Example of Peerto Peer Communication

Each layer of the OSI or TCP model has its own functions and interacts with the layer above it and layer below it. Furthermore, the communication between each layer's end devices also establishes peer-to-peer communication; this means that each layer of the OSI model communicates with the corresponding peer. For example, Layer 3 of Host A in Figure 1-3 will communicate with the corresponding Layer 3 (IP) device host B.

Consider the normal communication that occurs between two IP hosts over a WAN running Frame Relay, as displayed in Figure 1-3.

Figure 1-3 Peer-to-Peer Communication Example

Data Received by Application

Host A Host B

Application Data (Layer 7) Application Data (Layer 7)

Peer-to-Peer Communication

Data

TCP header

Data

Data

802.3 header

IP header

TCP header

IP header

Data

Layer 4 Peer-to-peer Layer 4 (TCP) communication (TCP)

TCP header

Data

Layer 3

Layer 3

Data

Layer 2

Data

TCP header

Layer 3

IP header

TCP header

Data

Layer 2

802.3 header

IP header

TCP header

(Binary Transmission)

Router A

Router A

Router B

0101011000

(Binary Transmission)

Router B

Frame

IP

TCP

Data

CRC

Relay

Header

Header

The data from Host A is encapsulated inside a TCP header and passed down to Layer 3 (the IP layer) for address configuration, where an IP header is also added. Information included here is the source IP address and destination address. Layer 3 (the network layer) passes the data to the local router acting as the gateway via the Ethernet connection in raw binary.

Router A strips the 802.3 header and encapsulates the IP, TCP, and data in a Frame Relay packet for delivery over the WAN. A CRC is added here to ensure the packet is not corrupted over the WAN. Because Frame Relay is connectionless, if an error occurs, it's up to the upper layers to retransmit; Frame Relay will not retransmit the packet. Similarly, HDLC (Layer 2 protocol) is connectionless and depends on upper layers to resubmit damaged data packets. PPP (connection-oriented), on the other hand, resubmits packets damaged in transmission over the WAN.

Router B receives the Layer 2 frames, strips the Frame Relay header/CRC, and encapsulates the IP, TCP, and data frame back into an 802.3 header (with its own CRC, Ethernet checks only for errors and cannot repair them; once more, upper layers, such as TCP, ensure data delivery) for binary transmission across the Ethernet to Host B. The data is passed up the layers through IP, TCP, and finally to the application, where the application layer reads and acts upon the data.

The good news for security candidates is that Token Ring and legacy technologies are not covered in the written exam, so this chapter concentrates only on Ethernet switching. Before covering switching, the next section summarizes the evolution of Ethernet so that you are aware of the standards that have developed since Xerox Corporation first introduced Ethernet.

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