Limitations of TCP and UDP

TCP provides reliable data transfer, but it transmits data in a sequence. However, some applications may need reliable data transfer, though not necessarily in a strict sequence. These applications prefer partial ordering of data, wherein ordering is maintained only within subflows of data. The strict sequence maintenance in TCP not only makes partial ordering of data impossible, it also causes unnecessary delay in the overall data delivery. Moreover, if a single packet is lost, delivery of subsequent packets is blocked until the lost TCP packet is delivered. This causes head-of-line (HOL) blocking.

TCP transmits data in a stream. This requires that applications add their own record marking, to delineate their messages. Applications must use the PUSH flag in the TCP header, to ensure that a complete message is transferred in reasonable time.

In a TCP connection, each host includes a single network interface, and a connection is established between the network interfaces of the two hosts. As a result, if the connection breaks because of a path failure, data becomes unavailable until the connection is re-established.

TCP is vulnerable to denial of service (DoS) attacks, such as SYN flood attacks. A DoS occurs when a malicious host forges an IP packet with a fake IP address and sends a large number of TCP SYN messages to the victim host. Each time the TCP stack, on the victim host, receives a new SYN message, the TCP stack allocates kernel resources to service the new SYN message. When the TCP stack is flooded with multiple SYN messages, the victim host can run out of resources and fail to service the new legitimate SYN messages.

In UDP, the transfer of data is unreliable, because it is a connectionless protocol. In a UDP connection, an application cannot verify if the packet has reached the destination.

UDP does not contain an in-built congestion control mechanism to detect path congestion. As a result, more data may be injected into an already congested network. This results in data loss.

If stringent rules for reliable data transfer are implemented in applications that use UDP, the implementation causes additional overhead and complexity in the applications.