Multimedia Communication in the Internet VoIP and SIP

To:

Contact:

Expires:

3600

SIP Registrar (domain iptel.org)

SIP/2.0 200 OK

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Basic SIP Call-Flow (Proxy Mode)

Location Server DNS SRV Query ? iptel.org

Reply: IP Address of iptel.org SIP Server

#0 INVITE

From:

To:

Call-ID: [email protected]

#1 #2 #6 OK 200 From:

To:

Call-ID:

#7 ACK sip:[email protected]

Proxy

#3 INVITE

From:

To:

Call-ID:

#4 OK 200 From:

#5 To:

Call-ID:

Media streams

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Proxy Server Functionality

• Serve as rendezvous point at which callees are globally reachable • Perform routing function, i.e., determine to which SIP hop (UA/proxy/redirect) signaling should be relayed • Allow for call processing logic.     user’s signaling preferences AAA firewall control etc.

• Forking: Several destinations may be tried for a request sequentially or in parallel.

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Proxy Chaining

• SIP proxies may be arbitrarily chained and add different values along the signaling path   least-cost gateway routing load distribution   processing users’ preferences etc.

• In some case a local outbound proxy may be involved  provides locally important call processing logic (e.g., identifying nearest 911)   manages firewall provides least-gateway-cost routing service  IP phones must know address of the proxy:may be configured manually or with a configuration protocol (DHCP, TFTP, ... ) • Servers have to avoid loops and recognize spirals

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Proxy Chaining - an Example

Caller’s administrative domain

Administrative domain of a PSTN gateway operator gw01.asia.pstn.com

pstn.com

asia.pstn.com

#3 #4 #1 Caller’s outbound proxy accomplishes firewall traversal.

Destination’s “first-hit proxy” identifies a proxy serving dialed Proxy in the target area distributes load in a gateway farm.

area.

Note: signaling (in red) may take a completely different path from media (in blue).

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Ability to Try Multiple Destinations: Forking

• • • • A proxy may fork a request to multiple destinations either in parallel (“reach me everywhere”) or serially (“forward no reply”).

A proxy can cancel pending parallel searches after a successful response is received. A proxy can iterate through redirection responses (“recursive forking”).

The first “OK” is taken.

#1 INVITE #2 Trying #3 INVITE #4 Ringing #5 CANCEL #6 OK #7 INVITE

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Stateful versus Stateless Proxy Operational Mode

• SIP Proxies may operate either in stateful or stateless mode; which of the modes is used depends on implementation or configuration.

• stateless mode: 

Usage:

good for heavy-load scenarios -- works well for example if they act as application-layer load distributors.



Behavior:

  proxies just receive messages, perform routing logic, send messages out and forget anything they knew; they should cache results of SIP routing logic as it is not able to distinguish between retransmissions and new requests - and would result in new execution of SIP routing logic for every retransmission

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Stateful versus Stateless Proxy Operational Mode

• stateful mode: 

Usage:

good for implementing some services (e.g., “forward on no reply”) 

Behavior:

   proxies maintain state during entire transaction; they remember outgoing requests as well as incoming requests that generated them until transaction is over; a forking proxy should be stateful reduce retransmission time by acting on behalf of sender closer to destination

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“Stateful” Proxy Refers to Transactions

SIP state forgotten as soon as transaction over • Frequently Misunderstood Issue SIP proxies deliver a “one-time rendezvous service” (as opposed to state storage service).

INVITE [email protected]

• Thus a stateful proxy just keeps state during a SIP “rendezvous transaction” and completely forgets it afterwards.

OK • A SIP proxy is not aware of existing calls. In case of failure, existing calls are NOT affected!

Legend SIP signaling SIP state media

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Tell Other Parties Where You Are: Contact Header Field

• • In general, it indicates where users may be reached for subsequent requests     Minimize proxy’s involvement in transactions: once all parties know each other’s contact address they can communicate directly and proxy server is no more needed used for the “next-try” in 3xx redirection responses may include multiple weighted addresses if behind a firewall, the Contact header field carries public server’s address In REGISTER method  indicates current SIP address of requestor  if different from current registration, it is added to current registration list   if equal, contact parameters including updated and expiration timer updated  2xx replies return a list of current registrations “Expires: 0” removes an entry, if along with “Contact: *” all entries are removed

Note: Multiple header fields with the same field-name may be present in a message if and only if the entire field-value for that header field is defined as a comma-separated list (i.e., #(values)). Tekelec Confidential / For Discussion Purposes Only / Tekelec Confidential

SIP Operation in Redirect Mode

#2 Location Server #3 [email protected]

#1 INVITE [email protected]

#4 302 moved temporarily Contact: [email protected]

Proxy #5 ACK [email protected]

#6 INVITE [email protected]

#7 OK 200 #8 ACK [email protected]

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