Providing Integrated Services over Low-bitrate Links
RFC 2689

Document Type RFC - Informational (September 1999; No errata)
Last updated 2013-03-02
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Network Working Group                                         C. Bormann
Request for Comments: 2689                       Universitaet Bremen TZI
Category: Informational                                   September 1999

          Providing Integrated Services over Low-bitrate Links

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

   This document describes an architecture for providing integrated
   services over low-bitrate links, such as modem lines, ISDN B-
   channels, and sub-T1 links.  It covers only the lower parts of the
   Internet Multimedia Conferencing Architecture [1]; additional
   components required for application services such as Internet
   Telephony (e.g., a session initiation protocol) are outside the scope
   of this document.  The main components of the architecture are: a
   real-time encapsulation format for asynchronous and synchronous low-
   bitrate links, a header compression architecture optimized for real-
   time flows, elements of negotiation protocols used between routers
   (or between hosts and routers), and announcement protocols used by
   applications to allow this negotiation to take place.

1.  Introduction

   As an extension to the "best-effort" services the Internet is well-
   known for, additional types of services ("integrated services") that
   support the transport of real-time multimedia information are being
   developed for, and deployed in the Internet.  Important elements of
   this development are:

   -  parameters for forwarding mechanisms that are appropriate for
      real-time information [11, 12],

   -  a setup protocol that allows establishing special forwarding
      treatment for real-time information flows (RSVP [4]),

   -  a transport protocol for real-time information (RTP/RTCP [6]).

Bormann                      Informational                      [Page 1]
RFC 2689       Integrated Services over Low-bitrate Links September 1999

   In addition to these elements at the network and transport levels of
   the Internet Multimedia Conferencing Architecture [1], further
   components are required to define application services such as
   Internet Telephony, e.g., protocols for session initiation and
   control.  These components are outside the scope of this document.

   Up to now, the newly developed services could not (or only very
   inefficiently) be used over forwarding paths that include low-bitrate
   links such as 14.4, 33.6, and 56 kbit/s modems, 56 and 64 kbit/s ISDN
   B-channels, or even sub-T1 links.  The encapsulation formats used on
   these links are not appropriate for the simultaneous transport of
   arbitrary data and real-time information that has to meet stringent
   delay requirements.  Transmission of a 1500 byte packet on a 28.8
   kbit/s modem link makes this link unavailable for the transmission of
   real-time information for about 400 ms.  This adds a worst-case delay
   that causes real-time applications to operate with round-trip delays
   on the order of at least a second -- unacceptable for real-time
   conversation.  In addition, the header overhead associated with the
   protocol stacks used is prohibitive on low-bitrate links, where
   compression down to a few dozen bytes per real-time information
   packet is often desirable.  E.g., the overhead of at least 44
   (4+20+8+12) bytes for HDLC/PPP, IP, UDP, and RTP completely
   overshadows typical audio payloads such as the 19.75 bytes needed for
   a G.723.1 ACELP audio frame -- a 14.4 kbit/s link is completely
   consumed by this header overhead alone at 40 real-time frames per
   second total (i.e., at 25 ms packetization delay for one stream or 50
   ms for two streams, with no space left for data, yet).  While the
   header overhead can be reduced by combining several real-time
   information frames into one packet, this increases the delay incurred
   while filling that packet and further detracts from the goal of
   real-time transfer of multi-media information over the Internet.

   This document describes an approach for addressing these problems.
   The main components of the architecture are:

   -  a real-time encapsulation format for asynchronous and synchronous
      low-bitrate links,

   -  a header compression architecture optimized for real-time flows,

   -  elements of negotiation protocols used between routers (or between
      hosts and routers), and

   -  announcement protocols used by applications to allow this
      negotiation to take place.

Bormann                      Informational                      [Page 2]
RFC 2689       Integrated Services over Low-bitrate Links September 1999

2.  Design Considerations

   The main design goal for an architecture that addresses real-time
   multimedia flows over low-bitrate links is that of minimizing the
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