Post DHCP/DNS ->
- Once the IP address is known through the DNS process, an HTTP GET request is prepared at the application layer. This request is then forwarded to the Transport layer.
- There are two protocols (TCP and UDP) that are majorly used at this layer. It is at this layer the requests are encapsulated in form of transport layer packets. If TCP is being used then it also takes care that packet size should not exceed lowest MTU in the path between source and destination. This is done to avoid fragmentation of packet somewhere in the middle of its journey. On the other hand if UDP is being used then this special care is not taken and as a result packets can get fragmented.
- Once the packet is formed at transport layer, it is pushed to the IP layer. This layer adds the information related to source and destination IP addresses and some other important information like TTL (time to live), fragmentation information etc. All this information is required while the packet is on its way to the destination.
- After this the packet enters the data link layer where the information related to MAC addresses is added and then the packet is pushed on to the physical layer. So a stream of 0’s and 1’s is sent out of your NIC onto the physical media.
If the destination of the packet is not directly connected to the source computer then through the routing information present on the source computer, the packet is transmitted to the nearest relevant computer node. There can be various nodes in a network like routers, bridges, gateways etc. Each entity has its own importance like a router is used for forwarding the packet, a bridge is used for connecting networks using same protocol while gateways are used for connecting networks with different protocols.
If we consider a basic network then routers are the main agents which play a vital role in forwarding the packet from source to destination. When the packet first leaves the source computer then the mac address of the relevant router (to which the packet is being transferred) is used as its destination mac address.
When the packet reaches to that router, then the router performs the following action :
- It decreases the TTL value and recomputes the check-sum of the packet.
- The router searches its routing information table for the complete host address as specified by the packet’s destination IP address. If found then router takes action to forward the packet to the relevant host.
- If no such entry is found then the table is searched for the network address derived from the destination IP. If found then router forwards the packet to that particular network.
- If above two checks fail then the packet is transferred to the the default router as derived from the default entry in its routing information table.
In any of the above cases, whenever the packet is transferred by router to some other router or to the destination, the destination mac address of the packet is changed to the immediate router or destination to which it is being sent. In this way the IP address information in the packet remains the same but the destination mac address changes from one router to another. So in this way, the packet travels from one router to another until it reaches the destination.
Now, at the destination:
- The packet is first received at the physical layer which issues an IRQ to the CPU to indicate that some data is arrived and is waiting to be processed.
- After this the data is sent up to the data link layer where MAC layer is checked to see if this packet is indeed for this computer only.
- If the above check is passed then this packet is passed to IP layer where some IP address checks and check-sum verifications are done and then it is passed on to the relevant transport layer protocol.
- Once this is done, then from the knowledge of the ports the information (or the HTTP GET request in our case) is passed on the application listening on that port.
- This way the request reaches the google web server.
After this the response is formed and transmitted back in the same way as described above.
References:
TCP/IP PROTOCOL SUITE
Communications between computers on a network is done through protocol suits. The most widely used and most widely available protocol suite is TCP/IP protocol suite. A protocol suit consists of a layered architecture where each layer depicts some functionality which can be carried out by a protocol. Each layer usually has more than one protocol options to carry out the responsibility that the layer adheres to. TCP/IP is normally considered to be a 4 layer system. The 4 layers are as follows :
- Application layer
- Transport layer
- Network layer
- Data link layer
1. Application layer
This is the top layer of TCP/IP protocol suite. This layer includes applications or processes that use transport layer protocols to deliver the data to destination computers.
At each layer there are certain protocol options to carry out the task designated to that particular layer. So, application layer also has various protocols that applications use to communicate with the second layer, the transport layer. Some of the popular application layer protocols are :
- HTTP (Hypertext transfer protocol)
- FTP (File transfer protocol)
- SMTP (Simple mail transfer protocol)
- SNMP (Simple network management protocol) etc
2. Transport Layer
This layer provides backbone to data flow between two hosts. This layer receives data from the application layer above it. There are many protocols that work at this layer but the two most commonly used protocols at transport layer are TCP and UDP.
TCP is used where a reliable connection is required while UDP is used in case of unreliable connections.
TCP divides the data(coming from the application layer) into proper sized chunks and then passes these chunks onto the network. It acknowledges received packets, waits for the acknowledgments of the packets it sent and sets timeout to resend the packets if acknowledgements are not received in time. The term ‘reliable connection’ is used where it is not desired to loose any information that is being transferred over the network through this connection. So, the protocol used for this type of connection must provide the mechanism to achieve this desired characteristic. For example, while downloading a file, it is not desired to loose any information(bytes) as it may lead to corruption of downloaded content.
UDP provides a comparatively simpler but unreliable service by sending packets from one host to another. UDP does not take any extra measures to ensure that the data sent is received by the target host or not. The term ‘unreliable connection’ are used where loss of some information does not hamper the task being fulfilled through this connection. For example while streaming a video, loss of few bytes of information due to some reason is acceptable as this does not harm the user experience much.
3. Network Layer
This layer is also known as Internet layer. The main purpose of this layer is to organize or handle the movement of data on network. By movement of data, we generally mean routing of data over the network. The main protocol used at this layer is IP. While ICMP(used by popular ‘ping’ command) and IGMP are also used at this layer.
4. Data Link Layer
This layer is also known as network interface layer. This layer normally consists of device drivers in the OS and the network interface card attached to the system. Both the device drivers and the network interface card take care of the communication details with the media being used to transfer the data over the network. In most of the cases, this media is in the form of cables. Some of the famous protocols that are used at this layer include ARP(Address resolution protocol), PPP(Point to point protocol) etc.
TCP/IP CONCEPT EXAMPLE
One thing which is worth taking note is that the interaction between two computers over the network through TCP/IP protocol suite takes place in the form of a client server architecture.
Client requests for a service while the server processes the request for client.
Now, since we have discussed the underlying layers which help that data flow from host to target over a network. Lets take a very simple example to make the concept more clear.
Consider the data flow when you open a website.
As seen in the above figure, the information flows downward through each layer on the host machine. At the first layer, since http protocol is being used, so an HTTP request is formed and sent to the transport layer.
Here the protocol TCP assigns some more information(like sequence number, source port number, destination port number etc) to the data coming from upper layer so that the communication remains reliable i.e, a track of sent data and received data could be maintained.
At the next lower layer, IP adds its own information over the data coming from transport layer. This information would help in packet travelling over the network. Lastly, the data link layer makes sure that the data transfer to/from the physical media is done properly. Here again the communication done at the data link layer can be reliable or unreliable.
This information travels on the physical media (like Ethernet) and reaches the target machine.
Now, at the target machine (which in our case is the machine at which the website is hosted) the same series of interactions happen, but in reverse order.
The packet is first received at the data link layer. At this layer the information (that was stuffed by the data link layer protocol of the host machine) is read and rest of the data is passed to the upper layer.
Similarly at the Network layer, the information set by the Network layer protocol of host machine is read and rest of the information is passed on the next upper layer. Same happens at the transport layer and finally the HTTP request sent by the host application(your browser) is received by the target application(Website server).
One would wonder what happens when information particular to each layer is read by the corresponding protocols at target machine or why is it required? Well, lets understand this by an example of TCP protocol present at transport layer. At the host machine this protocol adds information like sequence number to each packet sent by this layer.
At the target machine, when packet reaches at this layer, the TCP at this layer makes note of the sequence number of the packet and sends an acknowledgement (which is received seq number + 1).
Now, if the host TCP does not receive the acknowledgement within some specified time, it re sends the same packet. So this way TCP makes sure that no packet gets lost. So we see that protocol at every layer reads the information set by its counterpart to achieve the functionality of the layer it represents.
PORTS, SERVERS AND STANDARDS
On a particular machine, a port number coupled with the IP address of the machine is known as a socket. A combination of IP and port on both client and server is known as four tuple. This four tuple uniquely identifies a connection. In this section we will discuss how port numbers are chosen.
You already know that some of the very common services like FTP, telnet etc run on well known port numbers. While FTP server runs on port 21, Telent server runs on port 23. So, we see that some standard services that are provided by any implementation of TCP/IP have some standard ports on which they run. These standard port numbers are generally chosen from 1 to 1023. The well known ports are managed by Internet Assigned Numbers Authority(IANA).
While most standard servers (that are provided by the implementation of TCP/IP suite) run on standard port numbers, clients do not require any standard port to run on.
Client port numbers are known as ephemeral ports. By ephemeral we mean short lived. This is because a client may connect to server, do its work and then disconnect. So we used the term ‘short lived’ and hence no standard ports are required for them.
Also, since clients need to know the port numbers of the servers to connect to them, so most standard servers run on standard port numbers.
The ports reserved for clients generally range from 1024 to 5000. Port number higher than 5000 are reserved for those servers which are not standard or well known.
If we look at the file ‘/etc/services’, you will find most of the standard servers and the port on which they run.
$ cat /etc/services systat 11/tcp users daytime 13/udp netstat 15/tcp qotd 17/tcp quote msp 18/udp chargen 19/udp ttytst source ftp-data 20/tcp ftp 21/tcp ssh 22/tcp ssh 22/udp telnet 23/tcp ... ... ...
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