cnp3-ebook/principles/transport

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102 ../../principles/transport.rst:613

	English Compared to the connectionless network layer service, the transport layer service allows several applications running on a host to exchange SDUs with several other applications running on remote hosts. Let us consider two hosts, e.g. a client and a server. The network layer service allows the client to send information to the server, but if an application running on the client wants to contact a particular application running on the server, then an additional addressing mechanism is required. The network layer address identifies a host, but it is not sufficient to differentiate the applications running on a host. `Port numbers` provides this additional addressing. When a server application is launched on a host, it registers a `port number`. This `port number` will be used by the clients to contact the server process. 829/10000 · 829
103 ../../principles/transport.rst:615

	English The figure below shows a typical usage of port numbers. The client process uses port number `1234` while the server process uses port number `5678`. When the client sends a request, it is identified as originating from port number `1234` on the client host and destined to port number `5678` on the server host. When the server process replies to this request, the server's transport layer returns the reply as originating from port `5678` on the server host and destined to port `1234` on the client host. 507/10000 · 507
104 ../../principles/transport.rst:634

	English To support the connection-oriented service, the transport layer needs to include several mechanisms to enrich the connectionless network-layer service. We discuss these mechanisms in the following sections. 206/10000 · 206
105 ../../principles/transport.rst:638

	English Connection establishment 24/10000 · 24
106 ../../principles/transport.rst:640

	English Like the connectionless service, the connection-oriented service allows several applications running on a given host to exchange data with other hosts. The port numbers described above for the connectionless service are also used by the connection-oriented service to multiplex several applications. Similarly, connection-oriented protocols use checksums/CRCs to detect transmission errors and discard segments containing an invalid checksum/CRC. 446/10000 · 446
107 ../../principles/transport.rst:642

	English An important difference between the connectionless service and the connection-oriented one is that the transport entities in the latter maintain some state during lifetime of the connection. This state is created when a connection is established and is removed when it is released. 281/10000 · 281
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

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