" Firewall " & " Ethernet Frame "

It uses Sliding Window Mechanism... TCP Stuff...

A sliding window protocol is a feature of packet-based data transmission protocols. Sliding window protocols are used where reliable in-order delivery of packets is required, such as in the Data Link Layer (OSI model) as well as in the Transmission Control Protocol (TCP).
Conceptually, each portion of the transmission (packets in most data link layers, but bytes in TCP) is assigned a unique consecutive sequence number, and the receiver uses the numbers to place received packets in the correct order, discarding duplicate packets and identifying missing ones. The problem with this is that there is no limit of the size of the sequence numbers that can be required.
By placing limits on the number of packets that can be transmitted or received at any given time, a sliding window protocol allows an unlimited number of packets to be communicated using fixed-size sequence numbers.

Protocol operation

The transmitter and receiver each have a current sequence number nt and nr, respectively. They each also have a window size wt and wr. The window sizes may vary, but in simpler implementations they are fixed. The window size must be greater than zero for any progress to be made.
As typically implemented, nt is the next packet to be transmitted, i.e. the sequence number of the first packet not yet transmitted. Likewise, nr is the first packet not yet received. Both numbers are monotonically increasing with time; they only ever increase.
The receiver may also keep track of the highest sequence number not yet received; the variable ns is one more than the sequence number of the highest sequence number received. For simple receivers that only accept packets in order (wr=1), this is the same as nr, but can be greater if wr>1. Note the distinction: all packets below nr have been received, no packets above ns have been received, and between nr and ns, some packets have been received.
When the receiver receives a packet, it updates its variables appropriately and transmits an acknowledgment with the new nr. The transmitter keeps track of the highest acknowledgment it has received na. The transmitter knows that all packets up to, but not including na have been received, but is uncertain about packets between na and ns; i.e. na ≤ nr ≤ ns.
The sequence numbers always obey the rule that na ≤ nr ≤ ns ≤ nt ≤ na + wt. That is:
na ≤ nr: The highest acknowledgement received by the transmitter cannot be higher than the highest nr acknowledged by the receiver.
nr ≤ ns: The span of fully-received packets cannot extend beyond the end of the partially-received packets.
ns ≤ nt: The highest packet received cannot be higher than the highest packet sent.
nt ≤ na + wt: The highest packet sent is limited by the highest acknowledgement received and the transmit window size.

Transmitter operation
Whenever the transmitter has data to send, it may transmit up to wt packets ahead of the latest acknowledgment na. That is, it may transmit packet number nt as long as nt < na+wt.
In the absence of a communication error, the transmitter soon receives an acknowledgment for all the packets it has sent, leaving na equal to nt. If this does not happen after a reasonable delay, the transmitter must retransmit the packets between na and nt.
Techniques for defining "reasonable delay" can be extremely elaborate, but they only affect efficiency; the basic reliability of the sliding window protocol does not depend on the details.

Receiver operation
Every time a packet numbered x is received, the receiver checks to see if it falls in the receive window, nr ≤ x < ns+wr. (The simplest receivers only have to keep track of one value nr=ns.) If it falls within the window, the receiver accepts it. If it is numbered nr, the receive sequence number is increased by 1, and possibly more if further consecutive packets were previously received and stored. If x > nr, the packet is stored until all preceding packets have been received. If x≥ns, the latter is updated to ns=x+1.
If the packet's number is not within the receive window, the receiver discards it and does not modify nr or ns.
Whether the packet was accepted or not, the receiver transmits an acknowledgment containing the current nr. (The acknowledgment may also include information about additional packets received between nr or ns, but that only helps efficiency.)
Note that there is no point having the receive window wr larger than the transmit window wt, because there is no need to worry about receiving a packet that will never be transmitted; the useful range is 1 ≤ wr ≤ wt.



<a href="https://en.wikipedia.org/wiki/Sliding_window_protocol" target="_blank" rel="nofollow noopener noreferrer">Sliding Window Protocol</a>