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for routing incoming SIP requests and has no role in authorizing
outgoing requests. Authorization and authentication are handled in
SIP either on a request-by-request basis with a challenge/response
mechanism, or by using a lower layer scheme as discussed in Section
26.
The complete set of SIP message details for this registration example
is in Section 24.1.
Additional operations in SIP, such as querying for the capabilities
of a SIP server or client using OPTIONS, or canceling a pending
request using CANCEL, will be introduced in later sections.
5 Structure of the Protocol
SIP is structured as a layered protocol, which means that its
behavior is described in terms of a set of fairly independent
processing stages with only a loose coupling between each stage. The
protocol behavior is described as layers for the purpose of
presentation, allowing the description of functions common across
elements in a single section. It does not dictate an implementation
in any way. When we say that an element "contains" a layer, we mean
it is compliant to the set of rules defined by that layer.
Not every element specified by the protocol contains every layer.
Furthermore, the elements specified by SIP are logical elements, not
physical ones. A physical realization can choose to act as different
logical elements, perhaps even on a transaction-by-transaction basis.
The lowest layer of SIP is its syntax and encoding. Its encoding is
specified using an augmented Backus-Naur Form grammar (BNF). The
complete BNF is specified in Section 25; an overview of a SIP
message's structure can be found in Section 7.
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The second layer is the transport layer. It defines how a client
sends requests and receives responses and how a server receives
requests and sends responses over the network. All SIP elements
contain a transport layer. The transport layer is described in
Section 18.
The third layer is the transaction layer. Transactions are a
fundamental component of SIP. A transaction is a request sent by a
client transaction (using the transport layer) to a server
transaction, along with all responses to that request sent from the
server transaction back to the client. The transaction layer handles
application-layer retransmissions, matching of responses to requests,
and application-layer timeouts. Any task that a user agent client
(UAC) accomplishes takes place using a series of transactions.
Discussion of transactions can be found in Section 17. User agents
contain a transaction layer, as do stateful proxies. Stateless
proxies do not contain a transaction layer. The transaction layer
has a client component (re
ferred to as a client transaction) and a
server component (referred to as a server transaction), each of which
are represented by a finite state machine that is constructed to
process a particular request.
The layer above the transaction layer is called the transaction user
(TU). Each of the SIP entities, except the stateless proxy, is a
transaction user. When a TU wishes to send a request, it creates a
client transaction instance and passes it the request along with the
destination IP address, port, and transport to which to send the
request. A TU that creates a client transaction can also cancel it.
When a client cancels a transaction, it requests that the server stop
further processing, revert to the state that existed before the
transaction was initiated, and generate a specific error response to
that transaction. This is done with a CANCEL request, which
constitutes its own transaction, but references the transaction to be
cancelled (Section 9).
The SIP elements, that is, user agent clients and servers, stateless
and stateful proxies and registrars, contain a core that
distinguishes them from each other. Cores, except for the stateless
proxy, are transaction users. While the behavior of the UAC and UAS
cores depends on the method, there are some common rules for all
methods (Section 8). For a UAC, these rules govern the construction
of a request; for a UAS, they govern the processing of a request and
generating a response. Since registrations play an important role in
SIP, a UAS that handles a REGISTER is given the special name
registrar. Section 10 describes UAC and UAS core behavior for the
REGISTER method. Section 11 describes UAC and UAS core behavior for
the OPTIONS method, used for determining the capabilities of a UA.
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Certain other requests are sent within a dialog. A dialog is a
peer-to-peer SIP relationship between two user agents that persists
for some time. The dialog facilitates sequencing of messages and
proper routing of requests between the user agents. The INVITE
method is the only way defined in this specification to establish a
dialog. When a UAC sends a request that is within the context of a
dialog, it follows the common UAC rules as discussed in Section 8 but
also the rules for mid-dialog requests. Section 12 discusses dialogs
and presents the procedures for their construction and maintenance,
in addition to construction of requests within a dialog.
The most important method in SIP is the INVITE method, which is used
to establish a session between participants. A session is a
collection of participants, and streams of media between them, for
the purposes of communication. Secti
on 13 discusses how sessions are
initiated, resulting in one or more SIP dialogs. Section 14
discusses how characteristics of that session are modified through
the use of an INVITE request within a dialog. Finally, section 15
discusses how a session is terminated.
The procedures of Sections 8, 10, 11, 12, 13, 14, and 15 deal
entirely with the UA core (Section 9 describes cancellation, which
applies to both UA core and proxy core). Section 16 discusses the
proxy element, which facilitates routing of messages between user
agents.
6 Definitions
The following terms have special significance for SIP.
Address-of-Record: An address-of-record (AOR) is a SIP or SIPS URI
that points to a domain with a location service that can map
the URI to another URI where the user might be available.
Typically, the location service is populated through
registrations. An AOR is frequently thought of as the "public
address" of the user.
Back-to-Back User Agent: A back-to-back user agent (B2BUA) is a
logical entity that receives a request and processes it as a
user agent server (UAS). In order to determine how the request
should be answered, it acts as a user agent client (UAC) and
generates requests. Unlike a proxy server, it maintains dialog
state and must participate in all requests sent on the dialogs
it has established. Since it is a concatenation of a UAC and
UAS, no explicit definitions are needed for its behavior.
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Call: A call is an informal term that refers to some communication
between peers, generally set up for the purposes of a
multimedia conversation.
Call Leg: Another name for a dialog [31]; no longer used in this
specification.
Call Stateful: A proxy is call stateful if it retains state for a
dialog from the initiating INVITE to the terminating BYE
request. A call stateful proxy is always transaction stateful,
but the converse is not necessarily true.
Client: A client is any network element that sends SIP requests
and receives SIP responses. Clients may or may not interact
directly with a human user. User agent clients and proxies are
clients.
Conference: A multimedia session (see below) that contains
multiple participants.
Core: Core designates the functions specific to a particular type
of SIP entity, i.e., specific to either a stateful or stateless
proxy, a user agent or registrar. All cores, except those for
the stateless proxy, are transaction users.
Dialog: A dialog is a peer-to-peer SIP relationship betwee
n two
UAs that persists for some time. A dialog is established by
SIP messages, such as a 2xx response to an INVITE request. A
dialog is identified by a call identifier, local tag, and a
remote tag. A dialog was formerly known as a call leg in RFC
2543.
Downstream: A direction of message forwarding within a transaction
that refers to the direction that requests flow from the user
agent client to user agent server.
Final Response: A response that terminates a SIP transaction, as
opposed to a provisional response that does not. All 2xx, 3xx,
4xx, 5xx and 6xx responses are final.
Header: A header is a component of a SIP message that conveys
information about the message. It is structured as a sequence
of header fields.
Header Field: A header field is a component of the SIP message
header. A header field can appear as one or more header field
rows. Header field rows consist of a header field name and zero
or more header field values. Multiple header field values on a