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../../protocols/tcp.rst:159
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TCP connection establishment can be described as the four state Finite State Machine shown below. In this FSM, `!X` (resp. `?Y`) indicates the transmission of segment `X` (resp. reception of segment `Y`) during the corresponding transition. `Init` is the initial state.
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../../protocols/tcp.rst:177
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A client host starts in the `Init` state. It then sends a `SYN` segment and enters the `SYN Sent` state where it waits for a `SYN+ACK` segment. Then, it replies with an `ACK` segment and enters the `Established` state where data can be exchanged. On the other hand, a server host starts in the `Init` state. When a server process starts to listen to a destination port, the underlying TCP entity creates a TCP control block and a queue to process incoming `SYN` segments. Upon reception of a `SYN` segment, the server's TCP entity replies with a `SYN+ACK` and enters the `SYN RCVD` state. It remains in this state until it receives an `ACK` segment that acknowledges its `SYN+ACK` segment, with this it then enters the `Established` state.
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../../protocols/tcp.rst:179
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Apart from these two paths in the TCP connection establishment FSM, there is a third path that corresponds to the case when both the client and the server send a `SYN` segment to open a TCP connection [#ftcpboth]_. In this case, the client and the server send a `SYN` segment and enter the `SYN Sent` state. Upon reception of the `SYN` segment sent by the other host, they reply by sending a `SYN+ACK` segment and enter the `SYN RCVD` state. The `SYN+ACK` that arrives from the other host allows it to transition to the `Established` state. The figure below illustrates such a simultaneous establishment of a TCP connection.
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../../protocols/tcp.rst:185
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Simultaneous establishment of a TCP connection
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Denial of Service attacks
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../../protocols/tcp.rst:192
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When a TCP entity opens a TCP connection, it creates a Transmission Control Block (:term:`TCB`). The TCB contains the entire state that is maintained by the TCP entity for each TCP connection. During connection establishment, the TCB contains the local IP address, the remote IP address, the local port number, the remote port number, the current local sequence number and the last sequence number received from the remote entity. Until the mid 1990s, TCP implementations had a limit on the number of TCP connections that could be in the `SYN RCVD` state at a given time. Many implementations set this limit to about 100 TCBs. This limit was considered sufficient even for heavily load http servers given the small delay between the reception of a `SYN` segment and the reception of the `ACK` segment that terminates the establishment of the TCP connection. When the limit of 100 TCBs in the `SYN Rcvd` state is reached, the TCP entity discards all received TCP `SYN` segments that do not correspond to an existing TCB.
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../../protocols/tcp.rst:194
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This limit of 100 TCBs in the `SYN Rcvd` state was chosen to protect the TCP entity from the risk of overloading its memory with too many TCBs in the `SYN Rcvd` state. However, it was also the reason for a new type of Denial of Service (DoS) attack :rfc:`4987`. A DoS attack is defined as an attack where an attacker can render a resource unavailable in the network. For example, an attacker may cause a DoS attack on a 2 Mbps link used by a company by sending more than 2 Mbps of packets through this link. In this case, the DoS attack was more subtle. As a TCP entity discards all received `SYN` segments as soon as it has 100 TCBs in the `SYN Rcvd` state, an attacker simply had to send a few 100 `SYN` segments every second to a server and never reply to the received `SYN+ACK` segments. To avoid being caught, attackers were of course sending these `SYN` segments with a different address than their own IP address [#fspoofing]_. On most TCP implementations, once a TCB entered the `SYN Rcvd` state, it remained in this state for several seconds, waiting for a retransmission of the initial `SYN` segment. This attack was later called a `SYN flood` attack and the servers of the ISP named panix were among the first to `be affected <http://memex.org/meme2-12.html>`_ by this attack.
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../../protocols/tcp.rst:200
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To avoid the `SYN flood` attacks, recent TCP implementations no longer enter the `SYN Rcvd` state upon reception of a `SYN segment`. Instead, they reply directly with a `SYN+ACK` segment and wait until the reception of a valid `ACK`. This implementation trick is only possible if the TCP implementation is able to verify that the received `ACK` segment acknowledges the `SYN+ACK` segment sent earlier without storing the initial sequence number of this `SYN+ACK` segment in a TCB. The solution to solve this problem, which is known as `SYN cookies <http://cr.yp.to/syncookies.html>`_ is to compute the 32 bits of the `ISN` as follows :
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../../protocols/tcp.rst:202
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the high order bits contain the low order bits of a counter that is incremented slowly
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../../protocols/tcp.rst:203
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the low order bits contain a hash value computed over the local and remote IP addresses and ports and a random secret only known to the server
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The advantage of the `SYN cookies`_ is that by using them, the server does not need to create a :term:`TCB` upon reception of the `SYN` segment and can still check the returned `ACK` segment by recomputing the `SYN cookie`. The main disadvantage is that they are not fully compatible with the TCP options. This is why they are not enabled by default on a typical system.
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Retransmitting the first `SYN` segment
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../../protocols/tcp.rst:210
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As IP provides an unreliable connectionless service, the `SYN` and `SYN+ACK` segments sent to open a TCP connection could be lost. Current TCP implementations start a retransmission timer when they send the first `SYN` segment. This timer is often set to three seconds for the first retransmission and then doubles after each retransmission :rfc:`2988`. TCP implementations also enforce a maximum number of retransmissions for the initial `SYN` segment.
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../../protocols/tcp.rst:215
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As explained earlier, TCP segments may contain an optional header extension. In the `SYN` and `SYN+ACK` segments, these options are used to negotiate some parameters and the utilization of extensions to the basic TCP specification.
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../../protocols/tcp.rst:219
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The first parameter which is negotiated during the establishment of a TCP connection is the Maximum Segment Size (:term:`MSS`). The MSS is the size of the largest segment that a TCP entity is able to process. According to :rfc:`879`, all TCP implementations must be able to receive TCP segments containing 536 bytes of payload. However, most TCP implementations are able to process larger segments. Such TCP implementations use the TCP MSS Option in the `SYN`/`SYN+ACK` segment to indicate the largest segment they are able to process. The MSS value indicates the maximum size of the payload of the TCP segments. The client (resp. server) stores in its :term:`TCB` the MSS value announced by the server (resp. the client).
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../../protocols/tcp.rst:221
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Another utilization of TCP options during connection establishment is to enable TCP extensions. For example, consider :rfc:`1323` (which is discussed in :ref:`TCPReliable`). :rfc:`1323` defines TCP extensions to support timestamps and larger windows. If the client supports :rfc:`1323`, it adds a :rfc:`1323` option to its `SYN` segment. If the server understands this :rfc:`1323` option and wishes to use it, it replies with a :rfc:`1323` option in the `SYN+ACK` segment and the extension defined in :rfc:`1323` is used throughout the TCP connection. Otherwise, if the server's `SYN+ACK` does not contain the :rfc:`1323` option, the client is not allowed to use this extension and the corresponding TCP header options throughout the TCP connection. TCP's option mechanism is flexible and it allows the extension of TCP while maintaining compatibility with older implementations.
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The TCP options are encoded by using a Type Length Value format where :
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the first byte indicates the `type` of the option.
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the second byte indicates the total length of the option (including the first two bytes) in bytes
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the last bytes are specific for each type of option
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