WCF and other service-oriented technologies make a distinction between transport reliability and message reliability. Transport reliability (such as that offered by TCP/IP) offers point-to-point guaranteed delivery at the network packet level, and also guarantees in-order delivery of the packets. Transport reliability is not resilient to dropped network connections and a variety of other communication problems.
Message reliability, as the name implies, deals with reliability at the message level, independent of how many packets are required to deliver the message. Message reliability provides for end-to-end guaranteed delivery and order of messages, regardless of how many intermediaries are involved and how many network hops are required to deliver the message from the client to the service. Message reliability is based on an industry standard for reliable message-based communication that maintains a session at the transport level and supports retries in case of transport failures such as dropping a wireless connection. It automatically deals with congestion, message buffering, and flow control and can adjust the flow of messages accordingly. Message reliability also deals with connection management, verifying connections and cleaning them up when they are no longer needed.
Message reliability does not guarantee message delivery. All it provides is a guarantee that if the message does not reach its destination, the sender will know about it.
"Programming WCF Services, 2nd Edition"