keep-alive 

(Optional) Boolean that indicates whether the HTTP client should attempt to use persistent connections. The default is true.

keep-alive-timeout 

(Optional) The maximum number of seconds to keep a persistent connection open. The default is 29.

always-use-keep-alive 

(Optional) Boolean that indicates whether the HTTP client can reuse existing persistent connections for all types of requests. The default is false, meaning persistent connections will not be reused for non-GET requests nor for requests with a body.

protocol 

(Optional) HTTP protocol version string. By default, the HTTP client uses either "HTTP/1.0" or "HTTP/1.1" based on the contents of the HTTP request. Do not use the protocol parameter unless you encounter specific protocol interoperability problems. 

proxy-agent 

(Optional) Value of the Proxy-agent HTTP request header. The default is a string that contains the proxy server product name and version.