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`TCP/IP Protocol Fundamentals Explained
`with a Diagram
`
`by Himanshu Arora on November2, 2011
`Tweet
`
`Have you ever wondered how your computertalks to other computers on your local LAN orto
`other systems on the internet?
`
`Understanding the intricacies of how computers interact is an important part of networking and is
`of equal interest to a sysadmin as well as to a developer. In this article, we will make an attempt
`to discuss the concept of communication from the very basic fundamentallevel that needs to be
`understood by everybody.
`
`TCP/IP PROTOCOL SUITE
`
`Communications between computers on a networkis done through protocol suits. The most
`widely used and most widely available protocol suite is TCP/IP protocolsuite. A protocolsuit
`consists of a layered architecture where each layer depicts some functionality which can be
`carried out by a protocol. Each layer usually has more than one protocoloptions to carry out the
`responsibility that the layer adheres to. TCP/IP is normally considered to be a 4 layer system.
`The4 layers are as follows:
`
`1. Application layer
`2. Transport layer
`3. Network layer
`4. Data link layer
`
`Alacritech, Ex. 2043 Page 1
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`ALA11353476
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`Alacritech, Ex. 2043 Page 1
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`

`

`1. Application layer
`
`This is the top layer of TCP/IP protocol suite. This layer includes applications or processesthat
`use transport layer protocols to deliver the data to destination computers.
`
`At each layer there are certain protocol options to carry out the task designated to that particular
`layer. So, applicationlayer also has various protocols that applications use to communicate with
`the secondlayer, the transport layer. Some of the popular application layer protocols are:
`
`¢ HTTP (Hypertext transfer protocol)
`e
`FTP (File transfer protocol)
`e
`SMTP (Simple mailtransfer protocol)
`e
`SNMP (Simple network management protocol) etc
`
`2. Transport Layer
`
`This layer provides backbone to data flow between twohosts. This layer receives data from the
`application layer aboveit. There are many protocols that work at this layer but the two most
`commonly used protocols at transport layer are TCP and UDP.
`
`TCPis used where a reliable connection is required while UDPis used in case of unreliable
`connections.
`
`TCPdivides the data(coming from the application layer) into proper sized chunks and then
`passes these chunksonto the network. It acknowledges received packets, waits for
`the acknowledgmentsof the packets it sent and sets timeout to resend the packets if
`acknowledgements are not received in time. The term ‘reliable connection’ is used whereit is not
`desired to loose any information that is being transferred over the network throughthis
`connection. So, the protocol used for this type of connection must provide the mechanism to
`achieve this desired characteristic. For example, while downloadinga file, it is not desired to
`loose any information(bytes) as it may lead to corruption of downloaded content.
`
`UDPprovides a comparatively simpler but unreliable service by sending packets from one host
`to another. UDP doesnottake any extra measures to ensure that the data sent is received by the
`target host or not. The term ‘unreliable connection’ are used where loss of some information
`does not hamperthe task being fulfilled through this connection. For example while streaming a
`video, loss of few bytes of information due to somereasonis acceptable as this does not harm
`the user experience much.
`
`3. Network Layer
`
`This layer is also knownasInternet layer. The main purposeofthis layer is to organize or handle
`the movement ofdata on network. By movement ofdata, we generally mean routing ofdata over
`the network. The main protocolused at this layer is IP. While [CMP(used by popular ‘ping’
`command) and IGMParealso usedatthis layer.
`
`Alacritech, Ex. 2043 Page 2
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`ALA11353477
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`Alacritech, Ex. 2043 Page 2
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`

`

`4. Data Link Layer
`
`This layer is also known as networkinterface layer. This layer normally consists of device
`drivers in the OS and the network interface card attached to the system. Both the device drivers
`and the network interface card take care of the communication details with the media being used
`to transfer the data over the network. In most of the cases, this media is in the form of cables.
`Someofthe famous protocols that are used at this layer include ARP(Addressresolution
`protocol), PPP(Point to point protocol) etc.
`
`TCP/IP CONCEPT EXAMPLE
`
`Onething which is worth taking noteis that the interaction between two computers over the
`network through TCP/IP protocolsuite takes place in the form of a client server architecture.
`
`Client requests for a service while the server processes the request for client.
`
`Now,since we have discussed the underlying layers which help that data flow from host to target
`over a network. Lets take a very simple example to make the concept moreclear.
`
`Consider the data flow when youopen a website.
`
`Alacritech, Ex. 2043 Page 3
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`Alacritech, Ex. 2043 Page 3
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`

`

`Host (your laptop)
`
`Target (webserver) Media for data transfer (e.g. Ethernet)
`
`Asseen in the above figure, the information flows downward through each layer on the host
`machine. Atthe first layer, since http protocol is being used, so an HTTP request is formed and
`sent to the transport layer.
`
`Here the protocol TCP assigns some more information(like sequence number, source port
`number, destination port numberetc) to the data coming from upperlayerso that the
`communication remainsreliable i.e, a track of sent data and received data could be maintained.
`
`At the next lowerlayer, IP adds its own information over the data coming from transport layer.
`‘This information would help in packet travelling over the network. Lastly, the datalink layer
`makessure that the data transfer to/from the physical media is done properly. Here again the
`communication doneat the data link layer can bereliable or unreliable.
`
`This information travels on the physical media (like Ethernet) and reaches the target machine.
`
`Now,at the target machine (which in our case is the machine at which the website is hosted) the
`sameseries of interactions happen, but in reverse order.
`
`Alacritech, Ex. 2043 Page 4
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`Alacritech, Ex. 2043 Page 4
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`

`

`The packetis first recetved at the data link layer. At this layer the information (that was stuffed
`by the data link layer protocol of the host machine) is read and rest of the data is passed to the
`upperlayer.
`
`Similarly at the Networklayer, the information set by the Network layer protocol of host
`machineis read andrest of the information is passed on the next upper layer. Same happensat
`the transport layer and finally the HTTP request sent by the host application(your browser) is
`received by the target application(Website server).
`
`One would wonder what happens when information particular to each layer is read by the
`corresponding protocols at target machine or whyis it required? Well, lets understand this by an
`example of TCP protocolpresent at transport layer. At the host machine this protocol adds
`information like sequence numberto each packetsent by this layer.
`
`At the target machine, when packet reachesat this layer, the TCP at this layer makes note of the
`sequence numberof the packet and sends an acknowledgement (which is received seq number +
`1).
`
`Now,if the host TCP doesnot receive the acknowledgement within somespecified time, it re
`sends the same packet. So this way TCP makes sure that no packet gets lost. So we see that
`protocol at every layer reads the information set by its counterpart to achieve the functionality of
`the layerit represents.
`
`PORTS, SERVERS AND STANDARDS
`
`On a particular machine, a port number coupled with the IP address of the machine is known as a
`socket. A combination of IP and port on both client and server is knownas four tuple. This four
`tuple uniquely identifies a connection. In this section we will discuss how port numbers are
`chosen.
`
`You already know that some of the very common serviceslike FTP, telnet etc run on well known
`port numbers. While FTP server runs on port 21, Telent server runs on port 23. So, we seethat
`some standard services that are provided by any implementation of TCP/IP have some standard
`ports on which they run. These standard port numbers are generally chosen from 1 to 1023. The
`well known ports are managed by Internet Assigned Numbers Authority(IANA).
`
`While most standard servers (that are provided by the implementation of TCP/IP suite) run on
`standard port numbers, clients do not require any standard port to run on.
`
`Client port numbers are known as ephemeral ports. By ephemeral we meanshortlived. This is
`because a client may connect to server, do its work and then disconnect. So we used the term
`‘short lived’ and hence no standard ports are required for them.
`
`Also, since clients need to know the port numbers of the servers to connect to them, so most
`standard servers run on standard port numbers.
`
`Alacritech, Ex. 2043 Page 5
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`ALA11353480
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`Alacritech, Ex. 2043 Page 5
`
`

`

`The ports reserved for clients generally range from 1024 to 5000. Port numberhigher than 5000
`are reserved for those servers which are not standard or well known.
`
`If we lookat thefile ‘/etc/services’, you will find most of the standard servers and the port on
`whichthey run.
`
`$ cat /etc/services
`systat
`11/tcp
`daytime
`13/udp
`netstat
`15/tcp
`gotd
`17/tcp
`msp
`18/udp
`chargen
`19/udp
`ftp-data
`20/tcp
`ftp
`21/tcp
`ssh
`22/tcp
`ssh
`22/udp
`telnet
`23/tcp
`
`users
`
`quote
`
`ttytst source
`
`Asyou see from the/etc/services file, FTP has port number 21, telent has port number23etc.
`You can use ‘grep’ commandonthisfile to find any server and its associated port.
`
`Asfar as the standards are concerned, the following four organizations/groups managethe
`TCP/IP protocol suite. Both the IRTF and the IETF fall under the IAB.
`
`1. The Internet Society (ISOC)
`
`2. The Internet Architecture Board (IAB). The IAB falls under the ISOC.
`
`3. The Internet Engineering Task Force (IETF)
`4. The Internet Research Task Force (IRTF)
`
`Tweet
`> Add your comment
`
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`Alacritech, Ex. 2043 Page 6
`
`ALA11353481
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`Alacritech, Ex. 2043 Page 6
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`

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`{ 72 comments... add one }
`
`Devin November2, 2011, 9:02 am
`
`Youleft out the most popular application layer protocol — HTTP!
`
`Link
`
`krishnan November2, 2011, 9:09 am
`
`Excellent job. Short and informative
`
`Link
`
`jalal hajigholamali November2, 2011, 12:10 pm
`
`Hi,
`
`very nice article
`thanks a lot...
`
`Link
`
`Julio Hoffimann Mendes November2, 2011, 1:52 pm
`
`Thank you for the clear explanation! I’ve always wantedto learnalittle about TCP/IP.
`Keep the good work!
`
`Regards,
`Julio.
`
`Link
`
`Walyonaire November2, 2011, 3:17 pm
`
`Alacritech, Ex. 2043 Page 7
`
`ALA11353482
`
`Alacritech, Ex. 2043 Page 7
`
`

`

`This reminds me of my Cisco Lessons, thanks for the “refresh”.
`Excellent Article.
`
`Link
`
`Narendra Kangralkar November2, 2011, 10:21 pm
`
`Very good explanation !!!
`Keep writing.
`
`Link
`
`Ramesh Natarajan November2, 2011, 10:27 pm
`
`@Devin,
`
`Yes. HTTPis the most popular application layer protocol. This is why the “TCP/IP
`Concept Example”section explains it using the HTTP protocol.
`
`Anyway,thanks for pointing it out, HTTP is added to the “Application Layer”section to
`makethe list complete.
`
`Link
`
`Joe November3, 2011, 3:37 am
`
`Thank youforthe effort.
`
`Well written.
`
`Link
`
`Vonskippy November 3, 2011, 2:54 pm
`
`Seems redundant to the OSI model.
`
`Link
`
`ATUL KHACHANE November4, 2011, 8:32 pm
`
`
`Yes. Vonskippy
`
`Great articles here always................this is one stop resource.....and you will find
`almost everything you need...
`
`Link
`
`Alacritech, Ex. 2043 Page 8
`
`ALA11353483
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`Alacritech, Ex. 2043 Page 8
`
`

`

`Himanshu November5, 2011, 12:48 pm
`
`Thank youall for your valuable comments.
`
`Link
`
`grouchO November13, 2011, 9:15 pm
`
`Some books say the 5000 upperlimit for client (dynamic/ephemeral) ports was a misprint
`in W. Richard Stevens’ “TCP/IP Illustrated Vol. 1” and should have been 50000. The
`issue doesn’t seem to have affected the progress of the protocol in taking over the world.
`
`Link
`
`Himanshu November13, 2011, 11:30 pm
`
`@grouch0
`OK. Can you provide some references?
`
`Link
`
`Devin November 28, 2011, 8:37 pm
`
`You forgot the physical layer.
`
`Link
`
`Himanshu November29, 2011, 12:08 am
`
`Well, I think you are talking about the OSI model where physical layeris the last layer.
`In TCP/IP model Datalink layer is the layer 1. Sometimes this layer is also referred to as
`“Network Interface Layer” as as to avoid confusion with the data link layer of the OSI
`model. This Network interface Layer encapsulates physical layer also.
`
`Link
`
`Sravan December15, 2011, 2:48 am
`
`Thanks. Short and informativearticle.
`
`Link
`
`deepak kumar January 3, 2012, 2:23 pm
`
`nice .....very helpfull
`thnx
`
`Alacritech, Ex. 2043 Page 9
`
`ALA11353484
`
`Alacritech, Ex. 2043 Page 9
`
`

`

`Link
`
`Reece April 17, 2012, 5:04 am
`
`thanks for the information, it helped me understand TCP/IP
`
`Link
`
`Dharmesh May13, 2012, 9:42 pm
`
`Thanksa lot!!!
`
`It was a very short and good explanation...
`
`Link
`
`Asif May 13, 2012, 10:08 pm
`
`Simple nd Superbbbbbbbbbbb
`
`Link
`
`rohan June 16, 2012, 1:15 am
`
`an application uses ephemeralport and it uses http as application layer protocol. Now
`whenthe packetis sent to destination, will it contain both ephemeral destination port and
`standard http destination port no. Howthis packet will be handled by transport layer and
`application layer at destination?
`
`Link
`
`Tariq August 9, 2012, 3:02 pm
`
`Nice.
`
`A small demo with
`wireshark or any other tool could be used to show “how”this happensin real time.
`
`Link
`
`raju September 13, 2012, 8:54 am
`
`Thanksa pile
`It’s fabulousartical and very knowledgeable for me,
`
`Link
`
`Alacritech, Ex. 2043 Page 10
`
`ALA11353485
`
`Alacritech, Ex. 2043 Page 10
`
`

`

`malik usman October 3, 2012, 9:15 am
`
`Thanksa lot!!!
`
`It was a very short and good explanation...
`
`Link
`
`swathi November24, 2012, 10:13 am
`
`Its is short but soooo...... informative,thank u very much
`
`Link
`
`Ritu Kushwaha January 26, 2013, 11:12 am
`
`Useful Information in Short. Well Done!!
`Keep Writing...
`
`Link
`
`Umar Badu February 7, 2013, 10:28 am
`
`Useful information,i really appreciate this...... Thanks
`
`Link
`
`poonguzhali February 14, 2013, 7:37 am
`
`Its very easy to understand.
`
`Link
`
`Nagendra Bhat February 20, 2013, 8:20 pm
`
`Hi.
`Explanation is good andif there any blogsrelated to this subject please mentionit or
`please mail me..
`
`Link
`
`monica March 15, 2013, 3:10 am
`
`Very well explained in simple language
`
`Link
`
`Alacritech, Ex. 2043 Page 11
`
`ALA11353486
`
`Alacritech, Ex. 2043 Page 11
`
`

`

`nice thanksalot...
`
`pemo May13, 2013, 2:13 am
`
`Link
`
`Ameen June 12, 2013, 9:49 am
`
`tanx 4 ur explanation.
`
`Link
`
`anand July 31, 2013, 12:43 am
`
`very good to understand
`
`Link
`
`brig (retd) ashwani sharma August 20, 2013, 9:47 am
`
`An extremely clear, concise and lucid explanation of TCP/IP.
`Great effort. Keepit up.
`
`Best Wishes
`
`Link
`
`hana September 2, 2013, 12:05 am
`
`good understand
`thanks
`
`Link
`
`khanabusd September 28, 2013, 6:00 am
`
`very helpful........... ccc cece eee ee es
`
`Link
`
`zug October 16, 2013, 10:29 pm
`
`We'll written in a language that I find easy to read and understand. Thank you.
`
`Link
`
`Alacritech, Ex. 2043 Page 12
`
`ALA11353487
`
`Alacritech, Ex. 2043 Page 12
`
`

`

`justauser October 27, 2013, 2:41 pm
`
`great article! the perfect balance ofdetails and general view
`thanks!
`
`Link
`
`maurice November7, 2013, 2:51 am
`
`it very helpful....... in simple language
`
`Link
`
`jaco November8, 2013, 9:00 pm
`
`Thank u
`
`u manage to save me
`as idon’t yet have a text book to read
`this gave light into my understanding of TCP/IP im able to
`peruse my dreams ||
`
`Link
`
`Mehbub November19, 2013, 6:00 am
`
`very good explanation,u save me alsO............cceceeee esse encethank u vry much
`
`Link
`
`arun mehta December8, 2013, 8:28 pm
`
`thanks
`
`Link
`
`Mintu Krishna December 12, 2013, 12:09 pm
`
`very nice, in short & informative explanation. Useful for those who have keen interest in
`Internet.
`Thanksa lot.
`
`Link
`
`Ben January 24, 2014, 5:57 am
`
`
`I found this very useful! Thank you very much 0
`
`Alacritech, Ex. 2043 Page 13
`
`ALA11353488
`
`Alacritech, Ex. 2043 Page 13
`
`

`

`Link
`
`Karm Nath February 12, 2014, 3:23 am
`
`good explanation.......
`
`Link
`
`joe March 19, 2014, 2:22 am
`
`thanksfor this article. Quite good.
`
`Link
`
`amar March 24, 2014, 3:15 am
`
`Hi,
`Explanations are very easy to read and understand. I would like to know more depth
`about encapsulation and decapsulation process of pkts in source side and destination side.
`
`Thanks
`
`Link
`
`kamal April 9, 2014, 9:29 am
`
`I really found useful article coz...before the exam usually I don’t read the book I just
`look and search the info...in the net....
`
`Link
`
`ravi June 5, 2014, 6:25 am
`
`short n sweet 1]
`
`Link
`
`kul sidhu June 10, 2014, 8:11 am
`
`Plz give difference between osi and tcp/ip with diagram
`
`Link
`
`Asamoah Apau Daniel August 21, 2014, 6:55 am
`
`Alacritech, Ex. 2043 Page 14
`
`ALA11353489
`
`Alacritech, Ex. 2043 Page 14
`
`

`

`1. what is a transport protocol?
`a. explain with diagrams different between TCP and UDP
`2. Explain secure Protocol
`a. explain with diagrams different between FTP and SSH
`
`Link
`
`Anonymous October 22, 2014, 1:43 am
`
`Clearly explained
`
`Link
`
`Nasham November24, 2014, 12:15 pm
`
`It very education tanksalot
`
`Link
`
`INADRAJEET KUMAR VERMADecember21, 2014, 9:45 pm
`
`Very good explanation!!!
`Keep writing.
`
`Link
`
`Arun May 16, 2015, 7:00 am
`
`vrh well explained in simple langu
`
`Link
`
`Arun May16, 2015, 7:01 am
`
`thanksall
`
`Link
`
`Zubair Khalid May 25, 2015, 6:18 am
`
`well explained sir!
`
`most of my confusion is gone and my concepts are clear now. Thank your very much
`
`Link
`
`Alacritech, Ex. 2043 Page 15
`
`ALA11353490
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`Alacritech, Ex. 2043 Page 15
`
`

`

`RVT May 25, 2015, 10:22 am
`
`Can you compare Modbus TCPand IEC 61850? Whatare the advantages and
`disadvantages between these two communication protocols?
`
`Link
`
`jagrati sharma June 3, 2015, 8:25 pm
`
`dear friend thanks a lot u helped meto understand .thanks¥.
`
`Link
`
`Danish June 7, 2015, 1:22 am
`
`awesomearticle.. I had a 3 hour class yesterday but I learned more clearly from this
`article than I did in that class. So thanksa lot.. really appreciated!!
`
`Link
`
`Hum Tum!! June 8, 2015, 1:23 pm
`
`Canu just tell me the order of osi layer.
`
`1 > 7 or 7 <1 which 1 is correct?
`
`o >in!
`
`vignesh July 3, 2015, 4:25 am
`
`any possibility to increase the TCP/IP ports more than 65355.
`
`Link
`
`Aravind Reddy Solipuram August 21, 2015, 11:11 pm
`
`very informative.thanksa lot
`
`o 2In
`
`majulalief August 23, 2015, 8:31 am
`
`Nice and clearly explained
`
`Link
`
`gopi September 14, 2015, 11:20 am
`
`Alacritech, Ex. 2043 Page 16
`
`ALA11353491
`
`Alacritech, Ex. 2043 Page 16
`
`

`

`I like this article. Now I undrstnd hw datais trnsfr through layer. Thnxxx
`
`Link
`
`Mitravinda September 23, 2015, 12:20 am
`
`The internet model or TCP/IP reference modelspecifically applies to internet worked
`systems, and has 4 layers. The movement of data packets across a network would be
`managed by whichlayer?
`A) The link layer
`B) The(inter) network layer
`C) Thetransport layer
`D) The application layer
`
`Link
`
`Abdul Mohsin December30, 2015, 3:21 am
`
`Very nice explanation .ThankYou.
`
`Link
`
`Prakash January 25, 2016, 3:10 am
`
`nice details on TCP/IPbasics.
`
`Link
`
`Carol February 18, 2016, 12:27 pm
`
`Thanks for the short cut TCP/IP lesson. I’mabit of a hack with interoperability. Helped
`me to understand where to look when I’m stuck. Well done!
`
`Link
`
`DONADILIGA DUKUMarch 17, 2016, 12:55 am
`
`very interesting. Thanks alot
`
`Link
`
`Mark Aguilar December 25, 2016, 11:26 pm
`
`Very informative and an excellent refresher.
`
`Link
`
`Alacritech, Ex. 2043 Page 17
`
`ALA11353492
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`Alacritech, Ex. 2043 Page 17
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`

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`basavaraj P April 20, 2017, 1:00 am
`
`nice explanation man, many thanks
`
`Link
`
`Cancelrepl
`
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