Should Internet Service Providers Fear Peer-Assisted Content Distribution?
Thomas Karagiannis - U.C. Riverside
Pablo Rodriguez - Microsoft Research
Konstantina Papagiannaki - Intel Research Cambridge
Recently, peer-to-peer (P2P) networks have emerged as an
attractive solution to enable large-scale content distribution
without requiring major infrastructure investments. While such P2P
solutions appear highly beneficial for content providers and
end-users, there seems to be a growing concern among Internet
Service Providers (ISPs) that now need to support the distribution
cost. In this work, we explore the potential impact of future P2P
file delivery mechanisms as seen from three different
perspectives: i) the content provider, ii) the ISPs, and iii)
individual content consumers. Using a diverse set of measurements
including BitTorrent tracker logs and payload packet traces
collected at the edge of a 20,000 user access network, we quantify
the impact of peer-assisted file delivery on end-user experience
and resource consumption. We further compare it with the
performance expected from traditional distribution mechanisms
based on large server farms and Content Distribution Networks
While existing P2P content distribution solutions may provide
significant benefits for content providers and end-consumers in
terms of cost and performance, our results demonstrate that they
have an adverse impact on ISPs' costs by shifting the associated
capacity requirements from the content providers and CDNs to the
ISPs themselves. Further, we highlight how simple “locality-aware" P2P
delivery solutions can significantly alleviate the induced cost at
the ISPs, while providing an overall performance that approximates
that of a perfect world-wide caching infrastructure.
Peer-to-peer (P2P) networks, where commodity personal computers
form a cooperative network and share their resources (storage,
CPU, bandwidth), have recently emerged as a solution to large
scale content distribution without requiring major infrastructure
investments. By capitalizing on the bandwidth of end-nodes,
content providers that use peer-assisted solutions can benefit
from a cost-effective distribution of bandwidth-intensive content
to thousands of simultaneous users, both Internet-wide and in
Peer-assisted solutions are inherently self scalable, in that the
bandwidth capacity of the system increases as more nodes arrive:
each new node requests service from, but also provides service to,
the other nodes. The network can thus spontaneously adapt to the
demand by taking advantage of the resources provided by every
end-node, thus making it more resilient to “flash crowd" events,
which may challenge content distribution networks with hundreds of
]. Overall, the system's capacity grows at
the same rate as the demand, creating limitless scalability for a
The best example of such peer-assisted content distribution
architectures is BitTorrent, which has been embraced by several
content providers (Lindows, Blizzard) to reduce the load from
congested servers, minimize the distribution cost, and improve
download times of software and patch releases. However, while
such peer-assisted architectures can provide significant benefits
to end-users and content providers, there seems to be a growing
concern among Internet Service Providers (ISPs) regarding the cost
of supporting such solutions. Since demand is shifted from data
centers to end-nodes, peers become servers for other peers at the
network edge. This shift increases the amount of data
served by each ISP without a corresponding increase in revenue
from the peer-hosted data services provided.
In this paper, we explore the potential impact of future
peer-assisted mechanisms as seen from three different
perspectives: i) the content provider, ii) the ISPs, and iii) the
individual users. In particular, we focus on how peer-assisted
solutions affect ISP's traffic as load is shifted from the content
provider's data center to peers at the edge of the network. We
study how peer-assisted solutions affect ISPs over a range of
parameters, and compare them with other solutions such as
deploying large server farms or caching-based solutions. To this
extent, we use a diverse set of measurements including BitTorrent
tracker logs and payload packet traces collected on the Internet link
of an ISP network.
Despite the benefits for content providers and end-consumers, our
results demonstrate that peer-assisted content distribution
solutions have an adverse impact on ISPs' costs. In particular, we
show that current peer-assisted solutions roughly double the total
traffic on the ISPs access link as well as their peak load due to
the outbound network traffic. The reason is the lack of
consideration of peer-assisted algorithms toward optimizing ISP's
bandwidth. Such overhead is pushing a number of ISPs toward
regulating such traffic, e.g. placing downloading caps. On the
other hand, an ISP-friendly protocol that would minimize the ISPs'
cost could ease such concerns and prevent providers from blocking
or shaping P2P exchanges.
One way of providing an ISP-friendly system is by deploying caches
that store the files being requested by the peers. Such a cache
system would significantly reduce external network traffic for ISPs.
However, caching infrastructures need to be compatible with a wide
variety of P2P implementations and require extra hardware and
maintenance support. Instead, we consider the possibility of
adding small changes to existing peer-assisted distribution
algorithms to mimic the performance of a caching solution. In this
regard, we highlight how simple “locality-aware" peer-assisted
delivery solutions can significantly alleviate the induced cost at
the ISPs, while providing an overall performance that approximates
that of a world-wide caching infrastructure.
The highlights of our work can be summarized in the following
We provide a detailed study that sheds light on and
quantifies the impact of peer-assisted content distribution
solutions on ISPs based on real Internet traces.
- We present evidence that establish the potential for
locality-aware “peer-assisted" solutions. We estimate and
quantify file-availability and user-overlap in time where such
solutions are feasible.
- We describe easily deployable architectures for efficient
peer-assisted content distribution. For each case, we quantify the
benefits and highlight potential savings.
Overall, our work aims at providing ISPs and content providers
with a pragmatic, empirical cost-benefit analysis of current and
future possible peer-assisted solutions for content distribution.
The remainder of the paper is structured as follows. In Section 2
we make the case for BitTorrent-like systems as P2P-mechanisms
that could facilitate large scale content distribution and
describe BitTorrent's functionality. In section 3, we demonstrate
that current P2P systems and specifically BitTorrent do not
exploit locality and present the implications of such a practice
on a small ISP. In Section 4 we quantify the impact of P2P content
distribution on the capacity requirements of an ISP network. Our
findings motivate the need for a locality aware mechanism that
directs peers to obtain the requested content from other peers
located inside the same network, if they exist. We describe such
mechanisms and evaluate their performance in section 5.
In section 6, we describe related work. We discuss implications of
our findings in section 7 and finally conclude in
2 P2P as a mechanism for large scale content distribution
The P2P paradigm appears as an attractive alternative mechanism for
large scale distribution of legal content (e.g., Avalanche
Traditional content distribution mechanisms
typically require vast investments in terms of infrastructure, usually
in the form of CDNs and/or large server farms.
However, P2P content
distribution is only viable by satisfying the requirements of both the content providers and the
end-users.Any newly adopted mechanism
should not incur additional overhead on any of the involved parties,
i.e. the content provider, the users, and the users' ISPs.
We believe that BitTorrent-like P2P systems present a unique potential in achieving the aforementioned
goals. As a P2P protocol, BitTorrent enjoys the benefits of a distributed system that is inherently
more robust to events such as flash crowds, shown to be challenging even for CDNs with
hundreds of servers [10
], as well as cheaper in terms of infrastructure cost on the part of the
content provider. Content distribution using BitTorrent has been shown to offer outstanding performance in terms
of content delivery rates to the clients [22, 9
]. Lastly, BitTorrent features a unique policy across
P2P protocols, called the tit-for-tat policy, which is described below and ensures higher content availability.
In detail, the BitTorrent file distribution protocol specifies the
functionality of three main entities [28
]: a) a
which acts as a centralized server by coordinating
the distribution and receiving information from all connected
peers (over HTTP), b) a torrent
meta-info file with basic
description of the specific file (length, hash, name, etc.) and
the tracker (IP), and c) the actual peers downloading the file.
Peers only serving the file are referred to as “seeds", while
downloading peers are called “leechers". Peers connect first to
the tracker requesting a list of available peers in the network
and then randomly select a subset of them to download from. This
“peer" subset is periodically updated based on the contributions
of each individual uploader with each peer trying to achieve the
maximum available throughput. BitTorrent is characterized by a
“choke/unchoke" mechanism that encourages cooperation among peers
(peers upload more to the peers that offer them more data,
The “tit-for-tat" policy is a distinctive advantage of the BitTorrentsystem
which renders it ideal for a P2P content distribution scheme:
Peers are forced to always share the content during their
downloads while “free-riders" [2
] are indirectly banned from
the network. Given that unlike today's P2P file-sharing networks,
there is no notion of a “community” from the content
distribution perspective, the tit-for-tat incentive ensures
availability of the content as long as peers are requesting it.
However, BitTorrent users have no incentive of sharing the content once
the download is complete. This practice is in contrast to the
majority of P2P file-sharing networks where files are often made
available even long after the completion of the download.
Thus, availability may be compromised in a P2P content
distribution scheme if demand is not high enough to ensure a
sufficient number of active users. In the latter case, the
provider will then have to ensure a larger number of active
“seeds" in the network increasing its cost but facilitating
availability. Even in this case, the benefits of a peer-assisted
solution outweigh the cost compared to traditional content
In the following sections we study both the effect of peer-assisted content distribution on the resource requirements
of ISPs as well as the performance experienced by end-users. We use two different types of real measurement data. First, we study BitTorrent traffic from payload packet traces
collected at a 20,000 user access network. These traces provide the view of
file availability and potential savings from an edge-network view at small time scales.
In addition, we analyze the tracker log for the RedHat v9.0 distribution
that spans five months and offers a global perspective
to the same problem
at larger time scales.
3 P2P Content Distribution: the view from an edge network
In this section, we first look into the overhead of content
distribution through BitTorrent as experienced by an edge network. We
then quantify the savings that could be gained in the same
scenario if locality was exploited. We conclude by examining the
implications of locality-aware mechanisms on the user experience.
For this study we use three day-long packet traces collected with the high
box described in [17
] (Table 1
The monitor is installed on the access-link
(full-duplex Gigabit Ethernet link) of a residential university that hosts
numerous academic, research, and residential complexes with an
approximate population of 20,000 users (a population equivalent of a small
Our monitors capture the TCP/IP header and
adequate payload information from all packets crossing
the link in both directions to enable the identification of specific applications.
P2P traffic accounts for approximately a third of the
traffic in each one of the three traces
(identified by the methodology described in [11
13%-15% of the total traffic
(8%-11% of the total packets) in the link is due to BitTorrent flows. The large
fraction of BitTorrent traffic
reveals its growing popularity and offers a sufficient sample to study its
Description of full-payload packet traces
Studying the dynamics of the BitTorrent network in the traces involves identifying all BitTorrent packets,
determining their specific format and reconstructing all interactions across all BitTorrent flows.
To identify BitTorrent flows and messages, we have developed a BitTorrent protocol dissector.
While the Ethereal
protocol analyzer [6
] has a BitTorrent module, we encountered
several problems with missed packets (e.g., TCP/IP packets that contained BitTorrent messages in the payload but were not
identified by Ethereal), in the handling of out-of-order and fragmented BitTorrent messages
and with multiple messages in the same TCP/IP packet.
Furthermore, Ethereal's BitTorrent module cannot dissect tracker HTTP request/responses.
Specifically, we need to identify two types of messages for all BitTorrent flows:
the tracker request/responses and the messages between peers.
The nominal format for all packets can be found in [28
The tracker request consists -among other things- of the peer id, the file hash and the local IP
of the BitTorrent client (optional). The tracker response is usually a list of available clients for the requested
file with statistics regarding the number of seeds, leechers etc.
Regarding peer interactions, detecting the following BitTorrent messages is crucial to our analysis:
Handshake: The first BitTorrent message transmitted by the
initiator of the connection. It specifies the hash of the file and
the peer id.
- Piece: BitTorrent files are
divided into Pieces. The size of each piece is usually
262KB−1MB and pieces are further subdivided in blocks of
typically 16KB. Blocks constitute the byte-segments that are
actually transferred. The Piece message contains a data
block of a given piece. The first nine bytes of the message
specify the piece index,
the byte offset within the piece and the size of the block.
- BitField: BitField specifies which pieces of the file are available for upload.
It is a sequence of bits where set bits correspond to available pieces. It is typically the first message
after the handshake.
- Have: The Have message advertises that the sender has downloaded a piece and the piece
is now available for upload.
BitTorrent flows and messages are identified then through the following
steps: a) individual packets are classified into flows based on
the 5-tuple (source/destination IPs, source/destination TCP ports
and protocol), and b) our dissector looks for the BitTorrent handshake
message in the first two data packets of a flow (the BitTorrent handshake
should be the first data
packet of a BitTorrent flow but we allow
for malformed packets). If the packet contains a BitTorrent handshake
then the flow is flagged and all packets are examined by our
dissector. All packets of flagged flows are dissected by keeping
state of the interactions between the source and destination IPs.
Note that the file transferred between the two peers of a BitTorrent flow
is only specified at the Handshake
message. Thus, while
identifying a BitTorrent flow without the handshake message is possible,
distinguishing the file transferred is not. In our case, this
limitation only affects BitTorrent flows that are already active at the
beginning of our traces.
Information on all BitTorrent interactions among peers allows us to track user requests, the associated
downloads, the amount of time users request the same content at the same time, as well as the potential
impact of locality aware peer-assisted content distribution on the utilization of network resources.
We will look into each one of these metrics later in this section.
Identifying individual peers per file:
Our analysis depends
on robust identification of distinct peers in the BitTorrent network.
However, identifying individual peers from BitTorrent messages is far
We can identify individual peers using a) tracker request and b)
peer handshake messages. Limitations exist in both cases; while
some of these restrictions are common, others are specific to each
type of message. Thus each individual mechanism may be used to
reinforce the accuracy of the other. Consequently, our peer
identification relies on both methods.
Pitfalls that need to be taken into account are the following :
Network Address Translators (NATs):
Peers cannot be
identified based solely on the IP address because of NATs, in
which multiple peers appear to have the same IP address. To
overcome this limitation we couple
the IP with the observed peer id
: The peer id is not unique for the same peer
and varies with time and across flows. Typically the peer id
comprises two parts: a) the first 6−8 bytes of the peer id (out
of 20 total) reveal the user client and version of the client
(e.g, -AZ2202-, Azureus client, version 188.8.131.52), and b) random
bytes. The random portion of the peer id changes with time and
across flows. Thus, coupling the IP with the peer id, may result
in double-counting peers if the random part varies. To avoid this
pitfall, we only couple the IP with the non-random
the id (the peer can be safely assumed to use the same BitTorrent client
within the time scales of interest - in the rare case where
multiple NATed users use the same client and IP address for the
file, we will consider them as one, thus
underestimating locality which is the main theme of this work).
Thus, a distinct peer is now defined by the IP and
non-random portion of the peer id
. Note that the number of bytes
describing the non-random portion of the peer id varies with the
client (e.g., for Azureus
is 8 bytes while for BitComet is 6, etc.) [28
BitSpirit (BS) client
: Even the aforementioned
definition of a peer does not guarantee identification of distinct
peers due to the peer id assignment algorithm of the BS
BitTorrent client. BS
clients employ a function called “Adjust Client
Identify", that modifies the non-random portion of the peer id to
match the other peer's client in every flow! The BitTorrent client of
other peers is known through the tracker responses (tracker
responses include a list of peer IPs, ports and peer ids). On the
contrary, the random part of the peer id remains constant across
flows. This operation of BS
clients is only specific to
peer handshake messages and can be overcome by collapsing all
different peer ids from the same IP which present the same random
part of peer id into one user. Also, this restriction may be
overcome by correlating peers as found by peer handshake messages
with those shown by tracker messages for the specific IPs.
: The source IP of a tracker request does not
always correspond to the IP of the peer even when the peer is not
behind a NAT. A number of tracker requests are intercepted by
servers (tracker requests use HTTP) substituting the
source IP of the peer with the one of the proxy. We identify such
cases by the proxy_fwd_for
header field (when available) which also
reveals the original IP of the TCP packet. To avoid treating
proxies as peers, we replace the proxy IP with
the IP specified in the proxy_fwd_for
Random peer ids
: A number of clients assigns random
peer ids. This case affects mainly tracker requests and reports
where the peer id may vary with time. As with the BS
restriction, correlating peer ids from handshake and tracker
messages disambiguates individual users.
To achieve robust identification of distinct peers, we employ the
above methodology in both types of messages (handshakes and
tracker requests) separately and compare their outcomes. We
found that the agreement between the two cases was sufficient to
discriminate individual users per file.
In total, we observed
only 3 files out of a total of 360, where the produced list of
users per file was not the same across the two methods. In these
cases, we selected the list with the least number of users to
avoid overestimating potential benefits of locality.
3.2 Hit Ratios
We quantify locality in terms of hit ratios.
The hit ratio (analogous to caching hit ratio) refers to content
that has already been downloaded and is present locally within our monitored ISP.
We examine the hit ratio along three dimensions:
File hit ratio
, where we assume that the complete
file is “cached" locally after the first download. Local caching
would be the equivalent of a local-aware P2P system, where once a
full copy exists within the ISP (either one peer has the full
copy, or pieces of the file are spread across the ISP's
customers), requests are served locally (assuming always active
peers). Thus, the file hit ratio reveals the fraction of multiple
downloads of the same content for the ISP in terms of the total
number of downloaded files. Let Ni
, be the user population for
within the ISP with n
being the total number of files.
Then, the file hit ratio is defined as follows:
Byte hit ratio
, where we incorporate the
file size in the hit ratio. The byte hit ratio is defined by
multiplying each term in the sum of both the numerator and the
denominator in the previous fraction by the size of file i
file size can be inferred by the bitfield and piece
Piece hit ratio
, where we examine what
fraction of the incoming downloaded pieces for each file existed
locally at the time of the download. Local pieces can be inferred
by outgoing BitTorrent messages. Thus, there is a “Hit" for a downloaded
piece, if the specific piece was advertised earlier in the trace
by a local user through the BitField
messages. The total hit ratio is then the fraction of hits divided
by the total number of downloaded pieces. Note that while our
monitoring point disallows the capture of local interactions
(packets transferred within the ISP boundaries not crossing our
monitored link), our view of the status of each file is not
limited for two reasons: a) the BitField message reflects the
current status of the file with every new flow for the same file
(BitTorrent protocol is characterized by a large number of open
connections which yields a significant number of flows), and b)
once a new piece is acquired, the peer advertises the specific
piece to all its connected peers with the Have
Table 2: File, byte and piece hit ratios for the three traces.
|File Hit Ratio
|Byte Hit Ratio
|Piece Hit Ratio
presents the three hit ratios for all files across our three traces.
The file hit ratio ranges from 10%-18% with the byte hit ratio being slightly lower.
Taking into account specific piece and timing information reduces the hit ratio even more
(6%-12%). Given the short duration of the traces (one day) and the small size
of the monitored ISP the hit ratio is non negligible. Similar observation for one-day file and byte hit ratios
have also been presented in [8
] for the Kazaa network.
Requested files in the network are short-lived in accordance with
previous findings in other P2P networks [25
]. Only three
files existed in both the April and May traces, while the January
and April traces had only one file in common. Short-lived files
imply that a P2P caching solution would not require large amounts
of space and that if P2P nodes were to make their files available
for a short period of time after the download is over, nodes could
enjoy most of the possible sharing benefits.
3.3 Peer overlap in time
BitTorrent-like P2P systems assume the existence of a large number
end users for a single file; peers participate
in the sharing of the content by uploading
at the same rate approximately as they download (tit-for-tat). The
assumption of simultaneous active users, while valid globally,
needs to also be valid within the boundaries of individual ISPs so
that locality is beneficial. Note that an “active" peer in BitTorrent implies that the peer is currently downloading/uploading the
specific content and not just participating in the network.
We quantify user overlap in time by tracking peer dynamics for all files in the network.
We are interested in files that are downloaded/uploaded by at least two local peers
throughout our traces, since locality or caching would have no impact on files requested by a single peer.
Files with at least two users account for 10.5% (18/172), 8.7% (30/346) and 11.1% (34/306)
of the files for our January, April and May traces respectively.
Figure 1: Peer overlap in time. The lines
reflect files with at least two users over the whole trace. The
top line presents active files in time. The bottom line shows the
number of files with at least 2 active users in time.
Approximately 60% of the time multiple active users coexist and
could cooperate in the distribution of the content.
Peer overlap in time ranges from 30%-70% in our traces and is
defined as the time during which more than one active users exist
for at least one file versus the total time of the trace.
Fig. 1 presents time overlap of peers for the
April trace. The top line shows the number of “active" files in
time; by active, we refer to files for which we observe activity
at the specific time instance (download or upload). Note that we
only plot files with more than one request over the whole trace.
The bottom line shows the number of files with more than one
“active" user at each time instance. Active files and users
follow the known diurnal patterns while towards the beginning of
the second day the number of active users and files increases
significantly. Overall, there exists at least one file with at
least two active users 25%, 42% and 60% of the time for the
January, April and May traces. Accounting for the fact that a
number of already active BitTorrent flows at the beginning of our traces
may have been missed, the percentages increase by 5%-10% after
removing the first 5-10 hours of the traces.
Figure 2: Histogram of the maximum number of simultaneous users per file for all files
with at least two users in our traces. User-overlap is present for approximately 50% of the files, while
15% of the files have at least 3 simultaneous users at some point in time.
The maximum number of simultaneous active peers per file in our traces
is six. Fig. 2 shows the histogram of the maximum
number of active users per file for all files requested by at least two peers.
Locality could be exploited for 50% of the files where users coincide
(we consider time overlap of at least 10 minutes to regard peers as simultaneous).
Moreover, we observe at least three simultaneous peers for roughly 15% of the files.
Figure 3: Breakdown of downloaded bytes for files with at least 2
active users. 70%-90% of existing local pieces are downloaded
externally while 50%-90% of these pieces exist in active peers.
3.4 Potential Savings on ISP bandwidth
Having established the co-existence of active users for the same
file, we now quantify the percentage of “unnecessary" downloaded
bytes. To estimate potential savings, we assume two scenarios: a)
the caching case, where all local pieces are available once
downloaded irrespective of the availability of the peer having the
piece, and b) the peer-assisted case, where only local pieces in
active users are considered. Local pieces are inferred by the
BitField and Have messages as discussed previously.
70%-90% of existing local pieces and 50%-90% of existing local pieces in active users
are downloaded externally! Fig. 3 summarizes our findings by presenting a breakdown of all downloaded
bytes for files with Ni>1. Only a minimum portion of bytes is downloaded locally even though
more than 20% of the bytes exist in active users (with the exception of April). In an ideal caching scenario,
at least 40% of the content exists and could be downloaded locally.
3.5 Performance Implications for the User
Locality-aware peer-assisted content distribution mechanisms may
have further benefits in terms of
performance for individual peers.
networks may feature much wider bottlenecks than the global
Internet. In addition, the number of hops between
peers is likely to be
smaller and the associated propagation delays shorter,
if the traffic stays within the ISP. For instance, a
Gigabit Ethernet Local Area Network is likely to offer shorter,
higher-throughput paths, to local clients compared to the case where clients are
redirected to cross the Internet in order to retrieve the same content. To
test this assumption we proceed as follows.
Our packet traces allow us to observe the throughput obtained by
each user for each file retrieval at each 5 minute interval. The
aforementioned throughput value corresponds to the performance
experienced by the user using today's BitTorrent system. In a
locality-aware variation of BitTorrent the peer is going to be
served by a local peer whenever such a peer is active.
Consequently, for these periods of simultaneous activity the peer
is going to receive higher throughput than the one measured in our
trace. We assume that the throughput offered by local peers is
going to be at least as much as the maximum cumulative 5-minute
throughput the user achieves throughout the trace, i.e. the peer
is capable of matching its maximum upload/download rate. If we
call r(i,t) the rate measured in the trace for user i at time
interval t, and R(i) the maximum cumulative rate of user i
across the entire trace, then our simulation works as follows: 1)
while the cumulative number of bytes downloaded by all peers is
smaller than the file size, users download at their measured rates
r(i,t), 2) when the cumulative amount of bytes downloaded by the
active users exceeds the size of the file, then all active users
can download the rest of the file they need at R(i) (a rate
which is likely to be lower than the actual rate the user can
receive within his own network), 3) when user i finishes the
download of the entire file, it leaves the network - the number of
active users is reduced by one and the cumulative amount of bytes
in the system is reduced by the number of bytes downloaded by the
departing user (e.g. the file size); this means that when the last
user leaves the network the cumulative amount of bytes in the
system goes back to zero. At the end of the simulation we measure
the total number of bytes downloaded by each user i, and the
amount of time it took for user i to finish the download. We
then compute the average throughput each user i could achieve in
the locality-aware case.
We ran the simulation experiment described above for the three
most popular files in our packet traces. Notice that these files
had at least five user requests in a day and at least one user in
the Torrent downloads the file in its entirety during the one day
period. In Fig. 4, we plot the average latency in
the two tested scenarios (locality and no-locality) and the mean
rate improvement for each user. Approximately 70% of the peers
show increased mean rate in the locality scenario. The improvement
ranges from minimal to more than 100% for a particular client,
which experiences a mean rate improvement of up to 150%,
resulting in a reduction in download time exceeding one hour. 24%
of the clients experience more than 50% faster download rate,
while 30% of the clients see no improvement with locality since
no other user is active at the same time for that particular file
and thus the file needs to be downloaded from outside the ISP.
Admittedly, the sample provided by our packet traces is very
small. However, given that larger networks are more likely to
feature a larger pool of simultaneous active users, the associated
benefits in download times are likely to be even higher.
Figure 4: Implications of locality on user
performance. The upper plot presents the total download time per
user without and with a locality-aware p2p scheme. The lower plot
presents the percent improvement in the achieved mean download
Synopsizing our analysis of locality in our packet level traces we
observe the following:
File, byte and piece hit ratios range from 6%-18% in a day,
implying that approximately one in ten files is downloaded
more than once within the ISP in a day.
- Files are short-lived in the network with
only a minimum number of files being requested across months.
- Active users
coincide at least 30% of the time and could potentially cooperate
in a locality-aware P2P system.
- At least 20% of the
downloaded content existed locally in active users, while a
minimum of 40% existed locally in both active and nonactive
users in two of our traces.
- Peer download rates increase and download
completion times per peer decrease for at least 70% of the
clients in an idealized peer-assisted scenario.
4 Impact of Peer-Assisted Content Distribution on ISPs: A global perspective
In the previous section we established the benefits of locality
aware peer-assisted solutions for a small ISP both in terms of
ingress bandwidth as well as user experience. In this section we
study the impact of peer-assisted content distribution systems on
ISPs and content providers at a larger scale. We assess the
overall effects of such a system, by studying the BitTorrent
of the Redhat v9.0 distribution.
We analyze and compare
traditional client-server distribution approaches with existing
P2P and caching techniques, as well as locality-aware
In the remaining of the section, we first provide a description of the tracker log and the analysis methodology,
and then present our findings for the various content distribution scenarios.
4.1 Tracker log description and content distribution scenarios
The tracker log spans five months, from April to August 2003, and consists of
reports conveying the progress of individual peers in downloading the 1.855GB of the Redhat v9.0 image file.
As reported in  the user-population exhibits clear flash-crowd behavior.
Each peer sends periodic (typically every 15-30 minutes) updates to the tracker containing the amount
of uploaded and downloaded data for the specific interval.
Every entry of the log contains the IP address of the sender, a timestamp and the
peer report to the tracker. Peer reports typically contain the following fields:
file hash, peer id, port, downloaded bytes, uploaded
bytes, left bytes, event (started/completed/stopped), ip (optional)
and numwant (preferred length of the requested peer list).
Similar to our packet level analysis, identification of individual
peers through the tracker messages is the first essential element
in order to explore the impact of the Redhat distribution on the
ISPs through the BitTorrent network. However, the procedure of
distinguishing peers in the log messages poses further challenges
beyond the pitfalls highlighted in section 3. First, the
peer id is always random; user clients are not encoded in the peer
id as, at the time, there existed one major client, namely the
official BitTorrent client (http://www.bittorrent.com/).
Second, user-sessions, especially multi-session downloads cannot
be reconstructed in a straightforward manner (also discussed in
). A session is defined as a sequence of reports with the
same IP and peer id which is modified with every paused and
resumed session. Thus, calculating downloaded volumes per peer is
problematic as the downloaded-bytes
field refers to session and not overall bytes.
Finally, users may change IPs across or within the same session.
To disambiguate individual peers we employ the following methodology:
A user is defined by the IP and the peer id in agreement with our practices
in section 3. If the local IP exists
in the peer report, we replace the IP address of the sender with the local peer IP, only if it does not correspond
to private-address space (e.g., 192.168/24, etc.).
The source IP address may reflect NATs or proxies and thus the local address is preferred.
To track multi-session downloads originating from the
same IP as well as users behind NATs, we correlate the number of
left bytes across consecutive reports from the same IP
and varying peer ids. The left-bytes field signifies the number of
bytes left to complete the overall download of the file,
decreasing with time for individual peers. Thus, we regard as two
separate peers, peer ids for which the number of left bytes is
increasing across two consecutive reports with the same IP. On the
contrary, if the number of left bytes in the current report is
less or equal to the previous report, the two peer ids are
merged into one (the most recent), potentially underestimating the
population in some cases.
We track peers changing IPs within the same /24 subnet by maintaining a mapping of all peer ids to IPs.
All reports with the same peer id originating from the same /24 are treated as one peer (subject to our
left-bytes condition above). We observed approximately 50K such reports (approximately 2%).
Users are less likely to switch IPs outside /24 net boundaries (at least within the time-scale
however, note that users switching IPs across different ISPs (e.g., the download resumes
at a different location) do not affect our analysis, and in fact they
should be treated as distinct peers (downloaded/uploaded bytes affect the current ISP in each case).
After identifying the individual peers inside the network we group
IP addresses into ISPs using the ASFinder module from the Coral
Reef suite . For the mapping we use BGP tables
from RouteViews collected in May and August 20032.
Having a global view of where peers are located inside the network we study the
impact of the following content distribution scenarios:
A server, server farm or CDN is responsible for the content distribution (Scenario 1).
Peers receive the content directly from the server(s) that reside outside the ISP
network. Every peer request corresponds to a transfer across the ISP's Internet link.
- A distribution system based on a standard P2P system (Scenario 2). Here, content
is distributed across peers. Peers are matched based on random selections in the network.
However, we take into consideration the timing information existing in the tracker log. Peers are matched
only if both of them are active at a specific time interval. Peers become active at the time of their first
report in the log or with a start event. Similarly, we consider peers inactive after a stop event or
in the absence of a report for the specific peer within an hour. Timing information and stop/start
event reports are vital for two reasons: a) They minimize the probability of double-counting peers
(smooth transition to a new peer id),
and b) they allow us to reproduce the dynamics of today's popular P2P systems.
If matched peers appear both within the boundaries of the same ISP, their download/upload
volumes are not considered for the specific time-interval, as they do not incur any cost
for the ISP (the transfer is local).
- A distribution system based on a P2P BitTorrent-like system
(Scenarios 3 & 4), where peers periodically re-define
their peer list based on measurements of the upload throughput
offered by other peers in the network. In this case a peer obtains
the requested content selecting peers randomly from groups of
active users of comparable upload throughput (tit-for-tat policy).
The upload bands are based on a per-IP maximum upload throughput
as observed throughout the log (scenario 3), or on peer
instantaneous upload throughputs (scenario 4). Similar to
scenario 2, we consider all timing information (active/inactive
peers) from the log, and transfers within the boundaries of an ISP
are not taken into consideration.
- A peer-assisted content distribution system
exploiting locality within the ISP's boundaries (Scenario
In this scenario, a peer requesting content
will be redirected to any active peer (if available) within the same ISP (we discuss such mechanisms in Section 5).
Assuming that multiple active users within an ISP have the ability to cooperate,
we nominate a “leader" peer that first acquires the content and serves the rest of local users.
The leader is the peer with the most available content (inferred by the left-bytes field) at every interval.
Thus, when a leader is active within an ISP all other local peers are served by the leader.
Note that the leader is more of a conceptual entity for byte-tracking purposes in the log, than an
actual implementation of a peer-assisted solution. In reality, such a model assumes that peers have different,
overlapping or not pieces of the file and cooperate by serving each other; an assumption which should not be far
from reality since downloaded pieces are chosen randomly.
- A distributed caching architecture (Scenario 6)
resembling a perfect content distribution network with caches at the access link(s) of
each ISP. In this scenario, we assume infinite-space caches at the edge of the network,
resulting in only one downloaded-copy of the content per ISP.
Clients requesting the content are served locally by the cache.
Thus, only the first download for each ISP incurs cost and will be
considered. Caching would be the equivalent of a perfect
peer-assisted distribution scenario, where local peers are always
active and can satisfy all requests.
Figure 5: Graphical representation of the
examined scenarios. a) Each new download incurs additional cost
for the ISP while the content provider uploads as many copies as
individual users. b) The content provider uploads less copies
shifting the distribution cost to the ISPs through P2P. c) An
idealized peer-assisted solution ensures that one copy is
downloaded per ISP while users cooperate internally. d) A caching
solution where a copy is downloaded per ISP and all users are
served from the local cache.
graphically depicts the various scenarios. The
content provider shifts the cost of distribution to end-nodes of the
network with P2P (Fig. 5
(a)&(b)), while locality
mechanisms (peer-assisted Fig. 5
(c) or caching
(d)) provide savings for ingress ISP traffic.
4.2 Evaluation of content distribution scenarios
We now evaluate the cost and benefits for the content provider and
the ISPs for all scenarios described in the previous section.
First, we provide the evaluation metrics and a brief overview of our
findings, and then present extensive results for each case.
4.2.1 Metrics and Overview
We study the benefits and costs in terms of traffic volumes for the
content provider and the ISPs. All scenarios are evaluated under
the same set of metrics.
To quantify the ISP cost, we measure
the total ingress/egress bandwidth consumed, as well as the 95th
percentile of traffic
in hourly intervals as observed on their access links.
Such metrics are of significant interest to ISPs since they
typically translate to monetary costs. Scheme performance
is summarized across ISPs using the average value of the obtained distributions.
Content provider cost is measured in terms of total traffic served
which corresponds to the bandwidth requirements that the provider
should meet in each case. As with the ISP cost, we also measure the
percentile of the total traffic served per hourly interval.
Our findings can be summarized in the following points:
Peer-assisted content distribution reduces ISP downlink
bandwidth by a factor of two.
- A P2P locality case only
requires 1.5 times the peak capacity required by a perfect caching
- ISPs are required to upload just over only one
copy of the content satisfying at the same time a large number of
local peers with a local-aware solution.
- ISP savings in
the peer-assisted scenario increase roughly linearly to the
logarithm of active users.
- The overhead of a peer-assisted
approach in terms of peak load as defined by the 95th percentile,
is minimal and in some cases even less than the respective
overhead of a caching solution.
In the remainder of this section we expand on the above
4.2.2 Impact on Downloaded Traffic
In this section, we
consider the impact that peer-assisted content distribution has on
the ISP's downstream traffic.
A peer-assisted distribution scheme can lead to downlink bandwidth savings only if there are multiple
peers concurrently downloading the same file within the ISP.
If no active peers exist for the same file or the matching
algorithm does not have the ability to account for them,
then the benefits of peer-assisted distribution,
in terms of downstream bandwidth, can be significantly reduced.
presents the average value of the total and the 95th
of the data downloaded by each ISP for our
6 content distribution scenarios and for both May and August BGP tables
(AS1 and AS2 respectively).
P2P algorithms that match nodes at
random (scenario 2) provide very little benefit in terms of ISP
bandwidth savings compared to client/server distribution. In fact,
a P2P algorithm with random peer matching provides less than 2%
bandwidth savings over the case where the same file is distributed
once for each client.
Table 3: Downloaded data (in MB) by each
ISP. Percentages show
savings compared to the client/server model.
However, current P2P systems such as BitTorrent do not rely on a
completely random matching of nodes. As discussed in previous
sections, BitTorrent features an algorithm that leads to peers
clustering according to their upload capacities. Such a P2P algorithm may result in locality, if nodes within an ISP were to
have similar download/upload speeds.
shows that BitTorrent-like systems
improve the locality as more local clients are matched, however, the
extra benefits compared to a completely random selection are almost
negligible. A potential reason is that the user population for each
throughput band is quite large, ergo random peer selection within the band is more likely to lead to a
peer outside the ISP.
On the contrary, peer-assisted locality solutions offer high
potential benefits. Our results show that a peer-assisted
locality-aware scheme can reduce the ISP's ingress link
utilization by a factor of two
Finally, the perfect caching solution provides the best possible
benefits for the ISP since only one copy of the file is downloaded
regardless of the number of internal requests. Compared to the
caching infrastructure, the peer-assisted locality-aware solution
results in several times the optimal bandwidth requirements, since
peers are not always active resulting in multiple downloads per
ISP. Nevertheless, one should note that in absolute terms, the
amount of traffic generated by such a peer-assisted scheme is only
a small fraction of what a client/server solution would produce.
In terms of the 95th
percentile, Table 3
demonstrates that locality-aware solutions perform much closer to a
In most cases, a P2P solution
requires only approximately 1.5 times the amount of peak capacity
required by a caching solution
. P2P systems are most helpful when
demand is high since a large number of simultaneous users provides
increasing opportunities for cooperation. However, when user
populations is low, P2P systems like BitTorrent almost revert to a
client/server model. Thus, P2P systems even though they do not
operate at the optimal when decreasing the total amount of traffic,
they appear most beneficial when they are needed the most (e.g.
during peak loads).
4.2.3 Impact on Uploaded Traffic
Since peers become servers in the P2P paradigm, ISPs are bound to observe
increasing amounts of egress traffic, which may considerably impact
their bandwidth cost. This imposed cost is evident in
, where simple P2P content distribution results
in high upstream bandwidth requirements compared to the traditional
client server model (or the caching infrastructure) where local
users do not serve content.
On the contrary, locality keeps most of the traffic within the
ISP's boundaries while the amount of traffic uploaded externally is
reduced by more than a factor of 6. In fact most ISPs only need to
upload slightly over one copy of the file in order to satisfy a
large number of internal users
. Examination of the 95th
offers similar observations when comparing current P2P with locality
aware systems. However, cross-examination of the peak capacity
across tables 3
shows that the
peak upload capacity required is much smaller than the download
Average uploaded data (in MB) by each ISP.
4.2.4 Impact vs. ISP size
We now examine how peer-assisted content distribution affects ISPs
depending on their size. Fig. 6
savings in terms of total traffic and the 95th
by a perfect peer-assisted locality-aware solution compared to a
client-server solution depending on the maximum number of active
users inside an ISP (which is a rough indication of the ISP's
size). Fig. 6
(a) shows that savings from the
locality-aware solution increase almost linearly with the
logarithm of the number of active users. While small ISPs do not
experience high benefits as expected, medium and large-size ISPs
greatly benefit from the peer-assisted locality-aware system. In
fact, the benefits for ISPs with more than thirty maximum active
users are higher than 60%
. In terms of the 95th
the benefits are even higher for the same population size; for a
system with a maximum number of active users equal to thirty, the
benefits are approximately 80% (Fig. 6
Figure 6: Savings of a peer-assisted distribution
versus the traditional client/server model in terms of total
traffic and the 95th percentile with respect to the ISP size
(maximum number of active users). ISPs with more than thirty
active users experience savings of 60% and 80% for total and
peak traffic respectively.
4.2.5 P2P Locality-Aware vs Proxy Caches
We now examine how optimal the peer-assisted solution appears
compared to per-ISP caching.
presents the ratio of bytes
downloaded by a peer-assisted solution versus a caching solution.
The overhead of a peer-assisted solution can be quite high
compared to the caching solution for small ISPs, since the request
rate is low. As the ISP size increases, the overhead of the
peer-assisted solution also increases and large ISPs need to
download a larger number of copies compared to a caching solution.
However, the overhead appears to flatten out at large ISP sizes
since large number of active users ensures cooperation.
Figure 7: Comparison of the peer-assisted distribution
versus a perfect caching infrastructure. While in terms of total
traffic the overhead appears high, the two scenarios appear
equivalent in terms of peak traffic. The overhead in
terms of total traffic flattens out with increasing ISP size.
On the contrary the overhead of a a locality-aware solution is
small compared to caching in terms of the 95th
the 95th percentile of the peer-assisted solution is equal to or
even lower from the one of the caching scenario
. The lower 95th
percentile in the peer-assisted case results from the fact that
most peak rates observed in the peer-assisted solution are lower
than the peak rate of the caching scheme, thus pushing the
percentile closer to the mean rate.
4.2.6 Impact of Locality on the Content Provider
Finally, we consider the reduction in bandwidth enjoyed by the
content provider when a peer-assisted locality-aware solution is
used. To estimate the amount of data served by the content
provider in the peer-assisted case, we assume that requests which
are not satisfied within an ISP need to be satisfied by the
content provider. This is an upper bound of the amount of
bandwidth required from the content provider since a fraction of
the requests would also be satisfied by other ISPs in a P2P system.
demonstrates that locality results in less
than half the resource requirements when compared to the
client-server model both in terms of total egress traffic and 95th percentile
. Further, a caching solution reduces the
total egress capacity by one order of magnitude.
Table 5: Total egress server capacity
5 Locality Algorithms and their Performance
In this section we describe two simple potential implementations of a
peer-assisted content distribution mechanism and compare their performance.
Locality may be implemented either by ISPs or be imposed by the
content provider. ISPs may implement a locality-aware BitTorrent-like
system by deploying proxy-trackers
at the edges of their
network. Proxy-trackers will then intercept requests of local peers
and redirect them to any existing active users within the boundaries
of the ISP. This practice corresponds to our peer-assisted locality
scenario which we extensively studied throughout the previous
section. However, such infrastructure-based solutions are not always
possible to deploy and maintain.
Alternative locality-based algorithms may be supported by the
content provider without relying on extra infrastructure being
deployed. Such locality solutions could be implemented with small
modifications to BitTorrent trackers and could consider algorithms
based on simple prefix rules, domain names, or more sophisticated
hierarchical prefix matching rules, network-aware clustering [13
] or routing table lookups (the two
latter cases would lead to performance very similar to the proxy-tracker scenario).
To evaluate the performance of such algorithms, we assume a
certain division of clients among ISPs based on AS information
from our May BGP table (results are similar for August). Then, we
compare the performance of different locality algorithms versus a
perfect P2P locality-aware system that would utilize a
proxy-tracker at the edge of each ISP.
Note however that the resulting matching of peers in the peer-assisted scheme
will be inefficient when the imposed locality algorithm does not match AS boundaries
(for example a locality algorithm based on a /16 prefix matching).
amount of data downloaded and uploaded by each ISP depending on
the locality algorithm used. The percentage results in brackets
are the savings with respect to a client/server solution.
shows that locality solutions that try to
match clients within the same DNS domain are not as efficient as
proxy-tracker solutions, however, they provide roughly 50% of
the overall benefits. The limitations of such a solution arise
from the fact that different clients in the same domain may span
multiple ASes, thus, generating large amount of cross-over traffic
Downloaded data (in MB) by each ISP for different locality algorithms.
|P2P Locality (Avrg)
|P2P Locality (95th)
Uploaded data (in MB) for each ISP.
|P2P Loc. (Avrg)
|P2P Loc. (95th)
Static prefix-based solutions (e.g. /24, /16) that match users
within a certain prefix perform overall slightly worse than a DNS
based solution. Small prefixes (e.g. /24) result in creating small
groups with a limited population of active users, while on the
contrary, very large prefixes result in users being too spread
across multiple ISPs. Our results indicate that the best
prefix-based grouping is /13, which performs slightly better than
the domain-matching case.
Finally, we also examine the performance of a hierarchical-prefix
matching solution where users are first matched within a /24
prefix, then, /16, /14, and finally /13. The advantages of such a
hierarchical solution would be that a) peer-matching can
dynamically accommodate ISPs of different sizes, and b) nearby
peers within an ISP are matched first. The hierarchical-matching
algorithm provides the best performance after a proxy-tracker
matching scheme, although, it still fails to match a large number
of local peers. This may be due to the fact that ASes typically
own non-consecutive ranges of IP addresses, a practice that decreases the
effectiveness of any prefix-matching algorithm.
6 Related Work
Previous work on BitTorrent has focused on measurements
theoretical analysis [22
], and improvements [27
et al. analyze the log of a BitTorrent tracker
showing the flash-crowd effect of a single file, download speeds,
and the amount of time that peers stay after they have completed
the download [9
]. Pouwelse et al. present an
extensive analysis of BitTorrent showing availability, peer
up-times, and providing a better understanding of peer
inter-arrival times [21
Apart from BitTorrent, several measurement studies have addressed
the issues of availability [2, 3, 8
], integrity [29
], and download performance
] in other P2P systems. Saroiu
et al. use SProbe (sprobe.cs.washington.edu) to measure
the bandwidth of Gnutella peers [25
]. Liang et al. provide
an extensive study of the performance of the KaZaA network [15
analysis of Gnutella traces in terms of resource demand,
popularity of particular search terms, overlay topology, and node
latency was presented by Nogueira et al. [18
data, was also examined by Ripeanu and Foster [23
focusing on node connectivity, overlay topology, and protocol
message overhead. A trace analysis of the network traffic from the
perspective of traffic characterization and bandwidth provisioning
was presented by Sen and Wang [26
]. Markatos [16
conducted a study of Gnutella protocol traffic aimed at caching
query/response traffic to reduce network load. Leibowitz et al
] examined Kazaa traffic to determine proportion of total
network traffic by file popularity.
To the best of our knowledge, this paper presents the first rigorous study of the impact that peer-assisted
content distribution solutions have on ISPs from both a local and a global perspective. The fact that users
in peer-assisted solutions only form a sharing community only while downloading the same file, significantly differentiates them from other existing file-sharing applications
and has important implications in the potential benefit of locality-based solutions. For instance,
] provides an extensive analysis of content delivery systems, including CDN caching, KaZaa,
and Gnutella. However, their results do not carry over well to peer-assisted solutions such as BitTorrent
where cooperation only happens if clients are active and sharing the same file.
Regarding the locality analysis, previous studies have proposed new ways of clustering peers
]) and studied the potential benefits of locality in P2P file-sharing
systems such as KaZaa and Gnutella [8
In this work, we quantify the potential benefits of peer-assisted locality-aware solutions in a real setting
and study the benefits of very simple locality algorithms that require minor changes to existing solutions.
Moreover, we study the potential benefit that locality-based solutions have for the content provider and the
clients in terms of bandwidth reduction and decreased download times respectively.
Our analysis thus far has presented a detailed description of the cost and benefits
of locality-aware peer-assisted content distribution. However, we would like to stress here
a number of implications based on our findings.
Global vs. local benefits
: Our analysis presents two different
perspectives of the potential long-term benefits of peer-assisted content distribution. While in the global
case total savings appear significant, locality appears to only offer minimal savings in our monitored network.
This discrepancy is due to various factors, some inherent to each case, others
reflecting our measurement data. First, since savings depend on the ISP size (section 4),
benefits are limited for our monitored network (smaller than typical ISPs with a limited number of
simultaneous active users). Further, while the tracker log covers a period of six months
(including the initial flash crowd effect), the duration of all our packet traces is roughly a day thus
hiding potential benefits. Finally, locality analysis for our monitored ISP reflects
a large number of files (unpopular files or files past the flash crowd effect
reduce the observed hit ratios) versus one tracker in the Redhat log data.
Peer-assisted vs. existing content distribution solutions
Our analysis does not in any way suggest that peer-assisted
solutions should replace current content distribution practices
such as CDNs. Issues such as file availability with small user
population, limited end-to-end connectivity with NATs, security, reliability and service guarantees need to
be robustly resolved before such solutions can be deployed in a
commercial setting. In the meantime, our findings suggest that
peer-assisted schemes will definitely prove beneficial when
complementing current practices.
Impact of peer-assisted content distribution on internal ISP traffic
While locality-aware peer-assisted solutions appear attractive for ISPs, we would
like to stress here that decreasing egress traffic may create the need
for traffic re-engineering within ISP boundaries to account for the additional internal upload traffic.
Increased upload traffic may prove especially important for those ISPs that depend
on other carriers to deliver last-mile traffic (e.g., European Tier-2 ISPs).
In fact, for such ISPs, utilizing their transit egress capacity
might result in a more cost-effective solution rather than re-routing traffic internally.
Caching, on the contrary, fits naturally with the traditional asymmetric architecture of
today's ISPs, where the downstream channel is more heavily provisioned relative to the upstream,
driven by the assumption that customers download more than what they upload.
Based on payload packet traces as well as tracker-based logs,
we have studied the impact that local-aware peer-assisted content
distribution solutions can have on ISPs, content providers, and
end-users. In particular, we have identified that current P2P
solutions are very ISP-unfriendly, generating large amount of
unnecessary traffic both downstream as well as upstream.
We studied locality in the context of BitTorrent. Our traces
indicate that BitTorrent is locality-unaware, severely increasing
ISPs' bandwidth requirements. In particular, up to 70-90% of existing local
content was found to be downloaded from external
In BitTorrent, users typically only share content while their
download is active. Our results show that such a feature does not
significantly impact the benefits of a BitTorrent-like solution.
In fact, we found that users requesting the same file within an
ISP overlap 30%-70% of the time and could, therefore,
efficiently help each other if a locality algorithm were in place.
Furthermore, by having users stay longer after the download is
complete and share their content, the potential benefit of a
locality algorithm would be an extra 20%-40% in terms of reduced
downloaded bytes by the ISP.
Peer-assisted content distribution incurs significant upstream
capacity costs for the transit links (roughly doubling the
bandwidth requirements). However, simple locality based mechanisms
can rectify this effect, approximating the performance of a
perfect caching architecture. Overall, locality-aware peer-assisted algorithms
bandwidth of the content provider's egress link by more than a
factor of two. Strategies such as those used by BitTorrent trying
to match users with similar capacities provide little locality
The benefits of a peer-assisted locality solution increase with
the logarithm of the number of active users. Our findings show
that as soon as there are more than 30 active users within an
ISP, a peer-assisted locality solution provides more than 60%
savings in terms of ISP's ingress traffic compared to a
On the contrary, a peer-assisted locality-aware solution
generates five times more traffic on
average through the ISP's link than a perfect caching solution.
However, in absolute terms this represents only a small fraction
of the traffic generated by a client/server solution.
The benefits of a peer-assisted solution are always much more
pronounced in terms of 95th
percentile, thus, absorbing peak loads
and reducing the monetary impact on ISPs and content providers.
Simple locality-aware mechanisms based on domain-name grouping, or
prefix grouping provide roughly 50% of the potential benefits.
Our study shows that while current peer-assisted content
distribution solutions are ISP-unfriendly, this is not a
fundamental limitation and that minor modifications can indeed
significantly reduce the costs of all parties involved in the
content distribution process. Such simple modifications to
peer-assisted protocols can provide a cost-effective solution that
can be exploited by content providers to scale and accelerate the
delivery of content to millions of users without pushing ISPs
towards regulating or blocking such traffic.
The authors are thankful to Ernst W. Biersack for providing the RedHat tracker log and to Andrew Moore for facilitating this study.
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- FileSize = Number of bits in the BitField
× (maximum byte offset in a Piece message + block
size of the Piece message)
- The use of different routing tables does not impact our findings.
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