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The TIME-WAIT state in TCP and its Effect on Busy Servers Theodore Faber University of Southern California Xindian Long.

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Presentation on theme: "The TIME-WAIT state in TCP and its Effect on Busy Servers Theodore Faber University of Southern California Xindian Long."— Presentation transcript:

1 The TIME-WAIT state in TCP and its Effect on Busy Servers Theodore Faber University of Southern California Xindian Long

2 Outline TIME-WAIT state and its function Performance problems on busy web server Solution: move the TIME-WAIT to clients End points negotiation TCP, HTTP modification Experiments Conclusion

3 Delayed Packet Problem

4 TIME-WAIT State Blocking connections between the same address/port pair Holding a Time-Wait State at one endpoint 2MSL The end doing active close holds the TIME-WAIT State


6 TIME-WAIT state in the server The server actively closes the connection, and maintains the TIME-WAIT state Server sending data TCP connection closure as end-of-transaction marker Simple protocol, fast response Otherwise: Knowing the content length Extra end-of-transaction marker requires masking & restoring the marker

7 Performance Problems on Busy Server Too many connections in TIME-WAIT State TCB Consumes memory Slows active connections A shorter MSL weaken the protection Solutions like persistent connection are not enough

8 Solution: TIME-WAIT in Clients Blocking at one end is enough Move the TIME-WAIT to the Client End points negotiating TCP Modification HTTP Modification Scales better than persistent connection

9 TIME-WAIT Negotiation SYN (TW-Negotiate) ClientServer SYN, Ack (TW-Negotiate set to its choice) Ack (TW-Negotiate set to the same value) If Acceptable If not Acceptable RST Non-simultaneous connection establishment

10 TIME-WAIT Negotiation SYN (TW-Negotiate) endpoint1endpoint2 Simultaneous connection establishment SYN (TW-Negotiate) SYN-RCVD Value KnownValue To Use Either Contains No OptionNo Option Same IP AddressThat IP Address Different IP AddressLarger IP Address

11 TIME-WAIT Negotiation Advantage Makes post-connection memory requirement explict Transparent to applications No hidden performance penalty Disadvantage Significant change to TCP state machine Information from connection establishment affects the closure Negotiating at closure reduces endpoints control over their resource

12 TCP Level Solution Modify the clients TCP implementation Sends a RST packet to the server Puts itself into a TIME-WAIT state ClientServer Final Ack RST TW closed

13 Modified TCP State Machine

14 TCP Level Solution Only works for system that accepts in TIME-WAIT state Performance limited by the way is processed Changes the meaning of packet

15 Application Level Solution Decouple the end-of-connection with end- of-transaction indication HTTP 1.1 Modification Content Length as the end of transaction New Extension Request: CLIENT_CLOSE 1. The client opens a connection 2. Sending series of requests 3. Collecting the response 4. Sending a Client-Close, closing connection 5. The server closes its end without responding

16 CLIENT_CLOSE Extention ClientServer data CLIENT_CLOSE Detect the end of data close FIN...... ClientServer data close...... [Conection: close] line Last request: Connection: close FIN

17 Application Level Solution CLIENT-WAIT notify the server about the client close Requires change only on the client side Conforms to HTTP 1.1 Requires no change to other protocols Creates no new security vulnerability ONLY reduces the load by HTTP

18 Experiments Environment Implemented under SUNOS 4.1.3 Evaluation using custom benchmark program and WebSTONE benchmark 640 Mb/s Myrinet LAN Three Tests Demonstration of Worst-case Server Loading HTTP Experiment Time wait avoidance and Persistent Connections

19 Demonstration of Worst-Case Server Loading Test Configuration A Server Two clients doing simultaneous bulk transfer Server loaded with TIME-WAIT state by a fourth machine Constructing a worst case scenario Client connections are put at the end of the list of TCBs Two clients are used to neutralize the simple caching behavior Two distinct clients allow bursts from different clients to interleave

20 Demonstration of Worst-Case Server Loading

21 HTTP Load Experiments SystemThroughput Mb/sConn/sTCP Mem (Kb) Unmodified20.9749.09722.7 TCP Mods26.4062.0223.1 HTTP Mods31.7374.7023.4 Two workstations each running 20 clients File Size: 9KB-5MB SystemThroughput Mb/sConn/sTCP Mem (Kb) UnmodifiedFails TCP Mods1.14223.816.1 HTTP Mods1.14222.416.1 8 clients on 4 workstations, small files

22 TIME-WAIT Avoidance and Persistent Connection 2 Client machine, 1 Server 5 HTTP request bursts 10 request / Connection Throughput vs. Clients

23 TIME-WAIT Avoidance and Persistent Connection Connection Rate vs. Clients

24 TIME-WAIT Avoidance and Persistent Connection Memory vs. Clients

25 Conclusion TCP With TCP With CLIENT_CLOSE TIME-WAIT Client HTTP Negotiation Extension Reduces TIME-WAIT Loading Yes Yes Yes Compatible With Current Protocols Yes Yes Yes Changes Are Effective If Only No Yes Yes The Client Is Modified Allows System To Prevent Yes No Yes TIME-WAIT Assassination No Changes To Transport Protocol No No Yes No Changes To Application Protocols Yes Yes No Adds No Packet Exchanges To Yes No No Modified Protocol TIME-WAIT Allocation Is A Requirement Yes No No of Connection Establishment

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