ALTO Y. Gu
Internet-Draft Huawei
Intended status: BCP R. Alimi
Expires: January 13, 2011 Yale University
R. Even
Huawei
July 12, 2010
ALTO Information Redistribution
draft-gu-alto-redistribution-03
Abstract
The ALTO protocol proposes several mechanisms to increase
scalability. One of the proposed mechanisms is ALTO information
redistribution. This document concretely defines ALTO Information
Redistribution, indicates suggested extensions to the ALTO Protocol
to support redistribution, and shows how redistribution could be used
in practice.
Status of this Memo
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This Internet-Draft will expire on January 13, 2011.
Copyright Notice
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminologies and concepts . . . . . . . . . . . . . . . . . . 4
3. Redistribution Framework . . . . . . . . . . . . . . . . . . . 4
3.1. Basic Requirements . . . . . . . . . . . . . . . . . . . . 5
3.2. ALTO Information Types . . . . . . . . . . . . . . . . . . 5
3.3. Redistribution Scheme Design . . . . . . . . . . . . . . . 6
4. ALTO Redistribution Solutions Analysis . . . . . . . . . . . . 6
4.1. PUSH Information into Overlay . . . . . . . . . . . . . . 7
4.1.1. General Requirements for PUSH . . . . . . . . . . . . 7
4.1.2. Tracker Acts as Redistribution Proxy . . . . . . . . . 8
4.1.3. Supernode Acts as Redistribution Proxy . . . . . . . . 8
4.1.4. Advantages of PUSH . . . . . . . . . . . . . . . . . . 8
4.2. PULL Inforamtion into Overlay . . . . . . . . . . . . . . 9
4.2.1. PULL Use Cases . . . . . . . . . . . . . . . . . . . . 9
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.1. Normative References . . . . . . . . . . . . . . . . . . . 13
6.2. Informative References . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Introduction
When providing an ALTO service, Network Providers offer information
to clients with the goal of helping P2P applications to perform
better peer selection and improving network efficiency. The ALTO
Service becomes a distribution point of network information for ALTO
Clients within its network. A Network Provider may deploy an ALTO
Service using techniques such as load balancing and caching to handle
a large number of subscribers. In this document, we discuss an
additional mechanism to distribute ALTO information to ALTO Clients:
ALTO Information Redistribution. Consider a scenario where a large
number (e.g., millions) of users start their P2P live streaming
clients just before the start of a popular event. Each client
requests ALTO information directly from the ALTO Service within their
ISP if it has not yet been retrieved or needs to be refreshed. For
certain content (e.g., content with broad interest), even the number
of streaming clients within a single ISP may be extremely large. For
example, tens of millions of people watched the United States
Presidential Inauguration in Jan. 2009 via the Internet through such
sites as CNN. During the 2009 Spring Festival Evening in China, an
audience of about 24 million watched the program on the Internet. In
such a case, an ISP's ALTO Service may not be able to handle the load
and provide ALTO information directly to each client.
Many mechanisms, such as load balancing, can be used to increase
scalability of an ALTO Service. [I-D.penno-alto-protocol] proposes
ALTO Information Redistribution as one technique. Using ALTO
Information Redistribution, ALTO Clients may distribute ALTO
information to each other instead of requesting directly from the
ALTO Server. For example, a P2P infrastructure can be used to
distribute ALTO information to a large number of ALTO Clients with
small load on the ALTO Server.
This document serves three primary purposes:
1) Defines basic requirements for redistributing ALTO information,
and considerations for developing a redistribution scheme.
2) Propose extensions to the ALTO Protocol to support ALTO
Information redistribution to meet the defined requirements.
3) Provide use cases showing how redistribution may be applied in
practice.
We envision that multiple redistribution schemes are possible, and
the design may depend on the particular setting, such as scalability
requirements and existing application protocols. Thus,
standardization of a redistribution scheme for all kinds of scenario
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is not an object. This BCP intends to extract the fundamental for
real-world practice, and to provide use cases for most common
scenario of ALTO Redistribution.
Note that certain design changes during the development of the ALTO
Protocol may affect ALTO information redistribution. This document
will be updated to track the progress of the ALTO Protocol.
2. Terminologies and concepts
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in[RFC2119].
The document uses terms defined in
[I-D.ietf-alto-problem-statement]and [I-D.penno-alto-protocol].
3. Redistribution Framework
Before defining requirements for ALTO Information Redistribution, we
first give a concrete model for ALTO Information Redistribution that
we use in this document:
1. A set of ALTO Servers {S1, S2, S3, ...} are deployed, each
possibly serving a different network region.
2. A set of ALTO Clients are running. We assume that each ALTO
Client can be mapped to one of the available ALTO Servers by the
ALTO discovery process [I-D.song-alto-server-discovery]. Let
Clients(Si) denote the set of ALTO Clients mapped to ALTO Server
Si.
3. An ALTO Client wishes to obtain a particular set of information
from the ALTO Server. The desired information is fully specified
by: (1) the ALTO Server (hostname and port), and (2) the query
and input parameters that would be sent to the ALTO Server. See
Section 6 for a discussion of behavior when criteria other than
the input parameters are used by an ALTO Server to generate a
response.
4. An ALTO Client may obtain the information either directly from
its ALTO Server Si, or it may obtain the information from a
member of Clients(Si).
5. For a particular query to ALTO Server Si, at least one member of
Clients(Si) directly fetches from Si. The remaining members of
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Clients(Si) obtain the information from some other member in
Clients(Si).
3.1. Basic Requirements
A redistribution scheme should satisfy some basic requirements in
order to successfully redistribute an ALTO Server's response to a set
of ALTO Client.
1. The ALTO Client should be able to identify the desired
redistributed data based only on the ALTO Server hostname and
port, and the input parameters.
2. The ALTO Client should be able to check the validity of the
information once it is retrieved. That is, the ALTO Client
should be able to determine if the retrieved information was
indeed generated by its ALTO Server, , and is generated based on
the particular input parameters.
3.2. ALTO Information Types
In general, it should be possible to redistribute the response from a
particular ALTO Server that does not depend on anything except query
input parameters. However, redistribution may only be worthwhile for
queries that are made by a large number of ALTO Clients. In the
context of [I-D.penno-alto-protocol], we provide an example list of
information types that may be commonly used across many ALTO Clients,
and hence benefit from redistribution. The example list the most
obvious examples to our best knowledge, and there might be other
information types that are suitable for redistribution.
o Server Capability which indicates the capabilities implemented by
an ALTO Server.
o Full Network Map which lists the PIDs and IP prefixes that are
contained within each PID.
o Full Path Cost Map among all PIDs, which indicates the Path Cost
between each pair of PIDs.
[I-D.penno-alto-protocol]also specifies certain queries that may not
benefit from redistribution. For example, if a peer requests ordinal
Path Costs (i.e., a ranked list) for a set of individual endpoint
addresses (i.e., Resource Providers), the response may not be useful
to other ALTO Clients. This is because other ALTO Clients may be
running different applications or have a different set of available
peers (e.g., participating in a different swarm, or be in contact
with a different set of peers within the same swarm).
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3.3. Redistribution Scheme Design
While this document does not fully specify a particular
redistribution scheme, we provide a sampling of decisions that should
be considered when designing an implementing a redistribution scheme.
This list can be used as a guide for implementers when designing a
redistribution scheme for a particular setting. Considerations for a
redistribution scheme include:
o Who places the ALTO Information onto overlay, and how the ALTO
Information is published on the overlay
o How could peers decide whether to get ALTO Information from ALTO
Server or to get from Overlay?
o What method is used for peers to locate redistributed ALTO
information?
o What protocol is used for retrieving redistributed ALTO
information?
o How could peers verify the Redistributed Information it obtained.
o How to update the Redistributed Information on time and who is
responsible for updating.
o What naming scheme is used to specify the ALTO Server hostname,
port, and input parameters in the protocol for locating
redistributed ALTO information? For example, the naming scheme
could define how to compute the 'key' in a DHT.
o How is the redistributed information encoded? Note that the
original response from the ALTO Service may be reformatted (e.g.,
compressed) for redistribution. Note that if this approach is
taken and ALTO Clients still wish to verify received information,
ALTO Clients should be able to reconstruct the ALTO Service's
original response (e.g., via decompression). If a lossy
transformation is used (e.g., filtering), ALTO Clients may not be
able to verify the received information.
4. ALTO Redistribution Solutions Analysis
In this section, we present our considerations on Redistribution
Implementation. In the previous section, several questions have been
enumerated, and the answer to the first question is the baseline of
ALTO Redistribution. There are two distinct ways to place the ALTO
Information into the overlay: Redistribution Proxy PUSH the
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Information into the overlay or components of the overlay PULL the
Information into the Overlay.
The main difference betwen PUSH and PULL is as follows.
o In PUSH, the Redistribution Proxy can publish the updated
redistribution Information into the overlay. Since the
Redistribution Proxy can be much intelligent than a normal peer,
e.g. Redistribution Proxy has overall understanding of the
overlay, it can make decision on how to distribute the Information
in order to utilize the overlay much efficient. For example,
integration with DECADE, distribution via a CDN (many P2P apps are
integrating CDNs now). We will add more use case for PUSH in our
later version.
o While In PULL, peers individually pull/request updated ALTO
information from an overlay or ALTO server. Considering the
general size of a redistributable information, there might be an
out-of-control flooding through the overlay.
In the following text, we introduces some consideration of both PUSH
and PULL, and conclude other features of PUSH and PULL.
4.1. PUSH Information into Overlay
In PUSH method, we introduce a new terminology, Redistribution Proxy.
A Redistribution Proxy can be a Tracker in Tracker-based P2P Overlay
or a supernode either in Tracker-based or Trackerless P2P Overlay.
The duty of Redistribution Proxy to accecpt the redistributable
information from ALTO Server and push the information into the
overlay. There are two options on obtaining redistributable
inforamtion from ALTO Server. One is Redistribution Proxy request
and ALTO Server answers or ALTO Server positively update the
information to the Redistribution Proxy. Since the number of
Redistribution Proxy is much fewer than the total number of ALTO
Clients, so the latter option does not occupy much connection
resources on ALTO Server. In our draft, we introduce the PUSH based
on the latter option, which is ALTO Server pushes the uptaed
information to Redistribution Proxy positivly.
4.1.1. General Requirements for PUSH
To make PUSH feasible, the following requirements must be satisfied.
o Redistribution Proxy MUST has public IP Addresse, so that both
ALTO server and Peers can connect with it without NAT issue.
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o ALTO server MUST know Redistribution proxy's IP Address.
o Redistribution Proxy MUST be reliable.
4.1.2. Tracker Acts as Redistribution Proxy
If Tracker is deployed as Redistribution Proxy, it has two choices.
Tracker can publish the ALTO Information into overlay, then peers can
search and obtain the Information through Overlay.
Or Tracker can store the Information on itself, and answer peers'
particular request based on the Information. For those information
that is not redistributable, peers can request directly from ALTO
server or ask Tracker to request from ALTO server and answer.
In both case, an indication is necessary for telling peers where to
get ALTO Service. For example, peers should know that for those
redistributable type of Information, peers should first request from
Tracker, and for those unredistributable, they should request from
ALTO Sever. Which can be configured by application. The definition
of indication configuration is out of the scope of ALTO.
4.1.3. Supernode Acts as Redistribution Proxy
If Supernode is deployed as Redistribution Proxy, it has two choices
as well.
Supernode store the Information and answer peers request. In this
case, peers should learn Supernode's IP Address in advance.
Or Supernode just publish the Information into the overlay and then
peers search and get the Information through the overlay.
In both case, an indication is necessary as well.
4.1.4. Advantages of PUSH
The advantages of PUSH is obvious.
1 Intelligence. Refer to the text that explain the main difference
between PUSH and PULL.
2 Scalability. PUSH can significantly reduce the request directly
sent to ALTO Server, which can reduce the load on ALTO Server.
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3 Quick updating. Once the ALTO Information is updated in Server,
ALTO server can notify the Redistribution Proxy with new
Information.
4 Good Consistency, because redistributed Informaiton can be updated
in time.
5 Never expiration, because the redistributed Information is always
as new as those in the server.
6 Safe. Redistribution Proxy is reliable.
7 Low latency, because peers do not need to search and locate the
redistribution Information on the overlay, which is extremely
meaningful for time sensitive application, e.g. Live streaming.
4.2. PULL Inforamtion into Overlay
There could be Tracker or peers who PULL the ALTO Information into
the overlay.
If it's peer that pull ALTO information into the overlay, several
aspects should be considered.
1 Where to request for ALTO Information first, ALTO server or
overlay? This can be configured by application, e.g. always
request redistributable type of ALTO Information through Overlay
first. If appliction configuration recommends peer to request
from ALTO Server first, this won't be any good to ALTO
Redistribution.
2 Publishing mechanism. Should peer store the Information on itself
and publish the resource into the overlay, or should peer store
the Information on the corresponding responsible peer on the
overlay? However, this won't make much difference.
3 How to authenticate the Information? We have introduce a security
method with Signature, which is now in ALTP Protocol, while the
question is where to get the Public Key.
4.2.1. PULL Use Cases
The architecture of a particular P2P application will affect the
redistribution mechanism. Generally speaking, there are two kinds of
P2P applications: trackerless, and those using a tracker. In
Tracker-based applications, a resource directory is maintained on a
tracker, and peers contact the tracker to learn about new peers. In
a Trackerless overlay, peers are organized through a particular
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algorithm, e.g. DHT, and they publish or find resources by routing
through the overlay.
4.2.1.1. Tracker-based redistribution
1. The Tracker finds the ALTO server on behalf of a peer, queries
the necessary ALTO information and replies to the peer with the
ALTO information as well as the candidate list.
[I-D.kiesel-alto-3pdisc] describes several methods by which
Tracker can find the right ALTO server. Note that the Tracker
might omit the 'more preferred' peers when making the original
selection. However, the ALTO Information can be applied to peers
learned from other sources (e.g., Peer Exchange and/or DHT).
2. A peer asks for a Resource and Tracker replies without any ALTO
information. The peer queries the ALTO server for ALTO
information, and selects peers. In order to help lessen the
burden on ALTO server, as well as to help other peers who want
the same ALTO information, the peer publishes the ALTO
information on the Tracker (if the Tracker allows this behavior).
Peers may then distribute the ALTO information just as any other
Resource. The method introduced here can be regard as a
complementary process to (1).
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+-------------+
| |
| ALTO |
| Server |
| |
+-------------+
|
| (1) Query
| and
| Response
^^^^^^^^^^^^^^^|^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
^ +-----------------+ +----------------+ ^
^ | | | | ^
^ | Peer A | | Peer B | ^
^ | (ALTO Client) | | (ALTO Client) | ^
^ +-----------------+ +----------------+ ^
^ * (3) * o ^
^ * Redistribution * o (2) ^
^ ************************** o peer ^
^ o request ^
^ o ^
^ o ^
^ +---------------+ ^
^ | | ^
^ | Tracker | ^
^ | | ^
^ +---------------+ ^
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
--- ALTO query and response protocol
ooo peer request protocol in p2p overlay (out of scope)
*** ALTO information redistribution in overlay
Information redistribution among peers in Tracker-based P2P
Application
4.2.1.2. Trackerless redistribution
In a Trackerless overlay, peers obtain the ALTO information, then
publish it via a P2P protocol (e.g., in a DHT). Peers can also
locate and retrieve ALTO information through the protocol.
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+------------+
| |
| ALTO |
| Server |
| |
+------------+
|
| (1) Query
| and
| Response
^^^^^^^^^^^^^^^|^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
^ +------------------+ +-------------------+ ^
^ | | | | ^
^ | Peer A | | Peer B | ^
^ | (ALTO Client) | | (ALTO Client) | ^
^ +------------------+ +--------------------+ ^
^ * (2) * ^
^ * Overlay redistribution * ^
^ *************************** ^
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
--- ALTO query and response protocol
*** ALTO information searching and redistribution
by using P2P Protocol
Information redistribution among peers in Trackerless P2P Overlay
4.2.1.2.1. Lookup in DHT
When searching for a piece of data in a DHT, a node constructs an
identifier for the desired data. We propose here a simple naming
scheme that can be used to lookup ALTO Information in a DHT. This is
provided as a suggestion for an implementation technique, and is not
a requirement on redistribution implementations employing a DHT.
Also note that the ALTO Information does not need to be included
directly in the DHT. A mechanism such as the Distributed Tracker
implemented in Vuze [http://www.vuze.com] could be used to locate an
ALTO Client that in turn provides access to the ALTO Information.
This naming scheme allows redistribution of ALTO information
requested using HTTP GET requests in [I-D.penno-alto-protocol] Since
REST-style URLs are used, input parameters are included directly in
the URL (along with ALTO Server hostname and port). Thus, we compute
a hash of the form:
hash("alto:REQUEST_URL")
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hash("alto:REQUEST_URL")
where REQUEST_URL is the full HTTP URL that would have been used to
request ALTO information from the ALTO Server directly. The
resulting string can be used as the lookup key in the DHT.
The following simple examples show how the scheme applies to the
Information Types in Section 3.2:
1) Server Capability
hash("alto:http://alto.example.com:80/capability").
2) Full Network Map.
hash("alto:http://alto.example.com:80/prop/pid/map").
3) Full Path Cost among all PIDs
hash("alto:http://alto.example.com:80/cost/pid/map").
5. Acknowledgments
The authors would like to give special thanks to Jan Seedorf and many
others for the illuminative discussion in the mailing list. The
authors also thank David Bryan for providing comments on preliminary
versions of the draft.
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[I-D.ietf-alto-problem-statement]
Seedorf, J. and E. Burger, "Application-Layer Traffic
Optimization (ALTO) Problem Statement",
draft-ietf-alto-problem-statement-04 (work in progress),
September 2009.
6.2. Informative References
[I-D.penno-alto-protocol]
Penno, R. and Y. Yang, "ALTO Protocol",
draft-penno-alto-protocol-04 (work in progress),
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October 2009.
[I-D.kiesel-alto-3pdisc]
Kiesel, S., Tomsu, M., Schwan, N., Scharf, M., and M.
Stiemerling, "Third-party ALTO server discovery",
draft-kiesel-alto-3pdisc-03 (work in progress), July 2010.
[I-D.song-alto-server-discovery]
Yongchao, S., Tomsu, M., Garcia, G., Wang, Y., and V.
Avila, "ALTO Service Discovery",
draft-song-alto-server-discovery-03 (work in progress),
July 2010.
Authors' Addresses
Gu Yingjie
Huawei
Baixia Road No. 91
Nanjing, Jiangsu Province 210001
P.R.China
Phone: +86-25-84565868
Fax: +86-25-84565888
Email: guyingjie@huawei.com
Richard Alimi
Yale University
Email: richard.alimi@yale.edu
Roni Even
Huawei
Email: ron.even.tlv@gmail.com
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