Question

Consider two network entities, \(\mathrm{A}\) and \(\mathrm{B}\), which are connected by a perfect bidirectional channel (i.e., any message sent will be received correctly; the channel will not corrupt, lose, or re-order packets). A and B are to deliver data messages to each other in an alternating manner: First, A must deliver a message to \(\mathrm{B}\), then \(\mathrm{B}\) must deliver a message to \(\mathrm{A}\), then \(\mathrm{A}\) must deliver a message to \(\mathrm{B}\) and so on. If an entity is in a state where it should not attempt to deliver a message to the other side, and there is an event like rdt_send (data) call from above that attempts to pass data down for transmission to the other side, this call from above can simply be ignored with a call to rdt_unable_to_send (data), which informs the higher layer that it is currently not able to send data. [Note: This simplifying assumption is made so you don't have to worry about buffering data.] Draw a FSM specification for this protocol (one FSM for A, and one FSM for B!). Note that you do not have to worry about a reliability mechanism here; the main point of this question is to create a FSM specification that reflects the synchronized behavior of the two entities. You should use the following events and actions that have the same meaning as protocol rdt \(1.0\) in Figure 3.9: rdt_send(data), packet = make_pkt(data), udt_send (packet), rdt_rcv (packet), extract (packet, data), deliver_data (data). Make sure your protocol reflects the strict alternation of sending between \(\mathrm{A}\) and \(\mathrm{B}\). Also, make sure to indicate the initial states for A and B in your FSM descriptions.

Short answer

Entities A and B have alternating states ensuring strict sending and receiving turns.

Step-by-step solution

Understanding the Problem

We are asked to draw a Finite State Machine (FSM) for two entities, A and B, that communicate over a reliable channel in turns. Each has to wait for the other to send a message before it can send again.

Defining States and Initial State

For Entity A: - Initial state: A is ready to send a message to B (State: A_READY_SEND). - Next state: A is waiting to receive a message from B (State: A_WAIT_RECEIVE). For Entity B: - Initial state: B is waiting to receive a message from A (State: B_WAIT_RECEIVE). - Next state: B is ready to send a message to A (State: B_READY_SEND).

Event and Actions for A

- In A_READY_SEND: Upon rdt_send(data), A makes a packet (make_pkt(data)), sends it using udt_send(packet), and transitions to A_WAIT_RECEIVE. - In A_WAIT_RECEIVE: Upon rdt_rcv(packet), A extracts data (extract(packet, data)), delivers data (deliver_data(data)), and transitions back to A_READY_SEND.

Event and Actions for B

- In B_WAIT_RECEIVE: Upon rdt_rcv(packet), B extracts data (extract(packet, data)), delivers data (deliver_data(data)), and transitions to B_READY_SEND. - In B_READY_SEND: Upon rdt_send(data), B makes a packet (make_pkt(data)), sends it using udt_send(packet), and transitions back to B_WAIT_RECEIVE.

Synchronization and Alternation

The FSMs appear synchronized by construction, as A can only send after receiving from B, and vice-versa. Each FSM transitions ensure strict alternation.

Key concepts

Networking Protocols

Networking protocols are fundamental rules and conventions that govern communication between devices in a computer network. Imagine trying to have a conversation in a room where everyone speaks different languages. Networking protocols are like the universal translator that makes sure everyone understands each other.

They determine the format of data packets, how devices identify each other, and how connections are initiated and terminated. There are many types of networking protocols, each serving different purposes:

• Communication Protocols like TCP/IP, which dictate how data is packaged and transmitted across the network.
• Security Protocols such as HTTPS, which ensure data is securely transferred.
• Management Protocols like SNMP, that monitor and control network devices.
• Application Protocols such as HTTP and FTP, which facilitate specific tasks like browsing the web or transferring files.

These protocols ensure that devices, no matter where they are in the world, can communicate effectively without losing data integrity.

Alternating Bit Protocol

The Alternating Bit Protocol (ABP) is a simple and classic mechanism used in data communication to ensure all transmitted data packets are correctly received, even in unreliable networks. It's primarily used for reliable data transfer over systems with a simple error-checking method.

The ABP operates with the help of a single bit flag, alternately switching between 0 and 1, attached to each packet. This bit effectively tracks which packet is expected next by the receiver:

• Sender Side: Sends a packet with a bit (0 or 1). It waits for an acknowledgment (ACK) from the receiver confirming that the packet was received correctly.
• Receiver Side: When receiving a packet, it checks the bit. If it matches the next expected bit, it processes the packet and sends an ACK with the bit.



• If the packet is lost or the ACK does not arrive at the sender, the sender will retransmit the packet.

ABP ensures that even on channels where packets might be lost or duplicated, the system remains synchronized and each packet is delivered only once.

Reliable Data Transfer

Reliable data transfer is crucial in network communications to ensure that information sent from one end reaches the other intact and uncorrupted. Reliable data transfer mechanisms guard against packet loss, duplication, and reordering.

Fundamentally, a reliable data transfer system must ensure:

• Correctness: All packets arrive at the destination as intended and in the correct order.
• Robustness: The system should handle errors like packet loss or corruption and retransmit lost packets efficiently.
• Feedback: Acknowledgments (ACKs) are used to confirm the successful delivery of packets, providing feedback to the sender about the delivery status.

Protocols like TCP use complex techniques involving sequence numbers and acknowledgments to manage reliable data transfer and to differentiate from simple protocols like the Alternating Bit Protocol that is mostly used in theoretical and academic contexts. By ensuring all data arrives correctly, reliable data transfer allows applications such as video streams, file transfers, and internet connectivity to function smoothly and effectively.