Transcript 588 Section 1

588 Section 7
Neil Spring
May 18, 1999
Schedule
• Homework 2 review
• DNS
• Active Naming
Reminders & Notes
•
•
•
•
Programming Assignment 2 due May 24
Homework 3 will be due June 1
Project 3 will be due June 7
Time to talk to Tom about the final
project
Homework 2 -1
• A forwarding packets through C?
– whenever cost of A-B is higher than A-C-B
• Asymmetric cost info
– link state - everyone will get consistent
– distance vector - asymmetric routes
• Link inconsistency affecting routes
– if it’s in the middle of the network
Homework 2 - 2
• Number the leaves 0..n
• The route to 010 is (from the root): left
right left.
• The route from 001 to 010 is:
– up (to the 00? router)
– up (to the 0?? router)
– right (to the 01? Router)
– left (to the 010 host)
Homework 2 - 3
• Key realizations:
• Hierarchical routing
– end nodes are dumb
– routers know the topology
• Loose source routing
– end nodes know the topology
– routers don’t know much
– router failure is less transparent
2-4
• In slow start,
each ack
increases cwnd
by one.
• Delayed acks
not really shown
here.
Homework 2 - 5
• I don’t know yet. :)
All you need to know about
DNS
• Host to IP address mapping
– poplar.cs.washington.edu = 128.95.2.24
• IP address to host mapping
– 24.2.95.128.in-addr.arpa = poplar…
• Caching
– updates aren’t immediately visible
– negative caching important too
• UDP
All you might want to know
• Variable depth hierarchy
– .edu
– .washington.edu
– .cs.washington.edu
– .porcupine.cs.washington.edu
• Protection against bad data
– TTL/data value reversal example
• Caching doesn’t extend to hosts
– To speed things up?
DNS Attributes
• A: IP Address
– ip from name
• CNAME: Canonical name
– name to other name
• NS: Authoritative Name Server
• PTR: Pointer record
– name from ip
• MX: Mail Exchange
– gets mail destined for this machine
How to get em (host)
• host poplar:
• poplar.cs.washington.edu
A
128.95.2.24
• host -t CNAME poplar
• poplar.cs.washington.edu has no CNAME record
(Authoritative answer)
• host -t CNAME www.cs.washington.edu
• www.cs.washington.edu CNAME
bauhaus.cs.washington.edu
More host stuff
• host -v 128.95.2.24
• Query about 128.95.2.24 for record types PTR
• Name: poplar.cs.washington.edu
• Address: 128.95.2.24
• host -t mx poplar
• poplar.cs.washington.edu
june.cs.washington.edu
• poplar.cs.washington.edu
trout.cs.washington.edu
MX
10
MX
20
Active Names
• Richer interface for name lookup
• Motivation:
– client side web replica search
– name the service provided by
• active networks
• active services
• network caches / transformation agents
• Insight
– We name services: IBM’s web service
– We don’t care which host gives us the service
What do we normally do to
make big web servers?
• Split it up, statically
– java.sun.com
• DNS round robin
– www.microsoft.com (?)
– can add new servers without telling anyone
• Distributed director
– connection level splitting
• Neither consider locality of users
• IP address and service not equivalent
What could we do to make big
web servers?
• Help the user pick a close server
• Download netscape or IE:
– get a list of different places you can
download from
– how should you pick?
– Shouldn’t the machine figure it out?