Feature Interaction in Internet Telephony

Wissam Itani March 26, 2002

Outline

Feature Interaction DefinitionInternet Telephony Architectural ModelDifferences from the PSTNApplicability of Existing FI WorkExamples of New Interactions in ITNew Approaches for Managing Internet

InteractionsConclusion

Feature Interaction Definition

Feature interactions refer to situations in which instances of the same features or different service features affect each other.

Bad feature interaction (FI) is one that causes the system behavior to be undesirable.

Good FI

Internet Telephony Architectural Model

Within Internet telephony network, we find 3 types of devices : end sytems gateways signaling servers

Internet Telephony Architectural Model(Cont.)

End sytems : devices on which users place and receive calls.

Initiate and respond to signalingTransmit and receive mediaSmart : aware of call stateCall waiting, Multiple Line services ...

Internet Telephony Architectural Model(Cont.)

Gateways : devices which allow calls to be placed to and from other telephone networks.

Other devices see Gateways as end devices.

Networks behind Gateways are transparent to other devices.

Internet Telephony Architectural Model(Cont.)

Signaling servers : handle the application-level control of the routing of signaling messages

perform user location services

Differences from the PSTN

These differences affect what sorts of features are possible, how these features are created, and how their interactions are managed.

Internet telephony allows more flexibility and new possibilities, it also introduces new challenges.

Advantages of Internet Telephony

Advantages can be divided into 3 categories:

ProtocolNetworkConceptual

Protocol Issues

Internet telephony signaling protocols are more expressive

Internet telephony signaling can be extended while maintaining compatibility. The network would be able to support advanced features without undergoing universal upgrades of an entire system.

Protocol Issues (Cont.)

Adding another control protocol, for example, RTSP for voice mail, can be done independently of the telephony signaling protocol

Internet telephony does not make a strong distinction between user devices and network devices.

Internet model eliminates user-level address scarcity.

Network Issues

Internet telephony takes advantage of the nature of the Internet network itself

The Internet is inherently self-routing. Both signaling and media are sent off into the network through the same mechanism

Network Issues (Cont.)

The Internet environment supports a number of means of strong encryption and authentication, such as the IPsec suite of protocols.

Conceptual Issues

The conceptual framework of Internet services add new characteristics to Internet telephony environment.

Internet is already a distributed environment where multiple providers interwork and compete.

A customer can proxy all his calls through a service which, for example, automatically blocks calls from known telemarketers.

Conceptual Issues(Cont.)

The Internet environment enables programmability on a scale not seen in the telephone network.

Service providers motivated to create services which will distinguish them from competitors

New Complications

While the new characteristics of the Internet enable new possibilities, they also increase the complexity of creating features.

Distributed nature of the Internet: Features can be implemented and deployed at network devices, end systems, and signaling servers.

New Complications (Cont.)

User programmability is now possible : features could be created by amateur feature designers.

End systems have control of call state. 911 calls.

Several new features of Internet telephony protocols also have the potential for dramatic FI consequences with existing protocols.

New Complications (Cont.)

Forking proxy : call request transmitted to several devices.

Request expiration. User wishes may conflict with signaling servers responses.

Internet’s lack of address scarcity. With throw-away addresses now available one can easily evade a block on her addresses.

New Complications (Cont.)

Assumptions, based on the trust model in the PSTN, break down when end-to-end connectivity is introduced and anybody can become an Internet Service Provider.

Feature like ‘caller I-D blocking’ become much more difficult when users cannot trust the network not to reveal calling information to recipients.

Applicability of Existing FI Work

Single-component interactions are largely the same in the Internet environment as they are in the PSTN. The techniques developed to resolve these interactions should still be valid in the new environment.

Applicability of Existing FI Work (Cont.)

Multiple-component interactions, are much more complicated for Internet telephony. Features are designed and deployed by providers who do not cooperate. FI resolution techniques, depending on features designed globally and resolving their interactions at design time, are no longer practically applicable.

Examples of New Interactions in Internet Telephony

FI in Internet telephony may be categorized into 2 types of interactions: Cooperative : correspond with (SUMC)

interactions. Adversarial : correspond with (MUMC) or

(CUSY) interactions.

Cooperative Interactions (Example 1)

Request Forking (RF) and Call Forward to Voicemail : an RF allows a proxy server P to attempt to locate a user by forwarding a request to multiple destinations, A and B. The call will be connected to the first destination to pick up. The interaction arises when the callee is currently located at A, and B has had its calls forwarded to a voicemail system.

Figure from : Henning Schulzrinne, Industrial-Strength Internet Telephony

Request Forking

CALL bob@cse.psu.edu

run

alice

bob@jump

cse.pu.edu

Proxy Serverbob

Location Server

run.cse.psu.edujump.cse.psu.edu

INVITE bob@run200 O

K

jump

INV

ITE bob@jum

p

Cooperative Interactions (Example 2)

Camp-on (CO) and Call Forward on Busy: CO allows a caller to re-try calling a busy destination periodically until the line becomes free. If the destination has Call Forward on Busy,

the call is forwarded to some alternate destination; in this case CO can’t be triggered

this interaction can occur in the PSTN but it is more serious in Internet telephony.

Adversarial Interactions (Example 1)

Outgoing Call Screening and Call Forwarding: this interaction also appears in the PSTN, of course, but the ability to easily change addresses and get easy call forwarding on the Internet makes this problem much more significant in the Internet environment.

Adversarial Interactions (Example 2)

Outgoing Call Screening (OCS) and End-to-End connectivity : a signaling server cannot force calls to be placed through it because any device can talk to any other, and an end system can communicate directly with the remote party, bypassing local administrative controls entirely. Enforcement of OCS policy is difficult

New Approaches for Managing Internet Interactions

Explicitness: some interactions can be prevented or made less likely by making explicit the actions being taken, and their desired effects. For instance, a call wishing to camp on to the actual user to be contacted could specify “do not forward” so as to get back a busy response rather than have the call be forwarded against their wishes.

New Approaches for Managing Internet Interactions (Cont.)

Universal authentication: many of the problems introduced by polymorphic identities and identity forging can be resolved by insisting on strong authentication of request.

New Approaches for Managing Internet Interactions (Cont.)

Network-level administrative restrictions: such as firewalls must be used to limit end-to-end connectivity in order to impose administrative controls.

Verification testing: ensuring correct operation of features by testing them directly.

Conclusion

Learn from prior experiences of creation of telephony services

Some existing FI management techniques are impractical in IT

DFC, Agent’s negotiation, transaction processing, Service layer

References

[1] J. Lennox and H. Schulzrinne, “Feature Interaction in Internet Telephony” in proc. of Feature Interaction in Telecommunications and Software Systems VI, Glasgow, United Kingdom, May 2000.

[2] H. Schulzrinne, “The IETF Internet Telephony Architecture and Protocols” IEEE Network, pp. 18-23, May/June 1999.

[3] D. Zhang "The Deployment of Features in Internet Telephony" http://lotos.csi.uottawa.ca/ftp/pub/Lotos/Theses, Msc Thesis Carleton University, January 2002.

[4] L. Blair and J. Pang, “Feature Interactions-Life Beyond Traditional Telephony” Feature Interactions in Telecommunications and Software Systems VI., IOS Press, pp. 83-93, Amsterdam, 2000

[5] J. Cameron , “Feature Interaction in the New World” Feature Interactions in Telecommunications and Software Systems V, IOS Press, pp. 3-9, 1998

[6] K. Kimbler, “Service Interaction in Next Generation Networks” Feature Interactions in Telecommunications and Software Systems VI, IOS Press, pp. 14-20, 2000