Network Working Group Seisho Yasukawa
Category: Informational NTT
Expires: August 2006
Adrian Farrel
Old Dog Consulting
February 2006
An analysis of scaling issues in MPLS-TE backbone networks
draft-yasukawa-mpls-scaling-analysis-00.txt
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Abstract
Traffic engineered Multiprotocol Label Switching (MPLS-TE) is being
deployed in provider's backbone networks. The providers wish to grow
these networks, and need to discover whether existing protocols and
implementations can support the network sizes that they are planning.
This document presents an analysis of some of the scaling concerns
for MPLS-TE backbone networks, and examines the value of two
techniques for improving scaling.
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Contents
1. Introduction ................................................... 2
2. Network configurations ......................................... 3
2.1 Commercial drivers for selected configurations ................ 5
3. Required network sizes ......................................... 6
4. Issues of concern for scaling .................................. 6
4.1 LSP state ..................................................... 6
4.2 Processing overhead ........................................... 6
4.3 RSVP-TE implications .......................................... 7
4.4 Management .................................................... 7
4.5 Practical Numbers .... ........................................ 8
5. Scaling in flat networks ....................................... 8
6. Scaling improvements through Forwarding Adjacencies ............ 9
6.1 Two layer hierarchy ........................................... 9
6.1.1 Tuning the network topology to suit the two layer hierarchy. 10
6.2 Alternative two layer hierarchy .............................. 11
6.3 Three layer hierarchy ........................................ 11
6.4 Issues with hierarchical LSPs ................................ 12
7. Scaling improvements through multipoint-to-point LSPs ......... 13
7.1 Overview of MP2P LSPs ........................................ 13
7.2 Scaling improvements ......................................... 14
7.2.1 Comparison with other scenarios ............................ 15
7.3 Issues with MP2P LSPs ........................................ 16
8. Combined models ............................................... 17
9. Management Considerations ..................................... 17
10. Security Considerations ...................................... 18
11. Recommendations .............................................. 18
12. IANA Considerations .......................................... 18
13. Acknowledgements ............................................. 18
14. Intellectual Property Consideration .......................... 18
15. Normative References ......................................... 19
16. Informational References ..................................... 19
17. Authors' Addresses ........................................... 20
18. Disclaimer of Validity ....................................... 20
19. Full Copyright Statement ..................................... 20
1. Introduction
As traffic engineered Multiprotocol Label Switching (MPLS-TE) grows
in popularity, providers are looking at how they could deploy it in
networks that are larger than the more simple and experimental
networks that have been seen so far. This is leading them to examine
the number of LSPs that may need to be supported at various points
within the network, and at the consequent impact on control plane and
management plane software as well as on the constraints placed by the
physical limitations of the routers in the network.
Physical scaling topology concerns are addressed by building networks
that are not fully meshed. Network topologies tend to be meshed in
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the core, but tree-shaped at the edges giving rise to a snow-flake
design.
MPLS-TE, however, establishes a logical full mesh between all edge
points in the network, and this is where the scaling problems arise
since the tree structure of the network tends to focus a large number
of LSPs within the core of the network.
This document presents a generic network topology and introduces
terminology for the different scaling parameters. It then examines
how many LSPs might be carried within the core of a network.
Two techniques (hierarchical and multipoint-to-point LSPs) are
introduced and an examination is made of the scaling benefits that
they offer as well as of some of the concerns with using these
techniques.
Of necessity, this document makes very many generalizations. Not
least among these are a set of assumptions about the symmetry and
connectedness of the physical network. It is hoped that these
generalizations will not impinge on the usefulness of the overview of
the scaling properties that this document attmepts to give.
2. Network configurations
The network configurations considered in this document are based on a
hierarchy of connectivity within the core network. PE nodes have
connectivity to P nodes as shown in figure 1. There may be
interconnection between the PEs that are connected to a single P
node, but there is no direct connectivity between PEs that are
connected to different P-nodes. Dual homing of PEs to multiple P
nodes is not considered in this document although it may be a
valuable addition to a network configuration.
P
/|\
/ | \
/ | \
/ | \
PE PE PE
Figure 1 : PE to P node connectivity
The relationship between P-nodes is also structured in a hierarchical
way. Thus, as shown in figure 2, multiple P-nodes at one level are
connected to a P-node at a higher level. We number the levels such
that level 1 is the top level and level (n) is immediately above
level (n+1), and we denote a P-node at level n as a P(n). There may
be interconnection between P(n+1) nodes connected to a single P(n),
but there is no direct connectivity between P(n+1) nodes connected to
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different P(n) nodes. Again, dual homing of P(n+1) nodes to multiple
P(n) nodes is not considered in this document although it may be a
valuable addition to a network configuration.
P(n)
/|\
/ | \
/ | \
/ | \
P(n+1) P(n+1) P(n+1)
Figure 2 : Relationship between P-nodes
At the top level, P(1) nodes are connected in a full mesh. In
reality, the level 1 part of the network may be slightly less well
connected than this, but assuming a full mesh provides for
generality.
The key multipliers for scalability are the number of P(1) nodes, and
the multiplier relationship between P(n) and P(n+1) at each level
including down to PEs.
We define the multiplier M(n) as the number of nodes attached to any
one P(n).
We define S(n) as the number of nodes at level (n).
Thus: S(n) = S(1)*M(1)*M(2)*...*M(n+1)
So the number of PEs can be expressed as:
S(PE) = S(1)*M(1)*M(2)*...*M(n)
where the network has (n) layers of P-nodes.
Thus we may depict an example network as shown in figure 3. In this
case:
S(1) = 3
M(1) = 3
S(2) = S(1)*M(1) = 9
M(2) = 2
S(PE) = S(1)*M(1)*M(2) = 18
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PE PE PE PE PE PE
\ \/ \/ /
PE--P(2) P(2) P(2) P(2)--PE
\ | | /
\| |/
PE--P(2)---P(1)------P(1)---P(2)--PE
/ \ / \
PE \ / PE
\/
P(1)
/|\
/ | \
/ | \
PE--P(2) P(2) P(2)--PE
/ /\ \
PE PE PE PE
Figure 3 : An example network
2.1 Commercial drivers for selected configurations
It is reasonable to ask why this particular network connectivity
structure has been chosen.
The consideration must be physical scalability. Each LSR is only able
to support a limited number of physical interfaces. This necessarily
reduces the ability to fully mesh a network and leads to the
tree-like structure of the network towards the PEs.
A realistic commercial consideration for an operator is the fact that
the only revenue-generating nodes in the network are the PEs. Other
nodes are needed only to support connectivity and scalability.
Therefore, there is a desire to maximize S(PE) while minimizing the
sum of S(n) for all values of (n). This could be achieved by
minimizing the number of levels, and by maximizing the connectivity
at each layer, M(n). Ultimately, however, this would produce a
network of just interconnected PEs, which is clearly in conflict with
the physical scaling situation.
Therefore, the solution calls for a "few" levels with "relatively
large" connectivity at each level. We might say that the
cost-effectiveness of the network can be stated as:
K = S(PE)/(S(1)+S(2) + ... + S(n)) where n is the level above the PEs
This document examines the implications for control plane and data
plane scalability of this type of network when MPLS-TE LSPs are used
to provide full connectivity between all PEs.
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3. Required network sizes
An important question for this evaluation and analysis is the size of
the network that operators require. How many PEs are required? What
ratio of P to PE is acceptable. How many ports do devices have for
physical connectivity? What type of MPLS-TE connectivity between PEs
is required?
Although presentation of figures for desired network sizes is
ultimately pointless because history shows that networks grow beyond
all projections, it is useful to set some acceptable lower bounds.
That is, we can state that we already know that networks will be at
least of a certain size.
The most important features are:
- The network should have at least 1000 PEs.
- Each pair of PEs should be connected by at least one LSP in each
direction.
4. Issues of concern for scaling
This section presents some of the issues associated with the support
of LSPs at an LSR or within the network that may mean that there is a
a limit to the number LSPs that can be supported.
4.1 LSP state
LSP state is the data (information) that must be stored at an LSR in
order to maintain an LSP. While the size of the LSP state is
implementation-dependent, it is clear that any implementation will
require some data in order to maintain LSP state.
Thus LSP state becomes a scaling concern because as the number of
LSPs at an LSR increases, so the amount of memory required to
maintain the LSPs increases in direct proportion. Since the memory
capacity of an LSR is limited, there is a related limit placed on the
number LSPs that can be supported.
Note that techniques to reduce the memory requirements (such as data
compression) may serve to increase the number of LSPs that can be
supported, but this will only achieve a moderate multiplier and may
significantly decrease the ability to process the state rapidly.
4.2 Processing overhead
Depending largely on implementation issues, the number of LSPs
supported by an LSR may impact the processing speed for each LSP. For
example, control block search times can increase with the number of
control blocks, and even excellent implementations cannot completely
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mitigate this fact. Thus, since CPU power is constrained in any LSR,
there may be a practical limit to the number of LSPs that can be
supported.
Further processing overhead considerations depend on issues specific
to the control plane protocols and discussed in the next section.
4.3 RSVP-TE implications
Like many connection-oriented signaling protocols, RSVP-TE requires
that state is held within the network in order to maintain LSPs. The
impact of this is described in section 4.1. Note that RSVP-TE
requires that separate information is maintained for upstream and
downstream relationships, but does not require any specific
implementation of that state.
RSVP-TE is a soft-state protocol which means that protocol messages
(refresh messages) must be regularly exchanged between signaling
neighbors in order to maintain the state for each LSP that runs
between the neighbors. A common period for the transmission (and
receipt) of refresh messages is 30 seconds meaning that each LSR must
send and receive one message in each direction (upstream and
downstream) for every LSP it supports every 30 seconds. This has the
potential to be a significant constraint on the scaling of the
network, but various improvements [RFC2961] mean that this refresh
processing can be significantly reduced allowing an implementation to
be optimized to nearly remove all concerns about soft state scaling
in a stable network.
RSVP-TE also requires that signaling adjacencies are maintained
through the use of Hello message exchanges. Although [RFC3209]
suggests that Hello messages should be retransmitted every 5ms, in
practice values of around 3 seconds are more common. Nevertheless,
the support of Hello messages can represent a scaling limitation on
an RSVP-TE implementation since one message must be sent and received
to/from each signaling adjacency every time period. This can impose
limits on the number of neighbors (physical or logical) that an LSR
supports.
4.4 Management
Another practical concern for the scalability of large MPLS-TE
networks is the ability to manage the network. This may be
constrained by the available tools, the practicality of managing
large numbers of LSPs, and the management protocols in use.
Management tools are software implementations. Although such
implementations should not constrain the control plane protocols, it
is realistic to appreciate that network deployments will be limited
by the scalability available tools. In practice, most existing tools
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have a limit to the number of LSPs that they can support. While an
NMS may be able to support a large number of LSPs, the number that
can be supported by an EMS (or the number supported by an NMS
per-LSR) is more likely to be limited.
Similarly, practical constraints may be imposed by the operation of
management protocols. For example, an LSR may be swamped by
management protocol requests to read information about the LSPs that
it supports and this might impact its ability to sustain those LSPs
in the control plane. OAM, alarms and notifications can further add
to the burden placed on an LSR and limit the number of LSPs it can
support.
All of these consideration encourage a reduction in the number of
LSPs supported within the network and at any particular LSR.
4.5 Practical Numbers
In practice, reasonable target numbers are as follows.
S(PE) >= 1000
Number of levels is 3. That is: 1, 2 and PE.
M(2) <= 20
M(1) <= 20
S(1) <= 100
5. Scaling in flat networks
Before proceeding to examine potential scaling improvements, we need
to examine how well the flat network described in figure 3 scales.
Consider the requirement for a full mesh of LSPs linking all PEs.
That is, each PE has an LSP to and from each other LSP. Thus, if
there are S(PE) PEs in the network, there are S(PE)*(S(PE) - 1) LSPs.
Define L(n) as the number of LSPs handled by a level (n) LSR.
L(PE) = 2*(S(PE) - 1)
L(2) can be computed as the sum of all LSPs for all attached PEs
including the LSPs between the attached PEs (but being careful to
only count those LSPs once). Thus, since the number of attached PEs
is M(2), we have:
L(2) = 2*M(2)*(S(PE) - 1) - M(2)*(M(2) - 1)
L(1) can be computed as the sum of the number of LSPs for all
downstream PEs less those that can be served exclusively by the
attached P(2) nodes, and only counting the LSPs routed through two
attached P(2) nodes once. So:
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L(1) = 2*M(1)*M(2)*(S(PE) - 1) -
2*M(1)*M(2) -
M(2)*(M(1) - 1)
= 2*M(1)*M(2)*[S(PE) - 2.5] - M(2)
So, for example, with S(1) = 5, M(1) = 10, and M(2) = 20, we see:
S(PE) = 1000
L(PE) = 1998
L(2) = 39580
L(1) = 398980
Alternatively, with S(1) = 10, M(1) = 10, and M(2) = 20, we see:
S(PE) = 2000
L(PE) = 3998
L(2) = 79580
L(1) = 798980
In both examples, the number of LSPs at the core (P(1)) nodes is
probably unacceptably large even though there are only a relatively
modest number of PEs. In fact, L(2) may even be too large in the
second example.
6. Scaling improvements through Forwarding Adjacencies
One of the purposes of LSP hierarchies [RFC4206] is to improve the
scaling properties of MPLS-TE networks. LSP tunnels (sometimes known
as Forwarding Adjacencies (FAs)) may be established to provide
connectivity over the core of the network and multiple edge-to-edge
LSPs may be tunneled down a single FA LSP.
In our network it is natural to consider a mesh of FA LSPs between
all core nodes at the same level. We consider two possibilities here.
In the first all P(2) nodes are connected to all other P(2) nodes,
and the PE-to-PE LSPs are tunneled across the core of the network. In
the second, an extra layer of hierarchy is introduced by connecting
all P(1) nodes in a mesh and tunneling the P(2)-to-P(2) tunnels
through these.
6.1 Two layer hierarchy
In this hierarchy model, the P(2) nodes are connected by a mesh of
tunnels. This means that the P(1) nodes do not see the PE-to-PE LSPs.
It remains the case that:
L(PE) = 2*(S(PE) - 1)
L(2) is slightly increased. It can be computed as the sum of all LSPs
for all attached PEs including the LSPs between the attached PE plus
the number of FA LSPs providing a mesh to the other P(2) nodes. Thus,
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since the number of attached PEs is M(2) and the number of P(2) nodes
is S(2), we have:
L(2) = 2*M(2)*(S(PE) - 1) - M(2)*(M(2) - 1) + 2*(S(2) - 1)
L(1), however, is significantly reduced and can be computed as the
sum of the number of FA LSPs to and from each attached P(2) to each
other P(2) in the network, including (but counting only once) the FA
LSPs between attached P(2) nodes. So:
L(1) = 2*M(1)*(S(2) - 1) - M(1)*(M(1) - 1)
So, for example, with S(1) = 5, M(1) = 10, and M(2) = 20, we see:
S(PE) = 1000
S(2) = 50
L(PE) = 1998
L(2) = 39678
L(1) = 890
Alternatively, with S(1) = 10, M(1) = 10, and M(2) = 20, we see:
S(PE) = 2000
S(2) = 100
L(PE) = 3998
L(2) = 79778
L(1) = 1890
So, in both examples, the problem of the number of LSPs at the core
(P(1)) nodes is solved, but any problem with L(2) is made slightly
worse.
6.1.1 Tuning the network topology to suit the two layer hierarchy
Clearly we can reduce L(2) by selecting appropriate values of S(1),
M(1) and M(2). We can do this pretty much with immunity since no
change will affect L(PE), and since L(1) is now so small.
Observe that:
L(2) = 2*M(2)*(S(PE) - 1) - M(2)*(M(2) - 1) + 2*(S(2) - 1)
where S(PE) = S(1)*M(1)*M(2) and S(2) = S(1)*M(1).
So L(2) scales with M(2)^2 and we can have the most impact by
reducing M(2).
For example, with S(1) = 10, M(1) = 10, and M(2) = 10, we see:
S(PE) = 1000
S(2) = 100
L(PE) = 1998
L(2) = 20088
L(1) = 1890
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And similarly, with S(1) = 20, M(1) = 20, and M(2) = 5, we see:
S(PE) = 2000
S(2) = 400
L(PE) = 3998
L(2) = 20768
L(1) = 15580
These considerable scaling benefits must be offset against the cost
effectiveness of the network. Recall that
K = S(PE)/(S(1)+S(2) ... + S(n))
where n is the level above the PEs, so that for our network:
K = S(PE) / (S(1) + S(2))
Thus, in the first example the cost-effectiveness has been halved
from 1000/55 to 1000/110. And in the second example it has been
reduced to roughly one quarter, changing from 2000/110 to 2000/420.
So, although the tuning changes may be necessary to reach the desired
network size, they come at a considerable cost to the operator.
6.2 Alternative two layer hierarchy
An alternative to the two layer hierarchy presented in section 6.1 is
to provide a full mesh of FA LSPs between P(1) nodes. This technique
is only of benefit to any nodes in the core of the level 1 network.
Otherwise it makes no difference to the PE and P(2) nodes and
increases the burden at the P(1) nodes since they have to support all
of the PE-to-PE LSPs as in the flat model, plus the additional
2*(S(1) - 1) P(1)-to-P(1) FA LSPs. Thus, this approach should only be
considered where there is a mesh of P-nodes wihtin the ring of P(1)
nodes, and it is not considered further in this document.
6.3 Three layer hierarchy
As demonstrated by section 6.2, introducing a mesh of FA LSPs at the
top level (P(1)) has no benefit, but if we introduce an additional
level in the network (P(3) between P(2) and PE) we can introduce a
new layer of FA LSPs so that we have a full mesh of FA LSPs between
all P(3) nodes to carry the PE-to-PE LSPs, and a full mesh of FA LSPs
between all P(2) nodes to carry the P(3)-to-P(3) LSPs.
The math starts to get a little less pretty!
The number of PEs S(PE) = S(1)*M(1)*M(2)*M(3) and the number of
PE-to-PE LSPs at a PE remains as L(PE) = 2*(S(PE) - 1).
The number of LSPs at a P(3) can be deduced from section 6.1. It is
the sum of all LSPs for all attached PEs including the LSPs between
the attached PE plus the number of FA LSPs providing a mesh to the
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other P(3) nodes.
L(3) = 2*M(3)*(S(PE) - 1) - M(3)*(M(3) - 1) + 2*(S(3) - 1)
The number of LSPs at P(2) can also be deduced from section 6.1 since
it is the sum of all LSPs for all attached P(3) nodes including the
LSPs between the attached PE plus the number of FA LSPs providing a
mesh to the other P(2) nodes.
L(2) = 2*M(2)*(S(3) - 1) - M(2)*(M(2) - 1) + 2*(S(2) - 1)
Finally, L(1) can be copied straight from 6.1.
L(1) = 2*M(1)*(S(2) - 1) - M(1)*(M(1) - 1)
For example, with S(1) = 5, M(1) = 5, M(2) = 5, and M(3) = 8 we see:
S(PE) = 1000
S(3) = 125
S(2) = 25
L(PE) = 1998
L(3) = 16176
L(2) = 1268
L(1) = 220
Similarly, with S(1) = 5, M(1) = 5, M(2) = 8, and M(3) = 10 we see:
S(PE) = 2000
S(3) = 200
S(2) = 25
L(PE) = 3998
L(3) = 40038
L(2) = 3184
L(1) = 220
Of course, the extra level in the network tends to reduce the cost
effectiveness of the networks with values of K = 1000/155 and
K = 2000/230 (from 1000/55 and 2000/110) for the examples above.
6.4 Issues with hierarchical LSPs
A basic observation for hierarchical scaling techniques is that it is
hard to have any impact on the number of LSPs that must be supported
by the level of P(n) nodes adjacent to the PEs (for example, it is
hard to reduce L(3) in section 6.3). In fact, the only way we can
change the number of LSPs supported by these nodes is to change the
scaling ratio M(n) in the network, in other words to change the
number of PEs subtended to any P(n). But such a change has a direct
effect on the number of PEs in the network and so the
cost-effectiveness is impacted.
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Another concern with the hierarchical approach is that it must be
configured and managed. This may not seem like a large burden, but it
must be recalled that the P(n) nodes are not at the edge of the
network - they are a set of nodes that must be identified so that the
FA LSPs can be configured and provisioned. Although certain
techniques (such as IGP auto-mesh [AUTO-MESH]) can help with this
procedure, there is still a configuration and management overhead.
Finally, observe that we have been explaining these techniques using
conveniently symmetrical networks. Consider how we would arrange the
hierarchical LSPs in a network where some PEs are connected closer to
the center of the network than others.
7. Scaling improvements through multipoint-to-point LSPs
An alternative (or complementary) scaling technique has been proposed
using multipoint-to-point (MP2P) LSPs. The fundamental improvement in
this case is achieved by reducing the number of LSPs towards the
destination as LSPs towards the same destination are merged.
This section presents an overview of MP2P LSPs and describes their
applicability and scaling benefits.
7.1 Overview of MP2P LSPs
Note that the MP2P LSPs discussed here are for MPLS-TE and are not
the same concept familiar in the Label Distribution Protocol (LDP)
described in [RFC3036].
Traffic flows generally converge toward their destination and this
can be utilized by MPLS in constructing an MP2P LSP. With such an
LSP, the LFIB mappings at each LSR are many-to-one so that multiple
pairs {incoming interface, incoming label} are mapped to a single
pair {outgoing interface, outgoing label}. Obviously, if global
labels are used this mapping may be optimized within an
implementation.
It is important to note that with MP2P MPLS-TE LSPs the traffic flows
themselves are not merged. That is, the flows are labeled in their
own right as point-to-point (P2P) flows, and the MP2P LSPs are used
as tunnels. This means that traffic can be disambiguated at the
egress of the MP2P LSPs without the need to look at the payload.
Techniques for establishing MP2P MPLS-TE LSPs and for assigning the
correct bandwidth downstream of LSP merge points are out of the scope
of this document.
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7.2 Scaling improvements
Consider the network topology shown in figure 3. Suppose that we
establish MP2P LSP tunnels such that there is one tunnel terminating
at each PE, and that that tunnel has every other PE as an ingress.
Thus, a PE-to-PE MP2P LSP tunnel would have S(PE)-1 ingresses and one
egress, and there would be S(PE) such tunnels.
Note that there still remain 2*(S(PE) - 1) PE-to-PE P2P LSPs that are
carried through these tunnels.
Let's consider the number of LSPs handled at each node in the
network.
The PEs continue to handle the same number of PE-to-PE P2P LSPs, and
must also handle the MP2P LSPs. So:
L(PE) = 2*(S(PE) - 1) + S(PE)
But all P(n) nodes in the network only handle the MP2P LSP tunnels.
Nominally, this means that L(n) = S(PE) for all values of n, but life
is not really that simple. The number of LSPs is no longer the only
issue (although it may have some impact for some of the scaling
concerns listed in section 4). We are now interested more directly in
the amount of LSP state that is maintained by an LSR. We can quantify
this according to the number of LSP segments managed by an LSR. So,
in the case of a P2P LSP, an ingress or egress has one segment to
maintain, while a transit has two segments. Similarly, for an MP2P
LSP, an LSR must maintain one state for each upstream segment (which
will we can assume is in a one-to-one relationship with the number of
upstream neighbors) and exactly one downstream segment - ingresses
obviously have no upstream neighbors, and egresses have no downstream
segments.
So we can start again on our examination of the scaling properties,
using X(n) to represent the amount of state held at each P(n).
At the PEs, there is only connectivity to one other network node, the
P(2) node. But note that we have required that P2P LSPs be used
tunneled within the MP2P LSPs to allow disambiguation of data at the
egresses. So X(PE) is:
X(PE) = 4*(S(PE) - 1)
Each P(2) node has M(2) downstream PEs to each of which a single MP2P
LSP must be delivered coming from the one upstream P(1), and from
which one LSP segment converges towards each other PE in the network.
Note that not all MP2P LSPs initiated at subtended PEs are routed out
through the parent P(1) since some terminate at local PEs. Thus:
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X(2) = 2*M(2) + M(2)*(S(PE) - 1) + S(PE) - M(2)
= S(PE)*(M(2) + 1)
Similarly, at each P(1) node there are M(1) downstream P(2) nodes and
so a total of M(1)*M(2) downstream PEs. Each P(1) is connected in a
full mesh with the other P(1) nodes so has (S(1) - 1) neighbors. So
each P(1) must handle an LSP segment coming from each downstream P(2)
headed to each non-downstream PE and an LSP segment to another P(1)
for each non-downstream PE. At the same time, it must handle an LSP
segment coming from each other P(1) headed to each downstream PE, and
an LSP segment towards each downstream PE. Thus:
X(1) = (M(1) + 1)*(S(PE) - M(1)*M(2)) +
(S(1) - 1)*(S(PE) - M(1)*M(2)) + M(1)(M(2)
= M(1)*(S(PE) + M(2)*(M(1) - S(1) + 1)) +S(1)*S(PE)
So, for example, with
For example, with S(1) = 10, M(1) = 10, and M(2) = 10, we see:
S(PE) = 1000
S(2) = 100
X(PE) = 3996
X(2) = 11000
X(1) = 20100
And similarly, with S(1) = 20, M(1) = 20, and M(2) = 5, we see:
S(PE) = 2000
S(2) = 400
X(PE) = 5996
X(2) = 12000
X(1) = 80100
7.2.1 Comparison with other scenarios
For comparison with the examples in sections 5 and 6, we need to
convert those LSP-based figures to our new measure of LSP state.
Observe that each LSP in sections 5 and 6 generates two state units
at a transit LSR and one at an ingress or egress. So we can provide
conversions as follows:
Section 5
L(PE) = 2*(S(PE) - 1)
L(2) = 2*M(2)*(S(PE) - 1) - M(2)*(M(2) - 1)
L(1) = 2*M(1)*M(2)*[S(PE) - 2.5] - M(2)
X(PE) = 2*(S(PE) - 1)
X(2) = 4*M(2)*(S(PE) - 1) - 2*M(2)*(M(2) - 1)
X(1) = 4*M(1)*M(2)*[S(PE) - 2.5] - 2*M(2)
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So that using the example figures in section 7.1 (S(1) = 10,
M(1) = 10, and M(2) = 10) we see:
X(PE) = 1998
X(2) = 39780
X(1) = 398980
Clearly this technique is a significant improvement over the flat
network within the network, although the PEs are more heavily
stressed.
Section 6.1
L(PE) = 2*(S(PE) - 1)
L(2) = 2*M(2)*(S(PE) - 1) - M(2)*(M(2) - 1) + 2*(S(2) - 1)
L(1) = 2*M(1)*(S(2) - 1) - M(1)*(M(1) - 1)
S(2) = S(1)*M(1)
X(PE) = 2*(S(PE) - 1)
X(2) = 4*M(2)*(S(PE) - 1) - 2*M(2)*(M(2) - 1) + 2*(S(2) - 1)
X(1) = 4*M(1)*(S(2) - 1) - 2*M(1)*(M(1) - 1)
So that using the example figures in section 7.1 (S(1) = 10,
M(1) = 10, and M(2) = 10) we see:
X(PE) = 1998
X(2) = 39881
X(1) = 3780
And we can observe that the MP2P model is better at P(2), but the
hierarchical model is better at P(1).
In fact, this can be generalized to observe that the MP2P model
produces best effects towards the edge of the network, while the
hierarchical model makes most impression at the core. However, the
requirement for P2P LSPs tunneled within the MP2P LSPs does cause a
double burden at the PEs.
7.3 Issues with MP2P LSPs
The biggest challenges for MP2P LSPs are the provision of support in
the control and data planes. To some extent support must also be
provided in the management plane.
Control plane support is just a matter of defining the protocols and
procedures [MP2P-RSVP], although it must be clearly understood that
this will introduce some complexity to the control plane.
Hardware issues may be a little more tricky. For example, the
capacity of the upstream segments must never (allowing for
statistical over-subscription) exceed the capacity of the downstream
segment. Similarly, data planes must be equipped with sufficient
buffers to handle incoming packet collisions.
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The management plane will be impacted in several ways. Firstly the
management applications will need to handle LSPs with multiple
senders. This means that, although the applications need to process
fewer LSPs, they will be more complicated and will, in fact, need to
process the same number of ingresses and egresses. Other issues like
diagnostics and OAM would also need to be enhanced to support MP2P,
but might be borrowed heavily from LDP networks.
Lastly, note that the MP2P solution requires support of twice as many
LSPs at the PEs and requires tunneling to be introduced at the PEs.
Since PEs are not usually as fully specified as P-routers, this may
cause some concern although the use of previous hop popping on the
MP2P LSPs might help to reduce this issue.
In all cases, care must be taken not to confuse the reduction in the
number of LSPs with a reduction in the LSP state that is required. In
fact, the discussion in section 7.1 is slightly over-optimistic since
LSP state towards the destination will probably need to include
sender information and so will increase depending on the number of
sender for the MP2P LSP (the calculations do not take this into
account).
As a final note, observe that the MP2P scenario presented in this
section may be optimistic. MP2P LSP merging may be hard to achieve
between LSPs with significantly different traffic and QoS parameters.
Therefore, it may be necessary to increase the number of MP2P LSPs
arriving at an egress.
8. Combined models
There is nothing to prevent the combination of hierarchical and MP2P
solutions within a network. Note, however, that if MP2P LSPs are
tunneled through P2P FA LSPs across the core, none of the benefit of
LSP merging is seen for the hops during which the MP2P LSPs are
tunneled.
It is possible to construct solutions where FA LSPs are managed as
MP2P LSPs and additional significant savings are made.
9. Management Considerations
The management issues of the two models presented in this document
have been discussed in line. Neither solution is without its
management overhead.
Note, however, that scalability of management tools is one of the
motivators for this work and that network scaling solutions that
reduce the active management of LSPs at the cost of additional effort
to manage the more static elements of the network represent a
benefit. That is, it is worth the additional effort to set up MP2P or
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FA LSPs if it means that the network can be scaled to a larger size
without being constrained by the management tools.
10. Security Considerations
The techniques described in this document use existing or
yet-to-be-defined signaling protocol extensions and are subject to
the security provided by those extensions. Note that we are talking
about tunneling techniques used within the network and both
approaches are vulnerable to the creation of bogus tunnels that
deliver data to an egress or consume network resources.
The MP2P technique may prove harder to debug through OAM methods than
the FA LSP approach.
11. Recommendations
At this stage, no recommendations are made, but it would be valuable
to consult more widely to discover:
- The concerns of other service providers with respect to network
scalability.
- More opinions on the realistic constraints to the network
parameters listed in section 4.
- Desirable values for the cost-effectiveness of the network
(parameter K)
- The applicability, manageability and support for the two techniques
described
- The feasibility of combining the two techniques as discussed in
section 8.
12. IANA Considerations
This document makes no requests for IANA action.
13. Acknowledgements
TBD
14. Intellectual Property Consideration
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
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Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
15. Normative References
[RFC4206] Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP)
Hierarchy with Generalized Multi-Protocol Label
Switching (GMPLS) Traffic Engineering (TE)", RFC 4206,
October 2005.
16. Informational References
[RFC2961] Berger, L., Gan, D., Swallow, G., Pan, P., Tommasi,
F. and S. Molendini, "RSVP Refresh Overhead
Reduction Extensions", RFC 2961, April 2001.
[RFC3036] Andersson, L., Doolan, P., Feldman, N., Fredette, A.
and B. Thomas, "LDP Specification", RFC 3036,
January 2001.
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T.,
Srinivasan, V. and G. Swallow, "RSVP-TE: Extensions
to RSVP for LSP Tunnels", RFC 3209, December 2001.
[AUTO-MESH] Vasseur, JP., and Le Roux, JL., "Routing extensions
for discovery of Multiprotocol (MPLS) Label Switch
Router (LSR) Traffic Engineering (TE) mesh
membership", draft-ietf-ccamp-automesh, work in
progress.
[MP2P-RSVP] Yasukawa, Y., "Supporting Multipoint-to-Point Label
Switched Paths in Multiprotocol Label Switching
Traffic Engineering",
draft-yasukawa-mpls-mp2p-rsvpte, work in progress.
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17. Authors' Addresses
Seisho Yasukawa
NTT Corporation
9-11, Midori-Cho 3-Chome
Musashino-Shi, Tokyo 180-8585 Japan
Phone: +81 422 59 4769
EMail: yasukawa.seisho@lab.ntt.co.jp
Adrian Farrel
Old Dog Consulting
EMail: adrian@olddog.co.uk
18. Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
19. Full Copyright Statement
Copyright (C) The Internet Society (2006). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
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