Network Working Group L. Berger
Request for Comments: 2379 FORE Systems
BCP: 24 August 1998
Category: Best Current Practice
RSVP over ATM Implementation Guidelines
Status of this Memo
This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
Abstract
This memo presents specific implementation guidelines for running
RSVP over ATM switched virtual circuits (SVCs). The general problem
is discussed in [6]. Implementation requirements are discussed in
[2]. Integrated Services to ATM service mappings are covered in [3].
The full set of documents present the background and information
needed to implement Integrated Services and RSVP over ATM.
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 methods for supporting IP over ATM. The general issues related
to running RSVP[4] over ATM have been covered in several papers
including [6] and other earlier work. This document is intended as a
companion to [6,2] and as a guide to implementers. The reader should
be familiar with both documents.
This document provides a recommended set of functionality for
implementations using ATM UNI3.x and 4.0, while allowing for more
sophisticated approaches. We eXPect some vendors to additionally
provide some of the more sophisticated approaches described in [6],
and some networks to only make use of such approaches. The
recommended set of functionality is defined to ensure predictability
and interoperability between different implementations. Requirements
for RSVP over ATM implementations are provided in [2].
This document uses the same terms and assumption stated in [2].
Additionally, 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 RFC
2119 [5].
2. Implementation Recommendations
This section provides implementation guidelines for implementation of
RSVP over ATM. Several recommendations are common for all, RSVP
sessions, both unicast and multicast. There are also recommendations
that are unique to unicast and multicast session types.
2.1 RSVP Message VC Usage
The general issues related to which VC should be used for RSVP
messages is covered in [6]. It discussed several implementation
options including: mixed control and data, single control VC per
session, single control VC multiplexed among sessions, and multiple
VCs multiplexed among sessions. QoS for control VCs was also
discussed. The general discussion is not repeated here and [6]
should be reviewed for detailed information.
RSVP over ATM implementations SHOULD send RSVP control (messages)
over the best effort data path, see figure 1. It is permissible to
allow a user to override this behavior. The stated approach
minimizes VC requirements since the best effort data path will need
to exist in order for RSVP sessions to be established and in order
for RSVP reservations to be initiated. The specific best effort
paths that will be used by RSVP are: for unicast, the same VC used to
reach the unicast destination; and for multicast, the same VC that is
used for best effort traffic destined to the IP multicast group.
Note that for multicast there may be another best effort VC that is
used to carry session data traffic, i.e., for data that is both in
the multicast group and matching a sessions protocol and port.
Data Flow ==========>
QoS VCs
+-----+ --------------> +----+
-------------->
Src R1
Best Effort VC(s)
+-----+ <-----------------> +----+
/


RSVP Control
Messages
Figure 1: RSVP Control Message VC Usage
The disadvantage of this approach is that best effort VCs may not
provide the reliability that RSVP needs. However the best effort
path is expected to satisfy RSVP reliability requirements in most
networks. Especially since RSVP allows for a certain amount of packet
loss without any loss of state synchronization.
2.2 Aggregation
As discussed in [6], data associated with multiple RSVP sessions can
be sent using the same shared VCs. Implementation of such
"aggregation" models is still a matter for research. Therefore, RSVP
over ATM implementations SHOULD use independent VCs for each RSVP
reservation.
2.3 Short-Cuts
Short-cuts 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. Short-cut support for unicast traffic has
been defined in [7] and [1]. The ability for short-cuts and RSVP to
interoperate has been raised as a general question. The area of
concern is the ability to handle asymmetric short-cuts. Specifically
how RSVP can handle the case where a downstream short-cut may not
have a matching upstream short-cut. In this case, which is shown in
figure 2, PATH and RESV messages following different paths.
______
/
+-------- / Router <-------+
/ <....... RESVs Follow
______/ Hop-by-hop Path


V QoS VCs
+-----+ ==============> +----+
==============>
Src R1
Best Effort VC(s)
+-----+ <=================> +----+
/
:: Data Paths:
:: ----> Hop-by-hop (routed)
PATHs and Data ====> Short-cut
Follow Short-cut
Path
Figure 2: Asymmetric RSVP Message Forwarding With ATM Short-Cuts
Examination of RSVP shows that the protocol already includes
mechanisms that allows support of the asymmetric paths. The
mechanism is the same one used to support RESV messages arriving at
the wrong router and the wrong interface. RSVP messages are only
processed when they arrive at the proper interface. When messages
arrive on the wrong interface, they are forwarded by RSVP. The
proper interface is indicated in the NHOP object of the message. So,
existing RSVP mechanisms will support the asymmetric paths that can
occur when using 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 RSVP over ATM implementations is
when to establish a short-cut for a QoS data flow. RSVP over ATM
implementations SHOULD 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.
2.4 Data VC Management for Heterogeneous Sessions
Heterogeneous sessions can only occur with multicast RSVP sessions.
The issues relating to data VC management of heterogeneous sessions
are covered in detail in [6] and are not repeated in this document.
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
3.
+----+
+------> R1
+----+

+----+
+-----+ -----+ +--> R2
---------+ +----+ Receiver Request Types:
Src ----> QoS 1 and QoS 2
.........+ +----+ ....> Best-Effort
+-----+ .....+ +..> R3
: +----+
/ :
: +----+
+......> R4
+----+
Single
IP Mulicast
Group
Figure 3: Types of Multicast Receivers
[6] provides four models for dealing with heterogeneity: full
heterogeneity, limited heterogeneity, homogeneous, and modified
homogeneous models. The key issue to be addressed by an
implementation is providing requested QoS downstream. One of, or some
combination of, the discussed models [6] may be used to provide the
requested QoS. Unfortunately, none of the described models is the
right answer for all cases. For some networks, e.g. public WANs, it
is likely that the limited heterogeneous model or a hybrid limited-
full heterogeneous model will be desired. In other networks, e.g.
LANs, it is likely that a the modified homogeneous model will be
desired.
Since there is not one model that satisfies all cases,
implementations SHOULD implement one of either the limited
heterogeneity model or the modified homogeneous model.
Implementations SHOULD support both approaches and provide the
ability to select which method is actually used, but are not required
to do so.
3. Security Considerations
The same considerations stated in [4] and [8] apply to this document.
There are no additional security issues raised in this document.
4. 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.
5. 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] The ATM Forum, "MPOA Baseline Version 1", May 1997.
[2] Berger, L., "RSVP over ATM Implementation Requirements",
RFC2380, August 1998.
[3] Borden, M., and M. Garrett, "Interoperation of Controlled-Load
and Guaranteed-Service with ATM", RFC2381, August 1998.
[4] Braden, R., Zhang, L., Berson, S., Herzog, S., and S. Jamin,
"Resource ReSerVation Protocol (RSVP) -- Version 1 Functional
Specification", RFC2205, September 1997.
[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.
[6] 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.
[7] Luciani, J., Katz, D., Piscitello, D., and B. Cole, "NBMA Next
Hop Resolution Protocol (NHRP)", RFC2332, April 1998.
[8] Perez, M., Liaw, F., Grossman, D., Mankin, A., Hoffman, E., and
A. Malis, "ATM Signalling Support for IP over ATM", RFC1755,
February 1995.
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