Lecture 7: Network Layer

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Lecture 7 Network Layer

• Prev. summary
• IP Addressing
• Classful
• Classless
• IPv6

Application
Transport
Network
Link

• Todays lecture
• Address Resolution Protocol
• Mobile IP

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Overview
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LAN Addresses and ARP

• 32-bit IP address
• network-layer address
• used to get datagram to destination IP network
  (recall IP network definition)
• LAN (or MAC or physical or Ethernet) address
• used to get datagram from one interface to
  another physically-connected interface (same
  network)
• 48 bit MAC address (for most LANs) burned in the
  adapter ROM

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LAN Addresses and ARP
Each adapter on LAN has unique LAN address
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LAN Address (more)

• MAC address allocation administered by IEEE
• manufacturer buys portion of MAC address space
  (to assure uniqueness)
• Analogy
• (a) MAC address like Social Security
  Number
• (b) IP address like postal address
• MAC flat address gt portability
• can move LAN card from one LAN to another
• IP hierarchical address NOT portable
• depends on IP network to which node is attached

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ARP Address Resolution Protocol

• Each IP node (Host, Router) on LAN has ARP table
• ARP Table IP/MAC address mappings for some LAN
  nodes
• lt IP address MAC address TTLgt
• TTL (Time To Live) time after which address
  mapping will be forgotten (typically 20 min)

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Example
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ARP protocol

• A caches (saves) IP-to-MAC address pair in its
  ARP table until information becomes old (times
  out)
• soft state information that times out (goes
  away) unless refreshed
• ARP is plug-and-play
• nodes create their ARP tables without
  intervention from net administrator

• A wants to send datagram to B, and A knows Bs IP
  address.
• Suppose Bs MAC address is not in As ARP table.
• A broadcasts ARP query packet, containing B's IP
  address
• all machines on LAN receive ARP query
• B receives ARP packet, replies to A with its
  (B's) MAC address
• frame sent to As MAC address (unicast)

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Proxy ARP
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Gratuitous ARP

• A host send ARP query looking for itself
• It lets a host determine if another host is
  already configured with the same IP address
• If the host sending the gratuitous ARP has just
  changed its hardware address (perhaps the host
  was shut down, the interface card replaced, and
  then the host was rebooted), this packet causes
  any other host on the cable that has an entry in
  its cache for the old hardware address to update
  its ARP cache entry accordingly.

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Routing to another LAN

• walkthrough send datagram from A to B via R
• assume A knows B IP
  address
• Two ARP tables in router R, one for each IP
  network (LAN)
• In routing table at source Host, find router
  111.111.111.110
• In ARP table at source, find MAC address
  E6-E9-00-17-BB-4B, etc

A
R
B
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Routing to another LAN

• A creates datagram with source A, destination B
• A uses ARP to get Rs MAC address for
  111.111.111.110
• A creates link-layer frame with R's MAC address
  as dest, frame contains A-to-B IP datagram
• As data link layer sends frame
• Rs data link layer receives frame
• R removes IP datagram from Ethernet frame, sees
  its destined to B
• R uses ARP to get Bs physical layer address
• R creates frame containing A-to-B IP datagram
  sends to B

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How do you contact a mobile friend
Consider friend frequently changing addresses,
how do you find her?

• search all phone books?
• call her parents?
• expect her to let you know where he/she is?

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Mobility approaches

• Let routing handle it routers advertise
  permanent address of mobile-nodes-in-residence
  via usual routing table exchange.
• routing tables indicate where each mobile located
• no changes to end-systems

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Mobility approaches

• Let routing handle it routers advertise
  permanent address of mobile-nodes-in-residence
  via usual routing table exchange.
• routing tables indicate where each mobile located
• no changes to end-systems
• let end-systems handle it
• indirect routing communication from
  correspondent to mobile goes through home agent,
  then forwarded to remote
• direct routing correspondent gets foreign
  address of mobile, sends directly to mobile

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Indirect Routing moving between networks

• suppose mobile user moves to another network
• registers with new foreign agent
• new foreign agent registers with home agent
• home agent update care-of-address for mobile
• packets continue to be forwarded to mobile (but
  with new care-of-address)
• Mobility, changing foreign networks transparent
  on going connections can be maintained!

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Mobile IP

• RFC 3220
• has many features weve seen
• home agents, foreign agents, foreign-agent
  registration, care-of-addresses, encapsulation
  (packet-within-a-packet)
• three components to standard
• agent discovery
• registration with home agent
• indirect routing of datagrams

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Mobile IP registration example