1. 30-1 Computer Networking

2. 30-2 The Internet Hourglass Model The physical layer is how machines are physically connected to each other... FTP HTTPNVTFTP TCPUDP IP Token Ring Wi-fi (802.11 ) Ethern et InterNetwor k Transport Applicatio n Physical

3. 30-3 The inter-network layer The inter-network layer is what allows all machines on the Internet to communicate with each other –(Notice that many different protocols exist above/below IP, but they all converge to IP – this is how all computers can communicate over the Internet!) FTP HTTPNVTFTP TCPUDP IP Token Ring Wi-fi (802.11 ) Etherne t InterNetwor k Transport Application Physical

4. 30-4 IP Addresses Internet Addressing is done via IP Addresses. Like a phone number, it’s your computer’s unique hierarchical number on the Internet The IP Address is structured (like a phone number) : –example: 128.196.3.21 –Network = 128.196.3 –Host = 21 Network indicates the network (LAN) of this address Host indicates which specific node on that network –IP Address assignments are not Geographic –Each network gets a block of addresses based on assignment by InterNIC You get an IP address from your Network Provider

5. 30-5 Packets (like envelopes) The Internet is referred to as a packet-switching network. –Packet: A logical block of information consisting of a header part and a payload part for transmitting the payload part over a network: –(just like Ethernet frames) Packets exist at all layers of the network hierarchy –We’ve already seen Ethernet packets The contents of the header and the payload are dictated by the protocol Each packet has a header containing information regarding content, where it came from, where it is supposed to go –Depending on the specific protocol, there will be more information in the packet header. –Let’s look at an IP packet… (remember, when these are sent over an Ethernet, they will be the contents of the “payload” part of an Ethernet frame)

6. 30-6 Sending data across networks As the packet travels through the Internet from network to network: –Each packet may not travel through the same path through the Internet to its destination. Routes can change through-out the same transmission Will packets always arrive in order? –TTL (Time to Live) and forwarding: The number is originally set to something large (64). Each time it is received by a router, it decrements by 1. If the TTL number becomes 0 before reaching its destination: –The router throws away the packet and the router where this happened sends back an error message (time exceeded) with the address of the router. –Why throw away packets? –IP is a best-effort service – packets may get lost!

7. 30-7 IP Addresses Do I always get the same IP address? –Not necessarily... DHCP (dynamic host configuration protocol) allows you to get a different IP address every time you connect to the network –The network part is the same, but the host address changes –Example: »Let’s imagine I use Verizon dial-up. »Verizon has a fixed number IP addresses that it could assign to me (all of them are in the same network space). »I’m not guaranteed to get the same IP address every time I connect … maybe they’ve over sold their capabilities Clearly web servers and such need the same IP address all the time, so that people can know where there are. –Having the same IP address every time you connect is called a “static IP address” – static; e.g. it doesn’t change. –When it changes, it is called a Dynamic IP Address. –So, if you want to call a specific computer, you need to know the specific number, right? Yes. –Do I need to remember the number of every computer I want to contact?

8. 30-8 IP Addresses and DNS Mapping IP Addresses to names. –Humans aren’t good at memorizing numbers, we fair better with names. –Like the phone book, DNS (domain name system) associates hostnames with IP addresses www.google.com  216.239.37.99www.google.com So we only need to remember www.google.com, and DNS will translate the name into a number.www.google.com Like IP Addresses, hostnames have a network part and a host part –Unlike IP Addresses, in DNS, the second part is the Network Part (domain), the first part is the host part. –www.google.comwww.google.com google.com = domain www = host part –virgil.bu.edu bu.edu = domain virgil = host part

9. 30-9 Hostnames The right-most portion of the domain is the top-level domain. –There are two types of top-level domains: Organizational –.edu – educational –.com – commercial –.mil - military –.net - network –.org - organization –.gov - government Geographic –.uk –.fr –.usa

10. 30-10 DNS When nodes have the same domain, they are in the same network: –virgil.bu.edu –mail.bu.edu –csa.bu.edu Hostnames tell us a lot; but we still need the IP address of a node DNS is a network program that will give us the IP address of a specific hostname. Imagine looking for the IP address of virgil.bu.edu. We ask the closest domain name server “do you know the IP address of virgil.bu.edu?” Could every DNS server know every machine in every domain? How do we approach this problem?

11. 30-11 DNS Imagine looking for virgil.bu.edu. We ask the closest domain name server “do you know the IP address of virgil.bu.edu?” I don’t know Virgil, but I do know someone who knows about.bu.edu … let me ask that server. (That server is called a resolver) The request gets passed along to the bu nameserver That machine responds with the IP Address of Virgil: (128.197.56.79) With the IP Address, I can get data to you.

12. 30-12 Physical Layer concerns What happens if a frame gets corrupted some other way? –Suppose I hold a magnet over the wire that your transmission is going over, in an attempt to flip your bits. –If the data is not the same frame the sender intended to send, it’s not very useful to the recipiant This is not a collision so the NIC doesn’t auto-detect this –We need a simple way of letting the receiver check if this frame is intact How can a recipient tell if a frame has errors from the frame it has received? –CheckSum

13. 30-13 Checksum –We need a simple way of letting the receiver check if this frame is intact How can a recipient tell if a frame has errors from the frame it has received? CheckSum construction algorithm: –Sender: Add all the bits in the frame, which will yield another string of bits (we can calculate this quickly) CheckSum/Packet verification algorithm –Receiver: Add all the bits in the packet and compare to the checksum in the packet. »Match? Packet is good. »No? Throw the packet away. –Can we fix flipped bits using a checksum?

14. 30-14 The inter-network layer FTP HTTPNVTFTP TCPUDP IP Token Ring Wi-fi (802.11 ) Ethern et InterNetwork Transport Application Physical

15. 30-15 Motivating the Transport Layer The Physical layer allows us to move bits between two computers The Internet Protocol (IP) builds on top of this, and enables data to travel across networks on the Internet However there is a lot of things that IP doesn’t do... –What order are the packets coming in? –As a sender, are there other features we might like? Knowledge of whether a packet actually made it to the target Other communication about the target’s ability to receive more data Concerns about not flooding (over-utilizing) the network... –Yet, would every network application need all of the above? Does it make sense to “force” this functionality on all network communications? What would be the apparent cost of adding such features? –More bits = more data to send = time

16. 30-16 Motivating the Transport Layer Web Browser File Transfer Instant Messenger IP Packe t IP allows us move packets from one computer on the Internet to any other computer on the Internet. What if multiple programs want to use the network ? IP Packe t Which packets go to which application !?! Packe t

17. 30-17 Motivating the Transport Layer Web Browser File Transfer Instant Messenger IP Packe t IP allows us move packets from one computer on the Internet to any other computer on the Internet. What if multiple programs want to use the network ? IP Packe t Which packets go to which application !?! Packe t

18. 30-18 Transport The transport layer provides “end-to-end” connectivity –allows transfer of packets from one specific application on one computer another specific application on another computer. –provides each application on the connected computer the shared use of the network through a unique interface (ports) In this context, ports are conceptual. All network traffic is coming to the machine in the same way, but the separation into ports allows different applications to share the network. The purpose of ports and the transport layer: –Each application only cares about its own network traffic, and should only receive that traffic Additionally “some” transport-layer protocols provide features to: –Ensure messages are “arriving” in order –Make sure that all messages arrive at the receiver –and other legitimately useful features

19. 30-19 Transport Layer How bytes move from one application on a computer another application on another computer. FTP HTTPNVTFTP TCPUDP IP Token Ring wi-fi (802.11 ) Ethern et InterNetwork Transport Application Physical

20. 30-20 Transport Layer Web Browser File Transfer Instant Messenger Pack et for port 6734 Through the use of ports, packets can be addressed to an individual port on a given machine That port will be associated with a specific program So you can send a packet not only to a specific computer, but to a specific program! Pack et for port 6736 Pack et for port 6734 Transport Layer pack et buffe r at port 6734 pack et buffe r at port 6735 pack et buffe r at port 6736