CCNA Routing and Switching Complete Study Guide. Todd Lammle

Чтение книги онлайн.

      Okay, now while the information is being transferred between hosts, the two machines periodically check in with each other, communicating through their protocol software to ensure that all is going well and that the data is being received properly.

      Here’s a summary of the steps in the connection-oriented session – that three-way handshake – pictured in Figure 1.9:

      Sounds pretty simple, but things don’t always flow so smoothly. Sometimes during a transfer, congestion can occur because a high-speed computer is generating data traffic a lot faster than the network itself can process it! And a whole bunch of computers simultaneously sending datagrams through a single gateway or destination can also jam things up pretty badly. In the latter case, a gateway or destination can become congested even though no single source caused the problem. Either way, the problem is basically akin to a freeway bottleneck – too much traffic for too small a capacity. It’s not usually one car that’s the problem; it’s just that there are way too many cars on that freeway at once!

      But what actually happens when a machine receives a flood of datagrams too quickly for it to process? It stores them in a memory section called a buffer. Sounds great; it’s just that this buffering action can solve the problem only if the datagrams are part of a small burst. If the datagram deluge continues, eventually exhausting the device’s memory, its flood capacity will be exceeded and it will dump any and all additional datagrams it receives just like an inundated overflowing bucket!

      Flow Control

      Since floods and losing data can both be tragic, we have a fail-safe solution in place known as flow control. Its job is to ensure data integrity at the Transport layer by allowing applications to request reliable data transport between systems. Flow control prevents a sending host on one side of the connection from overflowing the buffers in the receiving host. Reliable data transport employs a connection-oriented communications session between systems, and the protocols involved ensure that the following will be achieved:

Because of the transport function, network flood control systems really work well. Instead of dumping and losing data, the Transport layer can issue a “not ready” indicator to the sender, or potential source of the flood. This mechanism works kind of like a stoplight, signaling the sending device to stop transmitting segment traffic to its overwhelmed peer. After the peer receiver processes the segments already in its memory reservoir – its buffer – it sends out a “ready” transport indicator. When the machine waiting to transmit the rest of its datagrams receives this “go” indicator, it resumes its transmission. The process is pictured in Figure 1.11.

FIGURE 1.11 Transmitting segments with flow control

      In a reliable, connection-oriented data transfer, datagrams are delivered to the receiving host hopefully in the same sequence they’re transmitted. A failure will occur if any data segments are lost, duplicated, or damaged along the way – a problem solved by having the receiving host acknowledge that it has received each and every data segment.

      A service is considered connection-oriented if it has the following characteristics:

      ■ A virtual circuit, or “three-way handshake,” is set up.

      ■ It uses sequencing.

      ■ It uses acknowledgments.

      ■ It uses flow control.

       The types of flow control are buffering, windowing, and congestion avoidance.

      Windowing

      Ideally, data throughput happens quickly and efficiently. And as you can imagine, it would be painfully slow if the transmitting СКАЧАТЬ