ForCES Working Group                 Jamal Hadi Salim
Internet Draft                       Znyx Networks
                                     Hormuzd Khosravi
                                     Andi Kleen
                                     Alexey Kuznetsov
                                     June 2002

                   Netlink as an IP Services Protocol

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as ``work in progress.''

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

Conventions used in this document

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

1.  Abstract

     This document describes Linux Netlink, which is used in Linux both
     as an intra-kernel messaging system as well as between kernel and

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     user space.  This document is intended as informational in the con-
     text of prior art for the ForCES IETF working group.  The focus of
      document is to describe Netlink from a perspective of a protocol
     between a Forwarding Engine Component (FEC) and a Control Plane
     Component (CPC), the two components that define an IP service.

     The document ignores the ability of Netlink as a intra-kernel mes-
     saging system, as an inter-process communication scheme (IPC), or
     as a configuration tool for other non-networking or non-IP network
     services (such as decnet, etc.).

2.  Introduction

     The concept of IP Service control-forwarding separation was first
     introduced in the early 1980s by the BSD 4.4 routing sockets
     [Stevens].  The focus at that time was a simple IP(v4) forwarding
     service and how the CPC, either via a command line configuration
     tool or a dynamic route daemon, could control forwarding tables for
     that IPv4 forwarding service.

     The IP world has evolved considerably since those days.  Linux
     Netlink, when observed from a service provisioning and management
     point of view, takes routing sockets one step further by breaking
     the barrier of focus around IPv4 forwarding.  Since the Linux 2.1
     kernel, Netlink has been providing the IP service abstraction to a
     few services other than the classical RFC 1812 IPv4 forwarding.

     The motivation for this document is not to list every possible ser-
     vice for which Netlink is applied.  In fact, we leave out a lot of
     services (multicast routing, tunnelling, policy routing, etc.).
     Neither is this document intended to be a tutorial on Netlink.  The
     idea is to explain the overall Netlink view with a special focus on
     the mandatory building blocks within the ForCES charter (i.e., IPv4
     and QoS).  This document also serves to capture prior art to many
     mechanisms that are useful within the context of ForCES.  The text
     is limited to a subset of what is available in kernel 2.4.6, the
     newest kernel when this document was first written.  It is also
     limited to IPv4 functionality.

     We first give some concept definitions and then describe how
     Netlink fits in.

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2.1.  Definitions

     A Control Plane (CP) is an execution environment that may have sev-
     eral sub-components, which we refer to as CPCs.  Each CPC provides
     control for a different IP service being executed by a Forwarding
     Engine (FE) component.  This relationship means that there might be
     several CPCs on a physical CP, if it is controlling several IP ser-
     vices.  In essence, the cohesion between a CP component and an FE
     component is the service abstraction.

2.1.1.  Control Plane Components (CPCs)

     Control Plane Components encompass signalling protocols, with
     diversity ranging from dynamic routing protocols, such as OSPF
     [RFC2328], to tag distribution protocols, such as CR-LDP [RFC3036].
     Classical management protocols and activities also fall under this
     category.  These include SNMP [RFC1157], COPS [RFC2748], and pro-
     prietary CLI/GUI configuration mechanisms.

     The purpose of the control plane is to provide an execution envi-
     ronment for the above-mentioned activities with the ultimate goal
     being to configure and manage the second Network Element (NE) com-
     ponent: the FE.  The result of the configuration defines the way
     that packets traversing the FE are treated.

2.1.2.  Forwarding Engine Components (FECs)

     The FE is the entity of the NE that incoming packets (from the net-
     work into the NE) first encounter.

     The FE's service-specific component massages the packet to provide
     it with a treatment to achieve an IP service, as defined by the
     Control Plane Components for that IP service.  Different services
     will utilize different FECs.  Service modules may be chained to
     achieve a more complex service (refer to the Linux FE model,
     described later).  When built for providing a specific service, the
     FE service component will adhere to a forwarding model.

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jhs_hk_ak_ank                                draft-forces-Netlink-03.txt  Linux IP Forwarding Engine Model

                         ____      +---------------+
                    +->-| FW |---> | TCP, UDP, ... |
                    |   +----+     +---------------+
                    |                   |
                    ^                   v
                    |                  _|_
                    +----<----+       | FW |
                              |       +----+
                              ^         |
                              |         Y
                            To host    From host
                             stack     stack
                              ^         |
                              |_____    |
 Ingress                            ^   Y
 device   ____    +-------+        +|---|--+   ____   +--------+ Egress
 ->----->| FW |-->|Ingress|-->---->| Forw- |->| FW |->| Egress | device
         +----+   |  TC   |        |  ard  |  +----+  |   TC   |-->
                  +-------+        +-------+          +--------+

     The figure above shows the Linux FE model per device.  The only
     mandatory part of the datapath is the Forwarding module, which is
     RFC 1812 conformant.  The different Firewall (FW), Ingress Traffic
     Control, and Egress Traffic Control building blocks are not manda-
     tory in the datapath and may even be used to bypass the RFC 1812
     module.  These modules are shown as simple blocks in the datapath
     but, in fact, could be multiple cascaded, independent submodules
     within the indicated blocks.  More information can be found at
     [Netfilter] and [Diffserv].

     Packets arriving at the ingress device first pass through a fire-
     wall module.  Packets may be dropped, munged, etc., by the firewall
     module.  The incoming packet, depending on set policy, may then be
     passed via an Ingress Traffic Control module.  Metering and polic-
     ing activities are contained within the Ingress TC module.  Packets
     may be dropped, depending on metering results and policing poli-
     cies, at this module.  Next, the packet is subjected to the only
     non-optional module, the RFC 1812-conformant Forwarding module.
     The packet may be dropped if it is nonconformant (to the many RFCs
     complementing 1812 and 1122).  This module is a juncture point at
     which packets destined to the forwarding NE may be sent up to the
     host stack.

     Packets that are not for the NE may further traverse a policy rout-
     ing submodule (within the forwarding module), if so provisioned.

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     Another firewall module is walked next.  The firewall module can
     drop or munge/transform packets, depending on the configured sub-
     modules encountered and their policies.  If all goes well, the
     Egress TC module is accessed next.

     The Egress TC may drop packets for policing, scheduling, congestion
     control, or rate control reasons.  Egress queues exist at this
     point and any of the drops or delays may happen before or after the
     packet is queued.  All is dependent on configured module algorithms
     and policies.

2.1.3.  IP Services

     An IP service is the treatment of an IP packet within the NE.  This
     treatment is provided by a combination of both the CPC and the FEC.

     The time span of the service is from the moment when the packet
     arrives at the NE to the moment that it departs.  In essence, an IP
     service in this context is a Per-Hop Behavior.  CP components run-
     ning on NEs define the end-to-end path control for a service by
     running control/signaling protocol/management-applications.  These
     distributed CPCs unify the end-to-end view of the IP service.  As
     noted above, these CP components then define the behavior of the FE
     (and therefore the NE) for a described packet.

     A simple example of an IP service is the classical IPv4 Forwarding.
     In this case, control components, such as routing protocols (OSPF,
     RIP, etc.) and proprietary CLI/GUI configurations, modify the FE's
     forwarding tables in order to offer the simple service of forward-
     ing packets to the next hop.  Traditionally, NEs offering this sim-
     ple service are known as routers.  In the diagram below, we show a
     simple FE<->CP setup to provide an example of the classical IPv4
     service with an extension to do some basic QoS egress scheduling
     and illustrate how the setup fits in this described model.

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                               Control Plane (CP)
                              |    /^^^^^^\      /^^^^^^\         |
                              |   |        |    | COPS  |-\       |
                              |   | ospfd  |    |  PEP  |  \      |
                              |   \       /      \_____/    |     |
                            /------\_____/         |       /      |
                            | |        |           |     /        |
                            | |_________\__________|____|_________|
                            |           |          |    |
             Forwarding    ************* Netlink  layer ************
             Engine (FE)   *****************************************
              |       IPv4 forwading    |              |             |
              |       FE Service       /               /             |
              |       Component       /               /              |
              |       ---------------/---------------/---------      |
              |       |             |               /         |      |
       packet |       |     --------|--        ----|-----     |   packet
       in     |       |     |  IPv4    |      | Egress   |    |    out
       -->--->|------>|---->|Forwading |----->| QoS      |--->| ---->|->
              |       |     |          |      | Scheduler|    |      |
              |       |     -----------        ----------     |      |
              |       |                                       |      |
              |        ---------------------------------------       |
              |                                                      |

     The above diagram illustrates ospfd, an OSPF protocol control dae-
     mon, and a COPS Policy Enforcement Point (PEP) as distinct CPCs.
     The IPv4 FE component includes the IPv4 Forwarding service module
     as well as the Egress Scheduling service module.  Another service
     might add a policy forwarder between the IPv4 forwarder and the QoS
     egress scheduler.  A simpler classical service would have consti-
     tuted only the IPv4 forwarder.

     Over the years, it has become important to add aditional services
     to routers to meet emerging requirements.  More complex services
     extending classical forwarding have been added and standardized.
     These newer services might go beyond the layer 3 contents of the
     packet header.  However, the name "router," although a misnomer, is
     still used to describe these NEs.  Services (which may look beyond
     the classical L3 service headers) include firewalling, QoS in Diff-
     serv and RSVP, NAT, policy based routing, etc.  Newer control pro-
     tocols or management activities are introduced with these new ser-

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     One extreme definition of a IP service is something for which a
     service provider would be able to charge.

3.  Netlink Architecture

     Control of IP service components is defined by using templates.

     The FEC and CPC participate to deliver the IP service by communi-
     cating using these templates.  The FEC might continously get
     updates from the Control Plane Component on how to operate the ser-
     vice (e.g., for v4 forwarding or for route additions or deletions).

     The interaction between the FEC and the CPC, in the Netlink con-
     text, defines a protocol.  Netlink provides mechanisms for the CPC
     (residing in user space) and the FEC (residing in kernel space) to
     have their own protocol definition--kernel space and user space
     just mean different protection domains.  Therefore, a wire protocol
     is needed to communicate.  The wire protocol is normally provided
     by some privileged service that is able to copy between multiple
     protection domains.  We will refer to this service as the Netlink
     service.  The Netlink service can also be encapsulated in a differ-
     ent transport layer, if the CPC executes on a different node than
     the FEC.  The FEC and CPC, using Netlink mechanisms, may choose to
     define a reliable protocol between each other.  By default, how-
     ever, Netlink provides an unreliable communication.

     Note that the FEC and CPC can both live in the same memory protec-
     tion domain and use the connect() system call to create a path to
     the peer and talk to each other.  We will not discuss this mecha-
     nism further other than to say that it is available.  Throughout
     this document, we will refer interchangebly to the FEC to mean ker-
     nel space and the CPC to mean user space.  This denomination is not
     meant, however, to restrict the two components to these protection
     domains or to the same compute node.

     Note: Netlink allows participation in IP services by both service

3.1.  Netlink Logical Model

     In the diagram below we show a simple FEC<->CPC logical relation-
     ship.  We use the IPv4 forwarding FEC (NETLINK_ROUTE, which is

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     discussed further below) as an example.

                               Control Plane (CP)
                              |    /^^^^^\      /^^^^^\          |
                              |   |       |     / CPC-2 \         |
                              |   | CPC-1 |     | COPS  |          |
                              |   | ospfd |     |  PEP  |          |
                              |          /      _____/           |
                              |    _____/           |             |
                              |        |             |             |
                           ************* BROADCAST WIRE  ************
              FE---------- *****************************************.
              |       IPv4 forwading |    |           |             |
              |       FEC          |    |           |             |
              |       --------------/ ----|-----------|--------     |
              |       |            /      |           |       |     |
              |       |     .-------.  .-------.   .------.   |     |
              |       |     |Ingress|  | IPv4  |   |Egress|   |     |
              |       |     |police |  |Forward|   | QoS  |   |     |
              |       |     |_______|  |_______|   |Sched |   |     |
              |       |                             ------    |     |
              |        ---------------------------------------      |
              |                                                     |

     Netlink logically models FECs and CPCs in the form of nodes inter-
     connected to each other via a broadcast wire.

     The wire is specific to a service.  The example above shows the
     broadcast wire belonging to the extended IPv4 forwarding service.

     Nodes (CPCs or FECs as illustrated above) connect to the wire and
     register to receive specific messages.  CPCs may connect to multi-
     ple wires if it helps them to control the service better.  All
     nodes (CPCs and FECs) dump packets on the broadcast wire.  Packets
     can be discarded by the wire if they are malformed or not specifi-
     cally formatted for the wire.  Dropped packets are not seen by any
     of the nodes.  The Netlink service MAY signal an error to the
     sender if it detects a malformatted Netlink packet.

     Packets sent on the wire can be broadcast, multicast, or unicast.
     FECs or CPCs register for specific messages of interest for pro-
     cessing or just monitoring purposes.

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     Appendices 1 and 2 have a high level overview of this interaction.

3.2.  Message Format

     There are three levels to a Netlink message: The general Netlink
     message header, the IP service specific template, and the IP ser-
     vice specific data.

       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
      |                                                               |
      |                   Netlink message header                      |
      |                                                               |
      |                                                               |
      |                  IP Service Template                          |
      |                                                               |
      |                                                               |
      |                  IP Service specific data in TLVs             |
      |                                                               |

     The Netlink message is used to communicate between the FEC and CPC
     for parametrization of the FECs, asynchoronous event notification
     of FEC events to the CPCs, and statistics querying/gathering (typi-
     cally by a CPC).

     The Netlink message header is generic for all services, whereas the
     IP Service Template header is specific to a service.  Each IP Ser-
     vice then carries parametrization data (CPC->FEC direction) or
     response (FEC->CPC direction).  These parametrizations are in TLV
     (Type-Length-Value) format and are unique to the service.

3.3.  Protocol Model

     This section expands on how Netlink provides the mechanism for ser-
     vice-oriented FEC and CPC interaction.

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3.3.1.  Service Addressing

     Access is provided by first connecting to the service on the FE.
     The connection is achieved by making a socket() system call to the
     PF_NETLINK domain.  Each FEC is identified by a protocol number.
     One may open either SOCK_RAW or SOCK_DGRAM type sockets, although
     Netlink does not distinguish between the two.  The socket connec-
     tion provides the basis for the FE<->CP addressing.

     Connecting to a service is followed (at any point during the life
     of the connection) by either issuing a service-specific command
     (from the CPC to the FEC, mostly for configuration purposes), issu-
     ing a statistics-collection command, or subscribing/unsubscribing
     to service events.  Closing the socket terminates the transaction.
     Refer to Appendices 1 and 2 for examples.

3.3.2.  Netlink Message Header

     Netlink messages consist of a byte stream with one or multiple
     Netlink headers and an associated payload.  If the payload is too
     big to fit into a single message it, can be split over multiple
     Netlink messages, collectively called a multipart message.  For
     multipart messages, the first and all following headers have the
     NLM_F_MULTI Netlink header flag set, except for the last header
     which has the Netlink header type NLMSG_DONE.

     The Netlink message header is shown below.

    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
                    0               1               2             3
    |                          Length                             |
    |            Type              |           Flags              |
    |                      Sequence Number                        |
    |                      Process ID (PID)                       |

   The fields in the header are:

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          Length: 32 bits
          The length of the message in bytes, including the header.

          Type: 16 bits
          This field describes the message content.
          It can be one of the standard message types:
               NLMSG_NOOP  Message is ignored.
               NLMSG_ERROR The message signals an error and the payload
                           contains a nlmsgerr structure.  This can be looked
                           at as a NACK and typically it is from FEC to CPC.
               NLMSG_DONE  Message terminates a multipart message.

          Individual IP services specify more message types, e.g.,
          NETLINK_ROUTE service specifies several types, such as RTM_NEWLINK,
          RTM_DELROUTE, etc.

          Flags: 16 bits
          The standard flag bits used in Netlink are
                 NLM_F_REQUEST   Must be set on all request messages (typically
                                 from user space to kernel space)
                 NLM_F_MULTI     Indicates the message is part of a multipart
                                 message terminated by NLMSG_DONE
                 NLM_F_ACK       Request for an acknowledgment on success.
                                 Typical direction of request is from user
                                 space (CPC) to kernel space (FEC).
                 NLM_F_ECHO      Echo this request.  Typical direction of
                                 request is from user space (CPC) to kernel
                                 space (FEC).

          Additional flag bits for GET requests on config information in
          the FEC.
                 NLM_F_ROOT     Return the complete table instead of a
                                single entry.
                 NLM_F_MATCH    Return all entries matching criteria passed in
                                message content.
                 NLM_F_ATOMIC   Return an atomic snapshot of the table being
                                referenced.  This may require special privileges
                                because it has the potential to interrupt
                                service in the FE for a longer time.

          Convenience macros for flag bits:
                 NLM_F_DUMP     This is NLM_F_ROOT or'ed with NLM_F_MATCH

          Additional flag bits for NEW requests
                 NLM_F_REPLACE   Replace existing matching config object with
                                 this request.
                 NLM_F_EXCL      Don't replace the config object if it already

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                 NLM_F_CREATE    Create config object if it doesn't already
                 NLM_F_APPEND    Add to the end of the object list.

          For those familiar with BSDish use of such operations in route
          sockets, the equivalent translations are:

                    - BSD ADD operation equates to NLM_F_CREATE or-ed
                      with NLM_F_EXCL
                    - BSD CHANGE operation equates to NLM_F_REPLACE
                    - BSD Check operation equates to NLM_F_EXCL
                    - BSD APPEND equivalent is actually mapped to

          Sequence Number: 32 bits
          The sequence number of the message.

          Process ID (PID): 32 bits
          The PID of the process sending the message.  The PID is used by the
          kernel to multiplex to the correct sockets.  A PID of zero is used
          when sending messages to user space from the kernel.  Mechanisms for Creating Protocols

     One could create a reliable protocol between an FEC and a CPC by
     using the combination of sequence numbers, ACKs, and retransmit
     timers.  Both sequence numbers and ACKs are provided by Netlink;
     timers are provided by Linux.

     One could create a heartbeat protocol between the FEC and CPC by
     using the ECHO flags and the NLMSG_NOOP message.  The ACK Netlink Message

     This message is actually used to denote both an ACK and a NACK.
     Typically, the direction is from FEC to CPC (in response to an ACK
     request message).  However, the CPC should be able to send ACKs
     back to FEC when requested.  The semantics for this are IP service-

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       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
       0               1               2               3
      |                       Netlink message header                  |
      |                       type = NLMSG_ERROR                      |
      |                          Error code                           |
      |                       OLD Netlink message header              |

     Error code: integer (typically 32 bits)

     An error code of zero indicates that the message is an ACK
     response.  An ACK response message contains the original Netlink
     message header, which can be used to compare against (sent sequence
     numbers, etc).

     A non-zero error code message is equivalent to a Negative ACK
     (NACK).  In such a situation, the Netlink data that was sent down
     to the kernel is returned appended to the original Netlink message
     header.  An error code printable via the perror() is also set (not
     in the message header, rather in the executing environment state

3.3.3.  FE System Services' Templates

     These are services that are offered by the system for general use
     by other services.  They include the ability to configure, gather
     statistics and listen to changes in shared resources.  IP address
     management, link events, etc. fit here.  We create this section for
     these services for logical separation, despite the fact that they
     are accessed via the NETLINK_ROUTE FEC. The reason that they exist
     within NETLINK_ROUTE is due to historical cruft: the BSD 4.4 Route
     Sockets implemented them as part of the IPv4 forwarding sockets.

Network Interface Service Module

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     This service provides the ability to create, remove, or get infor-
     mation about a specific network interface. The network interface
     can be either physical or virtual and is network protocol indepen-
     dent (e.g., an x.25 interface can be defined via this message).
     The Interface service message template is shown below.

       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
       0               1               2               3
      |   Family    |   Reserved  |          Device Type              |
      |                     Interface Index                           |
      |                      Device Flags                             |
      |                      Change Mask                              |

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          Family: 8 bits
          This is always set to AF_UNSPEC.

          Device Type: 16 bits
          This defines the type of the link.  The link could be Ethernet, a
          tunnel, etc.  We are interested only in IPv4, although the link type
          is L3 protocol-independent.

          Interface Index: 32 bits
          Uniquely identifies interface.

          Device Flags: 32 bits

                 IFF_UP            Interface is administrativel up.
                 IFF_BROADCAST     Valid broadcast address set.
                 IFF_DEBUG         Internal debugging flag.
                 IFF_LOOPBACK      Interface is a loopback interface.
                 IFF_POINTOPOINT   Interface is a point-to-point link.
                 IFF_RUNNING       Interface is operationally up.
                 IFF_NOARP         No ARP protocol needed for this interface.
                 IFF_PROMISC       Interface is in promiscuous mode.
                 IFF_NOTRAILERS    Avoid use of trailers.
                 IFF_ALLMULTI      Receive all multicast packets.
                 IFF_MASTER        Master of a load balancing bundle.
                 IFF_SLAVE         Slave of a load balancing bundle.
                 IFF_MULTICAST     Supports multicast
                 IFF_PORTSEL       Is able to select media type via ifmap.
                 IFF_AUTOMEDIA     Auto media selection active.
                 IFF_DYNAMIC       Interface was dynamically created.

           Change Mask: 32 bits
           Reserved for future use.  Must be set to 0xFFFFFFFF.

           Applicable attributes:
                  Attribute            Description
                  IFLA_UNSPEC          Unspecified.
                  IFLA_ADDRESS         Hardware address interface L2 address.
                  IFLA_BROADCAST       Hardware address L2 broadcast
                  IFLA_IFNAME          ASCII string device name.
                  IFLA_MTU             MTU of the device.
                  IFLA_LINK            ifindex of link to which this device
                                       is bound.
                  IFLA_QDISC           ASCII string defining egress root
                                       queueing discipline.
                  IFLA_STATS           Interface statistics.

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          Netlink message types specific to this service:
          RTM_NEWLINK, RTM_DELLINK, and RTM_GETLINK  IP Address Service Module

This service provides the ability to add, remove, or receive information
about an IP address associated with an interface.  The address provi-
sioning service message template is shown below.

 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
        0             1              2             3
 |   Family    |     Length    |     Flags     |    Scope      |
 |                     Interface Index                         |

 Family: 8 bits
 Address Family: AF_INET for IPv4; and AF_INET6 for IPV4.

 Length: 8 bits
 The length of the address mask.

 Flags: 8 bits
 IFA_F_SECONDARY  For secondary address (alias interface).
 IFA_F_PERMANENT  For a permanent address set by the user.
                  When this is not set, it means the address
                  was dynamically created (e.g., by stateless
 IFA_F_DEPRECATED Defines deprecated (IPV4) address.
 IFA_F_TENTATIVE  Defines tentative (IPV4) address (duplicate
                  address detection is still in progress).

 Scope: 8 bits
 The address scope in which the address stays valid.
        SCOPE_UNIVERSE: Global scope.
        SCOPE_SITE (IPv6 only): Only valid within this site.
        SCOPE_LINK: Valid only on this device.
        SCOPE_HOST: Valid only on this host.

     Applicable attributes:

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             Attribute             Description
                   IFA_UNSPEC      Unspecified.
                   IFA_ADDRESS     Raw protocol address of interface.
                   IFA_LOCAL       Raw protocol local address.
                   IFA_LABEL       ASCII string name of the interface.
                   IFA_BROADCAST   Raw protocol broadcast address.
                   IFA_ANYCAST     Raw protocol anycast address.
                   IFA_CACHEINFO   Cache address information.

     Netlink messages specific to this service: RTM_NEWADDR,

4.  Currently Defined Netlink IP Services

     Although there are many other IP services defined that are using
     Netlink, as mentioned earlier, we will talk only about a handful of
     those integrated into kernel version 2.4.6.  These are:


4.1.  IP Service NETLINK_ROUTE

     This service allows CPCs to modify the IPv4 routing table in the
     Forwarding Engine.  It can also be used by CPCs to receive routing
     updates, as well as to collect statistics.

4.1.1.  Network Route Service Module

     This service provides the ability to create, remove or receive
     information about a network route.  The service message template is
     shown below.

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      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
                      0               1               2             3
      |   Family    |  Src length   |  Dest length  |     TOS       |
      |  Table ID   |   Protocol    |     Scope     |     Type      |
      |                          Flags                              |

      Family: 8 bits
      Address Family: AF_INET for IPv4; and AF_INET6 for IPV4.

      Src length: 8 bits
      Prefix length of source IP address.

      Dest length: 8 bits
      Prefix length of destination IP address.

      TOS: 8 bits
      The 8-bit TOS (should be deprecated to make room for DSCP).

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      Table ID: 8 bits
      Table identifier.  Up to 255 route tables are supported.
                    RT_TABLE_UNSPEC    An unspecified routing table.
                    RT_TABLE_DEFAULT   The default table.
                    RT_TABLE_MAIN      The main table.
                    RT_TABLE_LOCAL     The local table.

                    The user may assign arbitary values between
                    RT_TABLE_UNSPEC(0) and RT_TABLE_DEFAULT(253).

      Protocol: 8 bits
      Identifies what/who added the route.
                    Protocol          Route origin.
                    RTPROT_UNSPEC     Unknown.
                    RTPROT_REDIRECT   By an ICMP redirect.
                    RTPROT_KERNEL     By the kernel.
                    RTPROT_BOOT       During bootup.
                    RTPROT_STATIC     By the administrator.

      Values larger than RTPROT_STATIC(4) are not interpreted by the
      kernel, they are just for user information.  They may be used to
      tag the source of a routing information or to distingush between
      multiple routing daemons.  See <linux/rtnetlink.h> for the
      routing daemon identifiers that are already assigned.

      Scope: 8 bits
      Route scope (valid distance to destination).
                    RT_SCOPE_UNIVERSE   Global route.
                    RT_SCOPE_SITE       Interior route in the
                                        local autonomous system.
                    RT_SCOPE_LINK       Route on this link.
                    RT_SCOPE_HOST       Route on the local host.
                    RT_SCOPE_NOWHERE    Destination does not exist.

      The values between RT_SCOPE_UNIVERSE(0) and RT_SCOPE_SITE(200)
      are available to the user.

      Type: 8 bits
      The type of route.

                    Route type        Description
                    RTN_UNSPEC        Unknown route.
                    RTN_UNICAST       A gateway or direct route.
                    RTN_LOCAL         A local interface route.
                    RTN_BROADCAST     A local broadcast route

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                                      (sent as a broadcast).
                    RTN_ANYCAST       An anycast route.
                    RTN_MULTICAST     A multicast route.
                    RTN_BLACKHOLE     A silent packet dropping route.
                    RTN_UNREACHABLE   An unreachable destination.
                                      Packets dropped and host
                                      unreachable ICMPs are sent to the
                    RTN_PROHIBIT      A packet rejection route.  Packets
                                      are dropped and communication
                                      prohibited ICMPs are sent to the
                    RTN_THROW         When used with policy routing,
                                      continue routing lookup in another
                                      table.  Under normal routing,
                                      packets are dropped and net
                                      unreachable ICMPs are sent to the
                    RTN_NAT           A network address translation
                    RTN_XRESOLVE      Refer to an external resolver (not

      Flags: 32 bits
      Further qualify the route.
                    RTM_F_NOTIFY     If the route changes, notify the
                    RTM_F_CLONED     Route is cloned from another route.
                    RTM_F_EQUALIZE   Allow randomization of next hop
                                     path in multi-path routing
                                     (currently not implemented).

      Attributes applicable to this service:
                    Attribute       Description
                    RTA_UNSPEC      Ignored.
                    RTA_DST         Protocol address for route
                                    destination address.
                    RTA_SRC         Protocol address for route source
                    RTA_IIF         Input interface index.
                    RTA_OIF         Output interface index.
                    RTA_GATEWAY     Protocol address for the gateway of
                                    the route
                    RTA_PRIORITY    Priority of route.
                    RTA_PREFSRC     Preferred source address in cases

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                                    where more than one source address
                                    could be used.
                    RTA_METRICS     Route metrics attributed to route
                                    and associated protocols (e.g.,
                                    RTT, initial TCP window, etc.).
                    RTA_MULTIPATH   Multipath route next hop's
                    RTA_PROTOINFO   Firewall based policy routing
                    RTA_FLOW        Route realm.
                    RTA_CACHEINFO   Cached route information.

     Additional Netlink message types applicable to this service:

4.1.2.  Neighbour Setup Service Module

     This service provides the ability to add, remove, or receive infor-
     mation about a neighbour table entry (e.g., an ARP entry or an IPv4
     neighbour solicitation, etc.).  The service message template is
     shown below.

      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
      0               1               2               3
      |   Family    |    Reserved1  |           Reserved2           |
      |                     Interface Index                         |
      |           State             |     Flags     |     Type      |

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      Family: 8 bits
      Address Family: AF_INET for IPv4; and AF_INET6 for IPV4.

      Interface Index: 32 bits
      The unique interface index.

      State: 16 bits
      A bitmask of the following states:
                    NUD_INCOMPLETE   Still attempting to resolve.
                    NUD_REACHABLE    A confirmed working cache entry
                    NUD_STALE        an expired cache entry.
                    NUD_DELAY        Neighbour no longer reachable.
                                     Traffic sent, waiting for
                    NUD_PROBE        A cache entry that is currently
                                     being re-solicited.
                    NUD_FAILED       An invalid cache entry.
                    NUD_NOARP        A device which does not do neighbor
                                     discovery (ARP).
                    NUD_PERMANENT    A static entry.

      Flags: 8 bits
                    NTF_PROXY        A proxy ARP entry.
                    NTF_ROUTER       An IPv6 router.

      Attributes applicable to this service:
                    Attributes      Description
                    NDA_UNSPEC      Unknown type.
                    NDA_DST         A neighbour cache network.
                                    layer destination address
                    NDA_LLADDR      A neighbour cache link layer
                    NDA_CACHEINFO   Cache statistics.

     Additional Netlink message types applicable to this service:

4.1.3.  Traffic Control Service

     This service provides the ability to provision, query or listen to
     events under the auspicies of traffic control.  These include
     queueing disciplines, (schedulers and queue treatment

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     algorithms--e.g., priority-based scheduler or the RED algorithm)
     and classifiers.  Linux Traffic Control Service is very flexible
     and allows for hierachical cascading of the different blocks for
     traffic resource sharing.

            ++    ++                 +-----+   +-------+   ++     ++ .++
            || .  ||     +------+    |     |-->| Qdisc |-->||     ||  ||
            ||    ||---->|Filter|--->|Class|   +-------+   ||-+   ||  ||
            ||    ||  |  +------+    |     +---------------+| |   ||  ||
            || .  ||  |              +----------------------+ |   || .||
            || .  ||  |  +------+                             |   ||  ||
            ||    ||  +->|Filter|-_  +-----+   +-------+   ++ |   || .||
            || -->||  |  +------+  ->|     |-->| Qdisc |-->|| |   ||->||
            || .  ||  |              |Class|   +-------+   ||-+-->|| .||
     ->dev->||    ||  |  +------+ _->|     +---------------+|     ||  ||
            ||    ||  +->|Filter|-   +----------------------+     || .||
            ||    ||     +------+                                 || .||
            || .  |+----------------------------------------------+|  ||
            ||    |          Parent Queuing discipline             | .||
            || .  +------------------------------------------------+ .||
            || . . .. . . .. . .                 . .. .. .. .      .. ||
            |                 Parent Queuing discipline                |
            |                  (attached to egress device)             |

     The above diagram shows an example of the Egress TC block.  We try
     to be very brief here.  For more information, please refer to
     [Diffserv].  A packet first goes through a filter that is used to
     identify a class to which the packet may belong.  A class is essen-
     tially a terminal queueing discipline and has a queue associated
     with it.  The queue may be subject to a simple algorithm, like
     FIFO, or a more complex one, like RED or a token bucket.  The out-
     ermost queueing discipline, which is refered to as the parent is
     typically associated with a scheduler.  Within this scheduler hier-
     archy, however, may be other scheduling algorithms, making the
     Linux Egress TC very flexible.

     The service message template that makes this possible is shown
     below.  This template is used in both the ingress and the egress
     queueing disciplines (refer to the egress traffic control model in
     the FE model section).  Each of the specific components of the
     model has unique attributes that describe it best.  The common
     attributes are described below.

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      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
      0               1               2               3
      |   Family    |  Reserved1    |         Reserved2             |
      |                     Interface Index                         |
      |                      Qdisc handle                           |
      |                     Parent Qdisc                            |
      |                        TCM Info                             |

      Family: 8 bits
      Address Family: AF_INET for IPv4; and AF_INET6 for IPV4.

      Interface Index: 32 bits
      The unique interface index.

      Qdisc handle: 32 bits
      Unique identifier for instance of queueing discipline.  Typically,
      this is split into major:minor of 16 bits each.  The major number
      would also be the major number of the parent of this instance.

      Parent Qdisc: 32 bits
      Used in hierarchical layering of queueing disciplines.  If this
      value and the Qdisc handle are the same and equal to TC_H_ROOT,
      then the defined qdisc is the top most layer known as the root

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      TCM Info: 32 bits
      Set by the FE to 1 typically, except when the Qdisc instance is in
      use, in which case it is set to imply a reference count.  From the
      CPC towards the direction of the FEC, this is typically set to 0
      except when used in the context of filters.  In that case, this
      32-bit field is split into a 16-bit priority field and 16-bit
      protocol field.  The protocol is defined in kernel source
      <include/linux/if_ether.h>, however, the most commonly used one
      is ETH_P_IP (the IP protocol).

      The priority is used for conflict resolution when filters
      intersect in their expressions.

      Generic attributes applicable to this service:

                   Attribute        Description
                   TCA_KIND         Canonical name of FE component.
                   TCA_STATS        Generic usage statistics of FEC
                   TCA_RATE         rate estimator being attached to
                                    FEC.  Takes snapshots of stats to
                                    compute rate.
                   TCA_XSTATS       Specific statistics of FEC.
                   TCA_OPTIONS      Nested FEC-specific attributes.

     Appendix 3 has an example of configuring an FE component for a FIFO

     Additional Netlink message types applicable to this service:


     This service allows CPCs to receive, manipulate, and re-inject
     packets via the IPv4 firewall service modules in the FE.  A fire-
     wall rule is first inserted to activate packet redirection.  The
     CPC informs the FEC whether it would like to receive just the meta-
     data on the packet or the actual data and, if the metadata is
     desired, what is the maximum data length to be redirected.  The
     redirected packets are still stored in the FEC, waiting a verdict

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     from the CPC.  The verdict could constitute a simple accept or drop
     decision of the packet, in which case the verdict is imposed on the
     packet still sitting on the FEC.  The verdict may also include a
     modified packet to be sent on as a replacement.

     Two types of messages exist that can be sent from CPC to FEC.
     These are: Mode messages and Verdict messages.  Mode messages are
     sent immediately to the FEC to describe what the CPC would like to
     receive.  Verdict messages are sent to the FEC after a decision has
     been made on the fate of a received packet.  The formats are
     described below.

      The mode message is described first.

      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
      0             1               2               3
      |   Mode    |    Reserved1  |           Reserved2             |
      |                         Range                               |

      Mode: 8 bits
      Control information on the packet to be sent to the CPC.  The
      different types are:

             IPQ_COPY_META   Copy only packet metadata to CPC.
             IPQ_COPY_PACKET Copy packet metadata and packet payloads
                             to CPC.

      Range: 32 bits
      If IPQ_COPY_PACKET, this defines the maximum length to copy.

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      A packet and associated metadata received from user space looks
      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
                    0               1               2               3
      |                       Packet ID                             |
      |                          Mark                               |
      |                       timestamp_m                           |
      |                       timestamp_u                           |
      |                          hook                               |
      |                       indev_name                            |
      |                       outdev_name                           |
      |           hw_protocol       |        hw_type                |
      |         hw_addrlen          |           Reserved            |
      |                       hw_addr                               |
      |                       data_len                              |
      |                      Payload . . .                          |

      Packet ID: 32 bits
      The unique packet identifier as passed to the CPC by the FEC.

      Mark: 32 bits
      The internal metadata value set to describe the rule in which
      the packet was picked.

      timestamp_m: 32 bits
      Packet arrival time (seconds)

      timestamp_u: 32 bits
      Packet arrival time (useconds in addition to the seconds in

      hook: 32 bits

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      The firewall module from which the packet was picked.

      indev_name: 128 bits
      ASCII name of incoming interface.

      outdev_name: 128 bits
      ASCII name of outgoing interface.

      hw_protocol: 16 bits
      Hardware protocol, in network order.

      hw_type: 16 bits
      Hardware type.

      hw_addrlen: 8 bits
      Hardware address length.

      hw_addr: 64 bits
      Hardware address.

      data_len: 32 bits
      Length of packet data.

      Payload: size defined by data_len
      The payload of the packet received.

      The Verdict message format 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
                   0               1               2               3
      |                         Value                               |
      |                       Packet ID                             |
      |                      Data Length                            |
      |                      Payload . . .                          |

      Value: 32 bits
      This is the verdict to be imposed on the packet still sitting
      in the FEC. Verdicts could be:
              NF_ACCEPT   Accept the packet and let it continue its
              NF_DROP     Drop the packet.

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      Packet ID: 32 bits
      The packet identifier as passed to the CPC by the FEC.

      Data Length: 32 bits
      The data length of the modified packet (in bytes). If you dont
      modify the packet just set it to 0.

      Size as defined by the Data Length field.

4.3.  IP Service NETLINK_ARPD

     This service is used by CPCs for managing the neighbor table in the
     FE.  The message format used between the FEC and CPC is described
     in the section on the Neighbour Setup Service Module.

     The CPC service is expected to participate in neighbor solicitation

     A neighbor message of type RTM_NEWNEIGH is sent towards the CPC by
     the FE to inform the CPC of changes that might have happened on
     that neighbour's entry (e.g., a neighbor being perceived as

     RTM_GETNEIGH is used to solicit the CPC for information on a spe-
     cific neighbor.

5.  Security Considerations

     Netlink lives in a trusted environment of a single host separated
     by kernel and user space.  Linux capabilities ensure that only
     someone with CAP_NET_ADMIN capability (typically, the root user) is
     allowed to open sockets.

6.  References

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        [RFC1633]  R. Braden, D. Clark, and S. Shenker, "Integrated
     Services in the Internet Architecture: an Overview", RFC 1633,
     ISI, MIT, and PARC, June 1994.

        [RFC1812]  F. Baker, "Requirements for IP Version 4
     Routers", RFC 1812, June 1995.

        [RFC2475]  M. Carlson, W. Weiss, S. Blake, Z. Wang, D.
     Black, and E.  Davies, "An Architecture for Differentiated
     Services", RFC 2475, December 1998.

        [RFC2748] J. Boyle, R. Cohen, D. Durham, S. Herzog, R.
     Rajan, A. Sastry, "The COPS (Common Open Policy Service) Pro-
     tocol", RFC 2748, January 2000.

        [RFC2328] J. Moy, "OSPF Version 2", RFC 2328, April 1998.

        [RFC1157] J.D. Case, M. Fedor, M.L. Schoffstall, C. Davin,
     "Simple Network Management Protocol (SNMP)", RFC 1157, May

        [RFC3036] L. Andersson, P. Doolan, N. Feldman, A. Fredette,
     B. Thomas "LDP Specification", RFC 3036, January 2001.

        [Stevens] G.R Wright, W. Richard Stevens.  "TCP/IP Illus-
     trated Volume 2, Chapter 20", June 1995



7.  Acknowledgements

1)   Andi Kleen, for man pages on netlink and rtnetlink.

2)   Alexey Kuznetsov is credited for extending Netlink to the IP ser-
     vice delivery model.  The original Netlink character device was

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     written by Alan Cox.

3)   Jeremy Ethridge for taking the role of someone who did not under-
     stand Netlink and reviewing the document to make sure that it made

8.  Author's  Address:

   Jamal Hadi Salim
   Znyx Networks
   Ottawa, Ontario

   Hormuzd M Khosravi
   2111 N.E. 25th Avenue JF3-206
   Hillsboro OR 97124-5961
   1 503 264 0334

   Andi Kleen
   Stahlgruberring 28
   81829 Muenchen

   Alexey Kuznetsov

9.  Appendix 1: Sample Service Hierachy

     In the diagram below we show a simple IP service, foo, and the
     interaction it has between CP and FE components for the service
     (labels 1-3).

     The diagram is also used to demonstrate CP<->FE addressing.  In
     this section, we illustrate only the addressing semantics.  In
     Apendix 2, the diagram is referenced again to define the protocol
     interaction between service foo's CPC and FEC (labels 4-10).

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      |   .-----.                                              |
      |  |                        . -------.                  |
      |  |  CLI   |               /                           |
      |  |        |              | CP protocol                |
      |         /->> -.         |  component  | <-.           |
      |    __ _/      |         |   For       |   |           |
      |                |         | IP service  |   ^           |
      |                Y         |    foo      |   |           |
      |                |          ___________/    ^           |
      |                Y   1,4,6,8,9 /  ^ 2,5,10   | 3,7       |
       --------------- Y------------/---|----------|-----------
                       |           ^    |          ^
                     ************* Netlink  layer ************
             FE        |           |    ^          ^
             .-------- Y-----------Y----|--------- |----.
             |                    |              /     |
             |                    Y            /       |
             |          . --------^-------.  /         |
             |          |FE component/module|/          |
             |          |  for IP Service   |           |
      --->---|------>---|     foo           |----->-----|------>--
             |           -------------------            |
             |                                          |
             |                                          |

     The control plane protocol for IP service foo does the following to
     connect to its FE counterpart.  The steps below are also numbered
     above in the diagram.

1)   Connect to the IP service foo through a socket connect.  A typical
     connection would be via a call to: socket(AF_NETLINK, SOCK_RAW,

2)   Bind to listen to specific asynchronous events for service foo.

3)   Bind to listen to specific asynchronous FE events.

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10.  Appendix 2: Sample Protocol for the Foo IP Service

     Our example IP service foo is used again to demonstrate how one can
     deploy a simple IP service control using Netlink.

     These steps are continued from Appendix 1 (hence the numbering).

4)   Query for current config of FE component.

5)   Receive response to (4) via channel on (3).

6)   Query for current state of IP service foo.

7)   Receive response to (6) via channel on (2).

9)   Register the protocol-specific packets you would like the FE to
     forward to you.

10)  Send service-specific foo commands and receive responses for them,
     if needed.

10.1.  Interacting with Other IP services

     The diagram in Appendix 1 shows another control component configur-
     ing the same service.  In this case, it is a proprietary Command
     Line Interface.  The CLI may or may not be using the Netlink proto-
     col to communicate to the foo component.  If the CLI issues com-
     mands that will affect the policy of the FEC for service foo then,
     then the foo CPC is notified.  It could then make algorithmic deci-
     sions based on this input.  For example, if an FE allowed another
     service to delete policies installed by a different service and a
     policy that foo installed was deleted by service bar, there might
     be a need to propagate this to all the peers of service foo.

11.  Appendix 3: Examples

     In this example, we show a simple configuration Netlink message
     sent from a TC CPC to an egress TC FIFO queue.  This queue algo-
     rithm is based on packet counting and drops packets when the limit
     exceeds 100 packets.  We assume that the queue is in a hierachical
     setup with a parent 100:0 and a classid of 100:1 and that it is to
     be installed on a device with an ifindex of 4.

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      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
                      0               1               2             3
      |                          Length (52)                        |
      | Type (RTM_NEWQDISC)           | Flags (NLM_F_EXCL |         |
      |                               |NLM_F_CREATE | NLM_F_REQUEST)|
      |                      Sequence Number(arbitrary number)      |
      |                        Process ID (0)                       |
      |Family(AF_INET)|  Reserved1    |         Reserved1           |
      |                     Interface Index  (4)                    |
      |                      Qdisc handle  (0x1000001)              |
      |                     Parent Qdisc   (0x1000000)              |
      |                        TCM Info  (0)                        |
      |            Type (TCA_KIND)   |           Length(4)          |
      |                        Value ("pfifo")                      |
      |            Type (TCA_OPTIONS) |          Length(4)          |
      |                        Value (limit=100)                    |

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