Preface

In relation to the RTIPA project which focusses on a bit lower level
details in the hypermedia chain (most notably networks and quality of
service(qos)) i decided to look into some recent standards (rfc's)
about this stuff and was pointed to the Differentiated Services
architecture. This is a fast summary of these protocols, what they are
and what they mean for us.

Issues:
	- DiffServ is a rather coarse grained system. It has only 8
	bits for a dscp (DiffServ Code Point, which annotates the
	packet allowing the core network nodes to choose the right
	forwarding stategy for it). It doesn't do micro (application
	to application) flows, since that is not scalable (it would
	require far to much state in core network 
	routers). However, something like rsvp could provide services
	to allocate parts of these big flow aggregates to micro flows,
	by selecting the microflows at the ingres of the DS Domain.

	- it can't give any guarantee over networks which don't
	implement any DS forwarding. This meens that for a packet flow
	between to hosts belonging to 2 different ds domains which
	dont have a direct connection, no guarantees can be given. If
	the DS domains are directly connected, it all depends on the
	Service level agreement(s) between the domains. For expedited
	forwarding (EF) this is all that can be done, for AF which
	doesn't give any guarantees about network bandwith anyway, it
	at least allows some control over the priority of the packets
	inside the delivering network (the service provider) which
	should have some idea about the total bandwith available to
	him.

Need to find the issues connecting this to the rtipa project
requirements.

Need to find out more about RSVP (resource reservation protocol) and
its possible interaction with DiffServ

Need a better understanding of the differences between Assured
Forwarding (gives no real guarantees, it seems, but allows for a
better distinction between traffic classes) and Expedited
forwarding (which can provide some guarantees ?).

Need to search for literature about dynamic network information and
adaptive multimedia 

Maybe another phb would serve multimedia delivery better. What should
be the properties of such a protocol? Or does the AF match multimedia
requirements well enough?

Introduction.

	The Differentiated Services Architecture tries to provide
network users with a model of QOS without putting a huge burden on all
core network services. This is done by not looking at individual
data streams from applications, but by aggregating these streams into
classes, whereby the core network infrastructure only has to look
at a certain field (the dscp or DiffServ Code Point) in each ip packet
to make a decision wrt qos. 
   
The burden of the qos is put on the in-- and out-- gateways/routers of
the differentiated services network. The in-- interface can be a host
from which a data stream originates or a gateway to another network
which doesn't use the diffserv architecture. Here the decission is
made which PHB the incoming packet should receive, and the regulation of
the packet streams is performed (regulation means dropping packets which
are above a certain agreed upon rate, or shaping packets in a node to
smooth the rate of packet flow). This regulation is based on the state
of the stream (fe rate which should be metrered) and assigned phb.

    The diffserv architecture seems focussed on things like ISP's and
service level agreements they have with their clients. See [rfc2475]
for the complete specification of the architecture.


Per Hop Behaviours
1) Expedited Forwarding

This per hop behaviour tries to implement a virtual leased line, with
the properties of low loss, latency, jitter and an assured
bandwith. This gives an end to end connection between two nodes. This
is accomplished by 
   a) Each node has a well defined departure rate (which can be
   guaranteed under all load circumstances).
   b) arrival reate of packets at any node is always less than the
   minimal departure rate of that node

The expedited forwarding makes sure of the a) part, while the network
boundary controllers provide the b) part through policing (dropping
packets if they are out of profile) and shaping (delaying).


2) Assured forwarding

Assured forwarding allows discrimination between the probability of
the dropping of the packet. Packets with a high drop precedence will
be dropped with a higher probability than packets with a low drop
precedence. It allows packets to be put in 4  classes (there
is a possibility to have more classes in a DS domain, but 4 classes
are predefined by [rfc2597]) each with 3 drop precedence levels.
Each class in each DS node is assigned a certain amount of forwarding
resources (buffer space and bandwith). The level of forwarding
assurence for an ip packet depends on, in order of importance, the
forwarding resources assigned to the AF class of this packet, the
current load of the AF class, and in case of congestion, the drop
precedence of the packet.

Example service given in [rfc2598] is the `olympic service' with gold,
silver and bronze service classes. Packets are assigned to these three
classes so that packets in the gold class experience lighter load and
thus have a greater probability for timely forwarding than packets
assigned to the silver class. The same relations would hold between
the silver and the bronze class. 

References:
[rfc2475]	"An Architecture for differentiated services", Network
		working group, december 1998
[rfc2597]	"Assured Forwarding PHB Group", June 1999
[rfc2598]	"An Expidited Forwarding PHB", June 1999
[EXPLICIT]	"Explicit Allocation of Best Effort Packet Delivery
		Service", David D. Clark & Wenjia Fang, IEEE/ACM
		transactions on Networking, vol 6. no 4, august 1998,
		pp 362-373
[AXI460]	"Differentiated Services for the AXI460 Router
		Series", third draft, Rune Hylsberg Jacobsen, Ericsson 
		(confidential)