Journey of a Packet in Internet

Introduction

Understanding how packets travel across networks is fundamental to understanding modern networking. This post explores the journey of a data packet from its creation to its destination.

Overview of a packet's journey through the internet

Packet Creation and Encapsulation

Application creates a packet to be forwarded on the network and passes it on to lower layers following the OSI model:

Transport Layer

The second is called transport layer that is responsible for end-to-end communication between two computers. The protocol that can be used is TCP or UDP. How these two protocols differ is a discussion in itself.

Network Layer

Once a packet is generated at the transport layer, it gets passed on to the network layer having source and destination IP within the packet. Most critical field added in IP or network layer is Time To Live (TTL) which is used by intermediate switches/routers to decide if packet has to be forwarded or not.

Data Link Layer

Packet is moved to data link or MAC layer after network layer, where destination and source MAC address of machines are appended. We shall observe how these fields are modified after each two neighbors.

Physical Layer

Data link layer sends this packet to physical layer and it is delivered as stream of "0" and "1" over available physical medium.

Packet encapsulation through the network layers

Processing a Packet at Router

Now packet has reached an intermediate router that lies in between source and destination. Router accepts the packet and does three basic operations:

Routing

Whenever router receives a packet, it strips out first of all MAC layer header and looks at the IP header containing destination IP address. Upon receiving destination IP, router glances into its database to identify where it should forward this packet so that packet reaches destination. These databases are referred to as routing tables.

There are three possibilities that can occur when router checks routing table for destination IP:

Router performing routing table lookup

Forwarding

After the completion of routing process, packet is forwarded from ingress interface to egress interface, which is typically referred to as forwarding. Process switching, fast switching and CEF switching are three methods of forwarding.

Before the third step, router decrements the TTL and re-computes checksum of packet and inserts it back.

Encapsulation

Third process is encapsulation. Please note that L3 or layer 3 or network layer destination IP address never ever changes in the journey of IP packet, except in cases like NAT or VPN.

Only one thing that does change is source and destination MAC addresses at data link layer.

Router keeps next hops' MAC address, which it needs to send the packet to, in it, it replaces destination and source MAC address in it and send it to physical layer.

Packet transformation between ingress and egress interfaces

Processing Packet at Destination Host

When the packet finally reaches its destination, these steps occur:

Conclusion

This journey of a packet from source to destination showcases the elegance of layered network architecture. Each layer serves a specific purpose, adding necessary information for successful delivery of data across networks..',