Same Layer and Adjacent-Layer Interactions

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Adjacent Layer Interaction

Adjacent-Layer interaction refers to how the adjacent networking layers in the same computer interact to each other. The higher-layer protocol uses the next lower-layer protocol to perform the service it needed; the lower layer provides a service to the layer above it.

We have seen a few examples so far:

• Application layer protocol HTTP uses the Transport layer protocol TCP to perform the error recovery service it needed. The Transport layer provides an error recovery service to the Application layer above it.
• Transport layer protocol UDP uses the Internet layer protocol IP to perform the IP addressing and routing service it needed. The Internet layer provides an IP addressing and routing service to the Transport layer above it.
• Internet layer protocol IP uses the Link layer protocol Ethernet to perform the host to host physical communication service it needed. The Link layer provides an physical connection and or link service to the Internet layer above it.

Newer TCP/IP model divides Link layer into two layers: Data Link Layer and Physical Layer. 

While Data Link layer focuses on Encapsulation and Mac Addressing, the Physical layer handles Bit Transmission.

• The Data Link layer protocol uses the Physical Layer protocol to perform the services it needed. The Physical layer provides services to the Data Link Layer above it.

Same Layer Interaction

Same layer interaction refers to the following scenario. 

When a particular layer on one computer needs to communicate with the same layer on another computer in the network, an encapsulation header for that particular layer needs to be used to carry the communication information. A particular layer at the sending computer adds the header of a certain protocol and the same layer at the receiving computer processes the header in order to get the communication information containing in the header.

We also has a few examples so far:

• When application layer on one machine want to communicate with application layer on the other machine, an HTTP header is added at the sending side, the receiving side processes the HTTP header to get information such as request type GET, request file path index.html. The receiver then reply with an HTTP OK message, which is the HTTP header wrapping around a HTML file content. The replied message then processed at the other side, the 200 status is extracted from the HTTP header and the HTML file content is retrieved from the message body.
• When transport layers on two machines want to communicate with TCP protocol. The transport layer of the sending machine adds an TCP header, wrapping the HTTP message inside the TCP segment body. The TCP header carries the error recovery information in the Sequence field. The transport layer at the receiving machine uses the SEQ field in the TCP header to perform error recovery service, also removes the TCP header and retrieves the HTTP message in the TCP segment body. 
• When Internet layers on two machines want to communicate with IP protocol. The IP layer of the sending machine adds an IP header, wrapping the TCP segment inside the IP packet body. The IP header carries the information about source IP and destination IP. The Internet layer at the receiving machine uses the IP header to perform IP addressing and routing service, also removes the IP header and retrieves the IP message in the IP packet body.