OSI Model and TCP IP Stack Questions

Comprehensive knowledge of the seven layer Open Systems Interconnection model, including Layer One Physical, Layer Two Data Link, Layer Three Network, Layer Four Transport, Layer Five Session, Layer Six Presentation, and Layer Seven Application. Understand the primary responsibilities and services at each layer, how data is packaged and transformed as it moves down and up the stack through encapsulation and decapsulation, and the unit of data at each stage such as bits at the physical layer, frames at the data link layer, packets at the network layer, and segments at the transport layer. Be able to identify common protocols and services that operate at each layer, for example Ethernet and link layer protocols at the data link layer, Internet Protocol at the network layer, Transmission Control Protocol and User Datagram Protocol at the transport layer, and application layer protocols such as Hypertext Transfer Protocol and Domain Name System at the application layer. Understand which hardware devices operate at which layers, such as cabling and transceivers at the physical layer, switches and bridges at the data link layer, and routers at the network layer, and how these devices affect forwarding and inspection. Know how the Open Systems Interconnection model maps to and differs from the four layer Transmission Control Protocol and Internet Protocol stack, including which functions are combined or abstracted differently, and how layering choices affect security placement, encryption strategy, performance, and troubleshooting. Be able to apply this knowledge to diagnose faults by mapping symptoms to layer specific causes and to reason about header fields, addressing and port schemes, segmentation and retransmission behavior, and cross layer interactions.