Transport Layer: Basics
Outline
Homework #2 solutions
Intro to transport
UDP
Congestion control basics
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Homework #2 - Solutions
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8.8: Q: Why is server unsure of client’s ID until it receives 3rd message? What attack is
server exposed to without 3rd message?
– A: A bad guy could replay message exchange of prior transaction.
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8:19: Q: How can DES be used to encrypt Telnet sessions?
– A: Use padding. But, padding with eg. seven zero bytes is vulnerable to code breaking. So,
you could pad with random bytes or pad with zero’s and then do cipher block chaining.
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8:24: Q: Assume filtering routers given in the diagram. How must they be configured so
that outsiders can Telent to net 2 but not net 1?
– A: R1 blocks inbound traffic to the Telent port unless it is bound for net 2. R2 blocks all Telent
traffic from net 2 to net 1.
Outside World
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Net 1
R2
Net 2
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Homework #2 Solutions contd.
• Pings:
– Berkeley: ~56ms, Germany: ~124ms, Singapore: ~300ms
– Berkeley: 0% loss, Germany: 0% loss, Singapore: 10% loss
• Traceroute (outgoing)
• Berkeley: 13 hops (Abilene),
• Germany: 19 hops (BBN, Sprint, Ebone)
• Singapore: 17 hops (Abovenet)
• Traceroute (incoming)
• Berkeley: 14 hops (Abilene)
• Germany: 19 hops (Ebone, Spring, BBN)
• Singapore: 19 hops (BBN)
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Context – up to now…
• Basics of the Internet
– Layering and protocols
• Packet switching vs. circuit switching
– Internet vs. telephone network
– Packet switching/statistical multiplexing improves utilization
– Packets can be lost or corrupted
• Applications generate data transferred over networks
– We have talked about a few…
• Measurement and performance
– What does traffic look like?
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To Boldly Go Where We Have Yet to Go
• 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 down to layer 4
Application
Transport
Network
Link
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Physical
– This layer is tricky!
– Goal at the end of the next two weeks is an
understanding of the Reno version of TCP
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Layering and Encapsulation Revisited
• Each layer in the Internet architecture relies on layers
below to provide services in black box fashion
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Layers make complex system easier to understand and specify
Makes implementation more flexible
Can make implementation a bigger and less efficient
Layers are implemented by protocols – rules for communication
• Data from applications moves up and down protocol stack
– Application level data is chopped into packets (segments)
– Encapsulation deals with attaching headers at layers 2, 3, 4
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Machinery
• Encapsulation (header/body)
Host 1
Host 2
Application
program
Application
program
Data
Data
RRP
RRP
RRP Data
RRP Data
HHP
HHP
HHP RRP Data
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End-to-End Protocols
• Underlying network is best-effort
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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
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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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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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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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User Datagram Protocol (UDP)
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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
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SrcPort
DstPort
Checksum
Length
Data
• Optional checksum
– Computed over psuedo header + UDP header + data
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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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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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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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UDP in practice
• Minimal specification makes UDP very flexible
– 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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Congestion in the Internet
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Statistical multiplexing of ON/OFF sources
Heavy-tailed file sizes
Routers have limited buffer capacity
Packets received after buffers are full are dropped
Throughput/delay
• Checksums are effective for detecting bit errors but
bit errors are not the only problem…
• We know that traffic has bursty characteristics
• Buffers do protect from short bursts
• Congestion lengthens delays and lowers throughput
Optimal
load
Load
– Standard throughput/load curve
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How can we deal with congestion?
• Over-provision networks
– Very expensive
– Commonly done
• Networks designed to normally operate at 5% capacity
• Develop protocols to respond to congestion
– Route away from congestion
• Good idea
• How can we do it?
– Retransmit in the face of loss
• This is the state of the art
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Congestion Control Basics
• UDP will send packets at a specified rate
– Does not have any mechanism for dealing with congestion
• Issues:
– Detecting congestion
– Reacting to congestion
– Avoding congestion
• Shaping traffic
• QoS mechanisms
• Transport protocol will deal with congestion…
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