Considerations in the development of a QoS Architecture for CCNx-like ICN protocols
draft-oran-icnrg-qosarch-01

Abstract

This is a position paper. It documents the author's personal views on how Quality of Service (QoS) capabilities ought to be accommodated in ICN protocols like CCNx or NDN which employ flow-balanced Interest/Data exchanges and hop-by-hop forwarding state as their fundamental machinery. It argues that such protocols demand a substantially different approach to QoS from that taken in TCP/IP, and proposes specific design patterns to achieve both classification and differentiated QoS treatment on both a flow and aggregate basis. It also considers the effect of caches as a resource in addition to memory, CPU and link bandwidth that should be subject to explicitly un-fair resource allocation. The proposed methods are intended to operate purely at the network layer, providing the primitives needed to achieve both transport and higher layer QoS objectives. It explicitly excludes any discussion of Quality of Experience (QoE) which can only be assessed and controlled at the application layer or above.

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Table of Contents

• 1. Introduction
• 2. Requirements Language
• 3. Some background on the nature and properties of Quality of Service in network protocols
• 3.1. Congestion Control basics relevant to ICN
• 4. What can we control to achieve QoS in ICN?
• 5. How does this relate to QoS in TCP/IP?
• 6. Why is ICN Different? Can we do Better?
• 7. A strawman set of principles to guide QoS architecture for ICN
• 8. IANA Considerations
• 9. Security Considerations
• 10. References
• 10.1. Normative References
• 10.2. Informative References
• Author's Address

1. Introduction

The TCP/IP protocol suite used on today's Internet has over 30 years of accumulated research and engineering into the provision of Quality of Service machinery, employed with varying success in different environments. ICN protocols like Named Data Networking (NDN [NDN]) and Content-Centric Networking (CCNx [RFC8569],[RFC8609]) have an accumulated 10 years of research and very little deployment. We therefore have the opportunity to either recapitulate the approaches take with TCP/IP (e.g. IntServ [RFC2998] and Diffserv [RFC2474]) or design a new architecture and associated mechanisms aligned with the properties of ICN protocols which differ substantially from those of TCP/IP. This position paper advocates the latter approach and comprises the author's personal views on how Quality of Service (QoS) capabilities ought to be accommodated in ICN protocols like CCNx or NDN. Specifically, these protocols differ in fundamental ways from TCP/IP. These differences are summarized in the following table:

This document proposes specific design patterns to achieve both flow classification and differentiated QoS treatment for ICN on both a flow and aggregate basis. It also considers the effect of caches as a resource in addition to memory, CPU and link bandwidth that should be subject to explicitly un-fair resource allocation. The proposed methods are intended to operate purely at the network layer, providing the primitives needed to achieve both transport and higher layer QoS objectives. It does not propose detailed protocol machinery to achieve these goals; it leaves these to supplementary specifications, such as [I-D.moiseenko-icnrg-flowclass]. It explicitly excludes any discussion of Quality of Experience (QoE) which can only be assessed and controlled at the application layer or above.

Much of this document is derived from presentations the author has given at ICNRG meetings over the last few years that are available through the IETF datatracker (see, for example [Oran2018QoSslides]).

2. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

3. Some background on the nature and properties of Quality of Service in network protocols

Much of this background material is tutorial, and can be simply skipped by readers familiar with the long and checkered history of quality of service in packet networks. Other parts of it are polemical yet serve to illuminate the author's personal biases and technical views.

All networking systems provide some degree of "quality of service" in that they exhibit non-zero utility when offered traffic to carry. The term therefore is used to describe systems that control the allocation of various resources in order to achieve managed unfairness. Absent explicit mechanisms to decide what traffic to be unfair to, most systems try to achieve some form of "fairness" in the allocation of resources, optimizing the overall utility delivered to all demand under the constraint of available resources. From this it should be obvious that you cannot use QoS mechanisms to create or otherwise increase resource capacity! In fact, all known QoS schemes have non-zero overhead and hence may (albeit slightly) decrease to total esources available to carry user traffic.

Further, accumulated experience seems to indicate that QoS is helpful in a fairly narrow range of network conditions:

• If your resources are lightly loaded, you don't need it, as neither congestive loss nor substantial queueing delay occurs
• If your resources are heavily oversubscribed, it doesn't save you. So many users will be unhappy that you are probably not delivering a viable service
• Failures can rapidly shift your state from the first above to the second, in which case either:
  • your QoS machinery cannot respond quickly enough to maintain the advertised service quality continuously, or
  • resource allocations are sufficiently conservative to result in substantial wasted capacity under non-failure conditions

Nevertheless, though not universally deployed, QoS is advantageous at least for some applications and some network environments. Some examples include:

• applications with steep utility functions [Shenker2006], such as real-time multimedia
• applications with safety-critical operational constraints, such as avionics or industrial automation
• dedicated or tightly managed networks whose economics depend on strict adherence to challenging service level agreements (SLAs)

Another factor in the design and deployment of QoS is the scalability and scope over which the desired service can be achieved. Here there are two major considerations, one technical, the other economic/political:

• Some signaled QoS schemes, such as RSVP [RFC2205], maintain state in routers for each flow, which scales linearly with the number of flows. For core routers through which pass millions to billions of flows, the memory required is infeasible to provide.
• The Internet is comprised of many minimally cooperating autonomous systems [AS]. There are practically no successful examples of QoS deployments crossing the AS boundaries of multiple service providers. This in almost all cases limits the applicability of QoS capabilities to be intra-domain.

Finally, the relationship between QoS and either accounting or billing is murky. Some schemes can accurately account for resource consumption and ascertain to which user to allocate the usage. Others cannot. While the choice of mechanism may have important practical economic and political consequences for cost and workable business models, this document considers none of those things and discusses QoS in the context of providing managed unfairness only.

Some further background on congestion control for ICN is below.

3.1. Congestion Control basics relevant to ICN

Congestion control is necessary in any packet network that multiplexes traffic among multiple sources and destinations in order to:

1. Prevent collapse of utility due to overload, where the total offered service declines as load increases, perhaps precipitously, rather than increasing or remaining flat.
2. Avoid starvation of some traffic due to excessive demand by other traffic.
3. Beyond the basic protections against starvation, achieve "fairness" among competing traffic. Two common objective functions are [minmaxfairness] and [proportionalfairness] both of which have been implemented and deployed successfully on packet networks for many years.

Before moving on to QoS, it is useful to consider how congestion control works in NDN or CCNx. Unlike the IP protocol family, which relies exclusively on end-to-end congestion control (e.g. TCP[RFC0793], DCCP[RFC4340], SCTP[RFC4960], QUIC[I-D.ietf-quic-transport]), CCNx and NDN employ hop-by-hop congestion control. There is per-Interest/Data state at every hop of the path and therefore for each outstanding Interest, bandwidth for data returning on the inverse path can be allocated. In current designs, this allocation is often done using Interest counting. By accepting one Interest packet from a downstream node, implicitly this provides a guarantee (either hard or soft) that there is sufficient bandwidth on the inverse direction of the link to send back one Data packet. A number of congestion control schemes have been developed for ICN that operate in this fashion, for example [Wang2013], [Mahdian2016], [Song2018], [Carofiglio2012]. Other schemes, like [Schneider2016] neither count nor police interests, but instead monitor queues using AQM (active queue management) to mark returning Data packets that have experienced congestion. This later class of schemes is similar to those used on IP in the sense that it depends on consumers adequately reducing their rate of Interest injection to avoid Data packet drops due to buffer overflow in forwarders. The former class of schemes is (arguably) more robust against mis-behavior by consumers.

4. What can we control to achieve QoS in ICN?

QoS is achieved through managed unfairness in the allocation of resources in network elements, particularly in the routers doing forwarding of ICN packets. So, a first order question is what resources need to be allocated, and how to ascertain which traffic gets what allocations. In the case of CCNx or NDN the important network element resources are:

For these resources, any QoS scheme has to specify two things:

1. How do you create equivalence classes (a.k.a. flows) of traffic to which different QoS treatments are applied?
2. What are the possible treatments and how are those mapped to the resource allocation algorithms?

Two critical facts of life come into play when designing a QoS scheme. First, the number of equivalence classes that can be simultaneously tracked in a network element is bounded by both memory and processing capacity to do the necessary lookups. One can allow very fine-grained equivalence classes, but not be able to employ them globally because of scaling limits of core routers. That means it is wise to either restrict the range of equivalence classes, or allow them to be aggregated, trading off accuracy in policing traffic against ability to scale.

The second is how flexible is the set of treatments that are defined. The ability to encode the treatment requests in the protocol can be limited (as it is for IP - there are only 6 of the TOS bits available for Diffserv treatments), but as or more important is whether there are practical traffic policing, queuing, and pacing algorithms that can be combined to support a rich set of QoS treatments.

The two considerations above in combination can easily be substantially more expressive than can be achieved in practice with the available number of queues on real network interfaces or the amount of per-packet computation needed to enque or dequeue a packet.

5. How does this relate to QoS in TCP/IP?

TCP/IP has fewer resource types to manage than ICN, and in some cases the allocation methods are simpler, as shown in the following table:

For these resources, IP has specified three fundamental things, as shown in the following table:

Equivalence classes for IP can be pairwise, by matching against both source and destination address+port, pure group using only destination address+port, or source-specific multicast with source adress+port and destination multicast address+port.

With Intserv, the signaling protocol RSVP [RFC2205] carries two data structures, the FLOWSPEC and the TSPEC. The former fulfills the requirement to identify the equivalence class to which the QoS being signaled applies. The latter comprises the desired QoS treatment along with a description of the dynamic character of the traffic (e.g. average bandwidth and delay, peak bandwidth, etc.). Both of these encounter substantial scaling limits, which has meant that Intserv has historically been limited to confined topologies, and/or high-value usages, like traffic engineering.

With Diffserv, the protocol encoding (6 bits in the TOS field of the IP header) artificially limits the number of classes one can specify. These are documented in [RFC4594]. Nonetheless, when used with fine-grained equivalence classes, one still runs into limits on the number of queues required.

6. Why is ICN Different? Can we do Better?

While one could adopt an approach to QoS mirroring the extensive experience with TCP/IP, this would, in the author's view, be a mistake. The implementation and deployment of QoS in IP networks has been spotty at best. There are of course economic and political reasons as well as technical reasons for these mixed results, but there are several architectural choices in ICN that make it a potentially much better protocol base to enhance with QoS machinery. This section discusses those difference and their consequences.

First and foremost, hierarchical names are a much richer basis for specifying equivalence classes than IP 5-tuples. The IP address (or prefix) can only separate traffic by topology to the granularity of hosts, and not express actual computational instances nor sets of data. Ports give some degree of per-instance demultiplexing, but this tends to be both coarse and ephemeral, while confounding the demultiplexing function with the assignment of QoS treatments to particular subsets of the data. Some degree of finer granularity is possible with IPv6 by exploiting the ability to use up to 64 bits of address for classifying traffic. In fact, the hICN project ([I-D.muscariello-intarea-hicn]), while adopting the request-response model of CCNx, uses IPv6 addresses as the available namespace, and IPv6 packets (plus "fake" TCP headers) as the wire format.

Nonetheless, the flexibility of tokenized, variable length, hierarchical names allows one to directly associate classes of traffic for QoS purposes with the structure of an application namespace. The classification can be as coarse or fine-grained as desired by the application. While not always the case, there is typically a straightforward association between how objects are named, and how they are grouped together for common treatment. Examples abound; a number can be conveniently found in [I-D.moiseenko-icnrg-flowclass].

In ICN, QoS is not pre-bound to topology since names are non-topological, unlike unicast IP addresses. This allows QoS to be applied to multi-destination and multi-path environments in a straightforward manner, rather than requiring either multicast with coarse class-based scheduling or complex signaling like that in RSVP-TE [RFC3209] that is needed to make point-to-multipoint MPLS work.

Because of IP's stateless forwarding model, complicated by the ubiquity of asymmetric routes, any flow-based QoS requires state that is decoupled from the actual arrival of traffic and hence must be maintained, at least as soft-state, even during quiescent periods. Intserv, for example, requires flow signaling with state O(#flows). ICN, even worst case, requires state O(#active interest/data exchanges), since state can be instantiated on arrival of an Interest, and removed lazily once the data hase been returned.

Unlike Intserv, Difserv eschews signaling in favor of class-based configuration of resources and queues in network elements. However, Diffserv limits traffic treatments to a few bits taken from the ToS field of IP. No such wire encoding limitations exist for NDN or CCNx, as the protocol is completely TLV-based, and one (or even more than one) new field can be easily defined to carry QoS treatment information.

Therefore, there are greenfield possibilities for more powerful QoS treatment options in ICN. For example, IP has no way to express a QoS treatment like "try hard to deliver reliably, even at the expense of delay or bandwidth". Such a QoS treatment for ICN could invoke native ICN mechanisms, none of which are present in IP, such as:

• In-network retransmission in response to hop-by-hop errors returned from upstream forwarders
• Trying multiple paths to multiple content sources either in parallel or serially
• Higher precedence for short-term caching to recover from downstream errors

Such mechanisms are typically described in NDN and CCNx as forwarding strategies. However, little or no guidance is given for what application actions or protocol machinery is used to decide which forwarding strategy to use for which Interests that arrive at a forwarder. See [BenAbraham2018] for an investigation of these issues. Associating forwarding strategies with the equivalence classes and QoS treatments directly can make them more accessible and useful to implement and deploy.

IP has three forwarding semantics, with different QoS needs (Unicast, Anycast, Multicast). ICN has the single forwarding semantic, so any QoS machinery can be uniformly applied across any request/response invocation, whether it employs dynamic destination routing, multi-destination parallel requests, or even localized flooding (e.g. directly on L2 multicast mechanisms). Additionally, the pull-based model of ICN avoids a number of thorny multicast QoS problems that IP has ([Wang2000], [RFC3170], [Tseng2003]).

The Multi-destination/multi-path forwarding model in ICN changes resource allocation needs in a fairly deep way. IP treats all endpoints as open-loop packet sources, whereas NDN and CCNx have strong asymmetry between producers and consumers as packet sources.

IP has no caching in routers, whereas ICN needs ways to allocate cache resources. Treatments to control caching operation are unlikely to look much like the treatments used to control link resources. NDN and CCNx already have useful cache control directives associated with Data messages. The CCNx controls include

ExpiryTime

time after which a cached Content Object is considered expired and MUST no longer be used to respond to an Interest from a cache.

Recommended Cache Time

time after which the publisher considers the Content Object to be of low value to cache.

(See [RFC8569] for the formal definitions)

ICN flow classifiers, such as those in [I-D.moiseenko-icnrg-flowclass] can be used to achieve soft or hard partitioning of cache resources in the content store of an ICN forwarder. For example, cached content for a given equivalence class can be considered fate shared in a cache whereby objects from the same equivalence class are purged as a group rather than individually. This can recover cache space more quickly and at lower overhead than pure per-object replacement. In addition, since the forwarder remembers the QoS treatment for each pending Interest in its PIT, the above cache controls can be augmented by policy to prefer retention of cached content for some equivalence classes as part of the cache replacement algorithm.

Stateless forwarding and asymmetric routing in IP limits available state/feedback to manage link resources. In contrast, NDN or CCNx forwarding allows all link resource allocation to occur as part of Interest forwarding, potentially simplifying things considerably. For example, with symmetric routing, producers have no control over the paths their data packets traverse, and hence any QoS treatments intended to affect routing paths will have no effect.

7. A strawman set of principles to guide QoS architecture for ICN

Based on the observations made in the earlier sections, this summary section captures the author's ideas for clear and actionable architectural principals for how to incorporate QoS machinery into ICN protocols like NDN and CCNx. Hopefully, they can guide further work and focus effort on portions of the giant design space for QoS that have the best tradeoffs in terms of flexibility, simplicity, and deployability.

Define equivalence classes using the name hierarchy rather than creating an independent traffic class definition. This directly associates the specification of equivalence classes of traffic with the structure of the application namespace. It can allow hierarchical decomposition of equivalence classes in a natural way because of the way hierarchical ICN names are constructed. Two practical mechanisms are presented in [I-D.moiseenko-icnrg-flowclass] with different tradeoffs between security and the ability to aggregate flows. Either prefix-based (EC3) or explicit name component based (ECNT) or both could be adopted as the part of the QoS architecture for defining equivalence classes.

Put consumers in control of Link and Forwarding resource allocation. Do all link buffering and forwarding (both memory and CPU) resource allocations based on Interest arrivals. This is attractive because it provides early congestion feedback to consumers, and allows scheduling the reverse link direction ahead of time for carrying the matching data. This makes enforcement of QoS treatments a single-ended rather than a double-ended problem and can avoid wasting resources on fetching data that will wind up dropped when it arrives at a bottleneck link.

Allow producers to influence the allocation of of cache resources. Producers want to affect caching decisions in order to:

• Shed load by having Interests served by content stores in forwarders before reaching the producer itself.
• Survive transient outages of either the producer or links close to the producer.

For caching to be effective, individual Data objects in an equivalence classes need to have similar treatment; otherwise well-known cache thrashing pathologies due to self-interference emerge. Producers have the most direct control over caching policies through the caching directives in Data messages. It therefore makes sense to put the producer, rather than the consumer or network operator in charge of specifying these equivalence classes.

See [I-D.moiseenko-icnrg-flowclass] for specific mechanisms to achieve this.

Allow consumers to influence the allocation of of cache resources. Consumers want to affect caching decisions in order to:

• Reduce latency for retrieving data
• Survive transient outages of either a producer or links close to the consumer

Consumers can have indirect control over caching by specifying QoS treatments in their Interests. Consider the following potential QoS treatments by consumers that can drive caching policies:

• A QoS treatment requesting better robustness against transient disconnection can be used by a forwarder close to the consumer (or downstream of an unreliable link) to preferentially cache the corresponding data.
• Conversely a QoS treatment together with, or in addition to a request for short latency, to indicate that new data will be requested soon enough that caching the current data being requested would be ineffective and hence to only pay attention to the caching preferences of the producer.
• A QoS treatment indicating a mobile consumer likely to incur a mobility event within an RTT (or a few RTTs). Such a treatment would allow a mobile network operator to preferentially cache the data at a forwarder positioned at a join point or rendezvous point of their topology

Network operators, whether closely tied administratively to producer or consumer, or constituting an independent transit administration, provide the storage resources in the ICN forwarders. Therefore, they are the ultimate arbiters of how the cache resources are managed. In addition to any local policies they may enforce, the cache behavior from the QoS standpoint emerges from how the producer-specified equivalence classes map onto cache space availability, including whether cache entries are treated individually, or fate-shared. Forwarders also determine how the consumer-specified QoS treatments map to the precedence used for retaining Data objects in the cache.

Besides utilizing cache resources to meet the QoS goals of individual producers and consumers, network operators also want to manage their cache resources in order to:

• Amerliorate congestion hotspots by reducing load converging on producers they host on their network.
• Improve Interest satisfaction rates by utilizing caches as short-term retransmission buffers to recover from link errors or outages.
• Improve both latency and reliability in environments when consumers move in the operator's topology.

Re-think how to specify traffic treatments - don't just copy Diffserv. Some of the Diffserv classes may form a good starting point, as their mapping onto queuing algorithms for managing link buffering are well understood. However, Diffserv alone does not allow one to express latency versus reliability tradeoffs or other useful QoS treatments. Nor does it permit "TSPEC"-style traffic descriptions as are allowed in a signaled QoS scheme (despite neither having nor needing a separate signaling protocol given the state carried in the ICN data plane by forwarders). Here are some examples:

• A "burst" treatment, where an initial Interest gives an aggregate data size to request allocation of link capacity for a large burst of Interest/Data exchanges. The Interest can be rejected at any hop if the resources are not available. Such a treatment can also accomodate Data implosion produced by the discovery procedures of management protocols like [I-D.irtf-icnrg-ccninfo].
• A "reliable" treatment, which affects preference for allocation of PIT space for the Interest and Content Store space for the data in order to improve the robustness of IoT data delivery in constrained environment, as is described in [I-D.gundogan-icnrg-iotqos].
• A "search" treatment, which, within the specified Interest Lifetime, tries many paths either in parallel or serial to potentially many content sources, to maximize the probability that the requested item will be found. This is done at the expense of the extra bandwidth of both forwarding Interests and receiving multiple responses upstream of an aggregation point. The treatment can encode a value expressing tradeoffs like breadth-first versus depth-first search, and bounds on the total resource expenditure. Such a treatment would be useful for instrumentation protocols like [I-D.mastorakis-icnrg-icntraceroute].

As an aside, loose latency control can be achieved by bounding Interest Lifetime as long as it is not also used as an application mechanism to provide subscriptions or establish path traces for producer mobility. See [Krol2018] for a discussion of the network versus application timescale issues in ICN protocols.

8. IANA Considerations

This document does not require any IANA actions

9. Security Considerations

There are a few ways in which QoS for ICN interacts with security and privacy issues. Since QoS addresses relationships among traffic rather than the inherent characteristics of traffic, it neither enhances nor deteriorates the security and privacy properties of the data being carried, as long as the machinery does not alter or otherwise compromise the basic security properties of the associated protocols. The QoS approaches advocated here for ICN can serve to amplify existing threats to network traffic however:

• An attacker able to manipulate the QoS treatments of traffic can mount a more focused (and potentially more effective) denial of service attack by suppressing performance on traffic the attacker is targeting. Since the architcture here assumes QoS treatments are manipulable hop-by-hop, any on-path adversary can wreak havoc. Note however, that in basic ICN, and on-path attacker can do this and more by dropping, delaying, or mis-routing traffic independent of any particular QoS machinery in use.
• By explicitly revealing equivalence classes of traffic via either names or other fields in packets, an attacker has yet one more handle to use to discover linkability of multiple requests.

10. References

10.1. Normative References

10.2. Informative References

Author's Address