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Fixes unused MTU settings at TcpSocket dispatch #384

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Oct 25, 2020
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Fixes unused MTU settings at TcpSocket dispatch #384

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b933bc3
Fixes unused MTU settings at TcpSocket dispatch
Oct 23, 2020
b767861
Taking into account TCP and IP header
Oct 23, 2020
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3 changes: 2 additions & 1 deletion src/socket/tcp.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1558,7 +1558,8 @@ impl<'a> TcpSocket<'a> {
// Extract as much data as the remote side can receive in this packet
// from the transmit buffer.
let offset = self.remote_last_seq - self.local_seq_no;
let size = cmp::min(self.remote_win_len, self.remote_mss);
let size = cmp::min(cmp::min(self.remote_win_len, self.remote_mss),
caps.max_transmission_unit);
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I'm not convinced this is correct. MTU is IP packet size (IP header + TCP header + payload). size here is only payload size.

You have to subtract the IP and TCP header sizes, as in here :

smoltcp/src/socket/tcp.rs

Line 1624 in 1268fb3

let mut max_segment_size = caps.max_transmission_unit;

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@mustermeiszer mustermeiszer Oct 23, 2020 •
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Ah, so much for a quick fix on my part. Haha.

Is this fine?

let size = cmp::min(cmp::min(self.remote_win_len, self.remote_mss),
                     caps.max_transmission_unit - ip_repr.buffer_len() - repr.mss_header_len());
repr.payload = self.tx_buffer.get_allocated(offset, size);

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Sure!

repr.payload = self.tx_buffer.get_allocated(offset, size);
// If we've sent everything we had in the buffer, follow it with the PSH or FIN
// flags, depending on whether the transmit half of the connection is open.
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