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Internet Draft S. Berson
Expiration: September 1997 ISI
File: draft-ietf-issll-atm-support-03.txt L. Berger
FORE Systems
IP Integrated Services with RSVP over ATM
March 26, 1997
Status of Memo
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Abstract
This draft describes a method for providing IP Integrated Services
with RSVP over ATM switched virtual circuits (SVCs). It provides an
overall approach to the problem as well as a specific method for
running over today's ATM networks. There are two parts of this
problem. This draft provides guidelines for using ATM VCs with QoS
as part of an Integrated Services Internet. A related draft[6]
describes service mappings between IP Integrated Services and ATM
services.
Authors' Note
The postscript version of this document contains figures that are not
included in the text version, so it is best to use the postscript
version. Figures will be converted to ASCII in a future version.
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Table of Contents
1. Introduction ........................................................3
1.1 Terms ...........................................................4
1.2 Assumptions .....................................................4
2. Policy ..............................................................6
3. Data VC Management ..................................................7
3.1 Reservation to VC Mapping .......................................7
3.2 Heterogeneity ...................................................8
3.3 Multicast End-Point Identification ..............................12
3.4 Multicast Data Distribution .....................................13
3.5 Receiver Transitions ............................................14
3.6 Dynamic QoS .....................................................14
4. Tear down old VC ............................................16
5. Activate timer ..............................................16
6. Security ............................................................21
7. Future Work .........................................................21
8. Authors' Addresses ..................................................23
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1. Introduction
The Internet currently has one class of service normally referred to
as "best effort." This service is typified by first-come, first-
serve scheduling at each hop in the network. Best effort service has
worked well for electronic mail, World Wide Web (WWW) access, file
transfer (e.g. ftp), etc. For real-time traffic such as voice and
video, the current Internet has performed well only across unloaded
portions of the network. In order to provide guaranteed quality
real-time traffic, new classes of service and a QoS signalling
protocol are being introduced in the Internet[7,18,17], while
retaining the existing best effort service. The QoS signalling
protocol is RSVP[8,19], the Resource ReSerVation Protocol.
ATM is rapidly becoming an important link layer technology. One of
the important features of ATM technology is the ability to request a
point-to-point Virtual Circuit (VC) with a specified Quality of
Service (QoS). An additional feature of ATM technology is the ability
to request point-to-multipoint VCs with a specified QoS. Point-to-
multipoint VCs allows leaf nodes to be added and removed from the VC
dynamically and so provide a mechanism for supporting IP multicast.
It is only natural that RSVP and the Internet Integrated Services
(IIS) model would like to utilize the QoS properties of any
underlying link layer including ATM.
Classical IP over ATM[11] has solved part of this problem, supporting
IP unicast best effort traffic over ATM. Classical IP over ATM is
based on a Logical IP Subnetwork (LIS), which is a separately
administered IP sub-network. Hosts within a LIS communicate using
the ATM network, while hosts from different sub-nets communicate only
by going through an IP router (even though it may be possible to open
a direct VC between the two hosts over the ATM network). Classical
IP over ATM provides an Address Resolution Protocol (ATMARP) for ATM
edge devices to resolve IP addresses to native ATM addresses. For
any pair of IP/ATM edge devices (i.e. hosts or routers), a single VC
is created on demand and shared for all traffic between the two
devices. A second part of the RSVP and IIS over ATM problem, IP
multicast, is close to being solved with MARS[1], the Multicast
Address Resolution Server. MARS compliments ATMARP by allowing an IP
address to resolve into a list of native ATM addresses, rather than
just a single address.
A key remaining issue for IP over ATM is the integration of RSVP
signalling and ATM signalling in support of the Internet Integrated
Services (IIS) model. There are two main areas involved in
supporting the IIS model, QoS translation and VC management. QoS
translation concerns mapping a QoS from the IIS model to a proper ATM
QoS, while VC management concentrates on how many VCs are needed and
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which traffic flows are routed over which VCs. Mapping of IP QoS to
ATM QoS is the subject of a companion draft[6].
This draft concentrates on VC management (and we assume in this draft
that the QoS for a single reserved flow can be acceptably translated
to an ATM QoS). Two types of VCs need to be managed, data VCs which
handle the actual data traffic, and control VCs which handle the RSVP
signalling traffic. Several VC management schemes for both data and
control VCs are described in this draft. For each scheme, there are
two major issues - (1) heterogeneity and (2) dynamic behavior.
Heterogeneity refers to how requests for different QoS's are handled,
while dynamic behavior refers to how changes in QoS and changes in
multicast group membership are handled. These schemes will be
evaluated in terms of the following metrics - (1) number of VCs
needed to implement the scheme, (2) bandwidth wasted due to duplicate
packets, and (3) flexibility in handling heterogeneity and dynamic
behavior. Examples where each scheme is most applicable are given.
1.1 Terms
The terms "reservation" and "flow" are used in many contexts,
often with different meaning. These terms are used in this
document with the following meaning:
o Reservation is used in this document to refer to an RSVP
initiated request for resources. RSVP initiates requests for
resources based on RESV message processing. RESV messages
that simply refresh state do not trigger resource requests.
Resource requests may be made based on RSVP sessions and RSVP
reservation styles. RSVP styles dictate whether the reserved
resources are used by one sender or shared by multiple
senders. See [8] for details of each. Each new request is
referred to in this document as an RSVP reservation, or
simply reservation.
o Flow is used to refer to the data traffic associated with a
particular reservation. The specific meaning of flow is RSVP
style dependent. For shared style reservations, there is one
flow per session. For distinct style reservations, there is
one flow per sender (per session).
1.2 Assumptions
The following assumptions are made:
o Support for IPv4 and IPv6 best effort in addition to QoS
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o Use RSVP with policy control as signalling protocol
o Assume UNI 3.x and 4.0 ATM services
o VCs initiation by sub-net senders
1.2.1 IPv4 and IPv6
Currently IPv4 is the standard protocol of the Internet which
now provides only best effort service. We assume that best
effort service will continue to be supported while introducing
new types of service according to the IP Integrated Services
model. We also assume that IPv6 will be supported as well as
IPv4.
1.2.2 RSVP and Policy
We assume RSVP as the Internet signalling protocol which is
described in [19]. The reader is assumed to be familiar with
[19].
IP Integrated Services discriminates between users by providing
some users better service at the expense of others. For ATM,
preferential service reflects when a new VC is created for a
user rather than joining an existing VC. Policy determines how
preferential services are allocated while allowing network
operators maximum flexibility to provide value-added services
for the marketplace. Mechanisms need to be be provided to
enforce access policies. These mechanisms may include such
things as permissions and/or billing.
For scaling reasons, policies based on bilateral agreements
between neighboring providers are considered. The bilateral
model has similar scaling properties to multicast while
maintaining no global information. Policy control is currently
being developed for RSVP (see [10] for details).
1.2.3 ATM
We assume ATM defined by UNI 3.x and 4.0, plus TM 4.0. ATM
provides both point-to-point and point-to-multipoint Virtual
Circuits (VCs) with a specified Quality of Service (QoS). ATM
provides both Permanent Virtual Circuits (PVCs) and Switched
Virtual Circuits (SVCs). In the Permanent Virtual Circuit
(PVC) environment, PVCs are typically used as point-to-point
link replacements. So the Integrated Services support issues
are similar to point-to-point links. This draft describes
schemes for supporting Integrated Services using SVCs.
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1.2.4 VC Initiation
There is an apparent mismatch between RSVP and ATM.
Specifically, RSVP control is receiver oriented and ATM control
is sender oriented. This initially may seem like a major
issue, but really is not. While RSVP reservation (RESV)
requests are generated at the receiver, actual allocation of
resources takes place at the sub-net sender.
For data flows, this means that sub-net senders will establish
all QoS VCs and the sub-net receiver must be able to accept
incoming QoS VCs. These restrictions are consistent with RSVP
version 1 processing rules and allow senders to use different
flow to VC mappings and even different QoS renegotiation
techniques without interoperability problems. All RSVP over
ATM approaches that have VCs initiated and controlled by the
sub-net senders will interoperate. Figure shows this model of
data flow VC initiation.
[Figure goes here]
Figure 1: Data Flow VC Initiation
The use of the reverse path provided by point-to-point VCs by
receivers is for further study. There are two related issues.
The first is that use of the reverse path requires the VC
initiator to set appropriate reverse path QoS parameters. The
second issue is that reverse paths are not available with
point-to-multipoint VCs, so reverse paths could only be used to
support unicast RSVP reservations. Receivers initiating VCs
via the reverse path mechanism provided by point-to-point VCs
is also for future study.
2. Policy
RSVP allows for local policy control [10] as well as admission
control. Thus a user can request a reservation with a specific QoS
and with a policy object that, for example, offers to pay for
additional costs setting up a new reservation. The policy module at
the entry to a provider can decide how to satisfy that request -
either by merging the request in with an existing reservation or by
creating a new reservation for this (and perhaps other) users. This
policy can be on a per user-provider basis where a user and a
provider have an agreement on the type of service offered, or on a
provider-provider basis, where two providers have such an agreement.
With the ability to do local policy control, providers can offer
services best suited to their own resources and their customers
needs.
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Policy is expected to be provided as a generic API which will return
values indicating what action should be taken for a specific
reservation request. The API is expected to have access to the
reservation tables with the QoS for each reservation. The RSVP
Policy and Integrity objects will be passed to the policy() call.
Four possible return values are expected. The request can be
rejected. The request can be accepted as is. The request can be
accepted but at a different QoS. The request can cause a change of
QoS of an existing reservation. The information returned from this
call will be used to call the admission control interface. As this
area is critical to deployment, progress will need to be made in this
area.
3. Data VC Management
Any RSVP over ATM implementation must map RSVP and RSVP associated
data flows to ATM Virtual Circuits (VCs). LAN Emulation [2],
Classical IP [11] and, more recently, NHRP [12] discuss mapping IP
traffic onto ATM SVCs, but they only cover a single QoS class, i.e.,
best effort traffic. When QoS is introduced, VC mapping must be
revisited. For RSVP controlled QoS flows, one issue is VCs to use for
QoS data flows.
In the Classic IP over ATM and current NHRP models, a single point-
to-point VC is used for all traffic between two ATM attached hosts
(routers and end-stations). It is likely that such a single VC will
not be adequate or optimal when supporting data flows with multiple
QoS types. RSVP's basic purpose is to install support for flows with
multiple QoS types, so it is essential for any RSVP over ATM solution
to address VC usage for QoS data flows.
This section describes issues and methods for management of VCs
associated with QoS data flows. When establishing and maintaining
VCs, the sub-net sender will need to deal with several complicating
factors including multiple QoS reservations, requests for QoS
changes, ATM short-cuts, and several multicast specific issues. The
multicast specific issues result from the nature of ATM connections.
The key multicast related issues are heterogeneity, data
distribution, receiver transitions, and end-point identification.
3.1 Reservation to VC Mapping
There are various approaches available for mapping reservations on
to VCs. A distinguishing attribute of all approaches is how
reservations are combined on to individual VCs. When mapping
reservations on to VCs, individual VCs can be used to support a
single reservation, or reservation can be combined with others on
to "aggregate" VCs. In the first case, each reservation will be
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supported by one or more VCs. Multicast reservation requests may
translate into the setup of multiple VCs as is described in more
detail in section 3.2. Unicast reservation requests will always
translate into the setup of a single QoS VC. In both cases, each
VC will only carry data associated with a single reservation. The
greatest benefit if this approach is ease of implementation, but
it comes at the cost of increased (VC) setup time and the
consumption of greater number of VC and associated resources.
We refer to the other case, when reservations are not combined,
as the "aggregation" model. With this model, large VCs could be
set up between IP routers and hosts in an ATM network. These VCs
could be managed much like IP Integrated Service (IIS) point-to-
point links (e.g. T-1, DS-3) are managed now. Traffic from
multiple sources over multiple RSVP sessions might be multiplexed
on the same VC. This approach has a number of advantages. First,
there is typically no signalling latency as VCs would be in
existence when the traffic started flowing, so no time is wasted
in setting up VCs. Second, the heterogeneity problem (section
3.2) in full over ATM has been reduced to a solved problem.
Finally, the dynamic QoS problem (section 3.6) for ATM has also
been reduced to a solved problem. This approach can be used with
point-to-point and point-to-multipoint VCs. The problem with the
aggregation approach is that the choice of what QoS to use for
which of the VCs is difficult, but is made easier since the VCs
can be changed as needed. The advantages of this scheme makes
this approach an item for high priority study.
3.2 Heterogeneity
Heterogeneity occurs when receivers request different QoS's within
a single session. This means that the amount of requested
resources differs on a per next hop basis. A related type of
heterogeneity occurs due to best-effort receivers. In any IP
multicast group, it is possible that some receivers will request
QoS (via RSVP) and some receivers will not. Both types of
heterogeneity are shown in figure . In shared media, like
Ethernet, receivers that have not requested resources can
typically be given identical service to those that have without
complications. This is not the case with ATM. In ATM networks,
any additional end-points of a VC must be explicitly added. There
may be costs associated with adding the best-effort receiver, and
there might not be adequate resources. An RSVP over ATM solution
will need to support heterogeneous receivers even though ATM does
not currently provide such support directly.
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[Figure goes here]
Figure 2: Types of Multicast Receivers
RSVP heterogeneity is supported over ATM in the way RSVP
reservations are mapped into ATM VCs. There are multiple models
for supporting RSVP heterogeneity over ATM. Section 3.2.1
examines the multiple VCs per RSVP reservation (or full
heterogeneity) model where a single reservation can be forwarded
into several VCs each with a different QoS. Section 3.2.2
presents a limited heterogeneity model where exactly one QoS VC is
used along with a best effort VC. Section 3.2.3 examines the VC
per RSVP reservation (or homogeneous) model, where each RSVP
reservation is mapped to a single ATM VC. Section 3.2.4 describes
the aggregation model allowing aggregation of multiple RSVP
reservations into a single VC. Further study is being done on the
aggregation model.
3.2.1 Full Heterogeneity Model
We define the "full heterogeneity" model as providing a
separate VC for each distinct QoS for a multicast session
including best effort and one or more QoS's. This is shown in
figure where S1 is a sender, R1-R3 are receivers, r1-r4 are IP
routers, and s1-s2 are ATM switches. Receivers R1 and R3 make
reservations with different QoS while R2 is a best effort
receiver. Three point-to-multipoint VCs are created for this
situation, each with the requested QoS. Note that any leafs
requesting QoS 1 or QoS 2 would be added to the existing QoS
VC.
[Figure goes here]
Figure 3: Full heterogeneity
Note that while full heterogeneity gives users exactly what
they request, it requires more resources of the network than
other possible approaches. In figure , three copies of each
packet are sent on the link from r1 to s1. Two copies of each
packet are then sent from s1 to s2. The exact amount of
bandwidth used for duplicate traffic depends on the network
topology and group membership.
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3.2.2 Limited Heterogeneity Model
An important special case of full heterogeneity is limited
heterogeneity. We define the "limited heterogeneity" model as
the case where the receivers of a multicast session are limited
to use either best effort service or a single alternate quality
of service. The alternate QoS can be chosen either by higher
level protocols or by dynamic renegotiation of QoS as described
below.
[Figure goes here]
Figure 4: Limited heterogeneity
In order to support limited heterogeneity, each ATM edge device
participating in a session would need at most two VCs. One VC
would be a point-to-multipoint best effort service VC and would
serve all best effort service IP destinations for this RSVP
session. The other VC would be a point to multipoint VC with
QoS and would serve all IP destinations for this RSVP session
that have an RSVP reservation established. This is shown in
figure where there are three receivers, R2 requesting best
effort service, while R1 and R3 request distinct reservations.
Whereas, in figure , R1 and R3 have a separate VC, so each
receives precisely the resources requested, in figure , R1 and
R3 share the same VC (using the maximum of R1 and R3 QoS)
across the ATM network. Note that though the VC and hence the
QoS for R1 and R3 are the same within the ATM cloud, the
reservation outside the ATM cloud (from router r4 to receiver
R3) uses the QoS actually requested by R3.
As with full heterogeneity, a disadvantage of the limited
heterogeneity scheme is that each packet will need to be
duplicated at the network layer and one copy sent into each of
the 2 VCs. Again, the exact amount of excess traffic will
depend on the network topology and group membership. Looking
at figure , there are two VCs going from router r1 to switch
s1. Two copies of every packet will traverse the r1-s1 link.
Another disadvantage of limited heterogeneity is that a
reservation request can be rejected even when the resources are
available. This occurs when a new receiver requests a larger
QoS. If any of the existing QoS VC end-points cannot upgrade
to the new QoS, then the new reservation fails though the
resources exist for the new receiver.
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3.2.3 Homogeneous and Modified Homogeneous Models
We define the "homogeneous" model as the case where all
receivers of a multicast session use a single quality of
service VC. Best-effort receivers also use the single RSVP
triggered QoS VC. The single VC can be a point-to-point or
point-to-multipoint as appropriate. The QoS VC is sized to
provide the maximum resources requested by all RSVP next-hops.
This model matches the way the current RSVP specification
addresses heterogeneous requests. The current processing rules
and traffic control interface describe a model where the
largest requested reservation for a specific outgoing interface
is used in resource allocation, and traffic is transmitted at
the higher rate to all next-hops. This approach would be the
simplest method for RSVP over ATM implementations.
While this approach is simple to implement, providing better
than best-effort service may actually be the opposite of what
the user desires since in providing ATM QoS. There may be
charges incurred or resources that are wrongfully allocated.
There are two specific problems. The first problem is that a
user making a small or no reservation would share a QoS VC
resources without making (and perhaps paying for) an RSVP
reservation. The second problem is that a receiver may not
receive any data. This may occur when there is insufficient
resources to add a receiver. The rejected user would not be
added to the single VC and it would not even receive traffic on
a best effort basis.
Not sending data traffic to best-effort receivers because of
another receiver's RSVP request is clearly unacceptable. The
previously described limited heterogeneous model ensures that
data is always sent to both QoS and best-effort receivers, but
it does so by requiring replication of data at the sender in
all cases. It is possible to extend the homogeneous model to
both ensure that data is always sent to best-effort receivers
and also to avoid replication in the normal case. This
extension is to add special handling for the case where a
best-effort receiver cannot be added to the QoS VC. In this
case, a best-effort VC can be established to any receivers that
could not be added to the QoS VC. Only in this special error
case would senders be required to replicate data. We define
this approach as the "modified homogeneous" model.
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3.2.4 Aggregation
The last scheme is the multiple RSVP reservations per VC (or
aggregation) model. With this model, large VCs could be set up
between IP routers and hosts in an ATM network. These VCs
could be managed much like IP Integrated Service (IIS) point-
to-point links (e.g. T-1, DS-3) are managed now. Traffic from
multiple sources over multiple RSVP sessions might be
multiplexed on the same VC. This approach has a number of
advantages. First, there is typically no signalling latency as
VCs would be in existence when the traffic started flowing, so
no time is wasted in setting up VCs. Second, the heterogeneity
problem in full over ATM has been reduced to a solved problem.
Finally, the dynamic QoS problem for ATM has also been reduced
to a solved problem. This approach can be used with point-to-
point and point-to-multipoint VCs. The problem with the
aggregation approach is that the choice of what QoS to use for
which of the VCs is difficult, but is made easier since the VCs
can be changed as needed. The advantages of this scheme makes
this approach an item for high priority study.
Multiple options for mapping reservations onto VCs have been
discussed. No matter which model or combination of models is
used by an implementation, implementations must not normally
send more than one copy of a particular data packet to a
particular next-hop (ATM end-point). Some transient over
transmission is acceptable, but only during VC setup and
transition. Implementations must also ensure that data traffic
is sent to best-effort receivers. Data traffic may be sent to
best-effort receivers via best-effort or QoS VCs as is
appropriate for the implemented model. In all cases,
implementations must not create VCs in such a way that data
cannot be sent to best-effort receivers. This includes the
case of not being able to add a best-effort receiver to a QoS
VC, but does not include the case where best-effort VCs cannot
be setup. The failure to establish best-effort VCs is
considered to be a general IP over ATM failure and is therefore
beyond the scope of this document.
3.3 Multicast End-Point Identification
Implementations must be able to identify ATM end-points
participating in an IP multicast group. The ATM end-points will
be IP multicast receivers and/or next-hops. Both QoS and best-
effort end-points must be identified. RSVP next-hop information
will provide QoS end-points, but not best-effort end-points.
Another issue is identifying end-points of multicast traffic
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handled by non-RSVP capable next-hops. In this case a PATH
message travels through a non-RSVP egress router on the way to the
next hop RSVP node. When the next hop RSVP node sends a RESV
message it may arrive at the source over a different route than
what the data is using. The source will get the RESV message, but
will not know which egress router needs the QoS. For unicast
sessions, there is no problem since the ATM end-point will be the
IP next-hop router. Unfortunately, multicast routing may not be
able to uniquely identify the IP next-hop router. So it is
possible that a multicast end-point can not be identified.
In the most common case, MARS will be used to identify all end-
points of a multicast group. In the router to router case, a
multicast routing protocol may provide all next-hops for a
particular multicast group. In either case, RSVP over ATM
implementations must obtain a full list of end-points, both QoS
and non-QoS, using the appropriate mechanisms. The full list can
be compared against the RSVP identified end-points to determine
the list of best-effort receivers.
There is no straightforward solution to uniquely identifying end-
points of multicast traffic handled by non-RSVP next hops. The
preferred solution is to use multicast routing protocols that
support unique end-point identification. In cases where such
routing protocols are unavailable, all IP routers that will be
used to support RSVP over ATM should support RSVP.
3.4 Multicast Data Distribution
Two models are planned for IP multicast data distribution over
ATM. In one model, senders establish point-to-multipoint VCs to
all ATM attached destinations, and data is then sent over these
VCs. This model is often called "multicast mesh" or "VC mesh"
mode distribution. In the second model, senders send data over
point-to-point VCs to a central point and the central point relays
the data onto point-to-multipoint VCs that have been established
to all receivers of the IP multicast group. This model is often
referred to as "multicast server" mode distribution. Figure shows
data flow for both modes of IP multicast data distribution. RSVP
over ATM solutions must ensure that IP multicast data is
distributed with appropriate QoS.
[Figure goes here]
Figure 5: IP Multicast Data Distribution Over ATM
In the Classical IP context, multicast server support is provided
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via MARS[1]. MARS does not currently provide a way to communicate
QoS requirements to a MARS multicast server (MCS). When using
multicast servers that do not support QoS requests, a sender must
set the service, not global, break bit(s).
3.5 Receiver Transitions
When setting up a point-to-multipoint VCs there will be a time
when some receivers have been added to a QoS VC and some have not.
During such transition times it is possible to start sending data
on the newly established VC. The issue is when to start send data
on the new VC. If data is sent both on the new VC and the old VC,
then data will be delivered with proper QoS to some receivers and
with the old QoS to all receivers. This means the QoS receivers
would get duplicate data. If data is sent just on the new QoS VC,
the receivers that have not yet been added will lose information.
So, the issue comes down to whether to send to both the old and
new VCs, or to send to just one of the VCs. In one case duplicate
information will be received, in the other some information may
not be received. This issue needs to be considered for three
cases: when establishing the first QoS VC, when establishing a VC
to support a QoS change, and when adding a new end-point to an
already established QoS VC.
The first two cases are very similar. It both, it is possible to
send data on the partially completed new VC, and the issue of
duplicate versus lost information is the same.
The last case is when an end-point must be added to an existing
QoS VC. In this case the end-point must be both added to the QoS
VC and dropped from a best-effort VC. The issue is which to do
first. If the add is first requested, then the end-point may get
duplicate information. If the drop is requested first, then the
end-point may loose information.
In order to ensure predictable behavior and delivery of data to
all receivers, data can only be sent on a new VCs once all parties
have been added. This will ensure that all data is only delivered
once to all receivers. This approach does not quite apply for the
last case. In the last case, the add should be completed first,
then the drop. This means that receivers must be prepared to
receive some duplicate packets at times of QoS setup.
3.6 Dynamic QoS
RSVP provides dynamic quality of service (QoS) in that the
resources that are requested may change at any time. There are
several common reasons for a change of reservation QoS. First, an
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existing receiver can request a new larger (or smaller) QoS if the
current received quality is unacceptable. Second, a sender may
change its traffic specification (TSpec), which can trigger a
change in the reservation requests of the receivers. Third, a new
sender can start sending to a multicast group with a larger
traffic specification than existing senders, triggering larger
reservations. Finally, a new receiver can make a reservation that
is larger than existing reservations. If the limited heterogeneity
model is being used and the merge node for the larger reservation
is an ATM edge device, a new larger reservation must be set up
across the ATM network.
Since ATM service, as currently defined in UNI 3.x and UNI 4.0,
does not allow renegotiating the QoS of a VC, dynamically changing
the reservation means creating a new VC with the new QoS, and
tearing down an established VC. Tearing down a VC and setting up
a new VC in ATM are complex operations that involve a non-trivial
amount of processor time, and may have a substantial latency.
There are several options for dealing with this mismatch in
service. A specific approach will need to be a part of any RSVP
over ATM solution.
One method for supporting changes in RSVP reservations is to
attempt to replace an existing VC with a new appropriately sized
VC. During setup of the replacement VC, the old VC must be left in
place unmodified. The old VC is left unmodified to minimize
interruption of QoS data delivery. Once the replacement VC is
established, data transmission is shifted to the new VC, and the
old VC is then closed.
If setup of the replacement VC fails, then the old QoS VC should
continue to be used. When the new reservation is greater than the
old reservation, the reservation request should be answered with
an error. When the new reservation is less than the old
reservation, the request should be treated as if the modification
was successful. While leaving the larger allocation in place is
suboptimal, it maximizes delivery of service to the user.
Implementations should retry replacing the too large VC after some
appropriate elapsed time.
One additional issue is that only one QoS change can be processed
at one time per reservation. If the (RSVP) requested QoS is
changed while the first replacement VC is still being setup, then
the replacement VC is released and the whole VC replacement
process is restarted.
To limit the number of changes and to avoid excessive signalling
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load, implementations may limit the number of changes that will be
processed in a given period. One implementation approach would
have each ATM edge device configured with a time parameter tau
(which can change over time) that gives the minimum amount of time
the edge device will wait between successive changes of the QoS of
a particular VC. Thus if the QoS of a VC is changed at time t,
all messages that would change the QoS of that VC that arrive
before time t+tau would be queued. If several messages changing
the QoS of a VC arrive during the interval, redundant messages can
be discarded. At time t+tau, the remaining change(s) of QoS, if
any, can be executed.
The sequence of events for a single VC would be
1. Wait if timer is active
2. Establish VC with new QoS
3. Remap data traffic to new VC
4. Tear down old VC
5. Activate timer
There is an interesting interaction between heterogeneous
reservations and dynamic QoS. In the case where a RESV message is
received from a new next-hop and the requested resources are
larger than any existing reservation, both dynamic QoS and
heterogeneity need to be addressed. A key issue is whether to
first add the new next-hop or to change to the new QoS. This is a
fairly straight forward special case. Since the older, smaller
reservation does not support the new next-hop, the dynamic QoS
process should be initiated first. Since the new QoS is only
needed by the new next-hop, it should be the first end-point of
the new VC. This way signalling is minimized when the setup to
the new next-hop fails.
3.7 Short-Cuts
Short-cuts [12] allow ATM attached routers and hosts to directly
establish point-to-point VCs across LIS boundaries, i.e., the VC
end-points are on different IP sub-nets. The ability for short-
cuts and RSVP to interoperate has been raised as a general
question. The area of concern is the ability to handle asymmetric
short-cuts. Specifically how RSVP can handle the case where a
downstream short-cut may not have a matching upstream short-cut.
In this case, which is shown in figure , PATH and RESV messages
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following different paths.
[Figure goes here]
Figure 6: Asymmetric RSVP Message Forwarding With ATM Short-Cuts
Examination of RSVP shows that the protocol already includes
mechanisms that will support short-cuts. The mechanism is the
same one used to support RESV messages arriving at the wrong
router and the wrong interface. The key aspect of this mechanism
is RSVP only processing messages that arrive at the proper
interface and RSVP forwarding of messages that arrive on the wrong
interface. The proper interface is indicated in the NHOP object
of the message. So, existing RSVP mechanisms will support
asymmetric short-cuts.
The short-cut model of VC establishment still poses several issues
when running with RSVP. The major issues are dealing with
established best-effort short-cuts, when to establish short-cuts,
and QoS only short-cuts. These issues will need to be addressed by
RSVP implementations.
3.8 VC Teardown
RSVP can identify from either explicit messages or timeouts when a
data VC is no longer needed. Therefore, data VCs set up to
support RSVP controlled flows should only be released at the
direction of RSVP. VCs must not be timed out due to inactivity by
either the VC initiator or the VC receiver. This conflicts with
VCs timing out as described in RFC 1755[14], section 3.4 on VC
Teardown. RFC 1755 recommends tearing down a VC that is inactive
for a certain length of time. Twenty minutes is recommended. This
timeout is typically implemented at both the VC initiator and the
VC receiver. Although, section 3.1 of the update to RFC 1755[15]
states that inactivity timers must not be used at the VC receiver.
When this timeout occurs for an RSVP initiated VC, a valid VC with
QoS will be torn down unexpectedly. While this behavior is
acceptable for best-effort traffic, it is important that RSVP
controlled VCs not be torn down. If there is no choice about the
VC being torn down, the RSVP daemon must be notified, so a
reservation failure message can be sent.
4. RSVP Control VC Management
One last important issue is providing a data path for the RSVP
messages themselves. There are two main types of messages in RSVP,
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PATH and RESV. PATH messages are sent to a multicast address, while
RESV messages are sent to a unicast address. Other RSVP messages are
handled similar to either PATH or RESV [Note 1] So ATM VCs used for
RSVP signalling messages need to provide both unicast and multicast
functionality.
There are several different approaches for how to assign VCs to use
for RSVP signalling messages. The main approaches are:
o use same VC as data
o single VC per session
o single point-to-multipoint VC multiplexed among sessions
o multiple point-to-point VCs multiplexed among sessions
There are several different issues that affect the choice of how to
assign VCs for RSVP signalling. One issue is the number of
additional VCs needed for RSVP signalling. Related to this issue is
the degree of multiplexing on the RSVP VCs. In general more
multiplexing means less VCs. An additional issue is the latency in
dynamically setting up new RSVP signalling VCs. A final issue is
complexity of implementation. The remainder of this section
discusses the issues and tradeoffs among these different approaches
and suggests guidelines for when to use which alternative.
4.1 Mixed data and control traffic
In this scheme RSVP signalling messages are sent on the same VCs
as is the data traffic. The main advantage of this scheme is that
no additional VCs are needed beyond what is needed for the data
traffic. An additional advantage is that there is no ATM
signalling latency for PATH messages (which follow the same
routing as the data messages). However there can be a major
problem when data traffic on a VC is nonconforming. With
nonconforming traffic, RSVP signalling messages may be dropped.
While RSVP is resilient to a moderate level of dropped messages,
excessive drops would lead to repeated tearing down and re-
establishing QoS VCs, a very undesirable behavior for ATM. Due to
these problems, this is not a good choice for providing RSVP
signalling messages, even though the number of VCs needed for this
scheme is minimized.
One variation of this scheme is to use the best effort data path
_________________________
[Note 1] This can be slightly more complicated for RERR messages
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for signalling traffic. In this scheme, there is no issue with
nonconforming traffic, but there is an issue with congestion in
the ATM network.
RSVP provides some resiliency to message loss due to congestion,
but RSVP control messages should be offered a preferred class of
service. A related variation of this scheme that is hopeful but
requires further study is to have a packet scheduling algorithm
(before entering the ATM network) that gives priority to the RSVP
signalling traffic. This can be difficult to do at the IP layer.
One possible approach at the ATM layer would be to use the Cell
Loss Priority (CLP) bit for RSVP signalling traffic to ensure
better service.
4.2 Single RSVP VC per RSVP Reservation
In this scheme, there is a parallel RSVP signalling VC for each
RSVP reservation. This scheme results in twice the minimum number
of VCs, but means that RSVP signalling messages have the advantage
of a separate VC. This separate VC means that RSVP signalling
messages have their own traffic contract and compliant signalling
messages are not subject to dropping due to other noncompliant
traffic (such as can happen with the scheme in section 4.1). The
advantage of this scheme is its simplicity - whenever a data VC is
created, a separate RSVP signalling VC is created. The
disadvantage of the extra VC is that extra ATM signalling needs to
be done.
Additionally, this scheme requires twice the minimum number of VCs
and also additional latency, but is quite simple. This approach
would tend to work well on hosts.
4.3 Multiplexed point-to-multipoint RSVP VCs
In this scheme, there is a single point-to-multipoint RSVP
signalling VC for each unique ingress router and unique set of
egress routers. This scheme allows multiplexing of RSVP
signalling traffic that shares the same ingress router and the
same egress routers. This can save on the number of VCs, by
multiplexing, but there are problems when the destinations of the
multiplexed point-to-multipoint VCs are changing. Several
alternatives exist in these cases, that have applicability in
different situations. First, when the egress routers change, the
ingress router can check if it already has a point-to-multipoint
RSVP signalling VC for the new list of egress routers. If the
RSVP signalling VC already exists, then the RSVP signalling
traffic can be switched to this existing VC. If no such VC
exists, one approach would be to create a new VC with the new list
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of egress routers. Other approaches include modifying the
existing VC to add an egress router or using a separate new VC for
the new egress routers. When a destination drops out of a group,
an alternative would be to keep sending to the existing VC even
though some traffic is wasted.
The number of VCs used in this scheme is a function of traffic
patterns across the ATM network, but is always less than the
number used with the Single RSVP VC per data VC. In addition,
existing best effort data VCs could be used for RSVP signalling.
Reusing best effort VCs saves on the number of VCs at the cost of
higher probability of RSVP signalling packet loss. One possible
place where this scheme will work well is in the core of the
network where there is the most opportunity to take advantage of
the savings due to multiplexing. The exact savings depend on the
patterns of traffic and the topology of the ATM network.
4.4 Multiplexed point-to-point RSVP VCs
In this scheme, multiple point-to-point RSVP signalling VCs are
used for a single point-to-multipoint data VC. This scheme allows
multiplexing of RSVP signalling traffic but requires the same
traffic to be sent on each of several VCs. This scheme is quite
flexible and allows a large amount of multiplexing. Since point-
to-point VCs can set up a reverse channel at the same time as
setting up the forward channel, this scheme could save
substantially on signalling cost. In addition, signalling traffic
could share existing best effort VCs. Sharing existing best
effort VCs reduces the total number of VCs needed, but might cause
signalling traffic drops if there is congestion in the ATM
network.
This point-to-point scheme would work well in the core of the
network where there is much opportunity for multiplexing. Also in
the core of the network, RSVP VCs can stay permanently established
either as Permanent Virtual Circuits (PVCs) or as long lived
Switched Virtual Circuits (SVCs). The number of VCs in this
scheme will depend on traffic patterns, but in the core of a
network would be approximately n(n-1)/2 where n is the number of
IP nodes in the network. In the core of the network, this will
typically be small compared to the total number of VCs.
4.5 QoS for RSVP VCs
There is an issue for what QoS, if any, to assign to the RSVP VCs.
Three solutions have been covered in section 4.1 and in the shared
best effort VC variations in sections 4.4 and 4.3. For other RSVP
VC schemes, a QoS (possibly best effort) will be needed. What QoS
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to use partially depends on the expected level of multiplexing
that is being done on the VCs, and the expected reliability of
best effort VCs. Since RSVP signalling is infrequent (typically
every 30 seconds), only a relatively small QoS should be needed.
This is important since using a larger QoS risks the VC setup
being rejected for lack of resources. Falling back to best effort
when a QoS call is rejected is possible, but if the ATM net is
congested, there will likely be problems with RSVP packet loss on
the best effort VC also. Additional experimentation is needed in
this area.
5. Encapsulation
Since RSVP is a signalling protocol used to control flows of IP data
packets, encapsulation for both RSVP packets and associated IP data
packets must be defined. There are currently two encapsulation
options for running IP over ATM, RFC 1483 and LANE. There is also
the possibility of future encapsulation options, such as MPOA[3].
The first option is described in RFC 1483[9] and is currently used
for "Classical" IP over ATM and NHRP.
The second option is LAN Emulation, as described in [2]. LANE
encapsulation does not currently include a QoS signalling interface.
If LANE encapsulation is needed, LANE QoS signalling would first need
to be defined by the ATM Forum. It is possible that LANE 2.0 will
include the required QoS support.
6. Security
The same considerations stated in [8] and [14] apply to this
document. There are no additional security issues raised in this
document.
7. Future Work
We have described a set of schemes for deploying RSVP over IP over
ATM. There are a number of other issues that are subjects of
continuing research. These issues (and others) are covered in [5],
and are briefly repeated here.
A major issue is providing policy control for ATM VC creation. There
is work going on in the RSVP working group [8] on defining an
architecture for policy support. Further work is needed in defining
an API and policy objects. As this area is critical to deployment,
progress will need to be made in this area.
NHRP provides advantages in allowing short-cuts across 2 or more
LIS's. Short cutting router hops can lead to more efficient data
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delivery. Work on NHRP is on-going, but currently provides only a
unicast delivery service. Further study is needed to determine how
NHRP can be used with RSVP and ATM. Future work depends on the
development of NHRP for multicast.
Furthermore, when using RSVP it may be desirable to establish
multiple short-cut VCs, to use these VCs for specific QoS flows, and
to use the hop-by-hop path for other QoS and non-QoS flows. The
current NHRP specification [12] does not preclude such an approach,
but nor does it explicitly support it. We believe that explicit
support of flow based short-cuts would improve RSVP over ATM
solutions. We also believe that such support may require the ability
to include flow information in the NHRP request.
There is work in the ION working group on MultiCast Server (MCS)
architectures for MARS. An MCS provides savings in the number of VCs
in certain situations. When using a multicast server, the sub-
network sender could establish a point-to-point VC with a specific
QoS to the server, but there is not current mechanism to relay QoS
requirements to the MCS. Future work includes providing RSVP and ATM
support over MARS MCS's.
Unicast ATM VCs are inherently bi-directional and have the capability
of supporting a "reverse channel". By using the reverse channel for
unicast VCs, the number of VCs used can potentially be reduced.
Future work includes examining how the reverse VCs can be used most
effectively.
Current work in the ATM Forum and ITU promises additional advantages
for RSVP and ATM including renegotiating QoS parameters and
variegated VCs. QoS renegotiation would be particularly beneficial
since the only option available today for changing VC QoS parameters
is replacing the VC. It is important to keep current with changes in
ATM, and to keep this document up-to-date.
Scaling of the number of sessions is an issue. The key ATM related
implication of a large number of sessions is the number of VCs and
associated (buffer and queue) memory. The approach to solve this
problem is aggregation either at the RSVP layer or at the ISSLL layer
(or both).
This document describes approaches that can be used with ATM UNI4.0,
but does not make use of the available leaf-initiated join, or LIJ,
capability. The use of LIJ may be useful in addressing scaling
issues. The coordination of RSVP with LIJ remains a research issue.
Lastly, it is likely that LANE 2.0 will provide some QoS support
mechanisms, including proper QoS allocation for multicast traffic.
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It is important to track developments, and develop suitable RSVP over
ATM LANE at the appropriate time.
8. Authors' Addresses
Steven Berson
USC Information Sciences Institute
4676 Admiralty Way
Marina del Rey, CA 90292
Phone: +1 310 822 1511
EMail: berson@isi.edu
Lou Berger
FORE Systems
6905 Rockledge Drive
Suite 800
Bethesda, MD 20817
Phone: +1 301 571 2534
EMail: lberger@fore.com
REFERENCES
[1] Armitage, G., "Support for Multicast over UNI 3.0/3.1 based ATM
Networks," Internet Draft, February 1996.
[2] The ATM Forum, "LAN Emulation Over ATM Specification", Version 1.0.
[3] The ATM Forum, "MPOA Baseline Version 1", 95-0824r9, September 1996.
[4] Berson, S., "`Classical' RSVP and IP over ATM," INET '96, July 1996.
[5] Borden, M., Crawley, E., Krawczyk, J, Baker, F., and Berson, S.,
"Issues for RSVP and Integrated Services over ATM," Internet Draft,
February 1996.
[6] Borden, M., and Garrett, M., "Interoperation of Controlled-Load and
Guaranteed-Service with ATM," Internet Draft, June 1996.
[7] Braden, R., Clark, D., Shenker, S. "Integrated Services in the
Internet Architecture: an Overview," RFC 1633, June 1994.
[8] Braden, R., Zhang, L., Berson, S., Herzog, S., and Jamin, S.,
"Resource ReSerVation Protocol (RSVP) -- Version 1 Functional
Specification," Internet Draft, November 1996.
[9] Heinanen, J., "Multiprotocol Encapsulation over ATM Adaptation Layer
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5," RFC 1483.
[10] Herzog, S., "Accounting and Access Control Policies for Resource
Reservation Protocols," Internet Draft, June 1996.
[11] Laubach, M., "Classical IP and ARP over ATM," RFC 1577, January
1994.
[12] Luciani, J., Katz, D., Piscitello, D., Cole, B., "NBMA Next Hop
Resolution Protocol (NHRP)," Internet Draft, June 1996.
[13] Onvural, R., Srinivasan, V., "A Framework for Supporting RSVP Flows
Over ATM Networks," Internet Draft, March 1996.
[14] Perez, M., Liaw, F., Grossman, D., Mankin, A., Hoffman, E., and
Malis, A., "ATM Signalling Support for IP over ATM," RFC 1755.
[15] Perez, M., Mankin, A. "ATM Signalling Support for IP over ATM -
UNI 4.0 Update" Internet Draft, November 1996.
[16] "ATM User-Network Interface (UNI) Specification - Version 3.1",
Prentice Hall.
[17] Shenker, S., Partridge, C., Guerin, R., "Specification of
Guaranteed Quality of Service," Internet Draft, August 1996.
[18] Wroclawski, J., "Specification of the Controlled-Load Network
Element Service," Internet Draft, August, 1996.
[19] Zhang, L., Deering, S., Estrin, D., Shenker, S., Zappala, D.,
"RSVP: A New Resource ReSerVation Protocol," IEEE Network, September
1993.
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