Cisco CCENT / CCNA Certification Exam Tutorial: Comparing TCP And UDP Headers

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?

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.  While the TCP header has a window field, sequence number, ack number, and quite a few other fields, the only other fields the UDP header has are the Length and Data fields. (There is an illustrated version of this article on my website.)

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.