github.com/SagerNet/gvisor@v0.0.0-20210707092255-7731c139d75c/pkg/tcpip/transport/tcp/reno.go (about)

     1  // Copyright 2018 The gVisor Authors.
     2  //
     3  // Licensed under the Apache License, Version 2.0 (the "License");
     4  // you may not use this file except in compliance with the License.
     5  // You may obtain a copy of the License at
     6  //
     7  //     http://www.apache.org/licenses/LICENSE-2.0
     8  //
     9  // Unless required by applicable law or agreed to in writing, software
    10  // distributed under the License is distributed on an "AS IS" BASIS,
    11  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    12  // See the License for the specific language governing permissions and
    13  // limitations under the License.
    14  
    15  package tcp
    16  
    17  // renoState stores the variables related to TCP New Reno congestion
    18  // control algorithm.
    19  //
    20  // +stateify savable
    21  type renoState struct {
    22  	s *sender
    23  }
    24  
    25  // newRenoCC initializes the state for the NewReno congestion control algorithm.
    26  func newRenoCC(s *sender) *renoState {
    27  	return &renoState{s: s}
    28  }
    29  
    30  // updateSlowStart will update the congestion window as per the slow-start
    31  // algorithm used by NewReno. If after adjusting the congestion window
    32  // we cross the SSthreshold then it will return the number of packets that
    33  // must be consumed in congestion avoidance mode.
    34  func (r *renoState) updateSlowStart(packetsAcked int) int {
    35  	// Don't let the congestion window cross into the congestion
    36  	// avoidance range.
    37  	newcwnd := r.s.SndCwnd + packetsAcked
    38  	if newcwnd >= r.s.Ssthresh {
    39  		newcwnd = r.s.Ssthresh
    40  		r.s.SndCAAckCount = 0
    41  	}
    42  
    43  	packetsAcked -= newcwnd - r.s.SndCwnd
    44  	r.s.SndCwnd = newcwnd
    45  	return packetsAcked
    46  }
    47  
    48  // updateCongestionAvoidance will update congestion window in congestion
    49  // avoidance mode as described in RFC5681 section 3.1
    50  func (r *renoState) updateCongestionAvoidance(packetsAcked int) {
    51  	// Consume the packets in congestion avoidance mode.
    52  	r.s.SndCAAckCount += packetsAcked
    53  	if r.s.SndCAAckCount >= r.s.SndCwnd {
    54  		r.s.SndCwnd += r.s.SndCAAckCount / r.s.SndCwnd
    55  		r.s.SndCAAckCount = r.s.SndCAAckCount % r.s.SndCwnd
    56  	}
    57  }
    58  
    59  // reduceSlowStartThreshold reduces the slow-start threshold per RFC 5681,
    60  // page 6, eq. 4. It is called when we detect congestion in the network.
    61  func (r *renoState) reduceSlowStartThreshold() {
    62  	r.s.Ssthresh = r.s.Outstanding / 2
    63  	if r.s.Ssthresh < 2 {
    64  		r.s.Ssthresh = 2
    65  	}
    66  
    67  }
    68  
    69  // Update updates the congestion state based on the number of packets that
    70  // were acknowledged.
    71  // Update implements congestionControl.Update.
    72  func (r *renoState) Update(packetsAcked int) {
    73  	if r.s.SndCwnd < r.s.Ssthresh {
    74  		packetsAcked = r.updateSlowStart(packetsAcked)
    75  		if packetsAcked == 0 {
    76  			return
    77  		}
    78  	}
    79  	r.updateCongestionAvoidance(packetsAcked)
    80  }
    81  
    82  // HandleLossDetected implements congestionControl.HandleLossDetected.
    83  func (r *renoState) HandleLossDetected() {
    84  	// A retransmit was triggered due to nDupAckThreshold or when RACK
    85  	// detected loss. Reduce our slow start threshold.
    86  	r.reduceSlowStartThreshold()
    87  }
    88  
    89  // HandleRTOExpired implements congestionControl.HandleRTOExpired.
    90  func (r *renoState) HandleRTOExpired() {
    91  	// We lost a packet, so reduce ssthresh.
    92  	r.reduceSlowStartThreshold()
    93  
    94  	// Reduce the congestion window to 1, i.e., enter slow-start. Per
    95  	// RFC 5681, page 7, we must use 1 regardless of the value of the
    96  	// initial congestion window.
    97  	r.s.SndCwnd = 1
    98  }
    99  
   100  // PostRecovery implements congestionControl.PostRecovery.
   101  func (r *renoState) PostRecovery() {
   102  	// noop.
   103  }