Mapping to ATM classes of service for
                    Differentiated Services Architecture

draft-ayandeh-diffserv-atm-00.txt               S. Ayandeh
Expiration: April 2000                          A. Krishnamurthy
                                                A. Malis
                                                Lucent Technologies

          1.  Status of this Memo

          This document is an Internet-Draft and is in full conformance
          with all provisions of Section 10 of RFC 2026. Internet-Drafts
          are working documents of the Internet Engineering Task Force
          (IETF), its areas, and its working groups.  Note that other
          groups may also distribute working documents as Internet-
          Drafts.

          Internet-Drafts are draft documents valid for a maximum of six
          months and may be updated, replaced, or obsoleted by other
          documents at any time.  It is inappropriate to use Internet
          Drafts as reference material or to cite them other than as
          "work in progress."

          The list of current Internet-Drafts can be accessed at
          http://www.ietf.org/ietf/1id-abstracts.txt

          The list of Internet-Draft Shadow Directories can be accessed
          at http://www.ietf.org/shadow.html.

                             Abstract

The guidelines for PHB specifications contained in the Differentiated
Services (DS) Architecture [1] require descriptions of:

. How a PHB would map to different link layers
. How a PHB would inter-work with non-DS compliant nodes and networks

This draft includes the mapping to ATM classes of service for EF [2] and
AF PHBs [3].

65
2.0  Introduction  3
3.0  Inter-working issues of DiffServ and ATM  3
4.0  Differentiated Services Requirements for ATM Classes of Service  4
5.0  Recommended Mapping of DiffServ PHBs to ATM Classes of Service  4
5.1  Mapping of EF PHB to CBR Class of Service  4
5.2  Mapping of AF PHB to ABR Class of Service  5
6.0  Example Use of Virtual Circuits  6
7.0  Interactions of TCP and UDP  6
8.0  Security Considerations  7
9.0  References  7
10.0  Authors' Addresses  7

65

2.0  Introduction
Multi-service networks form part of the mosaic of existing and emerging
data networks.  As such there is a need to ensure that characteristics
of IP services based on Differentiated Services (DS) architecture are
maintained end to end across (intermediate) ATM networks.  This is no
easy task, as ATM networks standardize a service at the user network
interface while allowing for adjustments to a set of well-defined
traffic parameters.  DS, on the other hand, supplies the supporting
traffic conditioning and per-hop behaviors and leaves the service
specification to be defined by the service provider.

The goal of this document is to specify the mapping of differentiated
services PHBs to existing ATM classes of service.  The ATM traffic
class, its descriptors, and QoS parameters, required to carry a given
PHB are discussed.  Circuit aggregation issues are limited to a
discussion of virtual path connections.

The motivation for this approach is:

a) To allow for traffic conditioning functions to occur at frame
   boundaries while the ATM network delivers a given PHB using existing
   ATM classes of service.  Note that ATM Forum's draft addendum [5],
   which proposes extensions to ATM service categories and signaling
   scheme, deals with new mechanisms that are in process of being
   specified.  These mechanisms in effect make an ATM switch DiffServ
   compliant.
b) To ensure that the resulting mappings do indeed meet the requirements
   of the EF and AF PHBs outlined in RFCs 2598 & 2597.  For example as
   we show further on, the approach also recommended by the ATM Forum's
   draft addendum [5] to their Traffic Management 4.1 Specification [4],
   of mapping IP services to ATM classes of service may at best
   approximate the expected behaviors and the resulting IP service.
c) To simplify the resulting deployments by offering two existing ATM
   classes of service that would correspond to the EF and AF per hop
   behaviors.  We label this as the "PHB-mapping" approach.  This is in
   contrast to the "service-mapping" approach taken in the addendum to
   TM 4.1 [5].  In the latter approach, services resulting from the AF
   PHB alone may be mapped to five ATM classes of service: rt-VBR, nrt-
   VBR, ABR, GFR, and Differentiated UBR.

3.0 DiffServ and ATM inter-working issues
The goal is to meet the requirements of a given differentiated services
PHB using a minimum set of resources in an ATM network.  Some
clarification of the terms used to describe the behavior of the traffic
source & QoS parameters in ATM networks is in order. This clarification
is intended to highlight the main differences between cell and packet
based metrics currently in use:

Cell rates:  Cell rates have to take into account the adaptation layer
overheads of ATM such as padding. Adaptation overheads vary with the
size of the packets.  Therefore conversion of a given bit rate at a
packet interface to cell rate has to take into account the statistical
nature of the packet length. Some additional bandwidth may have to be
allocated in order to account for the statistical nature of the packet
length.

Cell Transfer Delay (CTD):  This parameter in TM 4.1 is measured as the

65

interval between a pair of cell entry and exit events.  CTD does not
cover the transmission time at the egress port.  In DS measurements the
packet transmission time, which varies with packet size, is often
included.

Peak-to-peak CDVT:  This parameter is measured as the difference CTDx-
CTDd, where subscript x refers to an arbitrary cell and d to a defined
reference cell [ITU-T I.356] (injected at idle time).  The EF-PHB, for
example, uses the absolute value of the difference in nodal CTD for two
adjacent packets as a measure of jitter experienced at a node.

Cell Loss Ratio (CLR):  CLR represents a lower bound on the loss
probability of the corresponding packets.  Settings of CLR would require
some understanding of the cell loss process and how it relates to the
packet loss ratio.  For example, partial and early packet discard
mechanisms can realize this lower bound.

4.0  Differentiated Services Requirements for ATM Classes of Service
Currently defined DiffServ architecture places three basic requirements
on the ATM network: marking of packet drop precedence, as well as,
minimum cell rate control and active queue management.

. Marking of Packet Drop Precedence:  ATM offers a cell loss priority
(CLP=0 or 1) mechanism.  Cells resulting from in profile packets are
marked with CLP=0, while cells resulting from out of profile packets
are marked with CLP=1.  Marking of cells should therefore be applied
to entire frames and occur at frame boundaries.  Indiscriminate
marking of cells would lead to unacceptable throughput behavior in
the face of congestion in the cell network [6].

. Minimum Cell Rate [cell/s]:  Needs to be configured for both the EF
and AF PHBs.  For EF it ensures that the aggregate has a well-defined
minimum departure rate over a time interval equal to or greater than
the time it takes to send an output link MTU sized packet at the
configured rate of the EF-PHB.  This together with policing action at
the edge leads to a low loss, delay, and jitter per-hop behavior.

. Active Queue Management:  The assured forwarding PHB and hence the
resulting services, require the properties of a RED like algorithm
for active queue management [7].  ATM services can offer cell discard
for CLP=1 cells.  However, indiscriminate cell drops are not of any
use in support of AF PHB.  Both partial and early packet discard
mechanisms in ATM lead to a tail packet drop behavior.  Tail drop is
undesirable for its adverse effects on TCP flows. ATM traffic classes
[4] are being enhanced through local policy to offer active queue
management.  However, any solution to this problem is further
aggravated by virtual path connections where individual circuit and
frame delineation are not visible by definition.

5.0  Recommended Mapping of DiffServ PHBs to ATM Classes of Service
The requirements outlined in the previous section are met by the
following mappings of differentiated services PHBs to existing ATM
classes of service.

5.1  Mapping of EF-PHB to the rt-VBR Class of Service
Source Traffic Descriptor (VBR.1)

65

Peak Cell Rate (PCR) (CLP=0+1)  = line rate of the connection with
                                  its inverse T0+1

Sustained Cell Rate (SCR)       = configured rate of EF PHB with its
                                  inverse Ts
Maximum Burst Size (MBS)        = max-PDU-size

Two conformance definitions apply GCRA(T0+1 , CDVT) to regulate the peak
and GCRA(Ts0 , BT0 + CDVT) to regulate the rate of the connection
averaged over the life of the connection [4].  Non conforming cells are
dropped.

QoS Parameters
CLR                = Cell loss is not expected along the ATM path,
                     however loss may occur at the ingress for non-
                     conforming packets
MaxCTD             = Is the transfer delay of the ATM network
Peak-to-peak CDV   = a derivative of MaxCTD is equal to MaxCTD-(fixed
                     propagation, transmission, and switching delays)

Note that the network through the traffic descriptor provisions adequate
resources to accommodate cells bursting at line rate at the configured
rate of the EF-PHB.  By using an MBS equal to the max-PDU-size, the
network over provisions bandwidth.  However since EF-PHB is a policed
service, the excess bandwidth would be available for use by other
connections.

Mapping of EF-PHB to the CBR class of service would introduce shaping
induced delays at egress when re-assembling a packet.  Low delay
requirement of EF-PHB may therefore not be well served with CBR class of
traffic.

5.2  Mapping of AF PHB to ABR Class of Service
Source Traffic Descriptor (ABR)
PCR (CLP=0+1)     = minimum line rate along the path of the VC with its
                    inverse T0+1
MCR               = minimum bandwidth allocated to an AF class with its
                    inverse Tm0

The conformance definitions is GCRA(Tm0 , CDVT), or based on ideal
transmission time of CLP=0 cells resulting from the "Allowed Cell Rate"
in response to the feedback mechanism.  All data cells are transmitted
with CLP=0.  Congestion feedback is supplied as an explicit rate,
congestion indication, or no increase parameters contained in resource
management cells.

QoS Parameters
CLR              = no value needs to be specified.  The target value is
                   however intended for conforming connections and is
                   network specific.

The use of ABR traffic class is recommended as it allows all of the
requirements of the AF-PHB to be met.  Note that VBR, GFR, and relative
UBR only partially meet the packet drop precedence requirement, and do
not offer active queue management.  Furthermore, these traffic classes
do not support virtual path circuits for reasons already discussed in
section-3.

65

With ABR, the minimum throughput and buffer requirements of the AF PHB
can be provisioned.  With a minimum amount of cell loss, congestion in

the cell network is pushed back to the frame boundaries where active
queue management is applied.  Any mechanisms to implement back pressure
at the frame boundary are implementation specific.

Active queue management is applied to the full range of the packet drop
precedence markings i.e. AFx1, AFx2, and AFx3.  This is in contrast to
the three to two levels mapping which would occur with the use of Cell
Loss Priority (CLP) in the service-mapping approach.

The requirements of the AF PHB are therefore fully met.  Given that only
a few ABR connections would be carrying large aggregates of AF traffic,
any possible concern with the volume of resource management cells does
not arise.  For list of numerous references regarding IP traffic and ABR
see references [8, 9, 10].

6.0  Example Use of Virtual Circuits
Please note that the code points are only mentioned for illustrative
purpose.  The idea is to map PHBs to ATM classes of service and not DS
code points.

Diffserv code point                 VC-type

                  EF          rt-VBR VCb

                  AF11        ABR VCc   CLP=0
                  AF12                  CLP=0
                  AF13                  CLP=0

                  AF21        ABR VCd   CLP=0
                  AF22                  CLP=0
                  AF23                  CLP=0

                  AF31        ABR Vce   CLP=0
                  AF32                  CLP=0
                  AF33                  CLP=0

                  AF41        ABR VCf   CLP=0
                  AF42                  CLP=0
                  AF43                  CLP=0

                  BE          UBR VCg

7.0  Interactions of TCP and UDP
There are three alternatives available to handle the interactions of TCP
and UDP traffic [11, 12]:

a) Integrated treatment with two drop precedence:
TCP/UDP In Profile  ->   AFx1
TCP/UDP Out Profile ->   AFx2

In this scenario both TCP and UDP would achieve their committed rates.
In the face of persistent congestion UDP would consume the entire excess
bandwidth.

65

b) Integrated treatment with three drop precedence:
This scenario can not be supported by the "service-mapping" approach as
ATM is limited to two drop precedence levels.  However the "PHB-mapping"

approach can support this scenario:

TCP/UDP   In Profile  ->   AFx1
TCP       Out Profile ->   AFx2
UDP       Out Profile ->   AFx3

In this scenario both TCP and UDP would achieve their committed rates.
In the face of persistent congestion, TCP and UDP would share the excess
bandwidth.  This assumes that some excess bandwidth has been
provisioned.  WRED algorithms that maintain per drop level packet counts
further aid the fair allocation of the excess bandwidth.

c) Separate treatment (with two drop precedence)
TCP and UDP may each be treated as a separate class, given the
inadequate sharing of excess bandwidth in scenario "a" above. For
example:

TCP in profile  ->   AFx1
TCP out profile ->   AFx2

UDP in profile  ->   AFy1
UDP out profile ->   AFy2

Use of ABR to support the AF-PHB enables scenario "b" the "integrated
treatment with three drop precedence".  This saves on the number of AF
classes in use and hence the number of virtual circuits required.

8.0  Security Considerations

As with any other provisioned services, the ATM network must make use of
its capabilities for call admission control and police against sources
attempting to utilize more network resources than their service contract
allows.  In addition, when the ABR service class is used, the network
provides flow control feedback to sources.  This allows these sources to
avoid data loss on the ATM network if congestion occurs.  Congestion may
be due to denial of service attacks from other sources or temporary
network outages.

9.0  References
   [1] Blake, S., Black, D., Carlson, M., Davis, E., Wang, W., Weiss,
W., "An Architecture for Differentiated Services",  RFC 2475, December
1998.

   [] Jacobson, V., Nichols, K., Poduri, K., "An Expedited Forwarding
PHB", RFC 2598, June 1999.

   [] Heinanen, J., Baker, F., Weiss, W., Wroclawski, J, "Assured
Forwarding PHB Group", RFC 2597, June 1999.

   [] ATM Forum Traffic Management 4.1 Specification, ATM Forum/af-tm-
0121.000, March 1999.

   [] Addendum to TM 4.1: Enhancements to Support IP Differentiated

65

Services and IEEE 802.1D over ATM, ATM Forum BTD-TM-DIFF-01.02 (work in
progress), November/December 1999.

   [] Romanow, A., Floyd, S., "Dynamics of TCP traffic over ATM

networks", Proceedings of SIGCOMM'94, September 94.

   [] Braden, B., Clark, D., Crowcroft, J. , Davie, B. , Deering, S. ,
Estrin, D., Floyd, S., Jacobson, V., Minshall, G., Partridge, C.,
Peterson, L., Ramakrishnan, K., Shenker, S., Wroclawski, J., Zhang, L.,
Recommendations on Queue Management and Congestion Avoidance in the
Internet, April 1998. Available as RFC 2309 ( text) as an Informational
RFC

   [] Fahmy, S., Jain, R., Rabie, S., Goyal, R.,Vandalore, B., "Quality
of Service for Internet Traffic over ATM Service Categories," Journal of
Computer Communications special issue on enterprise networks, 1999

   [] Vandalore, B., Kalyanaraman, S., Jain, R., Goyal, R., Fahmy, S.,
"Simulation Study of World Wide Web traffic over the ATM ABR Service,"
Proceedings of SPIE Symposium on Voice, Video and Data Communications,
Vol. 3530, Conference on Performance and Control of Network Systems II,
Boston, MA, November 1998, pp. 415-422

   [] Pazos, C. M. D., Signore, V. A., Cavendish Jr., D., Gerla, M.,
"Performance of TCP over ATM for Various ABR Control Policies,"
Proceedings of ICCCN'96, Rockville, MD. October, 1996.

   [] Goyal, M., Durresi, A., Jain, R., Chunlei, L., "Effect of Number
of Drop Precedence in Assured Forwarding", draft-goyal-diffserv-dpstdy-
02, July 1999

   [] Elloumi, O., Cnodder, S., Pauwels, K., "Usefulness of three drop
precedence in Assured Forwarding service", draft-elloumi-diffserv-
threevstwo-00.txt, July 1999

10.0  Authors' Addresses
Siamack Ayandeh
Lucent Technologies
 Robbins Road,
Westford, MA, 01886
(978) 952 7866
sayandeh@ascend.com

Anand Krishnamurthy
Lucent Technologies
 Robbins Road,
Westford, MA, 01886
(978) 952 1448
ak26@lucent.com

Andrew Malis
Lucent Technologies
 Robbins Road,
Westford, MA, 01886
(978) 952 7414
amalis@lucent.com