the time-wait state in tcp and its effect on busy servers theodore faber university of southern...
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The TIME-WAIT state in TCP and its Effect on Busy Servers Theodore Faber University of Southern California
Xindian Long
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
Delayed Packet Problem
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
TIME-WAIT State
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 lengthExtra end-of-transaction marker requires masking & restoring the marker
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
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
TIME-WAIT Negotiation
SYN (TW-Negotiate)
Client Server
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
TIME-WAIT Negotiation
SYN (TW-Negotiate)
endpoint1
endpoint2
Simultaneous connection establishment SYN (TW-Negotiate)
SYN-RCVD SYN-RCVD
Value Known Value To Use
Either Contains No Option
No Option
Same IP Address That IP Address
Different IP Address Larger IP Address
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
TCP Level Solution Modify the clients’ TCP implementation
Sends a RST packet to the server Puts itself into a TIME-WAIT state
Client
Server
Final Ack
RSTTW
TW
closed
Modified TCP State Machine
TCP Level Solution Only works for system that accepts
<RST> in TIME-WAIT state Performance limited by the way
<RST> is processed Changes the meaning of <RST>
packet
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 connection2. Sending series of requests3. Collecting the response4. Sending a Client-Close, closing connection5. The server closes its end without responding
CLIENT_CLOSE Extention
Client
Server
data
CLIENT_CLOSE
Detect the end of data
closeFIN
.
.
.
Client
Server
data
close
.
.
.
[Conection: close] line
Last request:Connection: close
FIN
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
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
Demonstration of Worst-CaseServer 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
Demonstration of Worst-CaseServer Loading
HTTP Load Experiments
System Throughput Mb/s
Conn/s TCP Mem (Kb)
Unmodified
20.97 49.09 722.7
TCP Mods 26.40 62.02 23.1
HTTP Mods
31.73 74.70 23.4
Two workstations each running 20 clientsFile Size: 9KB-5MB
System Throughput Mb/s
Conn/s TCP Mem (Kb)
Unmodified
Fails Fails Fails
TCP Mods 1.14 223.8 16.1
HTTP Mods
1.14 222.4 16.1
8 clients on 4 workstations, small files
TIME-WAIT Avoidance andPersistent Connection
2 Client machine, 1 Server5 HTTP request bursts10 request / Connection
Throughput vs. Clients
TIME-WAIT Avoidance andPersistent Connection
Connection Rate vs. Clients
TIME-WAIT Avoidance andPersistent Connection
Memory vs. Clients
Conclusion TCP With TCP With CLIENT_CLOSE TIME-WAIT Client HTTP Negotiation <RST> ExtensionReduces TIME-WAIT Loading Yes Yes YesCompatible With Current Protocols Yes Yes YesChanges Are Effective If Only No Yes YesThe Client Is ModifiedAllows System To Prevent Yes No YesTIME-WAIT AssassinationNo Changes To Transport Protocol No No YesNo Changes To Application Protocols Yes Yes NoAdds No Packet Exchanges To Yes No NoModified ProtocolTIME-WAIT Allocation Is A Requirement Yes No Noof Connection Establishment