11.3.6
Notes
Note on Differentiated Services: Differentiated Services (DiffServ, or DS) is a method for
adding QoS (Quality of Service) to IP networks. In routed networks, critical network traffic
such as video and audio streams, which require a relatively uninterrupted flow of data, can
get blocked due to other traffic. DiffServ can be used to classify network traffic and give
precedence - i.e. low-latency, guaranteed service - to high-priority traffic, while offering
best-effort service to non-critical traffic such as file transfers or web traffic.
Each stream has a DSCP (Differentiated Services Code Point) field in the IP header. Routers
will identify the network service type in the DSCP field and provide the appropriate level of
service. Low-latency service can be realized, for example, through priority queuing,
bandwidth allocation, or by assigning dedicated routes.
Note on RTP and RTCP: The Real-time Transport Protocol (RTP) is designed for end-to-end
real-time, audio or video data flow transport. It is regarded as the primary standard for
video/audio transport over multicast or unicast network services. RTP does not provide
guaranteed delivery, but sequencing of the data makes it possible to detect missing packets.
It allows the recipient to compensate for breaks in sequence that may occur during the
transfer on an IP network. Error concealment can make the loss of packets unnoticeable.
RTP is usually used in conjunction with the Real-time Transport Control Protocol (RTCP). RTP
carries the media streams. RTCP provides reception quality feedback, participant
identification and synchronization between media streams.
Note on the Session Announcement Protocol (SAP): SAP, defined in RFC 2974 (see RFC
2974 - http://www.ietf.org/rfc/rfc2974.txt), is a protocol for advertising multicast session
information. A SAP announcer periodically broadcasts announcement packets which include
the session description information of multicast sessions presented by the announcer. SAP
uses the Session Description Protocol (SDP) as the format of the session descriptions. The
announcement is multicast with the same scope as the session it is announcing, ensuring
that the recipients of the announcement are within the scope of the session the
announcement describes. SAP listening applications can listen to the announcements and
use the information to construct a guide of all advertised sessions. This guide can be used to
select and start a particular session. The SAP announcer is not aware of the presence or
absence of SAP listeners.
Note on FloodGuard: FloodGuard is a TKH Security proprietary stream control mechanism
that can be enabled/disabled independently for each video and sampled data transmitter.
FloodGuard throttles the transmitter when it no longer receives control messages from the
receiver, thereby preventing the transmitter from flooding the network.
FloodGuard only works when enabled on both the transmitter and the receiver, and when
the transmitter sends to a unicast address.
When a transmitter is enabled, it opens a control receive port with the port number equal to
its source port 1. This port listens for control packets from the destination
receiver. When no FloodGuard packets come in during the time set for the FloodGuard
throttle delay, the receiver is expected to have disappeared (powered off, receiver disabled,
network problem, etc.) and the stream is 'throttled'. In throttled mode the transmitter - in
order to contact the intended receiver (again) - sends empty packets into the network at an
interval determined by the FloodGuard throttle interval parameter. After reception of a valid
FloodGuard packet the transmitter immediately resumes streaming.
Video
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