Secondly, early implementations rejected all TCP options (such as large windows or timestamps), because the server discarded the SYN queue entry where that information would otherwise be stored. Firstly, the server is limited to only 8 unique MSS values, as that is all that can be encoded in 3 bits. There are, however, two caveats that take effect when SYN cookies are in use. The use of SYN cookies does not break any protocol specifications, and therefore should be compatible with all TCP implementations. Decodes the value m from the 3-bit encoding in the SYN cookie, which it then can use to reconstruct the SYN queue entry.įrom this point forward, the connection proceeds as normal.Recomputes s to determine whether this is, indeed, a valid SYN cookie.Checks the value t against the current time to see if the connection has expired.The server then performs the following operations. The server then subtracts 1 from the acknowledgement number to reveal the SYN cookie sent to the client. When a client sends back a TCP ACK packet to the server in response to the server's SYN+ACK packet, the client must (according to the TCP spec) use n+1 in the packet's Acknowledgement number, where n is the initial sequence number sent by the server. (Note: since m must be encoded using 3 bits, the server is restricted to sending up to 8 unique values for m when SYN cookies are in use.) Middle 3 bits: an encoded value representing m.The returned value s must be a 24-bit value. let s be the result of a cryptographic hash function computed over the server IP address and port number, the client IP address and port number, and the value t.let m be the maximum segment size (MSS) value that the server would have stored in the SYN queue entry.let t be a slowly incrementing timestamp (typically time() logically right-shifted 6 positions, which gives a resolution of 64 seconds).SYN cookies are initial sequence numbers that are carefully constructed according to the following rules: The following describes one possible implementation, however as there is no public standard to follow, the order, length, and semantics of the fields may differ between SYN cookie implementations. As the sequence number is chosen by the sender, returned by the recipient, and has no otherwise-defined internal structure, it can be overloaded to carry additional data. According to the TCP specification, that first sequence number sent by an endpoint can be any value as decided by that endpoint. One of the values in this packet is a sequence number, which is used by the TCP to reassemble the data stream. In response, the server sends a TCP SYN+ACK packet back to the client. In order to initiate a TCP connection, the client sends a TCP SYN packet to the server. ( October 2021) ( Learn how and when to remove this template message) Unsourced material may be challenged and removed. Please help improve this section by adding citations to reliable sources.
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