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// Copyright 2018 Google LLC
//
// 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)
}
// HandleNDupAcks implements congestionControl.HandleNDupAcks.
func (r *renoState) HandleNDupAcks() {
// A retransmit was triggered due to nDupAckThreshold
// being hit. 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.
}
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