CSCI 5273 Computer Networks Stevens, Chapter 4

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CSCI 5273Computer NetworksStevens, Chapter 4
5ARP, RARP ICMP

• Dirk GrunwaldAssoc. ProfessorDept. of Computer
  ScienceUniversity of Colorado, Boulder

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Outline

• ARP is a protocol for mapping and IP address to a
  MAC address
• RARP is a protocol for managing a machine --
  telling a machine what its IP address should be,
  based on the MAC address
• ICMP is the internet control message protocol and
  is used to manage ( measure) many aspects of IP

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ARP - The Problem

• Once a packet has been routed to a specific
  network, we need to deliver it to the appropriate
  host
• The host Ethernet only listens to an ethernet MAC
  address
• We only have an IP address
• Thus, we need to know how to map the IP address
  to a MAC address

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ARP - Example

• FTP uses gethostbyname to determine the IP
  address of an FTP server
• FTP asks TCP to establish a connection
• TCP send a connection request to that IP address,
  which is on the local network
• The O/S uses ARP to determine the Ethernet MAC
  address
• The destination O/S replies the reply is
  received
• The IP layer can now send the packet

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The sequence
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Format of an ARP request
HardwareSize
ProtocolType
SizeType
HardwareType
EthenetDest. Address
EthernetSrc Address
SenderEnet Addr
SenderIP Addr
TargetEnet Addr
TargetIP Addr
Notice this!Used by Proxy ARP
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Notes

• ARP uses a physical (Ethernet) broadcast to the
  network
• A unicast response is used to inform the sender
  of the appropriate MAC address
• ARP responses are cache by the kernel
• Everyone listens to the Sender message and caches
  that response

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You can use arp to see the ARP table

• foobar-22 arp -a
• mx6-0.cs.colorado.edu (128.138.241.90) at
  00-00-f8-75-4b-38
• cs-gw3-esl.cs.colorado.edu (128.138.241.65) at
  00-e0-f7-94-05-80
• ragtop.cs.colorado.edu (128.138.241.81) at
  00-00-f8-75-5b-8c
• foobar-23

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Example ARP exchange
Destination MAC
Sender MAC

• 110754.537688 00f80a3f2 ffffffffffff
  arp 42 arp who-has ragtop.cs.colorado.edu tell
  foobar.cs.colorado.edu
• 110754.538665 00f8755b8c 00f80a3f2
  arp 60 arp reply ragtop.cs.colorado.edu is-at
  00f8755b8c

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Proxy ARP

• ARP Packets reply to the Sender Hardware
  Address and cache the Target Hardware Address
• This can be different than the Ethernet Source
  Address of the reply!
• Thus, host A can reply for host B, and all IP
  packets destined for B will be sent to A
• Host A can then insure they get to host B

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Using Proxy ARP
gateway
.1.4
Enet, 140.252.1
.1.183
netb
.1.29
bsdi
sun
192.252.1.183
.13.35
.13.33
Enet, 140.252.13
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Using ARP to spot configuration problems

• At boot-up, many systems issue an ARP request for
  their own IP address.
• If anyone responds, something is mis-configured
• You can also use gratituous ARP for rapid
  fail-over.
• Everyone (usually) snoops the sending hardware
  address
• Server A B have same internal IP address, but A
  is dormant
• Server A listens for a death song from Server B
• Server A immediately send an ARP request
• Everyone now thinks that A is the specified IP
  address

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RARP

• RARP is a reverse ARP request
• A host knows its MAC address, but not the
  specified IP address
• Broadcasts an RARP who-is request
• An RARP server looks up the MAC address in a
  table (/etc/ethers) and replys with the IP address

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Sample /etc/ethers table

• Anchor and clients
• 080020085866 anchor
• 0800200632a0 littletime
• 08002007a5ce chimay
• 08002002b274 duvel
• 08002000e68b alltime
• 080020000c7a pilsner
• 08002002a4c1 white
• 08002001d5a4 twig

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Implementation of RARP servers

• RARP entries are typically stored in a file
• Rather than implement this as a kernel
  process/module, most systems use a (privileged)
  daemon
• That daemon listens on the interface for all
  WHO-IS packets

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ICMP - Internet Control Message Protocol

• Communicates error and exceptional conditions
• Some ICMP messages cause errors to be returned to
  the use process

IPHeader
(contents depend on type code)
8-bit type
Checksum of ICMP
8-bitcode
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ICMP Types
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Error Reporting

• ICMP never returns errors (e.g. destination
  unreachable) for
• ICMP error messages
• A datagram destined for an IP broadcast address
• A datagram sent as a link-layer broadcast
• A fragment other than the first
• A datagram whose source address does not define a
  single host (zero, loopback, broadcast or
  multicast)
• Avoids broadcast storm
• Implies that protocols must be able to deal with
  dropped ICMP packets

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ICMP Destination Unreachable Codes
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Sample ICMP Types

• ICMP address mask reply informs a host about
  the subnet mask that should be used
• As stevens points out
• Not reliable (e.g., Unix SVR4 reply was
  incorrect)
• Alternate protocols exist (e.g. BOOTP DHCP)

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Sample ICMP Types

• ICMP Timestamp Request Reply
• Used to return the current time from another host
• Could be used for (primitive) time
  synchronization protocol, but NTP and XNTP do a
  much better job