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cnp3-ebook / principles/transport — English

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cnp3-ebook / principles/transport — English

The first solution to release a transport connection is to define a new control segment (e.g. the `DR` segment for Disconnection Request) and consider the connection to be released once this segment has been sent or received. This is illustrated in the figure below.
The first solution to release a transport connection is to define a new control segment (e.g. the `DR` segment for Disconnection Request) and consider the connection to be released once this segment has been sent or received. This is illustrated in the figure below.
2 years ago
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cnp3-ebook / principles/transport — English

Compared to reliable protocols in the datalink layer, reliable transport protocols encode their sequence numbers using more bits. 32 bits and 64 bits sequence numbers are frequent in the transport layer while some datalink layer protocols encode their sequence numbers in an 8 bits field. This large sequence number space is motivated by two reasons. First, since the sequence number is incremented for each transmitted byte, a single segment may consume one or several thousands of sequence numbers. Second, a reliable transport protocol must be able to detect delayed segments. This can only be done if the number of bytes transmitted during the MSL period is smaller than the sequence number space. Otherwise, there is a risk of accepting duplicate segments.
Compared to reliable protocols in the datalink layer, reliable transport protocols encode their sequence numbers using more bits. 32 bits and 64 bits sequence numbers are frequent in the transport layer while some datalink layer protocols encode their sequence numbers in an 8 bits field. This large sequence number space is motivated by two reasons. First, since the sequence number is incremented for each transmitted byte, a single segment may consume one or several thousands of sequence numbers. Second, a reliable transport protocol must be able to detect delayed segments. This can only be done if the number of bytes transmitted during the MSL period is smaller than the sequence number space. Otherwise, there is a risk of accepting duplicate segments.
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cnp3-ebook / principles/transport — English

Reliable transport protocols also use sequence numbers and acknowledgment numbers. While reliable protocols in the datalink layer use one sequence number per frame, reliable transport protocols consider all the data transmitted as a stream of bytes. In these protocols, the sequence number placed in the segment header corresponds to the position of the first byte of the payload in the bytestream. This sequence number allows detecting losses but also enables the receiver to reorder the out-of-sequence segments. This is illustrated in the figure below.
Reliable transport protocols also use sequence numbers and acknowledgment numbers. While reliable protocols in the datalink layer use one sequence number per frame, reliable transport protocols consider all the data transmitted as a stream of bytes. In these protocols, the sequence number placed in the segment header corresponds to the position of the first byte of the payload in the bytestream. This sequence number allows detecting losses but also enables the receiver to reorder the out-of-sequence segments. This is illustrated in the figure below.
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cnp3-ebook / principles/transport — English

The first point which is common between the two layers is that both use CRCs or checksums to detect transmission errors. Each segment contains a CRC/checksum which is computed over the entire segment (header and payload) by the sender and inserted in the header. The receiver recomputes the CRC/checksum for each received segment and discards all segments with an invalid CRC.
The first point which is common between the two layers is that both use CRCs or checksums to detect transmission errors. Each segment contains a CRC/checksum which is computed over the entire segment (header and payload) by the sender and inserted in the header. The receiver recomputes the CRC/checksum for each received segment and discards all segments with an invalid CRC.
2 years ago
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cnp3-ebook / principles/transport — English

The last scenario is less likely, but it is important to consider it as well. The remote entity receives an old `CR` segment. It notes the connection attempt and acknowledges it by sending a `CA` segment. The initiating entity does not have a matching connection attempt and replies by sending a `REJECT`. Unfortunately, this segment never reaches the remote entity. Instead, the remote entity receives a retransmission of an older `CA` segment that contains the same sequence number as the first `CR` segment. This `CA` segment cannot be accepted by the remote entity as a confirmation of the transport connection as its acknowledgment number cannot have the same value as the sequence number of the first `CA` segment.
The last scenario is less likely, but it is important to consider it as well. The remote entity receives an old `CR` segment. It notes the connection attempt and acknowledges it by sending a `CA` segment. The initiating entity does not have a matching connection attempt and replies by sending a `REJECT`. Unfortunately, this segment never reaches the remote entity. Instead, the remote entity receives a retransmission of an older `CA` segment that contains the same sequence number as the first `CR` segment. This `CA` segment cannot be accepted by the remote entity as a confirmation of the transport connection as its acknowledgment number cannot have the same value as the sequence number of the first `CA` segment.
2 years ago
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cnp3-ebook / principles/transport — 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.
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.
2 years ago
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cnp3-ebook / principles/transport — 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.
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.
2 years ago
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cnp3-ebook / principles/transport — 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.
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.
2 years ago
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cnp3-ebook / principles/transport — 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.
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.
2 years ago
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cnp3-ebook / principles/transport — English

an `error detection` mechanism that allows detecting corrupted data
an `error detection` mechanism that allows detecting corrupted data
2 years ago
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cnp3-ebook / principles/transport — English

the network layer may duplicate data.
the network layer may duplicate data.
2 years ago
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cnp3-ebook / principles/transport — English

the network layer has an upper bound on maximum length of the data;
the network layer has an upper bound on maximum length of the data;
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cnp3-ebook / principles/transport — English

the network layer may not deliver data in-order;
the network layer may not deliver data in-order;
2 years ago
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cnp3-ebook / principles/transport — English

the network layer may loose data;
the network layer may loose data;
2 years ago
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the network layer may corrupt data;
the network layer may corrupt data;
2 years ago
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cnp3-ebook / principles/transport — English

The transport layer entity interacts with both a user in the application layer and the network layer. It improves the network layer service to make it usable by applications. From the application's viewpoint, the main limitations of the network layer service come from its unreliable service:
The transport layer entity interacts with both a user in the application layer and the network layer. It improves the network layer service to make it usable by applications. From the application's viewpoint, the main limitations of the network layer service come from its unreliable service:
2 years ago
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cnp3-ebook / principles/transport — English

An important point to note about the connection-oriented service is its reliability. A `connection-oriented` service can only guarantee the correct delivery of all SDUs provided that the connection has been released gracefully. This implies that while the connection is active, there is no guarantee for the actual delivery of the SDUs exchanged as the connection may need to be abruptly released at any time.
An important point to note about the connection-oriented service is its reliability. A `connection-oriented` service can only guarantee the correct delivery of all SDUs provided that the connection has been released gracefully. This implies that while the connection is active, there is no guarantee for the actual delivery of the SDUs exchanged as the connection may need to be abruptly released at any time.
2 years ago

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