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In the previous lesson, we learned some tricks we can employ as operating system designers for optimizing the RPC communication software which powers client-server communication in the local area network from the point of view of reducing communication latency. Of course, user interactions go beyond the local area network to the wide area Internet. The primary issue, once a packet leaves your local node, is to route the packet reliably and quickly to the destination. Routing is part of the functionality of the network layer of the protocol stack of an operating system. What happens to a packet once it leaves your node? Well, the intermediate hardware routers between your node and the destination have routing tables. That help them to move the packet towards the desired destination node by doing a table look-up. The routing tables evolve over time, since the Internet itself is evolving continually. That's the big picture. And there are lots of fascinating details which you can learn in a course that is dedicated to computer networking. For the next part of the lesson on distributed systems, we want to ask the question, what can be done in the intermediate routers, to accommodate the quality of service needs of individual packet flows through the network? Or in other words, can we make the routers en route to the destinations smart? The specific thought experiment we are going to discuss is called active networks. And then, we will connect the dots from active networks to the current state of the art, which is referred to as software defined networking. Thus far in the course, we've been focusing on specializing operating system services for a single processor, or a multi-core or a parallel system, or a local area network. In this lesson, we will take this idea of specializing to the the wide area network. Specifically, we will study the idea of providing quality of service for network communication in an operating system by making the network active.

Routing on the Internet

Normally, when we think about routing of packets on the internet, typically what happens is. At the source node, you create a network packet and go through the layers of software stack on the sending node, and send the packet out on the network. And this network packet has a desired destination. And of course, it has to go through a whole number of intermediate routers in order to get to its eventual destination. And the routers on the Internet that are intermediate between the source and the destination, they don't inspect the packet for the contents or anything like that. All that they're doing is when the packet comes in, they're looking at the destination known for that packet and figuring out what is the next hub that I have to send the packet to. So each router is making the determination of the next hub for the package, and it makes the determination by doing a table lookup. So every router has a routing table, and the routing table is telling. Given a particular destination, what is the next hop? And that's how the packet flows from source to destination through a whole bunch of intermediate routers and finally gets to the destination. So in other words, the routers en route to the destination from the source, are simply forwarding packets. That is, the nodes are passive. And they're just doing a table lookup in order to figure out what is the next hop that I have to send this packet to? Now, what does it mean to make the nodes active? What we mean by making the node active is that the next hop for sending this package towards a destination is not simply a table look up. But it is actually determined by the router executing code that is actively as opposed to doing just a passive table look up. So, in other words, the packet in addition to the payload that is intended for the destination also carries code with it. And the code is being executed by the router in order to make a determination as to what to do with this packet in terms of routing it towards the desired destination. This sounds really clever because it can provide customized service for networks flows that are going through the network. And every network can have its own way of choosing what may be the desired route, in terms of going from source to destination. And so in other words we're saying. Well, this is an opportunity to virtualize the traffic flow from my network traffic independent of other network flows. This should be very familiar to you all because we've been talking about customizing operating system services in the SPIN operating system, and the XO kernel and so on. But, of course, the problem that we're talking about here, much, much, harder because the network is wide open. Our network traffic flow is going through the public internet infrastructure ,and we are talking about specializing the network flow for every network flow independent of others. There are lots of challenges to this values of active network. In particular, how can we write such code that we can distribute and send it over the wire so that routers can execute it. And who can write such code? And how can we be sure that the injected code does not break the network? Or in other words, for a particular network flow, there is a code that is going to be centered on. How do we make sure that is not going to in some way [UNKNOWN] other network flows? These are things that we have to worry about and sort of opening up the router and saying that we're going to take network flow specific decisions in each of the routers.

An Example

Let me give you an example to motivate why this vision of active networks is both intriguing and interesting. You may all know that Diwali is a big festival in India, just like Christmas is in the Western world. And let's say that I am sending Diwali greetings electronically to my siblings, who are in India. What I can do, is I can send individually a greeting message to each of my siblings. And so, there'll be N messages going out on the Internet from source to destination. So I send out N messages, and, you know, to reach any of my siblings. That's one way of doing it. A nicer approach would be, given that all my siblings are clustered in one corner of the globe, it would be nice if I could send just one message, traverses the Internet, gets to close to the destination of where my siblings are, and the router that is at this end, demultiplexes my message and sends it to all my siblings. Obviously, the second method is more frugal in terms of using network resources. I don't have to send N messages. I can send one message, and finally at or close to the destination, an active node takes this one message, recognizes, oh, this is intended for multiple recipients, and demultiplexes them, and sends it to all the eventual recipients of this message. Of course, we can generalize this idea and say that this idea of active router is going to be spread out throughout the Internet, so that even if my siblings, let's say, are distributed all over the world, then I could still send a single message from my source, and it gets demultiplexed along the way, depending on where all the eventual recipients are for this particular message that starts from me. So in other words, we can sprinkle this intelligence that is in this one particular router to all the routers in the Internet and that way we are making the entire Internet an active network. That's the vision behind active networks where the nodes in the internet will become, not just passive entities, but actually active, in looking at the message, and figuring out what to do with it, in terms of routing decisions.

How to Implement the Vision

Now that we've motivated the vision, let's see how we can implement the vision. In order to implement this vision, the operating system should provide quality of service APIs to the application. And these quality of service APIs could be things like, oh, this particular network flow that I'm creating has certain real time constraints, because it has video data and so on and so forth. And those are the hints that the operating system is going to use, in terms of synthesizing code that corresponds to the API that the operating system is providing you, to give hints to the network. So, the code that the operating system is going to synthesize is essentially taking the quality of service constraints, and putting them as executable code that can be, then passed it on as part of the packet. So in other words, the protocol stack of the operating system has to be enhanced to service these quality of service requirements, and generally to synthesize the code that has to be part of the payload. So the application is not only providing a payload, but it is giving quality of service constraints. And the operating system, in addition to the payload, generates or synthesizes code corresponding to this quality of service instructions. And this slaps on the IP header for where this particular message is eventually to be delivered, and hands it over to the Internet. And in the Internet, if we assume that the Internet routers are capable of executing this specialized code, then depending on the nature of what is being requested, a particular order may say, oh this particular packet I have to send it to multiple destinations, so let me send this down this link, down this link, and similarly when it comes over here, this router may say, oh, this packet has to go to multiple destinations. And so on and that we can see that intelligent routing decisions can be taken in the network. That's out of the road map of how we can take this vision and try to implement it. But the problem with carrying out the vision, in terms of this implementation that I just sketched, is that changing the operating system is non-trivilous, especially the protocol stackers, they have already mentioned TCP IP has several hundred thousand lines of code, so it is non trivial to change the protocol stack of every node in the entire universe. To handle active networks. And also, the second part of the challenge is that the network routers are not open. So, in other words, we cannot expect that every router on the Internet is capable of processing the code that I'm going on slap on to this payload and be able to make intelligent routing decisions. So there is a impedance mismatch between the vision and the implementation that I've sketched right here.

ANTS Toolkit

So, the ANTS toolkit, ANTS stands for Active Node Transfer System, took a different approach to show the utility of the vision. Since modifying the protocol stack is nontrivial, instead, the ANTS toolkit is really an application-level package. And this toolkit is available for the application programmer to say, here is my pay load and quality of service constraints. And what the ANTS toolkit does is to create an ANTS header to this payload. So, the new payload looks like this. And this is what is called a capsule. And a capsule consists of an ANTS header and the actual payload. And this is what is given to a normal operation system protocol stack. And so, this normal operating system protocol stack looks at this as the payload that has been given to it and it knows the destination address, where this has to go sticks on the IP header for it. So, the new packet that is generated by the protocol stack, looks like this. It has the IP header, and the rest is payload so long as This protocol stack is concerned, but we know this payload consists of two parts. One is the normal payload that the application generated, and in addition to that there is the ANTS header that have been slapped on by the ANTS tool kit and this is what traverses the network and when it traverses the network if a node in the network is a normal node, meaning it is not a smart node, but it is a normal IP router. Then it simply uses the IP header to say, well, here is what I have to do in terms of sending the packet to the next hop, towards the destination. On the other hand, if a node that receives this packet is an active node. Then, it can actually process this ANTS header, and say, oh, this particular packet needs to be. Demuliplexed, and sent to two different routes. And it might take that intelligent routing decision based on the nature of that node. So that's the idea, that we can push one of the paint points out of the operating system, into an enhanced toolkit that lives above the operating system. So that's sort of the ANTS toolkit vision. That's one part. Now, the second part, and of course the fact that the internet may not be open to opening up all of the routers to to be processing the specialized code that comes in the capsule. So, what we do is we keep the active nodes only at the edge of the network. In other words, the core IP network is unchanged, and all of the magic happens only at the edge of the network. So, once again, if I go back to my example of sending greetings to my siblings, then only the edge nodes have to do the magic in order to take my original message and process the code to deliver it to multiple destinations. So the rest of the network can remain unchanged. So the core of the IP network can, can be unchanged, and intelligence can be at the edge of the network. So this is, sort of, allowing, sort of, matting this active network vision with the core IP network being unchanged.

ANTS Capsule and API

So having given you the high level description of what ANTS toolkit does, let's dig a little deeper and look at the structure of the ANTS capsule as well as the APIs provided by ANTS in order to do capsule processing. First of all, the header as I told you consists of three parts. The original IP header, which is important for routing the package towards the destination if a node is a normal node, not an active node. And this is of course the payload that was generated by the application. And in the middle is this ANTS header, and there are two fields in this ANTS header that are particularly important. One is a type field, the other is a prev field. The type field is a way by which you can identify the code that has to be executed to process this capsule. And this type field is really an MD5 hash of the code that needs to be executed on this capsule, and we'll come back to that in a minute. And the second field that I said is important is the prev field. And this prev field is the identity of the upstream node that successfully processed the capsule of this type. And this information is going to be useful for us in terms of identifying the code that needs to be executed in order to process this capsule. We'll come back to how these two fields are actually used in processing a capsule once this capsule arrives at an active node. The short hint that I'll give you is that the capsule itself, as you see, does not contain the code that needs to be executed to process this capsule, but it only contains a type field. And this type field is a vehicle by which we can identify the code that needs to be executed to process this capsule. More on that in a minute. First, let's talk about the API that ANTS toolkit provides you. The most important function that we want to accomplish using the ANTS toolkit is forwarding packets through the network intelligently. So routing the capsule is the most important function that needs to be done. And that's most of what this ANTS API is all about. And that part is contained right here, saying that, well, route this packet in this manner, and deliver the packet to an application. And this is the set of API calls that allows you to do routing of the capsule through the network. This is where what I said about virtualizing the network comes in. Regardless of the actual topology, physical topology, I can take routing decisions commensurate with the network flow requirements contained in the capsule that arrives at a node. So the second part of the API is API for manipulating what is called a soft-store. Now, soft-store is storage that's available in every routing node for personalizing the network flow with respect to a particular type of capsule. And I mentioned earlier that the type is only a pointer to the code, not the code itself. And the soft-store is a place where we can store the code that corresponds to a particular capsule type. So the primitives that are available for manipulating the soft-store are things like put object and get object. The soft-store is basically key value store and in this key value store, you can store whatever is important for personalizing the network flow for capsules of this type. An obvious candidate for storing in the soft-store is the code that is associated with this type. So you can store the code that is associated with this type, so that future capsules of the same type, when it arrives at a particular node, they can retrieve the code from the soft-store and execute the code that needs to be executed for processing capsules of this type. Other interesting things that you might put into this soft-store are things like computed hints about the state of the network, which can be used for future capsule processing for capsules of the same type. And the third category of API that's available is querying the node for interesting tidbits about the state of the network or details about that node itself, for instance, what is the identity of the node that I'm currently at, and what the local time is at this node, at this node, and so on and so forth. So these are the kinds of things that are available. So the key thing that I want you to get out of looking at this ANTS API is that it is a very, very minimal setup API. So the number of API calls fits in this little table here. So that's the idea. Remember that routers are in the public Internet. And if you're talking about executing code in the router that is part of the public Internet, the router program that we're executing at a router node has to have certain important characteristics. Number one, it has to be easy to program. Number two, it should be easy to debug, maintain, and understand. And number three, it should be very quick, because we are talking about routing packets, and so the router program should not take a long time to do its router processing. So the API, this very simple API, allows you to generate very simple router programs that are easy to program because the APIs are simple, easy to debug, easy to maintain and understand. And the program itself is pretty small, that it's going to take not a humongous amount of time to do the packet processing.

Capsule Implementation

Now let's talk about implementation of the capsule, and in particular what are the actions taken on capsule arrival at a particular node. Now, I mentioned that the capsule does not contain code, but it is passed by reference. Or in other words what the capsule contains is the type identify which is really a fingerprint for the capsule code. And the way this type is generated it's basically a cryptographic fingerprint of the original capsule code. In the original implementation of Ann's tool kit they used an MD5 hash of the code and used that as a fingerprint. And it was the case that at the time that this particular research was done, MD5 hash was a cryptographically strong hash function, which was not broken. But subsequently MD5 hash has been broken. But nevertheless, the key thing that I want you to remember is that this type field is a cryptographically strong fingerprint that is derived from the capsule code. And that serves as a reference for the code itself. So when a node receives a capsule, one of two things are possible. The first possibility is that this node has seen capsules of this type before. If that is the case, then it is quite likely that in the soft store of this node the code that corresponds to this type is already existing. In which case, it's a simple thing for the current node to retrieve the code from its soft store, and execute the capsule and proceed with forwarding this capsule on to where its desired destination. On the other hand, if this capsule that arrived at this node is the first time that this node is seeing a capsule of this type, then, obviously, it's not going to have the code that corresponds to this type. In this case, what this current node is going to do is use the previous node field of the capsule and send a request to the previous node saying that, hey I got this capsule of this type, I don't have the code. Do you have the code? If you have, please send it to me. And when this request comes in, the previous node, obviously it has processed this capsule before. It's quite likely that this previous node has the capsule code residing in its soft store. And so it retrieves it from its soft store and sends it to the next node so that the next node has the capsule code now, can execute it, and also store it now locally in its own soft store so that future capsules of the same type when they arrive here, it can be processed using the code that is now stored in the soft store. And the key point to take away is that typically when we are talking about network flows, we are sending a whole bunch of packets one after another. And therefore, even though the first packet that comes to a node may not find the code that is associated with that particular type, and we have to do a little bit of heavy lifting in, in terms of going and reaching back to the previous node to retrieve the code. Because network flow is a whole bunch of packets, there's going to be a whole lot of other packets that come down the pike. They're all going to be processed using the soft store and the code that is stored in the soft store. In other words, we are exploiting the locality for capsule processing by storing the code that arrives in response to our request back to a previous node in the local soft store. One concern that we may have is that how do I believe that the code that I got from the previous node is actually the code that corresponds to this type or not. Well, this is where the cryptograpically strong fingerprint comes into play. What this node is going to do is, when it retrieves the code from the previous node, and when the code arrives, it is going to compute the fingerprint of the code that it just got. And see if that fingerprint matches the type field of the capsule. If it does, then it knows that this code is genuine. If it is not, then obviously somebody is trying to spoof my node by giving bogus codes so I'm going to reject it. So code spoofing can be avoided by having a fingerprint that is cryptographically strong, so that I can recompute the fingerprint match it against the type, and know that the demand loaded code that I got is actually the code that is associated with this particular capsule. And as I mentioned already, once I get this code, because I'm going to most likely see capsules of this type in the future as part of this particular network flow, I'm going to save the code in the soft store for future use. So when a capsule arrives at a node one of two things will happen. One is I will reach into my soft store and see if I have the code that matches this particular type in the capsule. If it isn't then I don't have the code, I'm going to reach back and get it from the previous node. But what if I go back to the previous node and the previous node does not have the code that corresponds to this type. So the action of a node when it cannot find the code that corresponds to a type either locally in its soft store or retrieving it from the previous node, is to simply drop the capsule. Because what's going to happen is that if this capsule is dropped, higher-level acknowledgements that are happening in that particular network flow is going to indicate to the source that something did not get through, and that source node is going to retransmit that capsule. This is exactly the same thing that happens with IP routing on the Internet. That is, if a node cannot process all the packet that it gets, it simply drops the packet. That's the same semantic that is used for capsule processing also because we're relying on higher level protocols, the transfer protocol that sits on top of the network protocol do end to end acknowledgement to make sure that all the packets that they're expecting have actually reached. And therefore at the level of capsule processing, we don't have to worry if we cannot process the capsule either locally by using the code in the soft store or by retrieving the code from the previous node. Simply drop the capsule. Now, is it likely that, when we reach into the previous node, that node does not have the code for it? Because, after all, it did process this capsule, and send this capsule on to me. It can happen, because the soft store is limited. Every router node has only a finite capacity, and all of its capacity it's not going to give to a single network flow. It's going to give only a part of its storage for the network flow that is corresponding to a particular capsule type. And therefore, I have to live within my means. So, the capsule code may have to throw away stuff every once in a while if it is storing more than what its capacity is in the soft store. So it is possible that the code that it originally stored in the soft store, it had to replace it. Because it is like a cache and therefore, it may have replaced it and therefore, it may not have it. And this is particularly possible if this request comes at a much later time because one of the things that you associate with this kind of routing code is that it is going to be timely. So there is going to be a time associated with the validity of things that I want to keep in my soft store. So if I, I get a request at this node at a much later point in time because the capsule arrived at this node traversing all over the network, it may not have the code that corresponds to the type. So it is possible, so if it happens, simply drop the capsule and let the higher level entities in the protocol stack take care of retransmitting the capsule if need be.

Potential Apps

So, how useful is active network? There are lots of potential applications that can be built using this active network's paradigm. In particular, what we want to do is, when ever we desire certain ways to visualize the behavior of the network. Then active networks become very useful. So, for instance, for implementing protocol independent multicast reliable multicast, or noticing congestions in the network and notifying the source and the destination about the congestions, private IP, any casting. These are all the kinds of things that are useful to implement using active networks. And as you can see from this list, the kinds of things that you want to do, using active networks, are things that are related to network functionality. Not high-level application, but network functionality. In particular, it is useful for building applications that are difficult to deploy in the internet. So, when you rely on the routing and the internet, it is entirely an administrative set up, and the administrative set up tends to mirror the physical set up. But, for your particular network flow, you may want a set up that is different from the physical set up. And the example I give you of me sending a greeting that will be a single message for most of the traverse through the internet, but at some point, it may actually get demultiplexed and get sent to several different destinations. Those kinds of multicasting and things like that, are specific to network flow. So, we are in some sense, overlaying our own desire on topology, on top of the physical topography of the internet, by using the active network paradigm. But, the key properties that you want to have for applications that you want to build using the active networks paradigm, is that the applications should be expressible and it should be compact and it should be fast and it should not rely on all the notes being active. These are key things to note in building applications that live on top of the active networks. So all of these suggest, once again, what I said already and that is, it is for providing network layer functionality, not end application functionality. So, what you want in the network layer, that is something that again orchestrate using the active networks paradigm.

Pros and Cons of Active Networks

So having talked about the vision and the practical implementation details of active networks, lets talk about the pros and cons. The pro is something that we have been stressing all along and that is the flexibility from the point if you have an application perspective that fact that you can ignore the physical layout of the network and slap on your own visualization of what you want to accomplish for your netowork flow. On top of the physical infrastructure is the key selling point for active networks. But this selling point comes at the cost of perhaps certain cons. What are all the cons? Well, one concern could be protection threats. What I mean by that is that eroding infrastructures carrying network flows. Not really mine but yours and the third person and so on and so forth. And just like in an operating system when we have a process, we want to make sure that that process is not doing anything malicious to other processes in the same node. Same way, my network flow should not do anything that is detrimental to your network flow on the internet. So that's what we mean by protection threats. So there are some safeguards in the ANTS toolkit to address these protection threats. The first is the run time safety of ANTS program that is running on a router node. And the way they ensure that is by first of all implementing ANTS itself in Java and using Java sandboxing technique on the router node, so that anything that a router code is doing for capsule processing Is limited to the Java's hand box that is executing in. And so, it cannot effect the flows of other network flows that are flowing through the same routing fabric. That's the first thing. The code spoofing that can happen. We are talking about cold being injected into the router, and of course there was a good reason for doing that, that I wanted a certain behavior to be observed by the network routers, in response to packets flowing through the network that belongs to me. But I want to be sure that the code that is being executed is the code that I wrote not some malicious code that is being spoofed to that node. Well here, here the safeguard is making sure that you have a robust, a robust fingerprint associated with the code. And so what you do is you generate a tight field for the capsule that is a crytographically strong finger print of the original code. And it always matching when you get the code from a previous known In response to your requst, you're goint to compute the fingerprint once more. Check it against the fingerprint that is contained in the capsule, ensure that there is no code spoofing happening. That's how you can overcome this protection threat. And the third concern can be integrity of the soft state. And what I mean by that is, the soft store that's available at an ordered node is limited in size, and you don't want any particular network flow to arbitrarily consume all of the soft state. And yet again, there is a restricted API that is provided in ANTS, that's the safeguard. For this protection threat. So these protection threats are concerns. But at least in the ANTS tool kit, they offer solutions to ensure that these protection threats are not show stoppers for active networks. The second concern that one might have is Resource Management threats. Now what I mean by that is Because we are executing code at a router, the result of that code execution could be that I proliferate packets in the internet. I'll give you an example of resending a message to my siblings I send one message, and at some point That one message becomes n message. So in some sense, we can start flooding the network with capsule processing. Is that a threat? Well it is, but internet is already susceptible to this kind of resource management threat and yes capsule adds to it, but it is not anything new that it is adding in terms of resource management threat. On the other hand, we can ask the question at each node is it going to consume more resources than it should. And this again comes back to the safegaurd that they have in the ANTS toolkit that the api is a restricted api and therefore the amount of resources that you can consume at a node It's fairly restricted, so there is sort of a mixed answer to this resource management concern. At a given node, the resource management concern doesn't quite exist so long as you adhere to the restricted API of ANTS. And the second concern that the capsules may flood the network can happen, but it already happens, we all experience spam on The internet so this is not adding any new problem but it is perhaps exasterbating an existing problem. So having looked at division and looking at division and the practicality of active networks, time for a quiz.


In your opinion, what can be roadblocks to the Active Network's Vision? And I'm going to give you multiple choices. And you can pick more than one choice if you think it fits the answer to this question. The first choice is that the active network's vision needs buy-in from the router vendors. Router vendors are vendors like Cisco. Who make the routing fabric and the first choice is saying it need, you, you need buy in from the router vendors, the second choice is saying that because the routing was active network is happening in software, the software routing speed cannot match the throughput needed in the internet core. That's the second choice, the third choice says that. It makes the internet more vulnerable, and the fourth choice says that it makes the router more susceptible to code spoofing.


The right answers are the first two boxes here. And let's talk through this. Now clearly, if we want to do anything in the router, we need buy in from the router vendors. So this is a big challenge and convincing the router makers that, yes please open up the network so that I can dump some code in it and execute. The code, so that is a big challenge. The second is also a big challenge and that is, if you look at the traffic on the internet today it's just humongous. There's so much traffic on the internet, and this is the reason why routers are dumb animals. All that the do is when a packet comes in, it's all happening in hardware, they do a table look up to figure out. Given the destination, what is the next stop that I send this package to? So the internet core, the routing fabric is operating at huge speeds, because even at the edge of the network today, we are already seeing gigabit speeds. Which means that the core of the network, you have several hundreds of gigabytes of packet processing that needs to be done and therefore it is important that the core of the network be blazingly fast. And software routing is not going to be able to match the speed that is needed in the core of the network for packet processing. So these two choices are good choices. Now, does active network's make the Internet more vulnerable? Not really because the internet is already vulnerable. Perhaps, it adds to it but not particularly making it more vulnerable than what it is already. And the last choice regarding code spoofing, so long as we make sure that the fingerprint that we generate to associate with the code, that is going to be used for processing the capsule when it arrives at a node. Is, cryptographically strong, then we can make sure that this code spoofing does not happen, so I would say, these are the two choices that apply for this question.


So let's talk about the feasibility of the vision of active networks. The reality is, router makers like Cisco are loath to opening of the network. So while the idea of active networks is very fascinating, that we can be frugal about the resources that we use in the Internet for different network flows and we can actually virtualize the physical infrastructure by slapping on. Our own idea of what should be the, the kind of network flow that I want for my packets, seems very attractive, but really it's not going to be feasible given that we have to open up the network. So it's going to be feasible only at the edge of the network. Secondly, when we are using active networks, we are talking about executing code in a router in order to determine the routing decision at that node. Or in other words, we're doing software routing. Software routing cannot match the hardware routing, because at the core of the network, there's so much of traffic being handled that you really want to do this in hardware, and doing this at software speed is not going to match the hardware speed of packet processing in the core of the network. So once again, this argues that active network is only feasible at the edge of the network and finally there are social and psychological reasons why active networks is maybe a little bit hard to digest. It is hard for the user community to accept arbitrary code executing in a public routing fabric. If my traffic is flowing through the network and if the router is going to actually execute some code in order to do the processing of my packet, that worries me. Already, we talk a lot about privacy and the fact that in corporate networks, in university networks, we are losing a lot of privacy. People are watching what's going on. And now, saying that the routers are going to do something intelligent, a smart processing packets, that might be a socially and psychologically unacceptable proposition. So these are reasons why it would make it difficult to sell the idea of active networks to the wide area internet. On the other hand, the idea of virtualizing the network flow is very appealing. And if you put together the two thoughts that I had, one is the idea that we can virtualize the network and the second that active networks is only feasible at the edge of the network, that brings up a very interesting proposition, which I am going to mention in my concluding remarks.


Active networks was way ahead of its time, and there was not a killer app to justify this particular line of thought. Further, active networks focused more on safety and less on performance, so in the 90s it seemed more like a solution looking for a problem. But difficulties with network management, rise of virtualization, the right hardware support and data center and cloud computing have all given active networks a new lease of life in the form of Software Defined Networking or SDN for short. Specifically, cloud computing promotes a model of utility computing where multiple tenants, by that I mean businesses, can host their respective corporate networks simultaneously on the same computational resources of a data center. Not that this is going to ever happen but imagine Coke and Pepsi, corporate networks running on the same data center resources. What this means is there is a need for perfect isolation, of the network traffic of one business from another, even though each of the network traffic is flowing on the same physical infrastructure. This calls for virtualization of the physical network itself, and hence the term, software defined networking. You will learn more about SDN if you take a companion course on networking that is offered in this same program.