we know that transport layer protocols like tcp control the flow and take care of the reliability by slide window and acknowledges ...etc. the data link layer with LLC sub layer has the same functionality for reliable connections also. the first question : is this means that both layers do the same functions twice? or when we use tcp in transport layer there is no need for LLC reliability functions ?how is it working ?
the second question: since IP layer is unreliable when it sends and receives packets, is this means that routers witch are layer 3 devices with no tcp protocol above it depends on LLC sub layer to takes care about the reliability "I mean between two routers" ?
I dont think you need other reliability, in words of disruption, both IP and Physical protocls (such as Ethernet) have CRC and Checksum in order to prevent disruption, but in terms of packet loss, TCP is what's called a stream protocol - it transfers you a stream of bytes, Sequence / Acknowledgment number server that purpose and help you keep track of bytes sent/read by client, if some weird "jump" in those numbers would be made, an ACK message wont be sent by receiving side, and protocol's sender would re-send lost packets hence I don't think that you need some reliability in terms of packet loss as TCP covers that for all downwards layers.... not sure I fully understood your question though, thats when we use TCP
about using pure IP and physical layer protocols, I honestly have no idea how to prevent packet-loss yet disruption is prevented by as mentioned earlier - checksums
TCP has a greater computation overhead to ensure reliable delivery of packets. But, since modern networks are fast, is there any scenario in which performance of UDP outweighs the reliability of TCP?
Is there any other particular advantage of UDP over TCP?
I can see two cases, where UDP would have an upper hand over TCP.
First, one of the attractive features of UDP is that since it does not need to retransmit lost packets nor does it do any connection setup, sending data incurs less delay. This lower delay makes UDP an appealing choice for delay-sensitive applications like audio and video.
Second, multicast applications are built on top of UDP since they have to do point to multipoint. Using TCP for multicast applications would be hard since now the sender would have to keep track of retransmissions/sending rate for multiple receivers.
It depends on your usage. If your application is time sensitive, like Voice over IP, then you don't care about missing packets. What you care about is the delay in that case.
You should have a look at this answer: What are examples of TCP and UDP in real life?
You could also look at the Wikipedia related section: http://en.wikipedia.org/wiki/User_Datagram_Protocol#Comparison_of_UDP_and_TCP
Applications that require constant data flow, bulk data and which require fastness than reliability uses UDP over TCP.
udp provides better application level control over what data is sent....since the data is packaged in a udp segment and immediately passed over to the network layer......hence no-frills segment delivery service is observed.
There is no need for connection establishment hence no delay(unlike tcp...which requires handshaking before the actual data transfer)
There is no need to maintain connection state in the end systems(ie no need for send and receive buffers,congestion control parameters and sequence and acknowledgement number parameters)..hence more active clients could be supported
Small packet header overhead for udp(only 8 bytes) where as tcp has 20 bytes of header
Facebook uses UDP connections instead of TCP/IP to connect to theirs Memcached Servers
There are couple of differences of UDP over TCP.
First, TCP is connection-based whereas UDP is connectionless.
Connection-based: Make sure that all messages will arrive and arrive in the correct order.
Connectionless: It does not guarantee order or completeness.
Second, Here is why UDP is faster over TCP:
UDP does not require ACK message back
UDP has no flow control
No duplication verification at the receiving end
Shorter header
I know the difference between the two on a technical level.
But in real life, can anyone provide examples (the more the better) of applications (uses) of TCP and UDP to demonstrate the difference?
UDP: Anything where you don't care too much if you get all data always
Tunneling/VPN (lost packets are ok - the tunneled protocol takes care of it)
Media streaming (lost frames are ok)
Games that don't care if you get every update
Local broadcast mechanisms (same application running on different machines "discovering" each other)
TCP: Almost anything where you have to get all transmitted data
Web
SSH, FTP, telnet
SMTP, sending mail
IMAP/POP, receiving mail
EDIT: I'm not going to bother explaining the differences, since you state that you already know and every other answer explains it anyway :)
UDP is mailing a letter at the post office.
TCP is mailing a letter with a return receipt at the post office, except that the post master will organize the letters in-order-of mailing and only deliver them in-order.
Well, it was an attempt anyway.
TCP:
World Wide Web(HTTP)
E-mail (SMTP TCP)
File Transfer Protocol (FTP)
Secure Shell (SSH)
UDP:
Domain Name System (DNS)
Streaming media applications such as movies
Online multiplayer games
Voice over IP (VoIP)
Trivial File Transfer Protocol (TFTP)
REAL TIME APPLICATION FOR TCP:
Email:
Reason: suppose if some packet(words/statement) is missing we cannot understand the content.It should be reliable.
REAL TIME APPLICATION FOR UDP:
video streaming:
* **Reason: ***suppose if some packet(frame/sequence) is missing we can understand the content.Because video is collection of frames.For 1 second video there should be
25 frames(image).Even though we can understand some frames are missing due to our imagination skills. Thats why UDP is used for video streaming.
The classic standpoint is to consider TCP as safe and UDP as unreliable.
But when TCP-IP protocols are used in safety critical applications,
TCP is not recommended because it can stop on error for multiple reasons.
Whereas UDP lets the application software deal with errors, retransmission timers, etc.
Moreover, TCP has more processing overhead than UDP.
Currently, UDP is used in aircraft controls and flight instruments,
in the ARINC 664 standard also named AFDX (Avionics Full-Duplex Switched Ethernet).
In ARINC 664, TCP is optional but UDP is used with the RTOS (real time operating systems) designed for the ARINC 653 standard (high reliability control software in civil aircrafts).
For more information about real time controls using IP and UDP in AFDX,
you can read the pages 27 to 50 in
http://www.afdx.com/pdf/AFDX_Training_October_2010_Full.pdf
TCP
I will not send data anymore until i get an acknowledgment.
this process is slow
It is used for security purpose
example: web, sending mail, receiving mail etc
UDP
Here i have no headache with acknowledgment.
this process is faster but here data can be lost .
example : video streaming , online games etc
TCP + UDP = SMTP(example : mobile,telephone)
TCP guarantees (in-order) packet delivery. UDP doesn't.
TCP - used for traffic that you need all the data for. i.e HTML, pictures, etc.
UDP - used for traffic that doesn't suffer much if a packet is dropped, i.e. video & voice streaming, some data channels of online games, etc.
TCP is a connection oriented protocol, It establishes a path, or a virtual connection all the way through switches routers proxies etc and then starts any communication. Various mechanisms like routing djikstras shortest path algorithm exist to establish the virtual end to end connection. So it finds itself used while browsing HTML and other pages, making payments and web applications in general.
UDP is a connectionless protocol - it simply has a destination and nodes simply pass it along if it comes as best as they can. So packets arriving out of order, along various routes etc are common. So Instant messengers and similar software developers think UDP an ideal solution.
In real life if you want to throw data in the net, without worrying about time taken to reach, order of reaching use UDP. If you want a solid path before you start throwing packets, and want same order and latency for your data packets use TCP - I will use UDP for Torrents and TCP for PayPal!
TCP :
Transmission Control Protocol is a connection-oriented protocol, which means that it requires handshaking to set up end-to-end communications. Once a connection is set up, user data may be sent bi-directionally over the connection.
Reliable – Strictly only at transport layer, TCP manages message acknowledgment, retransmission and timeout. Multiple attempts to deliver the message are made. If it gets lost along the way, the server will re-request the lost part. In TCP, there's either no missing data, or, in case of multiple timeouts, the connection is dropped. (This reliability however does not cover application layer, at which a separate acknowledgement flow control is still necessary)
Ordered – If two messages are sent over a connection in sequence, the first message will reach the receiving application first. When data segments arrive in the wrong order, TCP buffers delay the out-of-order data until all data can be properly re-ordered and delivered to the application.
Heavyweight – TCP requires three packets to set up a socket connection, before any user data can be sent. TCP handles reliability and congestion control.
Streaming – Data is read as a byte stream, no distinguishing indications are transmitted to signal message (segment) boundaries.
Applications of TCP
World Wide Web, email, remote administration, and file transfer rely on TCP.
UDP :
User Datagram Protocol is a simpler message-based connectionless protocol. Connectionless protocols do not set up a dedicated end-to-end connection. Communication is achieved by transmitting information in one direction from source to destination without verifying the readiness or state of the receiver.
Unreliable – When a UDP message is sent, it cannot be known if it will reach its destination; it could get lost along the way. There is no concept of acknowledgment, retransmission, or timeout.
Not ordered – If two messages are sent to the same recipient, the order in which they arrive cannot be predicted.
Lightweight – There is no ordering of messages, no tracking connections, etc. It is a small transport layer designed on top of IP.
Datagrams – Packets are sent individually and are checked for integrity only if they arrive. Packets have definite boundaries which are honored upon receipt, meaning a read operation at the receiver socket will yield an entire message as it was originally sent.
No congestion control – UDP itself does not avoid congestion. Congestion control measures must be implemented at the application level.
Broadcasts – being connectionless, UDP can broadcast - sent packets can be addressed to be receivable by all devices on the subnet.
Multicast – a multicast mode of operation is supported whereby a single datagram packet can be automatically routed without duplication to very large numbers of subscribers.
Applications of UDP
Numerous key Internet applications use UDP, including: the Domain Name System (DNS), where queries must be fast and only consist of a single request followed by a single reply packet, the Simple Network Management Protocol (SNMP), the Routing Information Protocol (RIP) and the Dynamic Host Configuration Protocol (DHCP).
Voice and video traffic is generally transmitted using UDP. Real-time video and audio streaming protocols are designed to handle occasional lost packets, so only slight degradation in quality occurs, rather than large delays if lost packets were retransmitted. Because both TCP and UDP run over the same network, many businesses are finding that a recent increase in UDP traffic from these real-time applications is hindering the performance of applications using TCP, such as point of sale, accounting, and database systems. When TCP detects packet loss, it will throttle back its data rate usage. Since both real-time and business applications are important to businesses, developing quality of service solutions is seen as crucial by some.
Some VPN systems such as OpenVPN may use UDP while implementing reliable connections and error checking at the application level.
TCP is appropriate when you have to move a decent amount of data (> ~1 kB), and you require all of it to be delivered. Almost all data that moves across the internet does so via TCP - HTTP, SMTP, BitTorrent, SSH, etc, all use TCP.
UDP is appropriate when you have small messages which you can afford to lose, and would like to send them as efficiently as possible. One reason you might be able to afford to lose them is because you can re-send them if they get lost. The main example on the internet is DNS - DNS consists of small queries saying things like "what is the IP number for stackoverflow.com?", and the responses are correspondingly small. Computers make a lot of these queries, so they should be made efficiently, but if they get lost en route, it's easy to time out and re-send them.
TCP guarantees packet delivery AND order. Order is almost as important as the delivery in the first place when reconstructing data for files such as executables, etc.
UDP does not guarantee delivery NOR order. Packets can arrive (or not!) in any order.
Common uses for TCP include file transfer where the integrity of the packets is paramount. Voice/video applications can afford to lose some data while still maintaining acceptable quality, and so usually use UDP.
One additional thought on some of the comments above that talks about ordered delivery.... It must be clarified that the destination computer may receive packets out of order on the wire, but the TCP at the destination is responsible for "rearranging out-of-order data" before passing it on to the upper layers of the stack. When you say TCP guarantees ordered packet delivery, what that means is it will deliver packets in correct order to the upper layers of the stack.
SCTP vs TCP vs UDPServices/Features SCTP TCP UDP
Connection-oriented yes yes no
Full duplex yes yes yes
Reliable data transfer yes yes no
Partial-reliable data transfer optional no no
Ordered data delivery yes yes no
Unordered data delivery yes no yes
Flow control yes yes no
Congestion control yes yes no
ECN capable yes yes no
Selective ACKs yes optional no
Preservation of message boundaries yes no yes
Path MTU discovery yes yes no
Application PDU fragmentation yes yes no
Application PDU bundling yes yes no
Multistreaming yes no no
Multihoming yes no no
Protection against SYN flooding attacks yes no n/a
Allows half-closed connections no yes n/a
Reachability check yes yes no
Psuedo-header for checksum no (vtags) yes yes
Time wait state vtags 4-tuple n/a
Since tcp usages are pretty straightforward from other answers, I'll mention some interesting UDP use-cases:
1)DHCP - Dynamic Host Configuration Protocol, which is being used in order to dynamically assign IP address and some other network configuration to the connecting devices. In simple words, this protocol allows you just connect to the network cable(or wifi) and start using the internet, without any additional configurations. DHCP uses UDP protocol. Since the settings request message is being broadcasted from the host and there is no way to establish a TCP connection with DHCP server(you don't know it's address) it's impossible to use TCP instead.
2)Traceroute - well-known network diagnostic tool which allows you to explore which path in the network your datagram passes to reach it's destination(and how much time it takes). By default, it works by sending UDP datagram with unlikely destination port number(ranging from 33434 to 33534) to the destination with the ttl(time-to-live) field set to 1. When the router somewhere in the network gets such datagram - it finds out that the datagram is expired. Then, the router drops the datagram and sends to the origin of the datagram an ICMP(Internet Control Message Protocol) error message indicating that the datagram's ttl was expired and containing router's name and IP address. Each time the host sends datagrams with higher and higher TTL, thus increasing the network part which it succeeds to overcome and getting new ICMP messages from new routers. When it eventually reaches it's destination(datagrams TTL is big enough to allow it),- the destination host sends 'Destination port unreachable' ICMP message to the origin host. This way, Traceroute knows that the destination was reached. Since the TCP guarantees segments delivery it would be at least inefficient to use it instead of UDP which, in turn, allows datagram to be just dropped without any resend attempts(resend is implemented on the higher level, with continuously increasing TTL as described above).
TCP: will get there in meaningful order
UDP: god knows (maybe)
UDP is applied a lot in games or other Peer-to-peer setups because it's faster and most of the time you don't need the protocol itself to make sure everything gets to the destination in the original order (UDP does not garantee packet delivery or delivery order).
Web traffic on the other hand is over TCP. (I'm not sure here but I think it has to do with the way the HTTP protocol is built)
Edited because I failed at UDP.
Real life examples of both TCP and UDP
tcp -> a phone call, sms or anything specific to destination
UDP -> a FM radio channel (AM), Wi-Fi.