





Internet Research Task Force (IRTF)                             I. Rimac
Request for Comments: 6029                                       V. Hilt
Category: Informational                                         M. Tomsu
ISSN: 2070-1721                                               V. Gurbani
                                               Bell Labs, Alcatel-Lucent
                                                              E. Marocco
                                                          Telecom Italia
                                                            October 2010


                        A Survey on Research on
       the Application-Layer Traffic Optimization (ALTO) Problem

Abstract

   A significant part of the Internet traffic today is generated by
   peer-to-peer (P2P) applications used originally for file sharing, and
   more recently for real-time communications and live media streaming.
   Such applications discover a route to each other through an overlay
   network with little knowledge of the underlying network topology.  As
   a result, they may choose peers based on information deduced from
   empirical measurements, which can lead to suboptimal choices.  This
   document, a product of the P2P Research Group, presents a survey of
   existing literature on discovering and using network topology
   information for Application-Layer Traffic Optimization.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Research Task Force
   (IRTF).  The IRTF publishes the results of Internet-related research
   and development activities.  These results might not be suitable for
   deployment.  This RFC represents the consensus of the Peer-to-Peer
   Research Group of the Internet Research Task Force (IRTF).  Documents
   approved for publication by the IRSG are not a candidate for any
   level of Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6029.









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Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Survey of Existing Literature  . . . . . . . . . . . . . . . .  4
     2.1.  Application-Level Topology Estimation  . . . . . . . . . .  5
     2.2.  Topology Estimation through Layer Cooperation  . . . . . .  8
       2.2.1.  P4P Architecture . . . . . . . . . . . . . . . . . . .  9
       2.2.2.  Oracle-Based ISP-P2P Collaboration . . . . . . . . . .  9
       2.2.3.  ISP-Driven Informed Path Selection (IDIPS) Service . . 10
   3.  Application-Level Topology Estimation and the ALTO Problem . . 10
   4.  Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . . 12
     4.1.  Coordinate Estimation or Path Latencies? . . . . . . . . . 12
     4.2.  Malicious Nodes  . . . . . . . . . . . . . . . . . . . . . 12
     4.3.  Information Integrity  . . . . . . . . . . . . . . . . . . 12
     4.4.  Richness of Topological Information  . . . . . . . . . . . 13
     4.5.  Hybrid Solutions . . . . . . . . . . . . . . . . . . . . . 13
     4.6.  Negative Impact of Over-Localization . . . . . . . . . . . 13
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 14
   7.  Informative References . . . . . . . . . . . . . . . . . . . . 14


















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1.  Introduction

   A significant part of today's Internet traffic is generated by peer-
   to-peer (P2P) applications, used originally for file sharing, and
   more recently for real-time multimedia communications and live media
   streaming.  P2P applications pose serious challenges to the Internet
   infrastructure; by some estimates, P2P systems are so popular that
   they make up anywhere between 40% and 85% of the entire Internet
   traffic [Karagiannis], [LightReading], [LinuxReviews], [Parker],
   [Glasner].

   P2P systems ensure that popular content is replicated at multiple
   instances in the overlay.  But perhaps ironically, a peer searching
   for that content may ignore the topology of the latent overlay
   network and instead select among available instances based on
   information it deduces from empirical measurements, which in some
   particular situations may lead to suboptimal choices.  For example, a
   shorter round-trip time estimation is not indicative of the bandwidth
   and reliability of the underlying links, which have more of an
   influence than delay for large file transfer P2P applications.

   Most Distributed Hash Tables (DHT) -- the data structures that impose
   a specific ordering for P2P overlays -- use greedy forwarding
   algorithms to reach their destination, making locally optimal
   decisions that may not turn out to be globally optimized [Gummadi].
   This naturally leads to the Application-Layer Traffic Optimization
   (ALTO) problem [RFC5693]: how to best provide the topology of the
   underlying network while at the same time allowing the requesting
   node to use such information to effectively reach the node on which
   the content resides.  Thus, it would appear that P2P networks with
   their application-layer routing strategies based on overlay
   topologies are in direct competition against the Internet routing and
   topology.

   One way to solve the ALTO problem is to build distributed
   application-level services for location and path selection [Francis],
   [Ng], [Dabek], [Costa], [Wong], [Madhyastha] in order to enable peers
   to estimate their position in the network and to efficiently select
   their neighbors.  Similar solutions have been embedded into P2P
   applications such as Vuze [Vuze].  A slightly different approach is
   to have the Internet service provider (ISP) take a proactive role in
   the routing of P2P application traffic; the means by which this can
   be achieved have been proposed [Aggarwal], [Xie], [Saucez].  There is
   an intrinsic struggle between the layers -- P2P overlay and network
   underlay -- when performing the same service (routing); however,
   there are strategies to mitigate this dichotomy [Seetharaman].





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   This document, initially intended as a complement to RFC 5693
   [RFC5693] and discussed during the creation of the IETF ALTO Working
   Group, has been completed and refined in the IRTF P2P Research Group.
   Its goal is to summarize the contemporary research activities on the
   Application-Layer Traffic Optimization problem as input to the ALTO
   working group protocol designers.

1.1.  Terminology

   Terminology adopted in this document includes terms such as "ring
   geometry", "tree structure", and "butterfly network", borrowed from
   P2P scientific literature.  [RFC4981] provides an exhaustive
   definition of such terminology.

   Certain security-related terms are to be understood in the sense
   defined in [RFC4949]; such terms include, but are not limited to,
   "attack", "authentication", "confidentiality", "encryption",
   "identity", and "integrity".  Other security-related terms (for
   example, "denial of service") are to be understood in the sense
   defined in the referenced specifications.

2.  Survey of Existing Literature

   Gummadi et al. [Gummadi] compare popular DHT algorithms, and besides
   analyzing their resilience, provide an accurate evaluation of how
   well the logical overlay topology maps on the physical network layer.
   In their paper, relying only on measurements independently performed
   by overlay nodes without the support of additional location
   information provided by external entities, they demonstrate that the
   most efficient algorithms in terms of resilience and proximity
   performance are those based on the simplest geometric concept (i.e.,
   the ring geometry, rather than tree structures, butterfly networks,
   and hybrid geometries).

   Regardless of the geometrical properties of the distributed data
   structures involved, interactions between application-layer overlays
   and the underlying networks are a rich area of investigation.  The
   available literature in this field can be divided into two categories
   (Figure 1): using application-level techniques to estimate topology,
   and using some kind of layer cooperation to estimate topology.











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     Application-layer traffic optimization
       |
       +--> Application-level topology estimation
       |      |
       |      +--> Coordinates-based systems
       |      |      |
       |      |      +--> GNP
       |      |      |
       |      |      +--> Vivaldi
       |      |      |
       |      |      +--> PIC
       |      |
       |      +--> Path selection services
       |      |      |
       |      |      +--> IDMaps
       |      |      |
       |      |      +--> Meridian
       |      |      |
       |      |      +--> Ono
       |      |
       |      +--> Link-layer Internet maps
       |             |
       |             +--> iPlane
       |
       +--> Topology estimation through layer cooperation
              |
              +--> P4P: Provider portal for applications
              |
              +--> Oracle-based ISPs and P2P cooperation
              |
              +--> ISP-driven informed path selection

     Figure 1: Taxonomy of Solutions for the Application-Layer Traffic
                           Optimization Problem

2.1.  Application-Level Topology Estimation

   Estimating network topology information on the application layer has
   been an area of active research.  Early systems used triangulation
   techniques to bound the distance between two hosts using a common
   landmark host.  In such a technique, given a cost function C, a set
   of vertexes V and their corresponding edges, the triangle inequality
   holds if for any triple {a, b, c} in V, C(a, c) is always less than
   or equal to C(a, g) + C(b, c).  The cost function C could be
   expressed in terms of desirable metrics such as bandwidth or latency.

   We note that the techniques presented in this section are only
   representative of the sizable research in this area.  Rather than



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   trying to enumerate an exhaustive list, we have chosen certain
   techniques because they represent an advance in the area that further
   led to derivative works.

   Francis et al. proposed IDMaps [Francis], a system where one or more
   special hosts called tracers are deployed near an autonomous system.
   The distance measured in round-trip time (RTT) between hosts A and B
   is estimated as the cumulative distance between A and its nearest
   tracer Ta, plus the distance between B and its nearest tracer Tb,
   plus the shortest distance from Ta to Tb.  To aid in scalability
   beyond that provided by the client-server design of IDMaps, Ng
   et al. proposed a P2P-based Global Network Positioning (GNP)
   architecture [Ng].  GNP was a network coordinate system based on
   absolute coordinates computed from modeling the Internet as a
   geometric space.  It proposed a two-part architecture: in the first
   part, a small set of finite distributed hosts called landmarks
   compute their own coordinates in a fixed geometric space.  In the
   second part, a host wishing to participate computes its own
   coordinates relative to those of the landmark hosts.  Thus, armed
   with the computed coordinates, hosts can then determine interhost
   distance as soon as they discover each other.

   Both IDMaps and GNP require fixed network infrastructure support in
   the form of tracers or landmark hosts; this often introduces a single
   point of failure and inhibits scalability.  To combat this, new
   techniques were developed that embedded the network topology in a
   low-dimensional coordinate space to enable network distance
   estimation through vector analysis.  Costa et al. introduced
   Practical Internet Coordinates (PIC) [Costa].  While PIC used the
   notion of landmark hosts, it did not require explicit network support
   to designate specific landmark hosts.  Any node whose coordinates
   have been computed could act as a landmark host.  When a node joined
   the system, it probed the network distance to some landmark hosts.
   Then, it obtained the coordinates of each landmark host and computed
   its own coordinates relative to each landmark host, subject to the
   constraint of minimizing the error in the predicted distance and
   computed distance.

   Like PIC, Vivaldi [Dabek] proposed a fully distributed network
   coordinate system without any distinguished hosts.  Whenever a node A
   communicates with another node B, it measures the RTT to that node
   and learns that node's current coordinates.  Node A subsequently
   adjusts its coordinates such that it is closer to, or further from, B
   by computing new coordinates that minimize the squared error.  A
   Vivaldi node is thus constantly adjusting its position based on a
   simulation of interconnected mass springs.  Vivaldi is now being used
   in the popular P2P application Vuze, and studies indicate that it
   scales well to very large networks [Ledlie].



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   Network coordinate systems require the embedding of the Internet
   topology into a coordinate system.  This is not always possible
   without errors, which impacts the accuracy of distance estimations.
   In particular, it has proved to be difficult to embed the triangular
   inequalities found in Internet path distances [Ledlie].  Thus,
   Meridian [Wong] abandons the generality of network coordinate systems
   and provides specific distance evaluation services.  In Meridian,
   each node keeps track of a small fixed number of neighbors and
   organizes them in concentric rings, ordered by distance from the
   node.  Meridian locates the closest node by performing a multi-hop
   search where each hop exponentially reduces the distance to the
   target.  Although less general than virtual coordinates, Meridian
   incurs significantly less error for closest node discovery.

   The Ono project [Ono] takes a different approach and uses network
   measurements from Content Distribution Networks (CDNs) such as Akamai
   to find nearby peers.  Used as a plugin to the Vuze bittorrent
   client, Ono provides 31% average download rate improvement [Su].

    Comparison of application-level topology estimation techniques, as
    reported in literature.  Results in terms of number of (D)imensions
             and (L)andmarks, 90th percentile relative error.

   +----------------+---------------+----------------+-----------------+
   | GNP vs.        | PIC(b) vs.    | Vivaldi vs.    | Meridian vs.    |
   | IDMaps(a) (7D, | GNP (8D, 16L) | GNP (2D, 32L)  | GNP (8D, 15L)   |
   | 15L)           |               |                |                 |
   +----------------+---------------+----------------+-----------------+
   | GNP: 0.50,     | PIC: 0.38,    | Vivaldi: 0.65, | Meridian: 0.78, |
   | IDMaps: 0.97   | GNP: 0.37     | GNP: 0.65      | GNP: 1.18       |
   +----------------+---------------+----------------+-----------------+

                 (a) Does not use dimensions or landmarks.
            (b) Uses results from the hybrid strategy for PIC.

                                  Table 1

   Table 1 summarizes the application-level topology estimation
   techniques.  The salient performance metric is the relative error.
   While all approaches define this metric a bit differently, it can be
   generalized as how close a predicted distance comes to the
   corresponding measured distance.  A value of zero implies perfect
   prediction, and a value of 1 implies that the predicted distance is
   in error by a factor of two.  PIC, Vivaldi, and Meridian compare
   their results with that of GNP, while GNP itself compares its results
   with a precursor technique, IDMaps.  Because each of the techniques
   uses a different Internet topology and a varying number of landmarks
   and dimensions to interpret the data set, it is impossible to



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   normalize the relative error across all techniques uniformly.  Thus,
   we present the relative error data in pairs, as reported in the
   literature describing the specific technique.  Readers are urged to
   compare the relative error performance in each column on its own and
   not draw any conclusions by comparing the data across columns.

   Most of the work on estimating topology information focuses on
   predicting network distance in terms of latency and does not provide
   estimates for other metrics such as throughput or packet loss rate.
   However, for many P2P applications latency is not the most important
   performance metric, and these applications could benefit from a
   richer information plane.  Sophisticated methods of active network
   probing and passive traffic monitoring are generally very powerful
   and can generate network statistics indirectly related to performance
   measures of interest, such as delay and loss rate on link-level
   granularity.  Extraction of these hidden attributes can be achieved
   by applying statistical inference techniques developed in the field
   of inferential network monitoring or network tomography subsequent to
   sampling of the network state.  Thus, network tomography enables the
   extraction of a richer set of topology information, but at the same
   time inherently increases complexity of a potential information plane
   and introduces estimation errors.  For both active and passive
   methods, statistical models for the measurement process need to be
   developed, and the spatial and temporal dependence of the
   measurements should be assessed.  Moreover, measurement methodology
   and statistical inference strategy must be considered jointly.  For a
   deeper discussion of network tomography and recent developments in
   the field, we refer the reader to [Coates].

   One system providing such a service is iPlane [Madhyastha], which
   aims at creating an annotated atlas of the Internet that contains
   information about latency, bandwidth, capacity, and loss rate.  To
   determine features of the Internet topology, iPlane bridges and
   builds upon different ideas, such as active probing based on packet
   dispersion techniques to infer available bandwidth along path
   segments.  These ideas are drawn from different fields, including
   network measurement as described by Dovrolis et al. in [Dovrolis] and
   network tomography [Coates].

2.2.  Topology Estimation through Layer Cooperation

   Instead of estimating topology information on the application level
   through distributed measurements, this information could be provided
   by the entities running the physical networks -- usually ISPs or
   network operators.  In fact, they have full knowledge of the topology
   of the networks they administer and, in order to avoid congestion on
   critical links, are interested in helping applications to optimize
   the traffic they generate.  The remainder of this section briefly



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   describes three recently proposed solutions that follow such an
   approach to address the ALTO problem.

2.2.1.  P4P Architecture

   The architecture proposed by Xie et al. [Xie] has been adopted by the
   Distributed Computing Industry Association (DCIA) P4P working group
   [P4P], an open group established by ISPs, P2P software distributors,
   and technology researchers, with the dual goal of defining mechanisms
   to (1) accelerate content distribution and (2) optimize utilization
   of network resources.

   The main role in the P4P architecture is played by servers called
   "iTrackers", deployed by network providers and accessed by P2P
   applications (or, in general, by elements of the P2P system) in order
   to make optimal decisions when selecting a peer to which the element
   will connect.  An iTracker may offer three interfaces:

   1.  Info: Allows P2P elements (e.g., peers or trackers) to get opaque
       information associated to an IP address.  Such information is
       kept opaque to hide the actual network topology, but can be used
       to compute the network distance between IP addresses.

   2.  Policy: Allows P2P elements to obtain policies and guidelines of
       the network, which specify how a network provider would like its
       networks to be utilized at a high level, regardless of P2P
       applications.

   3.  Capability: Allows P2P elements to request network providers'
       capabilities.

   The P4P architecture is under evaluation with simulations,
   experiments on the PlanetLab distributed testbed, and in field tests
   with real users.  Initial simulations and PlanetLab experiment
   results [P4P] indicate that improvements in BitTorrent download
   completion time and link utilization in the range of 50-70% are
   possible.  Results observed on Comcast's network during a field test
   trial conducted with a modified version of the software used by the
   Pando content delivery network (documented in RFC 5632 [RFC5632])
   show average improvements in download rate in different scenarios
   varying between 57% and 85%, and a 34% to 80% drop in the cross-
   domain traffic generated by such an application.

2.2.2.  Oracle-Based ISP-P2P Collaboration

   In the general solution proposed by Aggarwal et al. [Aggarwal],
   network providers offer host servers, called "oracles", that help P2P
   users choose optimal neighbors.



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   The oracle concept uses the following mechanism: a P2P client sends
   the list of potential peers to the oracle hosted by its ISP and
   receives a re-arranged peer list, ordered according to the ISP's
   local routing policies and preferences.  For instance, to keep the
   traffic local, the ISP may prefer peers within its network, or it may
   pick links with higher bandwidth or peers that are geographically
   closer to improve application performance.  Once the client has
   obtained this ordered list, it has enough information to perform
   better-than-random initial peer selection.

   Such a solution has been evaluated with simulations and experiments
   run on the PlanetLab testbed, and the results show both improvements
   in content download time and a reduction of overall P2P traffic, even
   when only a subset of the applications actually query the oracle to
   make their decisions.

2.2.3.  ISP-Driven Informed Path Selection (IDIPS) Service

   The solution proposed by Saucez et al. [Saucez] is essentially a
   modified version of the oracle-based approach described in
   Section 2.2.2, intended to provide a network-layer service for
   finding the best source and destination addresses when establishing a
   connection between two endpoints in multi-homed environments (which
   are common in IPv6 networking).  Peer selection optimization in P2P
   systems -- the ALTO problem in today's Internet -- can be addressed
   by the IDIPS solution as a specific sub-case where the options for
   the destination address consist of all the peers sharing a desired
   resource, while the choice of the source address is fixed.  An
   evaluation performed on IDIPS shows that costs for both providing and
   accessing the service are negligible.

3.  Application-Level Topology Estimation and the ALTO Problem

   The application-level techniques described in Section 2.1 provide
   tools for peer-to-peer applications to estimate parameters of the
   underlying network topology.  Although these techniques can improve
   application performance, there are limitations of what can be
   achieved by operating only on the application level.

   Topology estimation techniques use abstractions of the network
   topology, which often hide features that would be of interest to the
   application.  Network coordinate systems, for example, are unable to
   detect overlay paths shorter than the direct path in the Internet
   topology.  However, these paths frequently exist in the Internet
   [Wang].  Similarly, application-level techniques may not accurately
   estimate topologies with multipath routing.





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   When using network coordinates to estimate topology information, the
   underlying assumption is that distance in terms of latency determines
   performance.  However, for file sharing and content distribution
   applications, there is more to performance than just the network
   latency between nodes.  The utility of a long-lived data transfer is
   determined by the throughput of the underlying TCP protocol, which
   depends on the round-trip time as well as the loss rate experienced
   on the corresponding path [Padhye].  Hence, these applications
   benefit from a richer set of topology information that goes beyond
   latency, including loss rate, capacity, and available bandwidth.

   Some of the topology estimation techniques used by P2P applications
   need time to converge to a result.  For example, current BitTorrent
   clients implement local, passive traffic measurements and a tit-for-
   tat bandwidth reciprocity mechanism to optimize peer selection at a
   local level.  Peers eventually settle on a set of neighbors that
   maximizes their download rate, but because peers cannot reason about
   the value of neighbors without actively exchanging data with them,
   and because the number of concurrent data transfers is limited
   (typically to 5-7), convergence is delayed and easily can be
   sub-optimal.

   Skype's P2P Voice over IP (VoIP) application chooses a relay node in
   cases where two peers are behind NATs and cannot connect directly.
   Measurements taken by Ren et al. [Ren] showed that the relay
   selection mechanism of Skype (1) is not able to discover the best
   possible relay nodes in terms of minimum RTT, (2) requires a long
   setup and stabilization time, which degrades the end user experience,
   and (3) is creating a non-negligible amount of overhead traffic due
   to probing a large number of nodes.  They further showed that the
   quality of the relay paths could be improved when the underlying
   network Autonomous System (AS) topology is considered.

   Some features of the network topology are hard to infer through
   application-level techniques, and it may not be possible to infer
   them at all, e.g., service-provider policies and preferences such as
   the state and cost associated with interdomain peering and transit
   links.  Another example is the traffic engineering policy of a
   service provider, which may counteract the routing objective of the
   overlay network, leading to a poor overall performance [Seetharaman].

   Finally, application-level techniques often require applications to
   perform measurements on the topology.  These measurements create
   traffic overhead, in particular, if measurements are performed
   individually by all applications interested in estimating topology.






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4.  Open Issues

   Beyond a significant amount of research work on the topic, we believe
   that there are sizable open issues to address in an infrastructure-
   based approach to traffic optimization.  The following is not an
   exhaustive list, but a representative sample of the pertinent issues.

4.1.  Coordinate Estimation or Path Latencies?

   Despite the many solutions that have been proposed for providing
   applications with topology information in a fully distributed manner,
   there is currently an ongoing debate in the research community
   whether such solutions should focus on estimating nodes' coordinates
   or path latencies.  Such a debate has recently been fed by studies
   showing that the triangle inequality on which coordinate systems are
   based is often proved false in the Internet [Ledlie].  Proposed
   systems following both approaches -- in particular, Vivaldi [Dabek]
   and PIC [Costa] following the former, and Meridian [Wong] and iPlane
   [Madhyastha] the latter -- have been simulated, implemented, and
   studied in real-world trials, each one showing different points of
   strength and weaknesses.  Concentrated work will be needed to
   determine which of the two solutions will be conducive to the ALTO
   problem.

4.2.  Malicious Nodes

   Another open issue common in most distributed environments consisting
   of a large number of peers is the resistance against malicious nodes.
   Security mechanisms to identify misbehavior are based on triangle
   inequality checks [Costa], which, however, tend to fail and thus
   return false positives in the presence of measurement inaccuracies
   induced, for example, by traffic fluctuations that occur quite often
   in large networks [Ledlie].  Beyond the issue of using triangle
   inequality checks, authoritatively authenticating the identity of an
   oracle, and preventing an oracle from attacks are also important.
   Existing techniques -- such as Public Key Infrastructure (PKI)
   [RFC5280] or identity-based encryption [Boneh] for authenticating the
   identity and the use of secure multi-party computation techniques to
   prevent an oracle from collusion attacks -- need to be explored and
   studied for judicious use in ALTO-type solutions.

4.3.  Information Integrity

   Similarly, even in controlled architectures deployed by network
   operators where system elements may be authenticated [Xie],
   [Aggarwal],[Saucez], it is still possible that the information
   returned to applications is deliberately altered, for example,
   assigning higher priority to financially inexpensive links instead of



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   neutrally applying proximity criteria.  What are the effects of such
   deliberate alterations if multiple peers collude to determine a
   different route to the target, one that is not provided by an oracle?
   Similarly, what are the consequences if an oracle targets a
   particular node in another AS by redirecting an inordinate number of
   querying peers to it causing, essentially, a Distributed Denial-of-
   Service (DDoS) [RFC4732] attack on the node?  Furthermore, does an
   oracle broadcast or multicast a response to a query?  If so,
   techniques to protect the confidentiality of the multicast stream
   will need to be investigated to thwart "free riding" peers.

4.4.  Richness of Topological Information

   Many systems already use RTT to account for delay when establishing
   connections with peers (e.g., Content-Addressable Network (CAN)
   [Ratnasamy], Bamboo [Rhea]).  An operator can provide not only the
   delay metric but other metrics that the peer cannot figure out on its
   own.  These metrics may include the characteristics of the access
   links to other peers, bandwidth available to peers (based on
   operators' engineering of the network), network policies, preferences
   such as state and cost associated with intradomain peering links, and
   so on.  Exactly what kinds of metrics an operator can provide to
   stabilize the network throughput will also need to be investigated.

4.5.  Hybrid Solutions

   It is conceivable that P2P users may not be comfortable with operator
   intervention to provide topology information.  To eliminate this
   intervention, alternative schemes to estimate topological distance
   can be used.  For instance, Ono uses client redirections generated by
   Akamai CDN servers as an approximation for estimating distance to
   peers; Vivaldi, GNP, and PIC use synthetic coordinate systems.  A
   neutral third party can make available a hybrid layer-cooperation
   service -- without the active participation of the ISP -- that uses
   alternative techniques discussed in Section 2.1 to create a
   topological map.  This map can be subsequently used by a subset of
   users who may not trust the ISP.

4.6.  Negative Impact of Over-Localization

   The literature presented in Section 2 shows that a certain level of
   locality-awareness in the peer selection process of P2P algorithms is
   usually beneficial to application performance.  However, an excessive
   localization of the traffic might cause partitioning in the overlay
   interconnecting these peers, which will negatively affect the
   performance experienced by the peers themselves.





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   Finding the right balance between localization and randomness in peer
   selection is an open issue.  At the time of writing, it seems that
   different applications have different levels of tolerance and should
   be addressed separately.  Le Blond et al. [LeBlond] have studied the
   specific case of BitTorrent, proposing a simple mechanism to prevent
   partitioning in the overlay, yet reach a high level of cross-domain
   traffic reduction without adversely impacting peers.

5.  Security Considerations

   This document is a survey of existing literature on topology
   estimation.  As such, it does not introduce any new security
   considerations to be taken into account beyond what is already
   discussed in each paper surveyed.

   Insofar as topology estimation is used to provide a solution to the
   ALTO problem, the issues in Sections 4.2 and 4.3 deserve special
   attention.  There are efforts underway in the IETF ALTO working group
   to design a protocol that protects the privacy of the peer-to-peer
   users as well as the service providers.  [Chen] provides an overview
   of ALTO security issues, Section 11 of [Alimi] is an exhaustive
   overview of ALTO security, and Section 6 of RFC 5693 [RFC5693] also
   lists the privacy and confidentiality aspects of an ALTO solution.

   The following references provide a starting point for general peer-
   to-peer security issues: [Wallach], [Sit], [Douceur], [Castro], and
   [Friedman].

6.  Acknowledgments

   This document is a derivative work of a position paper submitted at
   the IETF RAI area/MIT workshop held on May 28th, 2008 on the topic of
   Peer-to-Peer Infrastructure (P2Pi) [RFC5594].  The article on a
   similar topic, also written by the authors of this document and
   published in IEEE Communications [Gurbani], was also partially
   derived from the same position paper.  The authors thank profusely
   Arnaud Legout, Richard Yang, Richard Woundy, Stefano Previdi, and the
   many people that have participated in discussions and provided
   insightful feedback at any stage of this work.

7.  Informative References

   [Aggarwal]      Aggarwal, V., Feldmann, A., and C. Scheideler, "Can
                   ISPs and P2P users cooperate for improved
                   performance?", in ACM SIGCOMM Computer Communications
                   Review, vol. 37, no. 3.





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   [Alimi]         Alimi, R., Ed., Penno, R., Ed., and Y. Yang, Ed.,
                   "ALTO Protocol", Work in Progress, July 2010.

   [Boneh]         Boneh, D. and M. Franklin, "Identity-Based Encryption
                   from the Weil Pairing", in Proceedings of the 21st
                   Annual International Cryptology Conference on
                   Advances in Cryptology, August 2001.

   [Castro]        Castro, M., Druschelw, P., Ganesh, A., Rowstron, A.,
                   and D. Wallach, "Security for Structured Peer-to-peer
                   Overlay Networks", in Proceedings of Symposium on
                   Operating Systems Design and Implementation
                   (OSDI'02), December 2002.

   [Chen]          Chen, S., Gao, F., Beijing, X., and M. Xiong,
                   "Overview for ALTO Security Issues", Work
                   in Progress, February 2010.

   [Coates]        Coates, M., Hero, A., Nowak, R., and B. Yu, "Internet
                   Tomography", in IEEE Signal Processing Magazine,
                   vol. 19, no. 3.

   [Costa]         Costa, M., Castro, M., Rowstron, A., and P. Key,
                   "PIC: Practical Internet coordinates for distance
                   estimation", in Proceedings of International
                   Conference on Distributed Systems 2003.

   [Dabek]         Dabek, F., Cox, R., Kaashoek, F., and R. Morris,
                   "Vivaldi: A Decentralized Network Coordinate System",
                   in ACM SIGCOMM: Proceedings of the 2004 conference on
                   Applications, technologies, architectures, and
                   protocols for computer communications, vol. 34,
                   no. 4.

   [Douceur]       Douceur, J., "The Sybil Attack", in Proceedings of
                   the First International Workshop on Peer-to-Peer
                   Systems, March 2002.

   [Dovrolis]      Dovrolis, C., Ramanathan, P., and D. Moore, "What do
                   packet dispersion techniques measure?",
                   in Proceedings of IEEE INFOCOM 2001.

   [Francis]       Francis, P., Jamin, S., Jin, C., Jin, Y., Raz, D.,
                   Shavitt, Y., and L. Zhang, "IDMaps: A global Internet
                   host distance estimation service", in Proceedings of
                   IEEE INFOCOM 2001.





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   [Friedman]      Friedman, A. and A. Camp, "Peer-to-Peer Security",
                   in The Handbook of Information Security, J. Wiley &
                   Sons, 2005.

   [Glasner]       Glasner, J., "P2P fuels global bandwidth binge",
                   available from http://www.wired.com/.

   [Gummadi]       Gummadi, K., Gummadi, R., Gribble, S., Ratnasamy, S.,
                   Shenker, S., and I. Stoica, "The impact of DHT
                   routing geometry on resilience and proximity", in ACM
                   SIGCOMM: Proceedings of the 2003 conference on
                   Applications, technologies, architectures, and
                   protocols for computer communications.

   [Gurbani]       Gurbani, V., Hilt, V., Rimac, I., Tomsu, M., and E.
                   Marocco, "A Survey of Research on the Application-
                   Layer Traffic Optimization Problem and the Need for
                   Layer Cooperation", in IEEE Communications, vol. 47,
                   no. 8.

   [Karagiannis]   Karagiannis, T., Broido, A., Brownlee, N., Claffy,
                   K., and M. Faloutsos, "Is P2P dying or just hiding?",
                   in Proceedings of IEEE GLOBECOM 2004 Conference.

   [LeBlond]       Le Blond, S., Legout, A., and W. Dabbous, "Pushing
                   BitTorrent Locality to the Limit", available
                   at http://hal.inria.fr/.

   [Ledlie]        Ledlie, J., Gardner, P., and M. Seltzer, "Network
                   Coordinates in the Wild", in USENIX: Proceedings of
                   NSDI 2007.

   [LightReading]  LightReading, "Controlling P2P traffic", available
                   from http://www.lightreading.com/.

   [LinuxReviews]  linuxReviews.org, "Peer to peer network traffic may
                   account for up to 85% of Internet's bandwidth usage",
                   available from http://linuxreviews.org/.

   [Madhyastha]    Madhyastha, H., Isdal, T., Piatek, M., Dixon, C.,
                   Anderson, T., Krishnamurthy, A., and A.
                   Venkataramani, "iPlane: an information plane for
                   distributed services", in USENIX: Proceedings of the
                   7th symposium on Operating systems design and
                   implementation.






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   [Ng]            Ng, T. and H. Zhang, "Predicting internet network
                   distance with coordinates-based approaches",
                   in Proceedings of INFOCOM 2002.

   [Ono]           "Northwestern University Ono Project", <http://
                   www.aqualab.cs.northwestern.edu/projects/Ono.html>.

   [P4P]           "DCIA P4P Working group",
                   <http://www.dcia.info/activities/#P4P>.

   [Padhye]        Padhye, J., Firoiu, V., Towsley, D., and J. Kurose,
                   "Modeling TCP throughput: A simple model and its
                   empirical validation", in Technical Report UM-CS-
                   1998-008, University of Massachusetts 1998.

   [Parker]        Parker, A., "The true picture of peer-to-peer
                   filesharing", available
                   from http://www.cachelogic.com/.

   [RFC4732]       Handley, M., Ed., Rescorla, E., Ed., and IAB,
                   "Internet Denial-of-Service Considerations",
                   RFC 4732, December 2006.

   [RFC4949]       Shirey, R., "Internet Security Glossary, Version 2",
                   FYI 36, RFC 4949, August 2007.

   [RFC4981]       Risson, J. and T. Moors, "Survey of Research towards
                   Robust Peer-to-Peer Networks: Search Methods",
                   RFC 4981, September 2007.

   [RFC5280]       Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
                   Housley, R., and W. Polk, "Internet X.509 Public Key
                   Infrastructure Certificate and Certificate Revocation
                   List (CRL) Profile", RFC 5280, May 2008.

   [RFC5594]       Peterson, J. and A. Cooper, "Report from the IETF
                   Workshop on Peer-to-Peer (P2P) Infrastructure, May
                   28, 2008", RFC 5594, July 2009.

   [RFC5632]       Griffiths, C., Livingood, J., Popkin, L., Woundy, R.,
                   and Y. Yang, "Comcast's ISP Experiences in a
                   Proactive Network Provider Participation for P2P
                   (P4P) Technical Trial", RFC 5632, September 2009.

   [RFC5693]       Seedorf, J. and E. Burger, "Application-Layer Traffic
                   Optimization (ALTO) Problem Statement", RFC 5693,
                   October 2009.




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   [Ratnasamy]     Ratnasamy, S., Francis, P., Handley, M., Karp, R.,
                   and S. Shenker, "A Scalable Content-Addressable
                   Network", in ACM SIGCOMM: Proceedings of the 2001
                   conference on Applications, technologies,
                   architectures, and protocols for computer
                   communications, January 2001.

   [Ren]           Ren, S., Guo, L., and X. Zhang, "ASAP: An AS-aware
                   peer-relay protocol for high quality VoIP",
                   in Proceedings of IEEE ICDCS 2006.

   [Rhea]          Rhea, S., Godfrey, B., Karp, B., Kubiatowicz, J.,
                   Ratnasamy, S., Shenker, S., Stoica, I., and H. Yu,
                   "OpenDHT: a public DHT service and its uses", in ACM
                   SIGCOMM: Proceedings of the 2005 conference on
                   Applications, technologies, architectures, and
                   protocols for computer communications, August 2005.

   [Saucez]        Saucez, D., Donnet, B., and O. Bonaventure,
                   "Implementation and Preliminary Evaluation of an ISP-
                   Driven Informed Path Selection", in Proceedings of
                   ACM CoNEXT 2007.

   [Seetharaman]   Seetharaman, S., Hilt, V., Hofmann, M., and M. Ammar,
                   "Preemptive Strategies to Improve Routing Performance
                   of Native and Overlay Layers", in Proceedings of IEEE
                   INFOCOM 2007.

   [Sit]           Sit, E. and R. Morris, "Security Considerations for
                   Peer-to-Peer Distributed Hash Tables, Revised Papers
                   from the First", in Proceedings of the First
                   International Workshop on Peer-to-Peer Systems,
                   March 2002.

   [Su]            Su, A., Choffnes, D., Kuzmanovic, A., and F.
                   Bustamante, "Drafting behind Akamai (travelocity-
                   based detouring)", in ACM SIGCOMM: Proceedings of the
                   2006 conference on Applications, technologies,
                   architectures, and protocols for computer
                   communications.

   [Vuze]          "Vuze bittorrent client", <http://www.vuze.com/>.

   [Wallach]       Wallach, D., "A survey of peer-to-peer security
                   issues", in Proceedings of International Symposium on
                   Software Security, 2002.





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   [Wang]          Wang, G., Zhang, B., and T. Ng, "Towards Network
                   Triangle Inequality Violation Aware Distributed
                   Systems", in ACM SIGCOMM: Proceedings of the 7th
                   conference on Internet measurement.

   [Wong]          Wong, B., Slivkins, A., and E. Sirer, "Meridian: A
                   lightweight network location service without virtual
                   coordinates", in ACM SIGCOMM: Proceedings of the 2005
                   conference on Applications, technologies,
                   architectures, and protocols for computer
                   communications.

   [Xie]           Xie, H., Krishnamurthy, A., Silberschatz, A., and Y.
                   Yang, "P4P: Explicit Communications for Cooperative
                   Control Between P2P and Network Providers", in ACM
                   SIGCOMM Computer Communication Review, vol. 38,
                   no. 4.

Authors' Addresses

   Ivica Rimac
   Bell Labs, Alcatel-Lucent
   EMail: rimac@bell-labs.com

   Volker Hilt
   Bell Labs, Alcatel-Lucent
   EMail: volkerh@bell-labs.com

   Marco Tomsu
   Bell Labs, Alcatel-Lucent
   EMail: marco.tomsu@alcatel-lucent.com

   Vijay K. Gurbani
   Bell Labs, Alcatel-Lucent
   EMail: vkg@bell-labs.com

   Enrico Marocco
   Telecom Italia
   EMail: enrico.marocco@telecomitalia.it












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