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INTERNET DRAFT
draft-ietf-qosr-framework-01.txt July, 28, 1997
A Framework for QoS-based Routing in the Internet
Eric Crawley Raj Nair Bala Rajagopalan Hal Sandick
Gigapacket Networks Arrowpoint NEC USA IBM
Status of this 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. Internet Drafts may be updated, replaced, or obsoleted by
other documents at any time. It is not appropriate to use Internet
Drafts as reference material or to cite them other than as a "working
draft" or "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, nic.nordu.net, ftp.nisc.sri.com or
munnari.oz.au.
Distribution of this memo is unlimited.
This Internet Draft expires on January, 28, 1998.
ABSTRACT
QoS-based routing is being recognized as the missing piece in the evolution
of QoS-based service offerings in the Internet. This document describes
some of the QoS-based routing issues and requirements, and proposes a framework for
QoS-based routing in the Internet. This framework is based on extending the current
Internet routing model of intra and interdomain routing to support QoS. The ideas
expressed in this document are subject to discussion and expected to evolve based on
inputs received over time.
1. SCOPE OF DOCUMENT & PHILOSOPHY
This document proposes a framework for QoS-based routing, with the
objective of fostering the development of an Internet-wide solution while
encouraging innovations in solving the many problems that arise. QoS-based
routing has many complex facets and it is recommended that the following
two-pronged approach be employed towards its development:
draft-ietf-qosr-framework-01.txt [Page 1]
1. Encourage the growth and evolution of novel intradomain QoS-based routing
architectures. This is to allow the development of independent, innovative
solutions that address the many QoS-based routing issues. Such solutions may
be deployed in autonomous systems (ASs), large and small, based on their
specific needs.
2. Specify simple, consistent and stable interactions between ASs implementing
routing solutions developed as above.
This approach follows the traditional separation between intra and interdomain
routing. It allows solutions like QOSPF [PGW97, ZSSC96], Integrated PNNI
[IPNNI] or other schemes to be deployed for intradomain routing without any
restriction, other than their ability to interact with a common, and perhaps
simple, interdomain routing protocol. The need to develop a single, all
encompassing solution to the complex problem of QoS-based routing is therefore
obviated. As a practical matter, there are many different views on how QoS-based
routing should be done. Much overall progress can be made if an opportunity
exists for various ideas to be developed and deployed concurrently, while some
consensus on the interdomain routing architecture is being developed.
Finally, this routing model is perhaps the most practical from an evolution
point of view. It is superfluous to say that the eventual success of a
QoS-based Internet routing architecture would depend on the ease of evolution.
The aim of this draft is to describe the QoS-based routing issues, identify
basic requirements on intra and interdomain routing, and describe an extension
of the current interdomain routing model to support QoS. It is not an objective
of this draft to specify the details of intradomain QoS-based routing architectures.
This is left up to the various intradomain routing efforts that might follow. Nor
is it an objective to specify the details of the interface between reservation protocols
such as RSVP and QoS-based routing. The specific interface functionality needed,
however, would be clear from the intra and interdomain routing solutions devised.
In the intradomain area, the goal is to develop the basic routing
requirements while allowing maximum freedom for the development of solutions. In
the interdomain area, the objectives are to identify the QoS-based
routing functions, and facilitate the development of a routing protocol that
allows relatively simple interaction between domains. The views presented in
this draft are expected to evolve as consensus emerges on Internet-wide QoS- based
routing needs.
In the next section, a glossary of relevant terminology is given. In Section 3,
the objectives of QoS-based routing are described and the issues that must be dealt
with by QoS-based Internet routing efforts are outlined. In Section 4, some
requirements on intradomain routing are defined. These requirements are
purposely broad, putting few constraints on solution approaches. The
interdomain routing model and issues are described in Section 5 and QoS-based
multicast routing is discussed in Section 6. The interaction between QoS-based
routing and resource reservation protocols is briefly considered in Section 7.
Related work is described in Section 8. Finally, summary and conclusions are
presented in Section 9.
draft-ietf-qosr-framework-01.txt [Page 2]
2. GLOSSARY
The following glossary lists the terminology used in this draft and an explanation
of what is meant. Some of these terms may have different connotations, but when
used in this draft, their meaning is as given.
Alternate Path Routing : A routing technique where multiple paths, rather
than just the shortest path, between a source and a destination are utilized to
route traffic. One of the objectives of alternate path routing is to distribute
load among multiple paths in the network.
Autonomous System (AS): A routing domain which has a common intradomain routing
plan and administrative authority.
Source: A host or router that can be identified by a unique unicast IP address.
Unicast destination: A host or router that can be identified by a unique unicast
IP address.
Multicast destination: A multicast IP address indicating all hosts and routers
that are members of the corresponding group.
IP flow (or simply "flow"): An IP packet stream from a source to a destination
(unicast or multicast) with an associated Quality of Service (QoS) (see below)
and higher level demultiplexing information. The associated QoS could be
"best-effort".
Quality-of-Service (QoS): A set of service requirements to be met by the
network while transporting a flow.
Service class: The definitions of the semantics and parameters of a specific
type of QoS.
Integrated services: The Integrated Services model for the Internet
defined in RFC 1633 allows for integration of QoS services with the best
effort services of the Internet. The Integrated Services (IntServ)
working group in the IETF has defined two service classes, Controlled
Load Service [W96] and Guaranteed Service [SPG97].
RSVP: The ReSerVation Protocol [BZBH96]. A QoS signaling protocol for the
Internet.
Path: A unicast or multicast path.
Unicast path: A sequence of links from an IP source to a unicast IP destination,
determined by the routing scheme for forwarding packets.
Multicast path (or Multicast Tree): A subtree of the network topology in which
all the leaves and zero or more interior nodes are members of the same multicast
group. A multicast path may be per-source, in which case the subtree is rooted
at the source.
draft-ietf-qosr-framework-01.txt [Page 3]
Flow set-up: The act of establishing state in routers along a path to satisfy the
QoS requirement of a flow.
Crankback: A technique where a flow setup is recursively backtracked along the
partial flow path up to the first node that can determine an alternative path
to the destination.
QoS-based routing: A routing mechanism under which paths for flows are
determined based on some knowledge of resource availability in the network as
well as the QoS requirement of flows.
Route pinning: A mechanism to keep a flow path fixed for a duration of time.
Flow Admission Control (FAC): A process by which it is determined whether a
link or a node has sufficient resources to satisfy the QoS required for a flow.
FAC is typically applied by each node in the path of a flow during flow set-up
to check local resource availability.
Higher-level admission control: A process by which it is determined whether or
not a flow set-up should proceed, based on estimates of the overall resource
usage by the flow. Higher-level admission control may result in the failure of
a flow set-up even when FAC at each node along the flow path indicates resource
availability.
3. QOS-BASED ROUTING ISSUES
Under QoS-based routing, paths for flows would be determined based on
some knowledge of resource availability in the network, as well as the QoS
requirement of flows. The main objectives of QoS-based routing are:
1. Dynamic determination of feasible paths: QoS-based routing can determine
a path, from among possibly many choices, that has a good chance of
accommodating the QoS of the given flow. Feasible path selection may be
subject to policy constraints, such as path cost, provider selection, etc.
2. Optimization of resource usage: A network state-dependent QoS-based
routing scheme can aid in the efficient utilization of network resources
by improving the total network throughput. Such a routing scheme can be the
basis for efficient network engineering.
3. Graceful performance degradation: State-dependent routing can compensate
for transient inadequacies in network engineering (e.g., during focused
overload conditions), giving better throughput and a more graceful
performance degradation as compared to a state-insensitive routing
scheme [A84].
QoS-based routing in the Internet, however, raises many issues:
- How do routers determine the QoS capability of each outgoing link and reserve
link resources? Note that some of these links may be virtual, over ATM
networks and others may be broadcast multi-access links.
draft-ietf-qosr-framework-01.txt [Page 4]
- What is the granularity of routing decision (i.e., destination-based, source
and destination-based, or flow-based)?
- What routing metrics are used and how are QoS-accommodating paths computed for
unicast flows?
- How are QoS-accommodating paths computed for multicast flows with different
reservation styles and receiver heterogeneity?
- What are the performance objectives while computing QoS-based paths?
- What are the administrative control issues?
- What factors affect the routing overheads?, and
- How is scalability achieved?
Some of these issues are discussed briefly next. Interdomain routing is discussed in
Section 5.
3.1 QoS Determination and Resource Reservation
------------------------------------------
To determine whether the QoS requirements of a flow can be accommodated
on a link, a router must be able to determine the QoS available on the link. It
is still an open issue as to how the QoS availability is determined for
broadcast multiple access links (e.g., Ethernet). A related problem is the
reservation of resources over such links. The ISSLL working group and the IEEE
802.1 group are attempting to resolve these issues.
Similar problems arise when a router is connected to a large non-broadcast
multiple access network, such as ATM. In this case, if the destination of a flow is
outside the ATM network, the router may have multiple egress choices.
Furthermore, the QoS availability on the ATM paths to each egress point may be
different. The issues then are,
o how does a router determine all the egress choices across the ATM network?
o how does it determine what QoS is available over the path to each egress
point?, and
o what QoS value does the router advertise for the ATM link.
Typically, IP routing over ATM (e.g., NHRP) allows the selection of a single egress
point in the ATM network, and the procedure does not incorporate any knowledge
of the QoS required over the path. An approach like I-PNNI [IPNNI] would be
helpful here, although with some complexity.
draft-ietf-qosr-framework-01.txt [Page 5]
3.2 Granularity of Routing Decision
-------------------------------
Routing in the Internet is currently based only on the destination
address of a packet. Many multicast routing protocols require routing
based on the source and destination of a packet. The Integrated
Services architecture and RSVP allow QoS determination for an
individual flow between a source and destination. This set of routing
granularities presents a problem for QoS routing solutions.
If routing based only on destination address is considered, then all
flows between any source and the destination will be routed over the
same path. This is acceptable if the path has adequate capacity but it can
be a problem if there are multiple flows to a destination that exceed
the capacity of the link.
One version of QOSPF [ZSSC96] determines QoS routes based on source
and destination address. This implies that all traffic between a given source
and destination, regardless of the flow, will travel down the same route.
Again, the route must have capacity for all the QoS traffic for the
source/destination pair. The amount of routing state is also increased
since the routing tables must include source/destination pairs instead
of just destination. This amount of state increases rapidly as the
traditional routes are summarized.
The best granularity is found when routing is based on individual flows
but this has a tremendous cost for routing state. Each QoS flow can be
routed separately between any source and destination. Use of the IPv6
flow label can help in identifying or classifying flows.
Both source/destination and flow based routing also have a dangerous
property when it comes to route loop detection. If a node along a flow
or source/destination based path loses the state information for the
flow and the flow based route is different from the destination only
based routing, the potential exists for a route loop to form when the
node forwards the packet based on destination routing towards a node
earlier on the path.
3.3 Metrics and Path Computation
----------------------------
3.3.1 Metric Selection and Representation
There are some considerations in defining suitable link and node metrics [WC96].
First, the metrics must represent the basic network properties of interest. Such
metrics include residual bandwidth, delay and jitter. Since the flow QoS requirements
have to be mapped onto path metrics, the metrics define the types
of QoS guarantees the network can support. Alternatively, QoS routing cannot support
QoS requirements that cannot be meaningfully mapped onto a reasonable combination of
path metrics. Second, path computation based on a metric or a combination of metrics
must not be too complex as to render them impractical. In this regard, it is worthwhile
draft-ietf-qosr-framework-01.txt [Page 6]
to note that path computation based on certain combinations of metrics
(e.g., delay and jitter) is theoretically hard. Thus, the allowable combinations
of metrics must be determined taking into account the complexity of computing paths
based on these metrics and the QoS needs of flows. A common strategy to allow
flexible combinations of metrics while at the same time reduce the path computation
complexity is to utilize "sequential filtering". Under this approach, a combination
of metrics is ordered in some fashion, reflecting the importance of different metrics
(e.g., cost followed by delay, etc.). Paths based on the primary metric are
computed first (using a simple algorithm, e.g., shortest path) and a subset of them are
eliminated based on the secondary metric and so forth until a single path is found.
This is an approximation technique and it trades off global optimality for
path computation complexity.
Now, once suitable link and node metrics are defined, a uniform representation of them
is required across independent domains, employing possibly different routing
schemes, in order to derive path metrics consistently
(path metrics are obtained by the composition of link and node metrics).
Encoding of the maximum, minimum, range, and granularity of the metrics are needed.
Also, the definitions of comparison and accumulation operators are required.
In addition, suitable triggers must be defined for indicating a significant
change from a minor change. The former will cause a routing update to be
generated. The stability of the QoS routes would depend on the ability to
control the generation of updates. With interdomain routing, it is essential to obtain
a fairly stable view of the interconnection among the ASs.
3.3.2 Metric Hierarchy
A hierarchy can be defined among various classes of service based on the
degree to which traffic from one class can potentially degrade service of
traffic from lower classes that traverse the same link. In this hierarchy,
guaranteed constant bit rate traffic is at the top and "best-effort" datagram
traffic at the bottom. Classes providing service higher in the hierarchy
impact classes providing service in lower levels. The same situation is not
true in the other direction. For example, a datagram flow cannot affect a real-
time service. Thus, it may be necessary to distribute and update different
metrics for each type of service in the worst case. But, several advantages
result by identifying a single default metric. For example, one could derive a
single metric combining the availability of datagram and real-time service
over a common substrate.
3.3.3 Datagram Flows
A delay-sensitive metric is the probably the most obvious type of metric
suitable for datagram flows. However, it requires careful analysis to avoid
instabilities and to reduce storage and bandwidth requirements. For example,
we could use a recursive filtering technique that is based on a simple and
efficient weighted averaging algorithm [NC94]. This filter is used to stabilize
the metric. While it is adequate for smoothing most loading patterns, it will
not distinguish between patterns consisting of regular bursts of traffic and
random loading. Among other stabilizing tools, is a minimum time between
updates that can help filter out high-frequency oscillations.
draft-ietf-qosr-framework-01.txt [Page 7]
3.3.4 Real-time Flows
In real-time quality-of-service, delay variation is generally more critical than
delay as long as the delay is not too high. Clearly, voice-based applications
cannot tolerate more than a certain level of delay. The condition of varying
delays may be expected to a greater degree in a shared medium environment with
datagrams, than in a network implemented over a switched substrate. Routing a
real-time flow therefore reduces to an exercise in allocating the required
network resources while minimizing fragmentation of bandwidth. The resulting
situation is a bandwidth-limited minimum hop path from a source to the destination.
In other words, the router performs an ordered search through paths of increasing
hop count until it finds one that meets all the bandwidth needs of the flow.
To reduce contention and the probability of false probes (due to inaccuracy in
route tables), the router could select a path randomly from a "window" of paths
which meet the needs of the flow and satisfy one of three additional criteria:
best-fit, first-fit or worst-fit. Note that there is a similarity between the
allocation of bandwidth and the allocation of memory in a multiprocessing system.
First-fit seems to be appropriate for a system with a high real-time flow arrival
rates; and worst-fit is ideal for real-time flows with high holding times.
This rather nonintuitive result was shown in [NC94].
3.3.5 Path Properties
Path computation by itself is merely a search technique, e.g., Shortest Path
First (SPF) is a search technique based on dynamic programming. The
usefulness of the paths computed depends to a large extent on the metrics
used in evaluating the cost of a path with respect to a flow.
Each link considered by the path computation engine must be evaluated
against the requirements of the flow, i.e., the cost of providing the services
required by the flow must be estimated with respect to the capabilities of the
link. This requires a uniform method of combining features such as delay,
bandwidth, priority and other service features. Furthermore, the costs must
reflect the lost opportunity of using each link after routing the flow.
3.3.6 Performance Objectives
One common objective during path computation is to improve the total network throughput.
In this regard, merely routing a flow on any path that accommodates its QoS
requirement is not a good strategy. In fact, this corresponds to uncontrolled
alternate routing and may adversely impact performance at higher traffic loads.
It is therefore necessary to consider the total resource allocation for a flow
along a path, in relation to available resources, to determine whether or not
the flow should be routed on the path [RSR95]. Such a mechanism is referred to
in this draft as "higher level admission control". The goal of this is to ensure
that the "cost" incurred by the network in routing a flow with a given QoS is
never more than the revenue gained. The routing cost in this regard may be the
lost revenue in potentially blocking other flows that contend for the same
resources. The formulation of the higher level admission control strategy, with
suitable administrative hooks and with fairness to all flows desiring entry to
the network, is an issue. The fairness problem arises because flows
draft-ietf-qosr-framework-01.txt [Page 8]
with smaller reservations tend to be more successfully routed than flows with
large reservations, for a given engineered capacity. To guarantee a certain
level of acceptance rate for "larger" flows, without over-engineering the
network, requires a fair higher level admission control mechanism. The application
of higher level admission control to multicast routing is discussed later.
3.4 Administrative Control
----------------------
There are several administrative control issues. First, within an AS employing
state-dependent routing, administrative control of routing behavior may be
necessary. One example discussed earlier was higher level admission control.
Some others are described in this section. Second, the control of interdomain
routing based on policy is an issue. The discussion of interdomain routing is
defered to Section 5.
Two areas that need administrative control, in addition to appropriate routing
mechanisms, are handling flow priority with preemption, and resource allocation
for multiple service classes.
3.4.1 Flow Priorities and Preemption
If there are critical flows that must be accorded higher priority than other
types of flows, a mechanism must be implemented in the network to
recognize flow priorities. There are two aspects to prioritizing flows. First,
there must be a policy to decide how different users are allowed to set
priorities for flows they originate. The network must be able to verify that a
given flow is allowed to claim a priority level signaled for it. Second, the
routing scheme must ensure that a path with the requested QoS will be found
for a flow with a probability that increases with the priority of the flow. In
other words, for a given network load, a high priority flow should be more
likely to get a certain QoS from the network than a lower priority flow
requesting the same QoS. Routing procedures for flow prioritization can be
complex. Identification and evaluation of different procedures are areas that
require investigation.
3.4.2 Resource Control
If there are multiple service classes, it is necessary to engineer a network to
carry the forecasted traffic demands of each class. To do this, router and link
resources may be logically partitioned among various service classes. It is
desirable to have dynamic partitioning whereby unused resources in various
partitions are dynamically shifted to other partitions on demand [ACFH92].
Dynamic sharing, however, must be done in a controlled fashion in order to
prevent traffic under some service class from taking up more resources than
what was engineered for it for prolonged periods of time. The design of such
a resource sharing scheme, and its incorporation into the QoS-based routing
scheme are significant issues.
draft-ietf-qosr-framework-01.txt [Page 9]
3.5 QoS-Based Routing for Multicast Flows
-------------------------------------
QoS-based multicast routing is an important problem, especially if the notion
of higher level admission control is included. The dynamism in the receiver
set allowed by IP multicast, and receiver heterogeneity add to the problem.
With straightforward implementation of distributed heuristic algorithms for
multicast path computation [W88, C91], the difficulty is essentially one of
scalability. To accommodate QoS, multicast path computation at a router
must have knowledge of not only the id of subnets where group members are
present, but also the identity of branches in the existing tree. In other words,
routers must keep flow-specific state information. Also, computing optimal
shared trees based on the shared reservation style [BZBH96], may require
new algorithms. Multicast routing is discussed in some detail in Section 6.
3.6 Routing Overheads
-----------------
The overheads incurred by a routing scheme depend on the type of the routing
scheme, as well as the implementation. There are three types of overheads to be
considered: computation, storage and communication. It is necessary to understand
the implications of choosing a routing mechanism in terms of these overheads.
For example, considering link state routing, the choice of the update propagation
mechanism is important since network state is dynamic and changes relatively
frequently. Specifically, a flooding mechanism would result in many unnecessary
message transmissions and processing. Alternative techniques, such as tree-based
forwarding [R96], have to be considered. A related issue is the quantization of
state information to prevent frequent updating of dynamic state. While coarse
quantization reduces updating overheads, it may affect the performance of the
routing scheme. The tradeoff has to be carefully evaluated.
QoS-based routing incurs certain overheads during flow establishment, for example,
computing a source route. Whether this overhead is disproportionate compared to
the length of the sessions is an issue. In general, techniques for the
minimization of routing-related overheads during flow establishment must be
investigated. Approaches that are useful include pre-computation of routes,
caching recently used routes, and TOS routing based on hints in packets
(e.g., the TOS field).
3.7 Scaling by Hierarchical Aggregation
-----------------------------------
QoS-based routing should be scalable, and hierarchical aggregation is a
common technique for scaling (e.g., [PNNI96]). But this introduces problems with
regard to the accuracy of the aggregated state information [L95]. Also, the
aggregation of paths under multiple constraints is difficult. One of the
difficulties is the risk of accepting a flow based on inaccurate information,
but not being able to support the QoS requirements of flow because the
capabilities of the actual paths that are aggregated are not known during
route computation. Performance impacts of aggregating path metric
draft-ietf-qosr-framework-01.txt [Page 10]
information must therefore be understood. A way to compensate for
inaccuracies is to use crankback, i.e., dynamic search for alternate paths as a
flow is being routed. But crankback increases the time to
set up a flow, and may adversely affect the performance of the routing
scheme under some circumstances. Thus, crankback must be used
judiciously, if at all, along with a higher level admission control mechanism.
4. INTRADOMAIN ROUTING REQUIREMENTS
At the intradomain level, the objective is to allow as much latitude as
possible in addressing the QoS-based routing issues. Indeed, there are many
ideas about how QoS-based routing services can be provisioned within ASs.
These range from on-demand path computation based on current state
information, to statically provisioned paths supporting a few service classes.
Another aspect that might invite differing solutions is performance
optimization. Based on the technique used for this, intradomain routing could
be very sophisticated or rather simple. Finally, the service classes supported,
as well as the specific QoS engineered for a service class, could differ from
AS to AS. For instance, some ASs may not support guaranteed service, while others
may. Also, some ASs supporting the service may be engineered for a better delay
bound than others. Thus, it requires considerable thought to determine the high
level requirements for intradomain routing that both supports the overall view
of QoS-based routing in the Internet and allows maximum autonomy in developing
solutions.
Our view is that certain minimum requirements must be satisfied by
intradomain routing in order to be qualified as "QoS-based" routing. These
are:
- The routing scheme must route a flow along a path that can accommodate
its QoS requirements, or indicate that the flow cannot be admitted with the
QoS currently being requested.
- The routing scheme must indicate disruptions to the current route of a flow
due to topological changes.
- The routing scheme must accommodate best-effort flows without any resource reservation
requirements. That is, present best effort applications and protocol stacks need not
have to change to run in a domain employing QoS-based routing.
- The routing scheme may optionally support QoS-based multicasting with receiver
heterogeneity and shared reservation styles.
In addition, the following capabilities are also recommended:
- Capabilities to optimize resource usage.
- Implementation of higher level admission control procedures to limit
the overall resource utilization by individual flows.
draft-ietf-qosr-framework-01.txt [Page 11]
Further requirements along these lines may be specified. The requirements
should capture the consensus view of QoS-based routing, but should not
preclude particular approaches (e.g., TOS-based routing) from being
implemented. Thus, the intradomain requirements are expected to be rather
broad.
5. INTERDOMAIN ROUTING
The fundamental requirement on interdomain QoS-based routing is scalability.
This implies that interdomain routing cannot be based on highly dynamic network
state information. Rather, such routing must be aided by sound network engineering
and relatively sparse information exchange between independent routing domains.
This approach has the advantage that it can be realized by straightforward extensions
of the present Internet interdomain routing model. A number of issues, however, need
to be addressed to achieve this, as discussed below.
5.1 Interdomain QoS-Based Routing Model
-----------------------------------
The interdomain QoS-based routing model is depicted below:
AS1 AS2 AS3
___________ _____________ ____________
| | | | | |
| B------B B----B |
| | | | | |
-----B----- B------------- --B---------
\ / /
\ / /
____B_____B____ _________B______
| | | |
| B-------B |
| | | |
| B-------B |
--------------- ----------------
AS4 AS5
Here, ASs exchange standardized routing information via border nodes B.
Under this model, each AS can itself consist of a set of interconnected ASs,
with standardized routing interaction. Thus, the interdomain routing model
is hierarchical. Also, each lowest level AS employs an intradomain QoS-
based routing scheme (proprietary or standardized by intradomain routing
efforts such as QOSPF). Given this structure, some questions that arise are:
- What information is exchanged between ASs?
- What routing capabilities does the information exchange lead to? (E.g., source
routing, on-demand path computation, etc.)
- How is the external routing information represented within an AS?
draft-ietf-qosr-framework-01.txt [Page 12]
- How are interdomain paths computed?
- What sort of policy controls may be exerted on interdomain path computation
and flow routing?, and
- How is interdomain QoS-based multicast routing accomplished?
At a high level, the answers to these questions depend on the routing paradigm.
Specifically, considering link state routing, the information exchanged between
domains would consist of an abstract representation of the domains in the form of
logical nodes and links, along with metrics that quantify their properties and resource
availability. The hierarchical structure of the ASs may be handled by a hierarchical link
state representation, with appropriate metric aggregation.
Link state routing may not necessarily be advantageous for interdomain routing for
the following reasons:
- One advantage of intradomain link state routing is that it would allow fairly
detailed link state information be used to compute paths on demand for
flows requiring QoS. The state and metric aggregation used in interdomain
routing, on the other hand, erodes this property to a great degree.
- The usefulness of keeping track of the abstract topology and metrics of a
remote domain, or the interconnection between remote domains is not obvious.
This is especially the case when the remote topology and metric encoding are
lossy.
- ASs may not want to advertise any details of their internal topology or
resource availability.
- Scalability in interdomain routing can be achieved only if information
exchange between domains is relatively infrequent. Thus, it seems practical
to limit information flow between domains as much as possible.
Compact information flow allows the implementation QoS-enhanced versions of existing
interdomain protocols such as BGP-4. We look at the interdomain routing issues in this
context.
5.2 Interdomain Information Flow
----------------------------
The information flow between routing domains must enable certain basic functions:
1. Determination of reachability to various destinations
2. Loop-free flow routes
3. Address aggregation whenever possible
draft-ietf-qosr-framework-01.txt [Page 13]
4. Determination of the QoS that will be supported on the path to a destination.
The QoS information should be relatively static, determined from the engineered
topology and capacity of an AS rather than ephemeral fluctuations in traffic
load through the AS. Ideally, the QoS supported in a transit AS should be
allowed to vary significantly only under exceptional circumstances, such as
failures or focused overload.
5. Determination, optionally, of multiple paths for a given destination, based on
service classes.
6. Expression of routing policies, including monetary cost, as a function of flow
parameters, usage and administrative factors.
Items 1-3 are already part of existing interdomain routing. Item 5 is also a straightfoward
extension of the current model. The main problem areas are therefore items 4 and 6.
The QoS of an end-to-end path is obtained by composing the QoS available in each transit AS.
Thus, border routers must first determine what the locally available QoS is in order to
advertise routes to both internal and external destinations. The determination of local
"AS metrics" (corresponding to link metrics in the intradomain case) should not be
subject to too much dynamism. Thus, the issue is how to define such metrics and what
triggers an occasional change that results in re-advertisements of routes.
The approach suggested in this draft is not to compute paths based on residual or
instantaneous values of AS metics (which can be dynamic), but utilize only the QoS
capabilities engineered for aggregate transit flows. Such engineering may be based on
the knowledge of traffic to be expected from each neighboring ASs and the corresponding
QOS needs. This information may be obtained based on contracts agreed upon prior to the
provisioning of services. The AS metric then corresponds to the QoS capabilities of the
"virtual path" engineered through the AS (for transit traffic) and a different metric
may be used for different neighbors. This is illustrated in the following figure.
AS1 AS2 AS3
___________ _____________ ____________
| | | | | |
| B------B1 B2----B |
| | | | | |
-----B----- B3------------ --B---------
\ /
\ /
____B_____B____
| |
| |
| |
| |
---------------
AS4
draft-ietf-qosr-framework-01.txt [Page 14]
Here, B1 may utlise an AS metric specific for AS1 when computing path metrics to be
advertised to AS1. This metric is based on the resources engineered in AS2 for transit
traffic from AS1. Similarly, B3 may utilize a different metric when computing path metrics
to be advertised to AS4. Now, it is assumed that as long as traffic flow into AS2 from
AS1 or AS4 does not exceed the engineered values, these path metrics would hold.
Excess traffic due to transient fluctuations, however, may be handled as best effort.
Thus, this model is different from the intradomain model, where end nodes pick
a path dynamically based on the QoS needs of the flow to be routed. Here, paths
within ASs are engineered based on presumed, measured or declared traffic and QoS
requirements. Under this model, an AS can contract for routes via
multiple transit ASs with different QoS requirements. For instance, AS4 above can
use both AS1 and AS2 as transits for same or different destinations. Also, a QoS
contract between one AS and another may generate another contract between the second
and a third AS and so forth.
An issue is what triggers the recomputation of path metrics within an AS. Failures
or other events that prevent engineered resource allocation should certainly trigger
recomputation. Recomputation should not be triggered in response to arrival of flows
within the engineered limit.
5.3 Path Computation
----------------
Path computation for an external destination at a border node is based on
reachability, path metrics and local policies of selection. If there are multiple
selection criteria (e.g., delay, bandwidth, cost, etc.), mutiple alternaives may have to be
maintained as well as propagated by border nodes. Selection of a path from among many
alternatives would depend on the QoS requests of flows, as well as policies. Path computation
may also utilze any heuristics for optimizing resource usage.
5.4 Flow Aggregation
----------------
An important issue in interdomain routing is the amount of flow state to be processed
by transit ASs. Reducing the flow state by aggregation techniques must therefore be
seriously considered. Flow aggregation means that transit traffic through an
AS is classified into a few aggregated streams rather than being routed at the individual
flow level. For example, an entry border router may classify various transit flows entering an
AS into a few coarse categories, based on the egress node and QoS requirements of the flows.
Then, the aggregated stream for a given traffic class may be routed as a single
flow inside the AS to the exit border router. This router may then present individual
flows to different neighboring ASs and the process repeats at each entry border
router. Under this scenario, it is essential that entry border routers keep track of
the resource requirements for each transit flow and apply admission control to determine
whether the aggregate requirement from any neighbor exceeds the engineered limit. If so,
some policy must be invoked to deal with the excess traffic. Otherwise, it may be assumed
that aggregated flows are routed over paths that have adequate resources to guarantee QoS for
the member flows. Finally, it is possible that entry border routers at a transit AS
may prefer not to aggregate flows if finer grain routing within the AS may be more
efficient (e.g., to aid load balancing within the AS).
draft-ietf-qosr-framework-01.txt [Page 15]
5.5 Path Cost Determination
-----------------------
It is hoped that the integrated services Internet architecture would allow
providers to charge for IP flows based on their QoS requirements. A QoS-
based routing architecture can aid in distributing information on expected
costs of routing flows to various destinations via different domains. Clearly,
from a provider's point of view, there is a cost incurred in guaranteeing QoS
to flows. This cost could be a function of several parameters, some related to
flow parameters, others based on policy. From a user's point of view, the
consequence of requesting a particular QoS for a flow is the cost incurred,
and hence the selection of providers may be based on cost. A routing scheme
can aid a provider in distributing the costs in routing to various destinations,
as a function of several parameters, to other providers or to end users. In
the interdomain routing model described earlier, the costs to a destination will
change as routing updates are passed through a transit domain. One of the
goals of the routing scheme should be to maintain a uniform semantics for
cost values (or functions) as they are handled by intermediate domains. As an
example, consider the cost function generated by border node B1 in domain
A and passed to node B2 in domain B below. The routing update may be
injected into domain B by B2 and finally passed to B4 in domain C by router
B3. Domain B may interpret the cost value received from domain A in any
way it wants, for instance, adding a locally significant component to it. But
when this cost value is passed to domain C, the meaning of it must be what
domain A intended, plus the incremental cost of transiting domain B, but not
what domain B uses internally.
Domain A Domain B Domain C
____________ ___________ ____________
| | | | | |
| B1------B2 B3---B4 |
| | | | | |
------------ ----------- ------------
A problem with charging for a flow is the determination of the cost when the QoS
promised for the flow was not actually delivered. Clearly, when a flow is
routed via multiple domains, it must be determined whether each domain delivers
the QoS it declares possible for traffic through it.
6. QOS-BASED MULTICAST ROUTING
The goals of QoS-based multicast routing are as follows:
- Scalability to large groups with dynamic membership
- Robustness in the presence of topological changes
- Support for receiver-initiated, heterogeneous reservations
- Support for shared reservation styles, and
draft-ietf-qosr-framework-01.txt [Page 16]
- Support for "global" admission control, i.e., administrative control of
resource consumption by the multicast flow.
The RSVP multicast flow model is as follows. The sender of a multicast
flow advertises the traffic characteristics periodically to the receivers. On
receipt of an advertisement, a receiver may generate a message to reserve
resources along the flow path from the sender. Receiver reservations may be
heterogeneous. Other multicast models may be considered.
The multicast routing scheme attempts to determine a path from the sender to
each receiver that can accommodate the requested reservation. The routing
scheme may attempt to maximize network resource utilization by minimizing
the total bandwidth allocated to the multicast flow, or by optimizing some
other measure.
6.1 Scalability, Robustness and Heterogeneity
-----------------------------------------
When addressing scalability, two aspects must be considered:
1. The overheads associated with receiver discovery. This overhead is incurred
when determining the multicast tree for forwarding best-effort sender
traffic characterization to receivers.
2. The overheads associated with QoS-based multicast path computation.This
overhead is incurred when flow-specific state information has to be
collected by a router to determine QoS-accommodating paths to a receiver.
Depending on multicast routing scheme, one or both of these aspects become
important. For instance, under the present RSVP model, reservations are
established on the same path over which sender traffic characterizations are
sent, and hence there is no path computation overhead. On the other hand,
under the proposed QOSPF model [ZSSC96] of multicast source routing,
receiver discovery overheads are incurred by MOSPF [M94] receiver location
broadcasts, and additional path computation overheads are incurred due to
the need to keep track of existing flow paths. Scaling of QoS-based multicast
depends on both these scaling issues. However, scalable best-effort
multicasting is really not in the domain of QoS-based routing work (solutions
for this are being devised by the IDMR WG [BCF94, DEFV94]). QoS-based
multicast routing may build on these solutions to achieve overall scalability.
There are several options for QoS-based multicast routing. Multicast source
routing is one under which multicast trees are computed by the first-hop
router from the source, based on sender traffic advertisements. The advantage
of this is that it blends nicely with the present RSVP signaling model. Also,
this scheme works well when receiver reservations are homogeneous and the
same as the maximum reservation derived from sender advertisement. The
disadvantages of this scheme are the extra effort needed to accommodate
heterogeneous reservations and the difficulties in optimizing resource
allocation based on shared reservations.
draft-ietf-qosr-framework-01.txt [Page 17]
In these regards, a receiver-oriented multicast routing model seems to have
some advantage over multicast source routing. Under this model:
1. Sender traffic advertisements are multicast over a best-effort tree which
can be different from the QoS-accommodating tree for sender data.
2. Receiver discovery overheads are minimized by utilizing a scalable IDMR
scheme (e.g., PIM, CBT), to multicast sender traffic characterization.
3. Each receiver-side router independently computes a QoS-accommodating path
from the source, based on the receiver reservation. This path can be computed
based on unicast routing information only, or with additional multicast
flow-specific state information. In any case, multicast path computation is
broken up into multiple, concurrent unicast path computations.
4. Routers processing unicast reserve messages from receivers aggregate
resource reservations from multiple receivers.
Flow-specific state information may be limited in Step 3 to achieve scalability.
In general, limiting flow-specific information in making multicast
routing decisions is important in any routing model. The advantages of this
model are the ease with which heterogeneous reservations can be
accommodated, and the ability to handle shared reservations. The
disadvantages are the incompatibility with the present RSVP signaling model,
and the need to rely on reverse paths when link state routing is not used.
Both multicast source routing and the receiver-oriented routing model
described above utilize per-source trees to route multicast flows. Another
possibility is the utilization of shared, per-group trees for routing flows.
The computation and usage of such trees require further work.
Finally, scalability at the interdomain level may be achieved if QoS-based
multicast paths are computed independently in each domain. This principle is
illustrated by the QOSPF multicast source routing scheme which allows
independent path computation in different OSPF areas. It is easy to
incorporate this idea in the receiver-oriented model also. An evaluation of
multicast routing strategies must take into account the relative advantages
and disadvantages of various approaches, in terms of scalability features and
functionality supported.
6.2 Multicast Admission Control
---------------------------
Higher level admission control, as defined for unicast, prevents excessive
resource consumption by flows when traffic load is high . Such an admission
control strategy must be applied to multicast flows when the flow path
computation is receiver-oriented or sender-oriented. In essence, a router
computing a path to/ for a receiver must determine whether the incremental
resource allocation for the receiver is excessive under some administratively
determined admission control policy. Other admission control criteria, based
on the total resource consumption of a tree may be defined.
draft-ietf-qosr-framework-01.txt [Page 18]
7. QOS-BASED ROUTING AND RESOURCE RESERVATION PROTOCOLS
There must clearly be a well-defined interface between routing and resource reservation
protocols. The nature of this interface, and the interaction between routing and resource
reservation has to be determined carefully to avoid incompatibilities. The importance of
this can be readily illustrated in the case of RSVP.
RSVP has been designed to operate independent of the underlying routing scheme.
Under this model, RSVP PATH messages establish the reverse path for RESV
messages. In essence, this model is not compatible with QoS-based routing
schemes that compute paths after receiver reservations are received. While this
incompatibility can be resolved in a simple manner for unicast flows, multicast
with heterogeneous receiver requirements is a more difficult case [GKS97]. For this,
reconciliation between RSVP and QoS-based routing models is necessary. Such a
reconciliation, however, may require some changes to the RSVP model depending
on the QoS-based routing model. On the other hand, QoS-based routing schemes
may be designed with RSVP compatibility as a necessary goal. How this affects
scalability and other performance measures must be considered.
8. RELATED WORK
"Adaptive" routing, based on network state, has a long history,
especially in circuit-switched networks. Such routing has also been
implemented in early datagram and virtual circuit packet networks. More
recently, this type of routing has been the subject of study in the context of
ATM networks, where the traffic characteristics and topology are
substantially different from those of circuit-switched networks [MMR96]. It
is instructive to review the adaptive routing methodologies, both to
understand the problems encountered and possible solutions.
Fundamentally, there are two aspects to adaptive, network state-dependent
routing.
1. Measuring and gathering network state information, and
2. Computing routes based on the available information.
Depending on how these two steps are implemented, a variety of routing
techniques are possible. These differ in the following respects:
- what state information is used
- whether local or global state is used
- what triggers the propagation of state information
- whether routes are computed in a distributed or centralized manner
- whether routes are computed on-demand, pre-computed, or in a hybrid manner
- what optimization criteria, if any, are used in computing routes
- whether source routing or hop by hop routing is used, and
- how alternate route choices are explored
It should be noted that most of the adaptive routing work has focused on
unicast routing. Multicast routing is one of the areas that would be prominent
with Internet QoS-based routing. We treat this separately, and the following
review considers only unicast routing. This review is not exhaustive, but
gives a brief overview of some of the approaches.
draft-ietf-qosr-framework-01.txt [Page 19]
8.1 Optimization Criteria
---------------------
The most common optimization criteria used in adaptive routing is
throughput maximization or delay minimization. A general formulation of
the optimization problem is the one in which the network revenue is
maximized, given that there is a cost associated with routing a flow over a
given path [MMR96, K88]. In general, global optimization solutions are
difficult to implement, and they rely on a number of assumptions on the
characteristics of the traffic being routed [MMR96]. Thus, the practical
approach has been to treat the routing of each flow (VC, circuit or packet
stream to a given destination) independently of the routing of other flows.
Many such routing schemes have been implemented.
8.2 Circuit Switched Networks
-------------------------
Many adaptive routing concepts have been proposed for circuit-switched
networks. An example of a simple adaptive routing scheme is sequential
alternate routing [T88]. This is a hop-by-hop destination-based routing
scheme where only local state information is utilized. Under this scheme, a
routing table is computed for each node, which lists multiple output link
choices for each destination. When a call set-up request is received by a node,
it tries each output link choice in sequence, until it finds one that can
accommodate the call. Resources are reserved on this link, and the call set-up
is forwarded to the next node. The set-up either reaches the destination, or is
blocked at some node. In the latter case, the set-up can be cranked back to the
previous node or a failure declared. Crankback allows the previous node to
try an alternate path. The routing table under this scheme can be computed in
a centralized or distributed manner, based only on the topology of the
network. For instance, a k-shortest-path algorithm can be used to determine k
alternate paths from a node with distinct initial links [T88]. Some
mechanism must be implemented during path computation or call set-up to
prevent looping.
Performance studies of this scheme illustrate some of the pitfalls of alternate
routing in general, and crankback in particular [A84, M86, YS87].
Specifically, alternate routing improves the throughput when traffic load is
relatively light, but adversely affects the performance when traffic load is
heavy. Crankback could further degrade the performance under these
conditions. In general, uncontrolled alternate routing (with or without
crankback) can be harmful in a heavily utilized network, since circuits tend
to be routed along longer paths thereby utilizing more capacity. This is an
obvious, but important result that applies to QoS-based Internet routing also.
The problem with alternate routing is that both direct routed (i.e., over
shortest paths) and alternate routed calls compete for the same resource. At
higher loads, allocating these resources to alternate routed calls result in the
displacement of direct routed calls and hence the alternate routing of these
calls. Therefore, many approaches have been proposed to limit the flow of
alternate routed calls under high traffic loads. These schemes are designed
draft-ietf-qosr-framework-01.txt [Page 20]
for the fully-connected logical topology of long distance telephone networks
(i.e., there is a logical link between every pair of nodes). In this topology,
direct routed calls always traverse a 1-hop path to the destination and
alternate routed calls traverse at most a 2-hop path.
"Trunk reservation" is a scheme whereby on each link a certain bandwidth is
reserved for direct routed calls [MS91]. Alternate routed calls are allowed on
a trunk as long as the remaining trunk bandwidth is greater than the reserved
capacity. Thus, alternate routed calls cannot totally displace direct routed
calls on a trunk. This strategy has been shown to be very effective in
preventing the adverse effects of alternate routing.
"Dynamic alternate routing" (DAR) is a strategy whereby alternate routing is
controlled by limiting the number of choices, in addition to trunk reservation
[MS91]. Under DAR, the source first attempts to use the direct link to the
destination. When blocked, the source attempts to alternate route the call via
a pre-selected neighbor. If the call is still blocked, a different neighbor is
selected for alternate routing to this destination in the future. The present call
is dropped. DAR thus requires only local state information. Also, it "learns"
of good alternate paths by random sampling and sticks to them as long as possible.
More recent circuit-switched routing schemes utilize global state to select
routes for calls. An example is AT&T's Real-Time Network Routing (RTNR)
scheme [ACFH92]. Unlike schemes like DAR, RTNR handles multiple
classes of service, including voice and data at fixed rates. RTNR utilizes a
sophisticated per-class trunk reservation mechanism with dynamic bandwidth
sharing between classes. Also, when alternate routing a call, RTNR utilizes
the loading on all trunks in the network to select a path. Because of the fully-
connected topology, disseminating status information is simple under RTNR;
each node simply exchanges status information directly with all others.
From the point of view of designing QoS-based Internet routing schemes,
there is much to be learned from circuit-switched routing. For example,
alternate routing and its control, and dynamic resource sharing among
different classes of traffic. It is, however, not simple to apply some of the
results to a general topology network with heterogeneous multirate traffic.
Work in the area of ATM network routing described next illustrates this.
8.3 ATM Networks
------------
The VC routing problem in ATM networks presents issues similar to that
encountered in circuit-switched networks. Not surprisingly, some extensions
of circuit-switched routing have been proposed. The goal of these routing
schemes is to achieve higher throughput as compared to traditional shortest-
path routing. The flows considered usually have a single QoS requirement,
i.e., bandwidth.
The first idea is to extend alternate routing with trunk reservation to general
topologies [SD95]. Under this scheme, a distance vector routing protocol is
used to build routing tables at each node with multiple choices of increasing
draft-ietf-qosr-framework-01.txt [Page 21]
hop count to each destination. A VC set-up is first routed along the primary
("direct") path. If sufficient resources are not available along this path,
alternate paths are tried in the order of increasing hop count. A flag in the
VC set-up message indicates primary or alternate routing, and bandwidth on
links along an alternate path is allocated subject to trunk reservation. The
trunk reservation values are determined based on some assumptions on traffic
characteristics. Because the scheme works only for a single data rate, the
practical utility of it is limited.
The next idea is to import the notion of controlled alternate routing into
traditional link state QoS-based routing [RSR95, GKR96]. To do this, first
each VC is associated with a maximum permissible routing cost. This cost
can be set based on expected revenues in carrying the VC or simply based on
the length of the shortest path to the destination. Each link is associated with
a metric that increases exponentially with its utilization. A switch computing
a path for a VC simply determines a least-cost feasible path based on the link
metric and the VC's QoS requirement. The VC is admitted if the cost of the
path is less than or equal to the maximum permissible routing cost. This
routing scheme thus limits the extent of "detour" a VC experiences, thus
preventing excessive resource consumption. This is a practical scheme and
the basic idea can be extended to hierarchical routing. But the performance of
this scheme has not been analyzed thoroughly. A similar notion of admission
control based on the connection route was also incorporated in a routing
scheme presented in [ACG92].
Considering the ATM Forum PNNI protocol [PNNI96], a partial list of its stated
characteristics are as follows:
o Scales to very large networks
o Supports hierarchical routing
o Supports QoS
o Uses source routed connection setup
o Supports multiple metrics and attributes
o Provides dynamic routing
The PNNI specification is sub-divided into two protocols: a signaling and a
routing protocol. The PNNI signaling protocol is used to establish point-to-
point and point to multipoint connections and supports source routing,
crankback and alternate routing. PNNI source routing allows loop free paths.
Also, it allows each implementation to use its own path computation
algorithm. Furthermore, source routing is expected to support incremental
deployment of future enhancements such as policy routing.
The PNNI routing protocol is a dynamic, hierarchical link state protocol that
propagates topology information by flooding it through the network. The
topology information is the set of resources (e.g., nodes, links and addresses)
which define the network. Resources are qualified by defined sets of metrics
and attributes (delay, available bandwidth, jitter, etc.) which are grouped by
supported traffic class. Since some of the metrics used will change frequently
e.g., available bandwidth, threshold algorithms are used to determine if the
change in a metric or attribute is significant enough to require propagation of
updated information. Other features include, auto configuration of the
routing hierarchy, connection admission control (as part of path calculation)
and aggregation and summarization of topology and reachability information.
draft-ietf-qosr-framework-01.txt [Page 22]
Despite its functionality, the PNNI routing protocol does not address the
issues of multicast routing, policy routing and control of alternate routing. A
problem in general with link state QoS-based routing is that of efficient
broadcasting of state information. While flooding is a reasonable choice with
static link metrics it may impact the performance adversely with dynamic metrics.
Finally, Integrated PNNI [I-PNNI] has been designed from the start to take
advantage of the QoS Routing capabilities that are available in PNNI and integrate
them with routing for layer 3. This would provide an integrated layer 2
and layer 3 routing protocol for networks that include PNNI in the ATM
core. The I-PNNI specification has been under development in the ATM
Forum and, at this time, has not yet incorporated QoS routing mechanisms
for layer 3.
8.4 Packet Networks
---------------
Early attempts at adaptive routing in packet networks had the objective of
delay minimization by dynamically adapting to network congestion.
Alternate routing based on k-shortest path tables, with route selection based
on some local measure (e.g., shortest output queue) has been described [R76,
YS81]. The original ARPAnet routing scheme was a distance vector protocol
with delay-based cost metric [MW77]. Such a scheme was shown to be prone
to route oscillations [B82]. For this and other reasons, a link state delay-
based routing scheme was later developed for the ARPAnet [MRR80]. This
scheme demonstrated a number of techniques such as triggered updates,
flooding, etc., which are being used in OSPF and PNNI routing today.
Although none of these schemes can be called QoS-based routing schemes,
they had features that are relevant to QoS-based routing.
IBM's System Network Architecture (SNA) introduced the concept of Class
of Service (COS)-based routing [A79, GM79]. There were several classes of
service: interactive, batch, and network control. In addition, users could
define other classes. When starting a data session an application or device
would request a COS. Routing would then map the COS into a statically
configured route which marked a path across the physical network. Since
SNA is connection oriented, a session was set up along this path and the
application's or device's data would traverse this path for the life of the
session. Initially, the service delivered to a session was based on the network
engineering and current state of network congestion. Later, transmission
priority was added to subarea SNA. Transmission priority allowed more
important traffic (e.g. interactive) to proceed before less time-critical traffic
(e.g. batch) and improved link and network utilization. Transmission priority
of a session was based on its COS.
Subarea SNA later evolved to support multiple or alternate paths between
nodes. But, although assisted by network design tools, the network
administrator still had to statically configure routes. IBM later introduced
SNA's Advanced Peer to Peer Networking (APPN) [B85]. APPN added new
features to SNA including dynamic routing based on a link state database.
An applications would use COS to indicate it traffic requirements and APPN
draft-ietf-qosr-framework-01.txt [Page 23]
would calculate a path capable of meeting these requirements. Each COS
was mapped to a table of acceptable metrics and parameters that qualified the
nodes and links contained in the APPN topology Database. Metrics and
parameters used as part of the APPN route calculation include, but are not
limited to: delay, cost per minute, node congestion and security. The
dynamic nature of APPN allowed it to route around failures and reduce
network configuration.
The service delivered by APPN was still based on the network engineering,
transmission priority and network congestion. Then in 1995 IBM introduced
an extension to APPN, High Performance Routing (HPR)[IBM97]. HPR uses
a highly responsive congestion avoidance algorithm called adaptive rate
based (ARB) congestion control. Using predictive feedback methods, the
ARB algorithm prevents congestion and improves network utilization. Most
recently, an extension to the COS table has been defined so that HPR routing
could recognize and take advantage of ATM QoS capabilities.
Considering IP routing, both IDRP [R92] and OSPF support type of service (TOS)-
based routing. While the IP header has a TOS field, there is no standardized way of
utilizing it for TOS specification and routing. It seems possible to make use of the
IP TOS feature, along with TOS-based routing and proper network engineering, to
do QoS-based routing. Among the newer schemes, Source Demand Routing (SDR)
[ELRV96] allows on-demand path computation by routers and the implementation
of strict and loose source routing. The Nimrod architecture [CCM96] has a number
of concepts built in to handle scalability and specialized path computation.
9. SUMMARY AND CONCLUSIONS
In this draft, a framework for QoS-based Internet routing was defined. This
framework emphasizes the traditional separation between intra and
interdomain routing. This approach is especially meaningful in the case of
QoS-based routing, since there are many views on how QoS-based routing
should be accomplished and many different needs. The objective of this draft
was to encourage the development of different solution approaches for
intradomain routing, subject to some broad requirements, while consensus on
interdomain routing is achieved. To this end, the QoS-based routing issues
were described, and some broad intradomain routing requirements and an interdomain
routing model were defined. In addition, QoS-based multicast routing was discussed
and a detailed review of related work was presented.
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AUTHORS' ADDRESSES
Bala Rajagopalan Raj Nair
NEC USA, C&C Research Labs Arrowpoint
4 Independence Way 235 Littleton Rd.
Princeton, NJ 08540 Westford, MA 01886
U.S.A U.S.A
Ph: +1-609-951-2969 Ph: +1-508-692-5875, x29
Email: braja@ccrl.nj.nec.com Email: nair@arrowpoint.com
Hal Sandick Eric S. Crawley
IBM ND, E95/B664 Gigapacket Networks
800 Park Offices Drive 25 Porter Rd.
RTP, NC 27705 Littelton, MA 01460
U.S.A U.S.A
Ph: +1-919-254-4614 Ph: +1-508-486-0665
Email: sandick@vnet.ibm.com Email: esc@gigapacket.com
******* This draft expires on January, 28, 1998 ********
