CSCI 6433
Internet Protocols
Class 5
Dave Roberts
1
Internet Protocol Principles
0 Good Citizen Principle
0 When a resource is scarce, use less of it
0 Conservation of Router Time
0 Minimize the work done by routers
0 Soft State
0 Save information that takes traffic to obtain that might
be used again; age and replace it
2
Tonight
0 User Datagram Protocol (UDP)
0 Reliable Stream Transfer Service (TCP)
0 Network Address Translation (NAT)
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What Does IP Do?
0 Addressing
0 Data encapsulation
0 Fragmentation and reassembly
0 Routing/indirect delivery
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UDP and TCP
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Ports
0 Destination of a datagram is a program
0 Selection is by protocol port number
0 Messages carry source and destination port
numbers
0 UDP and TCP provide protocol ports, provide
ability to distinguish among destinations
within a given host
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UDP and TCP in Operation
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UDP--User Datagram Protocol
0 UDP provides unreliable, best-effort, connectionless
datagram delivery services to multiple destinations
within a single host computer, using IP to carry
messages
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UDP Message Format
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UDP Message Format
Question: How many
different UDP ports can there
be on a PC?
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UDP Layering
Application programs access UDP, and
UDP uses IP to send and receive
datagrams
UDP message is encapsulated in an IP
datagram
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UDP Demultiplexing
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Question
0 What good is data that’s not delivered correctly?
0 Why would we want errors in data?
0 What is UDP used for?
0 Does ICMP use UDP?
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Well-Known Port
Assignments
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Summary
0 UDP is a “thin” protocol
0 Unreliable
0 Best-effort
0 Distinguishes between applications
0 16-bit port number is used
0 UDP lies in the transport layer
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On to TCP!
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What TCP Does
0 Addressing and multiplexing
0 Connection establishment, management and
termination
0 Data handling and packaging
0 Provides reliability and transmission quality services
0 Provides flow control and congestion avoidance
features
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How TCP Does Its Job
0 Connection-oriented
0 Bidirectional
0 Multiply-connected
0 Reliable
0 Acknowledged
0 Stream-oriented
0 Data-unstructured
0 Data flow managed
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Why Stream Delivery?
0 Using stream delivery protocol frees the application
from any concern about network delivery of data
0 Packets may arrive out of order
0 Packet data may contain errors
0 Application program doesn’t want to know optimal
packet size
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TCP—Reliable Stream Transport
Service
0 Stream orientation—receiver gets the same sequence
of octets sent by the sender
0 Unstructured stream—no way to mark boundaries,
applications must use a stream
0 Virtual circuit connection—source and destination
operate at the same time, interact during the transfer
0 Buffered transfer—large blocks can be transferred,
fed to receiver as needed
0 Full duplex connection—communication can take
place in both directions
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Reliable Delivery, Unreliable Transport
0 Ethernet is unreliable
0 IP is unreliable
0 But TCP, that runs over IP, is reliable
0 How?
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Unreliable Transport
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Positive Acknowledgement with
Retransmission
0 Sender starts a timer when sending, saves data
0 Receiver sends acknowledgement (ACK) when data
arrives successfully
0 When ACK arrives, sender cancels timer, deletes data
0 If timer expires, sender retransmits data, starts
another timer
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Reliable Transport
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Delays
A positive acknowledgement protocol
introduces a number of delays because it
delays sending a new packet until it receives
an acknowledgement for the previous packet.
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Sliding Windows
To solve the problem, we use the sliding window
approach to decouple transmissions from reception of
acknowledgements.
The sender transmits multiple packets before waiting
for an acknowledgement. The number of packets is
said to be the window size.
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Sliding Window Approach
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Sliding Window Example
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Enhanced Positive
Acknowledgement
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Sliding Window Approach
Question: Suppose the IP link travels over a
very high-capacity satellite connection that
has ¼ second latency in each direction. What
is the effect on throughput if the window size
is set to 1? What if it is set to, say, 1,000?
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TCP Protocol Enhancements
0 There are a number of protocol extensions
and hardware products designed to speed up
TCP in the presence of delays
0 Generally they involve some form of providing
ACKs to the sender before the receiver has
received the packet
0 The extension then has to provide some form
of retransmission itself, since TCP counts
success when it receives an ACK
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TCP and UDP Layering
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Ports and Connections
0 UDP uses the port as a queue to process incoming
data
0 TCP uses port numbers, but the port number is part of
identifying a connection
0 A connection is identified as a pair of pairs in the form
<host, port>
• Question: can two programs both use the
same TCP port on the same machine at once?
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TCP Segments and Streams
Data stream divided into segments.
Window used to send multiple segments
before acknowledgement arrives. Allows
receiver to restrict transmission until it has
buffer space.
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Flow Control
0 Each ack includes a window advertisement, giving
the number of additional octets the receiver is
willing to accept
0 Sender increases or decreases its window size
based on the window advertisement
0 The sliding window provides end-to-end flow
control
0 Congestion, that can occur at intermediate points,
is dealt with by congestion control mechanisms
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MSS
0 MSS—maximum segment size is negotiated
0 MSS may be the MTU of a single connecting network,
if both ends are on the same network
0 MSS may be the MTU of path if sender and receiver
are on different networks
0 Risk of poor performance if MSS is too large or too
small
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Problems with Wrong MSS
0 If too large: segments will be fragmented,
requiring extra network messages and routing
0 If too small: fixed size of TCP header, IP
header, Ethernet header all are still required
for a small amount of data
Question: what’s the right size for a
TCP segment?
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TCP Segment Format
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Code Bits
Code bits determine purpose, contents of the
segment
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Urgent TCP Data
0 Programs may need to send “out of band” data
0 That is, data is treated as being sent through another
connection
0 In TCP, this can be done by setting the urgent flag
0 Receiving program processes urgent data ahead of the
rest of the data stream
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TCP Acknowledgements
0 TCP acknowledgements can’t refer to datagram or
segment numbers, since retransmission can change
message size
0 TCP ack specifies sequence number of the next octet
the receiver expects to receive
0 Called a cumulative acknowledgement
0 Both efficient and inefficient
42
Timeout, Retransmission
0 Great variation in transit times, from local net to
distant network
0 Adaptive retransmission algorithm—monitors the
performance of each connection, deduces
reasonable timeout value
0 Stores round trip time, uses new RTT to change
stored RTT slowly
0 Timeout is some constant times RTT
0 Note that acknowledgements are ambiguous in
case of retransmissions
43
Karn’s Algorithm
0 Timer backoff strategy
0 Compute an initial timeout
0 If retransmission, then increase timeout
0 Often, double the timeout
0 Timeout increases have an upper limit
0 Karn uses only unambiguous acknowledgements to
compute RTT
new_timeout = γ * old_timeout
44
Response to Congestion
0 No way for TCP to directly know about congestion, which
causes retransmission, increasing congestion even further
0 Multiplicative
decrease
congestion
avoidance—when
Karn’s
Algorithm
is an example
of
segment is lost,
sender
congestion
window
is reduced by half
which
principle
of Internet
protocols?
(starts same as receiver advertisement), ack timer is
increased
0 Allowed window=min(receiver_advertisement,
congestion_window)
0 Slow-start recovery—start congestion window at single
segment size, increase by one for each ack
45
Explicit And Explicit Notification
0 Most versions of TCP use implicit feedback
mechanisms—using timeout and multiple ACKs to
detect loss and congestion
0 Some versions of explicit feedback have been
developed for TCP/IP
0 Selective acknowledgement—receiver identifies
left edge, right edge of received segments
0 Explicit congestion notification—routers add
notification to TCP header, then receiver notifies
sender in next ACK
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Global Synchronization
0 When router is congested, early routers used tail-
drop strategy
0 Tail-drop—drop most recent datagram
0 This can cause drops from many TCP sources at
the same time, leads to global synchronization
0 Random Early Discard (RED)—uses min and max.
If fewer than min datagrams in queue, add it. If
more than min, randomly discard one datagram.
More than max, then drop new datagram.
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Establishing TCP Connections
Three-way handshake is used to establish
TCP connection.
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Close TCP Connection
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The TCP FSM
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Well-Known Port Assignments
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Silly-Window Syndrome
0 Receiver reports buffer space available as
WINDOW field in ACK
0 Advertising single byte window causes sender to
send one-octet segment
0 Avoid it by:
0 Receiver doesn’t advertise until space has accumulated
0 Sender doesn’t send unless enough data to fill medium-
size segment. But sends data when an ack arrives, even
if still accumulating
52
NAT
0 Developed in order to extend life of IP address pool
0 Used these characteristics of Internet connections:
0 Most hosts are clients that don’t need to be publicly
accessible
0 Not all hosts access the Internet at the same time
0 Internet connections are routed
0 NAT shares globally unique IP addresses among the
hosts that are actually using an Internet connection
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Analogy
0 Think of a NAT router as a company’s telephone
operator
0 Connects outside callers to local extensions
0 Connects local extensions to the outside network
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Summary
0 TCP provides an important service: Full duplex,
reliable connection between two machines
0 Segment as basic unit of interchange
0 Flow control through advertising, backoff
0 Many additional techniques to increase performance
Since TCP provides reliable communication, why do
we need UDP? Why not just use TCP for all Internet
communication?
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Network Address Translation
0 Extends the pool of addresses
0 Creates a form of two-level addressing
0 Could be used even more widely to extend the pool of
IPv4 addresses
0 Does not allow for a server inside the NAT network to
hosts outside
0 Can be port-based or not
56
What NAT Does
0 NAT maps IP addresses at a router
0 NAT allows a network to have local IP addresses that
do not conform to Internet standards
0 NAT has a pool of valid IP addresses, and shares them
among local hosts
0 NAT changes IP addresses in messages that it
exchanges through the router
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How NAT Works
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Limitation
0 Traditional NAT is said to be “unidirectional”, allows
only for transactions originated inside the network
0 No allowance for a server inside the network that
serves users across the Internet
0 If each host on a network has many sessions open,
pool of IP addresses may become exhausted
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Port-Mapped NAT
0 UDP and TCP offer ports; connection is named by IP
address and port number
0 NAT can map UDP and TCP messages to IP address
and port numbers
0 This greatly increases the number of simultaneous
connections that can be handled using one valid IP
address
Question: how many simultaneous connections can be
handled by a NAT box for each valid IP address?
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NAT Saving IP Addresses
0 ISPs, large organizations can solve the IP address
shortage
0 Every ISP gets just one IP address per 10,000
customers
0 Same for large organizations
0 Port-mapped NAT is used for ISP customers
0 Excess IP addresses are returned for reuse
So why do we need IPv6? Why not use NAT
to increase effective number of IP addresses
by 10,000 (or more)?
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The Problem with NAT
0 First, NAT changes messages, does not operate truly
end-to-end, violates a basic idea of Internet protocols
0 Some applications have IP addresses other than in the
IP header
0 These applications won’t work with NAT unless NAT
knows about them and changes the IP addresses
0 FTP is an example
0 NAT works only for sessions started from inside the
network
How can I run a server on a
network that is behind a NAT box?
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Summary
0 NAT can extend the pool of IP addresses
0 NAT has limitations
0 NAT must violate layering to be useful
0 Over time, NAT will become more and more complex
0 NAT does not operate end to end
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