refactor: remove redundant IpMtu option, use TCB MTU value instead.

Author: Orvaxis

src/stream/tcp.rs

@@ -53,9 +53,6 @@ pub struct TcpConfig {
 pub enum TcpOptions {
     /// Maximum segment size (MSS) for TCP connections. Default is 1460 bytes.
     MaximumSegmentSize(u16),
-
-    /// MTU option，helps to set the MSS (Maximum Segment Size) option in the TCP header.
-    IpMtu(u16),
 }
 
 impl Default for TcpConfig {
@@ -884,6 +881,7 @@ pub(crate) fn write_packet_to_device(
         seq,
         ack,
         window_size,
+        tcb.get_mtu(),
         payload.unwrap_or_default(),
         options,
     )?;
@@ -902,6 +900,7 @@ pub(crate) fn create_raw_packet(
     seq: u32,
     ack: u32,
     win: u16,
+    mtu: u16,
     mut payload: Vec<u8>,
     options: Option<&Vec<TcpOptions>>,
 ) -> std::io::Result<NetworkPacket> {
@@ -912,28 +911,24 @@ pub(crate) fn create_raw_packet(
     tcp_header.rst = flags & RST != 0;
     tcp_header.fin = flags & FIN != 0;
     tcp_header.psh = flags & PSH != 0;
-
+    let mut actual_mss = if src_addr.is_ipv6() || dst_addr.is_ipv6() {
+        mtu - 40 - TCP_HEADER_LEN // IPv6 header is 40 bytes
+    } else {
+        mtu - 20 - TCP_HEADER_LEN
+    };
+    let mut tcp_options = Vec::new();
     if let Some(opts) = options {
-        let mut tcp_options = Vec::new();
         for opt in opts {
             match opt {
-                TcpOptions::MaximumSegmentSize(mss) => tcp_options.push(etherparse::TcpOptionElement::MaximumSegmentSize(*mss)),
-                TcpOptions::IpMtu(mtu) => {
-                    tcp_options.push(etherparse::TcpOptionElement::MaximumSegmentSize(
-                        mtu - TCP_HEADER_LEN
-                            - if src_addr.is_ipv6() || dst_addr.is_ipv6() {
-                                40
-                            } else {
-                                20 // minimum IPv4 header size
-                            },
-                    ));
-                }
+                TcpOptions::MaximumSegmentSize(mss) => actual_mss = *mss,
             }
         }
-        tcp_header
-            .set_options(&tcp_options)
-            .map_err(|e| std::io::Error::new(InvalidInput, e))?;
     }
+    tcp_options.push(etherparse::TcpOptionElement::MaximumSegmentSize(actual_mss));
+    tcp_header
+        .set_options(&tcp_options)
+        .map_err(|e| std::io::Error::new(InvalidInput, e))?;
+
     let ip_header = match (src_addr.ip(), dst_addr.ip()) {
         (std::net::IpAddr::V4(src), std::net::IpAddr::V4(dst)) => {
             let mut ip_h =