|Page Contents||Off-site Links|
|Introduction||RFC 1883: IPng|
|ST2+||RFC 1819: RTP|
|RTP and RTCP||RFC 1889: RTP|
Existing phone call and video link software uses UDP packets sent over IP. These are just sent off and forgotten about; there is no attempt to get them in the right order, correct errors or cope if packets are missing.
To get better quality, we need more capacity and the means of reserving bandwidth Quality of Service (QoS). (NB The existing IP QoS bits are never used.)
Two approaches have been proposed:
ST2+ (also called ST, ST2 or RFC 1819) is a new version of the base Internet protocol IP; RFC 1190 is now superseded.
IP has a version number of 4. All the world's software and routers require this version to be used currently. There is a new generation of IP (RFC 1883: IPng) with a version number of 6.
However ST2+ is a version of IP with a version number of 5. Its packets only need a 12 byte header compared to IP's 24 bytes as there is no need for retransmission of packets. Higher levels of software will typically use compression, and CELP or MELP to inject some form of Reed-Solomon error correction.
ST2+ is one way, from the source, but there can be multiple targets.
The associated SCMP protocol acts as a bandwidth manager for ST2+. The requested QoS is known as a "flowspec", defining allowable packet delays and maximum packet sizes, etc.
If ST2+ packets need to pass through networks which do not support ST2+ then the ST2+ packets can be encapsulated with IP and sent on; obviously the QoS benefits may well be lost at this stage.
ST2+ has a lower overhead than RSVP and RTP, but may lose out
as its rivals are adopted.
The Resource Reservation Protocol (RSVP) similarly allows bandwidth to be reserved.
RSVP sends its packets using existing IP, ie RVSP is not a new version of IP and so works better with existing routers. However to be of any use, new routers (or router software) must still be installed.
Unlike ST2+, RSVP is initiated from the receiving, ie a user asks for the bandwidth.
RSVP works by telling the routers to inspect UDP and TCP traffic en route and gives priority to those packets that have requested bandwidth. Not surprisingly, this is a computationally intensive task - RSVP's main drawback.
RSVP could work with ST2+. Also if crossing boundaries between networks
that support one or the other, then traffic could be translated between protocols.
The Real Time Protocol (RTP) complements RSVP by allowing actual applications
to respond to the underlying network performance.
RTP packets are also sent using UDP.
Received packets have shelf life information (similar to IP's 'time to live')
The companion protocol RTCP reports on how well the underlying transport is doing.
ST2+ or RSVP/RTP will only be of use if there is the network capacity to
cope with phone, TV or videoconferencing.
New transmission methods and protocols are available. Asynchronous Transfer Mode (ATM) or Frame Relay protocols are designed with such applications in mind.
The Internet physical wiring could change in the future. In city areas, individual service providers could be linked to a high capacity Metropolitan Area Network (MAN). These MANs would be inter-connected using SDH links. This physical arrangement would suit ATM and Frame Relay and allow Quality of Service higher protocols to be implemented.