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../../principles/transport.rst:782
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To conclude our description of the basic mechanisms found in transport protocols, we still need to discuss the impact of segments arriving in the wrong order. If two consecutive segments are reordered, the receiver relies on their sequence numbers to reorder them in its receive buffer. Unfortunately, as transport protocols reuse the same sequence number for different segments, if a segment is delayed for a prolonged period of time, it might still be accepted by the receiver. This is illustrated in the figure below where segment `D(1,b)` is delayed.
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../../principles/transport.rst:789
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Ambiguities caused by excessive delays
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../../principles/transport.rst:793
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To deal with this problem, transport protocols combine two solutions. First, they use 32 bits or more to encode the sequence number in the segment header. This increases the overhead, but also increases the delay between the transmission of two different segments having the same sequence number. Second, transport protocols require the network layer to enforce a `Maximum Segment Lifetime (MSL)`. The network layer must ensure that no packet remains in the network for more than MSL seconds. In the Internet the MSL is assumed [#fmsl]_ to be 2 minutes :rfc:`793`. Note that this limits the maximum bandwidth of a transport protocol. If it uses `n` bits to encode its sequence numbers, then it cannot send more than :math:`2^n` segments every MSL seconds.
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../../principles/transport.rst:797
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Connection release
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../../principles/transport.rst:801
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When we discussed the connection-oriented service, we mentioned that there are two types of connection releases : `abrupt release` and `graceful release`.
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../../principles/transport.rst:810
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Abrupt connection release
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../../principles/transport.rst:812
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As the entity that sends the `DR` segment cannot know whether the other entity has already sent all its data on the connection, SDUs can be lost during such an `abrupt connection release`.
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../../principles/transport.rst:816
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The second method to release a transport connection is to release independently the two directions of data transfer. Once a user of the transport service has sent all its SDUs, it performs a `DISCONNECT.req` for its direction of data transfer. The transport entity sends a control segment to request the release of the connection *after* the delivery of all previous SDUs to the remote user. This is usually done by placing in the `DR` the next sequence number and by delivering the `DISCONNECT.ind` only after all previous `DATA.ind`. The remote entity confirms the reception of the `DR` segment and the release of the corresponding direction of data transfer by returning an acknowledgment. This is illustrated in the figure below.
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../../principles/transport.rst:822
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Graceful connection release
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../../principles/transport.rst:827
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Footnotes
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../../principles/transport.rst:828
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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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../../principles/transport.rst:830
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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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../../principles/transport.rst:834
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For a discussion on how the sending buffer can change, see e.g. [SMM1998]_
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../../principles/transport.rst:836
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Note that if the receive window shrinks, it might happen that the sender has already sent a segment that is not anymore inside its window. This segment will be discarded by the receiver and the sender will retransmit it later.
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../../principles/transport.rst:838
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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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