Voice Routing

1
Voice Routing

• Prof. Anirudha Sahoo
• KReSIT
• IIT Bombay

2
Outline

• Primer on Voice over IP (VOIP)
• Introduction to and current state of voice
  routing
• Telephony Routing over IP (TRIP)
• Telephony Gateway REgistration Protocol (TGREP)
• E.164 Number (ENUM)
• VOIP issues in NAT/Firewall

3
Voice Over IP (VOIP)

• Transmission of digitized voice in packet network
  (e.g. IP, ATM, Frame Relay)
• Enables telephone conversation to be carried over
  IP network (in part or end-to-end)
• Provides a toll bypass path for telephone calls
• Enables Telephony providers to provide cheaper
  service

4
VOIP System
PBX
PBX
(A typical PSTN system)
(A typical VOIP system)
5
VOIP System (cont.)
IP Network
CPE router
CPE router
LAN
LAN
SIP proxy
Soft phone
IP phone
IP phone
(Another VOIP system)
6
Outline

• Primer on Voice over IP (VOIP)
• Introduction to and current state of voice
  routing
• Telephony Routing over IP (TRIP)
• Telephony Gateway REgistration Protocol (TGREP)
• E.164 Number (ENUM)
• VOIP issues in NAT/Firewall

7
What is voice routing?

• The method of exchanging telephony route
  information between voice routing elements so
  that telephone calls can be made over IP network
• Basically it enables local voice gateways to
  obtain the IP address of the remote voice
  gateway(s) which can complete a given phone call

8
Static voice routing

• The routing information is obtained out of band
  and configured statically
• An example of static routing on Cisco voice
  gateway
• dial-peer voice 108 voip
• destination-pattern 1408T
• session target ipv4172.292.1.1

9
Limitations of Static routing

• Cannot react to the changes in routes quickly
• No dynamic information about resources at the
  remote gateway
• In the case where multiple routes exist choosing
  successful route at a particular time is not
  possible

10
Current State of voice routing

• Lots of configuration is static routing because
  of its simplicity.
• Limited dynamism within a domain (zone)
• Use Gatekeepers in a H323 network
• Gatekeepers can provide some dynamic resource
  based call routing
• Between two zones (or admin domains) the routes
  are exchanged statically

11
Routing in H323 network
GK
IP
Gateway
Gateway
(Single Zone)
12
Routing in H323 network
Single Zone
13
Routing in H323 network (single Zone)

• Gateways register H323 ID or phone numbers with
  the gatekeeper.
• Thus the GKs have the knowledge of which phone
  number can be terminated on which gateway.
• Gateways do not need to know where to send a
  call just need to get that info from GK

14
Routing in H323 network (multiple zones)
15
Routing in H323 network (multiple zones)

• Config on the GW_1 - ECV-2600-17
• h323-gateway voip id gk-zone1.test.com ipaddr
  10.52.218.47 1718
• Config on the GW_2 - ECV-2600-16
• h323-gateway voip id gk-zone2.test.com ipaddr
  10.52.218.46 1718

16
Routing in H323 network (multiple zones)

• Config on the GK_1 ECV-2600-15 (zone 1)
• gatekeeper zone local gk-zone1.test.com
  test.com 10.52.218.47
• zone remote gk-zone2.test.com test.com
  10.52.218.46 1719
• zone prefix gk-zone2.test.com 16..
• zone prefix gk-zone1.test.com 17..
  gw-priority 10 gw_1

17
Routing in H323 network (multiple zones)

• Config on the GK_2 ECV-2600-14 (zone 2)
• gatekeeper zone local gk-zone2.test.com
  test.com 10.52.218.46
• zone remote gk-zone1.test.com test.com
  10.52.218.47 1719
• zone prefix gk-zone2.test.com 16.. gw-priority
  10 gw_2
• zone prefix gk-zone1.test.com 17..

18
Routing in H323 network (multiple zones)

• So the routes between two zones are configured
  statically through configuration
• No way of knowing whether the endpoint
  (resources) is available

19
Routing in SIP network

• Within the same domain SIP gets the route from
  the location service
• Currently SIP routes are installed statically
  through DNS setup (between domains)
• sipuser_at_example.com is resolved to
• order pref flags service regexp
  replacement
• IN NAPTR 50 50 "s" "SIPSD2T" ""
  _sips._tcp.example.com.
• IN NAPTR 90 50 "s" "SIPD2T" ""
  _sip._tcp.example.com
• IN NAPTR 100 50 "s" "SIPD2U" ""
  _sip._udp.example.com.

20
Resource reporting in VOIP network

• Available resources is another aspect of voice
  routing, because the signaling entity (e.g. GK)
  can make better decisions based on it
• Given two routes to the same destination, the one
  with more available resources would have more
  chance of completing a call
• H323 provides limited (dynamic) resource
  reporting via RAI messages
• V4 has more flexible mechanism
• SIP does not have any mechanism to report
  resources dynamically

21
Resource reporting in H323

• Gateways use RAI message to report resources to
  GK
• almostOutOfResources field in RAI is used for the
  purpose
• V4 introduces capacity field to report resources
  quantitatively

22
Resource reporting in SIP

• No way of reporting resources to the SIP proxy
  dynamically
• But if information about resources is obtained
  out-of-band, SIP provides mechanism to
  incorporate that into routing decision
• order and preference are used to prefer one
  route over the other

23
Current Limitations in voice routing

• Voice routes have to be configured statically
  (out of band) between domains (zones)
• No standard way of propagating routes between SIP
  and H323 network.
• No way of dynamically reporting resources to the
  voice signaling node

24
Outline

• Primer on Voice over IP (VOIP)
• Introduction to and current state of voice
  routing
• Telephony Routing over IP (TRIP)
• Telephony Gateway REgistration Protocol (TGREP)
• E.164 Number (ENUM)
• VOIP issues in NAT/Firewall

25
Telephony Routing over IP (TRIP)

• Addresses the problem of dynamically getting
  voice routing information
• Given a phone number TRIP enables local voice
  gateway to find the IP address of remote gateway
  capable of completing a call to that phone number
• However, many factors influence the decision of
  which gateway to choose
• Calling party may be using signaling not
  supported by all the gateways
• Gateway capacity may be considered
• Service providers on the gateways may be
  considered

26
TRIP

• TRIP is designed after BGP so provides support
  for a rich policy based routing
• An ITSP may want to advertise some routes and
  suppress some other routes to its neighboring
  ITSP
• An ITSP can have policy to route some calls to
  its own gateway and some to business partner
  gateways.
• A telephony domain under one administration is
  known as ITAD
• Main component in an ITAD is the Location Server
  (LS) which has access to the TRIB

27
TRIP

• LS use TRIP protocol to distribute telephone
  routes across ITADs.
• TRIP is an application layer protocol whereas BGP
  is a network layer protocol
• It is possible that TRIP may run between two
  ITSPs which are not adjacent networks. But in BGP
  the speakers are usually adjacent to each other
• TRIP between two speakers in different ITADs is
  E-TRIP and that in the same ITAD is I-TRIP

28
TRIP Architecture
LS
TGREP
E-TRIP
TGREP
TGREP
LS
I-TRIP
TGREP
LS
ITAD1
ITAD2
29
TRIP Routes

• A combination of
• Destination address(es)
• Application protocol (e.g. SIP, H323)
• Many other attributes
• NexthopServer
• RoutedPath
• LocalPref
• TRIP routes are advertised between a pair of LSs
  in UPDATE message
• An LS may modify a route before advertising it to
  its neighbor (based on policy)

30
TRIP Routes

• To advertise a route, an UPDATE message with
  ReachableRoute attribute (along with other
  attributes) will be sent by an LS
• To indicate unavailability of a previously
  advertised route an UPDATE message with
  WithdrawRoute attribute
• A TRIP LS processes three types of routes
• External route received from external peer
• Internal route received from an internal LS in
  the same ITAD
• Local route locally injected into TRIP (may be
  by configuration or by route distribution from
  another protocol)

31
TRIP Routing Information Base (TRIB)
Loc-TRIB
Adj-TRIBs-In (Internal LSs)
Adj-TRIBs-out
Ext-TRIB
Adj-TRIBs-In (External LSs)
Local Routes
32
TRIB

• Adj-TRIBs-In contains unprocessed routes
  received from Internal and external peers
• Routes from each internal and external peer are
  stored independently
• Ext-TRIB Route selection algorithm run on all
  external routes and local routes and selects the
  best rout for a given destination and stored in
  Ext-TRIB
• One Ext-TRIB per LS
• Loc-TRIB contains local TRIP routing
  information that LS has selected by applying its
  local policies to Adj-TRIB-In (internal) and
  Ext-TRIB
• Adj-TRIBs-out stores information that the local
  LS has selected for advertisement to its external
  peers
• Aggregation may be applied here

33
Route Propagation
LS1
22
TGREP
LS2
E-TRIP
TGREP
22
TGREP
22
GW1
I-TRIP
TGREP
LS3
ITAD1
ITAD2
34
TRIP Route Format
Address family
Application Protocol
Length
Address
35
TRIP Attributes

• NexthopServer indicates the next hop that the
  voice signaling protocol for the destination
  should be sent to
• Advertisement Path the ITADs through which
  routing information carried in an advertisement
  has passed
• Used to detect looping
• RoutedPath identifies the ITADs through which
  voice signals sent using this route would pass
• LocalPref Intradomain attribute used to inform
  other LSs of the local LSs preference for a
  given route.

36
Advantages

• TRIP can be used to dynamically advertise
  telephony routes across domains
• It is voice signaling protocol agnostic
• Can be used in SIP as well as H323 networks
• Hence can be used to exchange routes between H323
  and SIP networks

37
Current Status

• No commercial product available from leading
  vendors
• No provider is currently using TRIP for voice
  routing
• Open source version available from vovida
  (www.vovida.org)

38
Outline

• Primer on Voice over IP (VOIP)
• Introduction to and current state of voice
  routing
• Telephony Routing over IP (TRIP)
• Telephony Gateway REgistration Protocol (TGREP)
• E.164 Number (ENUM)
• VOIP issues in NAT/Firewall

39
TGREP

• A route registration protocol for telephony
  destinations
• One way of injecting (originating) routes into a
  voice routing system
• Along with TRIP provides a complete dynamic
  routing solution in a VOIP system
• Shares a lot of similarity with the TRIP protocol
• Same format for messages
• Subset of attributes (along with some new
  attributes) used in TRIP
• Addresses the dynamic resource reporting problem
  in current H323 and SIP network

40
TGREP Architecture
PSTN
A G G R E G A T I O N
TGREP Session management
PSTN
TGREP
TRIP LS
PSTN
TGREP Receiver
LS/Proxy
41
TGREP Attributes

• TotalCircuitCapacity total number of circuits
  available for terminating calls through this
  advertised route
• Represents potentially achievable upper bound on
  the number of calls which can be terminated on
  this route in total
• Relatively static attribute
• AvailableCircuits number of PSTN circuits that
  are currently available on a route to complete
  calls
• A dynamic attribute

42
TGREP Attributes

• CallSuccess provides information about the
  number of normally terminated calls out of total
  number of attempted calls.
• Used by LS to keep track of failures in reaching
  certain telephony destinations through a gateway
• Trunkgroup A set of trunks grouped together to
  terminate calls to a particular destination
• Enables providers to route calls to destinations
  through preferred trunks

43
TGREP Attributes

• Carrier used to represent the list of carriers
  that gateway can complete calls to
• Enables providers to route calls to destinations
  through preferred carriers
• May group trunks belonging to a provider and
  identify that as a carrier

44
TGREP Address Family

• Introduces new address families
• Some attributes (e.g. AvailableCircuits) more
  naturally fit with trunkgroup than prefixes
• Some attributes (e.g. AvailableCircuits) can be
  specified more precisely for trunkgroups rather
  than prefixes.
• The attributes trunkgroup and carrier can be used
  as address family
• Enables providers to do carrier-sensitive routing

45
Current Status

• Standard is in a draft state
• Expected to become an RFC in 6 months
• Cisco voice gateways have the draft
  implementation available today
• No service provider is currently using it in
  their network

46
Outline

• Primer on Voice over IP (VOIP)
• Introduction to and current state of voice
  routing
• Telephony Routing over IP (TRIP)
• Telephony Gateway REgistration Protocol (TGREP)
• E.164 Number (ENUM)
• VOIP issues in NAT/Firewall

47
ENUM

• Enables use of DNS for telephony (E.164) numbers.
• A specification for identifying available
  services connected to a E.164 number
• A new domain e164.arpa has been introduced to
  provide infrastructure in DNS for storage of e164
  numbers
• Is the way to globally advertise the services
  through which an e.164 number can be reached (it
  need not always be a physical phone device
  anymore)
• Enables users to specify different ways to reach
  them

48
ENUM Example

• Uses NAPTR records in DNS
• ORIGIN 3.8.0.0.6.9.2.3.6.1.4.4.e164.arpa.
• NAPTR 10 100 "u" "E2Usip" "!.!sipinfo_at_exam
  ple.com!" .
• NAPTR 10 101 "u" "E2Uh323" "!.!h323info_at_ex
  ample.com!" .
• NAPTR 10 102 "u" "E2Umsg" !.!mailtoinfo_at_ex
  ample.com!" .

49
ENUM Call Flow
1. Alice dials Bobs phone number 44123456.
SIP INVITE sent to the SIP proxy 2. SIP Proxy
queries 6.5.4.3.2.1.4.4.e164.arpa to the DNS
server 3. DNS server sends response SIPE2U
sipbob_at_10.0.0.1 4. SIP proxy sends the call to
SIP proxy at 10.0.0.1 5. SIP proxy at 10.0.0.1
completes the call to Bobs IP phone.
DNS Server
2
4
10.0.0.1
5
Bob
Alice
50
Current State of ENUM

• Product commercially available
• Cisco voice gateways, Cisco SIP proxy, Neustar
  and NetZero
• European telecoms seems to be ahead of others
• Mostly in an experimental phase
• More widespread VOIP deployment is needed to make
  this a reality

51
Outline

• Primer on Voice over IP (VOIP)
• Introduction to and current state of voice
  routing
• Telephony Routing over IP (TRIP)
• Telephony Gateway REgistration Protocol (TGREP)
• E.164 Number (ENUM)
• VOIP issues in NAT/Firewall

52
VOIP issues in NAT/Firewalls

• VOIP is one of many IP applications that has
  problem traversing NAT and firewalls
• When ports are assigned dynamically (not
  well-known as with TCP, http), applications have
  problem with NAT/Firewalls
• In H.323, call setup message contains address and
  port information in the payload. So even if NAT
  changes the IP address and port in the packet
  itself, the called party will use the private IP
  in the payload and hence the application fails

53
VOIP issues in NAT/Firewalls

• H.323 and SIP exchange address and port
  information about data channel (RTP channels)
  within the control messages.
• If H.323 is using slow start, H.245 port
  information is conveyed in the h225 channel.
• Firewall needs to monitor H.225 channel for H.245
  port (because there cannot be any static rule
  that can allow this H.245 connection while
  blocking other undesired TCP connection)

54
Solutions to NAT/Firewall Issues

• Proxy placed at the border between two domains
  (private and public IP)
• Proxy terminates sessions with both the hosts and
  relays application signaling message as well as
  RTP media streams transparently between the two
  hosts
• Have ALG (Application Level Gateway) embedded
• ALG examines and modifies application payload
  content to allow VOIP traffic traverse the
  NAT/firewall
• Most common commercial solution
• ALG-enabled NAT/firewalls are quite expensive

55
Solutions to NAT/Firewall Issues

• Have a new type of firewall that dynamically
  opens pinholes to let a VOIP call go through it
• Module called Firewall Control Proxy (FCP) placed
  in the signaling path between private and public
  domains and monitors the call setup signals and
  commands the firewall to allow RTP streams
  destined to the appropriate IP addresses to pass
  through

56
References

• Cisco Proxy Server -http//www.cisco.com/en/US/
  products/sw/voicesw/ps2157/
• Understanding H323 gatekeepers
  -http//www.cisco.com/warp/public/788/voip/underst
  and-gatekeepers.html
• Basic two zones gateway-to-gatekeeper
  configuration -http//www.cisco.com/en/US/tech/tk
  652/tk701/technologies_configuration_example09186a
  00800a9a56.shtmlintro

57
References

• Telephony Routing over IP RFC 3219
• Voice over Internet Protocol (VOIP) B. Goode,
  Proceedings of the IEEE Vol. 90, No. 9, September
  2002.
• A Telephony Gateway REgistration Protocol
  (TGREP) Internet draft, http//www.ietf.org/int
  ernet-drafts/draft-ietf-iptel-tgrep-03.txt
• The E.164 to Uniform Resource Identifiers (URI)
  P. Faltstrom et. al., RFC 3761