The Internet Family of Protocols The TCP/IP protocol suite

TCP/IP protocol suite graphiicThe Internet Model of computer networking is not merely a reduced version of the OSI Reference Model with a straight line comparison of the four layers of the TCP/IP model to seven layers of the OSI model, as we explain here, and in the next page on the difference between the Internet and OSI reference model

The Internet protocol suite commonly known as TCP/IP is a set of communications protocols used for the Internet and similar networks. TCP/IP is not a single protocol, but rather an entire family of protocols.

The network concept of protocols establishes a set of rules for each system to speak the others language in order for them to communicate. Protocols describe both the format that a message must take as well as the way in which messages are exchanged between computers.

Transmission Control Protocol (TCP) and the Internet Protocol (IP), were the first two members of the family to be defined, consider them the parents of the family. Protocol stack describes a layered set of protocols working together to provide a set of network functions. Each protocol/layer services the layer above by using the layer below.

Internet Protocol (IP)

Internet Protocol (IP) envelopes and addresses the data, enables the network to read the envelope and forward the data to its destination and defines how much data can fit in a single packet. IP is responsible for routing of packets between computers.

Internet Protocol (IP) is a connectionless, unreliable datagram protocol, which means that a session is not created before sending data. An IP packet might be lost, delivered out of sequence, duplicated, or delayed. IP does not attempt to recover from these types of errors. The acknowledgment of packets delivered and the recovery of lost packets is the responsibility of a higher-layer protocol, such as TCP.

An IP packet, also known as an IP datagram, consists of an IP header and an IP payload. The IP header contains the following fields for addressing and routing: IP header field, Source IP address of the original source of the IP datagram, and the Destination IP address of the final destination of the IP datagram.

Time-to-Live (TTL) Designates the number of network segments on which the datagram is allowed to travel before being discarded by a router. The TTL is set by the sending host and is used to prevent packets from endlessly circulating on an IP internetwork. When forwarding an IP packet, routers are required to decrease the TTL by at least 1.

Transmission Control Protocol (TCP)

Transmission Control Protocol (TCP) breaks data up into packets that the network can handle efficiently, verifies that all the packets arrive at their destination, and reassembles the data. TCP is based on point-to-point communication between two network hosts. TCP receives data from programs and processes this data as a stream of bytes. Bytes are grouped into segments that TCP then numbers and sequences for delivery.

Transmission Control Protocol (TCP) is connection oriented, which means an acknowledgment (ACK) verifies that the host has received each segment of the message, reliable delivery service. Acknowledgments are sent by receiving computer, unacknowledged packets are resent. Sequence number are used with acknowledgments to track successful packet transfer

Before two TCP hosts can exchange data, they must first establish a session with each other. A TCP session is initialized through a process known as a three-way handshake. This process synchronizes sequence numbers and provides control information that is needed to establish a virtual connection between both hosts.

Once the initial three-way handshake completes, segments are sent and acknowledged in a sequential manner between both the sending and receiving hosts. A similar handshake process is used by TCP before closing a connection to verify that both hosts are finished sending and receiving all data.

TCP ports use a specific program port for delivery of data sent by using Transmission Control Protocol (TCP). TCP ports are more complex and operate differently from UDP ports.

While a UDP port operates as a single message queue and the network endpoint for UDP-based communication, the final endpoint for all TCP communication is a unique connection. Each TCP connection is uniquely identified by dual endpoints.

Comparison between the OSI and TCP/IP Models

TCP/IP Model Layer 4. Application Layer

Application layer is the top most layer of four layer TCP/IP model. Application layer is present on the top of the Transport layer. Application layer defines TCP/IP application protocols and how host programs interface with Transport layer services to use the network.

Application layer includes all the higher-level protocols:

  • DNS (Domain Naming System)
  • HTTP (Hypertext Transfer Protocol) is the protocol used to transport web pages.
  • FTP (File Transfer Protocol) used to upload and download files.
  • TFTP (Trivial File Transfer Protocol) used to upload and download files.
  • SNMP (Simple Network Management Protocol) designed to enable the analysis and troubleshooting of network hardware. For example, SNMP enables you to monitor workstations, servers, minicomputers, and mainframes, as well as connectivity devices such as bridges, routers, gateways, and wiring concentrators.
  • SMTP (Simple Mail Transfer Protocol) used for transferring email across the internet
  • DHCP (Dynamic Host Configuration Protocol) used to centrally administer the assignment of IP addresses, as well as other configuration information such as subnet masks and the address of the default gateway. When you use DHCP on a TCP/IP network, IP addresses are assigned to clients dynamically instead of manually.
  • X Windows, Telnet, SSH, RDP (Remote Desktop Protocol)

TCP/IP Model Layer 3. Transport Layer

Transport Layer is the third layer of the four layer TCP/IP model. The position of the Transport layer is between Application layer and Internet layer. The purpose of Transport layer is to permit devices on the source and destination hosts to carry on a conversation. Transport layer defines the level of service and status of the connection used when transporting data.

The main protocols included at Transport layer are TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).

TCP/IP Model Layer 2. Internet Layer

Internet Layer is the second layer of the four layer TCP/IP model. The position of Internet layer is between Network Access Layer and Transport layer. Internet layer pack data into data packets known as IP datagrams, which contain source and destination address (logical address or IP address) information that is used to forward the datagrams between hosts and across networks. The Internet layer is also responsible for routing of IP datagrams.

Packet switching network depends upon a connectionless internetwork layer. This layer is known as Internet layer. Its job is to allow hosts to insert packets into any network and have them to deliver independently to the destination. At the destination side data packets may appear in a different order than they were sent. It is the job of the higher layers to rearrange them in order to deliver them to proper network applications operating at the Application layer.

The main protocols included at Internet layer are IP (Internet Protocol), ICMP (Internet Control Message Protocol), ARP (Address Resolution Protocol), RARP (Reverse Address Resolution Protocol) and IGMP (Internet Group Management Protocol).

Reverse Address Resolution Protocol (RARP) adapted from the ARP protocol and provides reverse functionality. It determines a software address from a hardware (or MAC) address. A diskless workstation uses this protocol during bootup to determine its IP address.

Address Resolution Protocol (ARP) translates a host's software address to a hardware (or MAC) address (the node address that is set on the network interface card).

Internet Control Message Protocol (ICMP) enables systems on a TCP/IP network to share status and error information such as with the use of PING and TRACERT utilities.

TCP/IP Model Layer 1. Network Access Layer

Network Access Layer is the first layer of the four layer TCP/IP model. Network Access Layer defines details of how data is physically sent through the network, including how bits are electrically or optically signaled by hardware devices that interface directly with a network medium, such as coaxial cable, optical fiber, or twisted pair copper wire.

The protocols included in Network Access Layer are Ethernet, Token Ring, FDDI, X.25, Frame Relay etc.

The most popular LAN architecture among those listed above is Ethernet. Ethernet uses an Access Method called CSMA/CD (Carrier Sense Multiple Access/Collision Detection) to access the media, when Ethernet operates in a shared media. An Access Method determines how a host will place data on the medium.

IN CSMA/CD Access Method, every host has equal access to the medium and can place data on the wire when the wire is free from network traffic. When a host wants to place data on the wire, it will check the wire to find whether another host is already using the medium. If there is traffic already in the medium, the host will wait and if there is no traffic, it will place the data in the medium. But, if two systems place data on the medium at the same instance, they will collide with each other, destroying the data. If the data is destroyed during transmission, the data will need to be retransmitted. After collision, each host will wait for a small interval of time and again the data will be retransmitted.