Frames
For some codecs, interframes are not restricted to contain only P-frames. Another type of interframe is the B-frame, which uses two frames for prediction. The B-frame references a frame that occurs in the past and a frame that occurs in the future. The term B-frame is short for between-frame. Figure 3-23 shows a sequence of I-, P-, and B-frames. Figure 3-23 Sequence of I-, P-, and B-Frames Figure 3-23 Sequence of I-, P-, and B-Frames The arrows in Figure 3-23 show the dependencies between...
SIP Record Routing
When establishing a SIP call, the INVITE flows through one or more SIP proxies (if the caller does not know the address of the recipient) to the recipient. The subsequent SIP messages are sent back and forth between the caller and recipient directly, without the proxy getting involved in the signaling. However, in some cases, the proxy server may need to be present in the SIP signaling path to see all the messaging between the endpoints for the duration of the call. One example is the scenario...
Gatekeeper RAS Signaling
Gatekeepers communicate with endpoints, gateways, and MCUs using the RAS protocol. The following sections provide an overview of the basic concepts and messages used in RAS signaling but do not encompass the entire RAS message set. RAS signaling channels are the first to be opened between the gatekeeper and gatekeeper-managed devices and are separate from the call establishment and media channels. RAS signaling uses UDP port 1719 for H.225 messages and UDP port 1718 for multicast gatekeeper...
Depletion of Server Resources
DoS attacks do not always involve depleting the bandwidth on a link instead, DoS attacks can attempt to deplete resources inside a server or endpoint. In certain cases, servers allocate resources when they receive a packet from the network, and the attacker might seek to exhaust these resources by sending a flood of packets to the victim machine. The classic resource depletion attack is the SYN attack, which exploits the TCP protocol. In the TCP protocol, an endpoint requests a TCP connection...
DTMF Support
Endpoints that connect to a conference server via a PSTN gateway often must navigate through an IVR using DTMF tones, and therefore DTMF support in the endpoints and the conference server is important to the conferencing support. Endpoints can use three methods to send DTMF digits Voice-band DTMF tones are modulated as actual tones in the media. Endpoints that dial into a PSTN gateway must play DTMF tones in the media stream so that the PSTN gateway can hear the tones. Endpoints connecting via...
Bandwidth Information in the SDP
Bandwidth usage is specified with the attribute b < modifier> < bandwidth value> . Modifier should be application-specific (AS), conference type (CT), or transport-independent application-specific (TIAS), as defined in RFC 3890. The AS bandwidth includes the bandwidth that the RTP data traffic will consume, including the lower layers, down to the IP layer. Therefore, the bandwidth is in most cases calculated by considering the entire IP packet, which includes RTP payload, RTP header,...
Optional H323 Gatekeeper Features
Some of the optional gatekeeper services include advanced bandwidth management, special call authorization and routing features, and routing of H.225 and H.245 signaling messages. A partial list of optional gatekeeper features follows Call authorization Administrative control via access rules Call management Provides services such as call forwarding Bandwidth management More advanced control over network resource consumption Call control signaling Direct and gatekeeper routed signaling When an...
IPIP Gateway Inside the Firewall
Figure 8-15 shows a solution for NAT FW traversal using an IP-IP media gateway. Figure 8-15 NAT FW Traversal with an IP-IP Gateway Inside the Firewall Figure 8-15 NAT FW Traversal with an IP-IP Gateway Inside the Firewall In this approach, all media streams coming from or going to internal endpoints flow through the gateway. In addition, this topology has two gatekeepers An internal gatekeeper to facilitate connections between internal endpoints A gatekeeper in the DMZ to allow external...
Escalation and Deescalation
Escalation is a process that allows a video-capable endpoint to join the conference in the audio- only mode and later establish a video stream. This process occurs in response to one of two scenarios End users begin a call in audio-only mode and then decide to add a video connection, either through inserting the camera or enabling video in their video phone. An end user turns on a video camera while in an audio-only call, causing the endpoint to automatically establish a video connection. An...
Common H225 Message Types Used in H323 Signaling
This section describes some of the protocol data units (PDU) used in initiating, establishing, and disconnecting H.323 calls. The PDUs are transmitted over the H.225 signaling channel, and each packet is sent as a whole message. The message is defined using a structure defined by a Transport Protocol Data Unit Packet (TPKT). A TPKT format is defined by IETF RFC 2006 and is used to delimit individual messages within the TCP stream. The TPKT contains a one-octet version ID, followed by a...
Configuring Basic Security
Figure 8-4 shows a general configuration for video conferencing security. This configuration involves layers of security, with protection both at the edges of the network and inside the network. Figure 8-4 Basic Configuration for Video Conferencing Security Figure 8-4 Basic Configuration for Video Conferencing Security Internal Video Conferencing Endpoints Internal Video Conferencing Endpoints This topology shows a three-legged firewall. The firewall has connections for the enterprise, the...
H263 Compression Standard
The H.263 codec was developed by the ITU. H.263 and went through three iterations. The first version of the standard was finalized in 1995 and added many enhancements relative to H.261. In the following tables, this version is referred to as Base H.263. The next two iterations of H.263 were issued in 1998 and 2000, with the following further enhancements H.263v2 (aka, H.263+ or H.263 1998) Sixteen annexes were added, up to annex T. In addition, the specification added supplemental enhancement...
H263
As described in Chapter 3, Fundamentals of Video Compression, the H.263 codec has three commonly used versions. The RTP payload format for each version differs slightly and is addressed in two different RFCs RFC 2190 defines the payload format for H.263-1996, and RFC 2429 defines the payload format for H.263-1998 and H.263-2000. Figure 4-18 shows the basic format of an H.263 packet. The following sections describe H.263-1996, H.263-1998, and H.263-2000 in more detail. You also learn about key...
H323 Fast Connect Mode
The H.323 Fast Connect feature is an optimization added as part of H.323 Version 2. Fast Connect signaling can establish media streams in a point-to-point call with one round-trip message exchange, enabling immediate media exchange after the call has been connected. When Fast Connect mode is not used, the media connect is established using standard H.225 and H.245 signaling, called Slow Start mode. Users with endpoints that use H.323 Slow Start signaling may experience audio clipping because of...
H46017
Figure 8-16 illustrates NAT FW traversal with H.460.17. The DMZ contains a traversal server (TS) consisting of a modified gatekeeper. The DMZ GK operates only in GKRCS mode. Inside the enterprise, the diagram shows two types of endpoints those that support H.460.17 natively, and those that rely on a gateway proxy to incorporate the additional H.323 signaling required by the traversal protocol. The only firewall configuration necessary requires stateful bidirectional pinholes When a signaling...
H264
H.264 is a video codec that delivers visual quality superior to H.263 at the same bit rates. H.264 is also referred to as Advanced Video Coding (AVC). H.264 consists of two separate definitions The video coding layer (VCL) The network abstraction layer (NAL) The VCL represents the video content, and the NAL defines the packetization format for transport protocols such as RTP. All data is contained in NAL units. The H.264 bitstream can be of two formats NAL unit stream and byte stream format. We...
H323 Call Flow
Figure 8-5 shows the call flows for H.323. This diagram shows the original simple call flow specified in H.323v1. The basic H.323v1 case includes the following call flow 1. EP1 and the gatekeeper use the Registration, Admission, Status (RAS) protocol to pass highlevel connection commands. To discover a gatekeeper on the network, endpoints send the RAS Gatekeeper Request Message (GRQ) to UDP multicast address 224.0.1.41, on port 1718. In the process of defining the H.323 specification, the H.323...
Video SDP Extensions
The common video codecs used in video conferencing are H.261, H.263, and H.264. This section explains the syntax and semantics for describing parameters related to video codecs. Currently, no standard method exists to specify certain video-related parameters in the SDP offer answer. These parameters include the following Frame resolution (also called form factor) (Resolution means size, like 320x240.) Video endpoints and conference servers use the a fmtp attribute to carry codec-specific...
Video Specific H245 Messages
This section describes H.245 messages used specifically for video sessions. These commands may be initiated from either side after the video session has been established. These include commands for requesting a device change its video transmission rate, to stop and start video decoding, and to request that a sender transmit an intraframe. The Flow Control command is a directive from the decoder of the receiving side to the encoder of the transmitting side. The decoder sends the Flow Control...
H245 Messages
This section describes some of the H.245 PDUs frequently used in establishing H.323 media connections. The messages include PDUs for exchanging endpoint decoding capability, determining how endpoints should behave when conflicts arise, and for opening and closing logical channels. Logical channels are opened and closed depending on the decoding capability and requirements of the endpoints. Each H.245 logical channel is numbered. Logical channel 0 is used for the control channel, and it is...
Media Control Support
Two primary video-specific media control operations need to be supported on video conferences Video fast update (VFU also called fast video update FVU ) An endpoint issues a VFU if its decoder requires an I-frame to continue decoding the video stream. When the encoder receives the VFU, it encodes the next frame as an I-frame. The decoder can request a full update or can ask the encoder to update only a part of the frame. The frame is divided into smaller parts, each called a group of blocks...
