The Dot and OSI models
DoD Model OSI Model
A vast array of protocols combine at the DoD model’s Process / Aplications layer to integrate the various activities and duties spanning the focus of the OSI’s corresponding top three layers (Aplication, Presentation, and Session). (We’ll looking closely at those protocols in the next part of this chapter). The process/Aplication layer define protocols for node-to-node applications communication and also controls user-interface specifications.
The Host-to-Host layer paralles the functions of the OSI’s Transport layer, defining protocols for setting up level of transmission service for applications. It tackles issue like creating reliable end-to-end communication and ensuring the error-free delivery of data. It handle packet sequencing and maintains data integrity.
The internet layer correspondends to the OSI’s Network layer, designating the protocols relating to the logical transmission of packets over the entire network. It takes care of the addressing of hosts by giving them and IP (Internet
Protocol) address, and it handles the routing of packets among multiple networks. It also controls the communication flow between two host.
At the bottom of the model, the Network Access layer monitors the data exchange between the host and the network. The equivalent of Data Link and Physical layers of the OSI model, the Network Access layer oversees hardware addressing and define protocols for the physical transmission of data.
While the Dod and OSI models are alike in design and concept and have similar functions in similar places, how those functions occur is different. Figure 3.2 shows the TCP/IP protocol suite and how its protocols relate to the DoD Model Layers.
FIGURE 3.2 the TCP / IP protocol suite
DoD Model
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The Process/Application Layer Protocols
In this section, we will describe the different applications and services typically used in IP Network. The different protocols and application covered in the section include the following :
- Telnet
- FTP
- TFTP
- NFS
- SMTP
- LPD
- X Window
- SNMP
- DNS
- BootIP
- DHCP
Telnet
Telnet is the chameleon of protocols-its specialty is terminal emulation. It allows a user on a remote client, called the Telnet client, to access the resources of another machine, the Telnet server. Telnet achieves this by pulling a fast one on the Telnet server and making the client machine appear as though it were a terminal directly attached to the local network. This projection is actually a software image, a virtual terminal that can interact with the chosen remote host.
File Transfer Protocol (FTP)
The File Transfer Protocol (FTP) is the Protocol that actually lets us transfer files ; it can faciletate this between any two machine using it. But FTP isn’t just a protocol ; it’s also the program. Operating as a protocol, FTP is used by application.
Trivial File Transfer Protocol (TFTP)
The Trivial File Transfer Protocol (TFTP) is the stripped-down, stock version of FTP, but it’s the protocol of choice if you know exactly what you want and where to find it. It doesn’t give you the abundance of fucntion; that FTP does, though. TFTP has no directory-browsing abilities; it can do nothing but send and receive files.
Network File System (NFS)
Network File System (NFS) is a jewel of a protocol specializing in file sharing. It allows two different types of file system to interoparate. It works like this; Suppose the NFS server software is running on an NT server, and the NFS client software is running on a Unix host.
Simple Mail Transfer Protocol (SMTP)
Simple Mail Transfer Protocol (SMTP), answering our ubiquitous call to e-mail, uses a spooled, or queued, methode of mail delivery. Once a message has been sent to a destination, the message is spooled to a divice usually a disk.
Line Printer Daemon (LPD)
The Line Printer Daemon (LPD) protocol is designed for printer sharring. The LPD, along with the LPR (Line Printer) program, allows print jobs to be spooled and sent to the network’s printers using TCP/IP.
X Window
Designed for client-server operations, X Window defines a protocol for the writing of graphical user interface-based client/server applications. The idea is to allow a program, called a client, to run on one computer and have it display a program called a window server on another computer.
Domain Name Service (DNS)
Domain Name Service (DNS) resolves host name, specifically Internet names, like www.routersim.com. You don’t have to use DNS; you can just type in the IP address of any device you want to communicate with. An IP address identifies hosts on a network and the Internet as well.
Bootstrap Protocol (BootP)
BootP stands for Bootstrap Protocol. When a diskless workstation is powered on, it broadcasts a BootP request on the network
BootP is Used by a diskless machine to learn the following :
- Its own IP address
- The IP address and host name of a sever machine
- The boot filename of a file that is to be loaded into memory and execured at boot-up
Dynamic Host Configuration Protocol (DHCP)
The Dynamic Host Configuration Protocol (DHCP) gives IP address to hosts. It allows administration and works well in small-to-event-every large network environments. All types of hardware can be used as a DHCP server, including a Cisco router.
Notice all the information that can provided by DHCP sever :
- IP address
- Subnet mask
- Domain Name
- Default gateway (routers)
- DNS
- WINS information
A DHCP server can provided even more information, but the items in the buleted list are the most common.
The Host-to-Host Layer Protocol
The host-to-host layer’s main purpose is to shield the upper-layer applications from the complexities of the network. This layer says to the upper layer, “Just give me your data stream, with any instructions, and I’ll begin the process of getting your information ready to send”
The following sections describ the two protocols at this layer :
- Transmission Control Protocol (TCP)
- User Datagram Protocol (UDP)
Transmission Control Protocol (TCP)
The Transmission Control Protocol (TCP) takes large blocks of information from an application and breaks them into segment. It numbers and sequences each segment so that the destination’s TCP protocol can put the segment back into the order application intented.
Let’s take a look at a TCP segment copied from a network analyzer :
TCP – Transport Control Protocol
Source Port : 5973
Destination Port : 23
Sequence Number : 1456359907
Ack Number : 1242056456
Offset : 5
Reserved : %000000
Code : %011000
Ack Valid
Push Request
Window : 61320
Checksum : 0x61a6
Urgent Pointer : 0
No TCP Options
TCP Data Area :
Vl.5+.5.+5+. 76 4c 19 35 11 2b 19 35 11 2b 19 35 +. 11 2b 19
Frame Check Sequence : 0x0d0000f
Notice that everything talked about a bove is in the segment, as you can see from the number or field in the header, TCP has a lot of overhead.
User Datagram Protocol (UDP)
Application developers can use the User Datagram Protocol (UDP) in place of TCP. UDP is the scalde-down economy model and is considered a thin protocol. Like a thin person on a park bench, a thin protocol doesn’t take up a lot of room-or in this case, much bandwidth on a network.
Yoy need to understand what each field in the UDP segment is. The UDP segment contains the following fields :
Source port : port number of the host sending the data
Destination port : port number of the application request on the destination Notion host
Length of the segment : Length of UDP header and UDP data
CRC : Checksum of both the UDP header and UDP data fields
Data :
UDP, like TCP, doesn’t trust the lowers layers and runs its own CRC.
Port Number
TCP and UDP must use port numbers to communicate with the upper layers. Port numbers keep track of different conversation crossing the network simultaneously. Originating-source port number are dynamically assignet by the source host, which will be some number starting 1024. 1023 and below are defined in RFC 1700, which discusses what is called well-known port numbers.
Figure 3.5 illustrates how both TCP and UDP use port numbers.
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