Department of Computer Science Southern Illinois University Edwardsville Fall, 2013 Dr. Hiroshi Fujinoki Frame Relay Congestion Control CS 447 Networks and Data Communication

Frame_Relay3/002 Frame Relay as VC (Virtual Circuit) Since it is VC transmission, a path needs to be set up before transmission starts Network resources will be reserved for each path, network congestion can not happen for frame relay. Possible causes of network congestion in frame relay network  Transmitting hosts could transmit more than declared  Frame relay does allow senders to transmit more than they initially declare CS 447 Networks and Data Communication

Frame_Relay3/003 Example 1 NW application with constant transmission rate Time Transmission Rate Declared Peak Transmission Rate Unutilized link bandwidth (not much) CS 447 Networks and Data Communication

Frame_Relay3/004 Example 2 NW application with bursty transmission rate Time Transmission Rate Declared Peak Transmission Rate Unutilized link bandwidth Declared Peak Transmission Rate Unutilized link bandwidth CS 447 Networks and Data Communication

Frame_Relay3/005 Traffic Load Time 1/3 Link ( a wire) OKCongestion CS 447 Networks and Data Communication

Frame_Relay3/006 Traffic Load Throughput Traffic Load Delay A B C A B No Congestion Moderate Congestion Sever Congestion D C Knee CS 447 Networks and Data Communication

Frame_Relay3/007 Four Congestion Control Techniques  Traffic Policing (Frame Discard Control)  Congestion Avoidance (with Forward Explicit Congestion Notification - FECN)  Congestion Avoidance (with Backward Explicit Congestion Notification - BECN)  Congestion Recovery (with Implicit Congestion Notification) CS 447 Networks and Data Communication

Frame_Relay3/008 Traffic Load Throughput Traffic Load Delay A B C D A B C No Congestion Moderate Congestion Sever Congestion  Traffic Policing  FECN  BECN  Implicit CN CS 447 Networks and Data Communication

Frame_Relay3/009 Traffic Policing Sender R1R2R3 Receiver Traffic Policing Unit Data Transmission Monitor traffic rate from a sender If a sender starts transmitting more frames than declared, a decision should be made if the frames should be dropped or transferred - How much “more” should be transferred - For how long? Major Functions = to monitor traffic rate from each sender Drop frames if a sender transmits too much CS 447 Networks and Data Communication

Frame_Relay3/010 FECN Data Frame Slow-down message R1R2R3 0 1 DE Field DE Field set to 1 at a congested router At a congested router DE field is set to 1 The DE field can not be reset to 0 if once it is set to 1 When the receiver receives a frame with DE filed = 1, the receiver will sends “ slow down ” message to the sender Procedures DE = Discard Eligible A router in congestion CS 447 Networks and Data Communication

Frame_Relay3/011 BECN R1R2R3 0 0 Data Frame A congested router set DE field to 1 in an ACK message DE field can not be set to 0 again if it is once set to 1 The receiver starts slow-down when an ACK with DE filed = 1. Procedures 1 Slow-down message 1 DE Field set to 1 at a congested router A router in congestion CS 447 Networks and Data Communication

Frame_Relay3/012 Implicit CN ACK R1R2R3 Implicit Congestion Detection is Implicit Flow Control The sender detects a router congestion by detecting missing ACK from the receiver Blind time will be long, but the sender must have slowed-down since either FECD and BECD must have happened before Implicit CN On detecting missing ACK, the sender stops transmission No messaging overhead during heavy congestion Data Frame A router in congestion CS 447 Networks and Data Communication

Frame_Relay3/013 Different Motivations in the Four Congestion Control Techniques  Traffic Policing  FECN Congestion Avoidance  BECN Congestion Avoidance  Congestion Recovery To prevent network congestion (before it happens) To let the end host (receiver) make a decision for what should be done To avoid serious network congestion A congested router takes an immediate action to avoid serious network congestion With minimum extra messaging overhead To clear network traffic for recovery CS 447 Networks and Data Communication