Network Working Group F. Baker
Internet-Draft Cisco Systems
Expires: December 20, 2002 June 21, 2002
Recommended Packet Marking Policy
draft-ietf-ieprep-packet-marking-policy-00
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This paper summarizes a recommended correlation of applications to
Differentiated Service Code Points. There is no intrinsic
requirement that individual DSCPs correspond to given applications,
but as a policy it is useful if they can be applied consistently.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [2].
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1. Introduction
This paper summarizes a recommended correlation of applications to
Differentiated Service Code Points. There is no intrinsic
requirement that individual DSCPs correspond to given applications,
but as a policy it is useful if they can be applied consistently.
1.1 Expected use in the network
In the Internet today, corporate LANs and ISP WANs are generally not
heavily utilized - they are commonly 10% utilized at most. For this
reason, congestion, loss, and variation in delay and within corporate
and ISP backbones is virtually unknown. This clashes with ser
community perceptions, for three very good reasons.
o It has not always been thus, and will not always be thus. The
industry moves through cycles of bandwidth boom and bandwidth
bust, depending on prevailing market conditions and the periodic
deployment of new bandwidth-hungry applications.
o In access networks, the state is often different. This may be
because rates are artificially limited, or because of access
network design trade-offs.
o Other characteristics, such as database design on web servers, and
configuration of firewalls and routers, often looks externally
like a bandwidth limitation.
The intent of this document is to provide a consistent marking
strategy so that it can be configured and put into service on any
link which finds itself congested, typically access links.
1.2 Key Diffserv concepts
Someone seeking a deep understanding of the Differentiated Services
Architecture [6] would do well to read it. However, we recapitulate
key concepts here so save searching.
1.2.1 Queue or Class
A queue or class is a data structure which holds traffic which is
being subjected to delay due to lack of bandwidth. There are a
number of ways to implement a queue; in some of these, it is more
natural to discuss "classes in a queuing system" rather than "a set
of queues and a scheduler". In the literature, as a result, the
concepts are used somewhat interchangeably.
A simple model of a queuing system, however, is a set of data
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structures for packet data, which we will call queues or classes, and
a mechanism for selecting the next packet from among them.
1.2.1.1 Priority Queue
A priority queuing system is a combination of a set of queues and a
scheduler which empties them in priority sequence. When asked for a
packet, the scheduler inspects the first queue, and if theer is data
present returns a packet from that queue. Failing that, it inspects
the second queue, and so on. A freeway onramp with a stoplight for
one lane but allowing vehicles in the high occupancy vehicle lane to
pass is an example of a priority queue.
In a priority queuing system, a packet in the highest priority queue
will experience a readily calculated delay - it is proportional to
the amount of data remaining to be serialized when the packet arrived
plus the volume of the data already queued ahead of it in the same
queue. The technical reason for using a priority queue relates
exactly to this fact: it limits jitter and delay, and should be used
for traffic which has that requirement.
1.2.1.2 Rate Queues
Similarly, a rate-based queuing system is a combination of a set of
queues and a scheduler which empties each at a specified rate. An
example of a rate based queuing system is a road intersection with a
stoplight - the stoplight acts as a scheduler, giving each lane a
certain opportunity to pass traffic through the intersection.
In a rate-based queuing system, such as WFQ or WRR, the delay that a
packet in any given queue will experience is dependant on the
parameters and occupancy of its queue and the parameters and
occupancy of the queues it is competing with. A queue whose traffic
arrival rate is much less than the rate at which it lets traffic
depart will tend to be empty, and packets in it will experience
nominal delays. A packet whose arrival rate approximates or exceeds
its departure rate will tend to be full, and packets in it will
experience greater delay. Such a scheduler can impose a minimum
rate, a maximum rate, or both, on any queue it touches.
1.2.2 Active Queue Management
Active queue management is a generic name for any of a variety of
procedures that use packet dropping or marking to manage the depth of
a queue. The canonical example of such a procedure is RED93, in
which a queue is assigned a minimum and maximum threshold, and the
queuing algorithm maintains a moving average of the queue depth.
When the mean queue depth exceeds the maximum threshold, all traffic
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is marked or dropped; when the mean queue depth exceeds the minimum
threshold, a randomly selected subset if marked or dropped. This is
intended to communicate with the system emitting the traffic, causing
its congestion avoidance algorithms to kick in.
1.2.3 Policing of traffic
Additionally, at the first router in a network that a acket crosses,
arriving traffic may be measured, and dropped or marked according to
a policy. This may be used to bias feedback loops, such as is done
in AF [9], or to limit the amount of traffic in a system, as is done
in EF [11].
1.2.4 Differentiated Services Code Point (DSCP)
The DSCP is a number in the range 0..63, which is placed into an IP
packet to mark it according to the class of traffic it belongs in.
1.3 Per Hop Behavior (PHB)
In the end, the facilities just described are combined to form a
specified set of characteristics for handling different kinds of
traffic, depending on the needs of the application. This document
seeks to identify useful traffic aggregates and specify what PHB
should be applied to them.
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2. Specified Traffic Classes
Figure A shows eleven classes of traffic that are commonly specified
in enterprise networks or on access links. None of these is
mandatory for configuration; common experience is that a small subset
is useful in any given network configuration. This specification
recommends that if such a service is deployed, it be deployed in a
manner consistent with this table.
+=====+=======+====================+=====================+==============+
|PHB | DSCP | DSCP | Reference | Intended protocols | Configuration|
+=====+=======+========+===========+=====================+==============+
|EF | EF | 101110 | RFC 3246 | Interactive Voice |RSVP Admission|
| | | | | |Priority queue|
+-----+-------+--------+-----------+---------------------+--------------+
|AF1 | AF11, | 001010 | RFC 2597 | Bulk transfers, web,| drop/mark |
| | AF12, | 001100 | | general data service| AF13 <= AF12 |
| | AF13 | 001110 | | | <= AF11,|
| | | | | possible guaranteed minimum rate|
| | | | | possible guaranteed maximum rate|
+-----+-------+--------+-----------+---------------------+--------------+
|AF2 | AF21, | 010010 | RFC 2597 | Database access, | drop/mark |
| | AF22, | 010100 | | transaction services| AF23 <= AF22 |
| | AF23 | 010110 | | interactive traffic | <= AF21,|
| | | | | possible guaranteed minimum rate|
| | | | | possible guaranteed maximum rate|
+-----+-------+--------+-----------+---------------------+--------------+
|AF3 | AF31, | 011010 | RFC 2597 | Locally defined | drop/mark |
| | AF32, | 011100 | | mission-critical | AF33 <= AF32 |
| | AF33 | 011110 | | applications | <= AF31,|
| | | | | possible guaranteed minimum rate|
| | | | | possible guaranteed maximum rate|
+-----+-------+--------+-----------+---------------------+--------------+
|AF4 | AF41, | 100010 | RFC 2597 | Interactive video, | drop/mark |
| | AF42, | 100100 | | associated voice | AF43 <= AF42 |
| | AF43 | 100110 | | | <= AF41,|
| | | | | possible guaranteed minimum rate|
| | | | | possible guaranteed maximum rate|
| | | | | Bandwidth Signaling|
+-----+-------+--------+-----------+---------------------+--------------+
|IP |Class 6| 110000 | RFC 2474 | BGP, OSPF, etc | minimum rate |
|Routing | | section 4.2.2 |Deep Queue AQM|
+-----+-------+--------+-----------+---------------------+--------------+
|Streaming | 100000 | RFC 2474 | Often proprietary | minimum rate |
|Video|Class 4| | section 4.2.2 | AQM |
+-----+-------+--------+-----------+---------------------+--------------+
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+=====+=======+====================+=====================+==============+
|PHB | DSCP | DSCP | Reference | Intended protocols | Configuration|
+=====+=======+========+===========+=====================+==============+
| |Class 3| 011000 | RFC 2474 | SIP, H.323, etc | minimum rate |
|Telephony | | section 4.2.2 |Deep Queue AQM|
|Signaling | | | | |
|voice/video | | | | |
+-----+-------+--------+-----------+---------------------+--------------+
| |Class 2| 010000 | RFC 2474 | SNMP | minimum rate;|
|Network | | section 4.2.2 | AQM |
|Management | | | | |
+-----+-------+--------+-----------+---------------------+--------------+
| |class 1| 001000 |Internet II|User-selected service| AQM |
|Scavenger | | usage | | |
+-----+-------+--------+-----------+---------------------+--------------+
| |class 0| 000000 | RFC 2474 | Unspecified traffic | minimum rate |
|Default | | section 4.1 | AQM |
+=====+=======+========+===========+=====================+==============+
Figure A: Summary of specified diffserv classes
2.1 Voice on IP
The voice traffic class serves RP voice. It is specified in [11].
The fundamental service offered to voice traffic is best effort
service up to a specified upper bound with nominal delay. It is in
many respects similar to an ATM CBR VC; the circuit is guaranteed its
bandwidth, and if it stays within the negotiated rate it experiences
nominal loss and delay.
Typical configurations negotiate the use of Voice on IP using
protocols such as SIP and RSVP. When a user has been authorized to
send voice traffic, this admission procedure has verified that data
rates will be within the capacity of the network that it will use.
Since RTP voice does not respond to loss or marking in any
substantive way, the network must police at ingress to ensure that
the voice traffic stays within its negotiated bounds. Having thus
assured a predictable input rate, the network may use a priority
queue to ensure nominal delay and jitter.
2.2 File Transfer Applications
The File Transfer traffic class serves applications but which run
over TCP [1][7][7] or a transport with a consistent congestion
avoidance procedure, and normally drive as high a data rate as they
can obtain over a long period of time. The FTP protocol is a common
example. The PHB is specified in [9].
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The fundamental service offered to mission critical traffic is best
effort service with a specified minimum rate. One must assume that
this class will consume any available capacity, and on congested
links may experience queuing delay or loss.
Typical configurations use ECN [10] or random loss to implement
active queue management [4], and may impose a minimum or maximum
rate. In queues, the probability of loss of AF11 traffic may not
exceed the probability of loss of AF12 traffic, which in turn may not
exceed the probability of loss of AF13 traffic. Ingress policing
passes traffic in the class up to some specified threshold marked
AF11, additional traffic up to some secondary threshold marked as
AF32, and potentially passes additional traffic marked AF33. In such
a case, if one network customer is driving significant excess and
another seeks to use the link, any losses will be experienced by the
high rate user, causing him to reduce his rate.
2.3 Human-response Applications
The human response traffic class serves applications but which run
over TCP [1][7][7] or a transport with a consistent congestion
avoidance procedure, and serve transaction, database access, or
interactive protocols. Such applications might include telnet,
common ERP applications, instant messaging, or other applications
which hold a user waiting until they respond. The PHB is specified
in [9].
The fundamental service offered to human response traffic is best
effort service with a specified minimum rate. The rate should be
specified significantly in excess of actual measured rates, in order
to ensure that this traffic experiences only nominal delay or loss.
Typical configurations use ECN [10] or random loss to implement
active queue management [4], and may impose a minimum or maximum
rate. In queues, the probability of loss of AF21 traffic may not
exceed the probability of loss of AF22 traffic, which in turn may not
exceed the probability of loss of AF23 traffic.
2.4 Mission Specific and Critical Applications
The mission-specific traffic class serves applications but which run
over TCP [1][7][7] or a transport with a consistent congestion
avoidance procedure, and serve needs the network administrator deems
to need special support. For example, in a banking network, it might
support electronic banking protocols. The PHB is specified in [9].
The fundamental service offered to mission critical traffic is best
effort service with a specified minimum rate. The rate should be
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specified significantly in excess of actual measured rates, in order
to ensure that this traffic experiences only nominal delay or loss.
Typical configurations use ECN [10] or random loss to implement
active queue management [4], and may impose a minimum or maximum
rate. In queues, the probability of loss of AF31 traffic may not
exceed the probability of loss of AF32 traffic, which in turn may not
exceed the probability of loss of AF33 traffic.
2.5 Network Multimedia (video)
The Network Multimedia traffic class serves applications that carry
RTP data streams whose rate has been negotiated with the network
using a protocol such as RSVP [3]. If the mean rate is conceived as
Bc/frame interval and the difference between the mean and peak rate
is Be/frame interval, the first Bc packets in a frame are marked
AF41, the next Be packets are marked AF42, and any additional packets
may be summarily dropped, or marked AF43 and subjected to loss in any
but a queue of nominal depth. This PHB is specified in [9].
The fundamental service offered to network multimedia traffic is best
effort service with controlled rate and delay. This traffic does not
respond to loss or marking, and can be severely compromise by loss or
delays that exceed its framing interval. It can be assumed, however,
to have been initially transmitted in a manner roughly comparable to
[12]. As such, active queue management [4] serves primarily to deal
with extreme cases; ingress shaping or policing is depended on to
ensure rate compliance. In queues, the probability of loss of AF41
traffic may not exceed the probability of loss of AF42 traffic, which
in turn may not exceed the probability of loss of AF43 traffic if
any.
2.6 IP Routing Protocols
The IP Routing traffic class serves IP Routing Applications such as
BGP or OSPF. It is specified in [5].
The fundamental service offered to routing traffic is best effort
service with minimal loss, even at the cost of delays on the order of
tens of milliseconds. By placing it into a separate queue or
class, it may be ensured minima or maxima consistent with a specific
service level agreement. By placing it into a separate queue or
class, the routing it supports is helped to converge.
Typical configurations use ECN [10] or random loss to implement
active queue management [4], and may impose a minimum or maximum
rate.
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2.7 Streaming Video
The streaming video traffic class serves applications like Windows
Media Player or RealAudio. These may use proprietary protocols, or
may use TCP. It is specified in [5].
The fundamental service offered to streaming video is best effort
service. By placing it into a separate queue or class, it may be
ensured minima or maxima consistent with a specific service level
agreement.
Typical configurations use ECN [10] or random loss to implement
active queue management [4], and may impose a minimum or maximum
rate.
2.8 Telephony Signaling
The Telephony Signaling traffic class serves network control
applications like SIP and H.323 when used to route Voice on IP, Video
on IP, and related applications. It is specified in [5].
The fundamental service offered to Telephony Signaling traffic is
best effort service with minimize loss. The reason for this is to
maximize the speed of such routing, and avoid the poor user
experience that results from loss of control traffic. By placing it
into a separate queue or class, it may be ensured minima or maxima
consistent with a specific service level agreement.
Typical configurations use ECN [10] or random loss to implement
active queue management [4], and may impose a minimum or maximum
rate. The AQM parameters are specified in such a manner as to permit
relatively deep queues to form temporarily.
2.9 Network Management
The management traffic class serves applications that are necessary
to manage the network, such as SNMP servers, but which implement no
congestion avoidance procedure. It is specified in [5].
The fundamental service offered to the network traffic class is best
effort service with minimization of loss. By placing it into a
separate queue or class, it may be ensured minima or maxima
consistent with a specific service level agreement.
Typical configurations use random loss to implement active queue
management [4], to maximize the utility of network management
applications while protecting the network in the event of an
overload.
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2.10 Scavenger class
The scavenger traffic class serves applications which run over TCP
[1][7][7] or a transport with a consistent congestion avoidance
procedure, and which the user is willing to accept service without
guarantees. It is specified in [4].
The fundamental service offered to the scavenger traffic class is
best effort service. By placing it into a separate queue or class,
it may be ensured minima or maxima consistent with a specific service
level agreement.
Typical configurations use ECN [10] or random loss to implement
active queue management [4]. It generally does not impose a minimum
or maximum rate.
2.11 Default traffic class
The default traffic class serves applications which have not been
otherwise specified, but which run over TCP [1][7][7] or a transport
with a consistent congestion avoidance procedure. It is specified in
[5].
The fundamental service offered to the default traffic class is best
effort service with active queue management to limit over-all delay.
By placing it into a separate queue or class, it may be ensured
minima or maxima consistent with a specific service level agreement.
Typical configurations use ECN [10] or random loss to implement
[4]active queue management [4], and may impose a minimum or maximum
rate on the queue.
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3. Reflexive DSCP Policy
In reviewing the specific use of the Differentiated Services
Architecture for supporting the Internet Emergency Preparedness
System, we found what we believe is a general issue. This is that
even though a client or peer can connect to a server or peer with a
predictable DSCP value, the response does not have a predictable DSCP
value. We consider the issues, and recommend an approach to
application policy regarding the DSCP.
Figure 1 presents a connection being placed between two applications
across a differentiated services network.
. . . . . . . . . . . .
. . . . . .
. Client . . . . Server .
. /----------/ . . /------------/ . . /---------------/.
. Router -----/----- Router Router ----/----- Router .
. . . . . .
. . . . . .
. . . . . . . . . . . .
Figure 1: Connection across a network
A behavior aggregate originated in part by a certain client toward a
given server in a remote network may have certain application
requirements, such as requiring service appropriate to an ERP
application, video stream, or voice. One application may use
different aggregates for different purposes, and therefore have
different requirements. So the application may not be able to tell,
a priori, with what DSCP it should use or respond.
In addition, DSCPs have local significance in the Differentiated
Services Architecture. It is possible and perhaps likely that a
behavior aggregate might use different code points in different
networks.
There are a number of possible approaches to this issue. The
simplest, which we fear is currently standard in Differentiated
Services hosts, is to simply select a default value, such as "always
make TCP applications use AF11". For some applications, such as
voice (EF), this approach is appropriate, but for many it is not.
3.1 Default DSCP policy in a responder
When a system accepts sessions initiated from another system, and
there is no specific local policy, the responder SHOULD use the same
DSCP Group as its request. Thus, if a TCP SYN arrives using any of
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AF11, AF12, or AF13, the TCP SYN-ACK and subsequent messages SHOULD
use AF11 as the DSCP. When in doubt as to the set of DSCP code
points comprising a DSCP Group, it SHOULD respond with exactly the
same DSCP.
There has been interest of late in changing the quality of service
behavior for different portions of the same session, such as on a
per-URL basis. The requester could initiate this. Thus, if the DSCP
received on one TCP segment differs from the TCP used on a prior TCP
segment in a session, the new DSCP SHOULD be reflected unless local
policy prevents this.
One way to implement this requires the receiving transport (TCP,
SCTP, etc) to save the received DSCP and use an API to determine the
correct responding DSCP from a configuration file. The configuration
file lists the 64 possible DSCP values and the correct response. In
most cases, the two SHOULD be the same, but the twelve AFxy code
points map to AFx1. Local policy MAY update this mapping.
3.2 Application-directed DSCP policy
The originator of a session, which is to say the application that
opens it, SHOULD normally select the DSCP value used. This, of
course, needs to be consistent with local network policy, and may be
dictated entirely by that policy.
The application would do this through an API, ideally one that maps
the application to a DSCP value through local administrative policy.
Thus, the API could set the DSCP for signaling of voice calls to a
specific value, such as AF31. It would be better, though, if the API
were to set it to a key word such as "VoiceSignaling" or
"DatabaseAccess", and enable the network administration to interpret
the key word to an appropriate code point. One way to implement this
would be for the API code to look the key word up in a file or an
LDAP Policy.
It is possible for the responding application to use this same API.
For example, separate policies might apply to database records of one
type and database records of another type, something that only the
database access application could determine. It is also possible for
the application exchange to communicate a desired DSCP, and the
responding application to use the API accordingly. In such a case,
the application exchange MUST specify the key word rather than the
specific DSCP, as it cannot know the applicable policy in the
responder's network.
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4. Security Considerations
This document discusses policy, and describes a recommended default
policy, for the use of a Differentiated Services Code Point by
transports and applications. If implemented as described, it should
ask the network to do nothing that the network has not already
allowed. If that is the case, no new security issues should arise
from the use of such a policy.
It is possible, however, for the policy to be applied incorrectly, or
for another policy to be applied, which would be incorrect in the
network. In that case, a policy issue exists which the network must
detect, assess, and deal with. This is a known security issue in any
network dependent on policy-directed behavior.
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5. Acknowledgements
The author acknowledges a great many inputs, omst notably from Bruce
Davie, Dave Oran, and Rei Atarashi.
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Normative References
[1] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Zhang, L., Berson, S., Herzog, S. and S. Jamin, "Resource
ReSerVation Protocol (RSVP) -- Version 1 Functional
Specification", RFC 2205, September 1997.
[4] Howard, L., "An Approach for Using LDAP as a Network
Information Service", RFC 2307, March 1998.
[5] Nichols, K., Blake, S., Baker, F. and D. Black, "Definition of
the Differentiated Services Field (DS Field) in the IPv4 and
IPv6 Headers", RFC 2474, December 1998.
[6] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z. and W.
Weiss, "An Architecture for Differentiated Services", RFC 2475,
December 1998.
[7] Allman, M., Paxson, V. and W. Stevens, "TCP Congestion
Control", RFC 2581, April 1999.
[8] Floyd, S. and T. Henderson, "The NewReno Modification to TCP's
Fast Recovery Algorithm", RFC 2582, April 1999.
[9] Heinanen, J., Baker, F., Weiss, W. and J. Wroclawski, "Assured
Forwarding PHB Group", RFC 2597, June 1999.
[10] Ramakrishnan, K., Floyd, S. and D. Black, "The Addition of
Explicit Congestion Notification (ECN) to IP", RFC 3168,
September 2001.
[11] Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec, J.,
Courtney, W., Davari, S., Firoiu, V. and D. Stiliadis, "An
Expedited Forwarding PHB (Per-Hop Behavior)", RFC 3246, March
2002.
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Informative References
[12] Bonaventure, O. and S. De Cnodder, "A Rate Adaptive Shaper for
Differentiated Services", RFC 2963, October 2000.
[13] "International Emergency Preparedness Scheme", ITU E.106, March
2000.
[14] "Service Description for an International Emergency Multimedia
Service (Draft)", ITU-T F.706, August 2001.
Author's Address
Fred Baker
Cisco Systems
1121 Via Del Rey
Santa Barbara, CA 93117
US
Phone: +1-408-526-4257
Fax: +1-413-473-2403
EMail: fred@cisco.com
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