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Internet Draft L. Berger
Expires: January 1998 FORE Systems
File: draft-ietf-issll-atm-imp-req-00.txt
RSVP over ATM Implementation Requirements
July 11, 1997
Status of Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
To learn the current status of any Internet-Draft, please check the
"1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
Directories on ds.internic.net (US East Coast), nic.nordu.net
(Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific
Rim).
Abstract
This note 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 [6] provides specific guidelines
for running over today's ATM networks. The general problem is
discussed in [11]. Integrated Services to ATM service mappings are
covered in [9]. The full set of documents present the background and
information needed to implement Integrated Services and RSVP over
ATM.
Berger Expires: January 1998 [Page 1]
Internet Draft RSVP over ATM Requirements July 1997
Table of Contents
1. Introduction ........................................................3
1.1 Terms ...........................................................3
1.2 Assumptions .....................................................4
2. General RSVP Session Support ........................................4
2.1 VC Usage ........................................................4
2.2 VC Initiation ...................................................5
2.3 VC Teardown .....................................................5
2.4 Dynamic QoS .....................................................6
2.5 Encapsulation ...................................................7
3. Multicast RSVP Session Support ......................................7
3.1 Data VC Management for Heterogeneous Sessions ...................7
3.2 Multicast End-Point Identification ..............................9
3.3 Multicast Data Distribution .....................................10
3.4 Receiver Transitions ............................................11
4. Security ............................................................12
5. Acknowledgments .....................................................12
6. Author's Address ....................................................12
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Internet Draft RSVP over ATM Requirements July 1997
1. Introduction
This note 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[10] over ATM have been covered in several
papers including [11,7,5,8]. This document is intended as a
companion to [11,6]. The reader should be familiar with both
documents.
This document will define 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 [6].
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 [10] 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).
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1.2 Assumptions
The following assumptions are made:
o RSVP We assume RSVP as the internet signalling protocol which
is described in [10]. The reader is assumed to be familiar
with [10].
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 that best effort service
will continue to be a 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
draft 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 VC Usage
There are several options open to implementations on which VC to
use for RSVP messages and how to aggregate RSVP sessions over QoS
VCs. These options have been covered in [11] and some specific
implementation guidelines are stated in [6]. In order to ensure
interoperability between implementations that follow different
options, RSVP over ATM implementations MUST be able to receive
RSVP (control) messages on both QoS and best-effort VCs, and MUST
be able to receive multiple RSVP sessions per QoS VC.
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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
sub-net sender.
For data flows, this means that sub-net senders MUST establish all
QoS VCs and the RSVP enabled sub-net receiver MUST be able to
accept incoming QoS VCs. These restrictions are consistent with
RSVP version 1 processing rules and allow senders to use different
flow to VC mappings and even different QoS renegotiation
techniques without interoperability problems. All RSVP over ATM
approaches that have VCs initiated and controlled by the sub-net
senders will interoperate. Figure 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 a 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. Of course, 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
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Internet Draft RSVP over ATM Requirements July 1997
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 is not
needed. 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 RFC
1755[14], section 3.4 on VC Teardown. RFC 1755 recommends tearing
down a VC that is inactive for a certain length of time. Twenty
minutes is recommended. This timeout is typically implemented at
both the VC initiator and the VC receiver. Although, section 3.1
of the update to RFC 1755[15] states that inactivity timers must
not be used at the VC receiver.
In RSVP over ATM implementations, the configurable inactivity
timer mentioned in [14] 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 [15],
RSVP over ATM implementations MUST not use an inactivity timer to
clear any received connection.
2.4 Dynamic QoS
As stated in [11], 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 after VC setup. See
[11] 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
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required in order to conform to RSVP error handling as defined in
sections 2.5, 3.1.8 and 3.11.2 of [10]. 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. Implementations MAY also limit number of
changes processed in a time period per [11].
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 RFC 1483[12] on such QoS VCs.
Implementations MAY negotiate alternative encapsulations using the
B-LLI negotiation procedures defined in ATM Signalling, see [14]
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
particular to multicast sessions. These issues result from the
nature of ATM point-to-multipoint connections. 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 issues, but needs to be addressed in any RSVP
over ATM implementation in order to maintain expected 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 [11] and not repeated. 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.
Berger Expires: January 1998 [Page 7]
Internet Draft RSVP over ATM Requirements July 1997
+----+
+------> | 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
[11] 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 over
transmission is acceptable, but only during VC setup and
transition.
Implementations MUST also ensure that data traffic is sent to
best-effort receivers. Data traffic MAY be sent to best-effort
receivers via best-effort or QoS VCs as is appropriate for the
implemented model. In all cases, implementations MUST NOT create
VCs in such a way that data cannot be sent to best-effort
receivers. This includes the case of not being able to add a
best-effort receiver to a QoS VC, but does not include the case
where best-effort VCs cannot be setup. The failure to establish
best-effort VCs is considered to be a general IP over ATM failure
and is therefore beyond the scope of this document.
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
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Internet Draft RSVP over ATM Requirements July 1997
whether to first add the new next-hop or to change to the new QoS.
This is a fairly straight forward special case. Since the older,
smaller reservation does not support the new next-hop, the dynamic
QoS process SHOULD be initiated first. Since the new QoS is only
needed by the new next-hop, it SHOULD be the first end-point of
the new VC. This way signalling is minimized when the setup to
the new next-hop fails.
3.2 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-point. This occurs when the 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 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. It is
therefore possible that a multicast end-point can not be properly
identified.
In the host case, some multicast over ATM control mechanisms, such
as MARS, can 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, baseline RSVP over ATM implementations MUST only
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Internet Draft RSVP over ATM Requirements July 1997
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 models are planned for IP multicast data distribution over
ATM. In one model, senders establish point-to-multipoint VCs to
all ATM attached destinations, and data is then sent over these
VCs. This model is often called "multicast mesh" or "VC mesh"
mode distribution. In the second model, senders send data over
point-to-point VCs to a central point and the central point relays
the data onto point-to-multipoint VCs that have been established
to all receivers of the IP multicast group. This model is often
referred to as "multicast server" mode distribution. Figure 3
shows data flow for both modes of IP multicast data distribution.
The goal of RSVP over ATM solutions is to ensure that IP multicast
data is distributed with appropriate QoS.
_________
/ \
/ Multicast \
\ Server /
\_________/
^ | |
| | +--------+
+-----+ | | |
| | -------+ | | Data Flow:
| Src | ...+......|....+ V ----> Server
| | : | : +----+ ....> Mesh
+-----+ : | +...>| R1 |
: | +----+
: V
: +----+
+..> | R2 |
+----+
Figure 3: IP Multicast Data Distribution Over ATM
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.
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Internet Draft RSVP over ATM Requirements July 1997
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 BUS,
which means that the break bit MUST always be set. For LANE2.0
[3] there are provisions that allow for non-BUS 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. The issue is when to start send data
on the new VC. If data is sent both on the new VC and the old VC,
then data will be delivered with proper QoS to some receivers and
with the old QoS to all receivers. This means the QoS receivers
would get duplicate data. If data is sent just on the new QoS VC,
the receivers that have not yet been added will lose information.
So, the issue comes down to whether to send to both the old and
new VCs, or to send to just one of the VCs. In one case duplicate
information will be received, in the other some information may
not be received. This issue needs to be considered for three
cases: when establishing the first QoS VC, when establishing a VC
to support a QoS change, and when adding a new end-point to an
already established QoS VC.
The first two cases are essentially the same. In both, it is
possible to send data on the partially completed new VC, and the
issue of duplicate versus lost information is similar. The last
case 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 information. If the drop is requested first, then the
end-point may loose information.
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.
This approach does not quite apply for the last case. In the last
case, the add MUST be completed first, then the drop. This last
behavior requires receivers to be prepared to receive some
duplicate packets at times of QoS setup.
Berger Expires: January 1998 [Page 11]
Internet Draft RSVP over ATM Requirements July 1997
4. Security
The same considerations stated in [10] and [14] apply to this
document. There are no additional security issues raised in this
document.
5. Acknowledgments
This work is based on earlier drafts [5,7] and comments from the
ISSLL working group. The author would like to acknowledge their
contribution, most notably Steve Berson who coauthored [7].
6. Author's Address
Lou Berger
FORE Systems
6905 Rockledge Drive
Suite 800
Bethesda, MD 20817
Phone: +1 301 571 2534
EMail: lberger@fore.com
REFERENCES
[1] Armitage, G., "Support for Multicast over UNI 3.0/3.1 based ATM
Networks," RFC 2022, 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: Framework and UNI3.0/3.1 Method",
Internet Draft, June 1996.
[6] Berger, L., "RSVP over ATM Implementation Guidelines", Internet
Draft, July 1997.
[7] Berson, S., Berger, L., "IP Integrated Services with RSVP over ATM,"
Internet Draft, draft-ietf-issll-atm-support-02.txt, November 1996.
[8] Borden, M., Crawley, E., Krawczyk, J, Baker, F., and Berson, S.,
"Issues for RSVP and Integrated Services over ATM," Internet Draft,
February 1996.
Berger Expires: January 1998 [Page 12]
Internet Draft RSVP over ATM Requirements July 1997
[9] Borden, M., and Garrett, M., "Interoperation of Controlled-Load and
Guaranteed-Service with ATM," Internet Draft, March 1997.
[10] Braden, R., Zhang, L., Berson, S., Herzog, S., and Jamin, S.,
"Resource ReSerVation Protocol (RSVP) -- Version 1 Functional
Specification," Internet Draft, June 1997.
[11] Crawley, E., Berger, L., Berson, S., Baker, F., Borden, M., and
Krawczyk, J, "Issues for Integrated Services and RSVP over ATM,"
Internet Draft, July 1997.
[12] Heinanen, J., "Multiprotocol Encapsulation over ATM Adaptation
Layer 5," RFC 1483.
[13] Onvural, R., Srinivasan, V., "A Framework for Supporting RSVP Flows
Over ATM Networks," Internet Draft, March 1996.
[14] Perez, M., Liaw, F., Grossman, D., Mankin, A., Hoffman, E., and
Malis, A., "ATM Signalling Support for IP over ATM," RFC 1755.
[15] Maher, M., "ATM Signalling Support for IP over ATM - UNI 4.0
Update" Internet Draft, May 1997.
Berger Expires: January 1998 [Page 13]