Post on 01-Jan-2016
3: Transport Layer 3a-1
8: Principles of Reliable Data Transfer
Last Modified: 04/20/23 12:21 AM
Slides adapted from:J.F Kurose and K.W. Ross, 1996-2010
3: Transport Layer 3a-2
Roadmap
UDP is a very thin layer over IP multiplexing /demultiplexing error detection
TCP does these things also and then adds some other significant features
TCP is quite a bit more complicated and subtle
We are not going to jump right into TCP Start gently thinking about principles of
reliable message transfer in general
3: Transport Layer 3a-3
The Problem
Problem: send big message (broken into pieces) over unreliable channel such that it arrives on other side in its entirety and in the right order
No out of band communication! All communication sent along with the pieces of the message
Receiver allowed to send information back but only over the same unreliable channel!
3: Transport Layer 3a-4
Intuition: Faxing a document With Flaky Machine Can’t talk to person on the other side any other way Number the pages – so sender can put back together Let receiver send you a fax back saying what pages
they have and what they still need (include your fax number on the document!)
What if the receiver sends their responses with a flaky fax machine too?
What if it is a really big document? No point in overwhelming the receiver. Receiver might like to be able to tell you send first 10 pages then 10 more…
How does receiver know when they have it all? Special last page? Cover sheet that said how many to expect?
3: Transport Layer 3a-5
Principles of Reliable data transfer Solving this problem is one on top-10 list of
most important networking topics! important in application, transport, link layers
Characteristics of unreliable channel will determine complexity of reliable data transfer protocol– what is worst underlying channel can do? Drop packets/pages? Corrupt packet/pages (even special ones like
the cover sheet or the receiver’s answer?) Reorder packets/pages?
3: Transport Layer 3a-6
Reliable data transfer: getting started
sendside
receiveside
rdt_send(): called from above, (e.g., by app.). Passed data to deliver to receiver upper layer
udt_send(): called by rdt,to transfer packet over unreliable channel to
receiver
rdt_rcv(): called when packet arrives on rcv-side of channel
deliver_data(): called by rdt to deliver data to
upper
3: Transport Layer 3a-7
Reliable data transfer: getting startedWe’ll: incrementally develop sender, receiver
sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer
but control info will flow on both directions!
use finite state machines (FSM) to specify sender, receiver
state1
state2
event causing state transitionactions taken on state transition
state: when in this “state” next state
uniquely determined by
next event
eventactions
3: Transport Layer 3a-8
Rdt1.0: reliable transfer over a reliable channel
underlying channel perfectly reliable (so this should be easy ) no bit errors no loss of packets
separate FSMs for sender, receiver: sender sends data into underlying channel receiver read data from underlying channel
Transport Layer 3-9
Rdt2.0: channel with bit errors
underlying channel may flip bits in packet checksum to detect bit errors
the question: how to recover from 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
new mechanisms in rdt2.0 (beyond rdt1.0): error detection receiver feedback: control msgs (ACK,NAK) rcvr-
>sender
How do humans recover from “errors”during conversation?
3: Transport Layer 3a-10
Rdt2.0: channel with bit errors underlying channel may flip bits in packet (can’t drop or
reorder packets) recall: UDP checksum to detect bit errors
Once can have problems, the receiver must give the sender feedback (either that or the sender would just have to keep sending copy after copy forever to be sure)
After receiving a packet, the receiver could say one of two things: 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 human scenarios using ACKs, NAKs?
3: Transport Layer 3a-11
Rdt2.0: channel with bit errors
new mechanisms in rdt2.0 (beyond rdt1.0): receiver feedback: control msgs (ACK,NAK) rcvr-
>sender (let receiver fax you back info?) Possible retransmission – detection of duplicates
(number fax pages?) error detection (checksums? Cover sheet summary?)
3: Transport Layer 3a-15
rdt2.0 has a fatal flaw!
What happens if ACK/NAK corrupted? sender doesn’t know what happened at receiver! FSM implied could tell if it was and ACK or a NACK What if is a FLACK?
What to do? Assume it was an ACK and transmit next? What if it was a NACK?
Missing data Assume it was a NACK and retransmit; What if it was an ACK?
Duplicate data
Handling duplicates: To detect duplicate, sender adds sequence number to each pkt sender retransmits current pkt if ACK/NAK garbled If receiver has pkt with that number already it will discards (I.e.
not deliver up duplicate pkt)
3: Transport Layer 3a-17
rdt2.1: receiver, handles garbled ACK/NAKs
If not corrupt, alwayssend ACK, but only
Deliver_data first time
3: Transport Layer 3a-18
rdt2.1: discussionSender: seq # added to pkt two seq. #’s (0,1)
will suffice. Why? must check if
received ACK/NAK corrupted
twice as many states state must
“remember” whether “current” pkt has 0 or 1 seq. #
Receiver: must check if received
packet is duplicate state indicates whether 0 or
1 is expected pkt seq # Note: This protocol can also
handle if the channel can duplicate packets
note: when can sender and receiver safely exit? receiver can not know if its last ACK/NAK received OK at sender Missing connection
termination procedure
3: Transport Layer 3a-19
rdt2.2: a NAK-free protocol Less intuitive but getting us closer to TCP same functionality as rdt2.1, using NAKs only instead of NAK, receiver sends ACK for last pkt received
OK (or for other number on the first receive) receiver must explicitly include seq # of pkt being ACKed
duplicate (or unexpected) ACK at sender results in same action as NAK: retransmit current pkt
TCP really ACKS the next thing it wants
3: Transport Layer 3a-21
rdt3.0: channels with errors (and duplicates) and loss
New assumption: underlying channel can also lose packets (data or ACKs) How to deal with loss?
Retransmission plus seq # to detect duplicates
but not enough
Q: how to detect loss?
Approach: sender waits “reasonable” amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost): retransmission will be
duplicate, but use of seq. #’s already handles this
receiver must specify seq # of pkt being ACKed
requires countdown timer
3: Transport Layer 3a-25
Stop and Wait
Rdt3.0 also called Stop and Wait Sender sends one packet, then waits for
receiver response What is wrong with stop and wait?
Slow!! Must wait full round trip time between each send
Obvious Fix? Instead send lots, then stop and wait Call this a pipelined protocol because many
packets in the pipeline at the same time