cnp3-ebook/principles/transport

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108 ../../principles/transport.rst:644

	English The simplest approach to establish a transport connection would be to define two special control segments : `CR` (Connection Request) and `CA` (Connection Acknowledgment). The `CR` segment is sent by the transport entity that wishes to initiate a connection. If the remote entity wishes to accept the connection, it replies by sending a `CA` segment. The `CR` and `CA` segments contain `port numbers` that allow identifying the communicating applications. The transport connection is considered to be established once the `CA` segment has been received. At that point, data segments can be sent in both directions. 614/10000 · 614
109 ../../principles/transport.rst:669

	English Unfortunately, this scheme is not sufficient given the unreliable network layer. Since the network layer is imperfect, the `CR` or `CA` segments can be lost, delayed, or suffer from transmission errors. To deal with these problems, the control segments must be protected by a CRC or a checksum to detect transmission errors. Furthermore, since the `CA` segment acknowledges the reception of the `CR` segment, the `CR` segment can be protected using a retransmission timer. 472/10000 · 472
110 ../../principles/transport.rst:671

	English Unfortunately, this scheme is not sufficient to ensure the reliability of the transport service. Consider for example a short-lived transport connection where a single, but important transfer (e.g. money transfer from a bank account) is sent. Such a short-lived connection starts with a `CR` segment acknowledged by a `CA` segment, then the data segment is sent, acknowledged and the connection terminates. Unfortunately, as the network layer service is unreliable, delays combined to retransmissions may lead to the situation depicted in the figure below, where a delayed `CR` and data segments from a former connection are accepted by the receiving entity as valid segments, and the corresponding data is delivered to the user. Duplicating SDUs is not acceptable, and the transport protocol must solve this problem. 817/10000 · 817
111 ../../principles/transport.rst:711

	English To avoid these duplicates, transport protocols require the network layer to bound the `Maximum Segment Lifetime (MSL)`. The organization of the network must guarantee that no segment remains in the network for longer than `MSL` seconds. For example, on today's Internet, `MSL` is expected to be 2 minutes. To avoid duplicate transport connections, transport protocol entities must be able to safely distinguish between a duplicate `CR` segment and a new `CR` segment, without forcing each transport entity to remember all the transport connections that it has established in the past. 584/10000 · 584
112 ../../principles/transport.rst:713

	English A classical solution to avoid remembering the previous transport connections to detect duplicates is to use a clock inside each transport entity. This `transport clock` has the following characteristics : 204/10000 · 204
113 ../../principles/transport.rst:715

	English the `transport clock` is implemented as a `k` bits counter and its clock cycle is such that :math:`2^k \times cycle >> MSL`. Furthermore, the `transport clock` counter is incremented every clock cycle and after each connection establishment. This clock is illustrated in the figure below. 288/10000 · 288
114 ../../principles/transport.rst:716

	English the `transport clock` must continue to be incremented even if the transport entity stops or reboots 99/10000 · 99
115 ../../principles/transport.rst:722

	English Transport clock 15/10000 · 15
116 ../../principles/transport.rst:725

	English It should be noted that `transport clocks` do not need and usually are not synchronized to the real-time clock. Precisely synchronizing real-time clocks is an interesting problem, but it is outside the scope of this document. See [Mills2006]_ for a detailed discussion on synchronizing the real-time clock. 306/10000 · 306
117 ../../principles/transport.rst:727

	English This `transport clock` can now be combined with an exchange of three segments, called the `three way handshake`, to detect duplicates. This `three way handshake` occurs as follows : 181/10000 · 181
118 ../../principles/transport.rst:729

	English The initiating transport entity sends a `CR` segment. This segment requests the establishment of a transport connection. It contains a port number (not shown in the figure) and a sequence number (`seq=x` in the figure below) whose value is extracted from the `transport clock`. The transmission of the `CR` segment is protected by a retransmission timer. 354/10000 · 354
119 ../../principles/transport.rst:731

	English The remote transport entity processes the `CR` segment and creates state for the connection attempt. At this stage, the remote entity does not yet know whether this is a new connection attempt or a duplicate segment. It returns a `CA` segment that contains an acknowledgment number to confirm the reception of the `CR` segment (`ack=x` in the figure below) and a sequence number (`seq=y` in the figure below) whose value is extracted from its transport clock. At this stage, the connection is not yet established. 513/10000 · 513
120 ../../principles/transport.rst:733

	English The initiating entity receives the `CA` segment. The acknowledgment number of this segment confirms that the remote entity has correctly received the `CR` segment. The transport connection is considered to be established by the initiating entity and the numbering of the data segments starts at sequence number `x`. Before sending data segments, the initiating entity must acknowledge the received `CA` segments by sending another `CA` segment. 444/10000 · 444
121 ../../principles/transport.rst:735

	English The remote entity considers the transport connection to be established after having received the segment that acknowledges its `CA` segment. The numbering of the data segments sent by the remote entity starts at sequence number `y`. 232/10000 · 232
122 ../../principles/transport.rst:737

	English The three way handshake is illustrated in the figure below. 59/10000 · 59
123 ../../principles/transport.rst:743

	English Three-way handshake 19/10000 · 19
124 ../../principles/transport.rst:745

	English Thanks to the three way handshake, transport entities avoid duplicate transport connections. This is illustrated by considering the three scenarios below. 154/10000 · 154
125 ../../principles/transport.rst:747

	English The first scenario is when the remote entity receives an old `CR` segment. It considers this `CR` segment as a connection establishment attempt and replies by sending a `CA` segment. However, the initiating host cannot match the received `CA` segment with a previous connection attempt. It sends a control segment (`REJECT` in the figure below) to cancel the spurious connection attempt. The remote entity cancels the connection attempt upon reception of this control segment. 476/10000 · 476
126 ../../principles/transport.rst:753

	English Three-way handshake : recovery from a duplicate `CR` 52/10000 · 52
127 ../../principles/transport.rst:755

	English A second scenario is when the initiating entity sends a `CR` segment that does not reach the remote entity and receives a duplicate `CA` segment from a previous connection attempt. This duplicate `CA` segment cannot contain a valid acknowledgment for the `CR` segment as the sequence number of the `CR` segment was extracted from the transport clock of the initiating entity. The `CA` segment is thus rejected and the `CR` segment is retransmitted upon expiration of the retransmission timer. 492/10000 · 492

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