This chapter introduces to basics of computer networking and associated widely used essential networking communication protocols. The chapter provides the comparison of OSI and TCP model along with details of internet layer protocols including internet protocol (IP), IP addressing schemes, internet control messaging protocol (ICMP), etc. Next, the chapter discusses transport layer protocols transmission control protocol (TCP) and user datagram protocol (UDP) in detail. Application layer protocols including dynamic host control protocol (DHCP), secure shell (SSH), file transfer protocol (FTP), trivial FTP (TFTP), simple network management protocol (SNMP), hyper text transfer protocol secure (HTTPS), network time protocol (NTP), domain name system (DNS), and simple mail transfer protocol (SMTP) are also explained in this chapter. One just cannot attack a networking protocol without knowing how it works. Having a solid introduction about computer networking and network protocols is fundamental in the ethical hacking world. This chapter quickly revisits all essential concepts related to computer networking.
TopOsi And Tcpip Model (Meyer & Zobrist, 1990)
Open Systems Interconnection model (OSI model) is a conceptual model by International Organization for Standardization (ISO), which defines a networking framework to implement protocols in seven layers for providing the interoperability of diverse communication systems with standard protocols (Forouzan, 2017, Tanenbaum & Wetherall, 2013). TCP/IP is a four layered standard networking framework and can map to existing OSI model as shown in Figure 1. In both layered model, each layer serves the layer above it and is served by the layer below it.
As seen, In TCP/IP model, there does not exist presentation and session layers. Also the Network Access Layer combines the functions of Data link Layer and Physical Layer of OSI. Network Access Layer, first layer TCP/IP model defines details of physical transmission of bits on the network using 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 (most commonly used), Token Ring, FDDI, X.25, Frame Relay etc. Internet layer refers to network layer of OSI which deals with packet addressing, packaging, and routing function using IP addressing schemes. 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). The transport layer consists of TCP, UDP and RTP protocols and refers to end to end host connection. Application layer, top most layer of TCP/IP model includes all the higher-level protocols like DNS (Domain Naming System), HTTP (Hypertext Transfer Protocol), Telnet, SSH, FTP (File Transfer Protocol), TFTP (Trivial File Transfer Protocol), SNMP (Simple Network Management Protocol), SMTP (Simple Mail Transfer Protocol), DHCP (Dynamic Host Configuration Protocol), X Windows, RDP (Remote Desktop Protocol) etc.
OSI is a generic, protocol independent standard while TCP/IP model is based on standard protocols around which the Internet has developed. In TCP/IP model the transport layer does not guarantees delivery of packets as with that of OSI model and supports both Connection Oriented and Connection less transmission method. TCP/IP does not have a separate Presentation layer or Session layer as in case of OSI model. In OSI, network layer offers both Connection Oriented and Connection less transmission service but in TCPIP model, network layer supports connection less service method only. In TCP/IP, services, interfaces and protocols are not clearly separated. It is also protocol dependent.
TopInternet Layer Protocols
Internet layer deals with packet addressing, packaging, and routing function using IP addressing schemes. It consists of protocols like IP, ARP, ICMP, and IGMP.