Network Working Group T. Dreibholz
Internet-Draft University of Duisburg-Essen
Expires: Oktober 30, 2004 May 2004
An IPv4 Flowlabel Option
draft-dreibholz-ipv4-flowlabel-02
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Copyright Notice
Copyright (C) The Internet Society (2004). 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
4. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . 10
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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. 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, e.g. using a hash
function on the 3-tuple (source address, destination address, flow
label).
Using the IPv6 flow label, packets can be mapped easily to specific
flows, with the following features:
o Transport Layer 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.
o Support for Network Layer Encryption: The mapping is independent
of payload encryption (e.g. by IPsec).
o Support for Fragmentation: If fragmentation of a large IP packet
is necessary, all fragments contain the same flow label.
Therefore, fragmentation does not cause any flow-marking problem.
o 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.
o 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 used for layered
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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).
2.1.2 The Limitations of IntServ via IPv4
Using IntServ with IPv4, there are several problems that can only be
solved with high management effort:
o No Transport Layer 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!
o No Support for Network Layer Encryption: Since it is necessary to
read fields of the layer 4 protocol header, it may not be
encrypted. Therefore, e.g. the usage of IPsec is impossible.
o Support for Fragmentation: Only the first fragment of a large
packet contains the layer 4 header necessary to map the packet to
a flow. Mapping other fragments would require the hops to remember
packet identities and try to map fragments to packet identities.
Due to the management effort and memory requirements, this is not
realistic for high-bandwidth backbone routers; especially when
packet reordering must be considered. Furthermore, load sharing or
traffic distribution would be impossible.
o 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.
o 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 communication associations and the number
of flows, much more work is necessary compared to IPv6 using flow
labels.
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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 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.
Note, that the flow label option's length is 5 bytes. [2] requires
that padding must be used to end the IP header on a 32 bit boundary.
Therefore, the usual case with only the flowlabel option requires 3
padding bytes.
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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
4 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.
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[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.
Author's Address
Thomas Dreibholz
University of Duisburg-Essen, Institute for Experimental Mathematics
Ellernstrasse 29
45326 Essen, Nordrhein-Westfalen
Germany
Phone: +49 201 183-7637
Fax: +49 201 183-7673
EMail: dreibh@exp-math.uni-essen.de
URI: http://www.exp-math.uni-essen.de/~dreibh/
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