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// Copyright 2020 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
import (
"time"
"gvisor.dev/gvisor/pkg/tcpip/seqnum"
)
// RACK is a loss detection algorithm used in TCP to detect packet loss and
// reordering using transmission timestamp of the packets instead of packet or
// sequence counts. To use RACK, SACK should be enabled on the connection.
// rackControl stores the rack related fields.
// See: https://tools.ietf.org/html/draft-ietf-tcpm-rack-08#section-6.1
//
// +stateify savable
type rackControl struct {
// xmitTime is the latest transmission timestamp of rackControl.seg.
xmitTime time.Time `state:".(unixTime)"`
// endSequence is the ending TCP sequence number of rackControl.seg.
endSequence seqnum.Value
// fack is the highest selectively or cumulatively acknowledged
// sequence.
fack seqnum.Value
// rtt is the RTT of the most recently delivered packet on the
// connection (either cumulatively acknowledged or selectively
// acknowledged) that was not marked invalid as a possible spurious
// retransmission.
rtt time.Duration
}
// Update will update the RACK related fields when an ACK has been received.
// See: https://tools.ietf.org/html/draft-ietf-tcpm-rack-08#section-7.2
func (rc *rackControl) Update(seg *segment, ackSeg *segment, srtt time.Duration, offset uint32) {
rtt := time.Now().Sub(seg.xmitTime)
// If the ACK is for a retransmitted packet, do not update if it is a
// spurious inference which is determined by below checks:
// 1. When Timestamping option is available, if the TSVal is less than the
// transmit time of the most recent retransmitted packet.
// 2. When RTT calculated for the packet is less than the smoothed RTT
// for the connection.
// See: https://tools.ietf.org/html/draft-ietf-tcpm-rack-08#section-7.2
// step 2
if seg.xmitCount > 1 {
if ackSeg.parsedOptions.TS && ackSeg.parsedOptions.TSEcr != 0 {
if ackSeg.parsedOptions.TSEcr < tcpTimeStamp(seg.xmitTime, offset) {
return
}
}
if rtt < srtt {
return
}
}
rc.rtt = rtt
// Update rc.xmitTime and rc.endSequence to the transmit time and
// ending sequence number of the packet which has been acknowledged
// most recently.
endSeq := seg.sequenceNumber.Add(seqnum.Size(seg.data.Size()))
if rc.xmitTime.Before(seg.xmitTime) || (seg.xmitTime.Equal(rc.xmitTime) && rc.endSequence.LessThan(endSeq)) {
rc.xmitTime = seg.xmitTime
rc.endSequence = endSeq
}
}
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