Transmission Control Protocol (TCP)
Transmission Control Protocol (TCP) is a required TCP/IP standard protocol that provides a reliable, connection-oriented data delivery service between only two computers. Such a communication is known as a unicast. In connection oriented communication, the connection must be established before data can be transmitted between the two computers.
After the connection is established, data is transmitted over this single connection only. Connection-oriented communication is also referred to as reliable communication because it guarantees the delivery of the data at the destination. On the source computer, TCP organizes the data to be transmitted into packets. On the destination computer, TCP reorganizes the packets to recreate the original data.
User Datagram Protocol (UDP)
User Datagram Protocol (UDP) is a transport layer protocol that identifies the destination application in network communications. UDP provides a connectionless packet delivery service that offers fast but unreliable, best-effort delivery of the data.
UDP does not require an acknowledgment for the data received and does not attempt to retransmit data that is lost or corrupted. This means that less data is sent, but neither the arrival of packets nor the correct sequencing of delivered packets is acknowledged or guaranteed.
UDP is used by applications that transmit data to multiple computers by using broadcast or multicast transmissions. It is also used for transmitting small amounts of data or data that is not of high importance. Example uses of UDP include multicasting streaming media, such as during a live videoconference, and broadcasting a list of computer names, which are maintained for local communication.
To use UDP, the source application must supply its UDP port number as well as that of the destination application. It is important to note that UDP ports are distinct and separate from TCP ports, even though some of them use the same numbers.
Internet Protocol (IP)
Internet Protocol (IP) helps to identify the location of the destination computer in a network communication. IP is a connectionless, unreliable protocol that is primarily responsible for addressing packets and routing them between networked computers. Although IP always attempts to deliver a packet, a packet may be lost, corrupted, delivered out of sequence, duplicated, or delayed.
However, IP does not attempt to recover from these types of errors by requesting retransmission of the data. Acknowledging the delivery of packets and recovering lost packets is the responsibility of a higher-layer protocol, such as TCP, or of the application itself.
Activities Performed by IP
You can visualize IP as the mailroom of the TCP/IP stack, where packet sorting and delivery take place. The packets are passed down to IP by UDP or TCP from the transport layer or passed up from the network interface layer.
The primary function of IP is to route the packets until they reach their destination. Each packet includes the source IP address of the sender and the destination IP address of the intended recipient. These IP addresses in a packet remain the same throughout the packets journey across a network.
If IP identifies a destination address as an address from the same segment, it transmits the packet directly to that computer. If the destination IP address is not on the same segment, IP must use a router to send the information. IP is also responsible for ensuring that a packet does not remain on the network forever by limiting the number of networks across which the packet can travel.
This is done by assigning a Time to Live (TTL) number to every packet. A TTL specifies the maximum length of time that the packet can travel on the network before being discarded.
Internet Control Message Protocol (ICMP)
Internet Control Message Protocol (ICMP) provides troubleshooting facilities and error reporting for undeliverable packets. With ICMP, computers and routers that use IP communication can report errors and exchange limited control and status information. For example, if IP is unable to deliver a packet to a destination computer, ICMP sends a Destination Unreachable message to the source computer.
Although the IP protocol is used to move data across routers, ICMP reports errors and control messages on behalf of IP. ICMP does not attempt to make IP a reliable protocol, because ICMP messages are unacknowledged and therefore unreliable. It only attempts to report errors and provide feedback on specific conditions. Although this may not seem effective, it is much more efficient than using bandwidth to acknowledge each ICMP message.
Internet Group Management Protocol (IGMP)
Internet Group Management Protocol (IGMP) is a protocol that manages the membership lists for IP multicasting in a TCP/IP network. IP multicasting is a process by which a message is transmitted to a select group of recipients, known as a multicast group. IGMP maintains the list of members who subscribe to each multicast group.
All of the members of a multicast group listen for IP traffic directed to a specific multicast IP address and receive the packets sent to that IP address. However, because multicasting involves multiple computers, the packets are sent using the unreliable UDP protocol, which does not guarantee the delivery of the packets to the multicast group.
When multiple computers need to access information, such as streaming media, an IP address reserved for multicasting is used. Routers that are configured to process multicast IP addresses pick up this information and forward it to all subscribers of the multicast group associated with the multicast IP address.
Address Resolution Protocol (ARP)
Located in the Internet layer of the TCP/IP suite, Address Resolution Protocol (ARP) performs address resolution for outgoing packets. Address resolution is the process by which IP addresses are mapped to MAC addresses. The network adapters use the MAC address to determine if a packet is meant for that computer. Without the MAC address, the network adapters do not know if they are to pass the data to a higher layer for further processing. As the outgoing packets in the IP layer are being readied for transmission on the network, the source and destination MAC addresses must be added.
ARP Cache
ARP stores a table containing IP addresses and their corresponding MAC addresses. The area of memory where this table is stored is referred to as the ARP cache. The ARP cache for any computer contains the mappings for only computers and routers that reside on the same segment.
Physical Address Resolution
ARP compares every outbound packets destination IP address with the ARP cache to determine the MAC address to which the packet will be sent. If there is a matching entry, the MAC address is retrieved from the cache. If not, ARP broadcasts a request for the computer owning the IP address in question to reply with its MAC address.
Next, the computer with the corresponding IP address adds the initial computers MAC address to its cache and then replies with its own MAC address. When an ARP reply is received, the ARP cache is updated with the new information and the packet can then be sent.
If the packet is going to another segment, ARP resolves the MAC address for the router responsible for that segment, rather than resolving the address for the final destination computer. The router is then responsible for either finding the MAC address of the destination or forwarding the packet to another router.
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