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Unit 3 Of ACN

Unit 3 Of ACN

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  • 1. TCP performance problem 1
    • Standard TCP imposes two performance problems for transaction-oriented communication.
    • First, a TCP connection is opened with a "3-way handshake", which must complete successfully before data can be transferred.
    • The 3-way handshake adds an extra RTT (round trip time) to the latency of a transaction.
  • 2. 3 way handshake
  • 3. TCP performance problem 2
    • The second performance problem is that closing a TCP connection leaves one or both ends in TIME-WAIT state for a time 2*MSL
      • where MSL is the maximum segment lifetime (defined to be 120 seconds).
    • A new connection cannot be opened until the TIME-WAIT delay expires
  • 4. TCP close (connection termination)
  • 5. The TIME-WAIT State
    • The TIME-WAIT state is required for two main reasons.
      • The first is to provide enough time to ensure that the ACK is received by the other device, and to retransmit it if it is lost.
      • The second is to provide a “buffering period” between the end of this connection and any subsequent ones. If not for this period, it is possible that packets from different connections could be mixed, creating confusion.
    • The standard specifies that the client should wait double a particular length of time called the maximum segment lifetime (MSL) before finishing the close of the connection.
  • 6. TCP performance problem 3
    • Transaction rate:
      • the 16-bit port space limits the rate to 2**16/240 transactions per second
      • more practically, an excessive amount of kernel space would be occupied by TCP state blocks in TIME-WAIT state.
  • 7. Solutions for TCP performance problems
    • T/TCP solves these two performance problems for transactions, by
      • (1) bypassing the 3-way handshake (3WHS)
      • (2) shortening the delay in TIME-WAIT state.
  • 8. Bypassing the Three-Way Handshake
    • T/TCP introduces a 32-bit incarnation number, called a "connection count" (CC)
    • A distinct CC value is assigned to each direction of an open connection
    • A T/TCP implementation assigns monotonically increasing CC values to successive connections that it opens actively or passively
  • 9. TCP Accelerated Open (TAO)
    • A T/TCP server keeps a cache containing the last valid CC value
    • If an initial <SYN> segment from a particular client host carries a CC value larger than the corresponding cached value, the <SYN> segment must be new and can therefore be accepted immediately
    • Otherwise it executes a normal 3WHS
  • 10. new TCP options
    • CC – for bypassing 3WHS
    • CC.NEW
      • Receiving a CC.NEW causes the server to invalidate its cache entry and do a 3WHS
    • CC.ECHO
      • When a server sends a <SYN,ACK> segment, it echoes the connection count from the initial <SYN> in a CC.ECHO option, which is used by the client to validate the <SYN,ACK> segment.
  • 11. TCP Accelerated Open Client Server CC[A] = X SYN, data1, CC=x SYN, ACK(data1), data2, CC=y, CC.ECHO=x CC[A] = X0 X>X0, TAO OK
  • 12. Minimal TCP transaction Client Server CC[A] = X SYN, data1, FIN, CC=x SYN,ACK(FIN),data2,FIN,CC=y,CC.ECHO=x CC[A] = X0 X0>X, TAO OK ACK(FIN),CC=x closed Syn-sent Time-wait Listen Closed Close-wait
  • 13. Truncation of TIME- WAIT Client Server CC[A] = X SYN, data1, FIN, CC=x SYN,ACK(FIN),data2,FIN,CC=y,CC.ECHO=x CC[A] = X0 X0>X, TAO OK ACK(FIN),CC=x closed Syn-sent Time-wait Listen Closed Close-wait SYN,data3, FIN, CC=z Syn-sent