Network Working Group D. Grossman
Request for Comments: 3260 Motorola, Inc.
Updates: 2474, 2475, 2597 April 2002
Category: Informational
New Terminology and Clarifications for Diffserv
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This memo captures Diffserv working group agreements concerning new
and improved terminology, and provides minor technical
clarifications. It is intended to update RFC2474, RFC2475 and RFC
2597. When RFCs 2474 and 2597 advance on the standards track, and
RFC2475 is updated, it is intended that the revisions in this memo
will be incorporated, and that this memo will be obsoleted by the new
RFCs.
1. IntrodUCtion
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
RFC2474, RFC2475 and RFC2597. RFC2598 has been obsoleted by RFC
3246, and clarifications agreed by the group were incorporated in
that revision.
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 (as well as in the Policy WG [6]), 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
classifier 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.
Also note that the definition of a "Service Provisioning Policy" is
unchanged from RFC2475. RFC2475 defines a "Service Provisioning
Policy as "a policy which defines how traffic conditioners are
configured on DS boundary nodes and how traffic streams are mapped to
DS behavior aggregates to achieve a range of services." According to
one definition given in RFC3198 [6], a policy is "...a set of rules
to administer, manage, and control Access to network resources".
Therefore, the relationship between an SLS and a service provisioning
policy is that the latter is, in part, the set of rules that eXPress
the parameters and range of values that may be in the former.
Further note that this definition is more restrictive than that in
RFC3198.
3. Usage of PHB Group
RFC2475 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 RFC2597 [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).
RFC2597 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 RFC2597 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 RFC2475. 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
RFC2475 definition of PHB Group. RFC2475 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. RFC2474 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 subsequent to the
publication of RFC2474, they were assigned for explicit congestion
notification, as defined in RFC3168 [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 BCP 37, IANA Allocation
Guidelines for Values in the Internet Protocol and Related Headers
[5]. The IPV4 Type-of-Service (TOS) field and the IPV6 traffic class
field are superseded 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 RFC2474, as clarified in section 8 of this
memo.
The consensus of the DiffServ working group is that BCP 37 correctly
restates the structure of the former TOS and traffic class fields.
Therefore, for use in future documents, including the next update to
RFC2474, 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 used by Diffserv.
When RFC2474 is updated, consideration should be given to changing
the designation "currently unused (CU)" to "explicit congestion
notification (ECN)" and referencing RFC3168 (or its successor).
The update should also reference BCP 37.
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
Aggregate (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.
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 receives a packet
with a DSCP which it does not understand.
RFC2475 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, RFC2474 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."
RFC2474 is principally concerned with DS-interior nodes. However,
this behavior could also be performed in DS-ingress nodes AFTER the
traffic conditioning required by RFC2475 (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 RFC
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 imperative was
"SHOULD" rather than "SHALL".
The intent of RFC2475 clearly concerns DS-ingress nodes, or more
precisely, the ingress traffic conditioning function. This is
another context where the "SHOULD" in RFC2474 provides the
flexibility to do what the group intended. Such tortured readings
are not desirable.
Therefore, the statement in RFC2474 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 RFC2475 for that
case.
There was a similar issue, which manifested itself with the first
incarnation of Expedited Forwarding (EF). RFC2598 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 to 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 RFC2475 would appear to apply to this case. RFC3246
[7] clarifies this point.
7. No Backward Compatibility With RFC1349
At least one implementor has expressed confusion about the
relationship of the DSField, as defined in RFC2474, to the use of
the TOS bits, as described in RFC1349. The RFC1349 usage was
intended to interact with OSPF extensions in RFC1247. These were
never widely deployed and thus removed by standards action when STD
54, RFC2328, was published. The processing of the TOS bits is
described as a requirement in RFC1812 [8], RFC1122 [9] and RFC1123
[10]. RFC2474 states:
"No attempt is made to maintain backwards compatibility with the
"DTR" or TOS bits of the IPv4 TOS octet, as defined in [RFC791].",
In addition, RFC2474 obsoletes RFC1349 by IESG action. For
completeness, when RFC2474 is updated, the sentence should read:
"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 [RFC1812], [RFC1122] and [RFC1123] is also obsoleted
by this memo. Also see [RFC2780]."
8. IANA Considerations
IANA has requested clarification of a point in RFC2474, concerning
registration of experimental/local use DSCPs. When RFC2474 is
revised, the following should be added to Section 6:
IANA is requested to maintain a registry of RECOMMENDED DSCP
values assigned by standards action. EXP/LU values are not to be
registered.
9. 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 Standard (or if some issue arises that forces
recycling at Proposed). RFC2475 is expected to be updated at about
the same time as RFC2474. Those updates will also obsolete this
memo.
RFC2474: 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. Clarify that
only RECOMMENDED DSCPs assigned by standards action are to be
registered by IANA.
RFC2475: 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.
RFC2497: revise to reflect understanding that, AF classes are
instances of the AF PHB group, and are not collectively a PHB
group.
In addition, RFC3246 [7] has added a reference to RFC2475 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.
10. Security Considerations
Security considerations are addressed in RFC2475.
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. Mike Ayers, Mike
Heard and Andrea Westerinen provided valuable editorial comments.
Normative References
[1] Nichols, K., Blake, S., Baker, F. and D. Black, "Definition of
the Differentiated Services Field (DS Field) in the IPv4 and
IPv6 Headers", RFC2474, December 1998.
[2] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z. and W.
Weiss, "An Architecture for Differentiated Services", RFC2475,
December 1998.
[3] Heinanen, J., Baker, F., Weiss, W. and J. Wrocklawski, "Assured
Forwarding PHB Group", RFC2597, June 1999.
[4] Ramakrishnan, K., Floyd, S. and D. Black "The Addition of
Explicit Congestion Notification (ECN) to IP", RFC3168,
September 2001.
[5] Bradner, S. and V. Paxon, "IANA Allocation Guidelines for Values
in the Internet Protocol and Related Headers", BCP 37, RFC2780,
March 2000.
[6] Westerinen, A., Schnizlein, J., Strassner, J., Scherling, M.,
Quinn, B., Herzog, S., Huynh, A., Carlson, M., Perry, J. and S.
Waldbusser, "Terminology for Policy-Based Management", RFC3198,
November 2001.
[7] Davie, B., Charny, A., Baker, F., Bennett, J.C.R., Benson, K.,
Le Boudec, J., Chiu, A., Courtney, W., Cavari, S., Firoiu, V.,
Kalmanek, C., Ramakrishnam, K. and D. Stiliadis, "An Expedited
Forwarding PHB (Per-Hop Behavior)", RFC3246, March 2002.
[8] Baker, F., "Requirements for IP Version 4 Routers", RFC1812,
June 1995.
[9] Braden, R., "Requirements for Internet Hosts -- Communications
Layers", STD 3, RFC1122, October 1989.
[10] Braden, R., "Requirements for Internet Hosts -- Application and
Support", STD 3, RFC1123, October 1989.
Author's Address
Dan Grossman
Motorola, Inc.
20 Cabot Blvd.
Mansfield, MA 02048
EMail: dan@dma.isg.mot.com
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