Marshall, Ian W.

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Application Layer Active NetworksIan Marshall, BT Labs, Martlesham Heath, Ipswich, IP5 3REIan.W.Marshall@bt.comPositionThe deployment of new communication services is currently restricted by slowstandardisation, the difficulties of adding new capabilities to existing equipment, and thecomplexity of the existing network.  It has been proposed [1] that these difficulties couldbe overcome by adding programmes to packet headers which are intended to run onnetwork devices the packet encounters.  However service operators will never permitthird party programmes to run on their equipment without prior testing, and a strongguarantee that the programme will not degrade performance for other users.  Since it willbe extremely hard to create interesting programmes, in a language which is simpleenough to enable termination guarantees, we regard this approach as unrealistic.A somewhat different flavour of active networking in which the packets do not carryprogrammes, but flags indicating the desirability of running a programme has also beenproposed [2].  This would enable the service operator to choose a programme to load,which matches the indicated need, but is sourced from his own database of testedprogrammes.  We believe that this second approach may be valuable in the long term,however in order not to degrade router performance the number of flags will need to besmall and the number of possible programmes will thus also be small.  In addition it willnot resolve the immediate need for increased flexibility, as it will require standardisation,and will thus take time.We therefore propose a third alternative, which we call application layer networking[3].  In this system the network is populated with application level entities we refer to asservice nodes, or dynamic proxy servers.  These could be thought of as equivalent to thehttp caches currently deployed around the internet, but with a hugely increased and moredynamic set of capabilities.  This approach relies on redirecting selected packets into theapplication layer device, where programmes can be run to modify the content, or thecommunication mechanisms.  The programmes can be selected from a trusted datasource, and can be run without impacting router performance.  Packets are redirected on asession by session and protocol by protocol basis, so there is no need for additional flagsor standardisation.  Programmes are chosen using the mime type of the content, so againno additional data or standards are required.In our initial implementation, programmes to be run on the dynamic proxy server areimplemented as java beans for portability reasons, and the proxy server itself is a javaapplication. The beans can be loaded on demand and are referred to as proxylets (ananalogy with applets and serverlets).  A proxylet is a specific type of bean, whichsupports the operations Load, Run, Stop and Modify.  If stateful programmes are requiredit is necessary to add further operations to the list such as Suspend (stores state pendingrestart – possibly at another location).  The redirection of packets in a session carrying aparticular protocol (such as http) to the proxy is initiated either by the client of the sessionor by the server.  This avoids the issues associated with transparent redirect, and in ourview is preferable, but transparent redirect would be possible.We have shown, by implementing appropriate proxylets, that our approach offersbenefits for streaming of retreived audio and video files, for link compression, forenabling multicast content to propagate over unicast links, and for bridging tcp sessions.We are currently developing proxylets which perform authentication (for mobile access),content transcoding (for caching dynamic content and for adapting to low functionalityclients), dynamic redirection, and message queuing.  We anticipate demonstratingbenefits for all the proxylets under development.  We are aiming to release code for widearea experiments in 12-18 months, after we have designed a management andconfiguration system for the dynamic proxy server, and after we have created aperformance optimised implementation of the server.We believe that application layer active networking has a crucial role to play, bothrelatively soon and in the long term, particularly for applications where latencies of a fewhundred milliseconds are acceptable. If application layer active networking is deployedthe relatively small number of higher performance requirements could be handled atlower layers by placing flags in packets without requiring large numbers of flags andpotentially negating the benefitsReferences[1] D.Tennenhouse, D.Wetherall, “Towards an active network architecture” Computercommunication Review, 26, 2 (1996), pp5-18[2] Alexander, Shaw, Nettles and Smith “Active Bridging” Computer CommunicationReview, 27, 4 (1997), pp101-111[3] M.Fry and A.Ghosh “Application layer active networking”  HIPPARCH ’98Workshop

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