The Session Initiation Protocol Cheat Sheet by Steve Fowlkes

BASIC SIP CALL SCENARIO
1. The UE sends a request or "­INV­ITE­" to setup session with Proxy-CSCF
2. The Proxy-CSCF forwards this request to the Servin­g-CSCF.
3. The request is routed to the applic­ation servers found in the subscr­ibers profile for service handling.
4. After origin­ating services have been applied the Servii­ng-CSCF queries DNS (ENUM) for the address of the Interr­oga­tin­g-CSCF of the called UE and fowards the request to that Interr­oga­tin­g-CSCF.
5. The I-CSCF then gets the address of the respon­sible S-CSCF from the HSS, and forwards the request to it.
6. The request is given to the applic­ation server that is in the subscr­ibers HSS.
7. All termin­ating services are applied from the applic­ation server, the call is forwarded to the end users P-CSCF.
8. Request forwarded to termin­ating UE, and a voice path is establ­ished.

REGISTER If user device is not regist­ered, then this will be the first request sent. The caller's contact URI (sip:b­ob@­goo­gle.com) will be bound to an Address of Record in the Proxy-CSCF
INVITE If user device is already regist­ered, then this will be the first request sent. Indicates a client is being invited to partic­ipate in a call session. Sent to the Proxy-­CSCF, which forwards the request to the local S-CSCF, which in turn sends it to the 5420 Converged Telephony Server. It checks it's subscriber database for the called party and eventually passes the INVITE to the termin­ating S-CSCF
ACK An acknow­led­gement packet to confirm prior message was received.
BYE Terminates a call and can be sent by either the caller or the callee.
OPTIONS Queries the capabi­lities of connecting servers.
SUBSCRIBE Indicates that a user wishes to hear about changes in an applic­ation service. Examples of such services include automatic
callback services (based on device state events), buddy lists (based on user presence events), message waiting indica­tions (based on mailbox state change events). Ex:"­sip­:st­eve­@AL­U.c­om" would be an approp­riate URI to subscribe to for my presence state; it would also be an approp­riate URI to subscribe to the state of my voice mailbox. To distin­guish which, subscr­ibers MUST include exactly one "­Eve­nt" header in SUBSCRIBE requests.
NOTIFY The updates given to the subscribed user device of any changes or events that happened to the service that they are subscribed to.

SIP Ferries the signaling portion of the call setup and teardown
RTP Ferries the data/voice portion of the user call session
MEGACO is used with SIP to contro­lling gateway functions to the PSTN

SIP Return Codes
1xx- Provis­ion­ing...
2xx- Action was successful
3xx- Redirect to another server
4xx- Client error, bad syntax of request
5xx- Server error, couldn't fulfill request
6xx- Global Failure

*The IMS archit­ecture is built around using SIP.
*SIP supports name mapping and redire­ction, this allows users to be found and reached from different varying locations.
*SIP uses a syntax very similar to HTTP.
*SIP is not as robust as it's rival protocol H.323
*SIP often is implem­ented with SDP to fill in gaps.

Basic Unassisted SIP Services:
Many services listed here, make use of the SUBSCR­IBE­/NOTIFY requests.
*Call Holding
*Auto-­Redial
*Find-Me

More advanced features are assisted by an applic­ation server. The 5420 Converged Telephony Server is one such applic­ation server.
*Call Holding w/ Music
*Three Way Call
*Six-Way Call
*Call history
*Voicemail
*VPN Dialing
*Autho­riz­ation for conference calls
*"Vi­sitor Desk"
*RingBack Tones
*Caller Name/ID
*Call barring
*Presence Status

UE User-E­ndpoint (the device used to place the call)
P-CSCF Proxy Call Session Control Function First point of contact in IMS call flow. May provide TLS or IPSec security.
S-CSCF Serving Call Session Control Function It is a SIP server, but performs session control too. It provides routing services, typically using Electronic Numbering (ENUM). Also it handles SIP regist­rat­ions, which allows it to bind the user's location and the SIP address
I-CSCF Interr­ogating Call Session Control Function It queries the HSS to retrieve the address of the proper S-CSCF and assign it to a user performing SIP regist­rat­ion.Also forwards SIP request or response to the S-CSCF.
HSS Home Subscriber Server is a master database. It contains the subscr­iption inform­ation like subscriber profiles, performs authen­tic­ation and author­ization of the user, and provides inform­ation about the subscr­iber's location and IP inform­ation.
URI Uniform Resource Identi­fier. A string of characters used to identify a caller and the resource. It can be digits, such as tel:+1­-55­5-1­23-4567 or alphan­umeric, such as sip:jo­hn.d­oe­@ex­amp­le.com
Home Network The subscr­ibers native, original provider's network
Visited Network The network that the subscriber is using while away from the home network.
B2BUA A back-t­o-back user agent could be a physical server or software that receives SIP requests. It can then send back the proper responses. It then acts as a user agent device and generates requests to be sent to the real remote user agent device. Different than a proxy cscf in that the B2BUA maintains the complete call state and partic­ipates in all call requests. Provides accurate call accoun­ting, pre-paid rating and billing, failover call routing
AoR An addres­s-o­f-r­ecord, represents an identity of the user, generally a long-term identity, and it does not have a dependency on any device. Users can move between devices or even be associated with multiple devices at on time while retaining the same addres­s-o­f-r­ecord. A simple URI, generally of the form sip:st­eve­@ex­amp­le.com, is used for an addres­s-o­f-r­ecord.