RTP/RTCP/SIP/H.323

RTP/RTCP RFC 1889

RTP is application layer, doesn’t provide QoS. RTP header has packet seq#, timestamp, sync source. RTCP carries timestamp, SSRC, packet counts, lost packets, highest seq# received, and jitter.

RTP packet header is typically 12 octets.

0                   1                   2                   3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X|  CC   |M|     PT      |       sequence number         | RTP
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           timestamp                           | Header
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           synchronization source (SSRC) identifier            |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
|            contributing source (CSRC) identifiers             |
|                             ....                              |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
|                                                               | RTP
|                                                               | Pay-
|                                                               | load
|                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                               :...OPTIONAL RTP padding        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

RTCP packets are never transported individually, but always compounded. RTCP can have Receiver Reports (RR) vs. Sender Reports (SR), SDES, BYE, and APP. The compound must starts with RR or SR. SDES RTCP packet conveys user’s cname and other info.

RTP Compression - CRTP, ROHC (Robust Header Compression)

SIP

SIP Enabled Network – user agents, servers (SIP proxy, redirect, and registrar) and locations servers.

INVITE sip:UserB@there.com SIP/2.0
Via: SIP/2.0/UDP 4.3.2.1/5060
To: User B
From: User A
Call-ID:3654645675373567@4.3.2.1
CSeq: 1 INVITE
Contact:
Content-Length: 126

INVITE/200/ACK, media negotiation is done in this stage with SDP.

SIP message – INVITE, ACK, BYE, CANCEL, REGISTER, OPTIONS, INFO, PRACK, COMET, REFER SUBSCRIBE, UNSUBCRIBE, NOTIFY, MESSAGE.

Application Layer Gateway (ALG) is a SIP and RTP proxy trusted by firewall.

H.323

H.323 system includes endpoint (terminal, MCU, gateway), gatekeeper, AAA server, Address resolution server, feature server, and border element.

H.225.0-Q.931 - setup and release the call
H.245 - capabilities exchange and media control
H.225.0-RAS - endpoint and gatekeeper

Audio codec G.711(64Kbps), G.723.1, G.726, G.728, G.729, G.729A

Video H.261, H.263, H.263+, CIF (352x288)

H.225.0-Q.931 signaling port 1720
- call initiation
- call proceeding
- call altering
- call connection, now open second TCP connection for H.245 call control
- call termination

H.245
- TerminalCapabilitySet
- MasterSlaveDetermination
- OpenLogicalChannel
- Channel Ops
- CloseLogicalChannel

H.245 tunneling through H.225.0-Q.931

Fast Connect - to overcome the slience before H.245 TerminalCapabilitySet, use H.225.0-Q.931 fastStart to open logical channel before H.245.

H.225.0-RAS

Gatekeeper (RAS server) discovery - well known port 1719, or multicast 224.0.1.41/1718.