Service Disciplines for Guaranteed Performance Service Hui Zhang, “Service Disciplines for Guaranteed Performance Service in Packet-Switching Networks,” Proceedings of The IEEE, Vol. 83, no. 10, October Guaranteed performance service model: users specify traffic characteristics and performance requirements before transmission. Performance parameters: end-to-end delay bound, end-to-end delay jitter bound, loss probability.

Service Disciplines Work-conserving: A server is never idle when there is a packet to send. Nonwork-conserving: Each packet is assigned, either explicitly or implicitly, an eligibility time. Even when the server is idle, if no packets are eligible, none will be transmitted. - FCFS, Virtual Clock, WFQ, WF2Q, Delay-EDD - Jitter-EDD, Stop-and-Go, HRR, RCSP

FCFS vs. Virtual Clock

Department of Electrical Engineering Communications & Networking Group National Chung Cheng University WFQ and WF 2 Q u To approximate fluid fair queueing (FFQ) or generalized processor sharing (GPS) policy. Separate FIFO queue for each connection sharing the same output link. FFQ serves nonempty queues in proportion to their service shares. FFQ is impractical as it assumes that the server can serve all queued connections simultaneously and the traffic is infinitely divisible.

Department of Electrical Engineering Communications & Networking Group National Chung Cheng University Weighted Fair Queueing u When the server is ready to transmit the next packet at time t, it picks, among all the packets queued in the system at t, the first packet that would complete service in the corresponding FFQ system if no additional packets were to arrive after t. use only finish times of packets in the FFQ system. Also called Packetized GPS (PGPS).

Department of Electrical Engineering Communications & Networking Group National Chung Cheng University Worst-case Fair Weighted Fair Queueing u When the server is ready to transmit the next packet at time t, it picks, among all the packets that have started (and possibly finished) receiving service at time t, the first packet that would complete service in the corresponding FFQ system if no additional packets were to arrive after t. use both start times and finish times of packets in the FFQ system to achieve a more accurate emulation.

WFQ vs. WF 2 Q

Department of Electrical Engineering Communications & Networking Group National Chung Cheng University -EDD Delay-EDD u Earliest-due-date-first (EDD or EDF) Each packet from a periodic traffic stream is assigned a deadline and packets are sent in order of increasing deadlines. The deadline of a packet is the sum of its arrival time and the period of traffic stream. u Delay-EDD server negotiates a contract with each source: if a source obeys its promised traffic spec., the server will provide a delay bound A packet’s deadline = expected arrival time + delay bound

Department of Electrical Engineering Communications & Networking Group National Chung Cheng University Stop-and-go u Use a framing strategy; frame time = T. u The arriving frame of each incoming link is mapped to the departing frame of the output link by a constant delay. control capability: 2T bandwidth: peak rate allocation l’l’ l ’’ l Incoming link l ’ Outgoing link l Incoming link l ’’

Department of Electrical Engineering Communications & Networking Group National Chung Cheng University Department of Electrical Engineering Communications & Networking Group National Chung Cheng University Jitter-EDD u After a packet has been served at each server, a field in its header is stamped with the difference between its deadline and the actual finished time. u A regulator at the entrance of the next server holds the packet for this period before it is made eligible to be scheduled. u Provides delay jitter bound. Service Disciplines

Department of Electrical Engineering Communications & Networking Group National Chung Cheng University Department of Electrical Engineering Communications & Networking Group National Chung Cheng University Jitter-EDD u Scheduling algorithm Delay Bound served time switch n-1 switch n Arrival Deadline PreAhead Arrival hold Delay Bound Deadline Eligible Service Disciplines

Department of Electrical Engineering Communications & Networking Group National Chung Cheng University Department of Electrical Engineering Communications & Networking Group National Chung Cheng University Hierarchical Round Robin u Uses a multilevel framing strategy. u A slot in one level can either be allocated to a connection or to a lower level frame. u The server cycles through the frame and services packets according to the assignment of slots. u If the server cycles through a slot assigned to a connection, one packet from that connection is transmitted; if it cycles through a slot assigned to a lower level frame, it will service one slot from the lower level frame in the same fashion. Service Disciplines

Department of Electrical Engineering Communications & Networking Group National Chung Cheng University Department of Electrical Engineering Communications & Networking Group National Chung Cheng University Hierarchical Round Robin Service Disciplines