UDP is unreliable on packet level.
But, doesn't it provide one response message after recieving all(supposed) packets?
I understand that TCP provide ack for all packets whereas UDP provides one response message for all packets. Am I correct?
UDP is unreliable on packet level.
The qualification is meaningless. UDP is unreliable, period.
But, Doesn't is provide one response message after recieving all(supposed) packets?
No.
I understand that TCP provide ack for all packets
No. It provides enough ACKs so the sender knows where the receiver is up to. This can be accomplished by selective ACKs.
whereas UDP provides one response message for all packets.
No.
Am I correct?
No.
UDP doesn't send any responses at all. The application must do that if required.
TCP connection provides reliable communication between hosts on internet. To ensure reliable communication TCP maintains state information and sequencing of packets in client and server side leading to a duplex connection. This state information is used to provide acknowledgements for received packets. There are variety of techniques of syncing and requesting lost packets according to which number of acknowledgements vary.
UDP is used where reliability is not a requirement so maintaining state information is not required leading to a more light weight protocol operation. One example of its use is streaming applications. No acknowledgement is send in UDP.
Related
One of the key differences between HTTP/2 and HTTP/3 is the switch from TCP to UDP.
As I understand it, TCP verifies data integrity by verifying that no data packets have been lost. Any packets lost are requested again to ensure all data is correctly received.
For UDP, there is no such validation. If packets are lost then so be it.
That being said, if I make a request on HTTP/3 and a packet is lost, is there a mechanism to ensure I get all my data, or will there be a risk that my response will be missing data packets?
If packets are lost then so be it.
No, with UDP it is no "so be it" but it is up to the protocol on top of UDP to care about packet loss, duplication and reordering or not to care. For example with RTP (real time audio in VoIP etc) it is fine to have some packet loss, since there is no use for later arriving packets anyway (audio must be low latency). And reordering and duplication is handled in RTP with protocol-inherent sequence numbers.
For HTTP/3 instead data loss is not acceptable. HTTP/3 is build on top of QUIC which is build on top of UDP. Packet loss is handled within QUIC (see QUIC Loss Detection and Congestion Control). Thus HTTP/3 is basically build on top of a reliable transport (QUIC) the same as HTTP/1 and HTTP/2 are build on top of an reliable transport layer (TCP).
Does TCP and UDP protocol have a way to manage their saturation?
When I write saturation, I mean Network congestion: what happens if the buffer of the server is full and the client sends a datagram UDP/TCP to the server?
Have these protocols a way to handle this scenario, or data would be lost?
This is a question about TCP/UDP basics. For this reason this answer is not going to be a complete TCP and UDP guide.
Network congestion at low level protocols
In case of network congestions, the data sender will usually notice it because of the failure of data sending APIs (e.g. the BSD functions send() and sendto()).
For example I have personal experience of TCP/IP over GPRS, in which the network problems caused data sending APIs to fail. In that case it was up to the sender to preserve its data in order to send it as soon as possible.
Congestion at receiver's side
That's what the asker had actually in mind.
Let's start from UDP. Really short answer: by its own design, data sent to congested servers will be lost. From Wikipedia,EN:
[...]It has no handshaking dialogues, and thus exposes the user's
program to any unreliability of the underlying network; there is no
guarantee of delivery, ordering, or duplicate protection[...]
Finally TCP. It has been designed to provide what is missing in UDP. From Wikipedia, EN:
TCP provides reliable, ordered, and error-checked delivery of a stream
of octets (bytes) between applications running on hosts communicating
via an IP network
How are these features achieved? I cannot provide a full TCP tutorial in this answer, but I can list three TCP's fundamental traits for achieving reliability:
Packets are numbered (each packet, but we can say each byte has a specific sequence number)
Retransmissions. The receiver sends an acknowledge (ACK) for each packet (but we can say each byte) it receives. For this reason the sender understands that the packet has not been received and can retransmit it (the number of retransmissions allowed vary according to different implmentations and user settings)
Sliding window. Let's describe it in a simply way: each peer currently informs the remote peer about its window, the number of bytes it is able to receive. As soon as a congestion occurs, a peer can reduce the windows so that the sender will slow down until the congestion ends.
To answer OP's question: in case of server congestion in case of TCP connection, the protocol assure retransmissions and throughput dynamic management that preserve for a reasonable amount of time any sent data.
I hope this simple description helps. It probably has raised even more questions, and in this case I suggest to deepen your study at the real source:
RFC 768 (UDP)
RFC 793 (TCP)
If a TCP connection is established between a client and server, is sending data faster on this connection-oriented route compared to a connectionless given there is less header info in the packets? So a TCP connection is opened and bytes sent down the open connection as and when required. Or would UDP still be a better choice via a connectionless route where each packet contains the destination address?
Is sending packets via an established TCP connection (after all hand shaking has been done) a method to be faster than UDP?
I suggest you read a little bit more about this topic.
just as a quick answer. TCP makes sure that all the packages are delivered. So if one is dropped for whatever reason. The sender will continue sending it until the receiver gets it. However, UDP sends a packet and just forgets it, so you might loose some of the packets. As a result of this, UDP sends less number of packets over network.
This is why they use UDP for videos because first loosing small amount of data is not a big deal plus even if the sender sends it again it is too late for receiver to use it, so UDP is better. In contrast, you don't want your online banking be over UDP!
Edit: Remember, the speed of sending packets for both UDP and TCP is almost same, and depends on the network! However, after handshake is done in TCP, still receiver needs to send the acks, and sender has to wait for ack before sending new batch of data, so still would be a little slower.
In general, TCP is marginally slower, despite less header info, because the packets must arrive in order, and, in fact, must arrive. In a UDP situation, there is no checking of either.
Using the TCP/IP protocol, given a connection between a client and a server, are the packets sent by the client to the server always received in the same order they were sent?
For example, if the client sends 3 packets of data, A, B and C, will the server always receive A first followed by B and C or is it possible for the server to receive C first, followed by A and B?
At IP level, packets may arrive in any order (if they arrive). At TCP level, the data stream is guaranteed to be ordered in the same manner on both ends.
That means yes, the server will always receive A then B then C. As long as you are using TCP.
When using TCP, data is received by the destination application in the same order as it is sent by the source application.
See the following for more details:
http://en.wikipedia.org/wiki/Transmission_Control_Protocol#Data_transfer
TCP is a transmission protocol, and it transmits data by sending the data out in IP packets over the underlying IP network. TCP is responsible for ensuring the correct transmission of the data, which includes ordering the arriving packets, re-requesting missing ones and discarding duplicates.
TCP as such does not expose any notion of "packet" to the user; the fact that the data is chunked into IP packets is a detail of the "over IP" implementation. A different implementation, e.g. TCP-over-bicycle-courier, might employ an entirely different scheme.
It cannot happen that you receive data in a different order on the application side over a TCP socket.
It may happen that packets are received in a different order by the networking layer of the OS, but TCP makes it a requirement that the upper levels get data in order. It is the OS' role to ask again for unreceived fragments etc and assemble these fragments. So, you need not worry.
UDP, on the other hand, offers no such guarantee.
The server (as the physical NIC of the machine) might receive them in any order. Your OS might receive them in any order again - that will mostly (but not allways) be the order of physical reception. Your client application is guaranteed to receive them in correct order, thats a property of TCP
In general, packets will be received in the same order they are transmitted. But the network may drop or reorder packets. For example, packets may take different routes and arrive out of order. Packets may be lost or even duplicated on the network. The TCP implementation is responsible for retransmitting packets that are lost, acknowledging packets that are received, ignoring duplicated packets, all with the objective of accurately reconstructing the transmitted byte stream at the receiver.
At the application level, you send a stream of bytes and receive a stream of bytes. TCP does whatever is needed to ensure the received stream of bytes is identical to the sent stream of bytes, regardless of what happens to the packets on the network.
Why is the IP called a connectionless protocol? If so, what is the connection-oriented protocol then?
Thanks.
Update - 1 - 20:21 2010/12/26
I think, to better answer my question, it would be better to explain what "connection" actually means, both physically and logically.
Update - 2 - 9:59 AM 2/1/2013
Based on all the answers below, I come to the feeling that the 'connection' mentioned here should be considered as a set of actions/arrangements/disciplines. Thus it's more an abstract concept rather than a concrete object.
Update - 3 - 11:35 AM 6/18/2015
Here's a more physical explanation:
IP protocol is connectionless in that all packets in IP network are routed independently, they may not necessarily go through the same route, while in a virtual circuit network which is connection oriented, all packets go through the same route. This single route is what 'virtual circuit' means.
With connection, because there's only 1 route, all data packets will arrive in the same order as they are sent out.
Without connection, it is not guaranteed all data packets will arrive
in the same order as they are sent out.
Update - 4 - 9:55 AM 2016/1/20/Wed
One of the characteristics of connection-oriented is that the packet order is preserved. TCP use a sequence number to achieve that but IP has no such facility. Thus TCP is connection-oriented while IP is connection-less.
The basic idea is pretty simple: with IP (on its own -- no TCP, UDP, etc.) you're just sending a packet of data. You simply send some data onto the net with a destination address, but that's it. By itself, IP gives:
no assurance that it'll be delivered
no way to find out if it was
nothing to let the destination know to expect a packet
much of anything else
All it does is specify a minimal packet format so you can get some data from one point to another (e.g., routers know the packet format, so they can look at the destination and send the packet on its next hop).
TCP is connection oriented. Establishing a connection means that at the beginning of a TCP conversation, it does a "three way handshake" so (in particular) the destination knows that a connection with the source has been established. It keeps track of that address internally, so it can/will/does expect more packets from it, and be able to send replies to (for example) acknowledge each packet it receives. The source and destination also cooperate to serial number all the packets for the acknowledgment scheme, so each end knows whether packets it sent were received at the other end. This doesn't involve much physically, but logically it involves allocating some memory on both ends. That includes memory for metadata like the next packet serial number to use, as well as payload data for possible re-transmission until the other side acknowledges receipt of that packet.
TCP/IP means "TCP over IP".
TCP
--
IP
TCP provides the "connection-oriented" logic, ordering and control
IP provides getting packets from A to B however it can: "connectionless"
Notes:
UDP is connection less but at the same level as TCP
Other protocols such as ICMP (used by ping) can run over IP but have nothing to do with TCP
Edit:
"connection-oriented" mean established end to end connection. For example, you pick up the telephone, call someone = you have a connection.
"connection-less" means "send it, see what happens". For example, sending a letter via snail mail.a
So IP gets your packets from A to B, maybe, in any order, not always eventually. TCP sorts them out, acknowledges them, requests a resends and provides the "connection"
Connectionless means that no effort is made to set up a dedicated end-to-end connection, While Connection-Oriented means that when devices communicate, they perform handshaking to set up an end-to-end connection.
IP is an example of the Connectionless protocols , in this kind of protocols you usually send informations in one direction, from source to destination without checking to see if the destination is still there, or if it is prepared to receive the information . Connectionless protocols (Like IP and UDP) are used for example with the Video Conferencing when you don't care if some packets are lost , while you have to use a Connection-Oriented protocol (Like TCP) when you send a File because you want to insure that all the packets are sent successfully (actually we use FTP to transfer Files). Edit :
In telecommunication and computing in
general, a connection is the
successful completion of necessary
arrangements so that two or more
parties (for example, people or
programs) can communicate at a long
distance. In this usage, the term has
a strong physical (hardware)
connotation although logical
(software) elements are usually
involved as well.
The physical connection is layer 1 of
the OSI model, and is the medium
through which the data is transfered.
i.e., cables
The logical connection is layer 3 of
the OSI model, and is the network
portion. Using the Internetwork
Protocol (IP), each host is assigned a
32 bit IP address. e.g. 192.168.1.1
TCP is the connection part of TCP/IP. IP's the addressing.
Or, as an analogy, IP is the address written on the envelope, TCP is the postal system which uses the address as part of the work of getting the envelope from point A to point B.
When two hosts want to communicate using connection oriented protocol, one of them must first initiate a connection and the other must accept it. Logically a connection is made between a port in one host and other port in the other host. Software in one host must perform a connect socket operation, and the other must perform an accept socket operation. Physically the initiator host sends a SYN packet, which contains all four connection identifying numbers (source IP, source port, destination IP, destination port). The other receives it and sends SYN-ACK, the initiator sends an ACK, then the connection are established. After the connection established, then the data could be transferred, in both directions.
In the other hand, connectionless protocol means that we don't need to establish connection to send data. It means the first packet being sent from one host to another could contain data payloads. Of course for upper layer protocols such as UDP, the recipient must be ready first, (e.g.) it must perform a listen udp socket operation.
The connectionless IP became foundation for TCP in the layer above
In TCP, at minimal 2x round trip times are required to send just one packet of data. That is : a->b for SYN, b->a for SYN-ACK, a->b for ACK with DATA, b->a for ACK. For flow rate control, Nagle's algorithm is applied here.
In UDP, only 0.5 round trip times are required : a->b with DATA. But be prepared that some packets could be silently lost and there is no flow control being done. Packets could be sent in the rate that are larger than the capability of the receiving system.
In my knowledge, every layer makes a fool of the one above it. The TCP gets an HTTP message from the Application layer and breaks it into packets. Lets call them data packets. The IP gets these packets one by one from TCP and throws it towards the destination; also, it collects an incoming packet and delivers it to TCP. Now, TCP after sending a packet, waits for an acknowledgement packet from the other side. If it comes, it says the above layer, hey, I have established a connection and now we can communicate! The whole communication process goes on between the TCP layers on both the sides sending and receiving different types of packets with each other (such as data packet, acknowledgement packet, synchronization packet , blah blah packet). It uses other tricks (all packet sending) to ensure the actual data packets to be delivered in ordered as they were broken and assembled. After assembling, it transfers them to the above application layer. That fool thinks that it has got an HTTP message in an established connection but in reality, just packets are being transferred.
I just came across this question today. It was bouncing around in my head all day and didn't make any sense. IP doesn't handle transport. Why would anyone even think of IP as connectionless or connection oriented? It is technically connectionless because it offers no reliability, no guaranteed delivery. But so is my toaster. My toaster offers no guaranteed delivery, so why not call aa toaster connectionless too?
In the end, I found out it's just some stupid title that someone somewhere attached to IP and it stuck, and now everyone calls IP connectionless and has no good reason for it.
Calling IP connectionless implies there is another layer 3 protocol that is connection oriented, but as far as I know, there isn't and it is just plain stupid to specify that IP is connectionless. MAC is connectionless. LLC is connectionless. But that is useless, technically correct info.