What is the Internet made of?
Computers connected by links via switches / routers. Multiple links terminate at a switch/router, which enables connectivity between them.
Many switches/routers are present in an organization like IITB to connect all the end-hosts. End-hosts provide and consume useful services (e.g., web, email).
Internet Service providers (ISPs) connect multiple such end-user organizations. ISP networks have several routers that provide interconnections between end organizations, and form the “backbone” of the Internet.
Notion of IP addresses: every machine on the Internet has a unique address called the IP address (well, almost…). Assigned statically or dynamically (e.g., DHCP). Can be public or private (e.g., through a NAT). IP addresses have a hierarchical structure. Machines also have human-readable names that map to IP addresses.
Notion of a packet: unit of communication between machines.
Example: what happens when you open a web page from a computer by typing in a URL?
At the browser: The URL gets resolved into an IP address of the web server that hosts this page. The browser sends a special message (GET request) to the server, and the server sends back the HTML content. The browser then displays the page to the user.
At the end hosts: Browser opens a “TCP connection” to the web server. Messages between the browser and server are sent over this connection. TCP (transport control protocol) is a transport mechanism that reliably sends information back and forth between two end points. TCP modules (running in the end host operating systems, typically) make sure that messages are delivered reliably.
How are the TCP messages sent back and forth? The TCP data to the server must be routed to the server’s IP address via various “routers” running IP (internet protocol). Your computer sends the packet to an IP router or default gateway that sends it to another router, which sends it to another router, and so on. The IP routers “talk” amongst themselves and so they know how to get to any destination IP address. Thus, several routers along the way forward these TCP segments from one IP hop to the next based on the destination server’s IP address. At the final IP hop, the IP router next to the wbe server hands the packet to the web server over the LAN.
Several TCP segments and ACKs go back and forth this way to transfer the complete web page.
The above example illustrates the concept of layering. There are several functions in the networking “stack”, each handled by a layer. A layer at one host talks to its “peer” layer at the other host using a set of “protocols” or conventions that they both follow. Every layer provides a “service” or “abstraction” that the higher layer uses. For example, the application layer at client says GET a page and the application layer at server sends the page. This is the HTTP protocol. HTTP protocol uses the transport “service” provided by the TCP layer. That is, once it hands over the GET message to the TCP layer, the TCP layer takes care of actually transporting the bits in the message to the other end.
Why layering? Modularity. Each layer can evolve independently. Easier to build large networks over differing technologies.
There are 5/7 layers depending on which system you look (OSI vs Internet). The 5 layers are the most important ones: from top to bottom they are: application, transport, network, link, and physical.
Encapsulation and decpasulation. Every layer adds headers to the bytes it gets from the layer above. The headers are useful for operations of that layer. These headers are consumed and removed by the peer at the other end.
Where are these layers implemented? Application and transport layers run on end hosts - user programs, operating systems etc. IP routing protocols typically run in specialized routers. Link layer and physical layer run in network hardware (e.g., ethernet card) and accompanying device hardware.
Application layer: just concerned with the application-level semantics. GET a web page and display it. Browser issues requests and displays result. Web server holds all the pages on its disk/memory (or computes them dynamically), and serves them when a request arrives. Several application layer protocols. HTTP is the most popular one, but several others exist (SMTP for email), P2P protocols etc.
The transport layer (usually TCP, but sometimes UDP/RTP etc) is concerned with the transfer of the bits and bytes that make up the application data. This layer takes the application layer “messages”, makes them into TCP “segments”, transmits them at a suitable pace that the network can handle, makes sure they are received and acknowledged by the other end host, retransmits them when needed. TCP does not care about what the data is (that’s the job of the application layer), or how the data gets there (thats the job of the lower layers). That is, TCP provides a reliable in-order data stream abstraction. TCP performs congestion/flow control to ensure that it sends data at an optimum rate that the underlying network can handle (not too slow, not too fast). TCP is the most common, but UDP is used when reliability is not needed. RTP is used for real time services like VOIP applications. There are several variants of TCP also.
The application and transport layers are end-to-end, run at only the end hosts (in operating systems and user programs), they don’t care what the network does in between.
Next is the network or IP layer or Layer 3. TCP segments are converted to “IP datagrams”. The datagram has a source and destination IP address. The job of the network layer is to route the datagram to its destination IP address. Each IP router along the way looks up the destination address and decides which link it should forward the packet next.
Do IP routers have separate entries for every IP address? No. IP addresses have hierarchical structure. An aggregate of several IP addresses is called an IP prefix. Every IP router has several incoming links/ports (port is physical attachment point for an incoming/outgoing link) and several outgoing links/ports. For every incoming packet, IP router looks up a “forwarding table” and decides which port to send packet on. A forwarding table maps IP prefixes to next hop IP addresses (and hence to outgoing port that connects to the next hop IP router).
Forwarding tables are computed by routing protocols. Routing protocols help compute “routes” which decide how to forward a packet. A route is a specification of the path to reach a certain IP prefix. Minimally, for every IP prefix, its route tells us the next hop IP router on the path and the distance/metric to get to the destination along this path. Optionally, it can provide more details about the path. For every IP prefix, routing protocols gather information about several routes, pick the best one by some metric (e.g., shortest path), and insert this best route into the forwarding table. A routing table is this a superset of the forwarding table.
Intradomain routing (within an organization) and interdomain/wide-area routing (across the internet) are two different things. The former just deals with shortest paths while the latter has to deal with policy as well (i.e., the guy on the shortest path may not carry your data if he has no financial relationship with you). So separate routing protocols for intra and interdomain routing. We will study this in detail later.
Next is link layer (layer 2). Deals with delivering a link layer “frame” over an IP hop or “link”, for example, through a shared LAN or a switch or a point-to-point fiber link or a wireless link. Finally, the physical layer deals with transmitting the actual bits over a physical medium like a copper wire, optical fiber, or wireless radio. We won’t deal much with physical layer in this course.
Note that several link layer technologies can be connected together via IP. A host connected to an Ethernet network can communicate with another host connected to a wireless network, as long as they both speak IP and are connected by a path of IP routers. That is, the links between any two IP hops can be very different. Thus the Internet has grown as a network of networks due to the unifying IP protocol.