Internet DRAFT - draft-ietf-tsvwg-diffserv-intercon
draft-ietf-tsvwg-diffserv-intercon
TSVWG R. Geib, Ed.
Internet-Draft Deutsche Telekom
Intended status: Informational D. Black
Expires: June 18, 2017 Dell EMC
December 15, 2016
Diffserv-Interconnection classes and practice
draft-ietf-tsvwg-diffserv-intercon-14
Abstract
This document defines a limited common set of Diffserv Per Hop
Behaviours (PHBs) and codepoints (DSCPs) to be applied at
(inter)connections of two separately administered and operated
networks, and explains how this approach can simplify network
configuration and operation. Many network providers operate Multi
Protocol Label Switching (MPLS) using Treatment Aggregates for
traffic marked with different Diffserv Per Hop Behaviors, 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
and apply the Short-Pipe tunnel mode. While motivated by the
requirements of MPLS network operators that use Short-Pipe tunnels,
this document is applicable to other networks, both MPLS and non-
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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 June 18, 2017.
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Copyright Notice
Copyright (c) 2016 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Related work . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Applicability Statement . . . . . . . . . . . . . . . . . 4
1.3. Document Organization . . . . . . . . . . . . . . . . . . 5
2. MPLS and the Short Pipe tunnel model . . . . . . . . . . . . 5
3. Relationship to RFC5127 . . . . . . . . . . . . . . . . . . . 6
3.1. RFC5127 Background . . . . . . . . . . . . . . . . . . . 6
3.2. Differences from RFC5127 . . . . . . . . . . . . . . . . 7
4. The Diffserv-Intercon Interconnection Classes . . . . . . . . 8
4.1. Diffserv-Intercon Example . . . . . . . . . . . . . . . . 10
4.2. End-to-end PHB and DSCP Transparency . . . . . . . . . . 13
4.3. Treatment of Network Control traffic at carrier
interconnection interfaces . . . . . . . . . . . . . . . 14
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . 15
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
8.1. Normative References . . . . . . . . . . . . . . . . . . 16
8.2. Informative References . . . . . . . . . . . . . . . . . 16
Appendix A. Appendix A The MPLS Short Pipe Model and IP traffic 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction
Diffserv has been deployed in many networks; it provides
differentiated traffic forwarding based on the Diffserv Codepoint
(DSCP) field, which is part of the IP header [RFC2474]. This
document defines a set of common Diffserv classes (Per Hop Behaviors,
PHBs) and code points for use at interconnection points to which and
from which locally used classes and code points should be mapped.
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As described by section 2.3.4.2 of RFC2475, remarking of packets at
domain boundaries is a Diffserv feature [RFC2475]. 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 to better support incremental Diffserv
deployment in existing networks as well as with routers that do not
support Diffserv or are not configured to support it. Many networks
do not follow this recommendation, and instead remark unknown or
unexpected DSCPs to zero upon receipt for default (best effort)
forwarding in accordance with the guidance in RFC2475 [RFC2475] to
ensure that appropriate DSCPs are used within a Diffserv domain.
This draft is based on the latter approach, and defines additional
DSCPs that are known and expected at network interconnection
interfaces in order to reduce the amount of traffic whose DSCPs are
remarked to zero.
This document is motivated by requirements for IP network
interconnection with Diffserv support among providers that operate
Multi Protocol Label Switching (MPLS) in their backbones, but is also
applicable to other technologies. The operational simplifications
and methods in this document help align IP Diffserv functionality
with MPLS limitations resulting from the widely deployed Short Pipe
tunnel model for operation [RFC3270]. Further, limiting Diffserv to
a small number of Treatment Aggregates can enable network traffic to
leave a network with the DSCP value with which it was received, even
if a different DSCP is used within the network, thus providing an
opportunity to extend consistent Diffserv treatment across network
boundaries.
In isolation, use of a defined set of interconnection PHBs and DSCPs
may appear to be additional effort for a network operator. The
primary offsetting benefit is that 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.
Absent this approach, the PHBs and DSCPs have to be negotiated and
configured independently for each network interconnection, which has
poor administrative and operational scaling properties. Further,
consistent end-to-end Diffserv 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.
The interconnection approach described in this document (referred to
as Diffserv-Intercon) uses a set of PHBs (mapped to four
corresponding MPLS treatment aggregates) along with a set of
interconnection DSCPs allowing straightforward rewriting to domain-
internal DSCPs and defined DSCP markings for traffic forwarded to
interconnected domains. The solution described here can be used in
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other contexts benefitting from a defined Diffserv interconnection
interface.
The basic idea is that traffic sent with a Diffserv-Interconnect PHB
and DSCP is restored to that PHB and DSCP at each network
interconnection, even though a different PHB and DSCP may be used by
each network involved. 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. Traffic sent with other DSCPs can be remarked to an
interconnect DSCP or dealt with via additional agreement(s) among the
operators of the interconnected networks; use of the MPLS Short Pipe
model favors remarking unexpected DSCPs to zero in the absence of
additional agreement(s), as explained further in this document.
In addition to the common 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. RFC5160 suggests Meta-QoS-
Classes to help enabling deployment of standardized end to end QoS
classes [RFC5160]. The Diffserv-Intercon class- and codepoint scheme
is intended to complement that work (e.g., by enabling a defined set
of interconnection DSCPs and PHBs).
Border Gateway Protocol (BGP) signaling Class of Service at
interconnection interfaces by BGP [I-D.knoll-idr-cos-interconnect],
[ID.ietf-idr-sla] is complementary to Diffserv-Intercon. These two
BGP documents focus on exchanging Service Level Agreement (SLA) and
traffic conditioning parameters and assume that common PHBs
identified by the signaled DSCPs have been established (e.g., via use
of the Diffserv-Intercon DSCPs) prior to BGP signaling of PHB id
codes.
1.2. Applicability Statement
This document is applicable to use of Differentiated Services for
interconnection traffic between networks, and is particularly suited
to interconnection of MPLS-based networks that use MPLS Short-pipe
tunnels. This document is also applicable to other network
technologies, but it is not intended for use within an individual
network, where the approach specified in RFC5127 [RFC5127] is among
the possible alternatives; see Section 3 for further discussion.
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The Diffserv-Intercon approach described in this document simplifies
IP based interconnection to domains operating the MPLS Short Pipe
model for IP traffic, both terminating within the domain and
transiting onward to another domain. Transiting traffic is received
and sent with the same PHB and DSCP. Terminating traffic maintains
the PHB with which it was received, however the DSCP may change.
Diffserv-Intercon is also applicable to the Pipe tunneling model
[RFC2983], [RFC3270], but it is not applicable to the Uniform
tunneling model [RFC2983], [RFC3270].
The Diffserv-Intercon approach defines a set of four PHBs for support
at interconnections (or network boundaries in general).
Corresponding DSCPs for use at an interconnection interface are
added. Diffserv-intercon allows for a simple mapping of PHBs and
DSCPs to MPLS Treatment Aggregates. It is extensible by IETF
standardisation and this allows additional PHBs and DSCPs to be
specified for the Diffserv-intercon scheme. Coding space for private
interconnection agreements or provider internal services is left too.
1.3. Document Organization
This document is organized as follows: section 2 reviews the MPLS
Short Pipe tunnel model for Diffserv Tunnels [RFC3270], because
effective support for that model is a crucial goal of Diffserv-
Intercon. Section 3 provides background on RFC5127's approach to
traffic class aggregation within a Diffserv network domain and
contrasts it with the Diffserv-Intercon approach. Section 4
introduces Diffserv-Interconnection Treatment Aggregates, along with
the PHBs and DSCPs that they use, and explains how other PHBs (and
associated DSCPs) may be mapped to these Treatment Aggregates.
Section 4 also discusses treatment of IP traffic, MPLS VPN Diffserv
considerations and handling of high-priority Network Management
traffic. Appendix A describes how the MPLS Short Pipe model
(penultimate hop popping) impacts DSCP marking for IP
interconnections.
2. MPLS and the Short Pipe tunnel model
This section provides a summary of the implications of the MPLS Short
Pipe tunnels, and in particular their use of Penultimate Hop Popping
(PHP, see RFC3270) on the Diffserv tunnel framework described in
RFC2983. The Pipe and Uniform models for Differentiated Services and
Tunnels are defined in [RFC2983]. RFC3270 adds the Short Pipe model
to reflect the impact of MPLS PHP, primarily for MPLS-based IP
tunnels and VPNs. The Short Pipe model and PHP have subsequently
become popular with network providers that operate MPLS networks and
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are now widely used to transport unencapsulated IP traffic. This has
important implications for Diffserv functionality in MPLS networks.
RFC2474's recommendation to forward traffic with unrecognized DSCPs
with Default (best effort) service without rewriting the DSCP has not
been widely deployed in 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, and a common
practice is to remark to CS0 any 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 Traffic Class (TC) field and
allow that to differ from the PHB indicated by the DSCP in the MPLS-
encapsulated IP packet. If the MPLS label with the provider's TC
field is present at all hops within the provider network, this
approach would allow an unrecognized DSCP to be carried edge-to-edge
over an MPLS network, because the effective DSCP used by the
provider's MPLS network would be encoded in the MPLS label TC field
(and also carried edge-to-edge). Unfortunately this is only true for
the Pipe tunnel model.
The Short Pipe tunnel model and PHP behave differently because PHP
removes and discards the MPLS provider label carrying the provider's
TC field before the traffic exits the provider's network. 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, so that IP header cannot be used to carry a different DSCP
edge-to-edge. See Appendix A for a more detailed discussion.
3. Relationship to RFC5127
This document draws heavily upon RFC5127's approach to aggregation of
Diffserv traffic classes for use within a network, but there are
important differences caused by characteristics of network
interconnects that differ from links within a network.
3.1. RFC5127 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
function. Based on that foundation, [RFC5127] introduced the concept
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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.
RFC5127 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 Traffic Class information
(e.g., Diffserv PHB) in MPLS (and Ethernet) is only 3 bits in
size, and is intended for more than just Diffserv purposes (see,
e.g., [RFC5129]).
o The common interconnection DSCPs ought not to use all 8 possible
values. This leaves space for future standards, for private
bilateral agreements and for local use PHBs and DSCPs.
o Migrations from one Diffserv code point scheme to a different one
is another possible application of otherwise unused DSCPs.
3.2. Differences from RFC5127
Like RFC5127, this document also uses four traffic aggregates, but
differs from RFC5127 in some important ways:
o It follows RFC2475 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 if 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.
o Diffserv-Intercon suggests limiting the number of interconnection
PHBs per Treatment Aggregate to the minimum required. As further
discussed below, the number of PHBs per Treatment Aggregate is no
more than two. When two PHBs are specified for a Diffserv-
Intercon treatment aggregate, the expectation is that the provider
network supports DSCPs for both PHBs, but uses a single MPLS TC
for the Treatment Aggregate that contains the two PHBs.
o Diffserv-Intercon suggests mapping other PHBs and DSCPs into the
interconnection Treatment Aggregates as further discussed below.
o Diffserv-Intercon treats network control traffic as a special
case. Within a provider's network, the CS6 DSCP is used for local
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network control traffic (routing protocols and Operations,
Administration, and Maintenance (OAM) traffic that is essential to
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) 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;
rather, it is more likely to be specifically configured (e.g.,
most networks impose restrictions on use of BGP with other
networks for obvious reasons). See Section 4.2 for further
discussion.
o Because RFC5127 used a Treatment Aggregate for network control
traffic, Diffserv-Intercon can instead define a fourth traffic
aggregate to be defined for use at network interconnections
instead of the Network Control aggregate in RFC5127. Network
Control traffic may still be exchanged across network
interconnections as further discussed in Section 4.2. Diffserv-
Intercon uses this fourth traffic aggregate for VoIP traffic,
where network-provided service differentiation is crucial, as even
minor glitches are immediately apparent to the humans involved in
the conversation.
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 document defines a set of 4 interconnection 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 their internal scheme.
The interconnect SLA defines the set of DSCPs and PHBs supported
across the two interconnected domains and the treatment of PHBs and
DSCPs that are not recognized by the receiving domain.
Similar approaches that use 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]and MEF23.1 [MEF23.1].
The list of the four Diffserv-Interconnect traffic aggregates
follows, highlighting differences from RFC5127 and suggesting
mappings for all RFC4594 traffic classes to Diffserv-Intercon
Treatment Aggregates:
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Telephony Service Treatment Aggregate: PHB EF, DSCP 101 110 and PHB
VOICE-ADMIT, DSCP 101 100, see [RFC3246], [RFC4594] and
[RFC5865]. This Treatment Aggregate corresponds to RFC5127's
real time Treatment Aggregate definition regarding the
queuing (both delay and jitter should be minimized), but this
aggregate is restricted to transport Telephony Service Class
traffic in the sense of RFC4594 [RFC4594].
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 intended for Diffserv-Intercon network
interconnection of the portions of RFC5127's Real Time
Treatment Aggregate, that consume significant bandwidth.
This traffic is expected to consist of the RFC4594 classes
Broadcast Video, Real-Time Interactive and Multimedia
Conferencing. This treatment aggregate should be configured
with a rate queue (consistent with RFC4594's recommendation
for the transported traffic classes). By comparison to
RFC5127, the number of DSCPs has been reduced to one
(initially). The AF42 and AF43 PHBs could be added if there
is a need for three-color marked Multimedia Conferencing
traffic.
Assured Elastic Treatment Aggregate This Treatment Aggregate
consists of PHBs AF31 and AF32 ( i.e., DSCPs 011 010 and 011
100). By comparison to RFC5127, the number of DSCPs has been
reduced to two. This document suggests to transport
signaling marked by AF31 (e.g., as recommended by GSMA IR.34
[IR.34]). AF33 is reserved for extension of PHBs to be
aggregated by this TA. For Diffserv-Intercon network
interconnection, the following RFC4594 service classes should
be mapped to the Assured Elastic Treatment Aggregate: the
Signaling Service Class (being marked for lowest loss
probability), Multimedia Streaming Service Class, the Low-
Latency Data Service Class and the High-Throughput Data
Service Class.
Default / Elastic Treatment Aggregate: transports the default PHB,
CS0 with DSCP 000 000. RFC5127 example refers to this
Treatment Aggregate as Aggregate Elastic. An important
difference from RFC5127 is that any traffic with unrecognized
or unsupported DSCPs may be remarked to this DSCP. For
Diffserv-Intercon network interconnection, the RFC4594
standard service class and Low-priority Data service class
should be mapped to this Treatment Aggregate. This document
does not specify an interconnection class for RFC4594 Low-
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priority data. This data may be forwarded by a Lower Effort
PHB in one domain (like the PHB proposed by Informational
[RFC3662]), but using the methods specified in this document
will be remarked with DSCP CS0 at a Diffserv-Intercon network
interconnection. This has the effect that Low-priority data
is treated the same as data sent using the default class.
(Note: In a network that implements RFC2474, Low-priority
traffic marked as CS1 would otherwise receive better
treatment than traffic using the default class.)
RFC2475 states that Ingress nodes must condition all 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 CS0. 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; remarking to CS0 is a
widely deployed behavior.
During the process of setting up a Diffserv interconnection, both
networks should define the set of acceptable and unacceptable DSCPs
and specify the treatment of traffic marked with each DSCP.
While Diffserv-Intercon allows modification of unacceptable DSCPs, if
traffic using one or more of the PHBs in a PHB group (e.g., AF3x,
consisting of AF31, AF32 and AF33) is accepted as part of a supported
Diffserv-Intercon Treatment Aggregate, then traffic using other PHBs
from the same PHB group should not be modified to use PHBs outside of
that PHB group, and in particular should not be remarked to CS0
unless the entire PHB group is remarked to CS0. This avoids
unexpected forwarding behavior (and potential reordering, see also
[RFC7657]) when using Assured Forwarding (AF) PHBs [RFC2597].
4.1. Diffserv-Intercon Example
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 Diffserv
interconnection SLA.
Traffic of provider O terminates within provider T's network, while
provider W's traffic transits through the network of provider T to
provider F. This example assumes that all providers use their own
internal PHB and codepoint (DSCP) that correspond to the AF31 PHB in
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the Diffserv-Intercon Assured Elastic Treatment Aggregate (AF21 and
CS2 are used in the example).
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Provider O Provider W
| |
+----------+ +----------+
| AF21 | | CS2 |
+----------+ +----------+
V V
+~~~~~~~+ +~~~~~~~+
|Rtr PrO| |Rtr PrW| Rtr: Router
+~~~~~~~+ +~~~~~~~+ Pr[L]: Provider[L]
| DiffServ |
+----------+ +----------+
| AF31 | | AF31 |
+----------+ +----------+
V Intercon V
+~~~~~~~+ |
|RtrPrTI|------------------+ Router Provider T Ingress
+~~~~~~~+
| Provider T domain
+------------------+
| MPLS TC 2, AF21 |
+------------------+
| | +----------+ +~~~~~~~+
V `--->| AF21 |->-|RtrDstH| Router Destination Host
+----------+ +----------+ +~~~~~~~+
| AF21 | Local DSCPs Provider T
+----------+
|
+~~~~~~~+
|RtrPrTE| Router Provider T Egress
+~~~~~~~+
| DiffServ
+----------+
| AF31 |
+----------+
| Intercon
+~~~~~~~+
|RtrPrF | Router Provider F
+~~~~~~~+
|
+----------+
| AF11 | Provider F
+----------+
Diffserv-Intercon example
Figure 1
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Providers only need to deploy mappings of internal DSCPs to/from
Diffserv-Intercon DSCPs so that they can exchange traffic using the
desired PHBs. In the example, provider O has decided that the
properties of his internal class AF21 are best met by the Diffserv-
Intercon Assured Elastic Treatment Aggregate, PHB AF31. At the
outgoing peering interface connecting provider O with provider T the
former's peering router remarks AF21 traffic to AF31. The domain
internal PHB of provider T meeting the requirement of Diffserv-
Intercon Assured Elastic Treatment Aggregate are from AF2x PHB group.
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 Traffic Class value 2 for this aggregate. At the
penultimate MPLS node, the top MPLS label is removed and exposes the
IP header marked by the DSCP that has been set at the network
ingress. The peering router connecting domain T with domain F
classifies the packet by its domain-T-internal DSCP AF21. As the
packet leaves domain T on the interface to domain F, this causes the
packet's DSCP to be remarked to AF31. The peering router of domain F
classifies the packet for domain-F-internal PHB AF11, as this is the
PHB with properties matching Diffserv-Intercon's Assured Elastic
Treatment Aggregate.
This example can be extended. The figure shows Provider-W using CS2
for traffic that corresponds to Diffserv-Intercon Assured Elastic
Treatment Aggregate PHB AF31; that traffic is mapped to AF31 at the
Diffserv-Intercon interconnection to Provider-T. In addition,
suppose that Provider-O supports a PHB marked by AF22 and this PHB is
supposed to obtain Diffserv transport within Provider-T domain. Then
Provider-O will remark it with DSCP AF32 for interconnection to
Provider-T.
Finally suppose that Provider-W supports CS3 for internal use only.
Then no Diffserv- Intercon DSCP mapping needs to be configured at the
peering router. Traffic, sent by Provider-W to Provider-T marked by
CS3 due to a misconfiguration may be remarked to CS0 by Provider-T.
4.2. End-to-end PHB and DSCP Transparency
This section briefly discusses end-to-end Diffserv approaches related
to the Uniform, Pipe and Short Pipe tunnel models ([RFC2983],
[RFC3270]), when used edge-to-edge in a network.
o With the Uniform model, neither the DCSP nor the PHB change. This
implies that a network management packet received with a CS6 DSCP
would be forwarded with an MPLS Traffic Class corresponding to
CS6. The uniform model is outside the scope of this document.
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o With the Pipe model, the inner tunnel DCSP remains unchanged, but
an outer tunnel DSCP and the PHB could changed. For example a
packet received with a (network specific) CS1 DSCP would be
transported by default PHB and if MPLS is applicable, forwarded
with an MPLS Traffic Class corresponding to Default PHB. The CS1
DSCP is not rewritten. Transport of a large variety (much greater
than 4) DSCPs may be required across an interconnected network
operating MPLS Short pipe transport for IP traffic. In that case,
a tunnel based on the Pipe model is among the possible approaches.
The Pipe model is outside the scope of this document.
o With the Short Pipe model, the DCSP likely changes and the PHB
might change. This document describes a method to simplify
Diffserv network interconnection when a DSCP rewrite can't be
avoided.
4.3. Treatment of Network Control traffic at carrier interconnection
interfaces
As specified by RFC4594, section 3.2, Network Control (NC) traffic
marked by CS6 is 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 RFC4594. A specific example is that some CS6
traffic exchanged across carrier interconnections is terminated at
the network ingress node, e.g., when BGP is used between the 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 discussion in the previous section (4.2) is generally
inapplicable to network control traffic - network control traffic is
generally intended to control a network, not be transported between
networks. 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
forwarding that traffic within the network that provides transit.
Use of an IP tunnel is suggested in order to conceal the CS6 markings
on transiting network control traffic from the network that provides
the transit. In this case, Pipe model for Diffserv tunneling is
used.
If the MPLS Short Pipe model is deployed for unencapsulated 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.
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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 and that traffic's destination
is beyond the interconnected ingress node:
o classification of traffic that is network control traffic for both
domains. This traffic should be classified and marked for the CS6
DSCP.
o classification of traffic that is network control traffic for the
sending domain only. This traffic should be forwarded with a PHB
that is appropriate for the NC service class [RFC4594], e.g., AF31
as specified by this document. As an example GSMA IR.34
recommends an Interactive class / AF31 to carry SIP and DIAMETER
traffic. While this is service control traffic of high importance
to interconnected Mobile Network Operators, it is certainly not
Network Control traffic for a fixed network providing transit
among such operators, and hence should not receive CS6 treatment
in such a transit network.
o any other CS6 marked traffic should be remarked or dropped.
5. Acknowledgements
Bob Briscoe and Gorry Fairhurst reviewed the draft and provided rich
feedback. Brian Carpenter, Fred Baker, Al Morton and Sebastien
Jobert discussed the draft and helped improving it. Mohamed
Boucadair and Thomas Knoll helped adding awareness of related work.
James Polk's discussion during IETF 89 helped to improve the text on
the relation of this draft to RFC4594 and RFC5127.
6. IANA Considerations
This memo includes no request to IANA.
7. Security Considerations
The DSCP field in the IP header can expose additional traffic
classification information at network interconnections by comparison
to use of a zero DSCP for all interconnect traffic. If traffic
classification info is sensitive, the DSCP field could be remarked to
zero to hide the classification as a countermeasure, at the cost of
loss of Diffserv info and differentiated traffic handling on the
interconnect and subsequent networks. When AF PHBs are used, any
such remarking should respect AF PHB group boundaries as further
discussed at the end of Section 4.
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This document does not introduce new features; it describes how to
use existing ones. The Diffserv security considerations in [RFC2475]
and [RFC4594] apply.
8. References
8.1. Normative References
[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,
DOI 10.17487/RFC2474, December 1998,
<http://www.rfc-editor.org/info/rfc2474>.
[RFC2597] Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
"Assured Forwarding PHB Group", RFC 2597,
DOI 10.17487/RFC2597, June 1999,
<http://www.rfc-editor.org/info/rfc2597>.
[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, DOI 10.17487/RFC3246, March 2002,
<http://www.rfc-editor.org/info/rfc3246>.
[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, DOI 10.17487/RFC3270, May 2002,
<http://www.rfc-editor.org/info/rfc3270>.
[RFC5129] Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
Marking in MPLS", RFC 5129, DOI 10.17487/RFC5129, January
2008, <http://www.rfc-editor.org/info/rfc5129>.
[RFC5865] Baker, F., Polk, J., and M. Dolly, "A Differentiated
Services Code Point (DSCP) for Capacity-Admitted Traffic",
RFC 5865, DOI 10.17487/RFC5865, May 2010,
<http://www.rfc-editor.org/info/rfc5865>.
8.2. Informative References
[I-D.knoll-idr-cos-interconnect]
Knoll, T., "BGP Class of Service Interconnection", draft-
knoll-idr-cos-interconnect-17 (work in progress), November
2016.
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[ID.ietf-idr-sla]
IETF, "Inter-domain SLA Exchange", IETF,
http://datatracker.ietf.org/doc/
draft-ietf-idr-sla-exchange/, 2013.
[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.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
<http://www.rfc-editor.org/info/rfc2475>.
[RFC2983] Black, D., "Differentiated Services and Tunnels",
RFC 2983, DOI 10.17487/RFC2983, October 2000,
<http://www.rfc-editor.org/info/rfc2983>.
[RFC3662] Bless, R., Nichols, K., and K. Wehrle, "A Lower Effort
Per-Domain Behavior (PDB) for Differentiated Services",
RFC 3662, DOI 10.17487/RFC3662, December 2003,
<http://www.rfc-editor.org/info/rfc3662>.
[RFC4594] Babiarz, J., Chan, K., and F. Baker, "Configuration
Guidelines for DiffServ Service Classes", RFC 4594,
DOI 10.17487/RFC4594, August 2006,
<http://www.rfc-editor.org/info/rfc4594>.
[RFC5127] Chan, K., Babiarz, J., and F. Baker, "Aggregation of
Diffserv Service Classes", RFC 5127, DOI 10.17487/RFC5127,
February 2008, <http://www.rfc-editor.org/info/rfc5127>.
[RFC5160] Levis, P. and M. Boucadair, "Considerations of Provider-
to-Provider Agreements for Internet-Scale Quality of
Service (QoS)", RFC 5160, DOI 10.17487/RFC5160, March
2008, <http://www.rfc-editor.org/info/rfc5160>.
[RFC7657] Black, D., Ed. and P. Jones, "Differentiated Services
(Diffserv) and Real-Time Communication", RFC 7657,
DOI 10.17487/RFC7657, November 2015,
<http://www.rfc-editor.org/info/rfc7657>.
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[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 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 unencapsulated IP 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 encapsulated IP traffic, only the outer tunnel header is relevant
for 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; in this case all DSCPs
could be used in the inner IP header without affecting network
behavior based on the outer MPLS header. Here the Pipe model
applies.
Layer 2 and Layer 3 VPN traffic all use an additional MPLS label; in
this case, the MPLS tunnel follows the Pipe model. Classification
and queuing within an MPLS network is always based on an MPLS label,
as opposed to the outer IP header.
Carriers often select PHBs and DSCP without regard to
interconnection. As a result PHBs and DSCPs typically differ between
network carriers. With the exception of best effort traffic, a DSCP
change should be expected at an interconnection at least for
unencapsulated IP traffic, even if the PHB is suitably mapped by the
carriers involved.
Although RFC3270 suggests that the Short Pipe Model is only
applicable to VPNs, current networks also use it to transport non-
tunneled IPv4 traffic. This is shown in figure 2 where Diffserv-
Intercon is not used, resulting in exposure of the internal DSCPs of
the upstream network to the downstream network across the
interconnection.
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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 Diffserv context. The Core
\|/ > Router may require or enforce
V | that. 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 differentiated
forwarding behavior.
Without Diffserv-Intercon treatment, the traffic is likely to leave
each network marked with network-internal DSCP. DSCP_send in the
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figure above has to be remarked into the first network's Diffserv
scheme at the ingress MPLS Edge Router, to DSCP_d in the example.
For that reason, the traffic leaves this domain marked by the
network-internal 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 they 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
Authors' Addresses
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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
Dell EMC
176 South Street
Hopkinton, MA
USA
Phone: +1 (508) 293-7953
Email: david.black@dell.com
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