Lesson 4: OSI and TCP

Comp277: Data Communications and Networking

Summary of the OSI Model Layers

The TCP/IP Protocol Suite and OSI Model

TCP/IP is a hierarchical protocol made up of interactive modules, each of which provides a specific functionality; however, the modules are not necessarily interdependent. The TCP/IP, has four layers, namely:

  1. Host-to-network
  2. Internet
  3. Transport, and
  4. Application

However, for purposes of comparison with the OSI Model, we can say that the host-to-network is the same as the physical and data link; the internet layer is equivalent to the network; the transport layer is the same as the transport layer; and the application layers perform the combined jobs of session, presentation and application layers.

Host-to-network layer (Physical and Data Link Layers)

At the physical and data link layers, TCP/IP does not define any specific protocol. It supports all the standard and propriety protocols. A network in a TCP/IP internetwork can be a local-area network or a wide-area network.

Network Layer

At the network layer (or, more accurately, the internetwork layer), TCP/IP supports the Internetworking Protocol which uses four supporting protocols: (1) ARP- Address Resolution Protocol – used to associate a logical address with a physical address; (2)RARP – Reverse Address Resolution Protocol – allows the host to discover its Internet address when it knows only its physical address; (3) ICMP – Internet Control Message Protocol – is a mechanism used by hosts and gateways to send notification of datagram problems back to the sender ; and (4) IGMP – Internet Group Message Protocol – is used to facilitate the simultaneous transmission of a message to a group of recipients.

Internetworking Protocol (IP)

Internetworking Protocol is the transmission mechanism used by the TCP/IP. It transports data packets called datagram, each of which is transported separately. Datagrams can travel along different routes and can arrive out of sequence or be duplicated. IP does not keep track of the routes and has no mechanism to reorder datagrams once it has reached it destinations.

Transport Layer

Traditionally, the transport layer was represented in TCP/IP by two protocols: TCP and UDP, but a new transport layer protocol, SCTP, has been devised to meet the needs of some newer applications.

User Datagram Protocol (UDP)

The UDP is the simpler of the two standard TCP/IP transport protocols. It is a process-to-process protocol that adds only port addresses, checksum error control, and length information to the data from the upper layer.

Transmission Control Protocol (TCP)

The TCP provides full tansport-layer services to applications. TCP is a reliable stream transport protocol (where stream means there must be a established connection between both ends of a transmission before data can be transmitted).

At the sending end of each transmission, TCP divides a stream of data into smaller units called segments. Each segments includes a sequence number for reordering after receipt, together with acknowledgment number for the segment received. Segments are carried across the internet inside of IP datagrams.

At the receiving end, TCP collects each datagram as it comes in and reorders the transmission based on sequence numbers.

Stream Control Transmission Protocol

The SCTP provides support for newer applications such as voice over the Internet. It is a transport layer protocol that combines the best features of UDP and TCP.

Application Layer

The Application Layer in TCP/IP is equivalent to the combined session, presentation, and application layers in t

Addressing

Four levels of addresses are used in an internet employing the TCP/IP protocols:

  1. Physical (link) addresses
  2. Logical (IP) addresses
  3. Port addresses
  4. Specific addresses

Application LayerSpecific Addresses

Transport LayerPort Addresses

Network LayerLogical Addresses

Data Link Layer

Physical Addresses

Physical Layer