A core architectural building block of the Internet is the open protocol. A protocolis the rules that govern the transmission of data. The Internet consists of manysuch protocols, ranging from those that direct the physical transmission ofdata to those that govern the most common Internet applications, like emailor web browsing. Crucially, all these protocols are open, in that anyone canset up and operate a router, website, or email server without needing toregister with or get permission from a central authority.5 Open protocolswere key to the first phase of the Internet’s growth because they enabledunfettered access, removing barriers and bridging gaps between differentcommunities. This enabled and encouraged interactions between groupswith various interests and knowledge, resulting in immense creativity andidea-sharing.

This goal caused TCP and IP, which originally had beena single protocol in the architecture, to be separated intotwo layers. TCP provided one particular type of service,the reliable sequenceddata stream, while IP attempted toprovide a basic building block out of which a variety oftypes of service could be built. This building block wasthe datagram, which had also been adopted to supportsurvivability. Since the reliability associated with thedelivery of a datagram was not guaranteed, but “besteffort,” it was possible to build out of the datagram aservice that was reliable (by acknowledging andretransmitting at a higher level), or a service which tradedreliability for the primitive delay characteristics of theunderlying network substrate. The User DatagramProtocol (UDP)13 was created to provide a application-level interface to the basic datagram service of Internet.

A complex system that works is invariably found to have evolved from a simple system that worked. A complex system designed from scratch never works and cannot be patched up to make it work. You have to start over with a working simple system.

EvoArch suggests an additional reason that IPv4 has been so sta-ble over the last three decades. Recall that a large birth rate atthe layer above the waist can cause a lethal drop in the normalizedvalue of the kernel, if the latter is not chosen as substrate by thenew nodes. In the current Internet architecture, the waist is the net-work layer but the next higher layer (transport) is also very narrowand stable. So, the transport layer acts as an evolutionary shield forIPv4 because any new protocols at the transport layer are unlikelyto survive the competition with TCP and UDP. On the other hand,a large number of births at the layer above TCP or UDP (applica-tion protocols or specific applications) is unlikely to significantlyaffect the value of those two transport protocols because they al-ready have many products. In summary, the stability of the twotransport protocols adds to the stability of IPv4, by eliminating anypotential new transport protocols that could select a new networklayer protocol instead of IPv4.

The EvoArch model predicts the emergence of few powerful and old protocols in the middle layers, referred to as evolutionary kernels. The evolutionary kernels of the Internet architecture include IPv4 in the network layer, and TCP and the User Datagram Protocol (UDP) in the transport layer. These protocols provide a stable framework through which an always-expanding set of physical and data-link layer protocols, as well as new applications and services at the higher layers, can interoperate and grow. At the same time, however, those three kernel protocols have been difficult to replace, or even modify significantly.