TCP connection proliferation. Increasing concurrent connections through browser policy and sharding can improve page-load time but create other problems. Though highly concurrent connections circumvent an overly conservative slow start on a single TCP connection, they may (in aggregate) exceed total available bandwidth, inducing packet loss. Moreover, the likelihood of losing a critical control packet increases with the number of concurrent connections; for example, the TCP SYN packet, which initiates a TCP connection, has a retransmission timeout measured on the order of seconds if no acknowledgment is received.

Highly concurrent TCP connec-
tions also decrease the likelihood of
fast retransmit being invoked under
packet loss. Fast retransmit is a TCP
enhancement that immediately re-
sends a packet without waiting for
a fixed timeout delay if acknowledg-
ments for several packets subsequent
to the lost packet are received. High-
ly concurrent connections obtain
less bandwidth individually than a
single long-lived connection and are
therefore less likely to receive and ac-
knowledge enough packets in a short
enough duration to trigger fast re-
transmit. There is also less “body” in
each short-lived connection, increas-
ing the likelihood that any packet loss
would occur near the end of a connec-
tion where too few acknowledgments
exist to trigger fast retransmit.

to competing applications still using multiple TCP connections; for example, a backoff algorithm that responds to packet loss by reducing the available bandwidth of a connection by 50% will likewise halve the total bandwidth available to an application using a single SPDY TCP connection. The same backoff algorithm applied to an application using 12 concurrent TCP connections would reduce the total available bandwidth to the application by only 4% (1/24) of the connections. Connection proliferation should not be encouraged over the long term, though a short-term mitigation strategy would involve using a small number of concurrent SPDY connections. Long-term research may look to address this issue through smarter backoff algorithms providing equitable treatment to applications, independent of the number of TCP connections.

 

transitioning to sPDY SPDY has been implemented in several popular client browsers, most notably Chrome and Firefox. Though server support for SPDY continues to grow, it has yet to reach the maturity and adoption of client implementations. SPDY gateways are one way to acceler-

 

No SPDY Gateway
client
translateexample.com
example.com is 192.0.43.10

DNS name server
example.com
(192.0.43.10)

GETexample.com/home

200 OK
With SPDY Gateway
client
Client sends
request
without
translating
domain name
GETexample.com/home
SPDY gateway
DNS name server
translateexample.com
GETexample.com/home
example.com is 192.0.43.10

200 OK

200 OK december 2012 | vol. 55 | no. 12 | communications of the acm 71