This process is often delayed, however, because some of the packets get dropped along the way. When this happens, the receiving computer waits for the truant packets for a while, then asks for them to be resent. All in all, a single missing packet can really slow things down.
It's tough to eliminate packet loss completely, but an enhancement to the basic language of the Internet known as Explicit Congestion Notification will reduce it somewhat, hopefully giving a significant boost to certain types of transactions.
By adding a bit of information to the header, ECN will reduce the number of packets lost during transmission, but it's not clear how dramatic the effect will be.
For now, ECN applies only to conventional Web traffic such as email and Web pages. But researchers hope to incorporate ECN into streaming media transmissions, where packet loss poses the biggest problem.
The Internet Engineering Task Force, a standards body, is on the verge of making ECN a proposed official standard. Once a concept reaches proposed-standard status, vendors of Internet equipment start to implement it in their latest products. Cisco Systems is already testing ECN, and expects to make the technology available in its routers by early next year.
"The ECN idea is potentially very good, especially for emerging applications such as streaming applications," says Mike Luby, chief technical officer at Digital Fountain, a start-up with technology for speeding up streaming media transmissions. "However, it is not clear that wide-scale ECN deployment would dramatically improve the overall quality of the Internet, and it is hard to determine this without a significant deployment."
"It's felt that it makes a difference since you, the receiver, don't have to wait around for a packet that never shows up," says Scott Bradner, a leader of the IETF's Transport Area working group, the group working on ECN. "Whether that means a 2 percent difference or a 20 percent difference depends on the circumstances. The real world is a very unpredictable thing."
ECN could have a more dramatic effect when combined with Internet Protocol Multicast. Multicast is a way of efficiently sending the same stream of data to many users at once. So, with multicast, the effects of ECN are magnified with each router hop. However, numerous technical problems have kept multicast from being widely deployed. For now, says Sally Floyd, a researcher at the AT&T Centre for Internet Research, ECN is "just an improvement. It will be more useful later, for multicast and streaming media."
Traffic Rules on the Internet
Ordinary Internet traffic follows a basic set of traffic laws known as the Transmission Control Protocol (TCP).
Since the late 1980s, when rising Net traffic led to severe congestion problems, TCP has included a congestion-control mechanism that instructs a transmitting computer to slow the flow of data by half while the network is congested.
So, in effect, congestion, in the form of dropped packets, is the error signal that gives the sender the feedback to speed up or slow down its transmission rate.
Congestion occurs at routers, the traffic cops at the intersections of the Internet. When a router is deluged with packets, they start to pile up in its buffer, which will eventually overflow. "Buffer overflow means a lot of packets get dropped at once, and you didn't need to drop that many to tell people to slow down," Floyd says.
Since it's inefficient for routers to suddenly boil over and start dropping all packets, many routers use what's known as active queue management.
With active queue management, as a router's buffer gets close to capacity, it will start dropping some packets at random, with the number of packets being dropped increasing as the buffer limit approaches. The transmitting computer learns of the congestion when it fails to receive acknowledgments from the receiving computer within a certain time frame and this becomes the signal to slow its sending rate.
This system is inefficient, however, since the packet drops are necessary only as a signal for the transmitting computer to slow down. This is where ECN comes in.
ECN came about because of two, superfluous bits in the packet header left over from the implementation of quality of service for packets, where companies can pay extra to make their traffic high priority. Eight bits were set aside for QOS, but only six were actually used. The remaining two bits came up for grabs and ended up being used for ECN.
