Presented By Team Netgeeks

1
Presented ByTeam Netgeeks

• SIP
• Session Initiation Protocol

2
AGENDA

• Introduction
• Applications
• Overview of operation
• Protocol Elements
• Requests
• Responses
• SIP based services
• Security

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Introduction

• SIP ? Session Initiation Protocol, RFC 3261
• A signaling, presence and instant messaging
  protocol.
• Developed to set up, modify, and tear down
  multimedia sessions, request and deliver presence
  and instant messages over the Internet.
• SIP allows two end points to establish media
  sessions with each other.

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Introduction

• The main signaling functions of the protocol are
  as follows
• Location of an end point.
• Contacting an end point to determine willingness
  to establish a session.
• Exchange of media information to allow session to
  be established.
• Modification of existing media sessions.
• Tear-down of existing media sessions.

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Introduction

• Also been extended to request and deliver
  presence information (on-line/off-line status and
  location information such as that contained in a
  buddy list, as well as instant message
  sessions. These functions include
• Publishing and uploading of presence information.
• Requesting delivery of presence information.
• Presence and other event notification.
• Transporting of instant messages.

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SIP applications

• Setting up voice-over-IP calls
• Setting up multimedia conferences
• Event notification (subscribe/notify) a IM and
  presence
• Text and general messaging
• Signaling transport

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SIP

• SIP is based on an HTTP-like request/response
  transaction model.
• SIP works with both IPv4 and IPv6.
• SIP uses the well-known port number of 5060 or
  5061.
• It can use either TCP or UDP as transport
  protocol.

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Overview of operation

• The basic functions of SIP location of an end
  point, signal of a desire to communicate,
  negotiation of session parameters to establish
  the session, and teardown of the session once
  established.
• Incorporates elements of two widely used Internet
  protocols
• Hyper Text Transport Protocol (HTTP) and Simple
  Mail Transport Protocol (SMTP) used for e-mail.
• From HTTP, SIP borrowed a client-server design
  and the use of URLs and URIs.
• From SMTP, SIP borrowed a text-encoding scheme
  and header style.
• SIP identity
• ? Uniform Resource Identifier (URI) called a SIP
  URI
• 
  sipbob_at_macrosoft.com
• ? Secure URI, called a SIPS URI
• sipsbob_at_macrosoft.com

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Basic topology
INVITE
Wonder- Land
Macro- Soft
INVITE
INVITE
OK
OK
OK
Alice
Bob
RTP

• Note that the media is always end-to-end and not
  through the proxy.

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SIP Protocol Elements

• SIP has the following entities, each with
  different function.
• SIP Terminal - Supports the real time, two-way
• communication with other SIP entities.
• 2. SIP User Agent - user agents are the endpoints
  of the call.
• User Agent Client (UAC) initiating the call
• User Agent Server (UAS) answering the call.
• Receives and responds to SIP requests and can
  accept, refuse or redirect the call.
• The User Agent software switches between the UAC
  and UAS modes on a message-by-message basis
  depending on what is going on. The User Agents
  can be handsets or desktop applications.

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SIP Protocol Elements (contd)

• 3. SIP Network Server - Handles the signaling
  associated with multiple calls and allows
  peer-to-peer calls to be made using client-server
  protocol. Main function is to provide name
  resolution and user location, and to pass on
  messages to other servers using next-hop routing
  protocols.
• There is more than one type of server the Proxy
  Server , Redirect Server , and the Registrar
  Server .
• Proxy Servers - Network hosts acting as both
  clients and servers to other entities. The job is
  to ensure requests are routed to appropriate
  entity identified by a SIP Uniform Resource
  Identifier (URI). The Proxy servers can operate
  in two different modes
• The SIP stateful Proxy server and the SIP
  stateless Proxy server .
• Redirect Servers receive SIP requests and send
  response to zero or more addresses. The first
  location to answer takes the call. Redirect
  servers do not initiate SIP requests or accept
  SIP calls.
• Registrar Servers accept registration requests.
  These servers maintain the databases that contain
  location information of all user agents
  registered with a particular SIP domain, thereby
  enabling the users to update their location and
  policy information.

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(No Transcript)
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SIP - Requests

• INVITE - initiate call
• REGESTER - register with location service
• Other Methods
• ACK - confirm final response
• BYE - terminate (and transfer) call

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Session Initiation and Tear Down

Alices Proxy Server
Alices Softphone
Bobs Proxy Server
Bobs Softphone
INVITE F1
INVITE F2
INVITE F4
TRYING F3
TRYING F5
RINGING F6
RINGING F7
RINGING F8
200 OK F9
200 OK F10
200 OK F11
ACK F12
Media Stream
BYE F13
200 OK F14
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Sample Request Format
INVITE sipbob_at_macrosoft.com SIP/2.0 Via
SIP/2.0/UDP pc33.wonderland.com Max-Forwards 70
To Bob ltsipbob_at_macrosoft.comgt
FromAliceltsipalice_at_wonderland.comgttag19283017
74 Call-ID a84b4c76e66710_at_h3.wonderland.com
CSeq 314159 INVITE Contact ltsipalice_at_h3.wonde
rland.comgt Content-Type application/sdp
Content-Length 142
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Sample Response Format

• SIP/2.0 200 OK
• Via SIP/2.0/UDP server10.macrosoft.com
  branchz9hG4bKnashds8received192.0.2.3
• Via SIP/2.0/UDP bigbox3.site3.wonderland.comrece
  ived192.0.2.2
• Via SIP/2.0/UDP h3.wonderland.combranchz9hG4bK7
  76asdhds received192.0.2.1
• To Bob ltsipbob_at_m.macrosoft.comgttaga6c85cf
• From Alice ltsipalice_at_wonderland.comgttag1928301
  774
• Call-ID a84b4c76e66710_at_h3.wonderland.com
• CSeq 314159 INVITE
• Contact ltsipbob_at_192.0.2.4gt
• Content-Type application/sdp Content-Length 131

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SIP Responses
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SIP Response Routing
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SIP Transportation

• SIP operates over any packet network, reliable or
  unreliable
• choices
• UDP most common
• low state overhead
• small max. packet size
• TCP can combine multiple signaling flows over
  one link
• use with SSL
• connection setup overhead
• HOL blocking for trunks
• SCTP new protocol
• no HOL blocking
• fallback address (but SRV provides this
  already)
• connection setup overhead

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SIP based services

• Call forwarding basic INVITE behavior
  (proxy/redirect)
• Call transfer REFER method
• Call hold set media address to 0.0.0.0 can be
  done individually per media
• Caller id From, plus extensions

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SIP based services ( Call Tx )
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SIP presence protocol
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Security

• Threats
• spoofing From in REGISTER call redirection
• spoofing From in INVITE bypass call filtering
• snooping media packets
• billing confusion (identifier munging)
• denial-of-service attack

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Security (contd)
Basic (plaintext password) and digest
(challenge-response) are very similar to HTTP
security mechanisms.
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References

• http//www.faqs.org/rfcs/rfc3261.html
• SIP Understanding The Session Initiation
  Protocol by Alan B.Johnston
• http//www.cs.columbia.edu/hgs/teaching/ais/slide
  s/2003/sip_long.pdf