One document matched: draft-zhang-qos-ospf-00.txt
Quality of Service Extensions to OSPF
or
Quality Of Service Path First Routing
(QOSPF)
Status Of This Memo
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Abstract
This document describes a series of extensions for OSPF[1] and
MOSPF[2] that can be used to provide Quality of Service (QoS) routing
in conjunction with a resource reservation protocol such as RSVP[4]
or other mechanisms that can notify routing of the QoS needs of a
data flow. Advertisements indicating the resources available and the
resources used are advertised to the OSPF routing domain and paths
are computed based on topology in- formation, link resource
information, and the resource requirements of a particular data flow.
1.0 Introduction
QoS signalling protocols such as RSVP allow the instantiation of
network state to provide a specific service level to a data flow.
RSVP is specifically not a routing protocol but it does have
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interfaces to routing in order to determine the forwarding of its own
state messages.
Existing routing protocols are usually concerned only with topology
information and not network resources such as bandwidth, thus they
all have their limitations in providing integrated services. The
following figure is a simple illustration:
[Figure not in Text Version at this time.]
FIGURE 1. Example Topology
Suppose host H1 is sending data to host H3 at rate R. The routing
protocol in use gives the shortest path as defined by the metrics,
H1-->R1-->R3-->H3. However, even if R1 does not have adequate
resources on its interface to R3 to handle the flow at the rate R,
the route H1-->R2-->R3-->H3 that does have adequate resources
available, is not used because the routing protocol always uses the
shortest path.
One solution is to let the routing protocols consider network
resource information as well as topology information when they
calculate routes. With the OSPF protocol, complete topology
information is used to calculate routes; in QOSPF, network resource
information is added and used to calculate "QoS routes" that can
provide the resources needed for the flow even though the route may
not be strictly the shortest path.
2.0 Protocol Overview
2.1 Network Resource Information
In QOSPF, routers advertise network resource information as well as
topology information. A route for a data flow is calculated based on
topology, network resource information, and QoS requirements (e.g.
the TSpec of the RSVP PATH message) for the flow.
The network resource information includes available link resources on
a router as well as existing link resource reservations on the
router. The resource information is advertised in Link Resource
Advertisements (RES-LSAs) and Resource Reservation Advertisements
(RRAs). Another type of advertisement, Deterministic Area Border
Router Advertisements (DABRA), are needed for inter-area multicast
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QOSPF.
There are a lot of ways to represent network resource information. In
this document, we use Token Bucket parameters, as in the Controlled-
Load Service model[5]. It is expected that resource advertisements
that are related to other service models could be added over time.
The number of RRAs can easily get huge as the number of reserved
flows and network size grows, presenting a scaling issue. A solution
to this problem is addressed by Explicit Routing, discussed in
Section 6.0.
2.2 Route Pinning
Topology and network resource information not only make it possible
to calculate a shortest route that satisfies the required QoS for a
flow, but also makes Route Pinning very easy to achieve. Route
pinning means that an existing route with a reservation will not be
replaced by a better route unless the existing one is no longer
usable because of a topology change directly related to the existing
route.
2.3 Data-driven (Source, Destination) Route Computation
MOSPF uses data-driven (source, destination) routing. In other words,
a route is computed when the first packet for a (source, group) pair
is received. This is in contrast to unicast OSPF which pre-computes
routes based on destination only.
In QOSPF, routing for QoS flows is based on (source, destination),
and routing computations are triggered by external events regardless
of whether the flow is unicast or multicast. The initial trigger for
QoS routing computation comes from a resource reservation protocol
such as an RSVP PATH message.
There are two reasons for (source, destination) routing in unicast QOSPF:
- Resource reservations and RRAs are generally based on (source, destina-
tion);
- When (source, destination) routing is used, flows with the same destina-
tion but different sources can follow different paths when necessary.
Note the (source, destination) routing used in unicast QOSPF does not
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mean that the distribution tree must be rooted at the source. It only
means that the routing table lookup is based upon (source,
destination) rather than just the destination.
3.0 Resource Advertisements
Available and reserved network resources are advertised via Link
Resource Advertisements (RES-LSAs) and Resource Reserved
Advertisements (RRAs), respectively.
3.1 Link Resource Advertisement (RES-LSA)
A RES-LSA is very similar to a Router-LSA. The purpose of the RES-LSA
is to advertise the link resources available for each router in the
network. When calculating QoS routes, RES-LSAs are used instead of
Router-LSAs.
Each QOSPF router originates a RES-LSA for each area, listing the
largest amount of available resources for reservation on each of the
router's interfaces in the area, along with the link's delay metric.
This metric is roughly analogous to the standard OSPF cost metric but
is independent of the standard TOS metric to better characterize the
static delay properties of a link.
A new instance of RES-LSA is originated whenever a new Router-LSA
instance is originated for the area, or whenever the available
bandwidth resource or delay changes for a link in the area. An
algorithm may be used so that a new RES-LSA is originated only when
the available bandwidth resource changes significantly.
Like Router-LSAs, RES-LSAs are flooded throughout a single area.
The format of RES-LSAs is shown in Figure 2
[Figure not in Text Version at this time.]
FIGURE 2. Resource LSA
The RES-LSA header is the same as all other LSA headers.
The "rtype" field is the same as that in a Router LSA.
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The "number of links" is the count of links included in the LSA. For
each link the link type, link ID and link data are the same as for
the Router LSA.
The available link resource is represented by token bucket
parameters, in IEEE single precision floating point format, as in the
Controlled-Load Service model[5].
The link delay is a static delay metric for the link, in units of
milliseconds.
3.2 Resource Reservation Advertisement (RRA)
A Resource Reservation Advertisement describes a router's
reservations for a particular flow (source, destination) on its
interfaces within an area. The purpose of the RRA is to indicate the
resources used by a flow such that other routers are aware of the
resources and path used by the flow when calculate or recalculate the
tree for the flow. A new RRA is originated whenever one or more of
the reservations change.
Like RES-LSAs, RRAs are flooded throughout a single area.
The format of RRAs is shown in Figure 3.
[Figure not in Text Version at this time.]
FIGURE 3. Resource Reservation Advertisement with its Opaque LSA header
RRAs are encapsulated in Opaque LSAs with type = 11. The Opaque ID is
chosen by the advertising router and the flooding scope is "area-
local".
In RRAs, the Source is the IP address of the source of the data flow.
The number of links value is the count of links included in the RRA.
For each link, the link type, link ID and link data are identical to
the values used in the Router LSA.
The pin-flag is used for partial route-pinning discussed in Section
5.2.
The reserved bandwidth resource is represented by token bucket
parameters, in IEEE single precision floating point format, as in the
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Controlled-Load Service model[5].
The src_prefix_mask and dest_prefix_mask correspond to the network
mask length of the source and destination respectively.
The reservation information comes from a resource reservation
protocol, such as RSVP or some other mechanism for reserving
resources on the node. Whenever a reservation is made or canceled,
QOSPF will originate a new instance of the RRA for the flow. RSVP SE
style reservations can cause multiple RRAs to be originated depending
on the number of PATH state that is matched, and a RSVP WF style
reservation will cause a RRA with a wildcard source (0) to be
originated.
4.0 QOSPF Route Calculation
Input to the QOSPF Dijkstra calculation includes the source and
destination address and the QoS requirements for the flow, which are
currently the token bucket parameters from the RSVP PATH message but
could also come from other triggers.
The QOSPF Dijkstra calculation for an area is performed by processing
the area's RES-LSAs, Network-LSAs, RRAs, and Group-Membership-LSAs.
The latter is only used for the multicast case.
The key difference between the QOSPF Dijkstra and the normal
OSPF/MOSPF Dijkstra is that a router's RES-LSA rather than Router-LSA
is used to discover its neighbors, and links will be ignored if they
do not have sufficient resources (either available or already
reserved) for the flow.
To calculate the best or lowest-delay path, the static delay metric
is used in the same way OSPF uses the TOS zero cost metric of the
Router LSA.
4.1 Multicast QOSPF
4.1.1 Intra-area Multicast QOSPF
Like in normal MOSPF, the intra-area QoS SPF tree is forward-linked.
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This means that the best path is chosen based on the delay metrics
from the source to the target.
4.1.2 Inter-Area Multicast QOSPF
In MOSPF, for a (source, group) pair, a tree has to be calculated for
each area and then the trees are combined into a global tree. When
calculating a tree for an area, if the source is in another area, the
root of the tree is set to all the ABRs that support MOSPF and have
valid Summary LSAs containing the source.
As shown in Figure 4, suppose the source is in area 0.0.0.0. When R5
and R6 calculate their trees for area 0.0.0.1, they will root the
trees at R2, R3, and R4.
[Figure not in Text Version at this time.]
FIGURE 4. A Tree
In QOSPF, links without adequate resources for a data flow are not
considered. So, in Figure 1, suppose the link R1->R3 does not have
enough bandwidth, then R3 will not be on the multicast tree for area
0.0.0.0 so it will not get the packets. Now when R5 and R6 calculate
trees for area 0.0.0.1, they should root the trees only at R2 and R4.
For this reason, after R2 and R4 finishes calculation for area
0.0.0.0, they should notify routers in area 0.0.0.1 how to root the
tree via Deterministic ABR-Advertisements (DABRA).
[Figure not in Text Version at this time.]
FIGURE 5. DABRA with its Opaque LSA header
Each ABR on the QoS tree for the "source area" of a flow originates a
DABRA, listing all the ABRs on the tree, and floods it throughout all
"downstream areas". If the source of the flow is in one of the
router's directly attached areas, then the area is the "source area"
and all other areas are "downstream" areas; otherwise (the source is
in an area not directly attached to the router), the backbone area is
the "source area" and all non- backbone areas are "downstream areas".
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4.1.3 Inter-AS Multicast QOSPF
Similar to the inter-area case, there should be a notification about
how to root the tree. The details are not explored in this document.
4.1.4 Detailed Multicast QOSPF Dijkstra Calculation
The following is a modification to section 12.2 in the Multicast
Extensions to OSPF, RFC 1584.
1. Initialize the algorithm's data structures.
Same as RFC 1584.
2. Initialize the candidate list.
If the source is in another OSPF area, the candidate list is initialized
with the set of area border routers that are both in the DABRA (assuming
that a DABRA has been received at this point) and in the calculating
area. Otherwise, candidate list is initialized as discussed in RFC 1584.
3. If the candidate list is empty, the algorithm terminates.
Same as RFC 1584.
4. Move the closest candidate vertex to the shortest-path tree.
Same as RFC 1584.
5. Examine Vertex V's neighbors for possible inclusion in the candidate list.
If the vertex V just moved from the candidate list to the SPF tree is a
router vertex then lookup the router's Resource-LSA (the Resource-LSA is
used rather than the Router LSA to determine the router's neighbors). If
it is not found then go to step 4. Also lookup the router's Resource
Reservation Advertisement for the flow. This may or may not be found.
Each link (link L) in the RES-LSA describes a connection to a neighboring
vertex (Vertex W). A link is eligible if either an existing reservation for
the flow or the remaining bandwidth is enough to meet the required
bandwidth.
For the eligible links perform the following steps on vertex W.
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a. If W is already on the SPF tree, or if W's LSA does not contain a link
back to vertex V (if vertex W is a router vertex use vertex W's Router
LSA to make this determination as it is irrelevant whether or not there
is reserved bandwidth in the reverse direction), or if W's LSA has LS age
of MaxAge, or if W is not multicast capable (indicated by the MC- bit in
W's originators Router LSA or RES-LSA's options field), or if W does not
support QOSPF (indicated by the Q-bit [See Section 7.2] in W's
originators Router LSA or RES-LSA's options field), do not use this link.
b. If vertex V is a router vertex, the delay to associate with link L is
the delay metric from vertex V's RES-LSA. If both V and W are routers
there may be multiple links and the link with the least delay is used
(providing that the link meets the bandwidth requirements.) The delay
metric is always in the forward direction. If vertex V is a network
the delay is zero.
c. The delay from the source to W is the sum of the delay from the source
to V and the delay of the link from V to W. Let this sum be delay C. If
vertex W is not yet on the candidate list then install W on the
candidate list and modify it's parameters as described in step 5d
below. Otherwise W is already on the candidate list. In this case if:
1) C is less than W's current delay. In this case processing is the
same as in RFC1584.
2) C is equal to W's current delay. In this case processing is the
same as in RFC 1584.
3) C is greater than W's current delay. Examine the next link in V's
LSA.
d. This section is the same as RFC 1584 except that the delay is used
rather than the OSPF cost metric.
6. go to step 3.
After the tree for area A is built, the calculating router determines
if area A is used to determine the upstream node in the same was as
described by RFC 1548, and if the router is an ABR and area A is the
"source area" for the flow, a DABRA is originated and flooded
throughout "downstream areas".
4.2 Unicast QOSPF
In terms of adding to and moving from the candidate list, unicast
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QOSPF Dijkstra is very similar to multicast QOSPF so the Dijkstra
details are not discussed here.
4.2.1 Unicast QOSPF Dijkstra is needed in only one area
If the calculating router has multiple areas, then the best effort
route to the destination has to be found first to identify the area
that needs to run the Dijkstra:
1. If the route is an intra-area route, then the area that the route belongs to
needs to run the Dijkstra to find a QoS route to the destination network.
2. If the route is an inter-area route, then backbone area needs to run the
Dijkstra to find a QoS route to one of the ABRs that advertises the best
effort route.
3. Suppose the route is an external route. If the ASBR used by the external
route is within one of the router's directly attached areas, then that area
needs to run the Dijkstra to find out a QoS route to the ASBR; otherwise,
backbone area needs to run the Dijkstra to find out a QoS route to one of the
ABRs that advertise the ASBR.
Unlike best-effort Dijkstra, a complete tree for the area is not
needed. Once the shortest path to the destination network or the ABR
or the ASBR is found, the Dijkstra terminates.
4.2.2 Inter-area and Inter-AS Unicast QOSPF
In the case that the destination is not in a directly attached area,
things are more complicated because OSPF areas hide detailed topology
and network resource information. Using the topology in Figure 1
again; when R1 calculate a QoS route for (H, H2), it finds a QoS
route to ABR R2 that has a shortest best-effort route the
destination, but R2 can not find a QoS route to the destination. R3
has a QoS route to the destination but the QoS route from R1 to R3
was not calculated.
One way to solve the problem is let R2 send a "summary" to area
0.0.0.0 indicating that it does not have a QoS route for the
particular flow, so R1 will try to find a QoS route to R3. A router
should send the summary to each area that it sends the Type 3 Summary
LSAs for the destination network.
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However this may not be good idea because there may be a large number
of such summaries.
4.3 QOSPF Dijkstra Recalculation
Recalculation occurs upon one or more of the following situations:
- New instances of conventional OSPF/MOSPF LSAs, namely Network-LSAs,
Summary-LSAs, AS External LSAs and Group-Membership-LSAs in multicast
case.
- New instances of DABRAs in multicast case.
- New instances of RES-LSAs.
5.0 QOSPF Route Pinning
Route Pinning means that once reservations on a route from a source
to a destination have been made, the route will not be replaced with
a better route, unless the first route is no longer usable.
Therefore, a pinned path may not continue to be the shortest path or
the one with the most available resources. Control over route pinning
can be from a number of sources, such as configuration, flags in RSVP
RESV messages or other administrative controls.
Because Resource Reservation Advertisements describe existing
reservations, the route pinning algorithm can be accomplished with a
simple modification to the QOSPF Dijkstra algorithm:
When the Dijkstra is run for a flow, if the links from the RRAs for
the flow are used in preference to links from the RES-LSAs the
original path is automatically preserved when possible. This will
occur even if a new and better path is available.
5.1 Route Pinning Dijkstra Modification
Their are two changes that are made to the QOSPF Dijkstra algorithm
to implement route pinning.
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5.1.1 Adding vertices to the candidate list
When adding a vertex to the candidate list, if its parent has a
reservation for the flow on the link that leads to the vertex, the
vertex is marked as "reserved"; or, if its parent is a network vertex
marked as "reserved", it is also marked as "reserved".
If a neighbor W, of a vertex V that is just moved to the SPF tree, is
already on the candidate list and it would not be updated in the
normal OSPF/ MOSPF Dijkstra, it still is updated if it is not marked
as "reserved" and there is a reservation for the flow on the link
from V to W.
5.1.2 Moving a vertex from the candidate list to the SPF tree
A vertex marked as "reserved" is chosen with the smallest delay, even
if there is an un-reserved vertex with a smaller delay. Vertices that
are un-reserved are only moved to the SPF tree when there are no more
"reserved" vertices on the candidate list.
5.2 Partial Route Pinning
The previous section assumes that all QoS paths are to be pinned.
However, sometimes it is desirable that only part of a QoS
distribution tree is pinned because it is possible to have some
receivers that desire pinning and some that do not. This can also be
easily achieved if RSVP or some dynamic mechanism can signal the
desire for route pinning.
Suppose a router/host sends a RESV message to its previous hop router
A, and it indicates in the RESV message that it wants the path to be
pinned. Router A makes the reservation and notifies QOSPF that the
path should be pinned. When A originates an RRA for the flow, it sets
a pin-flag in the reservation for the link. When the route is
recalculated, instead of preferring all links with reservations, only
those links with "pinned" reservation are preferred, hence only part
of the route is pinned.
6.0 Explicit Routing OSPF (EROSPF)
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QOSPF needs both available resource information and existing resource
reservation information in addition to the normal topology and
membership information. When the size of a routing domain or the
number of QoS data flows increases, there is a scaling problem
because it takes a lot of bandwidth, memory and CPU power to flood,
store and process the resource reservation information even though
many of the routers may not be interested in the information.
To alleviate this scaling problem, Explicit Routing (ER) can be used:
for a flow (source, destination) only the source router(s) (see
Section 6.1.1 and Section 6.2) calculate a route, and then the
forwarding information is distributed to the downstream routers along
the path.
Because other routers do not need to perform the Dijkstra
calculation, they are saved from this possible CPU-intensive
computation. In the QOSPF case, the resource reservation information
only needs to be kept on the source routers, thus saving bandwidth,
memory, and CPU cycles. EROSPF is also applicable to standard MOSPF
to reduce the computational needs of the transit routers.
6.1 Multicast Explicit Routing
The following discussion is in terms of a single area. In the multi-
area case, each area maintains a forwarding table, and a global
forwarding table comes from the merge of all the areas' forwarding
tables.
6.1.1 Source Router Determination
The source router for a flow in an area is determined by one of the
following conditions:
- the source of a flow is on a directly connected network within the area.
- the router is an ABR and the source is not in the area.
In other words, explicit routes are only calculated by the source
router and the border routers that the flow travels through. It is
very possible to have multiple source routers for a (source,
destination) pair. In this case, each source router will calculate
the tree separately, and then distribute forwarding information
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(i.e., its subtree) to the downstream routers on its subtree.
6.1.2 Explicit Routing Advertisements (ERAs)
The forwarding information for a (source, destination) pair is
contained in an Explicit Routing Advertisement (ERA), which is passed
in an Opaque LSA along the subtree described by the ERA. The passing
scope is determined by information contained in the ERA.
There are two kinds of ERAs. One is an Installation-ERA, used to
distribute forwarding information and the other is a Flushing-ERA,
used to flush obsolete forwarding information.
6.1.2.1 Format of Installation-ERA
The Format of Installation-ERAs is shown in Figure 6:
[Figure not in Text Version at this time.]
FIGURE 6. Format of Installation-ERA
ERAs are carried in Opaque LSAs with the Opaque type 10. The Opaque
ID is chosen by its originator. The flooding scope is no-flooding,
meaning that the receiver should not flood it out. However, when the
receiver parses the ERA, it will build new ERA(s) off the received
one and send out the new ones with the same Opaque LSA header and ERA
header (see Section 6.1.6).
The no-flooding scope is not supported in the current Opaque LSA
specification, but we are seeking to include it in future
specification.
The source and destination masks are represented as prefix lengths.
Each ERA describes routers on a route tree. For each router, its
incoming interface and a list of outgoing interfaces are listed. The
interface type is the same as in OSPF Router LSAs. The interface is
represented as one of the following:
- for a numbered interface, it is the ip address of the upstream (for
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incoming interface) or downstream (for outgoing interface) neighbor.
- for an unnumbered point-to-point interface, it is the interface index.
The offset fields (adjust offset and child offset) are used to encode
the subtree into the ERA body, as explained in Section 6.1.3 and
Section 6.1.6.
6.1.2.2 Flushing-ERA
A Flushing-ERA is used to flush a previously advertised
Installation-ERA when the route changes (see Section 6.1.8). The
flushing-ERA uses the MaxAge instance of the previously advertised
ERA with an empty ERA body.
6.1.3 Creating Installation-ERAs
After a source router finishes a route calculation, it builds an ERA
to encode the subtree that has the router itself as the root. The
subtree is traversed in "preorder". In the example in Figure 7
(numbers are interface addresses or indices), the source router A
will build an ERA listing routers in the order of A,B,D,E,C.
[Figure not in text version at this time.]
FIGURE 7. An example
The "adjust offset" is set to 0 by the source router. Except for the
first router placed into the ERA, when a router is added to the ERA,
the "child offset" of the parent's outgoing interface leading to the
router is set to the offset of the router in the ERA body. Note that
all offsets are relative to the ERA body. After building the whole
ERA, the source router builds one ERA for each subtree under itself
and unicasts the ERA to the root of the subtree, which is the first
router listed in the ERA. For example, router A will build an ERA for
the subtree rooted at B and unicast it to B, and build an ERA for the
subtree rooted at C and unicasts it to C. This building process is
pretty simple and is described in Section 6.1.6. However, the source
router only stores the ERA for the whole tree and not the newly built
ERAs. The ERA for the subtree rooted at A is shown in Figure 8.
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[Figure not in Text Version at this time.]
FIGURE 8. The ERA for the subtree in Figure 7
6.1.4 Using Multiple ERAs for Long Routes
The structure and processing of the ERA allows the router computing
the route to encode as much of the route as can fit in a packet. The
source router can send an ERA to a downstream router that is not an
immediate neighbor providing the subtree that continues from the
downstream router. It is not likely that this facility would be used
often in many networks.
6.1.5 Transmitting, acknowledging, and storing of ERAs:
A source router stores in its database ERAs (together with their
Opaque LSA header) for trees with itself as the root. An ERA built
for an immediate downstream neighbor is unicast to the incoming
interface of the first router in the ERA (the first router in the ERA
is always the receiver), encapsulated in an Opaque LSA.
A router also stores in its database ERAs received from its parents,
but not those ERAs built for its downstream neighbors.
The acknowledgment and retransmission mechanism is the same as that
used for conventional LSAs. Since the transmission and
acknowledgment of OSPF LSAs are between adjacent neighbors while
sometimes ERAs and DABRAs need to be sent to non-adjacent routers, a
special pair of update/ack packets are needed for ERAs for DABRAs.
See Section 7.3
6.1.6 Processing of Installation-ERAs:
The first listed router in a received ERA is always the receiver
itself.
Upon ERA receipt, the forwarding entry for a (source, destination)
pair is installed (or updated) and associated with the ERA.
If there is a previous instance of the Installation-ERA, to each
immediate downstream neighbor listed in the previous instance of the
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ERA but not in the new ERA, send a Flushing-ERA with the same header
as that of the previous instance.
For each immediate downstream neighbor listed in the received ERA, a
new ERA is constructed from the received ERA and sent to the incoming
interface of the first listed router in the newly constructed ERA.
The Opaque LSA header and the ERA header remain the same, however.
The new ERA's "adjust offset" is set to the "child offset" associated
with the outgoing interface in the received ERA that leads to the
neighbor. The child offsets are not changed in the new ERA. The
subtree for the neighbor is then copied into the new ERA. The subtree
is in the following range of the RECEIVED ERA BODY:
[child offset - old adjust offset, next child offset - old adjust offset]
If there is no "next child", then the remaining portion of the ERA
body is copied. Notice that the encoding work done by the source, and
the offset fields make the downstream routers' job a matter of
copying and shifting.
In the example in Figure 7, A will build two ERAs from the ERA for
itself, one for B and the other for C. The two ERAs are illustrated
in Figure 9.
[Figure not in Text Version at this time.]
FIGURE 9. ERAs built by A for B and C
6.1.7 Processing of Flushing-ERAs
Upon receipt of a Flushing-ERA, the corresponding Installation-ERA is
found and a MaxAge Flushing-ERA is constructed and sent out with same
header as the existing ERA for each immediate downstream neighbor in
the Installation- ERA. If a forwarding entry exists for the
corresponding Installation-ERA, the forwarding entry's incoming
interface is set to NULL (so that no packets for the (source, group)
will be accepted on the interface) if there are no other
Installation-ERAs for the (s, g). If other Installation-ERAs exist, a
new forwarding entry is constructed for the (source, group) pair. If
there is no forwarding entry for the (source, group), forwarding
entry with a NULL incoming interface is installed to prevent
forwarding of any received packet for the (source, group) pair.
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6.1.8 Route Change
For all routers, if the upstream neighbor or interface of the first
router in an ERA goes down, a MaxAge Flushing-ERA is immediately sent
to each immediate downstream neighbor to flush the ERA. This does not
need to wait until the source finishes recalculation.
When there is a topology change, the source routers recalculate the
tree, and send updated ERAs along their subtrees. New ERAs are
carried in the Opaque LSAs with the same Opaque ID as in the old
ones, but with a larger sequence number.
For all routers, if a previous downstream neighbor is no longer
listed in a newer ERA, a Flushing-ERA with the same header of the
previous instance of the new ERA is sent to the neighbor to flush its
corresponding Installation-ERA.
6.2 Unicast Explicit Routing
While Multicast ER makes sense even if QOSPF is not used, Unicast
Explicit Routing is needed only for QoS routing.
A router is a source router for a unicast flow (source, destination)
when one of the following conditions exists:
- The source is on one of the router's directly connected networks in the
area that needs the Dijkstra, or
- The source is not in the area, and the router is an ABR.
The multicast ERA is also used for unicast, but in the unicast case,
the "MOSPF IL Type", "MOSPF Init Case", and incoming interface are
not used, and the number of outgoing interface is always 1.
6.3 Changes of behavior of QOSPF if Explicit Routing is used
Explicit Routing is introduced to address QOSPF's scaling problem,
but QOSPF does not logically depend on Explicit Routing. The
discussions in Section 3.0 and Section 4.0 have been assuming that no
ER is used.
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When ER is used, the following behaviors of QOSPF are changed:
6.3.1 Flooding scope of RRAs
RRAs are no longer flooded throughout an area. Instead, they are sent
to the source router(s) using opaque "no-flooding" scope. The source
routers then can use opaque "link-local" scope to flood the RRAs to
other routers on the source network.
6.3.2 Flooding scope of DABRAs
DABRAs are no longer flooded throughout "downstream areas" of the
"source area". Instead, a DABRA is sent to all the ABRs on the route
in the "source area".
6.4 Quick Scaling Performance Analysis
The Scaling problems with QOSPF are primarily caused by RRAs, so
let's do a scaling analysis in terms of number of RRAs flooded per
second, based on the following area configuration:
Number of routers in the area: R
Average number of routers on a multicast tree: M = abs(sqr_root(R))
Average number of flows that sources from a router: F
Period of time during which to set up all the flows: T = 10 seconds
For each flow, each router has to originate a RRA, so there will be
(R * M * F) RRAs originated.
If explicit routing (ER) is not used, each router will get all the
RRAs, so the R routers will receive (R * F * M - F) RRAs (a router
does not need to receive its own RRAs), i.e, (R * F * M - F)/10 RRAs
have to be transmitted per second.
If ER is used, only the source routers will receive the RRAs.
Assuming those RRAs are sent to the source router following the
reversed multicast path, then at most (1 + 2 +,,, + (M - 2) + (M -
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1)) transmissions are needed for each flow, or F * (1 + 2 +... + (M
-2) + (M - 1))/10 RRAs have to be transmitted per second.
Changing the value of R, we have the following result:
Number of routers (R): 9 16 25 36 49 64
Number of RRAs per sec w/ ER: 0.3F 0.6F 1.0F 1.5F 2.1F 2.8F
Number of RRAs per sec w/o ER: 2.6F 6.3F 9.9F 21.5F 34.2F 51.1F
It is clear that QOSPF does not scale without ER but it scales well with ER.
7.0 Changes to OSPF to accommodate QOSPF/ER
Because of the new functionality and new types of LSAs, the following
changes are needed to accommodate QOSPF or ER.
7.1 Database Exchange
In OSPF, when a router exchanges its database with a neighbor, it
only sends the neighbor those types of LSAs that the neighbor
understands. This is done by checking the Options field in the
neighbors' Database Description packets.
A new bit must be added to the Options field: the Q-bit. If set,
means the router supports QOSPF and understands RES-LSAs. The Options
field is almost full now. Fortunately there are two unused bytes
ahead of the Option field and they can be used for more options.
7.2 "Rtype" field in Router/RES LSAs
A new bit, P-bit, in the "Rtype" field of Router LSAs and RES-LSAs
must be set if the router is configured to do route pinning for
QMOSPF, so that all routers in the same area can agree on using or
disabling route pinning and to compute identical multicast trees.
Additionally, a Q-bit, indicating that a router supports QOSPF is
needed.
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7.3 New Types of OSPF packets
OSPF requires that any LSAs be exchanged between neighbors that are
supposed to become adjacent and a Link State Update/Ack packet would
simply be discarded if it is from a neighbor with a state less than
ExStart.
However, when ER is used, the RRAs and ERAs may be sent to non-
adjacent routers. The solution is to invent a new pair of update/ack
packets that do not require adjacency to transmit/acknowledge RRAs
and ERAs when ER is used. The same acknowledgment/retransmission
scheme as those between adjacent neighbors can be used to ensure
reliable transmission of RRAs and ERAs.
8.0 Security Considerations
Given that QOSPF could be triggered by RSVP, it is expected that the
security mechanisms for RSVP will provide authorization and access
control for QOSPF routing calculations. Additionally, the OSPF
security mechanisms for authenticating neighbors and data received
are very important for explicit routing since ER packets can change
forwarding state in a very direct manner. Especially, since an ERA
can be sent to a router on a different network, ERA packets'
authentication should be per area instead of per interface.
9.0 Acknowledgments
The authors gratefully acknowledge the following people/organizations
for making this protocol come together:
- Tim Trapp of Thompson International for the initial problem,
constraints, as well as constructive discussions.
- E-Systems, Inc. Particularly, Hai Nguyen, Gerry Rosen, and Thomas Grill
for their patience and perserverence during some of the difficult
design and development phases.
- Richard Over, Brad Noblet, and the Custom Services Software Engineering
team at Bay Networks for sheltering and nurturing the design and
development team.
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- The IP group at Bay Networks for lots of support and code modification
to multicast routing and RSVP to support QOSPF.
- John Krawczyk, Ross Callon, Mohd Bashar, Mike Davis, and Dennis Baker
for useful design comments.
10.0 Notice Regarding Intellectual Property Rights
Bay Networks may seek patent or other intellectual property
protection for some or all of the technologies disclosed in this
document. If any standards arising from this disclosure are or become
protected by one or more patents assigned to Bay Networks, Bay
Networks intends to disclose those patents and license them on
reasonable and non-discriminatory terms. Future revisions of this
draft may contain additional information regarding specific
intellectual property protection sought or received.
11.0 References
1. J. Moy, OSPF Version 2, Request for Comments (RFC) 1583
2. J. Moy, Multicast Extensions to OSPF, Request for Comments(RFC) 1584,
March 1994.
3. R. Coltun, The OSPF Opaque LSA Option, Internet Draft,
draft-coltun-ospf-opaque-01.txt
4. R. Braden, L. Zhang, S. Berson, S. Herzog, S. Jamin. Resource
ReSerVation Protocol (RSVP) - Version 1 Functional Specification,
Internet Draft, draft-ietf-rsvp-spec-12.txt, May 1996.
5. J. Wroclawski, Specification of the Controlled-Load Network Element
Service, Internet Draft, draft-ietf-intserv-ctrl-load-svc-01.txt,
November, 1995.
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12.0 Authors' Address
Zhaohui (Jeffrey) Zhang
Bay Networks, Inc.
2 Federal Street
Billerica, MA 01821
+1 508-670-8888
zzhang@baynetworks.com
Cheryl Sanchez
Bay Networks, Inc.
3 Federal Street
Billerica, MA 01821
+1 508-670-8888
csanchez@baynetworks.com
Bill Salkewicz
Bay Networks, Inc.
3 Federal Street
Billerica, MA 01821
+1 508-670-8888
bsalkewi@baynetworks.com
Eric S. Crawley
Bay Networks, Inc.
3 Federal Street
Billerica, MA 01821
+1 508-670-8888
esc@baynetworks.com
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