| // Copyright 2018 The gVisor Authors. |
| // |
| // Licensed under the Apache License, Version 2.0 (the "License"); |
| // you may not use this file except in compliance with the License. |
| // You may obtain a copy of the License at |
| // |
| // http://www.apache.org/licenses/LICENSE-2.0 |
| // |
| // Unless required by applicable law or agreed to in writing, software |
| // distributed under the License is distributed on an "AS IS" BASIS, |
| // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| // See the License for the specific language governing permissions and |
| // limitations under the License. |
| |
| package tcp |
| |
| // renoState stores the variables related to TCP New Reno congestion |
| // control algorithm. |
| // |
| // +stateify savable |
| type renoState struct { |
| s *sender |
| } |
| |
| // newRenoCC initializes the state for the NewReno congestion control algorithm. |
| func newRenoCC(s *sender) *renoState { |
| return &renoState{s: s} |
| } |
| |
| // updateSlowStart will update the congestion window as per the slow-start |
| // algorithm used by NewReno. If after adjusting the congestion window |
| // we cross the SSthreshold then it will return the number of packets that |
| // must be consumed in congestion avoidance mode. |
| func (r *renoState) updateSlowStart(packetsAcked int) int { |
| // Don't let the congestion window cross into the congestion |
| // avoidance range. |
| newcwnd := r.s.sndCwnd + packetsAcked |
| if newcwnd >= r.s.sndSsthresh { |
| newcwnd = r.s.sndSsthresh |
| r.s.sndCAAckCount = 0 |
| } |
| |
| packetsAcked -= newcwnd - r.s.sndCwnd |
| r.s.sndCwnd = newcwnd |
| return packetsAcked |
| } |
| |
| // updateCongestionAvoidance will update congestion window in congestion |
| // avoidance mode as described in RFC5681 section 3.1 |
| func (r *renoState) updateCongestionAvoidance(packetsAcked int) { |
| // Consume the packets in congestion avoidance mode. |
| r.s.sndCAAckCount += packetsAcked |
| if r.s.sndCAAckCount >= r.s.sndCwnd { |
| r.s.sndCwnd += r.s.sndCAAckCount / r.s.sndCwnd |
| r.s.sndCAAckCount = r.s.sndCAAckCount % r.s.sndCwnd |
| } |
| } |
| |
| // reduceSlowStartThreshold reduces the slow-start threshold per RFC 5681, |
| // page 6, eq. 4. It is called when we detect congestion in the network. |
| func (r *renoState) reduceSlowStartThreshold() { |
| r.s.sndSsthresh = r.s.outstanding / 2 |
| if r.s.sndSsthresh < 2 { |
| r.s.sndSsthresh = 2 |
| } |
| |
| } |
| |
| // Update updates the congestion state based on the number of packets that |
| // were acknowledged. |
| // Update implements congestionControl.Update. |
| func (r *renoState) Update(packetsAcked int) { |
| if r.s.sndCwnd < r.s.sndSsthresh { |
| packetsAcked = r.updateSlowStart(packetsAcked) |
| if packetsAcked == 0 { |
| return |
| } |
| } |
| r.updateCongestionAvoidance(packetsAcked) |
| } |
| |
| // HandleLossDetected implements congestionControl.HandleLossDetected. |
| func (r *renoState) HandleLossDetected() { |
| // A retransmit was triggered due to nDupAckThreshold or when RACK |
| // detected loss. Reduce our slow start threshold. |
| r.reduceSlowStartThreshold() |
| } |
| |
| // HandleRTOExpired implements congestionControl.HandleRTOExpired. |
| func (r *renoState) HandleRTOExpired() { |
| // We lost a packet, so reduce ssthresh. |
| r.reduceSlowStartThreshold() |
| |
| // Reduce the congestion window to 1, i.e., enter slow-start. Per |
| // RFC 5681, page 7, we must use 1 regardless of the value of the |
| // initial congestion window. |
| r.s.sndCwnd = 1 |
| } |
| |
| // PostRecovery implements congestionControl.PostRecovery. |
| func (r *renoState) PostRecovery() { |
| // noop. |
| } |