CS144 计算机网络实验 lab3 笔记

介绍

本实验中,我们将会在之前实验的基础上,实现一个TCP sender ----将字节流转换成数据报并发送.

TCP协议是一个在不可靠的协议上提供可靠的,流量控制的协议。

我们在本实验中会实现一个TCP发送端，负责将发送端应用层传入的比特流转换成一系列由发出的TCP报文段，在另一端，由TCP接收端将TCP报文段转换成原始比特流（也有可能什么都不做，在从未受到syn，或者多次受到的时候），并将ack和窗口大小返回给TCP发送端

TCP接收端和发送端都负责处理一部分TCP报文段。TCP发送端写入到报文中的每部分都会被TCP接收端解析，包括：seq，SYN，FIN，内容

但是，TCP发送端只会一部分报文的内容，包括：ackno和SYN

TCP发送端有以下任务:

跟踪window size,也就是处理ackno和window sizes
尽可能的填充 ( 自己的 ) windows,直到满了或者ByteStream空了为止
对已经发送但没有收到ackno的报文段 ( 未被确认的报文段) 保持跟踪
如果超出规定的时间,还没有收到对应的ack,就重新发送所有已经发送,但没有收到ackno的报文段

TCP发送端是如何知道报文段丢失呢?

TCP发送端会发送一系列的报文段. 每个报文段都是来自ByteStream的子串加上位置索引seq,如果是第一个报文段,需要加上syn,最后一个需要加上fin

为了发送这些报文段,TCP发送端会对所有已发送的报文段保持跟踪,直到收到响应报文段的ack

具体实现:

TCP发送端会定期的调用TCPSender::tick函数,来表明时间的流逝.
TCP发送端负责监管已发出报文段,如果最早的已发出但没有收到ack的报文段,超出了规定时间,它将会被重新发送

实现记录时间,以及计算时间

每隔几毫秒,TCP发送端的TCPSender’s tick方法将被调用，它告自上次调用此方法以来已经过多少毫秒,不需要自己处理时间
当TCP发送端初始化的时候,将会有一个retransmission timeout (RTO),这就是重传时间,重传时间是变化的,但是最初的超时时间都是一样的,需要调用_initial retransmission timeout
实现一个重传定时器: 当重传时间到达时发出警告,超过重传时间关闭警告,只可以依赖tick方法,不可以根据现实时间
每次一个数据报发送的时候,如果定时器没有开启,就要开启定时器
当确认所有数据时，停止重传计时器
如果调用tick后发现定时器过期:
- 重传最早的未收到ack的包
- 如果windows size大小不为零:
  - 你需要跟踪连续重传的数量,因为TCP连接需要据此判断连接是否出现问题,是否需要中断
  - 将重传定时器时长加倍,这叫做exponential backoff指数补偿,随着重传次数的增加，补偿的程度也会指数增长
- 在超过重传时间后重置重传定时器并开启
当成功收到数据
- 设置重传时间为初始值
- 如果发送重传数据,就需要重启定时器
- 将连续重传记录的数量置零

实现TCP发送端 Implementing the TCP sender

思路

void fill_window()
- TCP发送端被要求尽可能的读取ByteStream中的数据,并且形成数据报 ( 未发送 )
- 确保每次发送的数据报的量正好等于TCP接收端窗口的大小
- 使用TCPSegment::length in sequence space()得到seq,不要忘了SYN , FIN 也占用一个序列号,所以也占用窗口空间
void ack_received(const WrappingInt32 ackno, const uint16 t window size)
- 得到接收端TCP窗口的左沿和大小 , ackno理应大于所有已发送数据报的seq
void tick( const size t ms since last tick )
- 表示自从上一次发送后经过多少毫秒了.不需要我们来调用,参数的意义是距离上次调用经过了多少时间,这个不需要我们操心,我们需要实现每次调用tick()的时候函数做什么
void send empty segment()
- 生成空的数据报,只发一个ack

思路总结

void tick( const size t ms since last tick )
- 表示自从上一次发送后经过多少毫秒了.不需要我们来调用,参数的意义是距离上次调用经过了多少时间,这个不需要我们操心,我们需要实现每次调用tick()的时候函数做什么
我,们需要存储已发送但未被确认的报文段,进行累计确认,如果超时,只需要重传最早的报文段即可
TCPReceiver 调用 unwrap 时的 checkpoint 是上一个接收到的报文段的 absolute_seqno，TCPSender 调用unwrap时的 checkpoint 是 _next_seqno。
retransmission timeout(RTO)，具体实现是RFC6298的简化版
- 重传连续的之后double ；收到ackno后重置到_initial_RTO
- 可参考RFC 6298第5小节实现_timer

加入成员变量

private:bool _syn_sent = false;bool _fin_sent = false;uint64_t _bytes_in_flight = 0;  // Number of bytes in flight 就是未被确认的字节数uint16_t _receiver_window_size = 0;  //接收方的滑动窗口uint16_t _receiver_free_space = 0; //接收方的剩余空间uint16_t _consecutive_retransmissions = 0; //unsigned int _time_elapsed = 0;bool _timer_running = false;                    std::queue<TCPSegment> _segments_outstanding{};  // the segment has been sentvoid send_segment(TCPSegment &seg);              // send segmentbool _ack_valid(uint64_t abs_ackno);//! our initial sequence number, the number for our SYN.WrappingInt32 _isn;//! outbound queue of segments that the TCPSender wants sentstd::queue<TCPSegment> _segments_out{};//! retransmission timer for the connectionunsigned int _initial_retransmission_timeout;//! outgoing stream of bytes that have not yet been sentByteStream _stream;//! the (absolute) sequence number for the next byte to be sentuint64_t _next_seqno{0};unsigned int _rto = 0;
public:

函数实现

`fill_window()` 实现

如果syn未发送,发送并返回
如果syn未应答,返回
如果fin已发送,返回
如果 _receiver_window_size 不为 0
- 当 receiver_free_space 不为 0,尽可能地填充 payload
- 如果 _stream 已经 EOF，且 _receiver_free_space 仍不为 0，填上 FIN（fin 也会占用 _receiver_free_space）
- 如果 _receiver_free_space 还不为 0，且 _stream 还有内容，回到步骤 1 继续填充
如果 _receiver_window_size 为 0，则需要发送零窗口探测报文
- 如果 _receiver_free_space 为 0
  - 如果 _stream 已经 EOF，发送仅携带 FIN 的报文
  - 如果 _stream 还有内容，发送仅携带一位数据的报文

`send_segment()`实现

发送seq,不要忘记isn
记录接收方窗口大小,如果syn true,减去发送包的大小
如果计时器未开启,开启计时器

其他的都很好理解,看一下代码就能懂

总体来说,有点小复杂,总有一些地方不明了,最后借鉴了一位博主的实现成功完成

#include "tcp_sender.hh"#include "tcp_config.hh"#include <random>
#include <algorithm>
// Dummy implementation of a TCP sender// For Lab 3, please replace with a real implementation that passes the
// automated checks run by `make check_lab3`.template <typename... Targs>
void DUMMY_CODE(Targs &&.../* unused */) {}using namespace std;//! \param[in] capacity the capacity of the outgoing byte stream
//! \param[in] retx_timeout the initial amount of time to wait before retransmitting the oldest outstanding segment
//! \param[in] fixed_isn the Initial Sequence Number to use, if set (otherwise uses a random ISN)
TCPSender::TCPSender(const size_t capacity, const uint16_t retx_timeout, const std::optional<WrappingInt32> fixed_isn): _isn(fixed_isn.value_or(WrappingInt32{random_device()()})), _initial_retransmission_timeout{retx_timeout}, _stream(capacity), _rto{retx_timeout} {}
uint64_t TCPSender::bytes_in_flight() const { return _bytes_in_flight; }void TCPSender::fill_window() {if (!_syn_sent) {_syn_sent = true;TCPSegment seg;seg.header().syn = true;send_segment(seg);return;}if (!_segments_outstanding.empty() && _segments_outstanding.front().header().syn)return;if (!_stream.buffer_size() && !_stream.eof())return;if (_fin_sent)return;if (_receiver_window_size) {while (_receiver_free_space) {TCPSegment seg;size_t payload_size = min({_stream.buffer_size(),static_cast<size_t>(_receiver_free_space),static_cast<size_t>(TCPConfig::MAX_PAYLOAD_SIZE)});seg.payload() = _stream.read(payload_size);if (_stream.eof() && static_cast<size_t>(_receiver_free_space) > payload_size) {seg.header().fin = true;_fin_sent = true;}send_segment(seg);if (_stream.buffer_empty())break;}} else if (_receiver_free_space == 0) {// The zero-window-detect-segment should only be sent once (retransmition excute by tick function).// Before it is sent, _receiver_free_space is zero. Then it will be -1.TCPSegment seg;if (_stream.eof()) {seg.header().fin = true;_fin_sent = true;send_segment(seg);} else if (!_stream.buffer_empty()) {seg.payload() = _stream.read(1);send_segment(seg);}}
}void TCPSender::send_segment(TCPSegment &seg) {seg.header().seqno = wrap(_next_seqno, _isn);_next_seqno += seg.length_in_sequence_space();_bytes_in_flight += seg.length_in_sequence_space();if (_syn_sent)_receiver_free_space -= seg.length_in_sequence_space();_segments_out.push(seg);_segments_outstanding.push(seg);if (!_timer_running) {_timer_running = true;_time_elapsed = 0;}
}
// See test code send_window.cc line 113 why the commented code is wrong.
//! \param ackno The remote receiver's ackno (acknowledgment number)
//! \param window_size The remote receiver's advertised window size
void TCPSender::ack_received(const WrappingInt32 ackno, const uint16_t window_size) {uint64_t abs_ackno = unwrap(ackno, _isn, _next_seqno);if (!_ack_valid(abs_ackno)) {// cout << "invalid ackno!\n";return;}_receiver_window_size = window_size;_receiver_free_space = window_size;while (!_segments_outstanding.empty()) {TCPSegment seg = _segments_outstanding.front();if (unwrap(seg.header().seqno, _isn, _next_seqno) + seg.length_in_sequence_space() <= abs_ackno) {_bytes_in_flight -= seg.length_in_sequence_space();_segments_outstanding.pop();// Do not do the following operations outside while loop.// Because if the ack is not corresponding to any segment in the segment_outstanding,// we should not restart the timer._time_elapsed = 0;_rto = _initial_retransmission_timeout;_consecutive_retransmissions = 0;} else {break;}}if (!_segments_outstanding.empty()) {_receiver_free_space = static_cast<uint16_t>(abs_ackno + static_cast<uint64_t>(window_size) -unwrap(_segments_outstanding.front().header().seqno, _isn, _next_seqno) - _bytes_in_flight);}if (!_bytes_in_flight)_timer_running = false;// Note that test code will call it again.fill_window();
}// See test code send_window.cc line 113 why the commented code is wrong.
bool TCPSender::_ack_valid(uint64_t abs_ackno) {return abs_ackno <= _next_seqno &&//  abs_ackno >= unwrap(_segments_outstanding.front().header().seqno, _isn, _next_seqno) +//          _segments_outstanding.front().length_in_sequence_space();abs_ackno >= unwrap(_segments_outstanding.front().header().seqno, _isn, _next_seqno);
}
//! \param[in] ms_since_last_tick the number of milliseconds since the last call to this method
void TCPSender::tick(const size_t ms_since_last_tick) {if (!_timer_running)return;_time_elapsed += ms_since_last_tick;if (_time_elapsed >= _rto) {_segments_out.push(_segments_outstanding.front());if (_receiver_window_size || _segments_outstanding.front().header().syn) {++_consecutive_retransmissions;_rto <<= 1;}_time_elapsed = 0;}
}unsigned int TCPSender::consecutive_retransmissions() const { return _consecutive_retransmissions; }void TCPSender::send_empty_segment() {TCPSegment seg;seg.header().seqno = wrap(_next_seqno, _isn);_segments_out.push(seg);
}