TSVWG R. Geib, Ed.
Internet-Draft Deutsche Telekom
Intended status: Informational D. Black
Expires: September 10, 2015 EMC Corporation
March 9, 2015
DiffServ interconnection classes and practice
draft-ietf-tsvwg-diffserv-intercon-01
Abstract
This document proposes a limited set of DiffServ PHBs and codepoints
to be applied at (inter)connections of two separately administered
and operated networks. Many network providers operate MPLS using
Treatment Aggregates for traffic marked with different DiffServ PHBs,
and use MPLS for interconnection with other networks. This document
offers a simple interconnection approach that may simplify operation
of DiffServ for network interconnection among providers that use
MPLS.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 10, 2015.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Related work . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Applicability Statement . . . . . . . . . . . . . . . . . 4
1.3. Document Organization . . . . . . . . . . . . . . . . . . 4
2. MPLS and the Short Pipe tunnel model . . . . . . . . . . . . 4
3. Relationship to RFC 5127 . . . . . . . . . . . . . . . . . . 5
3.1. RFC 5127 Background . . . . . . . . . . . . . . . . . . . 6
3.2. Differences from RFC 5127 . . . . . . . . . . . . . . . . 6
4. The DiffServ-Intercon Interconnection Classes . . . . . . . . 7
4.1. End-to-end QoS: PHB and DS CodePoint Transparency . . . . 12
4.2. Treatment of Network Control traffic at carrier
interconnection interfaces . . . . . . . . . . . . . . . 13
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
7. Security Considerations . . . . . . . . . . . . . . . . . . . 14
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
8.1. Normative References . . . . . . . . . . . . . . . . . . 14
8.2. Informative References . . . . . . . . . . . . . . . . . 15
Appendix A. Appendix A Carrier interconnection related DiffServ
aspects . . . . . . . . . . . . . . . . . . . . . . 16
Appendix B. Appendix B The MPLS Short Pipe Model and IP traffic 18
Appendix C. Change log . . . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
DiffServ has been deployed in many networks. As described by section
2.3.4.2 of RFC 2475, remarking of packets at domain boundaries is a
DiffServ feature [RFC2475]. This draft proposes a set of standard
QoS classes and code points at interconnection points to which and
from which locally used classes and code points should be mapped.
RFC2474 specifies the DiffServ Codepoint Field [RFC2474].
Differentiated treatment is based on the specific DSCP. Once set, it
may change. If traffic marked with unknown or unexpected DSCPs is
received, RFC2474 recommends forwarding that traffic with default
(best effort) treatment without changing the DSCP markings. Many
networks do not follow this recommendation, and instead remark
unknown or unexpected DSCPs to the zero DSCP upon receipt for
consistency with default (best effort) forwarding in accordance with
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the guidance in RFC 2475 [RFC2474] to ensure that appropriate DSCPs
are used within a DiffServ domain.
This document is motivated by requirements for IP network
interconnection with DiffServ support among providers that operate
MPLS in their backbones, but is applicable to other technologies.
The operational simplifications and methods in this document help
align IP DiffServ functionality with MPLS limitations; further,
limiting DiffServ to a small number of Treatment Aggregates can
enable network traffic to leave a network with the same DSCPs that it
was received with, even if a different DSCP is used within the
network, thus providing an opportunity to extend consistent QoS
treatment across network boundaries.
In isolation, use of standard interconnection PHBs and DSCPs may
appear to be additional effort for a network operator. The primary
offsetting benefit is that the mapping from or to the interconnection
PHBs and DSCPs is specified once for all of the interconnections to
other networks that can use this approach. Otherwise, the PHBs and
DSCPs have to be negotiated and configured independently for each
network interconnection, which has poor scaling properties. Further,
end-to-end QoS treatment is more likely to result when an
interconnection code point scheme is used because traffic is remarked
to the same PHBs at all network interconnections. This document
envisions one-to-one DSCP remarking at network interconnections (not
n DSCP to one DSCP remarking).
In addition to the standard interconnecting PHBs and DSCPs,
interconnecting operators need to further agree on the tunneling
technology used for interconnection (e.g., MPLS, if used) and control
or mitigate the impacts of tunneling on reliability and MTU.
1.1. Related work
In addition to the activities that triggered this work, there are
additional RFCs and Internet-drafts that may benefit from an
interconnection PHB and DSCP scheme. RFC 5160 suggests Meta-QoS-
Classes to enable deployment of standardized end to end QoS classes
[RFC5160]. In private discussion, the authors of that RFC agree that
the proposed interconnection class- and codepoint scheme and its
enablement of standardised end to end classes would complement their
own work.
Work on signaling Class of Service at interconnection interfaces by
BGP [I-D.knoll-idr-cos-interconnect], [ID.idr-sla] is beyond the
scope of this draft. When the scheme in this document is used,
signaled access to QoS classes may be of interest. These two BGP
documents focus on exchanging SLA and traffic conditioning parameters
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and assume that common PHBs identified by the signaled DSCPs have
been established prior to BGP signaling of QoS.
1.2. Applicability Statement
This document is primarily applicable to use of Differentiated
Services for interconnection traffic between networks, and in
particular to interconnection of MPLS-based networks. The approach
described in this document is not intended for use within the
interconnected (or other) networks, where the approach specified in
RFC 5127 [RFC5127] is among the possible alternatives; see Section 3
for further discussion.
The Diffserv-Intercon approach described in this document simplifies
IP based interconnection to domains operating the MPLS Short Pipe
model to transport plain IP traffic terminating within or transiting
through the receiving domain. Transit traffic is reiceived and sent
with the same PHB and DSCP. Terminating traffic maintains the PHB
with which it was received, however the DSCP may change.
1.3. Document Organization
This document is organized as follows: section 2 reviews the MPLS
Short Pipe tunnel model for DiffServ Tunnels [RFC3270]; effective
support for that model is a crucial goal of this document. Section 3
provides background on RFC 5127's approach to traffic class
aggregation within a DiffServ network domain and explains why this
document uses a somewhat different approach. Section 4 introduces
DiffServ interconnection Treatment Aggregates, plus the PHBs and
DSCPs that are mapped to these Treatment Aggregates. Further,
section 4 discusses treatment of non-tunneled and tunneled IP traffic
and MPLS VPN QoS aspects. Finally Network Management PHB treatment
is described. Annex B describes the impact of the MPLS Short Pipe
model (penultimate hop popping) on QoS related IP interconnections.
2. MPLS and the Short Pipe tunnel model
The Pipe and Uniform models for Differentiated Services and Tunnels
are defined in [RFC2983]. RFC3270 adds the MPLS Short Pipe model in
order to support penultimate hop popping (PHP) of MPLS Labels,
primarily for IP tunnels and VPNs. The Short Pipe model and PHP have
become popular with many network providers that operate MPLS networks
and are now widely used to transport non-tunnelled IP traffic, not
just traffic encapsulated in IP tunnels and VPNs. This has important
implications for DiffServ functionality in MPLS networks.
RFC 2474's recommendation to forward traffic with unrecognized DSCPs
with Default (best effort) service without rewriting the DSCP has
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proven to be a poor operational practice. Network operation and
management are simplified when there is a 1-1 match between the DSCP
marked on the packet and the forwarding treatment (PHB) applied by
network nodes. When this is done, CS0 (the all-zero DSCP) is the
only DSCP used for Default forwarding of best effort traffic, so a
common practice is to use CS0 to remark traffic received with
unrecognized or unsupported DSCPs at network edges.
MPLS networks are more subtle in this regard, as it is possible to
encode the provider's DSCP in the MPLS TC field and allow that to
differ from the PHB indicated by the DSCP in the MPLS-encapsulated IP
packet. That would allow an unrecognized DSCP to be carried edge-to-
edge over an MPLS network, because the effective DSCP used by the
MPLS network would be encoded in the MPLS label TC field (and also
carried edge-to-edge); this approach assumes that a provider MPLS
label with the provider's TC field is present at all hops within the
provider's network.
The Short Pipe tunnel model and PHP violate that assumption because
PHP pops and discards the MPLS provider label carrying the provider's
TC field. That discard occurs one hop upstream of the MPLS tunnel
endpoint (which is usually at the network edge), resulting in no
provider TC info being available at tunnel egress. To ensure
consistent handling of traffic at the tunnel egress, the DSCP field
in the MPLS-encapsulated IP header has to contain a DSCP that is
valid for the provider's network; propagating another DSCP edge-to-
edge requires an IP tunnel of some form. See Annex B for a more
detailed discussion.
If transport of a large number (much greater than 4) DSCPs is
required across a network that supports this DiffServ interconnection
scheme, a tunnel or VPN can be provisioned for this purpose, so that
the inner IP header carries the DSCP that is to be preserved not to
be changed. From a network operations perspective, the customer
equipment (CE) is the preferred location for tunnel termination,
although a receiving domains Provider Edge router is another viable
option.
3. Relationship to RFC 5127
This document draws heavily upon RFC 5127's approach to aggregation
of DiffServ traffic classes for use within a network, but there are
some important differences caused by the characteristics of network
interconnects.
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3.1. RFC 5127 Background
Many providers operate MPLS-based backbones that employ backbone
traffic engineering to ensure that if a major link, switch, or router
fails, the result will be a routed network that continues to meet its
Service Level Agreements (SLAs). Based on that foundation, [RFC5127]
introduced the concept of DiffServ Treatment Aggregates, which enable
traffic marked with multiple DSCPs to be forwarded in a single MPLS
Traffic Class (TC) based on robust provider backbone traffic
engineering. This enables differentiated forwarding behaviors within
a domain in a fashion that does not consume a large number of MPLS
Traffic Classes.
RFC 5127 provides an example aggregation of DiffServ service classes
into 4 Treatment Aggregates. A small number of aggregates are used
because:
o The available coding space for carrying QoS information (e.g.,
DiffServ PHB) in MPLS and Ethernet is only 3 bits in size, and is
intended for more than just QoS purposes (see e.g. [RFC5129]).
o There should be unused codes for interconnection purposes. This
leaves space for future standards, for private bilateral
agreements and for local use PHBs and DSCPs.
o Migrations from one code point scheme to another may require spare
QoS code points.
RFC 5127 also follows RFC 2474 in recommending transmission of DSCPs
through a network as they are received at the network edge.
3.2. Differences from RFC 5127
Like RFC 5127, this document also uses four traffic aggregates, but
differs from RFC 5127 in three important ways:
o It follows RFC 2475 in allowing the DSCPs used within a network to
differ from those to exchange traffic with other networks (at
network edges), but provides support to restore ingress DSCP
values when one of the recommended interconnect DSCPs in this
draft is used. This results in DSCP remarking at both network
ingress and network egress, and this draft assumes that such
remarking at network edges is possible for all interface types.
As discussed in Section 2 above, the MPLS Short Pipe tunnel model
effectively requires use of a DSCP that is locally valid for the
network involved, leading to this remarking approach.
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o It treats network control traffic as a special case. Within a
network, the CS6 DSCP is used for local network control traffic
(routing protocols and OAM traffic that is essential for network
operation administration, control and management) that may be
destined for any node within the network. In contrast, network
control traffic exchanged between networks (e.g., BGP traffic)
usually terminates at or close to a network edge, and is not
forwarded through the network because it is not part of internal
routing or OAM for the receiving network. In addition, such
traffic is unlikely to be covered by standard interconnection
agreements; it is more likely to be specifically configured (e.g.,
most networks impose on exchange of BGP for obvious reasons). See
Section 4.2 for further discussion.
o Because network control traffic is treated as a special case, a
fourth traffic aggregate is defined for use at network
interconnections to replace the Network Control aggregate in RFC
5127. Network Control traffic may still be exchanged across
network interconnections as further discussed in Section 4.2
4. The DiffServ-Intercon Interconnection Classes
At an interconnection, the networks involved need to agree on the
PHBs used for interconnection and the specific DSCP for each PHB.
This may involve remarking for the interconnection; such remarking is
part of the DiffServ Architecture [RFC2475], at least for the network
edge nodes involved in interconnection. This draft proposes a
standard interconnection set of 4 Treatment Aggregates with well-
defined DSCPs to be aggregated by them. A sending party remarks
DSCPs from internal schemes to the interconnection code points. The
receiving party remarks DSCPs to her internal scheme. The set of
DSCPs and PHBs supported across the two interconnected domains and
the treatment of PHBs and DSCPs not recognized by the receiving
domain should be part of the interconnect SLA.
RFC 5127's four treatment aggregates include a Network Control
aggregate for routing protocols and OAM traffic that is essential for
network operation administration, control and management. Using this
aggregate as one of the four in RFC 5127 implicitly assumes that
network control traffic is forwarded in potential competition with
all other network traffic, and hence DiffServ must favor such traffic
(e.g., via use of the CS6 codepoint) for network stability. That is
a reasonable assumption for IP-based networks where routing and OAM
protocols are mixed with all other types of network traffic;
corporate networks are an example.
In contrast, mixing of all traffic is not a reasonable assumption for
MPLS-based provider or carrier networks, where customer traffic is
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usually segregated from network control (routing and OAM) traffic via
other means, e.g., network control traffic use of separate LSPs that
can be prioritized over customer LSPs (e.g., for VPN service) via
other means. This segregation of network control traffic from
customer traffic is also used for MPLS-based network
interconnections. In addition, many customers of a network provider
do not exchange Network Control traffic (e.g., routing) with the
network provider. For these reasons, a separate Network Control
traffic aggregate is not important for MPLS-based carrier or provider
networks; when such traffic is not segregated from other traffic, it
may reasonably share the Assured Elastic treatment aggregate (as RFC
5127 suggests for a situation in which only three treatment
aggregates are supported).
In contrast, VoIP is emerging as a valuable and important class of
network traffic for which network-provided QoS is crucial, as even
minor glitches are immediately apparent to the humans involved in the
conversation.
Similar approaches to use of a small number of traffic aggregates
(including recognition of the importance of VoIP traffic) have been
taken in related standards and recommendations from outside the IETF,
e.g., Y.1566 [Y.1566], GSMA IR.34 [IR.34] andMEF23.1 [MEF23.1].
The list of the four DiffServ Interconnect traffic aggregates
follows, highlighting differences from RFC 5127 and the specific
traffic classes from RFC 4594 that each class aggregates.
Telephony Service Treatment Aggregate: PHB EF, DSCP 101 110 and
VOICE-ADMIT, DSCP 101100, see [RFC3246] , [RFC4594][RFC5865].
This Treatment Aggregate corresponds to RFC 5127s real time
Treatment Aggregate definition regarding the queuing, but it
is restricted to transport Telephony Service Class traffic in
the sense of RFC 4594.
Bulk Real-Time Treatment Aggregate: This Treatment Aggregate is
designed to transport PHB AF41, DSCP 100 010 (the other AF4
PHB group PHBs and DSCPs may be used for future extension of
the set of DSCPs carried by this Treatment Aggregate). This
Treatment Aggregate is designed to transport the portions of
RFC 5127's Real Time Treatment Aggregate, which consume large
amounts of bandwidth, namely Broadcast Video, Real-Time
Interactive and Multimedia Conferencing. The treatment
aggregate should be configured with a rate queue (which is in
line with RFC 4594 for the mentioned traffic classes). As
compared to RFC 5127, the number of DSCPs has been reduced to
one (initially). The proposed queuing mechanism is in line
with RFC4594 definitions for Broadcast Video and Real-Time
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Interactive. If need for three-color marked Multimedia
Conferencing traffic arises, AF42 and AF43 PHBs may be added.
Assured Elastic Treatment Aggregate This Treatment Aggregate
consists of the entire AF3 PHB group AF3, i.e., DSCPs 011
010, 011 100 and 011 110. As compared to RFC5127, just the
number of DSCPs, which has been reduced. This document
suggests to transport signaling marked by AF31. RFC5127
suggests to map Network Management traffic into this
Treatment Aggregate, if no separate Network Control Treatment
Aggregate is supported (for a more detailed discussion of
Network Control PHB treatment see section 3.2). GSMA IR.34
proposes to transport signaling traffic by AF31 too.
Default / Elastic Treatment Aggregate: transports the default PHB,
CS0 with DSCP 000 000. RFC 5127 example refers to this
Treatment Aggregate as Aggregate Elastic. An important
difference as compared to RFC5127 is that any traffic with
unrecognized or unsupported DSCPs may be remarked to this
DSCP.
RFC 4594's Multimedia Streaming class has not been mapped to the
above scheme. By the time of writing, the most popular streaming
applications use TCP transport and adapt picture quality in the case
of congestion. These applications are proprietary and still change
behaviour frequently. Currently, the Bulk Real-Time Treatment
Aggregate or the Assured Elastic Treatment Aggregate may be a
reasonable match. NOTE: This paragraph would benefit from WG review
and discussion.
The overall approach to DSCP marking at network interconnections is
illustrated by the following example. Provider O and provider W are
peered with provider T. They have agreed upon a QoS interconnection
SLA.
Traffic of provider O terminates within provider Ts network, while
provider W's traffic transits through the network of provider T to
provider F. Assume all providers run their own internal codepoint
schemes for a PHB group with properties of the DiffServ Intercon
Assured Treatment Aggregate.
Provider-O Provider-W
RFC5127 GSMA 34.1
| |
+----------+ +----------+
|AF21, AF22| | CS3, CS2 |
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+----------+ +----------+
| |
V V
+++++++++ +++++++++
|Rtr PrO| |Rtr PrW| Rtr Pr:
+++++++++ +++++++++ Router Peering
| DiffServ |
+----------+ +----------+
|AF31, AF32| |AF31, AF32|
+----------+ +----------+
| Intercon |
V V
+++++++++ |
|RtrPrTI|------------------+
+++++++++
| Provider-T domain
+-----------+
| MPLS TC 2 |
| DSCP rew. | rew. -> rewrite
| AF21, AF22|
+-----------+
| | Local DSCPs Provider-T
| | +----------+ +++++++++
V +->|AF21, AF22|->-|RtrDstH|
| +----------+ +++++++++
+----------+ RtrDst:
|AF21, AF22| Router Destination
+----------+
|
+++++++++
|RtrPrTE|
+++++++++
| DiffServ
+----------+
|AF31, AF32|
+----------+
| Intercon
+++++++++
|RtrPrF|
+++++++++
|
+----------+
| CS4, CS3 |
+----------+
|
Provider-F
GSM IR.34
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DiffServ Intercon example
Figure 1
Providers only need to deploy internal DSCP to DiffServ Intercon DSCP
mappings to exchange traffic in the desired classes. Provider W has
decided that the properties of his internal classes CS3 and CS2 are
best met by the Diffserv Intercon Assured Elastic Treatment
Aggregate, PHBs AF31 and AF32 respectively. At the outgoing peering
interface connecting provider W with provider T the former's peering
router remarks CS3 traffic to AF31 and CS2 traffic to AF32. The
domain internal PHBs of provider T that meet the requirements of
Diffserv Intercon Assured Elastic Treatment Aggregate are AF2x.
Hence AF31 traffic received at the interconnection with provider T is
remarked to AF21 by the peering router of domain T, and domain T has
chosen to use MPLS TC value 2 for this aggregate. Traffic received
with AF32 is similarly remarked to AF22, but uses the same MPLS TC
for the Treatment Aggregate, i.e. TC 2. At the penultimate MPLS
node, the top MPLS label is removed. The packet should be forwarded
as determined by the incoming MPLS TC. The peering router connecting
domain T with domain F classifies the packet by it's domain T
internal DSCP AF21 for the Diffserv Intercon Assured Elastic
Treatment Aggregate. As it leaves domain T on the interface to
domain F, this causes the packet to be remarked to AF31. The peering
router of domain F classifies the packet for domain F internal PHB
CS4, as this is the PHB with properties matching DiffServ Intercon's
Assured Elastic Treatment Aggregate. Likewise, AF21 traffic is
remarked to AF32 by the peering router od domain T when leaving it
and from AF32 to CS3 by domain F's peering router when receiving it.
This example can be extended. Suppose Provider-O also supports a PHB
marked by CS2 and this PHB is supposed to be transported by QoS
within Provider-T domain. Then Provider-O will remark it with a DSCP
other than the AF31 DSCP in order to preserve the distinction from
CS2; AF11 is one possibility that might be private to the
interconnection between Provider-O and Provider-T; there's no
assumption that Provider-W can also use AF11, as it may not be in the
SLA with Provider-W.
Now suppose Provider-W supports CS2 for internal use only. Then no
DiffServ Intercon DSCP mapping may be configured at the peering
router. Traffic, sent by Provider-W to Provider-T marked by CS2 due
to a misconfiguration may be remarked to CS0 by Provider-T.
See section 4.1 for further discussion of this and DSCP transparency
in general.
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RFC2575 states that 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. As a consequence, an
interconnect SLA needs to specify not only the treatment of traffic
that arrives with a supported interconnect DSCP, but also the
treatment of traffic that arrives with unsupported or unexpected
DSCPs.
The proposed interconnect class and code point scheme is designed for
point to point IP layer interconnections among MPLS networks. Other
types of interconnections are out of scope of this document. The
basic class and code point scheme is applicable on Ethernet layer
too, if a provider e.g. supports Ethernet priorities like specified
by IEEE 802.1p.
4.1. End-to-end QoS: PHB and DS CodePoint Transparency
This section describes how the use of a common PHB and DSCP scheme
for interconnection can lead to end-to-end DiffServ-based QoS across
networks that do not have common policies or practices for PHB and
DSCP usage. This will initially be possible for PHBs and DSCPs
corresponding to at most 3 or 4 Treatment Aggregates due to the MPLS
considerations discussed previously.
Networks can be expected to differ in the number of PHBs available at
interconnections (for terminating or transit service) and the DSCP
values used within their domain. At an interconnection, Treatment
Aggregate and PHB properties are best described by SLAs and related
explanatory material. For the above reasons and the desire to
support interconnection among networks with different DiffServ
schemes, the DiffServ interconnection scheme supports a small number
of PHBs and DSCPs; this scheme is expandable.
The basic idea is that traffic sent with a DiffServ interconnect PHB
and DSCP is restored to that PHB and DSCP (or a PHB and DSCP within
the AF3 PHB group for the Assured Treatment Aggregate) at each
network interconnection, even though a different PHB and DSCP may be
used by each network involved. So, Bulk Inelastic traffic could be
sent with AF41, remarked to CS3 by the first network and back to AF41
at the interconnection with the second network, which could mark it
to CS5 and back to AF41 at the next interconnection, etc. The result
is end-to-end QoS treatment consistent with the Bulk Inelastic
Traffic Aggregate, and that is signaled or requested by the AF41 DSCP
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at each network interconnection in a fashion that allows each network
operator to use their own internal PHB and DSCP scheme.
The key requirement is that the network ingress interconnect DSCP be
restored at network egress, and a key observation is that this is
only feasible in general for a small number of DSCPs.
4.2. Treatment of Network Control traffic at carrier interconnection
interfaces
As specified by RFC4594, section 3.2, Network Control (NC) traffic
marked by CS6 is to be expected at some interconnection interfaces.
This document does not change RFC4594, but observes that network
control traffic received at network ingress is generally different
from network control traffic within a network that is the primary use
of CS6 envisioned by RFC 4594. A specific example is that some CS6
traffic exchanged across carrier interconnections is terminated at
the network ingress node, e.g. if BGP is running between two routers
on opposite ends of an interconnection link;in this case the
operators would enter into a bilateral agreement to use CS6 for that
BGP traffic.
The end-to-end QoS discussion in the previous section (4.1) is
generally inapplicable to network control traffic - network control
traffic is generally intended to control a network, not be
transported across it. One exception is that network control traffic
makes sense for a purchased transit agreement, and preservation of
the CS6 DSCP marking for network control traffic that is transited is
reasonable in some cases, although it is generally inappropriate to
use CS6 for transiting traffic, including transiting network control
traffic. Use of an IP tunnel is suggested in order to reduce the
risk of CS6 markings on transiting network control traffic being
interpreted by the network providing the transit.
If the MPLS Short Pipe model is deployed for non-tunneled IPv4
traffic, an IP network provider should limit access to the CS6 and
CS7 DSCPs so that they are only used for network control traffic for
the provider's own network.
Interconnecting carriers should specify treatment of CS6 marked
traffic received at a carrier interconnection which is to be
forwarded beyond the ingress node. An SLA covering the following
cases is recommended when a provider wishes to send CS6 marked
traffic across an interconnection link which isn't terminating at the
interconnected ingress node:
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o classification of traffic which is network control traffic for
both domains. This traffic should be classified and marked for
the NC PHB.
o classification of traffic which is network control traffic for the
sending domain only. This traffic should be classified for a PHB
offering similar properties as the NC class (e.g. AF31 as
specified by this document). As an example GSMA IR.34 proposes an
Interactive class / AF31 to carry SIP and DIAMETER traffic. While
this is service control traffic of high importance to the
interconnected Mobile Network Operators, it is certainly not
Network Control traffic for a fixed network providing transit
between such operators, and hence should not receive CS6 treatment
in such a network.
o any other CS6 marked traffic should be remarked or dropped.
5. Acknowledgements
Al Morton and Sebastien Jobert provided feedback on many aspects
during private discussions. Mohamed Boucadair and Thomas Knoll
helped adding awareness of related work. Fred Baker and Brian
Carpenter provided intensive feedback and discussion.
6. IANA Considerations
This memo includes no request to IANA.
7. Security Considerations
This document does not introduce new features, it describes how to
use existing ones. The security considerations of RFC 2475 [RFC2475]
and RFC 4594 [RFC4594] apply.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2474] 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.
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[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998.
[RFC2597] Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
"Assured Forwarding PHB Group", RFC 2597, June 1999.
[RFC3246] 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.
[RFC3260] Grossman, D., "New Terminology and Clarifications for
Diffserv", RFC 3260, April 2002.
[RFC3270] Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen,
P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi-
Protocol Label Switching (MPLS) Support of Differentiated
Services", RFC 3270, May 2002.
[RFC5129] Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
Marking in MPLS", RFC 5129, January 2008.
[RFC5462] Andersson, L. and R. Asati, "Multiprotocol Label Switching
(MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
Class" Field", RFC 5462, February 2009.
[RFC5865] Baker, F., Polk, J., and M. Dolly, "A Differentiated
Services Code Point (DSCP) for Capacity-Admitted Traffic",
RFC 5865, May 2010.
8.2. Informative References
[I-D.knoll-idr-cos-interconnect]
Knoll, T., "BGP Class of Service Interconnection", draft-
knoll-idr-cos-interconnect-13 (work in progress), November
2014.
[ID.idr-sla]
IETF, "Inter-domain SLA Exchange", IETF,
http://datatracker.ietf.org/doc/
draft-ietf-idr-sla-exchange/, 2013.
[IEEE802.1Q]
IEEE, "IEEE Standard for Local and Metropolitan Area
Networks - Virtual Bridged Local Area Networks", 2005.
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[IR.34] GSMA Association, "IR.34 Inter-Service Provider IP
Backbone Guidelines Version 7.0", GSMA, GSMA IR.34
http://www.gsma.com/newsroom/wp-content/uploads/2012/03/
ir.34.pdf, 2012.
[MEF23.1] MEF, "Implementation Agreement MEF 23.1 Carrier Ethernet
Class of Service Phase 2", MEF, MEF23.1
http://metroethernetforum.org/PDF_Documents/technical-
specifications/MEF_23.1.pdf, 2012.
[RFC2983] Black, D., "Differentiated Services and Tunnels", RFC
2983, October 2000.
[RFC4594] Babiarz, J., Chan, K., and 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.
[RFC5160] Levis, P. and M. Boucadair, "Considerations of Provider-
to-Provider Agreements for Internet-Scale Quality of
Service (QoS)", RFC 5160, March 2008.
[Y.1566] ITU-T, "Quality of service mapping and interconnection
between Ethernet, IP and multiprotocol label switching
networks", ITU,
http://www.itu.int/rec/T-REC-Y.1566-201207-I/en, 2012.
Appendix A. Appendix A Carrier interconnection related DiffServ aspects
NOTE: This Appendix is likely to be deleted in the next version of
this draft. The authors would appreciate comments on the value (or
lack thereof) of this text.
This apppendix provides a general discussion of PHB and DSCP mapping
at IP interconnection interfaces.
The following scenarios start from a domain sending non-tunneled IP
traffic using a PHB and a corresponding DSCP to an interconnected
domain. The receiving domain may:
o Support the PHB and offer the same corresponding DSCP.
o Not support the PHB and use the DSCP for a different PHB.
o Not support the PHB and not use the DSCP.
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o Support the PHB with a differing DSCP, and the DSCP of the sending
domain is not used for another PHB
o Support the PHB with a differing DSCP, and the DSCP of the sending
domain is used for another PHB.
RFC2475 allows for local use PHBs which are only available within a
domain. If any such a local use PHB is present, non-tunneled IP
traffic possibly cannot utilize 64 DSCPs end-to-end.
If a domain receives traffic for a PHB, which it does not support,
there are two general scenarios:
o The received DSCP is not available for usage within the domain.
o The received DSCP is available for usage within the domain.
RFC2474 suggests transporting packets received with unrecognized
DSCPs by the Default PHB and not changing the DSCP as received. Also
if a particular DSCP is unused within a domain, the network may
subsequently change its QoS design and assign a PHB to a formerly
unused DSCP, making transparent transport of that DSCP as an unknown
DSCP with the Default PHB no longer possible. Remarking to another
DSCP apart from the Default PHBs DSCP does not seem to be a good
option in the latter case, as it's not clear which other DSCP should
be used. If a domain interconnects with many other domains, the
concerns discussed here may have to be dealt with many times.
The scenarios above indicate, that reliably delivering a non-tunneled
IP packet by the same PHB and DSCP unchanged end-to-end is only
likely, if both domains support this DSCP and use the same
corresponding DSCP.
Limitations in the number of supported PHBs are to be expected if
DiffServ is applied across different domains. Unchanged end-to-end
DSCPs should only be expected for non-tunneled IP traffic, if the PHB
and DSCP are well specified and generally deployed. This is true for
Default Forwarding. EF PHB is a candidate. The Network Control PHB
is a local use only example, hence end-to-end support of CS6 for non-
tunneled IP traffic at interconnection points should only be
expected, if the receiving domain regards this traffic as Network
Control traffic relevant for the own domain too.
DiffServ Intercon proposes a set of PHBs and corresponding DSCPs at
interconnection points. A PHB to DSCPs correspondence is specified
for interconnection interfaces. Supported PHBs should be available
end-to-end, but domain internal DSCPs may change end-to-end, although
they are restored at network interconnection points.
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Appendix B. Appendix B The MPLS Short Pipe Model and IP traffic
The MPLS Short Pipe Model (or penultimate Hop Label Popping) is
widely deployed in carrier networks. If non-tunneled IPv4 traffic is
transported using MPLS Short Pipe, IP headers appear inside the last
section of the MPLS domain. This impacts the number of PHBs and
DSCPs that a network provider can reasonably support . See Figure 2
(below) for an example.
For tunneled IPv4 traffic, only the outer tunnel header is involved
in forwarding. If the tunnel does not terminate within the MPLS
network section, only the outer tunnel DSCP is involved, as the inner
DSCP does not affect forwarding behavior.
Non-tunneled IPv6 traffic as well as Layer 2 and Layer 3 VPN traffic
all use an additional MPLS label; forwarding within an MPLS network
is based on that label, as opposed to the outer IP header.
Carriers often select QoS PHBs and DSCP without regard to
interconnection. As a result PHBs and DSCPs typically differ between
network carriers. PHBs may be mapped. With the exception of best
effort traffic, a DSCP change should be expected at an
interconnection at least for plain IP traffic even if the PHB is
mapped across the carriers involved.
Beyond RFC3270's suggestions that the Short Pipe Model is only
applicable to VPNs, current network structures also use it to
transport non tunneled IPv4 traffic. This is shown in figure 2.
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|
\|/ IPv4, DSCP_send
V
|
Peering Router
|
\|/ IPv4, DSCP_send
V
|
MPLS Edge Router
| Mark MPLS Label, TC_internal
\|/ Remark DSCP to
V (Inner: IPv4, DSCP_d)
|
MPLS Core Router (penultimate hop label popping)
| \
| IPv4, DSCP_d | The DSCP needs to be in network-
| ^^^^^^^^| internal QoS context. The Core
\|/ > Router might require or enforce
V | it. The Edge Router may wrongly
| | classify, if the DSCP is not in
| / network-internal DiffServ context.
MPLS Edge Router
| \ Traffic leaves the network marked
\|/ IPv4, DSCP_d | with the network-internal
V > DSCP_d that must be dealt with
| | by the next network (downstream).
| /
Peer Router
| Remark DSCP to
\|/ IPv4, DSCP_send
V
|
Short-Pipe / penultimate hop popping example
Figure 2
The packets IP DSCP must be in a well understood Diffserv context for
schedulers and classifiers on the interfaces of the ultimate MPLS
link (last link traversed before leaving the network). The necessary
Diffserv context is network-internal and a network operating in this
mode enforces DSCP usage in order to obtain robust QoS behavior.
Without DiffServ-Intercon treatment, the traffic is likely to leave
each network marked with network-internal DSCP. DSCP_send of the
figure above is remarked to the receiving network's DiffServ scheme.
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It leaves the domain marked by the domains DSCP_d. This structure
requires that every carrier deploys per-peer PHB and DSCP mapping
schemes.
If DiffServ-Intercon is applied DSCPs for traffic transiting the
domain can be mapped from and remapped to an original DSCP. This is
shown in figure 3. Internal traffic may continue to use internal
DSCPs (e.g, DSCP_d) and those may also be used between a carrier and
its direct customers.
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Internal Router
|
| Outer Header
\|/ IPv4, DSCP_send
V
|
Peering Router
| Remark DSCP to
\|/ IPv4, DSCP_ds-int DiffServ Intercon DSCP and PHB
V
|
MPLS Edge Router
|
| Mark MPLS Label, TC_internal
\|/ Remark DSCP to
V (Inner: IPv4, DSCP_d) domain internal DSCP for
| the PHB
MPLS Core Router (penultimate hop label popping)
|
| IPv4, DSCP_d
| ^^^^^^
\|/
V
|
|
MPLS Edge Router--------------------+
| |
\|/ Remark DSCP to \|/ IPv4, DSCP_d
V IPv4, DSCP_ds-int V
| |
| |
Peer Router Domain internal Broadband
| Access Router
\|/ Remark DSCP to \|/
V IPv4, DSCP_send V IPv4, DSCP_d
| |
Short-Pipe example with Diffserv-Intercon
Figure 3
Appendix C. Change log
00 to 01 Added terminology and references. Added details and
information to interconnection class and codepoint scheme.
Editorial changes.
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01 to 02 Added some references regarding related work. Clarified
class definitions. Further editorial improvements.
02 to 03 Consistent terminology. Discussion of Network Management
PHB at interconnection interfaces. Editorial review.
03 to 04 Again improved terminology. Better wording of Network
Control PHB at interconnection interfaces.
04 to 05 Large rewrite and re-ordering of contents.
05 to 06 Description of IP and MPLS related requirements and
constraints on DSCP rewrites.
06 to 07 Largely rewrite, improved match and comparison with RFCs
4594 and 5127.
07 to 08 Added Annex A and B which where forgotten when putting
together -07
Authors' Addresses
Ruediger Geib (editor)
Deutsche Telekom
Heinrich Hertz Str. 3-7
Darmstadt 64295
Germany
Phone: +49 6151 5812747
Email: Ruediger.Geib@telekom.de
David L. Black
EMC Corporation
176 South Street
Hopkinton, MA
USA
Phone: +1 (508) 293-7953
Email: david.black@emc.com
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