which interfaces or connections mattered to it. Such features, when they were implemented, were a part of the routing software within the operating system. The only way programs could get access to them was through an obscure set of nonstandard kernel APIs called a routing socket.

On a system with multiple network interfaces it is not possible, using the standard sockets API, to write an application that can easily be multihomed— that is, take advantage of both interfaces so if one fails, or if the primary route over which the packets were flowing breaks, the application would not lose its connection to the server.

The recently developed Stream Control Transport Protocol (SCTP) ⁴ incorporates support for multihoming at the protocol level, but it is impossible to export this support through the sockets API. Several ad-hoc system calls were initially provided and are the only way to access this functionality. At the moment this is the only protocol that has both the capacity and user demand for this feature, so the API has not been standardized across more than a few operating systems. Table 2 shows the APIs that SCTP added.

While the list of functions in Table 2 contains more APIs than are strictly necessary, it is important to note that many are derivatives of preexisting APIs, such as send(), which need to be extended to work in a multihoming world. The set of APIs needs to be harmonized to make multihoming a first-class citizen in the sockets world. The problem now is that sockets are so successful and ubiquitous that it is very hard to change the existing API set for fear of confusing its users or the preexisting programs that use it.

As systems come to have more network interfaces built in, providing the ability to write applications that take advantage of multihoming will be an absolute necessity. One can easily imagine the use of such technology in a smartphone, which already has three network interfaces: its primary connection via the cellular network, a WiFi interface, and often a Bluetooth interface as well. There is no reason for an application to lose connectivity if even one of these network interfaces is working properly. The problem for application designers is that they want their code

as systems come
to have more
network interfaces
built in, providing
the ability to write
applications that
take advantage
of multihoming
will be an absolute
necessity.

to work, with few or no changes, across a plethora of devices, from cellphones, to laptops, to desktops, and so on. With properly defined APIs we would remove the artificial barrier that prevents this. It is only because of the history of the sockets API and the fact that it has been “good enough” to date that this need has not yet been addressed.

High bandwidth, low latency, and multihoming are driving the development of alternatives to the sockets API. With LANs now reaching 10Gbps, it is obvious that for many applications client/server style communication is far too inefficient to use the available bandwidth. The communication paradigms supported by the sockets API must be expanded to allow for memory sharing across the kernel boundary, as well as for lower-latency mechanisms to deliver data to applications. Multihoming must become a first-class feature of the sockets API because devices with multiple active interfaces are now becoming the norm for networked systems.

 

Related articles on queue.acm.org

Code Spelunking: Exploring

Cavernous Code Bases

George Neville-Neil

http://queue.acm.org/detail.cfm?id=945136

API Design Matters

Michi Henning

http://queue.acm.org/detail.cfm?id=1255422

You Don’t Know Jack about Network Performance

Kevin Fall and Steve McCanne

http://queue.acm.org/detail.cfm?id=1066069

References

1. balaji, P., bhagvat, s., Jin, h.-W., and Panda, D.k. asynchronous zero-copy communication for synchronous sockets in the sockets direct protocol (sdp) over infiniband journal. in Proceedings of the 20th IEEE International Parallel and Distributed Processing Symposium.

2. gilligan, r., thomson, s., bound, J., mccann, J., and stevens, W. basic socket interface extensions for iPv6. rfc 3493 (feb. 2003); http://www.rfc-editor. org/rfc/rfc3493.txt.

3. romanow, a., mogul, J., talpey, t., and bailey, s. remote Direct memory access (rDma) over iP Problem statement. rfc 4297 (Dec. 2005); http:// www.rfc-editor.org/rfc/rfc4297.txt.

44. stewart, r., et al. stream control transmission Protocol. rfc 2960 (oct. 2000); http://www.ietf.org/ rfc/rfc2960.txt.

George V. Neville-Neil ( [e-mail]) is the proprietor of neville-neil consulting. he works on networking and operating systems code and teaches courses on program-related topics.

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