CS 144: Introduction to Computer Networking, Fall 2020
https://cs144.github.io/
总体思路
tick 不需要我们来调用，参数的意义是距离上次 tick 被调用过去的时间，也不需要我们来设定。我们只需要在 tick 中实现，通过参数判断过去了多少时间，需要执行何种操作即可。
注意根据文档，我们要不需要实现选择重传，而是类似回退 N，需要存储已发送并且未被确认的段，进行累计确认，超时时只要重传这些段中最早的那一个即可。
TCPReceiver 调用 unwrap 时的 checkpoint 是上一个接收到的报文段的 absolute_seqno，TCPSender 调用 unwrap 时的 checkpoint 是 _next_seqno。
我的实现中计时器开关的处理：
- 发送新报文段时若计时器未打开，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），关闭计时器
- tick() 中，若计时器为关闭状态，直接返回，否则累加计时然后处理超时
添加的成员变量
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 _segments_outstanding{};
bool _ack_valid(uint64_t abs_ackno);
void _send_segment(TCPSegment &seg);
}; send_segment(TCPSegment &seg)只在fill_window()中被调用，重传只需要_segments_out.push(_segments_outstanding.front())_receiver_window_size保存收到有效（有效的含义见上面ack_valid()）确认报文段时，报文段携带的接收方窗口大小_receiver_free_space是在_receiver_window_size的基础上，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，发送然后返回
- 如果 SYN 未被应答，返回
- 如果 FIN 已经发送，返回
- 如果 _stream 暂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 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
- 当
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
- 之所以还需要判断
_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() 函数控制，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(_receiver_free_space),
static_cast(TCPConfig::MAX_PAYLOAD_SIZE)});
seg.payload() = _stream.read(payload_size);
if (_stream.eof() && static_cast(_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
代码比较直白，注意进行累计确认之后，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(
abs_ackno + static_cast(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);
} tick() 实现
注意，窗口大小为 0 时不需要增加 RTO。但是发送 SYN 时，窗口为初始值也为 0，而 SYN 超时是需要增加 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;
}
}其他代码
#include
TCPSender::TCPSender(const size_t capacity, const uint16_t retx_timeout, const std::optional 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);
}