Signaling III

1
Signaling - III
2
SIP

• Session Initiation Protocol
• First RFC 2543, now RFC 3261
• Standardized by the Internet Engineering Task
  Force(IETF)
• Considered to be of the Intelligent Endpoint
  model type
• Messages are text-based, easily readable
• Can work over TCP/UDP/SCTP/TLS

3
SIP Entities

• User Agent
• Registrar
• Proxy Server
• Redirect Server
• Entities are logical, and may be co-located

4
User Agent(UA)

• An endpoint or client, the entity attempting
  to initiate the session
• Phone, softclient etc
• User Agent Client(UAC)
• Sender of a SIP request
• User Agent Server(UAS)
• Receiver of a SIP request
• Note UAS/UAC roles can be interchanged in a
  single session

5
Registrar

• Server that UAs register to(like GK in H.323)
• Keeps a registration Database of User-gtIP-Address
• Typically performs an authentication function

6
Proxy Server

• A server that receives SIP requests and routes
  them to a UAS or another Proxy Server
• Based on routes/rules
• May stay in the path of responses to that
  request(typically does)
• May perform an authentication function

7
Redirect Server

• Receives a request and responds to it with a
  redirect response pointing to the location of
  the target

8
Functionality buckets

• Registration
• Locating or finding the user
• Contacting the user and creating a Session
• Authentication and (to some extent) access
  control
• Unlike H.323, all of these functions are
  performed by essentially one protocol - SIP

9
SIP addressing

• Uses URI(Uniform Resource Identifier) with the
  sip scheme
• sipalice_at_example.com
• Of the form user_at_domain, very much like email
  addresses
• Plain phone number addresses are not native to
  SIP, there is a tel scheme for that

10
SIP addressing

• So sip238155_at_example.com is legal, just
• sip238155 is not
• tel [phone] is a tel URL and legal in
  some SIP messages

11
Requests and Responses

• Not surprisingly, the protocol functions using a
  request-response model
• Typically a request is referred to as a Method
• The SIP message structure is based on HTTP, and
  is very similar to it
• An exchange of requests and responses influence
  different kinds of state on the endpoints or
  intermediaries, and that is key

12
The SIP state/layer model
Call/Session state
UA
Dialog state
Transaction state
Requests Responses
Syntax/Encoding
13
Getting a request to the right place

• Requests are targeted at a SIP URI
• Typically DNS is used to find the server
  responsible for the domain
• That server determines the location of the target
  from the registration database
• This could be a multi-hop process

14
DNS SRV records

• DNS Domain Name Service
• SRV Service record
• The most common DNS record in use is the A
  record.
• Maps hostname ? IP address
• SRV gives the IP address or hostname of a server
  providing a certain service in a certain domain

15
SIP Request structure

• METHOD URI Protocol-Version
• Header..
• Header
• Content-Type Header
• Content-Length Header
• Message-Body(type defined by Content-Type)

16
SIP Response structure

• Response-Code Response-String Protocol
• Header..
• Header..
• Content-Type Header
• Content-Length Header
• Message-Body(type defined by Content-Type)

17
Key SIP Requests

• REGISTER
• INVITE
• ACK
• BYE
• CANCEL
• There are many more, some of which we will look
  at later, and some we will not

18
Key SIP Headers

• From
• To
• Via
• Cseq
• CallId
• Contact
• Content-Type
• Content-Length

19
SIP Response codes

• 1xx
• Request in progress
• 2xx
• Request successful
• 3xx
• Request redirected
• 4xx
• Client error
• 5xx
• Server error
• 6xx
• Global failure
• Every class except 1xx is termed final response

20
REGISTER

• Sent by UA to Registrar
• Informs the Registrar about where the UA is(IP
  address etc)
• This process creates a Registration which is
  valid for a period of time determined by the
  Registrar
• After that time, the UA needs to register again
• REGISTER messages are typically authenticated by
  the Registrar

21
INVITE

• Sent by UA to initiate a session
• Sent to target UA(callee) typically through one
  or more proxies
• Basic INVITE sequence
• INVITE ?
• ? 100 Trying
• ? 180 Ringing
• ? 200 OK

22
ACK

• ACK acknowledges a final response to an INVITE
  request
• Sent by UAC
• 3-way handshake to complete INVITE transaction
• Basic ACK sequence
• INVITE ?
• ? 200 OK
• ? ACK

23
CANCEL

• Sent by UA to CANCEL pending request for a
  session
• Typical CANCEL sequence
• INVITE ?
• ? 100 Trying
• ? 180 Ringing
• CANCEL ?
• ? 200 OK (for the CANCEL)

24
BYE

• Sent by UA(either side) to terminate session
• Basic INVITE sequence
• INVITE ?
• ? 100 Trying
• ? 180 Ringing
• ? 200 OK (Session established !)
• ACK ?
• ? BYE
• ? 200 OK

25
The SIP From header

• From Anup Rao ltsipanup.v.rao_at_gmail.comgt
  tag3wdwf
• Logical sender of the request
• Should ideally contain the SIP Address of
  Record, or AOR, rather than an instance of a
  client
• ltsipanup.v.rao_at_10.1.1.1gt
• NOT a good idea
• Tag is a key used in identifying a dialog,
  which will come up soon
• The non-URI part(Anup Rao) is referred to as
  the display name

26
The SIP To Header

• To Anup Rao ltsipanup.v.rao_at_gmail.comgt
• Specifies intended recipient. Should be AOR
• May or may not be the final recipient
• Request retargetting by a proxy, say for a
  feature like call-forwarding
• Similar to From, a tag parameter will exist in
  the context of a dialog

27
The SIP Via Header

• Records the path the request has taken
• Where the response must be sent
• Last hop shows up as the last Via header
• Via SIP/2.0/UDP pc33.atlanta.combranchz9hG4bKna
  shds8
• branch parameter is used as a Transaction
  Identifier

28
The SIP Cseq header

• CSeq 4711 INVITE
• Serves to identify and order transactions
• Note that just a response code does not tell you
  which request the response is for
• The number part needs to be a 32 bit integer

29
The SIP Call-ID header

• Call-ID f81d4fae-7dec-11d0-a765-00a0c91e6bf6_at_foo.
  bar.com
• Needs to be globally unique across space and time
• Groups together a series of messages into a
  higher level construct(typically call or session)

30
The SIP Contact header

• Contact ltsipanup.v.rao_at_somehost.example.comgt
• An address(SIP URI) that an instance of the
  sender can be contacted for future requests
• sipuser_at_ipaddress is also typical
• This is what is used in the REGISTER message to
  convey where the instance is
• The expires parameter says how long the address
  is valid

31
The SIP Content-Type header

• Content-Type application/sdp
• Says how to interpret the message-body
• Similar to mime-type in http or email

32
The SIP Content-Length header

• Content-Length 132
• The message body for the purposes of transport is
  just a stream of bytes
• Content-Length says how many bytes

33
Basic SIP Trapezoid
domain1 Proxy
domain2 Proxy
user_at_domain1
user_at_domain2
34
Basic SIP trapezoid
alice
Proxy(atlanta.com)
bob
Proxy(biloxi.com)
REGISTER/200
REGISTER/200
INVITE
INVITE
100
INVITE
100
100
180 Ringing
180 Ringing
180 Ringing
200 OK
200 OK
200 OK
ACK
Media flow
BYE
200 OK
alice_at_atlanta.com calls bob_at_biloxi.com
35
Redirect flow
alice
Proxy(atlanta.com)
jane
REGISTER/200
REGISTER/200
INVITE
100
301
ACK
INVITE
180 Ringing
200 OK
ACK
Media flow
alice_at_atlanta.com calls jane_at_atlanta.com
36
State constructs

• Transaction
• The branch id in the Via header
• OR the number and Method name in the Cseq header
• State from Request to final Response
• Dialog(persists beyond single request-response)
• UAS Call-ID, remote-tagto-tag,
  local-tagfrom-tag
• UAC Call-ID, remote-tagfrom-tag,
  local-tagto-tag
• State elements
• Local sequence number
• Remote sequence number
• Route set
• Other stuff
• Call/Session
• 1 or more dialogs

37
Registration example from 3261

• Bob(bob_at_biloxi.com)s UA registers with the
  biloxi.com registrar/proxy

38
Registration request
REGISTER sipregistrar.biloxi.com SIP/2.0 Via
SIP/2.0/UDP bobspc.biloxi.com5060branchz9hG4bKn
ashds7 Max-Forwards 70 To Bob
ltsipbob_at_biloxi.comgt From Bob
ltsipbob_at_biloxi.comgttag456248 Call-ID
843817637684230_at_998sdasdh09 CSeq 1826 REGISTER
Contact ltsipbob_at_192.0.2.4gt Expires 7200
Content-Length 0
39
Registration response
SIP/2.0 200 OK Via SIP/2.0/UDP
bobspc.biloxi.com5060branchz9hG4bKnashds7
received192.0.2.4 To Bob ltsipbob_at_biloxi.comgt
tag2493k59kd From Bob ltsipbob_at_biloxi.comgttag
456248 Call-ID 843817637684230_at_998sdasdh09
CSeq 1826 REGISTER Contact ltsipbob_at_192.0.2.4gt
Expires 7200 Content-Length 0
40
Session example from RFC 3261

• Alice(alice_at_atlanta.com) calls Bob(bob_at_biloxi.com)
  
• Thru two proxies P(atlanta.com), and P(biloxi.com)

41
Alice ? Alices proxy
INVITE sipbob_at_biloxi.com SIP/2.0 Via
SIP/2.0/UDP pc33.atlanta.combranchz9hG4bKnashds8
Max-Forwards 70 To Bob ltsipbob_at_biloxi.comgt
From Alice ltsipalice_at_atlanta.comgttag192830177
4 Call-ID a84b4c76e66710 CSeq 314159 INVITE
Contact ltsipalice_at_pc33.atlanta.comgt
Content-Type application/sdp Content-Length
142 (Alice's SDP not shown)
42
Alices Proxy ? Alice
SIP/2.0 100 Trying Via SIP/2.0/UDP
pc33.atlanta.combranchz9hG4bKnashds8
received192.0.2.1 To Bob ltsipbob_at_biloxi.comgt
From Alice ltsipalice_at_atlanta.comgttag192830177
4 Call-ID a84b4c76e66710 CSeq 314159 INVITE
Content-Length 0
43
Alices Proxy ? Bobs Proxy
INVITE sipbob_at_biloxi.com SIP/2.0 Via
SIP/2.0/UDP bigbox3.site3.atlanta.combranchz9hG4
bK77ef4c2312983.1 Via SIP/2.0/UDP
pc33.atlanta.combranchz9hG4bKnashds8
received192.0.2.1 Max-Forwards 69 To Bob
ltsipbob_at_biloxi.comgt From Alice
ltsipalice_at_atlanta.comgttag1928301774 Call-ID
a84b4c76e66710 CSeq 314159 INVITE Contact
ltsipalice_at_pc33.atlanta.comgt Content-Type
application/sdp Content-Length 142 (Alice's SDP
not shown)
44
Bobs Proxy ? Alices Proxy
SIP/2.0 100 Trying Via SIP/2.0/UDP
bigbox3.site3.atlanta.combranchz9hG4bK77ef4c2312
983.1 received192.0.2.2 Via SIP/2.0/UDP
pc33.atlanta.combranchz9hG4bKnashds8
received192.0.2.1 To Bob ltsipbob_at_biloxi.comgt
From Alice ltsipalice_at_atlanta.comgttag192830177
4 Call-ID a84b4c76e66710 CSeq 314159 INVITE
Content-Length 0
45
Bobs Proxy to Bob
INVITE sipbob_at_192.0.2.4 SIP/2.0 Via
SIP/2.0/UDP server10.biloxi.combranchz9hG4bK4b43
c2ff8.1 Via SIP/2.0/UDP bigbox3.site3.atlanta.co
mbranchz9hG4bK77ef4c2312983.1
received192.0.2.2 Via SIP/2.0/UDP
pc33.atlanta.combranchz9hG4bKnashds8
received192.0.2.1 Max-Forwards 68 To Bob
ltsipbob_at_biloxi.comgt From Alice
ltsipalice_at_atlanta.comgttag1928301774 Call-ID
a84b4c76e66710 CSeq 314159 INVITE Contact
ltsipalice_at_pc33.atlanta.comgt Content-Type
application/sdp
46
Bob to Bobs Proxy
SIP/2.0 180 Ringing Via SIP/2.0/UDP
server10.biloxi.combranchz9hG4bK4b43c2ff8.1
received192.0.2.3 Via SIP/2.0/UDP
bigbox3.site3.atlanta.combranchz9hG4bK77ef4c2312
983.1 received192.0.2.2 Via SIP/2.0/UDP
pc33.atlanta.combranchz9hG4bKnashds8
received192.0.2.1 To Bob ltsipbob_at_biloxi.comgt
taga6c85cf From Alice ltsipalice_at_atlanta.comgtt
ag1928301774 Call-ID a84b4c76e66710 Contact
ltsipbob_at_192.0.2.4gt CSeq 314159 INVITE
Content-Length 0
47
Bobs proxy to Alices Proxy
SIP/2.0 180 Ringing Via SIP/2.0/UDP
bigbox3.site3.atlanta.combranchz9hG4bK77ef4c2312
983.1 received192.0.2.2 Via SIP/2.0/UDP
pc33.atlanta.combranchz9hG4bKnashds8
received192.0.2.1 To Bob ltsipbob_at_biloxi.comgt
taga6c85cf From Alice ltsipalice_at_atlanta.comgtt
ag1928301774 Call-ID a84b4c76e66710 Contact
ltsipbob_at_192.0.2.4gt CSeq 314159 INVITE
Content-Length 0
48
Alices Proxy to Alice
SIP/2.0 180 Ringing Via SIP/2.0/UDP
pc33.atlanta.combranchz9hG4bKnashds8
received192.0.2.1 To Bob ltsipbob_at_biloxi.comgt
taga6c85cf From Alice ltsipalice_at_atlanta.comgtt
ag1928301774 Call-ID a84b4c76e66710 Contact
ltsipbob_at_192.0.2.4gt CSeq 314159 INVITE
Content-Length 0
49
Bob to Bobs proxy
SIP/2.0 200 OK Via SIP/2.0/UDP
server10.biloxi.combranchz9hG4bK4b43c2ff8.1
received192.0.2.3 Via SIP/2.0/UDP
bigbox3.site3.atlanta.combranchz9hG4bK77ef4c2312
983.1 received192.0.2.2 Via SIP/2.0/UDP
pc33.atlanta.combranchz9hG4bKnashds8
received192.0.2.1 To Bob ltsipbob_at_biloxi.comgt
taga6c85cf From Alice ltsipalice_at_atlanta.comgtt
ag1928301774 Call-ID a84b4c76e66710 CSeq
314159 INVITE Contact ltsipbob_at_192.0.2.4gt
Content-Type application/sdp Content-Length
131
50
Bobs proxy to Alices proxy
SIP/2.0 200 OK Via SIP/2.0/UDP
bigbox3.site3.atlanta.combranchz9hG4bK77ef4c2312
983.1 received192.0.2.2 Via SIP/2.0/UDP
pc33.atlanta.combranchz9hG4bKnashds8
received192.0.2.1 To Bob ltsipbob_at_biloxi.comgt
taga6c85cf From Alice ltsipalice_at_atlanta.comgtt
ag1928301774 Call-ID a84b4c76e66710 CSeq
314159 INVITE Contact ltsipbob_at_192.0.2.4gt
Content-Type application/sdp Content-Length
131 (Bob's SDP not shown)
51
Alices proxy to Alice
SIP/2.0 200 OK Via SIP/2.0/UDP
pc33.atlanta.combranchz9hG4bKnashds8
received192.0.2.1 To Bob ltsipbob_at_biloxi.comgt
taga6c85cf From Alice ltsipalice_at_atlanta.comgtt
ag1928301774 Call-ID a84b4c76e66710 CSeq
314159 INVITE Contact ltsipbob_at_192.0.2.4gt
Content-Type application/sdp Content-Length
131 (Bob's SDP not shown)
52
Alice to Bob
ACK sipbob_at_192.0.2.4 SIP/2.0 Via SIP/2.0/UDP
pc33.atlanta.combranchz9hG4bKnashds9 Max-Forward
s 70 To Bob ltsipbob_at_biloxi.comgttaga6c85cf
From Alice ltsipalice_at_atlanta.comgttag192830177
4 Call-ID a84b4c76e66710 CSeq 314159 ACK
Content-Length 0
53
Bob hangs up(Bob to Alice)
BYE sipalice_at_pc33.atlanta.com SIP/2.0 Via
SIP/2.0/UDP 192.0.2.4branchz9hG4bKnashds10
Max-Forwards 70 From Bob ltsipbob_at_biloxi.comgt
taga6c85cf To Alice ltsipalice_at_atlanta.comgttag
1928301774 Call-ID a84b4c76e66710 CSeq 231
BYE Content-Length 0
54
Alice to Bob
SIP/2.0 200 OK Via SIP/2.0/UDP
192.0.2.4branchz9hG4bKnashds10 From Bob
ltsipbob_at_biloxi.comgttaga6c85cf To Alice
ltsipalice_at_atlanta.comgttag1928301774 Call-ID
a84b4c76e66710 CSeq 231 BYE Content-Length 0
55
Key takeaways

• INVITE transaction has some special cases, other
  than that all other requests are the same
• A proxy should be able to route a new SIP request
  without specifically knowing anything about it

56
SIP Authentication

• Basic/Digest, only Digest recommended
• Follows HTTP model
• Hop by Hop, limits use
• UAC sends request, UAS rejects with a 401
  Authorization Required with a WWW-Authenticate
  header
• UAC resends request with appropriate
  credentials(Authentication-Info header)

57
Authentication flow
alice
Proxy(atlanta.com)
REGISTER/200
INVITE
100
Transaction ends
407
ACK
INVITE(credentials)
58
Reading Exercise

• RFC 3665Sections 2.22.53.33.43.73.83.9