2 cissp ® Official Study Guide Eighth Edition

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Transport Layer
The 
Transport layer (layer 4)
is responsible for managing the integrity of a connection 
and controlling the session. It accepts a PDU (variably spelled out as Protocol Data Unit, 
Packet Data Unit, or Payload Data Unit—i.e., a container of information or data passed 
between network layers). A PDU coming from the Session layer is converted into a seg-
ment. The Transport layer, which controls how devices on the network are addressed or 
referenced, establishes communication connections between nodes (also known as 
devices
) 
and defines the rules of a session. Session rules specify how much data each segment can 
contain, how to verify the integrity of data transmitted, and how to determine whether 
data has been lost. Session rules are established through a handshaking process, so the 
communicating devices are in agreement on the rules. (Please see the section “Transport 
Layer Protocols” later in this chapter for the discussion of the SYN/ACK three-way hand-
shake of TCP.)
The Transport layer establishes a logical connection between two devices and provides 
end-to-end transport services to ensure data delivery. This layer includes mechanisms for 
segmentation, sequencing, error checking, controlling the flow of data, error correction, 
multiplexing, and network service optimization. The following protocols operate within the 
Transport layer:
■
Transmission Control Protocol (TCP)
■
User Datagram Protocol (UDP)
■
Sequenced Packet Exchange (SPX)
■
Secure Sockets Layer (SSL)
■
Transport Layer Security (TLS)
Session Layer
The 
Session layer (layer 5)
is responsible for establishing, maintaining, and terminating 
communication sessions between two computers. It manages dialogue discipline or dia-
logue control (simplex, half-duplex, full-duplex), establishes checkpoints for grouping and 
recovery, and retransmits PDUs that have failed or been lost since the last verified check-
point. The following protocols operate within the Session layer:
■
Network File System (NFS)
■
Structured Query Language (SQL)
■
Remote Procedure Call (RPC)
Communication sessions can operate in one of three different discipline or control 
modes: