Relative Path, but for Port?

Question

We are all familiar with relative paths: A relative path to ./images/hello.jpg from http://www.domain.com/hey links to http://www.domain.com/

Answer 1

You cannot change any part of the authority (i.e. the host:port part) in relative URLs. See the algorithm described in section 5.2.2 of RFC 3986 to see how relative URLs are interpreted. Important thing to notice is that authority is simply copied from base URL or from the URL being resolved and authority's structure is never interpreted. This implies that you cannot change any of its parts, including the port part.

Here's the algorithm in pseudo-code copied from the RFC:

  -- The URI reference is parsed into the five URI components
  --
  (R.scheme, R.authority, R.path, R.query, R.fragment) = parse(R);

  -- A non-strict parser may ignore a scheme in the reference
  -- if it is identical to the base URI's scheme.
  --
  if ((not strict) and (R.scheme == Base.scheme)) then
     undefine(R.scheme);
  endif;

  if defined(R.scheme) then
     T.scheme    = R.scheme;
     T.authority = R.authority;
     T.path      = remove_dot_segments(R.path);
     T.query     = R.query;
  else
     if defined(R.authority) then
        T.authority = R.authority;
        T.path      = remove_dot_segments(R.path);
        T.query     = R.query;
     else
        if (R.path == "") then
           T.path = Base.path;
           if defined(R.query) then
              T.query = R.query;
           else
              T.query = Base.query;
           endif;
        else
           if (R.path starts-with "/") then
              T.path = remove_dot_segments(R.path);
           else
              T.path = merge(Base.path, R.path);
              T.path = remove_dot_segments(T.path);
           endif;
           T.query = R.query;
        endif;
        T.authority = Base.authority;
     endif;
     T.scheme = Base.scheme;
  endif;

  T.fragment = R.fragment;