InternetEngineering TaskForce R.Guerin/S. Kamat/A. Orda
INTERNETDRAFT IBM/IBM/Technion
T.Przygienda/D. Williams
Fore/IBM
25 March 1997
QoS Routing Mechanismsand OSPFExtensions
draft-guerin-qos-routing-ospf-01.txt
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, and 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 (US East Coast), nic.nordu.net
(Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific
Rim).
Abstract
This memo describes extensions to the OSPF [Moy94] protocol to
support QoS routes. The focus of the document is on the algorithms
used to compute QoS routes and on the necessary modifications to
OSPF to support this function, e.g., the information needed, its
format, how it is distributed, and how it is used by the QoS path
selection process. Aspects related to how QoS routes are established
and managed are also discussed. The goal of this document is to
identify a framework and possible approaches to allow deployment of
QoS routing capabilities with the minimum possible impact to the
existing routing infrastructure.
Guerin, et al. Expires 30 September 1997 [Page i]
Internet Draft QoS Routing Mechanisms 25 March 1997
Contents
Status of This Memo i
Abstract i
1. Introduction 1
1.1. Overall Framework . . . . . . . . . . . . . . . . . . . . 1
1.2. Simplifying Assumptions . . . . . . . . . . . . . . . . . 2
2. Path Selection Information and Algorithms 4
2.1. Metrics . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Advertisement of Link State Information . . . . . . . . . 5
2.3. Path Selection Algorithms . . . . . . . . . . . . . . . . 6
2.3.1. Algorithm for exact pre-computed QoS paths . . . 7
2.3.2. Algorithm for on-demand computation of QoS paths 11
2.3.3. Algorithm for approximate pre-computed QoS paths 12
2.4. Extracting Forwarding Information from Routing Table . . 15
3. Establishment and Maintenance of QoS Routes 15
4. OSPF Protocol Enhancements 17
4.1. QoS -- Optional Capabilities . . . . . . . . . . . . . . 18
4.2. Encoding Resources as Extended TOS . . . . . . . . . . . 19
4.2.1. Encoding bandwidth resource . . . . . . . . . . . 20
4.2.2. Encoding Delay . . . . . . . . . . . . . . . . . 22
4.3. Packet Formats . . . . . . . . . . . . . . . . . . . . . 23
4.4. Calculating the Inter-area Routes . . . . . . . . . . . . 23
4.5. Open Issues . . . . . . . . . . . . . . . . . . . . . . . 23
A. Pseudocode for BF Algorithm 24
B. Zero-Hop Edges 26
C. Source Routing Support 27
D. Computational Complexity 29
E. Extension: Handling Propagation Delays 30
F. QoS Path Establishment and Management with RSVP 31
F.1. RSVP/Routing interface . . . . . . . . . . . . . . . . . 32
F.2. Path Management Rules . . . . . . . . . . . . . . . . . . 32
F.3. Impact of QoS Routing on the Data Path . . . . . . . . . 35
Guerin, et al. Expires 30 September 1997 [Page ii]
Internet Draft QoS Routing Mechanisms 25 March 1997
F.4. Alternatives and Extensions . . . . . . . . . . . . . . . 36
Guerin, et al. Expires 30 September 1997 [Page iii]
Internet Draft QoS Routing Mechanisms 25 March 1997
1. Introduction
In this document we describe a set of proposed additions to the
OSPF routing protocol (the additions are built on top of OSPF V2)
to support Quality-of-Service (QoS) routing in IP. In particular,
we discuss the metrics required to support QoS, the associated link
advertisement mechanisms, the path selection algorithm, as well
as aspects of route establishment (pinning and unpinning). Our
goals are to define an approach which while achieving the goals of
improving performance for QoS flows (likelihood to be routed on a
path capable of providing the requested QoS), does so with the least
possible impact on the existing OSPF protocol. Given the inherent
complexity of QoS routing, achieving this goal obviously implies
trading-off ``optimality'' for ``simplicity'', but we believe this
to be required in order to facilitate deployment of QoS routing
capabilities.
1.1. Overall Framework
We consider a network (1) that supports both best-effort packets and
packets with QoS guarantees. The way in which the network resources
are split between the two classes is irrelevant to our proposal,
except for the assumption that each QoS capable router in the network
is able to dedicate some of its resources to satisfy the requirements
of QoS packets. QoS capable routers are also assumed to be able to
identify and advertise the amount of their resources that remain
available for additional QoS flows. In addition, we limit ourselves
to the case where all the routers involved support the QoS extensions
described in this document, i.e., we do not consider the problem of
establishing a route in an heterogeneous environment with routers
that are QoS-capable and others that are not. Furthermore, in this
document we focus on the case of unicast flows, although many of the
additions we define are applicable to multicast flows as well.
We assume that a flow with QoS requirements will specify them
in some fashion that is accessible to the routing protocol. For
example, this could correspond to the arrival of an RSVP [RZB+96]
PATH message, whose TSpec is passed to routing together with the
destination address. After processing such a request, the routing
protocol returns a path that it deems the most suitable given the
flow's requirements. Depending on the scope of the path selection
----------------------------
1. In this document we commit the abuse of notation of calling a
``network'' the interconnection of routers and networks through which
we attempt to compute a QoS path.
Guerin, et al. Expires 30 September 1997 [Page 1]
Internet Draft QoS Routing Mechanisms 25 March 1997
process, this returned path could range from simply identifying the
best next hop, i.e., a traditional hop-by-hop routing, to specifying
all intermediate nodes to the destination, i.e., a source route.
Note that this decision impacts the operation of the path selection
algorithm as it translates into different requirements in order to
construct and return the appropriate path information. Note also
that extension to multicast paths will impact differently a source
routed and a hop-by-hop approach.
In this document, we will focus on hop-by-hop routing. The
algorithms solutions for path computation and establishment can be
easily modified for source routing and such extensions are discussed
in appendix C.
Once a suitable path has been identified, the flow is assigned to
it (pinning) and remains assigned to it until it either releases
the path (unpinning) or deems that it has become unsuitable, e.g.,
because of link failure or unavailability of the necessary resources.
Note that resource reservation and/or accounting should help limit
the frequency of the latter.
In this document, we focus on the aspect of selecting an appropriate
path based on information on link metrics and flow requirements.
There are obviously many other aspects that need to be specified in
order to define a complete proposal for QoS routing. Issues such as
specifying the frequency of updates and the granularity of advertised
changes to metrics, support for heterogenous areas with a mix of QoS
capable and incapable routers, etc., require further study. The
discussion of a complete solution to these problems is, however,
deferred to subsequent versions of this draft.
1.2. Simplifying Assumptions
In order to achieve our goal of a minimum impact to the existing
protocol, we impose certain restrictions on the range of requirements
the QoS path selection algorithm needs to deal with directly.
Specifically, a policy scheme is used to a priori prune from
the network, those portions that would be unsuitable given the
requirements of the flow. This limits the ``optimization'' performed
by the path selection to a containable set of parameters, which helps
keep complexity at an acceptable level. Specifically, the path
selection algorithm will focus on selecting a path that is capable of
satisfying the bandwidth requirement of the flow, while at the same
time trying to minimize the amount of network resources that need to
be allocated to support the flow, i.e., minimize the number of hops
used.
Guerin, et al. Expires 30 September 1997 [Page 2]
Internet Draft QoS Routing Mechanisms 25 March 1997
This focus on bandwidth is adequate in most instances, but does not
fully capture the complete range of potential QoS requirements. For
example, a delay-sensitive flow of an interactive application could
be put on a path using a satellite link, if that link provided a
direct path and had plenty of unused bandwidth. This would clearly
be an undesirable choice. Our approach to preventing such poor
choices, is to assign delay-sensitive flows to a policy that would
eliminate from the network all links with high propagation delay,
e.g., satellite links, before invoking the path selection algorithm.
In general, each existing policy would present to the path selection
algorithm its correspondingly pruned network topology, and the same
algorithm would then be used to generate an appropriate path.
Another important aspect in minimizing the impact of QoS routing
is to develop a solution that has the smallest possible computing
overhead. Additional computations are unavoidable, but it is
desirable to keep the total cost of QoS routing at a level comparable
to that of traditional routing algorithms. In this document, we
describe several alternatives to the path selection algorithm,
that represent different trade-offs between simplicity, accuracy,
and computational cost. In particular, we specify algorithms
that generate exact solutions based either on pre-computations or
on-demand computations. We also describe algorithms that allow
pre-computations at the cost of some loss in accuracy, but with
possibly lower complexity or greater ease of implementation. It
should be mentioned, that while several alternative algorithms are
described in this document, the same algorithm needs to be used
consistently within a given routing domain. This requirement can be
relaxed when a source routed approach is used as the responsibility
of selecting a QoS path lies with a single entity, the origin of
the request, which ensures consistency. Hence, it may then be
possible for each router to use a different path selection algorithm.
However, in general, the use of a common path selection algorithm is
recommended, if not necessary, for proper operation.
The rest of this document is structured as follows. In Section 2,
we describe the path computation process and the information it
relies on. In Section 3, we briefly review some issues associated
with path management and their implications. In Section 4, we go
over the extensions to OSPF that are needed in order to support the
path selection process of Section 2. Finally, several appendices
provide details on the different path selection algorithms described
in Section 2, elaborate on path management mechanisms, and outline
several additional work items.
Guerin, et al. Expires 30 September 1997 [Page 3]
Internet Draft QoS Routing Mechanisms 25 March 1997
2. Path Selection Information and Algorithms
This section describes several path selection algorithms that
can be used to generate QoS capable paths based on different
trade-offs between accuracy, computational complexity, and ease of
implementation. In addition, the section also covers aspects related
to the type of information, i.e., metrics, on which the algorithms
operate, and how that information is made available, i.e., link state
advertisements. The discussion on these topics is of a generic
nature, and OSPF specific details are provided in Section 4.
2.1. Metrics
As stated earlier, the process of selecting a path that can satisfy
the QoS requirements of a new flow relies on both the knowledge of
the flow's requirements and characteristics, and information about
the availability of resources in the network. In addition, for
purposes of efficiency, it is also important for the algorithm to
account for the amount of resources the network has to allocate in
order to support a new flow. In general, the network prefers to
select the ``cheapest'' path among all paths suitable for a new flow.
Furthermore, the network may also decide not to accept a new flow
for which it identified a feasible path, if it deems the cost of the
path to be too high. Accounting for these aspects involves several
metrics on which the path selection process is based. They include:
- Link available bandwidth: As mentioned earlier, we assume that
most QoS requirements are derivable from a rate-related quantity,
termed ``bandwidth''. We further assume that associated with
each link is a maximal bandwidth value, e.g., the link physical
bandwidth or some fraction thereof that has been set aside for
QoS flows. Since for a link to be capable of accepting a new
flow with given bandwidth requirements, at least that much
bandwidth must be still available on the link, the relevant link
metric is, therefore, the (current) amount of available (i.e.,
unallocated) bandwidth.
- Hop-count: This quantity is used as a measure of the path cost
to the network. A path with a smaller number of hops (that can
support a requested connection) is typically preferable, since
it consumes fewer network resources. While as a general rule
each edge in the graph on which the path is computed should be
counted as one hop, some edges, specifically those that connect
a transit network to a router, should not be taken into account.
(See Appendix B for a detailed explanation.)
Guerin, et al. Expires 30 September 1997 [Page 4]
Internet Draft QoS Routing Mechanisms 25 March 1997
- Policy: As previously discussed, policies are used to identify
the set of links in the network that need to be considered when
running the path selection algorithm. In particular, policies
are used to prune from the network links that are incompatible,
performance or characteristics wise, with the requirements of
a flow. A specific policy example of special importance, is
the elimination of high latency links when considering path
selection for delay sensitive flows. The use of policies to
handle specific requirements allows considerable simplification
in the optimization task to be performed by the path selection
algorithm.
2.2. Advertisement of Link State Information
It is assumed that each router maintains an updated database of the
network topology, including the current state (available bandwidth)
of each link. As described in Section 4, the distribution of link
state (metrics) information is based on extending OSPF mechanisms.
However, in addition to how link state information is distributed,
another important aspect is when such distribution is to take place.
Ideally, one would want routers to have the most current view
of the bandwidth available on all links in the network, so that
they can make the most accurate decision on which path to select.
Unfortunately, this then calls for very frequent updates, e.g.,
close to every time the available bandwidth of a link changes, which
is neither scalable nor practical. Alternatively, one may opt for
a simple mechanism based on periodic updates, where the period of
updates is determined based on a tolerable corresponding load on the
network and the routers. The main disadvantage of such an approach
is that major changes in the bandwidth available on a link could
remain unknown for a full period and, therefore, result in many
incorrect routing decisions.
As a result, we propose to use a simple hybrid update mechanism, that
attempts to reconcile accuracy of link state information with the
need for the smallest possible overhead. Periodic updates are used,
say every T seconds, to notify nodes of any change of more than ffi
in the available bandwidth of a link, and event-driven updates are
generated immediately whenever the change in available link bandwidth
since the last update exceeds . The values for T, ffi, and depend
on network size, link speed, processing capabilities, and overall
traffic patterns, but typical values would be: T 30sec, ffi 10%,
40%. Regardless of bandwidth changes, as in the current OSPF
specifications, we also impose a minimum interval between consecutive
updates, e.g., we do not allow any particular LSA to get updated more
Guerin, et al. Expires 30 September 1997 [Page 5]
Internet Draft QoS Routing Mechanisms 25 March 1997
than once every MinLSInterval seconds, e.g., 5, in order to prevent
the possibility of overload.
2.3. Path Selection Algorithms
There are several aspects to the path selection algorithms. The
main ones include the optimization criteria it relies on, the exact
topology on which it is run, and when it is invoked.
As mentioned before, invocation of the path selection algorithm can
be either per flow, or when warranted by changes in link states when
the algorithm used allows precomputation of paths (more on this
below).
The topology on which the algorithm is run is, as with the standard
OSPF path selection, a directed graph where vertices (2) consist of
routers and networks (transit vertices) as well as stub networks
(non-transit vertices). When computing a path, stub networks are
added as a post-processing step, which is essentially similar to
what is done with the current OSPF routing protocol. In addition,
for each policy supported on a router, the topology used by the
path selection algorithm is correspondingly pruned to reflect the
constraints imposed by the policy, and in some instances the flow
requirements.
The optimization criteria used by the path selection are reflected
in the costs associated with each interface in the topology and how
those costs are accounted for in the algorithm itself. As mentioned
before, the cost of a path is a function of both its hop count and
the amount of available bandwidth. As a result, each interface
has associated with it a metric, that corresponds to the amount of
bandwidth which remains available on this interface. This metric
is combined with hop count information to provide a cost value,
in a manner that depends on the exact form of the path selection
algorithm. It will, therefore, be detailed in the corresponding
sections below, but all the different alternatives that are described
share a common goal. That is, they all aim at picking a path with
the minimum possible number of hops among those that can support
the requested bandwidth. When several such paths are available,
the preference is for the path whose available bandwidth (i.e., the
smallest value on any of the links in the path) is maximal. The
rationale for the above rule is the following: we focus on feasible
paths (as accounted by the available bandwidth metric) that consume
----------------------------
2. In this document, we use the terms node and vertex interchangeably.
Guerin, et al. Expires 30 September 1997 [Page 6]
Internet Draft QoS Routing Mechanisms 25 March 1997
a minimal amount of network resources (as accounted by the hop-count
metric); and the rule for selecting among these paths aims at
balancing load as well as maximizing the likelihood that the required
bandwidth will indeed be available.
It should be noted that standard routing algorithms are typically
single objective optimizations, i.e., they may minimize the
hop-count, or maximize the path bandwidth, but not both. Double
objective path optimization is a more complex task, and, in
general, it is an intractable problem [GJ79]. Nevertheless, as
we will see, because of the specific nature of the two objectives
being optimized (bandwidth and hop count), the complexity of our
proposed algorithm(s) is competitive with even that of standard
single-objective algorithms.
2.3.1. Algorithm for exact pre-computed QoS paths
In this section, we describe a path selection algorithm, that for a
given network topology and link metrics (available bandwidth) allows
us to pre-compute all possible QoS paths, and also has a reasonably
low computational complexity. Specifically, the algorithm allows
us to pre-compute for any destination a minimum hop count path with
maximum bandwidth, and has a computational complexity comparable to
that of a standard shortest path algorithm (3).
The path selection algorithm is based on a Bellman-Ford (BF)
shortest path algorithm, which is adapted to compute paths of maximum
available bandwidth for all hop counts. It is a property of the BF
algorithm that, at its h-th iteration, it identifies the optimal (in
our context: maximal bandwidth) path between the source and each
destination, among paths of at most h hops. In other words, the
cost of a path is a function of its available bandwidth, i.e., the
smallest available bandwidth on all links of the path, and finding
a minimum cost path amounts to finding a maximum bandwidth path.
However, we also take advantage of the fact that the BF algorithm
progresses by increasing hop count, to essentially get for free the
hop count of a path as a second optimization criteria.
Specifically, at the kth (hop count) iteration of the algorithm,
the maximum bandwidth available to all destinations on a path of
no more than k hops is recorded (together with the corresponding
routing information). After the algorithm terminates, this
----------------------------
3. See Appendix D for a more comprehensive discussion on the aspect of
computational complexity.
Guerin, et al. Expires 30 September 1997 [Page 7]
Internet Draft QoS Routing Mechanisms 25 March 1997
information enables us to identify for all destinations and bandwidth
requirements, the path with the smallest possible number of hops and
sufficient bandwidth to accommodate the new request. Furthermore,
this path is also the one with the maximal available bandwidth among
all the feasible paths with this minimum number of hops. This is
because for any hop count, the algorithm always selects the one with
maximum available bandwidth.
We now proceed with a more detailed description of the algorithm
and the data structure used to record routing information, i.e.,
the QoS routing table that gets built as the algorithm progresses
(pseudo-code for the algorithm can be found in Appendix A). As
mentioned before, the algorithm operates on a directed graph
consisting only of transit vertices (routers and networks), with
stub-networks subsequently added to the path(s) generated by the
algorithm. The metric associated with each edge in the graph is the
bandwidth available on the corresponding interface. Let us denote
by bn;mthe available bandwidth on the edge between vertices n and
m. The vertex corresponding to the router where the algorithm is
being run, i.e., the computing router, is denoted as the ``source
node'' for the purpose of path selection. The algorithm proceeds to
pre-compute paths from this source node to all possible destination
networks and for all possible bandwidth values. At each (hop count)
iteration, intermediate results are recorded in a QoS routing table,
which has the following structure:
The QoS routing table:
- a Kx H matrix, where K is the number of destinations (vertices
in the graph) and H is the maximal allowed (or possible) number
of hops for a path.
- The (n;h) entry is built during the hth iteration (hop count
value) of the algorithm, and consists of two fields:
* bw: the maximum available bandwidth, on a path of at most h
hops between the source node (router) and destination node
n;
* neighbor: this is the routing information associated with
the h (or less) hops path to destination node n, whose
available bandwidth is bw. In the context of hop-by-hop
path selection (4), the neighbor information is simply the
identity of the node adjacent to the source node on that
----------------------------
4. Modifications to support source routing are discussed in Appendix C.
Guerin, et al. Expires 30 September 1997 [Page 8]
Internet Draft QoS Routing Mechanisms 25 March 1997
path. As a rule, the ``neighbor'' node must be a router and
not a network (see Appendix B), the only exception being
the case where the network is the destination node (and the
selected path is the single edge interconnecting the source
to it).
Next, we provide additional details on the operation of the algorithm
and how the entries in the routing table are being updated as the
algorithm proceeds. For simplicity, we first describe the simpler
case where all edges count as ``hops'', and later explain how
zero-hop edges (see Appendix B for further details) are handled.
When the algorithm is invoked, the routing table is first initialized
with all bw fields set to 0 and neighbor fields cleared. Next, the
entries in the first column (which corresponds to one-hop paths) of
the neighbors of the computing router are modified in the following
way: the bw field is set to the value of the available bandwidth
on the direct edge from the source. The neighbor field is set to
the identity of the neighbor of the computing router, i.e., the next
router on the selected path.
Afterwards, the algorithm iterates for at most H iterations
(considering the above initial iteration as the first). H can be
either the maximum possible hop count of any path, i.e., an implicit
value, or it can be set explicitly in order to limit path lengths to
some maximum value (5) to better control worst case complexity.
At iteration h, we first copy column h - 1 into column h. In
addition, the algorithm keeps a list of nodes that changed their bw
value in the previous iteration, i.e., during the h- 1-st iteration.
The algorithm then looks at each link (n;m) and checks the maximal
available bandwidth on an h-hop path to node m whose final hop is
that link. This amounts to taking the minimum between the bw field
in entry (n;h -1) and the link metric value bn;m kept in the topology
database. If this value is higher than the present value of the bw
field in entry (m;h), then a better (larger bw value) path has been
found for destination m and with h hops. The bw field of entry
(m;h) is then updated to reflect this new value. In the case of
hop-by-hop routing, the neighbor field of entry (m;h) is set to the
same value as in entry (n;h - 1). This records the identity of the
first hop (next hop from the source) on the best path identified thus
far for destination m and with h (or less) hops.
----------------------------
5. This maximum value should be larger than the length of the minimum
hop-count path to any node in the graph.
Guerin, et al. Expires 30 September 1997 [Page 9]
Internet Draft QoS Routing Mechanisms 25 March 1997
We conclude by outlining how zero-hop edges are handled. At each
iteration h (starting with the first), whenever an entry (m;h) is
modified, it is checked whether there are zero-cost edges (m;k)
emerging from node m, which is the case when m is a transit network
(see Appendix B). In that case, we attempt to improve the entry of
node k that corresponds to the h-th hop, i.e., entry (k;h) (rather
than entry (k;h + 1)), since the edge (m;k) should not count as an
additional hop. As with the regular operation of the algorithm, this
amounts to taking the minimum between the bw field in entry (m;h)
and the link metric value bm;kkept in the topology database. If
this value is higher than the present value of the bw field in entry
(k;h), then the bw field of entry (k;h) is updated to this new value.
In the case of hop-by-hop routing, the neighbor field of entry (k;h)
is set, as usual, to the same value as in entry (m;h) (which is also
the value in entry (n;h- 1)).
Note that while for simplicity of the exposition, the issue of equal
cost, i.e., same hop count and available bandwidth, is not detailed
in the above description, it is straightforward to add this support.
It only requires that the neighbor field be expanded to record the
list of next (previous) hops, when multiple equal cost paths are
present.
Addition of Stub Networks
As was mentioned earlier, the path selection algorithm is run
on a graph whose vertices consist only of routers and transit
networks and not stub networks. This is intended to keep the
computational complexity as low as possible as stub networks can
be added relatively easily through a post-processing step. This
second processing step is similar to the one used in the current OSPF
routing table calculation [Moy94][Section 16, p. 148], with some
differences to account for the QoS nature of routes.
Specifically, after the QoS routing table has been constructed, all
the router vertices are again considered. For each router, stub
networks whose link appears in the router's links advertisement will
be processed to determine QoS routes available to them. The QoS
routing information for a stub network is similar to that of routers
and transit networks and consists of an extension to the QoS routing
table in the form of an additional row. The columns in that new row
again correspond to paths of different hop counts, and contain both
bandwidth and next hop information. We also assume that an available
bandwidth value has been advertised for the stub network. As before,
how this value is determined is beyond the scope of this document.
The QoS routes for a stub network S are constructed as follows:
Guerin, et al. Expires 30 September 1997 [Page 10]
Internet Draft QoS Routing Mechanisms 25 March 1997
Each entry in the row corresponding to stub network S has its bws
field initialized to zero and its neighbor set to null. When stub
network S is found in the link advertisement of router V, the value
bw(S,h) in the hth column of the row corresponding to stub network S
is updated as follows:
bw(S,h) = min ( bw(S,h) ; min ( bw(V,h) , b(V,S) ) ),
where bw(V,h) is the bandwidth value of the corresponding column
for the QoS routing table row associated with router V, i.e.,
the bandwidth available on an h hop path to V, and b(V,S) is the
advertised available bandwidth on the link from V to S. The above
expression essentially states that the bandwidth of a h hop paths to
stub network S is updated using a path through router V, only if the
minimum of the bandwidth of the h hop path to V and the bandwidth on
the link between V and S is larger than the current value.
Update of the neighbor field proceeds similarly whenever the
bandwidth of a path through V is found to be larger than or equal
to the current value. If it is larger, then the neighbor field
of V in the corresponding column replaces the current neighbor
field of S. If it is equal, then the neighbor field of V in the
corresponding column is concatenated with the existing field for S,
i.e., the current set of neighbors for V is added to the current set
of neighbors for S.
2.3.2. Algorithm for on-demand computation of QoS paths
In the previous section, we described an algorithm that allows
pre-computation of QoS routes. However, it may be feasible in
some instances, e.g., limited number of requests for QoS routes,
to instead perform such computations on-demand, i.e., upon receipt
of a request for a QoS route. The benefit of such an approach is
that depending on how often recomputation of pre-computed routes is
triggered, on-demand route computation can yield better routes by
using the most recent link metrics available. Another benefit of
on-demand path computation is the associated storage saving, i.e.,
there is no need for a QoS routing table. This is essentially the
standard trade-off between memory and processing cycles.
In this section, we briefly describe how a standard Dijkstra
algorithm can, for a given destination and bandwidth requirement,
generate a minimum hop path that can accommodate the required
bandwidth and also has maximum bandwidth. Because the Dijkstra
algorithm is already used in the current OSPF route computation, only
differences from the standard algorithm are described. Also, while
Guerin, et al. Expires 30 September 1997 [Page 11]
Internet Draft QoS Routing Mechanisms 25 March 1997
for simplicity we do not consider here zero-hop edges (see Appendix
B), the modification required for supporting them is straightforward.
The algorithm essentially performs a minimum hop path computation,
on a graph from which all edges, whose available bandwidth is less
than that requested by the flow triggering the computation, have been
removed. This can be performed either through a pre-processing step,
or while running the algorithm by checking the available bandwidth
value for any edge that is being considered. Another modification
to a standard Dijkstra based minimum hop count path computation, is
that the list of equal cost next (previous) hops which is maintained
as the algorithm proceeds, needs to be sorted according to available
bandwidth. This is to allow selection of the minimum hop path with
maximum available bandwidth. Alternatively, the algorithm could also
be modified to, at each step, only keep among equal hop count paths
the one with maximum available bandwidth. This would essentially
amount to considering a cost that is function of both hop count and
available bandwidth.
2.3.3. Algorithm for approximate pre-computed QoS paths
This section outlines a Dijkstra-based algorithm that allows
pre-computation of QoS routes for all destinations and bandwidth
values. The benefit of using a Dijkstra-based algorithm is a greater
synergy with existing OSPF implementations. The cost is, however, a
loss in the ``accuracy'' of the pre-computed paths, i.e., the paths
being generated may be of a larger hop count than needed. This
loss in accuracy comes from the need to rely on quantized bandwidth
values, that are used when computing a minimum hop count path. In
other words, the range of possible bandwidth values that can be
requested by a new flow is mapped into a fixed number of quantized
values, and minimum hop count paths are generated for each quantized
value. For example, one could assume that bandwidth values are
quantized as low, medium, and high, and minimum hop count paths are
computed for each of these three values. A new flow is then assigned
to the minimum hop path that can carry the smallest quantized
value, i.e., low, medium, or high, larger than or equal to what it
requested.
Here too, we discuss the elementary case where all edges count as
``hops'', and note that the modification required for supporting
zero-hop edges is straightforward.
As with the BF algorithm, the algorithm relies on a routing table
that gets built as the algorithm progresses. The structure of the
routing table is as follows:
Guerin, et al. Expires 30 September 1997 [Page 12]
Internet Draft QoS Routing Mechanisms 25 March 1997
The QoS routing table:
- a K x Q matrix, where K is the number of vertices and Q is the
number of quantized bandwidth values.
- The (n;q) entry contains information that identifies the
minimum hop count path to destination n, that is capable of
accommodating a bandwidth request of at least bw[q] (the qth
quantized bandwidth value). It consists of two fields:
* hops: the minimal number of hops on a path between the
source node and destination n, that can accommodate a
request of at least bw[q] units of bandwidth.
* neighbor: this is the routing information associated with
the minimum hop count path to destination node n, whose
available bandwidth is at least bw[q]. With a hop-by-hop
routing approach, the neighbor information is simply the
identity of the node adjacent to the source node on that
path.
The algorithm operates again on a directed graph where vertices
correspond to routers and transit networks. The metric associated
with each edge in the graph is as before the bandwidth available on
the corresponding interface, where bn;mis the available bandwidth
on the edge between vertices n and m. The vertex corresponding to
the router where the algorithm is being run is selected as the source
node for the purpose of path selection, and the algorithm proceeds to
compute paths to all other nodes (destinations).
Starting with the highest quantization index, Q, the algorithm
considers the indices consecutively, in decreasing order. For each
index q, the algorithm deletes from the original network topology
all links (n;m) for which bn;m< bw[q], and then runs on the remaining
topology a Dijkstra-based minimum hop count algorithm (6) between
the source node and all other nodes (vertices) in the graph. Note
that as with the Dijkstra used for on-demand path computation, the
elimination of links such that bn;m < bw[q] could also be performed
while running the algorithm.
After the algorithm terminates, the q-th column in the routing table
is updated. This amounts to recording in the hops field the hop
----------------------------
6. Note that a Breadth-First-Search (BFS) algorithm
[CLR90] could also be used. It has a lower complexity, but would not
allow reuse of existing code in an OSPF implementation.
Guerin, et al. Expires 30 September 1997 [Page 13]
Internet Draft QoS Routing Mechanisms 25 March 1997
count value of the path that was generated by the algorithm, and by
updating the neighbor field. As before, the update of the neighbor
field depends on the scope of the path computation. In the case
of a hop-by-hop routing decision, the neighbor field is set to the
identity of the node adjacent to the source node (next hop) on the
path returned by the algorithm. However, note that in order to
ensure that the path with the maximal available bandwidth is always
chosen among all minimum hop paths that can accommodate a given
quantized bandwidth, a slightly different update mechanism of the
neighbor field needs to be used in some instances. Specifically,
when for a given row, i.e., destination node n, the value of the
hops field in column q is found equal to the value in column q + 1
(here we assume q < Q), i.e., paths that can accommodate bw[q] and
bw[q+ 1] have the same hop count, then the algorithm copies the value
of the neighbor field from entry (n;q+ 1) into that of entry (n;q).
Addition of Stub Networks
This proceeds in a manner very similar to that of Section 2.3.1,
except for some minor variations reflecting differences in the
structure of the QoS routing table. Specifically, the columns of
the QoS routing table now correspond to quantized bandwidth values,
and the bw field of a column entry has been replaced by a hops
field. Hence, the QoS routes for a stub network S are constructed
as follows:
Each entry in the row corresponding to stub network S has its hops
field initialized to zero and its neighbor set to null. When stub
network S is found in the link advertisement of router V, the value
hops(S,q) in the qth column of the row corresponding to stub network
S is updated as follows:
hops(S,q) = hops(V,q) IF (hops(V,q) <= hops(S,q) AND b(V ,S) >=
bw[q]),
where bw[q] is the qth quantized bandwidth value, and b(V,S) is
the advertised available bandwidth on the link from V to S. The
above expression essentially states that the hop count of the path
to stub network S capable of supporting a bandwidth allocation
of bw[q], is updated using a path through router V, only if the
corresponding path through V has fewer hops than the current one,
and the bandwidth on the link between V and S is larger than bw[q].
Update of the neighbor field proceeds similarly whenever the path
through router V capable of supporting a bandwidth allocation of
bw[q], is found to yield a hop count smaller than or equal to the
current value. If it is smaller, then the neighbor field of V in
the corresponding column replaces the current neighbor field of S.
Guerin, et al. Expires 30 September 1997 [Page 14]
Internet Draft QoS Routing Mechanisms 25 March 1997
If it is equal, then the neighbor field of V in the corresponding
column is concatenated with the existing field for S, i.e., the
current set of neighbors for V is added to the current set of
neighbors for S.
2.4. Extracting Forwarding Information from Routing Table
When the QoS paths are precomputed, the forwarding information for
a flow with given destination and bandwidth requirement needs to be
extracted from the routing table. The case of hop-by-hop routing is
much simpler compared to source routing. This is because, only the
next hop needs to be returned instead of a complete source route.
Specifically, assume a new request to destination, say, d, and with
bandwidth requirements B. The index of the destination vertex
identifies the row in the QoS routing table that needs to be checked
to generate a path. How the row is searched to identify a suitable
path depends on which algorithm was used to construct the QoS routing
table. If the Bellman-Ford algorithm of Section 2.3.1 is used, the
search proceeds by increasing index (hop) count until an entry is
found, say at hop count or column index of h, with a value of the
bw field which is greater than or larger than B. This entry points
to the initial information identifying the selected path. If the
Dijkstra algorithm of Section 2.3.3 is used, the first quantized
value bBsuch that Bb B is first identified, and the associated
column then determines the first entry in the QoS routing table that
identifies the selected path.
The next hop information is then directly retrieved from the neighbor
information of the first entry pointed to in the QoS routing table.
The case of source routing is discussed in Appendix C.
3. Establishment and Maintenance of QoS Routes
In this section, we briefly review issues related to how QoS paths
are established and maintained. For both, there are functional and
protocol aspects that need to be covered.
The goal of QoS routing is to select paths for flows with QoS
requirements, in such a manner as to increase the likelihood that the
network will indeed be capable of satisfying them. The use of QoS
routing algorithms such as the ones described in this document have a
number of implications above and beyond what is required when using
standard routing algorithms.
Guerin, et al. Expires 30 September 1997 [Page 15]
Internet Draft QoS Routing Mechanisms 25 March 1997
First, a specific mechanism needs to be used to identify flows with
QoS requirements, so that they can be assigned to the corresponding
QoS routing algorithm. In this section, we assume that the RSVP
protocol [RZB+96] is used for that purpose. Specifically, RSVP PATH
messages serve as the trigger to query QoS routing. Second, because
of variations in the availability of resources in the network, routes
between the same source and destination and for the same QoS, may
often differ depending on when the request is made. However, it is
important to ensure that such changes are not always reflected on
existing paths. This is to avoid potential oscillations between
paths and limit changes to cases where the initial selection turns
out to be inadequate.
As a result, some state information needs to be associated with a
QoS route to determines its current validity, i.e., should the QoS
routing algorithm be queried to generate a new and potentially better
route, or does the current one remain adequate. We say that a path
is ``pinned'' when its state specifies that QoS routing need not be
queried anew, while a path is considered ``unpinned'' otherwise.
The main issue is then to define how, when, and where route pinning
and unpinning is to take place. In our context, where the RSVP
protocol is used as the vehicle to request QoS routes, we also want
this process to be as synergetic as possible with the existing RSVP
state management. In particular, our goal is to support pinning and
unpinning of routes in a manner consistent with RSVP soft states
while requiring minimal changes to the RSVP processing rules.
It should be noted that some changes are unavoidable, especially
to the interface between RSVP and routing. Specifically,
QoS routing requires, in addition to the current source and
destination addresses, at a minimum, knowledge of the flow's traffic
characteristics (TSpec), and possibly also service types (as per
the information in the Adspec), PHOP, IP TTL value, etc. In this
document, we assume that the information provided by RSVP to QoS
routing includes at least the sender TSpec in addition to the source
and destination addresses. While such changes seem unavoidable, our
goal is again to keep them as small as possible and also to avoid any
change to the existing RSVP message format.
Specifically, we assume interactions between RSVP and routing that
are very similar to what is currently defined. During the processing
of RSVP PATH messages, RSVP queries (QoS) routing to obtain the
next hop(s) for forwarding the PATH message. The PATH message is
then forwarded on the interface(s) returned by (QoS) routing. As
mentioned before, the sender TSpec is part of the information made
available to routing, and a QoS routing algorithm selects the next
hop along a path to the destination that is most likely to support
the flow specified by the sender TSpec. Thus, forwarding the PATH
Guerin, et al. Expires 30 September 1997 [Page 16]
Internet Draft QoS Routing Mechanisms 25 March 1997
message along this next hop should improve the likelihood of the
reservation request succeeding during RESV message processing. In
particular, RESV messages, if any, propagate as before in the reverse
direction of the PATH messages and attempt to reserve the required
resources along the path delineated by PATH messages.
In the context of the above hop-by-hop routing, there are two main
issues associated with the pinning and unpinning of QoS paths.
1. Detection of loops that may be caused by inconsistencies in the
QoS routes returned by QoS routing at different nodes. Such
inconsistencies are typically transient, but it is important
that the pinning of a path does not result in the formation of
permanent loops .
2. Query of a new QoS route in case of failures. An example of
such failures is a reservation failure because the RESV message
arrived substantially later after the QoS route was initially
selected. Other failures include the usual link failures, and in
general it is important to allow QoS routing to become aware of
the failure and select a better route if one is available.
The QoS path management scheme proposed here addresses the above two
issues based on the following two design rules:
1. What is pinned is a ``path'' taken by a specific RSVP flow and
not a ``route'' as computed by the routing algorithm. Hence
pinning and unpinning are RSVP domain operations, and as a result
completely independent of the specific QoS routing algorithm
being used.
2. Path pinning and unpinning is kept ``soft'' by tying it to the
existing RSVP soft state mechanism. In other words, we rely on
existing RSVP refreshes and time-out mechanisms to detect the
state changes that trigger pinning and unpinning of paths. In
addition, such changes are triggered only on the basis of current
RSVP state information.
Appendix F specifies how the above two rules translate into the
conditions and processing rules for pinning and unpinning paths,
that address the problem of loops while also enabling reactions to
failures.
4. OSPF Protocol Enhancements
As stated above, a goal of this work is to limit the additions to the
existing OSPF V2 protocol, while still providing the required level
Guerin, et al. Expires 30 September 1997 [Page 17]
Internet Draft QoS Routing Mechanisms 25 March 1997
of support for QoS based routing. To this end, all of the existing
OSPF mechanisms, data structures, advertisements, and data formats
remain in place. The purpose of this section of the document is to
list the enhancements to the OSPF protocol to support QoS as outlined
in the previous sections.
4.1. QoS -- Optional Capabilities
The OSPF Options field is present in OSPF Hello packets, Database
Description packets and all LSAs. The Options field enables OSPF
routers to support (or not support) optional capabilities, and to
communicate their capability level to other OSPF routers. Through
this mechanism, routers of differing capabilities can be mixed with
an OSPF routing domain.
This document describes one of the Option bits: the Q-bit (for QoS
capability). The Q-bit is set in router, network, and summary links
advertisements, and is used to identify routers and networks that
support (or do not support) QoS routing as defined in this document.
When the Q-bit is set in a router or summary links link state
advertisement, it means that there are QoS fields to process in the
packet. When the Q-bit is set in a network link state advertisement
it means that the network described in the advertisement is QoS
capable.
-------------------------------
| * | Q | DC| EA|N/P| MC| E | T |
-------------------------------
The Q-bit is in its semantics very similar to the T-bit and QoS
capability can be viewed just as an extension of TOS-capabilities.
This similarity will be reinforced by the encoding introduced
further in this section. However, one important difference remains
between TOS and QOS capabilities. A router that does not have TOS
capabilities forwards packets along TOS 0 paths. Additionally,
TOS metrics that are not advertised are assumed to have the same
cost as TOS 0. In case of QoS resources such as bandwidth, this
does not make sense and could possibly introduce routing loops
due to different criteria for which the routes are optimized.
Therefore, if no route exists through routers all supporting QoS or
any of those does not provide the necessary metric for the resource
considered, communication with the requested quality of service is
not possible. Since each of the router link directions is described
in an independent LSA, even a uni-directional failure to communicate
Guerin, et al. Expires 30 September 1997 [Page 18]
Internet Draft QoS Routing Mechanisms 25 March 1997
is possible since the availability of a resource metric in one
direction does not guarantee its accessibility for the other one.
4.2. Encoding Resources as Extended TOS
Introduction of QoS should ideally not influence the compatibility
with existing OSPFv2 routers. To achieve this goal, necessary
extensions in packet formats must be defined in a way that either
is understood by OSPFv2 routers, ignored or in the worst case
``gracefully'' misinterpreted. Encoding of QoS metrics in the
TOS field which fortunately enough is longer in OSPF packets
than officially defined in [Alm92], allows us to mimic the new
facility as extended TOS capability. OSPFv2 routers will either
disregard these definitions or consider those unspecified. Specific
precautions are taken to prevent careless OSPF implementations
from influencing traditional TOS routing when misinterpreting the
extension introduced.
For QoS resources, 32 combinations are available through the use
of the fifth bit in TOS fields contained in different LSAs. Since
[Alm92] defines TOS as being four bits long, this definition never
conflicts with existing values. Additionally, to prevent naive
implementations that do not take all bits of the TOS field in OSPF
packets into considerations, the definitions of the `QoS encodings'
is aligned in their semantics with the TOS encoding. Only bandwidth
and delay are specified as of today and their values map onto
`maximize throughput' and `minimize delay' if the uppermost bit is
not taken into account. Accordingly, link reliability and jitter
could be defined later if necessary.
OSPF encoding RFC 1349 TOS values
___________________________________________
0 0000 normal service
2 0001 minimize monetary cost
4 0010 maximize reliability
6 0011
8 0100 maximize throughput
10 0101
12 0110
14 0111
16 1000 minimize delay
18 1001
20 1010
22 1011
24 1100
26 1101
Guerin, et al. Expires 30 September 1997 [Page 19]
Internet Draft QoS Routing Mechanisms 25 March 1997
28 1110
30 1111
OSPF encoding `QoS encoding values'
-------------------------------------------
32 10000
34 10001
36 10010
38 10011
40 10100 bandwidth
42 10101
44 10110
46 10111
48 11000 delay
50 11001
52 11010
54 11011
56 11100
58 11101
60 11110
62 11111
Representing TOS and QoS in OSPF.
4.2.1. Encoding bandwidth resource
Given the fact that the actual metric field in OSPF packets only
provides 16 bits to encode the value used and that links supporting
bandwidth ranging into Gbits/s are becoming reality, linear
representation of the available resource metric is not feasible. The
solution is exponential encoding using appropriately chosen implicit
base value and number bits for encoding mantissa and the exponent.
Detailed considerations leading to the solution described are not
presented here but can be found in [Prz95].
Given a base of 8, the 3 most significant bits should be reserved for
the exponent part and the remaining 13 for the mantissa. This allows
a simple comparison for two numbers encoded in this form, which is
often useful during implementation.
The following table shows bandwidth ranges covered when using
different exponents and the granularity of possible reservations.
exponent
Guerin, et al. Expires 30 September 1997 [Page 20]
Internet Draft QoS Routing Mechanisms 25 March 1997
value x range (2^13-1)*8^x step 8^x
-------------------------------------------------
0 8,191 1
1 65,528 8
2 524,224 64
3 4,193,792 512
4 33,550,336 4,096
5 268,402,688 32,768
6 2,147,221,504 262,144
7 17,177,772,032 2,097,152
Ranges of Exponent Values for 13 bits,
base 8 Encoding, in Bytes/s
The bandwidth encoding rule may be summarized as:
1. represent available bandwidth in 16 bit field as a 3 bit exponent
(with assumed base of 8) followed by a 13 bit mantissa as shown
below
0 8 16
| | |
-----------------
|EXP| MANT |
-----------------
2. advertise 2's complement of the above representation.
Thus, the above encoding advertises a numeric value that is
216- 1-(exponential encoding of the available bandwidth):
This has the property of advertising a higher numeric value for lower
available bandwidth, a notion that is consistent with that of cost.
Although it may seem slightly pedantic to insist on the property
that less bandwidth is expressed higher values, it has, besides
consistency, a robustness aspect in it. A router with a poor OSPF
implementation could misuse or misunderstand bandwidth metric as
normal administrative cost provided to it and compute spanning trees
with a ``normal'' Dijkstra. The effect of a heavily congested link
advertising numerically very low cost could be disastrous in such
a scenario. It would raise the link's attractiveness for future
traffic instead of lowering it. Evidence that such considerations
Guerin, et al. Expires 30 September 1997 [Page 21]
Internet Draft QoS Routing Mechanisms 25 March 1997
are not speculative, but similar scenarios have been encountered, can
be found in [Tan89].
Concluding with an example, assume a link with bandwidth of 8 Gbits/s
= 10243Bytes/s, its encoding would consist of an exponent value of 6
since 10243=4; 096*86, which would then have a granularity of 86 260
kBytes/s. The associated binary representation would then be %(110)
0 1000 0000 0000% or 53,248 (7). The bandwidth cost (advertised
value) of this link when it is idle, is then the 2's complement of
the above binary representation, i.e., %(001) 1 0111 1111 1111% which
corresponds to a decimal value of (216- 1)- 53;248 = 12;287. Assuming
now a current reservation level of of 6;400 Mbits/s = 200 * 10242,
there remains 1;600 Mbits/s of available bandwidth on the link. The
encoding of this available bandwidth of 1'600 Mbits/s is 6;400 * 85,
which corresponds to a granularity of 85 30 kBytes/s, and has a
binary representation of %(101) 1 1001 0000 0000% or decimal value
of 47,360. The advertised cost of the link with this load level, is
then %(010) 0 0110 1111 1111%, or (216- 1)- 47;360 =18;175.
Note that the cost function behaves as it should, i.e., the less
bandwidth is available on a link, the higher the cost and the less
attractive the link becomes. Furthermore, the targeted property of
better granularity for links with less bandwidth available is also
achieved. It should, however, be pointed out that the numbers given
in the above examples match exactly the resolution of the proposed
encoding, which is of course not always the case in practice. This
leaves open the question of how to encode available bandwidth
values when they do not exactly match the encoding. The standard
practice is to round it to the closest number. Because we are
ultimately interested in the cost value for which it may be better
to be pessimistic than optimistic, we choose to round costs up and,
therefore, bandwidth down.
4.2.2. Encoding Delay
Delay is encoded in microseconds using the same exponential method
as described for bandwidth except that the base is defined to be 4
instead of 8. Therefore the maximum delay that can be expressed is
(213 -1) *47 134 seconds.
----------------------------
7. exponent in parenthesis
Guerin, et al. Expires 30 September 1997 [Page 22]
Internet Draft QoS Routing Mechanisms 25 March 1997
4.3. Packet Formats
Given the extended TOS notation to account for QoS metrics, no
changes in packet formats are necessary except for the introduction
of Q-bit in the options field. Routers not understanding the Q-bit
should either not consider the QoS metrics distributed or consider
those as `unknown' TOS.
4.4. Calculating the Inter-area Routes
This document proposes a very limited use of OSPF areas, that is, it
is assumed that summary links advertisements exist for all networks
in the area. This document does not discuss the problem of providing
support for area address ranges and QoS metric aggregation. This is
left for further studies.
4.5. Open Issues
Support for AS External Links, Virtual Links, and incremental updates
for summary link advertisements are not addressed in this document
and are left for further study. For Virtual Links that do exist, it
is assumed for path selection that this links are non-QoS capable
even if the router advertises QoS capability. Also, as stated
earlier, this document does not address the issue of non-QoS routers
within a QoS domain.
Acknowledgments
We would like to thank the many people who have helped shape various
aspects of this document and the approaches it describes, either
through discussions or explicit suggestions. In particular, we would
like to acknowledge the help and inputs of John Moy, Sanjay Kamat,
Dilip Kandlur, and Dimitrios Pendarakis.
Guerin, et al. Expires 30 September 1997 [Page 23]
Internet Draft QoS Routing Mechanisms 25 March 1997
APPENDICES
A. Pseudocode for BF Algorithm
Note: The pseudocode below assumes a hop-by-hop forwarding approach in
updating the neighbor field. The modifications needed to support
a source routed approach are straightforward. The pseudocode also
does not handle equal cost multi-paths for simplicity, but the
modification needed to add this support are straightforward.
Input:
V = set of vertices, labeled by integers 1 to N.
L = set of edges, labeled by ordered pairs (n,m) of vertex labels.
s = source vertex (at which the algorithm is executed).
For all edges (n,m) in L:
* b(n,m) = available bandwidth (according to last received update)
on interface associated with the edge between vertices n and m.
* If(n,m) outgoing interface corresponding to edge (n,m) when n is
a router.
H = maximum hop-count (at most the graph diameter).
Type:
tab_entry: record
bw = integer,
neighbor = integer 1..N.
Variables:
TT[1..N, 1..H]: topology table, whose (n,h) entry is a tab_entry record, such
that:
TT[n,h].bw is the maximum available bandwidth (as known
thus far) on a path of at most h hops between
vertices s and n,
TT[n,h].neighbor is the first hop on that path (a neighbor
of s). It is either a router or the destination n.
S_prev: list of vertices that changed a bw value in the TT table
in the previous iteration.
S_new: list of vertices that changed a bw value (in the TT table etc.\ ) in t
he
current iteration.
The Algorithm:
begin;
for n:=1 to N do /* initialization */
begin;
Guerin, et al. Expires 30 September 1997 [Page 24]
Internet Draft QoS Routing Mechanisms 25 March 1997
TT[n,0].bw := 0;
TT[n,0].neighbor := null
TT[n,1].bw := 0;
TT[n,1].neighbor := null
end;
TT[s,0].bw := infinity;
reset S_prev;
for all neighbors n of s do
begin;
TT[n,1].bw := max( TT[n,1].bw, b[s,n]);
if (TT[n,1].bw = b[s,n]) then TT[n,1].neighbor := If(s,n);
/* need to make sure we are picking the maximum */
/* bandwidth path for routers that can be reached */
/* through both networks and point-to-point links */
if ( (n is a router) and ({n} not in S_prev) )
then S_prev := S_prev union {n}
/* only routers are added to S_prev, but we need to */
/* check they are not already included in S_prev */
else /* n is a network: */
/* proceed with network--router edges, without */
/* counting another hop */
for all (n,k) in L, k <> s, do
/* i.e., for all other neighboring routers of n */
begin;
TT[k,1].bw := max( min( TT[n,1].bw, b[n,k]), TT[k,1].bw );
/* In case k could be reached through another path */
/* (a point-to-point link or another network) with */
/* more bandwidth, we do not want to update TT[k,1].bw */
if (min( TT[n,1].bw, b[n,k]) = TT[k,1].bw )
/* If we have updated TT[k,1].bw by going through */
/* network n */
then TT[k,1].neighbor := If(s,n);
/* neighbor is interface to network n */
if ( {k} not in S_prev) then S_prev := S_prev union {k}
/* only routers are added to S_prev, but we again need */
/* to check they are not already included in S_prev */
end
end;
for h:=2 to H do /* consider all possible number of hops */
begin;
reset S_new;
for all vertices m in V do
begin;
TT[m,h].bw := TT[m,h-1].bw;
TT[m,h].neighbor := TT[m,h-1].neighbor
Guerin, et al. Expires 30 September 1997 [Page 25]
Internet Draft QoS Routing Mechanisms 25 March 1997
end;
for all vertices n in S_prev do
/* as shall become evident, S_prev contains only routers */
begin;
for all edges (n,m) in L do
if min( TT[n,h-1].bw, b[n,m]) > TT[m,h].bw then
begin;
TT[m,h].bw := min( TT[n,h-1].bw, b[n,m]);
TT[m,h].neighbor := TT[n,h-1].neighbor;
if m is a router then S_new := S_new union {m}
/* only routers are added to S_new */
else /* m is a network: */
/* proceed with network--router edges, without counting them as */
/* another hop */
for all (m,k) in L, k <> n,
/* i.e., for all other neighboring routers of m */
if min( TT[m,h].bw, b[m,k]) > TT[k,h].bw then
begin;
/* Note: still counting it as the h-th hop, as (m,k) is a */
/* network--router edge */
TT[k,h].bw := min( TT[m,h].bw, b[m,k]);
TT[k,h].neighbor := TT[m,h].neighbor;
S_new := S_new union {k}
/* only routers are added to S_new */
end
end
end;
S_prev := S_new;
/* the two lists can be handled by a toggle bit */
if S_prev=null then h=H+1 /* if no changes then exit */
end;
end.
B. Zero-Hop Edges
The need to handle zero-hop edges is due to the potential presence
of multiple access networks, e.g., T/R, E/N, or ATM, to interconnect
routers. Such entities are also represented by means of a vertex
in the current OSPF operation. Clearly, in such cases a network
connecting two routers should be considered as a single hop path
rather than a two hop path. For example, consider three routers
A, B, and C connected over an Ethernet network N, which the OSPF
topology represents as:
In the above example, although there are directed edges in both
directions, an edge from the network to any of the three routers
Guerin, et al. Expires 30 September 1997 [Page 26]
Internet Draft QoS Routing Mechanisms 25 March 1997
A----N----B
|
|
C
must have zero ``cost'', so that it is not counted twice. It should
be noted that when considering such environments in the context
of QoS routing, it is assumed that some entity is responsible
for determining the ``available bandwidth'' on the network. The
specification of the operation of such an entity is beyond the scope
of this document.
C. Source Routing Support
As mentioned before, the scope of the path selection process can
range from simply returning the next hop on the QoS path selected for
the flow, to specifying the complete path that was computed, i.e., a
source route. Obviously, the information being returned by the path
selection algorithm differs in these two cases, and constructing it
imposes different requirements on the path computation algorithm and
the data structures it relies on. While the presentation of the path
computation algorithms focused on the hop-by-hop routing approach,
the same algorithms can be applied to generate source routes with
minor modifications. These modifications and how they facilitate
constructing source routes are discussed next.
The general approach to facilitate construction of source routes is
to update the neighbor field differently from the way it is done
for hop-by-hop routing as described in Section 2. Recall that in
the path computation algorithms the neighbor field is updated to
reflect the identity of the node adjacent to the source node on the
partial path computed. This facilitates returning the next hop at
the source for the specific path. In the context of source routing,
the neighbor information is updated to reflect the identity of the
previous router on the path.
With this change, the basic approach used to extract the complete
list of verti ces on a path from the neighbor information in the
QoS routing table is to proceed recursively from the destination to
the origin vertex. The path is extracted by stepping through the
precomputed QoS routing table from vertex to vertex, and identifying
at each step the corresponding neighbor (precursor) information.
Once the source router is reached, the concatenation of all the
neighbor fields that have been extracted forms the desired source
Guerin, et al. Expires 30 September 1997 [Page 27]
Internet Draft QoS Routing Mechanisms 25 March 1997
route. This applies to the source-routed versions of both algorithms
of Sections 2.3.1 and 2.3.3.
Specifically, assume a new request to destination, say, d, and with
bandwidth requirements B. The index of the destination vertex
identifies the row in the QoS routing table that needs to be checked
to generate a path. How the row is searched to identify a suitable
path depends on which algorithm was used to construct the QoS routing
table. If the Bellman-Ford algorithm of Section 2.3.1 is used, the
search proceeds by increasing index (hop) count until an entry is
found, say at hop count or column index of h, with a value of the
bw field which is greater than or larger than B. This entry points
to the initial information identifying the selected path. If the
Dijkstra algorithm of Section 2.3.3 is used, the first quantized
value bBsuch that Bb B is first identified, and the associated
column then determines the first entry in the QoS routing table that
identifies the selected path.
Once this first entry has been identified, reconstruction of the
complete list of vertices on the path proceeds similarly, whether
the table was built using the algorithm of Sections 2.3.1 or 2.3.3.
Specifically, in both cases, the neighbor field in each entry points
to the previous node on the path from the source node and with the
same bandwidth capabilities as those associated with the current
entry. The complete path is, therefore, reconstructed by following
the pointers provided by the neighbor field of successive entries.
In the case of the Bellman-Ford algorithm of Section 2.3.1, this
means moving backwards in the table from column to column, using at
each step the row index pointed to by the neighbor field of the entry
in the previous column. Each time, the corresponding vertex index
specified in the neighbor field is pre-pended to the list of vertices
constructed so far. Since we start at column h, the process ends
when first column is reached, i.e., after h steps, at which point
the list of vertices making up the path has been reconstructed.
In the case of the Dijkstra algorithm of Section 2.3.3, the
backtracking process is similar although slightly different because
of the different relation between paths and columns in the routing
table, i.e., a column now corresponds to a quantized bandwidth value
instead of a hop count. The backtracking now proceeds along the
column corresponding to the quantized bandwidth value needed to
satisfy the bandwidth requirements of the flow. At each step, the
vertex index specified in the neighbor field is pre-pended to the
list of vertices constructed so far, and is used to identify the next
row index to move to. The process ends when an entry is reached
whose neighbor field specifies the origin vertex of the flow. Note
that since there are as many rows in the table as there are vertices
Guerin, et al. Expires 30 September 1997 [Page 28]
Internet Draft QoS Routing Mechanisms 25 March 1997
in the graph, i.e., N, it could take up to N steps before the
process terminates.
Note that the identification of the first entry in the routing table
is identical to what was described for the hop-by-hop routing case.
However, as described in this section, the update of the neighbor
fields while constructing the QoS routing tables, is being performed
differently in the source and hop-by-hop routing cases. Clearly, two
different neighbor fields can be kept in each entry and updates to
both could certainly be performed jointly, if support for both source
routing and hop-by-hop routing is needed.
D. Computational Complexity
One generic aspect of the algorithmic complexity of computing
QoS paths is the efficiency of the shortest path algorithm used.
Specifically, in this document, we have described approaches based on
both Bellman-Ford and Dijkstra shortest paths algorithms. Dijkstra's
algorithm has traditionally been considered more efficient for
standard shortest path computations because of its lower worst case
complexity. However, the answer is not as simple as may appear, and
in this section we briefly review a number of considerations, in
particular in the context of multi-criteria QoS paths, which indicate
that a BF approach may often provide a lower complexity solution.
The asymptotic worst-case complexity of the Dijkstra algorithm is
O(NlogN + M), where N is the number of vertices in the graph,
and M the number of edges. However, this bound is obtained
under the assumption of a Fibonnaci heap implementation of the
Dijkstra algorithm, which is impractical due to the large constants
involved [CLR90]. In practice, the Dijkstra algorithm is typically
implemented using binary heaps, for which the asymptotic worst-case
bound is O(MlogN).
The asymptotic worst-case bound for the BF algorithm is O(H . M),
where M is again the number of edges in the graph, and H, which is
the maximum number of iterations of the algorithm, is an upper-bound
on the number of hops in a shortest path. Although, theoretically,
H can be as large as N - 1, in practice it is usually considerably
smaller than N. Moreover, in some network scenarios an upper-bound
U of small size (i.e., U << N) is imposed on the allowed number
of hops; for example, it might be decided to exclude paths that
have more than, say, 16 hops, as part of a call admission scheme.
In such cases, the number of iterations of the BF algorithm can be
limited to U, thus bounding the number of operations to O(U . M),
i.e., effectively to O(M). As a consequence, as noted in [BG92],
in practical networking scenarios, the BF algorithm can offer an
Guerin, et al. Expires 30 September 1997 [Page 29]
Internet Draft QoS Routing Mechanisms 25 March 1997
efficient solution to the shortest path problem, one that often
outperforms the Dijkstra algorithm. (8)
In the context of QoS path selection, the potential benefits of the
BF algorithm are even stronger. As mentioned before, efficient
selection of a suitable path for flows with QoS requirements cannot
usually be handled using a single-objective optimization criterion.
While multi-objective path selection is known to be an intractable
problem [GJ79], the BF algorithm allows us to handle a second
objective, namely the hop-count, which is reflective of network
resources, at no additional cost in terms of complexity. The
Dijkstra algorithm requires some modifications (or approximations,
e.g., bandwidth quantization) in order to be able to deal with hop
count as a second objective.
Therefore, in the context of a QoS path selection algorithm,
where one objective is some QoS-oriented metric, such as available
bandwidth, whereas the second is a hop-count metric, a BF-based
algorithm provides an efficient scheme for pre-computing paths,
i.e., one with a worst case asymptotic complexity of O(H . M).
Alternatively, if QoS paths are pre-computed using a Dijkstra
algorithm with Q quantized bandwidth values, the corresponding worst
case asymptotic complexity is O(Q . (M logN)). Both approaches
provide solutions of comparable orders of complexity, whose exact
merits depend on the respective values of H, Q and N. If on-demand
computations of QoS paths are practical, then a standard Dijkstra
algorithm provides a solution of complexity O(MlogN).
E. Extension: Handling Propagation Delays
In general, the framework proposed for path selection does not allow
us to explicitly account for link propagation delays. As mentioned,
this aspect is dealt with through a policy mechanism, which for
delay-sensitive connections deletes from the topology database links
with high propagation delays, such as satellite links. However, it
is worth pointing out that a simple extension to the proposed path
selection algorithm allows us to directly account for delay in a
----------------------------
8. For example, in the experimental comparison reported in [CGR94], the
BF algorithm outperformed the Dijkstra algorithm in about one third
of the studied types of topology, and in several of the other
topologies it outperformed the Dijkstra algorithm for networks of up
to about 16,000 nodes. It should be noted that in those experiments
no upper bound on the number of hops in a shortest path was set.
Guerin, et al. Expires 30 September 1997 [Page 30]
Internet Draft QoS Routing Mechanisms 25 March 1997
number of special cases. We proceed to describe next this extension
and the case where it applies.
A common way to map delay guarantees into bandwidth guarantees
(e.g., consistent with the schedulers and corresponding delay
bounds presented in [GGPS96, PG94]) is according to the following
expression:
D(p) =A(h(p))=b +sum(l in p) d(l) (1)
where p is the path traversed, D(p) is the guaranteed (upper-bound)
end to end delay, h(p) is the number of hops, b is the reserved
bandwidth, d(l) is the (fixed) propagation delay of a link l, and A(h)
is a parameter that grows with h (a typical value is A(h)= B +h . c,
where B is the burst size and c is the maximum packet size).
Since we deal with intra-domain routing, and since links with
prohibitively high propagation delays are assumed to be filtered out
by means of policy, it can be assumed that typically there is some
value d which is a reasonable upper bound on the propagation delays
d(l) of all links. Expression (1) then implies that an end to end
delay requirement D can be translated into a bandwidth requirement
b(h) by the following expression:
b(h) =A(h)=(D -h. d) (2)
where h is the number of hops on the path established for the
connection.
F. QoS Path Establishment and Management with RSVP
In this section, we briefly illustrate the use of the QoS path
selection approach described in this document, for unicast RSVP
flows. The objective is to path set up QoS paths for RSVP flows
and keep them pinned a s long as it is desirable to do so, while
requiring minimal changes to RSVP. Clearly, some changes are
needed, particularly to RSVP's interface to routing and its message
processing rules. These will be detailed next. In addition, the
impact of this path management approach data path is considered and
alternative approaches and extensions are discussed.
Guerin, et al. Expires 30 September 1997 [Page 31]
Internet Draft QoS Routing Mechanisms 25 March 1997
F.1. RSVP/Routing interface
Currently, RSVP acquires routing entries using its asynchronous
query-response interface to routing [Zap96]. Route query is of the
form
Route_Query( [SrcAddress], DestAddress, Notify_flag )
and Routing responds with OutInterface (or OutInterface_list in case
of a multicast connection)
In order for RSVP to interact with a QoS routing algorithm,
QoS_Route_Query needs to also include (at a minimum) the
sender_TSpec, so that it is now of the form
Route_Query( [SrcAddress], DestAddress, TSpec, Notify_flag )
and again responds with OutInterface (or OutInterface_list in case of
a multicast connection).
Another small difference with the current interface is that the
Notify_flag should always be set to True. This is because there will
be no Route_Query to QoS routing in the case of pinned paths. Hence,
it is important that a trigger be provided to unpin the path in case
of failure. However, note that QoS routing will only generate an
asynchronous Route_Change callback to RSVP in the case of the failure
of a local (to the router) link currently used by the QoS path.
F.2. Path Management Rules
The state of a QoS path as maintained by RSVP consists of a flag
that is used to indicate whether the path is currently pinned or
not. Specifically, a pinned path means that QoS routing need not be
queried for a new path (next hop) for forwarding a PATH refresh. The
rules for pinning and unpinning routes are as follows:
1. Routes get pinned during processing of PATH messages.
2. Routes get unpinned when
(a) corresponding path states are removed (time-out or PATH
_TEAR),
(b) some of the parameters received in PATH messages change,
(c) a local admission control failure error is detected after
receiving a RESV message,
Guerin, et al. Expires 30 September 1997 [Page 32]
Internet Draft QoS Routing Mechanisms 25 March 1997
(d) a PATH _ERR with a specific error code is received, or
(e) failure notification of a local link belonging to the path is
received.
Minor changes to RSVP message processing rules are adequate to handle
pinning and unpinning of paths as needed. These specific changes are
described below.
PATH message processing:
When receiving the first PATH message, RSVP determines that no PATH
state exists for the flow. It then queries QoS routing to obtain the
next hop along the best available path. This next hop is stored as
part of the PATH state with its pinned flag set.
Upon receiving a PATH refresh, RSVP checks for changes in PATH state
that are of relevance to QoS routing. In particular, it checks for
changes in PHOP and the IP TTL value. If there are no changes and
the current next hop is indicated as pinned, it will be used to
forward the next PATH refresh. If the PATH state has changed or the
current next hop is marked as unpinned, RSVP queries QoS routing
again to obtain (and pin) a new next hop that is to be used when
forwarding the next PATH refresh. Similarly, at the time when a
PATH refresh is to be sent, RSVP checks if the current next hop is
pinned or not. If it is, it is used to forward the PATH refresh.
Otherwise, QoS routing is again queried to obtain (and pin) a new
next hop.
The unpinning of the path upon detecting changes in either the PHOP
or the IP TTL value of an incoming PATH message is used to ensure
that transient loops caused by inconsistent routing information are
eventually cleared [GKH97].
PATH_TEAR message processing:
Processing is similar to what is currently done. PATH and RESV
states are removed.
RESV message processing:
The only change needed is for the case when the resource reservation
attempt fails. As currently specified, a RESV_ERR message with
"admission control failure" error code is still sent downstream in
such instances. However, some additional processing is needed in
Guerin, et al. Expires 30 September 1997 [Page 33]
Internet Draft QoS Routing Mechanisms 25 March 1997
order to enable selection of a better path in case one exists. This
starts with the unpinning of the current next hop, and then proceeds
in either one of two ways: attempt *local* repair of the QoS path or
not.
In case local repair is attempted, RSVP queries again its local
QoS routing table. If a different next hop is returned, i.e., the
reservation may now succeed, then local repair is attempted by
pinning the new hop and sending a PATH message along the new route.
If the same next hop is returned, then local repair has failed. In
this case or when local repair is not attempted, the current next
hop is then unpinned in the PATH state (but kept). Furthermore, a
PATH _ERR message is sent upstream with a new QoS_Path_Failure Error
Code (the exact code point is tbd) and an associated Error Value
specifying that the type of error was "Requested QoS unavailable"
(the specific format of the Error Value field is tbd). As described
below, the receipt of a PATH _ERR message with the QoS_Path_Failure
Error Code triggers unpinning of the next hop information at upstream
router. This ensures that QoS routing will be queried at the time of
the next PATH refresh, so that a better path, if one exists, can be
identified.
Route_Change notification processing:
A Route_Change notification is triggered when QoS routing detects
that a local link currently used by a QoS path failed. Upon
receiving such a notification, RSVP immediately unpins the current
next hop. As in the case of reservation failure, RSVP can then
first attempt local repair, i.e., query QoS routing for a new next
hop. If a new next hop is returned by QoS routing, RSVP uses it
to replace the previous next hop, marks it as pinned, and forwards
a PATH message towards the new next hop. If QoS routing responds
that no path to the destination is available or if local repair is
not attempted, RSVP sends upstream a PATH _ERR message with the
QoS_Path_Failure Error Code and an Error Value specifying "Link
failure".
PATH_ERR message processing:
The only modification is, as mentioned above, to recognize the new
QoS_Path_failure Error Code and unpin the associated next hop. This
forces a fresh QoS route query during the processing of the next PATH
refresh.
Guerin, et al. Expires 30 September 1997 [Page 34]
Internet Draft QoS Routing Mechanisms 25 March 1997
RESV_ERR message processing:
There are no changes to RESV_ERR processing.
F.3. Impact of QoS Routing on the Data Path
The use of QoS routing only affects the choice of a data path and not
how the actual forwarding of data packet takes place. Nevertheless,
there is an important aspect that needs to be noted. Specifically,
while PATH messages are immediately forwarded onto the next hop
returned by QoS routing, the same need not apply to data packets.
This is because of the potential for transient loops in QoS paths.
Forwarding PATH messages on a QoS path that may contain loops has
minimal impact on the routers and is actually useful to detect and
eliminate loops (more on this below). However, depending on how fast
loops can be resolved, forwarding data packets on a QoS path may be
best deferred until the absence of a loop has been verified.
As a result, it is proposed that modification of the packet
classifier in the forwarding loop that will result in data packets
being sent towards the next hop specified by QoS routing, be deferred
until the time a RESV message is received. As discussed below, the
receipt of a RESV message also implies that loops are not present in
the QoS path. Note that the update of classifiers at the time of
receipt of a RESV message is consistent with when this is done using
the current default routing. The main difference is that the actual
flow of data packets may not start following the QoS path until after
the classifier has been updated in the first node where the default
and the QoS paths start differing.
There are some drawbacks with the above approach, e.g., inability to
take advantage of partial reservations in some instances, and they
can be addressed in a number of ways. One possibility, that may be
acceptable if transient loops are detected and removed quickly, is
to actually update classifiers upon receipt of a PATH message (or a
certain number of PATH messages, when it appears that the QoS path
is stable and loops are not present). Another more comprehensive
alternative is to couple this process with the handling of policy
information. Such a coupling is a natural step as the ability for
users to specify how much of a partial reservation is acceptable
to them, i.e., does one need to look for another path, is really
a policy issue. In order to support such a coupling, policy data
objects would have to be included in PATH, RESV, RESV_ERR, and
PATH_ERR messages, in order to enable the local policy control
module to assess the suitability of a QoS path. The discussion and
description of such an approach is the subject of future work.
Guerin, et al. Expires 30 September 1997 [Page 35]
Internet Draft QoS Routing Mechanisms 25 March 1997
For a detailed discussion of how these QoS path management rules
within RSVP prevent loops and handle race conditions, the reader is
referred to [GKH97].
F.4. Alternatives and Extensions
In the path management approach described here, bulk of the
responsibility for QoS path management, i.e., pinning and unpinning
of next hop information, lies with RSVP. This was motivated in part
by the need to couple path management with the RSVP soft state
management, and by the close relation to existing RSVP processing
rules. However, it is also possible to defer this responsibility to
routing itself. The cost of such an approach would be the need for
QoS routing to replicate some of the RSVP state information, e.g. ,
store PHOP, NHOP, TSpec, etc. , for each flow, and also by requiring
that this information be passed across the interface between RSVP and
routing.
Another different design approach is to rely on the inclusion of
source (explicit) route objects, that would be carried in RSVP PATH
messages as opaque objects and passed to QoS routing at each node.
Such a design affords some simplification as it avoids the problem
of loops altogether, but issues related to pinning and unpinning of
paths (at the source) remain for the cases of reservation and link
failures. The discussion of such a design is clearly of interest,
but it is beyond the scope of this document.
References
[Alm92] P. Almquist. Type of Service in the Internet Protocol
Suite. INTERNET-RFC, Internet Engineering Task Force, July
1992.
[BG92] D. Bertsekas and R. G. Gallager. Data Networks. Prentice
Hall, Englewood Cliffs, NJ, 2nd edition, 1992.
[CGR94] B. V. Cherkassky, A. V. Goldberg, and T. Radzik. Shortest
Paths Algorithms: Theory and Experimental Evaluation.
In Proceedings of the 5th Annual ACM SIAM Symposium on
Discrete Algorithms, pages 516--525, Arlington, VA, January
1994.
[CLR90] T. H. Cormen, C. E. Leiserson, and R. L. Rivest.
Introduction to Algorithms. MIT Press, Cambridge, MA,
1990.
Guerin, et al. Expires 30 September 1997 [Page 36]
Internet Draft QoS Routing Mechanisms 25 March 1997
[GGPS96] L. Georgiadis, R. Guerin, V. Peris, and K. N. Sivarajan.
Efficient Network QoS Provisioning Based on per Node
Traffic Shaping. IEEE/ACM Transactions on Networking,
4(4):482--501, August 1996.
[GJ79] M.R. Garey and D.S. Johnson. Computers and Intractability.
Freeman, San Francisco, 1979.
[GKH97] R. Guerin, S. Kamat, and S. Herzog. QoS Path Management
with RSVP (draft-ietf-guerin-qos-pathmgt-rsvp-00.txt).
INTERNET-DRAFT, Internet Engineering Task Force, March
1997.
[Moy94] J. Moy. OSPF Version 2 - RFC No. 1583 (rfc1583.ps,txt).
INTERNET-RFC, Internet Engineering Task Force, March 1994.
[PG94] A. K. Parekh and R. G. Gallager. A Generalized Processor
Sharing Approach to Flow Control in Integrated Services
Networks: the Multiple Node Case. IEEE/ACM Transactions
on Networking, 2:137--150, 1994.
[Prz95] A. Przygienda. Link State Routing with QoS in ATM
LANs. Ph.D. Thesis Nr. 11051, Swiss Federal Institute of
Technology, April 1995.
[RZB+96] R. Braden (Ed.), L. Zhang, S. Berson, S. Herzog, and
S. Jamin. Resource reSerVation Protocol (RSVP) version
1, functional specification (draft-ietf-rsvp-spec-13.ps).
INTERNET-DRAFT, Internet Engineering Task Force - RSVP WG,
July 1996.
[Tan89] A. Tannenbaum. Computer Networks. Addisson Wesley, 1989.
[Zap96] D. Zappala. RSRR: A Routing Interface for RSVP
(draft-ietf-rsvp-routing-01.ps). INTERNET-DRAFT, Internet
Engineering Task Force - RSVP WG, November 1996.
Authors' Address
Roch Guerin
IBM T.J. Watson Research Center
P.O. Box 704
Yorktown Heights, NY 10598
guerin@watson.ibm.com
VOICE +1 914 784-7038
Guerin, et al. Expires 30 September 1997 [Page 37]
Internet Draft QoS Routing Mechanisms 25 March 1997
FAX +1 914 784-6205
Sanjay Kamat
IBM T.J. Watson Research Center
P.O. Box 704
Yorktown Heights, NY 10598
sanjay@watson.ibm.com
VOICE +1 914 784-7402
FAX +1 914 784-6205
Ariel Orda
Dept. Electrical Engineering
Technion - I.I.T
Haifa, 32000 - ISRAEL
ariel@ee.technion.ac.il
VOICE +011 972-4-8294646
FAX +011 972-4-8323041
Tony Przygienda
FORE Systems Inc.
174 Thorn Hill Rd
Warrendale PA, 15086
prz@fore.com
Doug Williams
IBM T.J. Watson Research Center
P.O. Box 704
Yorktown Heights, NY 10598
dougw@watson.ibm.com
VOICE +1 914 784-5047
FAX +1 914 784-6318
Guerin, et al. Expires 30 September 1997 [Page 38]
