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Bob : `This is the correct password, you're welcome`
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Bob : `What is the secret password ?`
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`carriage return` (`CR`) : `0001101b`
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Connection establishment
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Connectionless transport
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Connection release
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Data transfer
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Dynamic receiving window
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Finally, some unreliable connectionless service providers may deliver to a destination a different SDU than the one that was supplied in the `Data.request`. This is illustrated in the figure below.
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For a discussion on how the sending buffer can change, see e.g. [SMM1998]_
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For example, the :manpage:`htonl(3)` (resp. :manpage:`ntohl(3)`) function the standard C library converts a 32-bits unsigned integer from the byte order used by the CPU to the network byte order (resp. from the network byte order to the CPU byte order). Similar functions exist in other programming languages.
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`Go-back-n` and `selective repeat` can be used in the transport layer as in the datalink layer. Since the network layer does not guarantee an in-order delivery of the packets, a transport entity should always store the segments that it receives out-of-sequence. For this reason, most transport protocols will opt for some form of selective repeat mechanism.
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Graceful connection release
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If Alice wants to enter the meeting room but does not know the password, her conversation could be as follows :
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In addition, the :term:`ASCII` table also defines several non-printable or control characters. These characters were designed to allow an application to control a printer or a terminal. These control characters include `CR` and `LF`, that are used to terminate a line, and the `Bell` character which causes the terminal to emit a sound.
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In practice, an `unreliable connectionless service` will usually deliver a large fraction of the SDUs. However, since the delivery of SDUs is not guaranteed, the user must be able to recover from the loss of any SDU.
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In reality, the Internet does not strictly enforce this MSL. However, it is reasonable to expect that most packets on the Internet will not remain in the network during more than 2 minutes. There are a few exceptions to this rule, such as :rfc:`1149` whose implementation is described in http://www.blug.linux.no/rfc1149/ but there are few real links supporting :rfc:`1149` in the Internet.
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Interactions between the transport layer, its user, and its network layer provider
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In the application layer, most servers are implemented as processes. The network and transport layer on the other hand are usually implemented inside the operating system and the amount of memory that they can use is limited by the amount of memory allocated to the entire kernel.
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In the datalink layer, the sliding window has usually a fixed size which depends on the amount of buffers allocated to the datalink layer entity. Such a datalink layer entity usually serves one or a few network layer entities. In the transport layer, the situation is different. A single transport layer entity serves a large and varying number of application processes. Each transport layer entity manages a pool of buffers that needs to be shared between all these processes. Transport entity are usually implemented inside the operating system kernel and shares memory with other parts of the system. Furthermore, a transport layer entity must support several (possibly hundreds or thousands) of transport connections at the same time. This implies that the memory which can be used to support the sending or the receiving buffer of a transport connection may change during the lifetime of the connection [#fautotune]_ . Thus, a transport protocol must allow the sender and the receiver to adjust their window sizes.
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