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Programming UNIX Sockets in C - Frequently Asked Questions: Writing UDP/SOCK_DGRAM applications Previous Next Table of Contents

5. Writing UDP/SOCK_DGRAM applications

5.1 When should I use UDP instead of TCP?

UDP is good for sending messages from one system to another when the order isn't important and you don't need all of the messages to get to the other machine. This is why I've only used UDP once to write the example code for the faq. Usually TCP is a better solution. It saves you having to write code to ensure that messages make it to the desired destination, or to ensure the message ordering. Keep in mind that every additional line of code you add to your project in another line that could contain a potentially expensive bug.

If you find that TCP is too slow for your needs you may be able to get better performance with UDP so long as you are willing to sacrifice message order and/or reliability.

UDP must be used to multicast messages to more than one other machine at the same time. With TCP an application would have to open separate connections to each of the destination machines and send the message once to each target machine. This limits your application to only communicate with machines that it already knows about.

5.2 What is the difference between "connected" and "unconnected" sockets?

From Andrew Gierth (

If a UDP socket is unconnected, which is the normal state after a bind() call, then send() or write() are not allowed, since no destination address is available; only sendto() can be used to send data.

Calling connect() on the socket simply records the specified address and port number as being the desired communications partner. That means that send() or write() are now allowed; they use the destination address and port given on the connect call as the destination of the packet.

5.3 Does doing a connect() call affect the receive behaviourof the socket?

From Richard Stevens (

Yes, in two ways. First, only datagrams from your "connected peer" are returned. All others arriving at your port are not delivered to you.

But most importantly, a UDP socket must be connected to receive ICMP errors. Pp. 748-749 of "TCP/IP Illustrated, Volume 2" give all the gory details on why this is so.

5.4 How can I read ICMP errors from "connected" UDP sockets?

If the target machine discards the message because there is no process reading on the requested port number, it sends an ICMP message to your machine which will cause the next system call on the socket to return ECONNREFUSED. Since delivery of ICMP messages is not guarenteed you may not recieve this notification on the first transaction.

Remember that your socket must be "connected" in order to receive the ICMP errors. I've been told, and Alan Cox has verified that Linux will return them on "unconnected" sockets. This may cause porting problems if your application isn't ready for it, so Alan tells me they've added a SO_BSDCOMPAT flag which can be set for Linux kernels after 2.0.0.

5.5 How can I be sure that a UDP message is received?

You have to design your protocol to expect a confirmation back from the destination when a message is received. Of course is the confirmation is sent by UDP, then it too is unreliable and may not make it back to the sender. If the sender does not get confirmation back by a certain time, it will have to re-transmit the message, maybe more than once. Now the receiver has a problem because it may have already received the message, so some way of dropping duplicates is required. Most protocols use a message numbering scheme so that the receiver can tell that it has already processed this message and return another confirmation. Confirmations will also have to reference the message number so that the sender can tell which message is being confirmed. Confused? That's why I stick with TCP.

5.6 How can I be sure that UDP messages are received in order?

You can't. What you can do is make sure that messages are processed in order by using a numbering system as mentioned in 5.5 How can I be sure that a UDP message is received?. If you need your messages to be received and be received in order you should really consider switching to TCP. It is unlikely that you will be able to do a better job implementing this sort of protocol than the TCP people already have, without a significant investment of time.

5.7 How often should I re-transmit un-acknowleged messages?

The simplest thing to do is simply pick a fairly small delay such as one second and stick with it. The problem is that this can congest your network with useless traffic if there is a problem on the lan or on the other machine, and this added traffic may only serve to make the problem worse.

A better technique, described with source code in "UNIX Network Programming" by Richard Stevens (see 1.5 Where can I get source code for the book [book title]?), is to use an adaptive timeout with an exponential backoff. This technique keeps statistical information on the time it is taking messages to reach a host and adjusts timeout values accordingly. It also doubles the timeout each time it is reached as to not flood the network with useless datagrams. Richard has been kind enough to post the source code for the book on the web. Check out his home page at

5.8 How come only the first part of my datagram is getting through?

This has to do with the maximum size of a datagram on the two machines involved. This depends on the sytems involved, and the MTU (Maximum Transmission Unit). According to "UNIX Network Programming", all TCP/IP implementations must support a minimum IP datagram size of 576 bytes, regardless of the MTU. Assuming a 20 byte IP header and 8 byte UDP header, this leaves 548 bytes as a safe maximum size for UDP messages. The maximum size is 65516 bytes. Some platforms support IP fragmentation which will allow datagrams to be broken up (because of MTU values) and then re-assembled on the other end, but not all implementations support this.

This information is taken from my reading of "UNIX Netowrk Programming" (see 1.5 Where can I get source code for the book [book title]?).

Andrew has pointed out the following regarding large UDP messages:

Another issue is fragmentation. If a datagram is sent which is too large for the network interface it is sent through, then the sending host will fragment it into smaller packets which are reassembled by the receiving host. Also, if there are intervening routers, then they may also need to fragment the packet(s), which greatly increases the chances of losing one or more fragments (which causes the entire datagram to be dropped). Thus, large UDP datagrams should be avoided for applications that are likely to operate over routed nets or the Internet proper.

5.9 Why does the socket's buffer fill up sooner than expected?

From Paul W. Nelson (

In the traditional BSD socket implementation, sockets that are atomic such as UDP keep received data in lists of mbufs. An mbuf is a fixed size buffer that is shared by various protocol stacks. When you set your receive buffer size, the protocol stack keeps track of how many bytes of mbuf space are on the receive buffer, not the number of actual bytes. This approach is used because the resource you are controlling is really how many mbufs are used, not how many bytes are being held in the socket buffer. (A socket buffer isn't really a buffer in the traditional sense, but a list of mbufs).

For example: Lets assume your UNIX has a small mbuf size of 256 bytes. If your receive socket buffer is set to 4096, you can fit 16 mbufs on the socket buffer. If you receive 16 UDP packets that are 10 bytes each, your socket buffer is full, and you have 160 bytes of data. If you receive 16 UDP packets that are 200 bytes each, your socket buffer is also full, but contains 3200 bytes of data. FIONREAD returns the total number of bytes, not the number of messages or bytes of mbufs. Because of this, it is not a good indicator of how full your receive buffer is.

Additionaly, if you receive UDP messages that are 260 bytes, you use up two mbufs, and can only recieve 8 packets before your socket buffer is full. In this case, only 2080 bytes of the 4096 are held in the socket buffer.

This example is greatly simplified, and the real socket buffer algorithm also takes into account some other parameters. Note that some older socket implementations use a 128 byte mbuf.

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