19. Common Message Components

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 [email protected] 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:[email protected];user=phone sips:[email protected] sip:alice@192.0.2.4 sip:atlanta.com;method=REGISTER?to=alice%40atlanta.com sip:alice;day=[email protected]

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:%[email protected];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=[email protected];user=phone

or sips:+358-555-1234567;postd=[email protected];user=phone

not sip:[email protected];postd=pp22;user=phone

sips:[email protected];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=[email protected];user=phone sip:+358-555-1234567;POSTD=[email protected];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=[email protected];user=phone sip:+358-555-1234567;isub=1411;postd=[email protected];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=[email protected];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=[email protected];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.