Internet Basics

1. Internet Basics (How your email gets to you) Richard G. Clegg

2. What we will cover • The "layers model" of the internet • Internet addresses • Internet protocols • The basics of rout(e)ing • The journey of email

3. For more information • Tanenbaum (Computer Networks) • Stevens (TCP/IP illustrated) • RFCs (requests for comments) • IETF (Internet Engineering Task Force)

4. The Internet - emailing a friend your computer university of york G/169 router JANET transatlantic cable LAN your friend's computer US backbone

5. Models of the Internet TCP/IP Reference Model OSI/ISO reference model Application Transport Internet Host-to-network Application Presentation Session Transport Network Data Link Physical Model Layers open systems interconnection (international standards office) transmission control protocol/internet protocol

6. Physical layer • Think "wires in the ground". • This is the physical hardware of the internet. • Wires/optical cables/wireless links and other technologies provide a way for transmission of raw bits (0s and 1s). • Routers and switches connect these cables and direct the traffic.

7. Data link layer • Groups raw bits into packets of data. • Basic error checking for lost data. • In TCP/IP the "Physical layer" and the "Data Link" layer are grouped together and called the host-to-network layer.

8. Network Layer/Internet Layer • Tells data which link to travel down. • Addresses the problem known as routing. • Deals with the question "where do I go next to get to my destination?" • Ensures packets get from source A to destination B.

9. Transport Layer • Accepts data – splits it into packets. • Ensures a connection between source and destination. • If necessary ensure that connection is lossless (resend missing data). • Provides flow control if necessary (send data faster or slower depending on the network conditions).

10. Session Layer (not TCP/IP) • Provides a single connection for one application. • This connection may be two way or may be synchronised. Presentation Layer (not TCP/IP) Provides extra functions which are used quite often. E.g. exchange of character strings.

11. Application layer • The computer programs which actually do things with the network. • For example, your email program which will talk to the email server at the other end. • At this layer, we have many protocols (http, snmp, smtp, ftp, telnet) which different bits of software use. • We often talk in terms of client and server architecture for the software.

12. TCP/IP model in summary

13. Internet (IP) addresses richard@manor.york.ac.uk (email) http://www.apoptygma.eu.org (www) ftp://ftp.uk.debian.org (file transfer) telnet://towel.blinkenlights.nl (telnet) 144.32.108.74 148.122.211.110 195.224.53.39 62.250.7.101 These are the “real” IP addresses of the above sites. IP addresses are 32 bits grouped into 4 octets. (Octet = 8 bits – a number from 0-255)

14. IP Networks(1) • IP addresses use less significant bits first to indicate sub-networks. • IP address: 123.45.67.89 • Netmask:255.255.255.0 • If two IP addresses are the same when bitwise OR’d against the inverse (bitwise NOT) of the netmask then they are on the same subnet • 123.45.67.?? is always on the same subnet in the above example.

15. IP Networks(2) • IP networks were originally subdivided into class A, B, C, D and E networks.

16. The IP header • IP packets all have a header as shown

17. About the IP header • Type of Service: (Best efforts, immediate delivery etc) • Total length (of whole packet) • Identification (number of packet for later reassembly) • Fragment offset – sometimes the network splits a packet into fragments. • Flags (information about fragments). DF= Dont Fragment MF= More Fragments to come

18. About the IP header (2) • Time To Live (TTL) – reduced by one every hop. When it reaches zero packet is killed. (This is to ensure that the network doesn’t fill up with lost packets). • Protocol – identified by a number (usually TCP or UDP). • Checksum – to ensure that the packet is not corrupted.

19. IPv6 • IPv4 allows over 4 billion computers (but not really) – inefficient subnetting is using these up. • IPv6 allows 16 octet addresses (4 octets in IPv4). • 3x1038 addresses (> Avogadro’s number). • 7x1023 IP addresses per square meter of the earth’s surface. • Why so many? Electrical devices may want IP addresses – your house could be its own subnetwork. Why NOT have so many?

20. IPv6(2) • Better security than current IP(v4). • Allow “roaming hosts”. • Permits the old and new protocols to co-exist. • Pay more attention to type of service (for real time data).

21. Domain Name System (DNS) • DNS takes the human readable name and converts it to octets. • On a unix machine you can try this using nslookup. manor.york.ac.uk 1% nslookup www.ntk.net Server: castle2.york.ac.uk Address: 144.32.128.5 Non-authoritative answer: Name: vwww.flirble.org Address: 195.40.6.34 Aliases: www.ntk.net Question Answer

22. DNS(2) generic/US national TLDs (Top Level Domains) net com org gov mil jp uk nl sun ac vnvnation co org eng www york musicnonstop ic manor www doc www src

23. DNS (3)

24. Routing Tables • How do packets know where to go? • This problem is known as routing. • The oldest (and easiest) solution is static routing. • Each computer has a table saying where to go to get to each other computer. • On a Local Area Network (LAN) list all machines on your subnet and the address of the external router for everything else. • Most machines only need to know how to get to their nearest router.

25. Dynamic Routing • Distance Vector Routing (Bellman-Ford) • Each router stores a distance metric to various network locations. • Routers exchange routing packets periodically to update their route information (routing table). • Each router measures how far their neighbours are away and learns how far the neighbours are from various destinations.

26. Dynamic Routing (2) A knows that it is 2 “units” from B and 4 from C. It also knows that C takes 4 “units” to get to D and B takes 5. Therefore, to get packets to D, A will first send them to B. 2 A B 1 4 C 4 D If things change, then the routing tables are updated. For example, if congestion increases the B to C cost to 3 then B will tell A the new “price” to get to D is 7. A will send things directly to C instead of via B. If the link A-C breaks A will send via B again.

27. Routing problems 1 • The count to infinity problem. • All links cost 1. • Link C-D breaks. • What happens? A B 1 1 C 1 D Assumed Cost to D * split horizon hack

28. Open Shortest Path First (OSPF) • Common internet routing algorithm. • Uses three metrics, throughput, delay and reliability. • Allows some “load balancing”. • Accounts for the fact that routers cannot know the entire internet. • Works within an “Autonomous System” (AS) assumed to be run by the same organisation. • Border Gateway Protocol (BGP) connects these (I will not describe BGP here).

29. TCP and UDP • Once we’ve got our IP packet safely to its destination what happens next? • Having stripped off the header, the first thing we find is another header. • The second header provides information on which port to enter the machine on and where to send the reply. • It also provides a checksum to check the data is valid. • UDP will do nothing else. TCP will ensure that the connection is lossless.

30. What are ports? • Ports are conceptual “points of entry” into a host computer. • They do not correspond with real hardware. • Usually a service is associated with a port (e.g. http on port 80). • Servers “listen on a port” for connection attempts. • Ports provide one level of internet security. • Generally, low level ports are reserved for special services.

31. Common Services and Ports Service Listens on Port ftp 21 telnet 23 smtp (mail) 25 finger 79 http 80 User configured services (your Half-Life server?) will listen on high numbered ports which are usually left open to all users.

32. UDP data • User Datagram Protocol – the header is shown below. • Length and checksum are as for IP.

33. About UDP • Provides a lossy connection (data may vanish). • Does not guarantee packets are delivered in order. • Useful for real time applications. (It is no use having your Quake III information arriving correctly but ten seconds late). • UDP applications can implement their own packet loss checking but it is best to use TCP for this.

34. The TCP header • The TCP header is shown below

35. About the TCP header • Sequence number (what is the “order” of this packet) incremented by 1 for every packet. • Acknowledgement number (what packet sequence number does this acknowledge). • Header length (how many 32 bit words are in options). • Flags: SYN = start connection, ACK = acknowledge packet, FIN= finish connection. • (Three other flags, URG, RST, PSH).

36. TCP header (2) • Window size will be described in more detail later (it sets how many unacknowledged packets may exist). • Checksum – is as for IP and UDP. • Urgent Pointer – points to part of the data that must be looked at by the receiver before the TCP session (rarely used).

37. About TCP • TCP provides a lossless connection (or flags an error when losses occur). • Data packets are given an order and can be reassembled. • TCP provides some limited congestion control. • TCP is most useful for applications where data validity is important but real-time is not critical (email, www, ftp). • TCP packets are part of a TCP session.

38. TCP connections • This diagram shows the start of a TCP connection. A sends packet X with SYN. “Hello I would like to talk”. B sends a SYN, ACK pair “I got your message. I would also like to talk” A sends an ACK (and some data) “I got your message, here is some data.”

39. TCP mechanisms • The window size is the number of outstanding (unacknowledged) packets that that a TCP session can send. • The window size provides a crude method for congestion control. • The window size increases to allow more packets to be sent (it increases throughput). • If a packet is lost then the window is reduced again.

40. TCP lost packets • When a packet is received out of sequence the receiver sends an ACK with the same number as the previous. • If the sender receives three duplicate ACKs then it assumes the packet has been lost and resends. • If the sender has not received an ACK for a packet within a certain amount of time then it times out and assumes the packet lost. • Packet loss causes the packet to be resent and the congestion window to be reduced.

41. TCP Window Increase/Decrease Congestion window The initial doubling of the window size is called “slow start”. Timeout Threshold Threshold Transmission no

42. ICMP • Internet Control Message Protocol packets are used for various control purposes. Here are some common ones: • Time exceeded: TTL hit 0. • Echo request: Can you hear me out there? • Echo reply: Yes I can hear you. • Source Quench: Stop sending so much data. • Timestamp request/reply (as echo but with times).

43. The story of ping • Ping is a handy utility for checking if a computer is alive using ICMP echo request/reply (or timestamp if we want). • Ping is a first test if a computer is networked. • We can even measure the speed of light using ping. http://xxx.lanl.gov/abs/physics/0201053 • Hacking makes it increasingly unused. manor.york.ac.uk 20% ping -s castle.york.ac.uk PING castle2.york.ac.uk: 56 data bytes 64 bytes from castle2.york.ac.uk (144.32.128.5): icmp_seq=0. time=1. ms 64 bytes from castle2.york.ac.uk (144.32.128.5): icmp_seq=1. time=1. ms 64 bytes from castle2.york.ac.uk (144.32.128.5): icmp_seq=2. time=1. ms 64 bytes from castle2.york.ac.uk (144.32.128.5): icmp_seq=3. time=1. ms

44. Traceroute • Traceroute neatly combines ping and the TTL flag to get a “route” to a computer. • If the TTL is one the the packet will “die” after one hop. • ICMP will return a Time exceeded flag. • This will tell us where the first “hop” of our journey is. • Increase the TTL by one to find the next “hop”.

45. ICMP tourism (with traceroute) This shows the trip from Estonia to my flat in Fulford via my Internet Service Provider (ISP) – V21 in Rochdale traceroute to host213-121-67-224: (213.121.67.224): 2-20 hops, 38 byte packets 2 213.180.11.162 tondi-CR.online.ee 1.62 ms (ttl=127) 3 213.180.25.1 liiva-CR.online.ee 1.82 ms (ttl=126) 4 213.180.11.189 tix-CR.online.ee 2.16 ms (ttl=125) 5 212.47.215.6 r1-Fa4-0-80-Tln-TIX.EE.KPNQwest.net 2.28 ms (ttl=251) 6 134.222.224.5 r5-AT3-1.105.sthm-KPN1.SE.kpnqwest.net 12.2 ms (ttl=250) 7 134.222.119.226 r2-Ge0-2-0-0.Sthm-KQ1.SE.KPNQwest.net 34.3 ms (ttl=246!) 8 134.222.230.157 r2-Se0-3-0.hmbg-KQ2.DE.KPNQwest.net 33.4 ms (ttl=247!) 9 134.222.230.117 r2-Se0-2-0.0.ffm-KQ1.DE.kpnqwest.net 34.1 ms (ttl=249!) 10 134.222.230.29 r2-Se0-3-0.0.ledn-KQ1.NL.kpnqwest.net 39.6 ms (ttl=248!) 11 134.222.230.169 r1-Se0-0-0.0.ldn-KQ1.UK.kpnqwest.net 43.7 ms (ttl=246!) 12 134.222.231.14 r1-Se0-0-0.0.Ldn-KQ4.UK.KPNQwest.net 44.9 ms (ttl=245!) 13 134.222.109.241 r13-Gi5-0.200.ldn-KQ4.UK.kpnqwest.net 45.4 ms (ttl=245!) 14 195.66.225.10 linx-l1.ukcore.bt.net 45.2 ms (ttl=244!) 15 194.74.65.126 core2-pos14-0.ilford.ukcore.bt.net 45.3 ms (ttl=243!) 16 194.74.65.222 core2-pos5-0.reading.ukcore.bt.net 46.7 ms (ttl=242!) 17 62.6.196.109 core2-pos8-0.birmingham.ukcore.bt.net 54.3 ms (ttl=241!) 18 194.74.16.194 core2-pos9-0.rochdale.ukcore.bt.net 51.0 ms (ttl=240!) 19 217.32.168.5 vhsaccess1-gig1-0.rochdale.fixed.bt.net 51.1 ms (ttl=239!) 20 213.121.156.22 ugint0066-p.vhsaccess1.rochdale.fixed-nte.bt.net 51.3 ms (ttl=238!)

46. The journey of email SYN SYN,ACK ACK Set up the TCP connection Look up IP name for distant.com Dav e, Gr eat to s To: dave@distant.com From: richard@manor Dave, Great to see you the other day... Packetise the data Dav Dav Get first hop from routing table Add IP header to front of that Add TCP header to first packet Send the first packet to its first hop Destination gets packet and returns ACK And so on for further hops. Start sending rest of data

47. How can we model this? • As mathematicians we want to be able to say something about these systems. • How can we apply what we have learned in this and other courses to the internet? • In a future lecture I hope to outline some open research questions about the net and show how mathematics can help solve some of these problems.