Transfer Protocols and Mechanisms › IP Multicast or MCAST › UDP Operation and Limitations

UDP Operation and Limitations

Multicast does not provide guaranteed delivery; it uses UDP to send its packets, which is less reliable than TCP. UDP is a simple, unreliable datagram protocol that supports the SOCK_DGRAM abstraction for the Internet protocol family. It is layered directly above the Protocol (IP). UDP sockets are connectionless and are typically used with the sendto, sendmsg, recvfrom, and recvmsg system calls. The following are some of the disadvantages and limitations of using multicast and UDP:

• UDP packets can be lost or discarded in several ways, including a failure of the underlying communication mechanism.
• UDP implements a checksum over the data portion of the packet. If the checksum of a received packet is in error, the packet is dropped without notifying the sender.
• UDP sockets store a limited queue of received packets. Thus, datagrams received outside of these limits are discarded without notification.
• UDP receives and processes Internet Control Message Protocol (ICMP) error messages in response to UDP packets with the following limitations:
  • ICMP “source quench” messages are ignored.
  • ICMP “destination unreachable,” “time exceeded,” and “parameter problem” messages disconnect the socket from its peer. So, later attempts to send packets using the same socket return an error.
• UDP does not guarantee that packets are delivered in the order they are sent.
• UDP may generate duplicate packets during the communication process.
• The application using UDP must verify delivery of a UDP packet. Guaranteed delivery is a key feature of DTS. The DTS agents on the initiator and responder work together to ensure proper delivery of data, even UDP packets.
• UDP packets are given low priority on a routed network. If a network is busy and needs to drop packets, the UDP packets are dropped first. When the router in a DTS environment drops a packet, the responding agent notifies the initiator that data was dropped and requests a resend. The initiator then resends the packet. This constant need for resends causes more network traffic, which causes more dropped packets.
• If a router sends multicast packets to all subnets more than once, network traffic increases drastically. Any multicast message is sent to all subnets. The routers have no way of knowing which subnets have responders to this data; therefore, the data is sent to all subnets.