Routing Area Working Group R. White
Internet-Draft Verisign
Intended status: Informational S. Hares
Expires: August 22, 2013 Hickory Hill Consulting
R. Fernando
Cisco Systems
February 18, 2013
Use Cases for an Interface to the Routing System
draft-white-i2rs-use-case-00
Abstract
Programmatic interfaces to provide control over individual forwarding
devices in a network promise to reduce operational costs while
improving scaling, control, and visibility into the operation of
large scale networks. To this end, several programmatic interfaces
have been proposed. OpenFlow, for instance, provides a mechanism to
replace the dynamic control plane processes on individual forwarding
devices throughout a network with off box processes that interact
with the forwarding tables on each device. Another example is
NETCONF, which provides a fast and flexible mechanism to interact
with device configuration and policy.
There is, however, no proposal which provides an interface to all
aspects of the routing systemas a system. Such a system would not
interact with the forwarding system on individual devices, but rather
with the control plane processes already used to discover the best
path to any given destination through the network, as well as
interact with the routing information base (RIB), which feeds the
forwarding table the information needed to actually switch traffic at
a local level.
This document describes a set of use cases such a system could
fulfill. It is designed to provide underlying support for the
framework, policy, and other drafts describing the Interface to the
Routing System (IRS).
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on August 22, 2013.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Optimized Exit Control . . . . . . . . . . . . . . . . . . . . 4
3. Distributed Reaction to Network Based Attacks . . . . . . . . 7
4. Remote Service Routing . . . . . . . . . . . . . . . . . . . . 8
5. Within Data Center Routing . . . . . . . . . . . . . . . . . . 10
6. Temporary Overlays between Data Centers . . . . . . . . . . . 12
7. Central membership computation for MPLS based VPNs . . . . . . 13
8. Normative References . . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
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1. Introduction
The Interface to the Routing System Framework [IRS] desribes a
mechanism where the distributed control plane can be augmented by an
outside control plane through an open, accessible interface,
including the Routing Information Base (RIB), in individual devices.
This represents a "halfway point" beteween completely replacing the
traditional distributed control plane and directly configuring
devices to distribute policy or modifications to routing through off-
board processes. This draft proposes a set of use cases that explain
where the work described in [IRS] will be useful. The goal is to
inform not only the community's understanding of where IRS fits in
the larger scheme of SDN proposals, but also to inform the
requirements, framework, and specification of IRS to provide the best
fit for the purposes which make the most sense for this type of
programmatic interface.
Towards this end the authors have searched for a number of different
use cases representing not only complex modifications of the control
plane, including interaction with applications and network
conditions, but also simpler use cases. The array of use cases
presented here should provide the reader with a solid understanding
of the power of an SDN solution that will augment, rather than
replace, traditional distributed control planes.
Each use case is presented in its own section.
2. Optimized Exit Control
At edges where traffic exits along two or more possible paths, it is
often desirable to choose a path based on more information the
dynamic control plane provides. For instance, a network operator may
want to take into account factos such as:
o Cost per unit of data sent, indluding time of day variations,
surcharges over a specific amount of data transmitted, and
surcharges for transmitting data to specific types of
destinations.
o Urgency of data traffic or flow.
o Exit point performance, including historical jitter, delay, and
available bandwidth, possibly on a per destination basis.
o Availability of a specific destination through a given link at the
per destination basis (more specific than the routing protocol
provides).
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A number of possible solutions have been proposed or deployed in the
past. For instance, the necessary metrics could be added to [BGP],
or any other routing protocol, to provide the necessary information,
and fine-tuned algorithms could be developed and deployed. Massive
changes to well known and understood distributed control plane
protocols to resolve a single use case, however, are not likely to be
productive for the community as a whole. It's often difficult to
justify the added complexity in the database and algorithms of
routing protools to solve what is considered a point case.
Another alternative has been the development of specific appliances
designed to monitor the information necessary to provide an optimal
edge decision, and then to use some automated configuration mechanism
to transmit the decision to the edge routers. An example is
illustrated in the figure below.
|-----------------R1-----------|
| | |
Internal Network Controller External Network
| | |
|-----------------R2-----------|
The controller in this network must:
o Discover the topology of the network from R1 and R2.
o Compare the current traffic flow information to policies set
administratively by the network operator.
o Monitor the flow of traffic from the perspective of R1 and R2.
o Inject forwarding information to directly impact the traffic flow
at the edge devices, or modify the policy of the existing
distributed (dynamic) control plane already running in the
network.
Many of these steps is challenging for currently available solutions.
To discover the topology at the edge rotuers, the controllers can
either participate in the control plane, or walk the local routing
table using a network management protocol. Neither of these options
are optimal in this case because the controlling process cannot
interact dynamically with the local topology information in near real
time through such mechanisms.
Injecting forwarding information directly into the RIB on the
individual devices in this network is possible today through the
configuration of static routes through some external mechanism, such
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as SNMP, NETCONF, or by direct external interaction with the devices'
CLI. None of these options are attractive because:
o They modify the actual configuration of the device (unlike a
dynamic routing process).
o They are too persistent (routes installed through static
configuration persist across device reboots).
o The controller cannot interact with the routing table in parallel
with other routing processes. For instance, when a routing
process attempts to install a new route in the routing table,
there is often a callback or other notification to the other
routing processes running on the same device; this notification
provides important information the controller can take into
account in its view of the current state of the routing table, and
the state of the device's routing table. Interface level events
also often trigger notifications from the RIB to local routing
processes; these notifications would be invaluble for the
controller to modify injected routing state in reaction to network
topology events.
o Routes installed through the an off box controller through the CLI
or XML interface are difficult to redistribute into other
protocols to draw traffic to a specific exit point, and it can be
difficult to fine tune how these injected routes interact with
routes learned through other routing processes.
IRS can resolve these issues by providing an open interface to the
local RIB on each device, allowing the controller to interact with
the RIB just as a local routing process would. This would allow the
controlling process to see the topology information in the RIB
dynamically, receiving near real time updates for route removals,
installs, and other events, and without relying on static
configuration to inject forwarding information each device can use.
Summary of IRS Capabilities and Interactions:
o IRS should provide the ability to read the local RIB of each
forwarding device, including the destination prefix (NLRI), a
table identifier (if the forwarding device has multiple forwarding
instances), the metric of each installed route, a route
preference, and an identifier indicating the installing process.
o The ability to monitor the available routes installed in the RIB
of each forwarding device, including near real time notification
of route installation and removal. This information must include
the destination prefix (NLRI), a table identifier (if the
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forwarding device has multiple forwarding instances), the metric
of the installed route, and an identifier indicating the
installing process.
o The ability to install destination based routes in the local RIB
of each forwarding device. This must include the ability to
supply the destination prefix (NLRI), a table identifier (if the
forwarding device has multiple forwarding instances), a route
preference, a route metric, a next hop, an outbound interface, and
a route process identifier.
o The ability to interact with various policies configured on the
forwarding devices, in order to inform the policies implemented by
the dynamic routing processes. This interaction SHOULD be through
existing configuration mechanisms, such as NETCONF, and SHOULD be
recorded in the configuration of the local device so operators are
aware of the full policy implemented in the network from the
running configuration.
o The ability to interact with traffic flow and other network
traffic level measurement protocols and systems, in order to
determine path performance, top talkers, and other information
required to make an informed path decision based on locally
configured policy.
3. Distributed Reaction to Network Based Attacks
Quickly modifying the control plane to reroute traffic for one
destination while leaving a standard configuration in place (filters,
metrics, and other policy mechanisms) is a challenge --but this is
precisely the challenge of a network engineer attempting to deal with
a network incursion. The ability to redirect specific flows of
information or specific classes of traffic into, through, and back
out of traffic analyzers on the fly is crucial in these situations.
The following network diagram provides an illustration of the
problem.
Valid Source---\ /--R2--------------------\
R1 R3---Valid Destination
Attack Source--/ \--Monitoring Device-----/
Modifying the cost of the link between R1 and R2 to draw the attack
traffic through the monitoring device in the distributed control
plane will, of necessity, also draw the valid traffic through the
monitoring device. Drawing valid traffic through a monitoring device
introduces delay, jitter, and other quality of service issues, as
well as posing a problem for the monitoring device itself in terms of
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traffic load and management.
An IRS controller could stand between the detection of the attack and
the control plane to facilitate the rapid modification of control and
forwarding planes to either block the traffic or redirect it to
analysis devices connected to the network.
Summary of IRS Capabilities and Interactions:
o The ability to monitor the available routes installed in the RIB
of each forwarding device, including near real time notification
of route installation and removal. This information must include
the destination prefix (NLRI), a table identifier (if the
forwarding device has multiple forwarding instances), the metric
of the installed route, and an identifier indicating the
installing process.
o The ability to install source and destination based routes in the
local RIB of each forwarding device. This must include the
ability to supply the destination prefix (NLRI), the source prefix
(NLRI), a table identifier (if the forwarding device has multiple
forwarding instances), a route preference, a route metric, a next
hop, an outbound interface, and a route process identifier.
o The ability to install a route to a null destination, effectively
filtering traffic to this destination.
o The ability to interact with various policies configured on the
forwarding devices, in order to inform the policies implemented by
the dynamic routing processes. This interaction SHOULD be through
existing configuration mechanisms, such as NETCONF, and SHOULD be
recorded in the configuration of the local device so operators are
aware of the full policy implemented in the network from the
running configuration.
o The ability to interact with traffic flow and other network
traffic level measurement protocols and systems, in order to
determine path performance, top talkers, and other information
required to make an informed path decision based on locally
configured policy.
4. Remote Service Routing
In hub and spoke overlay networks, there is always an issue with
balancing between the information held in the spoke routing table,
optimal routing through the network underlying the overlay, and
mobility. Most solutions in this space use some form of centralized
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route server that acts as a directory of all reachable destinations
and next hops, a protocol by which spoke devices and this route
server communicate, and caches at the remote sites.
An IRS solution would use the same elements, but with a different
control plane. Remote sites would register (or advertise through
some standard routing protocol, such as BGP), the reachable
destinations at each site, along with the address of the router (or
other device) used to reach that destination. These would, as
always, be stored in a route server (or several redundant route
servers) at a central location.
When a remote site sends a set of packets to the central location
that are eventually destined to some other remote site, the central
location can forward this traffic, but at the same time simply
directly insert the correct routing information into the remote
site's routing table. If the location of the destination changes,
the route server can directly modify the routing information at the
remote site as needed.
An interesting aspect of this solution is that no new and specialized
protocols are needed between the remote sites and the centralized
route server(s). Normal routing protocols can be used to notify the
centralized route server(s) of modifications in reachability
information, and the route server(s) can respond as needed, based on
local algorithms optimized for a particular application or network.
For instance, short lived flows might be allowed to simply pass
through the hub site with no reaction, while longer lived flows might
warrant a specific route to be installed in the remote router.
Algorithms can also be developed that would optimize traffic flow
through the overlay, and also to remove routing entries from remote
devices when they are no longer needed based on far greater
intelligence than simple non-use for some period of time.
Summary of IRS Capabilities and Interactions:
o The ability to read the local RIB of each forwarding device,
including the destination prefix (NLRI), a table identifier (if
the forwarding device has multiple forwarding instances), the
metric of each installed route, a route preference, and an
identifier indicating the installing process.
o The ability to monitor the available routes installed in the RIB
of each forwarding device, including near real time notification
of route installation and removal. This information must include
the destination prefix (NLRI), a table identifier (if the
forwarding device has multiple forwarding instances), the metric
of the installed route, and an identifier indicating the
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installing process.
o The ability to install destination based routes in the local RIB
of each forwarding device. This must include the ability to
supply the destination prefix (NLRI), a table identifier (if the
forwarding device has multiple forwarding instances), a route
preference, a route metric, a next hop, an outbound interface, and
a route process identifier.
5. Within Data Center Routing
Data Centers have evolved into massive topologies with thousands of
server racks and millions of hosts. Data Centers use BGP with ECMP,
ISIS (with multiple LAGs), or other protocols to tie the data center
together. Data centers are currently designed around a three or four
tier structure with: server, top-of-rack switches, aggregation
switches, and router interfacing the data center to the Internet.
Microsoft's usage of BGP in the data center, described in [Lapukh-
BGP], examines many of these elements of data center design.
One key element of these Data Center routing infrastructures is the
ability to quickly read topology information and excute configuration
from a centralized location. Key to this environment is the tight
feedback loop between learning about topology changes or loading
changes, and instantiating new routing policy. Without IRS, may Data
Centers are using extra physical topologies or logical topologies to
work around the features.
For example, Microsoft's network uses BGP because the topology state
could be read from BGP impementations in a consistent fashion.
Microsoft might have chosen a different routing protocol (such as
ISIS) if the routing protocol state had been easier to obtain.
Microsoft chose BGP for the data center because routers had a good
BGP interface with topology information.
An IRS solution would use the same in the elements, but with a
different control plane. The IRS enable control plane could provide
the Data Center 4 tier infrastructure the quick access to topology
and data flow information needed for traffic flow optimization.
Changes to the Data Center infrastructure done via the IRS could have
a tight feedback loop.
Again, this solution would reduce the need for new and specialized
protocols while giving the Data Center the control it desire. The
IRS routing interface could be extended to virtual routers.
Summary of IRS Capabilities and Interactions:
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o The ability to read the local RIB of each forwarding device,
including the destination prefix (NLRI), a table identifier (if
the forwarding device has multiple forwarding instances), the
metric of each installed route, a route preference, and an
identifier indicating the installing process.
o The ability to monitor the available routes installed in the RIB
of each forwarding device, including near real time notification
of route installation and removal. This information must include
the destination prefix (NLRI), a table identifier (if the
forwarding device has multiple forwarding instances), the metric
of the installed route, and an identifier indicating the
installing process.
o The ability to install destination based routes in the local RIB
of each forwarding device. This must include the ability to
supply the destination prefix (NLRI), a table identifier (if the
forwarding device has multiple forwarding instances), a route
preference, a route metric, a next hop, an outbound interface, and
a route process identifier.
o The ability to read the tables of other local protocol processes
running on the device. This reading action SHOULD be supported
through an import/export interface which can present the
information in a consistent manner across all protocol
implementations, rather than using a protocol specific model for
each type of available process.
o The ability to inject information directly into the local tables
of other protocol processes running on the forwarding device.
This injection SHOULD be supported through an import/export
interface which can inject routing information in a consistent
manner across all protocol implementations, rather than using a
protocol specific model for each type of available process.
o The ability to interact with various policies configured on the
forwarding devices, in order to inform the policies implemented by
the dynamic routing processes. This interaction SHOULD be through
existing configuration mechanisms, such as NETCONF, and SHOULD be
recorded in the configuration of the local device so operators are
aware of the full policy implemented in the network from the
running configuration.
o The ability to interact with traffic flow and other network
traffic level measurement protocols and systems, in order to
determine path performance, top talkers, and other information
required to make an informed path decision based on locally
configured policy.
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6. Temporary Overlays between Data Centers
Data Centers within one organization may operate as one single entity
even though the Data Centers are geographically distributed fashion.
Applications are load balanced within Data Centers and between data
centers to take advantage of cost economics in power, storage, and
server availability for compute resources. Applications are also
transfer to alternate data centers in case of failures within a data
center. To reduce time during failure, Data Centers often replicate
user storage between two or more data centers. During the tranfer of
stored information prior to a Data Center to Data Center move, the
Data Center controllers need to dynamically aquire a large amount of
inter-data center bandwidth through an overlay network, often during
off hours.
IRS could provide the connection between the overlay network
configuration, local policies, and the control plane to dynamically
bring a large bandwidth inter-data center overlay or channel into
use, and then to remove it from use when the data transfer is
completed.
Similarly, during a fail-over, a control process within data centers
interacts with a group host process and the network to seamless move
the processing to another data center. During the fail-over case,
additional process state may need to be moved as well to restart the
system. The difference between these data-to-data center moves is
immediate and urgent need to move systems. If an application (such
as medical or banking services) pays to have this type of fail-over,
it is likely the service will pay for preemption on network
bandwidth. IRS can allow the Data Center network and the Network
connecting the data center to prempt other best-effort traffic to
send this priority data flow. After the high priority data flow has
finished, networks can return to their previous condition
Summary of IRS Capabilities and Interactions:
o The ability to read the local RIB of each forwarding device,
including the destination prefix (NLRI), a table identifier (if
the forwarding device has multiple forwarding instances), the
metric of each installed route, a route preference, and an
identifier indicating the installing process.
o The ability to monitor the available routes installed in the RIB
of each forwarding device, including near real time notification
of route installation and removal. This information must include
the destination prefix (NLRI), a table identifier (if the
forwarding device has multiple forwarding instances), the metric
of the installed route, and an identifier indicating the
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installing process.
o The ability to install destination based routes in the local RIB
of each forwarding device. This must include the ability to
supply the destination prefix (NLRI), a table identifier (if the
forwarding device has multiple forwarding instances), a route
preference, a route metric, a next hop, an outbound interface, and
a route process identifier.
o The ability to interact with various policies configured on the
forwarding devices, in order to inform the policies implemented by
the dynamic routing processes. This interaction SHOULD be through
existing configuration mechanisms, such as NETCONF, and SHOULD be
recorded in the configuration of the local device so operators are
aware of the full policy implemented in the network from the
running configuration.
o The ability to interact with policies and configurations on the
forwarding devices using time based processing, either through
timed auto-rollback or some other mechanism. This interaction
SHOULD be through existing configuration mechanisms, such as
NETCONF, and SHOULD be recorded in the configuration of the local
device so operators are aware of the full policy implemented in
the network from the running configuration.
o The ability to interact with traffic flow and other network
traffic level measurement protocols and systems, in order to
determine path performance, top talkers, and other information
required to make an informed path decision based on locally
configured policy.
7. Central membership computation for MPLS based VPNs
MPLS based VPNs use route target extended communities to express
membership information. Every PE router holds incoming BGP NLRI and
processes them to determine membership and then import the NLRI into
the appropriate MPLS/VPN routing tables. This consumes resources,
both memory and compute on each of the PE devices.
An alternative approach is to monitor routing updates on every PE
from the attached CEs and then compute membership in a central
manner. Once computed the routes are pushed to the VPN RIBs of the
participating PEs.
This centralization of membership control has a few advantages.
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o The membership mechanism (route-targets) need not be configured in
each of the PEs and can be expressed once centrally.
o No resources in the PEs need to be spent to categorize routes into
the VRF tables that they belong and to filter out unwanted state.
o Doing it centrally means the availability of almost unlimited
compute capacity to compute membership and hence can be done in a
scaleable manner.
o More sophisticated routing policies and filters can be applied
during the central import/export process than can be expressed and
performed using the traditional route target mechanism.
o Routes can be selectively pushed only to the participating PE's
further reducing the memory load on the individual routers in the
network. This further obviates for a distributed mechanisms such
as rt constraints to reduce unnecessary path state in the routers.
Note that centrally compution of membership can be applied to other
scenarios as well such as VPLS, MVPNs, MAC VPNs etc. Depending on
the scenario, what gets monitored from the CE might vary. Central
computation will especially help VPLS where multi-homing and load
balancing using distributed techniques has particularly been a
challenge.
Also note that one of the biggest promises of central route
computation is simplification and reduction of computation and memory
load on all devices in the network. This use case is just one
example that illustrates these benefits of central computation very
well.
Summary of IRS Capabilities and Interactions:
o The ability to read the loc-RIB-In BGP table that gets all the
routes that the CE has provided to a PE router.
o The ability to install destination based routes in the local RIB
of the PE devices. This must include the ability to supply the
destination prefix (NLRI), a table identifier, a route preference,
a route metric, a next-hop tunnel through which traffic would be
carried
8. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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Authors' Addresses
Russ White
Verisign
12061 Bluemont Way
Reston, VA 20190
USA
Email: riwhite@verisign.com
Susan Hares
Hickory Hill Consulting
7453 Hickory Hill
Saline, MI 48176
USA
Email: shares@ndzh.com
Rex E. Fernando
Cisco Systems
170 W Tasman Dr
San Jose, CA 95134
USA
Email: rex@cisco.com
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