Frames is the last PDU used before information is put on the wire for transmission. Packets are formed of control information and the payload.
When working with addressing and information flow, I tend to spend a significant amount of time in layers two and three, which are data link and network, respectively. Layer two refers to communication between devices on a common switched infrastructure.
This layer is best known for switches and switching. Each device uses a media access control or MAC address. This layer deals only with switching, so frames don’t really care about the encapsulated IP addresses they are transferring. Layer three refers to sending information via IP address.
This layer is best known for routers and routing. As information passes from router to router on its way to a destination, it will encapsulate from layer three, to two, then to one as it is transmitted from one router to the other.When it enters a router, it will decapsulate back up from layer one, to two, and then to three. It will then do a lookup on the layer three information to see where to forward the packet, then repeat the process. As information moves up and down the OSI model, some layers have a special name for the data at one of those layers.
This is called Protocol Data Unit. As this information moves down, it is encapsulated into the lower PDU. As it arrives to a host and moves up the OSI model, it is decapsulated. Think of it as Russian nesting dolls, enclosing one PDU with another. While I am focusing on layer two and three, all layers are very important to the troubleshooting process. Since many of networking’s issues can be traced back to a physical issue, it must not be forgotten.
Packets and Frames
Moving to layer two, we have frames. Frames is the last PDU used before information is put on the wire for transmission or the first received off the wire. A frame consists of bits arranged as framing bits, packet payload, and the frame check sequence. Framing bits are used in conjunction with the layer two transmission method. The frame check sequence is an extra bit of code put in place for error detection. The PDUs at layer three are packets. Packets are formed of control information and the payload.
Control information will consist of things like source and destination IP,sequencing information, and error detection information. The payload consists of user data to be transported. Layer four consists of TCP segmentsand UDP datagrams. A TCP segment consists of a header and a data portion. The header includes information like source and destination IP,sequence number, window size, and so on. The UDP datagram is designed much the same, but the header contains fewer options, though it does retain the key pieces.
Layer five through seven is just considered data. It can be compressed, encrypted, or clear text. Common nomenclature is important when troubleshooting or simply having a functionality discussion. Be sure to add the various PDUs to your vernacular.
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