Internet DRAFT - draft-ietf-trill-directory-framework
draft-ietf-trill-directory-framework
INTERNET-DRAFT Linda Dunbar
Intended Status: Informational Donald Eastlake
Huawei
Radia Perlman
Intel
Igor Gashinsky
Yahoo
Expires: February 10, 2014 August 11, 2013
TRILL (Transparent Interconnection of Lots of Links):
Edge Directory Assistance Framework
<draft-ietf-trill-directory-framework-07.txt>
Abstract
Edge TRILL (Transparent Interconnection of Lots of Links) switches
currently learn the mapping between MAC addresses and their egress
TRILL switch by observing the data packets they ingress or egress or
by the TRILL ESADI (End Station Address Distribution Information)
protocol. When an ingress TRILL switch receives a data frame whose
destination address (MAC&Label) that switch does not know, the data
frame is flooded within the frame's Data Label across the TRILL
campus.
This document describes the framework for using directory services to
assist edge TRILL switches in reducing multi-destination frames,
particularly unknown unicast frames flooding, and ARP/ND, thus
improving TRILL network scalability and security.
Status of This Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Distribution of this document is unlimited. Comments should be sent
to the TRILL working group mailing list.
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."
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The list of current Internet-Drafts can be accessed at
http://www.ietf.org/1id-abstracts.html. The list of Internet-Draft
Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
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Table of Contents
1. Introduction............................................4
2. Terminology.............................................6
3. Impact of Massive Number of End Stations................7
3.1 Issues of Flooding Based Learning in Data Centers......7
3.2 Two Examples...........................................8
4. Benefits of Directory Assisted TRILL Edge...............9
5. Generic operation of Directory Assistance..............11
5.1 Information in Directory for Edge RBridges............11
5.2 Push Model and Requirements...........................11
5.3 Pull Model and Requirements...........................13
6. Recommendation.........................................15
7. Security Considerations................................15
8. IANA Considerations....................................17
9. Acknowledgements.......................................17
10. References............................................18
10.1 Normative References.................................18
10.2 Informative References...............................18
Authors' Addresses........................................19
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1. Introduction
Edge TRILL (Transparent Interconnection of Lots of Links) switches
(devices implementing [RFC6325], also known as RBridges) currently
learn the mapping between destination MAC addresses and their egress
TRILL switch by observing data packets or by the ESADI (End Station
Address Distribution Information) protocol. When an ingress RBridge
receives a data frame for a destination address (MAC&Label) that
RBridge does not know, the data frame is flooded within that Data
Label across the TRILL campus. (Data Labels are VLANs or FGLs (Fine
Grained Labels [FGL]).
This document describes a framework for using directory services in
environments where such services are available, such as typical data
centers, to assist edge TRILL switches. This assistance can reduce
multi-destination frames, particularly ARP [RFC826], ND [RFC4861],
and unknown unicast thus improving TRILL network scalability. In
addition, the information provided by a directory can be more secure
than that learned from the data plane (see Section 7).
Data centers, especially Internet and/or multi-tenant data centers
tend to have a large number of end stations with a wide variety of
applications. Their networks differ from enterprise campus networks
in several ways that make them attractive for the use of directory
assistance, in particular:
1. Data center topology is often based on racks and rows.
Furthermore, guest operating system assignment to Servers, Racks,
and Rows is orchestrated by a Server/VM (virtual machine)
Management system, not done at random. So the information
necessary for a directory is normally available from that
management system.
2. Rapid workload shifting in data centers can accelerate the
frequency of the physical servers being re-loaded with different
applications. Sometimes, the applications loaded into one physical
server at different times can belong to different subnets. When a
VM is moved to a new location or a server is loaded with a new
application with different IP/MAC addresses, it is more likely
that the destination address of data packets sent out from those
VMs are unknown to their attached edge RBridges.
3. With server virtualization, there is an increasing trend to
dynamically create or delete VMs when demand for resource changes,
to move VMs from overloaded servers to less loaded servers, or to
aggregate VMs onto fewer servers when demand is light. This
results in the more frequent occurrence of multiple subnets on the
same port at the same time and a higher change rate for VMs than
for physical servers.
Both items 2 and 3 above can lead to applications in one subnet being
placed in different locations (racks or rows) or one rack having
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applications belonging to different subnets.
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2. Terminology
The terms "VLAN" and "Data Label" are used interchangeably with
"Subnet" in this document because it is common to map one subnet to
one VLAN or FGL.
Bridge: IEEE Std 802.1Q-2011 compliant device [802.1Q]. In this
document, Bridge is used interchangeably with Layer 2
switch.
Data label: VLAN or FGL.
EoR: End of Row switches in data center. Also known as
aggregation switches.
End Station: Guest OS running on a physical server or on a virtual
machine. An end station in this document has at least one IP
address and at least one MAC address and is connected to a
network.
FGL: Fine Grained Label [FGL].
IS-IS: Intermediate System to Intermediate System routing protocol.
TRILL uses IS-IS [IS-IS] [RFC6326].
RBridge: "Routing Bridge", an alternative name for a TRILL switch.
ToR: Top of Rack Switch in a data center. Also known as access
switches in some data centers.
TRILL: Transparent Interconnection of Lots of Links [RFC6325]
TRILL switch: A device implementing the TRILL protocol [RFC6325]
VM: Virtual Machine
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3. Impact of Massive Number of End Stations
This section discusses the impact of a massive number of end stations
in a TRILL campus using Data Centers as an example.
3.1 Issues of Flooding Based Learning in Data Centers
It is common for Data Center networks to have multiple tiers of
switches, for example, one or two Access Switches for each server
rack (ToR), aggregation switches for some rows (or EoR switches), and
some core switches to interconnect the aggregation switches. Many
aggregation switches deployed in data centers have high port density.
It is not uncommon to see aggregation switches interconnecting
hundreds of ToR switches.
+-------+ +------+
+/------+ | +/-----+ |
| Aggr11| + ----- |AggrN1| + EoR Switches
+---+---+/ +------+/
/ \ / \
/ \ / \
+---+ +---+ +---+ +---+
|T11|... |T1x| |T21| .. |T2y| ToR switches
+---+ +---+ +---+ +---+
| | | |
+-|-+ +-|-+ +-|-+ +-|-+
| |... | | | | .. | |
+---+ +---+ +---+ +---+ Server racks
| |... | | | | .. | |
+---+ +---+ +---+ +---+
| |... | | | | .. | |
+---+ +---+ +---+ +---+
Figure 1: Typical Data Center Network Design
The following problems could occur when TRILL is deployed in a data
center with large number of end stations and the end stations in one
subnet/Label could be placed under multiple edge RBridges:
- Unnecessary filling of slots in the MAC address learning table
of edge RBridges, e.g. RBridge T11, due to T11 receiving
broadcast / multicast traffic (e.g. ARP/ND, cluster multicast,
etc.) from end stations under other edge RBridges that are not
actually communicating with any end stations attached to T11.
- Packets being flooded across TRILL campus when their
destination MAC addresses are not in ingress RBridge's MAC
address to egress RBridge cache.
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3.2 Two Examples
Consider a data center with 1,600 server racks. Each server rack has
at least one ToR switch. The ToR switches are further divided into 8
groups, with each group being connected by a set of aggregation
switches. There could be 4 to 8 aggregation switches in each set to
achieve load sharing for traffic to/from server racks. If TRILL is
deployed in this data center environment, let's consider the
following two scenarios for the TRILL campus boundary:
- Scenario #1: TRILL campus boundary starts at ToR switches:
If each server rack has one ToR, there are 1,600 edge RBridges.
If each rack has two ToR switches, then there will be 3,200
edge RBridges
In this scenario, the TRILL campus will have more than 1,600
(or 3,200) + 8*4 (or 8*8) nodes, which is a large IS-IS area.
Even though a mesh IS-IS area can scale up to thousands of
nodes, it is challenging for aggregation switches to handle IS-
IS link state advertisement among hundreds of parallel ports.
If each ToR has 40 downstream ports facing servers and each
server has 10 VMs, there could be 40*10 = 400 end stations
attached. If those end stations belong to 8 Labels then the
total number of MAC&Label entries learned by each edge RBridge
in the worst case might be 400*8 = 3,200, which is not a large
number.
- Scenario #2: TRILL campus boundary starts at the aggregation
switches:
With the same assumptions as before, the number of nodes in the
TRILL campus will be less than 100, and aggregation switches
don't have to handle IS-IS link state advisements among
hundreds of parallel ports.
However, the number of MAC&Label <-> Egress RBridge Mapping
entries to be learned and managed by RBridge edge node can be
very large. In the example above, each edge RBridge has 200
edge ports facing the ToR switches. If each ToR has 40
downstream ports facing servers and each server has 10 VMs,
there could be 200*40*10 = 80,000 end stations attached. If all
those end stations belong to 1,600 Labels (50 per Data Label)
and each Data Label has 200 end stations, then under the worst-
case scenario, the total number of MAC&Label entries to be
learned by each edge RBridge can be 1,600*200=320,000, which is
very large.
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4. Benefits of Directory Assisted TRILL Edge
In some environments, particularly data centers, the assignment of
applications to servers, including rack and row selection, is
orchestrated by Server (or VM) Management System(s). That is, there
is a database or multiple databases that have the knowledge of where
each application is placed. If the application location information
can be fed to RBridge edge nodes, through some form of Directory
Service, then there is much less chance of RBridge edge nodes
receiving unknown MAC destination address, therefore less chance of
flooding.
Avoiding unknown unicast address flooding to the TRILL campus is
especially valuable in the data center environment because there is a
higher chance of an edge RBridge receiving packets with unknown
unicast destination address and broadcast / multicast messages due to
VM migration and servers being loaded with different applications.
When a VM is moved to a new location or a server is loaded with a new
application with a different IP/MAC addresses, it is more likely that
the destination address of data packets sent out from those VMs are
unknown to their attached edge RBridges. In addition, gratuitous ARP
(IPv4, [RFC826]) or Unsolicited Neighbor Advertisement (IPv6,
[RFC4861]) sent out from those newly migrated or activated VMs have
to be flooded to other edge RBridges that have VMs in the same
subnets.
The benefits of using directory assistance include:
- Avoid flooding unknown unicast destination address across the
TRILL campus. The Directory enforced MAC&Label <-> Egress
RBridge mapping table can determine if a data packet needs to
be forwarded across TRILL campus.
When multiple RBridge edge ports are connected, possibly via
bridged LANs, to end stations (servers/VMs), a directory
assisted edge RBridge won't need to flood unknown unicast
destination data frames to all ports of the edge RBridges in
the frame's Data Label when it ingresses a frame. It can depend
on the directory to tell it where the destination is. When the
directory doesn't have the needed information, the frames can
be dropped or flooded depending on the policy configured.
- Reduce flooding of decapsulated Ethernet frames with unknown
MAC destination address to a bridged LAN connected to RBridge
edge ports.
When an RBridge receives a unicast TRILL data packet whose
destination Nickname matches with its own, the normal procedure
is for the RBridge to decapsulate it and forward the
decapsulated Ethernet frame to the directly attached bridged
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LAN. If the destination MAC is unknown, the RBridge floods the
decapsulated Ethernet frame out all ports in the fame's Data
Label. With directory assistance, the egress RBridge can
determine if the MAC destination address in a frame matches any
end stations attached via the bridged LAN. Frames can be
discarded if their destination addresses do not match.
- Reduce the amount of MAC&Label <-> Egress RBridge mapping
maintained by edge RBridges. There is no need for an edge
RBridge to keep MAC entries of remote end stations that don't
communicate with the end stations locally attached.
- Eliminate ARP/ND being broadcast or multicast through the TRILL
core.
- Some protection against spoofing of source addresses (see
Section 7).
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5. Generic operation of Directory Assistance
There are two different models for Directory assistance to edge
RBridges: Push Model and Pull Model. The Directory Information is
described in Section 5.1 below while Section 5.2 discusses Push Model
requirements and Section 5.3 Pull Model requirements.
5.1 Information in Directory for Edge RBridges
To achieve the benefits of directory assistance for TRILL, the
corresponding directory server entries will need, at a minimum, the
following logical data structure:
[IP, MAC, Data Label, {list of attached RBridge nicknames}, {list of
interested RBridges}]
The {list of attached RBridges} are the edge RBridges to which the
host (or VM) specified by the [IP, MAC, Data Label] in the entry is
attached. The {list of interested RBridges} are the remote RBridges
that might have attached hosts that communicate with the host in this
entry.
When a host has multiple IP addresses, there will be multiple
entries.
The {list of interested RBridges} could get populated when an RBridge
queries for information, or information is pushed from a directory
server. The list is used to notify those RBridges when the host
(specified by the [IP, MAC, Data Label]) in the entry changes its
RBridge attachment. An explicit list in the directory is not needed
as long as the interested RBridges can be determined.
5.2 Push Model and Requirements
Under this model, Directory Server(s) push the MAC&Label <-> Egress
RBridge mapping for all the end stations that might communicate with
end stations attached to an RBridge edge node. If the packet's
destination address can't be found in the MAC&Label <-> Egress
RBridge table, the Ingress RBridge could be configured to:
simply drop a data packet,
flood it to the TRILL campus, or
start the pull process to get information from pull directory
server(s)
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It may not be necessary that every edge RBridge gets the entire
mapping table for all the end stations in a campus. There are many
ways to narrow the full set down to a smaller set of remote end
stations that communicate with end stations attached to an edge
RBridge. A simple approach is to only push the mapping for the Data
Labels that have active end stations under an edge RBridge. This
approach can reduce the number of mapping entries being pushed.
However, the Push Model usually will push more entries of MAC&Label
<-> Egress RBridge mapping to an edge RBridges than needed. Under
the normal process of edge RBridge cache aging and unknown
destination address flooding, rarely used mapping entries would have
been removed. But it can be difficult for Directory Servers to
predict the communication patterns among applications within one Data
Label. Therefore, it is likely that the Directory Servers will push
down all the MAC&Label entries if there are end stations in the Data
Label attached to the edge RBridge. This is a disadvantage of the
Push Model compared with the Pull Model described below.
In the Push Model, it is necessary to have a way for an RBridge node
to request directory server(s) to push the mapping entries. This
method should at least include the Data Labels enabled on the
RBridge, so that directory server doesn't need to push down the
entire set of mapping entries for all the end stations in the campus.
An RBridge must be able to get mapping entries when it is initialized
or restarted.
The Push Model's detailed method and any handshake mechanism between
RBridge and Directory Server(s) is beyond the scope of this framework
document.
When a directory server needs to push a large number of entries to
edge RBridges, efficient data organization should be considered. For
example, with one edge RBridge nickname being associated with all
attached end stations' MAC addresses and Data Labels. As shown in
Table 1 below, to make the data more compact, a representation can be
used where a nickname need only occur once for a set of Labels, each
of which occurs only once and each of which is associated with a set
of multiple IP and MAC address pairs. It would be much more bulky to
have each IP and MAC address pair separately accompanied by its Label
and by the nickname of the RBridge by which it is reachable.
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+------------+---------+--------------------------------+
| Nickname1 |Label-1 | IP/MAC1, IP/MAC2, ,, IP/MACn |
| |-------- +--------------------------------+
| |Label-2 | IP/MAC1, IP/MAC2, ,, IP/MACn |
| |-------- +--------------------------------+
| | ...... | IP/MAC1, IP/MAC2, ,, IP/MACn |
+------------+-------- +--------------------------------+
| Nickname2 |Label-1 | IP/MAC1, IP/MAC2, ,, IP/MACn |
| |-------- +--------------------------------+
| |Label-2 | IP/MAC1, IP/MAC2, ,,IP/MACn |
| |-------- +--------------------------------+
| | | IP/MAC1, IP/MAC2, ,, IP/MACn |
+------------+-------- +--------------------------------+
| ------- |-------- +--------------------------------+
| | | IP/MAC1, IP/MAC2, ,, IP/MACn |
+------------+-------- +--------------------------------+
Table 1: Summarized table pushed down from directory
Whenever there is any change in MAC&Label <-> Egress RBridge mapping,
that can be triggered by end stations being added, moved, or de-
commissioned, an incremental update can be sent to the edge RBridges
which are impacted by the change. Therefore, something like a
sequence number has to be maintained by directory servers and
RBridges. Detailed mechanisms will be specified in a separate
document.
5.3 Pull Model and Requirements
Under this model, an RBridge pulls the MAC&Label <-> Egress RBridge
mapping entry from the directory server when its cache doesn't have
the entry. There are a couple of possibilities for triggering the
pulling process:
- The RBridge edge node can send a pull request whenever it
receives an unknown MAC destination, or
- The RBridge edge node can intercept all ARP/ND requests and
forward them or appropriate requests to the Directory Server(s)
that has the information on where the target end stations are
located.
The Pull Directory response could indicate that the address being
queried is unknown or that the requestor is administratively
prohibited from getting an informative response.
By using a Pull Directory, a frame with unknown MAC destination
address doesn't have to be flooded across the TRILL campus and the
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ARP/ND requests don't have to be broadcast or multicast across the
TRILL campus.
The ingress RBridge can cache the response pulled from the directory.
The timer for such a cache should be short in an environment where
VMs move frequently. The cache timer could be configured by the
management system or could be sent along with the Pulled reply by the
directory server(s). It is important that the cached information be
kept consistent with the actual placement of addresses in the campus;
therefore, there needs to be some mechanism by which RBridges that
have pulled information that has not expired can be informed when
that information changes or becomes invalid for other reasons.
One advantage of the Pull Model is that edge RBridges can age out
MAC&Label entries if they haven't been used for a certain configured
period of time or a period of time provided by the Directory.
Therefore, each edge RBridge will only keep the entries that are
frequently used, so its mapping table size will be smaller. Edge
RBridges would query the Directory Server(s) for unknown MAC
destination addresses in data frames or ARP/ND and cache the
response. When end stations attached to remote edge RBridges rarely
communicate with the locally attached end stations, the corresponding
MAC&VLAN entries would be aged out from the RBridge's cache.
An RBridge waiting for a response from Directory Servers upon
receiving a data frame with an unknown destination address is similar
to an L3/L2 boundary router waiting for an ARP or ND response upon
receiving an IP data packet whose destination IP is not in the
router's IP/MAC cache table. Most deployed routers today do hold the
packet and send ARP/ND requests to the target upon receiving a packet
with destination IP not in its IP to MAC cache. When ARP/ND replies
are received, the router will send the data packet to the target.
This practice minimizes flooding when targets don't exist in the
subnet.
When the target doesn't exist in the subnet, routers generally re-
send an ARP/ND request a few more times before dropping the packets.
So, the holding time by routers to wait for an ARP/ND response, if
the target doesn't exist in the subnet, can be longer than the time
taken by the Pull Model to get IP to MAC mapping from a directory
server.
For RBridges with mapping entries being pushed from a directory
server, they can be configured to use the Pull Model for targets
which don't exist in the mapping data pushed.
A separate document will specify the detailed messages and mechanism
for RBridges to pull information from directory server(s).
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6. Recommendation
TRILL should provide a directory assisted approach. This document
describes a basic framework for directory assistance to RBridge edge
nodes. More detailed mechanisms will be described in a separate
document or documents.
7. Security Considerations
For general TRILL security considerations, see Section 6 of
[RFC6325].
Accurate mapping of IP addresses into MAC addresses and of MAC
addresses to the RBridges from which they are reachable is important
to the correct delivery of information. The security of specific
directory assisted mechanisms for delivering such information will be
discussed in the document or documents specifying those mechanisms.
Directory assisted TRILL edge can be used to substantially improve
the security of a TRILL campus over TRILL's default MAC address
learning from the data plane. Assume S is an end station attached to
RB1 trying to spoof a target end station T and that T is attached to
RB2. Perhaps S wants to steal traffic intended for T or forge traffic
as if it was from T.
With that default TRILL data plane learning as described in
[RFC6325], S can impersonate T or any other end station in the same
Data Label (VLAN or FGL [FGL]) as S and possibly other Data Labels,
depending on how tightly VLAN admission and Appointed Forwarders
[RFC6439] are configured at the port by which S is connected to RB1.
S can just send native frames with the forged source MAC addresses of
T, perhaps broadcast frames for maximum effectiveness. With this
technique, S will frequently receive traffic intended for T. And S
can easily forge traffic as being from T.
Such spoofing can be prevented to the extent that the network
RBridges (1) use trusted directory services as described above in
this document, (2) discard native frames received from a local end
station when the directory says that end stations should be remote,
and, (3) when appropriate, intercept ARP and ND messages and respond
locally. Under these circumstances, S would be limited to spoofing
targets on the same RBridge as the ingress RBridge for S (that is,
RB1 = RB2). RB1 would still need to learn which local end stations
were attached to which port and S could confuse RB1 by sending frames
with the forged source MAC address of other end stations on RB1
although it would also still be restricted to frames in a VLAN that
both would be admitted by S's port of attachment and for which that
port is an Appointed Forwarder.
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Security against spoofing could be even further strengthened by
adding port of attachment information to the directory and discarding
native frames that are received on the wrong port. This would limit S
to spoofing targets that were on the same link as S and in a VLAN
admitted by the port of that link's attachment to RB1 and for which
that port is an Appointed Forwarder (or, if the link is multiply
connected, in the same way at all of the ports by which the link is
attached to an RBridge).
Even without directory services, secure ND (Secure Neighbor Discovery
[RFC3971]) or use of secure ESADI (as described in [ESADI]) may also
be helpful to security.
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8. IANA Considerations
This document requires no IANA actions. RFC Editor: please delete
this section before publication.
9. Acknowledgements
Thanks for comments and review from the following:
Sam Aldrin, David Black, Charlie Kaufman, Yizhou Li, and Erik
Nordmark
The document was prepared in raw nroff. All macros used were defined
within the source file.
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10. References
10.1 Normative References
As an Informational document, this draft has no Normative References.
10.2 Informative References
[802.1Q] - IEEE Std 802.1Q-2011, "IEEE Standard for Local and
metropolitan area networks - Virtual Bridged Local Area
Networks", May 2011.
[IS-IS] - ISO/IEC, "Intermediate system to Intermediate system
routeing information exchange protocol for use in conjunction
with the Protocol for providing the Connectionless-mode Network
Service (ISO 8473)", ISO/IEC 10589:2002.
[RFC826] - Plummer, D., "An Ethernet Address Resolution Protocol",
RFC 826, November 1982.
[RFC3971] - Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
"SEcure Neighbor Discovery (SEND)", RFC 3971, March 2005.
[RFC4861] - Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC6325] - Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A.
Ghanwani, "Routing Bridges (RBridges): Base Protocol
Specification", RFC 6325, July 2011.
[RFC6326] - Eastlake, D., Banerjee, A., Dutt, D., Perlman, R., and A.
Ghanwani, "Transparent Interconnection of Lots of Links (TRILL)
Use of IS-IS", RFC 6326, July 2011.
[RFC6439] - Perlman, R., Eastlake, D., Li, Y., Banerjee, A., and F.
Hu, "Routing Bridges (RBridges): Appointed Forwarders", RFC
6439, November 2011.
[ESADI] - H. Zhai, F. Hu, R. Perlman, D. Eastlake, O. Stokes, "TRILL:
ESADI Protocol", draft-ietf-trill-esadi, work in progress.
[FGL] - D. Eastlake, M. Zhang, P. Agarwal, R. Perlman, D. Dutt,
"TRILL (Transparent Interconnection of Lots of Links): Fine-
Grained Labeling", draft-ietf-trill-fine-labeling, in RFC
Editor's queue.
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Authors' Addresses
Linda Dunbar
Huawei Technologies
5430 Legacy Drive, Suite #175
Plano, TX 75024, USA
Phone: +1-469-277-5840
Email: ldunbar@huawei.com
Donald Eastlake
Huawei Technologies
155 Beaver Street
Milford, MA 01757 USA
Phone: +1-508-333-2270
Email: d3e3e3@gmail.com
Radia Perlman
Intel Labs
2200 Mission College Blvd.
Santa Clara, CA 95054-1549 USA
Phone: +1-408-765-8080
Email: Radia@alum.mit.edu
Igor Gashinsky
Yahoo
45 West 18th Street 6th floor
New York, NY 10011 USA
Email: igor@yahoo-inc.com
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L. Dunbar, et al [Page 20]
