cnp3-ebook/principles/transport

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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
128 ../../principles/transport.rst:762

	English Three-way handshake : recovery from a duplicate `CA` 52/10000 · 52
129 ../../principles/transport.rst:764

	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. 720/10000 · 720
130 ../../principles/transport.rst:770

	English Three-way handshake : recovery from duplicates `CR` and `CA` 60/10000 · 60

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