Network Working Group T. Dreibholz
Internet-Draft University of Essen
Expires: October 18, 2002 April 19, 2002
An IPv4 Flowlabel Option
draft-dreibholz-ipv4-flowlabel-00
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2 Abbreviations . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Conventions . . . . . . . . . . . . . . . . . . . . . . . . 4
2. A Flow Label Option for IPv4 . . . . . . . . . . . . . . . . 5
2.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.1 The Flow Label Field of IPv6 . . . . . . . . . . . . . . . . 5
2.1.2 The Limitations of IntServ via IPv4 . . . . . . . . . . . . 6
2.2 Definition of the Flow Label Option . . . . . . . . . . . . 7
3. Translation between IPv6 and IPv4 . . . . . . . . . . . . . 8
References . . . . . . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . 10
Full Copyright Statement . . . . . . . . . . . . . . . . . . 11
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Abstract
This draft defines an IPv4 option containing a flowlabel that is
compatible to IPv6. It is required for simplified usage of IntServ
and interoperability with IPv6.
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1. Introduction
1.1 Terminology
This document uses the following terms:
o IntServ (Integrated Services): Reservation of network resources
(bandwidth) on a per-flow basis. This introduces a scalability
problem, since it requires saving per-flow states on each router.
See [3], [6], [7], [8], [9], [10] and [11] for details.
o Flow: An IntServ reservation between two endpoints.
o Flow Label: The Flow Label field of the IPv6 header and the IPv4
option header defined in this draft. It is used for marking a
packet to use a specific IntServ reservation. See [4] for a
detailed description.
1.2 Abbreviations
o RSVP: ReSource Reservation Protocol
o TCP: Transmission Control Protocol
o QoS: Quality of Service
o UDP: User Datagram Protocol
1.3 Conventions
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD.
SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when
they appear in this document, are to be interpreted as described in
[5].
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2. A Flow Label Option for IPv4
2.1 Motivation
This section describes the motivation to add a flow label option to
the IPv4 protocol.
2.1.1 The Flow Label Field of IPv6
The Flow Label field of the IPv6 header (see [12] and [4]) is a 20-
bit pseudo-random number. All packets from the same source address
having the same flow label MUST contain the same destination address.
Therefore, the flow label combined with the source address is a
network- unique identification for a specific packet flow. The idea
behind the flow label is marking specific flows for IntServ. That
is, the routers on the path from source to destination keep e.g.
reservation states for the flows. The flow label provides easy
identification and utilizes efficient lookup. For example, a hash
function can be used to implement the state lookup, due to the
pseudo-random property of the flow label.
Using the IPv6 flow label, the following tasks are easily realizable:
1. Packets can be mapped easily to specific flows, with the
following features:
* Protocol Independence: Since the mapping is directly specified
in the IP header, all possible layer 4 protocols are
supported, even protocols to be specified in a far future
* Flow Sharing: By marking packets with a flow label, it is
possible to share a single flow (IntServ reservation) with
several communication associations from host A to host B. For
example, a video stream via UDP and a HTTP download via TCP
could share a single reservation. For the user, flow sharing
has the advantage that if one of its communication
associations temporarily requires lower bandwidth than
expected, other associations sharing the same flow may use the
remaining bandwidth. That is, his possibly expensive
reservation is fully utilized. Flow sharing also helps
keeping the total number of reservations a router has to
handle small, reducing their CPU and memory requirements and
therefore cost.
* Multi-Flow Connections: One communication association can
divide up its packets to several flows, simply by marking
packets with different flow labels. This technique can be
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used for layered transmission. That is, a stream (e.g. a
video) is divided up into several parts (called layers). For
example, the first layer (base layer) of a video contains a
low-quality version, the second (1st enhancement layer) the
data to generate a higher-quality version, etc. Now, the
first layer can be mapped to a high-quality reservation
(guaranteed bandwidth, low loss rate) at higher cost, but the
following layers can be mapped to lower-quality reservations
(e.g. higher loss rate) or even best effort at lower cost.
Research shows that the total transmission cost can be highly
reduced using layered transmission (see [1] for details).
2. Last but not least, efficient state lookup is possible using hash
functions in time O(1), since the flow label is pseudo-random.
This is important for routers having to manage many flows without
much CPU requirement and therefore cost.
2.1.2 The Limitations of IntServ via IPv4
Using IntServ with IPv4, there are several problems that can only be
solved with high management effort:
1. Complicated packet marking:
* No Protocol Independence: It is necessary to mark the packets
within the layer 4 protocol header. For example, the TCP or
UDP port numbers can be used to mark flows (with limitations,
see below). But for new protocols (e.g. experimental, new
standards, proprietary), software updates for *all* IntServ
routers are necessary to recognize the packet flow!
* No Flow Sharing: It is usually impossible for two different
communication associations to share the same flow, e.g. if
TCP flows are recognized using port numbers. This makes it
necessary to reserve an IntServ flow for each communication
association. This implies an increased number of flow states
for routers to keep and maintain. Furthermore, if one
association temporarily uses a lower bandwidth, the free
bandwidth of its flow cannot easily be borrowed to another
association.
* No Multi-Flow Connections: To use layered transmission, e.g.
a video via UDP, the transmission of every layer would require
own port numbers. In the case of connection-oriented
transmission protocols (e.g. TCP, SCTP), every layer would
even require its own connection setup and management.
Depending on the transport protocol, the number of
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communication associations and the number of flows, much more
work is necessary compared to IPv6 using flow labels.
2. More complicated flow lookup: Due to different protocols and non-
random flow identifications (e.g. port numbers), much more
management effort is necessary for flow lookups: Different
protocols may have flow identifications of differing length (e.g.
16-bit port numbers for TCP and UDP, n-bit association ID for
protocol X, m-bit for Y, ...). Due to the non-random property of
e.g. port numbers, hashing may be inefficient. Therefore,
lookup organization using trees and lookup in time O(log n) might
be necessary.
All in all, using IntServ flows with IPv4 requires much more work
compared to IPv6, where simply the flow label can be used. It is
therefore useful to add such a field to IPv4, too. An appropriate
place to add such a field is an IPv4 option header.
2.2 Definition of the Flow Label Option
IPv4 (see [2]) already defines an option header for a 16-bit SATNET
stream identifier. Since this identifier is not defined to be
pseudo-random and it would be incompatible to the 20-bit IPv6 flow
label, reuse of this existing option header is inappropriate.
Therefore, a new one is defined in the following.
Flow Label Option
+--------+--------+--------+--------+--------+
|10001111|00000010|0000 Flow Label |
+--------+--------+--------+--------+--------+
Type=143 Length=5
Flow Label: 20 bits
The 20-bit flow label. All definitions of [4] and [12] for the IPv6
flow label are also valid for this field. A value of zero denotes
that no flow label is used. In this case, the flow label option is
in fact unnecessary. Note, that the option header contains 3 bytes
and therefore 24 bits. The first 4 bits are unused and MUST be set
to 0.
The Flow Label option MUST be copied on fragmentation. It MAY NOT
appear more than once per IPv4 packet.
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3. Translation between IPv6 and IPv4
Since the new IPv4 flow label is fully compatible to the IPv6 flow
label, the field MAY be translated in the other protocol's one during
protocol translation. That is, a router can translate an IPv6 packet
set from an IPv6-only host to an IPv4-mapped address of an IPv4-only
host and the flow label may simply be copied. The same may also be
applied in the backwards direction.
Note, that copying the flow label during protocol translation is not
mandatory. There may be IntServ reservation reasons for not copying
but setting the flow label to zero. But a router MAY NOT set the
flow label to another value than the copy or 0, since the source is
responsible to ensure that the source address combined with the flow
label is network-unique
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References
[1] Dreibholz, T., "Management of Layered Variable Bitrate
Multimedia Streams Over DiffServ with A Priori Knowledge",
Master Thesis, February 2001.
[2] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
[3] Braden, B., Clark, D. and S. Shenker, "Integrated Services in
the Internet Architecture: an Overview", RFC 1633, June 1994.
[4] Partridge, C., "Using the Flow Label Field in IPv6", RFC 1809,
June 1995.
[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[6] Zhang, L., Berson, S., Herzog, S. and S. Jamin, "Resource
ReSerVation Protocol (RSVP) -- Version 1 Functional
Specification", RFC 2205, September 1997.
[7] "Resource ReSerVation Protocol (RSVP) Version 1 Applicability
Statement Some Guidelines on Deployment", RFC 2208, September
1997.
[8] Braden, B. and L. Zhang, "Resource ReSerVation Protocol (RSVP)
-- Version 1 Message Processing Rules", RFC 2209, September
1997.
[9] Wroclawski, J., "The Use of RSVP with IETF Integrated
Services", RFC 2210, September 1997.
[10] Wroclawski, J., "Specification of the Controlled-Load Network
Element Service", RFC 2211, September 1997.
[11] Shenker, S., Partridge, C. and R. Guerin, "Specification of
Guaranteed Quality of Service", RFC 2212, September 1997.
[12] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
[13] Hinden, R., Carpenter, B. and L. Masinter, "Format for Literal
IPv6 Addresses in URL's", RFC 2732, December 1999.
[14] Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer,
H., Taylor, T., Rytina, I., Kalla, M., Zhang, L. and V. Paxson,
"Stream Control Transmission Protocol", RFC 2960, October 2000.
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Author's Address
Thomas Dreibholz
University of Essen
Ellernstraße 29
D-45326 Essen
Germany
EMail: dreibh@exp-math.uni-essen.de
URI: http://www.exp-math.uni-essen.de/~dreibh
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