IS-IS Path Computation and Reservation
draft-farkas-isis-pcr-01

Abstract

IEEE 802.1Qca Path Control and Reservation (PCR) specifies explicit path control via IS-IS in Layer 2 networks in order to move beyond the shortest path capabilities provided by IEEE 802.1aq Shortest Path Bridging (SPB). IS-IS PCR provides capabilities for the establishment and control of explicit forwarding trees in a Layer 2 network domain. This document specifies the sub-TLVs for IS-IS PCR.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

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."

This Internet-Draft will expire on June 18, 2015.

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
• 2. Terminology and Definitions
• 3. Explicit Trees
• 4. Explicit ECT Algorithms
• 5. IS-IS PCR sub-TLVs
• 5.1. Topology sub-TLV
• 5.2. Hop sub-TLV
• 5.3. Bandwidth Constraint sub-TLV
• 5.4. Bandwidth Assignment sub-TLV
• 6. MRT-FRR Application
• 7. Summary
• 8. IANA Considerations
• 9. Security Considerations
• 10. Acknowledgements
• 11. References
• 11.1. Normative References
• 11.2. Informative References
• Authors' Addresses

1. Introduction

IEEE 802.1Qca Path Control and Reservation (PCR) [IEEE8021Qca] specifies extensions to IS-IS for the control of Explicit Trees (ETs). The PCR extensions are compatible with the Shortest Path Bridging (SPB) extensions to IS-IS specified by [RFC6329] and [IEEE8021aq]. Furthermore, IS-IS with PCR extensions relies on the SPB architecture and terminology; and some of the IS-IS SPB sub-TLVs are also leveraged. IS-IS PCR builds upon IS-IS and uses IS-IS in a similar way to SPB. IS-IS PCR only addresses point-to-point physical links, although IS-IS also supports shared media LANs.

This document specifies four IS-IS sub-TLVs for the control of explicit trees by IS-IS PCR in a Layer 2 network as specified by IEEE 802.1Qca. In addition to the sub-TLVs specified here, IS-IS PCR relies on the following IS-IS SPB sub-TLVs specified by [RFC6329]:

• SPB Base VLAN-Identifiers sub-TLV
• SPB Instance sub-TLV
• SPBV MAC address sub-TLV
• SPBM Service Identifier and Unicast Address sub-TLV

These sub-TLVs are used to provide the associations among bridges, MAC addresses, VIDs and I-SIDs within an IS-IS domain. The use of these SPB sub-TLVs for PCR is specified by IEEE 802.1Qca. Note that IS-IS PCR does not require the implementation of the full IS-IS SPB protocol but only the support of these SPB sub-TLVs. Nonetheless, if a bridge supports both IS-IS SPB and IS-IS PCR, then both of them are implemented by the same IS-IS entity.

The sub-TLVs specified here can be also applied for Fast ReRoute using Maximally Redundant Trees (MRT-FRR) [I-D.ietf-rtgwg-mrt-frr-architecture] in a Layer 2 network. MRTs are computed as specified in [I-D.ietf-rtgwg-mrt-frr-algorithm]. If MRT computation is split such that the Generalized Almost Directed Acyclic Graph (GADAG) is computed centrally, then these sub-TLVs can be used to distribute the GADAG, which is identical for each network node throughout a network domain.

PCR uses IS-IS, the SPB sub-TLVs listed above, and the new sub-TLVs defined here. IS-IS PCR has no impact to IETF protocols.

2. Terminology and Definitions

ADAG:

Almost Directed Acyclic Graph - a digraph that can be transformed into a DAG by removing all arcs incoming to the root.

B-VID:

Backbone VID.

Base VID:

The VID used to identify a VLAN in management operations.

BLCE:

Bridge Local Computation Engine - A computation engine in a bridge that performs path and routing computations. The BLCE implements e.g. SPF, CSPF, or the Maximally Redundant Trees Algorithm [I-D.ietf-rtgwg-mrt-frr-algorithm].

Constrained tree:

A tree meeting a certain constraint, e.g. providing a minimal available bandwidth.

Cut-node:

A node is a cut-node if removing it partitions the network.

Cut-link:

A link is a cut-link if removing it partitions the network.

DAG:

Directed Acyclic Graph - a digraph containing no directed cycle.

DEI:

Drop Eligible Indicator.

ECT Algorithm:

Equal Cost Tree Algorithm - The algorithm and mechanism that is used for the control of the active topology, i.e. forwarding trees. It can be one of the shortest path algorithms specified by IEEE 802.1aq. It can be also one of the explicit path control algorithms specified by IEEE 802.1Qca. Each ECT Algorithm has a 32-bit unique ID.

ET:

Explicit Tree - An explicitly defined tree, which is specified by its end points and the paths among the end points. If only the end points are specified but the paths are not, then it is a loose explicit tree. If the paths are also specified, then it is a strict explicit tree.

ETDB:

Explicit Tree Database - A database storing explicit trees.

FDB:

Filtering Database.

GADAG:

Generalized ADAG - a digraph, which has only ADAGs as all of its topology blocks.

Hop:

A hop is specified by two nodes. A strict hop has no intermediate nodes, whereas a loose hop can have one or more intermediate nodes. IS-IS PCR specifies an explicit tree by an ordered list of hops starting at the root, each successive hop being defined by the next element of the list.

I-SID:

Backbone Service Instance Identifier - A 24-bit ID.

Maximally Redundant Trees (MRTs):

A pair of trees with a common MRT Root where the path from any leaf node to the MRT Root along the first tree (MRT-Blue) and the path from the same leaf node along the second tree (MRT-Red) share the minimum number of nodes and the minimum number of links. Each such shared node is a cut-node. Any shared links are cut-links.

MRT-Blue:

MRT-Blue is one of the two MRTs; specifically, MRT-Blue is the increasing MRT where links in the GADAG are taken in the direction from a lower topologically ordered node to a higher one.

MRT-Red:

MRT-Red is one of the two MRTs; specifically, MRT-Red is the decreasing MRT where links in the GADAG are taken in the direction from a higher topologically ordered node to a lower one.

MRT Root:

The common root of the two MRTs: MRT-Blue and MRT-Red.

MSRP:

Multiple Stream Registration Protocol, standardized as IEEE 802.1Qat, already rolled into IEEE 802.1Q-2011.

PCA:

Path Control Agent - The agent that is part of the IS-IS domain and thus can perform IS-IS operations on behalf of a PCE, e.g. maintain the LSDB and send LSPs.

PCE:

Path Computation Element - An entity that is capable of computing a path through a network based on a representation of the topology of the network (obtained by undefined means external to the PCE).

PCP:

Priority Code Point.

Redundant trees:

A pair of trees with a common Root where the paths from any leaf node to the Root along the first tree and the second tree are disjoint.

SPBV:

SPB VID - The SPB mode where a unique VID is assigned to each SPT Root bridge and is used to identify an SPT.

SPBM:

SPB MAC - The SPB mode where a MAC or its shorthand is used to identify an SPT.

SPF:

Shortest Path First.

SPT:

Shortest Path Tree.

SRLG:

Shared Risk Link Group - A set of links that share a resource whose failure affects each link.

topology block:

Either a maximally two-connected (induced) subgraph, a cut-link with its endpoints, or an isolated node.

TED:

Traffic Engineering Database - A database storing the traffic engineering information propagated by IS-IS.

two-connected:

A graph that has no cut-nodes. This is a graph that requires at least two nodes to be removed before gets partitioned.

VID:

VLAN ID.

VLAN:

Virtual Local Area Network.

3. Explicit Trees

An explicit tree is determined by a Path Computation Element (PCE) [RFC4655] and is not required to follow the shortest path. A PCE is an entity that is capable of computing a topology for forwarding based on a network topology, its corresponding attributes, and potential constraints. A PCE explicitly describes a forwarding tree as described in Section 5.1. Either a single PCE or multiple PCEs determine explicit trees for a domain. Even if there are multiple PCEs in a domain, each explicit tree is determined by only one PCE, which is referred to as the owner PCE of the tree. PCEs and IS-IS PCR can be used in combination with IS-IS SPB shortest path routing.

The PCE interacts with the active topology control protocol, i.e. with IS-IS. The collaboration with IS-IS can be provided by a Path Control Agent (PCA) on behalf of a PCE. Either the PCE or the corresponding PCA is part of the IS-IS domain. If the PCE is not part of the IS-IS domain, then the PCE has to be associated with a PCA that is part of the IS-IS domain. The PCE or its PCA establishes IS-IS adjacency in order to receive all the LSPs transmitted by the bridges in the domain. The PCE, either on its own or via its PCA, can control the establishment of explicit trees in that domain by injecting an LSP conveying an explicit tree and thus instruct IS-IS to set up the explicit tree determined by the PCE. If instructed to do so by a PCE, IS-IS can also record and communicate bandwidth assignments, which can be only applied if no reservation protocol (e.g. Multiple Stream Registration Protocol (MSRP)) is used in the domain.

The operation details of the PCE are not specified by IEEE 802.1Qca. If the PCE is part of the IS-IS domain, then the PCE uses IS-IS PDUs to communicate with the IS-IS domain and the PCE has a live IS-IS LSDB, (i.e. the PCE implements the PCA functions too). A PCE can instead communicate with the IS-IS domain via a PCA, e.g. to retrieve the LSDB or instruct the creation of an explicit tree. However, the means of communication between the PCE and the PCA is not specified by IEEE 802.1Qca.

An Explicit Tree (ET) is an undirected loop-free topology, whose use is under the control of the owner PCE by means of associating VIDs and MAC addresses with it. As it is undirected, the ET contains no assumptions about the direction of any flows that use it; it can be used in either direction as specified by the VIDs and MAC addresses associated with it. It is the responsibility of the PCE to ensure reverse path congruency and multicast-unicast congruency if that is required.

An explicit tree is either strict or loose. A strict explicit tree specifies all bridges and paths it comprises. A loose tree only specifies the bridges as a list of hops that have a special role in the tree, e.g. a traffic end point, and no path or path segment is specified between the bridges, which are therefore loose hops even if traffic end points are adjacent neighbors. The special role of a hop can be: traffic end point, root, leaf, a bridge to be avoided, or a transit hop in case of a tree with a single leaf. The path for a loose hop is determined by the Bridge Local Computation Engine (BLCE) of the bridges. The shortest path is used for a loose hop unless specified otherwise by the descriptor (Section 5.1) of the tree or by the corresponding ECT Algorithm (Section 4).

A loose explicit tree is constrained if the tree descriptor includes one or more constraints, e.g. the administrative group that the links of the tree have to belong to. The BLCE of the bridges then apply the Constrained Shortest Path First (CSPF) algorithm, which is Shortest Path First (SPF) on the topology that only contains the links meeting the constraint(s).

An explicit tree is specified by a Topology sub-TLV (Section 5.1). The Topology sub-TLV associates one or more VIDs with an explicit tree. The Topology sub-TLV includes two or more Hop sub-TLVs (Section 5.2), and a hop is specified by an IS-IS System ID. A Hop sub-TLV can include a delay constraint for a loose hop. A Topology sub-TLV can also include further sub-TLVs to constrain loose hops. The bridges involved in an explicit tree store the corresponding Topology sub-TLVs in their Explicit Tree Database (ETDB).

Explicit trees are propagated and set-up by IS-IS PCR in a domain. The PCE or its PCA assembles the Topology sub-TLVs (Section 5.1), and adds it into an LSP, which is flooded throughout the domain. The Topology sub-TLV is flooded by the same techniques used for the SPB LSPs. The bridges then shall process the Topology sub-TLV upon reception. If the Topology sub-TLV specifies one or more loose trees, then the path for the loose hops is determined by the BLCE of the bridges. The bridges then install the appropriate FDB entries for frame forwarding along the tree described by the Topology sub-TLV, or the trees computed based on the Topology sub-TLV. Dynamic Filtering Entries are maintained by IS-IS for the VID, MAC address tuples associated with an ET.

Due to the LSP aging of IS-IS, the Topology sub-TLVs (Section 5.1) have to be refreshed similar to other IS-IS TLVs in order to keep the integrity of the LSDB. The corresponding Dynamic Filtering Entries are also refreshed in the FDB when a Topology sub-TLV is refreshed. Refreshing Topology sub-TLVs is the task of the entity being part of the IS-IS domain, i.e. either the PCE or the PCA.

If a conflict rises with respect to Topology sub-TLVs belonging to different PCEs in a domain with multiple PCEs, then IS-IS PCR resolves the conflict. IS-IS PCR grants the item in conflict, e.g. VID or bandwidth to be assigned, to one of the PCEs based on the LSPs conveying the Topology sub-TLVs. The item in conflict is granted to the PCE whose LSP ID modulo the number of Hop sub-TLVs conveyed by the Topology sub-TLV produces the numerically lowest value. If the modulo operation produces the same numerical result, then the item in conflict is granted to the PCE whose LSP has the numerically least LSP ID.

The bridges can support the following IS-IS features for the computation of explicit trees. The Extended IS Reachability TLV (type 22) specified in [RFC5305] provides the following link attribute IS-IS sub-TLVs:

• Administrative Group (color, resource class) (sub-TLV type 3),
• Maximum Link Bandwidth (sub-TLV type 9),
• Maximum Reservable Link bandwidth (sub-TLV type 10),
• Unreserved Bandwidth (sub-TLV type 11),
• Traffic Engineering Default Metric (sub-TLV type 18).

Further attributes are provided by the IS-IS TE Metric Extension link attribute sub-TLVs specified in [I-D.ietf-isis-te-metric-extensions]:

• Unidirectional Link Delay,
• Min/Max Unidirectional Link Delay,
• Unidirectional Delay Variation,
• Unidirectional Link Loss,
• Unidirectional Residual Bandwidth,
• Unidirectional Available Bandwidth,
• Unidirectional Utilized Bandwidth.

The Shared Risk Link Group (SRLG) information provided by the SRLG TLV (type 138) [RFC5307] can be also used. In order to indicate that the interface is unnumbered in this case, the corresponding flag takes value 0. The Link Local Identifier is an Extended Local Circuit Identifier and the Link Remote Identifier is a Neighbor Extended Local Circuit ID.

4. Explicit ECT Algorithms

The exact IS-IS control mode of operation is selected for a VLAN by associating its Base VID with the appropriate ECT Algorithm in the SPB Base VLAN-Identifiers sub-TLV [RFC6329], in addition to allocating the Base VID to IS-IS control. There are five distinct ECT Algorithms for the five explicit path control modes. The operation details of the explicit ECT Algorithms and their configuration is specified by IEEE 802.1Qca, a high level overview is given here.

The Strict Tree (ST) ECT Algorithm (00-80-C2-20) is used for a strict explicit tree. A strict ET is static as no other entity can update it but the owner PCE. In case of a topology change, it is the task of the owner PCE to detect the topology change, e.g. based on the changes in the LSDB, and to update the strict trees if needed. That is, the owner PCE computes the new tree, assembles its descriptor (Section 5.1), and then instructs IS-IS PCR to install it.

The Loose Tree (LT) ECT Algorithm (00-80-C2-21) is used for a single loose explicit tree. The path for loose hops is determined by the BLCE of the bridges; therefore, the Topology sub-TLV (Section 5.1) specifying the tree has to indicate which hop is the Root of the tree. The loose hops are maintained by IS-IS, i.e. restored upon a topology change if a loop-free path is available. If the tree computed by the BLCE visits the same bridge twice (implying that a loop or hairpin has been created), then that loop or hairpin shall be pruned from the tree even if it contains a hop specified by the Topology sub-TLV. It is a constraint if a bridge is not to be included, which can be specified by the Exclude flag of a Hop sub-TLV (Section 5.2) conveyed by the Topology sub-TLV specifying the tree.

The Loose Tree Set (LTS) ECT Algorithm (00-80-C2-22) is used if connectivity among the traffic end points specified by the Topology sub-TLV (Section 5.1) is to be provided by a set of loose trees such that one tree is rooted at each traffic end point. The BLCE of the bridges compute the loose trees, which are maintained by IS-IS, i.e. restored upon a topology change. One constraint can be to avoid some bridges in these trees, which can be specified by the Exclude flag (item c.6. in Section 5.2). Further constraints can be specified by the Topology sub-TLV.

The MRT ECT Algorithm (00-80-C2-23) is used for the establishment and maintenance of MRTs in a distributed fashion. The MRT Lowpoint Algorithm specified by [I-D.ietf-rtgwg-mrt-frr-algorithm] is to be used for the computation of MRTs. The MRT Lowpoint Algorithm first computes the GADAG then produces two MRTs for each MRT Root: MRT-Blue and MRT-Red. If the level of redundancy provided by each bridge being an MRT Root is not required, then the MRT Roots can be specified by a Topology sub-TLV (Section 5.1). Both the GADAG and the MRT computation steps are performed distributed, i.e. by each bridge.

The MRT GADAG (MRTG) ECT Algorithm (00-80-C2-24) splits the computation into two. As the GADAG is identical for each MRT within a domain, it is computed by a single entity, which is the GADAG Computer. The GADAG is then described in a Topology sub-TLV (Section 5.1), which is flooded in the domain. The bridges then compute the MRTs for the MRT Roots based on the GADAG received. Section 6 provides more details on the description of the GADAG.

MRTs are loose trees as bridges are involved in their computation and restoration. Thus both the MRT and the MRTG ECT Algorithms provide a set of loose trees: two MRTs for each MRT Root.

5. IS-IS PCR sub-TLVs

The following sub-TLVs are specified for IS-IS PCR. The Topology sub-TLV is carried in an MT-Capability TLV, the rest of the sub-TLVs are conveyed by Topology sub-TLV.

5.1. Topology sub-TLV

The variable length Topology sub-TLV shall be used to describe an explicit tree. The Topology sub-TLV may be also used for describing a Generalized Almost Directed Acyclic Graph (GADAG) as explained in Section 6 in detail. The Topology sub-TLV is carried in an MT-Capability TLV (type 144) [RFC6329] in a Link State PDU. A Topology sub-TLV specifying an explicit tree conveys one or more Base VIDs, two or more Hop sub-TLVs (Section 5.2). A Topology sub-TLV describing a loose tree specifies the shortest path algorithm to be used for the loose hops and it can also convey further sub-TLVs to specify constraints. Figure 1 shows the format of the Topology 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
+-+-+-+-+-+-+-+-+
|     Type      |                   (1 byte)
+-+-+-+-+-+-+-+-+
|    Length     |                   (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                   ECT Algorithm   (4 bytes)                   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Num Base VIDs |                   (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Res  |  Base VID 1 (12 bits) |   (0 or 2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       .................
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Res  |  Base VID n (12 bits) |   (0 or 2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        sub-TLV 1  (variable)                  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                         .................                       
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        sub-TLV m  (variable)                  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         +-----------+
                         |     A     |
                         +-----------+
                         |     I     | 
                         +-----------+
                         |     H     |
[B]---[A]---[I]          +-----------+
 |           |           |     G     |
 |           |           +-----------+
 |           |           |     E     |
[C]---[F]   [H]          +-----------+
 |           |           |     A     |
 |           |           +-----------+
 |           |           |     B     |
[D]   [E]---[G]          +-----------+
                         |     C     |
                         +-----------+
                         |     D     |
                         +-----------+
                         |     C     |
                         +-----------+
                         |     F     | 
                         +-----------+

 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
+-+-+-+-+-+-+-+-+
|     Type      |                       (1 byte)
+-+-+-+-+-+-+-+-+
|    Length     |                       (1 byte)
+-+-+-+-+-+-+-+-+
|C|V|T|R|L|E|Res|                       (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            System ID                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          System ID            |       (6 bytes)   
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Extended Local Circuit ID  (0 or 4 bytes)          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Num of VIDs  |                       (0 or 1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|T|R|Res|     VID 1   (12 bits) |       (0 or 2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       .................
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|T|R|Res|     VID n   (12 bits) |       (0 or 2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Delay Constraint                       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Delay Constraint       |       (0 or 6 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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
+-+-+-+-+-+-+-+-+
|     Type      |                   (1 byte)
+-+-+-+-+-+-+-+-+
|    Length     |                   (1 byte)
+-+-+-+-+-+-+-+-+
| PCP |D|P| Res |                   (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Available Bandwidth  (4 bytes)                   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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
+-+-+-+-+-+-+-+-+
|     Type      |                   (1 byte)
+-+-+-+-+-+-+-+-+
|    Length     |                   (1 byte)
+-+-+-+-+-+-+-+-+
| PCP |D|  Res  |                   (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Bandwidth  (4 bytes)                   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           +-----------+
                           |     A     |
                           +-----------+
                           |     B     |
                           +-----------+
                           |     C     |
                           +-----------+
                           |     F     |
[B]<---[A]<---[I]          +-----------+
 |      ^      ^           |     A     |
 |      |      |           +-----------+
 V      |      |           |     C     |
[C]--->[F]--->[H]          +-----------+
 |             ^           |     D     |
 |             |           +-----------+
 V             |           |     E     |
[D]--->[E]--->[G]          +-----------+
                           |     G     |
                           +-----------+
                           |     H     |
                           +-----------+
                           |     I     |
                           +-----------+
                           |     A     |
                           +-----------+
                           |     F     |
                           +-----------+
                           |     H     |
                           +-----------+