Active Network Vision and Reality: Lessons from a Capsule-Based System

D. Wetherall, "Active Network Vision and Reality: Lessons from a Capsule-Based System," 17th Symposium on Operating Systems Principles," (December 1999).

One line summary: This paper examines some of the points of debates concerning active networks by describing the experience of designing and implementing an active network toolkit called ANTS; it explores some of the benefits and potential uses of ANTS and also mentions the practical difficulties ANTS raises concerning performance and security.

Summary

This paper studies active networks by designing and implementing the ANTS active network toolkit. The paper first defines active networks as a new approach to network architecture in which customized code for applications is executed inside the network as opposed to at the end-hosts. They are interesting because of their potential to be used for creating new Internet services, but controversial due to performance and security concerns. In this paper the authors contrast their experience with ANTS with the original vision behind active networks specifically as it pertains to the following three areas: (1) the capsule model of programmability, (2) accessibility of that model, and (3) the applications that can be constructed with that model. The components of ANTS include capsules, which users send along the network like packets, but which contain code that is executed at intermediate nodes along the path, which are programmable routers called active nodes. Capsules implement a custom forwarding routine and so direct themselves and subsequent packets through the network using the routine they implement. The reference implementation of ANTS was written in Java. Any user can develop an application with ANTS, which provides an API for querying node state, routing via shortest paths, and placing data in a temporary soft-store. Once the code is written it is signed by a trusted authority and put in a directory service and registered with a local active node for distribution.

The paper then considers the three areas pertaining to the vision behind active networks in turn. (1) With respect to capsules, the authors argue that it is feasible to carry capsules by reference and load them on demand and that their intrinsic processing overhead is low, but that since node capabilities are heterogeneous, it is important that not all nodes be slowed by capsule processing, resulting in only a partially active network. However, forwarding performance by capsules is very bad. (2) With respect to the question of who can use active networks to introduce new code into the network, security is obviously a major concern. The authors demonstrate that they were able to isolate code and state in ANTS in a way that is similar or better to static protocols used today, but that they handle the problem of global resource allocation using a certification mechanism, which has several drawbacks. (3) Lastly, in terms of services that ANTS can be used to introduce, such services tend to be ones that are typically difficult to deploy, such as multicast, anycast, and explicit congestion notification. ANTS can most compellingly be used for rapid deployment and experimentation, but the authors have yet to find a “killer app” that necessitates the use of ANTS. In order to use ANTS, a service must work under the following constraints: it must be able to be expressed using a restricted API, and it must be compact, fast, and incrementally deployable. The authors offer other observations given their experiences, touching on several points that they note all protocols must deal with in some way. There observations are that ANTS can help to enable systematic changes to protocols in the network, that it handles heterogeneous node capabilities in a clean way, that contrary to common concerns, ANTS does not generally violate the end-to-end argument, and that changes to network services must be localized in order to be easily deployable.

Critique

One thing I liked about ANTS is that it is one solution that at least talks about and attempts to address this issue of changing current Internet protocols and introducing new ones. It is sad how much of networking research never actually gets put into use just because of the difficulty of making changes or additions to the existing architecture. However, while it’s a nice idea, in reality the performance and security issues are too big to imagine ANTS ever being used in reality. One thing that annoyed me about this paper is how several times the author tried to write off the poor performance of ANTS by blaming it on Java, but at the same time credited many of the security benefits to the use of Java. It kind of seemed like the author was trying to have it both ways by saying some performance problems could be overcome by not using Java, but failing to mention that not using Java or something like it may also introduce greater security problems. It was for the best that the paper was written from the perspective of trying to take a closer look at some of the criticisms that people have raised concerning the original vision behind active networks, rather than arguing that ANTS should actually be used in the real world, since the performance and potentially the security problems are just too bad to suggest otherwise. I approve of the author framing the motivation and contribution in the way he did.