SIP Tutorial/Workshop 1

SIP Basics
MÉSZÁROS Mihály
NIIF Budapest
03/16/10

Nemzeti Információs Infrastruktúra Fejlesztési Intézet
Slide 2 SIP Shop
IETF Working Groups
AVT
(RTP, SRTP, media)
ENUM
(E.164 lookup)
SIMPLE
(presence)
GEOPRIV
(geo + privacy)
IETF RAI
MMUSIC
(SDP, RTSP, ICE)
XCON
(konf. kontrol)
SPEERMINT
(peering)
SPEECHSC
(Speech Services)
BLISS
(interoperability)
MEDIACTRL
(media server)
P2PSIP
(Okos kliens)
SIP Core
(protocol)
ECRIT
(emergency)
Dispatch
DRINKS
XMPP

Slide 3 SIP Shop
SIP Short History
• Simple Conference Invitation Protocol
 Feb. 22, 1996 first drafts
• RFC 2543
 March 17, 1999 First approved RFC
• RFC 3261
 July 3, 2002 new RFC obsoleting 2543
• More details:
 http://www.cs.columbia.edu/sip/history.html

Slide 4 SIP Shop
Despite first draft name, currently the 'S' in SIP acronym
is no more for Simple!
• http://www.rfc3261.net/

Slide 5 SIP Shop
SIP Basics Concepts
• HTTP, SMTP like protocol over IPv4, IPv6 and IP Multicast
• SIP is a text-based protocol and uses the UTF-8 charset
• Distributed end2end design similar to Internet design
• Almost all intelligence in the endpoint
• Network use and know very basic information. Almost
only transaction state and routing information.
• Outcome:
 Flexibility
 New service can introduced without changing the network
 Scalability
 Network knows about only basic state information => network are
using less resource.
 Recovery from failure
 Basic small information can recovered more faster.
 Relatively expensive endpoints

Slide 6 SIP Shop
SIP functionality
• User Location
 determination of the end system to be used for communication
 Registrar
 DNS (RFC3263)
• User availability
 determination of the end system to be used for communication
 Location service (some kind of basic presence)
 OPTION method (ping)
• Session setup
 INVITE
• Session management
 ReINVITE
 retraget
 codec change (upgrade voice call to video + voice)
 hold
 change media ip address
 Tear down (Bye, Cancel)

Slide 7 SIP Shop
SIP Trapezoid
PROXY
2
PROXY
1
UA 2UA 1
Domain BDomain A

Slide 8 SIP Shop
RFC3261 example
• http://www.tech-invite.com/Ti-sip-ex3261.html

Slide 9 SIP Shop
Wireshark - Live sample capture, call flow

Slide 10 SIP Shop
Transaction
• request
• zero or more provisional (1XX)
response
• one final
 2XX
 3XX
 4XX
 5XX
 6XX
• If method is INVITE then ACK
 part of transaction if status code is
“negative” final (>=3XX)
 generated by transaction
 not part of transaction if status
code is “positive” final (2XX)
 generated by UA
• Transaction ID
 Via header, branch ID parameter

Slide 11 SIP Shop
Dialog (sequence of transactions)
• Dialog initiator methods
 INVITE,SUBSCRIBE,REFER
• peer-to peer relationship
between two UA
• sequencing (CSeq)
• dialog state
 dialog ID
 Call-ID
 local tag (From)
 remote tag (To)
 local sequence
 remote sequence
 local URI
 remote URI
 remote target(contact)
 secure flag (Boolean)
 Route-set
 ordered URI list

Slide 12 SIP Shop
Terminology (RFC3261)
• User Agent
• Servers
 Forwarding Proxy
 Registrar
 Redirect Server

Slide 13 SIP Shop
UA classification
• Equipment
 Soft Phone
 Hard Phone
• Media capability
 VoIP
 Audio only
 Videoconference, Telepresence
 audio and video
• Supported features
 Presence
 body list
 Presentation,
 second video stream
 screen sharing
 FECC
 Text messaging, chat
 File Transfer source: http://www.denphone.com/files/cisco-telepresence-1000.jpg

Slide 14 SIP Shop
User Agent
• User Agent (UA)
 UA Client (UAC): is
a logical entity that
creates a new
request, and then
uses the client
transaction state
machinery to send it.
 UA Server (UAS): is
a logical entity that
generates a response
to a SIP request.
UAUAS UAC
Network
(IP/IPv6)
Transport
(UDP,TCP,TLS,SCTP..)
Physical
Data Link

Slide 15 SIP Shop
SIP Trapezoid
PROXY
2
PROXY
1
UA 2UA 1
Domain BDomain A

Slide 16 SIP Shop
Transaction relationships
+---------+ +---------+ +---------+ +---------+
| +-+|Request |+-+ +-+|Request |+-+ +-+|Request |+-+ |
| |C||------->||S| |C||------->||S| |C||------->||S| |
| |l|| ||e| |l|| ||e| |l|| ||e| |
| |i|| ||r| |i|| ||r| |i|| ||r| |
| |e|| ||v| |e|| ||v| |e|| ||v| |
| |n|| ||e| |n|| ||e| |n|| ||e| |
| |t|| ||r| |t|| ||r| |t|| ||r| |
| | || || | | || || | | || || | |
| |T|| ||T| |T|| ||T| |T|| ||T| |
| |r|| ||r| |r|| ||r| |r|| ||r| |
| |a|| ||a| |a|| ||a| |a|| ||a| |
| |n|| ||n| |n|| ||n| |n|| ||n| |
| |s||Response||s| |s||Response||s| |s||Response||s| |
| +-+|<-------|+-+ +-+|<-------|+-+ +-+|<-------|+-+ |
+---------+ +---------+ +---------+ +---------+
UAC Outbound Inbound UAS
Proxy Proxy

Slide 17 SIP Shop
User Agent Graph (Fork example)
Source: www.iptel.org

SIP Shop
Transaction – Dialog - Session
• SIP Transaction: occurs between a client and a server
and comprises all messages from the first request sent
from the client to the server up to a final (non-1xx)
response sent from the server to the client. If the request
is INVITE and the final response is a non-2xx, the
transaction also includes an ACK to the response. The ACK
for a 2xx response to an INVITE request is a separate
transaction.
• Dialog: is a peer-to-peer SIP relationship between two UAs
that persists for some time. A dialog is established by SIP
messages, such as a 2xx response to an INVITE request. A
dialog is identified by a call identifier, local tag, and a
remote tag.
• Session:A multimedia session is a set of multimedia
senders and receivers and the data streams flowing from
senders to receivers.

Slide 19 SIP Shop
Structure of the Protocol
• TCP/IP
 Application Layer
• OSI
 Session Layer
• SIP is an application layer
protocol in TCP/IP reference
model
• SIP Structure:
 Transaction User (TU)
 Proxy core
 UA Core
 Transaction
 Transport
 SIP syntax and encoding
• Good help to understand SIP
Structure on tech-invite web
site.
 SIP Protocol Structure through an
Example
http://www.tech-invite.com/Ti-sip-archi.html
image source:http://www.hardwaresecrets.com/imageview.php?image=6731

Slide 20 SIP Shop
Terminology - Servers
• Proxy, Proxy Server: An intermediary entity that acts as
both a server and a client for the purpose of making
requests on behalf of other clients. A proxy server
primarily plays the role of routing, which means its job is to
ensure that a request is sent to another entity "closer" to
the targeted user. Proxies are also useful for enforcing
policy (for example, making sure a user is allowed to make
a call). A proxy interprets, and, if necessary, rewrites
specific parts of a request message before forwarding it.
• Redirect Server: A redirect server is a user agent server
that generates 3xx responses to requests it receives,
directing the client to contact an alternate set of URIs.
• Registrar: is a server that accepts REGISTER requests and
places the information it receives in those requests into the
location service for the domain it handles.

Slide 21 SIP Shop
Terminology - Location Service
• Location Service:A location service is used by a SIP
redirect or proxy server to obtain information about a
callee's possible location(s). It contains a list of
bindings of address-of-record keys to zero or more
contact addresses.
 Registrar is storing bindings in this DB service.
 Forwarding Proxy is utilizing the DB to make
routing decisions.
 Redirect Server using this DB to redirect UA.
Proxy, Registrar,
Redirect Server
Location Service
Database

Slide 22 SIP Shop
Terminology - Proxy
• Stateless:
is a logical entity that does not maintain the client or
server transaction state machines defined in this
specification when it processes requests. A stateless
proxy forwards every request it receives downstream
and every response it receives upstream.
• Transaction Stateful, Stateful Proxy:
is a logical entity that maintains the client and server
transaction state machines defined by this specification
during the processing of a request, also known as a
transaction stateful proxy.
• Call Stateful(!):
A proxy is call stateful if it retains state for a dialog from
the initiating INVITE to the terminating BYE request. A
call stateful proxy is always transaction stateful, but the
converse is not necessarily true.

Slide 23 SIP Shop
SIP Architecture
Source:www.tech-invite.com

Slide 24 SIP Shop
Identifying endpoints
• URI:
 Uniform Resource Identifier (URI) consists of a string of
characters used to identify or name a resource on the
Internet.
• UA URI schemes
 sip/sips:
 sip:alice@atlanta.test / sips:alice@atlanta.test
 Tel (E.164):
 tel:+3614503060
 The tel: uri scheme has no “user” or “domain” part!
• ISN/ITAD
 1234*256

Slide 25 SIP Shop
Terminology - AoR
• Address-of-Record: An address-of-record
(AOR) is a SIP or SIPS URI that points to a
domain with a location service that can map the
URI to another URI where the user might be
available. Typically, the location service is
populated through registrations. An AOR is
frequently thought of as the "public address" of
the user.
 Weel known uri
 sip:misi@niif.hu

Slide 26 SIP Shop
Message
generic-message = start-line
*message-header
CRLF
[ message-body ]
Start-line = Request-Line / Status-Line

Slide 27 SIP Shop
SIP Methods
Methods Reference
ACK [RFC3261]
BYE [RFC3261]
CANCEL [RFC3261]
INFO [RFC2976]
INVITE [RFC3261]
MESSAGE [RFC3428]
NOTIFY [RFC3265]
OPTIONS [RFC3261]
PRACK [RFC3262]
PUBLISH [RFC3903]
REFER [RFC3515]
REGISTER [RFC3261]
SUBSCRIBE [RFC3265]
UPDATE [RFC3311]

Slide 28 SIP Shop
SIP request message
INVITE sip:bob@biloxi.example.com SIP/2.0
Via: SIP/2.0/TCP client.atlanta.example.com:5060
;branch=z9hG4bK74bf9
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>
;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Contact: <sip:alice@client.atlanta.example.com
;transport=tcp>
Content-Type: application/sdp
Content-Length: 151
v=0
o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
s=-
c=IN IP4 192.0.2.101
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000

Slide 29 SIP Shop
SIP Response message
SIP/2.0 200 OK
Via: SIP/2.0/TCP client.atlanta.example.com:5060
;branch=z9hG4bK74bf9
;received=192.0.2.101
From: Alice <sip:alice@atlanta.example.com>
;tag=9fxced76sl
;tag=8321234356
CSeq: 1 INVITE
Contact: <sip:bob@client.biloxi.example.com;transport=tcp>
Content-Length: 147
v=0
o=bob 2890844527 2890844527 IN IP4
client.biloxi.example.com
s=-
c=IN IP4 192.0.2.201
t=0 0
m=audio 3456 RTP/AVP 0
a=rtpmap:0 PCMU/8000

Slide 30 SIP Shop
Minimal request - Request vs. To
• Mandatory fields
 start line
 request/response
 headers
 Via
 Max-Forwards
 From
 To
 Call-ID
 CSeq
• Difference between Request-URI and To header!
 To header
 identifies the original recipient, but maybe URI differ from the UAS
processing the request, due to call forwarding, or any other proxy operation.
 Proxy can't rewrite
 Request-URI
 identifies the UAS that is processing the request
 proxy can rewrite to a new URI what is different from original recipient
OPTION sip:bob@biloxi.example.com SIP/2.0
Via: SIP/2.0/TCP client.atlanta.test:5060;branch=z9hG4bK74bf9
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
CSeq: 1 INVITE

Slide 31 SIP Shop
Framing SIP messages / Transport
• Unreliable datagram protocol (UDP)
 One datagram is containing one message.
 If a request is within 200 bytes of the path MTU, or if it is larger
than 1300 bytes and the path MTU is unknown, the request
MUST be sent using an RFC 2914 congestion controlled transport
protocol, such as TCP.
 If an element sends a request over TCP because of these
message size constraints, and that request would have otherwise
been sent over UDP, if the attempt to establish the connection
generates either an ICMP Protocol Not Supported, or results in a
TCP reset, the element SHOULD retry the request, using UDP.
 If the transport user asks for a message to be sent over an
unreliable transport, and the result is an ICMP error, the
behavior depends on the type of ICMP error. Host, network,
port or protocol unreachable errors, or parameter problem errors
SHOULD cause the transport layer to inform the transport user
of a failure in sending.

SIP Shop
Framing SIP messages / Transport
• All SIP elements MUST implement UDP and TCP!
• Reliable stream protocol (TCP)
 In the case of stream-oriented transports such as TCP,
the Content-Length header field indicates the size of the
body. The Content-Length header field MUST be used
with stream oriented transports.
 If the transport user asks for a request to be sent over
a reliable transport, and the result is a connection
failure, the transport layer SHOULD inform the transport
user of a failure in sending.

SIP Shop
Cseq
• Cseq
 A CSeq header field in a request contains a single
decimal sequence number and the request method.
 The sequence number MUST be expressible as a 32-bit
unsigned integer. The method part of CSeq is case-
sensitive.
 The CSeq header field serves to order transactions
within a dialog, to provide a means to uniquely identify
transactions, and to differentiate between new requests
and request retransmissions. Two CSeq header fields
are considered equal if the sequence number and the
request method are identical.

SIP Shop
Dialog
• Dialog ID
 A dialog is identified at each UA with a dialog ID, which
consists of a Call-ID value, a local tag and a remote
tag.
• Dialog State
 A dialog contains certain pieces of state needed for
further message transmissions within the dialog. This
state consists of the dialog ID, a local sequence
number (used to order requests from the UA to its
peer), a remote sequence number (used to order
requests from its peer to the UA), a local URI, a
remote URI, remote target, a boolean flag called
"secure", and a route set, which is an ordered list of
URIs. The route set is the list of servers that need to be
traversed to send a request to the peer.

Slide 35 SIP Shop
Dialog example
• http://www.tech-invite.com/Ti-sip-dialog.html

SIP Shop
Record-Route
• Route Set definition:
 A route set is a collection of ordered SIP or SIPS URI
which represent a list of proxies that must be traversed
when sending a particular request. A route set can be
learned, through headers like Record-Route, or it can be
configured.
• Dialog
 The route set MUST be set to the list of URIs in the
Record-Route header field from the request, taken in
order and preserving all URI parameters.
 If no Record-Route header field is present in the
request, the route set MUST be set to the empty set.
 This route set, even if empty, overrides any pre-existing
route set for future requests in this dialog.

Slide 37 SIP Shop
SIP Methods
• RFC3261
 INVITE
 ACK
 OPTIONS
 BYE
 CANCEL
 REGISTER
• RFC2976
 INFO
• RFC3262
 PRACK
• RFC3265
 SUBSCRIBE
 NOTIFY
• RFC3311
 UPDATE
• RFC3428
 MESSAGE
• RFC3515
 REFER
• RFC3903
 PUBLISH

Slide 38 SIP Shop
SIP Response (Status Codes)
• 1XX Provisional
 100 Trying
 180 Ringing
 183 Session Progress
• 2XX Successful
 200 OK
• 3XX Redirection
 300 Multiple Choices
 301 Moved Permanently
 302 Moved Temporarily
• 4XX Request Failure
 400 Bad Request
 401 Unauthorized
 403 Forbidden
 404 Not Found
 etc.
• 5XX Server Failure
 500 Server Internal Error
 501 Not Implemented
 etc.
• 6XX Global Failure
 600 Busy Everywhere
 etc.

Slide 39 SIP Shop
SIP Trapezoid
PROXY
2
PROXY
1
UA 2UA 1
Domain BDomain A

Slide 40 SIP Shop
SIP Trapezoid II.
atlanta.com . . . biloxi.com
. proxy proxy .
. .
Alice's . . . . . . . . . . . . . . . . . . . . Bob's
softphone SIP Phone
| | | |
| INVITE F1 | | |
|--------------->| INVITE F2 | |
| 100 Trying F3 |--------------->| INVITE F4 |
|<---------------| 100 Trying F5 |--------------->|
| |<-------------- | 180 Ringing F6 |
| | 180 Ringing F7 |<---------------|
| 180 Ringing F8 |<---------------| 200 OK F9 |
|<---------------| 200 OK F10 |<---------------|
| 200 OK F11 |<---------------| |
|<---------------| | |
| ACK F12 |
|------------------------------------------------->|
| Media Session |
|<================================================>|
| BYE F13 |
|<-------------------------------------------------|
| 200 OK F14 |
|------------------------------------------------->|
| |

Slide 41 SIP Shop
Via, Routing
• Via
 The Via header field indicates the path taken by the
request so far and indicates the path that should be
followed in routing responses. The branch ID parameter
in the Via header field values serves as a transaction
identifier, and is used by proxies to detect loops.
• Routing
 Strict Routing (deprecated)
 Request URI always contain the next hop URI. (the next
hop proxy or a UA) Every hop is overwriting this with next
hop.
 Loose Routing (strongly preferred)
 Request URI always contain the destination User Agent URI

Slide 42 SIP Shop
Traversing a Strict-Routing Proxy (RFC3261)
In this scenario, a dialog is established across four proxies, each of
which adds Record-Route header field values.
The third proxy implements the strict-routing procedures specified
in RFC 2543 and many works in progress.
U1->P1->P2->P3->P4->U2
The INVITE arriving at U2 contains:
INVITE sip:callee@u2.domain.com SIP/2.0
Contact: sip:caller@u1.example.com
Record-Route: <sip:p4.domain.com;lr>
Record-Route: <sip:p3.middle.com>
Record-Route: <sip:p2.example.com;lr>
Record-Route: <sip:p1.example.com;lr>

Slide 43 SIP Shop
Traversing a Strict-Routing Proxy II.
Which U2 responds to with a 200 OK. Later, U2 sends the following BYE request
to P4 based on the first Route header field value.
BYE sip:caller@u1.example.com SIP/2.0
Route: <sip:p4.domain.com;lr>
Route: <sip:p3.middle.com>
Route: <sip:p2.example.com;lr>
P4 is not responsible for the resource indicated in the Request-URI so it will
leave it alone. It notices that it is the element in the first Route header field
value so it removes it. It then prepares to send the request based on the now
first Route header field value of sip:p3.middle.com, but it notices that this URI
does not contain the lr parameter, so before sending, it reformats the request to
be:
BYE sip:p3.middle.com SIP/2.0
Route: <sip:caller@u1.example.com>

Slide 44 SIP Shop
Traversing a Strict-Routing Proxy III.
P3 is a strict router, so it forwards the following to P2:
BYE sip:p2.example.com;lr SIP/2.0
Route: <sip:caller@u1.example.com>
P2 sees the request-URI is a value it placed into a Record-Route header field, so
before further processing, it rewrites the request to be:
P2 is not responsible for u1.example.com, so it sends the request to P1 based on
the resolution of the Route header field value.
P1 notices itself in the topmost Route header field value, so it removes it,
resulting in:
Since P1 is not responsible for u1.example.com and there is no Route header
field, P1 will forward the request to u1.example.com based on the Request-URI.

Slide 45 SIP Shop
Fork
• Parallel search (fork)
 http://www.tech-invite.com/Ti-sip-service-12.html
• Sequential search
 http://www.tech-invite.com/Ti-sip-service-13.html

Slide 46 SIP Shop
Options
• The SIP method OPTIONS allows a UA to query another UA or a proxy server as
to its capabilities. This allows a client to discover information about the
supported methods, content types, extensions, codecs, etc. without "ringing"
the other party.
• That is, a 200 (OK) would be returned if the UAS is ready to accept a call, a 486
(Busy Here) would be returned if the UAS is busy, etc. This allows an OPTIONS
request to be used to determine the basic state of a UAS, which can be an
indication of whether the UAS will accept an INVITE request.
• A forked OPTIONS will only result in a single 200 (OK) response.
OPTIONS sip:carol@chicago.com SIP/2.0
Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKhjhs8ass877
Max-Forwards: 70
To: <sip:carol@chicago.com>
From: Alice <sip:alice@atlanta.com>;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 63104 OPTIONS
Contact: <sip:alice@pc33.atlanta.com>
Accept: application/sdp
Content-Length: 0
SIP/2.0 200 OK
Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKhjhs8ass877
To: <sip:carol@chicago.com>;tag=93810874
From: Alice <sip:alice@atlanta.com>;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 63104 OPTIONS
Contact: <sip:carol@chicago.com>
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE
Accept: application/sdp
Accept-Encoding: gzip
Accept-Language: en
Supported: foo
Content-Length: 274
(SDP not shown)

SIP Shop
ACK
• General
 Cseq equal the requests being acknowledged
 The ACK MUST contain a single Via header field, and this MUST be
equal to the top Via header field of the original request.
 Servers MUST NOT attempt to challenge an ACK
 Different handling of Successful and Failure ACK!!
• 2xx
 end-to-end
 ACK is a new transaction
 Body presents in delayed offer case.
 2xx retransmission ACK generation is handled by UA core.
• 3xx-6xx
 hop-by-hop
 ACK belongs the transaction created by the request
 Placement of bodies in ACK for non-2xx is NOT RECOMMENDED.
 Retransmission is handled by transaction layer.

SIP Shop
Cancel
• Cancel in progrss request
• A UAS that receives a CANCEL request for an INVITE, but has not
yet sent a final response, would "stop ringing", and then respond
to the INVITE with a specific error response (a 487).
• CSeq equal the requests being.
• CANCEL is best for INVITE requests, which can take a long time to
generate a response.
• If no provisional response has been received, the CANCEL request
MUST NOT be sent; rather, the client MUST wait for the arrival of a
provisional response before sending the request.
• A stateful proxy responds to a CANCEL, rather than simply
forwarding a response it would receive from a downstream
element. For that reason, CANCEL is referred to as a "hop-by-hop"
request, since it is responded to at each stateful proxy hop.
• hop-by-hop and cannot be resubmitted, cannot be challenged.
• CANCEL request SHOULD be accepted by a server if it comes from
the same hop that sent the request being canceled.

SIP Shop
Transaction Layer
• FSM (Finite State
Machine)
 Client INVITE
 Client non-INVITE
 Server INVITE
 Server non-INVITE
• Look for all state
machines in RFC3261
• Timers
 T1,T2,T4
 Timer A,B,C,D,E,F,G,H,J,K
 Recommended default
values
 timeout, retransmit
|INVITE from TU
Timer A fires |INVITE sent
Reset A, V Timer B fires
INVITE sent +-----------+ or Transport Err.
+---------| |---------------+inform TU
| | Calling | |
+-------->| |-------------->|
+-----------+ 2xx |
| | 2xx to TU |
| |1xx |
300-699 +---------------+ |1xx to TU |
ACK sent | | |
resp. to TU | 1xx V |
| 1xx to TU -----------+ |
| +---------| | |
| | |Proceeding |-------------->|
| +-------->| | 2xx |
| +-----------+ 2xx to TU |
| 300-699 | |
| ACK sent, | |
| resp. to TU| |
| | | NOTE:
| 300-699 V |
| ACK sent +-----------+Transport Err. | transitions
| +---------| |Inform TU | labeled with
| | | Completed |-------------->| the event
| +-------->| | | over the action
| +-----------+ | to take
| ^ | |
| | | Timer D fires |
+--------------+ | - |
| |
V |
+-----------+ |
| | |
| Terminated|<--------------+
| |
+-----------+

SIP Shop
Authentication
• Basic authentication obsoleted and is not allowed
• Modified HTTP digest (MD5)
 How it works?
 HA1=MD5(A1)=MD5(username:relam:password)
 HA2=MD5(A2)=MD5(method:digestURI)
 response=(HA1:nonce:HA2)
 The password on AAA server can be stored only in from of clear text
or HA1 hash.
• ACK
 Problems come up for any requests that take no response, including
ACK. Any credentials in the INVITE that were accepted by a server
MUST be accepted by that server for the ACK.
 Servers MUST NOT attempt to challenge an ACK
• Cancel
 Although the CANCEL method does take a response (a 2xx), servers
MUST NOT attempt to challenge CANCEL requests since these
requests cannot be resubmitted.

SIP Shop
Auth & Identity
• http://www.tech-invite.com/Ti-sip-identity.html

SIP Shop
Locating endpoint
• How to find an UA from AoR?
• Location lookup sources
 Location Service / Location database
 Register
 used by proxies
 DNS
 RFC3263
 SRV, NAPTR
 Can be used by UA or proxy
 E164 URI mapping (ENUM)
 NAPTR
 Can be used by UA or Proxy
 Local database / Local policy
 Alias
 Local rewrite policy (normalization)
 white list / black list
 etc.

SIP Shop
Registration (binding)
• Registration creates bindings in a location service for a
particular domain that associates an address-of-record URI
with one or more contact addresses.
• expires: The "expires" parameter indicates how long the UA
would like the binding to be valid. The value is a number
indicating seconds.
• REGISTER requests add, remove, and query bindings.
 Add One or more contact header
example: Contact: sip:misi@alma.ki.niif.hu
 Expire is not 0
example: expire: 3600
 q parameter (default 1)
 Remove expire: 0
 Remove all expire: 0 + Contact: “*”
 Query A success response to any REGISTER request contains the
complete list of existing bindings, regardless of whether the request
contained a Contact header field. If no Contact header field is present in
a REGISTER request, the list of bindings is left unchanged.

SIP Shop
Registration example message
• http://www.tech-invite.com/Ti-sip-ex3261.html#figa

Slide 56 SIP Shop
SIP Trapezoid
PROXY
2
PROXY
1
UA 2UA 1
Domain BDomain A
DNS

Slide 57 SIP Shop
RFC3263
• From uri domain part
discovery of transport
protocol and socket
informations
 transport
 ip
 port
• Defaults
 default port: 5060
 default transport: udp
• DNS
 NATPR
 DDS
 SRV
 A/AAA
source:www.tekelc.com
SIP Pocket Guide
page17

Slide 58 SIP Shop
DNS SRV, NAPTR
• bind
 named-checkconf
• Dynamic Delegation Discovery System (DDDS)
 rfc3401, rfc3402, rfc3403, rfc3404
 Naming Authority Pointer (NAPTR) Resource Record (RR)
 key domain-names
• Example picece from DNS ZONE file
 NAPTR
;; order pref flags service regexp replacement
atlanta.test IN NAPTR 10 50 "s" "SIPS+D2T" "" _sips._tcp
atlanta.test IN NAPTR 30 50 "s" "SIP+D2U" "" _sip._udp
 SRV
;; priority Weight Port Target
_sip._tcp IN SRV 10 0 5060 a1
_sip._udp IN SRV 10 0 5060 a1

Slide 59 SIP Shop
DDDS Algorithm RFC3402
1. The First Well Known Rule is applied to the Application Unique
String which produces a Key.
2. The Application asks the Database for the ordered set of
Rules that are bound to that Key (see NOTE below on order
details).
3. The Substitution Expression for each Rule in the list is
applied, in order, to the Application Unique String until a non-
empty string is produced. The position in the list is noted and
the Rule that produced the non-empty string is used for the
next step. If the next step rejects this rule and returns to
this step then the Substitution Expression application process
continues at the point where it left off. If the list is
exhausted without a valid match then the application is
notified that no valid output was available.
4. If the Service description of the rule does not meet the
client's requirements, go back to step 3 and continue through
the already retrieved list of rules. If it does match the client's
requirements then this Rule is used for the next step. If and
only if the client is capable of handling it and if it is deemed
safe to do so by the Application's specification, the client may
make a note of the current Rule but still return to step 3 as
though it had rejected it. In either case, the output of this
step is one and only one Rule.
5. If the Flags part of the Rule designate that this Rule is NOT
Terminal, go back to step 2 with the substitution result as the
new Key.
6. Notify the Application that the process has finished and
provide the Application with the Flags and Services part of
the Rule along with the output of the last Substitution
Expression.
+--------- Application Unique String
| +-----+
| |input|
| +-------+ +---------+
| | First Well Known Rule |
| +-------+ +--------+
| |output|
| +------+
| First Key
| |
| +----<--------------<--------------+
| | |
| key (a DDDS database always |
| +-----+ takes a key and returns |
| |input| a rule) ^
| +---------+ +------------+ |
| | Lookup key in DDDS Database| |
| +---------+ +-----------+ |
| |output| |
| +------+ |
| rule set |
| | |
| | (the input to a rule |
| rule set is the rule and the AUS. ^
| +-----+ The output is always |
+---------------->|input| either a key or the result) |
+---------------+ +------------------+ |
| Apply Rules to Application Unique String| |
| until non-empty result are obtained | |
| that meet the applications requirements | |
+---------------+ +-----------------+ |
|output| |
+------+ ^
key |
| |
v |
+--------------------------------------+ |
| Was the last matching rule terminal? | No >------+
+--------------------------------------+
Yes (if the rule isn't terminal then
| its output is the new key which
| is used to find a new rule set)
+------------------------------------+
| The output of the last rule is the |
| result desired by the application |
+------------------------------------+

Slide 60 SIP Shop
RFC3263 DDDS
• The following is the information that DDDS
requests an application to provide:
 Application Unique String:
The Application Unique String (AUS) is the input to the
resolution service. For this application, it is the URI to
resolve.
 First Well Known Rule:
The first well known rule extracts a key from the AUS.
For this application, the first well known rule extracts the
host portion of the SIP or SIPS URI.
 Valid Databases:
The key resulting from the first well known rule is looked
up in a single database, the DNS.
 Expected Output:
The result of the application is an SRV record for the
server to contact.

Slide 61 SIP Shop
NAPTR example
atlanta.test IN NAPTR 10 10 "" "SIPS+D2T" "" redirect.atlanta.test
biloxi.test IN NAPTR 10 10 "" "SIPS+D2T" "" redirect.atlanta.test
redirect.atlanta.test IN NAPTR 5 10 "" "SIPS+D2T" "!^(biloxi.test)$!1!" .
redirect.atlanta.test IN NAPTR 10 10 "" "SIPS+D2T" "" redirect2.atlanta.test
redirect2.atlanta.test IN NAPTR 10 10 "s" "SIPS+D2T" "!^.*$!_sip._tcp.1!" .

Slide 62 SIP Shop
NAPTR
•
ORDER
A 16-bit unsigned integer specifying the order in which the NAPTR records MUST be processed
in order to accurately represent the ordered list of Rules. The ordering is from lowest to
highest. If two records have the same order value then they are considered to be the same rule
and should be selected based on the combination of the Preference values and Services offered.
•
PREFERENCE
Although it is called "preference" in deference to DNS terminology, this field is equivalent to the
Priority value in the DDDS Algorithm. It is a 16-bit unsigned integer that specifies the order in
which NAPTR records with equal Order values SHOULD be processed, low numbers being
processed before high numbers. This is similar to the preference field in an MX record, and is
used so domain administrators can direct clients towards more capable hosts or lighter weight
protocols. A client MAY look at records with higher preference values if it has a good reason to
do so such as not supporting some protocol or service very well.
DDS Priority: Simply a number used to show which of two otherwise equal rules may have
precedence. This allows the database to express rules that may offer roughly the same results
but one delegation path may be faster, better, cheaper than the other.
•
The important DIFFERENCE between ORDER and PREFERENCE is that once a match is
found the client MUST NOT consider records with a different Order but they MAY process
records with the same Order but different Preferences.
•
REPLACEMENT field and the REGEXP field together make up the Substitution Expression in
the DDDS Algorithm. It is simply a historical optimization specifically for DNS compression that
this field exists. The fields are also mutually exclusive.

Slide 63 SIP Shop
SRV (RFC2782)
• PRIORITY: A client MUST attempt to contact the target host with the lowest-numbered
priority it can reach; target hosts with the same priority SHOULD be tried in an order
defined by the weight field. The range is 0-65535.
• WEIGHT: The weight field specifies a relative weight for entries with the same priority.
Larger weights SHOULD be given a proportionately higher probability of being selected. The
range of this number is 0-65535.
Domain administrators SHOULD use Weight 0 when there isn't any server selection to
do, to make the RR easier to read for humans (less noisy). In the presence of records
containing weights greater than 0, records with weight 0 should have a very small chance
of being selected.
• Ordering algorithm
In the absence of a protocol whose specification calls for the use of other weighting
information, a client arranges the SRV RRs of the same Priority in the order in which target
hosts, specified by the SRV RRs, will be contacted. The following algorithm SHOULD be
used to order the SRV RRs of the same priority:
To select a target to be contacted next, arrange all SRV RRs (that have not been ordered
yet) in any order, except that all those with weight 0 are placed at the beginning of
the list. Compute the sum of the weights of those RRs, and with each RR associate the
running sum in the selected order. Then choose a uniform random number between 0
and the sum computed (inclusive), and select the RR whose running sum value is the
first in the selected order which is greater than or equal to the random number
selected. The target host specified in the selected SRV RR is the next one to be contacted
by the client. Remove this SRV RR from the set of the unordered SRV RRs and
apply the described algorithm to the unordered SRV RRs to select the next target
host. Continue the ordering process until there are no unordered SRV RRs. This process
is repeated for each Priority.

Slide 64 SIP Shop
RFC3263 (Details of RFC2782 process)
RFC 2782 spells out the details of how a set of SRV records are sorted and
then tried. However, it only states that the client should "try to connect to
the (protocol, address, service)" without giving any details on what
happens in the event of failure. Those details are described here for SIP.
For SIP requests, failure occurs if the transaction layer reports a 503 error
response or a transport failure of some sort (generally, due to fatal ICMP
errors in UDP or connection failures in TCP). Failure also occurs if the
transaction layer times out without ever having received any response,
provisional or final (i.e., timer B or timer F in RFC 3261 [1] fires). If a
failure occurs, the client SHOULD create a new request, which is identical
to the previous, but has a different value of the Via branch ID than the
previous (and therefore constitutes a new SIP transaction). That request is
sent to the next element in the list as specified by RFC 2782.

Slide 65 SIP Shop
RFC3263
• NAPTR
 First, a client resolving a SIPS URI MUST discard any services that do not contain "SIPS" as the
protocol in the service field. The converse is not true, however. A client resolving a SIP URI
SHOULD retain records with "SIPS" as the protocol, if the client supports TLS.
 Second, a client MUST discard any service fields that identify a resolution service whose value is
not "D2X", for values of X that indicate transport protocols supported by the client.
 For NAPTR records with SIPS protocol fields, (if the server is using a site certificate), the domain
name in the query and the domain name in the replacement field MUST both be valid based on
the site certificate handed out by the server in the TLS exchange. Similarly, the domain name in
the SRV query and the domain name in the target in the SRV record MUST both be valid based on
the same site certificate.
 fallback: If no NAPTR records are found, the client constructs SRV queries for those transport
protocols it supports, and does a query for each. Queries are done using the service identifier
"_sip" for SIP URIs and "_sips" for SIPS URIs.
• SRV
 If no SRV records are found, the client SHOULD use TCP for a SIPS URI, and UDP for a SIP URI.

Slide 66 SIP Shop
SRV weigth example
• zone file
• Step0: create a list with result. (Repeat this algorithm foreach priority!)
• Step1: Put RR-s with weight 0 to beginning of the list
• Step2: Sum weight 10+10+20=40
• Step3: Compute running sum associated each record
 a1 0
 a2 1-10
 a3 11-20
 a4 20-40
• Step4: random number between 0- sum(weight) /inclusive!/
• Step5: Find the (lucky) winner add to the end of the final result list
• Step6: Remove the winner from the list
• Step7: If we need the next RR go to step1 until list is not empty.

Slide 67 SIP Shop
DNS Zone Example
$TTL 60
$ORIGIN atlanta.test.
@ IN SOA a1.atlanta.test. root.a1.atlanta.test. (
2009071301 ; Serial
604800 ; Refresh
86400 ; Retry
2419200 ; Expire
604800 ) ; Negative Cache TTL
;
@ IN NS a1
@ IN NS a2
a1 IN A 195.111.158.41
a2 IN A 195.111.158.42
@ IN NAPTR 10 10 "s" "SIPS+D2T" "" _sips._tcp
@ IN NAPTR 20 10 "s" "SIP+D2T" "" _sip._tcp
@ IN NAPTR 30 10 "s" "SIP+D2U" "" _sip._udp
;; Priority Weight Port Target
_sips._tcp IN SRV 10 0 5061 a1
_sips._tcp IN SRV 20 0 5061 a2

Slide 68 SIP Shop
ENUM
(Photo: SWITCH)

Slide 69 SIP Shop
ENUM example
• Steps From E.164 to URI
 +36-1-4503060
 3614503060
 0.6.0.3.0.5.4.1.6.3
 0.6.0.3.0.5.4.1.6.3.e164.arpa
• NAPTR
6.8.4.6.1.1.4.1.6.3 IN NAPTR 10 10 "u" "e2u+sip" "!^(.*)$!sip:1@195.111.104.22!" .

Slide 70 SIP Shop
SUBSCRIBE-NOTIFY
• Subscription to
asynchronous evenet
• Notfiy everytime the
event occurs
• Limited lifetime, must be
preiodically refreshed.
• Usage
 Refer
 Presence
 Voicemail

Slide 71 SIP Shop
SIMPLE (Presence)
+---------------------------+
| PRESENCE SERVICE |
+---------------------------+
^ |
| PRESENCE PROTOCOL |
| v
+------------+ +------------+
| PRESENTITY | | WATCHER |
+------------+ +------------+
^ ^
| |
| |
o +--------------+ +-------------+ o
/| -->| PRESENCE UA | | WATCHER UA |<-- /|
X +--------------+ +-------------+ X
(PRINCIPAL) (PRINCIPAL)

Slide 72 SIP Shop
SIMPLE (Messaging)
+---------------------------+
| INSTANT MESSAGE SERVICE |
+---------------------------+
^ |
IM| INSTANT MESSAGE |IM
| PROTOCOL v
+------------+ +---------------+
| SENDER | | INSTANT INBOX |
+------------+ +---------------+
^ ^
| |
| |
o +-------------+ +------------------+ o
/| -->| SENDER UA | | INBOX UA |<-- /|
X +-------------+ +------------------+ X
(PRINCIPAL) (PRINCIPAL)

Slide 73 SIP Shop
Service Examples RFC3665
• http://www.tech-invite.com/Ti-sip-CF3665.html

Slide 74 SIP Shop
References
• http://www.tech-invite.com/
• http://www.iptel.org/files/sip_tutorial.pdf
• http://tools.ietf.org/html/rfc3261
• http://ftp.iptel.org/pub/ser/0.8.14/doc/html/sip_introduction.html
• http://www.switch.ch
• http://www.voip-info.org
• http://www.cs.columbia.edu/sip/
• http://www.jdrosen.net/
• http://en.wikipedia.org/wiki/Session_Initiation_Protocol
• http://www.iana.org/assignments/sip-parameters
• http://www.crt.realtors.org/projects/rets/variman/support/digest.php
• http://www.rfc3261.net/
• http://www.unc.edu/~cschlatt/docs/SIP_failover_and_redundancy.pdf
• http://nrenum.net/
• http://enumdata.org/
• http://www.iana.org/assignments/enum-services

SIP Tutorial/Workshop 1

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