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Problem Statement and Goals for Active-Active TRILL Edge

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7379.
Authors Yizhou Li , Hao Weiguo , Radia Perlman , Jon Hudson , Hongjun Zhai
Last updated 2014-04-02
Replaces draft-yizhou-trill-active-active-connection-prob
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state WG Document
Document shepherd Donald E. Eastlake 3rd
IESG IESG state Became RFC 7379 (Informational)
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TRILL Working Group                                            Yizhou Li
INTERNET-DRAFT                                                Weiguo Hao
Intended Status: Informational                       Huawei Technologies
                                                           Radia Perlman
                                                              Intel Labs
                                                              Jon Hudson
                                                            Hongjun Zhai
Expires: October 5, 2014                                   April 3, 2014

        Problem Statement and Goals for Active-Active TRILL Edge


   The IETF TRILL (Transparent Interconnection of Lots of Links)
   protocol provides support for flow level multi-pathing with rapid
   failover for both unicast and multi-destination traffic in networks
   with arbitrary topology. Active-active 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 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
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   The list of current Internet-Drafts can be accessed at

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Copyright and License 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
   ( in effect on the date of
   publication of this document. Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   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
   3. Problems in Active-Active at the TRILL Edge . . . . . . . . . .  6
     3.1 Frame Duplications . . . . . . . . . . . . . . . . . . . . .  7
     3.2 Loop . . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     3.2 Address Flip-Flop  . . . . . . . . . . . . . . . . . . . . .  7
     3.3 Unsynchronized Information Among Member RBridges . . . . . .  8
   4 High Level Requirements and Goals for Solutions  . . . . . . . .  8
   5 Security Considerations  . . . . . . . . . . . . . . . . . . . .  9
   6  IANA Considerations . . . . . . . . . . . . . . . . . . . . . .  9
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  9
   8  References  . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     8.1  Normative References  . . . . . . . . . . . . . . . . . . .  9
     8.2  Informative References  . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11


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1  Introduction

   The IETF TRILL (Transparent Interconnection of Lots of Links)
   [RFC6325] protocol provides loop free and per hop based multipath
   data forwarding with minimum configuration. TRILL uses [IS-IS]
   [RFC6165] [RFC6326bis] 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) load spread 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 re-configure to accommodate link or node failures or

   "Active-active" is the extension, to the extent practical, 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
   load spread end station traffic at the flow level across links to
   such multiple edge TRILL switches and rapidly re-configure to
   accommodate topology changes.

1.1  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

   The acronyms and terminology in [RFC6325] are used herein with the
   following additions:

   CE - As in [CMT], Classic Ethernet device (end station or bridge).
   The device can be either physical or virtual equipment.

   Data Label - VLAN or FGL (Fine Grained Label [RFCfgl]).

   MC-LAG - Multi-Chassis Link Aggregation. Proprietary extensions to
   [802.1AX] standard 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 MC-LAG. When multiple CEs attach to the exact

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   same set of edge RBridges, those edge RBridges can be considered as a
   single edge group. One RBridge can be in more than one edge group.

   TRILL switch - an alternative term for an RBridge.

2.  Target Scenario

   The TRILL appointed forwarder [RFC6325] [RFC6327bis] [RFC6439]
   mechanism provides per Data Label active-standby traffic spreading
   and loop avoidance at the same time. One and only one appointed
   RBridge can ingress/egress native frames into/from TRILL campus for a
   given VLAN among all edge RBridges connecting a legacy network to
   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 RBridge as appointed forwarder for
   different set of VLANs, load spreading over different edge RBidges
   across different Data Labels can be achieved.

   This section presents a typical scenario of active-active connections
   to TRILL campus via multiple edge RBridges where the current TRILL
   appointed forwarder mechanism is not working as expected. 

   The appointed forwarder mechanism [RFC6439] requires each of the edge
   RBridges to exchange TRILL IS-IS Hello packets from their access
   ports. As Figure 1 shows, when multiple access links of multiple edge
   RBridges are bundled as an MC-LAG (Multi-Chassis Link Aggregation
   Group), Hello messages sent by RB1 via access port to CE1 will not be
   forwarded to RB2 by CE1. RB2 (and other members of MC-LAG1) will not
   see that Hello from RB1 via the MC-LAG. Every member RBridge of MC-
   LAG1 thinks of itself as appointed forwarder on an MC-LAG1 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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               |                      |
               |   TRILL Campus       |
               |                      |
                    |       |    |
               -----        |     --------
              |             |             |
          +------+      +------+      +------+
          |      |      |      |      |      |
          |(RB1) |      |(RB2) |      | (RBk)|
          +------+      +------+      +------+
            |..|          |..|          |..|
            |  +----+     |  |          |  |
            |   +---|-----|--|----------+  |
            | +-|---|-----+  +-----------+ |
   MC-      | | |   +------------------+ | |       
   LAG1--->(| | |)                    (| | |) <---MC-LAGn
          +-------+    .  .  .       +-------+
          | CE1   |                  | CEn   |
          |       |                  |       |
          +-------+                  +-------+

        Figure 1 Active-Active connection to TRILL edge RBridges        

   Active-Active connection is useful when we want to achieve the
   following goals:

   - Flow rather than Data Label based load balancing is desired. 

   - More rapid failure recovery is desired. Current appointed forwarder
   mechanism relies on the Hello timer expiration to detect the
   unreachability of another edge RBridge connecting to the same local
   Ethernet link. Then re-appointing 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 BFD [RFCbfd]. Active-
   Active connection usually has a faster built-in mechanism for member
   node and/or link failure detection. Faster detection of failure would
   minimize the frame loss and recovery time.

   MC-LAG is a proprietary facility whose implementation varies by
   vendor. So, to be sure of MC-LAG operation across a group of edge
   RBridges, those edge RBridges will almost always be from the same
   vendor. In order to have a common understanding of active-active
   connection scenarios, the following assumptions are made:

   For a CE connecting to multiple edge RBs via active-active

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   a) the CE will forward a packet from an endnode to exactly one up-
   b) the CE will never forward packets it receives from one up-link to
   c) the CE will attempt to send all packets for a given flow on the
   same uplink
   d) packets are accepted from any of the uplinks and passed down to
   endnodes (if any exist)
   e) the CE has some unknown rule for which packets get sent to which
   uplinks (typically based on a simple hash function of Layer 2 through
   4 header fields)
   f) the CE cannot be assumed to give useful control information to the
   up-link 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 to which a CE is multiply attached:
   a) Any two RBs in the edge group are reachable from each other via
   TRILL campus.
   b) Each RB in the edge group is configured with an ID for each down-
   link to a CE  multiply attached to that group.  The ID will be
   consistent across the edge group.  For example, if CE1 attaches to
   RB1, RB2, ... RBn, then each of RBs will have been configured, for
   the port to CE1, that it is labeled "MC-LAG1"
   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
   d) When a RBridge fails, all the other RBridges having formed any MC-
   LAG with it know the information in a timely fashion.
   e) When a down-link of an edge group RBridge to an MC-LAG fails, all
   the other RBridges having formed any MC-LAG with that down-link know
   the information in a timely fashion
   f) The RBs in the edge group have some mechanism to exchange the
   state and information with each other, including the set of CEs they
   are connecting to or ID of MC-LAGs their down-links have joined. 

   Some MC-LAG implementations corresponding to the assumptions d,e and
   f above may only applicable to native Ethernet network or specific
   type of network. Then some extension for TRILL network may be
   required. It is not the assumption that RBridges in an edge group can
   naturally support them unless otherwise stated.  

3. Problems in Active-Active at the TRILL Edge

   This section presents the problems that need to be addressed in

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   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 sends a multi-destination TRILL Data packet in
   VLAN x, all edge group RBridges of MC-LAG1 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. 

   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 MC-LAG m connects to an edge group g and the edge
   group g consists of RB1, RB2 and RB3. The multi-destination frame
   ingressed from a port not connected to MC-LAG 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. 

   It should be noted that frame duplication is only a problem in multi-
   destination frame forwarding. Unicast forwarding does not have this

3.2 Loop 

   As shown in Figure 1, CE1 may send a native multi-destination frame
   to the TRILL campus via a member of the MC-LAG1 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 MC-LAG 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.  

3.2 Address Flip-Flop

   Consider RB1 and RB2 using their own nickname as ingress nickname for
   data into a TRILL campus. As shown by Figure 1, CE1 may send a data
   frame with the same VLAN and source MAC address to any member of the
   edge group MC-LAG1. If some egress RBridge receives TRILL data
   packets from different ingress RBridges but with same source Data
   Label and MAC address, it learns different Data Label and MAC to
   nickname address correspondences when decapsulating the data frames.

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   Address correspondence may keep flip-flopping among nicknames of the
   member RBridges of the MC-LAG for the same Data Label and MAC

   Most TRILL switches behave badly under these circumstances and, for
   example, interpret this as a severe network problem. It may also
   cause the returning traffic to go through the different paths to
   reach the destination resulting in persistent re-ordering of the

3.3 Unsynchronized Information Among Member RBridges

   A local Rbridge, say RB1 in MC-LAG1, may have learned a Data Label
   and MAC to nickname correspondence 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 MC-LAG1, for example RB2. RB2 may not have h1's
   Data Label and MAC to nickname correspondence stored. Therefore it
   has to do the flooding for unknown unicast. Such flooding is
   unnecessary since the returning traffic is almost always expected and
   RB1 had learned the address correspondence.

   Synchronization on the Data Label and MAC to nickname correspondence
   information among member RBridges will reduce such unnecessary

4 High Level Requirements and Goals for Solutions

   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 up-links of CE MUST be active; CE is free to choose any up-
   link on which to send packets; CE is able to receive the packet from
   any up-link of an edge group.
   2) Looping and frame duplication MUST be prevented. 
   3) Learning of Data Label and MAC to nickname correspondence 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
   6) Single up-link failure on CE to an edge group MUST not cause
   persistent packet delivery failure between TRILL campus and CE.


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   Control plane:
   1) No requirement for new information to be passed between edge
   RBridges and CE or between edge RBridges and endnodes.
   2) If there are any TRILL specific information required to be
   exchanged between RBridges in an edge group, for example data labels
   and MAC addresses binding to nicknames, solution SHOULD specify the
   mechanism to perform such exchange.
   3) RBridges SHOULD be able to discover other members in the same edge
   group by exchanging their MC-LAG 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 2 up to at least 4 active-
   active up-links on a multiply attached CE.
   5) Solution SHOULD not assume there is a dedicated physical line
   between any two of the edge RBridges in an edge group.

5 Security Considerations

   This draft does not introduce any extra security risks. Security
   risks introduced by any particular solutions to the problems
   presented here will be discussed in the separate document(s)
   describing such solutions. For general TRILL Security Considerations,
   see [RFC6325].

6  IANA Considerations

   No IANA action is required. RFC Editor: please delete this section
   before publication.

7.  Acknowledgments

   Special acknowledgments to Donald Eastlake and Mingui Zhang for their
   valuable comments.

8  References

8.1  Normative References

   [IS-IS]  ISO/IEC 10589:2002, Second Edition, "Intermediate System to

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              Intermediate System Intra-Domain Routing Exchange Protocol
              for use in Conjunction with the Protocol for Providing the
              Connectionless-mode Network Service (ISO 8473)", 2002.

   [RFC6165] Banerjee, A. and D. Ward, "Extensions to IS-IS for Layer-2
              Systems", RFC 6165, April 2011.

   [RFC6325] Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A.
              Ghanwani, "Routing Bridges (RBridges): Base Protocol
              Specification", RFC 6325, July 2011

   [RFC6326bis] Eastlake, D., Banerjee, A., Dutt, D., Perlman, R., and
              A. Ghanwani, "TRILL Use of IS-IS", draft-eastlake-isis-
              rfc6326bis, work in progress.

   [RFC6327bis] Eastlake 3rd, D., R. Perlman, A. Ghanwani, H. Yang, and
              V. Manral, "TRILL: Adjacency", draft-ietf-trill-
              rfc6327bis, work in progress.

   [RFC6439] Perlman, R., Eastlake, D., Li, Y., Banerjee, A., and F. Hu,
              "Routing Bridges (RBridges): Appointed Forwarders", RFC
              6439, November 2011 

   [RFCfgl] Eastlake, D., 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 Ediotr's queue.

8.2  Informative References

   [CMT] Senevirathne, T., Pathangi, J., and J. Hudson, "Coordinated
              Multicast Trees (CMT)for TRILL", draft-ietf-trill-cmt-
              02.txt, Work in Progress, October 2013.

   [RFCbfd] Manral, V., D. Eastlake, D. Ward, A. Banerjee, "TRILL
              (Transparent Interconnetion of Lots of Links):
              Bidirectional Forwarding Detection (BFD) Support", draft-
              ietf-trill-rbridge-bfd, in RFC Editor's queue.

   [TRILLPN] Zhai,H.,, "RBridge: Pseudonode Nickname", draft-hu-
              trill-pseudonode-nickname, Work in progress, November

   [802.1AX] IEEE, "Link Aggregration", 802.1AX-2008, 2008.

   [802.1Q] IEEE, "Media Access Control (MAC) Bridges and Virtual

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              Bridged Local Area Networks", IEEE Std 802.1Q-2011,
              August, 2011

Authors' Addresses

   Yizhou Li
   Huawei Technologies
   101 Software Avenue,
   Nanjing 210012

   Phone: +86-25-56625409

   Weiguo Hao
   Huawei Technologies
   101 Software Avenue,
   Nanjing 210012

   Phone: +86-25-56623144

   Radia Perlman
   Intel Labs
   2200 Mission College Blvd.
   Santa Clara, CA  95054-1549

   Phone: +1-408-765-8080

   Jon Hudson
   130 Holger Way
   San Jose, CA 95134 USA

   Phone: +1-408-333-4062

   Hongjun Zhai

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   68 Zijinghua Road, Yuhuatai District
   Nanjing, Jiangsu  210012

   Phone: +86 25 52877345

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