Difference between SIP and SIPS URI?

Question

I am reading SIP RFC 3261. I didn't get the difference between sip uri and sips uri. Can Some one help.

Answer 1

First note from rfc3261:

The format for a SIPS URI is the same, except that the scheme is
"sips" instead of sip.

Second note, which I guess is your question can be found in section 26.2.2 (SIPS URI Scheme), ie: it indicates that all hops needs to be secured, most commonly over TLS link.

When used as the Request-URI of a
request, the SIPS scheme signifies that each hop over which the
request is forwarded, until the request reaches the SIP entity
responsible for the domain portion of the Request-URI, must be
secured with TLS; once it reaches the domain in question it is
handled in accordance with local security and routing policy, quite
possibly using TLS for any last hop to a UAS.  When used by the
originator of a request (as would be the case if they employed a SIPS
URI as the address-of-record of the target), SIPS dictates that the
entire request path to the target domain be so secured.

Other rules apply when sips appears in a To or From header. For example, when sips appears in a To header, it indicates the "wish" to use secure links. When used in the Request-URI, it's a MUST.

When using sips, you may also needs to use sips in Contact headers. The sips URI will end up being used in Request-URI for other request within dialog, so you have to care about using the right sip/sips in Contact headers.