Computer Networks - Adoptive Flow control in TCP

1. Adaptive Flow Control

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