What is the Transport Layer?
The Transport Layer is layer four of the OSI model.
The Transport Layer resides above the Network Layer and below the Session Layer.
The Transport Layer is responsible for delivering messages between networked hosts.
As part of this, the Transport Layer is also responsible for fragmentation and reassembly.
In addition, some Transport Layer protocols also provide services to manage flow control and end-to-end error recovery.
Transport Layer Standards
Standards which operate at the Transport Layer include: TCP and UDP.
Books on the OSI Model

OSI Reference Model Pocket Guide
The pocket-sized reference provides the help you need to effectively prepare for the OSI related questions you'll encounter on Cisco's CCNA certification exam. It also serves as a practical tutorial explaining the confusing layered architecture of modern network design. The Open Systems Interconnection (OSI) Reference Model is the basis for much of modern networking. It is a layered model under which all networking protocols and services are defined. Many attempts at explaining OSI become too abstract and require a very high level of network design knowledge and experience to even begin to understand. This pocket guide presents a more practical, user-friendly examination of the topic. The author, Howard Berkowitz, CCSI, is an accomplished network designer and instructor who has written two books on network architecture. He effectively utilizes real-life experiences, analogies, and humor to clear up some of the most common confusions and misconceptions associated with the OSI Reference. Sample questions (with correct answers and complete explanations), similar to those found on Cisco's certification exams are included in this reference. These questions (with correct answers and complete explanations) allow readers to test their OSI knowledge level before attempting the actual tests.

OSI Reference Model for Telecommunications
The OSI (Open System Interconnection) Reference Model is a cornerstone of modern network design. Although the OSI model has become almost synonymous with data communications, it serves the public switched telephone network (PSTN) as well and is a productive way to organize and teach the building blocks of telecom systems. In OSI Reference Model for Telecommunications, hands-on expert Debbra Wetteroth provides telecom staffers the information they need to gain a working knowledge of this essential telecom service architecture and equipment. Her style that breaks down the barriers between data and voice vocabularies. This quick reference to the OSI model puts the data you need everyday at your fingertips.

OSI: A Model for Computer Communications Standards
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