Chapter 9 Caching, NAT

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Chapter 9Caching, NAT

• Professor Rick Han
• University of Colorado at Boulder
• rhan_at_cs.colorado.edu

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Announcements

• HW 4 due April 16
• Programming Assignment 3 tonight
• Next, Caching, NAT

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Recap of Previous Lecture

• HTTP
• Proxies
• Reasons to use Caching advantages, Security,
  Protocol translation
• Prefetching
• GET, POST, PUT
• Load Balancing
• HTTP Redirect for load balancing
• DNS
• Returns a list of IP addresses matching a name
• DNS round robin on order of IP addresses, Akamai
• Routing-based techniques
• SMTP, MIME

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Caching Policy at Proxy

• Relevant HTTP fields
• Request
• If-Modified-Since
• Response
• Last-Modified
• Expires
• Caching proxy doesnt cache pages with
• Pragma no-cache header field
• WWW-Authenticate or Authorization headers
• Server and proxy clocks must be reliable

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Caching Policy at Proxy (2)

• Browser has its own browser cache.
• Browser sends a conditional GET with
  If-Modified-Since header field when
• a user hits Reload, or
• a page expired in browser cache, or
• browser set to always ask for a page,
• A conditional GET will only succeed in returning
  a page if that page has been modified since the
  If-Modified-Since date.
• Otherwise, get back a status code 304 Not
  Modified
• Caching proxy receives a conditional GET what
  is its policy?

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Caching Policy at Proxy (3)

• If page not in proxys cache, or cached page has
  expired, or if cached page was Last-Modified
  earlier than requests If-Modified-Since date,
  then
• Forward Conditional GET to server. If server
  finds its page was Last-Modified earlier than
  If-Modified-Since date, then servers response to
  proxy is status 304 Not Modified
• Proxy returns status 304 Not Modified to client
  
• Else, server returned fresher page, so proxy
  caches it and returns it to client
• Else
• return cached page to client (its not expired
  and its freshly modified)

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Caching Policy at Proxy (4)

• Summary
• cached page returned only if not expired and new
  enough (recently modified), otherwise return
  fresher page from server or status 304 message
• Expires header may be missing, so proxy has
  to guess a probable expiration date
• If Last-Modified is recent, then guess the page
  is changing frequently, so choose a quick
  expiration date
• Chaining of caches is allowed, e.g. Browser cache
  chained with Proxy caches
• SQUID caching proxies are common freeware
• based on Harvest caches developed in part at the
  University of Colorado, see http//www.squid-cache
  .org

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More on MIME

• Displaying non-ASCII character sets
• augment MIME type and subtype with a qualifying
  parameter
• Content-type text/plain charset"utf-8"
• lthtmlgtltheadgtltmeta http-equiv"Content-Type"
  content"text/htmlcharsetbig5"gtlt/headgt
• Some other character set codes are
• korean for Korean KSC 5601, big5 for Chinese
  big5, x-euc-jp for Japanese EUC, iso-2022-jp for
  Japanese JIS, utf-8 for UniCode
• MIME also supports non-ASCII headers, RFC 2047

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Well-Known TCP Ports

• Many application-level protocols rely on
  client-server paradigm
• Server or daemon should be up and running on a
  well-known port
• Client then contacts server on well-known
  destination TCP port
• Clients source TCP port can be any above the
  reserved limit of 1023, for safety choose gt 5000
• IANA reserved/well-known TCP ports
• 80 http
• 20 ftp-data, 21 ftp control
• 23 telnet
• 25 smtp

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Network Address Translation (NAT)
LAN
External

• Enables reuse of IP addresses, gets around IPs
  limited 32-bit address space
• TCP-level semantic knowledge is required
• TCP/IP source
• Wants to send to cnn.com
• creates packet ltIP sender, TCP sender port, IP
  dest, TCP dest portgt
• NAT has a local IP LAN address and an external IP
  address

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NAT (2)
NAT Network Addr Trans
TCP/IP destination cnn.com
TCP/IP source
LAN
External

• NAT receives packet and
• Replaces IP sender addr with its own external IP
  address
• Replaces TCP sender port with its own TCP source
  port, chosen from a list of available source
  ports
• Sends packet to destination
• Stores a mapping of ltsender IP, sender TCP portgt
  to ltNAT IP, NAT TCP src portgt

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NAT (3)
NAT Network Addr Trans
TCP/IP destination cnn.com
TCP/IP source
LAN
External

• When a reply packet is received from cnn.com on
  NAT TCP src port
• NAT substitutes the original senders IP sender
  address and TCP sender port
• Even if two sources on same LAN choose the same
  TCP sending port, theyll differ in their IP
  addresses input/output mapping still unique
• Extends IP address spaces by qualifying addresses
  with TCP port s

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NAT (4)
LAN
External

• NAT needs to modify IP address and TCP port and
  TCP/IP checksums
• Adds latency
• The type of NAT described thus far is called a
  Dynamic NAT
• Internal hosts can use any non-reserved IP
  address to connect to external hosts
• External hosts dont know about internal IP
  addresses NATs as masquerading firewalls!

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NAT (5)

• Static NATs allow an external client to connect
  to an internal TCP/IP server, e.g. a Web server,
  at a specific address
• External host cant just send a Web request to
  NATs external IP address, port 80, because NAT
  wont know to which internal host to route the
  http request
• Static NAT allocates a fixed IP address for the
  Web server and constructs a static mapping from
  ISP-assigned external IP to internal IP
• Static NAT also called inbound mapping

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NAT (6)

• Static NATs work with FTP and other application
  layer protocols as well
• Static NATs introduce security risk allowing
  unknown outsiders to connect into a local
  computer
• Configure your NAT router to have two Ethernet
  LAN ports with separate IP addresses
• Place machines you wish to be public on one IP
  addresses, and keep private hosts on 2nd IP
  address

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NAT (7)
NAT Network Addr Trans
TCP/IP source
LAN

• NATs can be used a load balancers too
• TCP/IP Packets into a NAT are rewritten with the
  IP address and TCP port of the most lightly
  loaded destination
• Reply packets from destination also rewritten by
  NAT on way back to source

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NAT (8)
LAN
External

• Comparing NATs and Proxies
• Similar architecture of intermediary between
  client and server
• NATs are lower level (IP and TCP) vs. Proxies
  (HTTP)
• NATs are arguably faster than higher-layer
  proxies
• NATs are transparent to source and destination
  nodes, whereas HTTP clients are explicitly aware
  that there is an HTTP proxy intermediary in their
  path to the HTTP server