Diffserv Working Group Dan Grossman
Internet Draft Motorola, Inc.
Expires: Sepetember 2001
draft-ietf-diffserv-new-terms-05.txt
August, 2001
New Terminology for Diffserv
Status of this Memo
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Abstract
This memo captures Diffserv working group agreements concerning new
and improved terminology, and also provides minor technical
clarifications. It is intended to update RFC 2474, RFC 2475 and RFC
2597. When RFCs 2474 and 2475 advance on the standards track, and
RFC 2475 is updated, it is anticipated that the revisions in this
memo will be incorporated, and that this memo will be obsoleted by
the new RFCs.
Copyright Notice
Copyright (C) The Internet Society (1999, 2001). All Rights
Reserved.
1. Introduction
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As the Diffserv work has evolved, there have been several cases where
terminology has needed to be created or the definitions in Diffserv
standards track RFCs have needed to be refined. Some minor technical
clarifications were also found to be needed. This memo was created
to capture group agreements, rather than attempting to revise the
base RFCs and recycle them at proposed standard. It updates in part
RFC 2474, RFC 2475 and RFC 2597. RFC 2598 has been updated by RFC
XXXX (draft-ietf-diffserv-rfc2598bis), and clarifications agreed by
the group were incorporated in that update.
2. Terminology Related to Service Level Agreements (SLAs)
The Diffserv Architecture [2] uses the term "Service Level Agreement"
(SLA) to describe the "service contract... that specifies the
forwarding service a customer should receive". The SLA may include
traffic conditioning rules which (at least in part) constitute a
Traffic Conditioning Agreement (TCA). A TCA is "an agreement
specifying classifier rules and any corresponding traffic profiles
and metering, marking, discarding and/or shaping rules which are to
apply...."
As work progressed in Diffserv, it came to be believed that the
notion of an "agreement" implied considerations that were of a
pricing, contractual or other business nature, as well as those that
were strictly technical. There also could be other technical
considerations in such an agreement (e.g., service availability)
which are not addressed by Diffserv. It was therefore agreed that
the notions of SLAs and TCAs would be taken to represent the broader
context, and that new terminology would be used to describe those
elements of service and traffic conditioning that are addressed by
Diffserv.
- A Service Level Specification (SLS) is a set of parameters and
their values which together define the service offered to a traffic
stream by a DS domain.
- A Traffic Conditioning Specification (TCS) is a set of parameters
and their values which together specify a set of classfier rules
and a traffic profile. A TCS is an integral element of an SLS.
Note that the definition of "Traffic stream" is unchanged from RFC
2475. A traffic stream can be an individual microflow or a group of
microflows (i.e., in a source or destination DS domain) or it can
be a BA. Thus, an SLS may apply in the source or destination DS
domain to a single microflow or group of microflows, as well as to a
BA in any DS domain.
3. Usage of PHB Group
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RFC 2475 defines a Per-hop behavior (PHB) group to be:
"a set of one or more PHBs that can only be meaningfully specified
and implemented simultaneously, due to a common constraint applying
to all PHBs in the set such as a queue servicing or queue
management policy. A PHB group provides a service building block
that allows a set of related forwarding behaviors to be specified
together (e.g., four dropping priorities). A single PHB is a
special case of a PHB group."
One standards track PHB Group is defined in RFC 2597 [3], "Assured
Forwarding PHB Group". Assured Forwarding (AF) is a type of forwarding
behavior with some assigned level of queuing resources and three drop
precedences. An AF PHB Group consists of three PHBs, and uses three
Diffserv Codepoints (DSCPs).
RFC 2597 defines twelve DSCPs, corresponding to four independent AF
classes. The AF classes are referred to as AF1x, AF2x, AF3x, and AF4x
(where 'x' is 1, 2, or 3 to represent drop precedence). Each AF class
is one instance of an AF PHB Group.
There has been confusion expressed that RFC 2597 refers to all four AF
classes with their three drop precedences as being part of a single PHB
Group. However, since each AF class operates entirely independently of
the others, (and thus there is no common constraint among AF classes as
there is among drop precedences within an AF class) this usage is
inconsistent with RFC 2475. The inconsistency exists for historical
reasons and will be removed in future revisions of the AF specification.
It should now be understood that AF is a _type_ of PHB group, and each
AF class is an _instance_ of the AF type.
Authors of new PHB specifications should be careful to adhere to the RFC
2475 definition of PHB Group. RFC 2475 does not prohibit new PHB
specifications from assigning enough DSCPs to represent multiple
independent instances of their PHB Group. However, such a set of DSCPs
must not be referred to as a single PHB Group.
4. Definition of the DS Field
Diffserv uses six bits of the IPV4 or IPV6 header to convey the Diffserv
Codepoint (DSCP), which selects a PHB. RFC 2474 attempts to rename the
TOS octet of the IPV4 header, and Traffic Class octet of the IPV6
header, respectively, to the DS field. The DS Field has a six bit
Diffserv Codepoint and two "currently unused bits".
It has been pointed out that this leads to inconsistencies and
ambiguities. In particular, the "Currently Unused" (CU) bits of the DS
Field have not been assigned to Diffserv, and have been assigned an
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experimental use for an explicit congestion notification scheme [4].
In the current text, a DSCP is, depending on context, either an encoding
which selects a PHB or a sub-field in the DS field which contains that
encoding.
The present text is also inconsistent with the IANA allocation
guidelines [5]. The IPV4 Type-of-Service (TOS) field and the IPV6
traffic class field are superceded by the 6 bit DS field and a 2 bit CU
field. The IANA allocates values in the DS field following the IANA
considerations section in RFC 2474. Experimental uses of the CU field
are assigned after IESG approval processes. Permanent values in the CU
field are allocated following a Standards Action process.
The consensus of the DiffServ working group is that [5] correctly
restates the structure of the former TOS and traffic class fields.
Therefore, for use in future drafts, including the next update to RFC
2474, the following definitions should apply:
- the Differentiated Services Field (DSField) is the six most
significant bits of the (former) IPV4 TOS octet or the (former)
IPV6 Traffic Class octet.
- the Differentiated Services Codepoint (DSCP) is a value which is
encoded in the DS field, and which each DS Node MUST use to select
the PHB which is to be experienced by each packet it forwards.
The two least significant bits of the IPV4 TOS octet and the IPV6
Traffic Class octet are not presently used by Diffserv.
The update should also reference the IANA Allocation Guidelines,
assuming that they are published as an RFC.
5. Ordered Aggregates and PHB Scheduling Classes
Work on Diffserv support by MPLS Label Switched Routers (LSRs) led to
the realization that a concept was needed in Diffserv to capture the
notion of a set of BAs with a common ordering constraint. This
presently applies to AF behavior aggregates, since a DS node may not
reorder packets of the same microflow if they belong to the same AF
class. This would, for example, prevent an MPLS LSR which was also a
DS node from discriminating between packets of an AF Behavior
Agrregeate (BA) based on drop precedence and forwarding packets of
the same AF class but different drop precedence over different LSPs.
The following new terms are defined.
PHB Scheduling Class: A PHB group for which a common constraint is
that ordering of at least those packets belonging to the same
microflow must be preserved.
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Ordered Aggregate (OA): A set of Behavior Aggregates that share an
ordering constraint. The set of PHBs that are applied to this set
of Behavior Aggregates constitutes a PHB scheduling class.
6. Unknown/Improperly Mapped DSCPs
Several implementors have pointed out ambiguities or conflicts in the
Diffserv RFCs concerning behavior when a DS-node recieves a packet
with a DSCP which it does not understand.
RFC 2475 states:
"Ingress nodes must condition all other inbound traffic to ensure
that the DS codepoints are acceptable; packets found to have
unacceptable codepoints must either be discarded or must have their
DS codepoints modified to acceptable values before being forwarded.
For example, an ingress node receiving traffic from a domain with
which no enhanced service agreement exists may reset the DS
codepoint to the Default PHB codepoint [DSFIELD]."
On the other hand, RFC 2474 states:
"Packets received with an unrecognized codepoint SHOULD be
forwarded as if they were marked for the Default behavior (see Sec.
4), and their codepoints should not be changed."
The intent in RFC 2474 principally concerned DS-interior nodes.
However, this behavior could also be performed in DS-ingress nodes
AFTER the traffic conditioning required by RFC 2475 (in which case,
an unrecognized DSCP would occur only in the case of
misconfiguration). If a packet arrives with a DSCP that hadn't been
explicitly mapped to a particular PHB, it should be treated the same
way as a packet marked for Default. The alternatives were to assign
it another PHB, which could result in misallocation of provisioned
resources, or to drop it. Those are the only alternatives within the
framework of 2474. Neither alternative was considered desirable.
There has been discussion of a PHB which receives worse service than
the default; this might be a better alternative. Hence the
imperitive was"SHOULD" rather than "SHALL".
The intent in RFC 2475 clearly concerns DS-ingress nodes, or to be
more precise, the ingress traffic conditioning function. This is
another context where the "SHOULD" in RFC 2474 gives the flexibility
to do what the group intended. Such tortured readings are not
desirable.
Therefore, the statement in RFC 2474 will be clarified to indicate
that it is not intended to apply at the ingress traffic conditioning
function at a DS-ingress node, and cross reference RFC 2475 for that
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case.
There was a similar issue, which manifested itself with the first
incarnation of Expedited Forwarding (EF). RFC 2598 states:
To protect itself against denial of service attacks, the edge of a
DS domain MUST strictly police all EF marked packets to a rate
negotiated with the adjacent upstream domain. (This rate must be
<= the EF PHB configured rate.) Packets in excess of the
negotiated rate MUST be dropped. If two adjacent domains have not
negotiated an EF rate, the downstream domain MUST use 0 as the rate
(i.e., drop all EF marked packets).
The problem arose in the case of misconfiguration or routing
problems. An egress DS-node at the edge of one DS-domain forwards
packets an ingress DS-node at the edge of another DS domain. These
packets are marked with a DSCP that the egress node understands to
map to EF, but which the ingress node does not recognize. The
statement in RFC 2475 would appear to apply to this case. RFC XXXX
(draft-ietf-diffserv-rfc2598bis) clarifies this point.
7. No Backward Compatibility With RFC 1349
It has been pointed out that that RFC 2474 should have stated a bit
more explicitly that the TOS bit usage described in RFC 1349 is
obsoleted. This useage was intended to interact with OSPF extensions
in RFC 1247, which were never widely deployed. The processing of the
TOS bits is described as a requirement in RFC 1812. Although this is
a direct implication of the following sentence in RFC 2474:
"No attempt is made to maintain backwards compatibility with
the "DTR"
or TOS bits of the IPv4 TOS octet, as defined in [RFC791]."
Further clarification is needed. The previous sentence should be
augmented as follows when RFC 2474 is updated:
"No attempt is made to maintain backwards compatibility with the
"DTR/MBZ"
or TOS bits of the IPv4 TOS octet, as defined in [RFC791] and
[RFC1349]. This implies that TOS bit processing as described in
sections 5.2.4.3 and 5.3.2 of [RFC1812] is also obsoleted by this
memo. Also see [RFC2780]."
8. Summary of Pending Changes
The following standards track and informational RFCs are expected to
be updated to reflect the agreements captured in this memo. It is
intended that these updates occur when each standards track RFC
progresses to Draft (or if some issue arises that forces recycling at
Proposed). RFC 2475 is expected to be updated at about the same time
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as RFC 2474. These updates will also obsolete this memo.
RFC 2474: revise definition of DS field. Clarify that the
suggested default forwarding in the event of an unrecognized DSCP
is not intended to apply to ingress conditioning in DS-ingress
nodes. Clarify effects on RFC1349 and RFC1812.
RFC 2475: revise definition of DS field. Add SLS and TCS
definitions. Update body of document to use SLS and TCS
appropriately. Add definitions of PHB scheduling class and ordered
aggregate.
RFC 2497: revise to reflect understanding that AF classes are
instances of the AF PHB group, and are not collectively a PHB
group.
In addition, RFCXXXX (draft-ietf-diffserv-rfc2598bis) put a reference to
RFC 2475 in the security considerations section to cover the case of a
DS egress node receiving an unrecognized DSCP which maps to EF in the DS
ingress node.
7. Security Considerations
Security considerations are addressed in RFC 2475.
Acknowledgements This memo captures agreements of the Diffserv working
group. Many individuals contributed to the discussions on the Diffserv
list and in the meetings. The Diffserv chairs were Brian Carpenter and
Kathie Nichols. Among many who participated actively in these
discussions were Lloyd Wood, Juha Heinanen, Grenville Armitage, Scott
Brim, Sharam Davari, David Black, Gerard Gastaud, Joel Halpern, John
Schnizlein, Francois Le Faucheur, and Fred Baker [Author's note: who
have I forgotten? Please respond privately]. Mike Ayers provided
valuable editorial comments.
References
[1] Nichols, Blake, Baker, Black, "Defintion of the Differentiated
Services Field (DS Field) in the IPv4 and IPv6 Headers" RFC
2474, December 1998.
[2] Blake, Black, Carlson, Davies, Wang and Weiss "An Architecture
for Differentiated Services", RFC 2475, December 1998.
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[3] Heinanen, Baker, Weiss, Wrocklawski, "Assured Forwarding PHB
Group", RFC 2597
[4] Ramakrishnan and Floyd, "A proposal to add Explicit Congestion
Notification (ECN)" to IP, RFC 2481, January 1999
[5] Bradner and Paxon, "IANA Allocation Guidelines for Values in the
Internet Protocol and Related Headers", RFC2780, March 2000
Author's Address
Dan Grossman
Motorola, Inc.
20 Cabot Blvd.
Mansfield, MA 02048
Email: dan@dma.isg.mot.com
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