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?