1. DNS
6 November 2009
CS5229 Semester 1 2009/10
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2. root
.com
.org
.sg
yahoo
google
pbs
nus
ntu
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3. root
server
.sg
.nus
local
DNS
dig +trace www.nus.sg
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4. uses UDP
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5. Resource Record"
(name, TTL, class, type, value)"
ns1.nus.edu.sg. 6562 IN A 137.132.123.4
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6. TTL-based Caching
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7. Resource Type"
A NS PTR MX CNAME …"
ns1.nus.edu.sg. 6562 IN A 137.132.123.4
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8. DNS-based "
Load Balancing
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9. DNS-based "
Server Selection
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10. how fast?"
how efficient? "
is caching useful?"
how big should TTL be?
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11. “DNS Performance and
Effectiveness of Caching”"
J Jung et. al."
IEEE TON 2002
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CS5229 Semester 1 2009/10
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12. Logger
Internet
MIT Subnets
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13. 3 traces"
1 week each
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14. DNS query/respose"
TCP SYN/FIN/RST
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15. basic analysis
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16. DNS Query Types for mit-jan00
6.4
6.8
A
PTR
MX
24.6
ANY
60.4
Others
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CS5229 Semester 1 2009/10
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17. 496,802"
lookups followed by TCP
connections"
3,619,173"
TCP connections
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18. 80-87%"
DNS cache hit ratio"
for A lookup
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19. not very high since it includes
Web browser’s"
caching too
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20. how fast is DNS lookup?
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21. lookup latency ≈ "
time between first query"
and last response
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CS5229 Semester 1 2009/10
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22. mit
kaist
22

23. Number of referals per lookup
mit-jan00
1.16
0.15
24.07
0
1
74.62
2
> 2
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24. 0
1
2
24

25. NS Cache"
miss:
first query to gTLD or
root servers"
hit:
otherwise
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CS5229 Semester 1 2009/10
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26. hit
miss
26

27. 2,486,104"
iterative lookups"
6,039,582"
query packets
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28. significant number of
retransmissions
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29. ~24%"
of lookups are unanswered
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30. persistent retransmission"
referral loops
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31. answered
zero referral
31

32. no need to retransmit beyond
2-3 retransmissions!
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33. loops
33

34. each loop generated on average"
10"
query packets
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35. ~60%"
of all queries are generated by
unanswered lookup
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36. popularity of domain name
follows "
Zipf"
distribution
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37. Frequency of the x-th most
popular item is
37

38. popularity of web pages"
population of countries"
occurances of English words"
:"
:
38

39. 39

40. 40

41. 41

42. long tail :"
46%"
of domain names "
are accessed once
42

43. 43

44. 20%"
DNS responses are "
from root/gTLD servers
44

45. without caching NS records:"
5X"
loads on root/gTLD servers
45

46. kaist
mit
46

47. popular sites have shorter TTL"
TLL reduces over time
47

48. 6 November 2009
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49. sharing cache does not help
49

50. 50

51. large TTL does not help
51

52. “King: Estimating Latency
between Arbitrary End
Hosts”"
Gummadi et. al."
IMC 2002
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53. how to find alternate path "
with lower latency?
54

54. need to find latency between
pair of Internet hosts
55

55. assume name server "
is close to end host
56

56. authorative
authorative
NS
NS
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57. how to find a name server
close to an end host?"
look at domain name, IP prefix
58

58. how likely does a name server
allow recursive query?"
> 72% of name server tested
59

59. how accurate is the estimated
latency?
60

60. 61

61. 62

62. how close, really, is an end
host to its authorative NS?
63

63. 64

64. 65

65. 66

66. many diverged paths consist of
routers that are physically
adjacent
67

67. 75-80% of the disjoint paths
have latency less than 10ms"
in the Napster trace
68

68. Summary:"
Measurement study of DNS"
Use DNS to do measurement
69