An Investigation into Transport Protocols and Data Transport ...

4.2. Protocol Description 54
Receiver's Advertised Window
Network Window
1 2 3 4 5 6 7 8 9 10
Data Sent
and ack'd
Data Sent
and waiting for ack
Waiting for window to
move before sending data
Figure 4.3: Sliding Window of TCP.
4.2.2 Reliable Data Replication
To ensure reliable in-order data replication between machines, a TCP sender
must maintain sufficient history of what data has been received by the TCP
receiver. The implementation of an acknowledgment system in TCP offers
such a mechanism by explicitly notifying the sender of what the receiver has
received.
This history is provided by implementing a window of bytes on the sender
which are currently ‘in-situ’ or packets in flight. Each byte of data is assigned
a unique sequence number and the sequence number corresponding to the
start of the data payload of each TCP packet is embedded in the TCP header.
The left-hand side of the window, snd_una, is advanced when the sender
receives an acknowledgment from the receiver. acks are cumulative in the
sense that the acknowledgment number in the header of each ack confirms
all bytes up to the given sequence number.
As data is not always delivered to the TCP receiver in a continuous way
(the network 1 can lose, duplicate or re-order packets) any data packets that
are not sequentially received at the receiver are called out-of-order data.
1 And potentially any layer directly underneath the transport layer.