ALTO S. Kiesel
Internet-Draft University of Stuttgart
Intended status: Informational M. Tomsu
Expires: January 10, 2011 N. Schwan
M. Scharf
Alcatel-Lucent Bell Labs
M. Stiemerling
NEC Europe Ltd.
July 9, 2010
Third-party ALTO server discovery
draft-kiesel-alto-3pdisc-03
Abstract
The goal of Application-Layer Traffic Optimization (ALTO) is to
provide guidance to applications, which have to select one or several
hosts from a set of candidates that are able to provide a desired
resource.
Entities seeking guidance need to discover and possibly select an
ALTO server to ask. This is called ALTO server discovery. This memo
describes an ALTO server discovery mechanism based on DNS SRV
records.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 10, 2011.
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. DNS-based ALTO Server Discovery . . . . . . . . . . . . . . . 5
2.1. DNS SRV record definition . . . . . . . . . . . . . . . . 5
2.2. DNS SRV record lookup procedure . . . . . . . . . . . . . 6
2.2.1. Step 1: Finding the IP address . . . . . . . . . . . . 6
2.2.2. Step 2: Determining the DNS suffix . . . . . . . . . . 6
2.2.3. Step 3: Lookup SRV record . . . . . . . . . . . . . . 7
2.2.4. Step 4: Final lookup . . . . . . . . . . . . . . . . . 8
3. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1. Applicability for third party server discovery . . . . . . 9
3.2. Applicability for resource consumer server discovery . . . 9
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 12
6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.1. Normative References . . . . . . . . . . . . . . . . . . . 14
7.2. Informative References . . . . . . . . . . . . . . . . . . 14
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
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1. Introduction
The goal of Application-Layer Traffic Optimization (ALTO) is to
provide guidance to applications, which have to select one or several
hosts from a set of candidates, that are able to provide a desired
resource[RFC5693]. The requirements for ALTO are itemized in
[I-D.ietf-alto-reqs]. ALTO is realized by a client-server protocol.
ALTO clients send queries to ALTO servers, in order to solicit
guidance.
ALTO clients have to discover suitable ALTO servers. Therefore the
ALTO discovery client tells the ALTO client which ALTO servers to
send the queries to. The ALTO discovery client and the ALTO client
can be embedded in the resource consumer, which will eventually
access the desired resource. As an alternative, they can be embedded
in a resource directory, which assists resource consumers in finding
appropriate resource providers. In some specific peer-to-peer
application protocols these resource directories are called
"trackers". Finally the ALTO discovery client can be embedded in the
resource consumer, whereas the ALTO client is embedded in the
resource directory. ALTO queries, which are issued by a resource
directory on behalf of a resource consumer, are referred to as third-
party ALTO queries. The various possibilities to place ALTO servers
and the placement of ALTO clients is discussed in
[I-D.stiemerling-alto-deployments].
No matter where ALTO server and client are located, clients have to
first find out if there is an ALTO server deployed that is in charge
for them and second to get the contact information of that server,
i.e., the IP address, port number, and probably transport protocol
(which defaults to TCP for [I-D.ietf-alto-protocol]).
There exists a number of service location protocols, such as SLP
[RFC2608] or DHCP [RFC2131][RFC3315], which could in principle be
used for ALTO discovery. However, SLP is not widely deployed or used
within the Internet. DHCP is widely deployed but has several
limitations:
o Though widely deployed DHCP is not in use everywhere. For
important scenarios, such as PPPoE based DSL access networks one
would have to specify another mechanism.
o A DHCP-based discovery mechanism will always yield the addresses
of the ALTO servers that are provided by the network operator.
The user cannot influence the discovery and, e.g., select an
alternative ALTO service from an "independent" ALTO operator.
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o There are problems with residential gateways or broadband routers
with NAT. If the network operator gives information about ALTO
serves to the residential gateway via DHCP, the residential
gateway would have to forward this information to the hosts with
the (P2P) applications within the local network. This is not
supported by any of the already deployed residential gateways.
o DHCP poorly supports third-party ALTO server discovery, i.e., in
scenarios where the ALTO client is co-located with a resource
directory ("tracker"), which is located in a different
administrative domain than the client which will eventually access
the ressource.
The goal of this memo is to propose a uniform mechanism for all types
of ALTO client deployments that is implementable and deployable at a
fast pace, i.e., without creating other deployment dependecies for
ALTO. One way of fulfilling the previous mentioned requirements is
using the Service Records (SRV) of the Domain Name System (DNS), as
described in [RFC2782]. DNS SRVs have been defined and are used for
a number of protocols, such as SIP and XMPP.
Comments and discussions about this memo should be directed to the
ALTO working group: alto@ietf.org.
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2. DNS-based ALTO Server Discovery
2.1. DNS SRV record definition
We define a new service record for ALTO servers according to
[RFC2782]. The general format of the SRV RR, whose DNS type code is
33, is
_Service._Proto.Name TTL Class SRV Priority Weight Port Target
Where for the ALTO server discovery, we define:
Service alto
Proto tcp
Name "The domain this RR refers to. The SRV RR is unique in that
the name one searches for is not this name; the example near the
end shows this clearly."[RFC2782]
TTL Standard DNS meaning [RFC2782]
Class Standard DNS meaning[RFC2782]
Priority "The priority of this target host. A client MUST attempt
to contact the target host with the lowest-numbered priority it
can reach; target hosts with the same priority SHOULD be tried in
an order defined by the weight field. The range is 0-65535. This
is a 16 bit unsigned integer in network byte order." [RFC2782]
Weight "A server selection mechanism. The weight field specifies a
relative weight for entries with the same priority. Larger
weights SHOULD be given a proportionately higher probability of
being selected. The range of this number is 0-65535. This is a
16 bit unsigned integer in network byte order. Domain
administrators SHOULD use Weight 0 when there isn't any server
selection to do, to make the RR easier to read for humans (less
noisy). In the presence of records containing weights greater
than 0, records with weight 0 should have a very small chance of
being selected.[...]" [RFC2782]
Port "The port on this target host of this service. The range is 0-
65535. This is a 16 bit unsigned integer in network byte order.
This is often as specified in Assigned Numbers but need not
be."[RFC2782] It will be set to 80, if the standard TCP port for
HTTP is used, but this also allows to run the service on any other
port.
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Target "The domain name of the target host. There MUST be one or
more address records for this name, the name MUST NOT be an alias
(in the sense of RFC 1034 or RFC 2181). Implementors are urged,
but not required, to return the address record(s) in the
Additional Data section. Unless and until permitted by future
standards action, name compression is not to be used for this
field. A Target of "." means that the service is decidedly not
available at this domain. "[RFC2782]
An example for querying for such an ALTO service record running in
the domain myisp.net:
_alto._tcp.example.com IN SRV 1 0 80 alto-srv01.myisp.net
2.2. DNS SRV record lookup procedure
This section describes the algorithm that is applied to discover the
ALTO server. We differentiate between two use cases: In use case (a)
the user has no specific wish which ALTO service instance to use.
Here the ALTO service instance is provided by default by the user's
access network provider, thus the ALTO discovery client needs to
determine the correct domain name automatically. In case (b) the
user configures a specific ALTO service instance that he wants to
use. Here the ALTO discovery client already has the information
about which DNS suffix to use.
2.2.1. Step 1: Finding the IP address
The first step for the ALTO discovery client is to determine the IP
address or IP addresses of the resource consumer. The resource
consumer may have private IP addresses and public IP addresses and
depending on the deployment it might be necessary to determine for
all IP addresses the ALTO server in charge of. To determine its
public IP address the resource consumer may need to use STUN[RFC5389]
or BEP24[bep24]. For the following example we assume that the IP
address of the resource consumer is a.b.c.d
2.2.2. Step 2: Determining the DNS suffix
To get the DNS suffix in case (a) the ALTO discovery uses a DNS PTR
query for the IP address of the resource consumer as determined in
step 1. The local DNS server resolves the IP address to the FQDN
that also contains the DNS suffix for the respective IP address. A
possible answer for a PTR lookup for d.c.b.a.in-addr.apra might be,
for example:
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d-c-b-a.dsl.westcoast.myisp.net
In case (b) The user specifies the DNS suffix on its own, for example
in a config file option. Here the user wants to use an ALTO service
instance which is operated by a third party as for example the
tracker operator. A possible DNS suffix entered by the user may be:
myaltoprovider.org
2.2.3. Step 3: Lookup SRV record
In step 3 the ALTO discovery client uses the information that has
been determined in the previous steps to composes the domain name
that is used for the SRV queries. As the suffix part in not obvious
in all cases e.g., it can be for the above example
"westcoast.myisp.net" or "myisp.net", the ALTO discovery client might
need to perform multiple SRV lookups until it gets a PTR reply.
In case (a) the ALTO discovery client composes the domain name as
described in Section 2. If there is no response to the lookup the
DNS suffix is shortened by one part for the succeeding lookup. The
domain names used for the example as described above are:
_alto._tcp.d-c-b-a.dsl.westcoast.myisp.net.
_alto._tcp.dsl.westcoast.myisp.net.
_alto._tcp.westcoast.myisp.net.
_alto._tcp.myisp.net.
For case (b) the ALTO discovery client extends the DNS suffix by the
IP address of the resource consumer in reverse order to compose the
domain name. This is needed for the third party ALTO service
instance to direct the ALTO client to the ALTO server closest to the
client in case there are multiple ALTO servers deployed. The suffix
is then shortened as described before until a lookup is successful,
as for example
_alto._tcp.d.c.b.a.myaltoprovider.org.
_alto._tcp.c.b.a.myaltoprovider.org.
_alto._tcp.b.a.myaltoprovider.org.
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_alto._tcp.a.myaltoprovider.org.
_alto._tcp.myaltoprovider.org.
2.2.4. Step 4: Final lookup
After step 3 has been completed the ALTO discovery client processes
the PTR records and performs the final DNS lookup on the A record.
It then forwards the contact information to the ALTO client, which
can now contact the ALTO server to perform ALTO queries.
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3. Applicability
This section discusses the applicability of the proposed solution
with respect to the third party and the resource consumer server
discovery deployment scenarios. Each section discusses the proposed
steps that are needed to create the right domain name for the final
DNS lookup.
3.1. Applicability for third party server discovery
In case of the third party server discovery deployment scenario the
ALTO discovery client is a different entity than the resource
consumer. Typically the resource consumer is a peer wehereas the
ALTO (discovery) client is a resource directory which seeks for ALTO
guidance on behalf of the peer. Another use case for the third party
discovery is an application that looks for ALTO guidance
transparently to the resource consumer, for example a CDN.
Here the ALTO discovery client already knows the IP address of the
resource consumer which was used to establish the initial connection.
In general this IP address is a public address, either of the
resource consumer or of the last NAT on the path to the ALTO client.
This makes the IP address a good candidate for the next steps. In
case the resource consumer needs guidance for a different IP address,
for example one from a private network, we propose that the resource
consumer does the server discovery by itself and forwards the ALTO
server contact information directly to the ALTO client which in turn
can then do the third party ALTO query.
To determine the DNS suffix the ALTO discovery client uses a DNS PTR
query. As here the IP address is public we expect that the DNS query
will be successfully resolved to the FQDN of the domain where the
resource consumer is registered in.
To compose the right domain name from the FQDN the ALTO discovery
client follows the mechansims as described in Section 2.2.3.
Additionally the ALTO discovery client can cache domain names and
contact details of already discovered ALTO servers and compare them
with the FQDN by a longest suffix matching. If successful the client
can use the contact information and skip the final discovery step.
The fourth step of the procedure can be applied as described.
3.2. Applicability for resource consumer server discovery
In this scenario the ALTO discovery client that performs the
discovery is also the resource consumer, for example a peer in a P2P
system. After the discovery the peer does the ALTO query on its own,
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or it might share the ALTO server contact information with a third
party, for example a tracker, which then does the ALTO query on
behalf of the peer.
DNS SRV records can be used for resource consumer discovery, too.
Depending on the deployment scenario the resource consumer will have
multiple IP addresses which are possible candidates for a reverse
lookup to determine the FQDN in step 2. Usually the ALTO server is
responsible for a set of public IP addresses, thus in case the
resource consumer is behind a NAT or a residential gateway it needs
to determine the public IP address assigned to it. As discussed in
Section 2.2.1 this can be done by the use of STUN[RFC5389] or
BEP24[bep24].
In other deployment scenarios where internal guidance for a large
private domain is desired the ALTO server might be inside the same
private domain as the resource consumer. In this case the resource
consumer can either use a private IP address or it needs to find a
STUN server that is also inside the private domain in order to find
the right IP address.
To determine the DNS suffix for a public IP address the procedure is
as described in the respective section. In case of a private IP
address it has to be ensured that the DNS server that is used by the
discovery client is a local one that is capable of resolving the
private IP address.
The third and fourth step of the procedure can be applied as
described.
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4. IANA Considerations
This document does not mandate any immediate IANA actions. However,
such IANA considerations may arise from future ALTO discovery
specification documents which try to meet the requirements given
here.
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5. Security Considerations
This early version of this memo does not yet have any security
considerations, but they will be added in future revision.
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6. Conclusion
This document describes a general DNS SRV queries based ALTO server
discovery mechanism and discusses how ALTO discovery clients can find
the right domain name which has to be used for the query. In
addition this document discusses the applicability of the described
mechanism for the third party as well as the resource consumer
discovery.
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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.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
7.2. Informative References
[I-D.ietf-alto-protocol]
Alimi, R., Penno, R., and Y. Yang, "ALTO Protocol",
draft-ietf-alto-protocol-04 (work in progress), May 2010.
[I-D.ietf-alto-reqs]
Kiesel, S., Previdi, S., Stiemerling, M., Woundy, R., and
Y. Yang, "Application-Layer Traffic Optimization (ALTO)
Requirements", draft-ietf-alto-reqs-05 (work in progress),
June 2010.
[I-D.kiesel-alto-3pdisc]
Kiesel, S., Tomsu, M., Schwan, N., and M. Scharf, "Third-
party ALTO server discovery", draft-kiesel-alto-3pdisc-02
(work in progress), March 2010.
[I-D.stiemerling-alto-deployments]
Stiemerling, M. and S. Kiesel, "ALTO Deployment
Considerations", draft-stiemerling-alto-deployments-03
(work in progress), June 2010.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, March 1997.
[RFC2608] Guttman, E., Perkins, C., Veizades, J., and M. Day,
"Service Location Protocol, Version 2", RFC 2608,
June 1999.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
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[RFC5693] Seedorf, J. and E. Burger, "Application-Layer Traffic
Optimization (ALTO) Problem Statement", RFC 5693,
October 2009.
[bep22] Harrison, D., Shalunov, S., and G. Greg Hazel, "BitTorrent
Local Tracker Discovery Protocol",
BEP http://bittorrent.org/beps/bep_0022.html.
[bep24] Harrison, D., "Tracker Returns External IP",
BEP http://bittorrent.org/beps/bep_0024.html.
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Appendix A. Acknowledgments
The authors would like to thank Haibin Song, Richard Alimi, and Roni
Even for fruitful discussions during the 75th IETF meeting.
Marco Tomsu and Nico Schwan are partially supported by the ENVISION
project (http://www.envision-project.org), a research project
supported by the European Commission under its 7th Framework Program
(contract no. 248565). The views and conclusions contained herein
are those of the authors and should not be interpreted as necessarily
representing the official policies or endorsements, either expressed
or implied, of the ENVISION project or the European Commission.
Michael Scharf is supported by the German-Lab project
(http://www.german-lab.de) funded by the German Federal Ministry of
Education and Research (BMBF).
Martin Stiemerling is partially supported by the NAPA-WINE project
(Network-Aware P2P-TV Application over Wise Networks,
http://www.napa-wine.org), a research project supported by the
European Commission under its 7th Framework Program (contract no.
214412). The views and conclusions contained herein are those of the
authors and should not be interpreted as necessarily representing the
official policies or endorsements, either expressed or implied, of
the NAPA-WINE project or the European Commission.
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Authors' Addresses
Sebastian Kiesel
University of Stuttgart Computing Center
Allmandring 30
Stuttgart 70550
Germany
Email: ietf-alto@skiesel.de
URI: http://www.rus.uni-stuttgart.de/nks/
Marco Tomsu
Alcatel-Lucent Bell Labs
Lorenzstrasse 10
Stuttgart 70435
Germany
Email: marco.tomsu@alcatel-lucent.com
URI: www.alcatel-lucent.com/bell-labs
Nico Schwan
Alcatel-Lucent Bell Labs
Lorenzstrasse 10
Stuttgart 70435
Germany
Email: nico.schwan@alcatel-lucent.com
URI: www.alcatel-lucent.com/bell-labs
Michael Scharf
Alcatel-Lucent Bell Labs
Lorenzstrasse 10
Stuttgart 70435
Germany
Email: michael.scharf@alcatel-lucent.com
URI: www.alcatel-lucent.com/bell-labs
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Martin Stiemerling
NEC Laboratories Europe/University of Goettingen
Kurfuerstenanlage 36
Heidelberg 69115
Germany
Phone: +49 6221 4342 113
Email: martin.stiemerling@neclab.eu
URI: http://ietf.stiemerling.org
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