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rfc:rfc3549

Network Working Group J. Salim Request for Comments: 3549 Znyx Networks Category: Informational H. Khosravi

                                                                 Intel
                                                              A. Kleen
                                                                  Suse
                                                          A. Kuznetsov
                                                            INR/Swsoft
                                                             July 2003
              Linux Netlink as an IP Services Protocol

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

 This document describes Linux Netlink, which is used in Linux both as
 an intra-kernel messaging system as well as between kernel and user
 space.  The focus of this document is to describe Netlink's
 functionality as a protocol between a Forwarding Engine Component
 (FEC) and a Control Plane Component (CPC), the two components that
 define an IP service.  As a result of this focus, this document
 ignores other uses of Netlink, including its use as a intra-kernel
 messaging 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.).
 This document is intended as informational in the context of prior
 art for the ForCES IETF working group.

Salim, et. al. Informational [Page 1] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

Table of Contents

 1.  Introduction ...............................................  2
     1.1. Definitions ...........................................  3
          1.1.1.  Control Plane Components (CPCs)................  3
          1.1.2.  Forwarding Engine Components (FECs)............  3
          1.1.3.  IP Services ...................................  5
 2.  Netlink Architecture .......................................  7
     2.1. Netlink Logical Model .................................  8
     2.2. Message Format.........................................  9
     2.3. Protocol Model.........................................  9
          2.3.1.  Service Addressing............................. 10
          2.3.2.  Netlink Message Header......................... 10
          2.3.3.  FE System Services' Templates.................. 13
 3.  Currently Defined Netlink IP Services....................... 16
     3.1. IP Service NETLINK_ROUTE............................... 16
          3.1.1.  Network Route Service Module................... 16
          3.1.2.  Neighbor Setup Service Module.................. 20
          3.1.3.  Traffic Control Service........................ 21
     3.2. IP Service NETLINK_FIREWALL............................ 23
     3.3. IP Service NETLINK_ARPD................................ 27
 4.  References.................................................. 27
     4.1. Normative References................................... 27
     4.2. Informative References................................. 28
 5.  Security Considerations..................................... 28
 6.  Acknowledgements............................................ 28
 Appendix 1:  Sample Service Hierarchy .......................... 29
 Appendix 2:  Sample Protocol for the Foo IP Service............. 30
 Appendix 2a: Interacting with Other IP services................. 30
 Appendix 3:  Examples........................................... 31
 Authors' Addresses.............................................. 32
 Full Copyright Statement........................................ 33

1. Introduction

 The concept of IP Service control-forwarding separation was first
 introduced in the early 1990s by the BSD 4.4 routing sockets [9].
 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.

Salim, et. al. Informational [Page 2] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

 The motivation for this document is not to list every possible
 service for which Netlink is applied.  In fact, we leave out a lot of
 services (multicast routing, tunneling, 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.

1.1. Definitions

 A Control Plane (CP) is an execution environment that may have
 several 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
 services.  In essence, the cohesion between a CP component and an FE
 component is the service abstraction.

1.1.1. Control Plane Components (CPCs)

 Control Plane Components encompass signalling protocols, with
 diversity ranging from dynamic routing protocols, such as OSPF [5],
 to tag distribution protocols, such as CR-LDP [7]. Classical
 management protocols and activities also fall under this category.
 These include SNMP [6], COPS [4], and proprietary CLI/GUI
 configuration mechanisms.  The purpose of the control plane is to
 provide an execution environment for the above-mentioned activities
 with the ultimate goal being to configure and manage the second
 Network Element (NE) component: the FE.  The result of the
 configuration defines the way that packets traversing the FE are
 treated.

1.1.2. Forwarding Engine Components (FECs)

 The FE is the entity of the NE that incoming packets (from the
 network 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

Salim, et. al. Informational [Page 3] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

 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.

1.1.2.1. 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 mandatory
 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 [10] and [11].
 Packets arriving at the ingress device first pass through a firewall
 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 policing
 activities are contained within the Ingress TC module.  Packets may
 be dropped, depending on metering results and policing policies, 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.

Salim, et. al. Informational [Page 4] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

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

1.1.3. 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 running
 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 forwarding
 packets to the next hop.  Traditionally, NEs offering this simple
 service are known as routers.

Salim, et. al. Informational [Page 5] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

 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.
                         Control Plane (CP)
                        .------------------------------------
                        |    /^^^^^^\      /^^^^^^\         |
                        |   |        |    | COPS  |-\       |
                        |   | ospfd  |    |  PEP  |  \      |
                        |   \       /      \_____/    |     |
                      /------\_____/         |       /      |
                      | |        |           |     /        |
                      | |_________\__________|____|_________|
                      |           |          |    |
                     ******************************************
       Forwarding    ************* Netlink  layer ************
       Engine (FE)   *****************************************
        .-------------|-----------|----------|---|-------------
        |       IPv4 forwarding   |              |             |
        |       FE Service       /               /             |
        |       Component       /               /              |
        |       ---------------/---------------/---------      |
        |       |             |               /         |      |
 packet |       |     --------|--        ----|-----     |   packet
 in     |       |     |  IPv4    |      | Egress   |    |    out
 -->--->|------>|---->|Forwarding|----->| QoS      |--->| ---->|->
        |       |     |          |      | Scheduler|    |      |
        |       |     -----------        ----------     |      |
        |       |                                       |      |
        |        ---------------------------------------       |
        |                                                      |
        -------------------------------------------------------
 The above diagram illustrates ospfd, an OSPF protocol control daemon,
 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 constituted only
 the IPv4 forwarder.
 Over the years, it has become important to add additional 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

Salim, et. al. Informational [Page 6] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

 the classical L3 service headers) include firewalling, QoS in
 Diffserv and RSVP, NAT, policy based routing, etc.  Newer control
 protocols or management activities are introduced with these new
 services.
 One extreme definition of a IP service is something for which a
 service provider would be able to charge.

2. Netlink Architecture

 Control of IP service components is defined by using templates.
 The FEC and CPC participate to deliver the IP service by
 communicating using these templates.  The FEC might continuously get
 updates from the Control Plane Component on how to operate the
 service (e.g., for v4 forwarding or for route additions or
 deletions).
 The interaction between the FEC and the CPC, in the Netlink context,
 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 different
 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, however, Netlink
 provides an unreliable communication.
 Note that the FEC and CPC can both live in the same memory protection
 domain and use the connect() system call to create a path to the peer
 and talk to each other.  We will not discuss this mechanism further
 other than to say that it is available. Throughout this document, we
 will refer interchangeably to the FEC to mean kernel 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
 components.

Salim, et. al. Informational [Page 7] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

2.1. Netlink Logical Model

 In the diagram below we show a simple FEC<->CPC logical relationship.
 We use the IPv4 forwarding FEC (NETLINK_ROUTE, which is discussed
 further below) as an example.
                  Control Plane (CP)
                 .------------------------------------
                 |    /^^^^^\        /^^^^^\          |
                 |   |       |      / CPC-2 \         |
                 |   | CPC-1 |     | COPS   |         |
                 |   | ospfd |     |  PEP   |         |
                 |   |      /       \____ _/          |
                 |    \____/            |             |
                 |      |               |             |
              ****************************************|
              ************* BROADCAST WIRE  ************
 FE---------- *****************************************.
 |      IPv4 forwarding |    |           |             |
 |               FEC    |    |           |             |
 |       --------------/ ----|-----------|--------     |
 |       |            /      |           |       |     |
 |       |     .-------.  .-------.   .------.   |     |
 |       |     |Ingress|  | IPv4  |   |Egress|   |     |
 |       |     |police |  |Forward|   | QoS  |   |     |
 |       |     |_______|  |_______|   |Sched |   |     |
 |       |                             ------    |     |
 |        ---------------------------------------      |
 |                                                     |
  -----------------------------------------------------
 Netlink logically models FECs and CPCs in the form of nodes
 interconnected 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 multiple
 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 specifically
 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.

Salim, et. al. Informational [Page 8] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

 Packets sent on the wire can be broadcast, multicast, or unicast.
 FECs or CPCs register for specific messages of interest for
 processing or just monitoring purposes.
 Appendices 1 and 2 have a high level overview of this interaction.

2.2. Message Format

 There are three levels to a Netlink message: The general Netlink
 message header, the IP service specific template, and the IP service
 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 parameterization of the FECs, asynchronous event notification of
 FEC events to the CPCs, and statistics querying/gathering (typically
 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 Service
 then carries parameterization data (CPC->FEC direction) or response
 (FEC->CPC direction).  These parameterizations are in TLV (Type-
 Length-Value) format and are unique to the service.
 The different parts of the netlink message are discussed in the
 following sections.

2.3. Protocol Model

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

Salim, et. al. Informational [Page 9] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

2.3.1. 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 connection 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), issuing a
 statistics-collection command, or subscribing/unsubscribing to
 service events.  Closing the socket terminates the transaction.
 Refer to Appendices 1 and 2 for examples.

2.3.2. 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                          Length                             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Type              |           Flags              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      Sequence Number                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      Process ID (PID)                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Salim, et. al. Informational [Page 10] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

 The fields in the header are:
 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_DELLINK, RTM_GETLINK, RTM_NEWADDR, RTM_DELADDR, RTM_NEWROUTE,
 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

Salim, et. al. Informational [Page 11] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

 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
                        exists.
        NLM_F_CREATE    Create config object if it doesn't already
                        exist.
        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:
  1. BSD ADD operation equates to NLM_F_CREATE or-ed

with NLM_F_EXCL

  1. BSD CHANGE operation equates to NLM_F_REPLACE
  2. BSD Check operation equates to NLM_F_EXCL
  3. BSD APPEND equivalent is actually mapped to

NLM_F_CREATE

 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.

2.3.2.1. 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.

Salim, et. al. Informational [Page 12] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

2.3.2.2. 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
 specific.
  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                  |
 |                       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 variable).

2.3.3. 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.

Salim, et. al. Informational [Page 13] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

2.3.3.1. Network Interface Service Module

 This service provides the ability to create, remove, or get
 information about a specific network interface.  The network
 interface can be either physical or virtual and is network protocol
 independent (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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Family    |   Reserved  |          Device Type              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     Interface Index                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      Device Flags                             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      Change Mask                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 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 administratively 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.

Salim, et. al. Informational [Page 14] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

        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
                             address.
        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
                             queuing discipline.
        IFLA_STATS           Interface statistics.
 Netlink message types specific to this service:
 RTM_NEWLINK, RTM_DELLINK, and RTM_GETLINK

2.3.3.2. 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 provisioning 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Family    |     Length    |     Flags     |    Scope      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     Interface Index                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Family: 8 bits
 Address Family: AF_INET for IPv4; and AF_INET6 for IPV6.
 Length: 8 bits
 The length of the address mask.
 Flags: 8 bits
 IFA_F_SECONDARY  For secondary address (alias interface).

Salim, et. al. Informational [Page 15] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

 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
                  autoconfiguration).
 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.
 le attributes:
 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,
 RTM_DELADDR, and RTM_GETADDR.

3. 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:
    NETLINK_ROUTE, NETLINK_FIREWALL, and NETLINK_ARPD.

3.1. 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.

3.1.1. 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.

Salim, et. al. Informational [Page 16] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   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 IPV6.
 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).
 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 arbitrary 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 distinguish between
 multiple routing daemons.  See <linux/rtnetlink.h> for the
 routing daemon identifiers that are already assigned.

Salim, et. al. Informational [Page 17] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

 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
                                 (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
                                 originator.
               RTN_PROHIBIT      A packet rejection route.  Packets
                                 are dropped and communication
                                 prohibited ICMPs are sent to the
                                 originator.
               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
                                 originator.
               RTN_NAT           A network address translation
                                 rule.
               RTN_XRESOLVE      Refer to an external resolver (not
                                 implemented).

Salim, et. al. Informational [Page 18] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

 Flags: 32 bits
 Further qualify the route.
               RTM_F_NOTIFY     If the route changes, notify the
                                user.
               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
                               address.
               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
                               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
                               attributes.
               RTA_PROTOINFO   Firewall based policy routing
                               attribute.
               RTA_FLOW        Route realm.
               RTA_CACHEINFO   Cached route information.
 Additional Netlink message types applicable to this service:
 RTM_NEWROUTE, RTM_DELROUTE, and RTM_GETROUTE

Salim, et. al. Informational [Page 19] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

3.1.2. Neighbor Setup Service Module

 This service provides the ability to add, remove, or receive
 information about a neighbor table entry (e.g., an ARP entry or an
 IPv4 neighbor 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Family    |    Reserved1  |           Reserved2           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     Interface Index                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           State             |     Flags     |     Type      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Family: 8 bits
 Address Family: AF_INET for IPv4; and AF_INET6 for IPV6.
 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        Neighbor no longer reachable.
                                Traffic sent, waiting for
                                confirmation.
               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.

Salim, et. al. Informational [Page 20] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

 Attributes applicable to this service:
               Attributes      Description
               ------------------------------------
               NDA_UNSPEC      Unknown type.
               NDA_DST         A neighbour cache network.
                               layer destination address
               NDA_LLADDR      A neighbor cache link layer
                               address.
               NDA_CACHEINFO   Cache statistics.
 Additional Netlink message types applicable to this service:
 RTM_NEWNEIGH, RTM_DELNEIGH, and RTM_GETNEIGH.

3.1.3. Traffic Control Service

 This service provides the ability to provision, query or listen to
 events under the auspices of traffic control.  These include queuing
 disciplines, (schedulers and queue treatment algorithms -- e.g.,
 priority-based scheduler or the RED algorithm) and classifiers.
 Linux Traffic Control Service is very flexible and allows for
 hierarchical 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 [11].  A
 packet first goes through a filter that is used to identify a class
 to which the packet may belong.  A class is essentially a terminal

Salim, et. al. Informational [Page 21] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

 queuing 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 outermost queuing discipline, which
 is referred to as the parent is typically associated with a
 scheduler.  Within this scheduler hierarchy, 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 queuing
 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.
 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Family    |  Reserved1    |         Reserved2             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     Interface Index                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      Qdisc handle                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     Parent Qdisc                            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        TCM Info                             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Family: 8 bits
 Address Family: AF_INET for IPv4; and AF_INET6 for IPV6.
 Interface Index: 32 bits
 The unique interface index.
 Qdisc handle: 32 bits
 Unique identifier for instance of queuing 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 queuing 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 qdisc.
 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

Salim, et. al. Informational [Page 22] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

 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
 Qdisc.
 Additional Netlink message types applicable to this service:
 RTM_NEWQDISC, RTM_DELQDISC, RTM_GETQDISC, RTM_NEWTCLASS,
 RTM_DELTCLASS, RTM_GETTCLASS, RTM_NEWTFILTER, RTM_DELTFILTER, and
 RTM_GETTFILTER.

3.2. IP Service NETLINK_FIREWALL

 This service allows CPCs to receive, manipulate, and re-inject
 packets via the IPv4 firewall service modules in the FE.  A firewall
 rule is first inserted to activate packet redirection.  The CPC
 informs the FEC whether it would like to receive just the metadata 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 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.

Salim, et. al. Informational [Page 23] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   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.

Salim, et. al. Informational [Page 24] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       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
 timestamp_m)
 hook: 32 bits
 The firewall module from which the packet was picked.

Salim, et. al. Informational [Page 25] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         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
                     traversal.
         NF_DROP     Drop the packet.

Salim, et. al. Informational [Page 26] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

 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 don't
 modify the packet just set it to 0.
 Payload:
 Size as defined by the Data Length field.

3.3. 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 Neighbor Setup Service Module.
 The CPC service is expected to participate in neighbor solicitation
 protocol(s).
 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
 neighbor's entry (e.g., a neighbor being perceived as unreachable).
 RTM_GETNEIGH is used to solicit the CPC for information on a specific
 neighbor.

4. References

4.1. Normative References

 [1]  Braden, R., Clark, D. and S. Shenker, "Integrated Services in
      the Internet Architecture: an Overview", RFC 1633, June 1994.
 [2]  Baker, F., "Requirements for IP Version 4 Routers", RFC 1812,
      June 1995.
 [3]  Blake, S., Black, D., Carlson, M., Davies, E, Wang, Z. and W.
      Weiss, "An Architecture for Differentiated Services", RFC 2475,
      December 1998.
 [4]  Durham, D., Boyle, J., Cohen, R., Herzog, S., Rajan, R. and A.
      Sastry, "The COPS (Common Open Policy Service) Protocol", RFC
      2748, January 2000.
 [5]  Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
 [6]  Case, J., Fedor, M., Schoffstall, M. and C. Davin, "Simple
      Network Management Protocol (SNMP)", STD 15, RFC 1157, May 1990.

Salim, et. al. Informational [Page 27] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

 [7]  Andersson, L., Doolan, P., Feldman, N., Fredette, A. and B.
      Thomas, "LDP Specification", RFC 3036, January 2001.
 [8]  Bernet, Y., Blake, S., Grossman, D. and A. Smith, "An Informal
      Management Model for DiffServ Routers", RFC 3290, May 2002.

4.2. Informative References

 [9]  G. R. Wright, W. Richard Stevens.  "TCP/IP Illustrated Volume 2,
      Chapter 20", June 1995.
 [10] http://www.netfilter.org
 [11] http://diffserv.sourceforge.net

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. Acknowledgements

 1) Andi Kleen, for man pages on netlink and rtnetlink.
 2) Alexey Kuznetsov is credited for extending Netlink to the IP
    service delivery model.  The original Netlink character device was
    written by Alan Cox.
 3) Jeremy Ethridge for taking the role of someone who did not
    understand Netlink and reviewing the document to make sure that it
    made sense.

Salim, et. al. Informational [Page 28] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

Appendix 1: Sample Service Hierarchy

 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 Appendix 2,
 the diagram is referenced again to define the protocol interaction
 between service foo's CPC and FEC (labels 4-10).
   CP
  [--------------------------------------------------------.
  |   .-----.                                              |
  |  |                         . -------.                  |
  |  |  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,
    NETLINK_FOO).

Salim, et. al. Informational [Page 29] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

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

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).
 8) Register the protocol-specific packets you would like the FE to
    forward to you.
 9) Send service-specific foo commands and receive responses for them,
    if needed.

Appendix 2a: Interacting with Other IP services

 The diagram in Appendix 1 shows another control component configuring
 the same service.  In this case, it is a proprietary Command Line
 Interface.  The CLI may or may not be using the Netlink protocol to
 communicate to the foo component.  If the CLI issues commands that
 will affect the policy of the FEC for service foo then, then the foo
 CPC is notified.  It could then make algorithmic decisions 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.

Salim, et. al. Informational [Page 30] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

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 algorithm is
 based on packet counting and drops packets when the limit exceeds 100
 packets.  We assume that the queue is in a hierarchical 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.
 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                          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)                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Salim, et. al. Informational [Page 31] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

Authors' Addresses

 Jamal Hadi Salim
 Znyx Networks
 Ottawa, Ontario
 Canada
 EMail: hadi@znyx.com
 Hormuzd M Khosravi
 Intel
 2111 N.E. 25th Avenue JF3-206
 Hillsboro OR 97124-5961
 USA
 Phone: +1 503 264 0334
 EMail: hormuzd.m.khosravi@intel.com
 Andi Kleen
 SuSE
 Stahlgruberring 28
 81829 Muenchen
 Germany
 EMail: ak@suse.de
 Alexey Kuznetsov
 INR/Swsoft
 Moscow
 Russia
 EMail: kuznet@ms2.inr.ac.ru

Salim, et. al. Informational [Page 32] RFC 3549 Linux Netlink as an IP Services Protocol July 2003

Full Copyright Statement

 Copyright (C) The Internet Society (2003).  All Rights Reserved.
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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Salim, et. al. Informational [Page 33]

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