1. Programa IS Expert - NCE / UFRJ Clara Technical Meeting – Caracas 2005
Paulo Aguiar
Tel. (0xx21)
Núcleo de Computação Eletrônica e Departamento de Computação /IM da UFRJ
Redes de Computadores - Prof. Moacyr Azevedo

2. VoIP Call Steps User location Negotiation of call parameters
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

3. 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

4. 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

5. H.323 Elements PSTN Heritage Standardized longer
Telephone numbering addressing
ISDN signaling messages encapsulation
Standardized longer
Large number of products (especially among traditional telecommunications equipment manufacturers)
Gatekeeper: manages user registration, status and admission (RAS)
Gateway H.323/PBX: translates transmission formats and communication procedures between H.323 and the telephone network
H.323 Terminal: multimedia communication endpoint
Conference server: supports conferences involving three or more terminals

6. Programa IS Expert - NCE / UFRJ
ITU-T H.323
H.225 RAS
Call Establishment - Q.931/H.225
Media Control - H.245
Security - H.235
Redes de Computadores - Prof. Moacyr Azevedo

7. Programa IS Expert - NCE / UFRJ
H 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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8. Programa IS Expert - NCE / UFRJ
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
Redes de Computadores - Prof. Moacyr Azevedo

9. Programa IS Expert - NCE / UFRJ
H.245
Media control
Media description and control
Terminal capabilities transfer and negotiation
Control of logical channels used for media transportation
Redes de Computadores - Prof. Moacyr Azevedo

10. Programa IS Expert - NCE / UFRJ
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
Redes de Computadores - Prof. Moacyr Azevedo

11. Programa IS Expert - NCE / UFRJ
RAS Messages
Redes de Computadores - Prof. Moacyr Azevedo

12. Terminal Registration
Programa IS Expert - NCE / UFRJ
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
Redes de Computadores - Prof. Moacyr Azevedo

13. Terminal Registration in GK

14. Terminal Registration
Programa IS Expert - NCE / UFRJ
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
RCF (reg confirm)
RRJ
Redes de Computadores - Prof. Moacyr Azevedo

15. Authorization to Start or Accept a Call
Programa IS Expert - NCE / UFRJ
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)
Redes de Computadores - Prof. Moacyr Azevedo

16. Programa IS Expert - NCE / UFRJ
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
Redes de Computadores - Prof. Moacyr Azevedo

17. GK Signaling Models Direct Signaling
GK acts redirecting communication to allow terminals to establish call directly (point-to-point)

18. Direct Signaling Mode

19. 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

20. Direct Signaling Mode

21. 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

22. GK Routed Model

23. Programa IS Expert - NCE / UFRJ
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
Redes de Computadores - Prof. Moacyr Azevedo

24. Programa IS Expert - NCE / UFRJ
Call Basic Flow
Terminal A : Cesar
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, ...
H.225 : SETUP Call reference : 10 Call Identifier : H.323 ID of A : cesar_labvoip Source Type : PC CallType : Point To Point DestinationAddress : bill_microsoft
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, ...
SETUP
ALERTING
CONNECT
H.225 : CONNECT Call reference : 10 Call Identifier : EndPointType : PC H.245 Address (Ex : :8741)
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25. Programa IS Expert - NCE / UFRJ
SETUP Msg
Initial message for call establishment, sent in H.225 signaling TCP (default port 1720)
Examples of header fields follows
H.225 : SETUP Call reference : 10 Call Identifier : H.323 ID of A : cesar_labvoip Source Type : PC CallType : Point To Point DestinationAddress :
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26. Programa IS Expert - NCE / UFRJ
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 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
Redes de Computadores - Prof. Moacyr Azevedo

27. Programa IS Expert - NCE / UFRJ
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 << #*,”
H.323-ID, unicode string
url-ID
transport-ID (ex :1720)
-ID (ex.
Redes de Computadores - Prof. Moacyr Azevedo

28. Programa IS Expert - NCE / UFRJ
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 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
Redes de Computadores - Prof. Moacyr Azevedo

29. SETUP Header (IV) SourceCallSignalAddress EndpointIndentifier
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

30. Programa IS Expert - NCE / UFRJ
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 : SETUP Call reference : 10 Call Identifier : H.323 ID of A : cesar_labvoip Source Type : PC CallType : Point To Point DestinationAddress :
H.225 : CONNECT Call reference : 10 Call Identifier : EndPointType : PC H.245 Address (Ex : :8741)
Redes de Computadores - Prof. Moacyr Azevedo

31. Protocol Identification
Programa IS Expert - NCE / UFRJ
Protocol Identification
Campo de identificação do protocolo
Protocolos H.323 são objetos na árvore do ITU-T e os identificadores são decimais separados por pontos mostrando o caminho desde a raiz
ITU-T (0), ISO(1)
| Recomendação (0)
| H (8)
| 2250,245, etc
| versão (0)
| 1,2,3,4, etc
| msg
Exemplos
H versão 3:
H.245 versão 6:
Redes de Computadores - Prof. Moacyr Azevedo

32. Programa IS Expert - NCE / UFRJ
Como o TCP é orientado a fluxo (diferente do UDP), existe a necessidade de delimitar as PDUs
Isto é feito pelos cabeçalhos TPKT (RFC 1006)
ISO transport services on top of the TCP (TPKT)
the TPKT encapsulation is necessary in order to define message >boundaries on top of TCP which natively presents a data stream >abstraction to its higher layer. There was recently a discussion on the >h.323implementors forum mailing list around the fact that the reference >to TPKT is not well stated within H Not all of RFC1006 is adopted >by H.225.0, basically only the encapsulation with some fairly fixed >header values as I recall. I'm also pretty sure that H.245 also needs >the TPKT encapsulation. Since RAS is UDP based, I think it does not use >TPKT. The media of course is encapsulated in RTP on top of UDP. > >Hal Purdy >AT&T Laboratories >180 Park Avenue >Room E263, Bldg. 103 >Florham Park, NJ >(973) (w) >(973) (fax) >
You can filter TPKT protocols while capturing, as it's always using TCP port 102.
Redes de Computadores - Prof. Moacyr Azevedo

33. Programa IS Expert - NCE / UFRJ
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
Redes de Computadores - Prof. Moacyr Azevedo

34. Programa IS Expert - NCE / UFRJ
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
Redes de Computadores - Prof. Moacyr Azevedo

35. 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

36. 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
VideoFastUpdatePicture
VideoFastUpdateGOB
VideoTemporalSpatialTradeoff
VideoSendSyncEveryGOB
VideoSendSyncEveryGOB Cancel
VideoFastUpdateMB

37. Master/Slave Determination
Programa IS Expert - NCE / UFRJ
Master/Slave Determination
Useful when terminals can execute same action or function and a conflict may occur (ex. opening logical channels)
Master is always responsible
With H.235, Master is in charge of media channel cryptographic key distribution to the other terminals
masterSlaveDetermination messages contain a terminalType value reflecting its capabilities and a random number, to priorize MCU handling in relation to Gks, for example
Redes de Computadores - Prof. Moacyr Azevedo

38. Capability Negotiation
Programa IS Expert - NCE / UFRJ
Capability Negotiation
H.245 : TerminalCapabilitySet MultiplexCapability capabilityTable : H.261VideoCapability g711Alaw64k,g729 t120
Terminal A : Cesar
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, ...
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, ...
TerminalCapabilitySet
TerminalCapabilitySetAck
TerminalCapabilitySet
TerminalCapabilitySetAck
H.245 : TerminalCapabilitySet MultiplexCapability capabilityTable : H.261VideoCapability g711Alaw64k t120
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39. Opening Logical Channels
Programa IS Expert - NCE / UFRJ
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
Redes de Computadores - Prof. Moacyr Azevedo

40. Opening Logical Channel from A to B
Programa IS Expert - NCE / UFRJ
Opening Logical Channel from A to B
Terminal A : Cesar
Canal de Sinalização
de 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, ...
Terminal A : Bill
Canal de Sinalização
de Chamada TCP 1720
Canal de Controle H.245
TCP 8741
Canal(is) de Áudio
Canal(is) de Vídeo
Canal(is) de Dados
Canal RAS
Alias : Fone, ...
H.245 : OpenLogicalChannel
Logical Channel 1, RR RTCP 7771
g711Alaw 64k
Session number, payload type RTP
Supressão de silêncio
OpenLogicalChannel
OpenLogicalChannel
OpenLogicalChannelAck
OpenLogicalChannelAck
H.245 : OpenLogicalChannelAck
Logical Channel 1
SR RTCP 9345 / RTP 9344
Redes de Computadores - Prof. Moacyr Azevedo

41. Programa IS Expert - NCE / UFRJ
Conversação Ativa H.323
Terminal A : Cesar
Canal de Sinalização
de 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, ...
Terminal B : Bill
Canal de Sinalização
de Chamada TCP 1720
Canal de Controle H.245
TCP
Canal(is) de Áudio
RTP : UDP 9344
RTCP : UDP
RTCP : UDP 9345
Canal(is) de Vídeo
Canal(is) de Dados
Canal RAS
Alias : Fone, ...
Fluxo RTP vindo de A para B
RTCP RR
RTCP SR
Mensagens de Controle : LogicalChannel1
(RequestMode, Call Transfer, ... )
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42. Programa IS Expert - NCE / UFRJ
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
Redes de Computadores - Prof. Moacyr Azevedo

43. 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

44. 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

45. FastStart IN
SETUP (number= , fastStart : OpenLogicalChannel 1 -> receive G.711 RTP port 4432, RTCP port 4433,
OpenLogicalChannel 2 -> send G.711, RTCP port 3454 )
SETUP
PROCEEDING
PROCEEDING ( fastStart : OpenLogicalChannel 1 -> send, RTCP port 6554
OpenLogicalChannel 2 -> receive, RTP port 5634 RTCP port 5635)
Network messages (like “dial your code and destination number” can be sent thru logical channel 1
Can answer directly thru logical channel 2
CONNECT
CONNECT
Call was tranferred and answered, so charging can be started

46. 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

47. H.245 Tunneling Call Flow
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)
SETUP (number= ,
John’s terminal H245 capabilities)
SETUP (number= ,
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)
setup
FACILITY (OpenLogicalChannelACK, OpenLogicalChannel Terminal to GW)
FACILITY (OpenLogicalChannel ACK)
FACILITY (OpenLogicalChannel ACK)
ALERTING
ALERTING
CONNECT
CONNECT

48. Directory Gatekeeper

49. 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