| 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
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.
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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 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.
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 and assume that common PHBs identified by the signaled DSCPs have been established prior to BGP signaling of QoS.
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.
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.
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 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.
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.
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:
RFC 5127 also follows RFC 2474 in recommending transmission of DSCPs through a network as they are received at the network edge.
Like RFC 5127, this document also uses four traffic aggregates, but differs from RFC 5127 in three important ways:
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 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.
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 |
+----------+ +----------+
| |
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
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.
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.
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 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.
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:
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.
This memo includes no request to IANA.
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.
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:
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:
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.
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.
|
\|/ 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. 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.
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