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

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.

More on Memory

 A post almost after 2 years!!! One common question I get asked is, "what is the reference I follow for troubleshooting an issue at hand". I would not be able to give an answer to the question directly as most of the times, I won't have even a single reference material handy. It's not a self boasting article. It's an article describing how knowledge we gather at random places help during an issue. Let's dissect a memory usage issue in Linux I faced recently and see how the triage shaped up. One of our processes was getting repeated ENOMEM when it was trying to call malloc for some reason despite the box had plenty of unused RAM. Lets see how the triage went through I didn't understand in my Operating systems course what a virtual memory is. I did convincing myself that virtual memory is physical memory + swap(in a way correct but not completely) I attended an interview in 2013, where the Director of the division asked me when you do malloc do you get physi

Lessons from Memory

Started debugging an issue where Linux started calling OOM reaper despite tons of memory is used as Linux cached pages. My assumption was if there is a memory pressure, cache should shrink and leave way for the application to use. This is the documented and expected behavior. OOM reaper is called when few number of times page allocation has failed consequently. If for example mysql wants to grow its buffer and it asks for a page allocation and if the page allocation fails repeatedly, kernel invokes oom reaper. OOM reaper won't move out pages, it sleeps for some time and sees if kswapd or a program has freed up caches/application pages. If not it will start doing the dirty job of killing applications and freeing up memory. In our mysql setup, mysql is the application using most of the Used Memory, so no other application can free up memory for mysql to use. Cached pages are stored as 2 lists in Linux kernel viz active and inactive. More details here https://www.kernel.org/doc/gorma