Enhancements to TCP and IP Routers to Support Multimedia

1. Enhancements to TCP and IP Routers to Support Multimedia Based on paper: “Understanding and Improving TCP Performance over Networks with Minimum Guarantees”, co-authored by Wu-chang Feng et al. IEEE Transaction on Networking, Vol.7, No.2, April 1999 Presented in cs598kn by Klara Nahrstedt

2. Current Problems • Multimedia applications and their qualities vary from excellent to bad depending on the network load • Applications would benefit from a network service and end-to-end transport protocol that guarantees a minimum level of throughput all the times, but allows for higher throughput during periods of light load • One possible solution: Consider IntServ (Integrated Internet Services) with its “Controlled-load service” in the IP routers and modified TCP protocol

3. Integrated Services (IntServ) • Reservation specification: traffic envelope (Tspec) and service specification (Rspec) • Policing: Token Bucket • Marking at the source, classification at the network entry • Conformant controlled load (marked packets) and non-conformant control load and best effort (unmarked packets) • Use Weighted Fair Queuing scheduling • Selective packet discard mechanisms (RED, ERED)

4. IntServ (Discard Algorithms) • RED: Random Early Detection; • single FIFO queue is maintained for all packets and packets are dropped randomly with a given probability when the average queue length exceeds minimum threshold.If max. threshold is exceeded, all packets are dropped • ERED: Enhanced RED; • The thresholds apply only to unmarked packets. Marked packets are dropped only when the queue is full.

5. TCP Issues (Assumptions) • Assumptions: • At the source, tokens are generated at the service rate and are accumulated in the token bucket; • The peak rate is set to the link speed; • TCP segments belonging to reserved connections are transmitted as marked datagrams if there are sufficient tokens available in the token bucket at the time of transmission;

6. TCP Congestion Problem • TCP fails to fully exploit the benefits of the reservation and the compliant part of the throughput is less than the reservation levels; • Reason: • Since marked packets are not dropped by the network, it is apparent that the source is not generating sufficient number of marked packets to keep the reserved pipe full • Since the sender is a greedy source, it is TCP congestion control responsible for throttling the source due to cutting the congestion window; • Token loss happens

7. Acknowledgement Problem • Since TCP uses acknowledgement to trigger transmissions, any significant time gap between the receipt of successive acks causes the token bucket to overflow and results in a loss of transmission credits; • Change in congestion window causes token loss as well as gaps in acks, which again causes token losses

8. Solutions to Token Losses • Use deeper token bucket so that no token overflow happens; • Trade-off: large buffers needed because large burst can be supported; more congestion introduced because large bursts of marked packets are allowed into the routers, defeating service differentiation and ERED;

9. Solutions to Token Loss • Delayed or Timed Transmission: • a segment is held back for a random amount of time when there is not enough tokens to transmit it as a marked packet; • After every acknowledgement if (room under congestion and advertised window) if (token available > packet size) send packet as marked else send packet as unmarked

10. Solutions to Token Loss • Delayed or Timed Transmission: • a segment is held for a period and once the periodic timer expires, the connection examines the tokens in the token bucket. • If there are sufficient tokens and receiver’s window is large enough, sender sends packet as marked, ignoring the value of congestion window. • The timer is then reset to wake up another timer interval later. • After every TIMER expiry if (room under advertised window) if (token available) > packet size) send packet as marked reset TIMER

11. Solutions to Token Loss • Rate Adaptive Windowing – modified TCP windowing algorithm • The sender’s congestion window (CWND) consists of reserved part (RWND) =reserved rate*estimated round-trip time; and variable part= estimate of the residual capacity and shared by other connections; • Instead of reducing CWND by half at the beginning of fast recovery, the sender sets it to RWND + (CWND-RWND)/2 • Instead of reducing CWND to 1 at the beginning of slow start after detection of a lost segment through retransmission timeout, the sender sets it to RWND + 1 • At the receiver side congestion window (AWND) is also set to minimum of (RWND + (CWND-RWND)/2) and AWND

12. Summary • Multimedia Applications post new requirements • Minimal enhancements to the network infrastructure can help • Packet queuing and scheduling mechanisms • Assuming that the network supports minimum rate guarantees through end-to-end signaling, admission control, and resource reservation, TCP modifications to congestion control are useful and work; otherwise it doesn’t