Internet Engineering Task Force Mauricio Arango
INTERNET DRAFT RSL COM
Christian Huitema
Bellcore
<draft-huitema-sgcp-v1-02.txt> July 30, 1998
Simple Gateway Control Protocol (SGCP)
Mauricio Arango, Christian Huitema
Version 1.1 draft
July 30, 1998
Status of this document
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Abstract
This document describes an application programming interface (SGCI) and
a corresponding protocol (SGCP) for controlling Voice over IP (VoIP)
Gateways from external call control elements. The SGCP assumes a call
control architecture where the call control 'intelligence' is outside
the gateways and handled by external call control elements.
The document is structured in 5 main sections:
* The introduction presents the basic assumptions and the relation to
other protocols such as H.323, RTSP, SAP or SIP.
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* The interface section presents a conceptual overview of the SGCP,
presenting the naming conventions, the usage of the session
description protocol SDP, and the five procedurs that compose SGCP:
Notifications Request, Notification, Create Connection, Modify Con-
nection and Delete Connection.
* The protocol description section presents the SGCP encodings, which
are based on simple text formats, and the transmission procedure
over UDP.
* The security section presents the security requirement of SGCP, and
its usage of IP security services (IPSEC).
* An example section presents two detailed examples of call set up
procedures using SGCP.
* The description of the changes between version 1.0 and version 1.1
1. Introduction
This document describes an application programming interface (SGCI) and
a corresponding protocol (SGCP) for controlling Telephony Gateways from
external call control elements. A telephony gateway is a network element
that provides conversion between the audio signals carried on telephone
circuits and data packets carried over the Internet or over other packet
networks. Example of gateways are:
* Trunking gateways, that interface between the telephone network and
a Voice over IP network. Such gateways typically manage a large
number of digital circuits.
* Residential gateways, that provide a traditional analog (RJ11)
interface to a Voice over IP network.
* Network Access Servers, that can attach a "modem" to a telephone
circuit and provide data access to the Internet. We expect that, in
the future, the same gateways will combine Voice over IP services
and Network Access services.
The SGCP assumes a call control architecture where the call control
"intelligence" is outside the gateways and handled by external call con-
trol elements. The SGCP assumes that these call control elements, or
Call Agents, will synchronize with each other to send coherent commands
to the gateways under their control. The SGC defined in this document
does not define a mechanism for synchronizing Call Agents.
The SGCP assumes a connection model where the basic constructs are end-
points and connections. Endpoints are sources or sinks of data and could
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be physical or virtual. Examples of physical endpoints are:
* An interface on a gateway that terminates a trunk connected to a
PSTN switch (e.g., Class 5, Class 4, etc.). A gateway that ter-
minates trunks is called a trunk gateway.
* An interface on a gateway that terminates an analog POTS connection
to a phone, key system, PBX, etc. A gateway that terminates
residential POTS lines (to phones) is called a residential gateway.
An example of a virtual endpoint is an audio source in an audio-content
server. Creation of physical endpoints requires hardware installation,
while creation of virtual endpoints can be done by software.
Connections may be either point to point or multipoint. A point to point
connection is an association between two endpoints with the purpose of
transmitting data between these endpoints. Once this association is
established for both endpoints, data transfer between these endpoints
can take place. A multipoint connection is established by connecting the
endpoint to a multipoint session.
1.1. Relation with the H.323 standards
The SGCP is designed as an internal protocol within a distributed system
that appears to the outside as a single VoIP gateway. This system is
composed of a Call Agent, that may or may not be distributed over
several computer platforms, and of a set of gateways. In a typical con-
figuration, this distributed gateway system will interface on one side
with one or more telephony (i.e. circuit) switches, and on the other
side with H.323 conformant systems, as indicated in the following table:
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___________________________________________________________________
| Functional| Phone | Terminating | H.323 conformant |
| Plane | switch | Entity | systems |
|___________|____________|_________________|_______________________|
| Signaling | Signaling | Call agent | Signaling exchanges |
| Plane | exchanges | | with the call agent |
| | through | | through H.225/RAS and|
| | SS7/ISUP | | H.225/Q.931. |
|___________|____________|_________________|_______________________|
| | | | Possible negotiation |
| | | | of logical channels |
| | | | and transmission |
| | | | parameters through |
| | | | H.245 with the call |
| | | | agent. |
|___________|____________|_________________|_______________________|
| | | Internal | |
| | | synchronization| |
| | | through SGCP | |
|___________|____________|_________________|_______________________|
| Bearer | Connection| Telephony | Optional negotiation |
| Data | through | gateways | of logical channels |
| Transport | high speed| | and transmission |
| Plane | trunk | | parameters through |
| | groups | | H.245 directly with |
| | | | the telephony |
| | | | gateway. |
| | | | |
| | | | data using RTP, |
| | | | directly between the |
| | | | H.323 station and the|
| | | | gateway. |
|___________|____________|_________________|_______________________|
In the SGCP model, the gateways focus on the audio signal translation
function, while the Call Agent handles the signaling and call processing
functions. As a consequence, the Call Agent implements the "signaling"
layers of the H.323 standard, and presents itself as a "Gatekeeper" to
the H.323 systems. Calls are established using the "Gatekeeper Routed"
call model.
1.2. Relation with the IETF standards
While H.323 is the recognized standard for VoIP terminals, the IETF has
also produced specifications for other types of multi-media applica-
tions. These other specifications include:
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* the Session Description Protocol (SDP), RFC 2327,
* the Session Announcement Protocol (SAP),
* the Session Initiation Protocol (SIP),
* the Real Time Streaming Protocol (RTSP), RFC 2326.
The latter three specifications are in fact alternative signaling stan-
dards that allow for the transmission of a session description to an
interested party. SAP is used by multicast session managers to distri-
bute a multicast session description to a large group of recipients, SIP
is used to invite an individual user to take part in a point-to-point or
unicast session, RTSP is used to interface a server that provides real
time data. In all three cases, the session description is described
according to SDP; when audio is transmitted, it is transmitted through
the Real-time Transport Protocol, RTP.
The distributed gateway systems and SGCP will enable PSTN telephony
users to access sessions set up using SAP, SIP or RTSP. The Call Agent
provides for signaling conversion, according to the following table:
_________________________________________________________________
| Functional| Phone | Terminating| IETF conforming systems|
| Plane | switch | Entity | |
|___________|____________|_____________|_________________________|
| Signaling | Signaling | Call agent | Signaling exchanges |
| Plane | exchanges | | with the call agent |
| | through | | through SAP, SIP or |
| | SS7/ISUP | | RTSP. |
|___________|____________|_____________|_________________________|
| | | | Negotiation of session |
| | | | description parameters |
| | | | through SDP (telephony |
| | | | gateway terminated but |
| | | | passed via the call |
| | | | agent to and from the |
| | | | IETF conforming system)|
|___________|____________|_____________|_________________________|
| | | Internal | |
| Bearer | Connection| Telephony | Transmission of VoIP |
| Data | through | gateways | data using RTP, |
| Transport| high speed| | directly between the |
| Plane | trunk | | remote IP end system |
| | groups | | and the gateway. |
|___________|____________|_____________|_________________________|
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The SDP standard has a pivotal status in this architecture. We will see
in the following description that we also use it to carry session
descriptions in SGCP.
1.3. Definitions
Trunk: A communication channel between two switching systems. E.g., a
DS0 on a T1 or E1 line.
2. Simple Gateway Control Interface
The interface functions provide for connection control and endpoint con-
trol. Both use the same system model and the same naming conventions.
2.1. Model and naming conventions.
The SGCP assumes a connection model where the basic constructs are end-
points and connections. Connections are grouped in calls. One or more
connections can belong to one call. Connections and calls are set up at
the initiative of one or several Call Agents.
2.1.1. Names of endpoints
Endpoints names have two components:
* the domain name of the gateway that is managing the endpoint ,
* a local name within that gateway.
In the case of trunking gateways, endpoints are trunk circuits linking a
gateway to a telephone switch. These circuits are typically grouped into
a digital multiplex, that is connected to the gateway by a physical
interface. Such circuits are named in three contexts:
* In the ISUP protocol, trunks are grouped into trunk groups, identi-
fied by the SS7 point codes of the switches that the group con-
nects. Circuits within a trunk group are identified by a circuit
number (CIC in ISUP).
* In the gateway configuration files, physical interfaces are typi-
cally identified by the name of the interface, an arbitrary text
string. When the interface multiplexes several circuits, individual
circuits are typically identified by a circuit number.
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* In SGCP, the endpoints are identified by an endpoint
name.
The Call Agents use configuration databases to map ranges of circuit
numbers within an ISUP trunk group to corresponding ranges of circuits
in a multiplex connected to a gateway through a physical interface. The
gateway will be identified, in SGCP, by a domain name. The local name
will be structured to encode both the name of the physical interface,
for example X35V3+A4, and the circuit number within the multiplex con-
nected to the interface, for example 13. The circuit number will be
separated from the name of the interface by a fraction bar, as in:
X35V3+A4/13
The circuit number can be omitted when the physical interface only
includes one circuit, or if the Call Agent requests the gateway to
choose one available circuit within a multiplex.
Other types of endpoints will use different conventions. For example, an
endpoint that produces an "all lines busy" announcement could be named:
all-lines-busy/loop@announce-23.whatever.net
The exact syntax of such names should be specified in the corresponding
server specification.
2.1.2. Names of calls
Calls are identified by unique identifiers, independent of the underly-
ing platforms or agents. These identifiers are created by the Call
Agent. They are treated in SGCP as unstructured octet strings.
Call identifiers are expected to be unique within the system. When a
Call Agent builds several connections that pertain to the same call,
either on the same gateway or in different gateways, these connections
will all be linked to the same call through the globally unique identif-
ier. This identifier can then be used by accounting or management pro-
cedures, which are outside the scope of SGCP.
2.1.3. Names of connections
Connection identifiers are created by the gateway when it is requested
to create a connection. They identify the connection within the context
of an endpoint and a call. They are treated in SGCP as unstructured
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octet strings. The gateway should make sure that a proper waiting
period, at least 3 minutes, elapses between the end of a connection that
used this identifier and its use in a new connection.
2.1.4. Names of Call Agents and other entities
The simple gateway control protocol has been designed to allow the
implementation of redundant Call Agents, for enhanced network reliabil-
ity. This means that there is no fixed binding between entities and
hardware platforms or network interfaces.
Reliability can be improved by the following precautions:
* Entities such as endpoints or Call Agents are identified by their
domain name, not their network addresses. Several addresses can be
associated with a domain name. If a command or a response cannot be
forwarded to one of the network addresses, implementations should
retry the transmission using another address.
* Entities may move to another platform. The association between a
logical name (domain name) and the actual platform are kept in the
domain name service. Call Agents and Gateways should keep track of
the time-to-live of the record they read from the DNS. They should
query the DNS to refresh the information if the time to live has
expired.
2.1.5. Digit maps
The Call Agent can ask the gateway to collect digits dialed by the user.
This facility is intended to be used with residential gateways to col-
lect the numbers that a user dials; it may also be used with trunking
gateways and access gateways alike, to collect the access codes, credit
card numbers and other numbers requested by call control services.
An alternative procedure is for the gateway to notify the Call Agent of
the dialed digits, as soon as they are dialed. However, such a procedure
generates a large number of interactions. It is preferable to accumulate
the dialed numbers in a buffer, and to transmit them in a single mes-
sage.
The problem with this accumulation approach, however, is that it is hard
for the gateway to predict how many numbers it needs to accumulate
before transmission. For example, using the phone on our desk, we can
dial the following numbers:
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_______________________________________________________
| 0 | Local operator |
| 00 | Long distance operator |
| xxxx | Local extension number |
| 8xxxxxxx | Local number |
| #xxxxxxx | Shortcut to local number at|
| | other corporate sites |
| *xx | Star services |
| 91xxxxxxxxxx | Long distance number |
| 9011 + up to 14 digits| International number |
|________________________|_____________________________|
The solution to this problem is to load the gateway with a digit map
that correspond to the dial plan. This digit map is expressed using a
syntax derived from the Unix system command, egrep. For example, the
dial plan described above results in the following digit map:
(0T| 00T|[1-7]xxx|8xxxxxxx|#xxxxxxx|*xx|91xxxxxxxxxx|9011x.T)
The formal syntax of the digit map is described by the following BNF
notation:
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Digit ::= "0" | "1" | "2" | "3" | "4" | "5" | "6" |
"7" | "8" | "9"
Timer ::= "T" -- matches the detection of a timer
Letter::= Digit | Timer | "#" | "*" | "A" | "B" | "C"
| "D"
Range ::= "x" -- matches any digit
| "[" Letters "]" -- matches any of the specified letters
Letters ::= Subrange | Subrange Letters
Subrange ::= Letter -- matches the specified letter
| Digit "-" Digit -- matches any digit between first and
-- last
Position ::= Letter
| Range
StringElement::= Position -- matches an occurrence of
-- the position
| Position "." -- matches an arbitrary number of
-- occurrences
-- of the position, including 0
String::= StringElement | StringElement String
StringList::= String | String "|" StringList
DigitMap::= String | "(" StringList ")"
A DigitMap, according to this syntax, is defined either by a "string" or
by a list of strings. Each string in the list is an alternative number-
ing scheme. A gateway that detects digits, letters or timers will:
1. Add the event parameter code as a token to the end of an internal
state variable called the "current dial string"
2. Apply the current dial string to the digit map table, attempting a
match to each regular expression in the Digit Map in lexical order
3. If the result is under-qualified (partially matches at least one
entry in the digit map), do nothing further.
If the result matches, or is over-qualified (i.e. no further digits
could possibly produce a match), send the current digit string to the
Call Agent.
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Digit maps are provided to the gateway by the Call Agent, whenever the
Call Agent instructs the gateway to listen for digits.
2.2. Usage of SDP
The Call Agent uses the SGCP to provision the gateways with the descrip-
tion of connection parameters such as IP addresses, UDP port and RTP
profiles. These descriptions will follow the conventions delineated in
the Session Description Protocol which is now an IETF proposed standard,
documented in RFC 2327.
SDP allows for description of multimedia conferences. This version lim-
its SDP usage to the setting of audio circuits and data access circuits.
The initial session descriptions contain the description of exactly one
media, of type "audio" for audio connections, "nas" for data access.
2.3. Gateway Control Functions
This section describes the commands of the SGCP. The service consists of
connection handling and endpoint handling commands. There are five com-
mands in the protocol:
* The Call Agent can issue a NotificationRequest command to a gate-
way, instructing the gateway to watch for specific events such as
hook actions or DTMF tones on a specified endpoint .
* The gateway will then use the Notify command to inform the Call
Agent when the requested events occur.
* The Call Agent can use the CreateConnection command to create a
connection that terminates in an "endpoint" inside the gateway.
* The Call Agent can use the ModifyConnection command to change the
parameters associated to a previously established connection.
* The Call Agent can use the DeleteConnection command to delete an
existing connection. The DeleteConnection command may also be used
by a gateway to indicate that a connection can no longer be sus-
tained.
These services allow a controller (normally, the Call Agent) to instruct
a gateway on the creation of connections that terminate in an "endpoint"
attached to the gateway, and to be informed about events occurring at
the endpoint. An endpoint may be for example:
* A specific trunk circuit, within a trunk group terminating in a
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gateway,
* A specific announcement handled by an announcement server.
Connections are grouped into "calls". Several connections may belong to
the same call, terminating in the same endpoint . Each connection is
qualified by a "mode" parameter, which can be set to "send", "receive",
"send/receive", data, "inactive", "loopback" or "continuity test."
The handling of the audio signals received on these connections is
determined by the mode parameters:
* Audio signals received in data packets through connections in
"receive" or "send/receive" mode are mixed and sent to the end-
point.
* Audio signals originating from the endpoint are transmitted over
all the connections whose mode is "send" or "send/receive."
The "loopback" and "continuity test" modes are used during maintenance
and continuity test operations. There are two flavors of continuity
test, one specified by ITU and one used in the US. In the first case,
the test is a loopback test. The originating switch will send a tone
(the go tone) on the bearer circuit and expect the terminating switch to
loopback the circuit. If the originating switch sees the same tone
returned (the return tone), the COT has passed. If not, the COT has
failed. In the second case, the go and return tones are different. The
originating switch sends a certain go tone. The terminating switch
detects the go tone, it asserts a different return tone in the backwards
direction. When the originating switch detects the return tone, the COT
is passed. If the originating switch never detects the return tone, the
COT has failed.
If the mode is set to "loopback", the gateway is expected to return the
incoming signal from the endpoint back into that same endpoint. This
procedure will be used, typically, for testing the continuity of trunk
circuits according to the ITU specifications.
If the mode is set to "continuity test", the gateway is informed that
the other end of the circuit has initiated a continuity test procedure
according to the GR specification. The gateway will place the circuit in
the transponder mode required for dual-tone continuity tests.
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2.3.1. NotificationRequest
The NotificationRequest commands are used to request the gateway to send
notifications upon the occurrence of specified events in an endpoint.
For example, a notification may be requested for when a gateway detects
that an endpoint is receiving tones associated with fax communication.
The entity receiving this notification may decide to use a different
type of encoding method in the connections bound to this endpoint.
NotificationRequest( EndpointId,
NotifiedEntity,
RequestedEvents,
RequestIdentifier,
DigitMap,
SignalRequests)
EndpointId is the identifier for the endpoint in the gateway where Noti-
ficationRequest executes.
NotifiedEntity is an optional parameter that specifies where the notifi-
cations should be sent. When this parameter is absent, the notifications
should be sent to the originator of the NotificationRequest.
RequestIdentifier is used to correlate this request with the notifica-
tions that it triggers.
RequestedEvents is a list of events that the gateway is requested to
detect and report. Such events include, for example:
* fax tones,
* modem tones,
* continuity tone,
* continuity detection (as a result of a continuity test),
* on-hook transition (occurring in classic telephone sets when the
user hangs-up the handset),
* off-hook transition (occurring in classic telephone sets when the
user lifts the handset),
* flash hook (occurring in classic telephone sets when the user
briefly presses the hook that holds the handset),
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* wink,
* DTMF digits (or pulse digits).
To each event is associated an action, which can be:
* Notify the event immediately, together with the accumulated list of
observed events,
* Swap audio,
* Accumulate according to Digit Map,
* Ignore the event.
Events that are not specified in the list will, by default, be ignored.
The Swap Audio action can be used when a gateway handles more than one
active connection on an endpoint. This will be the case for three-way
calling, call waiting, and possibly other feature scenarios. In order to
avoid the round-trip to the Call Agent when just changing which connec-
tion is attached to the audio functions of the endpoint, the Notifica-
tionRequest can map an event (usually hook flash, but could be some
other event) to a local function swap audio, which selects the "next"
connection in a round robin fashion. If there is only one connection,
this action is effectively a no-op.
Hook transition events are normally observed only by access gateways.
Tone detection can be done by any gateway, including trunking gateways.
The Call Agent can send a NotificationRequest whose RequestedEvent list
is empty. It will do so, for example, to an access gateway when it does
not want to collect any more DTMF digits.
DigitMap is an optional parameter that allows the Call Agent to provi-
sion the gateways with a digit map according to which digits will be
accumulated. This parameter must be present if the RequestedEvent param-
eters contain an request to "accumulate according to the digit map." The
collection of these digits will result in a digit string. The digit
string is initialized to a null string upon reception of the Notifica-
tionRequest, so that a subsequent notification only returns the digits
that were collected after this request.
SignalRequests is a parameter that contains the set of actions that the
gateway is asked to perform on the endpoint , such as, for example:
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* Ringing,
* Distinctive ringing, which can occur in 8 variants numbered 0 to 7,
* Ring back tone,
* Dial tones,
* Intercept tone,
* Network Congestion tone,
* Busy tone,
* Confirm tone,
* Answer tone,
* Call waiting tone,
* Off hook warning tone,
* Preemption tone,
* Continuity tones,
* Continuity test,
* Announcement (completed by an announcement name and a parameter),
* ASDI display (and string to be displayed).
The action triggered by the SignalRequests is synchronized with the col-
lection of events specified in the RequestedEvents parameter. For exam-
ple, if the NotificationRequest mandates "ringing" and the event request
ask to look for an "off-hook" event, the ringing shall stop as soon as
the gateway detect an off hook event.
The specific definition of actions that are requested via these Signal-
Requests, such as the duration of and frequency of a DTMF digit, is out-
side the scope of SGC. This definition may vary from location to loca-
tion and hence from gateway to gateway.
The RequestedEvents and SignalRequests refer to the same events. In one
case, the gateway is asked to detect the occurrence of the event, and in
the other case it is asked to generate it. There are only a few excep-
tion to this rule, notably the ASDI display, which can only be signaled
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but not detected, and the fax and modem tones, which can be detected but
can not be signaled. However, we cannot necessarily expect all endpoint
s to detect all events. For example, a digital trunk interface would
normally not detect hook events, and may not be able to modulate DTMF
digit; signals such as "wink" only make sense in some types of trunks.
Gateways that receive a request to detect an event that they are not
equipped to detect, or to generate a signal that they are not equipped
to generate, should refuse the request and return an error code. Simi-
larly, gateways that are equipped to generate announcements but cannot
generate the specific announcement that the Call Agent requested, should
return an appropriate error code.
The Call Agent can send a NotificationRequest whose requested signal
list is empty. It will do so for example when tone generation should
stop.
2.3.2. Notifications
Notifications are sent via the Notify command and are sent by the gate-
way when the observed events occur.
Notify( EndpointId,
NotifiedEntity,
RequestIdentifier,
ObservedEvents)
EndpointId is the identifier for the endpoint in the gateway which is
issuing the Notify command.
NotifiedEntity is an optional parameter that identifies the entity to
which the notifications is sent. This parameter is equal to the Noti-
fiedEntity parameter of the NotificationRequest that triggered this
notification. The parameter is absent if there was no such parameter in
the triggering request. In this case, the notification is sent to the
entity from which the request was received.
RequestIdentifier is parameter that repeats the RequestIdentifier param-
eter of the NotificationRequest that triggered this notification. It is
used to correlate this notifications with the request that triggered it.
ObservedEvents is a list of events that the gateway detected. A single
notification may report a list of events that will be reported in the
order in which they were detected. The list may only contain the iden-
tification of events that were requested in the RequestedEvents parame-
ter of the triggering NotificationRequest. It will contain the events
that were either accumulated (but not notified) or treated according to
digit map (but no match yet), and the final event that triggered the
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detection or provided a final match in the digit map.
2.3.3. CreateConnection
This command is used to create a connection.
ConnectionId,
[SpecificEndPointId,]
[LocalConnectionDescriptor]
<--- CreateConnection(CallId,
EndpointId,
NotifiedEntity,
LocalConnectionOptions,
Mode,
RemoteConnectionDescriptor,
RequestedEvents,
RequestIdentifier,
DigitMap,
SignalRequests)
This function creates a connection between two endpoints. A connection
is defined by its endpoints. The input parameters in CreateConnection
provide the data necessary to build a gateway's "view" of a connection.
CallId is a globally unique parameter that identifies the call (or ses-
sion) to which this connection belongs. This parameter is unique within
the whole network of gateways; connections that belong to the same call
share the same call-id. The call-id can be used to identify calls for
reporting and accounting purposes.
EndpointId is the identifier for the connection endpoint in the gateway
where CreateConnection executes. The EndpointId can be fully-specified
by assigning a value to the parameter EndpointId in the function call or
it may be under-specified and the full value will be assigned by the
gateway and its complete value returned in the SpecificEndPointId param-
eter of the response.
The NotifiedEntity is an optional parameter that specifies where the
Notify or DeleteConnection commands should be sent. If the parameter is
absent, the Notify or DeleteConnection commands should be sent to the
originator of the CreateConnection command.
LocalConnectionOptions is a structure that describes the characteristics
of the data communications from the point of view of the gateway execut-
ing CreateConnection. The fields contained in LocalConnectionOptions are
the following:
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* Encoding Method
* Packetization period
* Bandwidth
* Type of Service,
* Usage of echo cancellation
The values of these fields are defined in the SDP standard. For each of
the first three fields, the Call Agent has three options:
* It may state exactly one value, which the gateway will then use for
the connection,
* It may provide a loose specification, such as a list of allowed
encoding methods or a range of packetization periods,
* It may simply provide a bandwidth indication, leaving the choice of
encoding method and packetization period to the gateway.
The bandwidth specification shall not contradict the specification of
encoding methods and packetization period. If an encoding method is
specified, then the gateway is authorized to use it, even if it results
in the usage of a larger bandwidth than specified.
The LocalConnectionOptions parameter may be absent in the case of a data
call.
The Type of Service specifies the class of service that will be used for
the connection. When the connection is transmitted over an IP network,
the parameters encodes the 8-bit type of service value parameter of the
IP header. When the Type of Service is not specified, the gateway shall
use a default or configured value.
By default, the telephony gateways always perform echo cancellation.
However, it is necessary, for some calls, to turn off these operations.
The echo cancellation parameter can have two values, "on" (when the echo
cancellation is requested) and "off" (when it is turned off.)
RemoteConnectionDescriptor is the connection descriptor for the remote
side of a connection, on the other side of the IP network. It includes
the same fields as in the LocalConnectionDescriptor, i.e. the fields
that describe a session according to the SDP standard. This parameter
may have a null value when the information for the remote end is not
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known yet. This occurs because the entity that builds a connection
starts by sending a CreateConnection to one of the two gateways involved
in it. For the first CreateConnection issued, there is no information
available about the other side of the connection. This information may
be provided later via a ModifyConnection call. In the case of data con-
nections (mode=data), this parameter describes the characteristics of
the data connection.
Mode indicates the mode of operation for this side of the connection.
The options are: FullDuplex, ReceiveOnly, SendOnly, Inactive, Loopback,
Continuity Test and Data
* FullDuplex indicates that this side of the connection sends data
generated by the associated endpoint and can receive data from the
remote end and feed it to the endpoint.
* ReceiveOnly indicates this connection end can only receive data
form the remote end and feed it to the associated endpoint.
* SendOnly indicates that this connection end can only send data from
its endpoint to the other end of the connection.
* Inactive indicates that this connection end does not send or
receive any data. The connection exists but is not active. It has
to be explicitly activated by a ModifyConnection command.
* Loopback indicates that the circuit to which the endpoint refers
should be placed in loopback mode, so that audio signals received
from the phone circuit are sent back on this same circuit.
* Continuity Test indicates that the other end of the circuit has
initiated a continuity test procedure according to the GR specifi-
cation. The gateway will place the circuit in the transponder mode
required for dual-tone continuity tests.
* Data indicates that the circuit to which the endpoint refers will
used for network access (such as Internet access) rather than for
Internet telephony.
The gateway returns a ConnectionId, that uniquely identifies the connec-
tion within one endpoint , and a LocalConnectionDescriptor, which is a
session description that contains information about addresses and RTP
ports, as defined in SDP. The LocalConnectionDescriptor is not returned
in the case of data connections. The SpecificEndPointId is an optional
parameter that identifies the responding endpoint. It can be used when
the EndpointId argument referred to a generic endpoint name. When a
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SpecificEndPointId is returned, the Call Agent should use it as the End-
pointId value is successive commands referring to this call.
After receiving a "CreateConnection" request that did not include a
RemoteConnectionDescriptor parameter, a gateway is in an ambiguous
situation. Because it has exported a LocalConnectionDescriptor parame-
ter, it can potentially receive packets. Because it has not yet received
the RemoteConnectionDescriptor parameter of the other gateway, it does
not know whether the packets that it receives have been authorized by
the Call Agent. It must thus navigate between two risks, i.e. clipping
some important announcements or listening to insane data. The behavior
of the gateway is determined by the value of the Mode parameter:
* If the mode was set to ReceiveOnly, the gateway should accept the
voice signals and transmit them through the endpoint .
* If the mode was set to Inactive, Loopback or Continuity Test, the
gateway should refuse the voice signals.
Note that the mode values FullDuplex and SendOnly don't make sense in
this situation. They should be treated as ReceiveOnly and Inactive.
The RequestedEvents, RequestIdentifier, DigitMap, and SignalRequests
parameters are optional. They can be used by the Call Agent to transmit
a NotificationRequest that is executed simultaneously with the creation
of the connection. For example, when the Call Agent wants to initiate a
call to an residential gateway, it should:
* ask the residential gateway to prepare a connection, in order to be
sure that the user can start speaking as soon as the phone goes off
hook,
* ask the residential gateway to start ringing,
* ask the residential gateway to notify the Call Agent when the phone
goes off-hook.
This can be accomplished in a single CreateConnection command, by also
transmitting the RequestedEvent parameters for the off hook event, and
the SignalRequest parameter for the ringing signal.
When these parameters are present, the creation and the NotificationRe-
quests should be synchronized, which means that both should be accepted,
or both refused. In our example, the CreateConnection may be refused if
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the gateway does not have sufficient resources, or cannot get adequate
resources from the local network access, and the off-hook Notification-
Request can be refused in the glare condition, if the user is already
off-hook. In this example, the phone should not ring if the connection
cannot be established, and the connection should not be established if
the user is already off hook.
2.3.4. ModifyConnection
This command is used to modify the characteristics of a gateway's "view"
of a connection. This "view" of the call includes both the local connec-
tion descriptors as well as the remote connection descriptor.
[LocalConnectionDescriptor]
<--- ModifyConnection(CallId,
EndpointId,
ConnectionId,
NotifiedEntity,
LocalConnectionOptions,
Mode,
RemoteConnectionDescriptor,
RequestedEvents,
RequestIdentifier,
DigitMap,
SignalRequests)
The parameters used are the same as in the CreateConnection command,
with the addition of a ConnectionId that identifies the connection
within the call. This parameter is returned by the CreateConnection
function, as part of the local connection descriptor. It uniquely iden-
tifies the connection within the call.
The ModifyConnection command can be used to affect parameters of a con-
nection in the following ways:
* Provide information on the other end of the connection, through the
RemoteConnectionDescriptor.
* Activate or deactivate the connection, by changing the value of the
Mode parameter. This can occur at any time during the connection,
with arbitrary parameter values.
* Change the sending parameters of the connection, for example by
switching to a different coding scheme, changing the packetization
period, or modifying the handling of echo cancellation.
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Connections can only be activated if the RemoteConnectionDescriptor has
been provided to the gateway.
The command will only return a LocalConnectionDescriptor if the local
connection parameters, such as RTP ports, were modified. (Usage of this
feature is actually for further study.)
The RequestedEvents, RequestIdentifier, DigitMap, and SignalRequests
parameters are optional. They can be used by the Call Agent to transmit
a NotificationRequest that is executed simultaneously with the modifica-
tion of the connection. For example, when a connection is accepted, the
calling gateway should be instructed to place the circuit in send-
receive mode and to stop providing ringing tones.
This can be accomplished in a single ModifyConnection command, by also
transmitting the RequestedEvent parameters, for the on hook event, and
an empty SignalRequest parameter, to stop the provision of ringing
tones.
When these parameters are present, the modification and the Notifica-
tionRequests should be synchronized, which means that both should be
accepted, or both refused.
2.3.5. DeleteConnection (from the Call Agent)
This command is used to terminate a connection. As a side effect, it
collects statistics on the execution of the connection.
Connection-parameters <-- DeleteConnection(CallId,
EndpointId,
ConnectionId,
NotifiedEntity,
RequestedEvents,
RequestIdentifier,
DigitMap,
SignalRequests)
In the general case where a connection has two ends, this command has to
be sent to both gateways involved in the connection. Some connections,
however, may use IP multicast. In this case, they can be deleted indivi-
dually.
After the connection has been deleted, the endpoint should be placed in
inactive mode. Any loopback that has been requested for the connection
should be cancelled.
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In response to the DeleteConnection command, the gateway returns a list
of parameters that describe the status of the connection. These parame-
ters are: <DIR>
Number of packets sent:
The total number of RTP data packets transmitted by the sender since
starting transmission on this connection. The count is not reset if the
sender changes its synchronization source identifier (SSRC, as defined
in RTP), for example as a result of a Modify command. The value is zero
if the connection was set in "receive only" mode.
Number of octets sent:
The total number of payload octets (i.e., not including header or pad-
ding) transmitted in RTP data packets by the sender since starting
transmission on this connection. The count is not reset if the sender
changes its SSRC identifier, for example as a result of a ModifyConnec-
tion command. The value is zero if the connection was set in "receive
only" mode.
Number of packets received:
The total number of RTP data packets received by the sender since start-
ing reception on this connection. The count includes packets received
from different SSRC, if the sender used several values. The value is
zero if the connection was set in "send only" mode.
Number of octets received:
The total number of payload octets (i.e., not including header or pad-
ding) transmitted in RTP data packets by the sender since starting
transmission on this connection. The count includes packets received
from different SSRC, if the sender used several values. The value is
zero if the connection was set in "send only" mode.
Number of packets lost:
The total number of RTP data packets that have been lost since the
beginning of reception. This number is defined to be the number of pack-
ets expected less the number of packets actually received, where the
number of packets received includes any which are late or duplicates.
The count includes packets received from different SSRC, if the sender
used several values. Thus packets that arrive late are not counted as
lost, and the loss may be negative if there are duplicates. The count
includes packets received from different SSRC, if the sender used
several values. The number of packets expected is defined to be the
extended last sequence number received, as defined next, less the
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initial sequence number received. The count includes packets received
from different SSRC, if the sender used several values. The value is
zero if the connection was set in "send only" mode. This parameter is
omitted if the connection was set in "data" mode.
Interarrival jitter:
An estimate of the statistical variance of the RTP data packet interar-
rival time measured in milliseconds and expressed as an unsigned
integer. The interarrival jitter J is defined to be the mean deviation
(smoothed absolute value) of the difference D in packet spacing at the
receiver compared to the sender for a pair of packets. Detailed computa-
tion algorithms are found in RFC 1889. The count includes packets
received from different SSRC, if the sender used several values. The
value is zero if the connection was set in "send only" mode. This param-
eter is omitted if the connection was set in "data" mode.
Average transmission delay:
An estimate of the network latency, expressed in milliseconds. This is
the average value of the difference between the NTP timestamp indicated
by the senders of the RTCP messages and the NTP timestamp of the
receivers, measured when this messages are received. The average is
obtained by summing all the estimates, then dividing by the number of
RTCP messages that have been received. This parameter is omitted if the
connection was set in "data" mode. </DIR>
For a detailed definition of these variables, refer to RFC 1889.
The NotifiedEntity, RequestedEvents, RequestIdentifier, DigitMap, and
SignalRequests parameters are optional. They can be used by the Call
Agent to transmit a NotificationRequest that is executed simultaneously
with the deletion of the connection. For example, when a user hangs up
is accepted, the gateway should be instructed to delete the connection
and to start looking for an off hook event.
This can be accomplished in a single DeleteConnection command, by also
transmitting the RequestedEvent parameters, for the off hook event, and
an empty SignalRequest parameter.
When these parameters are present, the DeleteConnection and the Notifi-
cationRequests should be synchronized, which means that both should be
accepted, or both refused.
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2.3.6. DeleteConnection (from the VoIP gateway)
In some circumstances, a gateway may have to clear a connection, for
example because it has lost the resource associated with the connection,
or because it has detected that the endpoint no longer is capable or
willing to send or receive voice. The gateway terminates the connection
by using a variant of the DeleteConnection command:
DeleteConnection( CallId,
EndpointId,
ConnectionId,
Reason-code,
Connection-parameters)
In addition to the call, endpoint and connection identifiers, the gate-
way will also send the call's parameters that would have been returned
to the Call Agent in response to a DeleteConnection command. The reason
code indicates the cause of the disconnection.
2.3.7. DeleteConnection (multiple connections, from the Call Agent)
A variation of the DeleteConnection function can be used by the Call
Agent to delete multiple connections at the same time. The command can
be used to delete all connections that relate to a Call for an endpoint:
DeleteConnection( CallId,
EndpointId)
It can also be used to delete all connections that terminate in a given
endpoint:
DeleteConnection( EndpointId)
Finally, Call Agents can take advantage of the hierarchical naming
structure of endoints to delete all the connections that belong to a
group of endpoints. For example, if endpoints names are structured as
the combination of a physical interface name and a circuit number, as in
"X35V3+A4/13", the Call Agent may replace the circuit number by a wild
card character "*", as in "X35V3+A4/*". This "wildcard" command
instructs the gateway to delete all the connections that where attached
to circuits connected to the physical interface "X35V3+A4".
After the connections have been deleted, the endpoint should be placed
in inactive mode. Any loopback that has been requested for the connec-
tions should be cancelled.
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This command does not return any individual statistics or call parame-
ters.
2.4. Race conditions
In order to implement proper call signalling, the Call Agent must keep
track of the state of the endpoint, and the gateway must make sure that
events are properly notified to the call agent. SGCP deals with race
conditions through the notion of a "quarantine list" and through expli-
cit detections of desynchronizations.
2.4.1. Quarantine list
SGCP controlled gateways will receive "notification requests" that ask
them to watch for a list of "events." The two protocol elements that
determine the handling of these events are the "Requested Events" list
and the "Digit Map."
When the endpoint is initialized, the requested events list and the
digit map are empty. After reception of a command, the gateway starts
observing the endpoint for occurences of the event mentioned in the
list.
The events are examined as they occur. The action that follows is deter-
mined by the "action" parameter associated to the event in the list of
requested events, and also by the digit map. The events that are
defined as "accumulate" are accumulated in a list of events, the events
that are marked as treated according to the digit map will be accumu-
lated in the dialed string. This will go on until one event is encoun-
tered that triggers a Notification to the "notified entity."
The gateway, at this point, will transmit the notification command and
will place the endpoint in a "notification" state. As long as the end-
point is in this notification state, the events that may occur on the
endpoint are accumulated, but not transmitted, even if they had normally
triggered a notification. The events are, in a sense, "quarantined." All
events for which the request list specifies actions such as Notify,
Accumulate or "Treat according to digit map" should be quarantined.
The endpoint exits the notification state when the acknowledgement of
the notification command is received. During that period, the notifica-
tion command may be retransmitted, as specified in section 3.5.
When the gateway receives the acknowledgement, it resets the "current
dial string" of the endpoint to a null value and starts processing the
list of quarantined events, using the already received list of requested
events and digit map. When processing these events, the gateway may
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encounter an event which requires a Notify command to be sent. If that
is the case, the gateway can adopt one of the two following behaviors:
* it can immediately transmit a Notify command that will report all
events that where accumulated until the triggering event, included,
leaving the unprocessed events in the quarantine list,
* or it can attempt to empty the quarantined list and transmit a sin-
gle Notify command reporting several sets of events and possibly
several dial strings. The dial string is reset to a null value
after each triggering event that satisfies the DigitMap. The events
that follow the last triggering event are left in the quarantine
list.
If the gateway transmits a Notify command, the end point will remain in
the notification state until the acknowledgement is received. If the
gateway does not find a quarantined event that requests a notification,
it places the end point in a normal state. Events are then processed as
they come, in exactly the same way as if a Notification Request command
had just been received.
A gateway may receive at any time a new Notification Request for the end
point. When a new notification request is received in the notification
state, the list of quarantined events is emptied, and the endpoint is
taken out of the notification state without waiting for the acknowledge-
ment of the notification command. The pending notification command will
not be repeated.
Whatever the state of the endpoint, the requested events list and digit
map are replaced by the newly received parameters, the accumulated
events are discarded, and the accumulated dial string is reset to a null
value.
2.4.2. Explicit detection
A key element of this state is the position of the hook. A race condi-
tion may occur when the user decides to go off-hook before the Call
Agent has the time to ask the gateway to notify an off hook event (the
"glare" condition well known in telephony), or if the user goes on-hook
before the Call Agent has the time to request the event's notification.
To avoid this race condition, the gateway should check the condition of
the endpoint before acknowledging a NotificationRequest. It should
return an error:
1- If the gateway is requested to notify an "off hook" transition
while the phone is already off hook,
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2- If the gateway is requested to notify an "on hook" or "flash hook"
condition while the phone is already on hook.
The other state variables of the gateway, such as the list of
RequestedEvent or list of requested signals, are entirely replaced after
each successful NotificationRequest, which prevents any long term
discrepancy between the Call Agent and the gateway.
When a NotificationRequest is unsuccessful, the list of RequestedEvents
and requested signals are emptied. They must be reinstated by a new
request.
Another race condition may occur when a notification is issued shortly
before the reception by the gateway of a NotificationRequest. The
RequestIdentifier is used to correlate Notify commands with Notifica-
tionRequest commands.
2.5. Return codes and error codes.
All SGCP commands are acknowledged. The acknowledgment carries a return
code, which indicates the status of the command. The return code is an
integer number, for which three ranges of values have been defined:
* values between 200 and 299 indicate a successful completion,
* values between 400 and 499 indicate a transient error,
* values between 500 and 599 indicate a permanent error.
The values that have been already defined are listed in the following
table:
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_________________________________________________________
| Code| Meaning |
|_____|__________________________________________________|
| 200 | The requested transaction was executed normally.|
| 250 | The connection was deleted. |
| 400 | The transaction could not be executed, |
| | due to a transient error. |
| 401 | The phone is already off hook |
| 402 | The phone is already on hook |
| 500 | The transaction could not be executed, |
| | because the endpoint is unknown. |
| 501 | The transaction could not be executed, |
| | because the endpoint is not ready. |
| 502 | The transaction could not be executed, |
| | because the endpoint does not have |
| | sufficient resources |
| 510 | The transaction could not be executed, |
| | because a protocol error was detected. |
| 511 | The transaction could not be executed, |
| | because the command contained an |
| | unrecognized extension. |
| 512 | The transaction could not be executed, |
| | because the gateway is not equipped to |
| | detect one of the requested events. |
| 513 | The transaction could not be executed, |
| | because the gateway is not equipped to |
| | generate one of the requested signals. |
| 514 | The transaction could not be executed, |
| | because the gateway cannot send the |
| | specified announcement. |
| 515 | The transaction refers to an incorrect |
| | connection-id (may have been already deleted) |
|_____|__________________________________________________|
3. Simple Gateway Control Protocol
The SGCP implements the simple gateway control interface as a set of
transactions. The transactions are composed of a command and a mandatory
response. There are five types of command:
* CreateConnection
* ModifyConnection
* DeleteConnection
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* NotificationRequest
* Notify
The first four commands are sent by the Call Agent to a gateway. The
Notify command is sent by the gateway to the Call Agent. The gateway may
also send a DeleteConnection as defined in 2.3.6.
3.1. General description
All commands are composed of a Command header, optionally followed by a
session description.
All responses are composed of a Response header, optionally followed by
a session description.
Headers and session descriptions are encoded as a set of text lines,
separated by a line feed character. The headers are separated from the
session description by an empty line.
SGCP uses a transaction identifier to correlate commands and responses.
The transaction identifier is encoded as a component of the command
header and repeated as a component of the response header (see section
3.2.1, 3.2.1.2 and 3.3).
Transaction identifiers have values between 1 and 999999999. An SGCP
entity shall not reuse an identifier sooner than 3 minutes after comple-
tion of the previous command in which the identifier was used.
3.2. Command Header
The command header is composed of:
* A command line, identifying the requested action or verb, the end-
point towards which the action is requested, and the SGC protocol
version,
* A set of parameter lines, composed of a parameter name followed by
a parameter value.
3.2.1. Command line
The command line is composed of:
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* The name of the requested verb,
* The identification of the transaction,
* The name of the endpoint that should execute the command (in notif-
ications, the name of the endpoint that is issuing the notifica-
tion),
* The protocol version.
These four items are encoded as strings of printable ASCII characters,
separated by white spaces, i.e. the ASCII space (0x20) or tabulation
(0x09) characters. It is recommended to use exactly one ASCII space
separator.
3.2.1.1. Coding of the requested verb
The five verbs that can be requested are encoded as four letter upper or
lower case ASCII codes (comparisons should be case insensitive) as
defined in the following table:
____________________________
| Verb | Code|
|____________________|______|
| CreateConnection | CRCX|
| ModifyConnection | MDCX|
| DeleteConnection | DLCX|
| NotificationRequest| RQNT|
| Notify | NTFY|
|____________________|______|
The transaction identifier is encoded as a string of up to 9 decimal
digits. In the command lines, it immediately follows the coding of the
verb.
New verbs may be defined in further versions of the protocol. It may be
necessary, for experimentation purposes, to use new verbs before they
are sanctioned in a published version of this protocol. Experimental
verbs should be identified by a four letter code starting with the
letter X, such as for example XPER.
3.2.1.2. Coding of the endpoint names
The endpoint names are encoded as e-mail addresses, as defined in RFC
821. In these addresses, the domain name identifies the system where the
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endpoint is attached, while the left side identifies a specific endpoint
on that system.
Examples of such addresses can be:
____________________________________________________________________
| 123456@gw23.whatever.net | Circuit number 123456 in |
| | the Gateway 23 of the "Whatever" |
| | network |
| Call-agent@ca.whatever.net | Call Agent for the |
| | "whatever" network |
| Busy-signal@ann12.whatever.net| The "busy signal" virtual |
| | endpoint in the announcement |
| | server number 12. |
|_______________________________|___________________________________|
The name of notified entities is expressed with the same syntax, with
the possible addition of a port number as in:
Call-agent@ca.whatever.net:5234
3.2.1.3. Coding of the protocol version
The protocol version is coded as the key word SGCP followed by a white
space and the version number. The version number is composed of a major
version, coded by a decimal number, a dot, and a minor version number,
coded as a decimal number. The version described in this document is
version 1.1.
In the initial messages, the version will be coded as:
SGCP 1.1
Version 1.1 is a superset of version 1.0. Gateways that implement ver-
sion 1.1 shall accept commands coded according to version 1.0. They
should refrain from using version 1.1 extensions (See the section on
compatibility with version 1.0).
3.2.2. Parameter lines
Parameter lines are composed of a parameter name, which in most cases is
composed of a single upper case character, followed by a colon, a white
space and the parameter value. The parameter that can be present in com-
mands are defined in the following table:
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_______________________________________________________________________
|Parameter name | Code| Parameter value |
|______________________|______|_______________________________________|
|CallId | C | Hexadecimal string, |
| | | at most 32 characters |
|ConnectionId | I | Hexadecimal string, |
| | | at most 32 characters |
|NotifiedEntity | N | An identifier, in RFC 821 format, |
| | | composed of an arbitrary string and |
| | | of the domain name of the requesting |
| | | entity, possibly completed by a port |
| | | number, as in: |
| | | Call-agent@ca.whatever.net:5234 |
|RequestIdentifier | X | Hexadecimal string, |
| | | at most 32 characters |
|LocalConnectionOptions| L | See |
|description | | |
|Connection Mode | M | See description |
|RequestedEvents | R | See description |
|SignalRequests | S | See description |
|DigitMap | D | A text encoding of a digit map |
|ObservedEvents | O | See description |
|ConnectionParameters | P | See description |
|ReasonCode | E | An arbitrary character string |
|SpecificEndpointID | Z | An identifier, in RFC 821 format, |
| | | composed of an arbitrary string, |
| | | followed by an "@" followed by the |
| | | domain name of the gateway to which |
| | | this endpoint is attached. |
|______________________|______|_______________________________________|
The parameters are not necessarily present in all commands. The follow-
ing table provides the association between parameters and commands. The
letter M stands for mandatory, O for optional and F for forbidden.
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___________________________________________________________
| Parameter name | CRCX| MDCX| DLCX| RQNT| NTFY|
|_______________________|______|______|______|______|______|
| CallId | M | M | O | F | F |
| ConnectionId | F | M | O | F | F |
| RequestIdentifier | O | O | O | M | M |
| LocalConnectionOptions| O | O | F | F | F |
| Connection Mode | M | M | F | F | F |
| RequestedEvents | O | O | O | O* | F |
| SignalRequests | O | O | O | O* | F |
| NotifiedEntity | O | O | O | O | O |
| ReasonCode | F | F | O | F | F |
| ObservedEvents | F | F | F | F | M |
| DigitMap | O | O | O | O | F |
| Connection parameters | F | F | O | F | F |
| Specific Endpoint ID | F | F | F | F | F |
|_______________________|______|______|______|______|______|
Note (*) that the RequestedEvents and SignalRequests parameters are
optional in the NotificationRequest. If these parameters are omitted,
the corresponding lists will be considered empty.
If implementers need to experiment with new parameters, for example when
developing a new application of SGCP, they should identify these parame-
ters by names that start with the string "X-", such as for example:
X-FlowerOfTheDay: Daisy
The parameters will only be recognized by applications that have been
upgraded to become part of the experiment. A gateway that receives an
extension that it cannot understand should refuse to execute the com-
mand. It should respond with an error code 511 (Unrecognized extension.)
3.2.2.1. Local connection options
The local connection options describe the operational parameters that
the Call Agent suggests to the gateway. These parameters are:
* The packetisation period in milliseconds, encoded as the keyword
"p", followed by a colon and a decimal number. If the Call Agent
specifies a range of values, the range will be specified as two
decimal numbers separated by an hyphen.
* The preferred type of compression algorithm, encoded as the keyword
"a", followed by a character string. If the Call Agent specifies a
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list of values, these values will be separated by a semicolon.
* The bandwidth in kilobits per second (1000 bits per second),
encoded as the keyword "b", followed by a colon and a decimal
number. If the Call Agent specifies a range of values, the range
will be specified as two decimal numbers separated by an hyphen.
* The echo cancellation parameter, encoded as the keyword "e", fol-
lowed by a colon and the value "on" or "off".
Each of the parameters is optional. When several parameters are present,
the values are separated by a comma.
Examples of connection descriptors are:
L: p:10, a:G.711
L: p:10, a:G.711;G.726-32
L: p:10-20, b: 64
L: b:32-64, e:off
3.2.2.2. Connection parameters
Connection parameters are encoded as a string of type and value pairs,
where the type is a two letter identifier of the parameter, and the
value a decimal integer. Types are separated from value by an `=' sign.
Parameters are encoded from each other by a comma.
The connection parameter types are specified in the following table:
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__________________________________________________________________
| Connection parameter| Code| Connection parameter |
| name | | value |
|_____________________|______|____________________________________|
| Packets sent | PS | The number of packets that |
| | | were sent on the connection. |
| Octets sent | OS | The number of octets that |
| | | were sent on the connection. |
| Packets received | PR | The number of packets that |
| | | were received on the connection. |
| Octets received | OR | The number of octets that |
| | | were received on the connection. |
| Packets lost | PL | The number of packets that |
| | | were not received on the |
| | | connection, as deduced from |
| | | gaps in the sequence number. |
| Jitter | JI | The average inter-packet arrival |
| | | jitter, in milliseconds, |
| | | expressed as an integer number. |
| Latency | LA | Average latency, in milliseconds, |
| | | expressed as an integer number. |
|_____________________|______|____________________________________|
An example of connection parameter encoding is:
P: PS=1245, OS=62345, PR=0, OR=0, PL=0, JI=0, LA=48
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3.2.2.3. Connection mode
The connection mode describes the mode of operation of the connection.
The possible values are:
_______________________________________________________
| Mode | Meaning |
|____________|_________________________________________|
| M: sendonly| The gateway should only send packets |
| M: recvonly| The gateway should only receive packets|
| M: sendrecv| The gateway should send |
| | and receive packets |
| M: inactive| The gateway should neither |
| | send nor receive packets |
| M: loopback| The gateway should place |
| | the circuit in loopback mode. |
| M: conttest| The gateway should place |
| | the circuit in test mode. |
| M: data | The gateway should use the circuit |
| | for network access for data |
| | (e.g., PPP, SLIP, etc.). |
|____________|_________________________________________|
3.2.2.4. Coding of event names
Event names are mentioned in the RequestedEvents, SignalRequest and
Observed Events parameter. Each event is identified by a code, as indi-
cated in the following table. These ASCII encodings are not case sensi-
tive. Values such as "hu", "Hu", "HU" or "hU" should be considered
equal.
The following codes are used to identify events:
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___________________________________________________________________
| Code | Event |
|_________________________________|________________________________|
| A string of digits [0-9], | DTMF tones |
| hash and star marks [*,#], | |
| the letters A, B, C, D and | |
| the timer indication T | |
| ann | Announcement |
| asdi | ASDI display |
| aw | Answer tone |
| bz | Busy tone |
| cf | Confirm tone |
| cbk | Call back request |
| cg | Network Congestion tone |
| co | Default continuity tone |
| co1 | Continuity tone (single tone) |
| co2 | Continuity test (go tone, |
| | in dual tone procedures) |
| cl | Carrier lost |
| cv | Continuity verified (response |
| | tone, in dual tone procedures)|
| dl | Dial tone |
| ft | Fax tones |
| hd | Off-hook transition |
| hf | Flash hook |
| hu | On-hook transition |
| it | Intercept tone |
| ld | Long duration connection |
| mt | Modem tones |
| oc | Operation Complete (e.g., |
| | end of announcement) |
| ot | Off hook warning tone |
| pa | Packet arrival |
| pt | Preemption tone |
| r0, r1, r2, r3, r4, r5, r6 or r7| Distinctive ringing |
| (0 .. | |
| 7) | |
| rg | Ringing |
| rt | Ring back tone |
| t | Timer |
| wk | Wink |
| wt | Call waiting tone |
|_________________________________|________________________________|
3.2.2.5. RequestedEvents
The RequestedEvent parameter provides the list of events that have been
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requested. The event codes are described in the previous section. In
addition to this coded events, the list may also include a parameter
specifying digit collection. This parameter could take one of the fol-
lowing forms:
* individual digits, pound sign (octothorpe), star sign or letters A,
B, C or D.
* timer mark T,
* ranges of digits, enclosed within square brackets, e.g. "[0-9]" or
"[0-9*#T]"
Each event can be qualified by a requested action, or by a list of
actions. The actions, when specified, are encoded as a list of keywords,
enclosed in parenthesis and separated by commas. The codes for the vari-
ous actions are:
_____________________________________
| Action | Code|
|_____________________________|______|
| Notify immediately | N |
| Accumulate | A |
| Treat according to digit map| D |
| Swap | S |
| Ignore | I |
|_____________________________|______|
When no action is specified, the default action is to notify the event.
This means that, for example, ft and ft(N) are equivalent. Events that
are not listed are ignored.
The digit-map action can only be specified for the digits, letters and
timers.
The requested list is encoded on a single line, with event/action groups
separated by commas. Examples of RequestedEvents encoding are:
R: hu(N), hf(S,N)
R: hu(N), [0-9#T](D)
3.2.2.6. SignalRequests
The SignalRequests parameter provides the name of the signals that have
been requested. Each signal is identified by a code, as indicated in the
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previous section.
Two events, announcement and ASDI display, can be qualified by addi-
tional parameters:
* the name and parameters of the announcement,
* the string that should be displayed.
These parameters will be enclosed within parenthesis, as in:
S: asdi(123456 Your friend)
S: ann(no-such-number, 1234567)
When several signals are requested, their codes are separated by a
comma, as in:
S: asdi(123456 Your friend), rg
3.2.2.7. ObservedEvent
The observed event parameters provides the list of events that have been
observed. The event codes are the same as those used in the Notifica-
tionRequest. Events that have been accumulated according to the digit
map are grouped in a single string. Examples of observed actions are:
O: hu
O: 8295555T
O: hf, hf, hu
The packet arrival event is used to notify that at least one packet was
recently sent to an Internet address that is observed by an endpoint.
The event report includes the Internet address, in standard ASCII encod-
ing, between parenthesis:
O: pa(192.96.41.1)
The call back event is used to notify that a call back has been
requested during the initial phase of a data connection. The event
report includes the identification of the user that should be called
back, between parenthesis:
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O: cbk(user25)
3.3. Format of response headers
The response header is composed of a response line, optionally followed
by headers that encode the response parameters.
The response line starts with the response code, which is a three digit
numeric value. The code is followed by a white space, the transaction
identifier, and an optional commentary.
In the case of a CreateConnection message, the response line is followed
by a Connection-Id parameter. It may also be followed a Specific-
Endpoint-Id parameter, if the creation request was sent to a generic
Endpoint-Id.
In the case of a DeleteConnection message, the response line is followed
by a Connection Parameters parameter, as defined in section 3.2.2.2.
A LocalConnectionDescriptor should be transmitted with a positive
response (code 200) to a CreateConnection. It may be transmitted in
response to a ModifyConnection command, if the modification resulted in
a modification of the session parameters. The LocalConnectionDescriptor
is encoded as a "session description," as defined in section 3.4. It is
separated from the response header by an empty line.
3.4. Encoding of the session description
The session description is encoded in conformance with the session
description protocol, SDP. SGCP implementations are expected to be fully
capable of parsing any conformant SDP message, and should send session
descriptions that strictly conform to the SDP standard. The usage of SDP
actually depends on the type of session that is being, as specified in
the "mode" parameter:
* if the mode is set to "data", the session description describes the
configuration of a data access service.
* if the mode is set to any other value, the session description is
for an audio service.
For an audio service, the gateway will consider the information provided
in SDP for the "audio" media. For a data service, the gateway will con-
sider the information provided for the "network-access" media.
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3.4.1. Usage of SDP for an audio service
In a telephony gateway, we only have to describe sessions that use
exactly one media, audio. The parameters of SDP that are relevant for
the telephony application are:
At the session description level:
* The IP address of the remote gateway (in commands) or of the
local gateway (in responses), or multicast address of the
audio conference, encoded as an SDP "connection data" parame-
ter. This parameter specifies the IP address that will be used
to exchange RTP packets.
For the audio media:
* Media description field (m) specifying the audio media, the
transport port used for receiving RTP packets by the remote
gateway (commands) or by the local gateway (responses) , the
RTP/AVP transport, and the list of formats that the gateway
will accept. This list should normally always include the code
0 (reserved for G.711).
* Optionally, RTPMAP attributes that define the encoding of
dynamic audio formats,
* Optionally, a packetization period (packet time) attribute
(Ptime) defining the duration of the packet,
* Optionally, an attribute defining the type of connection (sen-
donly, recvonly, sendrecv, inactive)
* The IP address of the remote gateway (in commands) or of the
local gateway (in responses), if it is not present at the ses-
sion level.
There is a request, in some environments, to use the SGCP to negotiate
connections that will use other transmission channels than RTP over UDP
and IP. This will be detailed in an extension to this document.
3.4.2. Usage of SDP in a network access service
The parameters of SDP that are relevant for a data network access appli-
cation are:
For the data media:
* Media description field (m) specifying the network access
media, identified by the code "m=nas/xxxx", where "xxxx"
describes the access control method that should be used for
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parametrizing the network access, as specified below. The
field may also specify the port that should be used for con-
tacting the server, as specified in the SDP syntax.
* Connection address parameter (c=) specifying the address, or
the domain name, of the server that implement the access con-
trol method. This parameter may also be specified at the ses-
sion level.
* Optionally, a bearer type attribute (a=bearer:) describing the
type of data connection to be used.
* Optionally, a framing type attribue (a=framing:) describing
the type of framing that will be used on the channel.
* Optionally, attributes describing the called number
(a=dialed:), the number to which the call was delivered
(a=called:) and the calling number (a=dialing:).
* Optionally, attributes describing the range of addresses that
could be used by the dialup client on its LAN (a=subnet:).
* Optionally, an encryption key, encoded as specified in the SDP
protocol (k=).
The connection address shall be encoded as specified in the SDP stan-
dard. It will be used in conjunction with the port specified in the
media line to access a server, whose type will one of:
__________________________________________________________
| Method name| Method description |
|____________|____________________________________________|
| radius | Authentication according |
| | to the Radius protocol. |
| tacacs | Authentication according |
| | to the TACACS+ protocol. |
| diameter | Authentication according |
| | to the Diameter protocol. |
| l2tp | Level 2 tunneling protocol. |
| | The address and port are those of the LNS.|
| login | Local login. (There is normally |
| | no server for that method.) |
| none | No authentication required. |
| | (The call was probably vetted |
| | by the Call Agent.) |
|____________|____________________________________________|
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If needed, the gateway may use the key specified in the announcement to
access the service. That key, in particular, may be used for the estab-
lishment of an L2TP tunnel.
The valid values of the bearer attribute are defined in the following
table:
___________________________________________________________________
| Type of bearer description | Example of values |
|_________________________________|________________________________|
| ITU modem standard | V.32, V.34, V.90. |
| ISDN transparent access, 64 kbps| ISDN64 |
| ISDN64 + V.110 | ISDN64/V.110 |
| ISDN64 + V.120 | ISDN64/V.120 |
| ISDN transparent access, 56 kbps| ISDN56 |
| Informal identification | (Requires coordination between|
| | |
| | the |
| Call Agent and the gateway) | |
|_________________________________|________________________________|
The valid values of the framing attribute are defined in the following
table:
_________________________________________________
| Type of framing description| Example of values|
|____________________________|___________________|
| PPP, asynchronous framing | ppp-asynch |
| PPP, HDLC framing | ppp-hdlc |
| SLIP, asynchronous | slip |
| Asynchronous, no framing | asynch |
|____________________________|___________________|
The network access authentication parameter provides instructions on the
access control that should be exercized for the data call. This optional
attribute is encoded as:
"a=subnet:" <network type> <address type>
<connection address> "/" <prefix length>
Where the parameters "network type", "address type", and "connection
address" are formatted as defined for the connection address parameter
(c=) in SDP, and where the "prefix length" is a decimal representation
of the number of bits in the prefix.
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Examples of SDP announcement for the network access service could be:
v=0
m=nas/radius
c=IN IP4 radius.example.net
a=bearer:v.34
a=framing:ppp-asynch
a=dialed:18001234567
a=called:12345678901
a=dialing:12340567890
v=0
m=nas/none
c=IN IP4 128.96.41.1
a=subnet:IN IP4 123.45.67.64/26
a=bearer:isdn64
a=framing:ppp-sync
a=dialed:18001234567
a=dialing:2345678901
v=0
c=IN IP4 access.example.net
m=nas/l2tp
k=clear:some-shared-secret
a=bearer:v.32
a=framing:ppp-asynch
a=dialed:18001234567
a=dialing:2345678901
3.5. Transmission over UDP
SGCP messages are transmitted over UDP. Commands are sent to one of the
IP addresses defined in the DNS for the specified endpoint . The
responses are sent back to the source address of the commands.
When no port is specified for the endpoint, the commands should be sent
to the default SGCP port, 2427.
SGCP messages, being carried over UDP, may be subject to losses. In the
absence of a timely response, commands are repeated. SGCP entities are
expected to keep in memory a list of the responses that they sent to
recent transactions, i.e. a list of all the responses they sent over the
last 30 seconds, and a list of the transactions that are currently being
executed. The transaction identifiers of incoming commands are compared
to the transaction identifiers of the recent responses. If a match is
found, the SGCP entity does not execute the transaction, but simply
repeats the response. The remaining commands will be compared to the
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list of current transaction. If a match is found, the SGCP entity does
not execute the transaction, which is simply ignored.
It is the responsibility of the requesting entity to provide suitable
time outs for all outstanding commands, and to retry commands when time
outs have been exceeded. Furthermore, when repeated commands fail to be
acknowledged, it is the responsibility of the
requesting entity to seek redundant services and/or clear existing or
pending connections.
The specification purposely avoids specifying any value for the
retransmission timers. These values are typically network dependent. The
retransmission timers should normally estimate the timer by measuring
the time spent between the sending of a command and the return of a
response. One possibility is to use the algorithm implemented in TCP-IP,
which uses two variables:
* the average acknowledgement delay, AAD, estimated through an
exponentially smoothed average of the observed delays,
* the average deviation, ADEV, estimated through an exponentially
smoothed average of the absolute value of the difference between
the observed delay and the current average
The retransmission timer, in TCP, is set to the sum of the average delay
plus N times the average deviation.
After the any retransmission, the SGCP entity should do the following:
* It should double the estimated value of the average delay, AAD
* It should compute a random value, uniformly distributed between 0.5
AAD and AAD
* It should set the retransmission timer to the sum of that random
value and N times the average deviation.
This procedure has two effects. Because it includes an exponentially
increasing component, it will automatically slow down the stream of mes-
sages in case of congestion. Because it includes a random component, it
will break the potential synchronization between notifications triggered
by the same external event.
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4. Security requirements
If unauthorized entities could use the SGCP, they would be able to set-
up unauthorized calls, or to interfere with authorized calls. We expect
that SGCP messages will always be carried over secure Internet connec-
tions, as defined in the IP security architecture as defined in RFC
1825, using either the IP Authentication Header, defined in RFC 1826, or
the IP Encapsulating Security Payload, defined in RFC 1827. The complete
SGCP protocol stack would thus include the following layers:
________________________________
| SGCP |
|_______________________________|
| UDP |
|_______________________________|
| IP security |
| (authentication or encryption)|
|_______________________________|
| IP |
|_______________________________|
| transmission media |
|_______________________________|
Adequate protection of the connections will be achieved if the gateways
and the Call Agents only accept messages for which IP security provided
an authentication service. An encryption service will provide additional
protection against eavesdropping, thus forbidding third parties from
monitoring the connections set up by a given endpoint
The encryption service will also be requested if the session descrip-
tions are used to carry session keys, as defined in SDP.
These procedure do not necessarily protect against denial of service
attacks by misbehaving gateways or misbehaving call agents. However,
they will provide an identification of these misbehaving entities, which
should then be deprived of their authorization through maintenance pro-
cedures.
5. Example of call flows
In order to understand the way the SGCP interface will be used, we have
described here two possible call flows between a TGW, which is a trunk-
ing gateway that implements SGCP, and an RGW, which is a residential
gateway that implements SGCP, as well as four call flows describing how
SGCP could be used to control a network access service.
The diagrams also show a Common Database (CDB) that can be queried for
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authorization and routing information, and an Accounting Gateway (ACC)
that collects accounting information at the start and the end of calls.
These diagrams are solely meant to exhibit the behavior of the SGCP, and
to help understanding this protocol. They are not meant as a tutorial on
the implementation of a Call Agent. They may very well include miscel-
laneous errors and imprecisions.
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5.1. Basic call, RGW to TGW
______________________________________________________________________
| Usr | RGW | CA | CDB| ACC| TGW | SS7/| CO |
| | | | | | | ISUP| |
|_______|__________|______________|_____|_____|__________|______|_____|
| | <- | Notification| | | | | |
| | | Request | | | | | |
| | Ack | -> | | | | | |
| Off | Notify | -> | | | | | |
| -hook | <- | Ack | | | | | |
| (Dial | <- | Notification| | | | | |
| -tone)| | Request | | | | | |
| | Ack | -> | | | | | |
| Digit | Notify | -> | | | | | |
| | <- | Ack | | | | | |
| (pro- | <- | Notification| | | | | |
| gress)| | Request | | | | | |
| | Ack | -> | | | | | |
| | <- | Create | | | | | |
| | | Connection | | | | | |
| | Ack | -> | | | | | |
| | | Query | | | | | |
| | | (E.164 S,D) | -> | | | | |
| | | <- | IP | | | | |
| | | Create | | | | | |
| | | Connection | - -| - -| -> | | |
| | | | | | (cut in)| | |
| | | <- | - -| - -| ack | | |
| | | IAM | - -| - -| - - | -> | |
| | <- | Modify | | | | IAM | -> |
| | | Connection | | | | | |
| | Ack | -> | | | | <- | ACM|
| | | <- | - -| - -| - - | ACM | |
| | <- | Notification| | | | | |
| | | Request | | | | | |
| | Ack | -> | | | | | |
| | | | | | | <- | ANM|
| | | <- | - -| - -| - - | ANM | |
| | <- | Notification| | | | | |
| | | Request | | | | | |
| | Ack | -> | | | | | |
| | <- | Modify | | | | | |
| | | Connection | | | | | |
| | Ack | -> | | | | | |
| | (cut in)| Call start | - -| -> | | | |
|_______|__________|______________|_____|_____|__________|______|_____|
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__________________________________________________________________
| Usr | RGW | CA | CDB| ACC| TGW | SS7/| CO |
| | | | | | | ISUP| |
|________|________|______________|_____|_____|_______|______|_____|
| | | | | | | <- | REL|
| | | <- | - -| - -| - - | REL | |
| | <- | Delete | | | | | |
| | | Connection | | | | | |
| | | Delete | | | | | |
| | | Connection | - -| - -| -> | | |
| | Perf | | | | | | |
| Data | -> | | | | | | |
| | | <- | - -| - -| perf | | |
| | | | | | data | | |
| | | Call end | - -| -> | | | |
| On-hook| Notify| -> | | | | | |
| | <- | Ack | | | | | |
| | <- | Notification| | | | | |
| | | Request | | | | | |
| | Ack | -> | | | | | |
|________|________|______________|_____|_____|_______|______|_____|
During these exchanges the SGCP is used by the Call Agent to control
both the TGW and the residential gateway. The exchanges occur on two
sides.
The first command is a NotificationRequest, sent by the Call Agent to
the residential gateway. The request will consist of the following
lines:
RQNT 1201 endpoint-1@rgw-2567.whatever.net SGCP 1.1
N: ca@ca1.whatever.net:5678
X: 0123456789AB
R: hd
The gateway, at that point, is instructed to look for an off-hook event,
and to report it. It will first acknowledge the command, repeating in
the acknowledgement message the transaction id that the Call Agent
attached to the query.
200 1201 OK
When the off hook event is noticed, the gateway initiates a
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NotificationRequest to the Call Agent:
NTFY 2001 endpoint-1@rgw-2567.whatever.net SGCP 1.1
N: ca@ca1.whatever.net:5678
X: 0123456789AB
O: hd
The Call Agent immediately acknowledges that notification.
200 2001 OK
The Call Agent examines the services associated to an off hook action
(it could take special actions in the case of a direct line). In most
cases, it will send a NotificationRequest, asking for more digits. The
current example provides the gateway with a permanent digit map, and
request the gateway to play a dialtone:
RQNT 1202 endpoint-1@rgw-2567.whatever.net SGCP 1.1
N: ca@ca1.whatever.net:5678
X: 0123456789AC
R: hu, [0-9#*T](D)
D: (0T|00T|[1-7]xxx|8xxxxxxx|#xxxxxxx|*xx|91xxxxxxxxxx|9011x.T)
S:
dt
The gateway immediately acknowledges that command.
200 1202 OK
The gateway will start accumulating digits according to that digit map.
When it has noticed a sufficient set of values, it will notify the
observed string to the Call Agent:
NTFY 2002 endpoint-1@rgw-2567.whatever.net SGCP 1.1
N: ca@ca1.whatever.net:5678
X: 0123456789AC
O: 912018294266
The Call Agent immediately acknowledges that notification.
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200 2002 OK
At this stage, the Call Agent will send a NotificationRequest, to stop
collecting digits yet continue watch for an on-hook transition:
RQNT 1203 endpoint-1@rgw-2567.whatever.net SGCP 1.1
X: 0123456789AD
R: hu
The Call Agent immediately acknowledges that command.
200 1203 OK
The Call Agent will then seize the incoming circuit, creating a connec-
tion:
CRCX 1204 endpoint-1@rgw-2567.whatever.net SGCP 1.1
C: A3C47F21456789F0
L: p:10, a:G.711;G.726-32
M: recvonly
The gateway immediately acknowledges the creation, sending back the
identification of the newly created connection and the session descrip-
tion used to receive audio data:
200 1204 OK
I:FDE234C8
v=0
c=IN IP4 128.96.41.1
m=audio 3456 RTP/AVP 0 96
a=rtpmap:96 G726-32/8000
The SDP announcement, in our example, specifies the address at which the
gateway is ready to receive audio data (128.96.41.1), the transport pro-
tocol (RTP), the RTP port (3456) and the audio profile (AVP). The audio
profile refers to RFC 1890, which defines that the payload type 0 has
been assigned for G.711 transmission. The gateway is also ready to use
ADPCM encoding at 32 kbps (G.726 4). There is no standard payload type
associated to ADPCM, so the gateway mentions its readiness to use a non
Arango, Huitema [Page 52]
Internet draft Simple Gateway Control Protocol (SGCP) 30 July 1998
standard payload associated to the dynamic type 96. The "rtpmap" attri-
bute entry associates the payload type 96 to G726-32/4.
The Call Agent, having seized the incoming trunk and completed a routing
look up to identify the outgoing gateway, must now seize the outgoing
trunk. It does so by sending a connection command to the e-gress gate-
way:
CRCX 1205 card23/21@trgw-7.whatever.net SGCP 1.1
C: A3C47F21456789F0
L: p:10, a:G.711;G.726-32
M: sendrecv
v=0
c=IN IP4 128.96.41.1
m=audio 3456 RTP/AVP 0 96
a=rtpmap:96 G726-32/8000
The CreateConnection command has the same parameters as the command sent
to the ingress gateway, with two differences:
* The EndpointId points towards the outgoing trunk,
* The message carries the session description returned by the ingress
gateway,
* Because the session description is present, the "mode" parameter is
set to "send/receive".
We observe that the call identifier is identical for the two connec-
tions. This is normal: the two connections belong to the same call,
which has a global identifier in our system.
The trunking gateway will acknowledge the connection command, sending in
the session description its own parameters such as address, ports and
RTP profile:
200 1205 OK
I:32F345E2
v=0
c=IN IP4 128.96.63.25
m=audio 1297 RTP/AVP 0 96
a=rtpmap:96 G726-32/8000
Arango, Huitema [Page 53]
Internet draft Simple Gateway Control Protocol (SGCP) 30 July 1998
The Call Agent will relay the information to the ingress gateway, using
a ModifyConnection command:
MDCX 1206 endpoint-1@rgw-2567.whatever.net SGCP 1.1
C: A3C47F21456789F0
I:FDE234C8
M: recvonly
v=0
c=IN IP4 128.96.63.25
m=audio 1297 RTP/AVP 0 96
a=rtpmap:96 G726-32/8000
The residential gateway immediately acknowledges the modification:
200 1206 OK
At this stage, the Call Agent has established a half duplex transmission
path. The phone attached to the residential gateway will be able to
receive the signals, such as tones or announcements, that the remote
switch may send through the trunking gateway.
When the call progresses, the Call Agent will receive from the remote
switch progress messages, for example an "address complete" message
(ACM). The Call Agent will analyze the message to determine whether sig-
nal are transmitted in band. If this is not the case, the Call Agent
will instruct the RGW to generate ringing tones by sending a Notifica-
tionRequest:
RQNT 1207 endpoint-1@rgw-2567.whatever.net SGCP 1.1
X: 0123456789AE
R: hu
S: rt
The gateway immediately acknowledges the command:
200 1207 OK
After the called user answers the call, the Call Agent will receive an
answering message (ANM) from the CO switch. At that point, it will send
a NotificationRequest to the residential gateway, to stop the ringing
tones, and a ModifyConnection command, to place the connection in full
Arango, Huitema [Page 54]
Internet draft Simple Gateway Control Protocol (SGCP) 30 July 1998
duplex mode:
RQNT 1208 endpoint-1@rgw-2567.whatever.net SGCP 1.1
X: 0123456789AF
R: hu
MDCX 1209 endpoint-1@rgw-2567.whatever.net SGCP 1.1
C: A3C47F21456789F0
I:FDE234C8
M: sendrecv
The residential gateway will acknowledge these two commands:
200 1208 OK
200 1209 OK
At this point, the connection is established.
When the Call Agent receives the REL message from the CO switch, it will
have to tear down the call. It will do so by sending to both gateways a
DeleteConnection command:
DLCX
1210 endpoint-1@rgw-2567.whatever.net SGCP 1.1
C: A3C47F21456789F0
I:FDE234C8
DLCX 1211 card23/21@trgw-7.whatever.net SGCP 1.1
C: A3C47F21456789F0
I:32F345E2
The gateways will respond with acknowledgements that should include a
"call parameters" header fields:
250 1210 OK
P: PS=1245, OS=62345, PR=780, OR=45123, PL=10, JI=27,
LA=48
250 1211 OK
P: PS=790, OS=45700, PR=1230, OR=61875, PL=15, JI=27,
LA=48
Arango, Huitema [Page 55]
Internet draft Simple Gateway Control Protocol (SGCP) 30 July 1998
At this point, phone attached to the residential gateway, in our
scenario, goes on-hook. This event is notified to the Call Agent,
according to the policy received in the last NotificationRequest by
sending a Notify command:
NTFY 2005 endpoint-1@rgw-2567.whatever.net SGCP 1.1
X: 0123456789AF
O: hu
After this notification, the Call Agent should send an acknowledgement:
200 2005 OK
It should then issue a new NotificationRequest, to be ready to receive
the next off-hook detected by the residential gateway:
RQNT 1212 endpoint-1@rgw-2567.whatever.net SGCP 1.1
X: 0123456789B0
R: hd
The gateway will acknowledge this message:
200 1212 OK
Both gateways, at this point, are ready for the next
call.
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Internet draft Simple Gateway Control Protocol (SGCP) 30 July 1998
5.2. Basic call, TGW to RGW
___________________________________________________________________
| CO | SS7/| TGW | CA | CDB| ACC| RGW | Usr |
| | ISUP| | | | | | |
|____|______|__________|_______________|_____|_____|________|______|
| IAM| -> | | | | | | |
| | IAM | - - | -> | | | | |
| | | | Check | -> | | | |
| | | | <- | IP | | | |
| | | <- | Create | | | | |
| | | | Connection | | | | |
| | | Ack | -> | | | | |
| | | | Create | | | | |
| | | | Connection | - -| - -| -> | |
| | | | <- | - -| - -| Ack | |
| | | <- | Modify | | | | |
| | | | Connection | | | | |
| | | Ack | -> | | | | |
| | | | Notification | | | | |
| | | | Request | - -| - -| -> | ring|
| | | | <- | - -| - -| Ack | |
| | <- | - - | ACM | | | | |
| <- | ACM | | | | | | |
| | | | | | | | off |
| | | | | | | | hook|
| | | | <- | - -| - -| Notify| |
| | | | Ack | - -| - -| -> | |
| | | | Notification | | | | |
| | | | Request | - -| - -| -> | |
| | | | <- | - -| - -| Ack | |
| | | <- | Modify | | | | |
| | | | Connection | | | | |
| | | Ack | -> | | | | |
| | | (cut-in)| Call start | - -| -> | | |
| | <- | - - | ANM | | | | |
| <- | ANM | | | | | | |
|____|______|__________|_______________|_____|_____|________|______|
Arango, Huitema [Page 57]
Internet draft Simple Gateway Control Protocol (SGCP) 30 July 1998
_____________________________________________________________
| CO| SS7/| TGW | CA | CDB| ACC| RGW | Usr |
| | ISUP| | | | | | |
|___|______|______|______________|_____|_____|________|______|
| | | | | | | | on |
| | | | | | | | hook|
| | | | <- | - -| - -| Notify| |
| | | | Ack | - -| - -| -> | |
| | | | Delete | | | | |
| | | | Connection | - -| - -| -> | |
| | | <- | Delete | | | | |
| | | | Connection | | | | |
| | <- | - - | REL | | | | |
| <-| REL | | | | | | |
| | | Perf| | | | | |
| | | data| -> | | | | |
| | | | <- | - -| - -| perf | |
| | | | | | | data | |
| | | | Call end | - -| -> | | |
| | | | Notification| | | | |
| | | | Request | - -| - -| -> | |
| | | | <- | - -| - -| Ack | |
|___|______|______|______________|_____|_____|________|______|
This diagram shows the various exchange of messages during a call from a
telephone user on the circuit-switched PSTN to a residential user con-
nected to a residential gateway. During these exchanges the Call Agent
uses SGCP to control both the TGW and the residential gateway. The
exchanges occur on two sides.
Upon reception of the IAM message, the Call Agent immediately sends a
CreateConnection request to the trunking gateway to connect to the
incoming trunk, creating a connection:
CRCX
1237 card23/21@trgw-7.whatever.net SGCP 1.1
C: A3C47F21456789F0
L: p:10, a:G.711;G.726-32
M: recvonly
The trunking gateway immediately acknowledges the creation, sending back
the identification of the newly created connection and the session
description used to receive audio data:
Arango, Huitema [Page 58]
Internet draft Simple Gateway Control Protocol (SGCP) 30 July 1998
200 1237 OK
I: FDE234C8
v=0
c=IN IP4 128.96.41.1
m=audio 3456 RTP/AVP 0 96
a=rtpmap:96 G726-32/8000
The SDP announcement, in our example, specifies the address at which the
gateway is ready to receive audio data (128.96.41.1), the transport pro-
tocol (RTP), the RTP port (3456) and the audio profile (AVP). The audio
profile refers to RFC 1890, which defines that the payload type 0 has
been assigned for G.711 transmission. The gateway is also ready to use
ADPCM encoding at 32 kbps (G.726 4). There is no standard payload type
associated to ADPCM, so the gateway mentions its readiness to use a non
standard payload associated to the dynamic type 96. The "rtpmap" attri-
bute entry associates the payload type 96 to G726/4.
The Call Agent, having seized the incoming trunk, must now reserve the
outgoing circuit. It does so by sending a connection command to the
residential gateway:
CRCX 1238 endpoint-1@rgw-2567.whatever.net SGCP 1.1
C: A3C47F21456789F0
L: p:10, a:G.711;G.726-32
M: sendrecv
v=0
c=IN IP4 128.96.41.1
m=audio 3456 RTP/AVP 0 96
a=rtpmap:96 G726-32/8000
The CreateConnection command has the same parameters as the command sent
to the ingress gateway, with two differences:
* The EndpointId points towards the outgoing trunk,
* The message carries the session description returned by the ingress
gateway,
* Because the session description is present, the "mode" parameter is
set to "send/receive".
We observe that the call identifier is identical for the two
Arango, Huitema [Page 59]
Internet draft Simple Gateway Control Protocol (SGCP) 30 July 1998
connections. This is normal: the two connection belong to the same call,
which has a global identifier in our system.
The trunking gateway will acknowledge the connection command, sending in
the session description its own parameters such as address, ports and
RTP profile:
200 1238 OK
I:32F345E2
v=0
c=IN IP4 128.96.63.25
m=audio 1297 RTP/AVP 0 96
a=rtpmap:96 G726-32/8000
The Call Agent will relay the information to the ingress gateway, using
a ModifyConnection command:
MDCX 1239 card23/21@trgw-7.whatever.net SGCP 1.1
C: A3C47F21456789F0
I:FDE234C8
M: recvonly
v=0
c=IN IP4 128.96.63.25
m=audio 1297 RTP/AVP 0 96
a=rtpmap:96 G726-32/8000
The trunking gateway immediately acknowledges the modification:
200 1239 OK
At this stage, the Call Agent has established a half-duplex transmission
path. The Call Agent must now tells the residential gateway to ring the
called line. It will send a NotificationRequest, consisting of the fol-
lowing lines:
RQNT
1240 endpoint-1@rgw-2567.whatever.net SGCP 1.1
X: 0123456789B1
R: hd
S: rg
Arango, Huitema [Page 60]
Internet draft Simple Gateway Control Protocol (SGCP) 30 July 1998
The residential gateway, at that point, is instructed to look for an
off-hook event, and to report it. It will first acknowledge the command,
repeating in the acknowledgement message the transaction id that the
Call Agent attached to the query.
200 1240 OK
Upon reception of this message, the Call Agent sends an address complete
message (ACM) to the calling switch, which will generate ringing tones
for the calling user.
When the gateway notices the off hook event, it sends a Notify command
to the Call Agent:
NTFY 2001 endpoint-1@rgw-2567.whatever.net SGCP 1.1X:
0123456789B0
O: hd
The Call Agent immediately acknowledges that notification.
200 2001 OK
The Call Agent now asks the residential gateway to send a Notify command
on the occurrence of an on-hook event. It does so by sending a Notifica-
tionRequest to the residential gateway:
RQNT 1241 endpoint-1@rgw-2567.whatever.net SGCP 1.1
X: 0123456789B1
R: hu
The gateway acknowledges that command:
200 1241 OK
In parallel, the Call Agent will send a ModifyConnection command to the
trunking gateway, to place the connection in full duplex mode:
MDCX 1242 card23/21@trgw-7.whatever.net SGCP 1.1
C: A3C47F21456789F0
I:FDE234C8
M: sendrecv
Arango, Huitema [Page 61]
Internet draft Simple Gateway Control Protocol (SGCP) 30 July 1998
The trunking gateway will acknowledge that command:
200 1242 OK
The Call Agent can now send an answer message (ANM) to the calling
switch.
After some time, the Call Agent will have to tear down the call. In our
example, this is triggered by the residential user, who hangs up. The
Notify command is sent to the Call Agent:
NTFY 2005 endpoint-1@rgw-2567.whatever.net SGCP 1.1
X: 0123456789B1
O: hu
The Call Agent acknowledges the notification.
200 2005 OK
It will then send to both gateways a DeleteConnection command:
DLCX
1243 endpoint-1@rgw-2567.whatever.net SGCP 1.1
C: A3C47F21456789F0
I:FDE234C8
DLCX 1244 card23/21@trgw-7.whatever.net SGCP 1.1
C: A3C47F21456789F0
I:32F345E2
The gateways will respond with a message that should include a "call
parameters" header fields:
250 1243 OK
P: PS=1245, OS=62345, PR=780, OR=45123, PL=10, JI=27,
LA=48
250 1244 OK
P: PS=790, OS=45700, PR=1230, OR=61875, PL=15, JI=27,
LA=48
The Call Agent should now issue a new NotificationRequest to the
Arango, Huitema [Page 62]
Internet draft Simple Gateway Control Protocol (SGCP) 30 July 1998
residential gateway to detect the next off-hook event:
RQNT 1245 endpoint-1@rgw-2567.whatever.net SGCP 1.1
X: 0123456789B2
R: hd
The residential gateway will acknowledge this command:
200 1245 OK
Both gateways, at this point, are ready for the next call.
Arango, Huitema [Page 63]
Internet draft Simple Gateway Control Protocol (SGCP) 30 July 1998
5.3. Data call to a TGW
_______________________________________________________________________
| PC | CO | SS7/| TGW | CA | ACC| Radius|
| | | ISUP| | | | |
|__________|_____|______|_____________|________________|_____|________|
| dials in | | | | | | |
| | IAM| -> | | | | |
| | | IAM | - - | -> | | |
| | | | | Check called | | |
| | | | | number. | | |
| | | | | Notices | | |
| | | | | data call. | | |
| | | | | Call start | -> | |
| | | | <- | Create | | |
| | | | | Connection | | |
| | | | | (data) | | |
| | | | Ack | -> | | |
| | | | | Connection | | |
| | | | | is completed. | | |
| | | | | Call | | |
| | | | | established | -> | |
| | | <- | - - | ANM | | |
| | <- | ANM | | | | |
| modem | - -| - - | -> | | | |
| <- | - -| - - | handshake | | | |
| PPP | - -| - - | -> | | | |
| | | | obtain | | | |
| | | | user-id, | | | |
| | | | password | | | |
| | | | Check | - - | - -| -> |
| | | | <- | - - | - -| Ack |
| <- | - -| - - | Validates | | | |
| | | | call, | | | |
| <- | - -| - - | procures | | | |
| | | | IP | | | |
| | | | address | | | |
|Connected | | | | | | |
| to | | | | | | |
| the | | | | | | |
| Internet | | | | | | |
|__________|_____|______|_____________|________________|_____|________|
Arango, Huitema [Page 64]
Internet draft Simple Gateway Control Protocol (SGCP) 30 July 1998
______________________________________________________________
| PC | CO | SS7/| TGW | CA | ACC| Radius|
| | | ISUP| | | | |
|____________|_____|______|_______|____________|_____|________|
| Closes | | | | | | |
| connection.| | | | | | |
| | REL| -> | | | | |
| | | REL | - - | -> | | |
| | | | <- | Delete | | |
| | | | | Connection| | |
| | | | Perf | | | |
| | | | data | -> | | |
| | | <- | - - | RLC | | |
| | <- | RLC | | | | |
| | | | | Call end | -> | |
|____________|_____|______|_______|____________|_____|________|
This diagram shows the exchange of messages during a call from a modem
user to an Internet Service Provider, using a trunking gateway that dou-
bles as a Network Access Server. During these exchanges the SGCP is used
by the Call Agent to control both the trunking gateway. Since there is
no "other end" of the call, only the trunk gateway is involved in the
call.
Upon reception of the IAM message, the Call Agent determines that the
call is a data call (e.g., by bearer capability, the called number,
etc.). Using configuration databases, the Call Agent selects the type of
modem parameters and authentication parameters that correspond to the
called number and to the calling number. It uses this knowledge to send
a CreateConnection command to the TGW, programming the incoming trunk:
CRCX 1237 card23/21@trgw-7.whatever.net SGCP 1.1
C: A3C47F21456789F0
M: data
X: 0123456789B1
R: cl
v=0
m=nas/radius
c=IN IP4 radius.example.net
a=bearer:v.32
a=framing:ppp-asynch
a=dialed:18001234567
a=dialing:2345678901
The trunking gateway checks that it has adequate resources for the call.
Arango, Huitema [Page 65]
Internet draft Simple Gateway Control Protocol (SGCP) 30 July 1998
If the trunking gateway did not have adequate resources, for example if
it could not support the requested modem type, it should refuse the
creation and send an error response to the Call Agent. If the gateway
has sufficient resources, it immediately acknowledges the creation,
sending back the identification of the newly created connection. (There
is no need to transmit a session description in the case of a data
call.)
200 1237 OK
I: FDE234C8
The Call Agent, knowing that this is a data call, can immediately ack-
nowledge the establishment of the connection, sending an ANM message
back to the calling switch.
The trunk gateway connects the incoming trunk to a DSP loaded with the
specified modem code. Once the call is established, the modem of the
calling PC will start a training sequence with the modem associated to
the trunk in the trunk gateway. The caller will then proceed to a normal
PPP synchronization, which probably implies a PPP login. The authentica-
tion parameters, in our example, are checked using Radius. The Radius
server that will be used is typically chosen as a function of the called
number, which identifies the data service that the calling modem
requested. In fact, the number can also be used to identify the specific
form of authentication that is requested (but not usually).
In our example, the call is completed when the calling modem hangs up.
This triggers an ISUP release message, which is forwarded to the Call
Agent. The Call Agent will request the TGW to delete the connection:
DLCX 1244 card23/21@trgw-7.whatever.net SGCP 1.1
C: A3C47F21456789F0
I: FDE234C8
The gateways will respond with a message that should include a "call
parameters" header fields:
250 1244 OK
P: PS=1245, OS=62345, PR=780, OR=45123
We should note that, because this is a data call, the call parameters
only include a count of the packets and octets that were sent and
received.
Arango, Huitema [Page 66]
Internet draft Simple Gateway Control Protocol (SGCP) 30 July 1998
5.4. Outgoing data call through a TGW
_____________________________________________________________________
| PC | CO | SS7/| TGW | CA | ACC| Router |
| | | ISUP| | | | |
|___________|_____|______|____________|_____________|_____|__________|
| | | | | | | notices |
| | | | | | | packet |
| | | | | | | to PC |
| | | | | <- | - -| NTFY |
| | | | | Ack | - -| -> |
| | | | | Decides to | | |
| | | | | place an | | |
| | | | | outgoing | | |
| | | | | call. | | |
| | | | | Call start | -> | |
| | | | <- | Create | | |
| | | | | Connection | | |
| | | | | (data) | | |
| | | | Ack | -> | | |
| | | <- | - - | IAM | | |
| (rings) | <- | IAM | | | | |
| | ACM| -> | | | | |
| | | ACM | - - | -> | | |
| (answer) | ANM| -> | | | | |
| | | ANM | - - | -> | | |
| | | | | Connection | | |
| | | | | complete. | | |
| | | | | Call | | |
| | | | | established| -> | |
| PPP | - -| - - | -> | | | |
| <- | - -| - - | Validates | | | |
| | | | call, | | | |
| | | | announces | | | |
| | | | IP address| - - | - -| -> |
| Connected | | | | | | |
| to the | | | | | | |
| Internet | | | | | | |
| | | | | | | |
|___________|_____|______|____________|_____________|_____|__________|
Arango, Huitema [Page 67]
Internet draft Simple Gateway Control Protocol (SGCP) 30 July 1998
___________________________________________________________________
| PC | CO | SS7/| TGW | CA | ACC| Router|
| | | ISUP| | | | |
|____________|_____|______|____________|____________|_____|________|
| Closes | | | | | | |
| connection.| | | | | | |
| | REL| -> | | | | |
| | | REL | - - | -> | | |
| | | | <- | Delete | | |
| | | | | Connection| | |
| | | | ceases | | | |
| | | | announcing| | | |
| | | | IP address| - - | - -| -> |
| | | | Perf | | | |
| data | -> | | | | | |
| | | <- | - - | RLC | | |
| | <- | RLC | | | | |
| | | | | Call end | -> | |
|____________|_____|______|____________|____________|_____|________|
This diagram shows the exchange of messages during a call from an an
Internet Service Provider to a modem, using a trunking gateway that dou-
bles as a Network Access Server. During these exchanges the SGCP is used
by the Call Agent to control both the TGW, and will also be used between
the Call Agent and a default router of the ISP.
In the example configuration, the calls are set on demand, when data
have to actually be sent from the Internet to the dial-up user. When no
connection is established, the local routing is configured to send the
packets towards a default router which may or may not be the same
machine as the TGW. In redundant configurations, there could be many
default routers. Each of these default routers has been programmed
(through a notification request) to send a notification to the Call
Agent when it receives a packet on the default route:
NTFY 2005 default-route@router25.whatever.net SGCP 1.1
X:
0123456789AF
O: pa(192.96.41.1)
After this notification, the Call Agent should send an acknowledgement:
200 2005 OK
(We should note here that using SGCP for this function is a stretch.
Arango, Huitema [Page 68]
Internet draft Simple Gateway Control Protocol (SGCP) 30 July 1998
There are other protocols, notably RMON, that already provide an ade-
quate service. These protocols could be used instead of SGCP without
affecting the discussion that follows.)
The Call Agent deduces from the notification that a circuit should be
established towards the dial-up user, or towards the dial-up router.
Using configuration databases, the Call Agent selects the number that
should be called, and also the type of modem parameters and authentica-
tion parameters that correspond to the called number. The Call Agent
uses its routing table to select an adequate TGW, with an available out-
going trunk. It uses a create connection command to seize this outgoing
trunk:
CRCX 1237 card23/21@trgw-7.whatever.net SGCP 1.1
C: A3C47F21456789F0
M: data
X: 0123456789B1
R: cl
v=0
m=nas/none
c=IN IP4 128.96.41.1
a=subnet:IN IP4 123.45.67.64/26
a=bearer:isdn64
a=framing:ppp-hdlc
a=dialed:18001234567
a=dialing:2345678901
The gateway immediately acknowledges the creation, sending back the
identification of the newly created connection. (There is no session
description in the case of a data call.)
200 1237 OK
I: FDE234C8
Once the trunk has been seized, the Call Agent will send an IAM message
to the switch that controls the trunk. The dialed PC will "ring" and
eventually take the call, triggering the arrival of progress messages
and then an answer message (ANM). At that point, the Call Agent knows
that the call is established.
The DSP associated to the incoming trunk has been loaded with the speci-
fied modem code a simple HDLC framing in our example. Once the call is
established, the calling PC will train with the modem associated with
the trunk. In our example, no authentication is requested: the Call
Agent has identified the dialed user through its called number.
Arango, Huitema [Page 69]
Internet draft Simple Gateway Control Protocol (SGCP) 30 July 1998
Once the association is established and the IP service is validated, the
gateway announces that it serves the local user. In our example, there
is no address configuration performed through PPP: the dialed user has a
permanent address, which has been programmed when it subscribed to the
service. However, one the circuit is validated, the gateway should start
announcing its access to this permanent address in the routing tables.
In our example, the dialed station is in fact an access point to a local
network, and the TGW should start announcing accessibility of that local
network (123.45.67.64/26) through the local routing procedures (an IGP
such as RIP, OSPF or EIGRP).
Note that the current design makes the hypothesis that the Call Agent
"tells" the address of the LAN to the TGW. This is a very debatable
design. If a secure IGP is used (e.g. using embedded keyed MD5 authenti-
cation, or using IPSEC) then the routing prefix will be naturally
exchanged through this IGP. On the other hand, some form of configura-
tion can provide a "double check" against user errors.
In our example, the call is completed when the called modem hangs up.
This triggers an ISUP release message, which is forwarded to the Call
Agent. The Call Agent will request the TGW to delete the connection:
DLCX 1244 card23/21@trgw-7.whatever.net SGCP 1.1
C: A3C47F21456789F0
I: FDE234C8
The gateways will respond with a message that should include a "call
parameters" header fields:
250 1244 OK
P: PS=1245, OS=62345, PR=780, OR=45123
We should note that, because this is a data call, the call parameters
only include a count of the packets and octets that were sent and
received.
5.5. Call back, using a TGW
There are three classic forms of call-back:
1- ANI-based Callback
2- PPP Callback (Microsoft Callback is a variant of this)
3- Login-based callback
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The ANI based call-back can be implemented entirely in the Call Agent,
as indicated in the following diagram:
______________________________________________________________
| PC | CO | SS7/| TGW| CA | ACC|
| | | ISUP| | | |
|________|_____|______|_____|___________________________|_____|
| dials | IAM| -> | | | |
| | | IAM | - -| -> | |
| | | | | Notices that the | |
| | | | | called number corresponds| |
| | | | | to a call back service, | |
| | | | | and that the calling | |
| | | | | number has subscribed | |
| | | | | to that service. | |
| | | | | Terminates the | |
| | | | | incoming call. | |
| | | <- | - -| REL | |
| | <- | REL | | | |
| | RLC| -> | | | |
| hangup | | RLC | - -| -> | |
| | | | | Decides to place | |
| | | | | an outgoing call. | |
| | | | | Call start | -> |
| | | | <- | Create | |
| | | | | Connection (data) | |
| | | | Ack| -> | |
| | | <- | - -| ANM | |
| (rings)| <- | ANM | | | |
|________|_____|______|_____|___________________________|_____|
The PPP callback suppose that the modem first establishes an incoming
connection, and go through the authentication exchange. The following
diagram provides an example of these exchanges:
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____________________________________________________________________
| PC | CO | SS7/| TGW | CA | ACC| Radius|
| | | ISUP| | | | |
|_________|_____|______|____________|________________|_____|________|
| dials in| | | | | | |
| | IAM| -> | | | | |
| | | IAM | - - | -> | | |
| | | | | Checks called | | |
| | | | | number. | | |
| | | | | Notices | | |
| | | | | data call. | | |
| | | | | Call start | -> | |
| | | | <- | Create | | |
| | | | | Connection | | |
| | | | | (data) | | |
| | | | Ack | -> | | |
| | | | | Connection | | |
| | | | | completed. | | |
| | | | | Call | | |
| | | | | established | -> | |
| | | <- | - - | ANM | | |
| | <- | ANM | | | | |
| modem | - -| - - | -> | | | |
| <- | - -| - - | handshake | | | |
| PPP | - -| - - | -> | | | |
| | | | obtain | | | |
| | | | user-id, | | | |
| | | | password | | | |
| | | | Check | - - | - -| -> |
| | | | <- | - - | - -| Ack |
| | | | reports | | | |
| | | | call back | | | |
| | | | condition | | | |
| | | | NTFY | -> | | |
| | | | <- | ACK | | |
| | | | | Decides | | |
| | | | | to place an | | |
| | | | | outgoing call.| | |
| | | | <- | Delete | | |
| | | | | Connection | | |
| | | | Perf | | | |
| | | | data | -> | | |
| | | <- | - - | REL | | |
| | <- | REL | | | | |
| | REL| -> | | | | |
| hangup | | REL | - - | -> | | |
|_________|_____|______|____________|________________|_____|________|
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__________________________________________________________________________
| PC | CO | SS7/| TGW | CA | ACC| Radius|
| | | ISUP| | | | |
|___________|_____|______|__________________|_____________|_____|________|
| | | | | Call start | -> | |
| | | | <- | Create | | |
| | | | | Connection | | |
| | | | | (data) | | |
| | | | Ack | -> | | |
| | | <- | - - | IAM | | |
| (rings) | <- | IAM | | | | |
| | ACM| -> | | | | |
| | | ACM | - - | -> | | |
| (answer) | ANM| -> | | | | |
| | | ANM | - - | -> | | |
| | | | | Connection | | |
| | | | | complete. | | |
| | | | | Call | | |
| | | | | established| -> | |
| PPP | - -| - - | -> | | | |
| <- | - -| - - | Validates call, | | | |
|Connected | | | | | | |
| to the | | | | | | |
| Internet | | | | | | |
| | | | | | | |
| Closes | | | | | | |
|connection.| | | | | | |
| | REL| -> | | | | |
| | | REL | - - | -> | | |
| | | | <- | Delete | | |
| | | | | Connection | | |
| | | | Perf data | -> | | |
| | | <- | - - | RLC | | |
| | <- | RLC | | | | |
| | | | | Call end | -> | |
|___________|_____|______|__________________|_____________|_____|________|
This diagram shows the exchange of messages during a call from a modem
user to an Internet Service Provider, using a trunking gateway that dou-
bles as a Network Access Server. During these exchanges the SGCP is used
by the Call Agent to control the TGW.
Upon reception of the IAM message, the Call Agent notices that the
called number corresponds to a data service. Using configuration data-
bases, the Call Agent selects the type of modem parameters and authenti-
cation parameters that correspond to the called number and to the
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Internet draft Simple Gateway Control Protocol (SGCP) 30 July 1998
calling number. It uses this knowledge to send a connection command to
the TGW, programming the incoming trunk:
CRCX 1237 card23/21@trgw-7.whatever.net SGCP 1.1
C: A3C47F21456789F0
M: data
X: 0123456789B1
R: cl, cbk
v=0
m=nas/radius
c=radius.example.net
a=bearer:v.32
a=framing:ppp-asynch
a=dialed:18001234567
a=dialing:2345678901
The gateway immediately acknowledges the creation, sending back the
identification of the newly created connection. (There is no session
description in the case of a data call.)
200 1237 OK
I: FDE234C8
The Call Agent, knowing that this is a data call, can immediately ack-
nowledge the establishment of the connection, sending an ANM message
back to the calling switch.
The DSP associated to the incoming trunk has been loaded with the speci-
fied modem code. Once the call is established, the modem of the calling
PC will be synchronized with the modem associated to the trunk. The
caller will then proceed to a normal PPP synchronization, which probably
implies a PPP login. The login parameters, in our example, are checked
using Radius. The Radius server that will be used is typically chosen as
a function of the called number, which identifies the data service that
the calling modem requested. In fact, the number can also be used to
identify the specific form of authentication that is requested.
In the call back example, the Radius server will indicate that the call
cannot be completed as such, and that the user should be called back
(for example, using a "Callback Framed" service type in its access-
accept response.) The TGW will thus send a Notify message to the Call
Agent, indicating that a call-back is requested:
NTFY 2005 card23/21@trgw-7.whatever.net SGCP 1.1
X: 0123456789B1
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Internet draft Simple Gateway Control Protocol (SGCP) 30 July 1998
O: cbk(user-id)
After this notification, the Call Agent should send an acknowledgement:
200 2005 OK
The Call Agent will check that the call back request can be followed
though. In its databases, it will find the regular address associated to
the "user-id," and prepare to set up a call to that address. It will
first clear the incoming call, sending a DeleteConnection command to the
TGW:
In our example, the call is completed when the calling modem hangs up.
This triggers an ISUP release message, which is forwarded to the Call
Agent. The Call Agent will request the TGW to delete the connection, and
reset the list of observed events:
DLCX 1244 card23/21@trgw-7.whatever.net SGCP 1.1
C: A3C47F21456789F0
I: FDE234C8
X: 0123456789B2
R:
The gateways will respond with a message that should include a "call
parameters" header fields:
250 1244 OK
P: PS=2, OS=345, PR=1, OR=123
We should note that, because this is a data call, the call parameters
only include a count of the packets and octets that were sent and
received.
The Call Agent will then proceed to set up an outgoing data call. This
call may be routed through the same TGW that received the incoming call,
but can also be routed through an entirely different endpoint , for
example if the calling user has moved out of its normal region.
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5.6. Data call to a TGW, using L2TP
________________________________________________________________________
| PC | CO | SS7/| TGW | CA | ACC| LNS |
| | | ISUP| | | | |
|__________|_____|______|_______________|______________|_____|_________|
| dials in | | | | | | |
| | IAM| -> | | | | |
| | | IAM | - - | -> | | |
| | | | | Check called| | |
| | | | | number. | | |
| | | | | Notices | | |
| | | | | data call. | | |
| | | | | Call start | -> | |
| | | | <- | Create | | |
| | | | | Connection | | |
| | | | | (data) | | |
| | | | Ack | -> | | |
| | | | | Connection | | |
| | | | | complete. | | |
| | | | | Call | | |
| | | | | established | -> | |
| | | <- | - - | ANM | | |
| | <- | ANM | | | | |
| modem | - -| - - | -> | | | |
| <- | - -| - - | handshake | | | |
| PPP | - -| - - | -> | | | |
| | | | obtain | | | |
| | | | user-id, | | | |
| | | | password | | | |
| | | | Establish | | | |
| | | | Tunnel | | | |
| | | | SCC-REQ | - - | - -| -> |
| | | | <- | - - | - -| SCC-REP|
| | | | <- | - - | - -| SCC-CON|
| | | | IC-REQ | - - | - -| -> |
| | | | <- | - - | - -| IC-REP |
| | | | <- | - - | - -| IC-CON |
| | | | Spoof PPP/LCP| - - | - -| -> |
| <- | - -| - - | Relays PPP | - - | - -| -> |
|Connected | | | | | | |
| to the | | | | | | |
| Internet | | | | | | |
|__________|_____|______|_______________|______________|_____|_________|
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_______________________________________________________________
| PC | CO | SS7/| TGW | CA | ACC| LNS|
| | | ISUP| | | | |
|____________|_____|______|___________|____________|_____|_____|
| Closes | | | | | | |
| connection.| | | | | | |
| | REL| -> | | | | |
| | | REL | - - | -> | | |
| | | | <- | Delete | | |
| | | | | Connection| | |
| | | | Perf | | | |
| | | | data | -> | | |
| | | <- | - - | RLC | | |
| | <- | RLC | | | | |
| | | | CDN | - - | - -| -> |
| | | | Stop-CC-N| - - | - -| -> |
| | | | | Call end | -> | |
|____________|_____|______|___________|____________|_____|_____|
This diagram shows the exchange of messages during a call from a modem
user to an Internet Service Provider, using a trunking gateway that dou-
bles as a Network Access Server. During these exchanges the SGCP is used
by the Call Agent to control the TGW. The PPP information is relayed to
a network server (LNS) using L2TP.
Upon reception of the IAM message, the Call Agent notices that the
called number corresponds to a data service. Using configuration data-
bases, the Call Agent selects the type of modem parameters and authenti-
cation parameters that correspond to the called number and to the cal-
ling number. It uses this knowledge to send a connection command to the
TGW, programming the incoming trunk:
CRCX 1237 card23/21@trgw-7.whatever.net SGCP 1.1
C: A3C47F21456789F0
M: data
X: 0123456789B1
R: cl
v=0
c=IN IP4 access.example.net
m=nas/l2tp
k=clear:some-shared-secret
a=bearer:v.32
a=framing:ppp-asynch
a=dialed:18001234567
a=dialing:2345678901
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The gateway immediately acknowledges the creation, sending back the
identification of the newly created connection. (There is no need to
transmit a session description in the case of a data call.)
200 1237 OK
I: FDE234C8
The Call Agent, knowing that this is a data call, can immediately ack-
nowledge the establishment of the connection, sending an ANM message
back to the calling switch.
The DSP associated to the incoming trunk has been loaded with the speci-
fied modem code. Once the call is established, the modem of the calling
PC will be synchronized with the modem associated to the trunk. The
caller will then proceed to a normal PPP synchronization, which probably
implies a PPP login.
Once PPP has been properly synchronized, the TGW establishes a tunnel
towards the LNS. Because L2TP is a two-layer protocol, the TGW must
first establish an L2TP control connection between itself and the LNS.
This connection may or may not have been established prior to the call
set-up.
Tunnel establishment requires a shared secret between the LNS and the
TGW; in our example, that secret is passed by the Call Agent, along with
the name of the LNS. Once the supporting tunnel is installed, the TGW
has to establish an L2TP tunnel, to relay the "incoming call." Once the
call is established, the PPP packets received on the trunk will be
relayed over the L2TP tunnel, and vice-versa.
In our example, the call is completed when the calling modem hangs up.
This triggers an ISUP release message, which is forwarded to the Call
Agent. The Call Agent will request the TGW to delete the connection:
DLCX 1244 card23/21@trgw-7.whatever.net SGCP 1.1
C: A3C47F21456789F0
I: FDE234C8
The gateways will respond with a message that should include a "call
parameters" header fields:
250 1244 OK
P: PS=1245, OS=62345, PR=780, OR=45123
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Internet draft Simple Gateway Control Protocol (SGCP) 30 July 1998
We should note that, because this is a data call, the call parameters
only include a count of the packets and octets that were sent and
received.
6. Versions and compatibility
Version 1.1 introduces the following changes from version 1.0:
* Extension parameters (X-??:)
* Error Code 511 (Unrecognized extension).
* All event codes can be used in RequestEvent, SignalRequest and
ObservedEvent parameters.
* Error Code 512 (Not equipped to detect requested event).
* Error Code 513 (Not equipped to generate requested signal).
* Error Code 514 (Unrecognized announcement).
* Specific Endpoint-ID can be returned in creation commands.
* Changed the code for the ASDI display from "ad" to "asdi" to avoid
conflict with the digits A and D.
* Changed the code for the answer tone from "at" to "aw" to avoid
conflict with the digit A and the timer mark T
* Changed the code for the busy tone from "bt" to "bz" to avoid con-
flict with the digit B and the timer mark T
* Specified that the continuity tone value is "co" (CT was
incorrectly used in several instances; CT conflicts with .)
* Changed the code for the dial tone from "dt" to "dl" to avoid con-
flict with the digit D and the timer mark T
* Added a code point for announcement requests.
* Added a code point for the "wink" event.
* Set the "octet received" code in the "Connection Parameters" to
"OR" (was set to RO, but then "OR" was used throughout all exam-
ples.)
* Added a "data" mode.
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Internet draft Simple Gateway Control Protocol (SGCP) 30 July 1998
* Added a description of SDP parameters for the network access mode
(NAS).
* Added four flow diagrams for the network access mode.
* Incorporated numerous editing suggestions to make the description
easier to understand. In particular, cleared the confusion between
requests, queries, functions and commands.
* Defined the continuity test mode as specifying a dual-tone tran-
sponder, while the loopback mode can be used for a single tone
test.
* Added event code "OC", operation completed.
* Added the specification of the "quarantine list", which clarifies
the expected handling of events and notifications.
* Added the specification of a "wildcard delete" operation.
7. Acknowledgements
We want to thank here the many reviewers who provided us with advice on
the design of SGCP, notably Flemming Andreasen, David Auerbach, Bob
Biskner, Barry Hoffner, Dave Oran, John Pickens, Lou Rubin, Chip Sharp,
Kurt Steinbrenner and Joe Stone.
8. References
* Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: A
Transport Protocol for Real-Time Applications", RFC 1889, January
1996.
* Schulzrinne, H., "RTP Profile for Audio and Video Conferences with
Minimal Control", RFC 1890, January 1996
* Handley, M, Jacobson, V., "SDP: Session Description Protocol", RFC
2327, April 1998.
* Handley, M., "SAP - Session Announcement Protocol", Work in Pro-
gress.
* Handley, M., Schooler, E., and H. Schulzrinne, "Session Initiation
Protocol (SIP)", Work in Progress.
* Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming
Protocol (RTSP)", RFC 2326, April 1998.
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Internet draft Simple Gateway Control Protocol (SGCP) 30 July 1998
* ITU-T, Recommendation Q.761, "FUNCTIONAL DESCRIPTION OF THE ISDN
USER PART OF SIGNALLING SYSTEM No. 7", (Malaga-Torremolinos, 1984;
modified at Helsinki, 1993)
* ITU-T, Recommendation Q.762, "GENERAL FUNCTION OF MESSAGES AND SIG-
NALS OF THE ISDN USER PART OF SIGNALLING SYSTEM No. 7", (Malaga-
Torremolinos, 1984; modified at Helsinki, 1993)
* ITU-T, Recommendation H.323, "VISUAL TELEPHONE SYSTEMS AND EQUIP-
MENT FOR LOCAL AREA NETWORKS WHICH PROVIDE A NON-GUARANTEED QUALITY
OF SERVICE."
* ITU-T, Recommendation H.225, "Call Signaling Protocols and Media
Stream Packetization for Packet Based Multimedia Communications
Systems."
* ITU-T, Recommendation H.245, "LINE TRANSMISSION OF NON-TELEPHONE
SIGNALS."
* Atkinson, R., "Security Architecture for the Internet Protocol."
RFC 1825, August 1995.
* Atkinson, R., "IP Authentication Header." RFC 1826, August 1995.
* Atkinson, R., "IP Encapsulating Security Payload (ESP)." RFC 1827,
August 1995.
9. Authors' Addresses
Mauricio Arango
RSL COM Latin America
6300 N.W. 5th Way, Suite 100
Ft. Lauderdale, FL 33309
Phone: (954) 489-3971
Email: marango@rslcom.com
Christian Huitema
Bellcore
MCC 1J236B
445 South Street
Morristown, NJ 07960
U.S.A.
Phone: +1 973-829-4266
EMail: huitema@bellcore.com
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Further information is available on the SGCP web site:
http://sgcp.bellcore.com
Arango, Huitema [Page 82]