Internet DRAFT - draft-ansari-forces-discovery
draft-ansari-forces-discovery
Furquan Ansari
Internet Draft Lucent Tech.
Document: draft-ansari-forces-discovery-01.txt Hormuzd Khosravi
Expires: April 24, 2005 Intel Corp.
Working Group: ForCES Jamal Hadi Salim
Znyx
October 25, 2004
ForCES Intra-NE Topology Discovery
draft-ansari-forces-discovery-01.txt
Status of this Memo
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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 [RFC-2119].
Abstract
This document describes a mechanism for discovering inter-FE
topology and topology maintenance. Such a mechanism is essential for
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all these elements in the set to behave as a single Network Element,
as required by the ForCES architecture as well as to perform certain
optimizations at the FE by making use of the topology. The discovery
mechanism only operates during post-association phase of ForCES
protocol.
Table of Contents
1. Definitions........................................................2
2. Introduction.......................................................2
2.1. Motivation....................................................4
3. Topology Discovery Mechanism.......................................5
3.1. Minimum requirements..........................................6
3.2. Protocol Details..............................................6
3.2.1. Topology Discovery and Maintenance.......................7
3.2.2. Full topology computation at the CE from partial
topologies......................................................8
3.3. Protocol and Message Headers..................................9
3.3.1. TLV definitions.........................................10
3.4. Inter-FE Topology Discovery Examples.........................10
3.4.1. Forwarding Elements connected in a daisy chain..........11
3.4.2. Forwarding Elements connected in a ring.................12
4. Security Considerations...........................................13
5. References........................................................13
5.1. Normative....................................................13
5.2. Informative..................................................14
6. Authors' Addresses................................................14
7. Full Copyright Notice.............................................14
8. Acknowledgements..................................................15
1. Definitions
Inter-FE topology discovery: Topology discovery relates to how the
FEs are interconnected with each other with respect to packet
forwarding. This is the complete view of the intra-NE network as
seen by the CE.
Inter-FE topology maintenance: Once the inter-FE topology has been
discovered, it has to be continuously monitored to ensure that any
changes to the topology are reported to the corresponding CE. This
represents the steady state and final phase of the protocol.
2. Introduction
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The ForCES framework document [RFC 3746] describes how a set of
control elements (CEs) and forwarding elements (FEs) interact with
each other to form a single network element (NE). It describes the
ForCES post-association phase protocol working across the Fp
reference point between CE and FE. This document describes an
important aspect of the ForCES operational infrastructure, that of
discovering the layout of the different elements within an NE.
We describe a mechanism for obtaining the Intra-NE/Inter-FE
topology.
The mechanism is divided into two distinct operational pieces:
. The FE side component that collects FE neighbor information.
. The CE side component that uses the neighbor information to
compute the NE topology.
Given the above split, we believe that this mechanism fits well
within a description of Topology Discovery LFB.
The mechanism at the FE can be divided into two modes or phases.
. Phase I corresponds to the actual discovery process wherein each
element discovers its neighbor and maintains a neighbor
relationship. Upon Joining an NE, the CE will instruct the FE to
start collecting this information. This happens when the FE is
administratively up and the associated neighbor links are deemed
to be provisioned and operationally up.
. Phase II corresponds to the topology maintenance phase, wherein
any changes to the inter-FE topology during normal operation is
reported to the corresponding CE so that an updated view is
available. The CE then makes all services it is controlling aware
of such details. As an example, based on policy, in a basic IPV4
service, the tables of associated FEs will need to be updated.
As noted above, both phases occur during the post-association phases
of the ForCES protocol. In other words, the ForCES protocol
association between the CE and the FEs should already have taken
place for the discovery mechanism to kick in. This is required
because the neighbor relationships maintained by the FEs are
reported back to the CE (or queried by the CE) over the ForCES
protocol.
The proposed discovery mechanism is required to scale to a very
large number of forwarding elements in the NE, with minimal impact
on the resources. The following list provides some of the features
and goals of the discovery mechanism.
. Determine connectivity between elements
. React to changes in link connectivity
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. Construct topology information from the collected partial topology
information
. Tolerant to protocol message losses
. Applicable to all inter-FE network topologies such as ring, mesh,
star etc.
. Cause minimal overhead
. Agnostic of the network interconnect technology
2.1. Motivation
The ForCES architecture defines a network element (NE) as a single
managed entity made up of a collection of FEs and CEs and is
indistinguishable from other network elements in the network. This
NE model definition leads to three types of links from the networkÆs
perspective: internal (or intra-NE) links and external (or inter-NE)
links and control links. Intra-NE links are purely internal to the
NE and are not exposed to the external world; whereas, inter-NE (or
external) links are exposed to the external world and over which
routing adjacencies (such as OSPF, IS-IS, BGP etc.) can be formed.
An NE can contain FEs that have zero or more internal/external links
¡ e.g. in Fig. 1, FE3 has two internal links and no external links
while FE1 and FE2 have two internal links and one external link
each. Control links are those links that are used for communication
between the CE and FE. If the CE and FE are a single Layer 3 hop
away as in Fig.1, the control link is typically a physical link e.g.
link A of FE1 in the figure. Control links can be logical as well.
A packet entering a ForCES NE may travel multiple FEs within the NE
before it exits onto the output link. This requires that the packet
be correctly forwarded from the ingress FE to the egress FE. This
internal forwarding requires knowledge of the physical FE inter-
connection topology so that the CE can appropriately setup internal
LFB tables at each FE to handle packet traversal in a sane manner.
We use a simple topology discovery mechanism that only operates on
internal links and provides the necessary routing information to
forward the packets from the ingress FE to the egress FE. Further,
the mechanism should be able to reroute around internal link
failures, if a path exists. This makes the NE highly available and
resilient.
NE 1
.....................................
. ----------------- .
. | CE | .
. ----------------- . ----------
. A ^ B ^ C ^ . | NE 1 |
. / | \ . | |
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. / A v \ . ----------
. / ------- B \ . ^ ^
. / +->| FE3 |<-+ \ . <====> / \
. / |C | | | \ . / \
. A v | ------- | v A . v v
. -------B | |D ------- . -------- ---------
. | FE1 |<-+ +->| FE2 | . | NE 2 |<---->| NE 3 |
. | |<--------------->| | . | | | |
. ------- C C ------- . -------- ---------
. D^ ^B .
.....|.......................|........
| |
V v
-------- --------
| NE 2 |<--------------->| NE 3 |
| | | |
-------- --------
(a) (b)
Figure 1:(a) illustrates the internal/external links and topology
within a NE. (b) Shows the network topology as seen by external
routing protocols
3. Topology Discovery Mechanism
The topology discovery mechanism described here will be restricted
to the case where the control and the forwarding elements are a
single layer 3 hop away. However, there is no restriction on the
number of layer 2 hops between the CE and the FEs. Although, the
mechanism can be extended to the multi-hop scenario, it is
considered beyond the scope of this document to describe it. The
mechanism is expected to work on point-to-point as well as multi-
access links.
In order to keep the discovery and maintenance mechanism as simple
as possible, the FEs only maintain relationships with the respective
neighbors to determine the status of the neighbors. No databases are
exchanged between the neighbors. This implies that the topology view
for each FE is only limited to the adjacent elements. This partial
topology information is reported back to the CE (or queried by the
CE) over the ForCES protocol. Since the CE receives such information
from all the FEs, it can easily construct the full topology from
individual partial topologies reported by each FE. Once the CE
constructs the full topology, such information can be passed to the
FEs, if needed (depending on policy).
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Topology information is needed by a lot of LFBs and associated
services that span multiple FEs within a NE. In the case where the
FE aids the CE in offloading the table updates, then it makes sense
for the FE to be topology aware. It is sometimes also helpful to
keep full topology information at the FEs for cases such as ômessage
snoopingö optimizations. For example, if an FE is aware of the
topology, it could snoop on messages sent to other FEs (e.g.
broadcasts, multicasts) and update its own tables dynamically
without involving the CE. Another example would be FE-FE primary-
backup handover scenario. With each FE being fully aware of the
complete topology, the backup FE can take over the responsibilities
of the primary without involving the CE for such a handover.
3.1. Minimum requirements
In order for the protocol to work as described, the following
assumptions are made.
. Each element has been configured with their respective IDs
(CEID, FEID)
. Element bindings process has already taken place. In other
words, the CE know all the FEs it wants to control and each FE
knows which CE is allowed to control it.
. The ForCES protocol association has already taken place between
the CE and the FE in question.
. The protocol is enabled on the required interfaces.
Note that these are configuration requirements and are satisfied by
the respective managers (CEM/FEM).
3.2. Protocol Details
Once the ForCES protocol association has been established between a
CE and a given FE ¡ i.e. it is in post-association phase, the CE
starts sending/advertising Hello/Probe messages to the FEÆs
neighbors such that the messages go through the given FE. In other
words, it looks like the given FE is generating probe messages to
the neighbor (except that these messages are coming from the CE over
the ForCES protocol first). However, this functionality of
generating probe messages by the CE can be offloaded to the FE
itself (to be more precise, to an FE LFB) ¡ so that the FE can
originate and terminate the probe messages. This provides better
scalability of the CE and itÆs resources. The CE can now simply
query each FEÆs neighbor relationship database and register for any
events related to topology changes.
All Hello/Probe messages travel a single PE hop and are not routed
to other elements beyond the first hop. This is ensured by using a
TTL of one on all Hello/Probe packets. The messages are sent as IP
datagrams (multicast/broadcast, where applicable, or unicast in
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general) to the neighboring elements over configured interfaces.
Each FE topology LFB component maintains the neighbor relationships
as long as the Hello messages are received from the neighbor. If it
does not receive a pre-determined number (configured) of back-to-
back Hello messages from a given neighbor, it deletes the entry from
the database and reports this change to the CE in the form of an
event-driven message over the ForCES protocol. This ensures that the
CE has the complete and up-to-date information of the underlying
topology of the Inter-FE network.
The Hello message contains information necessary for discovering and
maintaining neighbor relationships. It contains the PE ID, type of
protocol element (i.e. CE or FE), interval between any two messages,
interval for deeming a neighbor inactive, capability information
etc. This is, in some ways, similar to the capabilities of the OSPF
Hello protocol.
On receiving the Hello messages from a neighbor, the FE responds
back with its own Hello message in a packet format similar to the
one received from the neighbor. Essentially, both sides are
independently sending Hello messages to each other and listing their
neighbor table. Also, each neighbor will see itself listed on its
neighbors Hello message. This ensures bi-directionality of the link
between any two neighbors.
The operation is concisely described by the following steps:
. CE activates the topology LFB/component on the FE to initialize on
specific ports
. FE topology LFB/component sends neighbor probes/hellos
. CE queries FE for its neighbors
. FE continues to send these probes afterwards (maintenance) and
updates asynchronously any new updates
3.2.1. Topology Discovery and Maintenance
Since the CE needs to maintain consistent and up-to-date view of the
inter-FE topology, it needs to obtain real-time information of the
status of the internal links connecting the FEs. Since the topology
discovery and maintenance occurs during the post-association phase,
we make use of the event-notification and query/response messages
[ForCESP] of the ForCES protocol to provide this information to the
CE. It is important to note that each FE only maintains partial
topology information obtained through neighbor relationship
maintenance through Hello messages. The partial topology view seen
by each FE is only the neighbor connectivity information. The CE has
to derive the complete topology view of the interconnected FEs based
on the partial topology information reported by each FE (or queried
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by the CE). This ensures that that only the CE maintains all the
intelligence and the protocol operation on the FEs is very simple
and has minimal overhead. However, as mentioned above, if
optimizations can be performed by having the complete topology
information available at the FEs, the CE can push such information
to any FE interested in it (interest on the FE may be shown in the
form of policy configuration). This is an optional feature available
on each FE, which can be turned on or off through configuration or
during capability exchange negotiation at setup time. Each FE vendor
may decide to make use of this feature in different ways, so the
capability to obtain such topology information should exist.
The periodic Hello messages maintain PE neighbor relationships.
Any change in the link or neighbor status causes the FE to generate
an asynchronous/event-driven message to the CE indicating this
change. The mechanism defined in [ForCESP] is used for delivering
event-driven messages from the FE to the CE. This involves the CE
subscribing to such event-driven messages from the FE.
The CE aggregates the partial topology information received from
each FE and generates the inter-FE topology. With this complete
knowledge of the inter-FE topology, it can now make appropriate
updates to the LFB tables on each FE to move packets inside the NE ¡
from ingress FE to egress FE, assuming that the destination of the
packet is not the current NE itself. Any changes in the internal
link states (and hence the topology) requires that the CE
reconfigure the LFB tables on the FEs based on the most up-to-date
information to ensure that the packets do not end up in a black hole
or enter a loop.
3.2.2. Full topology computation at the CE from partial topologies
The CE receives neighbor relationships information from each FE that
it uses to construct the full topology of the internal network. Each
FEÆs neighbor relationship table contains information regarding the
local element ID, local port connecting the neighbor, the neighborÆs
ID, the neighborÆs port and any optional additional information.
Note that the fact that the FE already knows the neighborÆs port
information implies that it received the probe/hello messages from
the neighbor on that port in response to the hello sent and was,
therefore, able to establish bi-directionality of the link. If all
the links in the internal network are point-to-point links, the CE
simply has to aggregate all the neighbor relationship tables
obtained from all the FEs to generate the full topology. If we
assume the topology to be a graph, each edge of the graph will be
present twice ¡ essentially providing the same information from the
two endpoints of the graph. After deleting all the duplicate entries
(and thus reducing the table size by half), the CE now has accurate
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view of the full topology. Please refer section 3.4 [Fig. 3(b)] for
more details.
[Sub-section on generating full topology from partial topology
information for broadcast/multi-access, point-to-multipoint etc.
type of links]
3.3. Protocol and Message Headers
The protocol message consists of a fixed length header (16 bytes)
followed by one or more optional TLVs. The format of the message is
as follows.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version | Flags | Packet Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Port ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PE ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLV-Type | TLV-Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLV-Value ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure (2)
Version: Version number of this protocol. Currently acceptable value
is 0x01
Flags: These indicate whether the message is sent by a FE, (0x01) or
CE (0x02). More options may be defined in the future.
Packet Length: The length of the protocol message in bytes,
including the header and the following TLVs.
Checksum: 16-bit checksum for the protocol message. The checksum
calculation does not include the IP header.
Port ID: This indicates the port on which this packet was sent out
by the sender ¡ useful for topology construction.
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PE ID: This is the 32-bit identifier of the sender. It could either
be CE ID or FE ID, depending on the sender.
The protocol header is followed by one or many TLVs. The following
TLVs types are defined:
Hello TLV: Indicates the Hello message as exchanged by the
neighbors. The TLV defines the common hello parameters such as the
Hello Interval, Hold time, Uni-directional targeted Hellos, Sequence
space number, if needed etc.
Capabilities TLV: Provides the capabilities information - TBD
Vendor specific TLV: TBD
3.3.1. TLV definitions
TBD
3.4. Inter-FE Topology Discovery Examples
The following examples illustrate the topology discovery mechanism.
For sake of simplicity, we assume that there is only one CE per NE.
The FEIDs of the FEs in the topologies below are FE1, FE2, FE3, and
FE4. Each FE has port IDs labeled alphabetically. This is also the
case with the CE.
-----------------
| CE |
-----------------
A ^ B ^ C ^
/ | \
/ A v \
/ ------- B \
/ +->| FE3 |<-+ \
/ |C | | | \
A v | ------- | v A
-------B | |E -------
| FE1 |<-+ +->| FE2 |
| |<--------------->| |
------- C D -------
E ^ D| C ^ | B
| | | |
| v | v
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FE3 Control Element reachability Table
--------------------------------------
<Dest Addr> <local intf>
CE A
--------------------------------------
FE3 NEIGHBOR ASSOCIATION TABLE
-----------------------------------------------
<local intf> <neighbor_FEID> <neighbor_portID>
B FE2 E
C FE1 B
----------------------------------------------
Figure 3. (a) Full mesh among FE1, FE2, and FE3
During the element-binding phase, each FE sends out hello messages
with its FEID and Port ID (as outlined earlier) to all of its
neighbors. Since each neighboring FE also listens to such messages,
it receives the hello message and adds it to the neighbor
association table, which may look like that shown in Fig.4(a). In
the topology discovery phase, which is post ForCES association
stage, the CE queries each FE about its neighbor table. The FE
responds back with the partial topology information available
through its neighbor relationships. Both the query and the response
are carried by the ForCES protocol. The CE collects the partial
topology information from all the FEs in the NE and aggregates this
information to fully construct the inter-FE topology. Any changes to
the FE neighbor table, e.g. when a link state changes, generates a
trigger/update message to the CE. The new information is used to
recalculate the new topology and subsequently the CE takes
appropriate actions based on the new topology ¡ such as updating the
packet forwarding tables on the FEs.
The following examples show the neighbor association tables.
3.4.1. Forwarding Elements connected in a daisy chain
--------------
| CE |
--------------
A ^ ^ B ^ ^ D
/ | \ \
/------ | --\ -------\
A v A v v A v A
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-------B -------B -------B -------
| FE1 |<--->| FE2 |<--->| FE3 |<--->| FE4 |
------- E------- E------- D-------
D ^ |C D ^ |C D^ |C C^ |B
| | | | | | | |
| v | v | v | v
FE1 NBR ASSOCIATION TABLE FE2 NBR ASSOCIATION TABLE
-------------------------------- ------------------------------
<locl intf> <nbr_FEID> <nbr_port> <locl intf> <nbr_FEID> <nbr_port>
B FE2 E E FE1 B
B FE3 E
FE3 NBR ASSOCIATION TABLE FE4 NBR ASSOCIATION TABLE
-------------------------------- -----------------------------
<locl intf> <nbr_FEID> <nbr_port> <locl intf> <nbr_FEID> <nbr_port>
B FE4 D D FE3 B
E FE2 B
CE Topology (Aggregate)View CE Topology View
-------------------------------- ------------------------------
<Node> <Port> <Node> <Port> <Node> <Port> <Node> <Port>
FE1 B FE2 E\ FE1 B FE2 E
FE2 E FE1 B/ => FE2 B FE3 E
FE2 B FE3 E\ FE3 B FE4 D
FE3 E FE2 B/ ------------------------------
FE3 B FE4 D\
FE4 D FE3 B/
--------------------------------
Fig.3(b) Multiple FEs in a daisy chain
3.4.2. Forwarding Elements connected in a ring
^ |
D| v E
----------- A
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| FE1 |<-----------------------|
----------- |
C ^ ^ B |
/ \ |
| ^ / \ ^ | V A
B v |C v D C v D| v E ----------
--------- --------- D| |
| FE2 | | FE3 |<------------>| CE |
--------- --------- A | |
A ^ ^ E ^ B ----------
| \ / C ^ ^ B
| \ / | |
| D v E v | |
| ----------- A | |
| | FE4 |<----------------------| |
| ----------- |
| C | ^ B |
| v | |
| |
|----------------------------------------|
FE1 NBR ASSOCIATION TABLE FE2 NBR ASSOCIATION TABLE
-------------------------------- -----------------------------
<locl intf> <nbr_FEID> <nbr_port> <locl intf> <nbr_FEID> <nbr_port>
B FE3 C E FE4 D
C FE2 D D FE1 C
FE3 NBR ASSOCIATION TABLE FE4 NBR ASSOCIATION TABLE
-------------------------------- -----------------------------
<locl intf> <nbr_FEID> <nbr_port> <locl intf> <nbr_FEID> <nbr_port>
B FE4 E D FE2 E
C FE1 B E FE3 B
Fig. 3(c) Multiple FEs connected in a ring
4. Security Considerations
Like all protocols, this protocol will have security issues as well.
These issues will be researched in detail in future draft versions.
5. References
5.1. Normative
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[RFC3746] Yang, L., Dantu, R., Anderson, T. and R. Gopal,
"Forwarding and Control Element Separation (ForCES)
Framework", RFC 3746, April 2004.
[RFC3654] Khosravi, H. and T. Anderson, "Requirements for
Separation of IP Control and Forwarding", RFC 3654,
November 2003.
[ForCESP] F P Team, "ForCES protocol specification",
draft-ietf-forces-protocol-00.txt, Sept 2004.
5.2. Informative
[OSPF] J. Moy, ôOSPF Version 2ö, 1998, RFC 2328.
[BGP] Y. Rekhter, T. Li, ôA Border Gateway Protocol 4 (BGP-4)ö,
1995, RFC 1771.
[IS-IS] R. Collela et al., ôGuidelines for OSI NSAP Allocation in
the Internetö, 1994, RFC 1629.
6. Authors' Addresses
Furquan Ansari
Bell Labs Research, Lucent Tech.
101 Crawfords Corner Road
Holmdel, NJ 07733
USA
Phone: +1 732-949-5249
Email: furquan@lucent.com
Hormuzd Khosravi
Intel
2111 N.E. 25th Avenue JF3-206
Hillsboro, OR 97124-5961
USA
Phone: +1 503 264 0334
Email: hormuzd.m.khosravi@intel.com
Jamal Hadi Salim
ZNYX Networks
Ottawa, Ontario, Canada
Email: hadi@znyx.com
7. Full Copyright Notice
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"Copyright (C) The Internet Society (year). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights."
"This document and the information contained herein are provided on
an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."
8. Acknowledgements
We would like to thank Thyaga Nandagopal of Lucent Technologies for
his thoughts and contributions to the initial draft.
Funding for the RFC Editor function is currently provided by the
Internet Society.
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