Transcript ppt

Contents
 Causes and cost of congestion
 Three examples
 How to handle congestion
 End-to-end
 Network-assisted
 TCP congestion control
 ATM ABR congestion control
 Summary
What is congestion?
 Overflowing of routers buffers
 Packet retransmission
 Delay the problem?
 How to treat problem?
 Mechanisms to throttle senders
Causes and cost
 Example 1
 2 senders
 Router with infinite buffer space
 No retransmission
 Sending time infinte
 Link capacity (R)
Causes and cost
•Large queuing delays
Causes and cost
 Example 2
 2 senders
 Router with finite buffer space
 Retransmission
 Offered load
 Link capacity (R)
Causes and cost
•Retransmission due to buffer overflow
•Retrassmission of unneeded copies
Causes and cost
 Example 3
 4 senders
 4 routers with finite buffer space
 Multihop paths
 Link capacity (R)
 Offered load
Causes and cost
•Throughput goes to zero in the the limit of heavy traffic
•Wasted use of upstream links capacity
How to handle congestion
 End-to-end congestion control
 No explicit support from network
 IP layer provides no feedback
 Observed network behavior, ex. packet loss or delay
 TCP use end-to-end control
TCP congestion control
 Additive-increase, multiple-decrease (AIMD)
 Slow start (exponential)
 Congestion avoidance (linear)
 Fast recovery
 Timeout
 Tripple duplicat ACK’s
 TCP Tahoe
 TCP Reno (Newer versions of TCP)
How to handle congestion
 Network-assisted
 Explicit feedback
 Direct feedback
 Feedback via receiver
 Low impact on traffic
 ATM (Asynchronous Transfer Mode)
 ABR (Available Bit-Rate)
 XCP protocol
ATM ABR congestion control
 ATM takes a VC approach
 ATM is a kind of architecture
 Track behavior of individual senders
 ABR takes advantage of spare bandwidth
 Resource management cells (RM cells)
 RM cells convey congestion-related information
 Direct feedback or feedback via receiver
ABR
 Three mechanisms
 Explicit forward congestion indication (EFCI) bit
 Congestion indication and No increase (CI and NI) bit
 Expclicit rate (ER) field
Summary
 Why do we need to handle congestion?
 Minimize delays
 Make traffic more effective
 Maximize use of bandwidth
 How is congestion problem solved?
 End-to-end congestion control
 Network-assisted congestion control
Questions?