Why is Connection: keep-alive still being specified in http headers (isn't it deprecated)?

Question

According to "HTTP: The Definitive Guide", using

Connection: keep-alive

to specify a persistent connection is deprecated in HTTP/1.1, since HTTP/1.1 specifies that connections are persistent by default and must be closed manually by sending

Connection: close

Thus, my simple assumption is that "Connection: keep-alive" shouldn't really be used anymore. However, it still seems alive and well. For example, keep-alive is being returned in the following query:

curl -I https://foursquare.com

HTTP/1.1 200 OK
Server: nginx/0.8.52
Date: Thu, 11 Aug 2011 21:15:45 GMT
Content-Type: text/html; charset=utf-8
Connection: keep-alive
Expires: Thu, 11 Aug 2011 21:15:45 UTC
Set-Cookie: XSESSIONID=w19~kqtn4bpqmfq51p8qolstpk6ti;Path=/;Secure;HttpOnly
Set-Cookie: LOCATION=49.25::-123.13330078125::Hockeytown::CA;Path=/;Secure
Set-Cookie: bbhive=OQ32XATE0OQAEVCY0IVSWUDPQ1A2GT
Content-Length: 38815
Cache-Control: no-cache, private, no-store
Pragma: no-cache

My question is: Why is Connection: keep-alive still being specified in HTTP headers?

A corollary question is: Are there still (clients, servers, proxies, etc) that still only speak HTTP/1.0 and its variants, or are most such entities on HTTP/1.1 as of 2011?

Here are my working hypotheses:

1) HTTP/1.0 is no longer in use, b/c that was "many years" ago

2) Given (1), keep-alive shouldn't be used anymore, but is purely for vestigial reasons (that is, certain technologies haven't bothered to remove it, or keep it around as voodoo code, etc.)

If (1) is incorrect, and HTTP/1.0 is still in use, then sure it seems plausible to keep using keep-alive, despite follow-up questions on HTTP 1.0-1.1 interop.

Thanks in advance for any insights shared!

Answer 1

HTTP/1.0 have no headers like Connection, but there is many different implementation of HTTP/1.0 and HTTP/1.1.

so Connection: keep-alive is used 'Just in case'