TCOM 509 Internet Protocols TCPIP Lecture 03_a

1
TCOM 509 Internet Protocols (TCP/IP)Lecture
03_a

• Instructor Dr. Li-Chuan ChenDate 09/15/2003

• Based in part upon slides of Prof. J. Kurose (U
  Mass), Prof. B. Yener (Rensselaer Polytechnic
  Institute)

2
Outline

• Chapter 5 mapping Internet Addresses to
  Physical Addresses (ARP)
• Chapter 6
• Chapter 7

3
Address Resolution Problems

• Problem given an IP address, need to find its
  equivalent physical address
• Sender must map the intermediate routers IP
  address and destination IP address to their
  corresponding physical addresses.
• Solutions
• Direct Mapping
• Table lookup
• Dynamic Binding via ARP (Address Resolution
  Protocol)

4
Address Resolution Mechanisms

• Direct mappingMake the physical addresses equal
  to the host ID portion.
• Mapping is easy.
• Only possible if admin has power to choose both
  IP and physical address or when size of physical
  address lt IP address.
• Cannot apply to Ethernet addresses (Ethernet
  addresses are 48 bits vs. IP addresses which are
  32-bits).

5
Address Resolution Mechanisms

• Table LookupSearching or indexing to get MAC
  addresses
• Similar to lookup in /etc/hosts for names
• Problem change Ethernet card gt change table

IP Address
MAC Address
197.15.3.1
0A4B00000708
197.15.3.2
0B4B00000700
197.15.3.3
0A5B00010103
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Address Resolution Mechanisms

• Dynamic Binding (ARP)
• The host broadcasts a request What is the MAC
  address of 216.109.11.67?
• The host whose IP address is 216.109.11.67
  replies back The MAC address for 216.109.11.67
  is 8A-5F-3C-23-45-5616
• Broadcast is expensive.
• ARP responses are cached. Issues
• Broken hardware use timer.
• Table full least recently used (LRU)
• Each host updates its table when receiving an ARP
  broadcast.

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ARP Message Format
8
16
24
31
0
HW Type
Protocol Type
HW Len
Protocol Len
Operation
Sender HW Address (6 bytes)
Sender IP Address (4 bytes)
Sender HW Address
Target HW Address (6 bytes)
Sender IP Address
Target HW Address
Target IP Address (4 bytes)

• Hardware (HW) Type 1 for Ethernet
• Protocol Type 080016 IP address
• HW Len and Protocol Len allows arbitrary networks
  to be used
• Operation 1 ARP Request, 2 ARP Response
  3 RARP Request, 4 RARP
  Response
• ARP messages are sent directly to MAC layer
• ARP message is 28 octets long.

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ARP

• Let Ia IP address and pa physical address
• To send an internet packet across a physical net,
  the network software must map Ia to pa and use
  the pa to transmit the frame.
• If Ia lt pa, use direct mapping. Else, use ARP to
  perform dynamic mapping.
• Given an IP address, a host uses ARP to find the
  corresponding hardware address.
• To make ARP efficient, all hosts on the network
  receive the ARP request and update its cache.
• The host with the same Ia replies directly to the
  sender.

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Outline

• Chapter 4.1 4.18
• Chapter 5
• Chapter 6 Determining An Internet Address At
  Startup (RARP)
• Chapter 7

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RARP

• Problems Given pa, how do we find Ia ?
• Solution Reverse ARP (RARP)
• Use the same message format as ARP.
• Sender broadcast a RARP request (fills its pa in
  the target field).
• Only RARP server replies.
• Typically used in Ethernet LAN.
• If only one RARP server available on the network,
  use larger delay time before retransmit another
  request.
• More RARP servers?
• Pros reliable
• Cons overload the network. (assign primary and
  secondary server to solve this problem)

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RARP

• Diskless host needs to find its IP address at
  startup before it can communicate using TCP/IP.
• Give physical address, a host can use RARP to
  find its IP address from a RARP server on the
  network.

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Outline

• Chapter 4.1 4.18
• Chapter 5
• Chapter 6
• Chapter 7 Internet Protocol Connectionless
  Datagram Delivery

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Internet Services

• The three conceptual layers of internet services.

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IP Datagram Format
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IP Datagram Format

• Internet datagram basic transfer unit
• VERS - Version (4 bits) IPv4
• HLEN - Internet header length (4 bits) units of
  32-bit words. Min header is 20 bytes or 5 words.
• Total Length (16 bits) header data. Units of
  bytes. Total must be less than 64 K (216) octets.

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IP Header

• TOS - Type of service (8 bits)
• precedence (3 bits), delay, throughput, and
  reliability.
• Not widely supported.

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IP Header

• How big can a datagram be?
• What happens when a datagram is larger than the
  frame size of the underlying physical network?

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Maximum Transmission Unit (MTU)

• Each subnet has a maximum frame sizeEthernet
  1500 octetsFDDI 4470 octets per frameToken
  Ring 2K to 4K octets
• Transmission Unit IP datagram (data header)
• Each subnet has a maximum IP datagram length
  (header payload) MTU

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Fragmentation

• Datagrams larger than MTU are fragmented
• Original header is copied to each fragment and
  then modified (fragment flag, fragment offset,
  length,...)
• Fragments must be a multiple of 8-octets.

20
Reassembly

• Reassembly only at the final destination
• Partial datagrams are discarded after a timeout
• Fragments can be further fragmented along the
  path. Subfragments have a format similar to
  fragments.
• Minimum MTU along a path ? Path MTU

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IP Header

• Fragmentation
• Identifier (16 bits) used in reassembly to
  uniquely identify all the pieces of a fragment
  chain.
• Flags (3 bits) more fragments (MF), dont
  fragment (DF), and reserved bit.
• Fragment offset (13 bits) In units of 8 octets

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Fragmentation Example
Net 1 MTU 1500
Net 3 MTU 1500
Net 2 MTU 620
R1
R2
B
A

• Payload size 1400 bytes needs to be transmitted,
  Packet ID 2222
• Networks Ethernet (MTU1500) and Net2 (MTU620)
• Use smallest MTU size (620) to find payload size
  for the fragment packet.
• IP Header 20 bytes gt Payload MTU IP
  Header 600 bytes
• Fragments need to be multiples of 8-bytes.
• Nearest multiple to 600 is still 600 bytes
• Fragment offset length 600/8 75
• Number of fragments 1400/600 ?2.33? 3
• frag1 600, frag2 600, frag3 200, Packet ID
  2222 for all fragments
• Offset1 0, Offset2 75, Offset3 150MF1
  bit 1, MF2 bit 1, MF3 bit 0

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IP Header

• Time to live (TTL) 8 bits
• Specifies how long the datagram is allowed to
  live in the network (in seconds). Typically use
  number of hops visited.
• Protocol (8 bits)
• Next level protocol to receive the data, e.g.,
  ICMP (1), IGMP (2), TCP (6), UDP (17).
• Header checksum (16 bits)
• 1s complement sum of all 16-bit words in the
  header.

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IP Header

• Source Address (32 bits) Original source. Does
  not change along the path
• Destination Address (32 bits) Final destination.
  Does not change along the path.
• Options (variable length) security, source
  route, record route, stream id, timestamp
  recording
• Padding (variable length) Makes header length a
  multiple of 4
• Payload Data (variable length) Data header lt
  65,535 bytes

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IP Header

• Options for network testing or debugging
• Security - for military purpose and is only
  supported by some products.
• Source route a list of IP address that the
  datagram must take.
• Record route the nodes in the path must return
  their IP address.
• Stream id - used for voice for reserved resources
• Timestamp the time through the node is
  returned, so that delays may be measured.
• If entries in the options must be recorded by
  nodes, the sender must reserve sufficient space
  for the option data.

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Summary

• Internetworking Problem
• IP header supports connectionless delivery,
  variable length pkts/headers/options,
  fragmentation/reassembly,
• Fragmentation/Reassembly, Path MTU discovery.
• ARP, RARP address mapping
• Internet architectural principles