UNESCOCISM SECOND ADVANCED SCHOOL OF INFORMATICS UNESCO PROJECT Advanced Course on Networking TCPIP

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UNESCO/CISM SECOND ADVANCED SCHOOL OF
INFORMATICSUNESCO PROJECT Advanced Course on
Networking TCP/IP
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Content

• Internet Layer Protocols
• IP, ARP, RARP, ICMP.
• Transport Layer Protocols
• UDP
• TCP
• Applications
• DNS
• Some services

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TCP/IP Support Protocols
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IP Protocol

• Internet Prococol
• provides the packet delivery service for TCP, UDP
  and ICMP
• user processes do not normally explicitly
  generate IP datagrams
• Address Resolution Protocol maps an Internet
  address into a hardware address
• Reverse Address Resolution Protocol
• maps a hardware address into an Internet address.

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IP datagram structure

• an IP datagram consists of a header parts and a
  text part
• header has a 20 byte fixed part and a variable
  length optional part
• type of service field allows different
  combinations of reliability and speed to be
  chosen
• for digital speech IP can be told to emphasize
  fast delivery
• for file transfer is taking IP can be told that
  accuracy is paramount at the expense of speed.

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IP Datagram Fields
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IP Datagram Fields

• total length length of both header and data
• identification all fragments of a datagram
  contain the same id value host can determine
  which datagram an incoming fragment belongs
• DF do not fragment
• MF more fragments (All fragments except last
  one must have this bit set to true )
• Fragment offset must be a multiple of 8. Tells
  receiver where this fragment belongs in the
  datagram.
• Time to live in seconds. Decrements each second
  or each hop, when it reaches 0 it is thrown away
  
• Protocol field tells which of the various
  transport processes the datagram belongs, ie.,
  TCP or UDP
• Header checksum verifies header only
• Source and destination address indicate the
  network number

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IP addressing

• internet is a virtual structure
• implemented entirely in software
• packet frames and addresses were designed on
  merit
• addresses contained with 4 bytes
• conceptually the 32 bit number has two parts
• hostid
• netid
• three primary classes of IP addresses

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IP address classes
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IP address classes

• class A
• a handful of network which have more than 65536
  hosts
• class B
• addresses for intermediate size networks.
  256..65535 hosts. 14 bits for netid 16 bits for
  hostid
• class C
• networks which have less than 256 hosts
• class D
• multicast, hosts may dynamically join/leave
  multicast group
• hosts may be in many different multicast groups

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Network structure
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Network structure
210.112.1.5
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Network and Broadcast addresses

• two reserved hosted-s
• Internet addresses can be used to refer to
  networks as well as network cards. By convention
  the network address has hostid all bits 0
• a broadcast address conversely has hostid bits
  all 1
• one of the weakness of IP addressing is that if a
  machine changes network - its IP address must
  change

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Fragmentation and reassembly

• IP datagrams may be fragmented en route
• if intermediate nodes cannot cope with a large
  datagram (MTU (maximum transmission unit) is
  smaller than datagram size)
• IP datagrams may be reassembled en route
• although not a good idea as routing is dynamic.
  (So datagrams may not always travel the same
  route)
• to fragment a datagram into two a node creates
  two new datagrams with same fragment ids
• the first offset is 0, MF 1
• the second offset is n, MF 0

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Fragmentation and reassembly

• consider trying to send a 1420 byte datagram when
  the MTU is 620
• 1420 1400 data 20 IP header
• split into 3 packets
• first packet length 620 20 new IP header
  600 old data, offset 0
• second packet length 620 20 new IP header
  600 old data, offset 600
• third packet length 220 20 new IP header
  200 old data, offset 1200
• the new fragments have the same unique frag id as
  the original why?
• reassembly reverses this process

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IP Support Protocols

• ICMP (Internet Control Message Protocol)
• sends control information between the hosts
• routers generate most of this information
• routers use ICMP to
• inform hosts that a packet could not be delivered
  because of an error
• or a better route exists to a particular
  destination
• ICMP messages are send using IP frames
• ICMP messages use the IP protocol field and set
  it to 1

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Typical ICMP messages are

• destination unreachable - when a router cannot
  find a routing table entry for the destination of
  an IP packet
• routing redirect - a router sends a routing
  redirect message to inform a host that a better
  route exists via another router
• time expired - message indicates a packets ttl
  field has reached 0
• usually because of a configuration error
• malfunctioning router
• echo request and echo reply - echo request
  messages request that the destination return the
  data in an echo reply message (ping)

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Address Resolution Protocol

• IP address space is virtual and has no addressing
  relationship with the underlying datalink
  protocols
• every network interface has an IP address
• every network interface has a datalink address
• datalink addresses vary in format and size
• suppose IP is sending a packet to a remote host
  on the same Ethernet
• IP needs destination Ethernet address
• could manually keep track of hosts and their
  interface card datalink addresses

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Address Resolution Protocol

• clearly on a large network this becomes
  unmanageable
• ARP (Address Resolution Protocol) is an automatic
  method which maps any network level address (IP
  address) to datalink address
• ARP does this by exploiting the broadcast
  capability commonly found in most LAN datalink
  protocols

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RARP (Reverse ARP)

• ARP maps from network addresses to datalink
  addresses
• sometimes you require the opposite mapping
• many machines can read their datalink hardware to
  find out the datalink address
• but then needs to find out its IP address
• for example, disk less workstation, X terminal,
  printer

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RARP (Reverse ARP)

• at least one host on the IP network must contain
  a list of IP addresses with corresponding
  datalink addresses
• whereas ARP does not require that this list is
  present
• a RARP is a broadcast request - any host may
  reply
• the sender fills in its datalink address
• its network address is filled with zeros
• specifies the target datalink address (normally
  the same as sender)
• the RARP server fills in the requested IP
  (network) address
• RARP is normally the first step taken when a
  diskless workstation it powered up
• once it knows its own IP address it can then
  proceed to load its operating system from a
  network server by using a simple file transfer
  protocol (TFTP)

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Transport Layer Protocols

• TCP and UDP

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Transport Layer Protocols

• 2 main protocols TCP UDP
• TCP transmission control protocol
• Connection oriented
• Reliable sequence of numbered segments
  acknowledgments with any required
  re-transmissions
• Flow control sliding windows
• UDP user datagram protocol
• Connectionless
• Unreliable delivery of single segments
  (datagrams)
• Errors detected but not corrected (No acks)

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TCP/UDP analogies

• TCP as a telephone call
• Make a connection ring number
• Verifies connectivity
• Use connection communicate
• Reliability can you repeat that please?
• Flow control normal human courtesy
• Close connection receiver down
• UDP as a letter
• Write it, address it, post it, hope it gets there

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TCP vs UDP

• Relative merits
• TCP
• Reliable
• High network overheads
• Complex and large software
• UDP
• Unreliable (may not be a problem)
• Low network overheads
• Simple and small software

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When to use UDP

• Use TCP unless there is a reason for UDP
• Possible reasons to use UDP
• When the network efficiency is needed
• (SNMP)
• When the sw simplicity is needed
• (Bootstrap loading)
• When the reliability of TCP is counter-productive
  (Stream audio/video)

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User Datagram Protocol (UDP)

• UDP and the TCP/IP layered model
• UDP message format
• UDP and encapsulation
• UDP checksum

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UDP And The TCP/IP Layered Model
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Multiplexing and Demultiplexing 1
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Multiplexing and Demultiplexing 2
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How Do We Allocate Port Numbers?

• Well known port numbers
• Dynamic binding

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Port numbers

• 0 to 255 public port numbers
• 256 to 1023 assigned to companies for
• their own marketable apps
• 1024 unregulated

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Examples of public ports
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UDP Message Format
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UDP and Encapsulation
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UDP Checksum

• Optional checksum calculated on
• UDP datagram
• UDP pseudo-header
• Does this violate layering?

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UDP Checksum
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Differences between TCP UDP
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Properties of a Reliable Delivery Service

• Stream Orientation Application just transfers a
  stream of bytes
• Virtual Circuit Connection Is TCP VC?
• Buffered Transfer TCP decides what size TCP
  messages are, not user
• Unstructured Stream There is no structure in
  the stream of bytes as far as TCP is concerned
• Full Duplex Connection Can transfer data in
  both directions simultaneously and independently

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Positive Acknowledgement A Simple Example
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Positive Acknowledgement Packet Doesnt Arrive
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Positive Acknowledgement ACK Doesnt Arrive
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Round Trip Time (RTT)
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Round Trip Time (RTT)Estimate too low
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Round Trip Time (RTT)Estimate too high
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Sending Multiple Packets

• Receiver may not be able to process packets as
  fast as they arrive
• In fact ACKs serve at least 3 different purposes
• Recovery from lost packets
• Limit the rate at which sender can send packets
• Control congestion in the network
• Compromise solution is required
• The Sliding Window

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Sliding Windows Example

• Given a window size 6 packets
• A has sent packets 1 to 7
• A has received ACKs 1 to 4

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Sliding Windows ACK Received

• A receives ACK 5 and the window slides to the
  right

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Sliding Windows Packet 8 sent

• A sends Packet 8 and the window remain unchanged

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Sliding Windows Packet 9 sent

• A sends Packet 9 and the window remain unchanged

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Sliding Windows Packet 6 ACK-ed

• A receives ACK for Packet 6 and the window slides
  to the right

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Sliding Windows Packet 6 ACK-ed

• A sends packets 10, 11, and 12. No ACK received ?
  no more packets can be sent.

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Sliding WindowsWindow size?

• Different protocols fix the window size in
  different ways
• Fixed by protocol specification
• Agreed when connection established
• Adjusted whilst connection in progress
• (Demo is given later)

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Sliding Windows Missing ACKs ?

• With just one outstanding ACK it was easy. If A
  doesnt get an ACK it sends the packet again
• With several ACKs outstanding it gets much more
  complicated
• What does A do when it detects a missing ACK?
• What does B do when it detects a missing packet?
• Different protocols use different solutions

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Sliding Windows Missing ACKs ?

• There are two extreme positions
• When something goes missing A resends every
  packet starting with the first unacknowledged
  packet.
• This is called Go-Back-N.
• When something goes missing A B co-operate to
  just resend those packets B hasnt received.
• This is called Selective Retransmission.

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TCP Encapsulation
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TCP Header
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TCP Header Fields

• Source Port Port number of sender of segment
• Destination Port Port number of intended
  recipient
• Sequence Number TCP uses byte numbers not
  packet numbers
• Acknowledgement Number Number of the byte the
  sender of this segment expects next
• Window The size of the sliding window in bytes
• Checksum Just like the UDP checksum
• Control bits Special purpose bits
• Data User data

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Usage of SEQ-ACK-WIN Fields Example
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TCP is Complicated

• RFC 793 is over 90 pages long but doesnt cover
  everything
• There are a number of additional RFCs covering
  various aspects of TCP
• There are a number of reference implementations
  which have performance enhancing features
• There is no single TCP specification

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Next TCP continues

• Establishing A TCP Connection
• Timeouts and Retransmission
• Congestion
• Closing A TCP Connection

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TCP Connections

• There are obviously two end points to a
  connection
• An end point is identified by a combination of
  host IP address port number
• One end point that initiates the connection. This
  performs an active open
• One end point that accepts the connection. This
  performs a passive open
• The passive open must occur before the active
  open can succeed
• The passive end point can support multiple
  connections

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Establishing A TCP Connection
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Timeouts and Retransmission

• Selecting the correct value for the timeout is
  crucial to efficiently implementing TCP.

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Round-Trip Time (RTT)
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Round-Trip Time (RTT)

• RTT varies according to
• Different hosts
• Different times of day
• Even from second to second
• If we need to retransmit then what is the RTT?
• Time from original transmission to receiving ACK
• Time from last retransmission to receiving ACK

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Measuring RTT Karns Algorithm

• Ignore retransmission when trying to estimate RTT
• Increase timeout value until transmission
  succeeds
• Then recalculate RTT when retransmission no
  longer needed

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Estimating Timeout ValueOriginal Method

• rtt ALPHA rtt ( 1 ALPHA ) sample
• timeout BETA rtt
• 0 ALPHA
• BETA 1 typically 2

Last Measured value
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RTT trajectory using Karns Algorithm
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RTT Karns Algorithm
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Estimating Timeout Value Revised Method

• diff sample rtt
• rtt rtt DELTA diff
• dev dev RHO ( abs(diff) dev )
• timeout rtt ETA dev
• 0
• 0
• ETA 1 typically 3

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Slow-Start (Additive) Recovery

• When starting a new connection or increasing
  traffic after congestion is over
• congestion window one segment
• for each ACK received
• congestion window congestion window one
  segment

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Multiplicative Decrease Congestion Avoidance

• Sender also maintains a congestion window
• If a segment is lost then
• congestion window MIN (congestion window/2 ,
  one segment )
• allowed window MIN ( receive window, congestion
  window )
• ? increase retransmission timer for all segments
  in allowed window

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How does it work?
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How does it work?
Exponential Increase
Exponential Increase Increase CW with for each
received ACK
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How does it work?
Can be 64 Kbyte
Exponential Increase
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How does it work?
Linear Increase Increase CW with one for
each ACK-ed Window
Exponential Increase Increase CW with for each
received ACK
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How does it work?
Linear Increase
Probably due to loss or congestion
Exponential Increase
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Closing A TCP Connection
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Applications

• Naming and infrastructure
• there is a need for a mapping of textual domain
  names to numeric IP addresses
• difficult to remember 193.63.130.52 is the class
  C address for floppsie!
• also require services such as a consistent time
  between different machines

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Domain Name Service (DNS)

• the Internet standard DNS maps host names, such
  as floppsie.comp.glam.ac.uk to IP addresses such
  as 193.63.130.52
• DNS namespace is partitioned hierarchically into
  a tree
• glam.ac.uk - may map onto several class C
  networks
• floppsie.comp - indicates a machine within the
  computer studies network
• an interface card on class C network
  193.63.130.xx

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Naming and infrastructure

• we could have a simple lookup table that is
  manually updated
• soon becomes unmanageable
• use a dynamic mechanism, domain name service
• have a machine which will keep track of IP
  addresses and ASCII names
• if it cannot resolve a name it requests help from
  another machine higher up the tree
• the DNS protocol specifies how DNS clients ask
  DNS servers for mappings
• and how DNS servers communicate with each other.

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More Applications

• E-mail SMTP (MIME), POP3, IMAP
• FTP File Transfer Protocol
• Telnet Remote Login
• HTTP HyperText Transfer Protocol (WWW)
• NFS Network File System
• DHCP Dynamic Host Configuration Protocol