Flashcards
SCC 203 - Reliable Transport Protocols
Cards in this deck (35)
- 1What must be abstracted out during communication?– Feedback between sender and receiver
– Packet errors
– Packet loss - 2What other protocol is UDP known as?‘Fire and forget’ protocol
– No direct response to any datagram sent - 3What type of service does IP offer?• IP offers unreliable, best-effort service
- 4What must reliable transport protocol handle?– Reliable transp. protocol must handle possible errors
- 5What type of characteristics of unreliable channel determine complexity of a reliable protocol?– Packet loss
– Bit errors and data corruption
– Packet reordering
– Packet duplication? - 6Within acknowledgements, if a receiver gets the correct packet, what is returned?• Receiver gets correct packet
– Returns Acknowledgement (ACK) - 7Within acknowledgements, if a receiver gets the corrupt packet, what is returned?• Receiver gets corrupt packet
– Returns Negative Acknowledgement (NAK) - 8What do Automatic Repeat request have?– Error detection
• Checksums, etc.
– Receiver feedback
• Such as ACK and NAK
– Retransmission of corrupt packets
• For now we assume packets cannot be lost - 9What is a good way of describing protocol operation?State machines are a good way of describing protocol operation.
- 10Define the stop and wait protocol?• Sender waits for ACK before sending next packet
- 11What does a one bit sequence number allow both ends to spot?A one bit sequence number allows both ends to spot peer ‘misunderstanding’ last message
- 12Why do ACk and Nack not need sequence numbers?• Notice ACK and NAK don’t need seq. number
– Only a non-corrupt ACK allows sender to continue - 13Alternative to the use of both ACK and NACKs?• Alternative is to just have ACKn and no NAK
– As in, ACK0, ACK1
– ACK last packet sent, sender sends next packet
– ACK previous packet, sender resends last packet
• Thus a duplicate ACKn causes a resend - 14State a way to handle lost packets?The use of timeouts
- 15What do we need to do when we use timeouts?• Start timer when we send anything
• Stop timer when we receive expected response
• Re-start timer if timeout or unexpected response - 16What happens in the event of a timeout?• Resend last packet or ACK in event of a timeout
- 17What happens if anything unexpectedly received during a timeout?If anything unexpected received, it is ignored
– Again, the expected ACK causes us to move on - 18Adding a timeout to the stop and wait protocol is known asThis protocol is know as the Alternating Bit Protocol
– Sequence numbers alternate between 0 and 1 - 19Problem with too short and too long timeout values?Too short and we resend unnecessarily
– Duplicate packets or ACKs
– Not good, but protocol can handle this
• Too long and any problem gives long delay - 20What does the value of the timeout have to be?Value needs to be at least Round Trip Time (RTT)
– Or we’ll resend everything
– Remember: includes buffering and processing time - 21State the transmission delay formula?Transmission delay
dtrans = packet length / transmission rate - 22What does pipe lined transmission do to sequence numbers?• Increase range of sequence numbers
- 23What happens to the transportation of packets in pipeline transmission?Keep sending packets until out of sequence numbers
– ACKs free sequence numbers for re-use - 24The range of sequence number required depends on what?– Protocol error handling
– Transmission speed, faster -> more seq. numbers
– Round Trip Time, longer -> more seq. numbers - 25Can you describe the sliding window based system?– Window slides forward by one with each ACK
– Known as a Sliding Window Protocol
Each ACK frees a sequence number for re-use - 26What is the window size limited by?– Number of bits for sequence number
• n bits, seq. numbers: 0 .. 2n
-1
– Desire to limit number of un-ACK’d packets
• Maintain feedback to sender
• Congestion/ Flow control
• Error constraints - 27Describe what happens when a receiver discards out of order packets?– They’ll be resent
• Though could then be lost
– No need to handle re-ordering packets in receive buffer
e.g. Lost 2, discard 3, … - 28What alternative can we do in order to ensure we don't keep sending ACQs back to sender?• No need to send every ACK
– ACKn, ACKs all packets
up to and including n - 29How to avoid gaps during pipe-lined transmission?– Window size needs to be
big enough, and…
– ACKs often enough
• And don’t forget the timeout - 30Define the bandwidth delay produce formula?• Large bandwidth + large end-to-end delay
- 31What does the receiver do during selective repeat?– Individually ACKs all packets (in window) including resends
– Out of order packets accepted
– Slides window forward only when no missing messages - 32What does the sender do during selective repeat?– Slides window forward once all ACKs to that point received
– Need timeout for each sent packet as resent individually - 33Problem with large windows in selective repeat?• Sender may not distinguish retransmissions
– Remember end-point knows nothing of other
– Window size MUST be <= half seq. number space - 34Within out of order packets, why should sequence number space be large enough?• Sequence number space MUST be large enough to ensure packets cannot ‘live’ in network long enough to reappear
- 35What does GPRS meanThe GPRS core network is the central part of the general packet radio service (GPRS) which allows 2G, 3G and WCDMA mobile networks to transmit IP packets to external networks such as the Internet. The GPRS system is an integrated part of the GSM network switching subsystem.
Information
- Description
- Flashcards on SCC 203 - Reliable Transport Protocols, created by Ayo Olutobi on 15/04/2017.
- Created by
- Ayo Olutobi
- Views
- 4
- Rating