Internet DRAFT - draft-ietf-rtgwg-cl-use-cases
draft-ietf-rtgwg-cl-use-cases
RTGWG S. Ning
Internet-Draft Tata Communications
Intended status: Informational A. Malis
Expires: November 15, 2014 Consultant
D. McDysan
Verizon
L. Yong
Huawei USA
C. Villamizar
Outer Cape Cod Network Consulting
May 14, 2014
Advanced Multipath Use Cases and Design Considerations
draft-ietf-rtgwg-cl-use-cases-06
Abstract
Advanced Multipath is a formalization of multipath techniques
currently in use in IP and MPLS networks and a set of extensions to
existing multipath techniques.
This document provides a set of use cases and design considerations
for Advanced Multipath. Existing practices are described. Use cases
made possible through Advanced Multipath extensions are described.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on November 15, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Multipath Foundation Use Cases . . . . . . . . . . . . . . . 5
5. Advanced Multipath Use Cases . . . . . . . . . . . . . . . . 8
5.1. Delay Sensitive Applications . . . . . . . . . . . . . . 8
5.2. Large Volume of IP and LDP Traffic . . . . . . . . . . . 9
5.3. Multipath and Packet Ordering . . . . . . . . . . . . . . 9
5.3.1. MPLS-TP in network edges only . . . . . . . . . . . . 11
5.3.2. Multipath at core LSP ingress/egress . . . . . . . . 12
5.3.3. MPLS-TP as a MPLS client . . . . . . . . . . . . . . 13
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
7. Security Considerations . . . . . . . . . . . . . . . . . . . 14
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14
9. Informative References . . . . . . . . . . . . . . . . . . . 14
Appendix A. Network Operator Practices and Protocol Usage . . . 17
Appendix B. Existing Multipath Standards and Techniques . . . . 19
B.1. Common Multpath Load Spliting Techniques . . . . . . . . 19
B.2. Static and Dynamic Load Balancing Multipath . . . . . . . 20
B.3. Traffic Split over Parallel Links . . . . . . . . . . . . 21
B.4. Traffic Split over Multiple Paths . . . . . . . . . . . . 21
Appendix C. Characteristics of Transport in Core Networks . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
1. Introduction
Advanced Multipath requirements are specified in [RFC7226]. An
Advanced Multipath framework is defined in
[I-D.ietf-rtgwg-cl-framework].
Multipath techniques have been widely used in IP networks for over
two decades. The use of MPLS began more than a decade ago.
Multipath has been widely used in IP/MPLS networks for over a decade
with very little protocol support dedicated to effective use of
multipath.
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The state of the art in multipath prior to Advanced Multipath is
documented in Appendix B.
Both Ethernet Link Aggregation [IEEE-802.1AX] and MPLS link bundling
[RFC4201] have been widely used in today's MPLS networks. Advanced
Multipath differs in the following characteristics.
1. Advanced Multipath allows bundling of non-homogenous links
together as a single logical link.
2. Advanced Multipath provides more information in the TE-LSDB and
supports more explicit control over placement of LSP.
2. Assumptions
The supported services are, but not limited to, pseudowire (PW) based
services ([RFC3985]), including Virtual Private Network (VPN)
services, Internet traffic encapsulated by at least one MPLS label
([RFC3032]), and dynamically signaled MPLS ([RFC3209] or [RFC5036])
or MPLS-TP Label Switched Paths (LSPs) ([RFC5921]).
The MPLS LSPs supporting these services may be point-to-point, point-
to-multipoint, or multipoint-to-multipoint. The MPLS LSPs may be
signaled using RSVP-TE [RFC3209] or LDP [RFC5036]. With RSVP-TE,
extensions to Interior Gateway Protocols (IGPs) may be used,
specifically to OSPF-TE [RFC3630] or ISIS-TE [RFC5305].
The locations in a network where these requirements apply are a Label
Edge Router (LER) or a Label Switch Router (LSR) as defined in
[RFC3031].
The IP DSCP field [RFC2474] [RFC2475] cannot be used for flow
identification since L3VPN requires Diffserv transparency (see RFC
4031 5.5.2 [RFC4031]), and in general network operators do not rely
on the DSCP of Internet packets.
3. Terminology
Terminology defined in [RFC7226] and [RFC7190] is used in this
document.
In addition, the following terms are used:
classic multipath:
Classic multipath refers to the most common current practice in
implementation and deployment of multipath (see Appendix B). The
most common current practice when applied to MPLS traffic makes
use of a hash on the MPLS label stack, and if IPv4 or IPv6 are
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indicated under the label stack, makes use of the IP source and
destination addresses [RFC4385] [RFC4928].
classic link bundling:
Classic link bundling refers to the use of [RFC4201] where the
"all ones" component is not used. Where the "all ones" component
is used, link bundling behaves as classic multipath does.
Classic link bundling selects a single component link to carry
all of the traffic for a given LSP.
Among the important distinctions between classic multipath or classic
link bundling and Advanced Multipath are:
1. Classic multipath has no provision to retain packet order within
any specific LSP. Classic link bundling retains packet order
among any given LSP but as a result does a poor job of splitting
load among components and therefore is rarely (if ever) deployed.
Advanced Multipath allows per LSP control of load split
characteristics.
2. Classic multipath and classic link bundling do not provide a
means to put some LSP on component links with lower delay.
Advanced Multipath does.
3. Classic multipath will provide a load balance for IP and LDP
traffic. Classic link bundling will not. Neither classic
multipath or classic link bundling will measure IP and LDP
traffic and reduce the RSVP-TE advertised "Available Bandwidth"
as a result of that measurement. Advanced Multipath better
supports RSVP-TE used with significant traffic levels of native
IP and native LDP.
4. Classic link bundling cannot support an LSP that is greater in
capacity than any single component link. Classic multipath
supports this capability but may reorder traffic on such an LSP.
Advanced Multipath can retain order of an LSP that is carried
within an LSP that is greater in capacity than any single
component link if the contained LSP has such a requirement.
None of these techniques, classic multipath, classic link bundling,
or Advanced Multipath, will reorder traffic among IP microflows.
None of these techniques will reorder traffic among PW, if a PWE3
Control Word is used [RFC4385].
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4. Multipath Foundation Use Cases
A simple multipath composed entirely of physical links is illustrated
in Figure 1, where an multipath is configured between LSR1 and LSR2.
This multipath has three component links. Individual component links
in a multipath may be supported by different transport technologies
such as SONET, OTN, Ethernet, etc. Even if the transport technology
implementing the component links is identical, the characteristics
(e.g., bandwidth, latency) of the component links may differ.
The multipath in Figure 1 may carry LSP traffic flows and control
plane packets. Control plane packets may appear as IP packets or may
be carried within a generic associated channel (G-Ach) [RFC5586]. A
LSP may be established over the link by either RSVP-TE [RFC3209] or
LDP [RFC5036] signaling protocols. All component links in a
multipath are summarized in the same forwarding adjacency LSP (FA-
LSP) routing advertisement [RFC3945]. The multipath is summarized as
one TE-Link advertised into the IGP by the multipath end points (the
LER if the multipath is MPLS based). This information is used in
path computation when a full MPLS control plane is in use.
If Advanced Multipath techniques are used, then the individual
component links or groups of component links may optionally be
advertised into the IGP as sub-TLV of the multipath FA advertisement
to indicate capacity available with various characteristics, such as
a delay range.
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Management Plane
Configuration and Measurement <------------+
^ |
| |
+-------+-+ +-+-------+
| | | | | |
CP Packets V | | V CP Packets
| V | | Component Link 1 | | ^ |
| | |=|===========================|=| | |
| +----| | Component Link 2 | |----+ |
| |=|===========================|=| |
Aggregated LSPs | | | | |
~|~~~~~~>| | Component Link 3 | |~~~~>~~|~~
| |=|===========================|=| |
| | | | | |
| LSR1 | | LSR2 |
+---------+ +---------+
! !
! !
!<-------- Multipath ---------->!
Figure 1: a multipath constructed with multiple physical links
between two LSR
[RFC7226] specifies that component links may themselves be multipath.
This is true for most implementations even prior to the Advanced
Multipath work in [RFC7226]. For example, a component of a pre-
Advanced Multipath MPLS Link Bundle or ISIS or OSPF ECMP could be an
Ethernet LAG. In some implementations many other combinations or
even arbitrary combinations could be supported. Figure 2 shows three
three forms of component links which may be deployed in a network.
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+-------+ 1. Physical Link +-------+
| |-|----------------------------------------------|-| |
| | | | | |
| | | +------+ +------+ | | |
| | | | MPLS | 2. Logical Link | MPLS | | | |
| |.|.... |......|.....................|......|....|.| |
| | |-----| LSR3 |---------------------| LSR4 |----| | |
| | | +------+ +------+ | | |
| | | | | |
| | | | | |
| | | +------+ +------+ | | |
| | | |GMPLS | 3. Logical Link |GMPLS | | | |
| |.|. ...|......|.....................|......|....|.| |
| | |-----| LSR5 |---------------------| LSR6 |----| | |
| | +------+ +------+ | |
| LSR1 | | LSR2 |
+-------+ +-------+
|<---------------- Multipath --------------------->|
Figure 2: Illustration of Various Component Link Types
The three forms of component link shown in Figure 2 are:
1. The first component link is configured with direct physical media
plus a link layer protocol. This case also includes emulated
physical links, for example using pseudowire emulation.
2. The second component link is a TE tunnel that traverses LSR3 and
LSR4, where LSR3 and LSR4 are the nodes supporting MPLS, but
supporting few or no GMPLS extensions.
3. The third component link is formed by lower layer network that
has GMPLS enabled. In this case, LSR5 and LSR6 are not the nodes
controlled by the MPLS but provide the connectivity for the
component link.
A multipath forms one logical link between connected LSR (LSR1 and
LSR2 in Figure 1 and Figure 2) and is used to carry aggregated
traffic. Multipath relies on its component links to carry the
traffic but must distribute or load balance the traffic. The
endpoints of the multipath maps incoming traffic into the set of
component links.
For example, LSR1 in Figure 1 distributes the set of traffic flows
including control plane packets among the set of component links.
LSR2 in Figure 1 receives the packets from its component links and
sends them to MPLS forwarding engine with no attempt to reorder
packets arriving on different component links. The traffic in the
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opposite direction, from LSR2 to LSR1, is distributed across the set
of component links by the LSR2.
These three forms of component link are a limited set of very simple
examples. Many other examples are possible. A component link may
itself be a multipath. A segment of an LSP (single hop for that LSP)
may be a multipath.
5. Advanced Multipath Use Cases
The following subsections provide some uses of the Advanced Multipath
extensions. These are not the only uses, simply a set of examples.
5.1. Delay Sensitive Applications
Most applications benefit from lower delay. Some types of
applications are far more sensitive than others. For example, real
time bidirectional applications such as voice communication or two
way video conferencing are far more sensitive to delay than
unidirectional streaming audio or video. Non-interactive bulk
transfer is almost insensitive to delay if a large enough TCP window
is used.
Some applications are sensitive to delay but users of those
applications are unwilling to pay extra to insure lower delay. For
example, many SIP end users are willing to accept the delay offered
to best effort services as long as call quality is good most of the
time.
Other applications are sensitive to delay and willing to pay extra to
insure lower delay. For example, financial trading applications are
extremely sensitive to delay and with a lot at stake are willing to
go to great lengths to reduce delay.
Among the requirements of Advanced Multipath are requirements to
support non-homogeneous links. One solution in support of lower
delay links is to advertise capacity available within configured
ranges of delay within a given multipath and then support the ability
to place an LSP only on component links that meeting that LSP's delay
requirements.
The Advanced Multipath requirements to accommodate delay sensitive
applications are analogous to Diffserv requirements to accommodate
applications requiring higher quality of service on the same
infrastructure as applications with less demanding requirements. The
ability to share capacity with less demanding applications, with best
effort applications generally being the least demanding, can greatly
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reduce the cost of delivering service to the more demanding
applications.
5.2. Large Volume of IP and LDP Traffic
IP and LDP do not support traffic engineering. Both make use of a
shortest (lowest routing metric) path, with an option to use equal
cost multipath (ECMP). Note that though ECMP is prohibited in LDP
specifications, it is widely implemented. Where implemented for LDP,
ECMP is generally disabled by default for standards compliance, but
often enabled in LDP deployments.
Without traffic engineering capability, there must be sufficient
capacity to accommodate the IP and LDP traffic. If not, persistent
queuing delay and loss will occur. Unlike RSVP-TE, a subset of
traffic cannot be routed using constraint based routing to avoid a
congested portion of an infrastructure.
In existing networks which accommodate IP and/or LDP with RSVP-TE,
either the IP and LDP can be carried over RSVP-TE, or where the
traffic contribution of IP and LDP is small, IP and LDP can be
carried native and the effect on RSVP-TE can be ignored. Ignoring
the traffic contribution of IP is valid on high capacity networks
where a very low volume of native IP is used primarily for control
and network management and customer IP is carried within RSVP-TE.
Where it is desirable to carry native IP and/or LDP and IP and/or LDP
traffic volumes are not negligible, RSVP-TE needs improvement. An
enhancement offered by Advanced Multipath is an ability to measure
the IP and LDP, filter the measurements, and reduce the capacity
available to RSVP-TE to avoid congestion. The treatment given to the
IP or LDP traffic is similar to the treatment when using the "auto-
bandwidth" feature in some RSVP-TE implementations on that same
traffic, and giving a higher priority (numerically lower setup
priority and holding priority value) to the "auto-bandwidth" LSP.
The difference is that the measurement is made at each hop and the
reduction in advertised bandwidth is made more directly.
5.3. Multipath and Packet Ordering
A strong motivation for multipath is the need to provide LSP capacity
in IP backbones that exceeds the capacity of single wavelengths
provided by transport equipment and exceeds the practical capacity
limits achievable through inverse multiplexing. Appendix C describes
characteristics and limitations of transport systems today.
Section 3 defines the terms "classic multipath" and "classic link
bundling" used in this section.
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For purpose of discussion, consider two very large cities, city A and
city Z. For example, in the US high traffic cities might be New York
and Los Angeles and in Europe high traffic cities might be London and
Amsterdam. Two other high volume cities, city B and city Y may share
common provider core network infrastructure. Using the same
examples, the city B and Y may Washington DC and San Francisco or
Paris and Stockholm. In the US, the common infrastructure may span
Denver, Chicago, Detroit, and Cleveland. Other major traffic
contributors on either US coast include Boston, northern Virginia on
the east coast, and Seattle, and San Diego on the west coast. The
capacity of IP/MPLS links within the shared infrastructure, for
example city to city links in the Denver, Chicago, Detroit, and
Cleveland path in the US example, have capacities for most of the
2000s decade that greatly exceeded single circuits available in
transport networks.
For a case with four large traffic sources on either side of the
shared infrastructure, up to sixteen core city to core city traffic
flows in excess of transport circuit capacity may be accommodated on
the shared infrastructure.
Today the most common IP/MPLS core network design makes use of very
large links which consist of many smaller component links, but use
classic multipath techniques. A component link typically corresponds
to the largest circuit that the transport system is capable of
providing (or the largest cost effective circuit). IP source and
destination address hashing is used to distribute flows across the
set of component links as described in Appendix B.3.
Classic multipath can handle large LSP up to the total capacity of
the multipath (within limits, see Appendix B.2). A disadvantage of
classic multipath is the reordering among traffic within a given core
city to core city LSP. While there is no reordering within any
microflow and therefore no customer visible issue, MPLS-TP cannot be
used across an infrastructure where classic multipath is in use,
except within pseudowires.
Capacity issues force the use of classic multipath today. Classic
multipath excludes a direct use of MPLS-TP. The desire for OAM,
offered by MPLS-TP, is in conflict with the use of classic multipath.
There are a number of alternatives that satisfy both requirements.
Some alternatives are described below.
MPLS-TP in network edges only
A simple approach which requires no change to the core is to
disallow MPLS-TP across the core unless carried within a
pseudowire (PW). MPLS-TP may be used within edge domains where
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classic multipath is not used. PW may be signaled end to end
using single segment PW (SS-PW), or stitched across domains using
multisegment PW (MS-PW). The PW and anything carried within the
PW may use OAM as long as fat-PW [RFC6391] load splitting is not
used by the PW.
Advanced Multipath at core LSP ingress/egress
The interior of the core network may use classic link bundling,
with the limitation that no LSP can exceed the capacity of a
single circuit. Larger non-MPLS-TP LSP can be configured using
multiple ingress to egress component MPLS-TP LSP. This can be
accomplished using existing IP source and destination address
hashing configured at LSP ingress and egress. Each component
LSP, if constrained to be no larger than the capacity of a single
circuit, can make use of MPLS-TP and offer OAM for all top level
LSP across the core.
MPLS-TP as a MPLS client
A third approach involves making use of Entropy Labels [RFC6790]
on all MPLS-TP LSP such that the entire MPLS-TP LSP is treated as
a microflow by midpoint LSR, even if further encapsulated in very
large server layer MPLS LSP.
The above list of alternatives allow packet ordering within an LSP to
be maintained in some circumstances and allow very large LSP
capacities. Each of these alternatives are discussed further in the
following subsections.
5.3.1. MPLS-TP in network edges only
Classic MPLS link bundling is defined in [RFC4201] and has existed
since early in the 2000s decade. Classic MPLS link bundling place
any given LSP entirely on a single component link. Classic MPLS link
bundling is not in widespread use as the means to accommodate large
link capacities in core networks due to the simplicity and better
multiplexing gain, and therefore lower network cost of classic
multipath.
If MPLS-TP OAM capability in the IP/MPLS network core LSP is not
required, then there is no need to change existing network designs
which use classic multipath and both label stack and IP source and
destination address based hashing as a basis for load splitting.
If MPLS-TP is needed for a subset of LSP, then those LSP can be
carried within pseudowires. The pseudowires adds a thin layer of
encapsulation and therefore a small overhead. If only a subset of
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LSP need MPLS-TP OAM, then some LSP must make use of the pseudowires
and other LSP avoid them. A straightforward way to accomplish this
is with administrative attributes [RFC3209].
5.3.2. Multipath at core LSP ingress/egress
Multipath can be configured for large LSP that are made of smaller
MPLS-TP component LSP. Some implementations already support this
capability, though until Advanced Multipath no IETF document required
it. This approach is capable of supporting MPLS-TP OAM over the
entire set of component link LSP and therefore the entire set of top
level LSP traversing the core.
There are two primary disadvantage of this approach. One is the
number of top level LSP traversing the core can be dramatically
increased. The other disadvantage is the loss of multiplexing gain
that results from use of classic link bundling within the interior of
the core network.
If component LSP use MPLS-TP, then no component LSP can exceed the
capacity of a single circuit. For a given multipath LSP there can
either be a number of equal capacity component LSP or some number of
full capacity component links plus one LSP carrying the excess. For
example, a 350 Gb/s multipath LSP over a 100 Gb/s infrastructure may
use five 70 Gb/s component LSP or three 100 Gb/s LSP plus one 50 Gb/s
LSP. Classic MPLS link bundling is needed to support MPLS-TP and
suffers from a bin packing problem even if LSP traffic is completely
predictable, which it never is in practice.
The common means of setting very large LSP link bandwidth parameters
uses long term statistical measures. For example, at one time many
providers based their LSP bandwidth parameters on the 95th percentile
of carried traffic as measured over the prior one week period. It is
common to add 10-30% to the 95th percentile value measured over the
prior week and adjust bandwidth parameters of LSP weekly. It is also
possible to measure traffic flow at the LSR and adjust bandwidth
parameters somewhat more dynamically. This is less common in
deployments and where deployed, makes use of filtering to track very
long term trends in traffic levels. In either case, short term
variation of traffic levels relative to signaled LSP capacity are
common. Allowing a large over allocation of LSP bandwidth parameters
(ie: adding 30% or more) avoids over utilization of any given LSP,
but increases unused network capacity and increases network cost.
Allowing a small over allocation of LSP bandwidth parameters (ie:
10-20% or less) results in both underutilization and over utilization
but statistically results in a total utilization within the core that
is under capacity most or all of the time.
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The classic multipath solution accommodates the situation in which
some very large LSP are under utilizing their signaled capacity and
others are over utilizing their capacity with the need for far less
unused network capacity to accommodate variation in actual traffic
levels. If the actual traffic levels of LSP can be described by a
probability distribution, the variation of the sum of LSP is less
than the variation of any given LSP for all but a constant traffic
level (where the variation of the sum and the variation of the
components are both zero).
Splitting very large LSP at the ingress and carrying those large LSP
within smaller MPLS-TP component LSP and then using classic link
bundling to carry the MPLS-TP LSP is a viable approach. However this
approach loses the statistical gain discussed in the prior
paragraphs. Losing this statistical gain drives up network costs
necessary to acheive the same very low probability of only mild
congestion that is expected of provider networks.
There are two situations which can motivate the use of this approach.
This design is favored if the provider values MPLS-TP OAM across the
core more than efficiency (or is unaware of the efficiency issue).
This design can also make sense if transport equipment or very low
cost core LSR are available which support only classic link bundling
and regardless of loss of multiplexing gain, are more cost effective
at carrying transit traffic than using equipment which supports IP
source and destination address hashing.
5.3.3. MPLS-TP as a MPLS client
Accommodating MPLS-TP as a MPLS client requires the small change to
forwarding behavior necessary to support [RFC6790] and is therefore
most applicable to major network overbuilds or new deployments. This
approach is described in [RFC7190] and makes use of Entropy Labels
[RFC6790] to prevent reordering of MPLS-TP LSP or any other LSP which
requires that its traffic not be reordered for OAM or other reasons.
The advantage of this approach is an ability to accommodate MPLS-TP
as a client LSP but retain the high multiplexing gain and therefore
efficiency and low network cost of a pure MPLS deployment. The
disadvantage is the need for a small change in forwarding to support
[RFC6790].
6. IANA Considerations
This memo includes no request to IANA.
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7. Security Considerations
This document is a use cases document. Existing protocols are
referenced such as MPLS. Existing techniques such as MPLS link
bundling and multipath techniques are referenced. These protocols
and techniques are documented elsewhere and contain security
considerations which are unchanged by this document.
This document also describes use cases for multipath and Advanced
Multipath. Advanced Multipath requirements are defined in [RFC7226].
[I-D.ietf-rtgwg-cl-framework] defines a framework for Advanced
Multipath. Advanced Multipath bears many similarities to MPLS link
bundling and multipath techniques used with MPLS. Additional
security considerations, if any, beyond those already identified for
MPLS, MPLS link bundling and multipath techniques, will be documented
in the framework document if specific to the overall framework of
Advanced Multipath, or in protocol extensions if specific to a given
protocol extension defined later to support Advanced Multipath.
8. Acknowledgments
In the interest of full disclosure of affiliation and in the interest
of acknowledging sponsorship, past affiliations of authors are noted.
Much of the work done by Ning So occurred while Ning was at Verizon.
Much of the work done by Curtis Villamizar occurred while at
Infinera. Much of the work done by Andy Malis occurred while Andy
was at Verizon.
9. Informative References
[I-D.ietf-rtgwg-cl-framework]
Ning, S., McDysan, D., Osborne, E., Yong, L., and C.
Villamizar, "Advanced Multipath Framework in MPLS", draft-
ietf-rtgwg-cl-framework-04 (work in progress), July 2013.
[IEEE-802.1AX]
IEEE Standards Association, "IEEE Std 802.1AX-2008 IEEE
Standard for Local and Metropolitan Area Networks - Link
Aggregation", 2006, <http://standards.ieee.org/getieee802/
download/802.1AX-2008.pdf>.
[ITU-T.G.694.2]
ITU-T, "Spectral grids for WDM applications: CWDM
wavelength grid", 2003,
<http://www.itu.int/rec/T-REC-G.694.2-200312-I>.
[RFC1717] Sklower, K., Lloyd, B., McGregor, G., and D. Carr, "The
PPP Multilink Protocol (MP)", RFC 1717, November 1994.
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[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474, December
1998.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998.
[RFC2597] Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
"Assured Forwarding PHB Group", RFC 2597, June 1999.
[RFC2615] Malis, A. and W. Simpson, "PPP over SONET/SDH", RFC 2615,
June 1999.
[RFC2991] Thaler, D. and C. Hopps, "Multipath Issues in Unicast and
Multicast Next-Hop Selection", RFC 2991, November 2000.
[RFC2992] Hopps, C., "Analysis of an Equal-Cost Multi-Path
Algorithm", RFC 2992, November 2000.
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, 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.
[RFC3260] Grossman, D., "New Terminology and Clarifications for
Diffserv", RFC 3260, April 2002.
[RFC3270] Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen,
P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi-
Protocol Label Switching (MPLS) Support of Differentiated
Services", RFC 3270, May 2002.
[RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 3630, September
2003.
[RFC3809] Nagarajan, A., "Generic Requirements for Provider
Provisioned Virtual Private Networks (PPVPN)", RFC 3809,
June 2004.
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[RFC3945] Mannie, E., "Generalized Multi-Protocol Label Switching
(GMPLS) Architecture", RFC 3945, October 2004.
[RFC3985] Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
Edge (PWE3) Architecture", RFC 3985, March 2005.
[RFC4031] Carugi, M. and D. McDysan, "Service Requirements for Layer
3 Provider Provisioned Virtual Private Networks (PPVPNs)",
RFC 4031, April 2005.
[RFC4124] Le Faucheur, F., "Protocol Extensions for Support of
Diffserv-aware MPLS Traffic Engineering", RFC 4124, June
2005.
[RFC4201] Kompella, K., Rekhter, Y., and L. Berger, "Link Bundling
in MPLS Traffic Engineering (TE)", RFC 4201, October 2005.
[RFC4385] Bryant, S., Swallow, G., Martini, L., and D. McPherson,
"Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for
Use over an MPLS PSN", RFC 4385, February 2006.
[RFC4928] Swallow, G., Bryant, S., and L. Andersson, "Avoiding Equal
Cost Multipath Treatment in MPLS Networks", BCP 128, RFC
4928, June 2007.
[RFC5036] Andersson, L., Minei, I., and B. Thomas, "LDP
Specification", RFC 5036, October 2007.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, October 2008.
[RFC5586] Bocci, M., Vigoureux, M., and S. Bryant, "MPLS Generic
Associated Channel", RFC 5586, June 2009.
[RFC5921] Bocci, M., Bryant, S., Frost, D., Levrau, L., and L.
Berger, "A Framework for MPLS in Transport Networks", RFC
5921, July 2010.
[RFC6391] Bryant, S., Filsfils, C., Drafz, U., Kompella, V., Regan,
J., and S. Amante, "Flow-Aware Transport of Pseudowires
over an MPLS Packet Switched Network", RFC 6391, November
2011.
[RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and
L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
RFC 6790, November 2012.
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[RFC7190] Villamizar, C., "Use of Multipath with MPLS and MPLS
Transport Profile (MPLS-TP)", RFC 7190, March 2014.
[RFC7226] Villamizar, C., McDysan, D., Ning, S., Malis, A., and L.
Yong, "Requirements for Advanced Multipath in MPLS
Networks", RFC 7226, May 2014.
Appendix A. Network Operator Practices and Protocol Usage
Often, network operators have a contractual Service Level Agreement
(SLA) with customers for services that are comprised of numerical
values for performance measures, principally availability, latency,
delay variation. Additionally, network operators may have
performance objectives for internal use by the operator. See
RFC3809, Section 4.9 [RFC3809] for examples of the form of such SLA
and performance objective specifications. In this document we use
the term Performance Objective as defined in [RFC7226]. Applications
and acceptable user experience have an important relationship to
these performance parameters.
Consider latency as an example. In some cases, minimizing latency
relates directly to the best customer experience (for example, in
interactive applications closer is faster). In other cases, user
experience is relatively insensitive to latency, up to a specific
limit at which point user perception of quality degrades
significantly (e.g., interactive human voice and multimedia
conferencing). A number of Performance Objectives have a bound on
point-to-point latency and as long as this bound is met the
Performance Objective is met; decreasing the latency is not
necessary. In some Performance Objectives, if the specified latency
is not met, the user considers the service as unavailable. An
unprotected LSP can be manually provisioned on a set of links to meet
this type of Performance Objective, but this lowers availability
since an alternate route that meets the latency Performance Objective
cannot be determined.
Historically, when an IP/MPLS network was operated over a lower layer
circuit switched network (e.g., SONET rings), a change in latency
caused by the lower layer network (e.g., due to a maintenance action
or failure) was not known to the MPLS network. This resulted in
latency affecting end user experience, sometimes violating
Performance Objectives or resulting in user complaints.
A response to this problem was to provision IP/MPLS networks over
unprotected circuits and set the metric and/or TE-metric proportional
to latency. This resulted in traffic being directed over the least
latency path, even if this was not needed to meet an Performance
Objective or meet user experience objectives. This results in
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reduced flexibility and increased cost for network operators. Some
providers perfer to use lower layer networks to provide restoration
and grooming, but the inability to communicate performance
parameters, in particular latency, from the lower layer network to
the higher layer network is an important problem to be solved before
this can be done.
Latency Performance Objectives for point-to-point services are often
tied closely to geographic locations, while latency for multipoint
services may be based upon a worst case within a region.
The time frames for restoration (i.e., as implemented by
predetermined protection, convergence of routing protocols and/or
signaling) for services range from on the order of 100 ms or less
(e.g., for VPWS to emulate classical SDH/SONET protection switching),
to several minutes (e.g., to allow BGP to reconverge for L3VPN) and
may differ among the set of customers within a single service.
The presence of only three Traffic Class (TC) bits (previously known
as EXP bits) in the MPLS shim header is limiting when a network
operator needs to support QoS classes for multiple services (e.g.,
L2VPN VPWS, VPLS, L3VPN and Internet), each of which has a set of QoS
classes that need to be supported and where the operator prefers to
use only E-LSP [RFC3270]. In some cases one bit is used to indicate
conformance to some ingress traffic classification, leaving only two
bits for indicating the service QoS classes. One approach that has
been taken is to aggregate these QoS classes into similar sets on
LER-LSR and LSR-LSR links and continue to use only E-LSP. Another
approach is to use L-LSP as defined in [RFC3270] or use the Class-
Type as defined in [RFC4124] to support up to eight mappings of TC
into Per-Hop Behavior (PHB).
The IP DSCP cannot be used for flow identification. The use of IP
DSCP for flow identification is incompatible with Assured Forwarding
services [RFC2597] or any other service which may use more than one
DSCP code point to carry traffic for a given microflow. In general
network operators do not rely on the DSCP of Internet packets in core
networks but must preserve DSCP values for use closer to network
edges.
A label is pushed onto Internet packets when they are carried along
with L2VPN or L3VPN packets on the same link or lower layer network
provides a mean to distinguish between the QoS class for these
packets.
Operating an MPLS-TE network involves a different paradigm from
operating an IGP metric-based LDP signaled MPLS network. The
multipoint-to-point LDP signaled MPLS LSPs occur automatically, and
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balancing across parallel links occurs if the IGP metrics are set
"equally" (with equality a locally definable relation) and if ECMP is
enabled for LDP, which network operators generally do in large
networks.
Traffic is typically comprised of large (some very large) flows and a
much larger number of small flows. In some cases, separate LSPs are
established for very large flows. Very large microflows can occur
even if the IP header information is inspected by a LSR. For example
an IPsec tunnel that carries a large amount of traffic must be
carried as a single large flow. An important example of large flows
is that of a L2VPN or L3VPN customer who has an access line bandwidth
comparable to a client-client component link bandwidth -- there could
be flows that are on the order of the access line bandwidth.
Appendix B. Existing Multipath Standards and Techniques
Today the requirement to handle large aggregations of traffic, much
larger than a single component link, can be handled by a number of
techniques which we will collectively call multipath. Multipath
applied to parallel links between the same set of nodes includes
Ethernet Link Aggregation [IEEE-802.1AX], link bundling [RFC4201], or
other aggregation techniques some of which may be vendor specific.
Multipath applied to diverse paths rather than parallel links
includes Equal Cost MultiPath (ECMP) as applied to OSPF, ISIS, LDP,
or even BGP, and equal cost LSP, as described in Appendix B.4.
Various multipath techniques have strengths and weaknesses.
Existing multipath techniques solve the problem of large aggregations
of traffic, without addressing the other requirements outlined in
this document, particularly those described in Section 5.
B.1. Common Multpath Load Spliting Techniques
Identical load balancing techniques are used for multipath both over
parallel links and over diverse paths.
Large aggregates of IP traffic do not provide explicit signaling to
indicate the expected traffic loads. Large aggregates of MPLS
traffic are carried in MPLS tunnels supported by MPLS LSP. LSP which
are signaled using RSVP-TE extensions do provide explicit signaling
which includes the expected traffic load for the aggregate. LSP
which are signaled using LDP do not provide an expected traffic load.
MPLS LSP may contain other MPLS LSP arranged hierarchically. When an
MPLS LSR serves as a midpoint LSR in an LSP carrying client LSP as
payload, there is no signaling associated with these client LSP.
Therefore even when using RSVP-TE signaling there may be insufficient
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information provided by signaling to adequately distribute load based
solely on signaling.
Generally a set of label stack entries that is unique across the
ordered set of label numbers in the label stack can safely be assumed
to contain a group of flows. The reordering of traffic can therefore
be considered to be acceptable unless reordering occurs within
traffic containing a common unique set of label stack entries.
Existing load splitting techniques take advantage of this property in
addition to looking beyond the bottom of the label stack and
determining if the payload is IPv4 or IPv6 to load balance traffic
accordingly.
MPLS-TP OAM violates the assumption that it is safe to reorder
traffic within an LSP. If MPLS-TP OAM is to be accommodated, then
existing multipath techniques must be modified. [RFC6790] and
[RFC7190] provide a solution but require a small forwarding change.
For example, a large aggregate of IP traffic may be subdivided into a
large number of groups of flows using a hash on the IP source and
destination addresses. This is as described in [RFC2475] and
clarified in [RFC3260]. For MPLS traffic carrying IP, a similar hash
can be performed on the set of labels in the label stack. These
techniques are both examples of means to subdivide traffic into
groups of flows for the purpose of load balancing traffic across
aggregated link capacity. The means of identifying a group of flows
should not be confused with the definition of a flow.
Discussion of whether a hash based approach provides a sufficiently
even load balance using any particular hashing algorithm or method of
distributing traffic across a set of component links is outside of
the scope of this document.
The current load balancing techniques are referenced in [RFC4385] and
[RFC4928]. The use of three hash based approaches are described in
[RFC2991] and [RFC2992]. A mechanism to identify flows within PW is
described in [RFC6391]. The use of hash based approaches is
mentioned as an example of an existing set of techniques to
distribute traffic over a set of component links. Other techniques
are not precluded.
B.2. Static and Dynamic Load Balancing Multipath
Static multipath generally relies on the mathematical probability
that given a very large number of small microflows, these microflows
will tend to be distributed evenly across a hash space. Early very
static multipath implementations assumed that all component links are
of equal capacity and perform a modulo operation across the hashed
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value. An alternate static multipath technique uses a table
generally with a power of two size, and distributes the table entries
proportionally among component links according to the capacity of
each component link.
Static load balancing works well if there are a very large number of
small microflows (i.e., microflow rate is much less than component
link capacity). However, the case where there are even a few large
microflows is not handled well by static load balancing.
A dynamic load balancing multipath technique is one where the traffic
bound to each component link is measured and the load split is
adjusted accordingly. As long as the adjustment is done within a
single network element, then no protocol extensions are required and
there are no interoperability issues.
Note that if the load balancing algorithm and/or its parameters is
adjusted, then packets in some flows may be briefly delivered out of
sequence, however in practice such adjustments can be made very
infrequent.
B.3. Traffic Split over Parallel Links
The load splitting techniques defined in Appendix B.1 and
Appendix B.2 are both used in splitting traffic over parallel links
between the same pair of nodes. The best known technique, though far
from being the first, is Ethernet Link Aggregation [IEEE-802.1AX].
This same technique had been applied much earlier using OSPF or ISIS
Equal Cost MultiPath (ECMP) over parallel links between the same
nodes. Multilink PPP [RFC1717] uses a technique that provides
inverse multiplexing, however a number of vendors had provided
proprietary extensions to PPP over SONET/SDH [RFC2615] that predated
Ethernet Link Aggregation but are no longer used.
Link bundling [RFC4201] provides yet another means of handling
parallel LSP. RFC4201 explicitly allow a special value of all ones
to indicate a split across all members of the bundle. This "all
ones" component link is signaled in the MPLS RESV to indicate that
the link bundle is making use of classic multipath techniques.
B.4. Traffic Split over Multiple Paths
OSPF or ISIS Equal Cost MultiPath (ECMP) is a well known form of
traffic split over multiple paths that may traverse intermediate
nodes. ECMP is often incorrectly equated to only this case, and
multipath over multiple diverse paths is often incorrectly equated to
ECMP.
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Many implementations are able to create more than one LSP between a
pair of nodes, where these LSP are routed diversely to better make
use of available capacity. The load on these LSP can be distributed
proportionally to the reserved bandwidth of the LSP. These multiple
LSP may be advertised as a single PSC FA and any LSP making use of
the FA may be split over these multiple LSP.
Link bundling [RFC4201] component links may themselves be LSP. When
this technique is used, any LSP which specifies the link bundle may
be split across the multiple paths of the component LSP that comprise
the bundle.
Appendix C. Characteristics of Transport in Core Networks
The characteristics of primary interest are the capacity of a single
circuit and the use of wave division multiplexing (WDM) to provide a
large number of parallel circuits.
Wave division multiplexing (WDM) supports multiple independent
channels (independent ignoring crosstalk noise) at slightly different
wavelengths of light, multiplexed onto a single fiber. Typical in
the early 2000s was 40 wavelengths of 10 Gb/s capacity per
wavelength. These wavelengths are in the C-band range, which is
about 1530-1565 nm, though some work has been done using the L-band
1565-1625 nm.
The C-band has been carved up using a 100 GHz spacing from 191.7 THz
to 196.1 THz by [ITU-T.G.694.2]. This yields 44 channels. If the
outermost channels are not used, due to poorer transmission
characteristics, then typically 40 are used. For practical reasons,
a 50 GhZ or 25 GHz spacing is used by more recent equipment,
yielding. 80 or 160 channels in practice.
The early optical modulation techniques used within a single channel
yielded 2.5Gb/s and 10 Gb/s capacity per channel. As modulation
techniques have improved 40 Gb/s and 100 Gb/s per channel have been
achieved.
The 40 channels of 10 Gb/s common in the mid 2000s yields a total of
400 Gb/s. Tighter spacing and better modulations are yielding up to
8 Tb/s or more in more recent systems.
Over the optical modulation is an electrical encoding. In the 1990s
this was typically Synchronous Optical Networking (SONET) or
Synchronous Digital Hierarchy (SDH), with a maximum defined circuit
capacity of 40 Gb/s (OC-768), though the 10 Gb/s OC-192 is more
common. More recently the low level electrical encoding has been
Optical Transport Network (OTN) defined by ITU-T. OTN currently
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defines circuit capacities up to a nominal 100 Gb/s (ODU4). Both
SONET/SDH and OTN make use of time division multiplexing (TDM) where
the a higher capacity circuit such as a 100 Gb/s ODU4 in OTN may be
subdivided into lower fixed capacity circuits such as ten 10 Gb/s
ODU2.
In the 1990s, all IP and later IP/MPLS networks either used a
fraction of maximum circuit capacity, or at most the full circuit
capacity toward the end of the decade, when full circuit capacity was
2.5 Gb/s or 10 Gb/s. Beyond 2000, the TDM circuit multiplexing
capability of SONET/SDH or OTN was rarely used.
Early in the 2000s both transport equipment and core LSR offered 40
Gb/s SONET OC-768. However 10 Gb/s transport equipment was
predominantly deployed throughout the decade, partially because LSR
10GbE ports were far more cost effective than either OC-192 or OC-768
and 10GbE became practical in the second half of the decade.
Entering the 2010 decade, LSR 40GbE and 100GbE are expected to become
widely available and cost effective. Slightly preceding this
transport equipment making use of 40 Gb/s and 100 Gb/s modulations
are becoming available. This transport equipment is capable or
carrying 40 Gb/s ODU3 and 100 Gb/s ODU4 circuits.
Early in the 2000s decade IP/MPLS core networks were making use of
single 10 Gb/s circuits. Capacity grew quickly in the first half of
the decade but more IP/MPLS core networks had only a small number of
IP/MPLS links requiring 4-8 parallel 10 Gb/s circuits. However, the
use of multipath was necessary, was deemed the simplest and most cost
effective alternative, and became thoroughly entrenched. By the end
of the 2000s decade nearly all major IP/MPLS core service provider
networks and a few content provider networks had IP/MPLS links which
exceeded 100 Gb/s, long before 40GbE was available and 40 Gb/s
transport in widespread use.
It is less clear when IP/MPLS LSP exceeded 10 Gb/s, 40 Gb/s, and 100
Gb/s. By 2010, many service providers have LSP in excess of 100 Gb/
s, but few are willing to disclose how many LSP have reached this
capacity.
By 2012 40GbE and 100GbE LSR products had become available, but were
mostly still being evaluated or in trial use by service providers and
contect providers. The cost of components required to deliver 100GbE
products remained high making these products less cost effective.
This is expected to change within years.
The important point is that IP/MPLS core network links have long ago
exceeded 100 Gb/s and some may have already exceeded a Tb/s and a
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small number of IP/MPLS LSP exceed 100 Gb/s. By the time 100 Gb/s
circuits are widely deployed, many IP/MPLS core network links are
likely to exceed 1 Tb/s and many IP/MPLS LSP capacities are likely to
exceed 100 Gb/s. The growth in service provider traffic has
consistently outpaced growth in DWDM channel capacities and the
growth in capacity of single interfaces and is expected to continue
to do so. Therefore multipath techniques are likely here to stay.
Authors' Addresses
So Ning
Tata Communications
Email: ning.so@tatacommunications.com
Andrew Malis
Consultant
Email: agmalis@gmail.com
Dave McDysan
Verizon
22001 Loudoun County PKWY
Ashburn, VA 20147
USA
Email: dave.mcdysan@verizon.com
Lucy Yong
Huawei USA
5340 Legacy Dr.
Plano, TX 75025
USA
Phone: +1 469-277-5837
Email: lucy.yong@huawei.com
Curtis Villamizar
Outer Cape Cod Network Consulting
Email: curtis@occnc.com
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