User Datagram Protocol: Difference between revisions

The User Datagram Protocol (UDP) is one of the core protocols of the Internet protocol suite. Using UDP, programs on networked computers can send short messages sometimes known as datagrams (using Datagram Sockets) to one another.

UDP does not provide the reliability and ordering guarantees that TCP does. Datagrams may arrive out of order or go missing without notice. Without the overhead of checking if every packet actually arrived, UDP is faster and more efficient for many lightweight or time-sensitive purposes. Also, its stateless nature is useful for servers that answer small queries from huge numbers of clients. Compared to TCP, UDP is required for broadcast (send to all on local network) and multicast (send to all subscribers).

Common network applications that use UDP include the Domain Name System (DNS), streaming media applications such as IPTV, Voice over IP (VoIP), Trivial File Transfer Protocol (TFTP) and online games.

UDP utilizes ports to allow application-to-application communication. The port field is 16-bits so the valid range is 0 to 65,535. Port 0 is reserved, but is a permissible source port value if the sending process does not expect messages in response.

Ports 1 through 1023 are named "well-known" ports and on Unix-derived operating systems binding to one of these ports requires root access.

Ports 1024 through 49,151 are registered ports.

Ports 49,152 through 65,535 are ephemeral ports and are used as temporary ports primarily by clients when communicating to servers.

UDP is a minimal message-oriented transport layer protocol that is currently documented in IETF RFC 768.

In the Internet protocol suite, UDP provides a very simple interface between a network layer below (e.g., IPv4) and a session layer or application layer above.

UDP provides no guarantees to the upper layer protocol for message delivery and a UDP sender retains no state on UDP messages once sent (for this reason UDP is sometimes called the Unreliable Datagram Protocol). UDP adds only application multiplexing and checksumming of the header and payload. If any kind of reliability for the information transmitted is needed, it must be implemented in upper layers.

The UDP header consists of only 4 fields. The use of two of those are optional (pink background in table).

Source port

This field identifies the sending port when meaningful and should be assumed to be the port to reply to if needed. If not used then it should be zero.

Destination port

This field identifies the destination port and is required.

Length

A 16-bit field that specifies the length in bytes of the entire datagram: header and data. The minimum length is 8 bytes since that's the length of the header. The field size sets a theoretical limit of 65,527 bytes for the data carried by a single UDP datagram.

Checksum

The 16-bit checksum field is used for error-checking of the header and data.

With IPv4

When UDP runs over IPv4, the method used to compute the checksum is defined within RFC 768:

Checksum is the 16-bit one's complement of the one's complement sum of a pseudo header of information from the IP header, the UDP header, and the data, padded with zero octets at the end (if necessary) to make a multiple of two octets.

In other words, all 16-bit words are summed together using one's complement (with the checksum field set to zero). The sum is then one's complemented. This final value is then inserted as the checksum field. Algorithmically speaking, this is the same as for IPv4.

The difference is in the data used to make the checksum. Included is a pseudo-header that mimics the IPv4 header:

The source and destination addresses are those in the IPv4 header. The protocol is that for UDP (see List of IPv4 protocol numbers): 17. The UDP length field is the length of the UDP header and data.

If the checksum is calculated to be zero (all 0's) it should be sent as negative zero (all 1's). If a checksum is not used it should be sent as zero (all 0's) as zero indicates an unused checksum.

With IPv6

When UDP runs over IPv6, the checksum is no longer considered optional, and the method used to compute the checksum is changed, as per RFC 2460:

Any transport or other upper-layer protocol that includes the addresses from the IP header in its checksum computation must be modified for use over IPv6, to include the 128-bit IPv6 addresses instead of 32-bit IPv4 addresses.

When computing the checksum, a pseudo-header that mimics the IPv6 header is included:

The source address is the one in the IPv6 header. The destination address is the final destination; if the IPv6 packet doesn't contain a Routing header, that will be the destination address in the IPv6 header, otherwise, at the originating node, it will be the address in the last element of the Routing header, and, at the receiving node, it will be the destination address in the IPv6 header. The Next Header value is the protocol value for UDP: 17. The UDP length field is the length of the UDP header and data.

If the checksum is calculated to be zero (all 0's) it should be sent as negative zero (all 1's).

Lacking reliability, UDP applications must generally be willing to accept some loss, errors or duplication. Some applications such as TFTP may add rudimentary reliability mechanisms into the application layer as needed. Most often, UDP applications do not require reliability mechanisms and may even be hindered by them. Streaming media, real-time multiplayer games and voice over IP (VoIP) are examples of applications that often use UDP. If an application requires a high degree of reliability, a protocol such as the Transmission Control Protocol or erasure codes may be used instead.

Lacking any congestion avoidance and control mechanisms, network-based mechanisms are required to minimize potential congestion collapse effects of uncontrolled, high rate UDP traffic loads. In other words, since UDP senders cannot detect congestion, network-based elements such as routers using packet queuing and dropping techniques will often be the only tool available to slow down excessive UDP traffic. The Datagram Congestion Control Protocol (DCCP) is being designed as a partial solution to this potential problem by adding end host TCP-friendly congestion control behavior to high-rate UDP streams such as streaming media.

While the total amount of UDP traffic found on a typical network is often on the order of only a few percent, numerous key applications use UDP, including the Domain Name System (DNS), the simple network management protocol (SNMP), the Dynamic Host Configuration Protocol (DHCP) and the Routing Information Protocol (RIP), to name just a few.

The following, minimalistic example shows how to use UDP for client/server communication under Unix:

Common code for server and client to be saved in a udp_sample.h file:

#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>

#define BUFLEN 512
#define PORT 9930

void diep(char *s)
{
    perror(s);
    exit(1);
}

The server:

#include "udp_sample.h"

int main() {
    struct sockaddr_in sock_server;
    struct sockaddr_in sock_client;
    int s;
    socklen_t socklen = sizeof(struct sockaddr_in);
    char buf[BUFLEN];

    if ((s = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1) {
        diep("socket()");
    }
    sock_server.sin_family = AF_INET;
    sock_server.sin_port = htons(PORT);
    sock_server.sin_addr.s_addr = htonl(INADDR_ANY);

    if (bind(s, (struct sockaddr *) &sock_server, socklen) == -1) {
        diep("bind()");
    }

    for (;;) {
        if (recvfrom(s, buf, BUFLEN, 0,
          (struct sockaddr *) &sock_client, &socklen) == -1)
            diep("recvfrom()");

        printf("Received packet from %s:%d\nData: %s\n", 
               inet_ntoa(sock_client.sin_addr),
               ntohs(sock_client.sin_port), buf);

    }

    close(s);
    return 0;
}

The client (replace "127.0.0.1" by the IP address of the server):

#include "udp_sample.h"

int main(void)
{
    struct sockaddr_in sock_server;
    int s, i;
    socklen_t socklen = sizeof(struct sockaddr_in);
    char buf[BUFLEN];

    if ((s = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1) {
        diep("socket");
    }

    sock_server.sin_family = AF_INET;
    sock_server.sin_port = htons(PORT);

    if (inet_aton("127.0.0.1", &(sock_server.sin_addr)) == 0) {
        fprintf(stderr, "inet_aton() failed\n");
        exit(1);
    }

    for (i = 0; i < 3; i++) {
        printf("Sending packet %d\n", i);
        sprintf(buf, "This is packet %d\n", i);

        if (sendto(s, buf, BUFLEN, 0,
          (struct sockaddr *) &sock_server, socklen) == -1) {
            diep("sendto()");
        }
    }

    close(s);
    return 0;
}

• TCP and UDP port numbers for a complete (growing) list of ports/services
• Connectionless protocol
• UDP flood attack
• UDP Data Transport
• UDP-Lite
• Reliable User Datagram Protocol (RUDP)
• Transmission Control Protocol
• IP or Internet Protocol, on top of which rests UDP

• RFC 768
• IANA Port Assignments
• The Trouble with UDP Scanning (PDF)
• Breakdown of UDP frame
• UDP on MSDN Magazine Sockets and WCF