One tip on C macros

I am not a computer science graduate so when I started my career I knew only two languages Hindi and English. I later moved into IT industry as almost every other guy was doing. I went through a crash course on 'C' and before I could learn any other language, my company decided to move me into a client's project. I usually joke when I say I now know three language Hindi, English and 'C' but that is a fact. Since then I have worked on some more computer languages like C++, Perl and Python but I have always felt more comfortable with 'C'.

As I mentioned I learned the basic of 'C' in a 5 days crash course and most of what I now know, I learned from my peers of my various projects. During this time, I first learned what is the right way of writing 'C' macros and later found the explanation of why that is the right way. I decided to share some of those with you in multiple post. So here is the first one.

Let us assume you created a macro as follows:

#define  MULTIPLY_BY_TEN( x, y )                x = y * 10

Let us assume the usage of this macro is as mentioned below.

multiply_by_ten_function( one_time )
{
     unsigned long      ten_times;

     MULTIPLY_BY_TEN( ten_times, one_time );

     return ten_times;
}

Do you see anything wrong in this? No. I also don't see anything wrong. Actually in this usage, there is nothing wrong. But what do you think about the below usage?

multiply_by_ten_function( one_time, two_time )
{
     unsigned long      ten_times;

     MULTIPLY_BY_TEN( ten_times, one_time + two_time );

     return ten_times;
}

Yes. This will have a problem. Why? Because when the macro is expanded it will be :

multiply_by_ten_function( one_time, two_time )
{
     unsigned long      ten_times;

     ten_times = one_time + two_time * 10;

     return ten_times;
}

Instead of the addition of 'one_time' and 'two_time' first, it will multiply two_time with 10 and then add one_time to the result. So how do we fix such a thing? Very simple. We always wrap the arguments of a macro with '()'. Now let us look at the macro again:

#define  MULTIPLY_BY_TEN( x, y )                (x) = (y) * 10

Now this is how it will look after expanding:

multiply_by_ten_function( one_time, two_time )
{
     unsigned long      ten_times;

     (ten_times) = (one_time + two_time) * 10;

     return ten_times;
}

And you will get exactly what you expected. So I hope you will change your habit of not wrapping the arguments of macro with '()'. Please don't believe me and do try this out by writing a small 'C' code to validate the above.

I will put up more such tips in future post. If you want me to write about anything in particular, please leave me a comment.

 

Fast path and Slow path

Long back I debugged a problem where a video client was receiving some of the packets of a video stream out of order. During debugging, we found that some of the packets of the video stream were taking Fast Path while some of them were taking Slow Path.

So what exactly is Fast Path and Slow Path?

Before I get into that, let me tell you a little bit about router architecture. Routers usually have a general purpose processor (similar to CPU used in PCs) which is used for handling the control plane requirements like configuration of the device, networking protocols and packets destined to the router. In early routers, the same general purpose processor was used for forwarding the data packets as well. So as you can imagine the data forwarding was really slow.

Later as technology evolved, our routers needed to forward data packets with higher speed and a general purpose processor could not do this. So it was decided to move data forwarding part to a special purpose processor, usually known as Network Processor. Network Processor's sole responsibility is to forward the data packets. So current breed of routers use a general purpose processor for control plane requirements and a Network Processor to forward data packets at wire speed.

Now when a packet is forwarded at wire speed with minimal latency, the packet is said to be taking Fast Path. Why is this called Fast Path? Because a data packet is forwarded at the fastest speed. So who is forwarding this data traffic? What did you say? Network Processor. Yeah! You got this right. Now with this background, I am sure you can figure out which path is Slow Path. If you said, that when the packet is forwarded by the general purpose processor, I would say, Congratulations, you got this also right.

Someone might ask if Network Processor is suppose to forward data traffic when does general purpose processor forward the data traffic. A good question. It can happen in the following cases:

1. When ARP needs to be resolved for a gateway, Network Processor sends all the data packets to the control plane. Once ARP is resolved, control plane forwards these packets. Subsequent packets will be forwarded by Network Processor.
2. There could be an issue because of which Network Processor might not have enough information to forward a packet. In such a case, Network Processor sends these packets to control plane and control plane may forward it.

So the issue I was debugging was seen because of the 2nd case.

Please let me know if you have come across an issue where data traffic was taking Slow Path.

Writing Technical Blogs in routerfreak.com

When I started this blog, I had no idea what I am getting into. I just had this vision that I will write my blog posts in a simple language so that more people can read, understand and comment and we all learn in the process. But one of the immediate challenge was to popularize the blog so that more people reads my posts. I did following:

1. I shared my blog through mails with my friends and family. 
2. I updated my blog in Linked-In and shared every blog post in Linked-In.
3. As suggested by one of my colleague, I created a twitter account for this blog to get to wider audience. I started following some of my friends and collegues so that they can retweet my tweets to get my wider audience.

Even after doing the above, I find traffic to my blog was not as much as I wanted it to be. I thought that I should write guest posts for some popular blogs as a guest editor to get to wider audience.

When I searched through Internet, I found couple of good technical blogs. Most of these blogs are Network Administrator oriented. One such blog is Router Freak and it provides tools, tips, review etc. for Network Engineers. Router Freak also provide oppourtunity to other people to write for them. I contacted them through their web-site. They very kindly agreed for me to write for them.

I wrote my first blog post on IP TTL Security on 8th August. I followed it up with SSM and ASM on 2nd September. Please do read these two blog post and send me any comments you may have.

As I was showing enthusiasm in writing blog post and I think Router Freak liked the two blog post I wrote for them, we decided to get into an agreement. We discussed and decided that I shall write Technical Contents realted to Networking only for Router Freak.

I think its a win-win situation for both of us. I get the wider audience for my posts and Router Freak hopefully gets the good technical content. I request you to please subscribe to Router Freak's RSS feed or for Network Engineer's Newsletter or both so that you can be notified about new blog posts.

Contributing in IETF : where to start?

In one of my my earlier post 'India in IETF', I described India's contribution in IETF. In that post, I asked you to provide comments and suggestions on how we can improve the quality and quantity of India's contribution in IETF.

In multiple informal discussions, different people have pointed out that while they would like to contribute but they do not know how and where to start. So I decided to write this post to provide some information on the same.

First thing first. Do you know how IETF works? If not then you need to read through my previous post here on the same topic.

If you are reading this, I will assume that you know about the Areas and Working Groups in IETF. You need to identify the Working Group which interests you. It could be related to what you are currently working on or what you would be working on in near future or what you want to learn to expand your ever-growing knowledge. You can be interested in multiple Working Group at the same time.

So what next?

You go ahead and subscribe to the mailing list of each Working Group you are interested in. This is because most of the IETF technical work happens in mailing lists. How do you join a Working Group mailing list? You just need to send a mail to the Working Group mailing list with  'subscribe' in the subject line and body of the mail.

Now that you have joined the mailing list, you will start receiving mails sent by others. Read them to understand the discussion and people's point of view. If you feel like asking a question in any of the discussion, feel free to do that.

Scan all the Internet Drafts submitted to a working group. Pick one that interests you the most and read through it. While reading through it, take your time to understand the problem it is solving as well as the proposed solution. Note down all the questions you could not answer yourself. Also note down editorial changes like sentences which you think are not clear enough, incorrect spelling etc.. At the end of the reading if you think you have some questions and suggestions on a draft, do send it to the mailing list. One of the author will acknowledge your suggestions and will respond to them. In some cases, you will see that apart from authors other people also pitch in with their view on your suggestions. Finally all your suggestions will be addressed by authors. You can take a pat on your back because you have just helped enhancing the quality of a draft. Do this exercise with all the documents in the Working Group.

Please do not think that any question or suggestion you might have on a draft is a stupid one. You will know its worth only when you send it out on the mailing list. Also please do not get discouraged if nobody responds to your questions. Mostly this does not happen but if it does, do send your mail again. Also if your mail has some valid points, someone will surely reply.

What else?

You come across a problem which your customers are facing or a problem you see in a protocol you are working with or a problem you found when you were testing a protocol in a network configuration. You think that a solution to this problem does not exist.  You discuss this problem among your colleagues and come up with a solution. You are thinking that this solution will help in deployment. In such a case, you should send out a mail describing the problem on the relevant Working Group's mailing list. If others agree that such a problem exist, you should create a draft and submit it. You should then ask Working Group to review your draft. It is good to attend an IETF meeting to discuss this in the Working Group meeting as well as in the corridors of the meeting venue with as many people as possible. If the problem you are solving is a real problem, your draft will be accepted by the Working Group. It will then go through multiple rounds of review and will later become a RFC.

Just one suggestion from me. Please be patient when you have sent a mail on a mailing list. People might take some time to reply as they will be busy with their day-to-day work. But let me assure you that people do reply.

So what are you waiting for, go ahead and start contributing to make Internet better. Best of Luck!

Defects : Can they be avoided?

If you happen to be working in the IT industry, I can bet that you would have come across the word 'Defect' (or other popularly used word 'Bug') almost on daily basis. A 'defect' is nothing but a case of something not working as expected.

You can see defects usually in all walks of life. There are defects in the house you live, bus you take, car you drive, laptop you use. You would have seen various manufacturers recalling their products because they want to fix a defect.

Why one should worry about defects? Because defects are usually costly. Each defect has a cost associated to it. Cost of each defect depends upon which phase it got introduced and which phase it got caught. A defect that was introduced in design phase and caught in deployment phase will be the most costly defect. In the worst case, a defect could be life threatening. For example, a small defect brought down the space shuttle Columbia where all the astronauts were killed.

The big question is why and how defects get introduced?

In any product development, there are various stages. Let us take an example of software product as I am more familiar with this. We can divide the software product development in various stages like requirement collection, high level design, low level design, user-interface design, implementation, testing, deployment etc.. Defects get introduced in almost every stage.

Someone may ask, how come defects gets introduced in the testing phase. Actually testing is suppose to figure out all defects but as we know by experience that it is not the case. So this means that there is some defect in our testing because of which we are not able to detect all defects.

When you designs your product, it is almost impossible to test the design for all possible inputs. So there will always be some inputs for which you cannot test your design or implementation. So theoretically, there is a possibility that your design may fail for some such input. If I extend the above, we can safely say that in theory, it is almost impossible to avoid defects. Similarly, a developer without required level of skills, implementing the desgin will introduce more defects.

In my opinion, as mentioned above, in a completely new software, defects are introduced in all the stages. Once defects are seen, they are fixed. This is when more defects are introduced in the software. So this becomes a unavoidable cycle.

Why defects are introduced when fixing a defect? There could be various reasons like the developer does not know the big picture or there is not enough documentation about the implementation or there is no review process or the developer's skills are not good enough. In my opinion, review process can help in most of these cases. For example: even if the developer fixing the defect does not know the big picture, the chances of getting defects can be reduced if an expert review the fix. In one of my earlier post, I had talked about the necessity of code review. In that post, I argued in favour of code review which helps in reducing the defects in the system.

So defects are costly and seems unavoidable but there are means and processes with which we can reduce the occurrence of defects. I am a big supporter of review process. In my opinion, number of defects in the system can be reduced if there are reviews in each stage of development. Also, I think there must be more focus on better reviews in the preliminary stages like design. What do you think about this? It would be great if you can share the particular mean or process that has really helped you in reducing defects.

Multicast in Ethernet LANs

Now that we know why do we need multicast and how multicast addressing works, let us look at how Multicast works in Ethernet LANs.

As described in my earlier post on multicast addressing, Applications/Servers use a Multicast Address (Class D address) while sending out the contents. This multicast address is used as the destination address in the IP data packet.

Please take a look at figure 1, what do you think who all will receive the multicast traffic sent by the sender?

Did you say 'all-of-them'? If yes, you got it wrong. To tell you the truth, none of them will receive the multicast traffic as they did not indicated their interest beforehand.

So how do the receivers indicate their interest? They use a simple protocol known as IGMP (Internet Group Management Protocol). I will talk about how IGMP works in detail in a future post.  For the time being, just remember that a receiver uses IGMP membership report messages to indicate his interest in receiving multicast traffic for a specific Group Address.

Now look at figures 2 and tell me who all will receive the multicast traffic sent by the sender?

Did you say R1 and R2? If yes, you got it right. Congratulations!

With the figures, can you figure out what difference does the IGMP membership report messages made in this network so that R1 and R2 can now receive the multicast traffic? Actually speaking, nothing, IGMP messages do not change anything. So then question is, how does R1 and R2 can now receive multicast traffic by just sending IGMP messages? Answer lies in what Hosts does when they send IGMP messages?

I am sure most of you already know, how a hosts accept traffic for himself on a LAN. But let me explain it in my terms. Hosts uses Ethernet NICs to send/receive traffic in LANs. Every NIC card is configured with a unique 48 bits MAC address. When one hosts (or any network element) communicate with other hosts, it figure out the destination host's MAC address using ARP. It then prepare an Ethernet Packet with that MAC address as destination MAC and sends the packet on LAN. All Hosts NIC card receive this packet and compare the MAC address with their own MAC address and only that hosts whose MAC address matches, accept the packet. Look at figure - 3. Destination MAC in ethernet frame is of R1 so only R1 accept the packet.

Now there are two things that comes out of this. One is that what MAC address should the sender use when sender is sending a Multicast packet. Another is what should the receivers do to accept the multicast packet?

To keep things simple, an IP multicast address is mapped to an Ethernet MAC address. Here is how the mapping works. Out of 48 bits, first 25 bits are a constant (01 00 5E 0) while rest of the 23 bits are same as the lower 23 bits of the multicast IP address. For example: 224.1.1.1 will map to 01 00 5E 01 01 01.

Do you see a potential issue with this mapping? If you do, take a pat on the back. With this mapping scheme, two different Multicast IP address can map to the same MAC address. If you try to map IP address 224.1.1.1 and 235.1.1.1, they both will map to the same MAC address 01 00 5E 01 01 01. This happens because out of 28 unique bits of multicast address, only 23 bits are used in mapping. Take a look at figure - 4 to see how the mapping works.

Your next question might be that why we ended up with such a scheme. Why did not someone got it right? Actually there is a small story behind this. Steve Deering requested a block of MAC address to be reserved for IP multicast use such that he could create a unique MAC address for each multicast address. But he was allocated a smaller block which had  first 25 bits fixed and we ended up with this scheme.

Now that we know how a multicast address is mapped to Ethernet MAC address, let us see how the multicast communication works. When a receiver indicates its interest in receiving multicast traffic for a given group, apart from sending an IGMP membership report, receiver configure their NIC cards with multicast MAC address calculated based on the mapping of the multicast group address. It means if receivers is interested in multicast group address 224.1.1.1, it configures the NIC card to receive traffic with 01 00 5E 01 01 01. Because of this, any data traffic received with this multicast MAC address is accepted by the receiver. Similarly a sender populates the destination MAC address by calculate the multicast MAC address based on the mapping of the multicast group address. Please look at figure - 5 to understand what I am suggesting.


This is the first post where I have tried to add as many figures as possible. Please do let me know how you like this post in comparison to others where I avoided figures completely. I will leave you with a question now. If multiple multicast group address can map to single MAC address, how does the end hosts filter out packets he is not interested in?

How IETF works?

I illustrated India's contribution in IETF in my earlier 'India in IETF' post and ask you to provide comments and suggestions on how we can improve the quality and quantity of India's contribution in IETF.

In the past, during various informal discussions, people mentioned that they are not aware of how IETF works. I always pointed them to relevant links but I am sure not many have read through them. So I decided to give a brief run down on how IETF works in my own words. Please note that it is like a small crash course on this subject and if you want to get more information, please do go through the links posted at the end of this post.

IETF is a standard body that produces highly relevant technical documents which are used by people to design, develop, use and manage the Internet. IETF has organised its work in different Areas. At present, there are 8 active Areas in IETF. There are several Working Groups in each of these Areas. Each Working Group has a pre-defined charter that identifies the goals and milestones for the working group. There are currently more than 100 Working Groups in IETF. I will talk about the process of creating a Working Group in some future post.

Each Area is managed by one or more Area Directors (ADs) with the help of IESG (Internet Engineering Steering Group). IESG members are Area Directors who are selected by Nomination Committee. Each Working Group is managed by one or more Working Group Chair.

Most of the IETF work happens through mailing list. Each Working Group (WG) has its own mailing list. Anybody can subscribe to any Working Group's mailing lists for free and start contributing.

IETF produces RFCs (Request For Comments) which are implemented by various OEM vendors in their products to make Internet work. Authors write up a private draft which describe a specific problem and the proposed solution. Authors then submit their draft to the most relevant Working Group. Authors can post their draft to other relevant Working Groups as well. For example: A draft can be submitted to both PIM WG as well as L3VPN WG if the draft is related to both the Working Group. Working Group reviews all the proposals and accept those proposals on which Working Group like to work. These proposals are then submitted as IETF Internet Drafts. These drafts then go through extensive reviews in the Working Group. Once Working Group feels that a specific draft has gone through enough reviews, Working Group chair submit the draft to IESG. The draft goes through another review by IESG members and once it is approved by IESG, it is passed to RFC editor to convert it into a RFC.

The easiest way a person can contribute to a Working Group is by reviewing the various Internet Drafts submitted to that Working Group. Again, as mentioned above, its free. You just need to subscribe to the mailing list. This helps everybody as it enhances the quality of the drafts and in turn the output of IETF.

There are regular IETF face-to-face meetings which are organised three times a year. Two of these meetings usually happen in USA/Canada region while one meeting happens somewhere in rest-of-the-world. These meetings are as technical as you can think of. Each Working Group meets to discuss their on-going work. In these meetings, people discuss their current and future work, find co-authors, get feedback on their work and network with people.

For more information, please refer to the TAO and page setup for newcomers in IETF.