Internet DRAFT - draft-hares-lsr-grid-ring-convergence
draft-hares-lsr-grid-ring-convergence
LSR Working Group S. Hares
Internet-Draft Huawei
Intended status: Informational February 11, 2019
Expires: August 15, 2019
IPRAN Grid-Ring IGP convergence problems
draft-hares-lsr-grid-ring-convergence-00.txt
Abstract
This draft describes problems with IGP convergence time in some IPRAN
networks that use a physical topology of grid backbones that connect
rings of routers. Part of these IPRAN network topologies exist in
data centers with sufficient power and interconnections, but some
network equipment sits in remote sites impacted by power loss. In
some geographic areas these remote sites may be subject to rolling
blackouts. These rolling power blackouts could cause multiple
simultaneous node and link failures. In these remote networks with
blackouts, it is often critical that the IPRAN phone network re-
converge quickly.
The IGP running in these networks may run in a single level of the
IGP. This document seeks to briefly describe these problems to
determine if the emerging IGP technologies (flexible algorithms,
dynamic flooding, layers of hierarchy in IGPs) can be applied to help
reduce convergence times. It also seeks to determine if the
improvements of these algorithms or the IP-Fast re-route algorithms
are thwarted by the failure of multiple link and nodes.
Status of This Memo
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This Internet-Draft will expire on August 15, 2019.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. IPRAN Topologies . . . . . . . . . . . . . . . . . . . . . . 3
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Requirements language . . . . . . . . . . . . . . . . . . 7
4. Problem detection using theoretical IGP Convergence . . . . . 8
4.1. Equation applied to Data Center IGP Convergence . . . . . 9
4.2. Flooding Problem on the Rings . . . . . . . . . . . . . . 11
4.3. Flooding problem on the grid . . . . . . . . . . . . . . 12
5. Multiple simultaneous link and node failures . . . . . . . . 12
5.1. Multiple link failures on Ring . . . . . . . . . . . . . 13
5.2. Multiple link failures on Grid . . . . . . . . . . . . . 14
6. Problem with Flat ISIS areas . . . . . . . . . . . . . . . . 14
7. Problems with Dense Flooding Algorithm . . . . . . . . . . . 15
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
8.1. Normative References: . . . . . . . . . . . . . . . . . . 15
8.2. Informative References . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction
This draft describes problems with IGP convergence time in some IPRAN
networks. The physical topologies of these IPRAN networks combine a
grid backbone topology with a ring topology to support phone networks
(see figure 1). Routers are attached to the rings that route traffic
from the IPRAN devices (see figure 2). Each of the rings is attached
to two grid nodes in order to provide redundancy. All of the routers
in the IPRAN ring-grid network topology run a single IGP (IS-IS).
Some current deployments attach 10-30 routers per ring with a 20 by
20 grid of routers. In these deployments, a grid of 400 routers
supports between 10,000 - 15,000 routers on the IPRAN rings.
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Convergence of the IGP after a single link failure on one ring router
is over 1 second for these topologies. The desired convergence time
for a single link failure is less than 200 ms for phone networks.
Initial convergence of the full network may take on the order of
minutes.
Part of these IPRAN network topologies exist in data centers with
sufficient power and interconnections, but some network equipment
sits in remote sites impacted by power loss. In some geographic
regions, these remote sites may be subject to rolling blackouts.
These rolling power blackouts could cause multiple simultaneous link
or node failures. In these remote networks with blackouts, it is
often critical that the IPRAN network converge quickly to restore
what mobile phone service it can. Keeping isolated portions of the
network working may be critical to keep some phone service working.
Converging the isolated portions back into the network when repairs
are made also causes further disruptions.
Due to the topologies of the IPRAN network, this document examines
how the flooding of IGP informations causes the longer IGP
convergence times for single links. The potential multiple
simultaneous link and node failures mean that the assumptions in most
IGP and fast IP-Route algorithms do not apply.
This document seeks to briefly describe these problems to determine
if the following emerging IGP technologies an be applied to solve the
convergence problem:
flexible algorithms [I-D.ietf-lsr-flex-algo],
dynamic flooding [I-D.li-lsr-dynamic-flooding],
Level 1 abstraction for ISIS [I-D.li-area-abstraction]
hierarchical IS-IS [I-D.li-hierarchical-isis]
2. IPRAN Topologies
A bit of background on the IPRan sizes.
Grid topologies can be any size of square topologies. Figure 1 shows
a 3 router by 3 router topologies (3x3) with 9 nodes). Other sizes
could be 10 routers by 10 routers (10X10) with 100 nodes, 15 routers
by 15 routers (15X15) with 225 routers, or 50 nodes by 50 nodes
(20X20) with 400 routers. A grid with network topology of a 100x100
grid would have 10,000 gird-routers (grid only and ring-grid).
Suppose that for every two grid nodes, 3 rings would be attached and
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on each ring there are 50 nodes. This topology would result in
750,000 ring routers plus 10,000 grid routers. The size of this
topology rivals data center sizes, but the IPRAN network does not
have the infrastructure advantages of the data center.
+-----+ +-------+ +-----+
|Node |===Ring10==| Node | |Node |==Ring1======|
| A |===Ring11==| B | | C |==Ring2====| |
| |===Ring12==| | | |==Ring3==| | |
+-+-+-+ +-+-+-+-+ ++--+-+ | | |
| | | | | | | | | |
| +---------------+ | +-----+ | | | |
| | | | | |
+-----+ +---+---+ +---+-+ | | |
|Node |===Ring3===| Node | |Node |==Ring3==| | |
| H |===Ring4===| G |----+ I |==Ring2====+ |
| |===Ring5===| | | |==Ring1======+
+-+-+-+ +-+-+-+-+ +-+-+-+
| | | | | | |
| +---------------+ | +--------+ |
| | |
| +---------------+ | +--------+ |
| | | | | | |
+-|-+-+ +-+-+-+-+ +-+-+-+
|Node |===Ring20==| Node | |Node |
| F |===Ring21==| E | | D |
| |===Ring25==| | | |
+-+-+-+ +-+-+-+-+ +-+-+-+
| | | | | | |
Figure 1
Figure 1: Example IPRAN Grid-Ring Topology
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+----+ +----+ +-----+
|Ring| |Ring|... |Ring |
|Rtr1| |Rtr2|... |RTR30|
+--+-+ +--+-+ ++-+--+
+-----+ | | | +-------+
|Node |==Ring1==+=======+==========+====| Node |
| | | |
| A |==Ring2==+=======+===========+===| B |
| | | | | | |
+-----+ +--+-+ +--+-+ +--+--+ +-------+
|Ring| |Ring|... |Ring |
|Rtr1| |Rtr2|... |RTR50|
+--+-+ +--+-+ +--+--+
Figure 2
Figure 2: Example IPRAN Ring Topology
One characteristics of a grid is that a basic 3X3 square can be
overlaid on most grids. Figure 3 shows a 10 by 10 grid with 3 by 3.
Notice that the grid squares overlaid on column 10 and row 10 form
partial squares (see GS4, GS8, GS12, GS13, GS14, GS15, and GS16).
If additional connections were made most of column 10 could form a
single Grid (GS4, GS8, and GS12), and most of row 10 could form a
single grid (GS13, GS14, and GS15). Alternatively, with a single
connection, GS16 could merge with GS15 to form a partial grid of 4
nodes.
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X = Grid node
GS = Grid Square 1
GS1 GS2 GS3 GS4
+-------+-------+-------+---+
| X X X | X X X | X X X | X |
| X X X | X X X | X X X | X |
| X X X | X X X | X X X | X |
+-------+-------+-------+---+
GS5 GS6 GS7 GS8
+-------+-------+-------+---+
| X X X | X X X | X X X | X |
| X X X | X X X | X X X | X |
| X X X | X X X | X X X | X |
+-------+-------+-------+---+
GS9 GS10 GS11 GS12
+-------+-------+-------+---+
| X X X | X X X | X X X | X |
| X X X | X X X | X X X | X |
| X X X | X X X | X X X | X |
+-------+-------+-------+---+
GS13 GS14 GS15 GS16
+-------+-------+-------+---+
| X X X | X X X | X X X | X |
+-------+-------+-------+---+
Figure 3
Figure 3: Overlaying Grid Squares on IPRAN Grid
The grid topology is currently one flat IGP. However, logical grid
squares could form Level 1 areas within the IGP. If one desired to
create an L1 Area abstraction such as defined
[I-D.li-area-abstraction], then the grid-square areas could be
created as L1 areas and connected by 1-3 links to adjacent areas.
Figure 4 shows a logical topology for grid squares 1-8 from figure 2.
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X = Grid node
G = Grid node G, Area Leader
GSn = Grid Square n (1-8)
Layer 2 area (1-8)
GS1 GS2 GS3 GS4
+-------+-------+-------+---+
| X X X | X X X-|-E X X | X |
| X G E---E G E-|-E G E-|-G |
| E E E---E E E | X E E | X |
+-|-|---+-|-|-|-+---|-|-+-|-|
| E E X | E E E | X E E | E |
| X G X | X G X | X G X | G |
| X X X | X X X | X X X | X |
+-------+-------+-------+---+
GS5 GS6 GS7 GS8
Figure 4
Figure 4: Grid Squares Area Leaders and Area Edge Nodes
3. Definitions
This section provides definitions for nodes within the IPRAN routing
infrastructure:
ring router: a routing device only attach to a ring in an IPRAN
topology which routes end-system information
ring-grid router routing device attached to ring and the grid
topology
grid router: a routing device which is only attached to the IPRAN
Grid network
pseudo-node for grid area: a pseudo-node which summarizes for an
IGP a grid area at one level for a higher level.
3.1. Requirements language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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4. Problem detection using theoretical IGP Convergence
Theoretical "best" convergence times for a single link failure on
ring depths of 30 nodes suggests the flooding time is a major
component for the flat IGP. Estimates of theoretical best
convergence times may be based on set of equations shown in figure 5.
These equations show how network convergence is the maximum time for
the information on a link change (down (failure) or up) to spread to
all routers in the network. The change travels along a pathway of
routers from the change to any particular router. Therefore,
convergence is really topology dependent on the convergence time in
each router and the pathways.
The theoretical convergence equations in figure 5 include updating
the RIB/FIB (Trib) and forwarding elements (Tdd). Some IGPS may
forward IGP traffic after calculating the SPF (Tspf)and updating the
RIB/FIB, but before updating the FIB line cards (Tdd). In this case,
these factors would be zero in the equation.
If several factors are zero or a constant, then the convergence may
be determined by one element in the equation that dominates the
convergence per node.
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CT-Node = Td + To + Tf + Tspf + Trib + Tdd
CT-Node = Node convergence time
Td = link failure detection time
(or link up detection time)
To = time to originate LSP
describing the new topology
Tf = Time to flood the change
from this node to other nodes
that must perform a flood update
Tspf = Time for shortest path calculation
Trib = Time to update the RIB and FIB
Tdd = time to distribute the FIB to line cards
CT-path(i) = sum [CT-Node(j), .. CT-Node-(n))
where i = path through network
j = nodes on path (1..n)
CTnetwork = maximum (CT-path(i))
where i = 0..n paths
Figure 5
Figure 5: Convergence equations
[My first experience with an equation like this was Cengiz
Alaettinoglu research in IGP around 2000 at NANOG. (Please let me
know if you have a good scholarly reference or presentation reference
for these equations).]
4.1. Equation applied to Data Center IGP Convergence
Some early SPF implementations were slow with large IGP topologies.
In this case, IGP's SPF calculations dominates the convergence time
for all nodes. Thus the Tspf dominates the time for each network
path and the entire networks convergence time. One might summarize
the convergence as:
CT-network = (Tspf + constant) * maximum path-length
The maximum path length is often called the network depth. The
network depth of a full mesh network is 1. The network depth of a
dense mesh fat tree in a data center with 3 levels (top of rack,
aggregate, spine) is 3. If Tspf dominates the calculation then:
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CT-network = (Tspf + constant) * 3
Centralized algorithms might improve convergence time if Tspf is the
main factor. Rather than using routers with typically low
calculation power, centralized devices could be optimized for the
calculation. If the difference in network depth of sending the
information end-to-end on any network path and sending it to the
centralized processor and back is minimal, then centralized
processing may be more effective.
If flooding (Tf) dominates the per node convergence, the equation is:
CT-network = (Tf + constant) * 3
Many of the authors of the IGP flooding enhancements to reduce the
data flooded understand that the flooding depends on the maximum
pathway length for pathways in the IGP graph. (see 802.1aq
[I-D.allan-lsr-flooding-algorithm], Li et al.
[I-D.li-lsr-dynamic-flooding], Shen, Ginsberg, and Thyamagundalu
[I-D.shen-isis-spine-leaf-ext]). Others mention creating a sub-graph
of the entire topology to reduce the flooding traffic and reduce
convergence time (Chen et al. [I-D.cc-ospf-flooding-reduction]).
Some of the IGP flooding reductions are identifying and limiting the
number of global pathways without mentioning their concern for
length. (see Chunduri and Eckert [I-D.ce-lsr-ppr-graph]).
The point behind this is that each algorithm has a set of goals.
Those goals may impact other things that impact convergence. Some
questions one can ask are:
o Does the algorithm seek to reduced data flooded and stored?
o Does the algorithm seek to reduce convergence time?
o If the algorithm tries to both reduce the data flooded and stored,
what trade-offs did the algorithm make?
o what is the impact of the topology?
If one looks to adapt the algorithms developed for the dense
interconnections of the 3 tier data center to the IPRAN Grid-ring
network structure, these questions are important.
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4.2. Flooding Problem on the Rings
Putting 30 or 50 ring routers on a ring may help operational costs.
Within a city the higher density of rings may allow more cells for
the phone. In the rural networks, it may allow the cells to be
deployed over a larger physical area.
Every router one puts on a ring increases the network depth of the
path through a fully operational ring or a partitioned ring that is
still connected to the network. The network depth of a ring is
network depth = (n-ring-nodes + n-grid-ring)/2
where
n-ring-nodes = 30 to 50 nodes
n-grid-nodes = 2 nodes
A partitioned ring may have the full network depth if the link
between a grid-router and the ring router attached to it fails.
This flooding time is only for the on-ring path. For a network path
that involves the link failure of a ring router link the pathway is:
network depth = depth(failed-ring) +
depth(grid) +
depth(remote-ring)
depth(failed-ring)= network depth of ring with
failed link.
depth(grid) = network depth of pathway
through Grid
depth(remote-ring) = network depth of pathway
through remote ring
Figure 6
Figure 6: Convergence equations
The worse case IGP convergence time combines the worse case for each
of these network depths.
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4.3. Flooding problem on the grid
The network depth of grid topologies grows as the size of the grid
grows from 3X3 to 10X10 to 100X100. The network depth of the best
case pathway through the grid is a single hop as it is on the same
ring-grid router. The worse case path is the one from x1 to X2 in
figure 7. A network pathway that goes from x1 to X2 by using routers
in the following grid squares: pathway of GS2, GS3, GS4, GS8, GS12
could take 19 hops.
X = Grid node
GS = Grid Square 1
GS1 GS2 GS3 GS4
+-------+-------+-------+---+
|X1|X X | X X X | X X X | X |
|--+ | | | |
| X X X | X X X | X X X | X |
| X X X | X X X | X X X | X |
+-------+-------+-------+---+
GS5 GS6 GS7 GS8
+-------+-------+-------+---+
| X X X | X X X | X X X | X |
| X X X | X X X | X X X | X |
| X X X | X X X | X X X | X |
+-------+-------+-------+---+
GS9 GS10 GS11 GS12
+-------+-------+-------+---+
| X X X | X X X | X X X | X |
| X X X | X X X | X X X | X |
| X X X | X X X | X X X | X |
+-------+-------+-------+---+
GS13 GS14 GS15 GS16
+-------+-------+-------+---+
| X X X | X X X | X X X | X2|
+-------+-------+-------+---+
Figure 6
Figure 7: Worse Case for 10X10 Grid
5. Multiple simultaneous link and node failures
Part of these IPRAN network topologies exist in data centers with
power and connective, but some do not. Ring routers are more likely
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to be at remote sites where power loss can occur. However, some
ring-grid routers or grid-only routers may be in remote sites.
In some geographic locations, power losses can be rolling blackouts
that cause multiple link and node outages during the failure. These
outages may be unpredictable due to weather or natural disasters, or
semi-predictable due to brownouts. Upon attempts to restore power,
the restorations may have mixed combinations of links and nodes up.
Multiple simultaneous link and node failures may impact both the ring
topologies and the grid topologies in the IPRAN network.
For simplicity of this discussion, I will present the node outages as
the outages of all links. A node outage may take far longer if
rebooting the routers or reconfiguring spare ring routers takes a
long time. For this initial pass on this document, I will simply
treat node outages as failure of all links for a time period that
clear all valid paths.
Most fast re-route technology such LFA [RFC5286] or MRT [RFC7812]
set-up IP backup paths to route around a single link or node failure.
In fact, the MRT architecture explicitly states that
"MRT-FRR creates two alternative forwarding trees that ... are
maximally diverse from one another, providing link and node
protect for 100% of paths and failures as long as the failures do
not cut the network into multiple pieces"
5.1. Multiple link failures on Ring
Ring routers may be located at sites that may lose connection to the
ring or to a grid-ring router. A single link failure may cut the
ring, but leave all nodes attached if the failed link is between one
of the ring routers (single on ring) or between the a ring-grid
routers and a ring router.
Multiple link failures on a ring will cause the ring to partition,
isolating some nodes. One way to handle this is to ignore the
convergence on the partitioned rings. Since local phone service
during these outages may be useful, it may be important for the IGPs
on the isolated portions of the rings to continue to operate. During
the restoration phase, additional links may appear to go up and down
as the partitions heal. Several isolated portions of the ring may be
restored to form a larger isolated portion of the ring. Eventually,
the isolated parts should reconnect to a fully connected ring.
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5.2. Multiple link failures on Grid
Multiple link failures can occur on the ring-grid routers or grid-
only routers. These failures may dramatically impact the data
forwarding pathways through the grid and the flooding pathways. Fast
convergence of the grid depends on an algorithm tuned for the grid
topologies.
The failures on the grid can impact different parts of the IGP
convergence algorithm.
6. Problem with Flat ISIS areas
Abstraction in an IGP can provide a logical means to scale IGPs.
Creating 2 levels of topology in the IPRAN network based on ISIS
areas could reduce the network depth and the the size of the topology
database in level devices.
However, as Li states in [I-D.li-area-abstraction] the ISIS concepts
work well if:
o "the Level 1 area is tangential to the Level 2 area", or
o if "there are a number of routers in both level 1 and level 2 and
they are adjacent".
However it does not work well if Level 1 area needs to provide
transit for level 2 traffic.
Suppose all ring routers networks were placed in level 1 areas, and
grid-only routers were in level 2. The ring-grid routers are in both
level 1 and 2. This reduces the current topology to a topology
similar to the spine-leaf topology. While this reduces the amount of
LSP stored, it may not significantly improve IGP convergence. The
flooding topology must be examined to determine the maximum network
depth, and the router operations must be examined to determine the
per IGP flooding time.
It also restricts repair of an L2 Grid path via a L1 Ring. This
repair might be necessary in the multi-failure scenario.
The area abstraction described in [I-D.li-area-abstraction] could be
used to remove these restrictions.
Additional levels of hierarchy described by Li in
[I-D.li-hierarchical-isis] could be utilized in the grid to allow
additional levels of abstractions. These levels could reduce the
network depth that IGP flooding passes through.
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One difficulty with using abstraction provided by areas and levels is
the configuration of the appropriate network topology with multiple
levels, and reconfigurations of these levels. To be effective for
100X100 grids, it would be beneficial to automate the configuration
of areas.
7. Problems with Dense Flooding Algorithm
o spine-leaves - rings may be leaves, but grid is not spine-leave
topology.
o sparse link flooding - Grid may have too little or too much. Top
priority is fast convergence not reduced load of LSPF, but fast
convergence.
o preferred path graph - goal is preferred path reduction of the
number of preferred paths through network. Fast re-route also
sets up paths. The preferred path graph needs to be carefully
integrated with any fast reroute scheme.
o flooding of 802.1aq - is designed for dense mesh.
* The algorithm's two tree structure of 802.1aq provide complete
coverage in the presence of a single link failure while
constraining the number of LSAs.
* Both trees in the two structure have the same convergence
properties in the IPRAN ring and grid.
8. References
8.1. Normative References:
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
8.2. Informative References
[I-D.allan-lsr-flooding-algorithm]
Allan, D., "A Distributed Algorithm for Constrained
Flooding of IGP Advertisements", draft-allan-lsr-flooding-
algorithm-00 (work in progress), October 2018.
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[I-D.cc-ospf-flooding-reduction]
Chen, H., Cheng, D., Toy, M., and Y. Yang, "LS Flooding
Reduction", draft-cc-ospf-flooding-reduction-04 (work in
progress), September 2018.
[I-D.ce-lsr-ppr-graph]
Chunduri, U. and T. Eckert, "Preferred Path Route Graph
Structure", draft-ce-lsr-ppr-graph-01 (work in progress),
October 2018.
[I-D.ietf-lsr-flex-algo]
Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and
A. Gulko, "IGP Flexible Algorithm", draft-ietf-lsr-flex-
algo-01 (work in progress), November 2018.
[I-D.li-area-abstraction]
Li, T., "Level 1 Area Abstraction for IS-IS", draft-li-
area-abstraction-00 (work in progress), June 2018.
[I-D.li-hierarchical-isis]
Li, T., "Hierarchical IS-IS", draft-li-hierarchical-
isis-00 (work in progress), June 2018.
[I-D.li-lsr-dynamic-flooding]
Li, T., Psenak, P., Ginsberg, L., Przygienda, T., Cooper,
D., Jalil, L., and S. Dontula, "Dynamic Flooding on Dense
Graphs", draft-li-lsr-dynamic-flooding-02 (work in
progress), December 2018.
[I-D.shen-isis-spine-leaf-ext]
Shen, N., Ginsberg, L., and S. Thyamagundalu, "IS-IS
Routing for Spine-Leaf Topology", draft-shen-isis-spine-
leaf-ext-07 (work in progress), October 2018.
[RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
IP Fast Reroute: Loop-Free Alternates", RFC 5286,
DOI 10.17487/RFC5286, September 2008,
<https://www.rfc-editor.org/info/rfc5286>.
[RFC7812] Atlas, A., Bowers, C., and G. Enyedi, "An Architecture for
IP/LDP Fast Reroute Using Maximally Redundant Trees (MRT-
FRR)", RFC 7812, DOI 10.17487/RFC7812, June 2016,
<https://www.rfc-editor.org/info/rfc7812>.
Hares Expires August 15, 2019 [Page 16]
Internet-Draft IPRAN-IGP-Converge February 2019
Author's Address
Susan Hares
Huawei
Saline
US
Email: shares@ndzh.com
Hares Expires August 15, 2019 [Page 17]
