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Client/Server Networking

Client/Server Networking. Protocol Stack Summary. “turtles all the way down” each layer uses the services of the layer below and provides a service to the layer above Python lets you work at the layer of your choice programs are “cleaner” the higher the layer you use

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Client/Server Networking

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  1. Client/Server Networking

  2. Protocol Stack Summary • “turtles all the way down” • each layer uses the services of the layer below and provides a service to the layer above • Python lets you work at the layer of your choice • programs are “cleaner” the higher the layer you use • layers work by “hiding” the layers below behind function calls

  3. What lies under Socket? TCP/UDP IP “link layer” Internet Protocol Stack

  4. Networking • About “sharing” resources. • Compare to sharing of disk, IO devices, etc done by programs running on a computer • Computer Example: OS is the master controller • Network Example: Each participant “plays by the rules” but no “controller”

  5. Networking • All network cards share the same ethernet cable • all wireless transmitters share the same frequency channels • fundamental unit of sharing is “packet” • individual packets carry addressing info sufficient to arrive at final destination.

  6. Addressing • two layers: one hop at a time and end-to-end • single hop addressing performed by link layer • end-to-end addressing is IP • process addressing is TCP or UDP

  7. IP Addresses • a.b.c.d: 0 <= a,b,c,d <= 255 • New Paltz: 137.140.*.* • localhost: 127.0.0.1 • private networks: 10.*.*.*, 172.16-31.*.*, 192.168.*.*

  8. Domain Name Service • DNS converts host names into host IP addresses. • corresponds to directory assistance address = socket.gethostbyname(name);

  9. How DNS Works • gethostbyname() first looks in /etc/hosts • if this fails then it looks in /etc/resolv.conf for the address of “directory assistance”, also called a DNS Server. • sends the request to this address • Observation: If your DNS server is down, you won't get anywhere on the Internet.

  10. Routing • Each time a packet arrives at a new node a decision must be made at that node as to where to send the packet next. • Guiding principle of routing on the Internet is that each time a packet “hops” from one node to another it is always one hop closer to its final destination. • Exercise: Difference between host and node.

  11. Lots of Reading • The classic text is TCP/IP Illustrated: Vol I by Richard Stevens. • PDF file available on the web at books.google.com among other places • We will concentrate on Chapters 1-4, 9, 11, 14, 17-19.

  12. EXAMPLE: • Given: • Find longitude and latitude 207 N. Defiance St, Archbold, OH

  13. Protocol Stack: GoogleMaps ????? TCP IP Ethernet

  14. GoogleMaps URL HTTP Socket GoogleMaps • googlemaps library (3rd party) uses • urllib, uses • httplib, uses • Socket, uses • TCP, IP, Ethernet protocol stack inside the actual program itself TCP, IP and Ethernet make up the OS part of the protocol stack

  15. APIs vs Sockets: • well-tested • written by experts • common practice to use them • we still need to understand Sockets toappreciate things that depend upon them

  16. Wireshark: • lets you look at packets crossing the wire • needs root permissions • easy to filter out unneeded traffic • I saved some traffic and you can view it with Wireshark (see course web page).

  17. Highest Level API Example: • Fetch a JSON document without realizing it: #!/usr/bin/env python # Foundations of Python Network Programming - Chapter 1 - search1.py # Not even clear you are using a web service from googlemaps import GoogleMaps address = '207 N. Defiance St, Archbold, OH' print GoogleMaps().address_to_latlng(address)

  18. GET Syntax:

  19. #!/usr/bin/env python # Foundations of Python Network Programming - Chapter 1 - search2.py # HTML-level abstraction import urllib, urllib2 try: import json except ImportError: # for Python 2.5 import simplejson as json params = {'q': '207 N. Defiance St, Archbold, OH', 'output': 'json', 'oe': 'utf8'} url = 'http://maps.google.com/maps/geo?' + urllib.urlencode(params) rawreply = urllib2.urlopen(url).read() reply = json.loads(rawreply) print reply['Placemark'][0]['Point']['coordinates'][:-1]

  20. GET Syntax:

  21. #!/usr/bin/env python # Foundations of Python Network Programming - Chapter 1 - search3.py # HTTP level abstraction import httplib try: import json # json built in with Python 2.6 except ImportError: # for Python 2.5 import simplejson as json path = ('/maps/geo?q=207+N.+Defiance+St%2C+Archbold%2C+OH' '&output=json&oe=utf8') connection = httplib.HTTPConnection('maps.google.com') connection.request('GET', path) rawreply = connection.getresponse().read() reply = json.loads(rawreply) print reply['Placemark'][0]['Point']['coordinates'][:-1]

  22. GET Syntax:

  23. #!/usr/bin/env python # Foundations of Python Network Programming - Chapter 1 - search4.py import socket sock = socket.socket() # OS functionality sock.connect(('maps.google.com', 80)) sock.sendall( 'GET /maps/geo?q=207+N.+Defiance+St%2C+Archbold%2C+OH' '&output=json&oe=utf8&sensor=false HTTP/1.1\r\n' 'Host: maps.google.com:80\r\n' 'User-Agent: search4.py\r\n' 'Connection: close\r\n' '\r\n') rawreply = sock.recv(4096) print rawreply

  24. GET Syntax:

  25. # search4.py output HTTP/1.1 200 OK Content-Type: text/javascript; charset=UTF-8 Vary: Accept-Language Date: Wed, 21 Jul 2010 16:10:38 GMT Server: mafe Cache-Control: private, x-gzip-ok="" X-XSS-Protection: 1; mode=block Connection: close { "name": "207 N. Defiance St, Archbold, OH", "Status": { "code": 200, "request": "geocode" }, "Placemark": [ { ... "Point": { "coordinates": [ -84.3063479, 41.5228242, 0 ] } } ] } data transmitted by web server data read into program variable

  26. Things We've Seen: • protocols stacked on top of one another • higher level protocols using services of lower levels • programs get more specific and harder to maintain the lower down you go • the idea behind high-level protocols is precisely to hide lower levels • there's a whole lot going on below Socket.

  27. The Stack: • Fundamental unit of shared information is the packet. • Typical packet structure: • Transmitted as a single unit (but serially) • Routing is generally at the packet level • Things packets contain: data, addresses, layering, sequencing, protocol bytes, checksums • ethernet packets are called frames. IP header TCP/UDP header program data ethernet header

  28. Ethernet: • 14-byte header • addresses: two 6-byte addresses – source and destination • type: 2 bytes – 0800 == IP datagram • the two network cards involved can process the header without using the CPU, RAM, etc. • cable length (100m) and MTU • CSMA/CD • Some of the details: http://serverfault.com/questions/422158/ what-is-the-in-the-wire-size-of-a-ethernet-frame-1518-or-1542

  29. IP Addresses: • 32 bits: a.b.c.d • network address – n bits; host id – (32-n) bits • some times the network part has a subnet component; some times the subnet component is carved out of the hostID bits • a.b == 137.140 == New Paltz network address • a.b.c == 137.140.8 == CS subnet at New Paltz • the part of the network address that is not subnet identifies an organization like New Paltz.

  30. IP Address Classes:

  31. IP Special Addresses: • 127.*.*.*: local to the current machine • 10.*.*.*, 172.16-31.*.*, 192.168.*.*: private subnets. • none of these address found on the larger Internet.

  32. IP Routing: • Guiding principle: after each hop you are one step closer to your destination • typical local routing table contains a default entry pointing to the Internet together with one entry for each local subnet the host belongs to. [pletcha@archimedes PPT]$ netstat -nr Kernel IP routing table Destination Gateway Genmask Flags Iface 0.0.0.0 192.168.1.1 0.0.0.0 UG wlan0 192.168.1.0 0.0.0.0 255.255.255.0 U wlan0 192.168.122.0 0.0.0.0 255.255.255.0 U virbr0

  33. IP Routing Next Hop Algorithm: • Search Destination column of table entries with H-flag set which is an exact match to Destination IP in packet • If found and Flag is G or H then Gateway is next hop; otherwise Destination IP is next hop. • If not found then calculate Dest IP && Genmask for each entry that is not the default. If Dest IP && Genmask == Destination column entry then if Flag is G or H then Gateway is next hop; otherwise Destination IP is next hop. • Otherwise use the default entry. Flag is almost always G so Gateway is next hop IP.

  34. IP Routing Next Hop Algorithm: • Once you have the next hop IP you need to determine the next hop ethernet. • The Address Resolution Protocol (ARP) converts the next hop IP into a next hop ethernet. More recently replaced by the ip neigh command • Exercise: Read up on ARP in TCP/IP Illustrated. [pletcha@archimedes PPT]$ ip neigh 137.140.39.139 dev enp0s25 lladdr 00:c0:17:c2:14:f3 STALE 137.140.193.250 dev wlp3s0 lladdr 00:1f:29:07:e4:6a STALE 137.140.39.250 dev enp0s25 lladdr 00:21:a0:39:65:00 DELAY

  35. ARP Example • From my laptop (137.140.8.104) I try to locate joyous (137.140.8.101) • Because of my routing table I know it is locally connected so 137.140.8.101 is “next hop”. [pletcha@archimedes PPT]$ ping 137.140.8.101 PING 137.140.8.101 (137.140.8.101) 56(84) bytes of data. 64 bytes from 137.140.8.101: icmp_seq=1 ttl=64 time=0.266 ms ^C [pletcha@archimedes PPT]$ netstat -nr Kernel IP routing table Destination Gateway Genmask Flags MSS Window irtt Iface 0.0.0.0 137.140.8.250 0.0.0.0 UG 0 0 0 enp0s25 137.140.8.0 0.0.0.0 255.255.255.0 U 0 0 0 enp0s25 137.140.192.0 0.0.0.0 255.255.254.0 U 0 0 0 wlp3s0

  36. ARP Request

  37. ARP Reply

  38. Packet Fragmentation: • The Internet Protocol Suite supports 64k packets but specific IP networks support much smaller packets. • Ethernet networks support 1500 byte packets. • IP headers contain a Don't Fragment (DF) flag, set by sender. • DF not set, then a router can fragment a packet too large to be forwarded on a particular interface. • DF set, router sends an ICMP message to original sender so sender can fragment the original message and try again. • UDP: DF unset by OS • TCP: DF set by OS

  39. Packet Fragmentation (continued): • Each subnet has an MTU – Maximum Transmission Unit. • Path MTU = min hop MTU over all hops in a path • DSL providers make MTU = 1492. • Initially many service providers used MTU = 1500 and disabled ICMP so never knew their “large” traffic was being dropped. • TCP/IP Illustrated discusses how fragmentation actually happens (Read Section 11.5).

  40. TCP/IP Illustrated: • Pages to Look at 25, 38, 43, 44, 48, 58, 61, 63

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