Paulo AguiarTel. (0xx21) 2598-3165

e-mail: aguiar@nce.ufrj.brNúcleo de Computação Eletrônica e

Departamento de Computação /IM da UFRJ

H.323

Clara Technical Meeting – Caracas 2005

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VoIP Call

Steps– User location

• Find IP destination address based on E,164 number or alias

– Negotiation of call parameters • Type of CODEC, etc

– Establishment of voice media channels• UDP ports for sending RTP/RTCP

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VoIP Signaling Protocols

ITU-T H.323– Standardized by the telecommunications

community • Focus on interoperability and control

IETF SIP (Session Initiation Protocol)– Standardized by the Internet community

– Focus on flexibility and easiness of Web integration

Others– Skype, MGCP/Megaco/H.248, SIGTRAN

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H.323 History

Beginning: ITU-T SG-16 (May 1995) H.323 v1, June 1996 H.323 v2, Feb 1998

– H.323: Packet-based multimedia communication systems

H.323 v3, Feb 2000 H.323 v4, Mar 2001 H.323 v5, Jul 2003

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H.323 Elements

PSTN Heritage – Telephone

numbering addressing

– ISDN signaling messages encapsulation

Standardized longer

Large number of products (especially among traditional telecommunications equipment manufacturers)

Terminal H.323Terminal H.323

Terminal H.323

MCU

Gatekeeper GatewayPBX/H.323

CentralTelefônica (PBX)

Rede Local

Rede de Telefonia

Convencional

Gatekeeper: manages user registration, status and admission (RAS)

Gatekeeper: manages user registration, status and admission (RAS)

H.323 Terminal: multimedia communication endpoint

H.323 Terminal: multimedia communication endpoint

Gateway H.323/PBX: translates transmission formats and communication procedures between H.323 and the telephone network

Gateway H.323/PBX: translates transmission formats and communication procedures between H.323 and the telephone network

Conference server: supports conferences involving three or more terminals

Conference server: supports conferences involving three or more terminals

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ITU-T H.323

H.225 RAS Call Establishment - Q.931/H.225 Media Control - H.245 Security - H.235

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H.225.0 - RAS

Requisitions for Registration/Admission/Status Contemplates

– GK discovery and registration– Destination location – Call admission and bandwidth changes – Status information for resources and calls

RAS functionality was introduced in H.323 sometime later and uses UDP to avoid TCP latency

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Q.931/H.225.0

Uses some ISDN signaling messages:– Setup– Call Proceeding (optional)– Alerting– Connect– Release complete– Status, Status Enquiry (optional) – Facility (optional)

Messages are sent over a TCP connection

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H.245

Media control– Media description and control – Terminal capabilities transfer and negotiation – Control of logical channels used for media

transportation

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H.323 Gatekeeper

Server used for terminal registration and capable of:– RAS messages processing– Optionally route H.225/H.245 signaling – Optionally route media flows

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RAS Messages

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Terminal Registration

Once GK is found (thru RAS or statically configured), terminal registers sending RRQ (Registration Request)

RRQ may contain a list of aliases or telephone number the terminal wants to be associated with, and current IP address

Registration can be soft state and also indicate call signaling transport address for H.225

RRQ (reg request)

UDP port 1719

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Terminal Registration in GK

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Terminal Registration

GK sends a unique identifier to the terminal in a RCF (Registration Confirm) msg

If field “CanMapAlias” is set, GK can alter associated aliases, returning changes in RCF

GK can also redirect terminal to other GKs using RRJ (Registration Reject) to reject registration

RRJ

RCF (reg confirm)

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Authorization to Start or Accept a Call

ARQ contains– Bidirectional bandwidth estimate, including

audio and video if necessary– Call signaling model (if direct or routed by

GK)– Destination information

• E.164 or alias

– Call ID (global and unique)– Other information

ACF returns bandwidth to be used , IP transport address and port for Q.931/H.225 signaling Q.931

ARQ (admission req)

UDP port 1719

ACF (admission conf)

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Authorization

Terminal always asks for GK authorization when starting or receiving a call

GK performs basic admission control – Limit number of calls – Limit total bandwidth

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GK Signaling Models

Direct Signaling – GK acts redirecting communication to allow terminals to

establish call directly (point-to-point)

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Direct Signaling Mode

Terminal H.323Gatekeeper

10.1.1.2Gateway

IP 10.1.2.3Gatekeeper

10.1.2.2

ZONA ADMINISTRATIVA H.323 CESAR ZONA ADMINISTRATIVA H.323 PROVIDER

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Token

Optional parameter, transparently transferred if H.323 entity unaware of it

Token can contain a digital signature with call information (destination, gw, etc)

Token is copied from LCF and sent in the SETUP msg, allowing gw or GK at the destination to verify its authenticity and authorize operation

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GK Signaling Models

GK Routed– H.225 and H.245 signaling go thru GK, which acts as a proxy– GK sees and participates in all signaling, being able to change it, without

accessing media flows

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GK Routed Model

Terminal H.323Gatekeeper

10.1.1.2Gateway

IP 10.1.2.3Gatekeeper

10.1.2.2

ZONA ADMINISTRATIVA H.323 CESAR ZONA ADMINISTRATIVA H.323 PROVIDER

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TCP Connections

Q.931/H.225 connection or call signaling channel – Default port (1720)

H.245 connection for control and media negotiation– After has been established, the H.225 connection

can be released – This connection has to be maintained till the end of

call

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Call Basic Flow

Terminal B : Bill

Canal de Sinalização de Chamada TCP 1720

Canal de Controle H.245

Canal(is) de Áudio

Canal(is) de Vídeo

Canal(is) de Dados

Canal RAS

Alias : Fone, Email ...H.225 : SETUPCall reference : 10Call Identifier : 45442345H.323 ID of A : cesar_labvoipSource Type : PCCallType : Point To PointDestinationAddress : bill_microsoft

H.225 : CONNECTCall reference : 10Call Identifier : 45442345EndPointType : PCH.245 Address (Ex : 10.2.3.4:8741)

Terminal A : Cesar

Canal de Sinalizaçãode Chamada TCP 1720

Canal de Controle H.245

Canal(is) de Áudio

Canal(is) de Vídeo

Canal(is) de Dados

Canal RAS

Alias : Fone, Email ...

SETUP

CONNECT

ALERTING

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SETUP Msg

Initial message for call establishment, sent in H.225 signaling TCP (default port 1720)

Examples of header fields follows

H.225 : SETUPCall reference : 10Call Identifier : 45442345H.323 ID of A : cesar_labvoipSource Type : PCCallType : Point To PointDestinationAddress : Bill@dominio2.com.br

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SETUP Header (I)

Type of H.225 msg = (0x05) indicates SETUP Call Reference Value (CRV) (2 bytes)

– Local unique call reference value, copied in all subsequent messages related to this call

Call Identifier – 128 bits, globally unique, allowing TCP connection sharing among many

calls– Sent in UUI field (User-to-user IE) of all H.225.0-Q.931 msg

CID – Conference ID– Unique for all elements of a conference (used by MCU)

Source Type– Indicates nature of calling terminal– Ex.:PC, Gateway, MCU

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SETUP Header (II)

Source Address– List of aliases of caller – If caller has only an E.164 number, this number will also appear in the

Calling Party field

– Types of aliases defined in H.323• E.164 , a regular phone number using only characters from the set

<<0123456789#*,”

• H.323-ID, unicode string

• url-ID

• transport-ID (ex. 10.2.3.4:1720)

• Email-ID (ex. Bill@dominio2.com.br)

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SETUP Header (III)

Destination Address– E.164 address of destination– Also appears in the Called Party Number Information Element

DestExtraCallInfo– Additional list of E.164 addresses – For a H.320 128 kbps connection, the first E.164 address is given in the

Destination Address and the second E.164 goes here

RemoteExtesionAddress– alias(es) of called party

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SETUP Header (IV)

SourceCallSignalAddress– H.225 signaling transport address to be used by destination – Should not be used behind firewalls/NAT which change TCP

ports and IP addresses in packet headers• If this field is absent, destination extracts the transport address for

returning Q.932 responses from the headers of received packets, (point to NAT)

• When NAT gets responses to its own address, it replaces headers and forwards them to the source of SETUP, as it should be

EndpointIndentifier– Used when signaling is GK routed

CriptoTokens– Used for authentication and message integrity

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CONNECT Msg

Most important parameter: transport address for H.245 TCP connection – Can be sent earlier (Call Proceeding msg) or even in the SETUP

Call Reference field (from SETUP) is copied to CONNECT

H.225 : SETUPCall reference : 10Call Identifier : 45442345H.323 ID of A : cesar_labvoipSource Type : PCCallType : Point To PointDestinationAddress : Bill@dominio2.com.br

H.225 : CONNECTCall reference : 10Call Identifier : 45442345EndPointType : PCH.245 Address (Ex : 10.2.3.4:8741)

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H.225 Message Sequence

Call Proceeding, Alerting, Connect, or ReleaseComplete can be sent right immediately after receiving SETUP

One of them has to be received by caller before SETUP timer expires (4s)

After sending Alerting, user has 3 min to accept or reject call

Setup

Call Proceeding

Alerting

ReleaseComplete

Connect

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H.245 Control Channel

H.245 TCP connection used for – Master/slave determination– Capabilities determination – Opening of unidirectional logical channels – Closing of logical channels

H.225 TCP can be closed after CONNECT, but H.245 channel must stay open along the whole call

H.245 channel is unique for a call between two terminals: known as logical channel 0

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H.245 Messages

Request Response Command Indication

Capability MasterSlaveDetermination

MasterSlaveDeterminationAck

MasterSlaveDetermination Release

TerminalCapabilitySet TerminalCapabilitySetAck

TerminalCapabilitySetReject

TerminalCapabilitySetRelease

Channel Management

OpenLogicalChannel OpenLogicalChannelAck

OpenLogicalChannelReject

OpenLogicalChannelConfirm

CloseLogicalChannel CloseLogicalChannelAck

CloseLogicalChannelReject EndSession

Channel Operations

FlowControlCommand

Video commands

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Opening Logical Channels

Logical channels are unidirectional: from source to receiver– Necessary 2 channels for a normal audio conversation and

4 channels for videoconferencing Source sends OpenLogicalChannel with RTCP port

for receiving reports ACK from receiver specify UDP port for media and

RTCP/UDP port (default next odd port) Receiver Reports (RTCP RR) have to be sent back to

source in advertised RTCP port from source

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Opening Logical Channel from A to B

Terminal A : Cesar

Canal de Sinalizaçãode Chamada TCP 1720

Canal de Controle H.245 TCP

Canal(is) de Áudio

Canal(is) de Vídeo

Canal(is) de Dados

Canal RAS

Alias : Fone, Email ...

Terminal A : Bill

Canal de Sinalizaçãode Chamada TCP 1720

Canal de Controle H.245TCP 8741

Canal(is) de Áudio

Canal(is) de Vídeo

Canal(is) de Dados

Canal RAS

Alias : Fone, Email ...

OpenLogicalChannel

OpenLogicalChannel

OpenLogicalChannelAck

OpenLogicalChannelAck

H.245 : OpenLogicalChannelLogical Channel 1, RR RTCP 7771g711Alaw 64kSession number, payload type RTPSupressão de silêncio

H.245 : OpenLogicalChannelAckLogical Channel 1SR RTCP 9345 / RTP 9344

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Conversação Ativa H.323

Terminal A : Cesar

Canal de Sinalizaçãode Chamada TCP 1720

Canal de Controle H.245 TCP

Canal(is) de Áudio RTP : UDP RTCP : UDP 7771 RTCP : UDP

Canal(is) de Vídeo

Canal(is) de Dados

Canal RAS

Alias : Fone, Email ...

Fluxo RTP vindo de A para B

Mensagens de Controle : LogicalChannel1

RTCP RR

Terminal B : Bill

Canal de Sinalizaçãode Chamada TCP 1720

Canal de Controle H.245TCP

Canal(is) de Áudio RTP : UDP 9344 RTCP : UDP RTCP : UDP 9345

Canal(is) de Vídeo

Canal(is) de Dados

Canal RAS

Alias : Fone, Email ...

RTCP SR

(RequestMode, Call Transfer, ... )

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Call Ending

Close all logical channels with H.245 CloseLogicalChannel msg

Close H.245 channel with endSessionCommand Close other TCP connections

– Send H.225 ReleaseComplete In practice, terminals close only H.245 TCP

channel

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Question: Excessive Latency

Round trip time for each of the following interactions – ARQ/ACF– SETUP/CONNECT– H.245 Capability exchange – Master/Slave determination– Logical channel establishment

Additional latency to open H.225 and H.245 TCP connections

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FastStart Procedure

Send information for opening logical channels in Q.931 messages (Setup, Proceeding or Connect)

Allows: – Receiving audio before CONNECT– Reducing delays related to RTT

For calls interacting with PBX is necessary to keep H.245 channel open to get DTMF tones

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G atewayPSTN

FastStart

SETUP (number=+33 12345678, fastStart : OpenLogicalChannel 1 -> receive G.711 RTP port 4432, RTCP port 4433,OpenLogicalChannel 2 -> send G.711, RTCP port 3454 )

CONNECT

SETUP

PROCEEDING

CONNECT

PROCEEDING ( fastStart : OpenLogicalChannel 1 -> send, RTCP port 6554OpenLogicalChannel 2 -> receive, RTP port 5634 RTCP port 5635)

Can answer directly thru logical channel 2

IN

Call was tranferred and answered, so charging can be started

Terminal H .323

Telefone

RedePSTN

Network messages (like “dial your code and destination number” can

be sent thru logical channel 1

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H.245 tunneling

It is the encapsulation of one or more H.245 msgs in h245Control fields of any Q.931 message

If called terminal has the capability of handling H.245 encapsulation, all H.245 msgs can be exchanged in this way without the need to open an H.245 separated channel

On the contrary, the H.245 channel is open on the normal way

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H.245 Tunneling Call Flow

SETUP (number=+33 12345678,John’s terminal H245 capabilities)

ALERTING

CONNECT

SETUP (number=+33 12345678,John’s H245 terminal capabilities)

CALL PROCEEDING

CALL PROC. (GW H.245 capabilities, OpenLogicalChannel GW to Terminal)FACILITY (GW H.245 capabilities,

OpenLogicalChannel GW to Terminal)

FACILITY (OpenLogicalChannelACK, OpenLogicalChannel Terminal to GW) FACILITY (OpenLogicalChannelACK,

OpenLogicalChannel Terminal to GW)

FACILITY (OpenLogicalChannel ACK)FACILITY (OpenLogicalChannel ACK)

ALERTING

CONNECT

setup

Terminal H .323

G atekeeper10.1.1.2

G atewayPSTN

10.1.2.3

H.225 Facility can be used for sending encapsulation if no other

H.225 msgs is ready to be sent (very useful when GK is

operating as a signaling proxy)

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Directory Gatekeeper

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GnuGK Gatekeeper

Can operate in GK-Routed mode and as a media proxy

Can operate as a directory gatekeeper

Manipulates E.164 addressing (rewriting rules)

Supports users behind NAT Registration can be based

on H.235, LDAP or IP address

Ease block of calls to destinations starting with some forbidden digit (like mobile services)

Simple call detail record (CDR) generated for each call

Has a simple and efficient control mechanism acting on port 7000 that can be used for managing users and configurations

http://www.gnugk.org