The transport service are provided by transport protocols, which are distributed algorithms running on the hosts. The channels provided by the network layer between any two hosts can lose, duplicate and recorder messages in transit. The transport protocol is so designed such that they operate correctly inspite of unreliable network service and failure-prone networks and hosts.
The transport layer of a TCP/IP computer network, situated above the network layer and below the applications layer, provides transport service. The network layer provides unreliable packet transfer service between any two hosts. The transport layer uses this network service and provides transport services between any two applications in the network. Applications include email (SMTP), remote login (TELNET, SSH), file transfer (FTP), web browsers (HTTP), etc.
The ideal transport service is one that can transfer data packets between any two users and can do so reliably and with low-delay and low-jitter. Providing user-to-user service implies that the transport layer has to do user multiplexing at each host. Reliable data transfer means that data is delivered in the same sequence it was sent and without loss. Low-delay means that data sent is delivered within a specified (usually small ) time bound. Low jitter means that the time intervals between sending data is preserved at delivery within specified (usually small) time bounds. Achieving such ideal service requires the network to be capable of handling the worst-case load at any time, which, if the network is not to be incredibly expensive, means imposing severe restrictions on network access and data rates available to users ( as in telephony networks).
Fortunately, ideal transport service is not required for most applications. Thus the transport layer in TCP/IP networks does not strive for it. Instead it provides two separate services: a reliable service, which can suffer high delays and jitter and an unreliable service, which does no better than the network service. The reliable service, implemented by a transport protocol known as TCP, is used by applications where data integrity is essential, such as file transfer, email, remote login etc. The unreliable service, implemented by a transport protocol known as UDP, is used by applications where data loss can be tolerated but low-delay or low-jitter is desired, such as Internet telephony and voice/video streaming.
Thus reliable transport service is nothing but reliable data transfer between any two users. But reliable data transfer requires resources at the entities, such as buffers and processes for retransmitting data, reassembling data, etc. These resources typically cannot be maintained across failures. Furthermore, maintaining the resources continuously for every pair of users would be prohibitively inefficient, because only a very small fraction of user pairs in a network exchange data with any regularity, especially in a large network such as the Internet. Therefore a reliable transport service involves connection management and data transfer. Data transfer provides for the reliable exchange of data between connected users. Connection management provides for the establishment and termination of connections between users.
In general, reliable transport service (For e.g. TCP service) involves three aspects: user multiplexing, reliable connection management between users, and reliable data transfer between connected users. Unreliable transport service (For e.g. UDP service), on the other hand, involves two aspects: user multiplexing and unreliable data transfer between users.
The transport layer of a TCP/IP computer network, situated above the network layer and below the applications layer, provides transport service. The network layer provides unreliable packet transfer service between any two hosts. The transport layer uses this network service and provides transport services between any two applications in the network. Applications include email (SMTP), remote login (TELNET, SSH), file transfer (FTP), web browsers (HTTP), etc.
The ideal transport service is one that can transfer data packets between any two users and can do so reliably and with low-delay and low-jitter. Providing user-to-user service implies that the transport layer has to do user multiplexing at each host. Reliable data transfer means that data is delivered in the same sequence it was sent and without loss. Low-delay means that data sent is delivered within a specified (usually small ) time bound. Low jitter means that the time intervals between sending data is preserved at delivery within specified (usually small) time bounds. Achieving such ideal service requires the network to be capable of handling the worst-case load at any time, which, if the network is not to be incredibly expensive, means imposing severe restrictions on network access and data rates available to users ( as in telephony networks).
Fortunately, ideal transport service is not required for most applications. Thus the transport layer in TCP/IP networks does not strive for it. Instead it provides two separate services: a reliable service, which can suffer high delays and jitter and an unreliable service, which does no better than the network service. The reliable service, implemented by a transport protocol known as TCP, is used by applications where data integrity is essential, such as file transfer, email, remote login etc. The unreliable service, implemented by a transport protocol known as UDP, is used by applications where data loss can be tolerated but low-delay or low-jitter is desired, such as Internet telephony and voice/video streaming.
Thus reliable transport service is nothing but reliable data transfer between any two users. But reliable data transfer requires resources at the entities, such as buffers and processes for retransmitting data, reassembling data, etc. These resources typically cannot be maintained across failures. Furthermore, maintaining the resources continuously for every pair of users would be prohibitively inefficient, because only a very small fraction of user pairs in a network exchange data with any regularity, especially in a large network such as the Internet. Therefore a reliable transport service involves connection management and data transfer. Data transfer provides for the reliable exchange of data between connected users. Connection management provides for the establishment and termination of connections between users.
In general, reliable transport service (For e.g. TCP service) involves three aspects: user multiplexing, reliable connection management between users, and reliable data transfer between connected users. Unreliable transport service (For e.g. UDP service), on the other hand, involves two aspects: user multiplexing and unreliable data transfer between users.
No comments:
Post a Comment
Your feedback and comments are valuable for us: