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TCP Sliding Window

  • the purpose of the Sliding Window is not flow control; it is to keep track of bytes/packets sent, received, ACKd, written, read, expected,…on the sender and the receiver
  • uses the concept of pipelining: sending a bunch of segments without waiting for ACKs → improved performance
  • the Sliding window protocol improves on the Stop-and-Wait protocol
  • Window size is the number of bytes (or packets) that can be sent or received
  • Window size is advertised with the SYN segment
  • a sender maintains three variables:
    • SWS: number of segments the sender can send without waiting for ACK → the allowed number of unacknowledged segments to be sent
    • LAR
    • LSS
  • the receiver maintains these variables
    • RWS: number of out-of-order segments that it can receive before sending an ACK
    • LAS: the sequence number of the Last Acceptable Segment
    • LSR: the sequence number of the Last Segment Received
  • The sender makes sure this invariant is true: LSS – LAR <= SWS
  • the receiver makes sure this invariant is true: LAS – LSR <= RWS
  • Any time a host sends packets, it holds a SWS. Any time it has to receive data, it holds a RWS → since TCP is a full duplex protocol, each host maintains both SWS and RWS.
  • in terms of bytes:
    • the receiver can not buffer data that is more than ACK Seq Num + RWS
    • the sender must not send data more than ACK Seq Num + RWS
  • principle:
    • Sender side:
      • when a higher ACK is received:
        • the LAR pointer advances one slot
        • the window advances one slot, which means we can send one more segment
        • when the sender keeps receiving the same ACK, it means that one segment is lost. It must retransmit it and get an ACK for it before sliding the window.
    • Receiver side:
      • when a segment is received:
        • if seqnum <= LSR or seqnum > LAS, then the segment is discarded
        • otherwise it is accepted
        • if ACK is lost and ACK+1 is sent, then there is no need to send ACK, because ACK+1 acknowledges data up to ACK→ concept of TCP Cumulative ACKs
  • if a RTT = x ms, then the number of possible RTTs per one second is (1000/x)
  • SWS <= SendBuffer
  • RWS <= RcvBuffer
  • Advertized Window = the amount of bytes that the sender can send. This value is advertized by the receiver in the TCP header (Window field). This is also called RecvWindow.
tcp-sliding-window-1

Figure: The place of the Window field, in the TCP header

    • maxRcvBuffer – (LastByteRcvd – LastByteRead)    = Advertized Window
  •  Effective Window: how many bytes the sender can still send to a particular receiver. This value is relative to the Advertized Window.
    • Effective Window = Advertized Window – (LastByteSent – LastByteAckd)
  •  LastByteWritten – LastByteAcked <= maxSendBuffer
  • Bytes to know:
    • at the sender side: LastByteAckd < LastByteSent < LastByteWritten
    • at the receiver side: LastByteRead < NextExpectedByte < LastByteRecvd

tcp-sliding-window-2

Figure: Important bytes to know, from the sender side and the receiver side, in a TCP connection

  • the number of segments a sender can send is limited by two factors:
    • SWS
    • MSS
  • The window size is variable in time
  • The Sequence Number Space is the group of all the possible sequence numbers of bytes sent to a receiver. The sequence number space must be at least equal to SWS+RWS sequence numbers
  • if RWS == 1, then we talk about “Go-back-N” protocol. To illustrate the process of the Go back N protocol, let’s assume:
    • SWS = 3
    • Host A sends segments 1, 2, 3
    • segment 2 is lost
    • Since RWS ==1, then only one segment is allowed in the RecvBuffer, and host B can not buffer segment 3. So host B sends ACK2, ACK2, ACK2. At that time, host A times out segment 2 and resends it. Once received, host B sends ACK3 and host A resends segment 3.

 

References

TCP Congestion Control, Vern Paxon.

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