Internet Engineering Task Force Juha Heinanen
INTERNET DRAFT Telecom Finland
Expires April 1998 October 1997
Use of the IPv4 TOS Octet to Support Differential Services
<draft-heinanen-diff-tos-octet-00.txt>
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Abstract
This document describes how the TOS octet in the IPv4 header can be
used to support differential Internet services. The proposal is
based on the existing format of the TOS octet as defined in [RFC791]
and [RFC1349].
1. Introduction
In order to support differential services within the Internet, such
as those proposed by [Clark] and [Kilkki], the routers must be able
to distinguish, how they should treat IP packets in terms of Type of
Service (TOS) and Drop Preference (DP). The TOS usually indicates
the desired delay characteristics of the packet, whereas the DP
identifies how "important" the packet is if packets need to be
discarded due to congestion.
In a typical router implementation, each TOS has its own queue, which
it serves with such a policy that meets the characteristics of the
TOS. The DPs, on the other hand, map to queue thresholds so that an
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incoming packet with a DP value i may be discarded if the length of
the corresponding queue has exceeded a threshold value th(i) of that
queue.
The TOS of a packet is set by the application or, if the application
is unaware of differential Internet services, by the operating system
or a router. In the latter two cases, the TOS may be determined from
the TCP/UDP port numbers of the packet.
The DP of a packet can also be set by the application, the OS, or a
router, but the criteria based on which the DP is assigned can vary
widely. For example, a packet may be assigned a higher DP if it is
consider to be "outside" of a given user profile. For more
information on policies to set the DP, see [Clark] and [Kilkki].
2. TOS and DP in the IPv4 Header
Ideally, in order to support a wide range of TOSes and DPs, the IPv4
header should have several bits available for this purpose. Luckily,
the The IPv4 header has an 8 bit TOS octet, that can be used to carry
the TOS and DP information.
According to [RFC791], the IPv4 TOS octet is divided into a 3 bit
Precedence field and a 3 bit TOS field. The last two bits of the TOS
octet are reserved for future use:
Bits 0-2: Precedence.
Bit 3: 0 = Normal Delay, 1 = Low Delay.
Bits 4: 0 = Normal Throughput, 1 = High Throughput.
Bits 5: 0 = Normal Reliability, 1 = High Reliability.
Bit 6-7: Reserved for Future Use.
The following two sections propose how the TOS and DP information
needed to provide differential Internet services can be mapped to the
IPv4 TOS octet.
3. Drop Preference (DP)
The semantics of the 3 bit Precedence field doesn't contradict its
use as the DP field:
"Several networks offer service precedence, which somehow treats
high precedence traffic as more important than other traffic
(generally by accepting only traffic above a certain precedence
at time of high load)."
However, the actual meaning assigned in [RFC791] to the 3 bits of the
Precedence field is outdated and not very useful:
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INTERNET DRAFT October, 1997
111 - Network Control
110 - Internetwork Control
101 - CRITIC/ECP
100 - Flash Override
011 - Flash
010 - Immediate
001 - Priority
000 - Routine
On the other hand, [RFC791] further specifies that the Precedence
field has only local significance within a single network:
"The Network Control precedence designation is intended to be
used within a network only. The actual use and control of that
designation is up to each network. The Internetwork Control
designation is intended for use by gateway control originators
only. If the actual use of these precedence designations is of
concern to a particular network, it is the responsibility of that
network to control the access to, and use of, those precedence
designations."
This makes it easier to refine the precise semantic of the precedence
values without breaking possible existing usage of the Precedence
field.
It is proposed that differential Internet services use the Precedence
field of the IPv4 header to indicate the DP of the packet. The DP of
the packet can range from value 0, indicating the lowest DP, to value
7, indicating the highest DP.
A network does not need to implement all 8 DP levels. The table
below shows how a network can map a DP value given in the Precedence
field of an IPv4 packet to any number of DPs actually supported by
it:
DP Number of Supported DPs
value 2 3 4 5 6 7 8
-----------------------------------
0 0 0 0 0 0 0 0
1 0 0 0 0 0 0 1
2 0 0 1 1 1 1 2
3 0 0 1 1 2 2 3
4 1 1 2 2 3 3 4
5 1 1 2 3 4 4 5
6 1 2 3 4 5 5 6
7 1 2 3 4 5 6 7
The above mapping thus supports the 2 DP levels as proposed by
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INTERNET DRAFT October, 1997
[Clark] and the 4 DP levels as proposed by [Kilkki].
4. Type of Service (TOS)
The 3 bit TOS field can be used to indicate the actual Type of
Service that an IP packet expects to receive from a network that
supports differential services. As already shown above, [RFC791]
specifies the following semantics for these 3 bits:
Bit 3: 0 = Normal Delay, 1 = Low Delay.
Bits 4: 0 = Normal Throughput, 1 = High Throughput.
Bits 5: 0 = Normal Reliability, 1 = High Reliability.
In [RFC1349], the TOS field has been extended by one of the reserved
bits of [RFC791] resulting in the following semantics for the bits
3-6 of the IPv4 TOS octet:
1000 -- minimize delay
0100 -- maximize throughput
0010 -- maximize reliability
0001 -- minimize monetary cost
0000 -- normal service
It is not known to the author, what the status of [RFC1349] is in
terms of the standards' process or implementations.
Ideally, the whole TOS field of IPv4 TOS octet should be usable by
differential Internet services to support a wide variety of delay
characteristics. However, because it is not known yet, how many
delay levels will actually be needed and because the standardization
of the semantics of the TOS field is still at least somewhat open, it
is proposed that initially networks that support differential
Internet services only use the bit 3 of the TOS field to indicate the
desired delay treatment of a packet.
This is a safe choice, since the Bit 3 of the TOS octet is the delay
indicator bit according to both [RFC791] and [RFC1349]. By setting
the Bit 3 to 0 or 1, the packets requests a normal or low delay,
respectively. The possibility to request for low delay is believed
to be important in facilitating large scale deployment of the rapidly
emerging real-time applications, such as voice and video over IP.
The use of the Bits 4-7 of the TOS octet in connection with
differential Internet services is left for further study.
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5. Summary
This document has proposed how the IPv4 TOS octet can be used to
support differential services within the Internet. The proposal does
not include any syntactical changes to the IPv4 header and even the
semantic changes are kept to the very minimum.
The author hopes that this proposal could help to create a common
convention on the usage of the TOS octet for the support of
differential services within the Internet. Such services will be or,
more precisely, are being implemented anyhow and it is thus very
important that the implementations and service offerings will be
compatible with each other.
6. Security Considerations
Security is not addressed in the current version of this document.
References
[Clark] Clark, D. and Wroclawski, J., "An Approach to Service
Allocation in the Internet". draft-clark-diff-svc-alloc-00.txt, July
1997.
[Kilkki] Kilkki, K., "Simple Integrated Media Access (SIMA)".
<draft-kalevi-simple-media-access-01.txt>, June 1997.
[RFC791] Information Sciences Institute, "Internet Protocol". RFC
791, September 1981.
[RFC1349] Almquist, P., "Type of Service in the Internet Protocol
Suite". RFC 1349, July 1992.
Acknowledgements
I would like to thank Kalevi Kilkki for his comments on earlier
versions of this document.
Author Information
Juha Heinanen
Telecom Finland
PO Box 777
33101 Tampere
Finland
Phone +358 400 500 958
Email jh@tele.fi
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