ALTO Y. Gu
Internet-Draft Huawei
Intended status: BCP R. Alimi
Expires: September 9, 2010 Yale University
R. Even
Huawei
March 8, 2010
ALTO Information Redistribution
draft-gu-alto-redistribution-02
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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Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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 Protocol Requirements . . . . . . . . . . . . . . . . . . 6
4.1. Signature and Verification . . . . . . . . . . . . . . . . 6
4.2. Redistributable Indication and Expiration Time . . . . . . 6
5. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Tracker-based redistribution . . . . . . . . . . . . . . . 7
5.2. Trackerless redistribution . . . . . . . . . . . . . . . . 8
5.2.1. Lookup in DHT . . . . . . . . . . . . . . . . . . . . 9
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
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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:Which ALTO Client(s) directly query the
ALTO Server?
o What method is used for locating redistributed ALTO information?
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 What protocol is used for retrieving redistributed ALTO
information?
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 Protocol Requirements
4.1. Signature and Verification
We proposed to consider signature mechanism to ensure that an ALTO
client can verify that a redistributed information is generated from
the right ALTO server and is based on the correct input parameters in
-01 version. The basic idea has been accepted in [I-D.penno-alto-
protocol] and is described in section 7.5.3. [I-D.penno-alto-
protocol] also introduces how public key is obtained.
4.2. Redistributable Indication and Expiration Time
In -01 version, we proposed that ALTO server should using HTTP
Caching directives to indicate whether a response could be
redistributed and when a redistributed information should be updated.
In the lastest version of [I-D.penno-alto-protocol], An ALTO server
MAY indicate that a response is suitable fore redistribution by
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including a "redistribution" JSON object and the Expiration time is
also indicated as JSON object.
5. 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
algorithm, e.g. DHT, and they publish or find resources by routing
through the overlay.
5.1. 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
5.2. 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
5.2.1. 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").
6. 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.
7. References
7.1. 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.
7.2. 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. and M. Tomsu, "Third-party ALTO server
discovery", draft-kiesel-alto-3pdisc-01 (work in
progress), October 2009.
[I-D.song-alto-server-discovery]
Song, H., Tomsu, M., Garcia, G., Wang, Y., and V. Pascual,
"ALTO Service Discovery",
draft-song-alto-server-discovery-01 (work in progress),
July 2009.
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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