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// Copyright 2016 The Netstack Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package tcp
import (
"gvisor.googlesource.com/gvisor/pkg/tcpip/header"
"gvisor.googlesource.com/gvisor/pkg/tcpip/seqnum"
)
const (
// MaxSACKBlocks is the maximum number of SACK blocks stored
// at receiver side.
MaxSACKBlocks = 6
)
// UpdateSACKBlocks updates the list of SACK blocks to include the segment
// specified by segStart->segEnd. If the segment happens to be an out of order
// delivery then the first block in the sack.blocks always includes the
// segment identified by segStart->segEnd.
func UpdateSACKBlocks(sack *SACKInfo, segStart seqnum.Value, segEnd seqnum.Value, rcvNxt seqnum.Value) {
newSB := header.SACKBlock{Start: segStart, End: segEnd}
if sack.NumBlocks == 0 {
sack.Blocks[0] = newSB
sack.NumBlocks = 1
return
}
var n = 0
for i := 0; i < sack.NumBlocks; i++ {
start, end := sack.Blocks[i].Start, sack.Blocks[i].End
if end.LessThanEq(start) || start.LessThanEq(rcvNxt) {
// Discard any invalid blocks where end is before start
// and discard any sack blocks that are before rcvNxt as
// those have already been acked.
continue
}
if newSB.Start.LessThanEq(end) && start.LessThanEq(newSB.End) {
// Merge this SACK block into newSB and discard this SACK
// block.
if start.LessThan(newSB.Start) {
newSB.Start = start
}
if newSB.End.LessThan(end) {
newSB.End = end
}
} else {
// Save this block.
sack.Blocks[n] = sack.Blocks[i]
n++
}
}
if rcvNxt.LessThan(newSB.Start) {
// If this was an out of order segment then make sure that the
// first SACK block is the one that includes the segment.
//
// See the first bullet point in
// https://tools.ietf.org/html/rfc2018#section-4
if n == MaxSACKBlocks {
// If the number of SACK blocks is equal to
// MaxSACKBlocks then discard the last SACK block.
n--
}
for i := n - 1; i >= 0; i-- {
sack.Blocks[i+1] = sack.Blocks[i]
}
sack.Blocks[0] = newSB
n++
}
sack.NumBlocks = n
}
// TrimSACKBlockList updates the sack block list by removing/modifying any block
// where start is < rcvNxt.
func TrimSACKBlockList(sack *SACKInfo, rcvNxt seqnum.Value) {
n := 0
for i := 0; i < sack.NumBlocks; i++ {
if sack.Blocks[i].End.LessThanEq(rcvNxt) {
continue
}
if sack.Blocks[i].Start.LessThan(rcvNxt) {
// Shrink this SACK block.
sack.Blocks[i].Start = rcvNxt
}
sack.Blocks[n] = sack.Blocks[i]
n++
}
sack.NumBlocks = n
}
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