feat: started to make a TCP state machine

packets/src/lib.rs

@@ -577,7 +577,13 @@ impl TCPPacketBuilder {
         self
     }
 
-    pub fn build<I1, I2>(self, srcip: I1, dstip: I2, data: Vec<u8>) -> TCPPacket
+    pub fn build<I1, I2>(
+        self,
+        srcip: I1,
+        dstip: I2,
+        data: Vec<u8>,
+        checksum_offset: Option<u16>,
+    ) -> TCPPacket
     where
         I1: Into<Ipv4Addr>,
         I2: Into<Ipv4Addr>,
@@ -626,10 +632,7 @@ impl TCPPacketBuilder {
                 }
             })
             .fold(0u32, |acc, e| acc + e)
-            + 30;
-        // + 30 to intentionally deviate from
-        // the RFC, to make it so that I can identify the packets as sparse
-        // packets
+            + (checksum_offset.unwrap_or(0) as u32);
 
         let checksum = (checksum >> 16) + (checksum & 0xffff);
         let checksum = ((checksum >> 16) as u16) + (checksum as u16);
@@ -645,3 +648,33 @@ impl TCPPacketBuilder {
         }
     }
 }
+
+#[cfg(test)]
+mod test {
+    use std::net::Ipv4Addr;
+
+    use crate::TCPPacketBuilder;
+
+    #[test]
+    fn test_checksum() {
+        let tcp_packet = TCPPacketBuilder::default()
+            .srcport(57116)
+            .dstport(54248)
+            .seqnumber(0xaed77262)
+            .acknumber(0)
+            .syn(true)
+            .window(64240)
+            .options(vec![
+                0x02, 0x04, 0x05, 0xb4, 0x04, 0x02, 0x08, 0x0a, 0xd4, 0x06, 0xdf, 0x18, 0x00, 0x00,
+                0x00, 0x00, 0x01, 0x03, 0x03, 0x07,
+            ])
+            .build(
+                Ipv4Addr::new(10, 104, 18, 67),
+                Ipv4Addr::new(10, 104, 21, 16),
+                vec![],
+                None,
+            );
+
+        assert_eq!(tcp_packet.pkt().checksum(), 0x898d);
+    }
+}