RFC 2687 (rfc2687) - Page 2 of 13

PPP in a Real-time Oriented HDLC-like Framing

RFC 2687      PPP in Real-time Oriented HDLC-like Framing September 1999


   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.

   A true suspend/resume-oriented approach can only be implemented on a
   type-1 sender [1], but provides the best possible delay performance
   to this type of senders.  The format defined in this document may
   also be of interest to certain type-2-senders that want to exploit
   the better bit-efficiency of this format as compared to [5].  The
   format was designed so that it can be implemented by both type-1 and
   type-2 receivers.

1.1.  Specification Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [8].

2.  Requirements

   The requirements for this document are similar to those listed in
   [5].

   A suspend/resume-oriented solution can provide better worst-case
   latency than the pre-fragmenting-oriented solution defined in [5].
   Also, as this solution requires a new encapsulation scheme, there is
   an opportunity to provide a slightly more efficient format.

   Predictability, robustness, and cooperation with PPP and existing
   hard- and firmware installations are as important with suspend/resume
   as with pre-fragmenting.  A good suspend/resume solution achieves
   good performance even with type-2 receivers [1] and is able to work
   with PPP hardware such as async-to-sync converters.

   Finally, a partial non-requirement: While the format defined in this
   draft is based on the PPP multilink protocol ([2], also abbreviated
   as MP), operation over multiple links is in many cases not required.

3.  General Approach

   As in [5], the general approach is to start out from PPP multilink
   and add multiple classes to obtain multiple levels of suspension.
   However, in contrast to [5], more significant changes are required to
   be able to suspend the transmission of a packet at any point and
   inject a higher priority packet.




Bormann                     Standards Track