CS 144: Introduction to Computer Networking, Fall 2020
https://cs144.github.io/My Repo
https://github.com/wine99/cs1...
总体思路
tick 不须要咱们来调用，参数的意义是间隔上次 tick 被调用过来的工夫，也不须要咱们来设定。咱们只须要在 tick 中实现，通过参数判断过来了多少工夫，须要执行何种操作即可。
留神依据文档，咱们要不须要实现抉择重传，而是相似回退 N，须要存储已发送并且未被确认的段，进行累计确认，超时时只有重传这些段中最早的那一个即可。
TCPReceiver 调用 unwrap 时的 checkpoint 是上一个接管到的报文段的 absolute_seqno，TCPSender 调用 unwrap 时的 checkpoint 是 _next_seqno。
我的实现中计时器开关的解决：
- 发送新报文段时若计时器未关上，开启
- ack_received() 中，如果有报文段被正确地确认，重置计时器和 RTO，如果所有报文段均被确认（bytes in flight == 0），敞开计时器
- tick() 中，若计时器为敞开状态，间接返回，否则累加计时而后解决超时
增加的成员变量
class TCPSender { private: bool _syn_sent = false; bool _fin_sent = false; uint64_t _bytes_in_flight = 0; uint16_t _receiver_window_size = 0; uint16_t _receiver_free_space = 0; uint16_t _consecutive_retransmissions = 0; unsigned int _rto = 0; unsigned int _time_elapsed = 0; bool _timer_running = false; std::queue<TCPSegment> _segments_outstanding{}; // See test code send_window.cc line 113 why the commented code is wrong. bool 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); } public: void send_segment(TCPSegment &seg);};send_segment(TCPSegment &seg)只在fill_window()中被调用，重传只须要_segments_out.push(_segments_outstanding.front())_receiver_window_size保留收到无效（无效的含意见下面ack_valid()）确认报文段时，报文段携带的接管方窗口大小_receiver_free_space是在_receiver_window_size的根底上，再减去已发送的报文段可能占用的空间（_bytes_in_flight）
fill_window() 实现
- 如果 SYN 未发送，发送而后返回
- 如果 SYN 未被应答，返回
- 如果 FIN 曾经发送，返回
- 如果 _stream 临时没有内容但并没有 EOF，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 曾经 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 持续填充
- 当
如果
_receiver_window_size为 0，则须要发送零窗口探测报文如果
_receiver_free_space为 0- 如果 _stream 曾经 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
- 之所以还须要判断
_receiver_free_space为 0，是因为这些报文段在此处应该只发送一次，后续的重传由 tick() 函数管制，而当发送了零窗口报文段后_receiver_free_space的值就会从原来的与_receiver_window_size相等的 0 变成 -1
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; }}ack_received() 实现
代码比拟直白，留神进行累计确认之后，如果还有未被确认的报文段，_receiver_free_space 的值应为：收到的确认号绝对值 + 窗口大小 - 首个未确认报文的序号绝对值 - 未确认报文段的长度总和。
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();}tick() 实现
留神，窗口大小为 0 时不须要减少 RTO。然而发送 SYN 时，窗口为初始值也为 0，而 SYN 超时是须要减少 RTO 的。
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; }}其余代码
#include <algorithm>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; }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);}