20.2 Accept-Encoding

   The Accept-Encoding header field is similar to Accept, but restricts
   the content-codings [H3.5] that are acceptable in the response.  See
   [H14.3].  The semantics in SIP are identical to those defined in
   [H14.3].

   An empty Accept-Encoding header field is permissible.  It is
   equivalent to Accept-Encoding: identity, that is, only the identity
   encoding, meaning no encoding, is permissible.

   If no Accept-Encoding header field is present, the server SHOULD
   assume a default value of identity.

   This differs slightly from the HTTP definition, which indicates that
   when not present, any encoding can be used, but the identity encoding
   is preferred.

   Example:

      Accept-Encoding: gzip

20.3 Accept-Language

   The Accept-Language header field is used in requests to indicate the
   preferred languages for reason phrases, session descriptions, or
   status responses carried as message bodies in the response.  If no
   Accept-Language header field is present, the server SHOULD assume all
   languages are acceptable to the client.

   The Accept-Language header field follows the syntax defined in
   [H14.4].  The rules for ordering the languages based on the "q"
   parameter apply to SIP as well.

   Example:

      Accept-Language: da, en-gb;q=0.8, en;q=0.7

20.4 Alert-Info

   When present in an INVITE request, the Alert-Info header field
   specifies an alternative ring tone to the UAS.  When present in a 180
   (Ringing) response, the Alert-Info header field specifies an
   alternative ringback tone to the UAC.  A typical usage is for a proxy
   to insert this header field to provide a distinctive ring feature.

   The Alert-Info header field can introduce security risks.  These
   risks and the ways to handle them are discussed in Section 20.9,
   which discusses the Call-Info header field since the risks are
   identical.

   In addition, a user SHOULD be able to disable this feature
   selectively.

      This helps prevent disruptions that could result from the use of
      this header field by untrusted elements.

   Example:

      Alert-Info: <http://www.example.com/sounds/moo.wav>

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