Home > TCP and UDP Tutorial

TCP and UDP Tutorial

September 20th, 2016 Go to comments

TCP Features

Some popular TCP features we will learn here are: Multiplexing using port numbers, Flow control using windowing and Reliability (Error Detection and Error recovery)

Multiplexing using port numbers

Suppose you are using a laptop for web browsing, email communicating and FTP uploading at the same time. All of them require using TCP while your laptop only has one IP address (with one network card) so how your laptop knows which packets received from the Internet are dedicated for which application?

Above question is solved with port numbers. Each application will use a different and available port number to communicate with outside world. For example your laptop can choose port 50000 for web browsing, port 50001 for email communicating and port 50002 for FTP uploading.


Notice that your laptop can choose any available source port but it must use pre-defined destination ports for well-known services. Port numbers are defined in three ranges:
+ Well-known port numbers (0 through 1023): assigned to key or core services that systems offer
+ Registered port numbers (1024 through 49151): assigned to industry applications and processes. For example: 1433 is assigned for Microsoft SQL Server process)
+ Dynamic port numbers (49152 through 65535): used as temporary ports for specific communications. Our laptop can use these ports for communication

The table below lists TCP ports for well-known services:

TCP Service Description Port
FTP File Transfer Protocol 20, 21
SSH Secure shell 22
Telnet Terminal network 23
SMTP Simple Mail Transfer Protocol 25
DNS Domain Name Server 53
HTTP Hyper Text Transfer Protocol 80
HTTPS Hyper Text Transfer Protocol Secure 443

Note: There are some other well-known ports that are not listed here. The well-known ports are assigned by the Internet Assigned Numbers Authority (IANA) in the range of 0 to 1023.

Multiplexing relies on a concept called a socket. A socket consists of three things:

+ An IP address
+ A transport protocol
+ A port number

So suppose the IP address on our laptop is and use TCP to access web server with port 50000, we may write the socket (, TCP, 50000). For web server application running on the Web Server with IP the socket should be (, TCP, 80) as the web server uses the well-known port 80 for HTTP.

The socket on each computer is unique so the connection between two sockets on two computers identify a unique connection between them. Therefore you can use multiple applications on the same computer at the same time. Each application will use a unique source port so they cannot interfere with each other.

We only mentioned about source ports but notice TCP header requires both source port and destination port. That means if our laptop wants to connect to a Web Server it must include the destination port in TCP header as well. The destination port for Web Server in this case is 80. When the Web Server replies to our laptop, it uses the laptop’s source port as its destination port (50000 in this case).



Note: Both TCP and UDP use multiplexing with port numbers for their services.

Flow-control using windowing

In the TCP header there is a field called “Window” which plays an important role in the TCP transmission. A “Window” specifies the number of segments the sender can forward without receiving an acknowledgment. It is the key to transfer data and flow control efficiently. Let’s see how it works!

After the TCP connection has been established, both the client and server use this Window field to tell the other how many bytes of data it is willing to receive at one time before sending an acknowledgement to the sender. The larger the window size number (in bytes), the greater the amount of data that the host can transmit. For example, with a Window size of 1 (byte), every one byte must be acknowledged before sending the next one.


But waiting for ACK after each segment would be very inefficient. So TCP tries to increase the Window size to 3 (bytes), which means every three bytes can be received before sending the acknowledgement.


As you can see, the bigger the Window size, the fewer ACKs needed to be sent and the more efficient the transmission is. So the receiver will try to increase the Window size after each successful transmission so that the sender can send more. But the Window size cannot increase forever, TCP stops increasing the Window size when the receiver does not send an ACK (within a specific time period) or when the Window size reaches its maximum value. If a congestion occurs on the link then TCP may decrease the Window size.

The window size is variable during the lifetime of a connection so we often refer it as a “sliding window”.

If the sender does not receive the ACK in time, it knows that the segments should be resent, and that the transmission rate should be slowed down. Suppose Host A did not receive the expecting ACK 7 then it knows segments 4, 5, 6 should be resent.


Reliability (Error Detection and Error recovery)

This is the most important feature of TCP. TCP must recover from data that is damaged, lost, duplicated during the transmission. But please grasp the difference between error detection and error recovery first:
+ Error detection: the detection of errors during the transmission. Error detection does not repair corrupted data, it just detects it
+ Error recovery: the detection of errors and repair them

To achieve error detection, TCP adds some extra bits to the data, called checksum. A TCP sender computes the checksum value based on the contents of the TCP header and data fields. This 16-bit value will be compared with the value the receiver generates using the same computation. If the values match, the receiver can believe that segment arrived intact. If the values do not match, the receiver indicates an error occurred and the segment is discarded and a notification will be sent to the receiver depending on how the TCP stack is implemented on the receiver’s operating system.

To achieve error recovery, TCP uses the Sequence number (at the sender’s side) and Acknowledgement fields (at the receiver’s side) in the TCP header. These two fields are also used to find out lost, duplicated segments. Let’s see an example.

In the transmission below, host A sends three segments 1, 2, 3 to host B. Segment 2 was lost while segment 3 arrived to Host B. Then Host B replied with an ACK 2, implying that it is expecting segment 2 next. Host A can re-send another segment 2 to recover the lost segment. If Host B receive that segment it will ask for the segment 4 (because it already has segment 3).

TCP_Error_Recovery.jpgError recovery

You may ask “what will happen if the ACK 2 sent from Host B is also lost?” In fact, after sending each segment Host A sets a retransmission timer, just in case the ACK is lost (or all the sending segments are lost; Host B would not send ACK in this case because it did not receive anything). If this timer expires, Host A will send all the segments again.

Note: UDP does support error detection (via checksum) but it does not support error recovery. If UDP finds a corrupted segment, it just simply drop it.

Let’s sum up all things we learned about TCP and UDP so far.


+ Both TCP and UDP operate at Transport Layer
+ Both TCP and UDP use Multiplexing via port numbers


Reliable Unreliable
Connection-oriented Connectionless
Segment retransmission and flow control through windowing No retransmission or windowing
Segment sequence No sequencing
Acknowledge segment No acknowledgement
Start and end the communication by three-way handshake and four-way termination No action is required before and after sending real data
Support error recovery Only support error detection

Finally we show the TCP and UDP header in detail for your reference. There are some fields which are out of scope of this tutorial.

tcp_header.jpgTCP Header (20 bytes)

Notice about the FLAG fields (between the “Reserved” and “Window Size” fields). If SYN bit is turned on, it is a SYN message. If ACK bit is turned on, it is an ACK message. If both SYN and ACK bits are turned on, it is a SYN-ACK message.

And this is the UDP header:

UDP_header.jpgUDP Header (8 bytes)

Comments (28) Comments
Comment pages
1 4 5 6 2834
  1. Unknown
    June 28th, 2019

    Taking the ccna 200-125 please send me the dump to {email not allowed} I have a few weeks before the exam.

  2. Anonymous
    June 28th, 2019

    Skl3on318 gmail

  3. Anonym
    July 18th, 2019

    plz send me the dump to iwanencek15-gmail

  4. Anonymous
    July 20th, 2019

    can someone pls send the recent CCNA 210-260 dumps osuntobs (at) gmail. com exams is in 2 weeks

  5. Anonymous
    July 22nd, 2019

    hi everyone pls am in dear need of recent dumps for CCNA 210-260 exams is in less than 2 weeks osuntobs (at) gmail , would be extremely happy and gratefull to have this.

  6. Anonymous
    July 22nd, 2019

    hi everyone pls am in dear need of recent dumps for CCNA 200-125 exams is in less than 2 weeks osuntobs (at) gmail , would be extremely happy and gratefull to have this.

  7. Anonymous
    August 13th, 2019

    please send dumps

    chan_1609 @ yahoo . com

    {email not allowed}

  8. Frans
    September 8th, 2019

    Can someone send me recent dumps for ICND2 200-105 exam please. thanks in advance

  9. Frans
    September 8th, 2019

    Can someone send me recent dumps for ICND2 200-105 exam to migzola55gmail.com, please? thanks in advance

  10. peter Gicheru
    October 2nd, 2019

    kindly someone send me latest dumps for ICND2 200-105 to {email not allowed}

  11. peter Gicheru
    October 2nd, 2019

    my mail is komaspeter2gmail.com, kindly send me recent dumps

  12. maoca
    October 7th, 2019

    Kindly would anyone send me 200-125 test dumps to mariolopznet at hotmail.com? thank you in advance.

  13. Ben
    October 15th, 2019

    Kindly would anyone send me 200-125 test dumps to hang622 at gmail.com? thx!!

  14. uri
    October 18th, 2019

    can someone send me the latest dumps for ICND2 105-200 to {email not allowed}

  15. Que
    October 24th, 2019

    Hi, kindly please send me the lastest dump ccna200-125 please. my e mail is eskerka748 @ gmail.com

  16. dmitry
    October 24th, 2019

    Pls send me dumps 200-125, my mail dimon965316(dog)yandex(dot)ru

  17. dmitry
    October 24th, 2019

    Pls send me dumps 200-125, my mail dimon965316(dog)yandex(dot)ru )

  18. Yasin
    November 2nd, 2019

    i am giving ccna in two days, anyone help me sending the latest dumps.. email address {email not allowed}

  19. Yasin
    November 2nd, 2019

    i am giving ccna in two days, anyone help me sending the latest dumps.. email address yasin wasim 786 @ gmail.com

  20. Gabriel
    November 7th, 2019

    Im giving Ccna exam in a few weeks, pls send me the latest dumps. my email is dreadline(dot)oka @ gmail(dot)com

  21. Anonymous
    December 9th, 2019

    Can i am taking my CCNA 200-125 in 2 days… Can someone send me dumps? pilotishak @ gmail(dot)com

  22. Anonymous
    December 16th, 2019

    Kindly send CCNA latest dumps at haseebfarooq585 at gmail dot com

  23. Anonymous
    December 31st, 2019

    Please send me the recent dump for CCNA 200-125 beckz.t.r /gmail / com
    Test coming up next month.

  24. Mahesh
    January 8th, 2020

    please send me CCNA latest dumps {email not allowed}

  25. Mahesh
    January 8th, 2020


  26. Mat
    January 8th, 2020

    there is a very good explain about TCP/UDP protocols – I am impressed

  27. Amy
    January 18th, 2020

    Please send dumps for CCNA 200-125 to

    aspirantisee (at) gmail.com

  28. diana
    January 24th, 2020

    i recently bought dumps from surebraindumps, does anybody know about this company do they have valid dumps. here is the site to look and inform me it is urgent https://www.surebraindumps.com/200-125.htm i will be grateful if anybody helps me regarding this

Comment pages
1 4 5 6 2834
Add a Comment