The HTTP operation measures the round-trip time (RTT) between a Cisco device and an HTTP server to retrieve a web page. The
HTTP server response time measurements consist of three types:
DNS lookupRTT taken to perform domain name lookup.
TCP Connect--RTT taken to perform a TCP connection to the HTTP server.
HTTP transaction time--RTT taken to send a request and get a response from the HTTP server. The operation retrieves only the
home HTML page.
The DNS operation is performed first and the DNS RTT is measured. Once the domain name is found, a TCP Connect operation to
the appropriate HTTP server is performed and the RTT for this operation is measured. The final operation is an HTTP request
and the RTT to retrieve the home HTML page from the HTTP server is measured. One other measurement is made and called the
time to first byte which measures the time from the start of the TCP Connect operation to the first HTML byte retrieved by
the HTTP operation. The total HTTP RTT is a sum of the DNS RTT, the TCP Connect RTT, and the HTTP RTT.
For GET requests, IP SLAs will format the request based on the specified URL. For RAW requests, IP SLAs requires the entire
content of the HTTP request. When a RAW request is configured, the raw commands are specified in HTTP RAW configuration mode.
A RAW request is flexible and allows you to control fields such as authentication. An HTTP request can be made through a proxy
The results of an HTTP operation can be useful in monitoring your web server performance levels by determining the RTT taken
to retrieve a web page.
Regardless of the HTTP errors, the IP SLA works fine. Currently, the error codes are determined, and the IP SLA HTTP operation
goes down only if the return code is not 200.
The only time the SLA probe goes down is when the SLA is unable to establish a TCP connection or is unable to receive an answer
from the Remote server to its HTTP request.