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Session Initiation Protocol (SIP)

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Title: Session Initiation Protocol (SIP) Author: Onno W. Purbo Last modified by: Onno W Purbo Created Date: 1/18/2002 2:34:20 PM Document presentation format – PowerPoint PPT presentation

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Title: Session Initiation Protocol (SIP)


1
Session Initiation Protocol (SIP)
  • Onno W. Purbo
  • Onno_at_indo.net.id

2
Referensi
  • Henning Schulzrinne, Columbia University, New
    York, May 2001
  • Dr. Andreas Steffen, Komunikationsysteme

3
SIP in short ..
  • Dr. Andreas Steffen, Komunikationsysteme

4
SIP addressing ..
5
SIP processes
6
SIP format
7
Overview
  • Protocol architecture
  • Typical component architecture
  • Addressing and locating SIP entities.
  • Protocol operation and extensions.

8
Introduction ..
  • SIP core protocol for establishing sessions in
    the internet.
  • Transports session description information from
    initiator (caller) to callees.
  • Allows to exchange parameters in mid-session.
  • Terminate session.

9
VoIP protocol architecture
10
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11
Media Protocol Stack
12
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13
SIP Protocol Use
14
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15
SIP Applications ..
  • Setting up voice-over-IP calls.
  • Setting up multimedia conferences.
  • Event notification (subscribe / notify) ? IM and
    presence.
  • Text and general messaging.
  • Signaling transport.

16
SIP addressing
  • SIP uses e-mail style addressing to identify
    users.

17
Personal Mobility
18
SIP addressing ..
  • Typically, same as users e-mail address
  • djancuk_at_surabaya.com
  • 02114204701_at_gateway-r-us.com
  • Written as URL, e.g,
  • Sipdjancuk_at_surabaya.com
  • Can add parameters, such as, type (userphone)
    or transport protocol.

19
Tel URLs (RFC 2806)
  • Also can use tel URL for telephone numbers, e.g.,
  • Tel0811797677
  • Fax022.253.4677
  • Either global (tel001..) or local
    (tel0w00222534677 phone-context0222534677)
    number.
  • Allow post-dialing digits postdpp32.
  • Also modem52990000 typev32b?7e1 typev110.

20
SIP building blocks
21
Back-to-Back UA (B2BUA)
  • Two (or more) user agents, where incoming calls
    trigger outgoing calls to somebody else.
  • Also, third-party call control (later)
  • Useful for services and anonymity

22
Back-to-Back UA (B2BUA)
23
Maintaining state in SIP enitities.
  • Stateless
  • Each request and response handled independently.
  • (Transaction) Stateful
  • Remember a whole request/response transaction.
  • Call stateful
  • Remember a call from beginning to end.

24
SIP archirecture
  • Peer-to-peer
  • Outbound proxy
  • VoIP to PSTN
  • PSTN to VoIP

25
peer-to-peer
26
outbound proxy
27
VoIP to PSTN
28
PSTN to VoIP
29
SIP operation in proxy mode
30
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31
SIP operation in redirect mode
  • 302 redirection for a single call
  • 301 permanently

32
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33
Locating SIP users
34
Registrar and location servers
35
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36
Basic user location mechanism
  • Host (SIP URL) ? hostname of proxy
  • DNS hostname of proxy ?SIP server(s)
  • If SIP UAS alter user done.
  • If SIP proxy/redirect server map URLn ? URLn1,
    using any information in the request
  • Go to step 1
  • One minor exception ..

37
Basic SIP routing mechanism
  • Will fill in details later
  • Route using request URIs.
  • All but first request in call typically bypass
    proxies and go direct UAC-UAS.
  • However, can use record-routing to force
    certain proxies to be visited all the time.
  • Responses always traverse the same route as
    requests

38
Outbound proxies
  • Normally, proxy servers one or more domains.
  • Outbound proxies are used for all outbound
    requests from within a domain.
  • Typically, for managing corporate firewalls and
    policy enforcement.
  • May also provide dial plans or route tel/fax
    URLs.
  • Other uses lawyer client billing, ..

39
Locating users DNS SRV ..
  • e-mail DNS MX record mapping domain to mail
    server(s).
  • SIP use newer record for general purpose
    mapping, SRV (RFC 2782)
  • Mapping from service and transport protocol to
    one or more servers, including protocols.

40
Locating users DNS SRV ..
  • Allow priority (for back-up) and weight (for load
    balancing).

41
Using DNS SRV for scalable load balacing ..
42
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43
Aside SIP scaling ..
  • HTTP request director ?? SIP client-based
  • HTTP randomized DNS (short TTL!) ?? SRV weight
    and priorities.
  • Cant just distribute request randomly, since
    backend (registration) synchronization is needed.

44
Aside SIP scaling ..
  • Registration scaling request/second 3600 e.g.
    100 request/second ? 360.000 user/server.
  • Major bottleneck are logging and database
    updates.
  • Generally, higher registration than INVITE rates.

45
SIP operation ..
46
SIP requests and responses
  • Text, not binary, format.
  • Look very similar to HTTP/1.1
  • Requests and responses are similar except for
    first line.
  • Requests and responses can contain message
    bodies typically session descriptions, but also
    ASCII or HTML.

47
SIP syntax
48
SIP syntax
  • Field names and some tokens, e.g. media type, are
    case-insensitive.
  • Everything else is case-sensitive.
  • White space doesnt matter except in the first
    line.
  • Lines can be folded.
  • Multi-valued header fields can be combined as a
    comma-lists.

49
SIP methods
50
SIP invitation and media negotiation
51
Tagging To
  • After forking and merging, hard to tell who
    responded.
  • UAS responds with random tag added to
    disambiquate
  • To cuk ltsipcuk_at_sip-gtw.tel.co.idgt tag7gt8
  • Future requests ignored if they contain wrong tag.

52
SIP call legs
  • Call legs From, To, Call-ID
  • Requests from callee to caller reverse To and
    From.
  • Caller and callee keep their own Cseq space.
  • Either side can send more INVITEs or BYE

53
SIP responses
54
SIP response routing ..
  • Request are routed via URL
  • Response trace back request route without proxy
    server state
  • Forward to host, port in next Via
  • TCP re-use connection if possible, create new
    one if needed.
  • UDP may send responses to same port as requests.

55
SIP response routing
56
SIP spirals
57
SIP spirals
58
Forcing Request Path
  • Usually, bypass proxies on subsequent requests.
  • Some-proxies want to stay in path ?
    call-stateful
  • Firewalls
  • Anonymizer proxies.
  • Proxies controlling PSTN gateways.
  • Use Record-Route and Route

59
Request Routing
60
SIP request forking
61
SIP sequential request forking
62
SIP request forking
  • Branches tried in sequence or parallel (or some
    combination).
  • Recursion may try new branches if branch returns
    3xx.
  • Return best final answer lower status code.
  • Forward provisional responses.

63
Parallel forking call flow
64
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65
SIP transport issues
  • SIP 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

66
SIP transport issues ..
  • Choices
  • SCTP new protocol
  • No HOL blocking
  • Fall back address (but SVR provides this already)
  • Connection setup overhead

67
Other signaling approaches
68
Diff to classical signaling
69
Aside Alternative architecture
  • Master-slave MGC (media gateway controller)
    controls one or more gateways.
  • Allows splitting of signal and media
    functionality.
  • please send from circuit 42 to 10.1.2.3
  • Uses MGCP (implemented) or Megaco/H.248
    (standarized, but just beginning to be
    implemented).
  • Gateway can be residential.

70
Aside Alternative architecture
  • Basis of PacketCable NCS (Network Control System)
    architecture.
  • Service creation similar to digital PBX or
    switch.
  • End system has no semantic knowledge whats
    happening.
  • ? can charge for caller id, call waiting.

71
MGCP/SIP architecture
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