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Network Working Group J. Peterson
Request for Comments: 5594 NeuStar
Category: Informational A. Cooper
Center for Democracy & Technology
July 2009
Report from the IETF Workshop on Peer-to-Peer (P2P) Infrastructure,
May 28, 2008
Abstract
This document reports the outcome of a workshop organized by the
Real-time Applications and Infrastructure Area Directors of the IETF
to discuss network delay and congestion issues resulting from
increased Peer-to-Peer (P2P) traffic volumes. The workshop was held
on May 28, 2008 at MIT in Cambridge, MA, USA. The goals of the
workshop were twofold: to understand the technical problems that ISPs
and end users are experiencing as a result of high volumes of P2P
traffic, and to begin to understand how the IETF may be helpful in
addressing these problems. Gaining an understanding of where in the
IETF this work might be pursued and how to extract feasible work
items were highlighted as important tasks in pursuit of the latter
goal. The workshop was very well attended and produced several work
items that have since been taken up by members of the IETF community.
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (c) 2009 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 in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
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Table of Contents
1. Introduction ....................................................3
2. Scoping of the Problem and Solution Spaces ......................4
3. Service Provider Perspective ....................................4
3.1. DOCSIS Architecture and Upstream Contention ................4
3.2. TCP Flow Fairness and Service Flows ........................5
3.3. Service Provider Responses .................................6
4. Application Provider Perspective ................................7
5. Potential Solution Areas ........................................7
5.1. Improving Peer Selection: Information Sharing,
Localization, and Caches ...................................8
5.1.1. Leveraging AS Numbers ...............................9
5.1.2. P4P: Provider Portal for P2P Applications ...........9
5.1.3. Multi-Layer, Tracker-Based Architecture ............10
5.1.4. ISP-Aided Neighbor Selection .......................11
5.1.5. Caches .............................................12
5.1.6. Potential IETF Work ................................12
5.2. New Approaches to Congestion Control ......................14
5.2.1. End-to-End Congestion Control ......................15
5.2.2. Weighted Congestion Control ........................15
5.3. Quality of Service ........................................16
6. Applications Opening Multiple TCP Connections ..................17
7. Costs and Congestion ...........................................18
8. Next Steps .....................................................18
8.1. Transport Issues ..........................................19
8.2. Improved Peer Selection ...................................19
9. Security Considerations ........................................19
10. Acknowledgements ..............................................19
11. Informative References ........................................20
Appendix A. Program Committee ....................................21
Appendix B. Workshop Participants ................................21
Appendix C. Workshop Agenda ......................................24
Appendix D. Slides and Position Papers ..........................25
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1. Introduction
Increasingly, large ISPs are encountering issues with P2P traffic.
The transfer of static, delay-tolerant data between nodes on the
Internet is a well-understood problem, but traditional management of
fairness at the transport level is under strain from applications
designed to achieve the best end-user transfer rates. At peak times,
this results in networks running near absolute capacity, causing all
traffic to incur delays; the applications that bear the brunt of this
additional latency are real-time applications like Voice over IP
(VoIP) and Internet gaming. To explore how IETF standards work could
be useful in addressing these issues, the Real-time Applications and
Infrastructure area directors organized a "P2P Infrastructure"
workshop and invited contributions from subject matter experts in the
problem and solution spaces.
The goals of the workshop were twofold: to understand the technical
problems that ISPs and end users are experiencing as a result of high
volumes of P2P traffic, and to begin to understand how the IETF may
be helpful in addressing these problems. Gaining an understanding of
where in the IETF this work might be pursued and how to extract
feasible work items were highlighted as important tasks in pursuit of
the latter goal. The workshop's focus was on engineering solutions
that promise some imminent benefit to the Internet as a whole, as
opposed to longer-term research or closed proprietary solutions.
While public policy must inform work in this space, crafting or
debating public policy was outside the scope of the workshop.
Position papers were solicited in the weeks prior to the workshop,
and a limited number of speakers were invited to present their views
at the workshop based on these submissions. This report is a summary
of all participants' contributions. The program committee and
participant list are attached in Appendices A and B, respectively.
The agenda of the workshop can be found in Appendix C. A link to the
presentations given at the workshop and the position papers submitted
prior to the workshop is in Appendix D.
The workshop showcased the IETF community's recognition of the impact
of P2P and other high-volume applications on the Internet as a whole.
Participants welcomed the opportunity to discuss potential
standardization work that network operators, applications providers,
and end users would all find mutually beneficial. Two transport-
related work items gained significant traction: designing a protocol
for very deferential end-to-end congestion control for delay-tolerant
applications, and producing an informational document about the
reasoning behind and effects of applications opening multiple
transport connections at once. A separate area of interest that
emerged at the workshop focused on improving peer selection by having
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networks make more information available to applications. Finally,
presenters also covered traditional approaches to multiple service-
tier queuing such as Diffserv.
2. Scoping of the Problem and Solution Spaces
The genesis for the Peer-to-Peer Infrastructure (P2PI) workshop grew
in large part out of specific pain points that ISPs are experiencing
as a result of high volumes of P2P traffic. However, several
workshop participants felt that the IETF should approach a more
general space of problems, of which P2P-related congestion may be
merely one instance.
For example, high-volume applications besides P2P, whether they
already exist or have yet to be developed, could cause congestion
issues similar to those caused by P2P. The general class of
congestion problems attributable to always-on, high-volume
applications require the development of solutions that are reasonable
for operators, applications, and subscribers. And while much
attention has been paid to congestion on access links, increased
traffic volumes could impact other parts of the network. Although
the workshop focused primarily on the specific causes and effects of
current P2P traffic volumes, it will likely be useful in the future
for the IETF to consider how to pursue solutions to these larger
problems.
Obtaining more data about Internet congestion may also be a helpful
step before the IETF pursues solutions. This data collection could
focus on where in the network congestion is occurring, its duration
and frequency, its effects, and its root causes. Although individual
service providers expressed interest in sharing congestion data,
strategies for reliably and regularly obtaining and disseminating
such data on a broad scale remain elusive.
3. Service Provider Perspective
To help participants gain a fuller understanding of one specific
network operator's view of P2P-induced congestion, Jason Livingood
and Rich Woundy provided an overview of Comcast's network and
approach to management of P2P traffic.
3.1. DOCSIS Architecture and Upstream Contention
In the Data Over Cable Service Interface Specification (DOCSIS)
architecture [DOCSIS] that is used for many cable systems, there may
be a single Cable Modem Termination System (CMTS) serving hundreds or
thousands of residential cable customers. Each CMTS has multiple
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DOCSIS domains, each of which typically has a single downstream link
and a number of upstream links. Each CMTS is connected through a
hybrid fiber-coaxial (HFC) network to subscribers' cable modems.
The limiting resource in this architecture is usually bandwidth, so
bandwidth is typically the measure used for capacity planning. As
with all networks, congestion manifests itself when instantaneous
load exceeds available capacity.
In the upstream direction, any cable modem connected to a CMTS can
make a request to the CMTS to transmit, and requests are randomized
to minimize collisions. With many cable modems issuing requests at
once, the requests may collide, resulting in delays. DOCSIS does not
specify a size for cable modem buffers, but buffer delays of one to
four seconds have been observed with various cable modems from
different vendors.
Once the CMTS has granted a cable modem the ability to transmit its
data PDU, the modem can piggyback its next request on top of that
data PDU. In situations with a lot of upstream traffic, piggybacking
happens more often, which sends heavy upstream users to the front of
the CMTS queue, ahead of interactive but less-upstream-intensive
applications. For example, if the CMTS is granting requests
approximately every one to three milliseconds, then a cable modem
transmitting data for a service like VoIP with a packetization delay
of 20-30 milliseconds may get into contention with another modem on
the same CMTS that is constantly transmitting upstream and
piggybacking each new request. This may explain how heavy upstream
users ultimately dominate the pipe over more interactive
applications. Consequentially, it is imperative that assessments of
the problem space and potential solutions are mindful of the
influence that specific layer-2 networks may exert on the behavior of
Internet traffic, especially when considering the alleviation of
congestion in an access network.
3.2. TCP Flow Fairness and Service Flows
How TCP flow fairness applies to upstream requests to the CMTS is an
open question. A CMTS sees many service flows, each of which could
be a single TCP flow or many TCP flows (or UDP). The CMTS is not
aware of the source or destination IP address of a packet until it
has already been transmitted upstream, so those cannot be used to
impose flow fairness.
A particular cable modem can have multiple service flows defined.
For example, a modem that is also a VoIP endpoint can provision a
service flow for VoIP that would allow VoIP traffic to avoid the
upstream request process to the CMTS (and thereby avoid contention
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with other modems). The service flow would have upstream capacity
provisioned for it. The modem would have a separate service flow for
best efforts traffic. Some ISPs provision such a flow for their own
VoIP offerings; others allow subscribers to pay extra to have
particular traffic assigned to a provisioned service flow.
It may also be possible for an ISP to provision such a flow on the
fly when it recognizes the need for it. Diffserv [RFC2475] bits set
by the customer premises equipment could be used to classify flows,
for example.
3.3. Service Provider Responses
In 2005, ISP customers began increasingly complaining about the
performance of delay-sensitive traffic (VoIP and gaming), due in part
to the issues arising out of the DOCSIS architecture as described
above. At the same time, ISPs were seeing heavy growth in P2P
traffic and an increasing correlation between high levels of P2P
activity and packet loss.
In responding to this situation, cable ISPs have several avenues to
pursue. The newest generation of the DOCSIS specification, DOCSIS
3.0, enables faster transfer rates than most cable systems currently
support. While the rollout of DOCSIS 3.0 will provide additional
capacity, it will likely not obviate the need for congestion
management in an environment where client software is designed to
maximize bandwidth consumption regardless of available capacity.
Congestion management can take many forms; Jason and Rich explained
the new protocol-agnostic approach that Comcast is currently
trialing. Prior to these trials, all traffic was marked as "best
efforts". During the trials, all traffic is re-classified as
"priority". When a CMTS is approaching peak congestion on a
particular upstream or downstream port (the "Near Congestion State"),
some subscribers will have traffic re-classified as "best efforts".
Both the threshold for determining when a CMTS port is in Near
Congestion State and the number of minutes it remains in this state
are parameters being explored during the trials. To re-classify
upstream traffic, a new default DOCSIS service flow is used that has
the same provisioned bandwidth as the "priority" stream but that is
treated with lower priority.
The subscribers whose traffic is re-marked will be selected by
determining whether they have temporarily entered a "Long Duration
Bulk Consumption State". This state is achieved by consuming a
certain amount of bandwidth over a certain period of minutes (both
are tweakable parameters being explored during the trials). These
thresholds will depend on the subscriber's service tier --
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subscribers who pay for more bandwidth will have higher thresholds.
The re-marking will not distinguish between multiple users of the
same subscriber connection, so one family member's P2P usage could
cause another family member's Web browsing traffic to be lowered in
priority. There is no current mechanism for users to determine that
their traffic has been re-marked.
By temporarily reducing the traffic priority of subscribers who have
been consuming bandwidth in bulk for lengthy periods, this congestion
management technique aims to preserve a good user experience for
subscribers with burstier traffic patterns, including those using
real-time applications. As compared to an approach that reduces
particular subscribers' bandwidth during periods of congestion, this
technique eliminates the ability for applications to set their own
priority levels, but it also avoids the negative connotations that
some users may associate with bandwidth reductions.
This approach involves many tweakable parameters. A large part of
the trial process is aimed at determining the best settings for these
parameters, but there may also be opportunities to work with the
research community to identify the best way to adjust the thresholds
necessary to optimize the performance of the management technique.
4. Application Provider Perspective
Stanislav Shalunov provided an overview of BitTorrent's view of the
impact of increased P2P traffic volumes and potential mitigations.
The impact is described here; his proposed solutions (comprising the
bulk of his talk) are addressed in the appropriate subsections of
Section 5.
As uptake in P2P usage has grown, so has end-user latency. For
example, a user whose uplink capacity is 250-500 Kbps and whose modem
buffer has a capacity of 32-64 Kbps may easily fill the buffer
(unless the modem uses Adaptive Queue Management (AQM), which is
uncommon). This can result in delay on the order of seconds, with
disastrous effects on application performance. On a cable system
with shared capacity between neighbors, one neighbor could saturate
the buffer and affect the latency of another neighbor's traffic.
Even users with dedicated bandwidth can experience delays as their
own P2P traffic saturates the link and dominates their own more
latency-sensitive traffic.
5. Potential Solution Areas
The submissions received in advance of the workshop covered a broad
array of work addressing specific aspects of P2P traffic volume and
other related issues. Solution suggestions generally fell into one
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or more of three topic areas: improving peer selection, new
approaches to congestion control, and quality-of-service mechanisms.
The workshop discussions and outcomes in each area are described
below.
5.1. Improving Peer Selection: Information Sharing, Localization, and
Caches
Peer selection is an integral factor in determining the efficiency of
P2P networks from both the ISP and the P2P client points of view.
How peers are selected will determine both network load and client
performance.
The way that P2P clients select peers today varies from protocol to
protocol and client to client but, in general, peers are largely
oblivious to routing-level and network-topology information. This
results in P2P topologies that are agnostic of underlay topologies
and constraints.
Approaches to closing this gap generally involve an entity that has
knowledge of network topology, costs, or constraints (e.g., an ISP)
making some of this information available to P2P clients or trackers.
This information may be used to localize traffic based on some metric
of locality or to otherwise alter peer-selection decisions based on
the provided network information (hereafter referred to simply as
"localization"). One special case of this kind of approach would
help peers find caches containing the content they seek.
Any alteration to current peer-selection algorithms will have
engineering trade-offs. BitTorrent, for example, used randomized
peer selection by design. Choosing peers randomly out of a large
selection helps to average out problems among peers and allows for
connections to good peers that may be far away. Randomized peer
selection also supports "rarest first" piece selection, which allows
swarms to continue even when the original seed disappears and which
distributes pieces so that more peers are likely to have pieces of
interest to other peers. Any move away from randomized selection
would have to take these factors into account.
Although localization has the potential to improve peer selection,
the incentives for both parties to the information exchange are
complex. ISPs may want to move traffic off of their own networks,
which could motivate them to provide information to peers that has
the opposite effect of what the peers would expect. Likewise, peers
will want the use of the information they receive to result in
performance improvements; otherwise, they have no incentive to
consult with the network before selecting peers. Even when both
parties find the information sharing to be beneficial, user
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experiences will not necessarily be uniform depending on the scope of
the information provided and the peer's location. Localization
information could form one component of a peer-selection decision,
but it will likely need to be balanced against other factors.
Workshop participants discussed both current research efforts in this
area and how IETF standards work may be useful in furthering the
general concept of improved peer selection. Those discussions are
summarized below.
5.1.1. Leveraging AS Numbers
One simple way to potentially make peer selection more efficient
would be for a peer to prefer peers within its own Autonomous System
(AS). Transfers between peers within the same AS may be faster on
some networks, although more data is needed to determine the extent
of the potential improvement. On mobile networks, for example, the
utility of AS numbers is limited since they do not correlate to
geographic location. Peers may also see improvements by connecting
to other peers within a specific set of ASes or IP prefixes provided
by their ISPs. Some ISPs may have an incentive to expose this
granularity of information because it will potentially reduce their
transit costs.
A case study was conducted with the four most popular BitTorrent
torrents to determine what the effect of localizing to an AS might
be. The swarm sizes for the torrents were 9984, 3944, 2561, and
2023, with the size distributions appearing to be polynomial. With
more than 20 peers in a single AS, peers within an AS could trade
only with each other, avoiding interdomain traffic. More than half
(57%) of peers in the four swarms were in ASes like this. Thus, in
these cases connecting to peers within an AS could reduce transit
traffic by at least 57%. If the ASes have asymmetric upload and
download links, however, the resulting user experience may
deteriorate since each peer's download speed would be limited by
slower upload speeds.
With the largest swarm size at 9984, the probability of two peers
being in the same neighborhood is too low to make localization to the
neighborhood level worthwhile. Attempting a simple localization
scheme, such as the AS localization described above, and determining
its effectiveness likely makes more sense as a first step.
5.1.2. P4P: Provider Portal for P2P Applications
The Provider Portal for P2P Applications (P4P) project [P4P] aims to
design a framework to enable cooperation between providers and
applications (including P2P), where "providers" may be ISPs, content
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distribution networks, or caching services. In this architecture,
each provider can communicate information to P2P clients through a
portal known as an iTracker. An iTracker could be identified through
a DNS SRV record (perhaps with its own new record type), a whois
look-up, or a trusted third party.
An iTracker has different interfaces for different types of
information that the provider may want to share. The core interface
allows the provider to express the "virtual cost" of its intradomain
or interdomain links. Virtual cost may reflect any kind of provider
preferences and may be based on the provider's choice of metrics,
including utilization, transit costs, or geography. It is up to the
provider to decide how dynamic it wants to be in updating its virtual
cost determinations.
In tests of this framework, two parallel swarms were created with
approximately the same number of clients and similar geographical and
network distributions, both sharing the same file. One of the swarms
used the P4P framework, with the ISP's network topology map as input
to its iTracker, and the other swarm used traditional peer selection.
The swarm without P4P saw 98% of traffic to and from peers external
to the ISP, whereas with P4P that number was 50%. Download
completion times for the P4P-enabled swarm improved approximately 20%
on average.
5.1.3. Multi-Layer, Tracker-Based Architecture
The multi-layer, tracker-based P2P scheme described at the workshop
is a generic example of an architecture that demonstrates how
localization may be useful in principle.
In a traditional, tracker-based P2P system, trackers provide clients
with information about seeds and peers where clients can find the
content they seek. A multi-layered tracker architecture incorporates
additional "local" trackers that provide the same information, but
only for content located within their own local network scope.
Client queries are re-directed from the global tracker to the
appropriate local trackers. Local trackers may also exist on
multiple levels, in which case queries would be further re-directed.
This sort of architecture could also serve hybrid P2P/content
delivery networks, where the global tracker functions as both a
tracker and a content server, and local trackers track locally
provisioned caches in addition to seeds and peers.
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One challenge in this architecture is determining what "local" means
for trackers, seeds, and peers. Locality could be dependent on
traffic conditions, load balancing, static topology, policy, or some
other metric. These same considerations would also be crucial for
determining appropriate cache placement in a hybrid network.
This architecture presents in the abstract the problem of re-
directing from a global entity to a local entity. Client queries
need to find their way to the appropriate local tracker. This can be
accomplished through an off-path, explicit mechanism where local
trackers register with the global tracker in advance, or through an
on-path approach where the network proxies P2P requests. The off-
path tracker format approach is preferable for performance and
reliability reasons.
Inasmuch as the multi-layer scheme might require ISPs to aid peers in
finding optimal paths to unauthorized copies of copyrighted content,
ISPs may be concerned about the legal liability of participating.
5.1.4. ISP-Aided Neighbor Selection
ISPs have a lot of knowledge about their networks: everything from
the bandwidth, geography, and service class of particular nodes to
overarching routing policies, OSPF and BGP metrics, and distances to
peering points. The ISP-aided neighbor selection service described
below seeks to leverage this knowledge without requiring ISPs to
reveal any information that could not already be discerned through
reverse-engineering by client applications.
The service consists of an "oracle" hosted by an ISP. The oracle
receives a list of IP addresses from a network node, sorts the list
according to its own confidential criteria, and returns the sorted
list to the node. The peer ranking provided by the oracle could be
viewed as a special case of the virtual cost interface described in
the previous section.
This service could be used by P2P clients or trackers, or by any
other application that would benefit from learning its ISP's
connection preferences. The oracle could be run as a web server or
UDP service at a known location (perhaps similar to BIND).
For interdomain ranking, an ISP could rank its own peers first, or it
could base its ranking on the AS number of the IPs in the provided
list. Another option would be for multiple ISPs to work together to
have their oracles exchange lists with each other.
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The main challenge in implementing the oracle service is scalability.
If peers need to communicate to the oracle the IP address of every
peer they know, the size of oracle requests may be inordinately
large. Additionally, today's largest swarms approach 10000 peers,
and with every peer requesting a sorted list, the oracle request
volume will swell. With the growth of business models dependent upon
P2P for distribution of content, swarms in the future may be far
larger, further exacerbating the problem. Potential mitigations
include having trackers, instead of peers, issue oracle requests, and
using other peers' sorted lists as input, rather than always using an
unsorted list.
On the other hand, this approach is advantageous from a legal
liability perspective, because it does not require ISPs to have any
knowledge of where particular content might be located or to have any
role in directing peers to particular content.
5.1.5. Caches
Deploying caches as peers in P2P networks was suggested as a
component of multiple proposals put forth at the workshop. Caches
may help to ease network load by reducing the need for peers to
upload to each other and by localizing traffic.
The two main concerns about P2P caches relate to network capacity and
legal liability. For caches to be useful, they will likely need to
be large (one suggestion was that a 1 TB cache could service 30% of
requests within a single AS, and a 100 TB cache could service 80% of
requests). Large caches will require sizable bandwidth in order to
avoid contention among peers. Caches would not be usefully placed
within an HFC network on a cable system, for example.
The legal liability attached to hosting a P2P cache likely reduces
the incentives to do so. Even under legal regimes where liability
for caching may be unclear, ISPs and others may view hosting a cache
as too great of a legal risk to be worthwhile.
5.1.6. Potential IETF Work
Much of the localization work discussed at the workshop is still in
its initial stages, and many questions remain about the value that
localization provides for varying network and overlay architectures.
More data is needed to evaluate the effects on both traffic load and
client performance. Understanding swarm distributions is important;
swarms with long tails may not particularly benefit from
localization.
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Against this backdrop, the key task for the IETF (as identified at
the workshop) is to pinpoint incrementally beneficial work items in
the spaces discussed above. In the future, it may be possible to
standardize entire P2P mechanisms but, as a starting point, it makes
more sense to single out core manageable components for
standardization. The focus should be on items that are not so
specific to one ISP or P2P network that standardization is rendered
useless. Ideally, any mechanisms resulting from this work might
apply to future applications that exhibit the same bandwidth-
intensive properties as today's P2P file-sharing.
In considering any of these items, it will be necessary to ensure
that the information exchanged by networks and applications does not
harm any of the parties involved. Not every piece of information
exchanged will be beneficial or verifiable, and this fact must be
recognized and accounted for. Solutions that leave applications or
networks worse off than they already are today will not gain any
traction.
It should also not be assumed that a particular party will be best
suited to provide a particular kind of information. For example, an
ISP may not know what the connection costs are in other ISPs'
networks, whereas an overlay network that receives cost information
from several ISPs may have a better handle on this kind of data.
Standardization of information sharing should not assume the identity
of particular parties doing the sharing.
The list of potential work items discussed at the workshop is
provided below. Workshop participants showed particular interest in
pursuing the first three items further.
5.1.6.1. AS Numbers
P2P clients are currently reliant on IP-to-AS mapping tables when
they want to determine AS numbers. Providing a standard, easier way
for clients to obtain this information may help to make peer
selection more efficient on certain networks.
5.1.6.2. Querying for Preferred Peers
In situations where a peer or tracker can make requests in real time
to a service that expresses its ISP's peering preferences,
standardizing a format for requests and responses may be useful. The
focus would be on the communication of the information, not on the
criteria used to decide preferences. The information provided to
peers would have to be crafted to ensure that it protects the privacy
of other peers and safeguards proprietary network information.
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5.1.6.3. Local Tracker, iTracker, Oracle, or Cache Discovery
With the deployment of trackers, iTrackers, oracles, or other
mechanisms that provide some information specific to a node's
locality, nodes will need a way to find these resources. One task
for the IETF could be to explore a way to do discovery, potentially
by leveraging an existing discovery protocol (DNS, DHCP, anycast,
etc.). Depending on the resource to be discovered, discovery may
require only a simple look-up, or it may require a more complex
determination of which resource is "closest" to the node issuing the
request.
5.1.6.4. ISP Account Usage Information
Where ISP subscribers are bound by network usage policies or volume-
based quotas, it may be useful to have a standard way of
communicating the subscriber's current usage status. This would be
similar to information about how many minutes of cell phone airtime
are left in a subscriber's billing cycle. Applications could use
this information to make decisions about when and how to transfer
data. One challenge in implementing such a standard would be support
for potentially limitless different ISP business models. The level
of granularity that an ISP is able to provide may also be constrained
depending on the pricing model and how dynamic the information is
expected to be.
5.1.6.5. Tracker Formats
A multi-layered tracker approach could potentially be aided by a
standard tracker format for re-directing from a global tracker to a
local tracker. While the extent to which existing trackers will be
willing to consult with other trackers is unclear, the re-direction
format may have an analog in another context -- many HTTP servers
build their own indexes of mirror information for a similar purpose,
though these are not standardized. If the two problem spaces prove
to be similar enough, there may be room to standardize a format
across both.
5.2. New Approaches to Congestion Control
One recent informal survey presented at the workshop found that ISPs
perceive traffic volumes from heavy users to be a problem, but no
single congestion management tool has been put to wide use. Within
developer and research communities, congestion issues raised by
increased P2P traffic volumes have spurred new thinking about
congestion-control mechanisms at both the transport layer and the
application layer. The subsections below explore some of these new
ideas and highlight areas where IETF work may be appropriate.
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5.2.1. End-to-End Congestion Control
As noted previously, uptake in P2P usage can result in perceptible
end-user latency on the order of seconds for interactive
applications. One approach to resolving this "round-trip time (RTT)
in seconds" problem would be for P2P clients to implement better
congestion control that keeps the bottleneck full while yielding to
keep the delay of competing traffic low. Such an algorithm has been
implemented in BitTorrent's client by continuously sampling one-way
delay (separating propagation from queuing delay) and targeting a
small queuing delay value. This essentially approximates a scavenger
service class in an end-to-end congestion-control mechanism by
forcing bulk, elastic traffic to yield to competitors under
congestion.
In a similar vein, the P4P framework supports a component that allows
applications to mark traffic as "bulk data" (not time sensitive).
Applications adjust their behavior according to the feedback they
receive from such markings.
Experimenting with the standardization of these kinds of techniques
or any congestion-control framework with design goals that differ
from those of TCP may be helpful work for the IETF to pursue.
5.2.2. Weighted Congestion Control
Congestion control has typically been implemented at a protocol level
as an optional, cooperative effort between endpoints experiencing
congestion, but in looking for a long-term approach to congestion
control, we may need a more rigorous way for available bandwidth to
be allocated by and between the hosts using a network. The idea
behind weighted congestion control is to allow hosts to give more
weight to interactive applications during times of congestion.
Comparing such an approach with Diffserv showcases its strengths and
weaknesses. Unlike Diffserv, weighted congestion control could be
implemented on hosts with a simple extension to socket APIs (although
consensus among OSes would be necessary for portability). Through
weighted congestion control, control resides with the host, whereas
even when Diffserv APIs are available, it is difficult for a host to
know that the network is complying with its classifications. With
weighted congestion control, hosts need some disincentive to setting
their weights at maximum levels, whereas Diffserv was not designed
for individual users to employ. Both approaches must rely on traffic
senders to set policies, meaning that the congestion issues stemming
from P2P use on the receiver side are not aided by either mechanism.
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With Diffserv, a light user may waste his or her priority connecting
to a heavy user on another network, which is not a problem with host-
controlled weighting.
Weighted congestion control is just one example of a generalized set
of features that characterize useful approaches to congestion
control. These characteristics include full user control of
priorities within a user's own scope and no possibility of
interpreting ISP behavior as discriminatory. The former means that
ISPs should not override user decisions arbitrarily (though this does
not preclude an ISP from offering prioritization as an option). The
latter means that the metric for decision-making needs to obviate
suspicion of ISP motivations.
One metric that meets these criteria is a harm (cost) metric, where
cost is equal to the amount of data that was not served to its
destination. Using this metric, cost is greatest when traffic peaks
are greatest. It allows for a policy of not sending too much data
during times of congestion, without specifying exactly how much is
too much. The cost metric could be used either for a Diffserv
approach or for weighted congestion control.
One important limitation on ISPs from a congestion-control
perspective is that they do not have a window into congestion on
other ISPs' networks. Solving this problem requires a separate
mechanism to express congestion across domains.
One potential avenue for the IETF or IRTF to pursue would be to
establish a long-term design team to assess congestion problems in
general and the long-term effects of any proposed quick fixes. These
issues are not necessarily confined to P2P and should be viewed in
the broader context of massive increases in bandwidth use.
5.3. Quality of Service
Although ISPs have implemented a wide variety of short-term
approaches to dealing with congestion, several of these may not be
viable in the long term. For example, some ISPs have found that
using deep packet inspection to change the delivery characteristics
of certain traffic at times of congestion is more cost effective than
adding additional bandwidth. Over time, this approach could lead to
a cat-and-mouse game where applications providers continually adapt
to avoid being correctly classified by Deep Packet Inspection (DPI)
equipment. Similarly, ISPs implementing traffic analysis to identify
P2P traffic may find that, in the long run, the overhead required of
an effective classification scheme will be excessive.
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Quality of service (QoS) arrangements may be more suitable in the
long term. One approach that distinguishes certain classes of
traffic during times of congestion was described in Section 3.3. A
standardized mechanism that may be useful for implementing QoS is
Differentiated Services Code Points (DSCP) [RFC2474].
With DSCP, devices at the edge of the network mark packets with the
service level they should receive. Nodes within the network do not
need to remember anything about service flows, and applications do
not need to request a particular service level. Users effectively
avoid self-interference through service classification.
Although DSCP may have many uses, perhaps the most relevant to the
P2P congestion issue is its ability to facilitate usage-based
charging. User pricing agreements that charge a premium for real-
time traffic and best-effort traffic could potentially shape user
behavior, resulting in reduced congestion (although ISPs would need a
mechanism to mitigate the risk of charging subscribers for things
like unintentional malware downloads or DoS attacks). DSCP could
also be used to limit a user's supply of high-priority bandwidth,
resulting in a similar effect.
Equipment to support DSCP is already available. Although there has
been some concern about a perceived lack of DSCP deployment, it is
widely used by enterprise customers, and growth has been strong due
to uptake in VoIP at the enterprise level.
However, DSCP still faces deployment hurdles on many networks.
Perhaps the largest barrier of all to wide deployment is the lack of
uniform code points to be used across networks. For example, the
latest Windows Vista API marks voice traffic as CS7, above the
priority reserve for router traffic. To properly take advantage of
this change, every switch will need to re-mark all traffic. In
addition, disparate ISPs are currently without a means of verifying
each others' markings and thus may be unwilling to trust the markings
they receive.
6. Applications Opening Multiple TCP Connections
The workshop discussions about P2P congestion spurred a related
discussion about applications (P2P or otherwise) that open multiple
TCP connections. With multiple users sharing one link, TCP flow
fairness gives users with multiple open connections a larger
proportion of bandwidth. Since some P2P protocols use multiple open
connections for a single file transfer and users often pursue
multiple transfers at once, this can cause a P2P user to have many
more open connections at once than other users on the same link. The
same is true for users of other applications that open multiple
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connections. A single user with multiple open connections is not
necessarily a problem on its face, but the fact that fairness is
determined per flow rather than per user leaves that impression.
Workshop participants thought it may be useful for the IETF to
provide some information about such situations.
7. Costs and Congestion
Workshop participants expressed diverging opinions about how much the
cost of transferring data -- as experienced by ISPs and, by
extension, their subscribers -- should factor into IETF thinking on
P2P traffic issues.
On one hand, bandwidth costs may be significant, even when viewed in
isolation from congestion issues. Some estimates put the total cost
of shipping 1 GB between $0.10 and $2. The cost of transit bandwidth
in markets where subscribers are charged flat rates appears to have
leveled off and may no longer be getting cheaper. Thus, it may be
reasonable to expect more service providers to move to volume-based
pricing (where they have not already done so) as a means to control
congestion and increase revenues. This is only feasible if bandwidth
consumption is visible to end users, which argues for some mechanism
of exposing quotas and usage to applications. However, expressing
cost information may be outside of the technical purview of the IETF.
On the other hand, congestion can be viewed merely as a manifestation
of cost. An ISP that invests in capacity could be considered to be
paying to relieve congestion. Or, if subscribers are charged for
congesting the network, then cost and congestion could be viewed as
one and the same. The distinction between them may thus be
artificial.
Workshop participants felt that the issues highlighted here may be
useful fodder for IRTF work.
8. Next Steps
The IETF community recognizes the significance of both increasing P2P
traffic volumes and network load at large. The importance of
addressing the impact of high-volume, delay-tolerant data transfer on
end-user experiences was highly apparent at the workshop.
At the conclusion of the workshop and in the days following, it
became clear that the largest areas of interest fell into two
categories: transport-related issues and improved peer selection.
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8.1. Transport Issues
Two main transport-related work items evolved out of the workshop.
The first was the creation of a standardized, delay-based, end-to-end
congestion-control mechanism that applications such as P2P clients
could use to reduce their own impact on interactive applications in
use on shared links (as described in Section 5.2.1). The second was
an informational document that describes the current practice of P2P
applications opening multiple transport connections and that makes
recommendations about the best practices for doing so (as discussed
in Section 6).
8.2. Improved Peer Selection
Participants expressed strong interest in further pursuing the range
of concepts described in Section 5.1 that support mechanisms for
information sharing between networks and applications to help improve
peer selection. Adding to the appeal of this topic is its potential
utility for applications other than P2P that may also benefit from
information about the network. Because the scope of potential
solutions discussed at the workshop was broad, extracting out the
most feasible pieces to pursue is the necessary first step.
9. Security Considerations
The workshop discussions covered a range of potential engineering
activities, each with its own security considerations. For example,
if networks are to provide preference or topology information to
applications, the applications may desire some means of verifying the
authenticity of the information. As the IETF community begins to
pursue specific avenues arising out of this workshop, addressing
relevant security requirements will be crucial.
10. Acknowledgements
The IETF would like to thank MIT, which hosted the workshop, and all
those people at MIT and elsewhere who assisted with the organization
and logistics of the workshop.
The IETF is grateful to the program committee (listed in Appendix A)
for their time and energy in organizing the workshop, reviewing the
position papers, and crafting an event of value for all participants.
The IETF would also like to thank the scribes, Spencer Dawkins and
Alissa Cooper, who diligently recorded the proceedings during the
workshop.
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A special thanks to all the participants in the workshop (listed in
Appendix B) who took the time, came to the workshop to participate in
the discussions, and put in the effort to make this workshop a
success. The IETF especially appreciates the effort of those that
prepared and made presentations at the workshop.
11. Informative References
[DOCSIS] CableLabs, "DOCSIS Specifications - DOCSIS 2.0 Interface",
2008, <http://www.cablemodem.com/specifications/
specifications20.html>.
[P4P] Xie, H., Yang, Y., Krishnamurthy, A., and A. Silberschatz,
"P4P: Provider Portal for Applications", August 2008,
<http://uwnews.org/relatedcontent/2008/August/
rc_parentID43281_thisID43282.pdf>.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
December 1998.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998.
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Appendix A. Program Committee
Dave Clark, MIT
Lars Eggert, TSV AD
Cullen Jennings, RAI AD
John Morris, Center for Democracy and Technology
Jon Peterson, RAI AD
Danny Weitzner, MIT
Appendix B. Workshop Participants
Vinay Aggarwal, Deutsche Telekom Labs, TU Berlin
Marvin Ammori, Free Press
Loa Andersson, Acreo AB
Jari Arkko, Ericsson
Alan Arolovitch, PeerApp
Timothy Balcer
Mary Barnes, Nortel
Colby Barth, Cisco Systems
John Barlett, NetForecast
Salman Baset, Columbia University
Chris Bastian, Comcast
Matthew Bell, Charter Communications
Donald Bowman, Sandvine Inc.
Scott Bradner, Harvard University
Bob Briscoe, British Telecom
David Bryan, SIPeerior Technologies
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Rex Bullinger, National Cable & Telecommunications Association
Gonzalo Camarillo, Ericsson
Mary-Luc Champel, Thomson
William Check, NCTA
Alissa Cooper, Center for Democracy and Technology
Patrick Crowley, Washington University
Leslie Daigle, Internet Society
Spencer Dawkins
John Dickinson, Bright House Networks
Lisa Dusseault, CommerceNet
Lars Eggert, Nokia Research Center
Joe Godas, Cablevision
Vernon Groves, Microsoft
Daniel Grunberg, Immedia Semiconductor
Carmen Guerrero, University Carlos III Madrid
Vijay Gurbani, Bell Laboratories/Alcatel-Lucent
William Hawkins III, ITT
Volker Hilt, Bell Labs, Alcatel-Lucent
Russell Housley, Vigil Security, LLC
Robert Jackson, Camiant
Cullen Jennings, Cisco Systems
Paul Jessop, RIAA
XingFeng Jiang, Huawei
Michael Kelsen, Time Warner Cable
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Tom Klieber, Comcast
Eric Klinker, BitTorrent Inc.
Umesh Krishnaswamy
Gregory Lebovitz, Juniper
Erran Li, Bell-Labs
Jason Livingood, Comcast
Andrew Malis, Verizon
Enrico Marocco, Telecom Italia Lab
Marcin Matuszewski, Nokia
Danny McPherson, Arbor Networks, Inc.
Michael Merritt, AT&T
Lyle Moore, Bell Canada
John Morris, Center for Democracy and Technology
Jean-Francois Mule, Cablelabs
David Oran, Cisco Systems
Reinaldo Penno, Juniper Networks
Jon Peterson, NeuStar
Howard Pfeffer, Time Warner Cable
Laird Popkin, Pando Networks
Stefano Previdi, Cisco systems
Satish Putta
Eric Pescorla
Benny Rodrig, Avaya
Damien Saucez, UCLouvain (UCL)
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Henning Schulzrinne, Columbia University
Michael Sheehan, Juniper Networks
Don Shulzrinne, Immedia Semiconductor
David Sohn, Center for Democracy and Technology
Martin Stiemerling, NEC
Clint Summers, Cox Communications
Robert Topolski
Mark Townsley, Cisco Systems
Yushun Wang, Microsoft
Hao Wang, Yale University
Ye Wang, Yale University
David Ward, Cisco
Nicholas Weaver, ICSI
Daniel Weitzner, MIT
Magnus Westerlund, Ericsson
Thomas Woo, Bell Labs
Steve Worona, EDUCAUSE
Richard Woundy, Comcast
Haiyong Xie
Richard Yang, Yale University
Appendix C. Workshop Agenda
1. Welcome/Note Well/Intro Slides
Cullen Jennings
2. Service Provider Perspective (Comcast)
Rich Woundy and Jason Livingood
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3. Application Designer Perspective (BitTorrent)
Stanislav Shalunov
4. Lightning Talks & General Discussion
Robb Topoloski
Nick Weaver
Leslie Daigle
5. Localization and Caches
Laird Popkin and Haiyong Xie
Yu-Shun Wang
Vinay Aggrawal
6. New Approaches to Congestion
Bob Briscoe
Marcin Matuszewski
7. Quality of Service
Mary Barnes
Henning Schulzrinne
8. Conclusions & Wrap-Up
Appendix D. Slides and Position Papers
Slides and position papers are available at http://
trac.tools.ietf.org/area/rai/trac/wiki/PeerToPeerInfrastructure.
Position papers:
Nick Weaver - The case for "Ugly Now" User Fairness
Paul Jessop - Position paper of the RIAA
Nikloaos Laotaris, Pablo Rodriguez, Laurent Massoulie - ECHOES: Edge
Capacity Hosting Overlays of Nano Data Centers
Bruce Davie, Stefano Previdi, Jan Medved, Albert Tian - Peer
Selection Guidance
Marie-Jose Montpetit - Community Networks: getting P2P out of prison
- the next steps
D. Bryan, S. Dawkins, B. Lowekamp, E. Shim - Infrastructure-related
Attributes of App Scenarios for P2PSIP
Jiang XingFeng - Analysis of the Service Discovery in DHT network
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R. Penno - P2P Status and Requirements
Patrick Crowley and Shakir James - Symbiotic P2P: Resolving the
conflict between ISPs and BitTorrent through mutual cooperation
Robb Topolski - Framing Peer to Peer File Sharing
M. Stiemerling, S. Niccolini, S. Kiesel, J. Seedorf - A Network
Cooperative Overlay System
Y. Wang, S. Tan, R. Grove - Traffic Localization with Multi-Layer,
Tracker-Based Peer-to-Peer Content Distribution Architecture
Haiyong Xie, Y. Richard Yang, Avi Silberschatz, Arvind Krishnamurthy,
Laird Popkin - P4P: Provider Portal for P2P Applications
Michael Merritt, Doug Pasko, Laird Popkin - Network-Friendly Peer-to-
Peer Services
Camiant (Jackson) - Camiant Submission
Jason Livingood, Rich Woundy - Comcast Submission
Benny Rodrig - Enterprise IP Networks and the P2P Traffic Load Impact
Ted Hardie - Peer-to-Peer traffic and "Unattended Consequences"
Jiang XingFeng, Ning Zong - Content Replication for Internet P2P
Applications
Sandvine (Dundas) - Analysis of Traffic Demographics in Broadband
networks
Sandvine (Dundas) - Traffic Management in a World with Network
Neutrality
Stanislav Shalunov - Users want P2P, we make it work
R. Cuevas, A. Cuevas, I. Martinez-Yelmo, C. Guerrero - Internet scale
mobility service: a case study on building a DHT based service for
ISPs
M. Barnes, B. McCormick - Peer to Peer Infrastructure Considerations
Henning Schulzrinne - Encouraging Bandwidth Efficiency for Peer-to-
Peer Applications
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Damien Saucez, Benoit Donnet, Olivier Bonaventure, Dimitri
Papdimitriou - Towards an Open Path Selection Architecture
Eric Rescorla - Notes on P2P Blocking and Evasion
Vinay Aggrawal, Anja Feldmann - ISP-Aided Neighbor Selection in P2P
Systems
Enrico Marocco, Vijay K. Gurbani, Volker Hilt, Ivica Rimac, Marco
Tomsu - Peer-to-Peer Infrastructure: A Survey of Research on the
Application-Layer Traffic Optimization Problem and the Need for Layer
Cooperation
Tony Moncaster, Bob Briscoe, Louise Burness - Is There a Problem With
Peer-to-peer Traffic?
David Sohn, Alissa Cooper - Peer-to-Peer Infrastructure
Considerations
Bob Briscoe, Lou Burness, Tony Moncaster, Phil Eardley - Solving this
traffic management problem... and the next, and the next
Hannes Tschofenig, Marcin Matuszewski - Dealing with P2P Traffic in
an Operator Network
Jean-Francois Mule - CableLabs submission
Alan Arolovitch - Peer-to-peer infrastructure: Case for cooperative
P2P caching
Leslie Daigle - Defining Success: Questions for the Future of the
Internet and Bandwidth-Intensive Activities
William Check, Rex Bullinger -- NCTA Position Paper
Jari Arkko - Incentives and Deployment Considerations for P2PI
Solutions
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Authors' Addresses
Jon Peterson
NeuStar
USA
EMail: jon.peterson@neustar.biz
Alissa Cooper
Center for Democracy & Technology
1634 Eye St. NW, Suite 1100
Washington, DC 20006
USA
EMail: acooper@cdt.org
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