Soft QoS Service (SQS) in the Internet

摘要

The Internet is based on the best-effort (datagram) service. The IP (Internet Protocol) exchanges data in self-contained units, called packets, each of which is routed independently according to the destination address embedded in header. The datagram service is inherently unreliable as packets segmented from data may travel in different paths and are delivery out of sequence. Furthermore, some packets may be lost due to errors or congestion in the network. To enhance the delivery service, a stream service is provided using the TCP (Transmission Control Protocol). Installed at sender and receiver, TCP establishes a connection and maintains the connection state. TCP at the sender adds sequence numbers to packets so that they are reassembled at the receiver in order. TCP also estimates the status of the network congestion via acknowledgement delays and adjusts the transmission rate. As the Internet evolves and experiences an explosive growth, advanced services beyond the datagram and the stream are being considered. In order to meet the requirements of real-time video connections, the Integrated Services (IntServ) architecture introduces reservation service. Network resources are reserved along a path using the RSVP (Reservation Protocol). In RSVP, sources periodically generate path messages so that routers record the links leading to individual sources [Zhan 93]. A reservation request initiated by a receiver is forwarded toward the source as routers utilize the information on the location of the source derived from the path messages. The IntServ architecture parallels the ATM protocols in that RSVP provides the signaling for connection establishments and that the router maintain connection states (in soft state as opposed to hard state in ATM). In large networks, the maintenance of per-connection states may become significant.