Transcript IP Telephony

Microsoft ISA Server H.323
Gateway and Gatekeeper
Overview of IP Telephony, H.323, and ISA Server H.323
Support
Presentation Agenda
•
•
•
•
•
IP Telephony Overview
The ITU H.323 Specification
Microsoft ISA Server H.323 Gateway
Microsoft ISA Server H.323 Gatekeeper
Microsoft ISA Server Scenarios
IP Telephony Overview
Definition
• IP Telephony refers to the hardware and
software technologies that provide the
ability to place telephone calls over IP
based networks.
IP Telephony Overview
Traditional Voice Networks – PSTN
• The Public Switched Telephone Network
– The collection of networking equipment that belongs to
the carriers involved in providing telephone service.
• The PSTN is a Circuit Switched Network
– A virtual circuit is created in the PSTN “Cloud” for
each telephone call. The circuit is allocated (64k bps)
and maintained for the duration of the call, regardless
of the amount of traffic flowing over the circuit.
IP Telephony Overview
PSTN - Basic Network Topology
IP Telephony Overview
Traditional IP Networks – The Internet
• Packet Switched Networks
– Separate packets from the same communication
may take different paths through the cloud.
– More efficient use of network resources
– No inherent QoS or Security, without special a
special implementation to address these issues.
• Signaling and Media use the same network
IP Telephony Overview
Standards Bodies
• International Telecommunications Union (ITU)
– ITU-T division’s H SERIES specs define the
Transmission of non-telephone signals.
– Specifications must be licensed from the ITU
• Internet Engineering Task Force (IETF)
– RFC and Internet-Draft specifications are well-known
to most IT professionals
– Available in the public domain : http://www.ietf.org
IP Telephony Overview
The 3 competing signaling protocols
• H.323 (ITU)
– Umbrella specification defining the protocols and
codecs to be used by H.323 compliant devices.
• SIP (IETF)
– Session Initiation Protocol. New, up and coming
standard. Similar to H323 mechanically, but text-based
and simpler. More closely related to HTTP “on the
wire.”
• S/MGCP (IETF)
– Signaling Gateway Control Protocol / Media Gateway
Control Protocol.
IP Telephony Overview
Media Protocols
• RTP/RTCP (IETF)
– Real-Time Protocol/ Real-Time Control
Protocol.
– This is used almost universally for media
transport. Both H.323 and SIP specify RTP as
the media transport protocol of choice
IP Telephony Overview
Basic VoIP network diagram
The ITU H.323 Specification
• H.323 Specification
Title: Visual telephone systems
and equipment for local area
networks which provide a
non-guaranteed quality of service
The ITU H.323 Specification
Important Terms
• H.323 Entity: Any H.323 component, including
– Terminals
– Gateways
– Gatekeepers
– MCs, MPs, and MCUs.
• Endpoint: A Terminal, Gateway, or MCU.
• Call: Point-to-point multimedia communication between two
H.323 endpoints
• Multipoint Conference: A conference between three or more
terminals
The ITU H.323 Specification
H.323 Protocol Stack
The ITU H.323 Specification
Basic Call Model
A typical H.323 Call consists of 5 phases:
1. Call Setup (Phase A)
2. Initial communication between endpoints and
terminal capability exchange (Phase B)
3. Establishment of of Audio / Visual communication
between endpoints (Phase C)
4. Request and negotiation of Call Services (Phase D)
5. Call Termination (Phase E)
The ITU H.323 Specification
Basic Call in Action
ISA Server H.323 Gateway
Introduction
• The ISA Server H.323 Gateway is an
application layer H.323 Proxy.
• Traditional circuit-layer proxies (Winsock
Proxy, ISA Firewall Service) and
transparent proxies (NAT, SecureNAT) do
not properly handle H.323 traffic because of
the Protocol’s complexity.
ISA Server H.323 Gateway
Proxy History
• Proxy Server 2.0
– Winsock Proxy could handle only one outbound H.323
call at a time.
– No inbound H.323 calls were possible (No Server Proxy)
• Windows 2000 NAT
– H.323 / LDAP Protocol Editor allows outbound H.323
Calls (LDAP is needed for ILS lookup)
• ISA H.323 Gateway
– supports outbound H.323 calls and inbound calls with
Gatekeeper assistance
ISA Server H.323 Gateway
H.323 Gateway Implementation
• The ISA H.323 Proxy is implemented as an ISA
Application Filter.
– Application Filters can be externally developed using the
ISA SDK.
– Application filters plug-in to the ISA Firewall Service
– Application filters can perform
•
•
•
•
protocol editing, e.g., H.323 filter
Content inspection, e.g., SMTP filter
Virus scanning, e.g., 3rd Party filter
Other activities enabled by access to the application data stream
• Both SecureNAT Clients and Firewall (WSP) Clients
can use the H.323 Gateway
ISA Server H.323 Gateway
H.323 Gateway Implementation (cont.)
ISA Server H.323 Gatekeeper
Introduction
• ISA Gatekeeper Functionality
– Register Users (directory)
• The GK defines an H.323 zone and is referenced
when attempting to locate a user or terminal. The
GK provides alias to IP address resolution.
– Route Calls
• Terminals specify a GK if one exists for their zone.
The GK will route calls to the appropriate
destinations based on routing rules created by an
administrator.
ISA Server H.323 Gatekeeper
Scenario Example
ISA Server H.323 Gatekeeper
Server Properties
ISA Server H.323 Gatekeeper
Server Properties (cont.)
ISA Server H.323 Gatekeeper
Registering Users
ISA Server H.323 Gatekeeper
Call Routing - Destinations
ISA Server H.323 Gatekeeper
Call Routing Rules
• Rules are used to determine how a GK should help
the caller route the call.
• 3 Types of Call Routing Rules
– Phone Number Rules
– Email Address Rules
– IP Address Rules
• By matching the ID type to a destination,
– Phone# calls can be routed to a PSTN Gateway
– External IP Addresses, Email addresses, or Names can
be routed to external endpoints or GK’s.
ISA Server H.323 Gatekeeper
Routing Rule Precedence
• GK finds matching rules for each
destination type.
• Matching rules are then sorted by
– Quality of match (more matching elements)
– If Quality of match is equal, “exact” rule types
have precedence over “prefix” (ph#) or “suffix”
(domain/IP) rule types.
– If Quality and Type match, rule precedence
number is used.
ISA Server H.323 Gatekeeper
Routing Rule Precedence (cont)
• Now that rules have been sorted based on
matching, there may be equal rules with different
destinations. Each destination should be tried in
the case that a previous response is negative.
e.g., If ILS lookup fails, we should try Active
Directory for a match as well (assuming there are
rules for each of these destinations)
ISA Server H.323 Gatekeeper
Routing Rule Precedence (cont)
• Destinations are contacted in the following order:
– None. This is a “deny rule” and causes processing to
cease.
– Local Registration Database
– Gateway/Proxy
– Internet Locator Service (ILS)
– Gatekeeper
– Multicast Gatekeeper
– DNS
– Active Directory
– Local Network
ISA Server H.323 Gatekeeper
Routing Rule Precedence (cont)
Which Rules get applied?
What order are the applied rules processed?
Resources and References
• Books
– IP Telephony. (Bill Douskalis)
• Much of the VoIP and H.323 information in this presentation
came from this book
– IP Telephony: Packet-Based Multimedia
Communications Systems (Hersent, Gurle, Petit)
• Web Sites
– Databeam. This site has a good primer on H.323 and
T.120
• http://www.databeam.com/standards/index.html
– Intel. This page describes the problems and pitfalls of
getting H.323 through Firewalls
• http://support.intel.com/support/videophone/trial21/h323_wpr.
htm
Resources and References
(cont.)
• Specs
– ITU-T:
• H.323
• T.120
– IETF:
• RTP (RFC 1889) ftp://ftp.isi.edu/in-notes/rfc1889.txt