Internet Engineering Task Force (IETF) M. Zhang

Internet Engineering Task Force (IETF) M. Zhang
Request for Comments: 7727 H. Wen
Category: Standards Track Huawei
ISSN: 2070-1721 J. Hu
China Telecom
January 2016

Spanning Tree Protocol (STP) Application
of the Inter-Chassis Communication Protocol (ICCP)

Abstract

The Inter-Chassis Communication Protocol (ICCP) supports an inter-
chassis redundancy mechanism that is used to support high network
availability.

In this document, Provider Edge (PE) devices in a Redundancy Group
(RG) running ICCP are used to offer multihomed connectivity to
Spanning Tree Protocol (STP) networks to improve availability of the
STP networks. The ICCP TLVs and usage for the ICCP STP application
are defined.

Status of This Memo

This is an Internet Standards Track document.

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). Further information on
Internet Standards is available in 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/rfc7727.

Zhang, et al. Standards Track [Page 1]

RFC 7727 STP Application of ICCP January 2016

Copyright Notice

Copyright (c) 2016 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.

Zhang, et al. Standards Track [Page 2]

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Table of Contents

1. Introduction ....................................................4
1.1. Conventions Used in This Document ..........................4
1.2. Terminology ................................................4
2. Use Case ........................................................5
3. Spanning Tree Protocol Application TLVs .........................6
3.1. STP Connect TLV ............................................6
3.2. STP Disconnect TLV .........................................7
3.2.1. STP Disconnect Cause Sub-TLV ........................8
3.3. STP Configuration TLVs .....................................8
3.3.1. STP System Config ...................................9
3.3.2. STP Region Name ....................................10
3.3.3. STP Revision Level .................................10
3.3.4. STP Instance Priority ..............................11
3.3.5. STP Configuration Digest ...........................12
3.4. STP State TLVs ............................................12
3.4.1. STP Topology Changed Instances .....................12
3.4.2. STP CIST Root Time Parameters ......................14
3.4.3. STP MSTI Root Time Parameter .......................15
3.5. STP Synchronization Request TLV ...........................16
3.6. STP Synchronization Data TLV ..............................17
4. Operations .....................................................18
4.1. Common AC Procedures ......................................18
4.1.1. Remote PE Node Failure or Isolation ................19
4.1.2. Local PE Isolation .................................19
4.2. ICCP STP Application Procedures ...........................19
4.2.1. Initial Setup ......................................19
4.2.2. Configuration Synchronization ......................20
4.2.3. State Synchronization ..............................21
4.2.4. Failure and Recovery ...............................22
5. Security Considerations ........................................22
6. IANA Considerations ............................................23
7. References .....................................................23
7.1. Normative References ......................................23
7.2. Informative References ....................................24
Acknowledgements ................................................. 24
Authors' Addresses ............................................... 25

Zhang, et al. Standards Track [Page 3]

RFC 7727 STP Application of ICCP January 2016

1. Introduction

Inter-Chassis Communication Protocol (ICCP [RFC7275]) specifies a
multi-chassis redundancy mechanism that enables Provider Edge (PE)
devices located in a multi-chassis arrangement to act as a single
Redundancy Group (RG).

With the Spanning Tree Protocol (STP), a spanning tree will be formed
over connected bridges by blocking some links between these bridges
so that forwarding loops are avoided. This document introduces
support of STP as a new application of ICCP. When a bridged STP
network is connected to an RG, this STP application of ICCP enables
the RG members to act as a single root bridge participating in the
operations of STP.

STP-relevant information needs to be exchanged and synchronized among
the RG members. New ICCP TLVs for the ICCP STP application are
specified for this purpose.

From the point of view of the customer, the Service Provider is
providing a Virtual Private LAN Service (VPLS) [RFC4762].

1.1. 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 [RFC2119].

1.2. Terminology

ICCP: Inter-Chassis Communication Protocol
VPLS: Virtual Private LAN Service
STP: Spanning Tree Protocol
MSTP: Multiple Spanning Tree Protocol
MST: Multiple Spanning Trees
CIST: Common and Internal Spanning Tree ([802.1q], Section 3.27)
MSTI: Multiple Spanning Tree Instance ([802.1q], Section 3.138)
BPDU: Bridge Protocol Data Unit

In this document, unless otherwise explicitly noted, the term "STP"
also covers MSTP.

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2. Use Case

Customers widely use Ethernet as an access technology [RFC4762].
It's common that one customer's Local Area Network (LAN) has multiple
bridges connected to a carrier's network at different locations for
reliability purposes. Requirements for this use case are listed as
follows.

o Customers desire to balance the load among their available
connections to the carrier's network; therefore, all the
connections need to be active.

o When one connection to the carrier network fails, customers
require a connection in another location to continue to work after
the reconvergence of the STP rather than compromising the whole
STP network. The failure of the connection may be due to the
failure of the PE, the attachment circuit (AC), or even the
Customer Edge (CE) device itself.

In order to meet these requirements, the 'ICCP-STP' model is
proposed. It introduces STP as a new application of ICCP.

+--------------+ +=============+
| | | |
| | | |
| +---+ | | +-----+|<--|--Pseudowire-->|
| +---+CE1+<6>-------<5>+ PE1 || | |
| <1> +---+ | | +-----+|<--|--Pseudowire-->|
| +-+-+ | | || |
| |CE3| | | ||ICCP |--> Towards the Core
| +-+-+ | | || |
| <2> +---+ | | +-----+|<--|--Pseudowire-->|
| +---+CE2+<3>-------<4>+ PE2 || | |
| +---+ | | +-----+|<--|--Pseudowire-->|
| | | |
| Multihomed | | Redundancy |
| STP Network | | Group |
+--------------+ +=============+

Figure 1: A STP network is multihomed to an RG running ICCP

Figure 1 shows an example topology of this model. With ICCP, the
whole RG will be virtualized to be a single bridge. Each RG member
has its BridgeIdentifier (the MAC address). The numerically lowest
one is used as the BridgeIdentifier of the 'virtualized root bridge'.
The RG acts as if the ports connected to the STP network (ports <4>
and <5>) are for the same root bridge. All these ports send the
configuration BPDU with the highest root priority to trigger the

Zhang, et al. Standards Track [Page 5]

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construction of the spanning tree. The link between the peering PEs
is not visible to the bridge domains of the STP network. In this
way, the STP will always break a possible loop within the multihomed
STP network by breaking the whole network into separate islands so
that each is attached to one PE. That forces all PEs in the RG to be
active. This is different from a generic VPLS [RFC4762] where the
root bridge resides in the customer network and the multihomed PEs
act in the active-standby mode. Note that the specification of VPLS
remains unchanged other than for this operation. For instance, a
full-mesh of pseudowires (PWs) is established between PEs, and the
"split horizon" rule is still used to perform the loop-breaking
through the core.

3. Spanning Tree Protocol Application TLVs

This section specifies the ICCP TLVs for the ICCP STP application.
The Unknown TLV bit (U-bit) and the Forward unknown TLV bit (F-bit)
of the following TLVs MUST be sent as cleared and processed on
receipt as specified in [RFC7275].

3.1. STP Connect TLV

This TLV is included in the RG Connect Message to signal the
initiation of an ICCP STP application connection.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|F| Type=0x2000 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protocol Version |A| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optional Sub-TLVs |
~ ~
| |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

- U=F=0

- Type

Set to 0x2000 for "STP Connect TLV"

Zhang, et al. Standards Track [Page 6]

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- Length

Length of the TLV in octets excluding the U-bit, F-bit, Type,
and Length fields.

- Protocol Version

The version of ICCP STP application protocol. This document
defines version 0x0001.

- A bit

Acknowledgement Bit. Set to 1 if the sender has received a STP
Connect TLV from the recipient. Otherwise, set to 0.

- Reserved

Reserved for future use. These bits MUST be sent as 0 and
ignored on receipt.

- Optional Sub-TLVs

There are no optional Sub-TLVs defined for this version of the
protocol.

3.2. STP Disconnect TLV

This TLV is used in the RG Disconnect Message to indicate that the
connection for the ICCP STP application is to be terminated.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|F| Type=0x2001 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optional Sub-TLVs |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

- U=F=0

- Type

Set to 0x2001 for "STP Disconnect TLV"

- Length

Length of the TLV in octets excluding the U-bit, F-bit, Type,
and Length fields.

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- Optional Sub-TLVs

The only optional Sub-TLV defined for this version of the
protocol is the "STP Disconnect Cause" sub-TLV, defined below:

3.2.1. STP Disconnect Cause Sub-TLV

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|F| Type=0x200C | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Disconnect Cause String |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

- U=F=0

- Type

Set to 0x200C for "STP Disconnect Cause TLV"

- Length

Length of the TLV in octets excluding the U-bit, F-bit, Type,
and Length fields.

- Disconnect Cause String

Variable-length string specifying the reason for the disconnect,
encoded in UTF-8 [RFC3629] format. Used for operational
purposes.

3.3. STP Configuration TLVs

The STP Configuration TLVs are sent in the RG Application Data
Message. When an STP Config TLV is received by a peer RG member, the
member MUST synchronize with the configuration information contained
in the TLV. TLVs specified in Sections 3.3.1 to 3.3.5 define
specific configuration information.

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3.3.1. STP System Config

This TLV announces the local node's STP System Parameters to the RG
peers.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|F| Type=0x2002 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ROID |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAC Address |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

- U=F=0

- Type

Set to 0x2002 for "STP System Config TLV"

- Length

Length of the ROID plus the MAC address in octets. Always set
to 14.

- ROID

Redundant Object Identifier; format defined in Section 6.1.3 of
[RFC7275].

- MAC Address

The MAC address of the sender. This MAC address is set to the
BridgeIdentifier of the sender, as defined in [802.1q], Section
13.26.2. The numerically lowest 48-bit unsigned value of
BridgeIdentifier is used as the MAC address of the Virtual Root
Bridge mentioned in Section 2.

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3.3.2. STP Region Name

This TLV carries the value of Region Name.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|F| Type=0x2003 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Region Name |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

- U=F=0

- Type

Set to 0x2003 for "STP Region Name TLV"

- Length

Length of the TLV in octets excluding the U-bit, F-bit, Type,
and Length fields.

- Region Name

The Name of the MST Region as specified in [802.1q], Section
3.142.

3.3.3. STP Revision Level

This TLV carries the value of Revision Level.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|F| Type=0x2004 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Revision Level |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

- U=F=0

- Type

Set to 0x2004 for "STP Revision Level TLV".

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- Length

Length of the TLV in octets excluding the U-bit, F-bit, Type,
and Length fields. Always set to 2.

- Revision Level

The Revision Level as specified in [802.1q], Section 13.8, item
c.

3.3.4. STP Instance Priority

This TLV carries the value of Instance Priority to other members in
the RG.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|F| Type=0x2005 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pri | InstanceID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

- U=F=0

- Type

Set to 0x2005 for "STP Instance Priority TLV"

- Length

Length of the TLV in octets excluding the U-bit, F-bit, Type,
and Length fields.

- Pri

The Instance Priority. It is interpreted as unsigned integer
with higher value indicating a higher priority.

- InstanceID

The 12-bit Instance Identifier of the CIST or MSTI. This
parameter takes a value in the range 1 through 4094 for MSTI (as
defined in [802.1q], Section 12.8.1.2.2) and takes value of 0
for CIST.

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3.3.5. STP Configuration Digest

This TLV carries the value of STP VLAN Instance Mapping.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|F| Type=0x2006 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Configuration Digest |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

- U=F=0

- Type

Set to 0x2006 for "STP Configuration Digest TLV"

- Length

Length of the STP Configuration Digest in octets. Always set to
16.

- Configuration Digest

As specified in [802.1q], Section 13.8, item d.

3.4. STP State TLVs

The STP State TLVs are sent in the RG Application Data Message. They
are used by a PE device to report its STP status to other members in
the RG. Such TLVs are specified in the following subsections.

3.4.1. STP Topology Changed Instances

This TLV is used to report the Topology Changed Instances to other
members of the RG. The sender monitors Topology Change Notification
(TCN) messages and generates this list. The receiving RG member MUST
initiate the Topology Change event, including sending BPDU with the
Topology Change flag set to 1 out of the designated port(s) of the
Topology Changed bridge domains of the STP network, and flushing out
MAC addresses relevant to the instances listed in this TLV.

If the PE device supports MAC Address Withdrawal (see Section 6.2 of
[RFC4762]), it SHOULD send a Label Distribution Protocol (LDP)
Address Withdraw Message with the list of MAC addresses towards the
core over the corresponding LDP sessions. It is not necessary to

Zhang, et al. Standards Track [Page 12]

RFC 7727 STP Application of ICCP January 2016

send such a message to PEs of the same RG since the flushing of their
MAC address tables should have been performed upon receipt of the STP
Topology Changed Instances TLV.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|F| Type=0x2007 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| InstanceID List |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

- U=F=0

- Type

Set to 0x2007 for "STP Topology Changed Instances TLV"

- Length

Length of the TLV in octets excluding the U-bit, F-bit, Type,
and Length fields.

- InstanceID List

The list of the InstanceIDs of the CIST or MSTIs whose
topologies have changed as indicated by the TCN messages as
specified in [802.1q], Section 13.14. The list is formatted by
padding each InstanceID value to the 16-bit boundary as follows,
where the bits in the "R" fields MUST be sent as 0 and ignored
on receipt.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R|R|R|R| InstanceID#1 |R|R|R|R| InstanceID#2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ... ... ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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3.4.2. STP CIST Root Time Parameters

This TLV is used to report the Value of CIST Root Time Parameters
([802.1q], Section 13.26.7) to other members of the RG. All time
parameter values are in seconds with a granularity of 1. For ranges
and default values of these parameter values, refer to [802.1d1998],
Section 8.10.2, Table 8-3; [802.1d2004] Section 17.14, Table 17-1;
and [802.1q], Section 13.26.7.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|F| Type=0x2008 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MaxAge | MessageAge |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FwdDelay | HelloTime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RemainingHops |
+-+-+-+-+-+-+-+-+

- U=F=0

- Type

Set to 0x2008 for "STP CIST Root Time TLV"

- Length

Length of the TLV in octets excluding the U-bit, F-bit, Type,
and Length fields. Always set to 9.

- MaxAge

The Max Age of the CIST. It is the maximum age of the
information transmitted by the bridge when it is the Root Bridge
([802.1d2004], Section 17.13.8).

- MessageAge

The Message Age of the CIST (see [802.1q], Section 13.26.7).

- FwdDelay

The Forward Delay of the CIST. It is the delay used by STP
Bridges to transition Root and Designated Ports to Forwarding
([802.1d2004], Section 17.13.5).

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- HelloTime

The Hello Time of the CIST. It is the interval between periodic
transmissions of Configuration Messages by Designated Ports
([802.1d2004], Section 17.13.6).

- RemainingHops

The remainingHops of the CIST ([802.1q], Section 13.26.7).

3.4.3. STP MSTI Root Time Parameter

This TLV is used to report the parameter value of MSTI Root Time to
other members of the RG. As defined in [802.1q], Section 13.26.7, it
is the value of remainingHops for the given MSTI.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|F| Type=0x2009 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pri | InstanceID | RemainingHops |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

- U=F=0

- Type

Set to 0x2009 for "STP MSTI Root Time TLV"

- Length

Length of the TLV in octets excluding the U-bit, F-bit, Type,
and Length fields. Always set to 3.

- Pri

The Instance Priority. It is interpreted as an unsigned integer
with higher value indicating a higher priority.

- InstanceID

The 12-bit Instance Identifier of the Multiple Spanning Tree
Instance (MSTID). As defined in [802.1q], Section 12.8.1.2.2,
this parameter takes a value in the range 1 through 4094.

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- RemainingHops

The remainingHops of the MSTI. It is encoded in the same way as
in [802.1q], Section 14.4.1, item f.

3.5. STP Synchronization Request TLV

The STP Synchronization Request TLV is used in the RG Application
Data Message. This TLV is used by a device to request that its peer
retransmit configuration or operational state. The following
information can be requested:

- configuration and/or state of the STP system,
- configuration and/or state for a given list of instances.

The format of the TLV 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|F| Type=0x200A | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request Number |C|S| Request Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| InstanceID List |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

- U=F=0

- Type

Set to 0x200A for "STP Synchronization Request TLV"

- Length

Length of the TLV in octets excluding the U-bit, F-bit, Type,
and Length fields. Always set to 4.

- Request Number

2 octets. Unsigned integer uniquely identifying the request.
Used to match the request with a corresponding response. The
value of 0 is reserved for unsolicited synchronization, and it
MUST NOT be used in the STP Synchronization Request TLV. As
indicated in [RFC7275], given the use of TCP, there are no
issues associated with the wrap-around of the Request Number.

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- C-bit

Set to 1 if the request is for configuration data. Otherwise,
set to 0.

- S-bit

Set to 1 if the request is for running state data. Otherwise,
set to 0.

- Request Type

14 bits specifying the request type, encoded as follows:

0x00 Request System Data
0x01 Request data of the listed instances
0x3FFF Request System Data and data of all instances

- InstanceID List

The InstanceIDs of the CIST or MSTIs; format specified in
Section 3.4.1.

3.6. STP Synchronization Data TLV

The pair of STP Synchronization Data TLVs are used by the sender to
delimit a set of TLVs that are being transmitted in response to an
STP Synchronization Request TLV. The delimiting TLVs signal the
start and end of the synchronization data, and they associate the
response with its corresponding request via the Request Number field.
It's REQUIRED that each pair of STP Synchronization Data TLVs occur
in the same fragment. When the total size of the TLVs to be
transmitted exceeds the maximal size of a fragment, these TLVs MUST
be divided into multiple sets, delimited by multiple pairs of STP
Synchronization Data TLVs, and filled into multiple fragments. With
the Request Number, lost fragments can be identified and
re-requested.

The STP Synchronization Data TLVs are also used for unsolicited
advertisements of complete STP configuration and operational state
data. The Request Number field MUST be set to 0 in this case.

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STP Synchronization Data TLV has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|F| Type=0x200B | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request Number | Reserved |S|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

- U=F=0

- Type

set to 0x200B for "STP Synchronization Data TLV"

- Length

Length of the TLV in octets excluding the U-bit, F-bit, Type,
and Length fields. Always set to 4.

- Request Number

2 octets. Unsigned integer identifying the Request Number of
the "STP Synchronization Request TLV" that initiated this
synchronization data response.

- Reserved

Reserved bits for future use. These MUST be sent as 0 and
ignored on receipt.

- S

S = 0: Synchronization Data Start
S = 1: Synchronization Data End

4. Operations

Operational procedures for AC redundancy applications have been
specified in Section 9.2 of [RFC7275]. The operational procedures of
the ICCP STP application should follow those procedures, with the
changes presented in this section.

4.1. Common AC Procedures

The following changes are introduced to the generic procedures of AC
redundancy applications defined in Section 9.2.1 of [RFC7275].

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4.1.1. Remote PE Node Failure or Isolation

When a local PE device detects that a remote PE device that is a
member of the same RG is no longer reachable (using the mechanisms
described in Section 5 of [RFC7275]), the local PE device checks if
it has redundancy ACs for the affected services. If redundant ACs
are present, and if the local PE device has the new highest bridge
priority, the local PE device becomes the virtual root bridge for
corresponding ACs.

4.1.2. Local PE Isolation

When a PE device detects that it has been isolated from the core
network, then it needs to ensure that its AC redundancy mechanism
will change the status of all active ACs to standby. The AC
redundancy application SHOULD then send an RG Application Data
Message in order to trigger failover to another active PE device in
the RG. Note that this works only in the case of dedicated
interconnect (Sections 3.2.1 and 3.2.3), since ICCP will still have
the path to the peer, even though the PE device is isolated from the
MPLS core network.

4.2. ICCP STP Application Procedures

This section defines the procedures of the ICCP STP application that
are applicable for Ethernet ACs.

4.2.1. Initial Setup

When an RG is configured on a system that supports the ICCP STP
application, such systems MUST send an RG Connect Message with an STP
Connect TLV to each PE device that is a member of the RG. The
sending PE device MUST set the A bit to 1 in that TLV if it has
received a corresponding STP Connect TLV from its peer PE; otherwise,
the sending PE device MUST set the A bit to 0. If a PE device
receives an STP Connect TLV from its peer after sending its own TLV
with the A bit set to 0, it MUST resend the TLV with the A bit set to
1. A system considers the ICCP STP application connection to be
operational when it has both sent and received STP Connect TLVs with
the A bit set to 1. When the ICCP STP application connection between
a pair of PEs is operational, the two devices can start exchanging RG
Application Data Messages for the ICCP STP application. This
involves having each PE device advertise its STP configuration and
operational state in an unsolicited manner. A PE device SHOULD
follow the order below when advertising its STP state upon initial
application connection setup:

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- Advertise the STP System Config TLV
- Advertise remaining Configuration TLVs
- Advertise State TLVs

The update of the information contained in the State TLVs depends on
that in the Configuration TLVs. By sending the TLVs in the above
order, the two peers may begin to update STP state as early as
possible in the middle of exchanging these TLVs.

A PE device MUST use a pair of STP Synchronization Data TLVs to
delimit the entire set of TLVs that are being sent as part of this
unsolicited advertisement.

If a system receives an RG Connect Message with an STP Connect TLV
that has a differing Protocol Version, it MUST follow the procedures
outlined in the Section 4.4.1 ("Application Versioning") of
[RFC7275].

After the ICCP STP application connection has been established, every
PE device MUST communicate its system-level configuration to its
peers via the use of STP System Config TLV.

When the ICCP STP application is administratively disabled on the PE,
or on the particular RG, the system MUST send an RG Disconnect
Message containing STP Disconnect TLV.

4.2.2. Configuration Synchronization

A system that supports ICCP STP application MUST synchronize the
configuration with other RG members. This is achieved via the use of
STP Configuration TLVs. The PEs in the RG MUST all agree on the
common MAC address to be associated with the virtual root bridge. It
is possible to achieve this via consistent configuration on member
PEs. However, in order to protect against possible
misconfigurations, a virtual root bridge identifier MUST be set to
the MAC address advertised by the PE device with the numerically
lowest BridgeIdentifier (i.e., the MAC address of the bridge) in the
RG.

Furthermore, for a given ICCP STP application, an implementation MUST
advertise the configuration prior to advertising its corresponding
state. If a PE device receives any STP State TLV that it had not
learned of before via an appropriate STP Configuration TLV, then the
PE device MUST request synchronization of the configuration and state
from its peer. If during such synchronization a PE device receives a
State TLV that it has not learned before, then the PE device MUST
send a NAK TLV for that particular TLV. The PE device MUST NOT
request resynchronization in this case.

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4.2.3. State Synchronization

PEs within the RG need to synchronize their state for proper STP
operation. This is achieved by having each system advertise its
running state in STP State TLVs. Whenever any STP parameter either
on the CE or PE side is changed, the system MUST transmit an updated
TLV for the affected STP instances. Moreover, when the
administrative or operational state changes, the system MUST transmit
an updated State TLV to its peers.

A PE device MAY request its peer to retransmit previously advertised
state. This is useful in case the PE device is recovering from a
soft failure and attempting to relearn state. To request such
retransmissions, a PE device MUST send a set of one or more STP
Synchronization Request TLVs.

A PE device MUST respond to a STP Synchronization Request TLV by
sending the requested data in a set of one or more STP Configuration
or State TLVs delimited by a pair of STP Synchronization Data TLVs.

Note that the response may span across multiple RG Application Data
Messages, for example, when MTU limits are exceeded; however, the
above ordering MUST be retained across messages, and only a single
pair of Synchronization Data TLVs MUST be used to delimit the
response across all RG Application Data Messages.

A PE device MAY readvertise its STP state in an unsolicited manner.
This is done by sending the appropriate State TLVs delimited by a
pair of STP Synchronization Data TLVs and using a Request Number of
0.

While a PE device has sent out a synchronization request for a
particular PE device, it SHOULD silently ignore all TLVs that are
from that node, are received prior to the synchronization response,
and carry the same type of information being requested. This saves
the system from the burden of updating state that will ultimately be
overwritten by the synchronization response. Note that TLVs
pertaining to other systems should continue to be processed normally.

If a PE device receives a synchronization request for an instance
that doesn't exist or is not known to the PE, then it MUST trigger
the unsolicited synchronization of all information by restarting the
initialization.

If during the synchronization operation a PE device receives an
advertisement of a Node ID value that is different from the value
previously advertised, then the PE device MUST purge all state data
previously received from that peer prior to the last synchronization.

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4.2.4. Failure and Recovery

When a PE device that is active for the ICCP STP application
encounters a core isolation fault [RFC7275], it SHOULD attempt to
fail over to a peer PE device that hosts the same RG. The default
failover procedure is to have the failed PE device bring down the
link(s) towards the multihomed STP network. This will cause the STP
network to reconverge and to use the other links that are connected
to the other PE devices in the RG. Other procedures for triggering
failover are possible and are outside the scope of this document.

If the isolated PE device is the one that has the numerically lowest
BridgeIdentifier, PEs in the RG MUST synchronize STP Configuration
and State TLVs and determine a new virtual root bridge as specified
in Section 4.2.2.

Upon recovery from a previous fault, a PE device SHOULD NOT reclaim
the role of the virtual root for the STP network even if it has the
numerically lowest BridgeIdentifier among the RG. This minimizes
traffic disruption.

Whenever the virtual root bridge changes, the STP Topology Changed
Instances TLV lists the instances that are affected by the change.
These instances MUST undergo a STP reconvergence procedure when this
TLV is received as defined in Section 3.4.1.

5. Security Considerations

This document specifies an application running on the channel
provided by ICCP [RFC7275]. The security considerations on ICCP
apply in this document as well.

For the ICCP STP application, an attack on a channel (running in the
provider's network) can break not only the ability to deliver traffic
across the provider's network, but also the ability to route traffic
within the customer's network. That is, a careful attack on a
channel (such as the DoS attacks as described in [RFC7275]) can break
STP within the customer network. Implementations need to provide
mechanisms to mitigate these types of attacks. For example, the port
between the PE device and the malicious CE device may be blocked.

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6. IANA Considerations

The IANA maintains a top-level registry called "Pseudowire Name
Spaces (PWE3)". It has a subregistry called "ICC RG Parameter
Types".

IANA has made 13 allocations from this registry as shown below. IANA
has allocated the codepoints from the range marked for assignment by
IETF Review (0x2000-0x2FFF) [RFC5226]. Each assignment references
this document.

Parameter Type Description
-------------- ---------------------------------
0x2000 STP Connect TLV
0x2001 STP Disconnect TLV
0x2002 STP System Config TLV
0x2003 STP Region Name TLV
0x2004 STP Revision Level TLV
0x2005 STP Instance Priority TLV
0x2006 STP Configuration Digest TLV
0x2007 STP Topology Changed Instances TLV
0x2008 STP CIST Root Time TLV
0x2009 STP MSTI Root Time TLV
0x200A STP Synchronization Request TLV
0x200B STP Synchronization Data TLV
0x200C STP Disconnect Cause TLV

7. References

7.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <http://www.rfc-editor.org/info/rfc3629>.

[RFC4762] Lasserre, M., Ed., and V. Kompella, Ed., "Virtual
Private LAN Service (VPLS) Using Label Distribution
Protocol (LDP) Signaling", RFC 4762,
DOI 10.17487/RFC4762, January 2007,
<http://www.rfc-editor.org/info/rfc4762>.

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RFC 7727 STP Application of ICCP January 2016

[RFC7275] Martini, L., Salam, S., Sajassi, A., Bocci, M.,
Matsushima, S., and T. Nadeau, "Inter-Chassis
Communication Protocol for Layer 2 Virtual Private
Network (L2VPN) Provider Edge (PE) Redundancy",
RFC 7275, DOI 10.17487/RFC7275, June 2014,
<http://www.rfc-editor.org/info/rfc7275>.

[802.1q] IEEE, "IEEE Standard for Local and Metropolitan Area
Networks -- Bridges and Bridged Networks", IEEE Std
802.1Q-2014, DOI 10.1109/IEEESTD.2014.6991462, 2014.

[802.1d1998] IEEE, "Information technology -- Telecommunications and
information exchange between systems -- Local and
metropolitan area networks -- Common specifications --
Part 3: Media Access Control (MAC) Bridges", ANSI/IEEE
Std 802.1D-1998, DOI 10.1109/IEEESTD.1998.95619, 1998.

[802.1d2004] IEEE, "IEEE Standard for Local and metropolitan area
networks -- Media Access Control (MAC) Bridges", IEEE
Std 802.1D-2004, DOI 10.1109/ieeestd.2004.94569, 2004.

7.2. Informative References

[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.

Acknowledgements

The authors would like to thank the comments and suggestions from
Ignas Bagdonas, Adrian Farrel, Andrew G. Malis, Gregory Mirsky, and
Alexander Vainshtein.

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Authors' Addresses

Mingui Zhang
Huawei Technologies
No. 156 Beiqing Rd. Haidian District,
Beijing 100095
China

Email: [email protected]

Huafeng Wen
Huawei Technologies
101 Software Avenue,
Nanjing 210012
China

Email: [email protected]

Jie Hu
China Telecom
Beijing Information Science & Technology Innovation Park
Beiqijia Town Changping District,
Beijing 102209
China

Email: [email protected]

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