1. Transport Layer Outline Intro to transport UDP
Reliability (stop and wait, sliding window)
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2. To Boldly Go Where We Have Yet to Go
Application
Transport
Network
Link
Physical 5 4 3 2 1
Recall Internet Architecture
Layers used to define functionality
Our focus up to now has been layer 5
Applications demand reliable transport
Application may demand predictable delays
We are now going up to layer 4
This layer is tricky!
Goal at the end of the next few weeks is an understanding of the Reno version of TCP
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3. End-to-End Protocols Underlying network is best-effort
drop messages
re-orders messages
delivers duplicate copies of a given message
limits messages to some finite size
delivers messages after an arbitrarily long delay
Common end-to-end services
guarantee message delivery
deliver messages in the same order they are sent
deliver at most one copy of each message
support arbitrarily large messages
support synchronization
allow the receiver to flow control the sender
support multiple application processes on each host
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4. Basic function of transport layer
How can processes on different systems get the right messages?
Ports are numeric locators which enable messages to be demultiplexed to proper process.
Ports are addresses on individual hosts, not across the Internet.
Ports are established using well-know values first
Port 80 = http, port 53 = DNS
Ports are typically implemented as message queues
Simplest function of the transport layer: multiplexing/demultiplexing of messages
Enables processes on different systems to communicate
End-to-end since only processes on end hosts invoke this protocol
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5. Other transport layer functions
Connection control
Setting up and tearing down communication between processes
Error detection within packets – our first focus
Checksums
Reliable, in order delivery of packets – our second focus
Acknowledgement schemes
Flow control
Matching sending and receiving rates between end hosts
Congestion control
Managing congestion in the network
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6. User Datagram Protocol (UDP)
Unreliable and unordered datagram service
Adds multiplexing/demultiplexing
Adds reliability through optional checksum
No flow or congestion control
Endpoints identified by ports
servers have well-known ports
see /etc/services on Unix
Header format
Optional checksum
Computed over psuedo header + UDP header + data
SrcPort
DstPort
Checksum
Length
Data 16 31
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7. UDP Checksums Optional in current Internet
Psuedoheader consists of 3 fields from IP header: protocol number (TCP or UDP), IP src, IP dst and UDP length field
Psuedoheader enables verification that message was delivered between correct source and destination.
IP dest address was changed during delivery, checksum would reflect this
UDP uses the same checksum algorithm as IP
Internet checksum
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8. Basics of dealing with errors
Bit errors can be introduced in packets
This problem has been studied for a long time
Error detection (and correction) codes
Cyclic redundancy check (CRC) is a common error detection method
Basic idea of any scheme is to add redundant data
Extreme example – send two identical copies of data
Poor for many reasons
A primary goal is to send minimal amount of redundant data
CRC used in Ethernet has 32 bits for each 1500 byte packet
Another goal is to make generation of checksum fast
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9. Checksum basics contd.
Simple parity is the most basic method for error detection
Odd/even parity
Internet Checksum
Basic idea: sender adds up all words and transmit the sum
Add using 16 bit one’s complement arithmetic then take one’s complement of the result to get checksum
Receiver adds up all words and compares with checksum
It’s very simple and efficient to code this
Reason that this is used instead of CRC
Not really great detecting errors
CRC is much stronger
Forward error correction is another possibility
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10. UDP in practice Minimal specification makes UDP very flexible Examples
Any kind of end-to-end protocol can be implemented
See programming assignment #1
TCP can be implemented using UDP
Examples
Most commonly used in multimedia applications
These are frequently more robust to loss
RPC’s
Many others…
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11. Reliability We’re heading toward TCP
Baby steps first… lets start with looking at minimal support for reliability
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12. Methods of Reliability
Packets can be lost and/or corrupted during transmission
Bit level errors due to noise
Loss due to congestion
Use checksums to detect bit level errors
Internet Checksum is optionally used to detect errors in UDP
Uses 16 bits to encode one’s complement sum of data + headers
When bit level errors are detected, packets are dropped
Build reliability into the transmission protocol
Using acknowledgements and timeouts to signal lost or corrupt frame
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13. Acknowledgements & Timeouts
An acknowledgement (ACK) is a packet sent by one host in response to a packet it has received
Making a packet an ACK is simply a matter of changing a field in the transport header
Data can be piggybacked in ACKs
A timeout is a signal that an ACK to a packet that was sent has not yet been received within a specified timeframe
A timeout triggers a retransmission of the original packet from the sender
How are timers set?
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14. Acknowledgements & Timeouts
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15. Propagation Delay
Propagation delay is defined as the delay between transmission and receipt of packets between hosts
Propagation delay can be used to estimate timeout period
How can propagation delay be measured?
What else must be considered in the measurement?
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16. Exponentially weighted moving average RTT estimation
EWMA was original algorithm for TCP
Measure SampleRTT for each packet/ACK pair
Compute weighted average of RTT
EstRTT = a x EstimatedRTT + b x SampleRTT
where a + b = 1
a between 0.8 and 0.9
b between 0.1 and 0.2
Set timeout based on EstRTT
TimeOut = 2 x EstRTT
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17. Stop-and-Wait Process
Sender
Receiver
Sender doesn’t send next packet until he’s sure receiver has last packet
The packet/Ack sequence enables reliability
Sequence numbers help avoid problem of duplicate packets
Problem: keeping the pipe full
Example
1.5Mbps link x 45ms RTT = 67.5Kb (8KB)
1KB frames imples 1/8th link utilization
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18. Solution: Pipelining via Sliding Window
Allow multiple outstanding (un-ACKed) frames
Upper bound on un-ACKed frames, called window
Sender
Receiver T
ime …
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19. Buffering on Sender and Receiver
Sender needs to buffer data so that if data is lost, it can be resent
Receiver needs to buffer data so that if data is received out of order, it can be held until all packets are received
Flow control
How can we prevent sender overflowing receiver’s buffer?
Receiver tells sender its buffer size during connection setup
How can we insure reliability in pipelined transmissions?
Go-Back-N
Send all N unACKed packets when a loss is signaled
Inefficient
Selective repeat
Only send specifically unACKed packets
A bit trickier to implement
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20. Sliding Window: Sender
Assign sequence number to each frame (SeqNum)
Maintain three state variables:
send window size (SWS)
last acknowledgment received (LAR)
last frame sent (LFS)
Maintain invariant: LFS - LAR <= SWS
Advance LAR when ACK arrives
Buffer up to SWS frames
SWS
LAR
LFS …
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21. Sliding Window: Receiver
Maintain three state variables
receive window size (RWS)
largest frame acceptable (LFA)
last frame received (LFR)
Maintain invariant: LFA - LFR <= RWS
Frame SeqNum arrives:
if LFR < SeqNum < = LFA accept
if SeqNum < = LFR or SeqNum > LFA discarded
Send cumulative ACKs – send ACK for largest frame such that all frames less than this have been received
RWS
LFR
LFA …
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22. Sequence Number Space
SeqNum field is finite; sequence numbers wrap around
Sequence number space must be larger then number of outstanding frames
SWS <= MaxSeqNum-1 is not sufficient
suppose 3-bit SeqNum field (0..7)
SWS=RWS=7
sender transmit frames 0..6
arrive successfully, but ACKs lost
sender retransmits 0..6
receiver expecting 7, 0..5, but receives the original incarnation of 0..5
SWS < (MaxSeqNum+1)/2 is correct rule
Intuitively, SeqNum “slides” between two halves of sequence number space
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23. Stop & wait sequence numbers
Sender
Receiver
Sender
Receiver
Sender
Receiver
Frame 0
Frame 0
Frame 0
imeout
imeout
ACK 0 T T
ACK 0
ACK 0
Frame 0
Frame 1
Frame 0
imeout
imeout T
ACK 0
ACK 1
ACK 0 T
Frame 0
(c)
(d)
ACK 0
(e)
Simple sequence numbers enable the client to discard duplicate copies of the same frame
Stop & wait allows one outstanding frame, requires two distinct sequence numbers
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24. Sliding Window Example
Sender
Receiver 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 A3 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1 2 3 4 5 6 7 8 9 10 11 12 13 14 3 4 5 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1 2 3 4 5 6 7 8 9 10 11 12 13 14 6 A4 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1 2 3 4 5 6 7 8 9 10 11 12 13 14
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25. Sliding Window Summary
Sliding window is best known algorithm in networking
First role is to enable reliable delivery of packets
Timeouts and acknowledgements
Second role is to enable in order delivery of packets
Receiver doesn’t pass data up to app until it has packets in order
Third role is to enable flow control
Prevents server from overflowing receiver’s buffer
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