2.4.Layer 3: The Network Layer
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Although the data link layer uses physical addresses to make forwarding decisions, the network layer uses logical addressing to make forwarding decisions.
A variety of routed protocols (for example, AppleTalk and IPX) have their own logical addressing schemes, but by far, the most widely deployed routed protocol is Internet Protocol (IP).
The term switching is often associated with Layer 2 technologies; however, the concept of switching also exists at Layer 3.
Switching, at its essence, is making decisions about how data should be forwarded.
At Layer 3, three common switching techniques exist:
Packet switching:
With packet switching, a data stream is divided into packets. Each packet has a Layer 3 header, which includes a source and destination Layer 3 address.
Circuit switching:
Circuit switching dynamically brings up a dedicated communication link between two parties for those parties to communicate.
As a simple example of circuit switching, think of making a phone call from your home to a business. This interconnection (that is, circuit ) only exists for the duration of the phone call.
Message switching:
With message switching, a data stream is divided into messages. Each message is tagged with a destination address, and the messages travel from one network device to another network device on the way to their destination.
Because these devices might briefl y store the messages before forwarding them, a network using message switching is sometimes called a store-and-forward network.
You could visualize message switching like routing an e-mail message, where the e-mail message might be briefly stored on an e-mail server before being forwarded to the recipient.
Because Layer 3 devices make forwarding decisions based on logical network addresses, a Layer 3 device might need to know how to reach various network addresses.
For example, a common Layer 3 device is a router. A router can maintain a routing table indicating how to forward a packet based on the packet’s destination network address.
Connection services at the network layer can improve the communication reliability, in the event that the data link’s LLC sublayer is not performing connection services.
Helps prevent a sender from sending data more rapidly than the receiver is capable of receiving the data.
Allows packets to be placed in the appropriate sequence as they are sent to the receiver. This might be necessary because some networks support load balancing, where multiple links are used to send packets between two devices. Because multiple links are used, packets might arrive out of order.
