[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]
Versions: 00 01 02 03 04 05 06 07 08 09 10 11
RFC 6205
Network Working Group Tomohiro Otani(Ed.)
Internet Draft KDDI
Updates: 3471(if approved) Dan Li(Ed.)
Category: Standards Track Huawei
Expires: July 2011 January 11, 2011
Generalized Labels for Lambda-Switching Capable Label Switching
Routers
draft-ietf-ccamp-gmpls-g-694-lambda-labels-11.txt
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with
the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other
documents at any time. It is inappropriate to use Internet-Drafts
as reference material or to cite them other than as "work in
progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
This Internet-Draft will expire on July 11, 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
T. Otani et al. Expires July 2011 [Page 1]
draft-ietf-ccamp-gmpls-g-694-lambda-labels-11.txt January 2011
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.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before
November 10, 2008. The person(s) controlling the copyright in
some of this material may not have granted the IETF Trust the
right to allow modifications of such material outside the IETF
Standards Process. Without obtaining an adequate license from the
person(s) controlling the copyright in such materials, this
document may not be modified outside the IETF Standards Process,
and derivative works of it may not be created outside the IETF
Standards Process, except to format it for publication as an RFC
or to translate it into languages other than English.
Abstract
Technology in the optical domain is constantly evolving and as a
consequence new equipment providing lambda switching capability
has been developed and is currently being deployed.
Generalized MPLS (GMPLS) is a family of protocols that can be
used to operate networks built from a range of technologies
including wavelength (or lambda) switching. For this purpose,
GMPLS defined that a wavelength label only has significance
between two neighbors and global wavelength semantics are not
considered.
In order to facilitate interoperability in a network composed of
next generation lambda switch-capable equipment, this document
defines a standard lambda label format that is compliant with
Dense Wavelength Division Multiplexing and Coarse Wavelength
Division Multiplexing grids defined by the International
Telecommunication Union Telecommunication Standardization Sector.
The label format defined in this document can be used in GMPLS
signaling and routing protocols.
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
T. Otani et al. Expires July 2011 [Page 2]
draft-ietf-ccamp-gmpls-g-694-lambda-labels-11.txt January 2011
1. Introduction
As described in [RFC3945], Generalized MPLS (GMPLS) extends MPLS
from supporting only packet (Packet Switching Capable - PSC)
interfaces and switching to also include support for four new
classes of interfaces and switching:
o Layer-2 Switch Capable (L2SC)
o Time-Division Multiplex (TDM)
o Lambda Switch Capable (LSC)
o Fiber-Switch Capable (FSC).
A functional description of the extensions to MPLS signaling
needed to support new classes of interfaces and switching is
provided in [RFC3471].
This document presents details that are specific to the use of
GMPLS with Lambda Switch Capable (LSC) equipment. Technologies
such as Reconfigurable Optical Add/Drop Multiplex (ROADM) and
Wavelength Cross-Connect (WXC) operate at the wavelength
switching level. [RFC3471] has defined that a wavelength label
(section 3.2.1.1) "only has significance between two neighbors"
and global wavelength semantics is not considered. In order to
facilitate interoperability in a network composed of lambda
switch-capable equipment, this document defines a standard lambda
label format, which is compliant with both [G.694.1](Dense
Wavelength Division Multiplexing (DWDM)-grid) or [G.694.2](Coarse
Wavelength Division Multiplexing (CWDM)-grid).
2. Assumed Network Model and Related Problem Statement
Figure 1 depicts an all-optically switched network consisting of
different vendors' optical network domains. Vendor A's network
consists of ROADM or WXC, and vendor B's network consists of a
number of photonic cross-connect (PXC) and DWDM multiplexer &
demultiplexer, otherwise both vendors' networks might be based on
the same technology.
In this case, the use of standardized wavelength label
information is quite significant to establish a wavelength-based
LSP. It is also an important constraint when conducting CSPF
calculation for use by Generalized Multi-Protocol Label Switching
(GMPLS) RSVP-TE signaling, [RFC3473]. The way the Constrained
T. Otani et al. Expires July 2011 [Page 3]
draft-ietf-ccamp-gmpls-g-694-lambda-labels-11.txt January 2011
Shortest Path First (CSPF) is performed is outside the scope of
this document.
It is needless to say, an LSP must be appropriately provisioned
between a selected pair of ports not only within Domain A but
also over multiple domains satisfying wavelength constraints.
Figure 2 illustrates in detail the interconnection between Domain
A and Domain B.
|
Domain A (or Vendor A) | Domain B (or Vendor B)
|
Node-1 Node-2 | Node-6 Node-7
+--------+ +--------+ | +-------+ +-+ +-+ +-------+
| ROADM | | ROADM +---|------+ PXC +-+D| |D+-+ PXC |
| or WXC +========+ or WXC +---|------+ +-+W+=====+W+-+ |
| (LSC) | | (LSC) +---|------+ (LSC) +-+D| |D+-+ (LSC) |
+--------+ +--------+ | | +-|M| |M+-+ |
|| || | +++++++++ +-+ +-+ +++++++++
|| Node-3 || | ||||||| |||||||
|| +--------+ || | +++++++++ +++++++++
||===| WXC +===|| | | DWDM | | DWDM |
| (LSC) | | +--++---+ +--++---+
||===+ +===|| | || ||
|| +--------+ || | +--++---+ +--++---+
|| || | | DWDM | | DWDM |
+--------+ +--------+ | +++++++++ +++++++++
| ROADM | | ROADM | | ||||||| |||||||
| or WXC +========+ or WXC +=+ | +-+ +++++++++ +-+ +-+ +++++++++
| (LSC) | | (LSC) | | | |D|-| PXC +-+D| |D+-+ PXC |
+--------+ +--------+ +=|==+W|-| +-+W+=====+W+-+ |
Node-4 Node-5 | |D|-| (LSC) +-+D| |D+-+ (LSC) |
| |M|-| +-+M| |M+-+ |
| +-+ +-------+ +-+ +-+ +-------+
| Node-8 Node-9
Figure 1 Wavelength-based network model
T. Otani et al. Expires July 2011 [Page 4]
draft-ietf-ccamp-gmpls-g-694-lambda-labels-11.txt January 2011
+-------------------------------------------------------------+
| Domain A | Domain B |
| | |
| +---+ lambda 1 | +---+ |
| | |---------------|---------| | |
| WDM | N | lambda 2 | | N | WDM |
| =====| O |---------------|---------| O |===== |
| O | D | . | | D | O |
| T WDM | E | . | | E | WDM T |
| H =====| 2 | lambda n | | 6 |===== H |
| E | |---------------|---------| | E |
| R +---+ | +---+ R |
| | |
| N +---+ | +---+ N |
| O | | | | | O |
| D WDM | N | | | N | WDM D |
| E =====| O | WDM | | O |===== E |
| S | D |=========================| D | S |
| WDM | E | | | E | WDM |
| =====| 5 | | | 8 |===== |
| | | | | | |
| +---+ | +---+ |
+-------------------------------------------------------------+
Figure 2 Interconnecting details between two domains
In the scenario of Figure 1, consider the setting up of a
bidirectional LSP from ingress switch 1 to egress switch 9 using
GMPLS RSVP-TE. In order to satisfy wavelength continuity
constraint, a fixed wavelength (lambda 1) needs to be used in
domain A and domain B. A Path message will be used for signaling.
The Path message will contain the Upstream_Label object and a
Label_Set object; both containing the same value. The Label_set
object shall contain a single sub-channel that must be the same
as the Upstream_Label object. The Path setup will continue
downstream to switch 9 by configuring each lambda switch based on
the wavelength label. If a node has a tunable wavelength
transponder, the tuning wavelength is considered as a part of
wavelength switching operation.
Not using a standardized label would add undue burden on the
operator to enforce policy as each manufacturer may decide on a
different representation and therefore each domain may have its
own label formats. Moreover, manual provisioning may lead to
misconfiguration if domain-specific labels are used.
T. Otani et al. Expires July 2011 [Page 5]
draft-ietf-ccamp-gmpls-g-694-lambda-labels-11.txt January 2011
Therefore, a wavelength label should be standardized in order to
allow interoperability between multiple domains; otherwise
appropriate existing labels are identified in support of
wavelength availability. As identical wavelength information, the
ITU-T frequency grid specified in [G.694.1] for DWDM and
wavelength information in [G.694.2] for CWDM are used by Label
Switching Routers (LSRs) and should be followed as a wavelength
label.
3. Label Related Formats
To deal with the widening scope of MPLS into the optical and time
domains, several new forms of "label" have been defined in
[RFC3471]. This section contains a definition of a Wavelength
label based on [G.694.1] or [G.694.2] for use by LSC LSRs.
3.1. Wavelength Labels
In section 3.2.1.1 of [RFC3471], a Wavelength label is defined to
have significance between two neighbors, and the receiver may
need to convert the received value into a value that has local
significance.
We do not need to define a new type as the information stored is
either a port label or a wavelength label. Only the wavelength
label as below needs to be defined.
LSC equipment uses multiple wavelengths controlled by a single
control channel. In a case, the label indicates the wavelength to
be used for the LSP. This document defines a standardized
wavelength label format. As an example of wavelength values, the
reader is referred to [G.694.1] which lists the frequencies from
the ITU-T DWDM frequency grid. The same can be done for CWDM
technology by using the wavelength defined in [G.694.2].
Since the ITU-T DWDM grid is based on nominal central frequencies,
we need to indicate the appropriate table, the channel spacing in
the grid and a value n that allows the calculation of the
frequency. That value can be positive or negative.
The frequency is calculated as such in [G.694.1]:
Frequency (THz) = 193.1 THz + n * channel spacing (THz)
Where "n" is a two's-complement integer (positive, negative or 0)
and "channel spacing" is defined to be 0.0125, 0.025, 0.05 or 0.1
T. Otani et al. Expires July 2011 [Page 6]
draft-ietf-ccamp-gmpls-g-694-lambda-labels-11.txt January 2011
THz. When wider channel spacing such as 0.2 THz is utilized, the
combination of narrower channel spacing and the value "n" can
provide proper frequency with that channel spacing. Channel
spacing is not utilized to indicate the LSR capability but only
to specify a frequency in signaling.
For the other example of the case of the ITU-T CWDM grid, the
spacing between different channels was defined to be 20nm, so we
need to pass the wavelength value in nanometers(nm) in this case.
Examples of CWDM wavelengths are 1471, 1491, etc. nm.
The wavelength is calculated as follows
Wavelength (nm) = 1471 nm + n * 20 nm
Where "n" is a two's-complement integer (positive, negative or 0).
The grids listed in [G.694.1] and [G.694.2] are not numbered and
change with the changing frequency spacing as technology advances,
so an index is not appropriate in this case.
3.2. DWDM Wavelength Label
For the case of lambda switching (LSC) of DWDM, the information
carried in a Wavelength label is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Grid | C.S | Identifier | n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
(1) Grid: 3 bits
The value for grid is set to 1 for ITU-T DWDM Grid as defined in
[G.694.1].
T. Otani et al. Expires July 2011 [Page 7]
draft-ietf-ccamp-gmpls-g-694-lambda-labels-11.txt January 2011
+----------+---------+
| Grid | Value |
+----------+---------+
| Reserved | 0 |
+----------+---------+
|ITU-T DWDM| 1 |
+----------+---------+
|ITU-T CWDM| 2 |
+----------+---------+
|Future use| 3 - 7 |
+----------+---------+
(2) C.S.(channel spacing): 4 bits
DWDM channel spacing is defined as follows.
+----------+---------+
| C.S(GHz) | Value |
+----------+---------+
| Reserved | 0 |
+----------+---------+
| 100 | 1 |
+----------+---------+
| 50 | 2 |
+----------+---------+
| 25 | 3 |
+----------+---------+
| 12.5 | 4 |
+----------+---------+
|Future use| 5 - 15 |
+----------+---------+
(3) Identifier: 9 bits
The identifier field in lambda label format is used to
distinguish different lasers (in one node) when they can transmit
the same frequency lambda. The identifier field is a per-node
assigned and scoped value. This field MAY change on a per-hop
basis. In all cases but one, a node MAY select any value,
including zero (0), for this field. Once selected, the value MUST
NOT change until the LSP is torn down and the value MUST be used
in all LSP related messages, e.g., in Resv messages and label RRO
subobjects. The sole special case occurs when this label format
is used in a label ERO subobject. In this case, the special value
T. Otani et al. Expires July 2011 [Page 8]
draft-ietf-ccamp-gmpls-g-694-lambda-labels-11.txt January 2011
of zero (0) means that the referenced node MAY assign any
Identifier field value, including zero (0), when establishing the
corresponding LSP. When non-zero value is assigned to the
identifier field in a label ERO subobject, the referenced node
MUST use the assigned value for the identifier field in the
corresponding LSP related messages.
(4) n: 16 bits
n is a two's-complement integer to take either a negative, zero
or a positive value. The value used to compute the frequency as
shown above.
3.3. CWDM Wavelength Label
For the case of lambda switching (LSC) of CWDM, the information
carried in a Wavelength label is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Grid | C.S | Identifier | n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The structure of the label in the case of CWDM is the same as
that of DWDM case.
(1) Grid: 3 bits
The value for grid is set to 2 for ITU-T CWDM Grid as defined in
[G.694.2].
T. Otani et al. Expires July 2011 [Page 9]
draft-ietf-ccamp-gmpls-g-694-lambda-labels-11.txt January 2011
+----------+---------+
| Grid | Value |
+----------+---------+
| Reserved | 0 |
+----------+---------+
|ITU-T DWDM| 1 |
+----------+---------+
|ITU-T CWDM| 2 |
+----------+---------+
|Future use| 3 - 7 |
+----------+---------+
(2) C.S.(channel spacing): 4 bits
CWDM channel spacing is defined as follows.
+----------+---------+
| C.S(nm) | Value |
+----------+---------+
| Reserved | 0 |
+----------+---------+
| 20 | 1 |
+----------+---------+
|Future use| 2 - 15 |
+----------+---------+
(3) Identifier: 9 bits
The identifier field in lambda label format is used to
distinguish different lasers (in one node) when they can transmit
the same frequency lambda. The identifier field is a per-node
assigned and scoped value. This field MAY change on a per-hop
basis. In all cases but one, a node MAY select any value,
including zero (0), for this field. Once selected, the value MUST
NOT change until the LSP is torn down and the value MUST be used
in all LSP related messages, e.g., in Resv messages and label RRO
subobjects. The sole special case occurs when this label format
is used in a label ERO subobject. In this case, the special value
of zero (0) means that the referenced node MAY assign any
Identifier field value, including zero (0), when establishing the
corresponding LSP. When non-zero value is assigned to the
identifier field in a label ERO subobject, the referenced node
MUST use the assigned value for the identifier field in the
corresponding LSP related messages.
T. Otani et al. Expires July 2011 [Page 10]
draft-ietf-ccamp-gmpls-g-694-lambda-labels-11.txt January 2011
(4) n: 16 bits
n is a two's-complement integer. The value used to compute the
wavelength as shown above.
4. Security Considerations
This document introduces no new security considerations to
[RFC3471] and [RFC3473]. For a general discussion on MPLS and
GMPLS related security issues, see the MPLS/GMPLS security
framework [RFC5920].
5. IANA Considerations
IANA maintains the "Generalized Multi-Protocol Label Switching
(GMPLS) Signaling Parameters" registry. IANA is requested to add
three new subregistries to track the codepoints (Grid and C.S.)
used in the DWDM and CWDM Wavelength Labels, which are described
in the following sections.
5.1. Grid Subregistry
Initial entries in this subregistry are as follows:
Value Grid Reference
----- ------------------------- ----------
0 Reserved [This.I-D]
1 ITU-T DWDM [This.I-D]
2 ITU-T CWDM [This.I-D]
3-7 Not assigned at this time [This.I-D]
New values are assigned according to Standards Action.
5.2. DWDM Channel Spacing Subregistry
Initial entries in this subregistry are as follows:
T. Otani et al. Expires July 2011 [Page 11]
draft-ietf-ccamp-gmpls-g-694-lambda-labels-11.txt January 2011
Value Channel Spacing (GHz) Reference
----- ------------------------- ----------
0 Reserved [This.I-D]
1 100 [This.I-D]
2 50 [This.I-D]
3 25 [This.I-D]
4 12.5 [This.I-D]
5-15 Not assigned at this time [This.I-D]
New values are assigned according to Standards Action.
5.3. CWDM Channel Spacing Subregistry
Initial entries in this subregistry are as follows:
Value Channel Spacing (nm) Reference
----- ------------------------- ----------
0 Reserved [This.I-D]
1 20 [This.I-D]
2-15 Not assigned at this time [This.I-D]
New values are assigned according to Standards Action.
6. Acknowledgments
The authors would like to thank Adrian Farrel, Lou Berger,
Lawrence Mao, Zafar Ali and Daniele Ceccarelli for the discussion
and their comments.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3471] Berger, L., "Generalized Multi-Protocol Label Switching
(MPLS) Signaling Functional Description", RFC 3471,
January 2003.
[RFC3473] Berger, L., "Generalized Multi-Protocol Label Switching
(MPLS) Signaling - Resource ReserVation Protocol Traffic
Engineering (RSVP-TE) Extensions", RFC 3473, January
2003.
T. Otani et al. Expires July 2011 [Page 12]
draft-ietf-ccamp-gmpls-g-694-lambda-labels-11.txt January 2011
[RFC3945] Mannie, E., Ed., "Generalized Multiprotocol Label
Switching (GMPLS) Architecture", RFC 3945, October 2004.
7.2. Informative References
[G.694.1] ITU-T Recommendation G.694.1, "Spectral grids for WDM
applications: DWDM frequency grid", June 2002.
[G.694.2] ITU-T Recommendation G.694.2, "Spectral grids for WDM
applications: CWDM wavelength grid", December 2003.
[RFC5920] Fang, L., "Security Framework for MPLS and GMPLS
Networks", RFC 5920, July 2010.
T. Otani et al. Expires July 2011 [Page 13]
draft-ietf-ccamp-gmpls-g-694-lambda-labels-11.txt January 2011
8. Authors' Address
Tomohiro Otani
KDDI Corporation
2-3-2 Nishishinjuku Shinjuku-ku
Tokyo, 163-8003, Japan
Phone: +81-3-3347-6006
Email: tm-otani@kddi.com
Richard Rabbat
Google, Inc.
1600 Amphitheatre Pkwy
Mountain View, CA 94043
Email: rabbat@alum.mit.edu
Sidney Shiba
Email: sidney.shiba@att.net
Hongxiang Guo
Email: hongxiang.guo@gmail.com
Keiji Miyazaki
Fujitsu Laboratories Ltd
4-1-1 Kotanaka Nakahara-ku,
Kawasaki Kanagawa, 211-8588, Japan
Phone: +81-44-754-2765
Email: miyazaki.keiji@jp.fujitsu.com
Diego Caviglia
Ericsson
16153 Genova Cornigliano, ITALY
Phone: +390106003736
Email: diego.caviglia@ericsson.com
Dan Li
Huawei Technologies
F3-5-B R&D Center, Huawei Base,
Shenzhen 518129 China
Phone: +86 755-289-70230
Email: danli@huawei.com
Takehiro Tsuritani
KDDI R&D Laboratories Inc.
2-1-15 Ohara Fujimino-shi
Saitama, 356-8502, Japan
Phone: +81-49-278-7806
Email: tsuri@kddilabs.jp
T. Otani et al. Expires July 2011 [Page 14]
draft-ietf-ccamp-gmpls-g-694-lambda-labels-11.txt January 2011
9. Appendix A. DWDM Example
Considering the network displayed in figure 1 it is possible to
show an example of LSP set up using the lambda labels.
Node 1 receives the request for establishing an LSP from itself
to Node 9. The ITU-T grid to be used is the DWDM one, the channel
spacing is 50Ghz and the wavelength to be used is 193,35 THz.
Node 1 signals the LSP via a Path message including a Wavelength
Label structured as defined in section 3.2:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Grid | C.S | Identifier | n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Where:
Grid = 1 : ITU-T DWDM grid
C.S. = 2 : 50 GHz channel spacing
n = 5 :
Frequency (THz) = 193.1 THz + n * channel spacing (THz)
193.35 (THz) = 193.1 (THz) + n* 0.05 (THz)
n = (193.35-193.1)/0.05 = 5
10. Appendix B. CWDM Example
The network displayed in figure 1 can be used also to display an
example of signaling using the Wavelength Label in a CWDM
environment.
This time the signaling of an LSP from Node 4 to Node 7 is
considered. Such LSP exploits the CWDM ITU-T grid with a 20nm
channel spacing and is to established using wavelength equal to
1331 nm.
T. Otani et al. Expires July 2011 [Page 15]
draft-ietf-ccamp-gmpls-g-694-lambda-labels-11.txt January 2011
Node 4 signals the LSP via a Path message including a Wavelength
Label structured as defined in section 3.3:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Grid | C.S | Identifier | n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Where:
Grid = 2 : ITU-T CWDM grid
C.S. = 1 : 20 nm channel spacing
n = -7 :
Wavelength (nm) = 1471 nm + n * 20 nm
1331 (nm) = 1471 (nm) + n * 20 nm
n = (1331-1471)/20 = -7
T. Otani et al. Expires July 2011 [Page 16]
Html markup produced by rfcmarkup 1.128, available from
https://tools.ietf.org/tools/rfcmarkup/