| Rfc | 3222 |
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| Title | Terminology for Forwarding Information Base (FIB) based Router
Performance |
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| Author | G. Trotter |
|---|
| Date | December 2001 |
|---|
| Format: | TXT, HTML |
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| Status: | INFORMATIONAL |
|---|
|
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.
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Acknowledgement
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