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We start off this section by revisiting the security threats covered previously for layer 3. Thwarting layer 3 attacks, as you'll recall, is a simple matter of blocking ICMP traffic coming into a network. These safeguards protect against smurf, ping, and DDoS attacks. Easy peasy! But it's nowhere near as simple when operating at the layer 4 transport layer. The primary networking protocols at layer 4 are TCP nad UDP. A well-known layer 4 threat is the Fraggle attack which behaves similarly to attacks at layer 2 which exploit vulnerabilities with ICMP. However, simply blocking UDP traffic and TCP traffic for that matter, is usually not a prudent strategy. You'd might as well just disconnect your network from the Internet since it's next to impossible to get anything worthwhile accomplished without these vital protocols flowing both in and out of your network! We dig further into what TCP and UDP are in this section. The major distinction between the two is that TCP is a connection-oriented protocol with guaranteed delivery of data. Whereas, UDP is a connection-less protocol with unreliable data delivery. These two protocols are responsible for the end-to-end transport of data and are often referred to as the "Pony Express" protocols. We also point out that these protocols don't entirely live in layer 4, but in fact span layers 4-7. It should be noted, however, that for the exam, if they ask at which single layer TCP and UDP reside, then the answer is layer 4. We then examine how a TCP connection is established via a 3-way handshake: - SYN - SYN/ACK - ACK It is noted that this handshake is vulnerable to exploit by what is known as a SYN flood where excessive SYN requests are sent to a host without further acknowledgement. This is a form of DDoS attack that exhausts connection and memory resources on the host. The section concludes with mentioning the advantages and disadvantages of TCP and UDP along with the applications that employ them. A fast connection-less transport protocol in UDP is advantageous for applications such as real time gaming and instant messaging apps where speed takes precedence over reliability. Conversely, the slower connection-oriented transport protocol of TCP is required for apps where data loss is not tolerable such as email apps and transferring files using FTP. And finally, you'll want to be sure to watch all the way to the end for some great networking humor!