Figure 35-5: simplified TCP sender in Java

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Figure 35-5: simplified TCP sender
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Figure 35-5 shows the three major events related to data transmission/retransmission at a simplified TCP sender Let us consider a TCP connection between host A and B and focus on the data stream being sent from host A to host B At the sending host (A), TCP is passed application-layer data, which it frames into segments and then passes on to IP The passing of data from the application to TCP and the subsequent framing and transmission of a segment is the first important event that the TCP sender must handle Each time TCP releases a segment to IP, it starts a timer for that segment If
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Transmission Control Protocol
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this timer expires, an interrupt event is generated at host A TCP responds to the timeout event, the second major type of event that the TCP sender must handle, by retransmitting the segment that caused the timeout The third major event that must be handled by the TCP sender is the arrival of an acknowledgement segment (ACK) from the receiver (more specifically, a segment containing a valid ACK field value) Here, the sender's TCP must determine whether the ACK is a first-time ACK for a segment that the sender has yet to receive an acknowledgement for, or a so-called duplicate ACK that re-acknowledges a segment for which the sender has already received an earlier acknowledgement In the case of the arrival of a first-time ACK, the sender now knows that all data up to the byte being acknowledged has been received correctly at the receiver The sender can thus update its TCP state variable that tracks the sequence number of the last byte that is known to have been received correctly and in-order at the receiver To understand the sender's response to a duplicate ACK, we must look at why the receiver sends a duplicate ACK in the first place Table 35-1 summarizes the TCP receiver's ACK generation policy When a TCP receiver receives a segment with a sequence number that is larger than the next, expected, in-order sequence number, it detects a gap in the data stream - ie, a missing segment Since TCP does not use negative acknowledgements, the receiver can not send an explicit negative acknowledgement back to the sender Instead, it simply re-acknowledges (ie, generates a duplicate ACK for) the last in-order byte of data it has received If the TCP sender receives three duplicate ACKs for the same data, it takes this as an indication that the segment following the segment that has been ACKed three times has been lost In this case, TCP performs a fast retransmit [RFC 2581], retransmitting the missing segment before that segment's timer expires
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Event Arrival of in-order segment with expected sequence number All data up to up to expected sequence number already acknowledged No gaps in the received data Arrival of in-order segment with expected sequence number One other in-order segment waiting for ACK transmission No gaps in the received data Arrival of out-of-order segment with higherthan expected sequence number Gap detected Arrival of segment that partially or completely fills in gap in received data
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TCP receiver action Delayed ACK Wait up to 500 ms for arrival of another in-order segment If next in-order segment does not arrives in this interval, send an ACK
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Immediately send single cumulative ACK, ACKing both in-order segments Immediately send duplicate ACK, indicating sequence number of next expected byte Immediately send ACK, provided that segment starts at the lower end of gap
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Table 35-1: TCP ACK generation recommendations [RFC 1122, RFC 2581]
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A Few Interesting Scenarios
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We end this discussion by looking at a few simple scenarios Figure 35-6 depicts the scenario where host A sends one segment to host B Suppose that this segment has sequence number 92 and contains 8 bytes of data After sending this segment, host A waits for a segment from B with acknowledgment number 100 Although the segment from A is received at B, the acknowledgment from B to A gets lost In this case, the timer expires, and host A retransmits the same segment Of course, when host B receives the retransmission, it will observe that the bytes in the segment duplicate bytes it has already deposited in its receive buffer Thus TCP in host B will discard the bytes in the retransmitted segment
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