So we use the addTcpOptionAfter() method and give it a TcpOptionBuilder object. We'd like to add a fourth one of type MSS with MSS value of 1460 and we want it to appear first (before the existing TCP options). The packet we're editing already has 3 TCP options: NOP, NOP and Timestamp. TcpLayer exposes an API to read, add and remove TCP options. Now let's take a look at the 2 bottom lines in the code snippet above.
So we changed the source port to 12345 and set the URG flag. Again, like we saw in IPv4Layer, this method gives access to the actual packet bytes so every change we do changes the packet. We start by using the method getTcpHeader() which casts the raw packet bytes to a struct tpchdr* which contains all of the TCP fields.
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