Here you will get sliding window protocol program in C. In computer networks sliding window protocol is a method to transmit data on a network. Sliding window protocol is applied on the Data Link Layer of OSI model. At data link layer data is in the form of frames. In Networking, Window simply means. I know im supposed to use packet acknowledgements, so Im not sure if im supposed to use go-back-n arq or simply modify code to acknowledge, and use go-back-n differently. Could you please provide me an example of what you would change in the codes to make use of go-back-n. Go Back N protocol in c On-campus and online computer science courses to Learn the basic concepts of Computer Science.This tutorial will cover c,c++, java, data structure and algorithm,computer graphics,microprocessor,analysis of algorithms,Digital Logic Design and Analysis,computer architecture,computer networks,operating system.
Prerequisite : Sliding Window Protocol – Set 1 (Sender Side), Set 2 (Receiver Side)
Why Selective Repeat Protocol?
The go-back-n protocol works well if errors are less, but if the line is poor it wastes a lot of bandwidth on retransmitted frames. An alternative strategy, the selective repeat protocol, is to allow the receiver to accept and buffer the frames following a damaged or lost one.
Selective Repeat attempts to retransmit only those packets that are actually lost (due to errors) :
- Receiver must be able to accept packets out of order.
- Since receiver must release packets to higher layer in order, the receiver must be able to buffer some packets.
Retransmission requests :
- Implicit – The receiver acknowledges every good packet, packets that are not ACKed before a time-out are assumed lost or in error.Notice that this approach must be used to be sure that every packet is eventually received.
- Explicit – An explicit NAK (selective reject) can request retransmission of just one packet. This approach can expedite the retransmission but is not strictly needed.
- One or both approaches are used in practice.
Selective Repeat Protocol (SRP) :
This protocol(SRP) is mostly identical to GBN protocol, except that buffers are used and the receiver, and the sender, each maintain a window of size. SRP works better when the link is very unreliable. Because in this case, retransmission tends to happen more frequently, selectively retransmitting frames is more efficient than retransmitting all of them. SRP also requires full duplex link. backward acknowledgements are also in progress.
- Sender’s Windows ( Ws) = Receiver’s Windows ( Wr).
- Window size should be less than or equal to half the sequence number in SR protocol. This is to avoid packets being recognized incorrectly. If the windows size is greater than half the sequence number space, then if an ACK is lost, the sender may send new packets that the receiver believes are retransmissions.
- Sender can transmit new packets as long as their number is with W of all unACKed packets.
- Sender retransmit un-ACKed packets after a timeout – Or upon a NAK if NAK is employed.
- Receiver ACKs all correct packets.
- Receiver stores correct packets until they can be delivered in order to the higher layer.
- In Selective Repeat ARQ, the size of the sender and receiver window must be at most one-half of 2^m.
Efficiency of Selective Repeat Protocol (SRP) is same as GO-Back-N’s efficiency :
References –
Slideshare
Youtube
MIT article
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Characteristics
- Used in Connection-oriented communication.
- It offers error and flow control
- It is used in Data Link and Transport Layers
- Stop and Wait ARQ mainly implements Sliding Window Protocol concept with Window Size 1
Useful Terms:
- Propagation Delay: Amount of time taken by a packet to make a physical journey from one router to another router.
Propagation Delay = (Distance between routers) / (Velocity of propagation)
- RoundTripTime (RTT) = 2* Propagation Delay
- TimeOut (TO) = 2* RTT
- Time To Live (TTL) = 2* TimeOut. (Maximum TTL is 180 seconds)
Simple Stop and Wait
Sender:
Rule 1) Send one data packet at a time.
Rule 2) Send next packet only after receiving acknowledgement for previous.
Receiver:
Rule 1) Send acknowledgement after receiving and consuming of data packet.
Rule 2) After consuming packet acknowledgement need to be sent (Flow Control)
Problems :
1. Lost Data
2. Lost Acknowledgement:
3. Delayed Acknowledgement/Data: After timeout on sender side, a long delayed acknowledgement might be wrongly considered as acknowledgement of some other recent packet.
Stop and Wait ARQ (Automatic Repeat Request)
Above 3 problems are resolved by Stop and Wait ARQ (Automatic Repeat Request) that does both error control and flow control.
1. Time Out:
2. Sequence Number (Data)
3. Delayed Acknowledgement:
This is resolved by introducing sequence number for acknowledgement also.
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Working of Stop and Wait ARQ:
1) Sender A sends a data frame or packet with sequence number 0.
2) Receiver B, after receiving data frame, sends and acknowledgement with sequence number 1 (sequence number of next expected data frame or packet)
There is only one bit sequence number that implies that both sender and receiver have buffer for one frame or packet only.
C Program For Armstrong Number
Characteristics of Stop and Wait ARQ:
- It uses link between sender and receiver as half duplex link
- Throughput = 1 Data packet/frame per RTT
- If Bandwidth*Delay product is very high, then stop and wait protocol is not so useful. The sender has to keep waiting for acknowledgements before sending the processed next packet.
- It is an example for “Closed Loop OR connection oriented “ protocols
- It is an special category of SWP where its window size is 1
- Irrespective of number of packets sender is having stop and wait protocol requires only 2 sequence numbers 0 and 1
The Stop and Wait ARQ solves main three problems, but may cause big performance issues as sender always waits for acknowledgement even if it has next packet ready to send. Consider a situation where you have a high bandwidth connection and propagation delay is also high (you are connected to some server in some other country though a high speed connection). To solve this problem, we can send more than one packet at a time with a larger sequence numbers. We will be discussing these protocols in next articles.
So Stop and Wait ARQ may work fine where propagation delay is very less for example LAN connections, but performs badly for distant connections like satellite connection.
References:
http://users.ecs.soton.ac.uk/sqc/EL336/CNL-5.pdf
This Article is contributed by G. Shabharesh. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above
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