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IETF RFC 7379
Last modified on Friday, October 17th, 2014
 Permanent link to RFC 7379
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Internet Engineering Task Force (IETF) Y. Li
Request for Comments: 7379 W. Hao
Category: Informational Huawei Technologies
ISSN: 2070-1721 R. Perlman
EMC
J. Hudson
Brocade
H. Zhai
JIT
October 2014
Problem Statement and Goals for Active-Active Connection at the
Transparent Interconnection of Lots of Links (TRILL) Edge
 Abstract
The IETF TRILL (Transparent Interconnection of Lots of Links)
protocol provides support for flow-level multipathing with rapid
failover for both unicast and multi-destination traffic in networks
with arbitrary topology. Active-active connection at the TRILL edge
is the extension of these characteristics to end stations that are
multiply connected to a TRILL campus. This informational document
discusses the high-level problems and goals when providing active-
active connection at the TRILL edge.
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 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 7379.
Li, et al. Informational PAGE 1 
RFC 7379 Problems of Active-Active Connection October 2014
Copyright Notice
Copyright (c) 2014 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
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
1.1. Terminology ................................................3
2. Target Scenario .................................................4
2.1. LAALP and Edge Group Characteristics .......................6
3. Problems in Active-Active Connection at the TRILL Edge ..........7
3.1. Frame Duplications .........................................7
3.2. Loopback ...................................................8
3.3. Address Flip-Flop ..........................................8
3.4. Unsynchronized Information among Member RBridges ...........8
4. High-Level Requirements and Goals for Solutions .................9
5. Security Considerations ........................................10
6. References .....................................................11
6.1. Normative References ......................................11
6.2. Informative References ....................................12
Acknowledgments ...................................................12
Authors' Addresses ................................................13
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RFC 7379 Problems of Active-Active Connection October 2014
1. Introduction
The IETF TRILL (Transparent Interconnection of Lots of Links)
[RFC 6325] protocol provides loop-free and per-hop-based multipath
data forwarding with minimum configuration. TRILL uses IS-IS [IS-IS]
[RFC 6165] [RFC 7176] as its control-plane routing protocol and defines
a TRILL-specific header for user data. In a TRILL campus,
communications between TRILL switches can:
1) use multiple parallel links and/or paths,
2) spread load over different links and/or paths at a fine-grained
flow level through equal-cost multipathing of unicast traffic and
multiple distribution trees for multi-destination traffic, and
3) rapidly reconfigure to accommodate link or node failures or
additions.
To the degree practical, "active-active" is the extension of similar
load spreading and robustness to the connections between end stations
and the TRILL campus. Such end stations may have multiple ports and
will be connected, directly or via bridges, to multiple edge TRILL
switches. It must be possible, except in some failure conditions, to
spread end-station traffic load at the granularity of flows across
links to such multiple edge TRILL switches and rapidly reconfigure to
accommodate topology changes.
1.1. Terminology
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 [RFC 2119].
The acronyms and terminology in the RBridges base protocol [RFC 6325]
are used herein with the following additions:
CE: Customer Equipment (end station or bridge).
Data Label: VLAN or FGL (Fine-Grained Label [RFC 7172]).
LAALP: Local Active-Active Link Protocol. Any protocol
similar to MC-LAG that runs in a distributed fashion
on a CE, on the links from that CE to a set of edge
group RBridges, and on those RBridges.
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MC-LAG: Multi-Chassis Link Aggregation. Proprietary
extensions to IEEE Std 802.1AX-2011 [802.1AX] so that
the aggregated links can, at one end of the
aggregation, attach to different switches.
Edge group: a group of edge RBridges to which at least one CE is
multiply attached using an LAALP. When multiple CEs
attach to the exact same set of edge RBridges, those
edge RBridges can be considered as a single edge
group. An RBridge can be in more than one edge group.
RBridge: Routing Bridge. An alternative name for a TRILL
switch.
TRILL: Transparent Interconnection of Lots of Links.
TRILL switch: a device that implements the TRILL protocol; an
alternative term for an RBridge.
2. Target Scenario
This section presents a typical scenario of active-active connection
to a TRILL campus via multiple edge RBridges where the current TRILL
Appointed Forwarder mechanism does not work as expected.
The TRILL Appointed Forwarder mechanism [RFC 6439] can handle failover
(active-standby), provides loop avoidance, and, with administrative
configuration, provides load spreading based on VLAN. One and only
one appointed RBridge can ingress/egress native frames into/from the
TRILL campus for a given VLAN among all edge RBridges connecting a
legacy network to the TRILL campus. This is true whether the legacy
network is a simple point-to-point link or a complex bridged LAN or
anything in between. By carefully selecting different RBridges as
Appointed Forwarder for different sets of VLANs, load spreading over
different edge RBridges across different Data Labels can be achieved.
The Appointed Forwarder mechanism [RFC 6439] requires all of the edge
group RBridges to exchange TRILL IS-IS Hello packets through their
access ports. As Figure 1 shows, when multiple access links of
multiple edge RBridges are connected to a CE by an LAALP, Hello
messages sent by RB1 via access port to CE1 will not be forwarded to
RB2 by CE1. RB2 (and other members of LAALP1) will not see that
Hello from RB1 via the LAALP1. Every member RBridge of LAALP1 thinks
of itself as Appointed Forwarder on an LAALP1 link for all VLANs and
will ingress/egress frames. Hence, the Appointed Forwarder mechanism
cannot provide active-active or even active-standby service across
the edge group in such a scenario.
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RFC 7379 Problems of Active-Active Connection October 2014
----------------------
| |
| TRILL Campus |
| |
----------------------
| | |
----- | --------
| | |
+------+ +------+ +------+
| | | | | |
|(RB1) | |(RB2) | | (RBk)|
+------+ +------+ +------+
|..| |..| |..|
| +----+ | | | |
| +---|-----|--|----------+ |
| +-|---|-----+ +-----------+ |
| | | +------------------+ | |
LAALP1--->(| | |) (| | |) <---LAALPn
+-------+ . . . +-------+
| CE1 | | CEn |
| | | |
+-------+ +-------+
Figure 1: Active-Active Connection to TRILL Edge RBridges
Active-active connection is useful when we want to achieve the
following two goals:
- Flow-based rather than VLAN-based load balancing is desired.
- More rapid failure recovery is desired.
The current Appointed Forwarder mechanism relies on the TRILL Hello
timer expiration to detect the unreachability of another edge RBridge
connecting to the same local link. Then, reappointing the forwarder
for specific VLANs may be required. Such procedures take time on the
scale of seconds although this can be improved with TRILL use of
Bidirectional Forwarding Detection (BFD) [RFC 7175]. Active-active
connection usually has a faster built-in mechanism for member node
and/or link failure detection. Faster detection of failures
minimizes the frame loss and recovery time.
Today, LAALP is usually a proprietary facility whose implementation
varies by vendor. So, to be sure the LAALP operates successfully
across a group of edge RBridges, those edge RBridges will almost
always have to be from the same vendor. In the case where the LAALP
is an MC-LAG, the CE normally implements the logic described in IEEE
Std 802.1AX-2011 [802.1AX], so proprietary elements would only be at
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the end of the edge group. There is also a revision of IEEE Std
802.1AX-2011 [802.1AX] underway (802.1X-REV) to remove the
restriction in IEEE Std 802.1AX-2011 [802.1AX] that there be one box
at each end of the aggregation. So it is possible that, in the
future, an LAALP could be implemented through such a revised IEEE Std
802.1AX-2011 [802.1AX] with standards-conformant logic at the ends of
both the CE and edge group. In order to have a common understanding
of active-active connection scenarios, the assumptions in Section 2.1
are made about the characteristics of the LAALP and edge group of
RBridges.
2.1. LAALP and Edge Group Characteristics
For a CE connecting to multiple edge RBridges via an LAALP (active-
active connection), the following characteristics apply:
a) The LAALP will deliver a frame from an end node to TRILL at
exactly one edge group RBridge.
b) The LAALP will never forward frames it receives from one uplink to
another.
c) The LAALP will attempt to send all frames for a given flow on the
same uplink. To do this, it has some unknown rule for which
frames get sent to which uplinks (typically based on a simple hash
function of Layer 2 through 4 header fields).
d) Frames are accepted from any of the uplinks and passed down to end
nodes (if any exist).
e) The LAALP cannot be assumed to send useful control information to
the uplink such as "this is the set of other RBridges to which
this CE is attached" or "these are all the MAC addresses
attached".
For an edge group of RBridges to which a CE is multiply attached with
an LAALP:
a) Any two RBridges in the edge group are reachable from each other
via the TRILL campus.
b) Each RBridge in the edge group knows an ID for each LAALP instance
multiply attached to that group. The ID will be consistent across
the edge group and globally unique across the TRILL campus. For
example, if CE1 attaches to RB1, RB2, ... RBn using an LAALP, then
each of the RBs will know, for the port to CE1, that it has a
label such as "LAALP1".
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c) Each RB in the edge group can be configured with the set of
acceptable VLANs for the ports to any CE. The acceptable VLANs
configured for those ports should include all the VLANs the CE has
joined and be consistent for all the member RBridges of the edge
group.
d) When an RBridge fails, all the other RBridges that have formed an
LAALP instance with it learn of the failure in a timely fashion.
e) When a downlink of an edge group RBridge to an LAALP instance
fails, that RBridge and all the other RBridges participating in
the LAALP instance, including that downlink, learn of the failure
in a timely fashion.
f) The RBridges in the edge group have a mechanism to exchange
information with each other, information such as the set of CEs
they are connecting to or the IDs of the LAALP instances their
downlinks are part of.
Other than the applicable characteristics above, the internals of an
LAALP are out of the scope of TRILL.
3. Problems in Active-Active Connection at the TRILL Edge
This section presents the problems that need to be addressed in
active-active connection scenarios. The topology in Figure 1 is used
in the following sub-sections as the example scenario for
illustration purposes.
3.1. Frame Duplications
When a remote RBridge ingresses a multi-destination TRILL data packet
in VLAN x, all edge group RBridges of LAALP1 will receive the frame
if any local CE1 joins VLAN x. As each of them thinks it is the
Appointed Forwarder for VLAN x, without changes made for active-
active connection support, they would all forward the frame to CE1.
The bad consequence is that CE1 receives multiple copies of that
multi-destination frame from the remote end-host source.
Frame duplication may also occur when an ingress RBridge is non-
remote -- say, ingress and egress are two RBridges belonging to the
same edge group. Assume LAALP m connects to an edge group g, and the
edge group g consists of RB1, RB2, and RB3. The multi-destination
frames ingressed from a port not connected to LAALP m by RB1 can be
locally replicated to other ports on RB1 and also TRILL encapsulated
and forwarded to RB2 and RB3. CE1 will receive duplicate copies from
RB1, RB2, and RB3.
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Note that frame duplication is only a problem in multi-destination
frame forwarding. Unicast forwarding does not have this issue as
there is only ever one copy of the packet.
3.2. Loopback
As shown in Figure 1, CE1 may send a native multi-destination frame
to the TRILL campus via a member of the LAALP1 edge group (say RB1).
This frame will be TRILL encapsulated and then forwarded through the
campus to the multi-destination receivers. Other members (say RB2)
of the same LAALP edge group will receive this multicast packet as
well. In this case, without changes made for active-active
connection support, RB2 will decapsulate the frame and egress it.
The frame loops back to CE1.
3.3. Address Flip-Flop
Consider RB1 and RB2 using their own nickname as ingress nickname for
data into a TRILL campus. As shown in Figure 1, CE1 may send a data
frame with the same VLAN and source Media Access Control (MAC)
address to any member of the edge group LAALP1. If an egress RBridge
receives TRILL data packets from different ingress RBridges but with
the same source Data Label and MAC address, it learns different
correspondences between a {Data Label and MAC address} and nickname
when decapsulating the data frames. Address correspondence may keep
flip-flopping among nicknames of the member RBridges of the LAALP for
the same Data Label and MAC address. Existing hardware does not
support data-plane learning of multiple nicknames for the same MAC
address and Data Label -- when data-plane learning indicates
attachment of the MAC address to a new nickname, it overwrites the
old attachment nickname.
Implementers have stated that most current TRILL switch hardware,
when doing data-plane learning, behaves badly under these
circumstances and, for example, interprets address flip-flopping as a
severe network problem. It may also cause the returning traffic to
go through different paths to reach the destination, resulting in
persistent reordering of the frames.
3.4. Unsynchronized Information among Member RBridges
A local RBridge, say RB1 connected to LAALP1, may have learned a
correspondence between a {Data Label and MAC address} and nickname
for a remote host h1 when h1 sends a packet to CE1. The returning
traffic from CE1 may go to any other member RBridge of LAALP1, for
example, RB2. RB2 may not have h1's correspondence between a {Data
Label and MAC address} and nickname stored. Therefore, it has to do
the flooding for unknown unicast addresses [RFC 6325]. Such flooding
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is unnecessary since the returning traffic is almost always expected
and RB1 had learned the address correspondence. It is desirable to
avoid flooding; it imposes a greater burden on the network than known
destination unicast traffic because the flooded traffic is sent over
more links.
Synchronization of the correspondence between a {Data Label and MAC
address} and nickname information among member RBridges will reduce
such unnecessary flooding.
4. High-Level Requirements and Goals for Solutions
The problems identified in Section 3 should be solved in any solution
for active-active connection to edge RBridges. The following high-
level requirements and goals should be met.
Data plane:
1) All uplinks of a CE MUST be active: the LAALP is free to choose
any uplink on which to send packets, and the CE is able to receive
packets from any uplink of an edge group.
2) Loopback and frame duplication MUST be prevented.
3) Learning of correspondence between a {Data Label and MAC address}
and nickname by a remote RBridge MUST NOT flip-flop between the
local multiply attached edge RBridges.
4) Packets for a flow SHOULD stay in order.
5) The Reverse Path Forwarding Check MUST work properly as per the
RBridges base protocol [RFC 6325].
6) Single uplink failure on a CE to an edge group MUST NOT cause
persistent packet delivery failure between a TRILL campus and CE.
Control plane:
1) No requirement for new information to be passed between edge
RBridges and CEs or between edge RBridges and end nodes exists.
2) If there is any TRILL-specific information required to be
exchanged between RBridges in an edge group, for example, Data
Labels and MAC addresses binding to nicknames, a solution MUST
specify the mechanism to perform such exchange unless this is
handled internal to the LAALP.
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RFC 7379 Problems of Active-Active Connection October 2014
3) RBridges SHOULD be able to discover other members in the same edge
group by exchanging their LAALP attachment information.
Configuration, incremental deployment, and others:
1) Solution SHOULD require minimal configuration.
2) Solution SHOULD automatically detect misconfiguration of edge
RBridge group.
3) Solution SHOULD support incremental deployment, that is, not
require campus-wide upgrading for all RBridges, only changes to
the edge group RBridges.
4) Solution SHOULD be able to support from two up to at least four
active-active uplinks on a multiply attached CE.
5) Solution SHOULD NOT assume there is a dedicated physical link
between any two edge RBridges in an edge group.
5. Security Considerations
As an informational overview, this document does not introduce any
extra security risks. Security risks introduced by a particular
LAALP or other elements of solutions to the problems presented here
will be discussed in the separate document(s) describing such LAALP
or solutions.
End-station links in TRILL are Ethernet links, and consideration
should be given to securing them with link security as described in
IEEE Std 802.1AE-2006 [802.1AE] for the protection of end-station
data and link-level control messages, including any LAALP control
messages.
For general TRILL Security Considerations, see the RBridges base
protocol [RFC 6325].
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6. References
6.1. Normative References
[IS-IS] ISO/IEC, "Information technology -- Telecommunications and
information exchange between systems -- Intermediate
System to Intermediate System intra-domain routeing
information exchange protocol for use in conjunction with
the protocol for providing the connectionless-mode network
service (ISO 8473)", ISO/IEC 10589:2002, Second Edition,
2002.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[RFC 6165] Banerjee, A. and D. Ward, "Extensions to IS-IS for Layer-2
Systems", RFC 6165, April 2011,
<http://www.rfc-editor.org/info/RFC 6165>.
[RFC 6325] Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A.
Ghanwani, "Routing Bridges (RBridges): Base Protocol
Specification", RFC 6325, July 2011,
<http://www.rfc-editor.org/info/RFC 6325>.
[RFC 6439] Perlman, R., Eastlake, D., Li, Y., Banerjee, A., and F.
Hu, "Routing Bridges (RBridges): Appointed Forwarders",
RFC 6439, November 2011,
<http://www.rfc-editor.org/info/RFC 6439>.
[RFC 7172] Eastlake 3rd, D., Zhang, M., Agarwal, P., Perlman, R., and
D. Dutt, "Transparent Interconnection of Lots of Links
(TRILL): Fine-Grained Labeling", RFC 7172, May 2014,
<http://www.rfc-editor.org/info/RFC 7172>.
[RFC 7176] Eastlake 3rd, D., Senevirathne, T., Ghanwani, A., Dutt,
D., and A. Banerjee, "Transparent Interconnection of Lots
of Links (TRILL) Use of IS-IS", RFC 7176, May 2014,
<http://www.rfc-editor.org/info/RFC 7176>.
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RFC 7379 Problems of Active-Active Connection October 2014
6.2. Informative References
[802.1AE] IEEE, "IEEE Standard for Local and metropolitan area
networks -- Media Access Control (MAC) Security", IEEE Std
802.1AE-2006, 18 August 2006.
[802.1AX] IEEE, "IEEE Standard for Local and metropolitan area
networks -- Link Aggregration", IEEE Std 802.1AX-2008, 3
November 2008.
[RFC 7175] Manral, V., Eastlake 3rd, D., Ward, D., and A. Banerjee,
"Transparent Interconnection of Lots of Links (TRILL):
Bidirectional Forwarding Detection (BFD) Support", RFC
7175, May 2014, <http://www.rfc-editor.org/info/RFC 7175>.
Acknowledgments
Special acknowledgments to Donald Eastlake, Adrian Farrel, and Mingui
Zhang for their valuable comments.
Li, et al. Informational PAGE 12
RFC 7379 Problems of Active-Active Connection October 2014
Authors' Addresses
Yizhou Li
Huawei Technologies
101 Software Avenue,
Nanjing 210012
China
Phone: +86-25-56625409
EMail: liyizhou@huawei.com
Weiguo Hao
Huawei Technologies
101 Software Avenue,
Nanjing 210012
China
Phone: +86-25-56623144
EMail: haoweiguo@huawei.com
Radia Perlman
EMC
2010 256th Avenue NE, #200
Bellevue, WA 98007
United States
EMail: Radia@alum.mit.edu
Jon Hudson
Brocade
130 Holger Way
San Jose, CA 95134
United States
Phone: +1-408-333-4062
EMail: jon.hudson@gmail.com
Hongjun Zhai
JIT
EMail: honjun.zhai@tom.com
Li, et al. Informational PAGE 13
RFC TOTAL SIZE: 26764 bytes
PUBLICATION DATE: Friday, October 17th, 2014
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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