Internet DRAFT - draft-polk-tsvwg-diffserv-stds-problem-statement
draft-polk-tsvwg-diffserv-stds-problem-statement
Network WG James Polk
Internet-Draft Cisco
Intended status: Informational July 9, 2012
Expires: January 9, 2012
The Problem Statement for the Standard
Configuration of DiffServ Service Classes
draft-polk-tsvwg-diffserv-stds-problem-statement-00.txt
Abstract
This document describes the problem statement on two recently
proposed expansions to DiffServ. The first of these expansions
proposes updating the informational RFC 4594 document to standards
track status, while making the necessary changes to make it current;
for example, creating more granular traffic treatments, some with
new Per Hop Behaviors (PHB). The second proposal defines 6 new
DiffServ Codepoints necessary from these new PHBs in the proposal
within the first draft.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 9, 2012.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Brief Overview of RFC 4594 and RFC 5127 . . . . . . . . . . . 3
2.1 Brief Overview of RFC 4594 . . . . . . . . . . . . . . . . 3
2.2 Brief Overview of RFC 5127 . . . . . . . . . . . . . . . . 4
3. Brief Discussion of the RFC 4594 Update Draft . . . . . . . . 5
4. Conclusion and What's Next . . . . . . . . . . . . . . . . . 7
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1 Normative References . . . . . . . . . . . . . . . . . . 8
8.2 Informative References . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
Differentiated Services (DiffServ) [RFC2474] creates an IP header
marking or indicator with which intermediate nodes (i.e., routers
and switches) can make policy decisions. These 6-bit values are
called Differentiated Services Codepoint Point (DSCP) values. DSCP
values are used to differentiate packet treatment within an
intermediate node, not across a network, as the conditions affecting
that marking are different within each node. This is called Per Hop
Behavior (PHB). In other words, even though a packet has the same
DSCP from source to destination, it can and often does experience
different treatment depending on the conditions of the nodes it
traverses on its journey.
The DiffServ architecture allows for DSCP values within a packet to
be changed, or remarked, any number of times. In other words, a
packet can have its DSCP remarked at every layer-3 hop throughout
the life of that packet. This practice actually occurs infrequently,
but it is allowed.
At issue is a combination of the number of networks or endpoints
that are choosing to use DiffServ markings, and the number of
administrative domains (called "networks" in this document) a packet
traverses with different policies for how packet flows of a similar
type (e.g., a voice flow, or an email flow, etc.) are to be marked.
The community presently has RFC 4594 [RFC4594], which is an
informational guideline on how networks can or should mark certain
packet flows with differing traffic characteristics using DiffServ.
There are several reasons why this informational RFC lacks the
necessary clarity and strength to reach widespread adoption:
o confusion between RFC 4594 and RFC 5127 [RFC5127], the latter of
which is for aggregating many 6-bit DSCP values into a 3-bit (8
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value) field used specifically by service provider (SP) networks.
o some believe both RFCs are for SPs, while others ignore RFC 5127
and use RFC 4594 as if it were standards track or BCP.
o some believe RFC 5127 is for SPs only, and want RFC 4594 to
reduce the number of DSCPs within its guidelines to recommend
using only 3 or 4 DSCPs. This seems to stem from a manageability
and operational perspective.
o some know RFC 4594 is informational and do not follow its
guidelines specifically because it is informational.
o some use DSCP values that are not defined within RFC 4594, making
mapping between different networks using similar or identical
application flows difficult.
o some believe enterprise networks should not use either RFC except
at the edge of their networks, where they directly connect to SP
networks.
o some argue that the services classes guidance per class is too
broad and are therefore not sure in which service class a
particular application is to reside.
This document is not intended to reach RFC status. Rather, it is to
stimulate discussion on both RFC 4594 and 5127 to lessen existing
confusion within the community. It should be noted that RFC 4594 has
an offered update within TSVWG [ID-4594-UPDATE]. This draft has
created some heated discussions within that WG before and during the
Paris IETF meeting.
First, we'll discuss briefly RFCs 4594 and 5127 in Section 2. Then
we will discussion what the update to RFC 4594 proposes differently
and what we expect to happen to RFC 5127 in Section 3.
2. Brief Overview of RFC 4594 and RFC 5127
2.1 Brief Overview of RFC 4594
Essentially, RFC 4594 is a guideline for how to choose which DSCP to
use based on the traffic characteristics an application flow needs
to experience within a network for optimal performance. RFC 4594
specifically points to several existing standards-track DiffServ
RFCs to augment the text in each of those RFCs, without violating
any of the rules within each of those documents. RFC 4594:
o painstakingly lays out definitions and guidelines for each service
class.
o clearly indicates each service class's tolerance to delay, jitter
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and packet loss.
o details the conditioning treatments at the Differentiated Services
(DS) edge.
o categorizes traffic characteristics into 12 service classes
utilizing one or more DSCPs:
Network Control Broadcast Video
Telephony Low-Latency Data
Signaling OAM
Multimedia Conferencing High-throughput Data
Realtime Interactive Standard
Multimedia Streaming Low-priority Data
2.2 Brief Overview of RFC 5127
At its barest, RFC 5127 recommends that, of the many service classes
described within RFC 4594, each having different traffic
characteristics, similar service classes be grouped or aggregated
into 3, 4, or 5 markings for SP traversal. This limitation of the
number of individual service classes is partly to reduce the number
of separate distinctions traversing over their network because SPs
have difficulty managing what is deemed 'too many' different
classes. Another part for this reduction is customer expectations of
meeting contractual Service Level Agreements (SLAs).
To this end, and perhaps because of it, MPLS was designed with only
8 values of priority differentiation, i.e., the 3 EXP bits. To be
fair, LAN based IEEE has only a 3-bit priority field as well within
its specifications, known as the Priority Code Point (PCP), as part
of the 802.1Q header spec. IEEE 802.1e, which defines QoS over
Wi-Fi, also only defines 8 levels (called User Priority or UP
codes).
The result is to have the IETF within RFC 5127 recommend the
following (which is Figure 2 within that RFC):
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------------------------------------------------------------
|Treatment |Treatment || DSCP |
|Aggregate |Aggregate || |
| |Behavior || |
|==========+==========++=====================================|
| Network | CS || CS6 |
| Control |(RFC 2474)|| |
|==========+==========++=====================================|
| Real- | EF || EF, CS5, AF41, AF42, AF43, CS4, CS3 |
| Time |(RFC 3246)|| |
|==========+==========++=====================================|
| Assured | AF || CS2, AF31, AF21, AF11 |
| Elastic |(RFC 2597)||-------------------------------------|
| | || AF32, AF22, AF12 |
| | ||-------------------------------------|
| | || AF33, AF23, AF13 |
|==========+==========++=====================================|
| Elastic | Default || Default, (CS0) |
| |(RFC 2474)||-------------------------------------|
| | || CS1 |
------------------------------------------------------------
Figure 1: Treatment Aggregate Behavior
RFC 5127 goes on to recommend the marking and treatments on either
side of the provider edge remain the same. In other words, the DSCP
values remain the same and are used to determine which queue to
place the packets into within the aggregates, where the packets are
treated the same within that tunnel until the egress provider edge.
Many within enterprise networks do not pay attention to what RFC
5127 says because they are sufficiently removed from dealing with
the constraints of very few DSCP values or the need to aggregate
DSCP values into groups.
3. Brief Discussion of the RFC 4594 Update Draft
The RFC 4594 update draft [ID-4594-UPDATE] proposes to update what
has occurred since RFC 4594 was written (i.e., 2006), in which more
granular service classes can be differentiated by application
requirements. For example, Figure 2 within RFC 4594 identifies
"Telephony" as having 'Fixed-size small packets'. That is not true
for today's video flow, therefore it needs to be modified. The
update draft currently breaks out audio and video separately to
reflect this different, as well as the ability to treat each traffic
type differently within a network. Another example is gaming and
TCP. The two were believed by most, and it is still believed by many
that gaming requires a UDP delivery due to the requirements for
timely delivery of packets and that retransmissions would cause
delays and bad things to happen to gaming applications. This was
proved false within [ID-TCMTF], in which the author of that document
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had a presentation showing TCP was used and viable.
[RFC5865] created a new Expedited Forwarding (EF) DSCP value called
VOICE-ADMIT, the second time an application is identified within the
DiffServ realm. The first was the service class Broadcast Video,
which is poorly used within RFC 4594 because other types of flows
can be 'broadcast' other than video, such as audio. From this,
[ID-4594-UPDATE] moved in two directions:
o it called out two service classes (audio and video), even though
audio and video packets are not the only types of packets within
each traffic characteristic.
o it removed "Video" from the Broadcast service class name.
From the resistance to this proposal within [ID-4594-UPDATE],
perhaps other service class label names should be used.
The draft also recognizes the differences in video traffic, even
though it is always carried over RTP [RFC3550]. Aside from silence
suppression, video traffic varies far more than audio traffic. For
example, video is
o far more variable in bandwidth utilization within the same flow.
o far more variable in packet size.
o at different business priorities in some networks based on a
configuration. For example, desktop video often is of less
important than Telepresence video on the same network. Lacking
congestion, the two are treated the same. When congestion exists,
one is given priority over the other.
Consequently any service class that contains video needs to account
for larger packet size variation than audio, which was equally true
in 2006, but not contained in RFC 4594.
Further, with the publication of RFC 5865, the concept of 'capacity
admitted' traffic flows have been defined within DiffServ, and are
being expanded with the proposal within this new draft
[ID-NEW-DSCPS]. There are differing opinions as to whether the
realtime Treatment Aggregate in Figure 1 above should also contain
these capacity admitted flows, or if 'capacity admitted' traffic
flows should have their own Treatment Aggregate containing all
realtime capacity admitted traffic. Mixing capacity admitted traffic
with unbounded realtime traffic seems to be trouble from a
predictability point of view within routers believing they
individually understand exactly how much traffic will be traversing
each interface and at what rate.
All this said, there is a valid argument to constrain or prevent any
DSCP value from being assigned to a single application, mostly due
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to the limitation of the overall number of DSCP values available for
use. [ID-4594-UPDATE] provides at least several applications per
service class (or DSCP); a fact many have overlooked to date.
[ID-4594-UPDATE] is not only about or because of realtime traffic.
It is also an overall update to the ideas and guidelines within RFC
4594, with the intent to make that document a standards track
document for interoperability purposes.
4. Conclusion and What's Next
Without attempting to fundamentally change the guidelines within RFC
5127, this effort should not be as controversial as it has been, if
we understand that those networks that need more granular traffic
treatments can be configured with more granularity while not
violating the needs of other networks that do not wish to be made
aware of the increased treatment differences.
Everyone involved in this discussion needs to have a clear
understanding of the difference points of view within the RFC 4594
effort (i.e., the RFC and the update draft) as well as within RFC
5127. One focuses on defining each service class and the other
focuses on determining which of the existing service classes go into
which aggregate, if present.
We hope to form a BoF on this subject that will explicitly *not*
form a working group or produce any documents, or even drafts, but
will gather the community from several (if not all) areas, and not
just within the transport area. That is the purpose of this draft:
to stimulate discussion towards the goal of discussion within the
community on DiffServ. If the community does not believe a BoF is
necessary, the work will proceed, or not, in TSVWG. Knowing how many
within the community have attended TSVWG in each meeting for the
last 9 or so years, it is felt that a much wider audience is
necessary, given how much impact [ID-4594-UPDATE] can potentially
have.
5. Acknowledgements
The author would like to thank Gorry Fairhurst and David Black for
their positive discussions towards the formation of a BoF in
Vancouver IETF. The author would also like to thank Paul Jones for
doing a valuable proof read to catch points I didn't make clear, as
well as identify simple nits I should have caught the nth time I
reread this.
6. IANA Considerations
There are no IANA considerations as a result of this document.
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7. Security Considerations
There are no security considerations within this document because it
will not be progressed beyond this individual contributor stage, and
all the specifying will be done in other drafts that will wholly
contain all the security considerations for this goal/idea.
8. References
8.1 Normative References
There are no normative references within this document.
8.2. Informative References
[RFC2474] K. Nichols, S. Blake, F. Baker, D. Black, "Definition of the
Differentiated Services Field (DS Field) in the IPv4 and
IPv6 Headers ", RFC 2474, December 1998
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC4594] J. Babiarz, K. Chan, F Baker, "Configuration Guidelines for
Diffserv Service Classes", RFC 4594, August 2006
[RFC5127] Chan, K., Babiarz, J., and F. Baker, "Aggregation of
DiffServ Service Classes", RFC 5127, February 2008.
[RFC5865] F. Baker, J. Polk, M. Dolly, "A Differentiated Services Code
Point (DSCP) for Capacity-Admitted Traffic", RFC 5865,
May 2010
[ID-4594-UPDATE] J. Polk, "Standard Configuration of DiffServ Service
Classes", "work in progress", March 2012
[ID-NEW-DSCPS] J. Polk, "New Differentiated Services Code Point
Assignments for Rich Media Traffic", "work in progress",
March 2012
[ID-TCMTF] J. Saldana, D. Wing, J. Fernandez Navajas, Muthu A M.
Perumal, J. Ruiz Mas, "Tunneling Compressed Multiplexed
Traffic Flows (TCMTF)", "work in progress", March 2012
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Authors' Address
James Polk
3913 Treemont Circle
Colleyville, Texas 76034
Phone: +1.817.271.3552
Email: jmpolk@cisco.com
Polk Expires January 9, 2012 [Page 9]
