RTGWG C. Villamizar, Ed.
Internet-Draft Infinera Corporation
Intended status: Informational D. McDysan, Ed.
Expires: January 9, 2011 S. Ning
A. Malis
Verizon
L. Yong
Huawei USA
July 8, 2010
Requirements for MPLS Over a Composite Link
draft-ietf-rtgwg-cl-requirement-01
Abstract
There is often a need to provide large aggregates of bandwidth that
is best provided using parallel links between routers or MPLS LSR.
In core networks there is often no alternative since the aggregate
capacities of core networks today far exceed the capacity of a single
physical link or single packet processing element. Furthermore,
links may be composed of network elements operating across multiple
layers.
The presence of parallel links, potentially comprised of multiple
layers has resulted in a additional requirements. Certain services
may benefit from being restricted to a subset of the set of composite
link component links or a specific component link, where component
link characteristics, such as latency, differ. Certain services
require that LSP be treated as atomic and avoid reordering. Other
services will continue to require only that reordering not occur with
a microflow as is current practice.
Current practice related to multipath is described briefly in an
appendix.
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/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 9, 2011.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Network Operator Functional Requirements . . . . . . . . . . . 5
4.1. Availability, Stability and Transient Response . . . . . . 5
4.2. Component Links Provided by Lower Layer Networks . . . . . 6
4.3. Parallel Component Links with Different Characteristics . 7
5. Derived Requirements . . . . . . . . . . . . . . . . . . . . . 8
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . . 11
9.3. Appendix References . . . . . . . . . . . . . . . . . . . 11
Appendix A. More Details on Existing Network Operator
Practices and Protocol Usage . . . . . . . . . . . . 12
Appendix B. Existing Multipath Standards and Techniques . . . . . 14
B.1. Common Multpath Load Spliting Techniques . . . . . . . . . 15
B.2. Simple and Adaptive Load Balancing Multipath . . . . . . . 16
B.3. Traffic Split over Parallel Links . . . . . . . . . . . . 16
B.4. Traffic Split over Multiple Paths . . . . . . . . . . . . 17
Appendix C. ITU-T G.800 Composite Link Definitions and
Terminology . . . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
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1. Introduction
The purpose of this document is to describe why network operators
require certain functions in order to solve certain business problems
(Section 2). The intent is to first describe why things need to be
done in terms of functional requirements that are as independent as
possible of protocol specifications (Section 4). For certain
functional requirements this document describes a set of derived
protocol requirements (Section 5). Three appendices provide
supporting details as a summary of existing/prior operator
approaches, a summary of implementation techniques and relevant
protocol standards, and a summary of G.800 terminology used to define
the concept of a composite link. (Appendix B).
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Assumptions
The services supported include L3VPN, L2VPN (VPWS and VPLS), Internet
traffic encapsulated by at least one MPLS label, and dynamically
signaled MPLS-TP LSPs and pseudowires. The MPLS LSPs supporting
these services may be pt-pt, pt-mpt, or mpt-mpt.
The location in a network where these requirements apply are a Label
Edge Router (LER) or a Label Switch Router (LSR) as defined in RFC
3031 [RFC3031].
The IP DSCP 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. Definitions
Composite Link:
Section 6.9.2 of ITU-T-G.800 [ITU-T.G.800] defines composite link
as summarized in Appendix Appendix C. The following definitions
map the ITU-T G.800 terminology into IETF terminology which is
used in this document.
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Multiple parallel links: When multiple parallel component links
between the an LER/LSR and another LER/LSR.
Multi-layer Component Link: A component link that is formed by
other network elements at other layers.
Component Link: A physical link (e.g., Lambda, Ethernet PHY, SONET/
SDH, OTN, etc.) with packet transport capability, or a logical
link (e.g., MPLS LSP, Ethernet VLAN, MPLS-TP LSP, etc.)
Flow: A sequence of packets that must be transferred on one
component link.
Flow identification: The label stack and other information that
uniquely identifies a flow. Other information in flow
identification may include an IP header, PW control word,
Ethernet MAC address, etc. Note that an LSP may contain one or
more Flows or an LSP may be equivalent to a Flow. Flow
identification is used to locally select a component link, or a
path through the network toward the destination.
4. Network Operator Functional Requirements
The Functional Requirements in this section are grouped in
subsections starting with the highest priority.
4.1. Availability, Stability and Transient Response
Limiting the period of unavailability in response to failures or
transient events is extremely important as well as maintaining
stability. The transient period between some service disrupting
event and the convergence of the routing and/or signaling protocols
MUST occur within a time frame specified by SLA objectives. The
timeframes range from rapid restoration, on the order of 100 ms or
less (e.g., for VPWS), to several minutes (e.g., for L3VPN) and may
differ among the set of customers within a single service.
FR#1 The solution SHALL provide a means to summarize routing
advertisements regarding the characteristics of a composite
link such that the routing protocol convergence within the
timeframe needed to meet the SLA objective..
FR#2 The solution SHALL provide a means for aggregating signaling
such that in response to a failure in the worst case cross
section of the network that MPLS LSPs are restored within the
timeframe needed to meet the SLA objective.
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FR#3 The solution SHALL provide to select a path for a flow across a
network that contains a number of paths comprised of pairs of
nodes connected by composite links in such a way as to
automatically distribute the load over the network nodes
connected by composite links while meeting all of the other
mandatory requirements stated above. The solution SHOULD work
in a manner similar to that when the characteristics of the
individual component links are advertised.
FR#4 If extensions to existing protocols are specified and/or new
protocols are defined, then the solution SHOULD provide a means
for a network operator to migrate an existing deployment in a
minimally disruptive manner.
FR#5 Any automatic LSP routing and/or load balancing solutions MUST
not oscillate such that performance observed by users changes
such that an SLA is violated. Since oscillation may cause
reordering, there MUST be means to control the frequency of
changing the component link over which a flow is placed.
FR#6 Management and diagnostic protocols MUST be able to operate
over composite links.
4.2. Component Links Provided by Lower Layer Networks
Case 3 as defined in [ITU-T.G.800] involves a component link
supporting an MPLS layer network over another lower layer network
(e.g., circuit switched or another MPLS network (e.g., MPLS-TP)).
The lower layer network may change the latency (and/or other
performance parameters) seen by the MPLS layer network. Network
Operators have SLAs of which some components are based on performance
parameters. Currently, there is no protocol for the lower layer
network to inform the higher layer network of a change in a
performance parameter. Communication of the latency performance
parameter is a very important requirement. Communication of other
performance parameters (e.g., delay variation) is desirable.
FR#7 In order to support network SLAs and provide acceptable user
experience, the solution SHALL specify a protocol means to
allow a lower layer server network to communicate latency to
the higher layer client network.
FR#8 The precision of latency reporting SHOULD be at least 10% of
the one way latency for latency of 1 ms or more.
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FR#9 The solution SHALL provide a means to limit the latency on a
per LSP basis between nodes within a network to meet an SLA
target when the path between these nodes contains one or more
pairs of nodes connected via a composite link.
The SLAs differ across the services, and some services have
different SLAs for different QoS classes, for example, one QoS
class may have a much larger latency bound than another.
Overload can occur which would violate an SLA parameter (e.g.,
loss) and some remedy to handle this case for a composite
link.
FR#10 If the total demand offered by traffic flows exceeds the
capacity of the composite link, the solution SHOULD define a
means to cause the LSPs for some traffic flows to move to some
other point in the network that is not congested. These
"preempted LSPs" may not be restored if there is no
uncongested path in the network.
4.3. Parallel Component Links with Different Characteristics
Corresponding to Case 1 of [ITU-T.G.800], as one means to provide
high availability, network operators deploy a topology in the MPLS
network using lower layer networks that have a certain degree of
diversity at the lower layer(s). Many techniques have been developed
to balance the distribution of flows across component links that
connect the same pair of nodes (See Appendix B.3). When the path for
a flow can be chosen from a set of candidate nodes connected via
composite links, other techniques have been developed (See
Appendix B.4).
FR#11 The solution SHALL measure traffic on a labeled traffic flow
and dynamically select the component link on which to place
this flow in order to balance the load so that no component
link in the composite link between a pair of nodes is
overloaded.
FR#12 When a traffic flow is moved from one component link to
another in the same composite link between a set of nodes (or
sites), it MUST be done so in a minimally disruptive manner.
When a flow is moved from a current link to a target link with
different latency, reordering can occur if the target link
latency is less than that of the current or clumping can occur
if target link latency is greater than that of the current.
Therefore, some flows (e.g., timing distribution, PW circuit
emulation) are quite sensitive to these effects, which may be
specified in an SLA or are needed to meet a user experience
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objective (e.g. jitter buffer under/overrun).
FR#13 The solution SHALL provide a means to identify flows whose
rearrangement frequency needs to be bounded by a configured
value.
FR#14 The solution SHALL provide a means that communicates whether
the flows within an LSP can be split across multiple component
links. The solution SHOULD provide a means to indicate the
flow identification field(s) which can be used along the flow
path which can be used to perform this function.
FR#15 The solution SHALL provide a means to indicate that a traffic
flow shall select a component link with the minimum latency
value.
FR#16 The solution SHALL provide a means to indicate that a traffic
flow shall select a component link with a maximum acceptable
latency value as specified by protocol.
FR#17 The solution SHALL provide a means to indicate that a traffic
flow shall select a component link with a maximum acceptable
delay variation value as specified by protocol.
FR#18 The solution SHALL provide a local means to a node which
automatically distribute flows across the component links in
the composite link that connects to the other node such that
SLA objectives are met.
FR#19 The solution SHALL provide a means to distribute flows from a
single LSP across multiple component links to handle at least
the case where the traffic carried in an LSP exceeds that of
any component link in the composite link.
5. Derived Requirements
This section takes the next step and derives high-level requirements
on protocol specification from the functional requirements.
DR#1 The solution SHOULD attempt to extend existing protocols
wherever possible, developing a new protocol only if this adds
a significant set of capabilities.
The vast majority of network operators have provisioned L3VPN
services over LDP. Many have deployed L2VPN services over LDP
as well. TE extensions to IGP and RSVP-TE are viewed as being
overly complex by some operators.
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DR#2 A solution SHOULD extend LDP capabilities to meet functional
requirements (without using TE methods as decided in
[RFC3468]).
DR#3 Coexistence of LDP and RSVP-TE signaled LSPs MUST be supported
on a composite link. Other functional requirements should be
supported as independently of signaling protocol as possible.
DR#4 When the nodes connected via a composite link are in the same
MPLS network topology, the solution MAY define extensions to
the IGP.
DR#5 When the nodes are connected via a composite link are in
different MPLS network topologies, the solution SHALL NOT rely
on extensions to the IGP.
DR#6 When a worst case failure scenario occurs,the resulting number
of links advertised in the IGP causes IGP convergence to occur,
causing a period of unavailability as perceived by users. The
convergence time of the solution MUST meet the SLA objective
for the duration of unavailability.
DR#7 The Solution SHALL summarize the characteristics of the
component links as a composite link IGP advertisement that
results in convergence time better than that of advertising the
individual component links. This summary SHALL be designed so
that it represents the range of capabilities of the individual
component links such that functional requirements are met, and
also minimizes the frequency of advertisement updates which may
cause IGP convergence to occur. Examples of advertisement
update tiggering events to be considered include: LSP
establishment/release, changes in component link
characteristics (e.g., latency, up/down state), and/or
bandwidth utilization.
DR#8 When a worst case failure scenario occurs,the resulting number
of links advertised in the IGP causes IGP convergence to occur,
causing a period of unavailability as perceived by users. The
convergence time of the solution MUST meet the SLA objective
for the duration of unavailability.
DR#9 When a worst case failure scenario occurs, the number of
RSVP-TE LSPs to be resignaled will cause a period of
unavailability as perceived by users. The resignaling time of
the solution MUST meet the SLA objective for the duration of
unavailability. The resignaling time of the solution MUST not
increase significantly as compared with current methods.
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6. Acknowledgements
Frederic Jounay of France Telecom and Yuji Kamite of NTT
Communications Corporation co-authored a version of this document.
A rewrite of this document occurred after the IETF77 meeting.
Dimitri Papadimitriou, Lou Berger, Tony Li, the WG chairs John Scuder
and Alex Zinin, and others provided valuable guidance prior to and at
the IETF77 RTGWG meeting.
Tony Li and John Drake have made numerous valuable comments on the
RTGWG mailing list that are reflected in versions following the
IETF77 meeting.
7. IANA Considerations
This memo includes no request to IANA.
8. Security Considerations
This document specifies a set of requirements. The requirements
themselves do not pose a security threat. If these requirements are
met using MPLS signaling as commonly practiced today with
authenticated but unencrypted OSPF-TE, ISIS-TE, and RSVP-TE or LDP,
then the requirement to provide additional information in this
communication presents additional information that could conceivably
be gathered in a man-in-the-middle confidentiality breach. Such an
attack would require a capability to monitor this signaling either
through a provider breach or access to provider physical transmission
infrastructure. A provider breach already poses a threat of numerous
tpes of attacks which are of far more serious consequence. Encrption
of the signaling can prevent or render more difficult any
confidentiality breach that otherwise might occur by means of access
to provider physical transmission infrastructure.
9. References
9.1. 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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9.2. Informative References
[ITU-T.G.800]
ITU-T, "Unified functional architecture of transport
networks", 2007, <http://www.itu.int/rec/T-REC-G/
recommendation.asp?parent=T-REC-G.800>.
[RFC2702] Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M., and J.
McManus, "Requirements for Traffic Engineering Over MPLS",
RFC 2702, September 1999.
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
[RFC3468] Andersson, L. and G. Swallow, "The Multiprotocol Label
Switching (MPLS) Working Group decision on MPLS signaling
protocols", RFC 3468, February 2003.
[RFC3809] Nagarajan, A., "Generic Requirements for Provider
Provisioned Virtual Private Networks (PPVPN)", RFC 3809,
June 2004.
[RFC4031] Carugi, M. and D. McDysan, "Service Requirements for Layer
3 Provider Provisioned Virtual Private Networks (PPVPNs)",
RFC 4031, April 2005.
[RFC4665] Augustyn, W. and Y. Serbest, "Service Requirements for
Layer 2 Provider-Provisioned Virtual Private Networks",
RFC 4665, September 2006.
[RFC5254] Bitar, N., Bocci, M., and L. Martini, "Requirements for
Multi-Segment Pseudowire Emulation Edge-to-Edge (PWE3)",
RFC 5254, October 2008.
9.3. Appendix References
[I-D.ietf-pwe3-fat-pw]
Bryant, S., Filsfils, C., Drafz, U., Kompella, V., Regan,
J., and S. Amante, "Flow Aware Transport of Pseudowires
over an MPLS PSN", draft-ietf-pwe3-fat-pw-03 (work in
progress), January 2010.
[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>.
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[ITU-T.Y.1541]
ITU-T, "Network performance objectives for IP-based
services", 2006, <http://www.itu.int/rec/T-REC-Y.1541/en>.
[RFC1717] Sklower, K., Lloyd, B., McGregor, G., and D. Carr, "The
PPP Multilink Protocol (MP)", RFC 1717, November 1994.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998.
[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.
[RFC3260] Grossman, D., "New Terminology and Clarifications for
Diffserv", RFC 3260, April 2002.
[RFC4201] Kompella, K., Rekhter, Y., and L. Berger, "Link Bundling
in MPLS Traffic Engineering (TE)", RFC 4201, October 2005.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 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.
Appendix A. More Details on Existing Network Operator Practices and
Protocol Usage
Network operators have SLAs for services that are comprised of
numerical values for performance measures, principally availability,
latency, delay variation. See [ITU-T.Y.1541], RFC 3809, Section 4.9
[RFC3809] for examples of the form of such SLAs. Note that the
numerical values of Y.1541 span multiple networks and may be looser
than network operator SLAs. Applications and acceptable user
experience have a relationship to these performance parameters.
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Consider latency as an example. In some cases, minimizing latency
relates directly to the best customer experience (e.g., in TCP closer
is faster). I 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 SLAs have. a bound on point-
point latency, and as long as this bound is met, the SLA is met --
decreasing the latency is not necessary. In some SLAs, 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 to meet this type of SLA, but this lowers availability since an
alternate route that meets the latency SLA 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) this was not known to the MPLS network. This resulted in
latency affecting end user experience, sometimes violating SLAs 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 SLA or meet user
experience objectives. This results in reduced flexibility and
increased cost for network operators. Using lower layer networks to
provide restoration and grooming is expected to be more efficient,
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 SLAs for pt-pt services are often tied closely to geographic
locations, while latency for multipoint services may be based upon a
worst case within a region.
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. 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. The approach
that has been taken is to aggregate these QoS classes into similar
sets on LER-LSR and LSR-LSR links.
Labeled LSPs have been and use of link layer encapsulation have been
standardized in order to provide a means to meet these needs.
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The IP DSCP cannot be used for flow identification since RFC 4301
Section 5.5 [RFC4301] requires Diffserv transparency, and in general
network operators do not rely on the DSCP of Internet packets.
A label is pushed onto Internet packets when they are carried along
with L2/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 mpt-pt
LDP signaled MPLS LSPs occur automatically, and balancing across
parallel links occurs if the IGP metrics are set "equally" (with
equality a locally definable relation).
Traffic is typically comprised of a few large (some very large) flows
and many small flows. In some cases, separate LSPs are established
for very large flows. This can occur even if the IP header
information is inspected by a router, for example an IPsec tunnel
that carries a large amount of traffic. An important example of
large flows is that of a L2/L3 VPN customer who has an access line
bandwdith comparable to a client-client composite link bandwidth --
there could be flows that are on the order of the access line
bandwdith.
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, or
even BGP, and equal cost LSP, as described in Appendix B.4. Various
mutilpath techniques have strengths and weaknesses.
The term composite link is more general than terms such as link
aggregate which is generally considered to be specific to Ethernet
and its use here is consistent with the broad definition in
[ITU-T.G.800]. The term multipath excludes inverse multiplexing and
refers to techniques which only solve the problem of large
aggregations of traffic, without addressing the other requirements
outlined in this document.
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B.1. Common Multpath Load Spliting Techniques
Identical load balancing techniqes 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 other LSP as
payload, there is no signaling associated with these inner LSP.
Therefore even when using RSVP-TE signaling there may be insufficient
information provided by signaling to adequately distribute load
across a composite link.
Generally a set of label stack entries that is unique across the
ordered set of label numbers 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 multipth techniques must be modified. Such modifications
are outside the scope of this document.
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 flow 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.
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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 [I-D.ietf-pwe3-fat-pw]. 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. Simple and Adaptive Load Balancing Multipath
Simple 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. A common
simple multipath implementation assumes that all members (component
links) are of equal capacity and perform a modulo operation across
the hashed value. An alternate simple multipath technique uses a
table generally with a power of two size, and distributes the table
entries proportionally among members according to the capacity of
each member.
Simple 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 simple load balancing.
An adaptive multipath technique is one where the traffic bound to
each member (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 delivered out of
sequence.
B.3. Traffic Split over Parallel Links
The load spliting 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.
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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.
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.
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 LSP that comprise the
bundle.
Appendix C. ITU-T G.800 Composite Link Definitions and Terminology
Composite Link:
Section 6.9.2 of ITU-T-G.800 [ITU-T.G.800] defines composite link
in terms of three cases, of which the following two are relevant
(the one describing inverse (TDM) multiplexing does not apply).
Note that these case definitions are taken verbatim from section
6.9, "Layer Relationships".
Case 1: "Multiple parallel links between the same subnetworks
can be bundled together into a single composite link. Each
component of the 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 crossing this link may not be preserved.
This is illustrated in Figure 14."
Case 3: "A link can also be constructed by a concatenation of
component links and configured channel forwarding
relationships. The forwarding relationships must have a 1:1
correspondence to the link connections that will be provided
by the client link. In this case, it is not possible to
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fully infer the status of the link by observing the server
layer trails visible at the ends of the link. This is
illustrated in Figure 16."
Subnetwork: A set of one or more nodes (i.e., LER or LSR) and links.
As a special case it can represent a site comprised of multiple
nodes.
Forwarding Relationship: Configured forwarding between ports on a
subnetwork. It may be connectionless (e.g., IP, not considered
in this draft), or connection oriented (e.g., MPLS signaled or
configured).
Component Link: A topolological relationship between subnetworks
(i.e., a connection between nodes), which may be a wavelength,
circuit, virtual circuit or an MPLS LSP.
Authors' Addresses
Curtis Villamizar (editor)
Infinera Corporation
169 W. Java Drive
Sunnyvale, CA 94089
Email: cvillamizar@infinera.com
Dave McDysan (editor)
Verizon
22001 Loudoun County PKWY
Ashburn, VA 20147
Email: dave.mcdysan@verizon.com
So Ning
Verizon
2400 N. Glenville Ave.
Richardson, TX 75082
Phone: +1 972-729-7905
Email: ning.so@verizonbusiness.com
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Andrew Malis
Verizon
117 West St.
Waltham, MA 02451
Phone: +1 781-466-2362
Email: andrew.g.malis@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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