| Internet-Draft | HyStart++ | July 2021 |
| Balasubramanian, et al. | Expires 26 January 2022 | [Page] |
This doument describes HyStart++, a simple modification to the slow start phase of TCP congestion control algorithms. Traditional slow start can cause overshooting of the ideal send rate and cause large packet loss within a round-trip time which results in poor performance. HyStart++ uses a delay increase heuristic to exit slow start early while also mitigating poor performance which can result from false positives.¶
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[RFC5681] describes the slow start congestion control algorithm for TCP. The slow start algorithm is used when the congestion window (cwnd) is less than the slow start threshold (ssthresh). During slow start, in absence of packet loss signals, TCP increases cwnd exponentially to probe the network capacity. This fast growth can overshoot the ideal sending rate and cause significant packet loss which cannot always be recovered efficiently, impairing flow completion time.¶
HyStart++ first uses delay increase as a signal to exit slow start before any packet loss occurs. This is one of two algorithms specified in [HyStart]. After the HyStart delay algorithm finds an exit point, a novel Conservative Slow Start (CSS) phase is used to determine whether the slow start exit was spurious. This provides protection against jitter and prevents performance problems that result from early slow start exit due to false positives. HyStart++ reduces packet loss and retransmissions, and improves goodput in lab measurements as well as real world deployments.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].¶
We repeat here some definition from [RFC5681] to aid the reader.¶
SENDER MAXIMUM SEGMENT SIZE (SMSS): The SMSS is the size of the largest segment that the sender can transmit. This value can be based on the maximum transmission unit of the network, the path MTU discovery [RFC1191, RFC4821] algorithm, RMSS (see next item), or other factors. The size does not include the TCP/IP headers and options.¶
RECEIVER MAXIMUM SEGMENT SIZE (RMSS): The RMSS is the size of the largest segment the receiver is willing to accept. This is the value specified in the MSS option sent by the receiver during connection startup. Or, if the MSS option is not used, it is 536 bytes [RFC1122]. The size does not include the TCP/IP headers and options.¶
RECEIVER WINDOW (rwnd): The most recently advertised receiver window.¶
CONGESTION WINDOW (cwnd): A TCP state variable that limits the amount of data a TCP can send. At any given time, a TCP MUST NOT send data with a sequence number higher than the sum of the highest acknowledged sequence number and the minimum of cwnd and rwnd.¶
[HyStart] specifies two algorithms (a "Delay Increase" algorithm and an "Inter-Packet Arrival" algorithm) to be run in parallel to detect that the sending rate has reached capacity. In practice, the Inter-Packet Arrival algorithm does not perform well and is not able to detect congestion early, primarily due to ACK compression. The idea of the Delay Increase algorithm is to look for RTT spikes, which suggest that the bottleneck buffer is filling up.¶
In HyStart++, a TCP sender uses traditional slow start and then uses the "Delay Increase" algorithm to trigger an exit from slow start. But instead of going straight from slow start to congestion avoidance, the sender spends a number of RTTs in a Conservative Slow Start (CSS) phase to determine whether the exit was spurious. During CSS, the congestion window is grown exponentially like in regular slow start, but with a smaller exponential base, resulting in less aggressive growth. If the RTT shrinks at any time during CSS, it's concluded that the RTT spike was not related to congestion caused by the connection sending too fast (i.e. the exit was spurious), and the connection resumes slow start. If the RTT inflation persists throughout CSS, the connection enters congestion avoidance.¶
We assume that Appropriate Byte Counting (as described in [RFC3465]) is in use and L is the cwnd increase limit as discussed in RFC 3465.¶
A round is chosen to be approximately the Round-Trip Time (RTT). We recommend that rounds be measured using sequence numbers. Round can be approximated using sequence numbers as follows:¶
At the start of each round during standard slow start ([RFC5681]) and CSS:¶
For each arriving ACK in slow start, where N is the number of previously unacknowledged bytes acknowledged in the arriving ACK:¶
Update the cwnd¶
Keep track of minimum observed RTT¶
For rounds where cwnd is at or higher than LOW_CWND and N_RTT_SAMPLE RTT samples have been obtained, check if delay increase triggers slow start exit¶
CSS lasts at most CSS_ROUNDS rounds. If the transition into CSS happens in the middle of a round, that partial round counts towards the limit.¶
For each arriving ACK in CSS, where N is the number of previously unacknowledged bytes acknowledged in the arriving ACK:¶
Update the cwnd¶
Keep track of minimum observed RTT¶
For CSS rounds where N_RTT_SAMPLE RTT samples have been obtained, check if current round's minRTT drops below baseline indicating that HyStart exit was spurious.¶
If CSS_ROUNDS rounds are complete, enter congestion avoidance.¶
If loss or ECN-marking is observed anytime during standard slow start or CSS, enter congestion avoidance.¶
It is RECOMMENDED that a HyStart++ implementation use the following constants:¶
These constants have been determined with lab measurements and real world deployments. An implementation MAY tune them for different network characteristics.¶
Using smaller values of LOW_CWND will cause the algorithm to kick in before the last round RTT can be measured, particularly if the implementation uses an initial cwnd of 10 MSS. Higher values will delay the detection of delay increase and reduce the ability of HyStart++ to prevent overshoot problems.¶
The delay increase sensitivity is determined by MIN_RTT_THRESH and MAX_RTT_THRESH. Smaller values of MIN_RTT_THRESH may cause spurious exits from slow start. Larger values of MAX_RTT_THRESH may result in slow start not exiting until loss is encountered for connections on large RTT paths.¶
A TCP implementation is required to take at least one RTT sample each round. Using lower values of N_RTT_SAMPLE will lower the accuracy of the measured RTT for the round; higher values will improve accuracy at the cost of more processing.¶
The minimum value of CSS_GROWTH_DIVISOR MUST be at least 2. A value of 1 results in the same aggressive behavior as regular slow start. Values larger than 4 will cause the algorithm to be less aggressive and maybe less performant.¶
Smaller values of CSS_ROUNDS may miss detecting jitter and larger values may limit performance.¶
An implementation SHOULD use HyStart++ only for the initial slow start (when ssthresh is at its initial value of arbitrarily high per [RFC5681]) and fall back to using traditional slow start for the remainder of the connection lifetime. This is acceptable because subsequent slow starts will use the discovered ssthresh value to exit slow start and avoid the overshoot problem. An implementation MAY use HyStart++ to grow the restart window ([RFC5681]) after a long idle period.¶
As of the time of writing, HyStart++ draft 01 was default enabled for all TCP connections in Windows for two years. The original Hystart has been default-enabled for all TCP connections using the default congestion control module CUBIC ([RFC8312]) for a decade.¶
In lab measurements with Windows TCP, HyStart++ shows both goodput improvements as well as reductions in packet loss and retransmissions. For example across a variety of tests on a 100 Mbps link with a bottleneck buffer size of bandwidth-delay product, HyStart++ reduces bytes retransmitted by 50% and retransmission timeouts by 36%.¶
In an A/B test for HyStart++ draft 01 across a large Windows device population, out of 52 billion TCP connections, 0.7% of connections move from 1 RTO to 0 RTOs and another 0.7% connections move from 2 RTOs to 1 RTO with HyStart++. This test did not focus on send heavy connections and the impact on send heavy connections is likely much higher. We plan to conduct more such production experiments to gather more data in the future.¶
HyStart++ enhances slow start and inherits the general security considerations discussed in [RFC5681].¶
This document has no actions for IANA.¶