packet Numbers TCP conflates transmission order at the sender with delivery order at the receiver, which results in retransmissions of the same data carrying the same sequence number, and consequently leads to "retransmission ambiguity". QUIC separates the two. QUIC uses a packet number to indicate transmission order. Application data is sent in one or more streams and delivery order is determined by stream offsets encoded within STREAM frames. QUIC's packet number is strictly increasing within a packet number space, and directly encodes transmission order. A higher packet number signifies that the packet was sent later, and a lower packet number signifies that the packet was sent earlier. When a packet containing ack-eliciting frames is detected lost, QUIC includes 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.

Clearer Loss Epoch QUIC starts a loss epoch when a packet is lost and ends one when any packet sent after the epoch starts is acknowledged. TCP waits for the gap in the sequence number space to be filled, and so if a segment is lost multiple times in a row, the loss epoch may not end for several round trips. Because both should reduce their congestion windows only once per epoch, QUIC will do it once for every round trip that experiences loss, while TCP may only do it once across multiple round trips.

No Reneging QUIC ACKs contain information that is similar to TCP SACK, but QUIC does not allow any acknowledged packet to be reneged, greatly simplifying implementations on both sides and reducing memory pressure on the sender.

More ACK Ranges QUIC supports many ACK ranges, opposed to TCP's 3 SACK ranges. In high loss environments, this speeds recovery, reduces spurious retransmits, and ensures forward progress without relying on timeouts.

Explicit Correction For Delayed Acknowledgements QUIC endpoints measure the delay incurred between when a packet is received and when the corresponding acknowledgment is sent, allowing a peer to maintain a more accurate round-trip time estimate; see Section 13.2 of .

Probe Timeout Replaces RTO and TLP QUIC uses a probe timeout (PTO; see ), with a timer based on TCP's RTO computation. QUIC's PTO includes the peer's maximum expected acknowledgement delay instead of using a fixed minimum timeout. QUIC does not collapse the congestion window until persistent congestion () is declared, unlike TCP, which collapses the congestion window upon expiry of an RTO. Instead of collapsing the congestion window and declaring everything in-flight lost, QUIC allows probe packets to temporarily exceed the congestion window whenever the timer expires. In doing this, QUIC avoids unnecessary congestion window reductions, obviating the need for correcting mechanisms such as F-RTO (). Since QUIC does not collapse the congestion window on a PTO expiration, a QUIC sender is not limited from sending more in-flight packets after a PTO expiration if it still has available congestion window. This occurs when a sender is application-limited and the PTO timer expires. This is more aggressive than TCP's RTO mechanism when application-limited, but identical when not application-limited. A single packet loss at the tail does not indicate persistent congestion, so QUIC specifies a time-based definition to ensure one or more packets are sent prior to a dramatic decrease in congestion window; see .

The Minimum Congestion Window is Two Packets TCP uses a minimum congestion window of one packet. However, loss of that single packet means that the sender needs to waiting for a PTO () to recover, which can be much longer than a round-trip time. Sending a single ack-eliciting packet also increases the chances of incurring additional latency when a receiver delays its acknowledgement. QUIC therefore recommends that the minimum congestion window be two packets. While this increases network load, it is considered safe, since the sender will still reduce its sending rate exponentially under persistent congestion ().

Estimating the Round-Trip Time At a high level, an endpoint measures the time from when a packet was sent to when it is acknowledged as a round-trip time (RTT) sample. The endpoint uses RTT samples and peer-reported host delays (see Section 13.2 of ) to generate a statistical description of the network path's RTT. An endpoint computes the following three values for each path: the minimum value observed over the lifetime of the path (min_rtt), an exponentially-weighted moving average (smoothed_rtt), and the mean deviation (referred to as "variation" in the rest of this document) in the observed RTT samples (rttvar).

Generating RTT samples An endpoint generates an RTT sample on receiving an ACK frame that meets the following two conditions:

• the largest acknowledged packet number is newly acknowledged, and
• at least one of the newly acknowledged packets was ack-eliciting.

The RTT sample, latest_rtt, is generated as the time elapsed since the largest acknowledged packet was sent: An RTT sample is generated using only the largest acknowledged packet in the received ACK frame. This is because a peer reports acknowledgment delays for only the largest acknowledged packet in an ACK frame. While the reported acknowledgment delay is not used by the RTT sample measurement, it is used to adjust the RTT sample in subsequent computations of smoothed_rtt and rttvar (). To avoid generating multiple RTT samples for a single packet, an ACK frame SHOULD NOT be used to update RTT estimates if it does not newly acknowledge the largest acknowledged packet. An RTT sample MUST NOT be generated on receiving an ACK frame that does not newly acknowledge at least one ack-eliciting packet. A peer usually does not send an ACK frame when only non-ack-eliciting packets are received. Therefore an ACK frame that contains acknowledgements for only non-ack-eliciting packets could include an arbitrarily large ACK Delay value. Ignoring such ACK frames avoids complications in subsequent smoothed_rtt and rttvar computations. A sender might generate multiple RTT samples per RTT when multiple ACK frames are received within an RTT. As suggested in , doing so might result in inadequate history in smoothed_rtt and rttvar. Ensuring that RTT estimates retain sufficient history is an open research question.

Estimating min_rtt min_rtt is the sender's estimate of the minimum RTT observed for a given network path. In this document, min_rtt is used by loss detection to reject implausibly small rtt samples. min_rtt MUST be set to the latest_rtt on the first RTT sample. min_rtt MUST be set to the lesser of min_rtt and latest_rtt () on all other samples. An endpoint uses only locally observed times in computing the min_rtt and does not adjust for acknowledgment delays reported by the peer. Doing so allows the endpoint to set a lower bound for the smoothed_rtt based entirely on what it observes (see ), and limits potential underestimation due to erroneously-reported delays by the peer. The RTT for a network path may change over time. If a path's actual RTT decreases, the min_rtt will adapt immediately on the first low sample. If the path's actual RTT increases however, the min_rtt will not adapt to it, allowing future RTT samples that are smaller than the new RTT to be included in smoothed_rtt. Endpoints SHOULD set the min_rtt to the newest RTT sample after persistent congestion is established. This is to allow a connection to reset its estimate of min_rtt and smoothed_rtt () after a disruptive network event, and because it is possible that an increase in path delay resulted in persistent congestion being incorrectly declared. Endpoints MAY re-establish the min_rtt at other times in the connection, such as when traffic volume is low and an acknowledgement is received with a low acknowledgement delay. Implementations SHOULD NOT refresh the min_rtt value too often, since the actual minimum RTT of the path is not frequently observable.

Estimating smoothed_rtt and rttvar smoothed_rtt is an exponentially-weighted moving average of an endpoint's RTT samples, and rttvar is the variation in the RTT samples, estimated using a mean variation. The calculation of smoothed_rtt uses RTT samples after adjusting them for acknowledgement delays. These delays are decoded from the ACK Delay field of ACK frames as described in Section 19.3 of . The peer might report acknowledgement delays that are larger than the peer's max_ack_delay during the handshake (Section 13.2.1 of ). To account for this, the endpoint SHOULD ignore max_ack_delay until the handshake is confirmed (Section 4.1.2 of ). When they occur, these large acknowledgement delays are likely to be non-repeating and limited to the handshake. The endpoint can therefore use them without limiting them to the max_ack_delay, avoiding unnecessary inflation of the RTT estimate. Note however that a large acknowledgement delay can result in a substantially inflated smoothed_rtt, if there is either an error in the peer's reporting of the acknowledgement delay or in the endpoint's min_rtt estimate. Therefore, prior to handshake confirmation, an endpoint MAY ignore RTT samples if adjusting the RTT sample for acknowledgement delay causes the sample to be less than the min_rtt. After the handshake is confirmed, any acknowledgement delays reported by the peer that are greater than the peer's max_ack_delay are attributed to unintentional but potentially repeating delays, such as scheduler latency at the peer or loss of previous acknowledgements. Excess delays could also be due to a non-compliant receiver. Therefore, these extra delays are considered effectively part of path delay and incorporated into the RTT estimate. Therefore, when adjusting an RTT sample using peer-reported acknowledgement delays, an endpoint:

• MAY ignore the acknowledgement delay for Initial packets, since these acknowledgements are not delayed by the peer (Section 13.2.1 of );
• SHOULD ignore the peer's max_ack_delay until the handshake is confirmed;
• MUST use the lesser of the acknowledgement delay and the peer's max_ack_delay after the handshake is confirmed; and
• MUST NOT subtract the acknowledgement delay from the RTT sample if the resulting value is smaller than the min_rtt. This limits the underestimation of the smoothed_rtt due to a misreporting peer.

Additionally, an endpoint might postpone the processing of acknowledgements when the corresponding decryption keys are not immediately available. For example, a client might receive an acknowledgement for a 0-RTT packet that it cannot decrypt because 1-RTT packet protection keys are not yet available to it. In such cases, an endpoint SHOULD subtract such local delays from its RTT sample until the handshake is confirmed. Similar to , smoothed_rtt and rttvar are computed as follows. An endpoint initializes the RTT estimator during connection establishment and when the estimator is reset during connection migration; see Section 9.4 of . Before any RTT samples are available for a new path or when the estimator is reset, the estimator is initialized using the initial RTT; see . smoothed_rtt and rttvar are initialized as follows, where kInitialRtt contains the initial RTT value: RTT samples for the network path are recorded in latest_rtt; see . On the first RTT sample after initialization, the estimator is reset using that sample. This ensures that the estimator retains no history of past samples. On the first RTT sample after initialization, smoothed_rtt and rttvar are set as follows: On subsequent RTT samples, smoothed_rtt and rttvar evolve as follows:

Loss Detection QUIC senders use acknowledgements to detect lost packets, and a probe time out (see ) to ensure acknowledgements are received. This section provides a description of these algorithms. If a packet is lost, the QUIC transport needs to recover from that loss, such as by retransmitting the data, sending an updated frame, or discarding the frame. For more information, see Section 13.3 of . Loss detection is separate per packet number space, unlike RTT measurement and congestion control, because RTT and congestion control are properties of the path, whereas loss detection also relies upon key availability.

Acknowledgement-Based Detection Acknowledgement-based loss detection implements the spirit of TCP's Fast Retransmit (), Early Retransmit (), FACK (), SACK loss recovery (), and RACK (). This section provides an overview of how these algorithms are implemented in QUIC. A packet is declared lost if it meets all the following conditions:

• The packet is unacknowledged, in-flight, and was sent prior to an acknowledged packet.
• The packet was sent kPacketThreshold packets before an acknowledged packet (), or it was sent long enough in the past ().

The acknowledgement indicates that a packet sent later was delivered, and the packet and time thresholds provide some tolerance for packet reordering. Spuriously declaring packets as lost leads to unnecessary retransmissions and may result in degraded performance due to the actions of the congestion controller upon detecting loss. Implementations can detect spurious retransmissions and increase the reordering threshold in packets or time to reduce future spurious retransmissions and loss events. Implementations with adaptive time thresholds MAY choose to start with smaller initial reordering thresholds to minimize recovery latency.

Packet Threshold The RECOMMENDED initial value for the packet reordering threshold (kPacketThreshold) is 3, based on best practices for TCP loss detection (, ). In order to remain similar to TCP, implementations SHOULD NOT use a packet threshold less than 3; see . Some networks may exhibit higher degrees of packet reordering, causing a sender to detect spurious losses. Additionally, packet reordering could be more common with QUIC than TCP, because network elements that could observe and reorder TCP packets cannot do that for QUIC, because QUIC packet numbers are encrypted. Algorithms that increase the reordering threshold after spuriously detecting losses, such as RACK , have proven to be useful in TCP and are expected to be at least as useful in QUIC.

Time Threshold Once a later packet within the same packet number space has been acknowledged, an endpoint SHOULD declare an earlier packet lost if it was sent a threshold amount of time in the past. To avoid declaring packets as lost too early, this time threshold MUST be set to at least the local timer granularity, as indicated by the kGranularity constant. The time threshold is: If packets sent prior to the largest acknowledged packet cannot yet be declared lost, then a timer SHOULD be set for the remaining time. Using max(smoothed_rtt, latest_rtt) protects from the two following cases:

• the latest RTT sample is lower than the smoothed RTT, perhaps due to reordering where the acknowledgement encountered a shorter path;
• the latest RTT sample is higher than the smoothed RTT, perhaps due to a sustained increase in the actual RTT, but the smoothed RTT has not yet caught up.

The RECOMMENDED time threshold (kTimeThreshold), expressed as a round-trip time multiplier, is 9/8. The RECOMMENDED value of the timer granularity (kGranularity) is 1ms.

Note:

TCP's RACK () specifies a slightly larger threshold, equivalent to 5/4, for a similar purpose. Experience with QUIC shows that 9/8 works well.

Implementations MAY experiment with absolute thresholds, thresholds from previous connections, adaptive thresholds, or including RTT variation. Smaller thresholds reduce reordering resilience and increase spurious retransmissions, and larger thresholds increase loss detection delay.

Probe Timeout A Probe Timeout (PTO) triggers sending one or two probe datagrams when ack-eliciting packets are not acknowledged within the expected period of time or the server may not have validated the client's address. A PTO enables a connection to recover from loss of tail packets or acknowledgements. As with loss detection, the probe timeout is per packet number space. That is, a PTO value is computed per packet number space. A PTO timer expiration event does not indicate packet loss and MUST NOT cause prior unacknowledged packets to be marked as lost. When an acknowledgement is received that newly acknowledges packets, loss detection proceeds as dictated by packet and time threshold mechanisms; see . The PTO algorithm used in QUIC implements the reliability functions of Tail Loss Probe , RTO , and F-RTO algorithms for TCP . The timeout computation is based on TCP's retransmission timeout period .

Computing PTO When an ack-eliciting packet is transmitted, the sender schedules a timer for the PTO period as follows: The PTO period is the amount of time that a sender ought to wait for an acknowledgement of a sent packet. This time period includes the estimated network roundtrip-time (smoothed_rtt), the variation in the estimate (4*rttvar), and max_ack_delay, to account for the maximum time by which a receiver might delay sending an acknowledgement. When the PTO is armed for Initial or Handshake packet number spaces, the max_ack_delay in the PTO period computation is set to 0, since the peer is expected to not delay these packets intentionally; see 13.2.1 of . The PTO period MUST be at least kGranularity, to avoid the timer expiring immediately. When ack-eliciting packets in multiple packet number spaces are in flight, the timer MUST be set to the earlier value of the Initial and Handshake packet number spaces. An endpoint MUST NOT set its PTO timer for the application data packet number space until the handshake is confirmed. Doing so prevents the endpoint from retransmitting information in packets when either the peer does not yet have the keys to process them or the endpoint does not yet have the keys to process their acknowledgements. For example, this can happen when a client sends 0-RTT packets to the server; it does so without knowing whether the server will be able to decrypt them. Similarly, this can happen when a server sends 1-RTT packets before confirming that the client has verified the server's certificate and can therefore read these 1-RTT packets. A sender SHOULD restart its PTO timer every time an ack-eliciting packet is sent or acknowledged, when the handshake is confirmed (Section 4.1.2 of ), or when Initial or Handshake keys are discarded (Section 4.9 of ). This ensures the PTO is always set based on the latest estimate of the round-trip time and for the correct packet across packet number spaces. When a PTO timer expires, the PTO backoff MUST be increased, resulting in the PTO period being set to twice its current value. The PTO backoff factor is reset when an acknowledgement is received, except in the following case. A server might take longer to respond to packets during the handshake than otherwise. To protect such a server from repeated client probes, the PTO backoff is not reset at a client that is not yet certain that the server has finished validating the client's address. That is, a client does not reset the PTO backoff factor on receiving acknowledgements in Initial packets. This exponential reduction in the sender's rate is important because consecutive PTOs might be caused by loss of packets or acknowledgements due to severe congestion. Even when there are ack-eliciting packets in-flight in multiple packet number spaces, the exponential increase in probe timeout occurs across all spaces to prevent excess load on the network. For example, a timeout in the Initial packet number space doubles the length of the timeout in the Handshake packet number space. The total length of time over which consecutive PTOs expire is limited by the idle timeout. The PTO timer MUST NOT be set if a timer is set for time threshold loss detection; see . A timer that is set for time threshold loss detection will expire earlier than the PTO timer in most cases and is less likely to spuriously retransmit data.

Handshakes and New Paths Resumed connections over the same network MAY use the previous connection's final smoothed RTT value as the resumed connection's initial RTT. When no previous RTT is available, the initial RTT SHOULD be set to 333ms, resulting in a 1 second initial timeout, as recommended in . A connection MAY use the delay between sending a PATH_CHALLENGE and receiving a PATH_RESPONSE to set the initial RTT (see kInitialRtt in ) for a new path, but the delay SHOULD NOT be considered an RTT sample. Initial packets and Handshake packets could be never acknowledged, but they are removed from bytes in flight when the Initial and Handshake keys are discarded, as described below in . When Initial or Handshake keys are discarded, the PTO and loss detection timers MUST be reset, because discarding keys indicates forward progress and the loss detection timer might have been set for a now discarded packet number space.

Before Address Validation Until the server has validated the client's address on the path, the amount of data it can send is limited to three times the amount of data received, as specified in Section 8.1 of . If no additional data can be sent, the server's PTO timer MUST NOT be armed until datagrams have been received from the client, because packets sent on PTO count against the anti-amplification limit. Note that the server could fail to validate the client's address even if 0-RTT is accepted. Since the server could be blocked until more datagrams are received from the client, it is the client's responsibility to send packets to unblock the server until it is certain that the server has finished its address validation (see Section 8 of ). That is, the client MUST set the probe timer if the client has not received an acknowledgement for one of its Handshake packets and the handshake is not confirmed (see Section 4.1.2 of ), even if there are no packets in flight. When the PTO fires, the client MUST send a Handshake packet if it has Handshake keys, otherwise it MUST send an Initial packet in a UDP datagram with a payload of at least 1200 bytes.

Speeding Up Handshake Completion When a server receives an Initial packet containing duplicate CRYPTO data, it can assume the client did not receive all of the server's CRYPTO data sent in Initial packets, or the client's estimated RTT is too small. When a client receives Handshake or 1-RTT packets prior to obtaining Handshake keys, it may assume some or all of the server's Initial packets were lost. To speed up handshake completion under these conditions, an endpoint MAY, for a limited number of occasions per each connection, send a packet containing unacknowledged CRYPTO data earlier than the PTO expiry, subject to the address validation limits in Section 8.1 of . Doing so at most once for each connection is adequate to quickly recover from a single packet loss. Endpoints that do not cease retransmitting packets in response to unauthenticated data risk creating an infinite exchange of packets. Endpoints can also use coalesced packets (see Section 12.2 of ) to ensure that each datagram elicits at least one acknowledgement. For example, a client can coalesce an Initial packet containing PING and PADDING frames with a 0-RTT data packet and a server can coalesce an Initial packet containing a PING frame with one or more packets in its first flight.

Sending Probe Packets When a PTO timer expires, a sender MUST send at least one ack-eliciting packet in the packet number space as a probe. An endpoint MAY send up to two full-sized datagrams containing ack-eliciting packets, to avoid an expensive consecutive PTO expiration due to a single lost datagram or transmit data from multiple packet number spaces. All probe packets sent on a PTO MUST be ack-eliciting. In addition to sending data in the packet number space for which the timer expired, the sender SHOULD send ack-eliciting packets from other packet number spaces with in-flight data, coalescing packets if possible. This is particularly valuable when the server has both Initial and Handshake data in-flight or the client has both Handshake and Application Data in-flight, because the peer might only have receive keys for one of the two packet number spaces. If the sender wants to elicit a faster acknowledgement on PTO, it can skip a packet number to eliminate the acknowledgment delay. When the PTO timer expires, an ack-eliciting packet MUST be sent. An endpoint SHOULD include new data in this packet. Previously sent data MAY be sent if no new data can be sent. Implementations MAY use alternative strategies for determining the content of probe packets, including sending new or retransmitted data based on the application's priorities. It is possible the sender has no new or previously-sent data to send. As an example, consider the following sequence of events: new application data is sent in a STREAM frame, deemed lost, then retransmitted in a new packet, and then the original transmission is acknowledged. When there is no data to send, the sender SHOULD send a PING or other ack-eliciting frame in a single packet, re-arming the PTO timer. Alternatively, instead of sending an ack-eliciting packet, the sender MAY mark any packets still in flight as lost. Doing so avoids sending an additional packet, but increases the risk that loss is declared too aggressively, resulting in an unnecessary rate reduction by the congestion controller. Consecutive PTO periods increase exponentially, and as a result, connection recovery latency increases exponentially as packets continue to be dropped in the network. Sending two packets on PTO expiration increases resilience to packet drops, thus reducing the probability of consecutive PTO events. When the PTO timer expires multiple times and new data cannot be sent, implementations must choose between sending the same payload every time or sending different payloads. Sending the same payload may be simpler and ensures the highest priority frames arrive first. Sending different payloads each time reduces the chances of spurious retransmission.

Handling Retry Packets A Retry packet causes a client to send another Initial packet, effectively restarting the connection process. A Retry packet indicates that the Initial was received, but not processed. A Retry packet cannot be treated as an acknowledgment, because it does not indicate that a packet was processed or specify the packet number. Clients that receive a Retry packet reset congestion control and loss recovery state, including resetting any pending timers. Other connection state, in particular cryptographic handshake messages, is retained; see Section 17.2.5 of . The client MAY compute an RTT estimate to the server as the time period from when the first Initial was sent to when a Retry or a Version Negotiation packet is received. The client MAY use this value in place of its default for the initial RTT estimate.

Discarding Keys and Packet State When packet protection keys are discarded (see Section 4.9 of ), all packets that were sent with those keys can no longer be acknowledged because their acknowledgements cannot be processed anymore. The sender MUST discard all recovery state associated with those packets and MUST remove them from the count of bytes in flight. Endpoints stop sending and receiving Initial packets once they start exchanging Handshake packets; see Section 17.2.2.1 of . At this point, recovery state for all in-flight Initial packets is discarded. When 0-RTT is rejected, recovery state for all in-flight 0-RTT packets is discarded. If a server accepts 0-RTT, but does not buffer 0-RTT packets that arrive before Initial packets, early 0-RTT packets will be declared lost, but that is expected to be infrequent. It is expected that keys are discarded after packets encrypted with them would be acknowledged or declared lost. However, Initial secrets are discarded as soon as handshake keys are proven to be available to both client and server; see Section 4.9.1 of .

Congestion Control This document specifies a sender-side congestion controller for QUIC similar to TCP NewReno (). The signals QUIC provides for congestion control are generic and are designed to support different sender-side algorithms. A sender can unilaterally choose a different algorithm to use, such as Cubic (). If a sender uses a different controller than that specified in this document, the chosen controller MUST conform to the congestion control guidelines specified in Section 3.1 of . Similar to TCP, packets containing only ACK frames do not count towards bytes in flight and are not congestion controlled. Unlike TCP, QUIC can detect the loss of these packets and MAY use that information to adjust the congestion controller or the rate of ACK-only packets being sent, but this document does not describe a mechanism for doing so. The algorithm in this document specifies and uses the controller's congestion window in bytes. An endpoint MUST NOT send a packet if it would cause bytes_in_flight (see ) to be larger than the congestion window, unless the packet is sent on a PTO timer expiration (see ) or when entering recovery (see ).

Explicit Congestion Notification If a path has been validated to support ECN (, ), QUIC treats a Congestion Experienced (CE) codepoint in the IP header as a signal of congestion. This document specifies an endpoint's response when the peer-reported ECN-CE count increases; see Section 13.4.2 of .

Initial and Minimum Congestion Window QUIC begins every connection in slow start with the congestion window set to an initial value. Endpoints SHOULD use an initial congestion window of 10 times the maximum datagram size (max_datagram_size), limited to the larger of 14720 bytes or twice the maximum datagram size. This follows the analysis and recommendations in , increasing the byte limit to account for the smaller 8 byte overhead of UDP compared to the 20 byte overhead for TCP. If the maximum datagram size changes during the connection, the initial congestion window SHOULD be recalculated with the new size. If the maximum datagram size is decreased in order to complete the handshake, the congestion window SHOULD be set to the new initial congestion window. Prior to validating the client's address, the server can be further limited by the anti-amplification limit as specified in Section 8.1 of . Though the anti-amplification limit can prevent the congestion window from being fully utilized and therefore slow down the increase in congestion window, it does not directly affect the congestion window. The minimum congestion window is the smallest value the congestion window can decrease to as a response to loss, increase in the peer-reported ECN-CE count, or persistent congestion. The RECOMMENDED value is 2 * max_datagram_size.

Congestion Control States The NewReno congestion controller described in this document has three distinct states, as shown in .

Congestion Control States and Transitions | Start | +------------+ | Loss or | ECN-CE increase | v +------------+ Loss or +------------+ | Congestion | ECN-CE increase | Recovery | | Avoidance |------------------>| Period | +------------+ +------------+ ^ | | | +----------------------------+ Acknowledgment of packet sent during recovery ]]>

These states and the transitions between them are described in subsequent sections.

Slow Start A NewReno sender is in slow start any time the congestion window is below the slow start threshold. A sender begins in slow start because the slow start threshold is initialized to an infinite value. While a sender is in slow start, the congestion window increases by the number of bytes acknowledged when each acknowledgment is processed. This results in exponential growth of the congestion window. The sender MUST exit slow start and enter a recovery period when a packet is lost or when the ECN-CE count reported by its peer increases. A sender re-enters slow start any time the congestion window is less than the slow start threshold, which only occurs after persistent congestion is declared.

Recovery A NewReno sender enters a recovery period when it detects the loss of a packet or the ECN-CE count reported by its peer increases. A sender that is already in a recovery period stays in it and does not re-enter it. On entering a recovery period, a sender MUST set the slow start threshold to half the value of the congestion window when loss is detected. The congestion window MUST be set to the reduced value of the slow start threshold before exiting the recovery period. Implementations MAY reduce the congestion window immediately upon entering a recovery period or use other mechanisms, such as Proportional Rate Reduction (), to reduce the congestion window more gradually. If the congestion window is reduced immediately, a single packet can be sent prior to reduction. This speeds up loss recovery if the data in the lost packet is retransmitted and is similar to TCP as described in Section 5 of . The recovery period aims to limit congestion window reduction to once per round trip. Therefore during a recovery period, the congestion window does not change in response to new losses or increases in the ECN-CE count. A recovery period ends and the sender enters congestion avoidance when a packet sent during the recovery period is acknowledged. This is slightly different from TCP's definition of recovery, which ends when the lost segment that started recovery is acknowledged ().

Congestion Avoidance A NewReno sender is in congestion avoidance any time the congestion window is at or above the slow start threshold and not in a recovery period. A sender in congestion avoidance uses an Additive Increase Multiplicative Decrease (AIMD) approach that MUST limit the increase to the congestion window to at most one maximum datagram size for each congestion window that is acknowledged. The sender exits congestion avoidance and enters a recovery period when a packet is lost or when the ECN-CE count reported by its peer increases.

Ignoring Loss of Undecryptable Packets During the handshake, some packet protection keys might not be available when a packet arrives and the receiver can choose to drop the packet. In particular, Handshake and 0-RTT packets cannot be processed until the Initial packets arrive and 1-RTT packets cannot be processed until the handshake completes. Endpoints MAY ignore the loss of Handshake, 0-RTT, and 1-RTT packets that might have arrived before the peer had packet protection keys to process those packets. Endpoints MUST NOT ignore the loss of packets that were sent after the earliest acknowledged packet in a given packet number space.

Probe Timeout Probe packets MUST NOT be blocked by the congestion controller. A sender MUST however count these packets as being additionally in flight, since these packets add network load without establishing packet loss. Note that sending probe packets might cause the sender's bytes in flight to exceed the congestion window until an acknowledgement is received that establishes loss or delivery of packets.

Persistent Congestion When a sender establishes loss of all in-flight packets sent over a long enough duration, the network is considered to be experiencing persistent congestion.

Duration The persistent congestion duration is computed as follows: Unlike the PTO computation in , this duration includes the max_ack_delay irrespective of the packet number spaces in which losses are established. This duration allows a sender to send as many packets before establishing persistent congestion, including some in response to PTO expiration, as TCP does with Tail Loss Probes () and a Retransmission Timeout (). Larger values of kPersistentCongestionThreshold cause the sender to become less responsive to persistent congestion in the network, which can result in aggressive sending into a congested network. Too small a value can result in a sender declaring persistent congestion unnecessarily, resulting in reduced throughput for the sender. The RECOMMENDED value for kPersistentCongestionThreshold is 3, which results in behavior that is approximately equivalent to a TCP sender declaring an RTO after two TLPs. This design does not use consecutive PTO events to establish persistent congestion, since application patterns impact PTO expirations. For example, a sender that sends small amounts of data with silence periods between them restarts the PTO timer every time it sends, potentially preventing the PTO timer from expiring for a long period of time, even when no acknowledgments are being received. The use of a duration enables a sender to establish persistent congestion without depending on PTO expiration.

Establishing Persistent Congestion A sender establishes persistent congestion on receiving an acknowledgement if at least two ack-eliciting packets are declared lost, and:

• all packets, across all packet number spaces, sent between these two send times are declared lost;
• the duration between the send times of these two packets exceeds the persistent congestion duration (); and
• a prior RTT sample existed when both packets were sent.

The persistent congestion period SHOULD NOT start until there is at least one RTT sample. Before the first RTT sample, a sender arms its PTO timer based on the initial RTT (), which could be substantially larger than the actual RTT. Requiring a prior RTT sample prevents a sender from establishing persistent congestion with potentially too few probes. Since network congestion is not affected by packet number spaces, persistent congestion SHOULD consider packets sent across packet number spaces. A sender that does not have state for all packet number spaces or an implementation that cannot compare send times across packet number spaces MAY use state for just the packet number space that was acknowledged. When persistent congestion is declared, the sender's congestion window MUST be reduced to the minimum congestion window (kMinimumWindow), similar to a TCP sender's response on an RTO ().

Example The following example illustrates how a sender might establish persistent congestion. Assume: Consider the following sequence of events:

Time	Action
t=0	Send packet #1 (app data)
t=1	Send packet #2 (app data)
t=1.2	Recv acknowledgement of #1
t=2	Send packet #3 (app data)
t=3	Send packet #4 (app data)
t=4	Send packet #5 (app data)
t=5	Send packet #6 (app data)
t=6	Send packet #7 (app data)
t=8	Send packet #8 (PTO 1)
t=12	Send packet #9 (PTO 2)
t=12.2	Recv acknowledgement of #9

Packets 2 through 8 are declared lost when the acknowledgement for packet 9 is received at t = 12.2. The congestion period is calculated as the time between the oldest and newest lost packets: 8 - 1 = 7. The persistent congestion duration is: 2 * 3 = 6. Because the threshold was reached and because none of the packets between the oldest and the newest lost packets were acknowledged, the network is considered to have experienced persistent congestion. While this example shows PTO expiration, they are not required for persistent congestion to be established.

Pacing A sender SHOULD pace sending of all in-flight packets based on input from the congestion controller. Sending multiple packets into the network without any delay between them creates a packet burst that might cause short-term congestion and losses. Senders MUST either use pacing or limit such bursts. Senders SHOULD limit bursts to the initial congestion window; see . A sender with knowledge that the network path to the receiver can absorb larger bursts MAY use a higher limit. An implementation should take care to architect its congestion controller to work well with a pacer. For instance, a pacer might wrap the congestion controller and control the availability of the congestion window, or a pacer might pace out packets handed to it by the congestion controller. Timely delivery of ACK frames is important for efficient loss recovery. Packets containing only ACK frames SHOULD therefore not be paced, to avoid delaying their delivery to the peer. Endpoints can implement pacing as they choose. A perfectly paced sender spreads packets exactly evenly over time. For a window-based congestion controller, such as the one in this document, that rate can be computed by averaging the congestion window over the round-trip time. Expressed as a rate in bytes: Or, expressed as an inter-packet interval: Using a value for N that is small, but at least 1 (for example, 1.25) ensures that variations in round-trip time do not result in under-utilization of the congestion window. Practical considerations, such as packetization, scheduling delays, and computational efficiency, can cause a sender to deviate from this rate over time periods that are much shorter than a round-trip time. One possible implementation strategy for pacing uses a leaky bucket algorithm, where the capacity of the "bucket" is limited to the maximum burst size and the rate the "bucket" fills is determined by the above function.

Under-utilizing the Congestion Window When bytes in flight is smaller than the congestion window and sending is not pacing limited, the congestion window is under-utilized. When this occurs, the congestion window SHOULD NOT be increased in either slow start or congestion avoidance. This can happen due to insufficient application data or flow control limits. A sender that paces packets (see ) might delay sending packets and not fully utilize the congestion window due to this delay. A sender SHOULD NOT consider itself application limited if it would have fully utilized the congestion window without pacing delay. A sender MAY implement alternative mechanisms to update its congestion window after periods of under-utilization, such as those proposed for TCP in .

Security Considerations

Congestion Signals Congestion control fundamentally involves the consumption of signals - both loss and ECN codepoints - from unauthenticated entities. On-path attackers can spoof or alter these signals. An attacker can cause endpoints to reduce their sending rate by dropping packets, or alter send rate by changing ECN codepoints.

Traffic Analysis Packets that carry only ACK frames can be heuristically identified by observing packet size. Acknowledgement patterns may expose information about link characteristics or application behavior. To reduce leaked information, endpoints can bundle acknowledgments with other frames, or they can use PADDING frames at a potential cost to performance.

Misreporting ECN Markings A receiver can misreport ECN markings to alter the congestion response of a sender. Suppressing reports of ECN-CE markings could cause a sender to increase their send rate. This increase could result in congestion and loss. A sender can detect suppression of reports by marking occasional packets that it sends with an ECN-CE marking. If a packet sent with an ECN-CE marking is not reported as having been CE marked when the packet is acknowledged, then the sender can disable ECN for that path by not setting ECT codepoints in subsequent packets sent on that path . Reporting additional ECN-CE markings will cause a sender to reduce their sending rate, which is similar in effect to advertising reduced connection flow control limits and so no advantage is gained by doing so. Endpoints choose the congestion controller that they use. Congestion controllers respond to reports of ECN-CE by reducing their rate, but the response may vary. Markings can be treated as equivalent to loss (), but other responses can be specified, such as () or ().

IANA Considerations This document has no IANA actions.

References Normative References Fastly Mozilla Mozilla sn3rd In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. The User Datagram Protocol (UDP) provides a minimal message-passing transport that has no inherent congestion control mechanisms. This document provides guidelines on the use of UDP for the designers of applications, tunnels, and other protocols that use UDP. Congestion control guidelines are a primary focus, but the document also provides guidance on other topics, including message sizes, reliability, checksums, middlebox traversal, the use of Explicit Congestion Notification (ECN), Differentiated Services Code Points (DSCPs), and ports. Because congestion control is critical to the stable operation of the Internet, applications and other protocols that choose to use UDP as an Internet transport must employ mechanisms to prevent congestion collapse and to establish some degree of fairness with concurrent traffic. They may also need to implement additional mechanisms, depending on how they use UDP. Some guidance is also applicable to the design of other protocols (e.g., protocols layered directly on IP or via IP-based tunnels), especially when these protocols do not themselves provide congestion control. This document obsoletes RFC 5405 and adds guidelines for multicast UDP usage. This memo specifies the incorporation of ECN (Explicit Congestion Notification) to TCP and IP, including ECN's use of two bits in the IP header. [STANDARDS-TRACK] Informative References The purpose of this document is to move the F-RTO (Forward RTO-Recovery) functionality for TCP in RFC 4138 from Experimental to Standards Track status. The F-RTO support for Stream Control Transmission Protocol (SCTP) in RFC 4138 remains with Experimental status. See Appendix B for the differences between this document and RFC 4138. Spurious retransmission timeouts cause suboptimal TCP performance because they often result in unnecessary retransmission of the last window of data. This document describes the F-RTO detection algorithm for detecting spurious TCP retransmission timeouts. F-RTO is a TCP sender-only algorithm that does not require any TCP options to operate. After retransmitting the first unacknowledged segment triggered by a timeout, the F-RTO algorithm of the TCP sender monitors the incoming acknowledgments to determine whether the timeout was spurious. It then decides whether to send new segments or retransmit unacknowledged segments. The algorithm effectively helps to avoid additional unnecessary retransmissions and thereby improves TCP performance in the case of a spurious timeout. [STANDARDS-TRACK] This document defines the standard algorithm that Transmission Control Protocol (TCP) senders are required to use to compute and manage their retransmission timer. It expands on the discussion in Section 4.2.3.1 of RFC 1122 and upgrades the requirement of supporting the algorithm from a SHOULD to a MUST. This document obsoletes RFC 2988. [STANDARDS-TRACK] This document defines TCP's four intertwined congestion control algorithms: slow start, congestion avoidance, fast retransmit, and fast recovery. In addition, the document specifies how TCP should begin transmission after a relatively long idle period, as well as discussing various acknowledgment generation methods. This document obsoletes RFC 2581. [STANDARDS-TRACK] This document proposes a new mechanism for TCP and Stream Control Transmission Protocol (SCTP) that can be used to recover lost segments when a connection's congestion window is small. The "Early Retransmit" mechanism allows the transport to reduce, in certain special circumstances, the number of duplicate acknowledgments required to trigger a fast retransmission. This allows the transport to use fast retransmit to recover segment losses that would otherwise require a lengthy retransmission timeout. [STANDARDS-TRACK] This document presents a conservative loss recovery algorithm for TCP that is based on the use of the selective acknowledgment (SACK) TCP option. The algorithm presented in this document conforms to the spirit of the current congestion control specification (RFC 5681), but allows TCP senders to recover more effectively when multiple segments are lost from a single flight of data. This document obsoletes RFC 3517 and describes changes from it. [STANDARDS-TRACK] This document presents the RACK-TLP loss detection algorithm for TCP. RACK-TLP uses per-segment transmit timestamps and selective acknowledgements (SACK) [RFC2018] and has two parts: RACK ("Recent ACKnowledgment") starts fast recovery quickly using time-based inferences derived from ACK feedback. TLP ("Tail Loss Probe") leverages RACK and sends a probe packet to trigger ACK feedback to avoid retransmission timeout (RTO) events. Compared to the widely used DUPACK threshold approach, RACK-TLP detects losses more efficiently when there are application-limited flights of data, lost retransmissions, or data packet reordering events. It is intended to be an alternative to the DUPACK threshold approach in [RFC5681][RFC6675]. RFC 5681 documents the following four intertwined TCP congestion control algorithms: slow start, congestion avoidance, fast retransmit, and fast recovery. RFC 5681 explicitly allows certain modifications of these algorithms, including modifications that use the TCP Selective Acknowledgment (SACK) option (RFC 2883), and modifications that respond to "partial acknowledgments" (ACKs that cover new data, but not all the data outstanding when loss was detected) in the absence of SACK. This document describes a specific algorithm for responding to partial acknowledgments, referred to as "NewReno". This response to partial acknowledgments was first proposed by Janey Hoe. This document obsoletes RFC 3782. [STANDARDS-TRACK] CUBIC is an extension to the current TCP standards. It differs from the current TCP standards only in the congestion control algorithm on the sender side. In particular, it uses a cubic function instead of a linear window increase function of the current TCP standards to improve scalability and stability under fast and long-distance networks. CUBIC and its predecessor algorithm have been adopted as defaults by Linux and have been used for many years. This document provides a specification of CUBIC to enable third-party implementations and to solicit community feedback through experimentation on the performance of CUBIC. This memo updates RFC 3168, which specifies Explicit Congestion Notification (ECN) as an alternative to packet drops for indicating network congestion to endpoints. It relaxes restrictions in RFC 3168 that hinder experimentation towards benefits beyond just removal of loss. This memo summarizes the anticipated areas of experimentation and updates RFC 3168 to enable experimentation in these areas. An Experimental RFC in the IETF document stream is required to take advantage of any of these enabling updates. In addition, this memo makes related updates to the ECN specifications for RTP in RFC 6679 and for the Datagram Congestion Control Protocol (DCCP) in RFCs 4341, 4342, and 5622. This memo also records the conclusion of the ECN nonce experiment in RFC 3540 and provides the rationale for reclassification of RFC 3540 from Experimental to Historic; this reclassification enables new experimental use of the ECT(1) codepoint. This document proposes an experiment to increase the permitted TCP initial window (IW) from between 2 and 4 segments, as specified in RFC 3390, to 10 segments with a fallback to the existing recommendation when performance issues are detected. It discusses the motivation behind the increase, the advantages and disadvantages of the higher initial window, and presents results from several large-scale experiments showing that the higher initial window improves the overall performance of many web services without resulting in a congestion collapse. The document closes with a discussion of usage and deployment for further experimental purposes recommended by the IETF TCP Maintenance and Minor Extensions (TCPM) working group. This document describes an experimental Proportional Rate Reduction (PRR) algorithm as an alternative to the widely deployed Fast Recovery and Rate-Halving algorithms. These algorithms determine the amount of data sent by TCP during loss recovery. PRR minimizes excess window adjustments, and the actual window size at the end of recovery will be as close as possible to the ssthresh, as determined by the congestion control algorithm. This document provides a mechanism to address issues that arise when TCP is used for traffic that exhibits periods where the sending rate is limited by the application rather than the congestion window. It provides an experimental update to TCP that allows a TCP sender to restart quickly following a rate-limited interval. This method is expected to benefit applications that send rate-limited traffic using TCP while also providing an appropriate response if congestion is experienced. This document also evaluates the Experimental specification of TCP Congestion Window Validation (CWV) defined in RFC 2861 and concludes that RFC 2861 sought to address important issues but failed to deliver a widely used solution. This document therefore reclassifies the status of RFC 2861 from Experimental to Historic. This document obsoletes RFC 2861. Active Queue Management (AQM) mechanisms allow for burst tolerance while enforcing short queues to minimise the time that packets spend enqueued at a bottleneck. This can cause noticeable performance degradation for TCP connections traversing such a bottleneck, especially if there are only a few flows or their bandwidth-delay product (BDP) is large. The reception of a Congestion Experienced (CE) Explicit Congestion Notification (ECN) mark indicates that an AQM mechanism is used at the bottleneck, and the bottleneck network queue is therefore likely to be short. Feedback of this signal allows the TCP sender-side ECN reaction in congestion avoidance to reduce the Congestion Window (cwnd) by a smaller amount than the congestion control algorithm's reaction to inferred packet loss. Therefore, this specification defines an experimental change to the TCP reaction specified in RFC 3168, as permitted by RFC 8311. This document proposes a small modification to the way TCP increases its congestion window. Rather than the traditional method of increasing the congestion window by a constant amount for each arriving acknowledgment, the document suggests basing the increase on the number of previously unacknowledged bytes each ACK covers. This change improves the performance of TCP, as well as closes a security hole TCP receivers can use to induce the sender into increasing the sending rate too rapidly. This memo defines an Experimental Protocol for the Internet community.

Loss Recovery Pseudocode We now describe an example implementation of the loss detection mechanisms described in . The pseudocode segments in this section are licensed as Code Components; see the copyright notice.

Tracking Sent Packets To correctly implement congestion control, a QUIC sender tracks every ack-eliciting packet until the packet is acknowledged or lost. It is expected that implementations will be able to access this information by packet number and crypto context and store the per-packet fields () for loss recovery and congestion control. After a packet is declared lost, the endpoint can still maintain state for it for an amount of time to allow for packet reordering; see Section 13.3 of . This enables a sender to detect spurious retransmissions. Sent packets are tracked for each packet number space, and ACK processing only applies to a single space.

Sent Packet Fields

packet_number:

The packet number of the sent packet.

ack_eliciting:

A boolean that indicates whether a packet is ack-eliciting. If true, it is expected that an acknowledgement will be received, though the peer could delay sending the ACK frame containing it by up to the max_ack_delay.

in_flight:

A boolean that indicates whether the packet counts towards bytes in flight.

sent_bytes:

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

time_sent:

The time the packet was sent.

Constants of Interest Constants used in loss recovery are based on a combination of RFCs, papers, and common practice.

kPacketThreshold:

Maximum reordering in packets before packet threshold loss detection considers a packet lost. The value recommended in is 3.

kTimeThreshold:

Maximum reordering in time before time threshold loss detection considers a packet lost. Specified as an RTT multiplier. The value recommended in is 9/8.

kGranularity:

Timer granularity. This is a system-dependent value, and recommends a value of 1ms.

kInitialRtt:

The RTT used before an RTT sample is taken. The value recommended in is 333ms.

kPacketNumberSpace:

An enum to enumerate the three packet number spaces.

Variables of interest Variables required to implement the congestion control mechanisms are described in this section.

latest_rtt:

The most recent RTT measurement made when receiving an ack for a previously unacked packet.

smoothed_rtt:

The smoothed RTT of the connection, computed as described in .

rttvar:

The RTT variation, computed as described in .

min_rtt:

The minimum RTT seen in the connection, ignoring acknowledgment delay, as described in .

first_rtt_sample:

The time that the first RTT sample was obtained.

max_ack_delay:

The maximum amount of time by which the receiver intends to delay acknowledgments for packets in the Application Data packet number space, as defined by the eponymous transport parameter (Section 18.2 of ). Note that the actual ack_delay in a received ACK frame may be larger due to late timers, reordering, or loss.

loss_detection_timer:

Multi-modal timer used for loss detection.

pto_count:

The number of times a PTO has been sent without receiving an ack.

time_of_last_ack_eliciting_packet[kPacketNumberSpace]:

The time the most recent ack-eliciting packet was sent.

largest_acked_packet[kPacketNumberSpace]:

The largest packet number acknowledged in the packet number space so far.

loss_time[kPacketNumberSpace]:

The time at which the next packet in that packet number space will be considered lost based on exceeding the reordering window in time.

sent_packets[kPacketNumberSpace]:

An association of packet numbers in a packet number space to information about them. Described in detail above in .

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

On Sending a Packet After a packet is sent, information about the packet is stored. The parameters to OnPacketSent are described in detail above in . Pseudocode for OnPacketSent follows:

On Receiving a Datagram When a server is blocked by anti-amplification limits, receiving a datagram unblocks it, even if none of the packets in the datagram are successfully processed. In such a case, the PTO timer will need to be re-armed. Pseudocode for OnDatagramReceived follows:

On Receiving an Acknowledgment When an ACK frame is received, it may newly acknowledge any number of packets. Pseudocode for OnAckReceived and UpdateRtt follow: min_rtt + ack_delay): adjusted_rtt = latest_rtt - ack_delay rttvar = 3/4 * rttvar + 1/4 * abs(smoothed_rtt - adjusted_rtt) smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * adjusted_rtt ]]>

Setting the Loss Detection Timer QUIC loss detection uses a single timer for all timeout loss detection. The duration of the timer is based on the timer's mode, which is set in the packet and timer events further below. The function SetLossDetectionTimer defined below shows how the single timer is set. This algorithm may result in the timer being set in the past, particularly if timers wake up late. Timers set in the past fire immediately. Pseudocode for SetLossDetectionTimer follows:

On Timeout When the loss detection timer expires, the timer's mode determines the action to be performed. Pseudocode for OnLossDetectionTimeout follows: 0): // PTO. Send new data if available, else retransmit old data. // If neither is available, send a single PING frame. _, pn_space = GetPtoTimeAndSpace() SendOneOrTwoAckElicitingPackets(pn_space) else: assert(!PeerCompletedAddressValidation()) // Client sends an anti-deadlock packet: Initial is padded // to earn more anti-amplification credit, // a Handshake packet proves address ownership. if (has Handshake keys): SendOneAckElicitingHandshakePacket() else: SendOneAckElicitingPaddedInitialPacket() pto_count++ SetLossDetectionTimer() ]]>

Detecting Lost Packets DetectAndRemoveLostPackets is called every time an ACK is received or the time threshold loss detection timer expires. This function operates on the sent_packets for that packet number space and returns a list of packets newly detected as lost. Pseudocode for DetectAndRemoveLostPackets follows: largest_acked_packet[pn_space]): continue // Mark packet as lost, or set time when it should be marked. // Note: The use of kPacketThreshold here assumes that there // were no sender-induced gaps in the packet number space. if (unacked.time_sent <= lost_send_time || largest_acked_packet[pn_space] >= unacked.packet_number + kPacketThreshold): sent_packets[pn_space].remove(unacked.packet_number) if (unacked.in_flight): lost_packets.insert(unacked) else: if (loss_time[pn_space] == 0): loss_time[pn_space] = unacked.time_sent + loss_delay else: loss_time[pn_space] = min(loss_time[pn_space], unacked.time_sent + loss_delay) return lost_packets ]]>

Upon Dropping Initial or Handshake Keys When Initial or Handshake keys are discarded, packets from the space are discarded and loss detection state is updated. Pseudocode for OnPacketNumberSpaceDiscarded follows:

Congestion Control Pseudocode We now describe an example implementation of the congestion controller described in . The pseudocode segments in this section are licensed as Code Components; see the copyright notice.

Constants of interest Constants used in congestion control are based on a combination of RFCs, papers, and common practice.

kInitialWindow:

Default limit on the initial bytes in flight as described in .

kMinimumWindow:

Minimum congestion window in bytes as described in .

kLossReductionFactor:

Reduction in congestion window when a new loss event is detected. recommends a value is 0.5.

kPersistentCongestionThreshold:

Period of time for persistent congestion to be established, specified as a PTO multiplier. recommends a value of 3.

Variables of interest Variables required to implement the congestion control mechanisms are described in this section.

max_datagram_size:

The sender's current maximum payload size. Does not include UDP or IP overhead. The max datagram size is used for congestion window computations. An endpoint sets the value of this variable based on its Path Maximum Transmission Unit (PMTU; see Section 14.2 of ), with a minimum value of 1200 bytes.

ecn_ce_counters[kPacketNumberSpace]:

The highest value reported for the ECN-CE counter in the packet number space by the peer in an ACK frame. This value is used to detect increases in the reported ECN-CE counter.

bytes_in_flight:

The sum of the size in bytes of all sent packets that contain at least one ack-eliciting or PADDING frame, and have not been acknowledged or declared lost. The size does not include IP or UDP overhead, but does include the QUIC header and AEAD overhead. Packets only containing ACK frames do not count towards bytes_in_flight to ensure congestion control does not impede congestion feedback.

congestion_window:

Maximum number of bytes-in-flight that may be sent.

congestion_recovery_start_time:

The time when QUIC first detects congestion due to loss or ECN, causing it to enter congestion recovery. When a packet sent after this time is acknowledged, QUIC exits congestion recovery.

ssthresh:

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

The congestion control pseudocode also accesses some of the variables from the loss recovery pseudocode.

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

On Packet Sent Whenever a packet is sent, and it contains non-ACK frames, the packet increases bytes_in_flight.

On Packet Acknowledgement Invoked from loss detection's OnAckReceived and is supplied with the newly acked_packets from sent_packets. In congestion avoidance, implementers that use an integer representation for congestion_window should be careful with division, and can use the alternative approach suggested in Section 2.1 of .

On New Congestion Event Invoked from ProcessECN and OnPacketsLost when a new congestion event is detected. If not already in recovery, this starts a recovery period and reduces the slow start threshold and congestion window immediately.

Process ECN Information Invoked when an ACK frame with an ECN section is received from the peer. ecn_ce_counters[pn_space]): ecn_ce_counters[pn_space] = ack.ce_counter sent_time = sent_packets[ack.largest_acked].time_sent OnCongestionEvent(sent_time) ]]>

On Packets Lost Invoked when DetectAndRemoveLostPackets deems packets lost. first_rtt_sample: pc_lost.insert(lost) if (InPersistentCongestion(pc_lost)): congestion_window = kMinimumWindow congestion_recovery_start_time = 0 ]]>

Removing Discarded Packets From Bytes In Flight When Initial or Handshake keys are discarded, packets sent in that space no longer count toward bytes in flight. Pseudocode for RemoveFromBytesInFlight follows:

Change Log

• RFC Editor's Note: Please remove this section prior to publication of a final version of this document.

Issue and pull request numbers are listed with a leading octothorp.

Since draft-ietf-quic-recovery-31

• Limit the number of Initial packets sent in response to unauthenticated packets (#4183, #4188)

Since draft-ietf-quic-recovery-30 Editorial changes only.

Since draft-ietf-quic-recovery-29

• Allow caching of packets that can't be decrypted, by allowing the reported acknowledgment delay to exceed max_ack_delay prior to confirming the handshake (#3821, #3980, #4035, #3874)
• Persistent congestion cannot include packets sent before the first RTT sample for the path (#3875, #3889)
• Recommend reset of min_rtt in persistent congestion (#3927, #3975)
• Persistent congestion is independent of packet number space (#3939, #3961)
• Only limit bursts to the initial window without information about the path (#3892, #3936)
• Add normative requirements for increasing and reducing the congestion window (#3944, #3978, #3997, #3998)

Since draft-ietf-quic-recovery-28

• Refactored pseudocode to correct PTO calculation (#3564, #3674, #3681)

Since draft-ietf-quic-recovery-27

• Added recommendations for speeding up handshake under some loss conditions (#3078, #3080)
• PTO count is reset when handshake progress is made (#3272, #3415)
• PTO count is not reset by a client when the server might be awaiting address validation (#3546, #3551)
• Recommend repairing losses immediately after entering the recovery period (#3335, #3443)
• Clarified what loss conditions can be ignored during the handshake (#3456, #3450)
• Allow, but don't recommend, using RTT from previous connection to seed RTT (#3464, #3496)
• Recommend use of adaptive loss detection thresholds (#3571, #3572)

Since draft-ietf-quic-recovery-26 No changes.

Since draft-ietf-quic-recovery-25 No significant changes.

Since draft-ietf-quic-recovery-24

• Require congestion control of some sort (#3247, #3244, #3248)
• Set a minimum reordering threshold (#3256, #3240)
• PTO is specific to a packet number space (#3067, #3074, #3066)

Since draft-ietf-quic-recovery-23

• Define under-utilizing the congestion window (#2630, #2686, #2675)
• PTO MUST send data if possible (#3056, #3057)
• Connection Close is not ack-eliciting (#3097, #3098)
• MUST limit bursts to the initial congestion window (#3160)
• Define the current max_datagram_size for congestion control (#3041, #3167)

Since draft-ietf-quic-recovery-22

• PTO should always send an ack-eliciting packet (#2895)
• Unify the Handshake Timer with the PTO timer (#2648, #2658, #2886)
• Move ACK generation text to transport draft (#1860, #2916)

Since draft-ietf-quic-recovery-21

• No changes

Since draft-ietf-quic-recovery-20

• Path validation can be used as initial RTT value (#2644, #2687)
• max_ack_delay transport parameter defaults to 0 (#2638, #2646)
• ACK delay only measures intentional delays induced by the implementation (#2596, #2786)

Since draft-ietf-quic-recovery-19

• Change kPersistentThreshold from an exponent to a multiplier (#2557)
• Send a PING if the PTO timer fires and there's nothing to send (#2624)
• Set loss delay to at least kGranularity (#2617)
• Merge application limited and sending after idle sections. Always limit burst size instead of requiring resetting CWND to initial CWND after idle (#2605)
• Rewrite RTT estimation, allow RTT samples where a newly acked packet is ack-eliciting but the largest_acked is not (#2592)
• Don't arm the handshake timer if there is no handshake data (#2590)
• Clarify that the time threshold loss alarm takes precedence over the crypto handshake timer (#2590, #2620)
• Change initial RTT to 500ms to align with RFC6298 (#2184)

Since draft-ietf-quic-recovery-18

• Change IW byte limit to 14720 from 14600 (#2494)
• Update PTO calculation to match RFC6298 (#2480, #2489, #2490)
• Improve loss detection's description of multiple packet number spaces and pseudocode (#2485, #2451, #2417)
• Declare persistent congestion even if non-probe packets are sent and don't make persistent congestion more aggressive than RTO verified was (#2365, #2244)
• Move pseudocode to the appendices (#2408)
• What to send on multiple PTOs (#2380)

Since draft-ietf-quic-recovery-17

• After Probe Timeout discard in-flight packets or send another (#2212, #1965)
• Endpoints discard initial keys as soon as handshake keys are available (#1951, #2045)
• 0-RTT state is discarded when 0-RTT is rejected (#2300)
• Loss detection timer is cancelled when ack-eliciting frames are in flight (#2117, #2093)
• Packets are declared lost if they are in flight (#2104)
• After becoming idle, either pace packets or reset the congestion controller (#2138, 2187)
• Process ECN counts before marking packets lost (#2142)
• Mark packets lost before resetting crypto_count and pto_count (#2208, #2209)
• Congestion and loss recovery state are discarded when keys are discarded (#2327)

Since draft-ietf-quic-recovery-16

• Unify TLP and RTO into a single PTO; eliminate min RTO, min TLP and min crypto timeouts; eliminate timeout validation (#2114, #2166, #2168, #1017)
• Redefine how congestion avoidance in terms of when the period starts (#1928, #1930)
• Document what needs to be tracked for packets that are in flight (#765, #1724, #1939)
• Integrate both time and packet thresholds into loss detection (#1969, #1212, #934, #1974)
• Reduce congestion window after idle, unless pacing is used (#2007, #2023)
• Disable RTT calculation for packets that don't elicit acknowledgment (#2060, #2078)
• Limit ack_delay by max_ack_delay (#2060, #2099)
• Initial keys are discarded once Handshake keys are available (#1951, #2045)
• Reorder ECN and loss detection in pseudocode (#2142)
• Only cancel loss detection timer if ack-eliciting packets are in flight (#2093, #2117)

Since draft-ietf-quic-recovery-14

• Used max_ack_delay from transport params (#1796, #1782)
• Merge ACK and ACK_ECN (#1783)

Since draft-ietf-quic-recovery-13

• Corrected the lack of ssthresh reduction in CongestionEvent pseudocode (#1598)
• Considerations for ECN spoofing (#1426, #1626)
• Clarifications for PADDING and congestion control (#837, #838, #1517, #1531, #1540)
• Reduce early retransmission timer to RTT/8 (#945, #1581)
• Packets are declared lost after an RTO is verified (#935, #1582)

Since draft-ietf-quic-recovery-12

• Changes to manage separate packet number spaces and encryption levels (#1190, #1242, #1413, #1450)
• Added ECN feedback mechanisms and handling; new ACK_ECN frame (#804, #805, #1372)

Since draft-ietf-quic-recovery-11 No significant changes.

Since draft-ietf-quic-recovery-10

• Improved text on ack generation (#1139, #1159)
• Make references to TCP recovery mechanisms informational (#1195)
• Define time_of_last_sent_handshake_packet (#1171)
• Added signal from TLS the data it includes needs to be sent in a Retry packet (#1061, #1199)
• Minimum RTT (min_rtt) is initialized with an infinite value (#1169)

Since draft-ietf-quic-recovery-09 No significant changes.

Since draft-ietf-quic-recovery-08

• Clarified pacing and RTO (#967, #977)

Since draft-ietf-quic-recovery-07

• Include ACK delay in RTO(and TLP) computations (#981)
• ACK delay in SRTT computation (#961)
• Default RTT and Slow Start (#590)
• Many editorial fixes.

Since draft-ietf-quic-recovery-06 No significant changes.

Since draft-ietf-quic-recovery-05

• Add more congestion control text (#776)

Since draft-ietf-quic-recovery-04 No significant changes.

Since draft-ietf-quic-recovery-03 No significant changes.

Since draft-ietf-quic-recovery-02

• Integrate F-RTO (#544, #409)
• Add congestion control (#545, #395)
• Require connection abort if a skipped packet was acknowledged (#415)
• Simplify RTO calculations (#142, #417)

Since draft-ietf-quic-recovery-01

• Overview added to loss detection
• Changes initial default RTT to 100ms
• Added time-based loss detection and fixes early retransmit
• Clarified loss recovery for handshake packets
• Fixed references and made TCP references informative

Since draft-ietf-quic-recovery-00

• Improved description of constants and ACK behavior

Since draft-iyengar-quic-loss-recovery-01

• Adopted as base for draft-ietf-quic-recovery
• Updated authors/editors list
• Added table of contents

Contributors The IETF QUIC Working Group received an enormous amount of support from many people. The following people provided substantive contributions to this document:

• Alessandro Ghedini
• Benjamin Saunders
• Gorry Fairhurst
• Lars Eggert
• Magnus Westerlund
• Marten Seemann
• Martin Duke
• Martin Thomson
• Nick Banks
• Praveen Balasubramanian

Acknowledgments