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

     1  // Copyright 2020 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  // renoRecovery stores the variables related to TCP Reno loss recovery
    18  // algorithm.
    19  //
    20  // +stateify savable
    21  type renoRecovery struct {
    22  	s *sender
    23  }
    24  
    25  func newRenoRecovery(s *sender) *renoRecovery {
    26  	return &renoRecovery{s: s}
    27  }
    28  
    29  func (rr *renoRecovery) DoRecovery(rcvdSeg *segment, fastRetransmit bool) {
    30  	ack := rcvdSeg.ackNumber
    31  	snd := rr.s
    32  
    33  	// We are in fast recovery mode. Ignore the ack if it's out of range.
    34  	if !ack.InRange(snd.SndUna, snd.SndNxt+1) {
    35  		return
    36  	}
    37  
    38  	// Don't count this as a duplicate if it is carrying data or
    39  	// updating the window.
    40  	if rcvdSeg.logicalLen() != 0 || snd.SndWnd != rcvdSeg.window {
    41  		return
    42  	}
    43  
    44  	// Inflate the congestion window if we're getting duplicate acks
    45  	// for the packet we retransmitted.
    46  	if !fastRetransmit && ack == snd.FastRecovery.First {
    47  		// We received a dup, inflate the congestion window by 1 packet
    48  		// if we're not at the max yet. Only inflate the window if
    49  		// regular FastRecovery is in use, RFC6675 does not require
    50  		// inflating cwnd on duplicate ACKs.
    51  		if snd.SndCwnd < snd.FastRecovery.MaxCwnd {
    52  			snd.SndCwnd++
    53  		}
    54  		return
    55  	}
    56  
    57  	// A partial ack was received. Retransmit this packet and remember it
    58  	// so that we don't retransmit it again.
    59  	//
    60  	// We don't inflate the window because we're putting the same packet
    61  	// back onto the wire.
    62  	//
    63  	// N.B. The retransmit timer will be reset by the caller.
    64  	snd.FastRecovery.First = ack
    65  	snd.DupAckCount = 0
    66  	snd.resendSegment()
    67  }