RFC2380 - RSVP over ATM Implementation Requirements

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　　Network Working Group L. Berger
Request for Comments: 2380 FORE Systems
Category: Standards Track August 1998

RSVP over ATM Implementation Requirements

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1998). All Rights Reserved.

Abstract

This memo presents specific implementation requirements for running
RSVP over ATM switched virtual circuits (SVCs). It presents
requirements that ensure interoperability between multiple
implementations and conformance to the RSVP and Integrated Services
specifications. A separate document [5] provides specific guidelines
for running over today's ATM networks. The general problem is
discussed in [9]. Integrated Services to ATM service mappings are
covered in [6]. The full set of documents present the background and
information needed to implement Integrated Services and RSVP over
ATM.

Table of Contents

1. Introduction ................................................. 2
1.1 Terms .................................................... 2
1.2 Assumptions .............................................. 3
2. General RSVP Session Support ................................. 4
2.1 RSVP Message VC Usage .................................... 4
2.2 VC Initiation ............................................ 4
2.3 VC Teardown .............................................. 5
2.4 Dynamic QoS .............................................. 6
2.5 Encapsulation ............................................ 6
3. Multicast RSVP Session Support ............................... 7
3.1 Data VC Management for Heterogeneous Sessions ............ 7
3.2 Multicast End-Point Identification ....................... 8
3.3 Multicast Data Distribution .............................. 9
3.4 Receiver Transitions ..................................... 11

4. Security Considerations ...................................... 11
5. Acknowledgments .............................................. 11
6. Author's Address ............................................. 12
REFERENCES ...................................................... 13
FULL COPYRIGHT STATEMENT ........................................ 14

1. Introduction

This memo discusses running IP over ATM in an environment where SVCs
are used to support QoS flows and RSVP is used as the internet level
QoS signaling protocol. It applies when using CLIP/ION, LANE2.0 and
MPOA [4] methods for supporting IP over ATM. The general issues
related to running RSVP [8] over ATM have been covered in several
papers including [9] and other earlier work. This document is
intended as a companion to [9,5]. The reader should be familiar with
both documents.

This document defines the specific requirements for implementations
using ATM UNI3.x and 4.0. These requirements must be adhered to by
all RSVP over ATM implementations to ensure interoperability.
Further recommendations to guide implementers of RSVP over ATM are
provided in [5].

The rest of this section will define terms and assumptions. Section 2
will cover implementation guidelines common to all RSVP session.
Section 3 will cover implementation guidelines specific to multicast
sessions.

1.1 Terms

The terms "reservation" and "flow" are used in many contexts, often
with different meaning. These terms are used in this document with
the following meaning:

o Reservation is used in this document to refer to an RSVP
initiated request for resources. RSVP initiates requests for
resources based on RESV message processing. RESV messages that
simply refresh state do not trigger resource requests. Resource
requests may be made based on RSVP sessions and RSVP reservation
styles. RSVP styles dictate whether the reserved resources are
used by one sender or shared by multiple senders. See [8] for
details of each. Each new request is referred to in this
document as an RSVP reservation, or simply reservation.

o Flow is used to refer to the data traffic associated with a
particular reservation. The specific meaning of flow is RSVP
style dependent. For shared style reservations, there is one
flow per session. For distinct style reservations, there is one
flow per sender (per session).

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [7].

1.2 Assumptions

The following assumptions are made:

o RSVP

We assume RSVP as the internet signaling protocol which is
described in [8]. The reader is assumed to be familiar with
[8].

o IPv4 and IPv6

RSVP support has been defined for both IPv4 and IPv6. The
guidelines in this document are intended to be used to support
RSVP with either IPv4 or IPv6. This document does not require
one version over the other.

o Best effort service model

The current Internet only supports best effort service. We
assume that as additional components of the Integrated Services
model are defined, best effort service must continue to be
supported.

o ATM UNI 3.x and 4.0

We assume ATM service as defined by UNI 3.x and 4.0. ATM
provides both point-to-point and point-to-multipoint Virtual
Circuits (VCs) with a specified Quality of Service (QoS). ATM
provides both Permanent Virtual Circuits (PVCs) and Switched
Virtual Circuits (SVCs). In the Permanent Virtual Circuit (PVC)
environment, PVCs are typically used as point-to-point link
replacements. So the support issues are similar to point-to-
point links. This memo assumes that SVCs are used to support
RSVP over ATM.

2. General RSVP Session Support

This section provides implementation requirements that are common for
all (both unicast and multicast) RSVP sessions. The section covers
VC usage, QoS VC initiation, VC teardown, handling requested changes
in QoS, and encapsulation.

2.1 RSVP Message VC Usage

There are several RSVP Message VC Usage options available to
implementers. Implementers must select which VC to use for RSVP
messages and how to aggregate RSVP sessions over QoS VCs. These
options have been covered in [9] and some specific implementation
guidelines are stated in [5]. In order to ensure interoperability
between implementations that follow different options, RSVP over ATM
implementations MUST NOT send RSVP (control) messages on the same QoS
VC as RSVP associated data packets. RSVP over ATM implementations
MAY send RSVP messages on either the best effort data path or on a
separate control VC.

Since RSVP (control) messages and RSVP associated data packets are
not sent on the same VCs, it is possible for a VC supporting one type
of traffic to fail while the other remains in place. When the VC
associated with data packets fails and cannot be reestablished, RSVP
SHOULD treat this as an allocation failure. When the VC used to
forward RSVP control messages is abnormally released and cannot be
reestablished, the RSVP associated QoS VCs MUST also be released.
The release of the associated data VCs is required to maintain the
synchronization between forwarding and reservation states for the
associated data flows.

2.2 VC Initiation

There is an apparent mismatch between RSVP and ATM. Specifically,
RSVP control is receiver oriented and ATM control is sender oriented.
This initially may seem like a major issue but really is not. While
RSVP reservation (RESV) requests are generated at the receiver,
actual allocation of resources takes place at the subnet sender.

For data flows, this means that subnet senders MUST establish all QoS
VCs and the RSVP enabled subnet receiver MUST be able to accept
incoming QoS VCs. These restrictions are consistent with RSVP
version 1 processing rules and allow senders to use different flow to
VC mappings and even different QoS renegotiation techniques without
interoperability problems. All RSVP over ATM approaches that have
VCs initiated and controlled by the subnet senders will interoperate.
Figure 1 shows this model of data flow VC initiation.

Data Flow ==========>

+-----+
| | --------------> +----+
| Src | --------------> | R1 |
| *| --------------> +----+
+-----+ QoS VCs
/\
||
VC ||
Initiator

Figure 1: Data Flow VC Initiation

RSVP over ATM implementations MAY send data in the backwards
direction on an RSVP initiated QoS point-to-point VC. When sending
in the backwards data path, the sender MUST ensure that the data
conforms to the backwards direction traffic parameters. Since the
traffic parameters are set by the VC initiator, it is quite likely
that no resources will be requested for traffic originating at the
called party. It should be noted that the backwards data path is not
available with point-to-multipoint VCs.

2.3 VC Teardown

VCs supporting IP over ATM data are typically torndown based on
inactivity timers. This mechanism is used since IP is connectionless
and there is therefore no way to know when a VC is no longer needed.
Since RSVP provides explicit mechanisms (messages and timeouts) to
determine when an associated data VC is no longer needed, the
traditional VC timeout mechanisms are not needed. Additionally, under
normal operations RSVP implementations expect to be able to allocate
resources and have those resources remain allocated until released at
the direction of RSVP. Therefore, data VCs set up to support RSVP
controlled flows should only be released at the direction of RSVP.
Such 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 [12] states that inactivity
timers must not be used at the VC receiver.

In RSVP over ATM implementations, the configurable inactivity timer
mentioned in [11] MUST be set to "infinite" for VCs initiated at the
request of RSVP. 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 [12], RSVP over ATM implementations
MUST not use an inactivity timer to clear any received connections.

2.4 Dynamic QoS

As stated in [9], there is a mismatch in the service provided by RSVP
and that provided by ATM UNI3.x and 4.0. RSVP allows modifications
to QoS parameters at any time while ATM does not support any
modifications to QoS parameters post VC setup. See [9] for more
detail.

The 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 only then is the old VC
closed.

If setup of the replacement VC fails, then the old QoS VC MUST
continue to be used. When the new reservation is greater than the
old reservation, the reservation request MUST be answered with an
error. When the new reservation is less than the old reservation, the
request MUST 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. The behavior is also required in
order to conform to RSVP error handling as defined in sections 2.5,
3.1.8 and 3.11.2 of [8]. Implementations SHOULD retry replacing a
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 SHOULD BE released and the whole VC replacement
process is restarted. Implementations MAY also limit number of
changes processed in a time period per [9].

2.5 Encapsulation

There are multiple encapsulation options for data sent over RSVP
triggered QoS VCs. All RSVP over ATM implementations MUST be able to
support LLC encapsulation per RFC1483 [10] on such QoS VCs.
Implementations MAY negotiate alternative encapsulations using the
B-LLI negotiation procedures defined in ATM Signalling, see [11] for

details. When a QoS VC is only being used to carry IP packets,
implementations SHOULD negotiate VC based multiplexing to avoid
incurring the overhead of the LLC header.

3. Multicast RSVP Session Support

There are several aspects to running RSVP over ATM that are unique to
multicast sessions. This section addresses multicast end-point
identification, multicast data distribution, multicast receiver
transitions and next-hops requesting different QoS values
(heterogeneity) which includes the handling of multicast best effort
receivers. Handling of best effort receivers is not strictly an RSVP
issue, but needs to be addressed by any RSVP over ATM implementation
in order to maintain expected best effort internet service.

3.1 Data VC Management for Heterogeneous Sessions

The issues relating to data VC management of heterogeneous sessions
are covered in detail in [9]. In summary, heterogeneity occurs when
receivers request different levels of QoS within a single session,
and also when some receivers do not request any QoS. Both types of
heterogeneity are shown in figure 2.

+----+
+------> | R1 |
| +----+
|
| +----+
+-----+ -----+ +--> | R2 |
| | ---------+ +----+ Receiver Request Types:
| Src | ----> QoS 1 and QoS 2
| | .........+ +----+ ....> Best-Effort
+-----+ .....+ +..> | R3 |
: +----+
/\ :
|| : +----+
|| +......> | R4 |
|| +----+
Single
IP Mulicast
Group

Figure 2: Types of Multicast Receivers

[9] provides four models for dealing with heterogeneity: full
heterogeneity, limited heterogeneity, homogeneous, and modified
homogeneous models. No matter which model or combination of models
is used by an implementation, implementations MUST NOT normally send

more than one copy of a particular data packet to a particular next-
hop (ATM end-point). Some transient duplicate transmission is
acceptable, but only during VC setup and transition.

Implementations MUST also ensure that data traffic is sent to best
effort receivers. Data traffic MAY be sent to best effort receivers
via best effort or QoS VCs as is appropriate for the implemented
model. In all cases, implementations MUST NOT create VCs in such a
way that data cannot be sent to best effort receivers. This includes
the case of not being able to add a best effort receiver to a QoS VC,
but does not include the case where best effort VCs cannot be setup.
The failure to establish best effort VCs is considered to be a
general IP over ATM failure and is therefore beyond the scope of this
document.

There is an interesting interaction between dynamic QoS and
heterogeneous requests when using the limited heterogeneity,
homogeneous, or modified homogeneous models. 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 signaling is minimized when the setup to the new next-hop
fails.

3.2 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 usually provide QoS
end-points, but not best effort end-points.

There is a special case where RSVP next-hop information will not
provide the appropriate end-points. This occurs when a next-hop is
not RSVP capable and RSVP is being automatically tunneled. 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 via a different route than
used by the PATH message. The source will get the RESV message, but
will not know which ATM end-point should be associated with the
reservation. For unicast sessions, there is no problem since the ATM
end-point will be the IP next-hop router. There is a problem with

multicast, since multicast routing may not be able to uniquely
identify the IP next-hop router. It is therefore possible for a
multicast end-point to not be properly identified.

In certain cases it is also possible to identify the list of all best
effort end-points. Some multicast over ATM control mechanisms, such
as MARS in mesh mode, can be used to identify all end-points of a
multicast group. Also, some multicast routing protocols can provide
all next-hops for a particular multicast group. In both cases, RSVP
over ATM implementations can obtain a full list of end-points, both
QoS and non-QoS, using the appropriate mechanisms. The full list can
then be compared against the RSVP identified end-points to determine
the list of best effort receivers.

While there are cases where QoS and best effort end-points can be
identified, 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 control
mechanisms and routing protocols that support unique end-point
identification. In cases where such mechanisms and routing protocols
are unavailable, all IP routers that will be used to support RSVP
over ATM should support RSVP. To ensure proper behavior, baseline
RSVP over ATM implementations MUST only establish RSVP-initiated VCs
to RSVP capable end-points. It is permissible to allow a user to
override this behavior.

3.3 Multicast Data Distribution

Two basic models exist for IP multicast data distribution over ATM.
In one model, senders establish point-to-multipoint VCs to all ATM
attached destinations, and data is then sent over these VCs. This
model is often called "multicast mesh" or "VC mesh" mode
distribution. In the second model, senders send data over point-to-
point VCs to a central point and the central point relays the data
onto point-to-multipoint VCs that have been established to all
receivers of the IP multicast group. This model is often referred to
as "multicast server" mode distribution. Figure 3 shows data flow for
both modes of IP multicast data distribution.

_________
/ \
/ Multicast \
\ Server /
\_________/
^ | |
| | +--------+
+-----+ | | |
| | -------+ | | Data Flow:
| Src | ...+......|....+ V ----> Server
| | : | : +----+ ....> Mesh
+-----+ : | +...>| R1 |
: | +----+
: V
: +----+
+..> | R2 |
+----+

Figure 3: IP Multicast Data Distribution Over ATM

The goal of RSVP over ATM solutions is to ensure that IP multicast
data is distributed with appropriate QoS. Current multicast servers
[1,2] do not support any mechanisms for communicating QoS
requirements to a multicast server. For this reason, RSVP over ATM
implementations SHOULD 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). Use of the service-specific break bit tells the
receiver(s) that RSVP and Integrated Services are supported by the
router but that the service cannot be delivered over the ATM network
for the specific request.

In the case of MARS [1], the selection of distribution modes is
administratively controlled. Therefore network administrators that
desire proper RSVP over ATM operation MUST appropriately configure
their network to support mesh mode distribution for multicast groups
that will be used in RSVP sessions. For LANE1.0 networks the only
multicast distribution option is over the LANE Broadcast and Unknown
Server which means that the break bit MUST always be set. For
LANE2.0 [3] there are provisions that allow for non-server solutions
with which it may be possible to ensure proper QoS delivery.

3.4 Receiver Transitions

When setting up a point-to-multipoint VCs there will be a time when
some receivers have been added to a QoS VC and some have not.

During such transition times it is possible to start sending data on
the newly established VC. 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. Additionally, the QoS
receivers would get duplicate data. If data is sent just on the new
QoS VC, the receivers that have not yet been added will miss data.
So, the issue comes down to whether to send to both the old and new
VCs, or to just send to one of the VCs. In one case duplicate data
will be received, in the other some data may not be received. This
issue needs to be considered for three cases: when establishing the
first QoS VC, when establishing a VC to support a QoS change, and
when adding a new end-point to an already established QoS VC.

The first two cases are essentially the same. In both, it is
possible to send data on the partially completed new VC. In both,
there is the option of duplicate or lost data. In order to ensure
predictable behavior and to conform to the requirement to deliver
data to all receivers, data MUST NOT be sent on new VCs until all
parties have been added. This will ensure that all data is only
delivered once to all receivers.

The last case differs from the others and occurs 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 data. If the drop is requested first,
then the end-point may miss data. In order to avoid loss of data,
the add MUST be completed first and then followed by the drop. This
behavior requires receivers to be prepared to receive some duplicate
packets at times of QoS setup.

4. Security Considerations

The same considerations stated in [8] and [11] apply to this
document. There are no additional security issues raised in this
document.

5. Acknowledgments

This work is based on earlier drafts and comments from the ISSLL
working group. The author would like to acknowledge their
contribution, most notably Steve Berson who coauthored one of the
drafts.

6. Author's Address

Lou Berger
FORE Systems
1595 Spring Hill Road
5th Floor
Vienna, VA 22182

Phone: +1 703-245-4527
EMail: lberger@fore.com

REFERENCES

[1] Armitage, G., "Support for Multicast over UNI 3.0/3.1 based ATM
Networks," RFC2022, November 1996.

[2] The ATM Forum, "LAN Emulation Over ATM Specification", Version
1.0.

[3] The ATM Forum, "LAN Emulation over ATM Version 2 - LUNI
Specification", April 1997.

[4] The ATM Forum, "MPOA Baseline Version 1", May 1997.

[5] Berger, L., "RSVP over ATM Implementation Guidelines", BCP 24,
RFC2379, August 1998.

[6] Borden, M., and M. Garrett, "Interoperation of Controlled-Load
and Guaranteed-Service with ATM", RFC2381, August 1998.

[7] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.

[8] Braden, R., Zhang, L., Berson, S., Herzog, S., and S. Jamin,
"Resource ReSerVation Protocol (RSVP) -- Version 1 Functional
Specification", RFC2205, September 1997.

[9] Crawley, E., Berger, L., Berson, S., Baker, F., Borden, M., and
J. Krawczyk, "A Framework for Integrated Services and RSVP over
ATM", RFC2382, August 1998.

[10] Heinanen, J., "Multiprotocol Encapsulation over ATM Adaptation
Layer 5", RFC1483, July 1993.

[11] Perez, M., Liaw, F., Grossman, D., Mankin, A., Hoffman, E., and
A. Malis, "ATM Signalling Support for IP over ATM", RFC1755,
February 1995.

[12] Maher, M., "ATM Signalling Support for IP over ATM - UNI 4.0
Update", RFC2331, April 1998.

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