CSCE 515: Computer Network Programming

1
CSCE 515Computer Network Programming

• Chin-Tser Huang
• huangct_at_cse.sc.edu
• University of South Carolina

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Need for Address Resolution

• Need to map from an IP address to a hardware
  address
• Frames exchanged at hardware level must be
  addressed to correct interface
• Mapping between IP address and hardware address
  may change
• Need to map from a hardware address to an IP
  address
• For systems without a disk (diskless workstations
  or X terminals)

3
ARP and RARP

• Provide dynamic mapping between IP address and
  hardware address

32-bit Internet address
ARP
RARP
48-bit Ethernet address
4
ARP Request

• For computer i to get hardware address of
  computer j, i broadcasts a rqst message with IP
  address of j to the subnetwork

rqst(ipa.j)
i
default router
Internet
switch
r
j
5
ARP Reply

• If j sees a rqst message from i with its IP
  address, j sends a rply message with its IP
  address and hardware address to i

rply(ipa.j,hda.j)
i
default router
Internet
switch
r
j
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ARP and RARP Packet Format
hard size
prot size
Ethernet destination addr
Ethernet source addr
frame type
hard type
prot type
op
sender Ethernet addr
sender IP addr
target Ethernet addr
target IP addr
6
6
2
2
2
1
1
2
6
4
6
4
Ethernet header
28-byte ARP request/reply
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ARP Cache

• Maintain recent mappings from Internet addresses
  to hardware addresses
• Make ARP operations efficient
• Timeout is 20 minutes for completed entry and 3
  minutes for incomplete entry
• Timeout for an entry restarted every time it is
  referenced

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Proxy ARP

• A router answers ARP request on one of its
  networks for a host on another of its networks
• Sender of ARP request will regard the router as
  destination host and send IP datagram to the
  router
• Router acting as proxy agent will forward IP
  datagram to destination

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Gratuitous ARP

• A host sends an ARP request looking for its own
  IP address
• Usually done at bootstrap time
• Two features provided by gratuitous ARP
• Let a host determine if another host is already
  configured with same IP address
• Let other hosts update their cache entries

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arp Command

• -a option to display all entries in ARP cache
• -d option to delete an entry from ARP cache
  (superuser only)
• -s option to add a permanent entry to ARP cache
  (superuser only)
• Keyword temp
• Keyword pub

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Ethereal

• A free network protocol analyzer for Unix and
  Windows
• Examine data from a live network or from a
  capture file on disk
• Provide a rich display filter language and the
  ability to view the reconstructed stream of a TCP
  session
• http//www.ethereal.com

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Next Class

• IP
• Read TI Ch. 3
• Homework 1 will be passed out