A survey paper on how Linux implements the congestion control in TCP.

# TCP basics

The basics of TCP is specified in RFC793 and the dominant variant, NewReno, is specified in RFC2581.

TCP uses cwnd, the sender’s estimate of how much data can be outstanding in the network, to regulate transmission rate. It is initialized to 1-2. In slow start, it is increased by one for each incoming ACK. In congestion avoidance, it is increased by one in a round-trip time. The slow start threshold, ssthresh, determines whether the TCP is in slow start or congestion avoidance. CA iff cwnd ≥ ssthresh.

Lost of packet is detected by duplicate ACKs. After 3 duplicated ACKs, the sender retransmits a segment and set ssthresh to half of the currently outstanding data. Then, cwnd is set to ssthresh+3 segments, to account for the segments that have already left the network. In effect, the sender halves the Tx rate as packet loss is an indication of congestion.

Retransmission due to incoming dup ACKs is fast retransmit. After that, TCP is in fast recovery to wait until all segments in the last window have been acknowledged. During fast recovery, TCP sender maintains the number of outstanding segments, i.e. send one new segment for each incoming ACK as far as cwnd allows. Precisely, during fast recovery, cwnd increase by one per ACK to allow transmission of new segment. But once the fast recovery is over, the cwnd restored to the value at the beginning of fast recovery. Two ways to end fast recovery is suggested: (1) upon the receipt of the first new ACK; (2) after all segments in the last window has been ACKed. The latter is suggested in NewReno (RFC2582).

Retransmission timeout (RTO) also triggers retransmission. It expires when no new ACK received for a while. When RTO occurs, cwnd is reset to one segment. RTO value is determined by RTT. Precisely as follows (RFC2988):

RTTVAR = 0.75 * RTTVAR + 0.25 |SRTT - RTT|
SRTT = 0.875 * SRTT + 0.125 * RTT
RTO = max(SRTT + 4*RTTVAR, 1 second)


where RTT is the measured round trip time.

If cumulative ACK is used, recovery in TCP allows only one retransmission per round-trip time. SACK is proposed in RFC2018 to acknowledge scattered blocks of incoming data, so that the sender can retransmit more than one segment in an RTT. SACK needs consensus of both ends. There are two ways to interpret SACK: (1) all unacknowledged data are outstanding in the network; (2) Forward Acknowledgements, i.e. all unacknowledged packets are assumed lost. The latter is too aggressive if reordering is common. RFC2883 suggests an extended use of SACK, the duplicate SACK or DSACK. It reports also the duplicated segment so that sender can know about spurious retransmissions. The sender can therefore learn about the reordering behaviour and adapt to the network condition.

TCP timestamp options is proposed in RFC1323. It allows more accurate RTT measurements, especially when BDP is high. Timestamps are attached to each TCP segment and echoed by the receiver. Another reason for timestamp is to protect against old segments from previous incarnations of TCP. Moreover, timestamp can help detecting unnecessary retransmissions, by Eifel algorithm. Eifel algorithm suggests that if the ACK timestamp is earlier than the corresponding one in retransmission buffer, the retransmission is unnecessary. Then, the change to cwnd can be reverted.

RFC3168 suggests explicit congestion notification (ECN). It suggests routers to mark packets when they arrived a congested point. The TCP sender who received a echoed ECN should reduce its transmission rate.

# Linux TCP

RFCs said TCP can send when cwnd is smaller than by SND.NXT-SND.UNA, but Linux compares cwnd with the number of outstanding packets. That is, Linux is operated in packet units while RFC is in byte units.

Linux supports both interpretations of SACK. When SACK is used, Linux keeps track on the number of in-flight packets by

left_out = sacked_out + lost_out
in_flight = packets_out - left_out + retrans_out


where

• packets_out is the number of originally transmitted segments above SND.UNA
• sacked_out is the number of segments acknowledged by SACK
• lost_out is the estimated number of segments lost in the network (for FACK)
• retrans_out is the number of retransmitted segments

If no SACK, sacked_out is increased by one for every duplicated ACK, so that the behaviour of NewReno is conformed.

Linux keeps a scoreboard for segments. Segments can be marked as any combination of outstanding, acknowledged, retransmitted, or lost.

Besides the TCP state machine, Linux sender keeps another state:

• Open: Normal state in which the sender increases cwnd according to slow start of congestion avoidance upon ACK received.
• Disorder: Sender detected dup ACKs or SACKs. cwnd is not adjusted but each incoming ACK triggers transmission of a new segment. It follows the packet conservation principle that a new packet is sent only if an old packet has left the network. This behaviour is similar to that of RFC3042
• CWR (Congestion Window Reduced): Sender received congestion notifications (ECN or ICMP source quench or otherwise) but with no outstanding retransmissions. The cwnd is reduced by one segment for every second incoming ACK until the window is halved. CWR state is interrupted by Recovery or Loss state.
• Recovery: After sender received sufficient amount of successive dup ACKs and retransmitted the first unacknowledged segment. The cwnd is reduced by one segment for every second incoming ACK, until cwnd == ssthresh. The cwnd is not increased. Sender either retransmits segments that marked lost or makes forward transmission of new segment according to the packet conservation principle. Sender transit into Open state when all outstanding segments at the time recovery state was entered are acknowledged.
• Loss: When RTO expires. All outstanding segments are marked lost, and cwnd is set of one segment. Slow start is performed. In Loss state, cwnd is increased but Recovery state only allows cwnd decrease. Loss state transmit to Open state only after all segments marked loss are acknowledged.

There are some features in TCP that Linux implemented differently.

Linux RTO is in the granularity of 10ms. Some other implementation has granularity as large as 500ms. Linux allows the minimum RTO of 200ms instead of 1s in the specification. To avoid the problem of sudden decrease of RTT causing RTO to be overestimated, the calculation of mean deviation is adaptive:

if (RTT < SRTT && |SRTT-RTT| > RTTDEV) {
MDEV = 31/32 * MDEV + 1/32 * |SRTT-RTT|
} else {
MDEV = 0.75 * MDEV + 0.25 * |SRTT-RTT|
}


Then MDEV is used to calculate RTTDEV and RTO. RTTDEV and MDEV are separate variables: RTTDEV allows to decrease for once only in a round trip time but MDEV is a continuous estimate.

TCP specification suggests to avoid the silly window syndrome by delaying the ACK for a maximum of 500ms. Linux adjusts the delay timer dynamically to estimate the doubled packet inter-arrival time, with maximum limited to 200ms. But for the first incoming segments at the beginning of the connection, ACK is immediate. This behaviour, known as quick acknowledgement, is to speed up the transmission in the beginning of slow start. Quick ACK stops when half of the receiver’s advertised window is occupied.

RFC2861 suggests congestion window validation. It is triggered when the cwnd is not fully used, in which case may give an invalid estimate of network conditions. The TCP sender decreases cwnd to half way between the current cwnd and its utilization.

## Bibliographic data

@inproceedings{
title = "Congestion Control in TCP",
author = "Pasi Sarolahti and Alexey Kuznetsov",
howpublished = "Usenix",
booktitle = "Proceedings of Usenix 2002/Freenix Track",
pages = "49--62",
month = "June",
year = "2002",
}