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- In Figure 2-4B, the 2nd segment is lost. In order to recover the lost segment, the web client
replies a segment with acknowledge number equals to 100, which means it expecting byte
number 100 from the web server. The server then resends the data to the client (retransmission).
Since the client has already received bytes 200-299 without error, it is not necessary to request
again. Data is then reassembled back in order at the client end and passed to the application
layer. Finally, the client continues to request data from the web server by sending an ACK = 300.
- Positive Acknowledgment and Retransmission (PAR) uses a timer that is set to the
retransmission timeout interval and is being activated every time a sender sends a segment and
waiting for the ACK reply. The sender will resend all segments once the timer expired. This
provides a reliability mechanism that intends to overcome the following 2 problem scenarios:
i) The transmitted segment is lost or dropped.
ii) The ACK segment is failed to arrive at the sender.
- TCP segments may arrive out of order because routers can send data across different links to a
destination host. Hence the TCP stack running at the receiving end must reorder the out of order
segments before passing the data to the application layer.
- TCP Flow Control or Congestion Control provides a mechanism for the receiver to control the
sending rate of the sender with a windowing mechanism. It is achieved via SEQ, ACK and
Window fields in the TCP header. The receiver defines the Window size to tell the sender how
many bytes are allowed to send without waiting for an acknowledgement. It represents the
receiver’s available buffer. Buffer is used to temporarily store the received bytes before the
receiving application is free to process the received bytes. The sender will not send when the
receiver’s window is full. Increased Window size may result in increased throughput.
- The window size normally starts with small value and keeps on increment until an error occurs.
The window size is negotiated dynamically throughout a TCP session and it may slide up and
down, hence it is often being referred to as sliding window.
- Multiplexing allows multiple connections to be established between processes in 2 end systems.
Multiplexing is a feature that allows the transport layer at the receiving end to differentiate
between the various connections and decide the appropriate application layer applications to
hand over the received and reassembled data (similar to the concept of forming virtual circuits).
The source and destination port number fields in the TCP and UDP headers and a concept
called socket are being used for this purpose.
- Below lists some popular applications and their associated well-known port numbers:
Application Protocol Port Number
HTTP TCP 80
FTP TCP 20 (data) and 21 (control)
Telnet TCP 23
TFTP UDP 69
DNS TCP, UDP 53
DHCP UDP 67, 68
SMTP TCP 25
POP3 TCP 110
SNMP UDP 161
- Port numbers 0 – 1023 are well-known ports, port numbers 1024 – 49151 are registered ports,
and port numbers 49152 – 65535 are private vendor assigned and dynamic ports.
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