Host-to-Host Layer Protocols essentially shields the upper layer applications from the complex inner workings of the network. This layer takes the data from the application layer along with any specific instructions and prepares the information to be sent. There are two protocols at this layer: Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).
Transmission Control Protocol (TCP) is a connection-oriented protocol that takes large blocks of data from an application and breaks it into segments. Each segment is numbered and sequenced so that the destination TCP stack can reorder and properly sequence the information as was intended by the application layer. After the segments are sent, TCP (on the sending side) awaits acknowledgement by the receiving end within the TCP virtual circuit session, and any segments not acknowledged will be retransmitted.
User Datagram Protocol (UDP) is essentially the scaled down version of TCP, also known as the thin protocol. Unlike TCP, UDP does not include all of the bells and whistles; there is no sequencing, no acknowledgement, etc. The purpose is to send the data out and not worry about it. With TCP there is the necessity for sequencing and acknowledgement because everything is necessary for the data to be complete; on the other hand, with UDP, such as a phone call, not all of the data is necessary for you to understand the message being transmitted.
The information may sound jittery and chunky but it is understandable in the long run. UDP is classified as a connectionless protocol.
One important thing you need to keep in mind is the ability to differentiate between the two models.
Lammle, T. (2007). CCNA Cisco Certified Network Associate Study Guide. Indianapolis: Wiley Publishing, Inc.
Odom, W. (2012). Official Cert Guide ICND1 640-822. Indianapolis, IN: Cisco Press.
Odom, W. (2011). Official Cert Guide ICND2 640-816. Indianapolis, IN: Cisco Press.