Adoptive flowcontrol in TCP
- 2. TCP Sliding Window TCP Sliding Window provides, reliable delivery of data, data is delivered in order, and flow control between the sender and the receiver. Reliable delivery and Ordered Delivery:
- 3. TCP Sliding Window Sending Side LastByteAcked ≤ LastByteSent - Receiver cannot acknowledge a byte that has not been sent. LastByteSent ≤ LastByteWritten - TCP cannot send a byte that the application process has not yet written. Left side of the LastByteAcked need not to be saved in the buffer. Receiving Side LastByteRead < NextByteExpected - All preceding bytes are already received. - Byte cannot be read by application until it is received. NextByteExpected ≤ LastByteRcvd + 1 - If data has arrived in order, NextByteExpected points to the byte after LastByteRcvd - If data has arrives out of order, NextByteExpected points to the start of the first gap in the data. Bytes to the left of LastByteRead need not be buffered.
- 4. TCP Sliding Window Flow Control – Both sender and receiver buffer size is finite size, denoted by MaxSendBuffer, MaxRcvBuffer. – In sliding window, window size sets the amount of data that can be sent without waiting for acknowledgement. – Here receiver give the maximum window size for the sender by advertising a window that is no larger than the data that it can buffer. Receiver side must keep, LastByteRcvd – LastByteRead ≤ MaxRcvBuffer - To avoid overflowing its buffer (Receive Buffer). AdvertisedWindow = MaxRcvBuffer – ((NextByteExpected -1) – LastByteRead) - Represents the amount of free space remaining in the buffer Sender side must keep, LastByteSent – LastByteAcked ≤ AdvertisedWindow EffectiveWindow = AdvertisedWindow – (LastByteSent – LastByteAcked)
- 5. TCP Sliding Window To avoid the overflow of the send buffer by local application process, sender must make sure, LastByteWritten – LastByteAcked ≤ MaxSendBuffer Protecting against Wraparound SequenceNum – 32 bit long AdvertisedWindow – 16 bit long Sequence number space is twice as big as window size. Relevance of the 32-bit sequence number space • The sequence number used on a given connection might wraparound • A byte with sequence number x could be sent at one time, and then at a later time a second byte with the same sequence number x could be sent • Packets cannot survive in the Internet for longer than the MSL • MSL is set to 120 sec • We need to make sure that the sequence number does not wrap around within a 120- second period of time.
- 6. TCP Sliding Window Keeping the Pipe Full • 16-bit AdvertisedWindow field must be big enough to allow the sender to keep the pipe full.
- 7. TCP Sliding Window • Clearly the receiver is free not to open the window as large as the AdvertisedWindow field allows • If the receiver has enough buffer space – The window needs to be opened far enough to allow a full – delay × bandwidth product’s worth of data – Assuming an RTT of 100 ms Required Window Size for 100ms RTT