Application Layer Traffic Optimization (ALTO) Cross-Domain Server Discovery
draft-ietf-alto-xdom-disc-04
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (alto WG) | |
|---|---|---|---|
| Authors | Sebastian Kiesel , Martin Stiemerling | ||
| Last updated | 2018-12-20 (Latest revision 2018-11-13) | ||
| Replaces | draft-kiesel-alto-xdom-disc | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text htmlized pdfized bibtex | ||
| Reviews |
SECDIR Last Call review
Ready
|
||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Jan Seedorf | ||
| Shepherd write-up | Show Last changed 2018-10-29 | ||
| IESG | IESG state | IESG Evaluation - Defer::Revised I-D Needed | |
| Consensus boilerplate | Yes | ||
| Telechat date |
(None)
Needs a YES. Needs 8 more YES or NO OBJECTION positions to pass. |
||
| Responsible AD | Mirja Kühlewind | ||
| Send notices to | Jan Seedorf <jan.seedorf@hft-stuttgart.de> | ||
| IANA | IANA review state | IANA OK - No Actions Needed |
draft-ietf-alto-xdom-disc-04
ALTO S. Kiesel
Internet-Draft University of Stuttgart
Intended status: Standards Track M. Stiemerling
Expires: May 18, 2019 H-DA
November 14, 2018
Application Layer Traffic Optimization (ALTO) Cross-Domain Server
Discovery
draft-ietf-alto-xdom-disc-04
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.
In some deployment scenarios, in particular if the information about
the network topology is partitioned and distributed over several ALTO
servers, it may be needed to discover an ALTO server outside of the
own network domain, in order to get appropriate guidance. This
document details applicable scenarios, itemizes requirements, and
specifies a procedure for ALTO cross-domain server discovery.
Technically, the procedure specified in this document takes one
IP address or prefix and a U-NAPTR Service Parameter (typically,
"ALTO:https") as parameters. It performs DNS lookups (for NAPTR
resource records in the in-addr.arpa. or ip6.arpa. tree) and returns
one or more URI(s) of information resources related to that IP
address or prefix.
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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", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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 May 18, 2019.
Copyright Notice
Copyright (c) 2018 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. Overview on the ALTO Cross-Domain Server Discovery
Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. ALTO Cross-Domain Server Discovery Procedure Specification . . 6
3.1. Interface . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Step 1: Prepare Domain Name for Reverse DNS Lookup . . . . 6
3.3. Step 2: Prepare Shortened Domain Names . . . . . . . . . . 7
3.4. Step 3: Perform DNS U-NAPTR lookups . . . . . . . . . . . 8
3.5. Error Handling . . . . . . . . . . . . . . . . . . . . . . 9
4. Using the ALTO Protocol with Cross-Domain Server Discovery . . 10
4.1. Network and Cost Map Service . . . . . . . . . . . . . . . 10
4.2. Map-Filtering Service . . . . . . . . . . . . . . . . . . 11
4.3. Endpoint Property Service . . . . . . . . . . . . . . . . 11
4.4. Endpoint Cost Service . . . . . . . . . . . . . . . . . . 11
4.5. Summary and Further Extensions . . . . . . . . . . . . . . 13
5. Implementation, Deployment, and Operational Considerations . . 14
5.1. Considerations for ALTO Clients . . . . . . . . . . . . . 14
5.2. Deployment Considerations for Network Operators . . . . . 15
6. Security Considerations . . . . . . . . . . . . . . . . . . . 16
6.1. Integrity of the ALTO Server's URI . . . . . . . . . . . . 16
6.2. Availability of the ALTO Server Discovery Procedure . . . 17
6.3. Confidentiality of the ALTO Server's URI . . . . . . . . . 18
6.4. Privacy for ALTO Clients . . . . . . . . . . . . . . . . . 18
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
8.1. Normative References . . . . . . . . . . . . . . . . . . . 20
8.2. Informative References . . . . . . . . . . . . . . . . . . 20
Appendix A. Solution Approaches for Partitioned ALTO Knowledge . 22
A.1. Classification of Solution Approaches . . . . . . . . . . 22
A.2. Discussion of Solution Approaches . . . . . . . . . . . . 23
A.3. The Need for Cross-Domain ALTO Server Discovery . . . . . 23
A.4. Our Solution Approach . . . . . . . . . . . . . . . . . . 24
A.5. Relation to the ALTO Requirements . . . . . . . . . . . . 24
Appendix B. Requirements for ALTO Cross-Domain Server
Discovery . . . . . . . . . . . . . . . . . . . . . . 25
B.1. Discovery Client Application Programming Interface . . . . 25
B.2. Data Storage and Authority Requirements . . . . . . . . . 25
B.3. Cross-Domain Operations Requirements . . . . . . . . . . . 25
B.4. Protocol Requirements . . . . . . . . . . . . . . . . . . 26
B.5. Further Requirements . . . . . . . . . . . . . . . . . . . 26
Appendix C. ALTO and Tracker-based Peer-to-Peer Applications . . 27
C.1. Architectural Options . . . . . . . . . . . . . . . . . . 27
C.2. Evaluation . . . . . . . . . . . . . . . . . . . . . . . . 30
C.3. Example . . . . . . . . . . . . . . . . . . . . . . . . . 32
Appendix D. Contributors List and Acknowledgments . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 38
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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 an HTTP-based client-server
protocol [RFC7285], which can be used in various scenarios [RFC7971].
The ALTO base protocol document [RFC7285] specifies the communication
between an ALTO client and one ALTO server. In principle, the client
may send any ALTO query. For example, it might ask for the routing
cost between any two IP addresses, or it might request network and
cost maps for the whole network, which might be the worldwide
Internet. It is assumed that the server can answer any query,
possibly with some kind of default value if no exact data is known.
No special provisions were made for deployment scenarios with
multiple ALTO servers, with some servers having more accurate
information about some parts of the network topology while others
having better information about other parts of the network
("partitioned knowledge"). Various ALTO use cases have been studied
in the context of such scenarios. In some cases, one cannot assume
that a topologically nearby ALTO server (e.g., a server discovered
with the procedure specified in [RFC7286]) will always provide useful
information to the client. One such scenario is detailed in
Appendix C. Several solution approaches, such as redirecting a
client to a server that has more accurate information or forwarding
the request to it on behalf of the client, have been proposed and
analyzed (see Appendix A), but none has been specified so far.
Section 3 of this document specifies the "ALTO Cross-Domain Server
Discovery Procedure" for client-side usage in these scenarios. An
ALTO client that wants to send an ALTO query related to a specific IP
address or prefix X, may call this procedure with X as a paramenter.
It will use Domain Name System (DNS) lookups to find of one ore more
ALTO servers that can provide a competent answer. The above wording
"related to" was intentionally kept somewhat vague, as the exact
semantics depends on the ALTO service to be used; see Section 4.
Those who are in control of the "reverse DNS" for a given IP address
or prefix (i.e., the corresponding subdomain of in-addr.arpa. or
ip6.arpa.) - typically an Internet Service Provider (ISP), a
corporate IT department, or a university's computing center - may add
resource records to the DNS that point to one or more relevant ALTO
server(s). In many cases, it may be an ALTO server run by that ISP
or IT department, as they naturally have good insight into routing
costs from and to their networks. However, they may also refer to an
ALTO server provided by someone else, e.g., their upstream ISP.
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2. Overview on the ALTO Cross-Domain Server Discovery Procedure
This procedure was inspired by the "Location Information Server (LIS)
Discovery Using IP Addresses and Reverse DNS" [RFC7216] and re-uses
parts of the basic ALTO Server Discovery Procedure [RFC7286].
The basic idea is to use the Domain Name System (DNS), more
specifically the "in-addr.arpa." or "ip6.arpa." trees, which are
mostly used for "reverse mapping" of IP addresses to host names by
means of PTR resource records. There, URI-enabled Naming Authority
Pointer (U-NAPTR) resource records [RFC4848], which allow the mapping
of domain names to Uniform Resource Identifiers (URIs), are installed
as needed. Thereby, it is possible to store a mapping from an IP
address or prefix to one or more ALTO server URIs in the DNS.
The ALTO Cross-Domain Server Discovery Procedure is called with one
IP address or prefix and a U-NAPTR Service Parameter [RFC4848] as
parameters.
The service parameter SHOULD always be set to "ALTO:https". However,
other parameter values MAY be used in some scenarios, e.g.,
"ALTO:http" to search for a server that supports unencrypted
transmission for debugging purposes, or other application protocol or
service tags if applicable.
The procedure performs DNS lookups and returns one or more URI(s) of
information resources related to said IP address or prefix, usually
the URI(s) of one or more ALTO Information Resource Directory (IRD,
see Section 9 of [RFC7285]). The U-NAPTR records also provide
preference values, which should be considered if more than one URI is
returned.
The discovery procedure sequentially tries two different lookup
strategies: First, an ALTO-specific U-NAPTR record is searched in the
"reverse tree", i.e., in subdomains of in-addr.arpa. or ip6.arpa.
corresponding to the given IP address or prefix. If this lookup does
not yield a usable result, the procedure tries further lookups with
truncated domain names, which correspond to shorter prefix lengths.
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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3. ALTO Cross-Domain Server Discovery Procedure Specification
3.1. Interface
The procedure specified in this document takes two parameters, X and
SP, where X is an IP address or prefix and SP is a U-NAPTR Service
Parameter.
The parameter X may be an IPv4 or an IPv6 address or prefix in CIDR
notation (see [RFC4632] for the IPv4 CIDR notation and [RFC4291] for
IPv6). Consequently, the address type AT is either "IPv4" or "IPv6".
In both cases, X consists of an IP address A and a prefix length L.
For AT=IPv4, it holds: 0 <= L <= 32 and for AT=IPv6, it holds:
0 <= L <= 128.
For example, for X=198.51.100.0/24, we get AT=IPv4, A=198.51.100.0
and L=24. Similarly, for X=2001:0DB8::20/128, we get AT=IPv6,
A=2001:0DB8::20 and L=128.
In the intended usage scenario, the procedure SHOULD always be called
with the parameter SP set to "ALTO:https". However, for general
applicabiliy and in order to support future extensions, the procedure
MUST support being called with any valid U-NAPTR Service Parameter
(see Section 4.5. of [RFC4848] for the syntax of U-NAPTR Service
Parameters and Section 5. of the same document for information about
the IANA registries).
The procedure performs DNS lookups and returns one or more URI(s) of
information resources related to that IP address or prefix, usually
the URI(s) of one or more ALTO Information Resource Directory (IRD,
see Section 9 of [RFC7285]). For each URI, it also returns order and
preference values (see Section 4.1 of [RFC3403]), which should be
considered if more than one URI is returned.
During execution of this procedure, various error conditions may
occur and have to be reported to the caller; see Section 3.5.
For the remainder of the document, we use the following notation for
calling the ALTO Cross-Domain Server Discovery Procedure:
IRD_URIS_X = XDOMDISC(X,"ALTO:https")
3.2. Step 1: Prepare Domain Name for Reverse DNS Lookup
First, the procedure checks the prefix length L for unsupported
values: If AT=IPv4 (i.e., if A is an IPv4 address) and L < 8, the
procedure aborts and indicates an "invalid prefix length" error to
the caller. Similarly, if AT=IPv6 and L < 32, the procedure aborts
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and indicates an "invalid prefix length" error to the caller.
Otherwise, the procedure continues.
If AT=IPv4, a domain name R32 is constructed according to the rules
specified in Section 3.5 of [RFC1035] and it is rooted in the special
domain "IN-ADDR.ARPA.".
For example, A=198.51.100.3 yields R32="3.100.51.198.IN-ADDR.ARPA.".
If AT=IPv6, the domain name R128 is constructed according to the
rules specified in Section 2.5 of [RFC3596] and the special domain
"IP6.ARPA." is used.
For example (note: a line break was added after the second line),
A = 2001:0DB8::20 yields
R128 = "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."
3.3. Step 2: Prepare Shortened Domain Names
For this step, an auxiliary function "skip" is defined as follows:
skip(str,n) will skip all characters in the string str, up to and
including the n-th dot, and return the remaining part of str. For
example, skip("foo.bar.baz.qux.quux.",2) will return "baz.qux.quux.".
If AT=IPv4, the following additional domain names are generated from
the result of the previous step: R24=skip(R32,1), R16=skip(R32,2),
and R8=skip(R32,3). Removing one label from a domain name (i.e., one
number of the "dotted quad notation") corresponds to shortening the
prefix length by 8 bits.
For example, R32="3.100.51.198.IN-ADDR.ARPA." yields
R24="100.51.198.IN-ADDR.ARPA.", R16="51.198.IN-ADDR.ARPA.", and
R8="198.IN-ADDR.ARPA.".
If AT=IPv6, the following additional domain names are generated from
the result of the previous step: R64=skip(R128,16),
R56=skip(R128,18), R48=skip(R128,20), and R32=skip(R128,24).
Removing one label from a domain name (i.e., one hex digit)
corresponds to shortening the prefix length by 4 bits.
For example (note: a line break was added after the first line),
R128 = "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." yields
R64 = "0.0.0.0.0.0.0.0.8.B.D.0.1.0.0.2.IP6.ARPA.",
R56 = "0.0.0.0.0.0.8.B.D.0.1.0.0.2.IP6.ARPA.",
R48 = "0.0.0.0.8.B.D.0.1.0.0.2.IP6.ARPA.", and
R32 = "8.B.D.0.1.0.0.2.IP6.ARPA."
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3.4. Step 3: Perform DNS U-NAPTR lookups
The address type of A and the prefix length are matched against the
first and the second column of the following table, respectively:
+------------+------------+------------+----------------------------+
| 1: Address | 2: Prefix | 3: MUST do | 4: SHOULD do further |
| Type AT | Length L | 1st lookup | lookups in that order |
+------------+------------+------------+----------------------------+
| IPv4 | 32 | R32 | R24, R16, R8 |
| IPv4 | 24 .. 31 | R24 | R16, R8 |
| IPv4 | 16 .. 23 | R16 | R8 |
| IPv4 | 8 .. 15 | R8 | (none) |
| IPv4 | 0 .. 7 | (none, abort: invalid prefix length L) |
+------------+------------+------------+----------------------------+
| IPv6 | 128 | R128 | R64, R56, R48, R32 |
| IPv6 | 64 (..127) | R64 | R56, R48, R32 |
| IPv6 | 56 .. 63 | R56 | R48, R32 |
| IPv6 | 48 .. 55 | R48 | R32 |
| IPv6 | 32 .. 47 | R32 | (none) |
| IPv6 | 0 .. 31 | (none, abort: invalid prefix length L) |
+------------+------------+------------+----------------------------+
Then, the domain name given in the 3rd column and the U-NAPTR Service
Parameter SP the procedure was called with (usually "ALTO:https")
MUST be used for an U-NAPTR [RFC4848] lookup, in order to obtain one
or more URIs (indicating protocol, host, and possibly path elements)
for the ALTO server's Information Resource Directory (IRD). If such
URI(s) can be found, the ALTO Cross-Domain Server Discovery Procedure
returns that information to the caller and terminates successfully.
For example, the following two U-NAPTR resource records can be used
for mapping "100.51.198.IN-ADDR.ARPA." (i.e., R24 from the example in
the previous step) to the HTTPS URIs "https://alto1.example.net/ird"
and "https://alto2.example.net/ird", with the former being preferred.
100.51.198.IN-ADDR.ARPA. IN NAPTR 100 10 "u" "ALTO:https"
"!.*!https://alto1.example.net/ird!" ""
100.51.198.IN-ADDR.ARPA. IN NAPTR 100 20 "u" "ALTO:https"
"!.*!https://alto2.example.net/ird!" ""
If no matching U-NAPTR records can be found, the procedure SHOULD try
further lookups, using the domain names from the fourth column in the
indicated order, until one lookup succeeds. If no IRD URI could be
found after looking up all domain names from the 3rd and 4th column,
the procedure terminates unsuccessfully, returning an empty URI list.
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3.5. Error Handling
The ALTO Cross-Domain Server Discovery Procedure may fail for several
reasons.
If the procedure is called with syntactically invalid parameters or
unsupported parameter values (in particular the prefix length L, see
Section 3.2), the procedure aborts, no URI list will be returned and
the error has to be reported to the caller.
The procedure performs one or more DNS lookups in a well-defined
order (corresponding to descending prefix lenghts, see Section 3.4),
until one produces a usable result. Each of these DNS lookups might
not produce a usable result, either due to a normal condition (e.g.,
domain name exists, but no ALTO-specific NAPTR resource records are
associated with it), a permanent error (e.g., non-existent domain
name), or due to a temporary error (e.g., timeout). In all three
cases, and as long as there are further domain names that can be
looked up, the procedure SHOULD immediately try to lookup the next
domain name (from column 4 in the table given in Section 3.4). Only
after all domain names have been tried at least once, the procedure
MAY retry those domain names that had caused temporary lookup errors.
Generally speaking, ALTO provides advisory information for the
optimization of applications (e.g., peer-to-peer applications,
overlay networks, etc.), but applications should not rely on the
availability of such information for their basic functionality (see
Section 8.3.4.3 of RFC 7285 [RFC7285]). Consequently, the speedy
detection of an ALTO server, even though it may give less accurate
answers than other servers, or the quick realization that there is no
suitable ALTO server, is in general more preferable than causing long
delays by retrying failed queries. Nevertheless, the ALTO Cross-
Domain Server Discovery Procedure SHOULD inform its caller, if DNS
queries have failed due to temporary errors and that retrying the
discovery at a later point in time might give more accurate results.
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4. Using the ALTO Protocol with Cross-Domain Server Discovery
Based on a modular design principle, ALTO provides several ALTO
services, each consisting of a set of information resouces that can
be accessed using the ALTO protocol. The information resources that
are available at a specific ALTO server are listed in its Information
Resource Directory (IRD, see Section 9 of [RFC7285]). The ALTO
protocol specification defines the following ALTO services and their
corresponding information resouces:
o Network and Cost Map Service, see Section 11.2 of [RFC7285]
o Map-Filtering Service, see Section 11.3 of [RFC7285]
o Endpoint Property Service, see Section 11.4 of [RFC7285]
o Endpoint Cost Service, see Section 11.5 of [RFC7285]
The ALTO Cross-Domain Server Discovery Procedure is most useful in
conjunction with the Endpoint Property Service and the Endpoint Cost
Service. However, for the sake of completeness, possible interaction
with all four services is discussed below. Extension documents may
specify further information resources; however, these are out of
scope of this document.
4.1. Network and Cost Map Service
An ALTO client may invoke the ALTO Cross-Domain Server Discovery
Procedure (as specified in Section 3) for an IP address or prefix "X"
and get a list of one or more IRD URI(s), including order and
preference values: IRD_URIS_X = XDOMDISC(X,"ALTO:https"). These
IRD(s) will always contain a network and a cost map, as these are
mandatory information resources (see Section 11.2 of [RFC7285]).
However, the cost matrix may be very sparse. If, according to the
network map, PID_X is the PID that contains the IP address or prefix
X, and PID_1, PID_2, PID_3, ... are other PIDS, the cost map may look
like this:
From \ To PID_1 PID_2 PID_X PID_3
------+-----------------------------------
PID_1 | 92
PID_2 | 6
PID_X | 46 3 1 19
PID_3 | 38
In this example, all cells outside column "X" and row "X" are
unspecified. A cost map with this structure contains the same
information as what could be retrieved using the ECS, cases 1 and 2
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in Section 4.4. Accessing cells outside column "X" and row "X" may
not yield useful results.
Trying to assemble a more densely populated cost map from several
cost maps with this very sparse structure may be a non-trivial task,
as different ALTO servers may use different PID definitions (i.e.,
network maps) and incompatible scales for the costs, in particular
for the "routingcost" metric.
4.2. Map-Filtering Service
An ALTO client may invoke the ALTO Cross-Domain Server Discovery
Procedure (as specified in Section 3) for an IP address or prefix "X"
and get a list of one or more IRD URI(s), including order and
preference values: IRD_URIS_X = XDOMDISC(X,"ALTO:https"). These
IRD(s) may provide the optional Map-Filtering Service (see Section
11.3 of [RFC7285]). This service returns a subset of the full map,
as specified by the client. As discussed in Section 4.1, a cost map
may be very sparse in the envisioned deployment scenario. Therefore,
depending on the filtering criteria provided by the client, this
service may return results similar to the Endpoint Cost Service, or
it may not return any useful result.
4.3. Endpoint Property Service
If an ALTO client wants to query an Endpoint Property Service (see
Section 11.4 of RFC 7285 [RFC7285]) about an endpoint with IP address
"X" or a group of endpoints within IP prefix "X", respectively, it
has to invoke the ALTO Cross-Domain Server Discovery Procedure (as
specified in Section 3): IRD_URIS_X = XDOMDISC(X,"ALTO:https"). The
result IRD_URIS_X is a list of one or more URIs of Information
Resource Directories (IRD, see Section 9 of [RFC7285]). Considering
the order and preference values, the client has to check these IRDs
for a suitable Endpoint Property Service and query it.
If the ALTO client wants to do a similar Endpoint Property query for
a different IP address or prefix "Y", the whole procedure has to be
repeated, as IRD_URIS_Y = XDOMDISC(Y,"ALTO:https") may yield a
different list of IRD URIs. Of course, the results of individual DNS
queries may be cached as indicated by their respective time-to-live
(TTL) values.
4.4. Endpoint Cost Service
The optional ALTO Endpoint Cost Service (ECS, see Section 11.5 of RFC
7285 [RFC7285]) provides information about costs between individual
endpoints and it also supports ranking. The ECS allows that
endpoints may be denoted by IP addresses or prefixes. The ECS is
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called with a list of one or more source IP addresses or prefixes,
which we will call (S1, S2, S3, ...), and a list of one or more
destination IP addresses or prefixes, called (D1, D2, D3, ...).
This specification distinguishes several cases, regarding the number
of elements in the list of source and destination addresses,
respectively:
1. Exactly one source address S1 and more than one destination
addresses D1, D2, D3, ... In this case, the ALTO client has to
invoke the ALTO Cross-Domain Server Discovery Procedure (as
specified in Section 3) with that single source address as a
parameter: IRD_URIS_S1 = XDOMDISC(S1,"ALTO:https"). The result
IRD_URIS_S1 is a list of one or more URIs of Information Resource
Directories (IRD, see Section 9 of [RFC7285]). Considering the
order and preference values, the client has to check these IRDs
for a suitable Endpoint Cost Service and query it. The ECS is an
optional service (see Section 11.5.1 of RFC 7285 [RFC7285]) and
therefore, it may well be that an IRD does not refer to an ECS.
Calling the Cross-Domain Server Discovery Procedure only once
with the single source address as a parameter - as opposed to
multiple calls, e.g., one for each destination address - is not
only a matter of efficiency. In the given scenario, it is
advisable to send all ECS queries to the same ALTO server. This
ensures that the results can be compared (e.g., for sorting
candidate resource providers), even with cost metrics without a
well-defined base unit, e.g., the "routingcost" metric.
2. More than one source addresses S1, S2, S3, ... and exactly one
destination address D1. In this case, the ALTO client has to
invoke the ALTO Cross-Domain Server Discovery Procedure with that
single destination address as a parameter:
IRD_URIS_D1 = XDOMDISC(D1,"ALTO:https"). The result IRD_URIS_D1
is a list of one or more URIs of IRDs. Considering the order and
preference values, the client has to check these IRDs for a
suitable ECS and query it.
3. Exactly one source address S1 and exactly one destination address
D1. The ALTO client may perform the same steps as in case 1, as
specified above. As an alternative, it may also perform the same
steps as in case 2, as specified above.
4. More than one source addresses S1, S2, S3, ... and more than one
destination addresses D1, D2, D3, ... In this case, the ALTO
client should split the list of desired queries based on source
addresses and perform separately for each source address the same
steps as in case 1, as specified above. As an alternative, the
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ALTO client may also group the list based on destination
addresses and perform separately for each destination address the
same steps as in case 2, as specified above. However, comparing
results between these sub-queries may be difficult, in particular
if the cost metric is a relative preference without a well-
defined base unit (e.g., the "routingcost" metric).
See Appendix C for a detailed example showing the interaction of a
tracker-based peer-to-peer application, the ALTO Endpoint Cost
Service, and the ALTO Cross-Domain Server Discovery Procedure.
4.5. Summary and Further Extensions
Considering the four services defined in the ALTO base protocol
specification [RFC7285], the ALTO Cross-Domain Server Discovery
Procedure works best with the Endpoint Property Service (EPS) and the
Endpoint Cost Service (ECS). Both the EPS and the ECS take one or
more IP addresses as a parameter. The previous sections specify how
the parameter for calling the ALTO Cross-Domain Server Discovery
Procedure has to be derived from these IP adresses.
In contrast, the ALTO Cross-Domain Server Discovery Procedure seems
less useful if the goal is to retrieve network and cost maps that
cover the whole network topology. However, the procedure may be
useful if a map centered at a specific IP address is desired (i.e., a
map detailing the vicinity of said IP address or a map giving costs
from said IP address to all potential destinations).
The interaction between further ALTO services (and their
corresponding information resources) needs to be investigated and
defined once such further ALTO services are specified in an extension
document.
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5. Implementation, Deployment, and Operational Considerations
5.1. Considerations for ALTO Clients
5.1.1. 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 cross-domain ALTO server discovery. To that end, an ALTO
client embedded in a resource 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 [RFC7286].
5.1.2. IPv4/v6 Dual Stack, Multihoming, NAT, and Host Mobility
The procedure specified in this document can discover ALTO server
URIs for a given IP address or prefix. 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
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always use the same address for contacting the resource directory and
the resource providers, i.e., overriding the operating system's
automatic source IP address selection, or use resource consumer based
ALTO server discovery [RFC7286] 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.
5.2. Deployment Considerations for Network Operators
5.2.1. Separation of Interests
We assume that if two organizations share parts of their DNS
infrastructure, i.e., have common in-addr.arpa. and/or ip6.arpa.
subdomains, 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
redirect the client to another ALTO server using mechanisms of the
ALTO protocol (see Sect. 9 of [RFC7285]).
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6. 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 cross-domain 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 [RFC7285] apply.
6.1. 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 15.1. of [RFC7285]). 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 cross-domain ALTO
server discovery, (s)he could also launch significantly more
serious other attacks (e.g., redirecting various application
protocols).
Protection Strategies and Mechanisms
The cross-domain 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
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"wrong") 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 15.1 of
[RFC7285]. 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 cross-domain 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 15.2 of [RFC7285]).
6.2. Availability of the ALTO Server Discovery Procedure
Scenario Description
An attacker could compromise the cross-domain 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 15.5 of [RFC7285]) servers against Denial-
of-Service (DoS) attacks.
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6.3. Confidentiality of the ALTO Server's URI
Scenario Description
An unauthorized party could invoke the cross-domain 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 IP address.
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.
6.4. 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
in order to optimize traffic from/to a specific IP address.
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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7. IANA Considerations
This document does not require any IANA action.
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8. References
8.1. 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.
[RFC4848] Daigle, L., "Domain-Based Application Service Location
Using URIs and the Dynamic Delegation Discovery Service
(DDDS)", RFC 4848, April 2007.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
8.2. Informative References
[I-D.kiesel-alto-alto4alto]
Kiesel, S., "Using ALTO for ALTO server selection",
draft-kiesel-alto-alto4alto-00 (work in progress),
July 2010.
[I-D.kiesel-alto-ip-based-srv-disc]
Kiesel, S. and R. Penno, "Application-Layer Traffic
Optimization (ALTO) Anycast Address",
draft-kiesel-alto-ip-based-srv-disc-03 (work in progress),
July 2014.
[RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral
Self-Address Fixing (UNSAF) Across Network Address
Translation", RFC 3424, November 2002.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
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Architecture", RFC 4291, DOI 10.17487/RFC4291,
February 2006, <https://www.rfc-editor.org/info/rfc4291>.
[RFC4632] Fuller, V. and T. Li, "Classless Inter-domain Routing
(CIDR): The Internet Address Assignment and Aggregation
Plan", BCP 122, RFC 4632, DOI 10.17487/RFC4632,
August 2006, <http://www.rfc-editor.org/info/rfc4632>.
[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.
[RFC7216] Thomson, M. and R. Bellis, "Location Information Server
(LIS) Discovery Using IP Addresses and Reverse DNS",
RFC 7216, April 2014.
[RFC7285] Alimi, R., Penno, R., Yang, Y., Kiesel, S., Previdi, S.,
Roome, W., Shalunov, S., and R. Woundy, "Application-Layer
Traffic Optimization (ALTO) Protocol", RFC 7285,
September 2014.
[RFC7286] Kiesel, S., Stiemerling, M., Schwan, N., Scharf, M., and
H. Song, "Application-Layer Traffic Optimization (ALTO)
Server Discovery", RFC 7286, June 2014.
[RFC7971] Stiemerling, M., Kiesel, S., Scharf, M., Seidel, H., and
S. Previdi, "Application-Layer Traffic Optimization (ALTO)
Deployment Considerations", RFC 7971, DOI 10.17487/
RFC7971, October 2016,
<http://www.rfc-editor.org/info/rfc7971>.
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Appendix A. Solution Approaches for Partitioned ALTO Knowledge
The ALTO base protocol document [RFC7285] specifies the communication
between an ALTO client and a single ALTO server. It is implicitly
assumed that this server can answer any query, possibly with some
kind of default value if no exact data is known. No special
provisions were made for the case that the ALTO information
originates from multiple sources, which are possibly under the
control of different administrative entities (e.g., different ISPs)
or that the overall ALTO information is partitioned and stored on
several ALTO servers.
A.1. Classification of Solution Approaches
Various protocol extensions and other solutions have been proposed to
deal with multiple information sources and partitioned knowledge.
They can be classified as follows:
1 Ensure that all ALTO servers have the same knowledge
1.1 Ensure data replication and synchronization within the
provisioning protocol (cf. RFC 5693, Fig 1 [RFC5693]).
1.2 Use an Inter-ALTO-server data replication protocol. Possibly,
the ALTO protocol itself - maybe with some extensions - could be
used for that purpose; however, this has not been studied in
detail so far.
2 Accept that different ALTO servers (possibly operated by
different organizations, e.g., ISPs) do not have the same
knowledge
2.1 Allow ALTO clients to send arbitrary queries to any ALTO server
(e.g. the one discovered using [RFC7286]). If this server
cannot answer the query itself, it will fetch the data on behalf
of the client, using the ALTO protocol or a to-be-defined inter-
ALTO-server request forwarding protocol.
2.2 Allow ALTO clients to send arbitrary queries to any ALTO server
(e.g. the one discovered using [RFC7286]). If this server
cannot answer the query itself, it will redirect the client to
the "right" ALTO server that has the desired information, using
a small to-be-defined extension of the ALTO protocol.
2.3 ALTO clients need to use some kind of "search engine" that
indexes ALTO servers and redirects and/or gives cached results.
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2.4 ALTO clients need to use a new discovery mechanism to discover
the ALTO server that has the desired information and contact it
directly.
A.2. Discussion of Solution Approaches
The provisioning or initialization protocol for ALTO servers (cf. RFC
5693, Fig 1 [RFC5693]) is currently not standardized. It was a
conscious decision not to include this in the scope of the IETF ALTO
working group. The reason is that there are many different kinds of
information sources. This implementation specific protocol will
adapt them to the ALTO server, which offers a standardized protocol
to the ALTO clients. However, adding the task of synchronization
between ALTO servers to this protocol (i.e., approach 1.1) would
overload this protocol with a second functionality that requires
standardization for seamless multi-domain operation.
For the 1.? solution approaches, in addition to general technical
feasibility and issues like overhead and caching efficiency, another
aspect to consider is legal liability. Operator "A" might prefer not
to publish information about nodes in or paths between the networks
of operators "B" and "C" through A's ALTO server, even if A knew that
information. This is not only a question of map size and processing
load on A's ALTO server. Operator A could also face legal liability
issues if that information had a bad impact on the traffic
engineering between B's and C's networks, or on their business
models.
No specific actions to build a "search engine" based solution
(approach 2.3) are currently known and it is unclear what could be
the incentives to operate such an engine. Therefore, this approach
is not considered in the remainder of this document.
A.3. The Need for Cross-Domain ALTO Server Discovery
Approaches 1.1, 1.2, 2.1, and 2.2 do not only require the
specification of an ALTO protocol extension or a new protocol that
runs between ALTO servers. A large-scale, maybe Internet-wide,
multi-domain deployment would also need mechanisms by which an ALTO
server could discover other ALTO servers, learn which information is
available where, and ideally also who is authorized to publish
information related to a given part of the network. Approach 2.4
needs the same mechanisms, except that they are used on the client-
side instead of the server-side.
It is sometimes questioned whether there is a need for a solution
that allows clients to ask arbitrary queries, even if the ALTO
information is partitioned and stored on many ALTO servers. The main
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argument is, that clients are supposed to optimize the traffic from
and to themselves, and that the information needed for that is most
likely stored on a "nearby" ALTO server, i.e., the one that can be
discovered using [RFC7286]. However, there are scenarios where the
ALTO client is not co-located with an endpoint of the to-be-optimized
data transmission. Instead, the ALTO client is located at a third
party, which takes part in the application signaling, e.g., a so-
called "tracker" in a peer-to-peer application. One such scenario,
where it is advantageous to place the ALTO client not at an endpoint
of the user data transmission, is analyzed in Appendix C.
A.4. Our Solution Approach
Several solution approaches for cross-domain ALTO server discovery
have been evaluated, using the criteria documented in Appendix B.
One of them was to use the ALTO protocol itself for the exchange of
information availability [I-D.kiesel-alto-alto4alto]. However, the
drawback of that approach is that a new registration administration
authority would have to be established.
This document specifies a DNS-based procedure for cross-domain ALTO
server discovery, which was inspired by "Location Information Server
(LIS) Discovery Using IP Addresses and Reverse DNS" [RFC7216]. The
primary goal is that this procedure can be used on the client-side
(i.e., approach 2.4), but together with new protocols or protocol
extensions it could also be used to implement the other solution
approaches itemized above.
A.5. Relation to the ALTO Requirements
During the design phase of the overall ALTO solution, two different
server discovery scenarios have been identified and documented in the
ALTO requirements document [RFC6708]. The first scenario, documented
in Req. AR-32, can be supported using the discovery mechanisms
specified in [RFC7286]. An alternative approach, based on IP anycast
[I-D.kiesel-alto-ip-based-srv-disc], has also been studied. This
document, in contrast, tries to address Req. AR-33.
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Appendix B. Requirements for ALTO Cross-Domain Server Discovery
This appendix itemizes requirements that have been collected before
the design phase and that are reflected by the design of the ALTO
Cross-Domain Server Discovery Procedure.
B.1. Discovery Client Application Programming Interface
The discovery client will be called through some kind of application
programming interface (API) and the parameters will be an IP address
and, for purposes of extensibility, a service identifier such as
"ALTO". It will return one or more URI(s) that offers the requested
service ("ALTO") for the given IP address.
In other words, the client would be used to retrieve a mapping:
(IP address, "ALTO") -> IRD-URI(s)
where IRD-URI(s) is one or more URI(s) of Information Resource
Directories (IRD, see Section 9 of [RFC7285]) of ALTO server(s) that
can give reasonable guidance to a resource consumer with the
indicated IP address.
B.2. Data Storage and Authority Requirements
The information for mapping IP addresses and service parameters to
URIs should be stored in a - preferably distributed - database. It
must be possible to delegate administration of parts of this
database. Usually, the mapping from a specific IP address to an URI
is defined by the authority that has administrative control over this
IP address, e.g., the ISP in residential access networks or the IT
department in enterprise, university, or similar networks.
B.3. Cross-Domain Operations Requirements
The cross-domain server discovery mechanism should be designed in
such a way that it works across the public Internet and also in other
IP-based networks. This in turn means that such mechanisms cannot
rely on protocols that are not widely deployed across the Internet or
protocols that require special handling within participating
networks. An example is multicast, which is not generally available
across the Internet.
The ALTO Cross-Domain Server Discovery protocol must support gradual
deployment without a network-wide flag day. If the mechanism needs
some kind of well-known "rendezvous point", re-using an existing
infrastructure (such as the DNS root servers or the WHOIS database)
should be preferred over establishing a new one.
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B.4. Protocol Requirements
The protocol must be able to operate across middleboxes, especially
across NATs and firewalls.
The protocol shall not require any pre-knowledge from the client
other than any information that is known to a regular IP host on the
Internet.
B.5. Further Requirements
The ALTO cross domain server discovery cannot assume that the server
discovery client and the server discovery responding entity are under
the same administrative control.
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Appendix C. ALTO and Tracker-based Peer-to-Peer Applications
This appendix illustrates one ALTO use case and shows that ALTO
Cross-Domain Server Discovery is beneficial in that scenario.
C.1. Architectural Options
The ALTO protocol specification [RFC7285] 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 ALTO queries on behalf of remote resource consumers.
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. After receiving a query for a given resource
(F1) the resource directory invokes this ALTO client 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. ALTO cli. 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 ALTO query on behalf of
remote resource consumer
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
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server some time ago.
C.2. Evaluation
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%
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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
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 ALTO queries on behalf of remote
resource consumers. 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. This task can be solved using the ALTO Cross-
Domain Server Discovery Procedure.
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C.3. Example
This section provides a complete example of the ALTO Cross-Domain
Server Discovery Procedure in a tracker-based peer-to-peer scenario.
The example is based on the network topology shown in Figure 5. Five
access networks - Networks a, b, c, x, and t - are operated by five
different network operators. They are interconnected by a backbone
structure. Each network operator runs an ALTO server in their
network, i.e., ALTO_SRV_A, ALTO_SRV_B, ALTO_SRV_C, ALTO_SRV_X, and
ALTO_SRV_T, respectively.
_____ __ _____ __ _____ __
__( )__( )_ __( )__( )_ __( )__( )_
( Network a ) ( Network b ) ( Network c )
( Res. Provider A ) ( Res. Provider B ) ( Res. Provider C )
(__ ALTO_SRV_A __) (__ ALTO_SRV_B __) (__ ALTO_SRV_C __)
(___)--(____) \ (___)--(____) / (___)--(____)
\ / /
---+---------+-----------------+----
( Backbone )
------------+------------------+----
_____ __/ _____ \__
__( )__( )_ __( )__( )_
( Network x ) ( Network t )
( Res. Consumer X ) (Resource Directory)
(_ ALTO_SRV_X __) (_ ALTO_SRV_T __)
(___)--(____) (___)--(____)
Figure 5: Example network topology
A new peer of a peer-to-peer application wants to join a specific
swarm (overlay network), in order to access a specific resource.
This new peer will be called "Resource Consumer X" in accordance to
the terminology of [RFC6708] and it is located in Network x. It
contacts the tracker ("Resource Directory"), which is located in
Network t. The mechanism by which the new peer discovers the tracker
is out of the scope of this document. The tracker maintains a list
of peers that take part in the overlay network, and hence it can
determine that Resource Providers A, B, and C are candidate peers for
Resource Consumer X.
As shown in the previous section, a tracker-side ALTO optimization
(c.f. Figure 3 and Figure 4) is more efficient than a client-side
optimization. Consequently, the tracker wants to use the ALTO
Endpoint Cost Service (ECS) to learn the routing costs between X and
A, X and B, as well as X and C, in order to sort A, B, and C by their
respective routing costs to X.
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In theory, there are many options how the ALTO Cross-Domain Server
Discovery Procedure could be used. For example, the tracker could do
the following steps:
IRD_URIS_A = XDOMDISC(A,"ALTO:https")
COST_X_A = query the ECS(X,A,routingcost) found in IRD_URIS_A
IRD_URIS_B = XDOMDISC(B,"ALTO:https")
COST_X_B = query the ECS(X,B,routingcost) found in IRD_URIS_B
IRD_URIS_C = XDOMDISC(C,"ALTO:https")
COST_X_C = query the ECS(X,C,routingcost) found in IRD_URIS_C
Maybe, the ALTO Cross-Domain Server Discovery Procedure queries would
yield in this scenario: IRD_URIS_A = ALTO_SRV_A, IRD_URIS_B =
ALTO_SRV_B, and IRD_URIS_C = ALTO_SRV_C. That is, each ECS query
would be sent to a different ALTO server. The problem with this
approach is that we are not neccessarily able to compare COST_X_A,
COST_X_B, and COST_X_C with each other. The specification of the
routingcost metric mandates that "A lower value indicates a higher
preference", but "an ISP may internally compute routing cost using
any method that it chooses" (see section 6.1.1.1. of [RFC7285]).
Thus, COST_X_A could be 10 (milliseconds round-trip time), while
COST_X_B could be 200 (kilometers great circle distance between the
approximate geographic locations of the hosts) and COST_X_C could be
3 (router hops, corresponding to a decrease of the TTL field in the
IP header). Each of these metrics fulfills the "lower value is more
preferable" requirement on its own, but obviously, they cannot be
compared with each other. Even if there was a reasonable formula to
compare, for example, kilometers with milliseconds, we could not use
it, as the units of measurement (or any other information about the
computation method for the routingcost) are not sent along with the
value in the ECS reply.
To avoid this problem, the tracker tries to send all ECS queries to
the same ALTO server. As specified in Section 4.4 of this document,
case 2, it uses the IP address of Resource Consumer x as parameter to
the discovery procedure:
IRD_URIS_X = XDOMDISC(X,"ALTO:https")
COST_X_A = query the ECS(X,A,routingcost) found in IRD_URIS_X
COST_X_B = query the ECS(X,B,routingcost) found in IRD_URIS_X
COST_X_C = query the ECS(X,C,routingcost) found in IRD_URIS_X
This strategy ensures that COST_X_A, COST_X_B, and COST_X_C can be
compared with each other.
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As discussed above, the tracker calls the ALTO Cross-Domain Server
Discovery Procedure with IP address X as a parameter. For the
reminder of this example, we assume that X = 2001:DB8:1:2:227:eff:
fe6a:de42. Thus, the procedure call is
IRD_URIS_X = XDOMDISC(2001:DB8:1:2:227:eff:fe6a:de42,"ALTO:https").
The first parameter 2001:DB8:1:2:227:eff:fe6a:de42 is a single IPv6
address. Thus, we get AT = IPv6, A = 2001:DB8:1:2:227:eff:fe6a:de42,
L = 128, and SP = "ALTO:https".
The procedure constructs (see Step 1 in Section 3.2)
R128 = "2.4.E.D.A.6.E.F.F.F.E.0.7.2.2.0.2.0.0.0.1.0.0.0.
8.B.D.0.1.0.0.2.IP6.ARPA.", as well as (see Step 2 in Section 3.3)
R64 = "2.0.0.0.1.0.0.0.8.B.D.0.1.0.0.2.IP6.ARPA.",
R56 = "0.0.1.0.0.0.8.B.D.0.1.0.0.2.IP6.ARPA.",
R48 = "1.0.0.0.8.B.D.0.1.0.0.2.IP6.ARPA.", and
R32 = "8.B.D.0.1.0.0.2.IP6.ARPA.".
In order to illustrate the third step of the ALTO Cross-Domain Server
Discovery Procedure, we use the "dig" (domain information groper) DNS
lookup utility that is available for many operating systems (e.g.,
Linux). A real implementation of the ALTO Cross-Domain Server
Discovery Procedure would not be based on the "dig" utility, but use
appropriate libraries and/or operating system APIs. Please note that
the following steps have been performed in a controlled lab
environment with a appropriately configured name server. A suitable
DNS configuration will be needed to reproduce these results. Please
also note that the rather verbose output of the "dig" tool has been
shortened to the relevant lines.
Since AT = IPv6 and L = 128, in the table given in Section 3.4, the
sixth row (not counting the column headers) applies.
As mandated by the third column, we start with a lookup of R128,
looking for NAPTR resource records:
| user@labpc:~$ dig -tNAPTR 2.4.E.D.A.6.E.F.F.F.E.0.7.2.2.0.\
| 2.0.0.0.1.0.0.0.8.B.D.0.1.0.0.2.IP6.ARPA.
|
| ;; Got answer:
| ;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 26553
| ;; flags: qr aa rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADD'L: 0
The domain name R128 does not exist (status: NXDOMAIN), so we cannot
get a useful result. Therefore, we continue with the fourth column
of the table and do a lookup of R64:
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| user@labpc:~$ dig -tNAPTR 2.0.0.0.1.0.0.0.8.B.D.0.1.0.0.2.IP6.ARPA.
|
| ;; Got answer:
| ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 33193
| ;; flags: qr aa rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADD'L: 0
The domain name R64 could be looked up (status: NOERROR), but there
are no NAPTR resource records associated with it (ANSWER: 0). Maybe,
there are some other resource records such as PTR, NS, or SOA, but we
are not interested in them. Thus, we do not get a useful result and
we continue with looking up R56:
| user@labpc:~$ dig -tNAPTR 0.0.1.0.0.0.8.B.D.0.1.0.0.2.IP6.ARPA.
|
| ;; Got answer:
| ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 35966
| ;; flags: qr aa rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 1, ADD'L: 2
|
| ;; ANSWER SECTION:
| 0.0.1.0.0.0.8.B.D.0.1.0.0.2.IP6.ARPA. 604800 IN NAPTR 100 10 "u"
| "LIS:HELD" "!.*!https://lis1.example.org:4802/?c=ex!" .
| 0.0.1.0.0.0.8.B.D.0.1.0.0.2.IP6.ARPA. 604800 IN NAPTR 100 20 "u"
| "LIS:HELD" "!.*!https://lis2.example.org:4802/?c=ex!" .
The domain name R56 could be looked up and there are NAPTR resource
records associated with it. However, each of these records has a
service parameter that does not match our SP = "ALTO:https" (see
[RFC7216] for "LIS:HELD"), and therefore, we have to ignore them.
Consequently, we still do not have a useful result and continue with
a lookup of R48:
| user@labpc:~$ dig -tNAPTR 1.0.0.0.8.B.D.0.1.0.0.2.IP6.ARPA.
|
| ;; Got answer:
| ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 50459
| ;; flags: qr aa rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 1, ADD'L: 2
|
| ;; ANSWER SECTION:
| 1.0.0.0.8.B.D.0.1.0.0.2.IP6.ARPA. 604800 IN NAPTR 100 10 "u"
| "ALTO:https" "!.*!https://alto1.example.net/ird!" .
| 1.0.0.0.8.B.D.0.1.0.0.2.IP6.ARPA. 604800 IN NAPTR 100 10 "u"
| "LIS:HELD" "!.*!https://lis.example.net:4802/?c=ex!" .
This lookup yields two NAPTR resource records. We have to ignore the
second one as its service parameter does not match our SP, but the
first NAPTR resource record has a matching service parameter.
Therefore, the procedure terminates successfully and the final
outcome is: IRD_URIS_X = "https://alto1.example.net/ird".
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The ALTO client that is embedded in the tracker will access the ALTO
Information Resource Directory (IRD, see Section 9 of [RFC7285]) at
this URI, look for the Endpoint Cost Service (ECS, see Section 11.5
of [RFC7285]), and query the ECS for the costs between A and X, B and
X, as well as C and X, before returning an ALTO-optimized list of
candidate resource providers to resource consumer X.
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Appendix D. Contributors List and Acknowledgments
The initial version of this document was co-authored by Marco Tomsu
(Alcatel-Lucent).
This document borrows some text from [RFC7286], as historically, it
has been part of the draft that eventually became said RFC. 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.izus.uni-stuttgart.de
Martin Stiemerling
University of Applied Sciences Darmstadt, Computer Science Dept.
Haardtring 100
Darmstadt 64295
Germany
Phone: +49 6151 16 37938
Email: mls.ietf@gmail.com
URI: http://ietf.stiemerling.org
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