Skip to main content

Data Link Layer

Came across this question in Quora https://www.quora.com/TCP-IP/What-is-the-main-purpose-of-Data-link-layer-in-TCP-IP-or-OSI-layer.
So what DLL layer does when I know the ip and port from ip and tcp layer?
Actually ip layer just specifies the destination ip. Say if I need to route a packet from a network A to C via network B. A ip address of host in network C will be in the destination ip field of IP header.
The ip layer in the host finds the route from the routing table(use route -n to see the available routes)  and sends a ARP request to find the mac address of the host matching the route, then send the packet to the host by placing its mac address in segment header. Now the packet reaches the router of network A. Now the router finds the route for the host in network C from its routing table and place that mac address and send the packet to a router in B network which does the same to push the packet to router in network in C and finally to the destination host.
Routes can have ip only within the same subnet becoz only then arp can send broadcast messages within the subnet and find mac address or else it will be a infinite process to find routes to the ip of the routes specified(Try adding routes to different subnets using route add -n)
So in general ip address is used only to find the routes and its the mac address that actually transmits the packet. The destination ip does not change during transit.

Comments

Post a Comment

Popular posts from this blog

How we have systematically improved the roads our packets travel to help data imports and exports flourish

This blog post is an account of how we have toiled over the years to improve the throughput of our interDC tunnels. I joined this company around 2012. We were scaling aggressively then. We quickly expanded to 4 DCs with a mixture of AWS and colocation. Our primary DC is connected to all these new DCs via IPSEC tunnels established from SRX. The SRX model we had, had an IPSEC throughput of 350Mbps. Around December 2015 we saturated the SRX. Buying SRX was an option on the table. Buying one with 2Gbps throughput would have cut the story short. The tech team didn't see it happening. I don't have an answer to the question, "Is it worth spending time in solving a problem if a solution is already available out of box?" This project helped us in improving our critical thinking and in experiencing the theoretical network fundamentals on live traffic, but also caused us quite a bit of fatigue due to management overhead. Cutting short the philosophy, lets jump to the story. ...
8 comments
Read more

The FB outage

 This outage has caused considerable noise everywhere. It was quite discomforting for me because during the whole conversation nobody bothered to understand the gravity of the issue. I don't expect end users to understand the issue. But this is going to be a blogpost for all of those in the tech field, Such an event can happen how much ever chaos engineering, best of the tech jargon we implement in the stack To all my Site Reliability Engineer friends, Site Up is our first priority. I myself said many a times outage is news and SREs should prevent outage. But I'm afraid this is leading to a cult in the industry who despises outages and takes no learnings from it. I don't know what has happened in Facebook. I can explain a scenario which may or may not be right but that can definitely show the gravity of the issue. Let's draw a probable Facebook architecture Disclaimer I don't work at Facebook. So this might not be how facebook routes traffic. This is based on my exp...
2 comments
Read more

Ptrace

Ptrace is a nice setup ( some people call dirty setup) on linux to debug running processes. This ptrace in sys/ptrace.h is used by strace and gdb. To trace a child process, the child process should call PTRACE_TRACEME. The kernel during each system call(or execution of each instruction) checks if the process is traced. If it is traced, it issues a SIGTRAP, the parent process if in wait() state, will get a signal. The parent issues a SIGSTOP to hold current state of child and can access the registers and memory of child using PEEKDATA and alter the values in register and memory using POKEDATA. Once the required job is done, parent will allow the child to run with a SIGCONT signal. Since one can access registers, the next instruction to be executed can be easily found using instruction pointer, this comes in handy when we need to set breakpoints while debugging. The entire code base can also be changed using ptrace. PTRACE_ATTACH attaches a running process. It does some hack to become...
1 comment
Read more