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CS 656003 Lecture Note 16/10/2017 Middle term Time: 30/10/2017 1:00 pm 20 points

Principles of reliable data transfer What the function to output the data to the channel? Outstream.write however, write is not reliable.

Finite State Machine (FSM) We use FSM to specify sender, receiver. How many state for a telephone? 1. Of 2. Dial Tone 3. In Call 4. Busy Transfers: pick-up, hang-up, dial

rdt1.0 



underlying channel perfectly reliable 

no bit errors



no loss of packets

separate FSMs for sender, receiver: 

sender sends data into underlying channel



receiver reads data from underlying channel

Muliple packets while getting response in http

How to recover form errors? 

acknowledgements (ACKs): receiver explicitly tells sender that pkt received OK



negative acknowledgements (NAKs): receiver explicitly tells sender that pkt had errors



sender retransmits pkt on receipt of NAK

ACK: Send the next packet NAK: Resend the packet

rdt2.0 fatal flaw: What happens if ACK/NAK corrupted?

rdt2.1 Wait for call 0 from above Wait for ACK or NAK 0 Wait for call 1 from from above Wait for ACK or NAK 1

pattern: 1010 Why is rdt2.1 safe? It stop and wait.

How do receiver handles duplicates? corrupted ACK will trigger resending So, send ACK for the last packet again.

rdt2.2 is simplified, uses ACKs only. explicitly include seq # of pkt being ACKed

What to do when one receives a corrupted packet? send ACK for the previous packet

rdt3.0--channels with errors and loss The key is timer. What to do when ACKs get lost? resend the packet When do the receiver use a timer? Want to enhance throughput

rdt3.0 performance Send out multiple packets at the same time. pipelining: sender allows multiple, “in-flight”, yet-to-be-acknowledged pkts

Important IPs NJIT BANG 10.200.5.17 smtp.njit.edu 128.235.5.18 NJIT wireless 10.200.*.* NJIT wired 128.235.*.*

+The IP address of the same domain should have the same beginning. POP port 110 IMAP port 143 SMTP port 25

AFSCONNECT has 2 IP addresses 128.2.35.* 10.80.0.11 Why? virtual interfaces

FTP TCP control connection port 21 FTP TCP data connection port 20

+465 is the secured SMTP port, it needs SSL handshake.

https://kb.ucla.edu/articles/why-cant-i-send-e-mails-to-uclaedu-addresses above link for ucla mail server security facts....