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Internet Engineering Task Force Curtis Villamizar
INTERNET-DRAFT UUNET
draft-ietf-ospf-omp-02 February 24, 1999
OSPF Optimized Multipath (OSPF-OMP)
Status of this Memo
This document is an Internet-Draft and is in full conformance with all
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Copyright (C) The Internet Society (February 24, 1999). All Rights
Reserved.
Abstract
OSPF may form multiple equal cost paths between points. This is true
of any link state protocol. In the absense of any explicit support
to take advantage of this, a path may be chosen arbitrarily. Tech-
niques have been utilized to divide traffic somewhat evenly among the
available paths. These techniques have been referred to as Equal Cost
Multipath (ECMP). An unequal division of traffic among the available
paths is generally preferable. Routers generally have no knowledge
of traffic loading on distant links and therefore have no basis to
optimize the allocation of traffic.
INTERNET-DRAFT OSPF Optimized Multipath (OSPF-OMP) February 24, 1999
Optimized Mulitpath is a compatible extension to OSPF, utilizing the
Opaque LSA to distribute loading information, proposing a means to
adjust forwarding, and providing an algorithm to make the adjustments
gradually enough to insure stability yet provide reasonably fast ad-
justment when needed.
1 Overview
Networks running OSPF are often heavily loaded. Topologies often
evolve to include multiple paths. Multiple paths may be initially de-
signed to provide redundancy but also result from incremental addition
of circuits to accomodate traffic growth. The redundant paths provide
a potential to distribute traffic loading and reduce congestion. Op-
timized Mulitpath (OMP) provides a means for OSPF to make better use
of this potential to distribute loading.
1.1 Past Attempts
Early attempts to provide load sensitive routing involved changing
link costs according to loading. These attempts were doomed to fail-
ure because the adjustment increment was grossly course and oscilla-
tion was inevitable [2]. This early experience is largely responsible
for the common belief that any form of load sensitive routing will
fail due to severe oscillations resulting from instability.
Attempts to use a metric composed of weighted components of delay,
traffic, and fixed costs have also been met with very limited success.
The problem again is the granularity of adjustment. As the composi-
tion of weighted components switches favored paths large amounts of
traffic are suddenly moved, making the technique prone to oscillations
[3]. The oscillation is damped to some extent by providing a range
of composite metric differences in which composite metrics are con-
sidered equal and equal cost multipath techniques are used. Even then
the technique still suffers oscillations due to the course adjustments
made at equal/unequal metric boundaries.
1.2 Equal Cost Multipath
A widely utilized technique to improve loading is known as Equal Cost
Multipath (ECMP). ECMP is specified in [5]. In ECMP no attempt to
make dynamic adjustments to OSPF costs based on loading and therefore
ECMP is completely stable. If the topology is such that equal cost
paths exist, then an attempt is made to divide traffic equally among
the paths. At least three methods of splitting traffic have been
used.
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1. Per packet round robin forwarding.
2. Dividing destination prefixes among available next hops in the for-
warding entries.
3. Dividing traffic according to a hash function applied to the source
and desination pair.
The ``per packet round robin forwarding'' technique is only applicable
if the delays on the paths are almost equal. The delay difference
must be small relative to packet serialization time. Delay differ-
ences greater than three times the packet serialization time can cause
terrible TCP performance. For example, packet 2, 4, and 6 may arrive
before packet 1, triggering TCP fast retransmit. The result will be
limiting TCP to a very small window and very poor performance over
long delay paths.
The delay differences must be quite small. A 532 byte packet is seri-
alized onto a DS1 link in under 2.8 msec. At DS3 speed, serialization
is accomplished in under 100 usec. At OC12 it is under 7 usec. For
this reason ``per packet round robin forwarding'' is not applicable to
a high speed WAN.
Dividing destination prefixes among available next hops provides a
very course and unpredictable load split. Very short prefixes are
problematic. In reaching an end node, the majority of traffic is of-
ten destined to a single prefix. This technique is applicable to a
high speed WAN but with the drawbacks just mentioned better techniques
are needed.
The ``source/destination hash'' based technique was used as far back
as the T1-NSFNET in the IBM RT-PC based routers. A hash function,
such as CRC-16, is applied over the source address and destination ad-
dress. The hash space is then split evenly among the available paths
by either setting threshholds or performing a modulo operation. Traf-
fic between any given source and destination remain on the same path.
Because the technique is based on host addresses, and uses both the
source and destination address, it does not suffer the course gran-
ularity problem of the prefix based technique, even when forwarding
to a single prefix. Source/destination hash is the best technique
available for a high speed WAN.
The forwarding decision for the ``source/destination hash'' based
technique is quite simple. When a packet arrives, look up the for-
warding entry in the radix tree. The next hop entry can be an array
index into a set of structures, each containing one or more actual
next hops. If more than one next hop is present, compute a CRC16
value based on the source and destination addresses. The CRC16 can
be implemented in hardware and computed in parallel to the radix tree
lookup in high speed implementations, and discarded if not needed.
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.----.
/ \
| N2 |
\ /
`----'
// \\
// \\
// \\
.----. .----.
/ \ / \
| N1 | ======== | N3 |
\ / \ /
`----' `----'
Figure 1: A very simple application of ECMP
Each next hop entry in the structure must contain a boundary value
and the next hop itself. An integer ``less than'' comparison is made
against the boundary value determining whether to use this next hop
or move to the next a comparison. In hardware the full set of compar-
isons can be made simultaneously for up to some number of next hops or
a binary search can be performed. This yields the next hop to use.
1.3 Optimized Multipath differs from ECMP
For ECMP, the boundary values are set by first dividing one more than
the maximum value that the hash computation can return (65536 for
CRC16) by the number of available next hops and then setting the Nth
boundary to N times that number (with the Nth value fixed at one more
than the maximum value regardless of underflow caused by trucating
during division, 65536 for CRC16).
An equal load split is not always optimal. Consider the example in
Figure 1 with the offered traffic in Table 1. If all of the link
costs are set equally, then the link N1---N3 is significantly over-
loaded (135.75%) while the path N1---N2---N3 is lightly loaded (45.25%
and 22.62%). If the cost on the N1---N3 link is equal to the cost
of the N1---N2---N3 path, then N1 will try to split the load destined
toward N3 across the two paths.
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No ECMP OMP
Nodes Node Names Split Traffic Traffic
n3-n1 Node 3 -> Node 1 60 30 40
n1-n3 Node 1 -> Node 3 60 30 40
n3-n2 Node 3 -> Node 2 20 50 40
n2-n3 Node 2 -> Node 3 20 50 40
n2-n1 Node 2 -> Node 1 10 40 30
n1-n2 Node 1 -> Node 2 10 40 30
Table 1: Traffic loading for the example in Figure 1
Given the offered traffic in Table 1 the loading on N1---N3 is reduced
to 67.87% but the link loading on the path N2---N3 becomes 113.12%.
Ideally node N1 should put 1/3 of the traffic toward N3 on the path
N1---N2---N3 and 2/3 on the path N1---N3. To know to do this N1 must
know the loading on N2--N3.
This is where Optimized Multipath (OMP) provides additional benefit
over ECMP. Ignoring for the moment how node N1 knows to put 1/3 of the
traffic toward N3 on the path N1---N2---N3 (described later in Sec-
tion 2), the way the distribution of traffic is accomplished from a
forwarding standpoint is to move the boundary in the forwarding struc-
ture from the default value of 1/2 of 65536 to about 1/3 of 65536. If
there are a very large set of source and destination host addresses
pairs, then the traffic will be split among the 65536 possible hash
values. This provides the means for a very fine granularity of ad-
justment.
Having explained how a fine granularity of forwarding adjustment can
be accomplished, what remains is to define how nodes in a large topol-
ogy can know what the loading levels are elsewhere in the topology and
defining an algorithm which can allow autonomous unsyncronized deci-
sions on the parts of many routers in a topology to quickly converge
on a near optimal loading without the risk of oscillation. This is
covered in the following sections.
2 Flooding Loading Information
Loading information is flooded within an OSPF area using Opaque
LSAs [1]. Area local scope (link-state type 10) link state at-
tributes are flooded containing an ``Opaque Type'' of LSA_OMP_LINK_LOAD
or LSA_OMP_PATH_LOAD. The type LSA_OMP_LINK_LOAD Opaque LSA is
used to flood link loading information within an area. The
type LSA_OMP_PATH_LOAD Opaque LSA is used to flood loading informa-
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tion for use with inter-area routes. Loading information obtained
from an exterior routing protocol may also be considered if avail-
able. The means of passing loading information in an exterior routing
protocol is beyond the scope of this document.
2.1 Link Loading Information
Within an area link loading is flooded using the type LSA_OMP_LINK_LOAD
Opaque LSA. The format of this LSA is described in Appendix A.
The ``Opaque Information'' in the type LSA_OMP_LINK_LOAD Opaque LSA
contains the following.
1. a measure of link loading in each direction as a fraction of link
capacity,
2. a measure of packets dropped due to queue overflow in each direc-
tion (if known) expressed as a fraction,
3. the link capacity in kilobits per second (or unity if less than
1000 bytes per second).
Generally the number of ouput packets dropped will be known. In de-
signs where drops occur on the input, the rate of input queue drops
should be recorded. These measures of loading and drop are computed
using the interface counters generally maintained for SNMP purposes,
plus a running count of output queue drops if available. The coun-
ters are sampled every 15 seconds but generally flooded at longer time
intervals.
The previous value of each of the counters is substracted from the
current value. The counters required are 1) bytes out, 2) bytes in,
3) packets out, 4) packets in, 5) output queue drops, and 6) input
queue drops. These counters should already exist to satisfy SNMP
requirements.
A value of instantaneous load in each direction is based on byte count
and link capacity. An instantaneous output queue drop rate is based
on queue drops and packet count. Some of the values are filtered as
described in Appendix B.1.
The last time that a type LSA_OMP_LINK_LOAD Opaque LSA with the same
Opaque ID was sent is recorded and the values sent are recorded. For
the purpose of determining when to reflood, an equivalent loading fig-
ure is used. The computation of equivalent loading is described in
Section 2.3.
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The higher of the current equivalent loading computation and
the previous is used when determining whether to send the
type LSA_OMP_LINK_LOAD Opaque LSA. The type LSA_OMP_LINK_LOAD Opaque
LSA is flooded according to elapsed time since last flooded, the cur-
rent equivalent load, and the difference between the current equiva-
lent load and the previously flooded equivalent load. The reflooding
decision is described in detail in Appendix B.1.
The point of this graduated reflooding schedule is to reduce the
amount of flooding that is occurring unless links are in trouble or
undergoing a significant traffic shift. Change may occur in a qui-
escent network due to failure external to the network that causes
traffic to take alternate paths. In this case, the more frequent
flooding will trigger a faster convergence. Traffic shift may also
occur due to shedding of loading by the OMP algorithm itself as the
algorithm converges in response to an external change.
2.2 Path Loading Information
Path loading information regarding an adjacent area is flooded by an
Area Border Router (ABR) using the type LSA_OMP_PATH_LOAD Opaque LSA.
The format of this LSA is described in Appendix A.
The ``Opaque Information'' in the type LSA_OMP_PATH_LOAD Opaque LSA
contains the following.
1. the highest loading in the direction toward the destination as a
fraction of link capacity,
2. a measure of total packet drop due to queue overflow in the direc-
tion toward the destination expressed as a fraction,
3. the smallest link capacity on the path to the destination.
These values are taken from the link on the path from the ABR to the
destination of the summary LSA. The link with the highest loading may
not be the link with the lowest capacity. The queue drop value is one
minus the product of fraction of packets that are not dropped at each
measurement point on the path (input and output in the direction of
the path). The following computation is used.
path-loss = 1 - product(1 - link-loss)
The path loading and path loss rate are filtered according to the
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algorithm defined in Appendix B.1. Rather than polling a set of coun-
ters the current value of the path loading and path loss rate is used.
An equivalent load is calculated for each path to a summary LSA des-
tination as described in Section 2.3. A type LSA_OMP_PATH_LOAD Opaque
LSA is flooded according to the same rate schedule as described in the
prior section and Appendix B.1.
An ABR may be configured to not send type LSA_OMP_PATH_LOAD Opaque LSA
into any given area. See Appendix C.
2.3 Computing equivalent loading
The equivalent load is the actual fractional loading multiplied by a
factor that provides an estimate based on loss of the extent to which
TCP is expected to slow down to avoid congestion. This estimate is
based on the link bandwidth and loss rate, knowledge of TCP dynamics,
and some assumption about the characteristics of the TCP flows being
passed through the link. Some of the assumptions must be configured.
If loss is low or zero, the equivalent load will be equal to the ac-
tual fractional loading (link utilization expressed as a number be-
tween 0 and 1). If loss is high and loading is at or near 100%, then
the equivalent load calculation provides a means of deciding which
links are more heavily overloaded. The equivalent load figure is
not intended to be an accurate pridiction of offerred load, simply a
metric for use in deciding which link to offload.
Mathis et al provide the following estimate of loss given TCP window
size and round trip time [4].
p < (MSS / (BW * RTT))**2
The basis for the estimate is that TCP slows down roughly in propor-
tion to the inverse of the square root of loss. There is no way to
know how fast TCP would be going if no loss were present if there are
other bottlenecks. A somewhat arbitrary assumption is made that TCP
would go no faster than if loss were at 0.5%. If loss is greater than
0.5% then TCP performance would be reduced. The equivalent loading is
estimated using the following computation.
equiv-load = load * K * sqrt(loss)
The inverse of the square root of 0.1% is 10 so 10 may be used for the
value of ``K''.
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The conversion of loss to estimated loading is not at all accurate.
The non-linearity does affect the time to converge though convergence
still occurs as long as loss is positively correlated to loading.
This is discussed further in Appendix E.1.
3 Next hop structures
A ``next hop structure'' contains a set of complete paths to a des-
tination, some of which may share the same immediate next hop. The
name is not meant to imply a single next hop. A given route can ref-
erence only one next hop structure, which can contain multiple paths
and multiple next hops. Entries for paths that use the same next hop
are combined before moving information to the forwarding table. A
next hop structure contains the information nessecary to balance load
across a set of next hops.
For intra-area routes, a separate next hop structure must exist for
each destination router or network. For inter-area routes (summary
routes), at most one next hop structure is needed for each combination
of ABRs which announce summary routes that are considered equidis-
tant. Optimizing inter-area and external routing is discussed in
Section 3.2.
The set of intra-area next hop structures is initialized after the
OSPF SPF calculation is completed. An additional set of next hops is
then added by relaxing the best path criteria.
The use of the next hop structure and its contents is described in
Section 4.1.
3.1 Relaxing the Best Path Criteria
The exercise of setting link costs to produce the most beneficial set
of equal costs paths is tedious and very difficult for large topolo-
gies. OSPF as defined in RFC--2328 requires that only the best path
be considered. For the purpose of Optimized Multipath, this crite-
ria can be relaxed to allow a greater number of multipaths but not to
the point of creating routing loops. Any next hop which is closer in
terms of costs than the current hop and does not cross a virtual link
can be considered a viable next hop for multipath routing. If next
hops were used where the cost at the next hop is equal or greater,
routing loops would form.
In considering the paths beyond the next hop path, only the best paths
should be considered. There is no way to determine if subsequent
routers have relaxed the best path criteria. In addition, there is
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no need to consider the additional paths if the best path criteria
is relaxed downstream. If best path criteria is relaxed downstream,
the best paths must be part of the downstream next hop structure. If
there are additional paths the the downstream is able to use to fur-
ther distribute the load, the entire set of paths will still converge
toward optimal loading.
The best path criteria is relaxed only for intra-area routes. The
best path criteria can also be relaxed when considering the cost to
reach ABRs or ASBRs. The best path criteria should not be relaxed
when considering the total cost to reach a summary route or external
route.
3.2 Offloading Congestion Outside the OSPF Area
For inter-area routes or external routes, a separate next hop struc-
ture must exist for each such route if it is desireable to reduce
loading outside of the area and the loading within the area is suffi-
ciently low to safely allow this.
The existing procedures regarding selection of inter-area and exter-
nal routes outlined in [5] still apply. For inter-area routes the
intra-area cost and cost of the summary route are summed. For exter-
nal routes the intra-area cost is summed with a type 1 external cost
and considered before a type 2 external cost. The best path criteria
is not relaxed when applied to the sum of intra-area cost and summary
route cost or intra-area cost and type 1 external cost.
In order for an ABR or ASBR to be considered as a viable exit point to
the area for a given destination, it must be advertising an applicable
summary route or external route. The best summary route or external
route must still be choosen. If a single ABR or ASBR advertises the
best route, multiple paths to that ABR or ASBR may be used, but traf-
fic cannot be sent toward an ABR or ASBR advertising a higher cost
summary route or external route. If two or more ABR or ASBR advertise
a route at the same cost, then traffic load can be split among these
ABR or ASBR.
For intra-area routes if a type LSA_OMP_PATH_LOAD Opaque LSA exists
for the summary LSA and more than one ABR is advertising an equally
preferred summary route and the equivalent load for the summary LSA
is greater than 90% and the equivalent load within the area is suffi-
ciently smaller than the inter-area loading, then a next hop structure
can be created specifically to allow offloading of the intra-area
route. For external routes, if an equivalent loading exists, and more
than one ASBR is advertising an equally preferred external route, and
the equivalent load is greater than 95% and the equivalent load within
the area is sufficiently smaller than the external route loading, then
a separate structure is used.
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Hysterysis must be used in the algorithm for determining if an equiv-
alent load on a summary LSA or external route is considered suffi-
ciently larger than the intra-area equivalent load or if an external
route loading is considered sufficiently larger than the inter-area
equivalent load. For for the purpose of describing this algorithm one
equivalent load is referred to as the more external, and the other as
the more internal equivalent load.
If the more external equivalent load exceeds the more internal equiv-
alent load by 15% and the more internal equivalent load is under 85%,
then a separate next hop structure is created. If the more external
equivalent load falls below 20% of the more internal equivalent load
or the more internal equivalent load exceeds 98%, then an existing
separate next hop structure is marked for removal and combined with
the more internal next hop structure (see Section 3.3). The more ex-
ternal equivalent load should not fall significantly below the more
internal unless either the traffic toward the more external destina-
tion increases or the loading on the more internal increases, since
the more internal equivalent load will become the critical segment on
the separate next hop structure if the load is sufficiently shifted
but is unlikely to overshoot by 20%. These threshholds should be
configurable at least per type of routes (inter-AS or external).
The degree to which Summary LSA loading and external route loading
will be considered is limited. This serves two purposes. First, it
prevents compensating for external congestion to the point of loading
the internal network beyond a fixed threshhold. Second, it prevents
triggering the removal of the next hop structure, which if allowed to
occur could trigger a hysteresis loop. This mechanisms are described
in Section 3.4, and Appendix C.4.
3.3 Creating and destroying next hop structures
As described in Section 3.2 separate next hop structure is needed
if the loading indicated by the type LSA_OMP_PATH_LOAD Opaque LSA or
exterior routing protocol is sufficiently high to require separate
balancing for traffic to the summary-LSA or exterior route and the
intra-AS loading is sufficiently low.
When a separate next hop structure is created, the same available
paths appear in the structure, leading to the same set of ABR or ASBR.
The balance on these available paths should be copied from the exist-
ing more internal next hop structure. By initializing the new next
hop structure this way, a sudden change in loading is avoided if a
great deal of traffic is destined toward the summary route or external
route.
When a separate next hop structure can be destroyed, the traffic
should be transitioned gradually. The next hop structure must be
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marked for deletion. The traffic share in this separate next hop
structure should be gradually changed so that it exactly matches the
traffic share in the more internal next hop structure. The gradual
change should follow the adjustment rate schedule described in Sec-
tion 4.1 where the move increment is increased gradually as moves
continue in the same direction. The only difference is that there is
no need to overshoot when adjusting to match the more internal next
hop structure parameters. Once the separate next hop structure marked
for deletion matches the more internal next hop structure, the summary
route or external route can be changed to point to the more internal
next hop structure and the deletion can be made.
3.4 Critcally loaded segment
For every set of paths, one link or part of the path is identified as
the ``critcally loaded'' segment. This is the part of the path with
the highest equivalent load as defined in Section 2.3. For an inter-
area route with a separate next hop structure, the critically loaded
segment may be the critcally loaded segment for the intra-area set of
paths, or it may be the summary LSA if the equivalent load on the sum-
mary LSA is greater. For an external route with a separate next hop
structure, the critcally loaded segment may be the critcally loaded
segment for the internal route or it may be the external route if
the equivalent load on the external route is greater. In considering
loading reported for summary LSA or external routes, the loading may
be clamped to some configured ceiling (see Appendix C.4). If intra-
area loading exceeds this ceiling, the summary LSA loads or external
routes loads are in effect ignored.
Each next hop structure has exactly one ``critcally loaded'' segment.
There may be more than one path in the next hop structure sharing
this critcally loaded segment. A particular Opaque LSA may be the
critcally loaded segment for no next hop structures if it is lightly
loaded. Another Opaque LSA may be the critcally loaded segment for
many next hop structures if it is heavily loaded.
3.5 Optimizing Partial Paths
Under some circumstances multiple paths will exist to a destination
where all of the available paths share one or more links. In some
cases overall system convergence time can be substantially improved by
optimizing a partial path when the most heavily loaded link is shared
by all available paths to a destination.
Computations are actually reduced when partial paths are considered.
The next hop structures kept within the routing process must contain
the full paths used to reach a destination (this is already a require-
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ment). After an SPF calculation has changed the next hop structure
and before attempting any optimization the set of paths are examined
looking for intr-area links which are common to all paths. If any
such links are found, only intra-area links closer than any of these
links can be considered as candidates for the ``critcally loaded''
segment (Section 3.4). If there is only one immediate hop, no attempt
is made to load balance.
The change in load adjustment parameters should be applied to the
data structures for the full paths even though only a subset of the
links are eligible to be considered as the critcally loaded segment.
For the purpose of building type LSA_OMP_PATH_LOAD Opaque LSA loading
along the entire path must be considered including links shared by all
available paths.
4 Adjusting Equal Cost Path Loadings
Next hop structures are described in Section 3. A next hop structure
contains a set of complete paths to a destination.
Adjustments are made to a next hop structure to reflect differences in
loading on the paths as reported by the type LSA_OMP_LINK_LOAD Opaque
LSA and type LSA_OMP_PATH_LOAD Opaque LSA. Section 3.4 describes the
selection of a ``critically loaded segment'' which is used to de-
termine when to make adjustments and the size of the adjustments.
Section 3.5 describes conditions under which some links are excluded
from considerations as the ``critically loaded segment''.
An adjustment to loading of a given set of equal cost paths is made
when one of two conditions are true. Either the ``critically loaded
segment'' has been reflooded, or a criteria is met involving 1) the
difference between the equivalent load of the``critically loaded seg-
ment'' and the lightest loaded path, 2) the equivalent load of the
``critically loaded segment'', 3) the type of destination, intr-area,
inter-area, or external, and 4) the amount of time since the last
load adjustment. The details of this conditional are described in
Appendix B.
The reflooding algorithm is designed to be slightly less aggressive
than the adjustment algorithm. This reduces the need to continuously
flood small changes except in conditions of overload or substantial
change in loading. Some overshoot may occur due to adjustments made
in the absence of accurate knowledge of loading.
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4.1 Load Adjustment Rate
In order to assure stability the rate of adjustment must be suffi-
ciently limited. An adaptive adjustment rate method is used.
A ``critcally loaded'' segment for a next hop structure is determined
as described in Section 3.4. When the type LSA_OMP_LINK_LOAD Opaque
LSA or type LSA_OMP_PATH_LOAD Opaque LSA for this segment is updated
or the criteria in Appendix B is met, load is shed from all paths in
the next hop structure that include that segment toward all paths in
the next hop structure that do not include that segment. A separate
set of variables controlling rate of adjustment is kept for each path
receiving load.
The number of paths usually exceeds the number of next hops. The dis-
tinction between paths which share a next hop is important if one of
the paths sharing a next hop goes down (see Section 4.2). This dis-
tinction is only needed in making the computations. When moving the
next hop structure into the data structures used for forwarding, paths
which share a common next hop may be combined.
The following variables are kept for each path in a next hop struc-
ture.
1. The current ``traffic share'' (an integer, the range is 0 to 65355
for a CRC16 hash),
2. The current ``move increment'' used when moving traffic toward this
path (an integer, the range is 0 to 65355 for a CRC16 hash),
3. The number of moves in the same direction, referred to as the
``move count''.
If there is no prior history for a path, then the move increment is
initialized to a constant, typically about 1% (about 650 for CRC16).
The number of moves in the same direction is initialized to 0. No
loading adjustment is made on the first iteration.
If the critcally loaded segment has changed, all paths now containing
the critically loaded segment are first examined. The lowest move
increment of any one of these paths is noted.
The move increment is adjusted for each path before any traffic is
moved. One of the following actions is taken for each path.
1. If the path contains the critcally loaded segment its move incre-
ment is left unchanged.
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2. If the path does not contain the critcally loaded segment but the
critically loaded segment has changed and the path contains the
prior critcally loaded segment, then first the move increment is
replaced with the lowest move increment from any of the paths con-
taining the critically loaded segment unless the move increment is
already lower. Then in either case the move increment is cut in
half.
3. If the path does not contain the critcally loaded segment and ei-
ther the critically loaded segment has not changed, or the path
does not contain the prior critcally loaded segment, then the move
increment is increased.
The amount increase in the move increment is described in Ap-
pendix B.4. The increase is designed to minimize the possibility
of dramatic overshoot due to to great an increase in adjustment rate.
The move increment is never less than a configured minimum. The in-
crease in move increment is never less than one but generally is con-
strained to a higher number by virtue of being calculated based on the
prior move increment. The configured minimum for the move increment
is typically 0.1% (65 for CRC16). The move increment is never allowed
to exceed the size of the hash space divided by the number of equal
cost paths in the next hop structure.
The dramatic decrease in move increment when move direction is re-
versed and the slow increase in move increment when it remains in the
same direction keeps the algorithm stable. The exponential nature of
the increase allows the algorithm to track externally caused changes
in traffic loading.
The traffic share allocated to a path not containing the critcally
loaded segment is incremented by the move amount for that path and the
traffic share allocated to the path or paths containing the the crit-
cally loaded segment are reduced by this amount divided by the number
of paths containing the critcally loaded segment. This adjustment is
described in pseudocode in Appendix B.4.
This adjustment process is repeated for each path in a next hop struc-
ture. The new hash space boundaries are then moved to the forwarding
engine.
4.2 Dealing with Link Adjacency Changes
Link failures do occur for various reasons. OSPF routing will con-
verge to a new set of paths. Whatever load balance had previously
existed will be upset and the load balancing will have to converge to
a new load balanced state. Previous load balancing parameter should
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remain intact to the extent possible after the SPF calculation has
completed. Adjustments for new or deleted paths in the SPF result are
described here. These adjustments must be made after the best path
criteria is relaxed as described in Section 3.1.
4.2.1 Impact of Link Adjacency Changes
Links which are intermitent may be the most harmful. The OSPF
``Hello'' protocol is inadequate for handling intermitent links. When
such a link is up it may draw traffic during periods of high loss,
even brief periods of complete loss.
The inadequacies of the OSPF ``Hello'' protocol is well known and many
implementations provide lower level protocol state information to OSPF
to indicate a link in the ``down'' state. For example, indications
may include carrier loss, excessive framing errors, unavailable sec-
onds, or loss indications from PPP LQM.
Even where the use of a link is avoided by providing indication of
lower level link availability, intermitent links are still problem-
atic. During a brief period immediately after a link state attribute
is initially flooded OSPF state can be inconsistent among routers
within the OSPF area. This inconsistency can cause intermittent rout-
ing loops and have a severe short term impact on link loading. An
oscillating link can cause high levels of loss and is generally better
off held in the neighbor adjacency ``down'' state. The algorithm de-
scribed in the [7] can be used when advertising OSPF type 1 or type 2
LSA (router and network LSAs).
Regardless as to whether router and network LSAs are damped, neigh-
bor adjacency state changes will occur and router and network LSAs
will have to be handled. The LSA may indicate an up transition or
a down transition. In either an up or down transition, when the SPF
algorithm is applied, existing paths to specific destinations may no
longer be usable and new paths may become usable. In the case of an
up transition, some paths may no longer be usable because their cost
is no longer among those tied for the best. In the case of down tran-
sitions, new paths may become usable because they are now the best
path still available.
4.2.2 Handling the Loss of Paths
When a path becomes unusable, paths which previously had the same
cost may remain. This can only occur on an LSA down transition.
A new next hop entry should be created in which the proportion of
source/destination hash space allocated to the now infeasible path
is distributed to the remaining paths proportionally to their prior
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allocation. Very high loading percentages should result, trigger-
ing an increase in LSA_OMP_LINK_LOAD Opaque LSA flooding rate until
convergence is approached.
4.2.3 Handling the Addition of Paths
When a new path becomes usable it may be tied for best with paths car-
rying existing traffic. This can only occur on an LSA up transition.
A new next hop entry should be created in which the loading on the
new path is zero. If such a path were to oscillate, little or no load
would be affected. If the path remains usable, the shift of load to
this path will accellerate until a balance is reached.
If a completely new set of best paths becomes available, the load
should be split across the available paths. The split used in sim-
ulations was a share on a given link proportional to 10% of link
capacity plus the remaining link bandwidth as determined by prior
LSA_OMP_LINK_LOAD Opaque LSA values. The contribution of link capacity
in the weighting should be configurable. See Appendix C.5.
Acknowledgements
Numerous individual have provided valuable comments regarding this
work. Dave Ward made a very substantial contribution by pointing out
that the best path criteria could be relaxed. Geoffrey Cristallo pro-
vided comments on the handling of inter-area and external routes with
worked examples which resulted in corrections and clarifications to
this document. John Scudder, Tony Li, and Daniel Awduche have also
provided particularly valuable review and comments.
References
[1] R. Coltun. The ospf opaque lsa option. Technical Report RFC 2370,
Internet Engineering Task Force, 1998. ftp://ftp.isi.edu/in-
notes/rfc2370.txt.
[2] Atul Khanna and John Zinky. The revised ARPAnet routing met-
ric. In SIGCOMM Symposium on Communications Architectures and
Protocols, pages 45--56, Austin, Texas, September 1989. ACM.
[3] Steven H. Low and P. Varaiya. Dynamic behavior of a class of
adaptive routing protocols (IGRP). In Proceedings of the Confer-
ence on Computer Communications (IEEE Infocom), pages 610--616,
March/April 1993.
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INTERNET-DRAFT OSPF Optimized Multipath (OSPF-OMP) February 24, 1999
[4] M. Mathis, J. Semke, J. Mahdavi, and T. Ott. The macroscopic be-
havior of the TCP congestion avoidance algorithm. ACM Computer
Communication Review, 27(3), July 1997.
[5] J. Moy. Ospf version 2. Technical Report RFC 2328, Internet Engi-
neering Task Force, 1998. ftp://ftp.isi.edu/in-notes/rfc2328.txt.
[6] W. Stevens. Tcp slow start, congestion avoidance, fast retrans-
mit, and fast recovery algorithms. Technical Report RFC 2001,
Internet Engineering Task Force, 1997. ftp://ftp.isi.edu/in-
notes/rfc2001.txt.
[7] C. Villamizar, R. Chandra, and R. Govindan. Bgp route flap damp-
ing. Technical Report RFC 2439, Internet Engineering Task Force,
1998. ftp://ftp.isi.edu/in-notes/rfc2439.txt.
Security Considerations
The use of the Opaque LSAs described in this document do no impact
the security of OSPF deployments. In deployments which use a strong
OSPF authentication method, and require signatures on LSA from the
originating router, no leveraging of a partial compromise beyond a
localized disruption of service is possible. In deployments which
use a strong OSPF authentication method, but do not require signatures
on LSA from the originating router, compromise of a single router can
be leveraged to cause significant disruption of service with or with-
out the use of these Opaque LSA, but disruption of service cannot be
achieved without such a compromise. In deployments which use a weak
OSPF authentication method, significant disruption of service can be
caused through forged protocol interactions with or without the use of
these Opaque LSA.
Author's Addresses
Curtis Villamizar
UUNET Network Architecture Group
<curtis@uu.net>
Full Copyright Statement
Copyright (C) The Internet Society (February 24, 1999). All Rights
Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
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or assist in its implmentation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are in-
cluded on all such copies and derivative works. However, this doc-
ument itself may not be modified in any way, such as by removing the
copyright notice or references to the Internet Society or other In-
ternet organizations, except as needed for the purpose of developing
Internet standards in which case the procedures for copyrights defined
in the Internet Standards process must be followed, or as required to
translate it into languages other than English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
``AS IS'' basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEER-
ING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUD-
ING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MER-
CHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
A Data Formats
+----------------+----------------+----------------+----------------+
| Link State Advertisement Age | Options | LSA Type |
+----------------+----------------+----------------+----------------+
| Opaque Type | Opaque ID |
+----------------+----------------+----------------+----------------+
| Advertising Router |
+----------------+----------------+----------------+----------------+
| Link State Advertisement Sequence Number |
+----------------+----------------+----------------+----------------+
| LSA Checksum | LSA length |
+----------------+----------------+----------------+----------------+
| Version | Reference Type | Packing Method | BW Scale |
+----------------+----------------+----------------+----------------+
| Reference to a Type 1-5 LSA (32 or 64 bits, see below) |
+----------------+----------------+----------------+----------------+
| Packed Loading Information (variable length, see below) |
+----------------+----------------+----------------+----------------+
The ``LSA Age'', ``Options'', and ``LSA Type'' are part of the Link
State Advertisement Format described in Appendix A of RFC--2328. The
LSA Type is 10, signifying an Opaque LSA with Area local scope, as de-
fined in RFC--2370. RFC--2370 splits the Link State ID field into two
part, Opaque Type, and Opaque ID. The Opaque Type contains either the
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value LSA_OMP_LINK_LOAD or LSA_OMP_PATH_LOAD as described in Section 2.
The ``Advertising Router'', ``Link State Advertisement Sequence Num-
ber'', ``LSA Checksum'', and ``LSA length'' are part of the Link State
Advertisement Format described in Appendix A of RFC--2328. The re-
mainder of the packet is specific to Opaque Type LSA_OMP_LINK_LOAD or
LSA_OMP_PATH_LOAD LSAs.
Opaque Type The Opaque Type will contain the value LSA_OMP_LINK_LOAD
or LSA_OMP_PATH_LOAD as described in Section 2. Numeric values will
be requested from IANA.
Opaque ID The Opaque ID will contain an integer which will be unique
per router and interface, virtual interface, or MAC address for
which loading is reported. These numbers are only of significance
to the advertising router except as a means of identification of
subsequent LSAs.
For a LSA_OMP_LINK_LOAD Opaque LSA, the ``Opaque ID'' must contain
a 24 bit integer that is unique to the link or virtual link. The
method of assignment of these 24 bit integers is a local matter. A
router must be capable of uniquely identify an interface using a 24
bit number.
For a LSA_OMP_PATH_LOAD Opaque LSA, the ``Opaque ID'' must contain
a 24 bit integer that is unique to a summary LSA or AS-external LSA
advertised by the same router. The method of assignment of these
24 bit integers is a local matter. A router must be capable of
uniquely identify a summary LSA or AS-external LSA using a 24 bit
number.
Version The version number is 1.
Reference Type The Reference Type indicates the type of LSA that is
being referenced in the ``Reference to a Type 1-5 LSA'' field.
Packing Method The Packing Method is an integer that indicates how
the ``Packed Loading Information'' field is formated.
BW Scale Bandwidth numbers must be scale by shifting the 32 bit in-
teger left by this amount. If this value is non-zero, 64 bit inte-
gers or larger should be used to represent the bandwidth.
Reference to a Type 1-5 LSA This field contains the 32 bit ``Link
State ID'' field of a Type 1-5 LSA. Type 1-5 indicate:
1. Router-LSAs
2. Network-LSAs
3. Summary-LSAs (IP network)
4. Summary-LSAs (ASBR)
5. AS-external-LSAs
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Loading information for a Type 1 LSA, a Router-LSA, is sent as a
LSA_OMP_LINK_LOAD Opaque LSA. For a Type 1 LSA the ``Link State ID''
field is followed by a 32 bit ``Link ID''. This identifies a single
link. There are four types of Links.
1. Point-to-point connection to another router
2. Connection to a transit network
3. Connection to a stub network
4. Virtual link
Normally loading information is provided for a Link Type 1. Load-
ing information may also be provided for a Link Type 2 or 3. Load-
ing information cannot be provided to a Link Type 4.
Loading information is not provided for Type 2 LSAs, Network-LSAs.
Loading information for Type 3 and Type 4 LSAs, Summary-LSAs for
IP networks in another area and Summary-LSAs for ASBRs, is sent as
a LSA_OMP_PATH_LOAD Opaque LSA as described in Section 2.2. For
a Type 3 and Type 4 LSA there is no information in the Reference
following the ``Link State ID''.
Loading information for Type 5 LSAs, AS-external-LSAs, is sent as
a LSA_OMP_PATH_LOAD Opaque LSA as described in Section 2.2. For a
Type 5 LSA there is no information in the Reference following the
``Link State ID''.
Packed Loading Information The format of the Packed Loading Informa-
tion depends on the value of the Packing Method field. Currently
only the value 1 is defined.
The following format is used when the Packing Method field contains 1.
The LSA must be ignored if values other than 1 are found in Packing
Method.
+----------------+----------------+----------------+----------------+
| In Scaled Link Capacity in kilobits per second |
+----------------+----------------+----------------+----------------+
| In Link Loading Fraction | In Link Drop Fraction (packets) |
+----------------+----------------+----------------+----------------+
| Out Scaled Link Capacity in kilobits per second |
+----------------+----------------+----------------+----------------+
| Out Link Loading Fraction | Out Link Drop Fraction (packet) |
+----------------+----------------+----------------+----------------+
The Scaled Link Capacity is an unsigned integer in kilobits per sec-
ond. If this would be larger than a 32 bit integer, the value should
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be shifted to the right until the top bit is in the 32 bit number MSB
and the number of bit shifts recorded in the BW Scale field
The Link Loading Fraction is a 16 bit unsigned integer from 0 to
0xffff representing capacity in bytes being used relative to capac-
ity in bytes per the measurement interval. The hex number 0x10000
would represent unity, but if full loading has been acheived or due
to counting or truncation error, greater than full loading, substitute
0xffff. The Link Drop Fraction is a 16 bit unsigned integer from 0 to
0xffff representing number of packets dropped relative to the number
of packets received. This value can be derived from the change in two
MIB-2 counters (ifOutDiscard<<16)/ifInPacket. The hex number 0x10000
would represent unity (all of the packets being dropped) so 0xffff
must be substituted.
B Concise Statement of the Algorithms
An OSPF router may play one of two roles, or both. The two functions
are flooding loading information and load balancing. An interior
routers and edge routers will flood loading information. A router
may choose not to flood information if it is beleived that there is no
way that the interface could become congested or if it has no way to
measure the load, as is the case in a shared broadcast interface. An
ingress or interior router will process loading information and if it
has equal cost paths will balance load across those paths.
The description of algorithms is broken down into the following sub-
sections.
Flooding Loading Information Appendix B.1
Building Next Hop Structures Appendix B.2
Processing Loading Information Appendix B.3
Adjusting Loading Appendix B.4
The algorithms are defined in the following section in pseudocode.
B.1 Flooding Loading Information
It is assumed that counters are large enough to avoid multiple over-
flow (ie: 64 bit counters are used for high speed interfaces) and
that counter overflow is easily detected is compensated for in counter
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deltas. It is assumed that ifInDiscard and ifOutDiscard accurately
counts all queueing drops.
The following counters are sampled at 15 second intervals:
ifInOctets, ifOutOctets, ifInPacket, ifOutPacket, ifInDiscard and
ifOutDiscard. The value if ifInSpeed and ifOutSpeed is assumed to
be constant. Some state must be stored. The previously used value
of each raw counter is needed to compute deltas. State variables
InFilteredUtil, OutFilteredUtil, InLoss, OutLoss, InEquivLoad and Out-
EquivLoad must be saved. The last time a reflooding occurred must
also be stored.
The input and output utilizations are expressed as fractions using
ifInOctets, ifOutOctets, ifInSpeed, and ifOutSpeed. Call the raw 15
second fractional utilizations InRawUtil and OutRawUtil. Compute the
following filtered values for both In and Out, making sure to save the
previous values.
PrevFilteredUtil = FilteredUtil;
if (RawUtil < FilteredUtil) {
FilteredUtil -= (FilteredUtil >> 3);
FilteredUtil += (RawUtil >> 3);
} else if (RawUtil > FilteredUtil) {
FilteredUtil -= (FilteredUtil >> 1);
FilteredUtil += (RawUtil >> 1);
}
InLoss and OutLoss is computed using the ratio of the deltas of Dis-
card to Packet SNMP counters. Next compute an ``equivalent loading''
for the purpose of determining whether to reflood.
PrevEquivLoad = EquivLoad;
if (Loss < 0.005) {
EquivLoad = FilteredUtil;
} else {
if (Loss <= 0.09) {
LossComp = 10 * sqrt(Loss);
} else {
LossComp = 3;
}
EquivLoad = FilteredUtil * LossComp;
}
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A square root is somewhat time consuming to compute, so a table lookup
can be done to avoid this computation. Increments of 0.1% loss would
yield an 90 entry table. A 128-512 entry table would be adequate.
The table can be sized so a shift and mask can be used to index it.
The computation could then be done with a table lookup, a shift, and
an integer multiply. At most this needs to be done for links with
nonzero loss once every 15 seconds.
Then decide whether to flood based on the greater of the relative
change in InEquivLoad or OutEquivLoad and on the time elapsed since
the last reflooding (taking care not to divide by zero).
Diff = max(abs(InEquivLoad - InPrevEquivLoad)
/ InPrevEquivLoad,
abs(OutEquivLoad - OutPrevEquivLoad)
/ OutPrevEquivLoad);
Load = max(InEquivLoad, OutEquivLoad)
Elapsed = Now - LastReflood;
if (((Load > 1.00) && (Diff > 0.05) && (Elapsed >= 30))
|| ((Load > 1.00) && (Diff > 0.02) && (Elapsed >= 60))
|| ((Load > 1.00) && (Diff > 0.01) && (Elapsed >= 90))
|| ((Load > 1.00) && (Elapsed >= 180))
|| ((Load > 0.90) && (Diff > 0.05) && (Elapsed >= 60))
|| ((Load > 0.90) && (Diff > 0.02) && (Elapsed >= 240))
|| ((Load > 0.90) && (Diff > 0.01) && (Elapsed >= 480))
|| ((Load > 0.90) && (Elapsed >= 600))
|| ((Load > 0.70) && (Diff > 0.10) && (Elapsed >= 60))
|| ((Load > 0.70) && (Diff > 0.05) && (Elapsed >= 120))
|| ((Load > 0.70) && (Diff > 0.02) && (Elapsed >= 480))
|| ((Load > 0.70) && (Elapsed >= 900))
|| ((Load > 0.50) && (Diff > 0.10) && (Elapsed >= 60))
|| ((Load > 0.50) && (Diff > 0.05) && (Elapsed >= 300))
|| ((Load > 0.25) && (Diff > 0.25) && (Elapsed >= 120))
|| ((Load > 0.25) && (Elapsed >= 1200))
) {
/* we passed one of the tests so flood it */
LastReflood = Now;
...
If the decision is made to reflood an LSA according to the test above,
then input and output FilteredUtil and Loss must be packed into an LSA
and flooded.
The 15second timer interval should be jittered by a random number in
the range of plus or minus 5 seconds (obviously taking the actual time
interval into account in computing rates).
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B.2 Building Next Hop Structures
The OSPF SPF calculation is done as per RFC--2328. Minor differ-
ences in the outcome result from relaxing the best path criteria as
described in Section 3.1. Modification to the SPF algorithm is de-
scribed in Appendix D. The arrival of loading information does not
require new SPF calculations since neither reacheability or costs are
changed.
The SPF calculation yields the shortest paths from the given node
to all other routers and networks in the OSPF area. In some cases
multipaths will already exist. For all destinations, every feasible
hop is examined, and paths through next hops that simply provide an
improvement are added, as described in Section 3.1.
After completing the SPF calculation and relaxing the best path cri-
teria, intra-area multipath sets are recorded as next hop structures.
If a previous SPF had been in use, loadings are transfered to the new
set of data structures and links are added or removed as needed as
described in Section 4.2.
After recording the intra-area next hop structures, the existing set
of inter-area next hop structures and the set of external route next
hop structures is examined. Paths are added or removed from next
hop structures as needed, as described in Section 3, Section 3.3, and
Section 4.2.
Inter-area and external routes map onto the intra-area routing. These
therefore share the same set of paths and the same next hop structure
as the intra-area route to the nearest ABR or ASBR. Next hop struc-
tures may be needed to reach any one in a set of ABRs or ASBRs if 1)
there are multiple ABRs equally distant to a summary route or 2) mul-
tiple ASBRs equally distant advertising an external route at the same
cost, 3) relaxing the best path criteria for an intra-area destination
results in going to a next-hop which does not share the same closest
ABR or ASBR.
Next hop structures may also be needed to offload paths in adjacent
areas or external paths. The following conditional is used to deter-
mine whether a next hop structure should be added for a SummaryLSA.
if (IntraAreaLoad < 85%
&& SummaryLSALoad > 90%
&& SummaryLSALoad - IntraAreaLoad > 15%) {
/* add a next hop structure */
...
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The conditional for an external route is the same, except the intra-
area load would be a more internal load, intra-area, or Summary LSA,
and the 90% threshhold is increased to 95%.
The following conditional is used to determine is an existing sepa-
rate next hop structure for a Summary LSA or external route should be
deleted.
if (MoreInternalLoad > 98%
|| MoreInternalLoad - MoreExternalLoad > 20%) {
/* delete a next hop structure */
...
B.3 Processing Loading Information
Adjustments to loading may be triggerred by one of two events. When
a new loading LSA is received, if the LSA corresponds to the most
heavily loaded link for a next hop structure, then the next hop struc-
ture should be readjusted immediately. The last time each next hop
structure has been readjusted must be maintained and periodically
readjusted. Timer events are handled as follows.
foreach NextHopStruct ( AllNextHopStructs ) {
Elapsed = Now - LastReadjust[NextHopStruct];
MinLoaded = MinEquivLoad(NextHopStruct);
MaxLoaded = MaxEquivLoad(NextHopStruct);
AbsDiff = MaxLoaded - MinLoaded;
if (((Elapsed >= 60)
&& (AbsDiff > 0.045) && (MaxLoaded > 0.95))
|| ((Elapsed >= 90)
&& (AbsDiff > 0.03) && (MaxLoaded > 0.95))
|| ((Elapsed >= 120)
&& (AbsDiff > 0.01) && (MaxLoaded > 0.97))
|| ((Elapsed >= 240)
&& (AbsDiff > 0.005) && (MaxLoaded > 0.98))
|| ((Elapsed >= 90)
&& (AbsDiff > 0.05) && (MaxLoaded > 0.90))
|| ((Elapsed >= 120)
&& (AbsDiff > 0.03) && (MaxLoaded > 0.90))
|| ((Elapsed >= 180)
&& (AbsDiff > 0.01) && (MaxLoaded > 0.90))
|| (Elapsed >= 300)) {
/* we need to readjust this multipath set */
...
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This loop and conditional results in a subset of the available next
hop structures being adjusted based on the timer. The same effect may
be accomplished by determining when a next hop structure will need to
be adjusted if no further flooding changes arrive and queueing next
hop structures on lists according to how long they will remain idle.
B.4 Adjusting Loading
A next hop structure will need to be adjusted when one of the two cri-
teria in the prior section is met. The adjustment procedure itslef
relies upon the following stored state.
NextHopStruct {
LastReadjust;
PrevCriticalSegment;
TotalPaths;
SetofPaths (
Path;
bit HasCriticalSegment,
HasPrevCriticalSeg;
TrafficShare;
MoveIncrement;
MoveCount;
);
};
Before the path move increments are adjusted, a preliminary pass is
made over the next hop structure. This pass notes which paths con-
tain the prior critical segment, notes which paths contain the current
critical segment and counts the number of paths containing the current
critical segment.
NumberWithCritical = 0;
MinRateWithCritical = 65536;
foreach Path ( SetofPaths ) {
SetOrClear HasCriticalSegment;
SetOrClear HasPrevCriticalSeg;
if (HasCriticalSegment) {
++NumberWithCritical;
if (MoveIncrement < MinRateWithCritical)
MinRateWithCritical = MoveIncrement;
}
}
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Next the move increments for each path is adjusted.
foreach Path ( SetofPaths ) {
if (HasCriticalSegment)
continue;
if (!HasPrevCriticalSeg) {
++MoveCount;
if (MoveCount > 4) {
Increase = MoveIncrement
/ (2 * (1 + NumberWithCritical));
} else {
Increase = MoveIncrement
/ (4 * (1 + NumberWithCritical));
}
if (Increase < 1)
Increase = 1;
MoveIncrement += Increase;
} else {
if (MoveIncrement > MinRateWithCritical)
MoveIncrement = MinRateWithCritical;
MoveIncrement /= 2;
MoveCount = 0;
}
if (MoveIncrement < MinMoveIncrement)
MoveIncrement = MinMoveIncrement;
if (MoveIncrement > 65535)
MoveIncrement = 65535;
}
Then traffic share is adjusted.
foreach Path1 ( SetofPaths ) {
if (!Path1.HasCriticalSegment)
continue;
foreach Path2 ( SetofPaths ) {
if (Path2.HasCriticalSegment)
continue;
Move = Path2.MoveIncrement / NumberWithCritical;
if (Move < 1)
Move = 1;
if (Move > (65536 - Path2.TrafficShare)) {
Move = 65536 - Path2.TrafficShare;
Path2.MoveIncrement = Move;
}
if (Move > Path1.TrafficShare)
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Move = Path1.TrafficShare;
Path2.TrafficShare += Move;
Path1.TrafficShare -= Move;
}
}
The traffic shares for paths sharing a common next hop are then summed
and the appropriate information is transferred to the forwarding data
structures.
C Configuration Options
Many of the capabilities described here must be configurable. The
ability to enable and disable capability subsets is needed. Many
parameters used by the algorithm should also be configurable.
C.1 Capability Subsets
There should at least be the ability to enabled and disabled the fol-
lowing.
default description of capability
ON Flooding any loading information
ON Flooding loading information for specific links
- Relaxing best path criteria
- Adjusting traffic shares (default to even split)
OFF Flooding loading information for Summary LSA
OFF Flooding loading information for specific Summary LSA
OFF Flooding loading information for external routes
OFF Flooding loading information for specific external routes
OFF Considering loading information for Summary LSA
OFF Considering loading information for specific Summary LSA
OFF Considering loading information for external routes
OFF Considering loading information for specific exter-
nal routes
Flooding and considering Summary LSA and external route loading in-
formation should be disabled by default. Flooding loading information
should be enabled by default. Relaxing best path criteria and adjust-
ing traffic shares could be enabled or disabled by default, depending
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on vendor preference.
C.2 Parameters for Equivalent Load Calculation
The following values affect the computation of equivalent load.
default description of parameter
10 The value of K in ``equiv-load = load * K * sqrt(loss)''
0.5% The minimum loss rate at which to compensate for loss
9% The maximum loss rate above which compensation is fixed
C.3 Parameters for Relaxing the Best Path Criteria
The following parameter affects the number of next hops and paths
added as a result of relaxing the best path criteria. For example,
increasing the mtric difference to 2 would require the next hop to be
a metric of ``2'' closer than the current distance to the destination,
and reduce the number of paths added.
default description of parameter
1 The metric difference required to relaxing best path
C.4 Parameters for Loading Outside of the OSPF Area
The following parameters affect the creation of separate next hop
structures to compensate for loading on Summary LSA and external
routes when the those loadings are high and intra-AS loadings are
substantially lower.
default description of parameter
15% The loading difference to consider a more external load
over a more internal load
85% The maximum internal loading where a more external load
will become eligible for consideration
90% The minimum loading in which a Summary LSA will be
considered over a an intra-area loading
95% The minimum loading in which an external route will be
considered over a an intra-area loading
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20% The load difference at which an external load will be
removed from consideration due to being well under the
internal load.
94% The maximum value used for in place of loading for a
Summary LSA when performing traffic share adjustment.
98% The internal loading where a Summary LSA will be
removed from consideration over the internal load
90% The maximum value used for in place of loading for a
external route when performing traffic share adjustment.
98% The internal loading where a external route will be
removed from consideration over the internal load
Limiting the compensation that will be made to accommodate external
loading is consistent with the reason for considering external routes.
Rarely does a business go out of its way to improve the performance of
their competitor's product, a network service or otherwise. Avoiding
congestion in a peer's network is doing a favor for one's own cus-
tomers by not sending their traffic into known areas of congestion
but only if it does not significantly impact congestion in one's own
network.
Limiting the compensation for Summary LSA loading and external route
loading avoids triggering the hysteresis criteria where a separate
next hop structure is deleted if an internal loading exceeds a fixed
threshhold. In effect the loading on the Summary LSA loading and ex-
ternal route loading is ignored if internal loadings exceed a given
threshhold, since the Summary LSA loading or external route loading
will no longer be considered as the critical segment. If internal
loading reaches a point where even with load balancing internal paths
exceed the higher threshhold, the next hop structure will be removed.
C.5 Parameters for Initial Loading of Paths
When determining the initial loading on a new set of paths, where the
destination was previously unreachable, or none of the previous paths
appear in the new next hop structure, the following weighting is used.
weight = (link-capacity * 0.10)
+ (link-capacity * (1 - utilization))
The contribution of link capacity in the weighting should be config-
urable.
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default description of parameter
10% The fraction of total link capacity to consider in
addition to the reported unused link capacity.
C.6 Parameters associated with Flooding and Traffic Share
Parameters associated with flooding rate, move increment and traffic
share adjustment should not be configurable by the user or should be
well hidden so as only to be accessible to developers. Adjustment of
these parameters can slow convergence or increase overshoot. If the
parameters are sufficiently altered instability could result. While
it is likely that some improvements could result from further tuning,
experimentation on live networks is not encouraged.
D Modified SPF Calculation
The most common implementation of the SPF calculation is Dikstra's
algorithm. Most implementations do not yield the full path as a
consequence of the SPF calculation. Retaining the full path as the
algorithm procedes is a relatively minor modification.
If the best path criteria is relaxed, the information obtained from
a single Dikstra calculation is insufficient. Dikstra's algorithm
provides a very efficient single-source shortest path calculation.
For the relaxed best path criteria, the cost to any destination from
any immediately adjacent node is needed in addition to the set of best
paths and costs from the current node.
It is beleived to be more efficient to compute an SPF using Dikstra's
algorithm from the standpoint of each adjacent node in addition to an
SPF from the current node than it is to use an algorithm to compute
the costs from any node to any other node. The former runs in order
N squared while algorithms to accomplish the latter runs in order N
cubed, where N is the number of nodes in the graph. The amount of
computation would be expected to be about equal in the case where all
nodes are immediately adjacent to the current node.
There is likely to be more efficient methods of computing the costs
from a subset of nodes to all destinations than either using multiple
Dikstra calculations or computing the costs from all nodes to all oth-
ers and only making use of a subset of the results. This efficiency
consideration is left as an exercise for the implementor.
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E Algorithm Performance
A number of simulations have been performed to test the OSPF-OMP algo-
rithms. In these simulations the algorithm has been demonstrated to
be very stable. This work has not been formally published yet but is
currently available at http://engr.ans.net/ospf-omp.
The simulations done to date have only modeled behavior of load bal-
ancing intra-area routes. This would be applicable to networks in
which external routing was mapped onto IGP routing with a single OSPF
area. Passing loading information between areas, allowing loading in
one area affect an adjacent area, has not been simulated. Similarly
passing loading information with external routes and affecting loading
in a peer AS has not been simulated.
E.1 Conversion from Loss to Equivalent Load
The current adjustment for loss in the equivalent load is based on
the premise that traffic is dominated by TCP and that TCP flows suffi-
ciently unconstrained by other bottlenecks and of sufficient duration
exist to keep the aggrgate traffic flow elastic. This is considered
a very safe assumption for Internet traffic. Enterprise networks
may have a higher contribution of incompressible traffic (traffic not
conforming to a form of congestion avoidance such as TCP congestion
avoidance described in RFC--2001).
The assumed relationship between packet loss and link utilization is
based on the work of Mathis et al [4]. The constants in this rela-
tionship cannot be determined as they depend on delay bandwith product
of TCP flows, the number and duration of TCP flows, and whether TCP
flows are severely constrained elsewhere.
The objective is to estimate the offered load, which cannot be mea-
sured directly when links are at full utilization, using the link
utilization and loss. The load adjustment algorithm should remain
stable as long as the first derivative of the estimator over offered
load remains positive. If the first derivative is negative within
some region, then oscillation will occur within that range of opera-
tion. The greatest risk of this occurring is in routers where active
queue management is not used (ie: where drop-tail queues are used)
and in particular where buffering is too small. In such cases, as
offered load increases just beyond full utilization, loss increases
somewhat, but utilization can drop substantially (typically to about
90%) as offered load increases. In this region, as the offered load
increases, the estimator of offered load may decrease, causing the
link to appear less loaded than another. The rather aggresive com-
pensation for loss is intended to insure that this effect either does
not occur, or occurs only within a very narrow range of offerred load
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at just over full utiization. If the derivative is negative within a
narrow range, oscillations can occur only within that range, and the
oscillations are well bounded.
E.2 Performance as traffic loads change
This work has considerable advantages over other approaches, particu-
larly traffic engineering approaches that involve adjustment of vir-
tual circuit layouts based on historic traffic figures. The advantage
is the ability to adjust loading gradually as actual offerred traffic
deviates for expected. The advantage is even greater when compared to
dynamic virtual circuit layouts, using PNNI for example, since these
algorithms have proven to often converge on very suboptimal layouts.
Simulations demonstrating behavior in these particular cases can be
found at http://engr.ans.net/ospf-omp/ramp.html.
E.3 Convergence after a major perturbation
Simulations have been performed in which link failures are examined.
Without relaxing the best path criteria, OSPF OMP is quite dependant
of the set of link metrics to create a set of equal cost paths that
will allow the most heavily loaded portions of the topology to be
offloaded. When links fail, the set of metrics often are far from
ideal for the remaining topology. Relaxing the best path criteria
significantly improves the response to link failure.
Simulations are available at http://engr.ans.net/ospf-omp/fail.html.
F Examples
Figure 2 provides a simple topology. For the purpose of illustrating
how OMP works, only the traffic flow from left to right between a few
pair of dominant traffic contributors is considered.
The traffic mapped onto the topology in Figure 2 is dominated by the
ingress nodes A and F and the egress nodes E and G. The capacity of
the links are 1 excpt link E-G which has a capacity of 2. The load
contributed by the ingress-egress pairs A-E, F-G, and F-E are 0.5.
The node pair A-G contributes a load of 1. Link costs are all 2,
except F-D which is 3, and F-G which is 6.
If ECMP were used, all the traffic from F to E would take the path
F-D-E. All the traffic from F to G would take the link F-G. All the
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cost=2 cost=2 cost=2 cost=2
A ----------> B ----------> C ----------> E ----------> G
| load=1.5 | load=0.5 load=0.5 ^ load<25% ^
| | | |
| | cost=2 cost=2 | |
| \-----------> D ------------/ |
| load=0.5 ^ load=1.0 |
| | |
| cost=3 | load=0.5 |
| cost=2 | cost=6 |
\-------------------------> F --------------------------/
load=0.5 load=1.0
Figure 2: A Simple Example
traffic from A to E would take the hop from A to B and then at B it
would be split evenly between the paths B-C-E and B-D-E. Half of the
traffic from A to G would take the hop A-B and half would take the hop
A-G.
ingress-egress load and path
F-E 0.5 0.50 F-D-E
F-G 0.5 0.50 F-G
A-E 0.5 0.25 A-B-C-E
0.25 A-B-D-E
A-G 1.0 0.33 A-B-C-E-G
0.33 A-B-D-E-G
0.33 A-F-G
link loading status
A-B 1.16 overloaded
A-F 0.33 underutilized
B-C 0.58 underutilized
B-D 0.58 underutilized
C-E 0.58 underutilized
D-E 1.08 overloaded
E-G 0.66 underutilized
F-D 0.50 underutilized
F-G 0.83 near capacity
Above is the initial loading for OMP which differs slightly from ECMP.
In ECMP half the traffic from A to G would take the A-F, where OMP
starts out with one third of the A-G traffic on link A-F.
Note that using OMP the path F-D-E-G with cost 7 is considered close
enough to equal to the path F-G with cost 6. This is because the
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next hop D is closer to G with a cost of 4 than F is with a cost of
6. Initially node F would not move load over because link D-E at a
loading of 1.08 is in worse shape than node F-G at a loading of 0.83.
For illustrative purposes three opportunities for moving load are
considered separately. These are shown in Figure 3, Figure 4, and
Figure 5.
cost=2 cost=2 cost=2 cost=2
A ----------> B ----------> C ----------> E ----------> G
| load=1.25 | load=.75 load=.75 ^ load<25% ^
| | | |
| | cost=2 cost=2 | |
| \-----------> D ------------/ |
| load=.25 ^ load=.75 |
| | |
| cost=3 | load=0.5 |
| cost=2 | cost=6 |
\-------------------------> F --------------------------/
load=0.75 load=1.25
Figure 3: First Opportunity for Load Adjustment
Node B has a clear opportunity to reduce the load on the link D-E by
moving traffic from the next hop D to the next hop C. If this opti-
mization were to be applied alone, utilizations on the links B-C,
C-E, and D-E would approach 0.75 and utilization on the link B-D would
approach 0.25. This is illustrated in Figure 3.
Node A can reduce the loss on link A-B by putting more load on link
A-F. This will initially have the effect of lowering A-B to 1.0 and
raising F-G to 1.0. The links can only pass 100would just reduce loss
on link A-B at the expense of increasing loss on link F-G. The load on
link A-F would increase to 0.5.
After node B had moved enough traffic away from link D-E so that its
loading fell below the 1.0 loading of link F-G, node F would begin
to move traffic destined to G away from link F-G. Load would be added
to link D-E but node B would continue to move load away from D-E.
Utilizations of B-C, C-E, and D-E would rise. Utilizations of F-D
would also approach 0.5 and utilization on F-G would fall. This is
illustrated in Figure 4.
Node A would be faced with an overloaded link A-B and a better path to
G of A-F-G, with the worst loading being at link F-G, initially only
slightly over capacity. Both links A-B and F-G would be reporting
100% utilization but link A-B would be expected to report higher loss.
In addition, as the other optimizations proceed, the utilization of
link F-G would approach 100%.
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cost=2 cost=2 cost=2 cost=2
A ----------> B ----------> C ----------> E ----------> G
| load=1.25 | load=.92 load=.92 ^ load<25% ^
| | | |
| | cost=2 cost=2 | |
| \-----------> D ------------/ |
| load=.08 ^ load=.92 |
| | |
| cost=3 | load=0.92 |
| cost=2 | cost=6 |
\-------------------------> F --------------------------/
load=0.75 load=0.92
Figure 4: Second Opportunity for Load Adjustment
cost=2 cost=2 cost=2 cost=2
A ----------> B ----------> C ----------> E ----------> G
| load=1.0 | load=1.0 load=1.0 ^ load=25% ^
| | | |
| | cost=2 cost=2 | |
| \-----------> D ------------/ |
| load=0 ^ load=1.0 |
| | |
| cost=3 | load=1.0 |
| cost=2 | cost=6 |
\-------------------------> F --------------------------/
load=1.0 load=1.0
Figure 5: Another Opportunity for Load Adjustment
Node A would move traffic from the next hop of B to the next hop of
F. Node F will continue to move load from F-G to F-D-E. Node B will
continue to move load from B-D-E to B-C-E. The utilizations of links
A-B, B-C, C-E, D-E, F-D, and F-G will approach 0.83. Utilization of
link A-F will approach 0.63. Utilization of link B-D will approach
zero. This is illustrated in Figure 5.
The following table provides the approximate state of traffic loading
acheived in a brief simulation. Of 6 links that could be balanced at
about 0.83, 3 converged to about 0.85, and three to about 0.82. Note
that the path F-G-E was used to get from F to E in addition to the
lower cost F-D-E.
ingress-egress load and path
F-E 0.5 0.25 F-D-E
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0.25 F-G-E
F-G 0.5 0.25 F-G
0.25 F-D-E-G
A-E 0.5 0.25 A-B-C-E
0.00 A-B-D-E
0.12 A-F-D-E
0.12 A-F-G-E
A-G 1.0 0.60 A-B-C-E-G
0.00 A-B-D-E-G
0.20 A-F-G
0.20 A-F-D-E-G
link loading status
A-B 0.85
A-F 0.65
B-C 0.85
B-D 0.00
C-E 0.85
D-E 0.82
E-G 0.80
F-D 0.82
F-G 0.82
In this example, multiple links are balanced at 82% to 85% utiliza-
tion. Without using OMP it is difficult (it might be impossible using
only ECMP) to avoid applying an offerred load that exceeds link ca-
pacity on parts of the topology. This example is intended to provide
a more advanced tutorial than the trivial three node example in Fig-
ure 1.
This example is among the simulations at http://engr.ans.net/ospf-
omp/tutorial. More complex topologies and traffic patterns have been
simulated and are available at the same URL.
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