Cisco CCENT 640-822 Certification Exam Training : Why Doesn't UDP Offer All The Great Features That TCP Does?

During your Cisco CCENT and CCNA studies, you'll see that so many great TCP features - the three-way handshake, windowing, sequence numbering, error detection and recovery - are strictly TCP features. UDP doesn't use any of them. Two questions come to mind:

- Why doesn't UDP offer these features?
- Why in the world do we use UDP for anything?

A look at the TCP and UDP headers will answer both of those questions! Here's the TCP header...

.. and the UDP header.

Quite a difference! Take a few moments to compare the two and you'll see that UDP can't perform any of those TCP features because UDP literally can't offer them. The UDP header has no sequence number field, no ack number field, no ACK bit, no SYN bit, and no window field.

The TCP and UDP headers have only three values in common - the source port, the destination port, and the checksum.

Now that we've answered the question about why UDP doesn't offer the features that TCP does, let's answer the second question regarding why UDP is used in the first place. That question can really be answered with one word... overhead.

The TCP header is much larger than the UDP header. That header is being applied to every segment, and that adds up! UDP's advantage over TCP is that its header is much smaller than TCP's.