ALTO S. Kiesel
Internet-Draft K. Krause
Intended status: Experimental University of Stuttgart
Expires: July 17, 2014 M. Stiemerling
NEC Europe Ltd.
January 13, 2014
Third-Party ALTO Server Discovery (3pdisc)
draft-kist-alto-3pdisc-05
Abstract
The goal of Application-Layer Traffic Optimization (ALTO) is to
provide guidance to applications that have to select one or several
hosts from a set of candidates capable of providing a desired
resource. ALTO is realized by a client-server protocol. Before an
ALTO client can ask for guidance it needs to discover one or more
ALTO servers that can provide suitable guidance.
This document specifies a procedure for third-party ALTO server
discovery, which can be used if the ALTO client is not co-located
with the actual resource consumer, but instead embedded in a third
party such as a peer-to-peer tracker.
Technically, the algorithm specified in this document takes one
IP address and a U-NAPTR Service Parameter (i.e., "ALTO:http" or
"ALTO:https") as parameters. It performs several DNS lookups (for
U-NAPTR and SOA resource records) and returns one or more URI(s) of
information resources related to that IP address.
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Terminology and Requirements Language
This document makes use of the ALTO terminology defined in RFC 5693
[RFC5693].
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 RFC 2119 [RFC2119].
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 17, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Third-party ALTO Server Discovery Procedure Specification . . 5
2.1. Interface . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Basic Principle . . . . . . . . . . . . . . . . . . . . . 5
2.3. Overall Procedure . . . . . . . . . . . . . . . . . . . . 6
2.4. Specification of Tasks and Conditional Branches . . . . . 7
2.4.1. T1: Prepare Domain Name for Reverse DNS Lookup . . . . 7
2.4.2. T2/B1: U-NAPTR Lookup in Reverse Zone . . . . . . . . 7
2.4.3. B2/T3/B3: Acquire SOA Record for Reverse Zone . . . . 8
2.4.4. T4/B4: U-NAPTR Lookup on SOA-MNAME . . . . . . . . . . 9
3. Implementation, Deployment, and Operational Considerations . . 10
3.1. Considerations for ALTO Clients . . . . . . . . . . . . . 10
3.1.1. Resource Consumer Initiated Discovery . . . . . . . . 10
3.1.2. IPv4/v6 Dual Stack, Multihoming, NAT, and Host
Mobility . . . . . . . . . . . . . . . . . . . . . . . 10
3.2. Deployment Considerations for Network Operators . . . . . 11
3.2.1. NAPTR in Reverse Tree vs. SOA-based discovery . . . . 11
3.2.2. Separation of Interests . . . . . . . . . . . . . . . 11
3.3. Impact on DNS . . . . . . . . . . . . . . . . . . . . . . 12
3.3.1. Non-PTR Resource Records in Reverse Tree . . . . . . . 12
3.3.2. Usage with DNS Hidden Master Servers . . . . . . . . . 12
3.3.3. Load on the DNS . . . . . . . . . . . . . . . . . . . 12
4. Security Considerations . . . . . . . . . . . . . . . . . . . 13
4.1. Integrity of the ALTO Server's URI . . . . . . . . . . . . 13
4.2. Availability of the ALTO Server Discovery Procedure . . . 14
4.3. Confidentiality of the ALTO Server's URI . . . . . . . . . 15
4.4. Privacy for ALTO Clients . . . . . . . . . . . . . . . . . 15
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.1. Normative References . . . . . . . . . . . . . . . . . . . 17
6.2. Informative References . . . . . . . . . . . . . . . . . . 17
Appendix A. ALTO and Tracker-based Peer-to-Peer Applications . . 19
Appendix B. Contributors List and Acknowledgments . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
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1. Introduction
The goal of Application-Layer Traffic Optimization (ALTO) is to
provide guidance to applications that have to select one or several
hosts from a set of candidates capable of providing a desired
resource [RFC5693]. ALTO is realized by a client-server protocol;
see requirement AR-1 in [RFC6708]. Before an ALTO client can ask for
guidance it needs to discover one or more ALTO servers that can
provide suitable guidance. For applications that use a centralized
resource directory, such as tracker-based P2P applications, the
efficiency of ALTO is significantly improved if the ALTO client is
embedded in said resource directory instead of the resource consumer
(see Appendix A for a detailed example and analysis of such a
scenario). The ALTO client embedded into the resource directory asks
for guidance on behalf of the resource consumers. To that end, it
needs to discover ALTO servers that can give guidance suitable for
these resource consumers, respectively. This is called third-party
party ALTO server discovery.
This document specifies a procedure for third-party ALTO server
discovery. In other words, this document tries to meet requirement
AR-33 in [RFC6708].
The ALTO protocol specification [I-D.ietf-alto-protocol] is based on
HTTP and expects the discovery procedure to yield the HTTP(S) URI of
an ALTO server's information resource directory. Therefore, this
document specifies an algorithm that takes a resource consumer's IP
address as argument, performs several DNS lookups (for U-NAPTR
[RFC4848] and SOA resource records), and produces URIs of ALTO
servers that are able to give reasonable ALTO guidance to a resource
consumer willing to communicate using this IP address.
To some extent, AR-32, i.e., resource consumer initiated ALTO server
discovery, can be seen as a special case of third-party ALTO server
discovery. However, the considerations in Section 3.1.1 apply. Note
that a less versatile yet simpler approach for resource consumer
initiated ALTO server discovery is specified in
[I-D.ietf-alto-server-discovery].
A more detailed discussion of various options where to place the
functional entities comprising the overall ALTO architecture can be
found in [I-D.ietf-alto-deployments].
Comments and discussions about this memo should be directed to the
ALTO working group: alto@ietf.org.
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2. Third-party ALTO Server Discovery Procedure Specification
2.1. Interface
The algorithm specified in this document takes one IP address and a
U-NAPTR Service Parameter (i.e., "ALTO:http" or "ALTO:https") as
parameters. It performs several DNS lookups (for U-NAPTR and SOA
resource records) and returns one or more URI(s) of information
resources related to that IP address.
2.2. Basic Principle
The algorithm sequentially tries two different lookup strategies.
First, an ALTO-specific U-NAPTR lookup is performed in the "reverse
tree", i.e., in subdomains of in-addr.arpa. or ip6.arpa.,
respectively. If this lookup does not yield a usable result, the SOA
record for the reverse zone is acquired, its master name server
(MNAME) value is extracted and used for a further ALTO-specific
U-NAPTR lookup.
The goal is to allow deployment scenarios that require fine-grained
discovery on a per-IP basis, as well as large-scale scenarios where
discovery is to be enabled for a large number of IP addresses with a
small number of additional DNS resource records.
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2.3. Overall Procedure
This figure gives an overview on the third-party discovery procedure.
All tasks (T) and conditional branches (B) are specified below.
(---------------------------------------)
( START 3pdisc with parameters )
( IP_address IP, Service_Parameter SP )
(-------------------+-------------------)
V
+- T1 --------------+-------------------+
| R:=<IP>.in-addr.arpa. / <IP>.ip6.arpa.|
+-------------------+-------------------+
V
+- T2 --------------+-------------------+
| X:=DNSlookup(R,U-NAPTR,SP) |
+-------------------+-------------------+
V
/ B1 --------------+------------------\
/---------< One or more U-NAPTR results in X >
| yes \------------------+------------------/
| V no
| /- B2 -------------+------------------\
| /----< Authority sect. with SOA record in X >
| | yes \------------------+------------------/
| | V no
| | +- T3 --------------+-------------------+
| | | X:=DNSlookup(R,SOA) |
| | +-------------------+-------------------+
| | V
| | /- B3 -------------+------------------\
| | < Lookup OK, SOA record present in X >----\
| | \------------------+------------------/ no |
| | V yes |
| \----------------------->+ |
| V |
| +- T4 --------------+-------------------+ |
| | M:=extract MNAME from SOA record in X | |
| | X:=DNSlookup(M,U-NAPTR,SP) | |
| +-------------------+-------------------+ |
| V |
| /- B4 -------------+------------------\ V
\--->+<---< One or more U-NAPTR results in X >--->+
| yes \-------------------------------------/ no |
V V
(-------+-------) (-------+-------)
( END, result X ) ( END, failure )
(---------------) (---------------)
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2.4. Specification of Tasks and Conditional Branches
2.4.1. T1: Prepare Domain Name for Reverse DNS Lookup
Task T1 takes the IP address parameter the 3pdisc procedure was
called with and constructs a domain name, which is stored in variable
"R" for use in subsequent tasks.
If the IP address given as a parameter to the 3pdisc procedure is an
IPv4 address, the domain name is constructed according to the rules
specified in Section 3.5 of [RFC1035] and it is rooted in the in the
special domain "IN-ADDR.ARPA.". For IPv6 addresses, the construction
rules in Section 2.5 of [RFC3596] apply and the special domain
"IP6.ARPA." is used.
Example values for "R" for IPv4 and IPv6 addresses could be (Note: a
line break was added in the IPv6 example):
R:="3.100.51.198.in-addr.arpa."
R:="0.2.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.B.D.0.
1.0.0.2.ip6.arpa."
2.4.2. T2/B1: U-NAPTR Lookup in Reverse Zone
Task T1 performs a U-NAPTR lookup as specified in [RFC4848] on "R",
in order to get service-specific U-NAPTR resource records that are
directly associated with the IP address in question.
The ALTO protocol specification defines HTTP and HTTPS as transport
mechanisms and URI schemes for ALTO. Consequently, the U-NAPTR
lookup is performed with the "ALTO" Application Service Tag and
either the "http" or the "https" Application Protocol Tag.
Application Service Tag and Application Protocol Tag are concatenated
to form the Service Parameter SP, i.e., either "ALTO:http" or "ALTO:
https".
The goal of said U-NAPTR lookup is to obtain one or more URIs for the
ALTO server's Information Resource Directory. If two or more URIs
are found they are sorted according to their order and preference
fields as specified in [RFC4848] and [RFC3403].
The lookup result, including a SOA record that may or may not be
present in the authority section, is stored in variable "X".
As an example, the following two U-NAPTR resource records can be used
for mapping "3.100.51.198.in-addr.arpa." to the HTTPS URI
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https://altoserver.isp.example.net/secure/directory or the HTTP URI
http://altoserver.isp.example.net/directory, with the former being
preferred.
3.100.51.198.in-addr.arpa.
IN NAPTR 100 10 "u" "ALTO:https"
"!.*!https://altoserver.isp.example.net/secure/directory!" ""
IN NAPTR 200 10 "u" "ALTO:http"
"!.*!http://altoserver.isp.example.net/directory!" ""
Conditional Branch B1 checks whether at least one U-NAPTR record
matching the service parameter SP could be retrieved. If so, the
procedure ends successfully and the sorted list of U-NAPTR records is
the result. Otherwise, if no U-NAPTR records could be retrieved, we
continue with B2.
Note: The U-NAPTR lookup in Task T2 is identical to Step 2 specified
in [I-D.ietf-alto-server-discovery], which specifies with "manual
input" and "DHCP" two alternatives for acquiring the name to be
looked up. Therefore, it is possible to merge both documents into a
common ALTO server discovery framework.
2.4.3. B2/T3/B3: Acquire SOA Record for Reverse Zone
The task of B2/T3/B3 is to acquire the SOA record for the "reverse
zone", i.e., the zone in the in-addr.arpa. or ip6.arpa. domain that
contains the IP address in question.
A sample SOA record could be:
100.51.198.in-addr.arpa
IN SOA dns1.isp.example.net. hostmaster.isp.example.net. (
1 ; Serial
604800 ; Refresh
86400 ; Retry
2419200 ; Expire
604800 ) ; Negative Cache TTL
Conditional Branch B2 checks whether the SOA record was present in
the authority section of X, i.e., the result of Task T2. If not, an
explicit lookup is done in Task T3. If Conditional Branch B3
determines that this explicit lookup failed, the discovery procedure
is aborted without a result; otherwise we continue with T4.
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2.4.4. T4/B4: U-NAPTR Lookup on SOA-MNAME
Now that the SOA record is available, Task T4 first extracts the
MNAME field, i.e., the responsible master name server from the SOA
record. An example MNAME could be:
dns1.isp.example.net.
Then, a U-NAPTR lookup as specified in Task T2 is performed on this
MNAME and the result is stored in variable "X".
Conditional Branch B4 checks whether at least one U-NAPTR record
matching the service parameter SP could be retrieved. If so, the
procedure ends successfully and the sorted list of U-NAPTR records is
the result. Otherwise, if no U-NAPTR records could be retrieved, the
discovery procedure is aborted without a result.
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3. Implementation, Deployment, and Operational Considerations
3.1. Considerations for ALTO Clients
3.1.1. Resource Consumer Initiated Discovery
To some extent, ALTO requirement AR-32 [RFC6708], i.e., resource
consumer initiated ALTO server discovery, can be seen as a special
case of third-party ALTO server discovery. To that end, an ALTO
client embedded in a resouce consumer would have to figure out its
own "public" IP address and perform the procedures described in this
document on that address. However, due to the widespread deployment
of Network Address Translators (NAT), additional protocols and
mechanisms such as STUN [RFC5389] would be needed and considerations
for UNSAF [RFC3424] apply. Therefore, using the procedures specified
in this document for resource consumer based ALTO server discovery is
generally NOT RECOMMENDED. Note that a less versatile yet simpler
approach for resource consumer initiated ALTO server discovery is
specified in [I-D.ietf-alto-server-discovery].
3.1.2. IPv4/v6 Dual Stack, Multihoming, NAT, and Host Mobility
The algortihm specified in this document can discover ALTO server
URIs for a given IP address. The intention is, that a third party
(e.g., a resource directory) that receives query messages from a
resource consumer can use the source address in these messages to
discover suitable ALTO servers for this specific resource consumer.
However, resource consumers (as defined in Section 2 of [RFC5693])
may reside on hosts with more than one IP address, e.g., due to
IPv4/v6 dual stack operation and/or multihoming. IP packets sent
with different source addresses may be subject to different routing
policies and path costs. In some deployment scenarios, it may even
be required to ask different sets of ALTO servers for guidance.
Furthermore, source addresses in IP packets may be modified en-route
by Network Address Translators (NAT).
If a resource consumer queries a resource directory for candidate
resource providers, the locally selected (and possibly en-route
translated) source address of the query message - as observed by the
resource directory - will become the basis for the ALTO server
discovery and the subsequent optimization of the resource directory's
reply. If, however, the resource consumer then selects different
source addresses to contact returned resource providers, the desired
better-than-random "ALTO effect" may not occur.
Therefore, a dual stack or multihomed resource consumer SHOULD either
always use the same address for contacting the resource directory and
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the resource providers, i.e., overriding the operating system's
automatic source IP address selection, or use resource consumer based
ALTO server discovery [I-D.ietf-alto-server-discovery] to discover
suitable ALTO servers for every local address and then locally
perform ALTO-influenced resource consumer selection and source
address selection. Similarly, resource consumers on mobile hosts
SHOULD query the resource directory again after a change of IP
address, in order to get a list of candidate resource providers that
is optimized for the new IP address.
3.2. Deployment Considerations for Network Operators
3.2.1. NAPTR in Reverse Tree vs. SOA-based discovery
As already outlined in Section 2.2, the third-party discovery
procedure sequentially tries two different lookup strategies, thus
giving network operators the choice of two different deployment
options:
o Individual NAPTR records in the in-addr.arpa or ip6.arpa domains
allow very fine-grained discovery of ALTO "entry point" URIs on a
per-IP-address basis. This method also gives the fastest response
times and causes a comparatively low load on the DNS, as the
algorithm terminates successfully after the first DNS query. DNS
operators that already maintain reverse zones (e.g., for PTR
records) should prefer this option, possibly using DNS server
implementation-specific methods for mass deployment (e.g., BIND9's
$GENERATE statement).
o If a DNS operator considers the first option too cumbersome, or if
IPv6 privacy extensions is to be used without dynamic PTR updates,
setting up SOA records in the in-addr.arpa. or ip6.arpa.
subdomains plus setting up corresponding ALTO-specific U-NAPTR
records will also give reasonable, yet less fine-grained results
at the cost of slightly higher delay and load on the DNS.
3.2.2. Separation of Interests
We assume that if two organizations share parts of their DNS
infrastructure, i.e., have a common SOA record in their in-addr.arpa.
or ip6.arpa. subdomain(s), they will also be able to operate a common
ALTO server, which still may do redirections if desired or required
by policies.
Note that the ALTO server discovery procedure is supposed to produce
only a first URI of an ALTO server that can give reasonable guidance
to the client. An ALTO server can still return different results
based on the client's address (or other identifying properties) or
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redirect the client to another ALTO server using mechanisms of the
ALTO protocol (see Sect. 6.7 of [I-D.ietf-alto-protocol]).
3.3. Impact on DNS
3.3.1. Non-PTR Resource Records in Reverse Tree
Installing NAPTR records, i.e., a record type other than PTR records,
in the in-addr.arpa or ip6.arpa domain may seem uncommon, but it is
not a new concept. Earlier documents that specify the usage of Non-
PTR resource records in the reverse tree include RFC 4025 [RFC4025],
RFC 4255 [RFC4255], and RFC 4322 [RFC4322].
3.3.2. Usage with DNS Hidden Master Servers
In some deployment scenarios, the Master DNS server for a in-
addr.arpa. or ip6.arpa. subdomain, as indicated in the respective SOA
record, may not be reachable due to traffic restrictions ("hidden
master"). This does not cause any problems with the algorithm
described here, as the MNAME is only used for further DNS lookups;
but it is never attempted to contact this server directly.
3.3.3. Load on the DNS
The procedure described in this document features several nested
conditional branches, but no loops. Each time being called it
attempts one to three DNS lookups.
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4. Security Considerations
A high-level discussion of security issues related to ALTO is part of
the ALTO problem statement [RFC5693]. A classification of unwanted
information disclosure risks, as well as specific security-related
requirements can be found in the ALTO requirements document
[RFC6708].
The remainder of this section focuses on security threats and
protection mechanisms for the third-party ALTO server discovery
procedure as such. Once the ALTO server's URI has been discovered
and the communication between the ALTO client and the ALTO server
starts, the security threats and protection mechanisms discussed in
the ALTO protocol specification [I-D.ietf-alto-protocol] apply.
4.1. Integrity of the ALTO Server's URI
Scenario Description
An attacker could compromise the ALTO server discovery procedure
or infrastructure in a way that ALTO clients would discover a
"wrong" ALTO server URI.
Threat Discussion
This is probably the most serious security concern related to ALTO
server discovery. The discovered "wrong" ALTO server might not be
able to give guidance to a given ALTO client at all, or it might
give suboptimal or forged information. In the latter case, an
attacker could try to use ALTO to affect the traffic distribution
in the network or the performance of applications (see also
Section 14.1. of [I-D.ietf-alto-protocol]). Furthermore, a
hostile ALTO server could threaten user privacy (see also Section
5.2.1, case (5a) in [RFC6708]).
However, it should also be noted that, if an attacker was able to
compromise the DNS infrastructure used for third-party ALTO server
discovery (see below), (s)he could also launch significantly more
serious other attacks (e.g., redirecting various application
protocols).
Protection Strategies and Mechanisms
The third-party ALTO server discovery procedure relies on a series
of DNS lookups. If an attacker was able to modify or spoof any of
the DNS records, the resulting URI could be replaced by a forged
URI. The application of DNS security (DNSSEC) [RFC4033] provides
a means to limit attacks that rely on modification of the DNS
records while in transit. Additional operational precautions for
safely operating the DNS infrastructure are required in order to
ensure that name servers do not sign forged (or otherwise "wrong")
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resource records. Security considerations specific to U-NAPTR are
described in more detail in [RFC4848].
A related risk is the impersonation of the ALTO server (i.e.,
attacks after the correct URI has been discovered). This threat
and protection strategies are discussed in Section 14.1 of
[I-D.ietf-alto-protocol]. Note that if TLS is used to protect
ALTO, the server certificate will contain the host name (CN).
Consequently, only the host part of the HTTPS URI will be
authenticated, i.e., the result of the ALTO server discovery
procedure. The DNS/U-NAPTR based mapping within the third-party
ALTO server discovery procedure needs to be secured as described
above, e.g., by using DNSSEC.
In addition to active protection mechanisms, users and network
operators can monitor application performance and network traffic
patterns for poor performance or abnormalities. If it turns out
that relying on the guidance of a specific ALTO server does not
result in better-than-random results, the usage of the ALTO server
may be discontinued (see also Section 14.2 of
[I-D.ietf-alto-protocol]).
4.2. Availability of the ALTO Server Discovery Procedure
Scenario Description
An attacker could compromise the third-party ALTO server discovery
procedure or infrastructure in a way that ALTO clients would not
be able to discover any ALTO server.
Threat Discussion
If no ALTO server can be discovered (although a suitable one
exists) applications have to make their decisions without ALTO
guidance. As ALTO could be temporarily unavailable for many
reasons, applications must be prepared to do so. However, The
resulting application performance and traffic distribution will
correspond to a deployment scenario without ALTO.
Protection Strategies and Mechanisms
Operators should follow best current practices to secure their DNS
and ALTO (see Section 14.5 of [I-D.ietf-alto-protocol]) servers
against Denial-of-Service (DoS) attacks.
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4.3. Confidentiality of the ALTO Server's URI
Scenario Description
An unauthorized party could invoke the third-party ALTO server
discovery procedure, or intercept discovery messages between an
authorized ALTO client and the DNS servers, in order to acquire
knowledge of the ALTO server URI for a specific resource consumer.
Threat Discussion
In the ALTO use cases that have been described in the ALTO problem
statement [RFC5693] and/or discussed in the ALTO working group,
the ALTO server's URI as such has always been considered as public
information that does not need protection of confidentiality.
Protection Strategies and Mechanisms
No protection mechanisms for this scenario have been provided, as
it has not been identified as a relevant threat. However, if a
new use case is identified that requires this kind of protection,
the suitability of this ALTO server discovery procedure as well as
possible security extensions have to be re-evaluated thoroughly.
4.4. Privacy for ALTO Clients
Scenario Description
An unauthorized party could intercept messages between an ALTO
client and the DNS servers, and thereby find out the fact that
said ALTO client uses (or at least tries to use) the ALTO service
on behalf of a specific resource consumer.
Threat Discussion
In the ALTO use cases that have been described in the ALTO problem
statement [RFC5693] and/or discussed in the ALTO working group,
this scenario has not been identified as a relevant threat.
Protection Strategies and Mechanisms
No protection mechanisms for this scenario have been provided, as
it has not been identified as a relevant threat. However, if a
new use case is identified that requires this kind of protection,
the suitability of this ALTO server discovery procedure as well as
possible security extensions have to be re-evaluated thoroughly.
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5. IANA Considerations
This document does not require any IANA action.
This document specifies an algorithm that uses U-NAPTR lookups
[RFC4848] with the Application Service Tag "ALTO" and the Application
Protocol Tags "http" and "https". These tags have already been
registered with IANA. In particular, for the registration of the
Application Service Tag "ALTO", see [I-D.ietf-alto-server-discovery].
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6. References
6.1. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3403] Mealling, M., "Dynamic Delegation Discovery System (DDDS)
Part Three: The Domain Name System (DNS) Database",
RFC 3403, October 2002.
[RFC3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
"DNS Extensions to Support IP Version 6", RFC 3596,
October 2003.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
[RFC4848] Daigle, L., "Domain-Based Application Service Location
Using URIs and the Dynamic Delegation Discovery Service
(DDDS)", RFC 4848, April 2007.
6.2. Informative References
[I-D.ietf-alto-deployments]
Stiemerling, M., Kiesel, S., Previdi, S., and M. Scharf,
"ALTO Deployment Considerations",
draft-ietf-alto-deployments-08 (work in progress),
October 2013.
[I-D.ietf-alto-protocol]
Alimi, R., Penno, R., and Y. Yang, "ALTO Protocol",
draft-ietf-alto-protocol-21 (work in progress),
November 2013.
[I-D.ietf-alto-server-discovery]
Kiesel, S., Stiemerling, M., Schwan, N., Scharf, M., and
H. Song, "ALTO Server Discovery",
draft-ietf-alto-server-discovery-10 (work in progress),
September 2013.
[RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral
Self-Address Fixing (UNSAF) Across Network Address
Translation", RFC 3424, November 2002.
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[RFC4025] Richardson, M., "A Method for Storing IPsec Keying
Material in DNS", RFC 4025, March 2005.
[RFC4255] Schlyter, J. and W. Griffin, "Using DNS to Securely
Publish Secure Shell (SSH) Key Fingerprints", RFC 4255,
January 2006.
[RFC4322] Richardson, M. and D. Redelmeier, "Opportunistic
Encryption using the Internet Key Exchange (IKE)",
RFC 4322, December 2005.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
[RFC5693] Seedorf, J. and E. Burger, "Application-Layer Traffic
Optimization (ALTO) Problem Statement", RFC 5693,
October 2009.
[RFC6708] Kiesel, S., Previdi, S., Stiemerling, M., Woundy, R., and
Y. Yang, "Application-Layer Traffic Optimization (ALTO)
Requirements", RFC 6708, September 2012.
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Appendix A. ALTO and Tracker-based Peer-to-Peer Applications
The ALTO protocol specification [I-D.ietf-alto-protocol] details how
an ALTO client can query an ALTO server for guiding information and
receive the corresponding replies. However, in the considered
scenario of a tracker-based P2P application, there are two
fundamentally different possibilities where to place the ALTO client:
1. ALTO client in the resource consumer ("peer")
2. ALTO client in the resource directory ("tracker")
In the following, both scenarios are compared in order to explain the
need for third-party ALTO queries.
In the first scenario (see Figure 2), the resource consumer queries
the resource directory for the desired resource (F1). The resource
directory returns a list of potential resource providers without
considering ALTO (F2). It is then the duty of the resource consumer
to invoke ALTO (F3/F4), in order to solicit guidance regarding this
list.
In the second scenario (see Figure 4), the resource directory has an
embedded ALTO client, which we will refer to as 3PAC (Third-Party
ALTO Client) in this document. After receiving a query for a given
resource (F1) the resource directory invokes the 3PAC to evaluate all
resource providers it knows (F2/F3). Then it returns a, possibly
shortened, list containing the "best" resource providers to the
resource consumer (F4).
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............................. .............................
: Tracker : : Peer :
: ______ : : :
: +-______-+ : : k good :
: | | +--------+ : P2P App. : +--------+ peers +------+ :
: | N | | random | : Protocol : | ALTO- |------>| data | :
: | known |====>| pre- |*************>| biased | | ex- | :
: | peers, | | selec- | : transmit : | peer |------>| cha- | :
: | M good | | tion | : n peer : | select | n-k | nge | :
: +-______-+ +--------+ : IDs : +--------+ bad p.+------+ :
:...........................: :.....^.....................:
|
| ALTO
| client protocol
__|___
+-______-+
| |
| ALTO |
| server |
+-______-+
Figure 1: Tracker-based P2P Application with random peer preselection
Peer w. ALTO cli. Tracker ALTO Server
--------+-------- --------+-------- --------+--------
| F1 Tracker query | |
|======================>| |
| F2 Tracker reply | |
|<======================| |
| F3 ALTO client protocol query |
|---------------------------------------------->|
| F4 ALTO client protocol reply |
|<----------------------------------------------|
| | |
==== Application protocol (i.e., tracker-based P2P app protocol)
---- ALTO client protocol
Figure 2: Basic message sequence chart for resource consumer-
initiated ALTO query
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............................. .............................
: Tracker : : Peer :
: ______ : : :
: +-______-+ : : :
: | | +--------+ : P2P App. : k good peers & +------+ :
: | N | | ALTO- | : Protocol : n-k bad peers | data | :
: | known |====>| biased |******************************>| ex- | :
: | peers, | | peer | : transmit : | cha- | :
: | M good | | select | : n peer : | nge | :
: +-______-+ +--------+ : IDs : +------+ :
:.....................^.....: :...........................:
|
| ALTO
| client protocol
__|___
+-______-+
| |
| ALTO |
| server |
+-______-+
Figure 3: Tracker-based P2P Application with ALTO client in tracker
Peer Tracker w. 3PAC ALTO Server
--------+-------- --------+-------- --------+--------
| F1 Tracker query | |
|======================>| |
| | F2 ALTO cli. p. query |
| |---------------------->|
| | F3 ALTO cli. p. reply |
| |<----------------------|
| F4 Tracker reply | |
|<======================| |
| | |
==== Application protocol (i.e., tracker-based P2P app protocol)
---- ALTO client protocol
Figure 4: Basic message sequence chart for third-party ALTO query
Note: the message sequences depicted in Figure 2 and Figure 4 may
occur both in the target-aware and the target-independent query mode
(c.f. [RFC6708]). In the target-independent query mode no message
exchange with the ALTO server might be needed after the tracker
query, because the candidate resource providers could be evaluated
using a locally cached "map", which has been retrieved from the ALTO
server some time ago.
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The problem with the first approach is, that while the resource
directory might know thousands of peers taking part in a swarm, the
list returned to the resource consumer is usually shortened for
efficiency reasons. Therefore, the "best" (in the sense of ALTO)
potential resource providers might not be contained in that list
anymore, even before ALTO can consider them.
For illustration, consider a simple model of a swarm, in which all
peers fall into one of only two categories: assume that there are
"good" ("good" in the sense of ALTO's better-than-random peer
selection, based on an arbitrary desired rating criterion) and "bad'
peers only. Having more different categories makes the maths more
complex but does not change anything to the basic outcome of this
analysis. Assume that the swarm has a total number of N peers, out
of which are M "good" and N-M "bad" peers, which are all known to the
tracker. A new peer wants to join the swarm and therefore asks the
tracker for a list of peers.
If, according to the first approach, the tracker randomly picks n
peers from the N known peers, the result can be described with the
hypergeometric distribution. The probability that the tracker reply
contains exactly k "good" peers (and n-k "bad" peers) is:
/ m \ / N - m \
\ k / \ n - k /
P(X=k) = ---------------------
/ N \
\ n /
/ n \ n!
with \ k / = ----------- and n! = n * (n-1) * (n-2) * .. * 1
k! (n-k)!
The probability that the reply contains at most k "good" peers is:
P(X<=k)=P(X=0)+P(X=1)+..+P(X=k).
For example, consider a swarm with N=10,000 peers known to the
tracker, out of which M=100 are "good" peers. If the tracker
randomly selects n=100 peers, the formula yields for the reply:
P(X=0)=36%, P(X<=4)=99%. That is, with a probability of approx. 36%
this list does not contain a single "good" peer, and with 99%
probability there are only four or less of the "good" peers on the
list. Processing this list with the guiding ALTO information will
ensure that the few favorable peers are ranked to the top of the
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list; however, the benefit is rather limited as the number of
favorable peers in the list is just too small.
Much better traffic optimization could be achieved if the tracker
would evaluate all known peers using ALTO, and return a list of 100
peers afterwards. This list would then include a significantly
higher fraction of "good" peers. (Note, that if the tracker returned
"good" peers only, there might be a risk that the swarm might
disconnect and split into several disjunct partitions. However,
finding the right mix of ALTO-biased and random peer selection is out
of the scope of this document.)
Therefore, from an overall optimization perspective, the second
scenario with the ALTO client embedded in the resource directory is
advantageous, because it is ensured that the addresses of the "best"
resource providers are actually delivered to the resource consumer.
An architectural implication of this insight is that the ALTO server
discovery procedures must support third-party discovery. That is, as
the tracker issues ALTO queries on behalf of the peer which contacted
the tracker, the tracker must be able to discover an ALTO server that
can give guidance suitable for that respective peer.
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Appendix B. Contributors List and Acknowledgments
The initial version of this document was co-authored by Marco Tomsu
<marco.tomsu@alcatel-lucent.com>.
Hannes Tschofenig provided the initial input to the U-NAPTR solution
part. Hannes and Martin Thomson provided excellent feedback and
input to the server discovery.
This memo borrows some text from [I-D.ietf-alto-server-discovery], as
the 3pdisc was historically part of that memo. Special thanks to
Michael Scharf and Nico Schwan.
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Authors' Addresses
Sebastian Kiesel
University of Stuttgart Information Center
Allmandring 30
Stuttgart 70550
Germany
Email: ietf-alto@skiesel.de
URI: http://www.rus.uni-stuttgart.de/nks/
Kilian Krause
University of Stuttgart Information Center
Allmandring 30
Stuttgart 70550
Germany
Email: schreibt@normalerweise.net
URI: http://www.rus.uni-stuttgart.de/nks/
Martin Stiemerling
NEC Laboratories Europe
Kurfuerstenanlage 36
Heidelberg 69115
Germany
Phone: +49 6221 4342 113
Email: martin.stiemerling@neclab.eu
URI: http://ietf.stiemerling.org
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