Layer 4 (Toss Salad)

Note: The OSI (Open Systems Interconnection) model was created by the ISO (International Organization of Standardization - so weird - ISO is not an acronym but an abbreviation of the Greek word isos meaning "equal" and it was chosen by the organization)

The OSI reference model is a conceptual model, a general-purpose framework for how computers communicate over a network.

• The physical layer is concerned with the transmission and reception of the bitstream over a physical medium. It describes the electrical/optical, mechanical, and functional interfaces to the physical medium, and it carries the signals for all of the higher layers.
• The data link layer formats data into frames appropriate for transmission onto the physical medium. Defines rules for using the medium and also can recognize transmission errors. 
• The network layer controls logical addressing (subnetting), routing and path determination bases on network conditions, priority of service, and other factors.
• The transport layer ensures that messages are delivered error-free, in sequence, and with no losses or duplications; provides host to host flow control and segments large data blocks into smaller parts for transmission. 
• The session layer allows session establishment between processes running on different stations.
• The presentation layer formats the data to be presented to the application layer. It can be viewed as the translator for the network. This layer may translate data from a format used by the application layer into a common format at the sending station, then translate the common format to a format known by the application layer at the receiving station. Includes encryption.
• The application layer interfaces between network and application software. Also includes authentication services. 

The TCP/IP suite was created by the DoD in the 1960s and is a stack of protocols for "data transfer" used in production networks.

Host to Host Communication via the TCP/IP Stack

Common Transport Layer Protocols with application protocol port # 

• TCP (Transport Control Protocol) and UDP (User Datagram Protocol) are the most common Layer 4 protocols
• TCP is connection-oriented - once there is a connection then bidirectionally data can be sent
• TCP carries out sequencing to order segments
• TCP is reliable - receiver sends acknowledgments to sender and lost segments are resent
• TCP performs flow control

• UDP - sends traffic best effort
• UDP is not connection-oriented, no sequencing, no reliable, no flow control