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Clean up and document TCP MSS calculations.

This commit is contained in:
whitequark 2017-09-22 06:01:59 +00:00
parent ba2d7f483a
commit f23bfe2014
1 changed files with 25 additions and 26 deletions
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51
src/socket/tcp.rs
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@ -1172,6 +1172,17 @@ impl<'a> TcpSocket<'a> {
}
}
// Construct the lowered IP representation.
// We might need this to calculate the MSS, so do it early.
let mut ip_repr = IpRepr::Unspecified {
src_addr: self.local_endpoint.addr,
dst_addr: self.remote_endpoint.addr,
protocol: IpProtocol::Tcp,
payload_len: 0
}.lower(&[])?;
// Construct the basic TCP representation, an empty ACK packet.
// We'll adjust this to be more specific as needed.
let mut repr = TcpRepr {
src_port: self.local_endpoint.port,
dst_port: self.remote_endpoint.port,
@ -1290,33 +1301,21 @@ impl<'a> TcpSocket<'a> {
is_keep_alive = false;
}
// Remember the header length before enabling the MSS option, since that option
// only affects SYN packets.
let header_len = repr.header_len();
if repr.control == TcpControl::Syn {
// First enable the option, without assigning any value, to get a correct
// result for the payload_len field of ip_repr below.
repr.max_seg_size = Some(0);
}
// Then, construct the IP representation, since we know the final length
// of the TCP header.
let ip_repr = IpRepr::Unspecified {
src_addr: self.local_endpoint.addr,
dst_addr: self.remote_endpoint.addr,
protocol: IpProtocol::Tcp,
payload_len: repr.buffer_len()
}.lower(&[])?;
// Finally, compute the maximum segment size, deriving it from from the underlying
// maximum transmission unit and the header sizes we just determined.
let mut max_segment_size = limits.max_transmission_unit;
max_segment_size -= header_len;
max_segment_size -= ip_repr.buffer_len();
if repr.control == TcpControl::Syn {
// And fill in the actual option, if it's a SYN packet.
// Compute the maximum segment size, deriving it from from the underlying
// maximum transmission unit and the IP and TCP header sizes.
//
// Note that what we actually *want* is for the other party to limit
// the total length of the TCP segment, but what we *get* is limiting
// the amount of data in the TCP segment. As a result, if they interpret
// the requirement naively and send us a TCP packet with both some options
// and an MSS-sized payload, that packet's last few bytes will get split
// into a tiny fragment.
//
// TCP is not a well-designed protocol.
let mut max_segment_size = limits.max_transmission_unit;
max_segment_size -= ip_repr.buffer_len();
max_segment_size -= repr.header_len();
repr.max_seg_size = Some(max_segment_size as u16);
}