Semantic-Free Referencing

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At a logical level, the Web requires a Reference Resolution Service (RRS) to map from references (also known as links) to actual network locations. In the current Web, references are URLs with a hostname/pathname structure, and DNS serves as the RRS by mapping the hostname to an IP address where the target is stored. Our premise is that the Web would be better served by a new RRS, one that does not impose the limits of the current DNS. As others, most notably those in the URN community, have pointed out [1], an RRS for the Web should have at least the following two characteristics (neither of which DNS-based URLs have):


We believe that the simplest way to achieve persistence and freedom from contention is to use a reference namespace devoid of explicit semantics, meaning that a reference should neither embed information about the organization, administrative domain, or network provider it originated in or in which it is currently located, nor be human-friendly. We call such references semantic-free.

Unlike DNS-based URLs, flat, semantic-free references have no explicit structure to give resolution hints. Until recently, there was no way to resolve references scalably in such a namespace, which is largely why the URN literature chose to use a partitioned set of context-specific resolvers. However, the recently developed Distributed Hash Table (DHT) technology is exactly designed to map from an unstructured key to a network location responsible for the key. SFR uses DHTs to map each object reference to a machine that contains object meta-data, such as the object's current IP address and the pathname of the object. Once an application, such as a Web browser, has this meta-data, it can retrieve the actual object.

We imagine that SFR would be deployed on a managed infrastructure (as mentioned above, we believe the namespace should be unmanaged; the infrastructure, however, is a different story), not on the desktops of random cable modem users. To repeat: even though SFR uses DHTs, which are a so-called peer-to-peer technology, we are not relying on flaky personal machines connected via cable modems!

Benefits and Challenges

A version of the Web that used SFR instead of DNS would realize certain benefits, including:
resilient linking and seamless migration
avoid messages like "Please update the referrer that this Web page moved";
flexible replication
this includes allowing individuals to host content for each other without making clients select among mirror sites;
reliable pointer services
individuals can link to each other's links instead of having to link directly to objects;
extensibility and generality
because SFR provides a very general, application-independent interface, other applications besides the Web could get referencing functions.

SFR's features naturally do not come without cost since many of the desirable features of today's Web derive from DNS. As examples, DNS's hierarchical structure enforces URLs' uniqueness and provides fate sharing (a disconnected institution can still access local pages) while the human readability of DNS hostnames gives users some (perhaps misguided) confidence they have reached their desired data. Some of the challenges SFR must address include:

fast lookups
security and integrity
fate sharing
canonical names
users need easy ways to communicate references to each other;
users need to have confidence in the data they are viewing.
For more information, please see our papers or contact us.


[1] Besides the URN literature, similar observations have also been made by: (a) Michael O'Donnell, in his Proposal to separate Internet handles from names; (b) Bob Frankston in this essay; and (c) the Globe Project (see especially the paper Locating objects in wide-area systems, IEEE Communications Magazine, January 1998; here is ps or pdf).
[2] The current approach, in which individuals maintain Web pages with domains like, does not allow an individual Web object (such as a Web page or a directory of photographs) to separate from its original site. Also, even today, this approach might be awkward or inappropriate in contexts when content should not be named by a particular individual.


The following papers give more information about SFR. The first is a position paper for a workshop; it outlines an earlier version of our philosophy. The second is a report for a student workshop and contains a later version of our philosophy. The third is a full-length conference paper and contains the most refined statement of our philosophy along with design and implementation details.



We plan to release a prototype of SFR shortly; this prototype is layered on top of the Chord and DHash system.


Hari Balakrishnan   Scott Shenker   Michael Walfish  


This project is being conducted as part of the IRIS project, supported by the National Science Foundation under Cooperative Agreement No. ANI-0225660.

Contact Us

We welcome comments, questions, and feedback. Please send e-mail to sfr-n at


M. I. T. Laboratory for Computer Science · 200 Technology Square · Cambridge, MA 02139 · USA