Structured Streams: A New Transport Abstraction December 4, 2007

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Google Tech Talks
November, 27 2007

Internet applications currently have a choice between
stream and datagram transport abstractions. Datagrams
efficiently support small transactions and streams are
suited for long-running conversations, but neither
abstraction adequately supports applications like HTTP that
exhibit a mixture of transaction sizes, or applications like
FTP and SIP that use multiple transport instances.
Structured Stream Transport (SST) enhances the traditional
stream abstraction with a hierarchical hereditary structure,
allowing applications to create lightweight child streams
from any existing stream. Unlike TCP streams, these
lightweight streams incur neither 3-way handshaking delays
on startup nor TIME-WAIT periods on close. Each stream
offers independent data transfer and flow control, allowing
different transactions to proceed in parallel without
head-of-line blocking, but all streams share one congestion
control context. SST supports both reliable and best-effort
delivery in a way that semantically unifies datagrams with
streams and solves the classic "large datagram" problem,
where a datagram's loss probability increases exponentially
with fragment count. Finally, an application can prioritize
its streams relative to each other and adjust priorities
dynamically through out-of-band signaling. A user-space
prototype shows that SST is TCP-friendly to within 2%, and
performs comparably to a user-space TCP and to within 10% of
kernel TCP on a WiFi network.