Section 19 Common Message Components

There are certain components of SIP messages that appear in various
   places within SIP messages (and sometimes, outside of them) that
   merit separate discussion.
19.1 SIP and SIPS Uniform Resource Indicators
   A SIP or SIPS URI identifies a communications resource.  Like all
   URIs, SIP and SIPS URIs may be placed in web pages, email messages,
   or printed literature.  They contain sufficient information to
   initiate and maintain a communication session with the resource.
   Examples of communications resources include the following:
      o  a user of an online service
      o  an appearance on a multi-line phone
      o  a mailbox on a messaging system
      o  a PSTN number at a gateway service
      o  a group (such as "sales" or "helpdesk") in an organization
   A SIPS URI specifies that the resource be contacted securely.  This
   means, in particular, that TLS is to be used between the UAC and the
   domain that owns the URI.  From there, secure communications are used
   to reach the user, where the specific security mechanism depends on
   the policy of the domain.  Any resource described by a SIP URI can be
   "upgraded" to a SIPS URI by just changing the scheme, if it is
   desired to communicate with that resource securely.
19.1.1 SIP and SIPS URI Components
   The "sip:" and "sips:" schemes follow the guidelines in RFC 2396 [5].
   They use a form similar to the mailto URL, allowing the specification
   of SIP request-header fields and the SIP message-body.  This makes it
   possible to specify the subject, media type, or urgency of sessions
   initiated by using a URI on a web page or in an email message.  The
   formal syntax for a SIP or SIPS URI is presented in Section 25.  Its
   general form, in the case of a SIP URI, is:
      sip:user:password@host:port;uri-parameters?headers
   The format for a SIPS URI is the same, except that the scheme is
   "sips" instead of sip.  These tokens, and some of the tokens in their
   expansions, have the following meanings:
      user: The identifier of a particular resource at the host being
         addressed.  The term "host" in this context frequently refers
         to a domain.  The "userinfo" of a URI consists of this user
         field, the password field, and the @ sign following them.  The
         userinfo part of a URI is optional and MAY be absent when the
         destination host does not have a notion of users or when the
         host itself is the resource being identified.  If the @ sign is
         present in a SIP or SIPS URI, the user field MUST NOT be empty.
         If the host being addressed can process telephone numbers, for
         instance, an Internet telephony gateway, a telephone-
         subscriber field defined in RFC 2806 [9] MAY be used to
         populate the user field.  There are special escaping rules for
         encoding telephone-subscriber fields in SIP and SIPS URIs
         described in Section 19.1.2.
      password: A password associated with the user.  While the SIP and
         SIPS URI syntax allows this field to be present, its use is NOT
         RECOMMENDED, because the passing of authentication information
         in clear text (such as URIs) has proven to be a security risk
         in almost every case where it has been used.  For instance,
         transporting a PIN number in this field exposes the PIN.
         Note that the password field is just an extension of the user
         portion.  Implementations not wishing to give special
         significance to the password portion of the field MAY simply
         treat "user:password" as a single string.
      host: The host providing the SIP resource.  The host part contains
         either a fully-qualified domain name or numeric IPv4 or IPv6
         address.  Using the fully-qualified domain name form is
         RECOMMENDED whenever possible.
      port: The port number where the request is to be sent.
      URI parameters: Parameters affecting a request constructed from
         the URI.
         URI parameters are added after the hostport component and are
         separated by semi-colons.
         URI parameters take the form:
            parameter-name "=" parameter-value
         Even though an arbitrary number of URI parameters may be
         included in a URI, any given parameter-name MUST NOT appear
         more than once.
         This extensible mechanism includes the transport, maddr, ttl,
         user, method and lr parameters.
         The transport parameter determines the transport mechanism to
         be used for sending SIP messages, as specified in [4].  SIP can
         use any network transport protocol.  Parameter names are
         defined for UDP (RFC 768 [14]), TCP (RFC 761 [15]), and SCTP
         (RFC 2960 [16]).  For a SIPS URI, the transport parameter MUST
         indicate a reliable transport.
         The maddr parameter indicates the server address to be
         contacted for this user, overriding any address derived from
         the host field.  When an maddr parameter is present, the port
         and transport components of the URI apply to the address
         indicated in the maddr parameter value.  [4] describes the
         proper interpretation of the transport, maddr, and hostport in
         order to obtain the destination address, port, and transport
         for sending a request.
         The maddr field has been used as a simple form of loose source
         routing.  It allows a URI to specify a proxy that must be
         traversed en-route to the destination.  Continuing to use the
         maddr parameter this way is strongly discouraged (the
         mechanisms that enable it are deprecated).  Implementations
         should instead use the Route mechanism described in this
         document, establishing a pre-existing route set if necessary
         (see Section 8.1.1.1).  This provides a full URI to describe
         the node to be traversed.
         The ttl parameter determines the time-to-live value of the UDP
         multicast packet and MUST only be used if maddr is a multicast
         address and the transport protocol is UDP.  For example, to
         specify a call to alice@atlanta.com using multicast to
         239.255.255.1 with a ttl of 15, the following URI would be
         used:
            sip:alice@atlanta.com;maddr=239.255.255.1;ttl=15
         The set of valid telephone-subscriber strings is a subset of
         valid user strings.  The user URI parameter exists to
         distinguish telephone numbers from user names that happen to
         look like telephone numbers.  If the user string contains a
         telephone number formatted as a telephone-subscriber, the user
         parameter value "phone" SHOULD be present.  Even without this
         parameter, recipients of SIP and SIPS URIs MAY interpret the
         pre-@ part as a telephone number if local restrictions on the
         name space for user name allow it.
         The method of the SIP request constructed from the URI can be
         specified with the method parameter.
         The lr parameter, when present, indicates that the element
         responsible for this resource implements the routing mechanisms
         specified in this document.  This parameter will be used in the
         URIs proxies place into Record-Route header field values, and
         may appear in the URIs in a pre-existing route set.
         This parameter is used to achieve backwards compatibility with
         systems implementing the strict-routing mechanisms of RFC 2543
         and the rfc2543bis drafts up to bis-05.  An element preparing
         to send a request based on a URI not containing this parameter
         can assume the receiving element implements strict-routing and
         reformat the message to preserve the information in the
         Request-URI.
         Since the uri-parameter mechanism is extensible, SIP elements
         MUST silently ignore any uri-parameters that they do not
         understand.
      Headers: Header fields to be included in a request constructed
         from the URI.
         Headers fields in the SIP request can be specified with the "?"
         mechanism within a URI.  The header names and values are
         encoded in ampersand separated hname = hvalue pairs.  The
         special hname "body" indicates that the associated hvalue is
         the message-body of the SIP request.
   Table 1 summarizes the use of SIP and SIPS URI components based on
   the context in which the URI appears.  The external column describes
   URIs appearing anywhere outside of a SIP message, for instance on a
   web page or business card.  Entries marked "m" are mandatory, those
   marked "o" are optional, and those marked "-" are not allowed.
   Elements processing URIs SHOULD ignore any disallowed components if
   they are present.  The second column indicates the default value of
   an optional element if it is not present.  "--" indicates that the
   element is either not optional, or has no default value.
   URIs in Contact header fields have different restrictions depending
   on the context in which the header field appears.  One set applies to
   messages that establish and maintain dialogs (INVITE and its 200 (OK)
   response).  The other applies to registration and redirection
   messages (REGISTER, its 200 (OK) response, and 3xx class responses to
   any method).
19.1.2 Character Escaping Requirements
                                                       dialog
                                          reg./redir. Contact/
              default  Req.-URI  To  From  Contact   R-R/Route  external
user          --          o      o    o       o          o         o
password      --          o      o    o       o          o         o
host          --          m      m    m       m          m         m
port          (1)         o      -    -       o          o         o
user-param    ip          o      o    o       o          o         o
method        INVITE      -      -    -       -          -         o
maddr-param   --          o      -    -       o          o         o
ttl-param     1           o      -    -       o          -         o
transp.-param (2)         o      -    -       o          o         o
lr-param      --          o      -    -       -          o         o
other-param   --          o      o    o       o          o         o
headers       --          -      -    -       o          -         o
   (1): The default port value is transport and scheme dependent.  The
   default  is  5060  for  sip: using UDP, TCP, or SCTP.  The default is
   5061 for sip: using TLS over TCP and sips: over TCP.
   (2): The default transport is scheme dependent.  For sip:, it is UDP.
   For sips:, it is TCP.
   Table 1: Use and default values of URI components for SIP header
   field values, Request-URI and references
   SIP follows the requirements and guidelines of RFC 2396 [5] when
   defining the set of characters that must be escaped in a SIP URI, and
   uses its ""%" HEX HEX" mechanism for escaping.  From RFC 2396 [5]:
      The set of characters actually reserved within any given URI
      component is defined by that component.  In general, a character
      is reserved if the semantics of the URI changes if the character
      is replaced with its escaped US-ASCII encoding [5].  Excluded US-
      ASCII characters (RFC 2396 [5]), such as space and control
      characters and characters used as URI delimiters, also MUST be
      escaped.  URIs MUST NOT contain unescaped space and control
      characters.
   For each component, the set of valid BNF expansions defines exactly
   which characters may appear unescaped.  All other characters MUST be
   escaped.
   For example, "@" is not in the set of characters in the user
   component, so the user "j@s0n" must have at least the @ sign encoded,
   as in "j%40s0n".
   Expanding the hname and hvalue tokens in Section 25 show that all URI
   reserved characters in header field names and values MUST be escaped.
   The telephone-subscriber subset of the user component has special
   escaping considerations.  The set of characters not reserved in the
   RFC 2806 [9] description of telephone-subscriber contains a number of
   characters in various syntax elements that need to be escaped when
   used in SIP URIs.  Any characters occurring in a telephone-subscriber
   that do not appear in an expansion of the BNF for the user rule MUST
   be escaped.
   Note that character escaping is not allowed in the host component of
   a SIP or SIPS URI (the % character is not valid in its expansion).
   This is likely to change in the future as requirements for
   Internationalized Domain Names are finalized.  Current
   implementations MUST NOT attempt to improve robustness by treating
   received escaped characters in the host component as literally
   equivalent to their unescaped counterpart.  The behavior required to
   meet the requirements of IDN may be significantly different.
19.1.3 Example SIP and SIPS URIs
   sip:alice@atlanta.com
   sip:alice:secretword@atlanta.com;transport=tcp
   sips:alice@atlanta.com?subject=project%20x&priority=urgent
   sip:+1-212-555-1212:1234@gateway.com;user=phone
   sips:1212@gateway.com
   sip:alice@192.0.2.4
   sip:atlanta.com;method=REGISTER?to=alice%40atlanta.com
   sip:alice;day=tuesday@atlanta.com
   The last sample URI above has a user field value of
   "alice;day=tuesday".  The escaping rules defined above allow a
   semicolon to appear unescaped in this field.  For the purposes of
   this protocol, the field is opaque.  The structure of that value is
   only useful to the SIP element responsible for the resource.
19.1.4 URI Comparison
   Some operations in this specification require determining whether two
   SIP or SIPS URIs are equivalent.  In this specification, registrars
   need to compare bindings in Contact URIs in REGISTER requests (see
   Section 10.3.).  SIP and SIPS URIs are compared for equality
   according to the following rules:
      o  A SIP and SIPS URI are never equivalent.
      o  Comparison of the userinfo of SIP and SIPS URIs is case-
         sensitive.  This includes userinfo containing passwords or
         formatted as telephone-subscribers.  Comparison of all other
         components of the URI is case-insensitive unless explicitly
         defined otherwise.
      o  The ordering of parameters and header fields is not significant
         in comparing SIP and SIPS URIs.
      o  Characters other than those in the "reserved" set (see RFC 2396
         [5]) are equivalent to their ""%" HEX HEX" encoding.
      o  An IP address that is the result of a DNS lookup of a host name
         does not match that host name.
      o  For two URIs to be equal, the user, password, host, and port
         components must match.
         A URI omitting the user component will not match a URI that
         includes one.  A URI omitting the password component will not
         match a URI that includes one.
         A URI omitting any component with a default value will not
         match a URI explicitly containing that component with its
         default value.  For instance, a URI omitting the optional port
         component will not match a URI explicitly declaring port 5060.
         The same is true for the transport-parameter, ttl-parameter,
         user-parameter, and method components.
            Defining sip:user@host to not be equivalent to
            sip:user@host:5060 is a change from RFC 2543.  When deriving
            addresses from URIs, equivalent addresses are expected from
            equivalent URIs.  The URI sip:user@host:5060 will always
            resolve to port 5060.  The URI sip:user@host may resolve to
            other ports through the DNS SRV mechanisms detailed in [4].
      o  URI uri-parameter components are compared as follows:
         -  Any uri-parameter appearing in both URIs must match.
         -  A user, ttl, or method uri-parameter appearing in only one
            URI never matches, even if it contains the default value.
         -  A URI that includes an maddr parameter will not match a URI
            that contains no maddr parameter.
         -  All other uri-parameters appearing in only one URI are
            ignored when comparing the URIs.
      o  URI header components are never ignored.  Any present header
         component MUST be present in both URIs and match for the URIs
         to match.  The matching rules are defined for each header field
         in Section 20.
   The URIs within each of the following sets are equivalent:
   sip:%61lice@atlanta.com;transport=TCP
   sip:alice@AtLanTa.CoM;Transport=tcp
   sip:carol@chicago.com
   sip:carol@chicago.com;newparam=5
   sip:carol@chicago.com;security=on
   sip:biloxi.com;transport=tcp;method=REGISTER?to=sip:bob%40biloxi.com
   sip:biloxi.com;method=REGISTER;transport=tcp?to=sip:bob%40biloxi.com
   sip:alice@atlanta.com?subject=project%20x&priority=urgent
   sip:alice@atlanta.com?priority=urgent&subject=project%20x
   The URIs within each of the following sets are not equivalent:
   SIP:ALICE@AtLanTa.CoM;Transport=udp             (different usernames)
   sip:alice@AtLanTa.CoM;Transport=UDP
   sip:bob@biloxi.com                   (can resolve to different ports)
   sip:bob@biloxi.com:5060
   sip:bob@biloxi.com              (can resolve to different transports)
   sip:bob@biloxi.com;transport=udp
   sip:bob@biloxi.com     (can resolve to different port and transports)
   sip:bob@biloxi.com:6000;transport=tcp
   sip:carol@chicago.com                    (different header component)
   sip:carol@chicago.com?Subject=next%20meeting
   sip:bob@phone21.boxesbybob.com   (even though that's what
   sip:bob@192.0.2.4                 phone21.boxesbybob.com resolves to)
   Note that equality is not transitive:
      o  sip:carol@chicago.com and sip:carol@chicago.com;security=on are
         equivalent
      o  sip:carol@chicago.com and sip:carol@chicago.com;security=off
         are equivalent
      o  sip:carol@chicago.com;security=on and
         sip:carol@chicago.com;security=off are not equivalent
19.1.5 Forming Requests from a URI
   An implementation needs to take care when forming requests directly
   from a URI.  URIs from business cards, web pages, and even from
   sources inside the protocol such as registered contacts may contain
   inappropriate header fields or body parts.
   An implementation MUST include any provided transport, maddr, ttl, or
   user parameter in the Request-URI of the formed request.  If the URI
   contains a method parameter, its value MUST be used as the method of
   the request.  The method parameter MUST NOT be placed in the
   Request-URI.  Unknown URI parameters MUST be placed in the message's
   Request-URI.
   An implementation SHOULD treat the presence of any headers or body
   parts in the URI as a desire to include them in the message, and
   choose to honor the request on a per-component basis.
   An implementation SHOULD NOT honor these obviously dangerous header
   fields: From, Call-ID, CSeq, Via, and Record-Route.
   An implementation SHOULD NOT honor any requested Route header field
   values in order to not be used as an unwitting agent in malicious
   attacks.
   An implementation SHOULD NOT honor requests to include header fields
   that may cause it to falsely advertise its location or capabilities.
   These include: Accept, Accept-Encoding, Accept-Language, Allow,
   Contact (in its dialog usage), Organization, Supported, and User-
   Agent.
   An implementation SHOULD verify the accuracy of any requested
   descriptive header fields, including: Content-Disposition, Content-
   Encoding, Content-Language, Content-Length, Content-Type, Date,
   Mime-Version, and Timestamp.
   If the request formed from constructing a message from a given URI is
   not a valid SIP request, the URI is invalid.  An implementation MUST
   NOT proceed with transmitting the request.  It should instead pursue
   the course of action due an invalid URI in the context it occurs.
      The constructed request can be invalid in many ways.  These
      include, but are not limited to, syntax error in header fields,
      invalid combinations of URI parameters, or an incorrect
      description of the message body.
   Sending a request formed from a given URI may require capabilities
   unavailable to the implementation.  The URI might indicate use of an
   unimplemented transport or extension, for example.  An implementation
   SHOULD refuse to send these requests rather than modifying them to
   match their capabilities.  An implementation MUST NOT send a request
   requiring an extension that it does not support.
      For example, such a request can be formed through the presence of
      a Require header parameter or a method URI parameter with an
      unknown or explicitly unsupported value.
19.1.6 Relating SIP URIs and tel URLs
   When a tel URL (RFC 2806 [9]) is converted to a SIP or SIPS URI, the
   entire telephone-subscriber portion of the tel URL, including any
   parameters, is placed into the userinfo part of the SIP or SIPS URI.
   Thus, tel:+358-555-1234567;postd=pp22 becomes
      sip:+358-555-1234567;postd=pp22@foo.com;user=phone
   or
      sips:+358-555-1234567;postd=pp22@foo.com;user=phone
   not
      sip:+358-555-1234567@foo.com;postd=pp22;user=phone
   or
      sips:+358-555-1234567@foo.com;postd=pp22;user=phone
   In general, equivalent "tel" URLs converted to SIP or SIPS URIs in
   this fashion may not produce equivalent SIP or SIPS URIs.  The
   userinfo of SIP and SIPS URIs are compared as a case-sensitive
   string.  Variance in case-insensitive portions of tel URLs and
   reordering of tel URL parameters does not affect tel URL equivalence,
   but does affect the equivalence of SIP URIs formed from them.
   For example,
      tel:+358-555-1234567;postd=pp22
      tel:+358-555-1234567;POSTD=PP22
   are equivalent, while
      sip:+358-555-1234567;postd=pp22@foo.com;user=phone
      sip:+358-555-1234567;POSTD=PP22@foo.com;user=phone
   are not.
   Likewise,
      tel:+358-555-1234567;postd=pp22;isub=1411
      tel:+358-555-1234567;isub=1411;postd=pp22
   are equivalent, while
      sip:+358-555-1234567;postd=pp22;isub=1411@foo.com;user=phone
      sip:+358-555-1234567;isub=1411;postd=pp22@foo.com;user=phone
   are not.
   To mitigate this problem, elements constructing telephone-subscriber
   fields to place in the userinfo part of a SIP or SIPS URI SHOULD fold
   any case-insensitive portion of telephone-subscriber to lower case,
   and order the telephone-subscriber parameters lexically by parameter
   name, excepting isdn-subaddress and post-dial, which occur first and
   in that order.  (All components of a tel URL except for future-
   extension parameters are defined to be compared case-insensitive.)
   Following this suggestion, both
      tel:+358-555-1234567;postd=pp22
      tel:+358-555-1234567;POSTD=PP22
      become
        sip:+358-555-1234567;postd=pp22@foo.com;user=phone
   and both
        tel:+358-555-1234567;tsp=a.b;phone-context=5
        tel:+358-555-1234567;phone-context=5;tsp=a.b
      become
        sip:+358-555-1234567;phone-context=5;tsp=a.b@foo.com;user=phone
19.2 Option Tags
   Option tags are unique identifiers used to designate new options
   (extensions) in SIP.  These tags are used in Require (Section 20.32),
   Proxy-Require (Section 20.29), Supported (Section 20.37) and
   Unsupported (Section 20.40) header fields.  Note that these options
   appear as parameters in those header fields in an option-tag = token
   form (see Section 25 for the definition of token).
   Option tags are defined in standards track RFCs.  This is a change
   from past practice, and is instituted to ensure continuing multi-
   vendor interoperability (see discussion in Section 20.32 and Section
   20.37).  An IANA registry of option tags is used to ensure easy
   reference.
19.3 Tags
   The "tag" parameter is used in the To and From header fields of SIP
   messages.  It serves as a general mechanism to identify a dialog,
   which is the combination of the Call-ID along with two tags, one from
   each participant in the dialog.  When a UA sends a request outside of
   a dialog, it contains a From tag only, providing "half" of the dialog
   ID.  The dialog is completed from the response(s), each of which
   contributes the second half in the To header field.  The forking of
   SIP requests means that multiple dialogs can be established from a
   single request.  This also explains the need for the two-sided dialog
   identifier; without a contribution from the recipients, the
   originator could not disambiguate the multiple dialogs established
   from a single request.
   When a tag is generated by a UA for insertion into a request or
   response, it MUST be globally unique and cryptographically random
   with at least 32 bits of randomness.  A property of this selection
   requirement is that a UA will place a different tag into the From
   header of an INVITE than it would place into the To header of the
   response to the same INVITE.  This is needed in order for a UA to
   invite itself to a session, a common case for "hairpinning" of calls
   in PSTN gateways.  Similarly, two INVITEs for different calls will
   have different From tags, and two responses for different calls will
   have different To tags.
   Besides the requirement for global uniqueness, the algorithm for
   generating a tag is implementation-specific.  Tags are helpful in
   fault tolerant systems, where a dialog is to be recovered on an
   alternate server after a failure.  A UAS can select the tag in such a
   way that a backup can recognize a request as part of a dialog on the
   failed server, and therefore determine that it should attempt to
   recover the dialog and any other state associated with it.