| Rfc | 8256 |
|---|
| Title | Requirements for Hitless MPLS Path Segment Monitoring |
|---|
| Author | A.
D'Alessandro, L. Andersson, S. Ueno, K. Arai, Y. Koike |
|---|
| Date | October
2017 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | INFORMATIONAL |
|---|
|
Internet Engineering Task Force (IETF) A. D'Alessandro
Request for Comments: 8256 Telecom Italia
Category: Informational L. Andersson
ISSN: 2070-1721 Huawei Technologies
S. Ueno
NTT Communications
K. Arai
Y. Koike
NTT
October 2017
Requirements for Hitless MPLS Path Segment Monitoring
Abstract
One of the most important Operations, Administration, and Maintenance
(OAM) capabilities for transport-network operation is fault
localization. An in-service, on-demand path segment monitoring
function of a transport path is indispensable, particularly when the
service monitoring function is activated only between endpoints.
However, the current segment monitoring approach defined for MPLS
(including the MPLS Transport Profile (MPLS-TP)) in RFC 6371
"Operations, Administration, and Maintenance Framework for MPLS-Based
Transport Networks" has drawbacks. This document provides an
analysis of the existing MPLS-TP OAM mechanisms for the path segment
monitoring and provides requirements to guide the development of new
OAM tools to support Hitless Path Segment Monitoring (HPSM).
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8256.
Copyright Notice
Copyright (c) 2017 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
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4
4. Requirements for HPSM . . . . . . . . . . . . . . . . . . . . 8
4.1. Backward Compatibility . . . . . . . . . . . . . . . . . 8
4.2. Non-Intrusive Segment Monitoring . . . . . . . . . . . . 8
4.3. Monitoring Multiple Segments . . . . . . . . . . . . . . 9
4.4. Monitoring Single and Multiple Levels . . . . . . . . . . 9
4.5. HPSM and End-to-End Proactive Monitoring Independence . . 10
4.6. Monitoring an Arbitrary Segment . . . . . . . . . . . . . 10
4.7. Fault while HPSM Is Operational . . . . . . . . . . . . . 11
4.8. HPSM Manageability . . . . . . . . . . . . . . . . . . . 13
4.9. Supported OAM Functions . . . . . . . . . . . . . . . . . 13
5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6. Security Considerations . . . . . . . . . . . . . . . . . . . 14
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
8.1. Normative References . . . . . . . . . . . . . . . . . . 14
8.2. Informative References . . . . . . . . . . . . . . . . . 15
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction
According to the MPLS-TP OAM requirements [RFC5860], mechanisms MUST
be available for alerting service providers of faults or defects that
affect their services. In addition, to ensure that faults or service
degradation can be localized, operators need a function to diagnose
the detected problem. Using end-to-end monitoring for this purpose
is insufficient in that an operator will not be able to localize a
fault or service degradation accurately.
A segment monitoring function that can focus on a specific segment of
a transport path and that can provide a detailed analysis is
indispensable to promptly and accurately localize the fault. A
function for monitoring path segments has been defined to perform
this task for MPLS-TP. However, as noted in the MPLS-TP OAM
Framework [RFC6371], the current method for segment monitoring of a
transport path has implications that hinder the usage in an operator
network.
After elaborating on the problem statement for the path segment
monitoring function as it is currently defined, this document
provides requirements for an on-demand path segment monitoring
function without traffic disruption. Further works are required to
evaluate how proposed requirements match with current MPLS
architecture and to identify possible solutions.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2.1. Terminology
HPSM - Hitless Path Segment Monitoring
LSP - Label Switched Path
LSR - Label Switching Router
ME - Maintenance Entity
MEG - Maintenance Entity Group
MEP - Maintenance Entity Group End Point
MIP - Maintenance Entity Group Intermediate Point
OTN - Optical Transport Network
TCM - Tandem Connection Monitoring
SPME - Sub-Path Maintenance Element
3. Problem Statement
A Sub-Path Maintenance Element (SPME) function to monitor (and to
protect and/or manage) MPLS-TP network segments is defined in
[RFC5921]. The SPME is defined between the edges of the segment of a
transport path that needs to be monitored, protected, or managed.
SPME is created by stacking the shim header (MPLS header), according
to [RFC3031]; it is defined as the segment where the header is
stacked. OAM messages can be initiated at the edge of the SPME.
They can be sent to the peer edge of the SPME or to a MIP along the
SPME by setting the TTL value of the Label Stack Entry (LSE) and
interface identifier value at the corresponding hierarchical LSP
level in case of a per-node model.
According to Section 3.8 of [RFC6371], MPLS-TP segment monitoring
should satisfy two network objectives:
(N1) The monitoring and maintenance of current transport paths has
to be conducted in-service without traffic disruption.
(N2) Segment monitoring must not modify the forwarding of the
segment portion of the transport path.
The SPME function that is defined in [RFC5921] has the following
drawbacks:
(P1) It increases network management complexity, because a new sub-
layer and new MEPs and MIPs have to be configured for the SPME.
(P2) Original conditions of the path change.
(P3) The client traffic over a transport path is disrupted if the
SPME is configured on-demand.
Problem (P1) is related to the management of each additional sub-
layer required for segment monitoring in an MPLS-TP network. When an
SPME is applied to administer on-demand OAM functions in MPLS-TP
networks, a rule for operationally differentiating those SPMEs will
be required at least within an administrative domain. This forces
operators to implement at least an additional layer into the
management systems that will only be used for on-demand path segment
monitoring. From the perspective of operation, increasing the number
of managed layers and managed addresses/identifiers is not desirable
in view of keeping the management systems as simple as possible.
Moreover, using the currently defined methods, on-demand setting of
SPMEs causes problems (P2) and (P3) due to additional label stacking.
Problem (P2) arises because the MPLS-exposed label value and MPLS
frame length change. The monitoring function should monitor the
status without changing any condition of the target segment or of the
target transport path. Changing the settings of the original shim
header should not be allowed, because this change corresponds to
creating a new segment of the original transport path that differs
from the original one. When the conditions of the path change, the
measured values or observed data will also change. This may make the
monitoring meaningless because the result of the measurement would no
longer reflect the performance of the connection where the original
fault or degradation occurred. As an example, setting up an on-
demand SPME will result in the LSRs within the monitoring segment
only looking at the added (stacked) labels and not at the labels of
the original LSP. This means that problems stemming from incorrect
(or unexpected) treatment of labels of the original LSP by the nodes
within the monitored segment cannot be identified when setting up
SPME. This might include hardware problems during label lookup,
misconfiguration, etc. Therefore, operators have to pay extra
attention to correctly setting and checking the label values of the
original LSP in the configuration. Of course, the reverse of this
situation is also possible; for example, an incorrect or unexpected
treatment of SPME labels can result in false detection of a fault
where no problem existed originally.
Figure 1 shows an example of SPME settings. In the figure, "X" is
the label value of the original path expected at the tail end of node
D. "210" and "220" are label values allocated for SPME. The label
values of the original path are modified as are the values of the
stacked labels. As shown in Figure 1, SPME changes both the length
of MPLS frames and the label value(s). In particular, performance
monitoring measurements (e.g., Delay Measurement and Packet Loss
Measurement) are sensitive to these changes. As an example,
increasing the packet length may impact packet loss due to MTU
settings; modifying the label stack may introduce packet loss, or it
may fix packet loss depending on the configuration status. Such
changes influence packet delay, too, even if, from a practical point
of view, it is likely that only a few services will experience a
practical impact.
(Before SPME settings)
--- --- --- --- ---
| | | | | | | | | |
| | | | | | | | | |
--- --- --- --- ---
A--100--B--110--C--120--D--130--E <= transport path
MEP MEP
(After SPME settings)
--- --- --- --- ---
| | | | | | | | | |
| | | | | | | | | |
--- --- --- --- ---
A--100--B-----------X---D--130--E <= transport path
MEP MEP
210--C--220 <= SPME
MEP' MEP'
Figure 1: SPME Settings Example
Problem (P3) can be avoided if the operator sets SPMEs in advance and
maintains them until the end of life of a transport path: but this
does not support on-demand. Furthermore, SMPEs cannot be set
arbitrarily because overlapping of path segments is limited to
nesting relationships. As a result, possible SPME configurations of
segments of an original transport path are limited due to the
characteristic of the SPME shown in Figure 1, even if SPMEs are
preconfigured.
Although the make-before-break procedure in the survivability
document [RFC6372] supports configuration for monitoring according to
the framework document [RFC5921], without traffic disruption the
configuration of an SPME is not possible without violating the
network objective (N2). These concerns are described in Section 3.8
of [RFC6371].
Additionally, the make-before-break approach typically relies on a
control plane and requires additional functionalities for a
management system to properly support SPME creation and traffic
switching from the original transport path to the SPME.
As an example, the old and new transport resources (e.g., LSP
tunnels) might compete with each other for resources that they have
in common. Depending on availability of resources, this competition
can cause admission control to prevent the new LSP tunnel from being
established as this bandwidth accounting deviates from the
traditional (non-control plane) management-system operation. While
SPMEs can be applied in any network context (single-domain, multi-
domain, single-carrier, multi-carrier, etc.), the main applications
are in inter-carrier or inter-domain segment monitoring where they
are typically preconfigured or pre-instantiated. SPME instantiates a
hierarchical path (introducing MPLS-label stacking) through which OAM
packets can be sent. The SPME monitoring function is also mainly
important for protecting bundles of transport paths and the carriers'
carrier solutions within an administrative domain.
The analogy for SPME in other transport technologies is Tandem
Connection Monitoring (TCM). TCM is used in Optical Transport
Networks (OTNs) and Ethernet transport networks. It supports on-
demand but does not affect the path. For example, in OTNs, TCM
allows the insertion and removal of performance monitoring overhead
within the frame at intermediate points in the network. It is done
such that their insertion and removal do not change the conditions of
the path. Though, as the OAM overhead is part of the frame
(designated overhead bytes), it is constrained to a predefined number
of monitoring segments.
To summarize: the problem statement is that the current sub-path
maintenance based on a hierarchical LSP (SPME) is problematic for
preconfiguration in terms of increasing the number of managed objects
by layer stacking and identifiers/addresses. An on-demand
configuration of SPME is one of the possible approaches for
minimizing the impact of these issues. However, the current
procedure is unfavorable because the on-demand configuration for
monitoring changes the condition of the original monitored path. To
avoid or minimize the impact of the drawbacks discussed above, a more
efficient approach is required for the operation of an MPLS-TP
transport network. A monitoring mechanism, named "Hitless Path
Segment Monitoring" (HPSM), supporting on-demand path segment
monitoring without traffic disruption is needed.
4. Requirements for HPSM
In the following sections, mandatory (M) and optional (O)
requirements for the HPSM function are listed.
4.1. Backward Compatibility
HPSM would be an additional OAM tool that would not replace SPME. As
such:
(M1) HPSM MUST be compatible with the usage of SPME.
(O1) HPSM SHOULD be applicable at the SPME layer too.
(M2) HPSM MUST support both the per-node and per-interface model as
specified in [RFC6371].
4.2. Non-Intrusive Segment Monitoring
One of the major problems of legacy SPME highlighted in Section 3 is
that it may not monitor the original path and it could disrupt
service traffic when set up on demand.
(M3) HPSM MUST NOT change the original conditions of the transport
path (e.g., the length of MPLS frames, the exposed label
values, etc.).
(M4) HPSM MUST support on-demand provisioning without traffic
disruption.
4.3. Monitoring Multiple Segments
Along a transport path, there may be the need to support monitoring
multiple segments simultaneously.
(M5) HPSM MUST support configuration of multiple monitoring segments
along a transport path.
--- --- --- --- ---
| | | | | | | | | |
| A | | B | | C | | D | | E |
--- --- --- --- ---
MEP MEP <= ME of a transport path
*------* *----* *--------------* <=three HPSM monit. instances
Figure 2: Multiple HPSM Instances Example
4.4. Monitoring Single and Multiple Levels
HPSM would apply mainly for on-demand diagnostic purposes. With the
currently defined approach, the most serious problem is that there is
no way to locate the degraded segment of a path without changing the
conditions of the original path. Therefore, as a first step, a
single-level, single-segment monitoring not affecting the monitored
path is required for HPSM. Monitoring simultaneous segments on
multiple levels is the most powerful tool for accurately diagnosing
the performance of a transport path. However, in the field, a
single-level, multiple-segment approach would be less complex for
management and operations.
(M6) HPSM MUST support single-level segment monitoring.
(O2) HPSM MAY support multi-level segment monitoring.
--- --- --- --- ---
| | | | | | | | | |
| A | | B | | C | | D | | E |
--- --- --- --- ---
MEP MEP <= ME of a transport path
*-----------------* <=On-demand HPSM level 1
*-------------* <=On-demand HPSM level 2
*-* <=On-demand HPSM level 3
Figure 3: Multi-Level HPSM Example
4.5. HPSM and End-to-End Proactive Monitoring Independence
There is a need for simultaneously using existing end-to-end
proactive monitoring and on-demand path segment monitoring.
Normally, the on-demand path segment monitoring is configured on a
segment of a maintenance entity of a transport path. In such an
environment, on-demand single-level monitoring should be performed
without disrupting the proactive monitoring of the targeted end-to-
end transport path to avoid affecting monitoring of user traffic
performance.
(M7) HPSM MUST support the capability of being operated concurrently
to, and independently of, the OAM function on the end-to-end
path.
--- --- --- --- ---
| | | | | | | | | |
| A | | B | | C | | D | | E |
--- --- --- --- ---
MEP MEP <= ME of a transport path
+-----------------------------+ <= Proactive end-to-end mon.
*------------------* <= On-demand HPSM
Figure 4: Independence between Proactive End-to-End Monitoring and
On-Demand HPSM
4.6. Monitoring an Arbitrary Segment
The main objective for on-demand path segment monitoring is to
diagnose the fault locations. A possible realistic diagnostic
procedure is to fix one endpoint of a segment at the MEP of the
transport path under observation and progressively change the length
of the segments. It is, therefore, possible to monitor all the
paths, step-by-step, with a granularity that depends on equipment
implementations. For example, Figure 5 shows the case where the
granularity is at the interface level (i.e., monitoring is at each
input interface and output interface of each piece of equipment).
--- --- --- --- ---
| | | | | | | | | |
| A | | B | | C | | D | | E |
--- --- --- --- ---
MEP MEP <= ME of a transport path
+-----------------------------+ <= Proactive end-to-end mon.
*-----* <= 1st on-demand HPSM
*-------* <= 2nd on-demand HPSM
| |
| |
*-----------------------* <= 4th on-demand HPSM
*-----------------------------* <= 5th on-demand HPSM
Figure 5: Localization of a Defect by Consecutive On-Demand Path
Segment Monitoring Procedure
Another possible scenario is depicted in Figure 6. In this case, the
operator wants to diagnose a transport path starting at a transit
node because the end nodes (A and E) are located at customer sites
and consist of small boxes supporting only a subset of OAM functions.
In this case, where the source entities of the diagnostic packets are
limited to the position of MEPs, on-demand path segment monitoring
will be ineffective because not all the segments can be diagnosed
(e.g., segment monitoring HPSM 3 in Figure 6 is not available, and it
is not possible to determine the fault location exactly).
(M8) It SHALL be possible to provision HPSM on an arbitrary segment
of a transport path.
--- --- ---
--- | | | | | | ---
| A | | B | | C | | D | | E |
--- --- --- --- ---
MEP MEP <= ME of a transport path
+-----------------------------+ <= Proactive end-to-end mon.
*-----* <= On-demand HPSM 1
*-----------------------* <= On-demand HPSM 2
*---------* <= On-demand HPSM 3
Figure 6: HPSM Configuration at Arbitrary Segments
4.7. Fault while HPSM Is Operational
Node or link failures may occur while HPSM is active. In this case,
if no resiliency mechanism is set up on the subtended transport path,
there is no particular requirement for HPSM. If the transport path
is protected, the HPSM function may monitor unintended segments. The
following examples are provided for clarification.
Protection scenario A is shown in Figure 7. In this scenario, a
working LSP and a protection LSP are set up. HPSM is activated
between nodes A and E. When a fault occurs between nodes B and C,
the operation of HPSM is not affected by the protection switch and
continues on the active LSP.
A - B - C - D - E - F
\ /
G - H - I - L
Where:
- end-to-end LSP: A-B-C-D-E-F
- working LSP: A-B-C-D-E-F
- protection LSP: A-G-H-I-L-F
- HPSM: A-E
Figure 7: Protection Scenario A
Protection scenario B is shown in Figure 8. The difference with
scenario A is that only a portion of the transport path is protected.
In this case, when a fault occurs between nodes B and C on the
working sub-path B-C-D, traffic will be switched to protection sub-
path B-G-H-D. Assuming that OAM packet termination depends only on
the TTL value of the MPLS label header, the target node of the HPSM
changes from E to D due to the difference of hop counts between the
working path route (A-B-C-D-E: 4 hops) and protection path route
(A-B-G-H-D-E: 5 hops). In this case, the operation of HPSM is
affected.
A - B - C - D - E - F
\ /
G - H
- end-to-end LSP: A-B-C-D-E-F
- working sub-path: B-C-D
- protection sub-path: B-G-H-D
- HPSM: A-E
Figure 8: Protection Scenario B
(M9) The HPSM SHOULD avoid monitoring an unintended segment when one
or more failures occur.
There are potentially different solutions to satisfy such a
requirement. A possible solution may be to suspend HPSM monitoring
until network restoration takes place. Another possible approach may
be to compare the node/interface ID in the OAM packet with that at
the node reached at TTL termination and, if this does not match, a
suspension of HPSM monitoring should be triggered. The above
approaches are valid in any circumstance, both for protected and
unprotected networks LSPs. These examples should not be taken to
limit the design of a solution.
4.8. HPSM Manageability
From a managing perspective, increasing the number of managed layers
and managed addresses/identifiers is not desirable in view of keeping
the management systems as simple as possible.
(M10) HPSM SHOULD NOT be based on additional transport layers (e.g.,
hierarchical LSPs).
(M11) The same identifiers used for MIPs and/or MEPs SHOULD be
applied to maintenance points for the HPSM when they are
instantiated in the same place along a transport path.
Maintenance points for the HPSM may be different from the
functional components of MIPs and MEPs as defined in the OAM
framework document [RFC6371]. Investigating potential
solutions for satisfying HPSM requirements may lead to
identifying new functional components; these components need to
be backward compatible with MPLS architecture. Solutions are
outside the scope of this document.
4.9. Supported OAM Functions
A maintenance point supporting the HPSM function has to be able to
generate and inject OAM packets. OAM functions that may be
applicable for on-demand HPSM are basically the on-demand performance
monitoring functions that are defined in the OAM framework document
[RFC6371]. The "on-demand" attribute is typically temporary for
maintenance operation.
(M12) HPSM MUST support Packet Loss and Packet Delay measurement.
These functions are normally only supported at the endpoints of a
transport path. If a defect occurs, it might be quite hard to locate
the defect or degradation point without using the segment monitoring
function. If an operator cannot locate or narrow down the cause of
the fault, it is quite difficult to take prompt actions to solve the
problem.
Other on-demand monitoring functions (e.g., Delay Variation
measurement) are desirable but not as necessary as the functions
mentioned above.
(O3) HPSM MAY support Packet Delay variation, Throughput
measurement, and other performance monitoring and fault
management functions.
Support of out-of-service on-demand performance-management functions
(e.g., Throughput measurement) is not required for HPSM.
5. Summary
A new HPSM mechanism is required to provide on-demand path segment
monitoring without traffic disruption. It shall meet the two network
objectives described in Section 3.8 of [RFC6371] and summarized in
Section 3 of this document.
The mechanism should minimize the problems described in Section 3,
i.e., (P1), (P2), and (P3).
The solution for the on-demand path segment monitoring without
traffic disruption needs to cover both the per-node model and the
per-interface model specified in [RFC6371].
The on-demand path segment monitoring without traffic disruption
solution needs to support on-demand Packet Loss Measurement and
Packet Delay Measurement functions and optionally other performance
monitoring and fault management functions (e.g., Throughput
measurement, Packet Delay variation measurement, Diagnostic test,
etc.).
6. Security Considerations
Security is a significant requirement of the MPLS Transport Profile.
This document provides a problem statement and requirements to guide
the development of new OAM tools to support HPSM. Such new tools
must follow the security considerations provided in OAM Requirements
for MPLS-TP in [RFC5860].
7. IANA Considerations
This document does not require any IANA actions.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031,
DOI 10.17487/RFC3031, January 2001,
<https://www.rfc-editor.org/info/rfc3031>.
[RFC5860] Vigoureux, M., Ed., Ward, D., Ed., and M. Betts, Ed.,
"Requirements for Operations, Administration, and
Maintenance (OAM) in MPLS Transport Networks", RFC 5860,
DOI 10.17487/RFC5860, May 2010,
<https://www.rfc-editor.org/info/rfc5860>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
8.2. Informative References
[RFC5921] Bocci, M., Ed., Bryant, S., Ed., Frost, D., Ed., Levrau,
L., and L. Berger, "A Framework for MPLS in Transport
Networks", RFC 5921, DOI 10.17487/RFC5921, July 2010,
<https://www.rfc-editor.org/info/rfc5921>.
[RFC6371] Busi, I., Ed. and D. Allan, Ed., "Operations,
Administration, and Maintenance Framework for MPLS-Based
Transport Networks", RFC 6371, DOI 10.17487/RFC6371,
September 2011, <https://www.rfc-editor.org/info/rfc6371>.
[RFC6372] Sprecher, N., Ed. and A. Farrel, Ed., "MPLS Transport
Profile (MPLS-TP) Survivability Framework", RFC 6372,
DOI 10.17487/RFC6372, September 2011,
<https://www.rfc-editor.org/info/rfc6372>.
Contributors
Manuel Paul
Deutsche Telekom AG
Email: manuel.paul@telekom.de
Acknowledgements
The authors would also like to thank Alexander Vainshtein, Dave
Allan, Fei Zhang, Huub van Helvoort, Malcolm Betts, Italo Busi,
Maarten Vissers, Jia He, and Nurit Sprecher for their comments and
enhancements to the text.
Authors' Addresses
Alessandro D'Alessandro
Telecom Italia
Via Reiss Romoli, 274
Torino 10148
Italy
Email: alessandro.dalessandro@telecomitalia.it
Loa Andersson
Huawei Technologies
Email: loa@pi.nu
Satoshi Ueno
NTT Communications
Email: ueno@nttv6.jp
Kaoru Arai
NTT
Email: arai.kaoru@lab.ntt.co.jp
Yoshinori Koike
NTT
Email: y.koike@vcd.nttbiz.com