Signaling protocols- MGCP

Signaling architectures Distributed Call Control (a.k.a. Intelligent endpoint model) Centralized Call Control (a.k.a. Call Agent, Softswitch model)

Dumb endpoint Decomposed gateway (physically separate signaling and

media) media conversion close to the traffic source or sink smaller number of controller easy to roll out new applications

Call Agent model

Intelligence concentrated at Call Agent Routing decisions Policy decisions

Call state kept at Call Agent Call agent is the master, gateway or

endpoint is the “slave”

Examples:MGCP (Media Gateway Control Protocol)SCCP (Simple Client Control Protocol, Cisco)

Intelligent endpoint model

Routing decisions made in conjunction with server in cloud

Policy decisions usually made by a policy server

Call state kept at endpoint

Examples: SIP (Session Initiation Protocol), ITU H.323

Media Gateway Control Protocol (MGCP)

IETF RFC 3435 Text based protocol, easily readable Master slave protocol Two main entities +

Call Agent(CA), a.k.a. Media Gateway Controller

Media Gateway (MG) One end is circuit switch, the other is RTP port Trunking gateway(TGW) Residential gateway(RGW)

Signaling Gateway

MGC Requirements

establish media stream and negotiate format

tones and announcement QoS conditioning network monitoring billing and accounting non-stop, incremental equipment /

subsystem upgrade

Key MGCP terms

Endpoints Basically physical circuits/resources that

reside on the gateway Single endpoint can have multiple

connections Connections

A data path between an endpoint and a peer (address, port) on the IP network

Calls Digit maps Events/Signals

Endpoints

Analog endpoints Named aaln/* follows slot/port hardware hierarchy

Digital endpoints (ds0 channel) Named ds/* Specifically ds/<unit-type1>-<unit #>/<unit-type2>-

<unit #>/.../<channel #> Example ds/ds1/1

Virtual endpoints An audio source like an IVR server, Announcement

server Conference bridge Packet relay (e.g. Firewall) Wire-tap access point Named virtual-endpoint-type/channel Example ann/*, for “announcement resources”

Endpoint Identifier

GW’s Domain Name + Local Name Local Name

A hierarchical form: X/Y/Z trunk4/12/7@gateway.somenetwork.net

To identify DS0 number 7 within DS1 number 12 on DS3 number 4 at gateway.somenetwork.net

Wild-cards $, any; *, all e.g., trunk1/5/$@gateway.somenetwork.net

CA wants to create a connection on an endpoint in a gateway and does not really care which endpoint is used.

e.g., trunk1/5/*@gateway.somenetwork.net CA requests statistical information related to all

endpoints on a gateway.

Connection

Between an endpoint, and a peer on the IP network

Connection ID created by gateway, and is unique in its scope

Connection has a “session description”, defining Remote address, port Types of media Codec type

Call

Notion of Call, and corresponding Call-ID owned by the Call Agent(CA)

Commonly, a Call comprises a set of two Connections created by the CA on one or two Gateways

A conference call typically have many connections.

Digit Maps

Most calls require the collection of DTMF digits, and their lookup in a route database by the CA

It is inefficient to be sending up each digit from the Gateway to the CA, one at a time

A digit map is a pattern provided by the CA to the Gateway to collect digits Egrep based

Digits are collected by the Gateway until the pattern is matched.

At that point, the collected digits are sent up to the CA

Example: 408xxxxxxx(408 followed by 7 digits)

Events/Signals

There are telephony signals other than DTMF digits Example line states offhook/onhook etc Trunk signaling

These are communicated using Event/signal packages Packages are defined by companion RFCs, like RFC

3660(Basic MGCP packages) Generic media Line Trunk Dtmf Media Announcement

Basic MGCP call components

GW1 GW2

CA(Call)

IP NetworkIP Network

endpoint endpoint

Connection 1 Connection 2

Protocol structure

Request/Response Each request acknowledged by a

response There is a set of response codes that

convey the status of the request Can be run over UDP

MGCP Command Format

MGCP 1.0

MGCP Request

Text format ACTION TRANS-ID ENDPOINT

VERSION Parameter line Parameter line ….. Parameter line = NAME: Value Examplerqnt 4 aaln/1@rgw2.whatever.net mgcp 1.0 r: l/hd(n)

x: 234567892

MGCP Response

RESP-CODE TRANS-ID RESP-STRING Parameter line Parameter line ….. Parameter line = NAME: Value Example200 0 ok z: aaln/1@rgw1.whatever.net z: aaln/2@rgw1.whatever.net

MGCP Requests Actions

RQNT (Request notification) Sent from CA to MGW to request notification on change

of endpoint status NTFY (Notify)

Sent from MGW to CA in response to RQNT CRCX (Create Connection)

Sent from CA to MGW MDCX (Modify Connection)

Sent from CA to MGW, e.g. provide far end info i.e. update info about the connection

DLCX (Delete Connection) Sent from CA to MGW typically. Additionally from MGW to

CA in error conditions

MGCP Requests AUCX (Audit Connection)

Sent from CA to MGW to get status of a connection.(Useful in CA redundancy/failover)

AUEP (Audit Endpoint) Sent from CA to MGW to get status of an endpoint.

(Useful in CA redundancy/failover) RSIP (Restart in progress)

Sent from MGW to CA to inform the CA that it is “rebooting”

EPCF (Endpoint configuration) Sent by the CA to MGW to alert configuration of the

endpoint(things like gain level, codec, etc

Key parameter types

X: Request id, RQNT/NTFY correlator C: Callid, String N: Notified entity (addr of call agent to be notified) I: Connection id S: Signal request(dialtone, ringback etc) M : Connection mode (sendonly, recvonly,

sendrecv) T: Events to be detected (on/off hook, DTMF) O: Events detected R: Events requested D: Digit map B: Bearer Information (A-law, mu-law)

during a call?

MGCP Response codes

100-199 Provisional, in process

200-299 Successful

400-499 Unsuccessful, transient error

500-5xx Unsuccessful, permanent error

800-8xxPackage specific errors

000Response acknowledgement

Reason codes

The reason for a certain request or response

As opposed to the Response code which is the status of executing that request

000 (normal state, used for audit) 900-9xx(Package specific)

transactionID 1111

endpoint1@MGACallID 1234567Mode reconly transactionID

1111connectionID

AAAARTP format 0

G711mu

Setup for call flow

Rgw2567128.96.41.1

Rgw124128.96.63.25

CA1port 5678

IP NetworkIP Network

endpoint endpoint

Connection 1 Connection 2

Joe Alice

14083238155

Route server/DB

Call flow sequence

Rgw2567128.96.41.1

Rgw124128.96.63.25

CA1port 5678

IP NetworkIP Network

endpoint endpoint

Joe Alice

14083238155

Route server/DB

0(RQNT)

1(NTFY)

2(CRCX)

4(MDCX)

6(NTFY)

5(RQNT)3(CRCX))

7(MDCX)

MGCP call flow (0)

RQNT 1202 aaln/1@rgw-2567.whatever.net MGCP 1.0 N: ca@ca1.whatever.net:5678 Notified Entity – CA address

X: 0123456789AC Request ID

R: L/hd(A, E(S(L/dl),R(L/oc, L/hu, D/[0-9#*T](D)))) Requested Event

D: (0T|00T|#xxxxxxx|1xxxxxxxxxx|91xxxxxxxxxx|9011x.T) DigitMap

S: Signal request

Q: process T: G/ft DetectEvents

200 1202 OK

Call flow (1)

NTFY 2002 aaln/1@rgw-2567.whatever.net MGCP 1.0 N: ca@ca1.whatever.net:5678 X: 0123456789AC O: L/hd,D/1,D/4,D/0,D/8,D/3,D/2,D/3,D/8,D/1,D/5,D/5

ObservedEvents

200 2002 OK

Call flow(2)

CRCX 1204 aaln/1@rgw-2567.whatever.net MGCP 1.0C: A3C47F21456789F0 CallIDL: p:10, a:PCMU LocalConnectionOptions

M: recvonly

200 1204 OK I: FDE234C8

v=0 o=- 25678 753849 IN IP4 128.96.41.1 s=- c=IN IP4 128.96.41.1 t=0 0 m=audio 3456 RTP/AVP 0

Call flow(3)

CRCX 1205 aaln/1@rgw-124.whatever.net MGCP 1.0 C: A3C47F21456789F0 L: p:10, a:PCMU M: sendrecv X: 0123456789AD R: L/hd S: L/rg

v=0 o=- 25678 753849 IN IP4 128.96.41.1 s=- c=IN IP4 128.96.41.1 t=0 0 m=audio 3456 RTP/AVP 0

Call flow(3)

200 1205 OKK: I: DFE233D1

v=0 o=- 4723891 7428910 IN IP4 128.96.63.25 s=- c=IN IP4 128.96.63.25 t=0 0 m=audio 3456 RTP/AVP 0

Call flow(4)

MDCX 1210 aaln/1@rgw-2567.whatever.net MGCP 1.0 C: A3C47F21456789F0 I: FDE234C8 M: recvonly X: 0123456789AE

v=0 o=- 4723891 7428910 IN IP4 128.96.63.25 s=- c=IN IP4 128.96.63.25 t=0 0 m=audio 3456 RTP/AVP 0

200 1206 OK

Call flow(5)

RQNT 1201 aaln/1@rgw-124.whatever.net MGCP 1.0N: ca@ca1.whatever.net:5678X: 0123456789AC R: l/hd(N)S: l/rg

200 1201 OK

Call flow(6)

NTFY 2002 aaln/1@rgw-124.whatever.net MGCP 1.0 N: ca@ca1.whatever.net:5678 X: 0123456789AC O: L/hd

200 2002 OK

Call flow(7)

MDCX 1210 aaln/1@rgw-2567.whatever.net MGCP 1.0 C: A3C47F21456789F0 I: FDE234C8 M: sendrecv X: 0123456789AE

v=0 o=- 4723891 7428910 IN IP4 128.96.63.25 s=- c=IN IP4 128.96.63.25 t=0 0 m=audio 3456 RTP/AVP 0

200 1206 OK

Teardown(on hook, alice)

NTFY 2002 aaln/1@rgw-124.whatever.net MGCP 1.0 N: ca@ca1.whatever.net:5678 X: 0123456789AC O: L/hd

200 2002 OK

Teardown(Joe)

DLCX 1210 aaln/1@rgw-2567.whatever.net MGCP 1.0 C: A3C47F21456789F0 I: FDE234C8

200 1210 OK P: PS=1245, OS=62345, PR=780, OR=45123, PL=10,

JI=27, LA=48

Teardown(Alice)

DLCX 1210 aaln/1@rgw-124.whatever.net MGCP 1.0 C: A3C47F21456789F0 I: FDE234C8

250 1210 OK P: PS=1245, OS=62345, PR=780, OR=45123, PL=10,

JI=27, LA=48

Provisional responses/ acknowledgements

1xx class responses to requests are allowed

Eventually must be followed by a “final” response

Final response = 2/4/5xx In this case, requestor ACKs the final

response – that is what the 000 code is for

Thoughts on MGCP

MGCP is a protocol that provides low level primitives for a master(CA) to control a slave(MGW) Usually more than one way to implement a

certain call flow/feature Feature implementation is in the CA and

CAs are usually very complex

Variations

Callee busy Wrong number Etc. Two/Multiple Call Agents

Call Agent to Call Agent signaling is usually an “intelligent endpoint” model protocol, such as SIP.

A typical PSTN usage scenario

TDMSwitch

TDMSwitch

Signaling Controller

(Signaling plane)

Data plane(TDM)

Typical MGCP usage scenario

Switch Switch

MGCP CA(Signaling plane)

MGW MGW

SS7 GW

IP NetworkData Plane

IP NetworkData Plane

SCPSTP

Reading exercise

Read RFC 3435, Sections 1 to 2.1.7, and the example in G.2.1, G.3.1

SCCP(Cisco)

Another example of the call agent model

Skinny Client Control Protocol Binary, not text Even simpler than MGCP(no digit

maps, for example) Applicable to IP-PBX, low RTT

between phones and CA “Features” implemented on the CA,

not on the phone

Wrapping up the CA model

Sometimes the CA is also termed the MGC(Media gateway controller)

H.248(also known as MEGACO) is an ITU alternative to MGCP, and operates on the same principles