Difference between TCP and UDP

Question

What is the difference between TCP and UDP    I know that TCP is used in the case of non-time critical applications  and UDP is used for games or applications that require fast transmission of data  I know that TCP is used for HTTP  HTTPs  FTP  SMTP  and Telnet  I know that UDP is used for DNS and DHCP   But why  What characteristics of TCP and UDP make it useful for their respective use cases

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Short and simple differences between Tcp and Udp protocol:

1) Tcp - Transmission control protocol and Udp - User datagram protocol.

2) Tcp is reliable protocol, Where as Udp is a unreliable protocol.

3) Tcp is a stream oriented, where as Udp is a message oriented protocol.

4) Tcp is a slower than Udp.

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Run into this thread and let me try to express it in this way   TCP  3-way handshake  Bob  Hey Amy  I d like to tell you a secret Amy  OK  go ahead  I m ready Bob  OK  Communication Bob   I   this is the first letter Amy  First letter received  please send me the second letter Bob       this is the second letter Amy  Second letter received  please send me the third letter Bob   L   this is the third letter After a while Bob   L   this the third letter Amy  Third letter received  please send me the fourth letter Bob   O   this the forth letter Amy                4-way handshake Bob  My secret is exposed  now  you know my heart  Amy  OK  I have nothing to say  Bob  OK      UDP    Bob  I LOVE U Amy received   OVI L E     TCP is more reliable than UDP with even message order guaranteed  that s no doubt why UDP is more lightweight and efficient

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TCP establishes a connection before the actual data transmission takes place  UDP does not  In this way  UDP can provide faster delivery  Applications like DNS  time server access  therefore  use UDP    Unlike UDP  TCP uses congestion control  It responses to the network load  Unlike UDP  it  slows down when network congestion is imminent  So  applications like multimedia preferring constant throughput might go for UDP    Besides  UDP is unreliable  it doesn t react on packet losses  So loss sensitive applications like multimedia transmission prefer UDP  However  TCP is a reliable protocol  so  applications that require reliability such as web transfer  email  file download prefer TCP    Besides  in today s internet UDP is not as welcoming as TCP due to middle boxes  Some applications like skype fall down to TCP when UDP connection is assumed to be blocked

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Think of TCP as a dedicated scheduled UPS FedEx pickup dropoff of packages between two locations  while UDP is the equivalent of throwing a postcard in a mailbox   UPS FedEx will do their damndest to make sure that the package you mail off gets there  and get it there on time  With the post card  you re lucky if it arrives at all  and it may arrive out of order or late  how many times have you gotten a postcard from someone AFTER they ve gotten home from the vacation    TCP is as close to a guaranteed delivery protocol as you can get  while UDP is just  best effort

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TCP is a connection oriented stream over an IP network  It guarantees that all sent packets will reach the destination in the correct order  This imply the use of acknowledgement packets sent back to the sender  and automatic retransmission  causing additional delays and a general less efficient transmission than UDP   UDP is a connection-less protocol  Communication is datagram oriented  The integrity is guaranteed only on the single datagram  Datagrams reach destination and can arrive out of order or don t arrive at all  It is more efficient than TCP because it uses non ACK  It s generally used for real time communication  where a little percentage of packet loss rate is preferable to the overhead of a TCP connection    In certain situations UDP is used because it allows broadcast packet transmission  This is sometimes fundamental in cases like DHCP protocol  because the client machine hasn t still received an IP address  this is the DHCP negotiaton protocol purpose  and there won t be any way to establish a TCP stream without the IP address itself

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Simple Explanation by Analogy  TCP is like this    Imagine you have a pen-pal on Mars  we communicated with written letters back in the good ol  days before the internet     You need to send your pen pal the seven habits of highly effective people  So you decide to send it in seven separate letters    Letter 1 - Be proactive Letter 2 - Begin with the end in mind      etc   etc  Letter 7 - Sharpen the Saw  Requirements   You want to make sure that your pen pal receives all your letters - in order and that they arrive perfectly  If your pen pay receives letter 7 before letter 1 - that s no good  if your pen pal receives all letters except letter 3 - that also is no good   Here s how we ensure that our requirements are met    Confirmation Letter  So your pen pal sends a confirmation letter to say  I have received letter 1   That way you know that your pen pal has received it  If a letter does not arrive  or arrives out of order  then you have to stop  and go back and re-send that letter  and all subsequent letters  Flow Control  Around the time of Xmas you know that your pen pal will be receiving a lot of mail  so you slow down because you don t want to overwhelm your pen pal   Your pen pal sends you constant updates about the number of unread messages there are in penpal s mailbox - if your pen pal says that the inbox is about to explode because it is so full  then you slow down sending your letters - because your pen pal won t be able to read them  Perfect arrival  Sometimes while you send your letter in the mail  it can get torn  or a snail can eat half of it  How do you know that all your letter has arrived in perfect condition  Well your pen pal will give you a mechanism by which you can check whether they ve got the full letter and that it was the exactly the letter that you sent   e g  via a word count etc     a basic analogy

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The Law of Leaky Abstractions by Joel Spolsky  http   www joelonsoftware com articles LeakyAbstractions html

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One of the differences is in short  UDP   Send message and dont look back if it reached destination  Connectionless protocol  TCP   Send message and guarantee to reach destination  Connection-oriented protocol

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From the Skullbox article  TCP  Transmission Control Protocol  is the most commonly used protocol on the Internet   The reason for this is because TCP offers error correction  When the TCP protocol is used there is a  guaranteed delivery   This is due largely in part to a method called  flow control   Flow control determines when data needs to be re-sent  and stops the flow of data until previous packets are successfully transferred  This works because if a packet of data is sent  a collision may occur  When this happens  the client re-requests the packet from the server until the whole packet is complete and is identical to its original   UDP  User Datagram Protocol  is anther commonly used protocol on the Internet  However  UDP is never used to send important data such as webpages  database information  etc  UDP is commonly used for streaming audio and video  Streaming media such as Windows Media audio files   WMA    Real Player   RM   and others use UDP because it offers speed  The reason UDP is faster than TCP is because there is no form of flow control or error correction  The data sent over the Internet is affected by collisions  and errors will be present  Remember that UDP is only concerned with speed  This is the main reason why streaming media is not high quality   1  TCP is connection oriented and reliable where as UDP is connection less and unreliable   2  TCP needs more processing at network interface level where as in UDP it   s not   3  TCP uses  3 way handshake  congestion control  flow control and other mechanism to make sure the reliable transmission   4  UDP is mostly used in cases where the packet delay is more serious than packet loss

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This sentence is a UDP joke  but I m not sure that you ll get it  The below conversation is a TCP IP joke  A  Do you want to hear a TCP IP joke  B  Yes  I want to hear a TCP IP joke  A  Ok  are you ready to hear a TCP IP joke  B  Yes  I m ready to hear a TCP IP joke  A  Well  here is the TCP IP joke  A  Did you receive a TCP IP joke  B  Yes  I   did   receive a TCP IP joke

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TLDR   TCP - stream-oriented  requires a connection  reliable  slow UDP - message-oriented  connectionless  unreliable  fast  Before we start  remember that all disadvantages of something are a continuation of its advantages  There only a right tool for a job  no panacea  TCP UDP coexist for decades  and for a reason  TCP It was designed to be extremely reliable and it does its job very well  It s so complex because it accomplishes a hard task  providing a reliable transport over the unreliable IP protocol  Since all TCP s complex logic is encapsulated into the network stack  you are free from doing lots of laborious  error-prone low-level stuff in the application layer  When you send data over TCP  you write a stream of bytes to the socket at the sender side where it gets broken into packets  passed down the stack and sent over the wire  On the receiver side packets get reassembled again into a continous stream of bytes  Maintaining this nice abstraction has a cost in terms of complexity and performance  If the 1st packet from the byte stream is lost  the receiver will delay processing of subsequent packets even those have already arrived  the so-called  quot head of line blocking quot    In addition  in order to be reliable  TCP implements this   TCP requires an established connection  which requires 3 round-trips   quot infamous quot  3-way handshake  TCP has a feature called  quot slow start quot  when it gradually ramps up the transmission rate after establishing a connection to allow a receiver to keep up with data rate Every sent packet has to be acknowledged or else a sender will stop sending more data And on and on and on     All this is exacerbated in slow unreliable wireless networks because TCP was designed for wired networks where delays are predictable and packet loss is not so common  In addition  like many people already mentioned  for some things TCP just doesn t work at all  DHCP   However  where relevant  TCP still does its work exceptionally well  Using a mail analogy a TCP session is similar to telling a story to your secretary who breaks it into mails and sends over a crappy mail service to a publisher  On the other side another secretary assembles mails into a single piece of text  Some mails get lost  some get corrupted  so a very complex procedure is required for reliable delivery and your 10-page story can take a long time to reach your publisher  UDP UDP  on the other hand  is message-oriented  so a receiver writes a message  packet  to the socket and then it gets transmitted to a receiver as-is  without any splitting assembling in the transport layer  Compared to TCP  its specification is very straightforward  Essentially  all it does for you is adding a checksum to the packet so a receiver can detect its corruption  Everything else must be implemented by you  a software developer  Now read the voluminous TCP spec and try thinking of re-implementing even a small subset of it  Some people went this way and got very decent results  to the point that HTTP 3  uses QUIC - a protocol based on UDP  However  this is more of an exception  Common applications of UDP are audio video streaming and conferencing applications like Skype  Zoom or Google Hangout where loosing packets is not so important compared to a delay introduced by TCP

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Reasons UDP is used for DNS and DHCP   DNS - TCP requires more resources from the server  which listens for connections  than it does from the client   In particular  when the TCP connection is closed  the server is required to remember the connection s details  holding them in memory  for two minutes  during a state known as TIME WAIT 2   This is a feature which defends against erroneously repeated packets from a preceding connection being interpreted as part of a current connection   Maintaining TIME WAIT 2 uses up kernel memory on the server   DNS requests are small and arrive frequently from many different clients   This usage pattern exacerbates the load on the server compared with the clients   It was believed that using UDP  which has no connections and no state to maintain on either client or server  would ameliorate this problem   DHCP - DHCP is an extension of BOOTP   BOOTP is a protocol which client computers use to get configuration information from a server  while the client is booting   In order to locate the server  a broadcast is sent asking for BOOTP  or DHCP  servers   Broadcasts can only be sent via a connectionless protocol  such as UDP   Therefore  BOOTP required at least one UDP packet  for the server-locating broadcast   Furthermore  because BOOTP is running while the client    boots  and this is a time period when the client may not have its entire TCP IP stack loaded and running  UDP may be the only protocol the client is ready to handle at that time   Finally  some DHCP BOOTP clients have only UDP on board   For example  some IP thermostats only implement UDP   The reason is that they are built with such tiny processors and little memory that the are unable to perform TCP -- yet they still need to get an IP address when they boot   As others have mentioned  UDP is also useful for streaming media  especially audio   Conversations sound better under network lag if you simply drop the delayed packets   You can do that with UDP  but with TCP all you get during lag is a pause  followed by audio that will always be delayed by as much as it has already paused   For two-way phone-style conversations  this is unacceptable

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