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RFC 3222 - Terminology for Forwarding Information Base (FIB) bas


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Network Working Group                                         G. Trotter
Request for Comments: 3222                          Agilent Technologies
Category: Informational                                    December 2001

    Terminology for Forwarding Information Base (FIB) based Router
                              Performance

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 (2001).  All Rights Reserved.

Abstract

   This document describes the terms to be used in a methodology that
   determines the IP packet forwarding performance of IP routers as a
   function of the forwarding information base installed within a
   router.  The forwarding performance of an IP router may be dependent
   upon or may be linked to the composition and size of the forwarding
   information base installed within a router.

Table of Contents

   1. Introduction.................................................... 2
   2. Overview........................................................ 3
   3. Existing Definitions............................................ 3
   4. Definition Format............................................... 3
   5. Definitions - parameters........................................ 4
   5.1 Network Prefix................................................. 4
   5.2 Network Prefix Length.......................................... 4
   5.3 Forwarding Information Base (FIB).............................. 5
   5.4 Forwarding Information Base Entry.............................. 6
   5.5 Forwarding Information Base Size............................... 6
   5.6 Longest Length Prefix Match Algorithm.......................... 7
   5.7 Forwarding Information Base Prefix Distribution................ 7
   5.8 Per-Interface or Per-Card Forwarding Information Base.......... 8
   5.9 Per-Interface Forwarding Information Base Cache................ 9
   5.10 Route Aggregation............................................ 10
   6. Definitions - metrics.......................................... 10
   6.1 Maximum Forwarding Information Base Size...................... 11
   6.2 Forwarding Information Base Learning Time..................... 11
   6.3 Forwarding Information Base-dependent Throughput.............. 12
   6.4 Forwarding Information Base-dependent Latency................. 12
   6.5 Forwarding Information Base-dependent Frame Loss Rate......... 13
   7. Security Considerations........................................ 13
   8. References..................................................... 13
   9. Author's Address............................................... 14
   10. Full Copyright Statement...................................... 15

1. Introduction

   This document defines terms that are to be used in a methodology that
   determines the IP packet forwarding performance of IP routers as a
   function of the forwarding information base installed within the
   router.

   The objective of this methodology is to evaluate the performance
   levels of IP routers as forwarding information bases continue to grow
   in size and complexity of structure.

   This methodology utilizes the packet forwarding performance
   measurements described in [2]; reference will also be made to the
   associated terminology document [3] for these terms.

2. Overview

   In order to measure the forwarding information base-based router
   performance, different forwarding information bases (5.3) are
   installed in the router.  The two key elements describing the FIB are
   the FIB size (5.5) and FIB prefix distribution (5.7).  The forwarding
   performance of a router may be dependent upon these two primary
   factors, particularly if FIB prefix distributions tend towards longer
   network prefixes (5.1).  The FIB-dependent throughput, latency and
   frame loss rate (6.3, 6.4, 6.5), measured with fully meshed traffic
   flows [2], will reflect the change in performance of the router.
   Tests may need to be performed up to the maximum FIB size (6.1).

   When configuring the router for these measurements, the routes need
   to be manually entered into the router, or advertised via a routing
   protocol.  It may take some period of time (the FIB learning time
   (6.2)) before the router learns all the routes.

   When routes are advertised into the router, the routes should be
   advertised in such a way so that route aggregation (5.10) does not
   occur.  Also, the effect of a per-interface FIB cache (5.9) needs to
   be taken into account.

3. Existing Definitions

   [3] should be consulted before attempting to make use of this
   document.  [2] contains discussions of a number of terms relevant to
   the benchmarking of network interconnect devices and should also be
   consulted.

4. Definition Format

   The definition format is the equivalent to that defined in [3], and
   is repeated here for convenience:

   X.x Term to be defined. (e.g., Latency)

   Definition:
      The specific definition for the term.

   Discussion:
      A brief discussion about the term, it's application and any
      restrictions on measurement procedures.

   Measurement units:
      The units used to report measurements of this term, if applicable.

   Issues:
      List of issues or conditions that effect this term.

   See Also:
      List of other terms that are relevant to the discussion of this
      term.

5. Definitions - parameters

   This section defines parameters that would dictate the execution of
   methodology to determine the FIB based forwarding performance of a
   router.

5.1 Network Prefix

   Definition:
      "A network prefix is . . . a contiguous set of bits at the more
      significant end of the address that defines a set of systems; host
      numbers select among those systems."

      (This definition is taken directly from section 2.2.5.2,
      "Classless Inter Domain Routing (CIDR)", in [4].)

   Discussion:
      In the CIDR context, the network prefix is the network component
      of an IP address.  A common alternative to using a bitwise mask to
      communicate this component is the use of "slash (/) notation."
      Slash notation binds the notion of network prefix length (see 5.2)
      in bits to an IP address. E.g., 141.184.128.0/17 indicates the
      network component of this IPv4 address is 17 bits wide.

   Measurement units:
      <n/a>

   Issues:

   See Also:
      Network Prefix Length (5.2)

5.2 Network Prefix Length

   Definition:
      The number of bits used to define the network prefix. Network
      prefixes, using CIDR terminology, are typically referred to as
      15.35.128.0 /17, indicating that the network prefix is 17 bits
      long.

   Discussion:
      When referring to groups of addresses, the network prefix length
      is often used as a means of describing groups of addresses as an
      equivalence class.  For example, 100 /16 addresses refers to 100
      addresses whose network prefix length is 16 bits.

   Measurement units:
      bits

   Issues:

   See Also:
      network prefix (5.1)
      forwarding information base prefix distribution (5.7)

5.3 Forwarding Information Base (FIB)

   Definition:
      As according to the definition in Appendix B of [4]:

      "The table containing the information necessary to forward IP
      Datagrams, in this document, is called the Forwarding Information
      Base.  At minimum, this contains the interface identifier and next
      hop information for each reachable destination network prefix."

   Discussion:
      The forwarding information base describes a database indexing
      network prefixes versus router port identifiers.

      A forwarding information base consists of [FIB size (5.5)] FIB
      entries (5.4).

      The forwarding information base is distinct from the "routing
      table" (or, the Routing Information Base), which holds all routing
      information received from routing peers.

      The forwarding information base contains unique paths only (i.e.
      does not contain secondary paths).

   Measurement units:
      <none>

   Issues:

   See Also:
      forwarding information base entry (5.4)
      forwarding information base size (5.5)
      forwarding information base prefix distribution (5.7)
      maximum forwarding information base size (6.1)

5.4 Forwarding Information Base Entry

   Definition:
      A single entry within a forwarding information base.  This entry
      consists of the minimum amount of information necessary to make a
      forwarding decision on a particular packet.  The typical
      components within a forwarding information base entry are a
      network prefix, a router port identifier and next hop information.
      This is an entry that the router can and does use to forward
      packets.

   Discussion:
      See (5.3).

   Measurement units:
      <n/a>

   Issues:

   See Also:
      forwarding information base (5.3)
      forwarding information base size (5.5)
      forwarding information base prefix distribution (5.7)
      maximum forwarding information base size (6.1)

5.5 Forwarding Information Base Size

   Definition:
      Refers to the number of forwarding information base entries within
      a forwarding information base.

   Discussion:
      The number of entries within a forwarding information base is one
      of the key elements that may influence the forwarding performance
      of a router.  Generally, the more entries within the forwarding
      information base, the longer it could take to find the longest
      matching network prefix within the forwarding information base.

   Measurement units:
      Number of routes

   Issues:

   See Also:
      forwarding information base (5.3)
      forwarding information base entry (5.4)
      forwarding information base prefix distribution (5.7)
      maximum forwarding information base size (6.1)

5.6 Longest Length Prefix Match Algorithm

   Definition:
      An algorithm that a router uses to quickly match destination
      addresses within received IP packets to exit interfaces on the
      router.

   Discussion:

   Measurement Units:
      <none>

   Issues:

   See Also:

5.7 Forwarding Information Base Prefix Distribution

   Definition:
      The distribution of network prefix lengths within the forwarding
      information base.

   Discussion:
      Network prefixes within the forwarding information base could be
      all of a single network prefix length, but, more realistically,
      the network prefix lengths will be distributed across some range.

      Individual performance measurements will be made against FIBs
      populated with the same network prefix length, as well as against
      FIBs with some distribution of network prefix lengths.

      The distribution of network prefix lengths may have an impact on
      the forwarding performance of a router.  The longer the network
      prefix length, the longer it will take for a router to perform the
      longest length prefix match algorithm, and potentially the lower
      the performance of the router.

   Measurement units:
      The forwarding information base prefix distribution is expressed
      by a list of network prefix lengths and the percentage of entries
      within the forwarding information base with a particular network
      prefix length.  For example, a forwarding information base prefix
      distribution is represented as:

         {[/16, 100], [/20, 360], [/24, 540]}

      This indicates that 100 of the entries within the forwarding
      information base have a 16 bit network prefix length, 360 have a
      20 bit network prefix length, and 540 have a 24 bit network prefix
      length.

   Issues:

   See Also:
      forwarding information base (5.3)
      forwarding information base entry (5.4)
      forwarding information base size (5.5)
      maximum forwarding information base size (6.1)

5.8 Per-Interface or Per-Card Forwarding Information Base

   Definition:
      A complete copy of the forwarding information base, installed on a
      router's card or individual physical interface to speed the
      destination address to network prefix lookup process.

   Discussion:
      Router manufacturers have developed many optimizations for
      routers, of which one optimization is to copy the forwarding
      information base to every interface or interface card on the
      router.  By doing this, destination address / network prefix
      lookups can be performed on the interface or card, unloading a
      router's CPU.

   Measurement units:
      <n/a>

   Issues:

   See Also:
      forwarding information base (5.3)
      per-interface forwarding information base cache (5.9)

5.9 Per-Interface Forwarding Information Base Cache

   Definition:
      A subset of a forwarding information base, installed on a router's
      interface card to speed the destination address / network prefix
      lookup process.

   Discussion:
      Prior to installing a complete copy of the forwarding information
      base on each interface of a router, a popular technique for
      speeding destination address lookups is to install a cache of
      frequently used routes on a router's interface.

      The most frequently used routes are placed in the forwarding
      information base cache.  IP packets whose destination address does
      not match a network prefix within the per-interface forwarding
      information base cache are forwarded to a router's central
      processor for lookup in the complete forwarding information base.

      The implication for benchmarking the performance of a router as a
      function of the forwarding information base is significant.  IP
      packets whose destination address matches an entry within the
      per-interface forwarding information base cache could be forwarded
      more quickly than packets whose destination address does not match
      an entry within the per-interface forwarding information base
      cache.

      To create useful benchmarks, the role of a per-interface
      forwarding cache needs to be considered.  The nature of
      benchmarking tests to measure the impact of the forwarding
      performance of a router requires that the destination addresses
      within IP packets transmitted into the router be distributed
      amongst the total set of network prefixes advertised into the
      router.  This negates the role of a per-interface forwarding
      information base cache, but serves to stress the forwarding
      information base-based packet forwarding performance of the
      router.

   Measurement units:
      <n/a>

   Issues:

   See Also:
      forwarding information base (5.3)
      per-interface forwarding information base (5.8)

5.10 Route Aggregation

   Definition:
      The ability of a router to collapse many forwarding information
      base entries into a single entry.

   Discussion:
      A router may aggregate routes in a forwarding information base
      into a single entry to conserve space.

      When advertising routes into a router to perform benchmarking
      tests as a function of the forwarding information base installed
      within the router, it is necessary to ensure that a router does
      not aggregate routes.

      Thus, when routes are advertised to the router or installed
      statically, care must be taken to ensure that the router does not
      aggregate routes.

      For example, if advertising a set of /24 network prefixes into a
      particular port on the router, 256 consecutive /24 routes, sharing
      a common leading 16 bits, should not be advertised on a single
      port.  If this is done, then the router will install a single
      entry within the forwarding information base indicating that all
      networks matching a particular /16 network prefix are accessible
      through one particular entry.

      Route aggregation on a router can be turned off, but routes should
      still be advertised into the router in such a manner as to avoid
      route aggregation.

   Measurement units:
      <none>

   Issues:

   See Also:

6. Definitions - metrics

   This section defines the metrics, or results, that would
   characterized the FIB based forwarding performance of a router.

6.1 Maximum Forwarding Information Base Size

   Definition:
      The maximum number of forwarding information base entries that can
      be supported within the forwarding information base. The Maximum
      Forwarding Information Base Size is the size over which all
      entries can and are used to forward traffic.

   Discussion:
      It is useful to know the maximum forwarding information base size
      for a router as it will be an indicator of the ability of the
      router to function within the given application space, and whether
      the router will be able to handle projected network growth.

      As a benchmarking value, it is necessary to discover this value so
      that performance measurements can be made up to the maximum
      possible forwarding information base size.

   Measurement units:
      Number of routes

   Issues:
      Could this value vary with the forwarding information base prefix
      distribution?

   See Also:
      forwarding information base (5.3)
      forwarding information base entry (5.4)
      forwarding information base size (5.5)
      forwarding information base prefix distribution (5.7)

6.2 Forwarding Information Base Learning Time

   Definition:
      The time a router takes to process received routing messages, and
      to construct (and, possibly to distribute amongst the interface
      cards in the router) the forwarding information base.  This is
      measured from the time at which a router is presented with the
      first routing message, through to when it can forward packets
      using any entry in the forwarding information base.

   Discussion:
      It takes time for a router to construct its forwarding information
      base.  A router needs to process received routing packets, build
      the routing information database, select the best paths, build the
      forwarding information base and then possibly distribute the

      forwarding information base or a subset thereof to the interface
      cards.  This entire process can take several minutes with very
      large forwarding information bases.

      When performing benchmarking tests that take the forwarding
      information base into account, time must be allocated for the
      router to process the routing information and to install the
      complete forwarding information base within itself, before
      performance measurements are made.

   Measurement units:
      Prefixes per second.

   Issues:

   See Also:
      forwarding information base (5.3)

6.3 Forwarding Information Base-dependent Throughput

   Definition:
      Throughput, as defined in [3], used in a context where the
      forwarding information base influences the throughput.

   Discussion:
      This definition for FIB-dependent throughput is added to
      distinguish the context of this measurement from that defined in
      [3].

   Measurement units:
      See [3].

   Issues:

   See Also:
      forwarding information base-dependent latency (6.4)
      forwarding information base-dependent frame loss rate (6.5)

6.4 Forwarding Information Base-dependent Latency

   Definition:
      Latency, as defined in [3], used in a context where the forwarding
      information base influences the throughput.

   Discussion:
      This definition for FIB-dependent latency is added to distinguish
      the context of this measurement from that defined in [3].

   Measurement units:
      See [3].

   Issues:

   See Also:
      forwarding information base-dependent throughput (6.3)
      forwarding information base-dependent frame loss rate (6.5)

6.5 Forwarding Information Base-dependent Frame Loss Rate

   Definition:
      Frame Loss Rate, as defined in [3], used in a context where the
      forwarding information base influences the throughput.

   Discussion:
      This definition for FIB-dependent frame loss rate is added to
      distinguish the context of this measurement from that defined in
      [3].

   Measurement units:
      See [3].

   Issues:

   See Also:
      forwarding information base-dependent throughput (6.3)
      forwarding information base-dependent latency (6.4)

7. Security Considerations

   As this document is solely for the purpose of providing metric
   methodology and describes neither a protocol nor a protocols
   implementation, there are no security considerations associated with
   this document.

8. References

   [1] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
       9, RFC 2026, October 1996.

   [2] Bradner, S. and J. McQuaid, "Benchmarking Methodology for Network
       Interconnect Devices", RFC 2544, March 1999.

   [3] Bradner, S., "Benchmarking Terminology for Network
       Interconnection Devices", RFC 1242, July 1991.

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

9. Author's Address

   Guy Trotter
   Agilent Technologies (Canada) Inc.
   #2500 4710 Kingsway
   Burnaby, British Columbia
   Canada
   V5H 4M2

   Phone: +1 604 454 3516
   EMail: Guy_Trotter@agilent.com

10. Full Copyright Statement

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
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   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
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   document itself may not be modified in any way, such as by removing
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   The limited permissions granted above are perpetual and will not be
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   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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Acknowledgement

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

 

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