Online holiday spending, excluding travel and auctions, was $8.41 billion between Nov. 1 through Dec. 5, up 23 percent from the year-ago period and on track to meet estimates, comScore Networks (search) said Friday.

"More than half of all online spending that will take place during the holiday season is complete," comScore said.

The holiday forecast from comScore, which covers November and December, calls for online sales of up to $15.5 billion this year, a 26 percent increase from 2003.

The biggest online shopping day of 2003 was Dec. 9 — a Tuesday — when online retailers rang up sales of $428 million.

This year's peak is expected to happen late this week or early next week and sales should top $500 million, comScore said.

Meanwhile, the performance of top retail sites such as EddieBauer.com (search), Walmart.com (WMT) and Target.com (TGT) is about equal to last year, even as sites have become more complex, said Roopak Patel, senior Internet analyst the public services division at Keynote Systems Inc (search).

Those sites are slightly more reliable — meaning users can complete the tasks they set out to complete — this year than last. To that end, reliability has risen to 98 percent from 97 percent last year, Patel said.

Nevertheless, there have been bumps in the road.

Sites slowed down and some users had a hard time completing tasks on the Friday and Monday after Thanksgiving — which are among the busiest online shopping days.

Earlier this week, popular online retailer Amazon.com (AMZN) had intermittent outages that Keynote said lasted for more than half a day and made the site virtually unavailable to one in five people.