One document matched: draft-so-yong-mpls-ctg-framework-requirement-00.txt
Network Working Group N. So
Internet-Draft A. Malis
Intended status: Standards Track D. McDysan
Expires: April 24, 2009 Verizon
L. Yong
Huawei USA
October 21, 2008
Framework and Requirements for Composite Transport Group (CTG)
draft-so-yong-mpls-ctg-framework-requirement-00
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Abstract
This document states a traffic distribution problem in today's IP/
MPLS network when multiple links are configured between two routers.
The document presents a Composite Transport Group framework as the
solution for the problems and specifies a set of requirements for
Composite Transport Group(CTG).
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions used in this document . . . . . . . . . . . . . . 4
2.1. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Problem Statements . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Incomplete/Inefficient Utilization . . . . . . . . . . . . 5
3.2. Inefficiency/Inflexibility of Logical Interface
Bandwidth Allocation . . . . . . . . . . . . . . . . . . . 6
4. Composite Transport Group Framework . . . . . . . . . . . . . 8
4.1. CTG Framework . . . . . . . . . . . . . . . . . . . . . . 8
4.2. Difference between CTG and A Bundled Link . . . . . . . . 10
4.2.1. Virtual Routable Link vs. TE Link . . . . . . . . . . 10
4.2.2. Component Link Parameter Independence . . . . . . . . 11
5. Composite Transport Group Requirements . . . . . . . . . . . . 12
5.1. CTG Appearance as a Routable Virtual Interface . . . . . . 12
5.2. CTG mapping of traffic to Component Links . . . . . . . . 12
5.2.1. Mapping Using Router TE information . . . . . . . . . 12
5.2.2. Mapping When No Router TE Information is Available . . 12
5.3. Bandwidth Control for Connections with and without TE
information . . . . . . . . . . . . . . . . . . . . . . . 13
5.4. CTG Transport Resilience . . . . . . . . . . . . . . . . . 14
6. Security Considerations . . . . . . . . . . . . . . . . . . . 15
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
9. Normative References . . . . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
Intellectual Property and Copyright Statements . . . . . . . . . . 20
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1. Introduction
IP/MPLS network traffic growth forces carriers to deploy multiple
parallel physical links between two routers. The network is also
expected to carry some flows of a rate that can approach that of any
single link or be very small comparing to a single link rate. There
is not an existing technology today that allows carriers to
efficiently utilize all parallel transport resources in a complex IP/
MPLS network environment. Composite Transport Group (CTG) provides
the local traffic engineering management over multiple parallel links
that solves this problem in MPLS networks.
The primary function of Composite Transport Group is to efficiently
transport aggregated traffic flows over multiple parallel links. CTG
can take the flow TE information into account when distributing the
flows over individual links to gain local traffic engineering
management and link failure protection. Because all links have the
same ingress and egress point, CTG does not need to perform route
computation and forwarding based on the traffic unit end point
information, which brings a unique local transport traffic
engineering scheme. CTG also manages the flows that do not have TE
information and associates them with CTG connections that have
assigned TE information based on auto bandwidth measurement, and use
the TE information in component link selection.
This document contains the problem statements and the framework and a
set of requirements for a Composite Transport Group (CTG). The
necessity for protcol extensions to provide solutions is for future
study.
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2. Conventions used in this document
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 [RFC2119].
2.1. Acronyms
BW: BandWidth
CTG: Composite Transport Group
ECMP: Equal Cost Multi-Path
FRR: Fast Re-Route
LAG: Link Aggregation Group
LDP: Label Distributed Protocol
LR: Logical Router
LSP: Label Switched Path
MPLS: Multi-Protocol Label Switch
OAM: Operation, Administration, and Maintenance
PDU: Packet Data Units
PE: Provider Edge device
RSVP: ReSource reserVation Protocol
RTD: Real Time Delay
TE: Traffic engineering
VRF: Virtual Routing & Forwarding
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3. Problem Statements
Two applications are described here that encounter the problems when
multiple parallel links are deployed between two routers in today's
IP/MPLS networks.
3.1. Incomplete/Inefficient Utilization
An MPLS-TE network is deployed to carry traffic on RSVP-TE LSPs, i.e.
traffic engineered flows. When traffic volume exceeds the capacity
of a single physical link, multiple physical links are deployed
between two routers as a single backbone trunk. How to assign LSP
traffic over multiple links and maintain this backbone trunk as a
higher capacity and higher availability trunk than a single physical
link becomes an extremely difficult task for carriers today. Three
methods that are available today are described here.
1. A hashing method is a common practice for traffic distribution
over multiple paths. This is used by Equal Cost Multi-Path
(ECMP) for IP services, and IEEE-defined Link Aggregation Group
(LAG) for Ethernet traffic. However, the traffic granularity in
a MPLS-TE network is individual LSPs, and they typically contain
a high rate of traffic flow(s) and have large differences in the
rates; furthermore, the links may be of different speeds. In
these cases hashing can cause some links to be congested while
others are partially filled because hashing can only distinguish
the flows, not the flow rates.
2. Assigning individual LSPs to each link through constrained
routing. A planning tool can track the utilization of each link
and assignment of LSPs to the links. To gain high availability,
FRR [RFC4090] is used to create a bypass tunnel on a link to
protect traffic on another link or to create a detour LSP to
protect another LSP. If reserving BW for the bypass tunnels or
the detour LSPs, the network will reserve a large amount of
capacity for failure recovery, which reduces the capacity to
carry other traffic. If not reserving BW for the bypass tunnels
and the detour LSPs, the planning tool can not assign LSPs
properly to avoid the congestion during link failure when there
are more than two parallel links. This is because during the
link failure, the impacted traffic is simply put on a bypass
tunnel or detour LSPs which does not have enough reserved
bandwidth to carry the extra traffic during the failure recovery
phase.
3. Facility protection, also called 1:1 protection. Dedicate one
link to protect another link. Only assign traffic to one link in
the normal condition. When the working link fails, switch
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traffic to the protected link. This requires 50% capacity for
failure recovery. This works when there are only two links.
Under the multiple parallel link condition, this causes
inefficient use of network capacity because there is no
protection capacity sharing. In addition, due to traffic
burstiness, having one link fully loaded and another link idle
increases transport latency and packet loss, which lowers the
link performance quality for transport.
None of these methods satisfies carrier requirement either because of
poor link utilization or poor performance. This forces carriers to
go with the solution of deploying single higher capacity link
solution. However, a higher capacity link can be expensive as
compared with parallel low capacity links of equivalent aggregate
capacity; a high capacity link can not be deployed in some
circumstances due to physical impairments; or the highest capacity
link may not large enough for some carriers.
An LDP network can encounter the same issue as an MPLS-TE enabled
network when multiple parallel links are deployed as a backbone
trunk. An LDP network can have large variance in flow rates where,
for example, the small flows may be carrying stock tickers at a few
kbps per flow while the large flows can be near 10 Gbps per flow
carrying machine to machine and server to server traffic from
individual customers. Those large traffic flows often cannot be
broken into micro flows. Therefore, hashing would not work well for
the networks carrying such flows. Without per-flow TE information,
this type of network has even more difficulty to use multiple
parallel links and keep high link utilization.
3.2. Inefficiency/Inflexibility of Logical Interface Bandwidth
Allocation
Logically-separate routing instances in some implementations further
complicates the situation. Dedicating separate physical backbone
links to each routing instance is not efficient. An alternative is
to assign a logical interface and bandwidth on each of the parallel
physical links to each routing instance, which improves efficiency as
compared with dedicating physical links to each routing instance.
Inefficiency can result if bandwidth on a logical interface is
dedicated to each routing instance. For example, if there are 2
routing instances and 3 parallel links and half of each link
bandwidth is assigned to a routing instance, then neither routing
instance can support an LSP with bandwidth greater than half the link
bandwidth.
Note that the traffic flows and LSPs from these different routing
instances effectively operate in a Ships-in-the-Night mode, where
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they are unaware of each other. Inflexibility results if there are
multiple sets of LSPs (e.g., from different routing instances)
sharing a set of parallel links, and at least one set of LSPs can
preempt another, then more efficient sharing of the link set between
the routing instances is highly desirable.
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4. Composite Transport Group Framework
4.1. CTG Framework
Composite Transport Group (CTG) is the method to transport aggregated
traffic over a composite link. A composite link defined in ITU-T
[ITU-T G.800] is a single link that bundles multiple parallel links
between the two same subnetworks. Each of component links of a
composite link is independent in the sense that each component link
is supported by a separate server layer trail. The composite link
conveys communication information using different server layer trails
thus the sequence of symbols across this link may not be preserved.
Composite Transport Group (CTG) is primarily a local traffic
engineering and transport technology over multiple parallel links or
multiple paths. The objective is for a composite link to appear as a
virtual interface to the connected routers. The router provisions
incoming traffic over the CTG connection. CTG connections are
transported over parallel links called Component Links. CTG
Component Links can be either physical links or logical links such as
LSP tunnels. The CTG distribution function can locally determine
which component link CTG connections should traverse. The major
components of CTG and their relationships are illustrated in Figure 1
below.
+---------+ +-----------+
| +---+ +---+ |
| | |============================| | |
LSP,LDP,IP| | C |~~~~~~5 CTG Connections ~~~~| C | |
~~~|~~>~~| |============================| |~~~>~~~|~~~
~~~|~~>~~| T |============================| T |~~~>~~~|~~~
~~~|~~>~~| |~~~~~~3 CTG Connections ~~~~| |~~~>~~~|~~~
| | G |============================| G | |
| | |============================| | |
| | |~~~~~~9 CTG connections~~~~~| | |
| | |============================| | |
| R1 +---+ +---+ R2 |
+---------+ +-----------+
! ! ! !
! !<----Component Links ------>! !
!<------ Composite Link ----------->!
Figure 1: Composite Transport Group Architecture Model
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In Figure 1, a composite link is configured between router R1 and R2.
The composite link has three component links. CTG creates a CTG
connection and select a component link for the CTG connection. LSP,
LDP, and IP traffic are mapped to CTG connections. A CTG connection
only exists in the scope of a composite link. The traffic in a CTG
connection is transported over a single component link.
A CTG connection is a point-to-point logical connection over a
composite link. The connection rides on component link in a one-to-
one or many-to-one relationship. LSPs map to CTG connections in a
one-to-one or many-to-one relationship. The connection can have the
following traffic engineering parameters:
o bandwidth over-subscription
o factor placement
o priority
o holding priority
CTG connection TE parameters can be mapped directly from the LSP
parameters signaled in RSVP-TE or can be set at the CTG management
interface (CTG Logical Port). The connection bandwidth MUST be set.
If a LSP has no bandwidth information, the bandwidth will be
calculated at CTG ingress using automatic bandwidth measurement
function.
LDP LSPs can be mapped onto the connections per LDP label. Both
outer label (PE-PE label) and Inner label (VRF Label) can be used for
the connection mapping. CTG connection bandwidth MUST be set through
auto-bandwidth measurement function at the CTG ingress. When the
connection bandwidth tends to exceed the component link capacity, CTG
is able to reassign the flows in one connection into several
connections and assign other component links for the connections
without traffic disruption.
A CTG component link can be a physical link or logical link (LSP
Tunnel) between two routers. When component links are physical
links, there is no restriction to component link type, bandwidth, and
performance objectives (e.g., RTD and Jitter). Each component link
MUST maintain its own OAM. CTG is able to get component link status
from each link and take an action upon component link status changes.
Each component link can have its own Component Link Cost and
Component Link Bandwidth as its associated engineered parameters.
CTG uses component link parameters in the assignment of CTG
connections to component links.
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CTG provides local traffic engineering management over parallel links
based on CTG connection TE information and component link parameters.
Component link selection for CTG connections is determined locally
and may change without reconfiguring the traffic flows. Changing the
selection may be triggered by a component link condition change, a
new traffic flow configured or existing one modified, or operator
required optimization process. CTG component link selection for CTG
connections enables TE based traffic distribution and link failure
recovery with much less link capacity than current methods mentioned
in the section of the problem statements.
CTG connections are created for traffic management purpose on a
composite link. They do not change the forwarding schema. The
forwarding engine still forwards based on the LSP label created per
traffic LSP. Therefore, there is no change to the forwarding.
Since MPLS is built on the top of IP network, some IP PDUs are
carried over the MPLS network. CTG may designate one CTG connection
for such traffic or use hashing to distribute IP PDUs over component
links. The assumption is that such traffic volume is very small
compared to LSP or LDP traffic.
CTG techniques applies to the situation that the rate of the distinct
traffic flows are not higher than component link capacity in CTG.
4.2. Difference between CTG and A Bundled Link
4.2.1. Virtual Routable Link vs. TE Link
CTG is a data plan transport function over a composite link. A
composite link contains multiple component links that can carry
traffic independently. CTG is the method to transport aggregated
traffic over a composite link. The composite link appears as a
single routable virtual interface between the connected routers. The
network only maps LSP or LDP to a composite link, i.e. not to
individual component links. CTG will select component link for
individual LSP and LDP and merge them at composite link egress.
A bundled link [RFC4201] is a collection of TE links. It is a
logical construct that represents a way to group/map the information
about certain physical resources that interconnect routers. The
purpose of bundled link is to improve routing scalability by reducing
the amount of information that has to be handled by OSPF/IS-IS. Each
physical links in the bundled link are an IGP link in OSPF/IS-IS. A
bundled link only has the significance to router control plane. The
router has to map individual LSP to each component link in the
bundled link, which is different from CTG. A bundled link only
applies to RSVP-TE signaled traffic.
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4.2.2. Component Link Parameter Independence
CTG allows component links to have different costs, traffic
engineering metric and resource classes. CTG can derive the virtual
interface cost from component link costs based on operator policy.
CTG can derive the traffic engineering parameter for a virtual
interface from its component link traffic engineering parameters.
However, a bundled link [RFC4201] requires that all component links
in a bundle to have the same traffic engineering metric, and the same
set of resource classes.
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5. Composite Transport Group Requirements
Composite Transport Group (CTG) is about the method to transport
aggregated traffic over multiple parallel links. CTG can address the
problems existing in today IP/MPLS network. Here are some CTG
requirements:
5.1. CTG Appearance as a Routable Virtual Interface
The carrier needs a solution where multiple routing instances see a
separate "virtual interface" to a shared composite transport group
composed of parallel physical links between a pair of routers.
The CTG would communicate parameters (e.g., admin cost, available
bandwidth, maximum bandwidth, allowable bandwidth) for the "virtual
interface" associated with each routing instance.
The "virtual interface" shall appear as a fully-featured IP adjacency
to each routing instance, not just an FA [RFC3477] . In particular,
it needs to work with at least the following IP/MPLS control
protocols: IGP, LDP, IGP-TE, and RSVP-TE.
5.2. CTG mapping of traffic to Component Links
The objective of CTG is to solve the traffic sharing problem at a
virtual interface level by mapping traffic to component links (not
using hashing):
1. using TE information from the control planes of the routing
instances attached to the virtual interface when available, or
2. using traffic measurements when it is not.
5.2.1. Mapping Using Router TE information
CTG SHALL use RSVP-TE for bandwidth signaled by a routing instance to
explicitly assign a TE LSPs to CTG connection that is assigned to a
specific link in the CTG.
The CTG SHALL be able to receive, interpret and act upon at least the
following router signaled parameters: minimum bandwidth, maximum
bandwidth, preemption priority, and holding priority and apply them
to CTG connections where the LSP is mapped.
5.2.2. Mapping When No Router TE Information is Available
CTG SHALL map LDP-assigned labeled packets based upon local
configuration (e.g., label stack depth) to define a CTG connection
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that is mapped to one of the parallel links in the CTG between
routers.
CTG SHALL map LDP-assigned labeled packets that identify the source-
destination LER as a CTG connection to a specific link in the CTG.
CTG SHALL also handle IP traffic without MPLS labels. This could use
locally defined methods to assign sets of IP traffic to a CTG
connection.
In all of the above mapping cases, CTG SHALL place an entire
connection onto a single physical link.
In a mapping case, the CTG SHALL measure the bandwidth actually used
by a particular connection to determine which component link
(physical link) on the CTG that CTG connection should be transmitted.
The CTG SHALL support parameters that control the time period between
moving a CTG connection from one link to another since this could
cause reordering.
The CTG SHALL support parameters that define at least a minimum
bandwidth, maximum bandwidth, preemption priority, and holding
priority for connections without TE information.
5.3. Bandwidth Control for Connections with and without TE information
The following requirements apply to a virtual interface (i.e.,
composite link in section 4) that supports connections with TE
information in conjunction with connections that do not have TE
information.
A "bandwidth shortage" issue can arise in CTG if the total bandwidth
of the connections with TE information and those without TE
information exceeds the bandwidth of the composite link.
The CTG SHALL support a policy based preemption capability such that
in the event of such a "bandwidth shortage" that the signaled or
configured preemption and holding parameters can be applied to the
following treatments to the connections:
o For a connection that has RSVP-TE LSP(s), signal the router that
the TE-LSP has been preempted.
o For a connection that has LDP(s), where the CTG is aware of the
LDP signaling involved to the preempted label stack depth, signal
release of the label to the router
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o For a connection that has IP traffic without MPLS labels, indicate
congestion to the router (e.g., using ECN, PCN, or some local
method) or block IP traffic.
5.4. CTG Transport Resilience
Component link in CTG can fail independently. The failure of
component link can impact some CTG connections. The impacted CTG
connection SHALL be placed to other active component links by using
the same rules as of component link section for CTG connections.
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6. Security Considerations
CTG is a local function on the router to support traffic engineering
management over multiple parallel links. It does not introduce a
security risk for control plane and dada plane.
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7. IANA Considerations
There is no IANA actions requested in this specification.
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8. Acknowledgements
Authors would like to thank Frederic Jounay from France Telecom,
Adrian Farrel from Olddog, and Ron Bonica from Juniper for
the review and great suggestions.
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9. Normative References
[ITU-T G.800]
ITU-T Q12, "Unified Functional Architecture of Transport
Network", ITU-T G.800, February 2008.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[RFC3477] Kompella, K., "Signalling Unnumbered Links in Resource
ReSerVation Protocol - Traffic Engineering (RSVP-TE)",
RFC 3477, January 2003.
[RFC4090] Pan, P., "Fast Reroute Extensions to RSVP-TE for LSP
Tunnels", RFC 4090, May 2005.
[RFC4201] Kompella, K., "Link Bundle in MPLS Traffic Engineering",
RFC 4201, March 2005.
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Authors' Addresses
So Ning
Verizon
2400 N. Glem Ave.,
Richerson, TX 75082
Phone: +1 972-729-7905
Email: ning.so@verizonbusness.com
Andrew Malis
Verizon
117 West St.
Waltham, MA 02451
Phone: +1 781-466-2362
Email: andrew.g.malis@verizon.com
Dave McDysan
Verizon
22001 Loudoun County PKWY
Ashburn, VA 20147
Phone: +1 707-886-1891
Email: dave.mcdysan@verizon.com
Lucy Yong
Huawei USA
1700 Alma Dr. Suite 500
Plano, TX 75075
Phone: +1 469-229-5387
Email: lucyyong@huawei.com
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