The Session Initiation Protocol:Internet-Centric Signaling

네트워크 연구실류 준 우

Introduction (1)

• Initiates, modifies, and terminates network sessions

• Focus on interactive multimedia sessions (Internet phone calls or multimedia conferences)

• Used for instant messaging, event notification or managing other session type (ex. Distributed games)

Introduction (2)

• Peer-to-Peer signaling Protocol• SIP URL : e-mail 과 비슷한 형태의

identifier 를 이용하여 사용자를 식별• Personal mobility

같은 id 로 다른 위치나 다른 단말에서 사용이 가능

Components (1)

• UAC (User Agent Client)- 요청 Msg 로 SIP transaction 을 함- 요청 Msg 의 존속 기간동안 UAC 로 동작

• UAS (User Agent Server)- SIP 요청 Msg 에 응답- 요청 메시지의 수용 , 거절 , 리다이렉트를 처리함

Components (2)

• Registrars- domain 내의 user 정보를 주고 받는다 . - Register 요청 메시지를 수용하는 서버

• Proxy Server- application-layer 에서 SIP request, response 를 forwarding 함

• Redirect Server- request 를 받아서 다른 주소나 서버로 넘겨준다 .

• Registrars, Proxy Server, Redirect Server 는 같은 Program 에서 구현 가능하다 .

Protocol architecture

H.323 SIP RTSP RTCPRSVP

RTP

TCP UDP

IPv4, IPv6

PPP

SONET

AAL3/4 AAL5

ATM Ethernet

PPP

V.34

Signaling

• SPI request URI 는 email 주소와 비슷하게 생겼다 .user name + host name + parameters

• Signaling Msg 는 다른 미디어 전송과 전혀 다른 path 로 전송된다 .

• SIP Msg 는 transfer protocol 에 independent 하게 동작한다 .

Describing & Changing Session

• SIP use SDP to describe multimedia sessions.

• SDP expressed in a simple textual format

SIP Message Format

• SIP 의 Msg format 은 HTTP 처럼 plain text 로 되어 있다 .

• Request Msg. 와 Response Msg 로 볼 수 있다 .

• Request Msg.UAC -> UAS

• Response Msg.UAC <- UAS

SIP Messages

Request Messages

Response Messages

Forking

• SIP differs from other signaling protocols in that it allows a call request to fork

• Automatic call distribution (ACD)ex) same number can ring at home and at work.

Reliability

• SIP Msg can be transmitted over unreliable transport protocols

• SIP has to take care of reliability on its own.

• INVITE Msg응답이 올 때까지 Client 는 재전송하고 ACK 때 까지 서버는 Responses 재전송

• Response 가 올때까지 Request 재전송

Locating Users

• Location User 는 User Registration 을 통해서 location mapping 을 만드는 것이다 .

• User agents 주기적으로 SIP REGISTER request 을 사용해 Local registrar server 에 등록 한다 .

Session Characteristics

• Overall session caller’s name, address, organizationcallee’s name, addresssession’s urgency and subject

• Media sessionacceptable media format

Security

• Need to be secured against eavesdropping and alteration.

• SIP inherits the basic and digest authentication mechanisms from HTTP.

• Currently, public key cryptography using the PGP format has been defined

Security

• Confidentiality- End-to-end encryption

Shared secret 또는 Public key- Hop-by-hop encryption

기존의 security mechanism (TLS/IPSec)• Authentication

- HTTP authentication mechanism 사용 (RFC2617)- TLS/IPSec 인증 기능 사용

Integration with Other Internet Services

• SIP addresses are regular URLs and can be embedded in Web pages, e-mail, and any other context where URLs are allowed.

• The message body of requests and responses uses the MIME mechanism familiar from e-mail.

• SIP messages can thus carry any binary or text object, with servers treating the message body as opaque data.

Telephony Service

• 많은 부가 기능을 가질 수 있는 서비스가 전통적인 Telephony Service 의 기준이상으로 SIP 에서 제공할 수 있다 .

• 특정 서비스 제공을 위한 요소를 첨가하기 보단 새로 만들어질 많은 서비스를 제공할 수 있는 기본적인 요소를 제공하는 방향을 제시

Multiparty Sessions

• Full meshsends an individual copy of the media stream to the others.

• Mixerseveral media streams and replicates them to all participants.

• Network-layer multicastmost efficiently supported by network-layer multicast

Expression Caller Preference

• many of their communications tools and location under a single public identifier.

Quality of Service

• SIP messages and data streams are likely to traverse very different parts of the network

• SIP 를 set-up resource reservations 에 사용하는건 부적합하다 .

• Can used to negotiate the use of QoS mechanisms

Mobility

• One of the central tasks• SIP is to locate one or more IP addresse

s where a user can receive media streams, given only a generic, locationindependent address identifying a domain.

• makes it easy to offer precall terminal mobility

SIP and MGCP/Megaco

• MGCP 는 Media Gateway 와 이를 제어하기 위한 Media Gateway Controller 간에 사용하는 프로토콜입니다

Programming SIP Services

• telephony services have been either hard-coded into switches

• SIP-based telephony there are at least four choices to create SIP-based services.(language-based APIs, create dedicated languages, SIP servlet is a pice of JAVA code , includesjava applets)

The Applicability of SIP

• There are many functions SIP explicitly does not provide

• not a session management or conference control protocol.

• as media transport, voting and polling, virtual microphone passing, chair election, floor control, and feedback on session

Related work

• Telephony signaling protocolschannel-associated signaling -> out-of-band signaling.

• ISUP to set up connections between IP endpoints

• SIP is related to messaging mechanisms such as Internet e-mail.

• The ICEBERG project also considers setup of large-scale sessions using multicast, while Elliott discusses session membership propagation.