Ενεργά λέμε τα δίκτυα τα οποία επεξεργάζονται και τα περιεχόμενα (και όχι μόνο την επικεφαλίδα) των πακέτων που μετάγουν. Επικεντρωνόμαστε στα ενεργά δίκτυα ενθυλάκωσης, όπου ο προς εκτέλεση κώδικας συμπεριλαμβάνεται στα μεταγώμενα πακέτα, σε αντιδιαστολή με τους προγραμματιζόμενους μεταγωγείς. Παρουσιάζεται αναλυτικά το Active Networks Encapsulation Protocol (ANEP). Παρουσιάζονται δικτυακές εφαρμογές στις οποίες τα ενεργά δίκτυα βελτιώνουν την απόδοση της εφαρμογής και ταυτόχρονα μειώνουν τις απαιτήσεις σε δικτυακούς πόρους. Ακολούθως εστιάζουμε στην \"Ενεργή Αξιόπιστη Πολλαπλή Μετάδοση\", ένα πρωτόκολλο αξιόπιστης πολλαπλής μετάδοσης το οποίο χρησιμοποιεί την ενεργή μεταγωγή για να διαχειριστεί την ανάκτηση απωλειών πακέτων εντός του δικτύου (καταστολή NACK, λανθάνουσα μνήμη για πακέτα διόρθωσης, πολλαπλή μετάδοση περιορισμένης εμβάλειας). Παρέχονται αποτελέσματα προσομοιώσεων που υποστηρίζουν την θέση ότι ακόμα και με μικρό ποσοστό ενεργών κόμβων, ένα ενεργό δίκτυο μπορεί να βελτιώσει ουσιαστικά τις επιδόσεις της εφαρμογής και να μειώσει ταυτόχρονα την χρήση εύρους ζώνης. Κλείνοντας, κάνουμε κάποια τελικά σχόλια και εξάγουμε συμπεράσματα σχετικά με το υψηλό κόστος εγκατάστασης και συντήρησης των ενεργών δίκτυων, και πως αυτό αντιδιαστέλλεται με τα πλεονεκτήματα των τελευταίων σε σχέση με τις επιδόσεις των εφαρμογών και την χρήση των δικτυακών πόρων. / Active Networks are networks consisting (at least partially) of active nodes. A node is active if it doesn’t only processes a packet’s header in order to route it, but is also able to evaluate and process the packet’s payload. There are two kinds of active networks, depending on whether they are based on programmable switches or on capsules which bundle code together with the data. This dissertation focuses on the latter. The operational model of an active network of this kind comprises code execution models, network node management models and resource allocation policies. The Active Networks Encapsulation Protocol (ANEP) sets the mechanism for defining the platform required to evaluate the code that is encapsulated in the packet, as well the nodes’ behaviour when they do not support the required platform (drop the packet or simply forward it). This mechanism provides active networks with the flexibility to operate even when a very small percentage of the network’s node is actually active. There are various situations and where active networks can make better use of network resources. There are, for example, applications where different users might make similar, but different, requests resulting in unnecessary bandwith consumption when supported by conventional caching mechanisms. Active networks can provide smart caches that will dynamically synthesize pages from data cached by previous requests. Another situation where active networks can improve network performance is network applications like tele-conference, that depend heavily on new network services. Active networks allow for the faster deployment of new network services that enhance network speed and security and rationalise bandwidth usage through, for example multicast. Furthermore, active networks can support specialised applications, like for example on-line auctions, with custom-made network services. It is important to note that when measuring network performance, one should focus onto the network application’s performance, rather than network per-packet metrics like throughput and latency. In other words, intranetwork processing might increase both packet size and latency, but will improve the application’s end-to-end performance and reduce total network load. The protocols for three innovative network applications are presented: active reliable multicast, auctions over the network and remote sensor merging. For each of these we present network services that can be easily implemented and deployed in active networks to improve application performance. Finally, a more detailed analysis (by means of simulation) of an active reliable multicast protocol is presented. Active networks achieve two ends: on the one hand they push the idea of a network proxy to its logical end by effectively turning all network elements into smart proxies that provide caching, filtering, NACK suppression and other services. On the other hand they carry out part of the computation inside the network, bringing it closer to the data sources. When the computation is, for example, data merging this is beneficial to both the application and the network resources. This, however, can only be achieved at a cost. First of all in hardware, since network elements have to be upgraded from simple routers to full-blown computers capable of supporting Java and scripting languages. But also in latency, since packets have to undergo much more complex processing along the way that simple routing. In the applications presented here the costs associated with active transport are counter-balanced by the advantages the latter has to offer to the application as well as to the network. The bet that active networks have to win in order to get widely accepted, is to have enough active application protocols developed that their installation and maintenance cost can be justified.