TCP requirements

• Two key requirements of the protocol:o Reliability: Ensuring that data that is sent

actually arrives at its destination, and if not, detecting this and re-sending the data.

o Data Flow Control: Managing the rate at which data is sent so that it does not overwhelm the device that is receiving it.

PAR

• Reliability in communications follow a rule – a device to send back an acknowledgment each

time it successfully receives a transmission

• If a transmission is not acknowledged after a period of time, it is retransmitted by its sender

• This system is called positive acknowledgment with retransmission (PAR)

• One drawback: the transmitter cannot send next message until the previous is acknowledged.

PAR

TCP sliding window • A variation on the enhanced PAR system • To support TCP’s stream orientation• Each device keeps track of the status of the

byte stream• Dividing Data into four conceptual categories:

– Bytes sent and acknowledged– Bytes sent but not yet acknowledged– Bytes not yet sent but that can be sent

immediately– Bytes not yet sent that cannot be sent until the

recipient signals that it is ready for them.

Data Stream categories

Send, usable windows

• The send window is the key to the entire TCP sliding window system: – it represents the maximum number of

unacknowledged bytes a device is allowed to have outstanding at once.

• The usable window is the amount of the send window that the sender is still allowed to send at any point in time; – it is equal to the size of the send window less the

number of unacknowledged bytes already transmitted.

Send and usable window

Send and usable window

Implementing sliding window

• Three essential fields in the TCP segment– The Sequence Number field indicates the number

of the first byte of data being transmitted. – The Acknowledgment Number is used to

acknowledge data received by the device sending this segment.

– The Window field tells the recipient of the segment the size to which it should set its send window

Window slide

• When a device gets an acknowledgment for a range of bytes, it knows they have been successfully received by their destination.

• It moves them from the “sent but unacknowledged” to the “sent and acknowledged” category.

• This causes the send window to slide to the right, allowing the device to send more data.

TCP window size management

• The receiver on receipt of segment must– Send Acknowledgement– Transfer data from buffer to application

• Receiver delay in transfer can happen

• Danger of buffer overflow

• Varying window size to manage data flow

Flow control

• The TCP sliding window system is used not just for ensuring reliability through acknowledgments and retransmission

• it is also the basis for TCP’s flow control mechanism. • By increasing or reducing the size of its receive window

– a device can raise or lower the rate at which its connection partner sends it data.

– In the case where a device becomes extremely busy, it can even reduce the receive window to zero, closing it

– this will halt any further transmissions of data until the window is reopened

Shrinking window

Silly window syndrome

• Sliding window mechanism does not ensure a min size of segment

• Shrinking window can result in inefficient transmission of small size segment

Silly window syndrome

SWS avoidance algorithm

• Receiver SWS avoidance– Restrict moving right edge of window by too

small amount– Reduce window size to 0– Right edge be moved by half buffer size or

MSS whichever is less

Sender SWS avoidance algorithm

• Nagle’s algorithm – John Nagle– Data can be immediately sent as long as all sent

data is acknowledged– When there is unacknowledged data

• Do not send till all data acknowledged

• Send after accumulating data for full segment

TCP ACK & Retransmission• TCP acknowledgments are cumulative• Tell a transmitter that all the bytes up to the

sequence number indicated in the acknowledgment were received successfully.

• If bytes are received out of order, they cannot be acknowledged until all the preceding bytes are received.

• TCP includes a method for timing transmissions and retransmitting lost segments if necessary.

Managing Retransmissions • Each time a segment is sent, a copy is Placed On

Retransmission Queue• Timer Starts at a predetermined value• Counts down over time• If an acknowledgment is received for a segment before its

timer expires, the segment is removed from the retransmission queue

• If the timer expires before an acknowledgment is received, the segment is retransmitted

• No guarantee that a retransmitted segment will be received• If not, Retransmission timer is reset, the segment will be

retransmitted again and the process repeated

Policies For Dealing with Retransmission

• Retransmit Only Timed-Out Segments

• Retransmit All Outstanding Segments

• TCP selective acknowledgment

Retransmission Time

• Length of time for retransmission timer is very important

• If it is set too low– A segment actually received might be retransmitted

– didn't wait long enough for the acknowledgment

• if it is set too long– waste time waiting for an acknowledgment that will

never arrive

– reducing overall performance

Choosing Retransmission time

• Ideally, the retransmission timer should be of value just slightly larger than the round-trip time (RTT)

• How to determine RTT?– Differences in TCP Connection Distances. – Transient Delays and Variability: The amount of time it

takes to send data between any two devices will vary over time due to various happenings on the internetwork: fluctuations in traffic, router loads and so on.

Adaptive Retransmission Based RTT

• TCP uses a dynamic, or adaptive retransmission scheme

• Average RTT value for the connection• Asmoothing formula:

– New RTT = (a * Old RTT) + ( (1-a) * Newest RTT Measurement); 0 < a < 1

– a ~ 1 -> better smoothing, slow reaction– a ~ 0 -> fast reaction

RTT Calculation by Karn's Algorithm

• Karn's algorithm-Inventor, Phil Karn – Does not use measured round-trip times– Eliminates problem of acknowledgment ambiguity

• Start by setting the timer, based on the current average round-trip time

• On retransmission, the timer is not reset to the same value but is “backed off” (increased) using a multiplier (typically 2) to give the retransmission more time to be received

• The timer continues to be increased until a retransmission is successful, up to a certain maximum value

RTT Calculation by Karn's Algorithm

• The round-trip timer is kept at the longer (backed-off) value until a valid round-trip time can be measured on a segment that is sent and acknowledged without retransmission

• This permits a device to respond with longer timers temporarily, while eventually having the round-trip time settle back to a long-term average when normal conditions resume