RFC2386 - A Framework for QoS-based Routing in the Internet

Network Working Group E. Crawley

Request for Comments: 2386 Argon Networks

Category: Informational R. Nair

Arrowpoint

B. Rajagopalan

NEC USA

H. Sandick

Bay Networks

August 1998

A Framework for QoS-based Routing in the Internet

Status of this Memo

This memo provides information for the Internet community. It does

not specify an Internet standard of any kind. Distribution of this

memo is unlimited.

Copyright Notice

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

ABSTRACT

QoS-based routing has been recognized as a 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.

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:

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. Encourage 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 [GKOP98, ZSSC97],

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 document 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 document 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 or

enhancement of a routing protocol that allows relatively simple

interaction between domains.

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. Security considerations are listed in Section 8 and related work

is described in Section 9. Finally, summary and conclusions are

presented in Section 10.

2. GLOSSARY

The following glossary lists the terminology used in this document

and an eXPlanation of what is meant. Some of these terms may have

different connotations, but when used in this document, 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

administrative authority and consistent internal routing policy. An

AS may employ multiple intradomain routing protocols internally and

interfaces to other ASs via a common interdomain routing protocol.

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 RFC1633 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 [W97] and Guaranteed Service [SPG97].

RSVP: The ReSerVation Protocol [BZBH97]. 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.

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 and

policy requirements 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: BACKGROUND AND ISSUES

3.1 Best-Effort and QoS-Based Routing

Routing deployed in today's Internet is focused on connectivity and

typically supports only one type of datagram service called "best

effort" [WC96]. Current Internet routing protocols, e.g. OSPF, RIP,

use "shortest path routing", i.e. routing that is optimized for a

single arbitrary metric, administrative weight or hop count. These

routing protocols are also "opportunistic," using the current

shortest path or route to a destination. Alternate paths with

acceptable but non-optimal cost can not be used to route traffic

(shortest path routing protocols do allow a router to alternate among

several equal cost paths to a destination).

QoS-based routing must extend the current routing paradigm in three

basic ways. First, to support traffic using integrated-services

class of services, multiple paths between node pairs will have to be

calculated. Some of these new classes of service will require the

distribution of additional routing metrics, e.g. delay, and available

bandwidth. If any of these metrics change frequently, routing updates

can become more frequent thereby consuming network bandwidth and

router CPU cycles.

Second, today's opportunistic routing will shift traffic from one

path to another as soon as a "better" path is found. The traffic

will be shifted even if the existing path can meet the service

requirements of the existing traffic. If routing calculation is tied

to frequently changing consumable resources (e.g. available

bandwidth) this change will happen more often and can introduce

routing oscillations as traffic shifts back and forth between

alternate paths. Furthermore, frequently changing routes can increase

the variation in the delay and jitter experienced by the end users.

Third, as mentioned earlier, today's optimal path routing algorithms

do not support alternate routing. If the best existing path cannot

admit a new flow, the associated traffic cannot be forwarded even if

an adequate alternate path exists.

3.2 QoS-Based Routing and Resource Reservation

It is important to understand the difference between QoS-based

routing and resource reservation. While resource reservation

protocols such as RSVP [BZBH97] provide a method for requesting and

reserving network resources, they do not provide a mechanism for

determining a network path that has adequate resources to accommodate

the requested QoS. Conversely, QoS-based routing allows the

determination of a path that has a good chance of accommodating the

requested QoS, but it does not include a mechanism to reserve the

required resources.

Consequently, QoS-based routing is usually used in conjunction with

some form of resource reservation or resource allocation mechanism.

Simple forms of QoS-based routing have been used in the past for Type

of Service (TOS) routing [M98]. In the case of OSPF, a different

shortest-path tree can be computed for each of the 8 TOS values in

the IP header [ISI81]. Such mechanisms can be used to select

specially provisioned paths but do not completely assure that

resources are not overbooked along the path. As long as strict

resource management and control are not needed, mechanisms such as

TOS-based routing are useful for separating whole classes of traffic

over multiple routes. Such mechanisms might work well with the

emerging Differential Services efforts [BBCD98].

Combining a resource reservation protocol with QoS-based routing

allows fine control over the route and resources at the cost of

additional state and setup time. For example, a protocol such as RSVP

may be used to trigger QoS-based routing calculations to meet the

needs of a specific flow.

3.3 QoS-Based Routing: Objectives

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.

- 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.

3.4 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. Solutions to these problems are just emerging [GPSS98].

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 it

introduces some complexity.

An additional problem with resource reservation is how to determine

what resources have already been allocated to a multicast flow. The

availability of this information during path computation improves the

chances of finding a path to add a new receiver to a multicast flow.

QOSPF [ZSSC97] handles this problem by letting routers broadcast

reserved resource information to other routers in their area.

Alternate path routing [ZES97] deals with this issue by using probe

messages to find a path with sufficient resources. Path QoS

Computation (PQC) method, proposed in [GOA97], propagates bandwidth

allocation information in RSVP PATH messages. A router receiving the

PATH message gets an indication of the resource allocation only on

those links in the path to itself from the source. Allocation for

the same flow on other remote branches of the multicast tree is not

available. Thus, the PQC method may not be sufficient to find

feasible QoS-accommodating paths to all receivers.

3.5 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 a destination. This set of

routing granularities presents a problem for QoS routing solutions.

If routing based only on destination address is considered, then an

intermediate router will route all flows between different sources

and a given destination along the same path. This is acceptable if

the path has adequate capacity but a problem arises if there are

multiple flows to a destination that exceed the capacity of the link.

One version of QOSPF [ZSSC97] 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 also increases since the routing tables must include

source/destination pairs instead of just the destination.

The best granularity is found when routing is based on individual

flows but this incurs a tremendous cost in terms of the routing

state. Each QoS flow can be routed separately between any source and

destination. PQC [GOA97] and alternate path routing [ZES97], are

examples of solutions which operate at the flow level.

Both source/destination and flow-based routing may be susceptible to

packet looping under hop-by-hop forwarding. Suppose a node along a

flow or source/destination-based path loses the state information for

the flow. Also suppose that the flow-based route is different from

the regular destination-based route. The potential then exists for a

routing loop to form when the node forwards a packet belonging to the

flow using its destination-based routing table to a node that occurs

earlier on the flow-based path. This is because the latter node may

use its flow-based routing table to forward the packet again to the

former and this can go on indefinitely.

3.6 Metrics and Path Computation

3.6.1 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-based 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 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 while 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 simplicity (The filtering technique

may be simpler, depending on the set of metrics used. For example,

with bandwidth and cost as metrics, it is possible to first eliminate

the set of links that do not have the requested bandwidth and then

compute the least cost path using the remaining links.)

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.

3.6.2 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.

3.6.3 Datagram Flows

A delay-sensitive metric is 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, a recursive filtering technique based on a simple and

efficient weighted averaging algorithm [NC94] could be used. 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.

3.6.4 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].

3.6.5 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.

3.6.6 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 [SD95] 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. Such a

mechanism is referred to in this document 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 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.

3.7 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.

3.7.1 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.

3.7.2 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.

3.8 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

[BZBH97], may require new algorithms. Multicast routing is discussed

in some detail in Section 6.

3.9 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).

3.10 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

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.

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?

- 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

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 document 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

Here, B1 may utilize 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 or marked with a

discard bit.

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).

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

- 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 the 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 [ZSSC97] 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.

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 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 nunicast 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 [RN98]. 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 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.

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. 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 [ZES97,

ZSSC97, GOA97]. 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. SECURITY CONSIDERATIONS

Security issues that arise with routing in general are about

maintaining the integrity of the routing protocol in the presence of

unintentional or malicious introduction of information that may lead

to protocol failure [P88]. QoS-based routing requires additional

security measures both to validate QoS requests for flows and to

prevent resource-depletion type of threats that can arise when flows

are allowed to make arbitratry resource requests along various paths

in the network. Excessive resource consumption by an errant flow

results in denial of resources to legitimate flows. While these

situations may be prevented by setting up proper policy constraints,

charging models and policing at various points in the network, the

formalization of such protection requires work [BCCH94].

9. 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.

9.1 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.

9.2 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 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.

9.3 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 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 [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.

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.

9.4 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.

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 application would use COS to indicate it traffic

requirements and APPN 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. IBM later

introduced an extension to APPN, High Performance Routing

(HPR)[IBM97]. HPR uses a 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. The emerging differentiated services model is

generating renewed interest in TOS support. 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. Recently, some work has been done on QoS-based routing

schemes for the integrated services Internet. For example, in [M98],

heuristic schemes for efficient routing of flows with bandwidth

and/or delay constraints is described and evaluated.

9. SUMMARY AND CONCLUSIONS

In this document, a framework for QoS-based Internet routing was

defined. This framework adopts 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 document 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.

The deployment of QoS-based routing across multiple administrative

domains requires both the development of intradomain routing schemes

and a standard way for them to interact via a well-defined

interdomain routing mechanism. This document, while outlining the

issues that must be addressed, did not engage in the specification of

the actual features of the interdomain routing scheme. This would be

the next step in the evolution of wide-area, multidomain QoS-based

routing.

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AUTHORS' ADDRESSES

Bala Rajagopalan

NEC USA, C&C Research Labs

4 Independence Way

Princeton, NJ 08540

U.S.A

Phone: +1-609-951-2969

EMail:

braja@ccrl.nj.nec.com

Raj Nair

Arrowpoint

235 Littleton Rd.

Westford, MA 01886

U.S.A

Phone: +1-508-692-5875, x29

EMail: nair@arrowpoint.com

Hal Sandick

Bay Networks, Inc.

1009 Slater Rd., Suite 220

Durham, NC 27703

U.S.A

Phone: +1-919-941-1739

EMail: Hsandick@baynetworks.com

Eric S. Crawley

Argon Networks, Inc.

25 Porter Rd.

Littelton, MA 01460

U.S.A

Phone: +1-508-486-0665

EMail: esc@argon.com

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