mirror of
https://github.com/impact-eintr/netstack.git
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104 lines
3.4 KiB
Go
104 lines
3.4 KiB
Go
// Copyright 2018 Google LLC
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package tcp
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import (
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"tcpip/netstack/tcpip/header"
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"tcpip/netstack/tcpip/seqnum"
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)
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const (
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// MaxSACKBlocks is the maximum number of SACK blocks stored
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// at receiver side.
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// MaxSACKBlocks 是接收端存储的最大SACK块数。
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MaxSACKBlocks = 6
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)
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// UpdateSACKBlocks updates the list of SACK blocks to include the segment
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// specified by segStart->segEnd. If the segment happens to be an out of order
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// delivery then the first block in the sack.blocks always includes the
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// segment identified by segStart->segEnd.
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// tcp的可靠性：UpdateSACKBlocks 更新SACK块列表以包含 segStart-segEnd 指定的段，只有没有被消费掉的seg才会被用来更新sack。
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// 如果该段恰好是无序传递，那么sack.blocks中的第一个块总是包括由 segStart-segEnd 标识的段。
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func UpdateSACKBlocks(sack *SACKInfo, segStart seqnum.Value, segEnd seqnum.Value, rcvNxt seqnum.Value) {
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newSB := header.SACKBlock{Start: segStart, End: segEnd}
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if sack.NumBlocks == 0 {
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sack.Blocks[0] = newSB
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sack.NumBlocks = 1
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return
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}
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var n = 0
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for i := 0; i < sack.NumBlocks; i++ {
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start, end := sack.Blocks[i].Start, sack.Blocks[i].End
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if end.LessThanEq(start) || start.LessThanEq(rcvNxt) {
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// Discard any invalid blocks where end is before start
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// and discard any sack blocks that are before rcvNxt as
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// those have already been acked.
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continue
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}
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if newSB.Start.LessThanEq(end) && start.LessThanEq(newSB.End) {
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// Merge this SACK block into newSB and discard this SACK
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// block.
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if start.LessThan(newSB.Start) {
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newSB.Start = start
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}
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if newSB.End.LessThan(end) {
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newSB.End = end
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}
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} else {
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// Save this block.
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sack.Blocks[n] = sack.Blocks[i]
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n++
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}
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}
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if rcvNxt.LessThan(newSB.Start) {
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// If this was an out of order segment then make sure that the
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// first SACK block is the one that includes the segment.
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//
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// See the first bullet point in
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// https://tools.ietf.org/html/rfc2018#section-4
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if n == MaxSACKBlocks {
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// If the number of SACK blocks is equal to
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// MaxSACKBlocks then discard the last SACK block.
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n--
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}
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for i := n - 1; i >= 0; i-- {
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sack.Blocks[i+1] = sack.Blocks[i]
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}
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sack.Blocks[0] = newSB
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n++
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}
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sack.NumBlocks = n
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}
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// TrimSACKBlockList updates the sack block list by removing/modifying any block
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// where start is < rcvNxt.
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// tcp的可靠性：TrimSACKBlockList 通过删除/修改 start为 <rcvNxt 的任何块来更新sack块列表。
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func TrimSACKBlockList(sack *SACKInfo, rcvNxt seqnum.Value) {
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n := 0
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for i := 0; i < sack.NumBlocks; i++ {
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if sack.Blocks[i].End.LessThanEq(rcvNxt) {
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continue
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}
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if sack.Blocks[i].Start.LessThan(rcvNxt) {
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// Shrink this SACK block.
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sack.Blocks[i].Start = rcvNxt
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}
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sack.Blocks[n] = sack.Blocks[i]
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n++
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}
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sack.NumBlocks = n
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}
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