RFC2382 - A Framework for Integrated Services and RSVP over ATM

Network Working Group E. Crawley, Editor
Request for Comments: 2382 Argon Networks
Category: Informational L. Berger
Fore Systems
S. Berson
ISI
F. Baker
Cisco Systems
M. Borden
Bay Networks
J. Krawczyk
ArrowPoint Communications
August 1998
A Framework for Integrated Services and RSVP over ATM
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
Abstract
This document outlines the issues and framework related to providing
IP Integrated Services with RSVP over ATM. It provides an overall
approach to the problem(s) and related issues. These issues and
problems are to be addressed in further documents from the ISATM
subgroup of the ISSLL working group.
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 quality real-time
traffic, new classes of service and a QoS signalling protocol are
being introduced in the Internet [1,6,7], while retaining the
existing best effort service. The QoS signalling protocol is RSVP
[1], the Resource ReSerVation Protocol and the service models
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 provides 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, and this memo concentrates on
ATM.
Classical IP over ATM [10] 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 subnetwork. Hosts within an LIS
communicate using the ATM network, while hosts from different subnets
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 being solved with MARS [5],
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.
The ATM Forum"s LAN Emulation (LANE) [17, 20] and Multiprotocol Over
ATM (MPOA) [18] also address the support of IP best effort traffic
over ATM through similar means.
A key remaining issue for IP in an ATM environment 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
which traffic flows are routed over which VCs.
1.1 Structure and Related Documents
This document provides a guide to the issues for IIS over ATM. It is
intended to frame the problems that are to be addressed in further
documents. In this document, the modes and models for RSVP operation
over ATM will be discussed followed by a discussion of management of
ATM VCs for RSVP data and control. Lastly, the topic of
encapsulations will be discussed in relation to the models presented.
This document is part of a group of documents from the ISATM subgroup
of the ISSLL working group related to the operation of IntServ and
RSVP over ATM. [14] discusses the mapping of the IntServ models for
Controlled Load and Guaranteed Service to ATM. [15 and 16] discuss
detailed implementation requirements and guidelines for RSVP over
ATM, respectively. While these documents may not address all the
issues raised in this document, they should provide enough
information for development of solutions for IntServ and RSVP over
ATM.
1.2 Terms
Several term used in this document are used in many contexts, often
with different meaning. These terms are used in this document with
the following meaning:
- Sender is used in this document to mean the ingress point to the
ATM network or "cloud".
- Receiver is used in this document to refer to the egress point from
the ATM network or "cloud".
- 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 [1] for details of each. Each new
request is referred to in this document as an RSVP reservation, or
simply reservation.
- 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).
2. Issues Regarding the Operation of RSVP and IntServ over ATM
The issues related to RSVP and IntServ over ATM fall into several
general classes:
- How to make RSVP run over ATM now and in the future
- When to set up a virtual circuit (VC) for a specific Quality of
Service (QoS) related to RSVP
- How to map the IntServ models to ATM QoS models
- How to know that an ATM network is providing the QoS necessary for
a flow
- How to handle the many-to-many connectionless features of IP
multicast and RSVP in the one-to-many connection-oriented world of
ATM
2.1 Modes/Models for RSVP and IntServ over ATM
[3] Discusses several different models for running IP over ATM
networks. [17, 18, and 20] also provide models for IP in ATM
environments. Any one of these models would work as long as the RSVP
control packets (IP protocol 46) and data packets can follow the same
IP path through the network. It is important that the RSVP PATH
messages follow the same IP path as the data such that appropriate
PATH state may be installed in the routers along the path. For an
ATM subnetwork, this means the ingress and egress points must be the
same in both directions for the RSVP control and data messages. Note
that the RSVP protocol does not require symmetric routing. The PATH
state installed by RSVP allows the RESV messages to "retrace" the
hops that the PATH message crossed. Within each of the models for IP
over ATM, there are decisions about using different types of data
distribution in ATM as well as different connection initiation. The
following sections look at some of the different ways QoS connections
can be set up for RSVP.
2.1.1 UNI 3.x and 4.0
In the User Network Interface (UNI) 3.0 and 3.1 specifications [8,9]
and 4.0 specification, both permanent and switched virtual circuits
(PVC and SVC) may be established with a specified service category
(CBR, VBR, and UBR for UNI 3.x and VBR-rt and ABR for 4.0) and
specific traffic descriptors in point-to-point and point-to-
multipoint configurations. Additional QoS parameters are not
available in UNI 3.x and those that are available are vendor-
specific. Consequently, the level of QoS control available in
standard UNI 3.x networks is somewhat limited. However, using these
building blocks, it is possible to use RSVP and the IntServ models.
ATM 4.0 with the Traffic Management (TM) 4.0 specification [21]
allows much greater control of QoS. [14] provides the details of
mapping the IntServ models to UNI 3.x and 4.0 service categories and
traffic parameters.
2.1.1.1 Permanent Virtual Circuits (PVCs)
PVCs emulate dedicated point-to-point lines in a network, so the
operation of RSVP can be identical to the operation over any point-
to-point network. The QoS of the PVC must be consistent and
equivalent to the type of traffic and service model used. The
devices on either end of the PVC have to provide traffic control
services in order to multiplex multiple flows over the same PVC.
With PVCs, there is no issue of when or how long it takes to set up
VCs, since they are made in advance but the resources of the PVC are
limited to what has been pre-allocated. PVCs that are not fully
utilized can tie up ATM network resources that could be used for
SVCs.
An additional issue for using PVCs is one of network engineering.
Frequently, multiple PVCs are set up such that if all the PVCs were
running at full capacity, the link would be over-subscribed. This
frequently used "statistical multiplexing gain" makes providing IIS
over PVCs very difficult and unreliable. Any application of IIS over
PVCs has to be assured that the PVCs are able to receive all the
requested QoS.
2.1.1.2 Switched Virtual Circuits (SVCs)
SVCs allow paths in the ATM network to be set up "on demand". This
allows flexibility in the use of RSVP over ATM along with some
complexity. Parallel VCs can be set up to allow best-effort and
better service class paths through the network, as shown in Figure 1.
The cost and time to set up SVCs can impact their use. For example,
it may be better to initially route QoS traffic over existing VCs
until a SVC with the desired QoS can be set up for the flow. Scaling
issues can come into play if a single RSVP flow is used per VC, as
will be discussed in Section 4.3.1.1. The number of VCs in any ATM
device may also be limited so the number of RSVP flows that can be
supported by a device can be strictly limited to the number of VCs
available, if we assume one flow per VC. Section 4 discusses the
topic of VC management for RSVP in greater detail.
Data Flow ==========>
+-----+
--------------> +----+
Src --------------> R1
* --------------> +----+
+-----+ QoS VCs
/

VC
Initiator
Figure 1: Data Flow VC Initiation
While RSVP is receiver oriented, ATM is sender oriented. This might
seem like a problem but the sender or ingress point receives RSVP
RESV messages and can determine whether a new VC has to be set up to
the destination or egress point.
2.1.1.3 Point to MultiPoint
In order to provide QoS for IP multicast, an important feature of
RSVP, data flows must be distributed to multiple destinations from a
given source. Point-to-multipoint VCs provide such a mechanism. It
is important to map the actions of IP multicasting and RSVP (e.g.
IGMP JOIN/LEAVE and RSVP RESV/RESV TEAR) to add party and drop party
functions for ATM. Point-to-multipoint VCs as defined in UNI 3.x and
UNI 4.0 have a single service class for all destinations. This is
contrary to the RSVP "heterogeneous receiver" concept. It is
possible to set up a different VC to each receiver requesting a
different QoS, as shown in Figure 2. This again can run into scaling
and resource problems when managing multiple VCs on the same
interface to different destinations.
+----+
+------> R1
+----+

+----+
+-----+ -----+ +--> R2
---------+ +----+ Receiver Request Types:
Src ----> QoS 1 and QoS 2
.........+ +----+ ....> Best-Effort
+-----+ .....+ +..> R3
: +----+
/ :
: +----+
+......> R4
+----+
Single
IP Multicast
Group
Figure 2: Types of Multicast Receivers
RSVP sends messages both up and down the multicast distribution tree.
In the case of a large ATM cloud, this could result in a RSVP message
implosion at an ATM ingress point with many receivers.
ATM 4.0 eXPands on the point-to-multipoint VCs by adding a Leaf
Initiated Join (LIJ) capability. LIJ allows an ATM end point to join
into an existing point-to-multipoint VC without necessarily
contacting the source of the VC. This can reduce the burden on the
ATM source point for setting up new branches and more closely matches
the receiver-based model of RSVP and IP multicast. However, many of
the same scaling issues exist and the new branches added to a point-
to-multipoint VC must use the same QoS as existing branches.
2.1.1.4 Multicast Servers
IP-over-ATM has the concept of a multicast server or reflector that
can accept cells from multiple senders and send them via a point-to-
multipoint VC to a set of receivers. This moves the VC scaling
issues noted previously for point-to-multipoint VCs to the multicast
server. Additionally, the multicast server will need to know how to
interpret RSVP packets or receive instruction from another node so it
will be able to provide VCs of the appropriate QoS for the RSVP
flows.
2.1.2 Hop-by-Hop vs. Short Cut
If the ATM "cloud" is made up a number of logical IP subnets (LISs),
then it is possible to use "short cuts" from a node on one LIS
directly to a node on another LIS, avoiding router hops between the
LISs. NHRP [4], is one mechanism for determining the ATM address of
the egress point on the ATM network given a destination IP address.
It is a topic for further study to determine if significant benefit
is achieved from short cut routes vs. the extra state required.
2.1.3 Future Models
ATM is constantly evolving. If we assume that RSVP and IntServ
applications are going to be wide-spread, it makes sense to consider
changes to ATM that would improve the operation of RSVP and IntServ
over ATM. Similarly, the RSVP protocol and IntServ models will
continue to evolve and changes that affect them should also be
considered. The following are a few ideas that have been discussed
that would make the integration of the IntServ models and RSVP easier
or more complete. They are presented here to encourage continued
development and discussion of ideas that can help aid in the
integration of RSVP, IntServ, and ATM.
2.1.3.1 Heterogeneous Point-to-MultiPoint
The IntServ models and RSVP support the idea of "heterogeneous
receivers"; e.g., not all receivers of a particular multicast flow
are required to ask for the same QoS from the network, as shown in
Figure 2.
The most important scenario that can utilize this feature occurs when
some receivers in an RSVP session ask for a specific QoS while others
receive the flow with a best-effort service. In some cases where
there are multiple senders on a shared-reservation flow (e.g., an
audio conference), an individual receiver only needs to reserve
enough resources to receive one sender at a time. However, other
receivers may elect to reserve more resources, perhaps to allow for
some amount of "over-speaking" or in order to record the conference
(post processing during playback can separate the senders by their
source addresses).
In order to prevent denial-of-service attacks via reservations, the
service models do not allow the service elements to simply drop non-
conforming packets. For example, Controlled Load service model [7]
assigns non-conformant packets to best-effort status (which may
result in packet drops if there is congestion).
Emulating these behaviors over an ATM network is problematic and
needs to be studied. If a single maximum QoS is used over a point-
to-multipoint VC, resources could be wasted if cells are sent over
certain links where the reassembled packets will eventually be
dropped. In addition, the "maximum QoS" may actually cause a
degradation in service to the best-effort branches.
The term "variegated VC" has been coined to describe a point-to-
multipoint VC that allows a different QoS on each branch. This
approach seems to match the spirit of the Integrated Service and RSVP
models, but some thought has to be put into the cell drop strategy
when traversing from a "bigger" branch to a "smaller" one. The
"best-effort for non-conforming packets" behavior must also be
retained. Early Packet Discard (EPD) schemes must be used so that
all the cells for a given packet can be discarded at the same time
rather than discarding only a few cells from several packets making
all the packets useless to the receivers.
2.1.3.2 Lightweight Signalling
Q.2931 signalling is very complete and carries with it a significant
burden for signalling in all possible public and private connections.
It might be worth investigating a lighter weight signalling mechanism
for faster connection setup in private networks.
2.1.3.3 QoS Renegotiation
Another change that would help RSVP over ATM is the ability to
request a different QoS for an active VC. This would eliminate the
need to setup and tear down VCs as the QoS changed. RSVP allows
receivers to change their reservations and senders to change their
traffic descriptors dynamically. This, along with the merging of
reservations, can create a situation where the QoS needs of a VC can
change. Allowing changes to the QoS of an existing VC would allow
these features to work without creating a new VC. In the ITU-T ATM
specifications [24,25], some cell rates can be renegotiated or
changed. Specifically, the Peak Cell Rate (PCR) of an existing VC
can be changed and, in some cases, QoS parameters may be renegotiated
during the call setup phase. It is unclear if this is sufficient for
the QoS renegotiation needs of the IntServ models.
2.1.3.4 Group Addressing
The model of one-to-many communications provided by point-to-
multipoint VCs does not really match the many-to-many communications
provided by IP multicasting. A scaleable mapping from IP multicast
addresses to an ATM "group address" can address this problem.
2.1.3.5 Label Switching
The MultiProtocol Label Switching (MPLS) working group is discussing
methods for optimizing the use of ATM and other switched networks for
IP by encapsulating the data with a header that is used by the
interior switches to achieve faster forwarding lookups. [22]
discusses a framework for this work. It is unclear how this work
will affect IntServ and RSVP over label switched networks but there
may be some interactions.
2.1.4 QoS Routing
RSVP is explicitly not a routing protocol. However, since it conveys
QoS information, it may prove to be a valuable input to a routing
protocol that can make path determinations based on QoS and network
load information. In other Words, instead of aSKINg for just the IP
next hop for a given destination address, it might be worthwhile for
RSVP to provide information on the QoS needs of the flow if routing
has the ability to use this information in order to determine a
route. Other forms of QoS routing have existed in the past such as
using the IP TOS and Precedence bits to select a path through the
network. Some have discussed using these same bits to select one of
a set of parallel ATM VCs as a form of QoS routing. ATM routing has
also considered the problem of QoS routing through the Private
Network-to-Network Interface (PNNI) [26] routing protocol for routing
ATM VCs on a path that can support their needs. The work in this
area is just starting and there are numerous issues to consider.
[23], as part of the work of the QoSR working group frame the issues
for QoS Routing in the Internet.
2.2 Reliance on Unicast and Multicast Routing
RSVP was designed to support both unicast and IP multicast
applications. This means that RSVP needs to work closely with
multicast and unicast routing. Unicast routing over ATM has been
addressed [10] and [11]. MARS [5] provides multicast address
resolution for IP over ATM networks, an important part of the
solution for multicast but still relies on multicast routing
protocols to connect multicast senders and receivers on different
subnets.
2.3 Aggregation of Flows
Some of the scaling issues noted in previous sections can be
addressed by aggregating several RSVP flows over a single VC if the
destinations of the VC match for all the flows being aggregated.
However, this causes considerable complexity in the management of VCs
and in the scheduling of packets within each VC at the root point of
the VC. Note that the rescheduling of flows within a VC is not
possible in the switches in the core of the ATM network. Virtual
Paths (VPs) can be used for aggregating multiple VCs. This topic is
discussed in greater detail as it applies to multicast data
distribution in section 4.2.3.4
2.4 Mapping QoS Parameters
The mapping of QoS parameters from the IntServ models to the ATM
service classes is an important issue in making RSVP and IntServ work
over ATM. [14] addresses these issues very completely for the
Controlled Load and Guaranteed Service models. An additional issue
is that while some guidelines can be developed for mapping the
parameters of a given service model to the traffic descriptors of an
ATM traffic class, implementation variables, policy, and cost factors
can make strict mapping problematic. So, a set of workable mappings
that can be applied to different network requirements and scenarios
is needed as long as the mappings can satisfy the needs of the
service model(s).
2.5 Directly Connected ATM Hosts
It is obvious that the needs of hosts that are directly connected to
ATM networks must be considered for RSVP and IntServ over ATM.
Functionality for RSVP over ATM must not assume that an ATM host has
all the functionality of a router, but such things as MARS and NHRP
clients would be worthwhile features. A host must manage VCs just
like any other ATM sender or receiver as described later in section
4.
2.6 Accounting and Policy Issues
Since RSVP and IntServ create classes of preferential service, some
form of administrative control and/or cost allocation is needed to
control access. There are certain types of policies specific to ATM
and IP over ATM that need to be studied to determine how they
interoperate with the IP and IntServ policies being developed.
Typical IP policies would be that only certain users are allowed to
make reservations. This policy would translate well to IP over ATM
due to the similarity to the mechanisms used for Call Admission
Control (CAC).
There may be a need for policies specific to IP over ATM. For
example, since signalling costs in ATM are high relative to IP, an IP
over ATM specific policy might restrict the ability to change the
prevailing QoS in a VC. If VCs are relatively scarce, there also
might be specific accounting costs in creating a new VC. The work so
far has been preliminary, and much work remains to be done. The
policy mechanisms outlined in [12] and [13] provide the basic
mechanisms for implementing policies for RSVP and IntServ over any
media, not just ATM.
3. Framework for IntServ and RSVP over ATM
Now that we have defined some of the issues for IntServ and RSVP over
ATM, we can formulate a framework for solutions. The problem breaks
down to two very distinct areas; the mapping of IntServ models to ATM
service categories and QoS parameters and the operation of RSVP over
ATM.
Mapping IntServ models to ATM service categories and QoS parameters
is a matter of determining which categories can support the goals of
the service models and matching up the parameters and variables
between the IntServ description and the ATM description(s). Since
ATM has such a wide variety of service categories and parameters,
more than one ATM service category should be able to support each of
the two IntServ models. This will provide a good bit of flexibility
in configuration and deployment. [14] examines this topic
completely.
The operation of RSVP over ATM requires careful management of VCs in
order to match the dynamics of the RSVP protocol. VCs need to be
managed for both the RSVP QoS data and the RSVP signalling messages.
The remainder of this document will discuss several approaches to
managing VCs for RSVP and [15] and [16] discuss their application for
implementations in term of interoperability requirement and
implementation guidelines.
4. RSVP VC Management
This section provides more detail on the issues related to the
management of SVCs for RSVP and IntServ.
4.1 VC Initiation
As discussed in section 2.1.1.2, 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 subnet sender. For data flows, this
means that subnet senders will establish all QoS VCs and the subnet
receiver must be able to accept incoming QoS VCs, as illustrated in
Figure 1. 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.
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.
4.2 Data VC Management
Any RSVP over ATM implementation must map RSVP and RSVP associated
data flows to ATM Virtual Circuits (VCs). LAN Emulation [17],
Classical IP [10] and, more recently, NHRP [4] 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
.bp 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, as shown in Figure
1.
RSVP reservation styles must also be taken into account in any VC
usage strategy.
This section describes issues and methods for management of VCs
associated with QoS data flows. When establishing and maintaining
VCs, the subnet 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.
4.2.1 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
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 4.2.2. 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.
When multiple reservations are combined onto a single VC, it is
referred to 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 4.2.2) in full over ATM
has been reduced to a solved problem. Finally, the dynamic QoS
problem (section 4.2.7) for ATM has also been reduced to a solved
problem.
The aggregation model can be used with point-to-point and point-to-
multipoint VCs. The problem with the aggregation model is that the
choice of what QoS to use for the VCs may be difficult, without
knowledge of the likely reservation types and sizes but is made
easier since the VCs can be changed as needed.
4.2.2 Unicast Data VC Management
Unicast data VC management is much simpler than multicast data VC
management but there are still some similar issues. If one considers
unicast to be a devolved case of multicast, then implementing the
multicast solutions will cover unicast. However, some may want to
consider unicast-only implementations. In these situations, the
choice of using a single flow per VC or aggregation of flows onto a
single VC remains but the problem of heterogeneity discussed in the
following section is removed.
4.2.3 Multicast Heterogeneity
As mentioned in section 2.1.3.1 and shown in figure 2, multicast
heterogeneity occurs when receivers request different qualities of
service 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. In shared media networks,
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.
RSVP heterogeneity is supported over ATM in the way RSVP reservations
are mapped into ATM VCs. There are four alternative approaches this
mapping. There are multiple models for supporting RSVP heterogeneity
over ATM. Section 4.2.3.1 examines the multiple VCs per RSVP
reservation (or full heterogeneity) model where a single reservation
can be forwarded onto several VCs each with a different QoS. Section
4.2.3.2 presents a limited heterogeneity model where exactly one QoS
VC is used along with a best effort VC. Section 4.2.3.3 examines the
VC per RSVP reservation (or homogeneous) model, where each RSVP
reservation is mapped to a single ATM VC. Section 4.2.3.4 describes
the aggregation model allowing aggregation of multiple RSVP
reservations into a single VC.
4.2.3.1 Full Heterogeneity Model
RSVP supports heterogeneous QoS, meaning that different receivers of
the same multicast group can request a different QoS. But
importantly, some receivers might have no reservation at all and want
to receive the traffic on a best effort service basis. The IP model
allows receivers to join a multicast group at any time on a best
effort basis, and it is important that ATM as part of the Internet
continue to provide this service. 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 qualities of
service.
Note that while full heterogeneity gives users exactly what they
request, it requires more resources of the network than other
possible approaches. The exact amount of bandwidth used for duplicate
traffic depends on the network topology and group membership.
4.2.3.2 Limited Heterogeneity Model
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.
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.
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. 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.
4.2.3.3 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. 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.
4.2.3.4 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 if the VCs can be changed
as needed.
4.2.4 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 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. To ensure proper
behavior, implementations should, by default, only establish RSVP-
initiated VCs to RSVP capable end-points.
4.2.5 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. RSVP over ATM solutions must
ensure that IP multicast data is distributed with appropriate QoS.
In the Classical IP context, multicast server support is provided via
MARS [5]. MARS does not currently provide a way to communicate QoS
requirements to a MARS multicast server. Therefore, RSVP over ATM
implementations must, by default, support "mesh-mode" distribution
for RSVP controlled multicast flows. When using multicast servers
that do not support QoS requests, a sender must set the service, not
global, break bit(s).
4.2.6 Receiver Transitions
When setting up a point-to-multipoint VCs for multicast RSVP
sessions, 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 can 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
- 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 operation should be completed first, then
the drop operation. This means that receivers must be prepared to
receive some duplicate packets at times of QoS setup.
4.2.7 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.
1. An existing receiver can request a new larger (or smaller) QoS.
2. A sender may change its traffic specification (TSpec), which can
trigger a change in the reservation requests of the receivers.
3. A new sender can start sending to a multicast group with a larger
traffic specification than existing senders, triggering larger
reservations.
4. 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 processing
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.
The default 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 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 T
(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+T would be queued. If several messages changing the QoS of a VC
arrive during the interval, redundant messages can be discarded. At
time t+T, the remaining change(s) of QoS, if any, can be executed.
This timer approach would apply more generally to any network
structure, and might be worthwhile to incorporate into RSVP.
The sequence of events for a single VC would be
- Wait if timer is active
- Establish VC with new QoS
- Remap data traffic to new VC
- Tear down old VC
- 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.
4.2.8 Short-Cuts
Short-cuts [4] 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 subnets. The ability for short-cuts
and RSVP to interoperate has been raised as a general question. An
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, PATH
and RESV messages following different paths.
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.
The key issue to be addressed by any RSVP over ATM solution is when
to establish a short-cut for a QoS data flow. The default behavior is
to simply follow best-effort traffic. When a short-cut has been
established for best-effort traffic to a destination or next-hop,
that same end-point should be used when setting up RSVP triggered VCs
for QoS traffic to the same destination or next-hop. This will happen
naturally when PATH messages are forwarded over the best-effort
short-cut. Note that in this approach when best-effort short-cuts
are never established, RSVP triggered QoS short-cuts will also never
be established. More study is expected in this area.
4.2.9 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 RFC1755 [11], section 3.4 on VC Teardown. RFC1755
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 RFC1755 [11] 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.
For VCs initiated at the request of RSVP, the configurable inactivity
timer mentioned in [11] must be set to "infinite". Setting the
inactivity timer value at the VC initiator should not be problematic
since the proper value can be relayed internally at the originator.
Setting the inactivity timer at the VC receiver is more difficult,
and would require some mechanism to signal that an incoming VC was
RSVP initiated. To avoid this complexity and to conform to [11]
implementations must not use an inactivity timer to clear received
connections.
4.3 RSVP Control Management
One last important issue is providing a data path for the RSVP
messages themselves. There are two main types of messages in RSVP,
PATH and RESV. PATH messages are sent to unicast or multicast
addresses, while RESV messages are sent only to unicast addresses.
Other RSVP messages are handled similar to either PATH or RESV,
although this might be more complicated for RERR messages. 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:
- use same VC as data
- single VC per session
- single point-to-multipoint VC multiplexed among sessions
- 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
fewer 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.
4.3.1 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 of QoS VCs, a very undesirable behavior for
ATM. Due to these problems, this may not be 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 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.
4.3.1.1 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 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 4.3.1). 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.
4.3.1.2 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 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.
4.3.1.3 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.
4.3.2 QoS for RSVP VCs
There is an issue of what QoS, if any, to assign to the RSVP
signalling VCs. For other RSVP VC schemes, a QoS (possibly best
effort) will be needed. What QoS 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. The methods for transmitting IP packets over
ATM (Classical IP over ATM[10], LANE[17], and MPOA[18]) are all based
on the encapsulations defined in RFC1483 [19]. RFC1483 specifies two
encapsulations, LLC Encapsulation and VC-based multiplexing. The
former allows multiple protocols to be encapsulated over the same VC
and the latter requires different VCs for different protocols.
For the purposes of RSVP over ATM, any encapsulation can be used as
long as the VCs are managed in accordance to the methods outlined in
Section 4. Obviously, running multiple protocol data streams over
the same VC with LLC encapsulation can cause the same problems as
running multiple flows over the same VC.
While none of the transmission methods directly address the issue of
QoS, RFC1755 [11] does suggest some common values for VC setup for
best-effort traffic. [14] discusses the relationship of the RFC1755
setup parameters and those needed to support IntServ flows in greater
detail.
6. Security Considerations
The same considerations stated in [1] and [11] apply to this
document. There are no additional security issues raised in this
document.
7. References
[1] Braden, R., Zhang, L., Berson, S., Herzog, S., and S. Jamin,
"Resource ReSerVation Protocol (RSVP) -- Version 1 Functional
Specification", RFC2209, September 1997.
[2] Borden, M., Crawley, E., Davie, B., and S. Batsell, "Integration
of Realtime Services in an IP-ATM Network Architecture", RFC
1821, August 1995.
[3] Cole, R., Shur, D., and C. Villamizar, "IP over ATM: A Framework
Document", RFC1932, April 1996.
[4] Luciani, J., Katz, D., Piscitello, D., Cole, B., and N.
Doraswamy, "NBMA Next Hop Resolution Protocol (NHRP)", RFC2332,
April 1998.
[5] Armitage, G., "Support for Mult