Tag: IP

What is route recursion

We are going back to networking basics with this post. In few lines below you will find most important theory that makes network gear do its job.

The main router job is to making routing decisions to be able to route packets toward their destination. Sometimes that includes recursive lookup of routing table if the next-hop value is not available via connected interface.

Routing decision on end devices

Lets have a look at routing decision that happens if we presume that we have a PC connected on our Ethernet network.

If one device wants to send a packet to another device, it first needs to find an answer to these questions:

  • Is maybe the destination IP address chunk of local subnet IP range?
    • If that is true, packet will be forwarded to the neighbour device using Layer 2 in the ARP example below.
    • If that is not the case, does the device network card configuration include a router address through which that destination can be reached? (default gateway)
  • Device then looks at his local ARP table. Does it include a MAC address associated with the destination IP address?
    • If the destination is not part of the local subnet, does the local ARP table contain the MAC address of the nearest router? (MAC address to IP address mapping of default gateway router)

Nagle’s algorithm

How Nagle’s algorithm is making TCP/IP better and when is ok to use it. Truth be told, Nagle should be avoided in today’s high-speed networks.

This article it’s not about mathematics, don’t be afraid. I’m running a networking blog and it’s not my intention to speak or write about anything related to mathematics. Biggest math problem that I’ve done in last few years is some simple subneting, EIGRP metric calculation and that is where I stopped with math for now.

On the other hand, I love the theory behind algorithms, specially if the algorithm is used in networking and if it is so simple and powerful as Nagle’s algorithm.

You can guess, John Nagle is the name of the fellow who created the algorithm. He found a solution for TCP/IP efficiency issue also known as “small packet problem”.

Here’s what happens:

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How to generate network packets – Ostinato Packet/Traffic Generator

Network Packet Generator or Network Traffic Generator is a tool every network engineer will sooner or later want to use. Here’s one I found and it’s great!

First time I saw an Ethernet frame in details on my CCNA class back in 2010 I immediately got the idea about generating some packets on my own. It was logical next step to ask myself: “Ok, so how can I make one of those and see what happens when I send it out on the network?”. I was not really sure that there is a tool that would make it possible.

Don’t get me wrong, net surfers don’t need this!

I mean, Yeah, ok, I know I am generating a lot of packets right now by not doing anything because my Mac is surely syncing who knows what across the Internet. The thing is, you are not really in control of your machine’s applications network layer which is talking across the network, so you can not really make much changes in frames header format and whats inside headers. Apps are sending out standard packets with standardised header format (flags, addresses etc.). The thing that we control is only the data that we send, the payload of those packets, headers, they do their thing to make the transfer possible.

You can control the packet source IP address of course, maybe MAC address sometimes on some Linux machines by editing your NIC configuration but I am sure you know that if you are still here 🙂

Network engineers do need this!

But I am a network engineer and I usually want:

  • to test something
  • make something that does not exist so far or is not standardised.
  • I want to try to create a new protocol that will talk using IP.
  • I want to change protocol implementation bugs from some vendor.
  • I need a way to create test packets to investigate strange firewall packet drops.
  • I want to see what will happen if some packets header flags are changed in strange way, how will that affect the packet forwarding.
  • I want to send stuff across the network and see what happens.
  • I need other stuff too.

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How can router decide so fast?

Cisco created all sorts of different magic inside their boxes that optimize forwarding processing of packets.

CEFMain router function is fairly self-explanatory. Router performs IP forwarding more often called IP routing. IP routing is process of deciding where to send the packet after it was received.

 

 

 

IP Routing explained in detail

Logic behind IP forwarding is listed in steps here with the assumption it will be an IPv4 packet that was received. This is process switching explained in 11 steps:

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/31 subnet in point-to-point links. Is that possible?

subnet /31

Ok, this is strange! At least this was my first reaction when I saw that in one of my CCIE labs that I am trying to resolve all the links between routers are addresses with a subnet /31.

Isn’t that weird that something like this you see for this first time after couple of years in networking. For me it was. It blow my mind out. I asked my more experienced networking colleagues later but for them it seemed new too. They said at first: Ok men, that’s not possible!

Well, try to type it on router interface and you will se that it is possible. It strange for sure, but it’s possible. Router OS (Cisco IOS in this case) will try to be sure that you will use this kind of subneting only for Point-to-point links. That’s why it will issue a warning message if you apply this subnet mask on an Ethernet interface. For serial it will go without the warning.

The idea behind this is of course simple if you put it this way:

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