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RFC 1821
Network Working Group M. Borden
Request for Comments: 1821 E. Crawley
Category: Informational Bay Networks
B. Davie
Bellcore
S. Batsell
NRL
August 1995
Integration of Real-time Services in an IP-ATM Network Architecture
Status of the Memo
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
Abstract
The IETF is currently developing an integrated service model which is
designed to support real-time services on the Internet.
Concurrently, the ATM Forum is developing Asynchronous Transfer Mode
networking which similarly provides real-time networking support. The
use of ATM in the Internet as a link layer protocol is already
occurring, and both the IETF and the ATM Forum are producing
specifications for IP over ATM. The purpose of this paper is to
provide a clear statement of what issues need to be addressed in
interfacing the IP integrated services environment with an ATM
service environment so as to create a seamless interface between the
two in support of end users desiring real-time networking services.
Table of Contents
1.0 Introduction 2
2.0 Problem Space Overview 3
2.1 Initial Assumptions 3
2.2 Topologies Under Consideration 4
2.3 Providing QoS in IP over ATM - a walk-though 5
3.0 Service Model Issues 6
3.1 Traffic Characterization 7
3.2 QoS Characterization 8
4.0 Resource Reservation Styles 10
4.1 RSVP 10
4.2 ST-II 13
4.3 Mapping IP flows to ATM Connections 15
5.0 End System Issues 16
6.0 Routing Issues 16
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RFC 1821 Real-time Service in IP-ATM Networks August 1995
6.1 Multicast routing 17
6.2 QoS Routing 17
6.3 Mobile Routing 18
7.0 Security Issues 19
8.0 Future Directions 20
9.0 References 22
10.0 Authors' Addresses 24
1.0 Introduction
The traditional network service on the Internet is best-effort
datagram transmission. In this service, packets from a source are
sent to a destination, with no guarantee of delivery. For those
applications that require a guarantee of delivery, the TCP protocol
will trade packet delay for correct reception by retransmitting those
packets that fail to reach the destination. For traditional
computer-communication applications such as FTP and Telnet in which
correct delivery is more important than timeliness, this service is
satisfactory. However, a new class of application which uses multiple
media (voice, video, and computer data) has begun to appear on the
Internet. Examples of this class of application are video
teleconferencing, video-on-demand, and distributed simulation. While
these applications can operate to some extent using best-effort
delivery, trading packet delay for correct reception is not an
acceptable trade-off. Operating in the traditional mode for these
applications results in reduced quality of the received information
and, potentially, inefficient use of bandwidth. To remedy this
problem the IETF is developing a real-time service environment in
which multiple classes of service are offered [6]. This environment
will greatly extend the existing best-effort service model to meet
the needs of multimedia applications with real-time constraints.
At the same time that this effort is underway in the IETF,
Asynchronous Transfer Mode (ATM) is being developed, initially as a
replacement for the current telephone network protocols, but more
recently as a link-layer protocol for computer communications. As it
was developed from the beginning with telephone voice applications in
mind, a real-time service environment is an integral part of the
protocol. With the approval of UNI 3.1 by the ATM Forum, the ATM
standards now have several categories of service. Given the wide
acceptance of ATM by the long-line carriers, the use of ATM in the
Internet is, if not guaranteed, highly likely. The question now
becomes, how can we successfully interface between the real-time
services offered by ATM and the new,integrated service environment
soon to be available in the IP protocol suite. The current IP over
ATM standards assume no real-time IP protocols. It is the purpose of
this RFC to clearly delineate what the issues are in integrating
real-time services in an IP-over-ATM network [10,15,19,20,21].
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RFC 1821 Real-time Service in IP-ATM Networks August 1995
In the IP-over-ATM environment, as in many others, multicast routing
adds an additional set of challenges. While the major focus of this
paper is quality of service (QoS) issues, it is unwise at best to
ignore multicast when considering these issues, especially since so
many of the applications that motivate the provision of real time QoS
also require efficient multicast support. We will therefore try to
keep considerations of multicast in the foreground in the following
discussion.
One of the primary motivations for this document is a belief by the
authors that ATM should, if possible, be used as more than a leased
line replacement. That is to say, while it is possible for the
Internet to be overlaid on constant bit rate (CBR), permanent virtual
circuits (PVCs), thus reducing IP over ATM to a previously solved
problem, we believe that this is unlikely to be the most efficient
way to use ATM services as they are offered by carriers or as they
appear in LANs. For example, a carrier offering a CBR service must
assume that the peak bit rate can be used continuously with no
degradation in quality and so resources must be allocated to the
connection to provide that service, even if the peak rate is in fact
rarely used. This is likely to make a CBR service more expensive that
a variable bit rate service of the same peak capacity. Another way
to view this is that the new IP service model will allow us to
associate information about the bandwidth requirements of
applications with individual flows; surely it is not wise to discard
this information when we request a service from an ATM subnet.
While we believe that there is a range of capabilities in ATM
networks that can be effectively used by a real-time Internet, we do
not believe that just because ATM has a capability, the Internet must
use it. Thus, our goal in this RFC is to begin to explore how an
Internet with real time service capability might make most effective
use of ATM networks. Since there are a number of problems to be
resolved to achieve this effective use, our major goal at this point
is to describe the scope of the problems that need to be addressed.
2.0 Problem Space Overview
In this section we aim to describe in high level terms the scope of
the problem that will be explored in more detail in later sections.
2.1 Initial Assumptions
We begin by assuming that an Integrated Services Internet, i.e., an
Internet with a range of qualities of service to support both real-
time and non-real-time applications, will eventually happen. A number
of working groups are trying to make this happen, notably
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* the Integrated Services group (int-serv), which is working to define
a new IP service model, including a set of services suited to a
range of real-time applications;
* the Resource reservation Setup Protocol group (rsvp), which is
defining a resource reservation protocol [7] by which the
appropriate service for an application can be requested from the
network;
* the Internet Streams Protocol V2 group (ST-II), which is updating
[27], a stream-oriented internet protocol that provides a range of
service qualities.
In addition, the IETF IP over ATM working group and the ATM Forum
Multiprotocol over ATM group are working to define a model for
protocols to make use of the ATM layer.
Since these groups have not yet generated standards, we will need to
do some amount of extrapolation to predict the problems that may
arise for IP over ATM. We also assume that the standards being
developed in the ATM Forum will largely determine the service model
for ATM. Again, some extrapolation may be needed. Given these
assumptions, this paper aims to explore ways in which a future
Integrated Services Internet might make effective use of ATM as it
seems likely to be deployed.
2.2 Topologies Under Consideration
Figure 1 shows a generic internetwork that includes ATM and non-ATM
subnetworks. This paper aims to outline the problems that must be
addressed to enable suitable quality of service to be provided end-
to-end across such a network. The problem space, therefore, includes
* communication across an 'ATM-only' network between two hosts
directly connected to the ATM network;
* communication between ATM-connected hosts which involves traversing
some non-ATM subnets;
* communication between a host on a non-ATM subnet and a host directly
connected to ATM;
* communication between two hosts, neither of which has a direct ATM
connection, but which may make use of one or more ATM networks for
some part of the path.
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RFC 1821 Real-time Service in IP-ATM Networks August 1995
[H]
| [H]
________|________________________ |
| | |
________|__ ______|___|____
| | | |
| ATM [R] [R] ATM |
| Cloud | | Cloud |___[H]
| | Non-ATM Internet | |
| | [R] |
|________[R] |_____________|
| | |
| | |
[H] |________________________________|
|
|
[H]
[H] = Host
[R] = Router
Figure 1
In the last case, the entities connected to the ATM network are IP
routers, and it is their job to manage the QoS provided by the ATM
network(s) in such a way that the desired end-to-end QoS is provided
to the hosts. While we wish to describe the problem space in a way
that covers all of these scenarios, the last is perhaps the most
general, so we will use it for most illustrative purposes. In
particular, we are explicitly not interested in ways of providing QoS
that are applicable only to a subset of these situations. We claim
that addressing these four situations is sufficiently general to
cover other situations such as those in which several ATM and non-ATM
networks are traversed.
It is worth mentioning that the ATM networks in this case might be
local or wide area, private or public. In some cases, this
distinction may be significant, e.g., because there may be economic
implications to a particular approach to providing QoS.
2.3 Providing QoS in IP over ATM - a walk-through
To motivate the following discussion, this section walks through an
example of what might happen when an application with a certain set
of QoS needs starts up. For this example, we will use the fourth case
mentioned above, i.e., two hosts connected to non-ATM networks,
making use of an ATM backbone.
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A generic discussion of this situation is made difficult by the fact
that the reservation of resources in the Internet may be sender or
receiver initiated, depending on the specifics of the setup protocol.
We will attempt to gloss over this distinction for now, although we
will return to it in Section 4. We will assume a unicast application
and that the traffic characteristics and the QoS requirements (such
as delay, loss, throughput) of the application are known to at least
one host. That host launches a request for the desired QoS and a
description of the expected traffic into the network; at some point
this request hits a router at the edge of the ATM network. The router
must examine the request and decide if it can use an existing
connection over the ATM network to honor the request or whether it
must establish a new connection. In the latter case, it must use the
QoS and traffic characterizations to decide what sort of ATM
connection to open and to describe the desired service to the ATM
network. It must also decide where to open the connection to. Once
the connection is opened, the request is forwarded across the ATM
network to the exit router and then proceeds across the non-ATM part
of the network by the normal means.
We can see from the above description that there are several sets of
issues to be discussed:
* How does the IP service model, with certain service classes and
associated styles of traffic and QoS characterization, map onto
the ATM service model?
* How does the IP reservation model (whatever it turns out to be) map
onto ATM signalling?
* How does IP over ATM routing work when service quality is added to
the picture?
These issues will be discussed in the following sections.
3.0 Service Model Issues
There are several significant differences between the ways in which
IP and ATM will provide QoS. When IP commits to provide a certain
QoS to an application according to the Internet service model, it
must be able to request an appropriate QoS from the ATM network using
the ATM service model. Since these service models are by no means the
same, a potentially complex mapping must be performed for the IP
layer to meet its commitments. The details of the differences
between ATM and IP and the problems presented by these differences
are described below.
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We may think of a real-time service model as containing the following
components:
* a way to characterize traffic (sometimes called the Tspec);
* a way to characterize the desired quality of service (the Rspec).
We label these components as traffic characterization and QoS
characterization. Each of these components is discussed in turn in
the following sections.
As well as these aspects of the service model, both ATM and IP will
have a number of mechanisms by which the model is implemented. The
mechanisms include admission control, policing, and packet
scheduling. A particularly important mechanism is the one by which
end-nodes communicate their QoS needs and traffic characteristics to
the network, and the network communicates admission control decisions
to the end-nodes. This is referred to as resource reservation or
signalling, and is the subject of Section 4. In fact, it seems to be
the only mechanism where significant issues of IP/ATM integration
arise. The details of admission control, policing and packet
scheduling are largely internal to a single network element and we do
not foresee significant problems caused by the integration of IP and
ATM. For example, while there may be plenty of challenges in
designing effective approaches to admission control for both IP and
ATM, it is not apparent that there are any special challenges for the
IP over ATM environment. As the walk-through of Section 2.3
described, a reservation request from a host would at some point
encounter the edge of the ATM cloud. At this point, either a new
connection needs to be set up across the ATM cloud, or the router can
decide to carry the requested traffic over an existing virtual
circuit. If the ATM cloud cannot create a new connection as
requested, this would presumably result in an admission control
failure which would cause the router to deny the reservation request.
3.1 Traffic Characterization
The traffic characterization provided by an application or user is
used by the network to make decisions about how to provide the
desired quality of service to this application and to assess the
effect the new flow will have on the service provided to existing
flows. Clearly this information feeds into the admission control
decision process.
In the Internet community, it is assumed that traffic will in general
be bursty and that bursty traffic can be characterized by a `token
bucket'. While ATM does not expect all traffic to be bursty (the
Continuous Bit Rate class being defined specifically for non-bursty
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traffic), it uses an essentially equivalent formulation for the
characterization of traffic that is bursty, referred to as the
Generic Cell Rate Algorithm (GCRA). However, ATM in some classes also
requires specification of peak cell rate, whereas peak rates are not
currently included in the IP traffic characterizations. It may be
possible to use incoming interface speeds to determine an approximate
peak rate.
One of the functions that must be performed in order to carry IP
traffic over an ATM network is therefore a mapping from the
characterization of the traffic as supplied to IP to a
characterization that is acceptable for ATM. While the similarity of
the two characterizations suggests that this is straightforward,
there is considerable flexibility in the mapping of parameters from
IP to ATM. As an extreme example, a router at the edge of an ATM
cloud that expects to receive bursts of IP packets on a non-ATM
interface, with the bursts described by some token bucket parameters,
could actually inject ATM cells at a constant rate into the ATM
network. This may be achieved without significant buffering if the
ATM link speed is faster than the point-to-point link speed;
alternatively, it could be achieved by buffering out the burstiness
of the arriving traffic. It seems more reasonable to map an IP flow
(or a group of flows) with variable bandwidth requirements onto an
ATM connection that accommodates variable bit rate traffic.
Determining how best to map the IP traffic to ATM connections in this
way is an area that warrants investigation.
A potential complication to this process is the fact that the token
bucket parameters are specified at the edge of the IP network, but
that the specification of the GCRA parameters at the entry to an ATM
network will frequently happen at a router in the middle of an IP
network. Thus the actual burstiness that is encountered at the router
may differ from that described by the IP token bucket parameters, as
the burstiness changes as the traffic traverses a network. The
seriousness of this problem needs to be understood to permit
efficient resource utilization.
3.2 QoS Characterization
In addition to specifying the traffic that they will submit to the
network, applications must specify the QoS they require from the
network. Since the goal is to carry IP efficiently over ATM networks,
it is necessary to establish mechanisms by which QoS specifications
for IP traffic can be translated into QoS specifications that are
meaningful for an ATM network.
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The proposed method of QoS specification for the Internet is to
specify a `service class' and some set of parameters, depending on
the service class. The currently proposed service classes are
* guaranteed, which provides a mathematically guaranteed delay
bound [23];
* predictive delay, which provides a probabilistic delay bound
[24];and
* controlled delay, which merely tries to provide several levels of
delay which applications may choose between [25].
These are in addition to the existing `best-effort' class. More IP
service classes are expected in the future. ATM has five service
classes:
* CBR (constant bit rate), which emulates a leased line, providing
very tightly constrained delay and designed for applications which
can use a fixed bandwidth pipe;
* VBR (variable bit rate)-real-time which attempts to constrain delay
for applications whose bandwidth requirements vary;
* VBR-non-real-time, intended for variable bandwidth applications
without tight delay constraints;
* UBR (unspecified bit rate) which most closely approximates the best
effort service of traditional IP;
* ABR (available bit rate) which uses a complex feedback mechanism
to control loss.
Each class requires some associated parameters to be specified, e.g.,
CBR requires a peak rate. Observe that these classes are by no means
in direct correspondence with the IP classes. In some cases, ATM
classes require parameters which are not provided at the IP level,
such as loss rate, to be specified. It may be necessary to assume
reasonable default values in these cases.
The major problem here is this: given traffic in a particular IP
service class with certain QoS parameters, how should it be sent
across an ATM network in such a way that it both meets its service
commitments and makes efficient use of the ATM network's resources?
For example, it would be possible to transport any class of IP
traffic over an ATM network using the constant bit rate (CBR) ATM
class, thus using the ATM network like a point-to-point link. This
would allow IP to meet its service commitments, but would be an
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inefficient use of network resources in any case where the IP traffic
was at all bursty (which is likely to be most cases). A more
reasonable approach might be to map all IP traffic into a variable
bit rate (VBR) class; certainly this class has the flexibility to
accommodate bursty IP traffic more efficiently than CBR.
At present, the IETF is not working on any service classes in which
loss rate is considered as part of the QoS specification. As long as
that is the case, the fact that ATM allows target loss rates to be
specified is essentially not an issue. However, we may certainly
expect that as the IP service model is further refined, service
classes that include specifications of loss may be defined. At this
point, it will be necessary to be able to map between loss rates at
the IP level and loss rates at the ATM level. It has already been
shown that relatively small loss rates in an ATM network can
translate to high loss rates in IP due to the fact that each lost
cell can cause the loss of an entire IP packet. Schemes to mitigate
this problem, which include the proposed approach to implementing the
ABR class, as well as other solutions [22], have been proposed. This
is clearly likely to be an important issue in the future.
4.0 Resource Reservation Styles
ATM uses a signalling protocol (Q.2931) both to establish virtual
connections and to allocate resources to those connections. It has
many of the characteristics of a 'conventional' signalling protocol,
such as being sender-driven and relying on hard-state in switches to
maintain connections. Some of the key characteristics are listed in
the table below. In the current standards, the QoS associated with a
connection at setup time cannot be changed subsequently (i.e., it is
static); in a unicast connection, resources are allocated in both
directions along the path, while in the multicast case, they are
allocated only from the sender to the receivers. In this case, all
senders receive the same QoS.
Two protocols have been proposed for resource reservation in IP. The
first (chronologically) is ST-II, the other is RSVP. Each of these,
and its relationship to ATM, is discussed in the following sections.
4.1 RSVP
IP has traditionally provided connectionless service. To support
real-time services in a connectionless world, RSVP has been proposed
to enable network resources to be reserved for a connectionless data
stream. ATM, on the other hand, provides a connection-oriented
service, where resource reservations are made at connection setup
time, using a user-network interface (UNI) and a network-network
interface (NNI) signalling protocol.
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RFC 1821 Real-time Service in IP-ATM Networks August 1995
-----------------------------------------------------------------
| Category | RSVP | ATM (UNI 3.0) |
-----------------------------------------------------------------
| | | |
| Orientation | Receiver-based | Sender-based |
| | | |
----------------------------------------------------------------
| | | |
| State | Soft state | Hard state |
| | (refresh/time-out) | (explicit delete) |
-----------------------------------------------------------------
| | | |
|QoS SetupTime | Separate from | Concurrent with |
| | route establishment | route establishment |
-----------------------------------------------------------------
| | | |
|QoS Changes? | Dynamic QoS | Static QoS |
| | | (Fixed at setup time) |
-----------------------------------------------------------------
| | | Bidirectional allocation|
|Directionality| Unidirectional | for unicast |
| |resource allocation |Unidirectional allocation|
| | | for multicast |
-----------------------------------------------------------------
| | | |
|Heterogeneity | Receiver | Uniform QoS to |
| | heterogeneity | all receivers |
-----------------------------------------------------------------
The principles used in the design of RSVP differ from those of ATM in
the following respects:
* Resource reservations in IP hosts and routers are represented by
soft state, i.e., reservations are not permanent, but time out
after some period. Reservations must be refreshed to prevent
time-out, and may also be explicitly deleted. In ATM, resources are
reserved for the duration of a connection, which must be explicitly
and reliably deleted.
* The soft state approach of RSVP allows the QoS reserved for a flow
to be changed at any time, whereas ATM connections have a static
QoS that is fixed at setup time.
* RSVP is a simplex protocol, i.e., resources are reserved in one
direction only. In ATM, connections (and associated reservations)
are bi-directional in point-to-point calls and uni-directional in
point-to-multipoint calls.
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* Resource reservation is receiver-initiated in RSVP. In ATM,
resources are reserved by the end system setting up the connection.
In point-to-multipoint calls, connection setup (and hence resource
reservation) must be done by the sender.
* RSVP has explicit support for sessions containing multiple senders,
namely the ability to select a subset of senders, and to
dynamically switch between senders. No such support is provided
by ATM.
* RSVP has been designed independently of other architectural
components, in particular routing. Moreover, route setup and
resource reservation are done at different times. In ATM, resource
reservation and route setup are done at the same time (connection
setup time).
The differences between RSVP and ATM state establishment, as
described above, raise numerous problems. For example, since point-
to-point connections are bidirectional in ATM, and since reservations
can be made in both directions, receiver-initiated resource
reservations in RSVP can be simulated in ATM by having the receiver
set up the connection and reserve resources in the backward direction
only. However, this is potentially wasteful of connection resources
since connections are only ever used to transfer data in one
direction even though communication between the two parties may be
bidirectional. One option is to use a `point-to-multipoint' ATM
connection with only one receiver. Of course, the fact that the RSVP
reservation request is made by the receiver(s) means that this
request must be somehow communicated to the sender on the ATM
network. This is somewhat analogous to the receiver-oriented join
operation of IP multicast and the problems of implementing it over
ATM, as discussed in Section 6. In general, the efficiency of any
proposed connection management scheme needs to be investigated in
both unicast and multicast contexts for a range of application
requirements, especially at a large scale.
The use by RSVP of `soft state' as opposed to explicit connections
means that routers at the ATM network's edges need to manage the
opening and closing of ATM connections when RSVP reservations are
made and released (or time out). The optimal scheme for connection
setup and tear-down will depend on the cost of setting up a
connection versus the cost of keeping the connection open for
possible future use by another stream, and is likely to be service
class-dependent. For example, connections may be left open for reuse
by best-effort traffic (subject to sufficient connections being
available), since no resources are explicitly reserved. On the other
hand, connections supporting the real-time service classes are likely
to be expensive to leave open since resources may be allocated even
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when the connection is idle. Again, the cost incurred will depend on
the class. For example, the cost of an open, idle `guaranteed' QoS
connection is likely to be significantly more expensive than a
connection providing predictive or controlled delay service. Note
that connections can be reused for traffic of the same class with
compatible QoS requirements, and that it may sometimes be possible to
use a `higher quality' class to substitute for a lower quality one.
Another characteristic of RSVP which presents problems for ATM is the
use of PATH messages to convey information to receivers before any
reservation is made. This works in IP because routing is performed
independently of reservation. Delivery of PATH messages across an ATM
network is therefore likely to require a mechanism for setting up
connections without reservations being made. The connection also
needs to be of sufficient quality to deliver PATH messages fairly
reliably; in some circumstances, a low quality best effort service
may be inadequate for this task. A related issue is the problem of
advertising services prior to reservations. The OPWA model (one pass
with advertising) requires network elements to advertise the QoS that
they are able to provide so that receivers can decide what level of
reservation to request. Since these advertisements may be made prior
to any resources having been reserved in the ATM network, it is not
clear how to make meaningful advertisements of the QoS that might be
provided across the ATM cloud.
Finally, the multiparty model of communication is substantially
different in RSVP and ATM. Emulating RSVP receiver-initiation using
ATM point-to-multipoint connections is likely to cause severe scaling
problems as the number of receivers becomes large. Also, some
functions of RSVP are not currently provided by ATM. For example,
there is no support for different receiver requirements and
capabilities-all receivers in a session receive the same QoS, which
is fixed at the time the first receiver is added to the multicast
tree. It is likely that ATM support for multi-party sessions will be
enhanced in later versions of the standards. It is necessary for such
support to evolve in a manner compatible with RSVP and IP multicast
routing protocols if large ATM clouds are to be deployed
successfully.
4.2 ST-II
ST-II [27] and ST2+ [12] (referred to generically as ST hereafter)
have data distribution and resource reservation schemes that are
similar to ATM in many respects.
* ST is connection oriented using "hard state". Senders set up
simplex data flows to all receivers closely matching point-to-
multipoint connections in ATM. Routing decisions are made when
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the connection is made and are not changed unless there is a
failure in the path. Positive acknowledgment is required from all
receivers. ST2+ [12] adds a receiver-based JOIN mechanism that can
reduce the burden on senders to track all receivers.
* ST reserves network resources at connection setup time. The ST
CONNECT message contains a flowspec indicating the resources to be
reserved for the stream. Agents along the path may change the
flowspec based on restrictions they may need to impose on the
stream. The final flowspec is returned to the sender in the ACCEPT
message from each receiver or target.
-----------------------------------------------------------------
| Category | RSVP | ATM (UNI 3.0) |
-----------------------------------------------------------------
| | | |
| Orientation | Sender-based | Sender-based |
| | | |
----------------------------------------------------------------
| | | |
| State | Hard state | Hard state |
| | (explicit disconnect)| (explicit delete) |
-----------------------------------------------------------------
| | | |
|QoS SetupTime | Concurrent with | Concurrent with |
| | stream setup | route establishment |
-----------------------------------------------------------------
| | | |
|QoS Changes? | Dynamic QoS | Static QoS |
| | | (Fixed at setup time) |
-----------------------------------------------------------------
| | | Bidirectional allocation|
|Directionality| Unidirectional | for unicast |
| |resource allocation |Unidirectional allocation|
| | | for multicast |
-----------------------------------------------------------------
| | | |
|Heterogeneity | Receiver | Uniform QoS to |
| | heterogeneity | all receivers |
-----------------------------------------------------------------
These similarities make mapping ST services to ATM simpler than RSVP
but the mapping is still not trivial. The task of mapping the ST
flowspec into an ATM service class still has to be worked out. There
may be policy issues related to opening a new VC for each stream
versus aggregating flows over an existing VC.
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Additionally, ST has some differences with UNI 3.1 that can cause
problems when integrating the two protocols:
* In ST, changes to active stream reservations are allowed. For
example, if the flowspec received from the target is not sufficient
for the stream, the sender can send a CHANGE message, requesting a
different QoS. UNI 3.1 does not allow changes to the QoS of a VC
after it is set up. Future ATM UNI specifications are contemplating
allowing changes to a VC after set up but this is still preliminary.
In the meantime, policies for over reservation or aggregation onto
a larger VC may be needed.
* ST uses simplex streams that flow in only one direction. This is
fine for UNI 3.1 point-to-multipoint connections since the data flow
is only in one direction. When mapping a point-to-point ST
connection to a standard point-to-point ATM VC, the reverse flow
connection is wasted.
This can be solved simply by using only point-to-multipoint VCs, even
if there is only one receiver.
4.3 Mapping IP flows to ATM connections
In general, there will be a great deal of flexibility in how one maps
flows at the IP level to connections at the ATM level. For example,
one could imagine setting up an ATM connection when a reservation
message arrives at the edge of an ATM cloud and then tearing it down
as soon as the reservation times out. However, to minimize latency or
perhaps for economic reasons, it may be preferable to keep the ATM
connection up for some period in case it is needed. Similarly, it may
be possible or desirable to map multiple IP flows to a single ATM
connection or vice versa.
An interesting situation arises when a reservation request is
received for an existing route across the cloud but which, when added
to the existing reservations using that connection, would exceed the
capacity of that connection. Since the current ATM standards do not
allow the QoS of a connection to be changed, there are two options:
tear down the old connection and create a new one with the new,
larger allocation of resources, or simply add a new connection to
accommodate the extra traffic. It is possible that the former would
lead to more efficient resource utilization. However, one would not
wish to tear down the first connection before the second was
admitted, and the second might fail admission control because of the
resources allocated to the first. The difficulties of this situation
seem to argue for evolution of ATM standards to support QoS
modification on an existing connection.
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5.0 End System Issues
In developing an integrated IP-ATM environment the applications need
to be as oblivious as possible of the details of the environment: the
applications should not need to know about the network topology to
work properly. This can be facilitated first by a common application
programing interface (API) and secondly by common flow and filter
specifications [18].
An example of a common API that is gaining momentum is the BSD
sockets interface. This is a UNIX standard and, with Winsock2, has
also become a PC standard. With the IETF integrated service
environment just beginning to appear in the commercial marketplace,
the ability to standardize on one common interface for both IP and
ATM applications is still possible and must be seriously and quickly
pursued to insure interoperability.
Since the IP integrated service and ATM environments offer different
QoS service types, an application should specify sufficient
information in its flow specification so that regardless of the
topology of the network, the network can choose an acceptable QoS
type to meet the applicationUs needs. Making the application provide
sufficient information to quantify a QoS service and allowing the
network to choose the QoS service type is essential to freeing the
application from requiring a set network topology and allowing the
network to fully utilize the features of IP and ATM.
6.0 Routing Issues
There is a fundamental difference between the routing computations
for IP and ATM that can cause problems for real-time IP services.
ATM computes a route or path at connection setup time and leaves the
path in place until the connection is terminated or there is a
failure in the path. An ATM cell only carries information
identifying the connection and no information about the actual source
and destination of the cell. In order to forward cells, an ATM
device needs to consult a list of the established connections that
map to the next hop device, without checking the final destination.
In contrast, routing decisions in IP are based on the destination
address contained in every packet. This means that an IP router, as
it receives each packet, has to consult a table that contains the
routes to all possible destinations and the routing decision is made
based on the final destination of the packet. This makes IP routing
very robust in the face of path changes and link failures at the
expense of the extra header information and the potentially larger
table lookup. However, if an IP path has been selected for a given
QoS, changes in the route may mean a change in the QoS of the path.
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6.1 Multicast routing
Considerable research has gone into overlaying IP multicast models
onto ATM. In the MARS (Multicast Address Resolution Server) model
[1], a server is designated for the Logical IP Subnet (LIS) to supply
the ATM addresses of the hosts in the IP multicast group, much like
the ATM ARP server [15]. When a host or router wishes to send to a
multicast group on the LIS, a query is made to the MARS and a list of
the ATM address of the hosts or routers in the group is returned. The
sending host can then set up point-to-point or point-to-multipoint
VCs to the other group members. When a host or a router joins an IP
multicast group, it notified the MARS. Each of the current senders to
the group is then notified of the new group member so that the new
member can be added to the point to multipoint VC's.
As the number of LIS hosts and multicast groups grows, the number of
VCs needed for a one-to-one mapping of VCs to multicast groups can
get very large. Aggregation of multicast groups onto the same VC may
be necessary to avoid VC explosion. Aggregation is further
complicated by the QoS that may be needed for particular senders in a
multicast group. There may be a need to aggregate all the multicast
flows requiring a certain QoS to a set of VCs, and parallel VCs may
be necessary to add flows of the same QoS.
6.2 QoS Routing
Most unicast and multicast IP routing protocols compute the shortest
path to a destination based solely on a hop count or metric. OSPF
[16] and MOSPF [17] allow computation based on different IP Type of
Service (TOS) levels as well as link metrics, but no current IP
routing protocols take into consideration the wide range of levels of
quality of service that are available in ATM or in the Integrated
Services models. In many routing protocols, computing all the routes
for just the shortest path for a large network is computationally
expensive so repeating this process for multiple QoS levels might be
prohibitively expensive.
In ATM, the Private Network-to-Network Interface (PNNI) protocol [13]
communicates QoS information along with routing information, and the
network nodes can utilize this information to establish paths for the
required QoS. Integrated PNNI (I-PNNI) [9] has been proposed as a way
to pass the QoS information available in ATM to other routing
protocols in an IP environment.
Wang & Crowcroft [28] suggest that only bandwidth and delay metrics
are necessary for QoS routing and this would work well for computing
a route that required a particular QoS at some setup time, but this
goes against the connectionless Internet model. One possible solution
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to the exhaustive computation of all possible routes with all
possible QoS values would be to compute routes for a common set of
QoS values and only then compute routes for uncommon QoS values as
needed, extracting a performance penalty only on the first packets of
a flow with an uncommon QoS. Sparse multicast routing protocols that
compute a multicast path in advance or on the first packets from a
sender (such as CBT [5] and MOSPF [17]) could also use QoS routing
information to set up a delivery tree that will have adequate
resources.
However, no multicast routing protocols allow the communication of
QoS information at tree setup time. Obtaining a tree with suitable
QoS is intended to be handled by RSVP, usually after the distribution
tree has been set up, and may require recomputation of the
distribution tree to provide the requested QoS.One way to solve this
problem is to add some "hints" to the multicast routing protocols so
they can get an idea of the QoS that the multicast group will require
at group initiation time and set up a distribution tree to support
the desired QoS. The CBT protocol [5] has some TBD fields in its
control headers to support resource reservation. Such information
could also be added to a future IGMP [11] JOIN message that would
include information on the PIM Rendezvous Point (RP) or CBT Core.
Another alternative is to recompute the multicast distribution tree
based on the RSVP messages but this has the danger of losing data
during the recomputation. However, this can leave a timing window
where other reservations can come along during the tree recomputation
and use the resources of the new path as well as the old path,
leaving the user with no path to support the QoS desired.
If unicast routing is used to support multicast routing, we have the
same problem of only knowing a single path to a given destination
with no QoS information. If the path suggested by unicast routing
does not have the resources to support the QoS desired, there are few
choices available. Schemes that use an alternate route to "guess" at
a better path have been suggested and can work for certain topologies
but an underlying routing protocol that provides QoS information is
necessary for a complete solution. As mentioned earlier, I-PNNI has
the potential to provide enough information to compute paths for the
requested QoS.
6.3 Mobile Routing
In developing an integrated IP-ATM network, potential new growth
areas need to be included in the planning stages. One such area is
mobile networking. Under the heading of mobile networks are included
satellite extensions of the ATM cloud, mobile hosts that can join an
IP subnetwork at random, and a true mobile network in which all
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network components including routers and/or switches are mobile.
The IP-ATM real-time service environment must be extended to include
mobile networks so as to allow mobile users to access the same
services as fixed network users. In doing so, a number of problems
exist that need to be addressed. The principle problems are that
mobile networks have constrained bandwidth compared to fiber and
mobile links and are less stable than fixed fiber links. The impact
of these limitations affect IP and ATM differently. In introducing
one or more constrained components into the ATM cloud,the effects on
congestion control in the overall network are unknown. One can
envision significant buffering problems when a disadvantaged user on
a mobile link attempts to access information from a high speed data
stream. Likewise, as ATM uses out of band signalling to set up the
connection, the stability of the mobile links that may have
significant fading or complete loss of connectivity could have a
significant effect on ATM performance.
For QoS, fading on a link will appear as a varying channel capacity.
This will result in time-dependent fluctuations of available links to
support a level of service. Current routing protocols are not
designed to operate in a rapidly changing topology. QoS routing
protocols that can operate in a rapidly changing topology are
required and need to be developed.
7.0 Security Issues
In a quality of service environment where network resources are
reserved, hence potentially depriving other users access to these
resources for some time period, authentication of the requesting host
is essential. This problem is greatly increased in a combined IP-ATM
topology where the requesting host can access the network either
through the IP or the ATM portion of the network. Differences in the
security architectures between IP and ATM can lead to opportunities
to reserve resources without proper authorization to do so. A common
security framework over the combined IP-ATM topology would be
desirable. In lieu of this, the use of trusted edge devices
requesting the QoS services are required as a near term solution.
Significant progress in developing a common security framework for IP
is underway in the IETF [2]. The use of authentication headers in
conjunction with appropriate key management is currently being
considered as a long range solution to providing QoS security [3,8].
In developing this framework, the reality of ATM portions of the
Internet should be taken into account. Of equal importance, the ATM
Forum ad-hoc security group should take into account the current work
on an IP security architecture to ensure compatibility.
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8.0 Future Directions
Clearly, there are some challenging issues for real-time IP-ATM
services and some areas are better understood than others. For
example, mechanisms such as policing, admission control and packet or
cell scheduling can be dealt with mostly independently within IP or
ATM as appropriate. Thus, while there may be hard problems to be
solved in these areas that need to be addressed in either the IP or
ATM communities, there are few serious problems that arise
specifically in the IP over ATM environment. This is because IP does
not particularly care what mechanisms a network element (such as an
ATM network) uses to provide a certain QoS; what matters is whether
the ATM service model is capable of offering services that can
support the end-to-end IP service model. Most of the hard problems
for IP over ATM therefore revolve around the service models for IP
and ATM. The one piece of mechanism that is important in an IP/ATM
context is signalling or resource reservation, a topic we return to
below.
The following paragraphs enumerate some of the areas in which we
believe significant work is needed. The work falls into three areas:
extending the IP over ATM standards; extensions to the ATM service
model; and extensions to the IP service model. In general, we expect
that practical experience with providing IP QoS over ATM will suggest
more enhancements to the service models.
We need to define ways of mapping the QoS and traffic
characterizations (Tspecs and Rspecs) of IP flows to suitable
characterizations for ATM connections. An agreement is needed so
that some sort of uniform approach is taken. Whatever agreement is
made for such mappings, it needs to be done so that when traversing
several networks, the requested QoS is obtained end-to-end (when
admission is possible). Practical experience should be gained with
these mappings to establish that the ATM service classes can in fact
provide suitable QoS to IP flows in a reasonably efficient way.
Enhancement of the ATM service classes may be necessary, but
experience is needed to determine what is appropriate.
We need to determine how the resource reservation models of IP (RSVP
and ST-II) interact with ATM signalling. Mechanisms for establishing
appropriate connection state with suitable QoS in ATM networks that
are part of a larger integrated services Internet need to be defined.
It is possible that the current IP/ATM mechanisms such as ARP servers
and MARS can be extended to help to manage this state.
There is a need for better QoS routing. While this functionality is
needed even in the pure ATM or pure IP environment, there is also an
eventual need for integrated QoS routing between ATM and IP. Further
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research and practical experience is needed in the areas of QoS
routing in IP in order to support more than the shortest best-effort
path, especially when this path may traverse ATM networks. In many
IP networks, there are multiple paths between a given source and
destination pair but current routing technologies only pay attention
to the current shortest path. As resources on the shortest path are
reserved, it will be necessary and viable to explore other paths in
order to provide QoS to a flow.
Enrichment of the ATM model to support dynamic QoS would greatly help
the IP over ATM situation. At present, the QoS objectives for ATM are
established at call set-up and then fixed for the duration of a call.
It would be advantageous to have the ability to provide a dynamic QoS
in ATM, so that an existing call could be modified to provide altered
services.
Another possible area of enhancement to the ATM service model is in
the area of multicasting. The multicast QoS offered is equal for all
receivers, and thus may be determined by the least favorable path
through the tree or by the most demanding receiver. Furthermore,
there is no current provision for multipoint to multipoint
connections. This limitation may rule out some of the services
envisioned in the IP service model.
There are areas of potential enrichment of the IP model as well.
While the receiver-based approach of RSVP has nice scaling properties
and handles receiver heterogeneity well, it is not clear that it is
ideal for all applications or for establishing state in ATM networks.
It is possible that a sender-oriented mode for RSVP might ease the
IP/ATM integration task.
Since the widespread availability of QoS raises new security concerns
(e.g., denial of service by excessive resource reservation), it seems
prudent that the IP and ATM communities work closely to adopt
compatible approaches to handling these issues.
This list is almost certainly incomplete. As work progresses to
define IP over ATM standards to support QoS and to implement
integrated services internetworks that include ATM, more issues are
likely to arise. However, we believe that this paper has described
the major issues that need to be taken into consideration at this
time by those who are defining the standards and building
implementations.
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9.0 References
1. Armitage, G., "Support for Multicast over UNI 3.1 based ATM
Networks", Work in Progress, Bellcore, February 1995.
2. Atkinson, R., "Security Architecture for the Internet Protocol",
RFC 1825, NRL, August 1995.
3. Atkinson, R., "IP Authentication Header", RFC 1826, NRL,
August 1995.
4. Ballardie, A., and J. Crowcroft, "Multicast-Specific Security
Threats and Counter-Measures", Proceedings of ISOC Symposium on
Network and Distributed System Security, San Diego, Feb. 1995,
pp. 2-16.
5. Ballardie, T., Jain, N., Reeve, S. "Core Based Trees (CBT)
Multicast, Protocol Specification", Work In Progress, University
College London, Bay Networks, June, 1995.
6. Braden, R., Clark, D., and S. Shenker, "Integrated Services in
the Internet Architecture: an Overview", RFC 1633, ISI/MIT/Xerox
PARC, July 1994.
7. Braden, R., Zhang, L., Estrin, Herzog, D., and S. Jamin,
"Resource ReSerVation Protocol (RSVP) - Version 1 Functional
Specification", Work in Progress, ISI/PARC/UCS, July 1995.
8. Braden, R., Clark, D., Crocker, S., and C. Huitema, "Report of IAB
Workshop on Security in the Internet Architecture", RFC 1636, ISI,
MIT, TIS, INRIA, June 1994.
9. Callon, R., and B. Salkewicz, An Outline for Integrated PNNI for
IP Routing", ATM Forum/ 95-0649, Bay Networks, July 1995.
10. Cole, R., Shur, D., and C. Villamizar, "IP over ATM: A Framework
Document", Work in Progress, AT&T Bell Laboratories/ ANS, April
1995.
11. Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC
1112, Stanford University, August 1989.
12. Delgrossi, L., and L. Berger, Editors, "Internet Stream Protocol
Version 2 (ST-2) Protocol Specification - Version ST2+", RFC 1819,
ST2 Working Group, August 1995.
13. Dykeman, D., Ed., "PNNI Draft Specification", ATM Forum/94-0471R8,
IBM Zurich Research Lab, May 1995.
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RFC 1821 Real-time Service in IP-ATM Networks August 1995
14. Goyal, P., Lam, S., and Vin, H., "Determining End-to-End Delay
Bounds in Heterogeneous Networks," 5th International Workshop on
Network and Operating System Support for Digital Audio and Video,
April, 1995.(Available via URL http://www.cs.utexas.edu/users/dmcl)
15. Laubach, M., "Classical IP and ARP over ATM", RFC 1577, HP,
January 1994.
16. Moy, J., "OSPF Version 2", RFC 1583, Proteon, March 1994.
17. Moy, J., "Multicast Extensions to OSPF," RFC 1584, Proteon, March
1994.
18. Partridge, C., "A Proposed Flow Specification", RFC 1363, BBN,
September 1992.
19. Perez, M., Liaw, F., Mankin, A., Hoffman, E., Grossman, D. and
A. Malis, "ATM Signaling Support for IP over ATM", RFC 1755,
ISI, Fore, Motorola Codex, Ascom Timeplex, February 1995.
20. Perkins, D., and Liaw, Fong-Ching, "Beyond Classical IP-Integrated
IP and ATM Architecture Overview", ATM Forum/94-0935, Fore Systems,
September 1994.
21. Perkins, D. and Liaw, Fong-Ching, "Beyond Classical IP-Integrated
IP and ATM Protocol Specifications", ATM Forum/94-0936, Fore
Systems, September 1994.
22. Romanow, A., and S. Floyd, "The Dynamics of TCP Traffic over ATM
Networks", Proceedings of ACM SIGCOMM U94, London, August 1994,
pp.79-88.
23. Shenker, S., and C. Partridge. "Specification of Guaranteed Quality
of Service", Work in Progress, Xerox/BBN, July 1995.
24. Shenker, S., and C. Partridge. "Specification of Predictive Quality
of Service", Work in Progress, Xerox/BBN, March 1995.
25. Shenker, S., C. Partridge and J. Wroclawski. "Specification of
Controlled Delay Quality of Service", Work in Progress,
Xerox/BBN/MIT, June 1995.
26. Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP:
A Transport Protocol for Real-time Applications", Work in Progress,
GMD/ISI/Xerox/LBL, March 1995.
27. Topolcic, C., "Experimental Internet Stream Protocol, Version 2
(ST-II)", RFC 1190, BBN, October 1990.
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RFC 1821 Real-time Service in IP-ATM Networks August 1995
28. Wang, Z., and J. Crowcroft, "QoS Routing for Supporting Resource
Reservation", University College of London white paper, 1995.
10. Authors' Addresses
Eric S. Crawley
Marty Borden
Bay Networks
3 Federal Street
Billerica, Ma 01821
508-670-8888
esc@baynetworks.com
mborden@baynetworks.com
Bruce S. Davie
Bellcore
445 South Street
Morristown, New Jersey 07960-6438
201-829-4838
bsd@bellcore.com
Stephen G. Batsell
Naval Research Laboratory
Code 5521
Washington, DC 20375-5337
202-767-3834
sgb@saturn.nrl.navy.mil
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