Sec.6.1 Introduction 495 2.Packer discarding.When a node with no available buffer space receives a packet, it has no alternative but to discard the packet.More generally,however,packets may be discarded while buffer space is still available if they belong to sessions that are using more than their fair share of some resource.are likely to cause congestion for higher-priority sessions,are likely to be discarded eventually along their path, and so on.(Note that if a packet has to be discarded anyway,it might as well be discarded as early as possible to avoid wasting additional network resources unnecessarily.)When some of a session's packets are discarded,the session may need to take some corrective action,depending on its service requirements.For sessions where all packets carry essential information (e.g..file transfer sessions), discarded packets must be retransmitted by the source after a suitable timeout; such sessions require an acknowledgment mechanism to keep track of the pack- ets that failed to reach their destination.On the other hand,for sessions such as voice or video,where delayed information is useless,there is nothing to be done about discarded packets.In such cases,packets may be assigned different levels of priority.and the network may undertake the obligation never to discard the highest-priority packets-these are the packets that are sufficient to support the minimum acceptable quality of service for the session.The data rate of the highest-priority packets (the minimum guaranteed rate)may then be negotiated between the network and the source when the session is established.This rate may also be adjusted in real time,depending on the congestion level in the net- work. 3.Packet blocking.When a packet is discarded at some node.the network resources that were used to get the packet to that node are wasted.It is thus preferable to restrict a session's packets from entering the network if after entering they are to be discarded.If the packets carry essential information,they must wait in a queue outside the network:otherwise,they are discarded at the source.In the latter case,however,the flow control scheme must honor any agreement on a minimum guaranteed rate that the session may have negotiated when it was first established. 4.Packer scheduling.In addition to discarding packets.a subnetwork node can ex- ercise flow control by selectively expediting or delaying the transmission of the packets of various sessions.For example,a node may enforce a priority service discipline for transmitting packets of several different priorities on a given outgo- ing link.As another example,a node may use a(possibly weighted)round-robin scheduling strategy to ensure that various sessions can access transmission lines in a way that is consistent both with some fairess criterion and also with the minimum data rate required by the sessions.Finally,a node may receive infor- mation regarding congestion farther along the paths used by some sessions,in which case it may appropriately delay the transmission of the packets of those sessions. In subsequent sections we discuss specific strategies for throttling sources and for restricting traffic access to the network.Sec. 6.1 Introduction 495 2. Packet discarding. When a node with no available buffer space receives a packet, it has no alternative but to discard the packet. More generally, however, packets may be discarded while buffer space is still available if they belong to sessions that are using more than their fair share of some resource, are likely to cause congestion for higher-priority sessions, are likely to be discarded eventually along their path, and so on. (Note that if a packet has to be discarded anyway, it might as well be discarded as early as possible to avoid wasting additional network resources unnecessarily.) When some of a session's packets are discarded, the session may need to take some corrective action, depending on its service requirements. For sessions where all packets carry essential information (e.g., file transfer sessions), discarded packets must be retransmitted by the source after a suitable timeout; such sessions require an acknowledgment mechanism to keep track of the pack￾ets that failed to reach their destination. On the other hand, for sessions such as voice or video, where delayed information is useless, there is nothing to be done about discarded packets. In such cases, packets may be assigned different levels of priority, and the network may undertake the obligation never to discard the highest-priority packets-these are the packets that are sufficient to support the minimum acceptable quality of service for the session. The data rate of the highest-priority packets (the minimum guaranteed rate) may then be negotiated between the network and the source when the session is established. This rate may also be adjusted in real time, depending on the congestion level in the net￾work. 3. Packet blocking. When a packet is discarded at some node, the network resources that were used to get the packet to that node are wasted. It is thus preferable to restrict a session's packets from entering the network if after entering they are to be discarded. If the packets carry essential information, they must wait in a queue outside the network; otherwise, they are discarded at the source. In the latter case, however, the flow control scheme must honor any agreement on a minimum guaranteed rate that the session may have negotiated when it was first established. 4. Packet scheduling. In addition to discarding packets, a subnetwork node can ex￾ercise flow control by selectively expediting or delaying the transmission of the packets of various sessions. For example, a node may enforce a priority service discipline for transmitting packets of several different priorities on a given outgo￾ing link. As another example, a node may use a (possibly weighted) round-robin scheduling strategy to ensure that various sessions can access transmission lines in a way that is consistent both with some fairness criterion and also with the minimum data rate required by the sessions. Finally, a node may receive infor￾mation regarding congestion farther along the paths used by some sessions, in which case it may appropriately delay the transmission of the packets of those sessions. In subsequent sections we discuss specific strategies for throttling sources and for restricting traffic access to the network