headerResponseTime:

 The time between sending a request to the server, any processing of 

the request by the server and the receipt of the reply headers. However, if the request has an 

entity body this will impact on the time to send the request to the server. 

entityDownloadRate:

 Indicated how fast the server and the network connection between the 

server and client can return a reply message body to the client. Again some replies may not 

have an entity body as would for with a status code of 1xx (informational), 204 (no content), 

and 304 (not modified) responses. 

So as has been established, not all derived parameters can be used in all situations. However, 

the option to derive a suitable parameter for a particular situation still exists enabling useful 

information to be generated. 

3.4.2 Service Availability Parameters 

Availability parameters are parameters that are use to indicate the availability of a service 

delivered to the end user. These parameters can be broken down into three main types of 

parameters that can indicate service failure. 

Connection failures to the service such as a network failure or failure to resolve a 

server name. These result in the client not being able to open a HTTP connection to 

the server 

Server Errors occur when the client can make a connection a HTTP server but a error 

in the server occurs and results in a reply status code of the 5xx variety 

Service Errors where a backend component that the service depends on (such as a 

database), fails and results in the service failure. No mechanism is implemented in 

HTTP to indicate this as it is not really related to HTTP protocol itself, but more to 

the service that is being delivered over HTTP.