[Dart] Comments on draft-ietf-tsvwg-rtcweb-qos-00

"Bless, Roland (TM)" <roland.bless@kit.edu> Thu, 12 June 2014 13:39 UTC

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Date: Thu, 12 Jun 2014 15:39:11 +0200
From: "Bless, Roland (TM)" <roland.bless@kit.edu>
Organization: Institute of Telematics, Karlsruhe Institute of Technology
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Subject: [Dart] Comments on draft-ietf-tsvwg-rtcweb-qos-00
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some remarks on draft-ietf-tsvwg-rtcweb-qos-00:

Sec. 2:
   The mitigation for such action is through an authorization mechanism.

An authorization mechanism alone is probably not sufficient, because DS
boundary nodes may need to police (e.g., remark/drop) incoming traffic
according to some agreed/negotiated traffic profile for some of the
PHBs, e.g., the EF PHB does not work properly if its traffic class
is overloaded (see below).

Sec. 4:
   The below uses the concept of a media flow, however these are
   commonly not equivalent to a transport flow, i.e. as defined by a
   5-tuple (source address, destination address, source port,
   destination port, and protocol).

I think one should try to use definitions that do not depend
on the classifier type like 5-tuple, because as Brian already
mentioned, for IPv6 a 3-tuple (IPsrc, IPdst, Flow Label) may be
sufficient and header fields of the 5-tuple are not always accessible.
I liked the definition of microflow in DiffServ as being an
application-to-application flow of packets, usually being identified
by that well known 5-tuple. For elements within the network microflows
are usually the most fine grained distinction level for flows
(except when you apply multi-field classifier that do some DPI and
add the SSRC etc.).
(nit: is probably a word like "text" missing in the first cited sentence

Proper definitions for media flow, RTP session, transport flow etc.
may help to avoid confusion, so maybe one could try to combine
efforts with draft-york-dart-dscp-rtp-00?

Sec. 5:
   SHOULD use these values to mark the appropriate media packets.  More
   information on EF can be found in [RFC3246].  More information on AF
   can be found in [RFC2597].

The problem with using EF is that if too many EF marked packets are
injected into a DS domain, they probably void the EF properties for
other users within this domain (background is that the service rate of
EF packets on a given output interface must exceed their arrival rate
at that interface over long and short time intervals - this is usually
checked by admission control before use and policing at DS boundary
nodes during use). Therefore, maybe some
discussion w.r.t. RFC5865 http://tools.ietf.org/html/rfc5865 may be

   The above table assumes that packets marked with CS1 is treated as
   "less than best effort".  However, the treatment of CS1 is
   implementation dependent.  If an implementation treats CS1 as other
   than "less than best effort", then the priority of the packets may be
   changed from what is intended.

BTW, there is no "less than best effort" PHB defined,
but a Lower Effort Per-Domain Behavior (PDB) in RFC 3662
/html/rfc3662). This RFC is also the source for the recommendation of
using the CS-1 codepoint for that purpose, so you may considering
referencing it.

   If a packet enters a QoS domain that has no support for the above
   defined Data Types/Application (service) classes, then the network
   node at the edge will remark the DSCP value based on policies.

Yes, but even if there is support for these classes traffic conditioning
(e.g., remarking/dropping etc.) may be
necessary and applied. Remarking to different PHBs may also lead to
packet reordering within a media flow?!