CCNA 2.1 TCP/IP model and the DoD model

• DoD model composed of four layers:

Process/Application layer
Host-to-host layer
Internet layer
Network access layer

• Process/Application layer includes protocols such as Telnet, FTP, TFTP, NFS, SMTP, LPD, X Window, SNMP, DNS, DHCP/BootP
• Telnet, specializes in terminal emulation, allows user on a remote device to access the resource of another device
• File transfer protocol (FTP) allows user to transfer, rename files and list, relocate file directories between hosts
• TFTP (Trivial) is the lite version of FTP (no directory browsing and authentication functions)
• Network File System (NFS) allows two differents type of file systems to interoperate (file sharing etc)
• Simple Mail Transfer Protocol (SMTP) utilizes a spooled/queued method for sending mail
• Post Office Protocol (POP) is used for receiving mail
• Line Printer Daemon (LPD) allows print jobs to be spooled and sent to network's printer via TCP/IP (printer sharing)
• X windows allows a client PC to display on another remote PC
• Simple Network Management Protocol (SNMP) collects network information(baseline), alerts (traps) from network devices (agents) and monitor them through a management station.
• Domain Name Service (DNS) allows user to resolve a fully qualified domain name (FQDN) to an IP address
• Dynamic Host Configuration Protocol (DHCP)/Bootstrap (BootP) assigns IP addresses to hosts. (connectionless/UDP)
• Host-to-host layer includes protocols such as Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)

• TCP segment format:

• UDP segment format:

• Port numbers are used by TCP & UDP to communicate with upper layers:

• Port numbers below 1024 are considered well-known (RFC-3232)
• DNS use both TCP and UDP:

• Internet Layer includes protocols such as Internet protocol (IP), Internet Control Message Protocol (ICMP), Address Resolution Protocol (ARP), Reverse Address Resolution Protocol (RARP) and Proxy ARP
• IP fragments segments from Host-to-host layer into packets. (RFC 791)
• IP Header

• Protocol/Type field in an IP header allows IP to communicate with protocols at Host-to-host layer

• Popular protocol numbers:

• ICMP is a management protocol and messaging service provider for IP
• ICMP is deployed on following events: destination unreachable, buffer full, hops limit, ping and traceroute
• ARP finds the hardware address of a host from a known IP address
• RARP is used by diskless device to discover its IP address by sending out a packet that includes its MAC address
• Proxy ARP allows devices on a subnet to reach remote subnets without configuring routing or default gateway