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Versions: (draft-stiemerling-alto-deployments)
00 01 02 03 04 05 06 07 08 09 10 11
12 13 14 15 16 RFC 7971
ALTO M. Stiemerling, Ed.
Internet-Draft NEC Europe Ltd.
Intended status: Informational S. Kiesel, Ed.
Expires: September 3, 2012 University of Stuttgart
S. Previdi
Cisco Systems, Inc.
March 2, 2012
ALTO Deployment Considerations
draft-ietf-alto-deployments-04
Abstract
Many Internet applications are used to access resources, such as
pieces of information or server processes, which are available in
several equivalent replicas on different hosts. This includes, but
is not limited to, peer-to-peer file sharing applications. The goal
of Application-Layer Traffic Optimization (ALTO) is to provide
guidance to these applications, which have to select one or several
hosts from a set of candidates, that are able to provide a desired
resource. The protocol is under specification in the ALTO working
group. This memo discusses deployment related issues of ALTO for
peer-to-peer and CDNs, some preliminary security considerations, and
also initial guidance for application designers using ALTO.
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
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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 September 3, 2012.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. General Considerations . . . . . . . . . . . . . . . . . . . . 5
2.1. General Placement of ALTO . . . . . . . . . . . . . . . . 5
2.2. Relationship between ALTO and Applications . . . . . . . . 7
2.3. Provided Guidance . . . . . . . . . . . . . . . . . . . . 7
2.3.1. Keeping Traffic Local in Network . . . . . . . . . . . 8
2.3.2. Off-Loading Traffic from Network . . . . . . . . . . . 8
2.3.3. Intra-Network Localization/Bottleneck Off-Loading . . 9
2.4. Provisiong ALTO Maps . . . . . . . . . . . . . . . . . . . 11
3. Deployment Considerations by ISPs . . . . . . . . . . . . . . 12
3.1. Requirement for Traffic Optimization by ISPs . . . . . . . 12
3.2. Considerations for ISPs . . . . . . . . . . . . . . . . . 13
3.2.1. Very small ISPs with simple Network Structure . . . . 13
3.2.2. Large ISPs with layered fixed Network Structure . . . 13
3.2.3. ISPs with Mobile Network . . . . . . . . . . . . . . . 15
4. Using ALTO for P2P . . . . . . . . . . . . . . . . . . . . . . 17
4.1. Using ALTO for Tracker-based Peer-to-Peer Applications . . 19
4.2. Expectations of ALTO . . . . . . . . . . . . . . . . . . . 24
5. Using ALTO for CDNs . . . . . . . . . . . . . . . . . . . . . 25
5.1. Request Routing using the Endpoint Cost Service . . . . . 25
5.1.1. ALTO Topology Vs. Network Topology . . . . . . . . . . 26
5.1.2. Topology Computation and ECS Delivery . . . . . . . . 26
5.1.3. Ranking Service . . . . . . . . . . . . . . . . . . . 26
5.1.4. Ranking and Network Events . . . . . . . . . . . . . . 27
5.1.5. Caching and Lifetime . . . . . . . . . . . . . . . . . 27
5.1.6. Redirection . . . . . . . . . . . . . . . . . . . . . 28
5.1.7. Groups and Costs . . . . . . . . . . . . . . . . . . . 28
6. Advanced Features . . . . . . . . . . . . . . . . . . . . . . 29
6.1. Cascading ALTO Servers . . . . . . . . . . . . . . . . . . 29
6.2. ALTO for IPv4 and IPv6 . . . . . . . . . . . . . . . . . . 30
6.3. Monitoring ALTO . . . . . . . . . . . . . . . . . . . . . 30
6.3.1. Monitoring Metrics Definition . . . . . . . . . . . . 30
6.3.2. Monitoring Data Sources . . . . . . . . . . . . . . . 31
6.3.3. Monitoring Structure . . . . . . . . . . . . . . . . . 31
7. Known Limitations of ALTO . . . . . . . . . . . . . . . . . . 33
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7.1. Limitations of Map-based Approaches . . . . . . . . . . . 33
7.2. Limitiations of Non-Map-based Approaches . . . . . . . . . 34
7.3. General Challenges . . . . . . . . . . . . . . . . . . . . 34
8. Extensions to the ALTO Protocol . . . . . . . . . . . . . . . 36
8.1. Host Group Descriptors . . . . . . . . . . . . . . . . . . 36
8.2. Rating Criteria . . . . . . . . . . . . . . . . . . . . . 36
8.2.1. Distance-related Rating Criteria . . . . . . . . . . . 36
8.2.2. Charging-related Rating Criteria . . . . . . . . . . . 37
8.2.3. Performance-related Rating Criteria . . . . . . . . . 37
8.2.4. Inappropriate Rating Criteria . . . . . . . . . . . . 38
9. API between ALTO Client and Application . . . . . . . . . . . 39
10. Security Considerations . . . . . . . . . . . . . . . . . . . 40
10.1. Information Leakage from the ALTO Server . . . . . . . . . 40
10.2. ALTO Server Access . . . . . . . . . . . . . . . . . . . . 40
10.3. Faking ALTO Guidance . . . . . . . . . . . . . . . . . . . 41
11. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 42
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 43
12.1. Normative References . . . . . . . . . . . . . . . . . . . 43
12.2. Informative References . . . . . . . . . . . . . . . . . . 43
Appendix A. Contributors List and Acknowledgments . . . . . . . . 45
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 46
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1. Introduction
Many Internet applications are used to access resources, such as
pieces of information or server processes, which are available in
several equivalent replicas on different hosts. This includes, but
is not limited to, peer-to-peer file sharing applications and Content
Delivery Networks (CDNs). The goal of Application-Layer Traffic
Optimization (ALTO) is to provide guidance to applications, which
have to select one or several hosts from a set of candidates, that
are able to provide a desired resource. The basic ideas of ALTO are
described in the problem space of ALTO is described in [RFC5693] and
the set of requirements is discussed in [I-D.ietf-alto-reqs].
However, there are no considerations about what operational issues
are to be expected once ALTO will be deployed. This includes, but is
not limited to, location of the ALTO server, imposed load to the ALTO
server, or from whom the queries are performed.
Comments and discussions about this memo should be directed to the
ALTO working group: alto@ietf.org.
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2. General Considerations
The ALTO protocol is a client/server protocol, operating between a
number of ALTO clients and an ALTO server, as sketched in Figure 1.
The ALTO working groups defines the ALTO protocol
[I-D.ietf-alto-protocol].
+----------+
| ALTO |
| Server |
+----------+
^
_.-----|------.
,-'' | `--.
,' | `.
( Network | )
`. | ,'
`--. | _.-'
`------|-----''
v
+----------+ +----------+ +----------+
| ALTO | | ALTO |...| ALTO |
| Client | | Client | | Client |
+----------+ +----------+ +----------+
Figure 1: Network Overview of ALTO Protocol
2.1. General Placement of ALTO
The ALTO server and ALTO clients can be situated at various entities
in a network deployment. The first differentiation is whether the
ALTO client is located on the actual host that runs the application,
as shown in Figure 2, (e.g., peer-to-peer filesharing application) or
if the ALTO client is located on resource directory, as shown in
Figure 3 (e.g., a tracker in peer-to-peer filesharing).
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+-----+
=====| |**
==== +-----+ *
==== * *
==== * *
+-----+ +------+===== +-----+ *
| |.....| |======================| | *
+-----+ +------+===== +-----+ *
Source of ALTO ==== * *
topological service ==== * *
information ==== +-----+ *
=====| |**
+-----+
Legend:
=== ALTO client protocol
*** Application protocol
... Provisioning protocol
Figure 2: Overview of protocol interaction between ALTO
elements,scenario without tracker
Figure 2 shows the operational model for applications that do not use
a tracker, such as, edonky, or in if the tracker should be the
querying party. This use case also holds true for CDNs. The ALTO
server can also be queried by CDNs to get a guidance about where the
a particular client accessing data in the CDN is exactly located in
the ISP's network.
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+-----+
**| |**
** +-----+ *
** * *
** * *
+-----+ +------+ +-----+** +-----+ *
| |.....| |=====| |**********| | *
+-----+ +------+ +-----+** +-----+ *
Source of ALTO Resource ** * *
topological service directory ** * *
information ("tracker") ** +-----+ *
**| |**
+-----+
Peers
Legend:
=== ALTO client protocol
*** Application protocol
... Provisioning protocol
Figure 3: Overview of protocol interaction between ALTO elements,
scenario with tracker
However, Figure 3 does not denote where the ALTO elements are
actually located, i.e., if the tracker and the ALTO server are in the
same ISP's domain, or if the tracker and the ALTO server are managed/
owned/located in different domains. The latter is the typical use
case, e.g., taking Pirate Bay as example that serves Bittorrent peers
world-wide.
2.2. Relationship between ALTO and Applications
ALTO is intended to be used by a wide-range of applications.
However, any application using ALTO must also work if no ALTO servers
can be found or if no responses to ALTO queries are received, e.g.,
due to connectivity problems or overload situation (see also
[I-D.ietf-alto-reqs]). (Editor's note: better text needed here!)
2.3. Provided Guidance
ALTO gives guidance to applications on what IP addresses or IP
prefixes, and such which hosts are to be preferred according to the
operator of the ALTO server. The general assumption of the ALTO WG
is that a network operator would always express to prefer hosts in
its own network while hosts located outside its own network are to be
avoided (are undesired to be considered by the applications). This
might be applicable in some cases but may not be applicable in the
general case. The ALTO protocol gives only the means to let the ALTO
server operator to express is preference, whatever this preference
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is. This section explores this space.
2.3.1. Keeping Traffic Local in Network
ALTO guidance can be used to let applications prefer other peers
within the same network operator's network instead of randomly
connecting to other peers which are located in another operator's
network. Figure 4 shows such a scenario where peers prefer peers in
the same network (e.g., Peer 1 and Peer 2 in ISP1 and Peer 3 and Peer
4 in ISP2).
,-------. +-----------+
,---. ,-' `-. | Peer 1 |
,-' `-. / ISP 1 ########|ALTO Client|
/ \ / # \ +-----------+
/ ISP X \ | # | +-----------+
/ \ \ ########| Peer 2 |
; +----------------------------|ALTO Client|
| | | `-. ,-' +-----------+
| | | `-------'
| | | ,-------. +-----------+
: | ; ,-' `########| Peer 3 |
\ | / / ISP 2 # \ |ALTO Client|
\ | / / # \ +-----------+
\ +---------+ # | +-----------+
`-. ,-' \ | ########| Peer 4 |
`---' \ +------------------|ALTO Client|
`-. ,-' +-----------+
`-------'
Legend:
### preferred "connections"
--- non-preferred "connections"
Figure 4: ALTO Traffic Network Localization
TBD: Describes limits of this approach (e.g., traffic localization
guidance is of less use if the peers cannot upload); describe how
maps would look like.
2.3.2. Off-Loading Traffic from Network
Another scenario where the use of ALTO can be beneficial is in mobile
broadband networks, e.g., CDMA200 or UMTS, but where the network
operator may have the desire to guide peers in its own network to use
peers in remote networks. One reason can be that the wireless
network is not made for the load cause by, e.g., peer-to-peer
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applications, and the operator has the need that peers fetch their
data from remote peers in other parts of the Internet.
,-------. +-----------+
,---. ,-' `-. | Peer 1 |
,-' `-. / ISP 1 +-------|ALTO Client|
/ \ / | \ +-----------+
/ ISP X \ | | | +-----------+
/ \ \ +-------| Peer 2 |
; #-###########################|ALTO Client|
| # | `-. ,-' +-----------+
| # | `-------'
| # | ,-------. +-----------+
: # ; ,-' `+-------| Peer 3 |
\ # / / ISP 2 | \ |ALTO Client|
\ # / / | \ +-----------+
\ ########### | | +-----------+
`-. ,-' \ # +-------| Peer 4 |
`---' \ ###################|ALTO Client|
`-. ,-' +-----------+
`-------'
Legend:
=== preferred "connections"
--- non-preferred "connections"
Figure 5: ALTO Traffic Network De-Localization
Figure 5 shows the result of such a guidance process where Peer 2
prefers a connection with Peer4 instead of Peer 1, as shown in
Figure 4.
TBD: Limits of this approach in general and with respect to p2p.
describe how maps would look like.
2.3.3. Intra-Network Localization/Bottleneck Off-Loading
The above sections described the results of the ALTO guidance on an
inter-network level. However, ALTO can also be used to guide peers
on which internal peers are to be preferred. For instance, to guide
Peers on a remote network side to prefer to connect to each other,
instead of crossing a bottleneck link, a backhaul link to connect the
side to the network core. Figure 6 shows such a scenario where Peer
1 and Peer 2 are located in Net 2 of ISP1 and connect via a low
capacity link to the core (Net 1) of the same ISP1. Peer1 and Peer 2
would both exchange their data with remote peers, probably clogging
the bottleneck link.
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,-------. +-----------+
,---. ,-' `-. | Peer 1 |
,-' `-. / ISP 1 #########|ALTO Client|
/ \ / Net 2 # \ +-----------+
/ ISP 1 \ | ######### | +-----------+
/ Net 1 \ \ # / | Peer 2 |
; ###; \ # ##########|ALTO Client|
| X~~~~~~~~~~~~X#######,-' +-----------+
| ### | ^ `-------'
| | |
: ; |
\ / Bottleneck
\ /
\ /
`-. ,-'
`---'
Legend:
### peer "connections"
~~~ bottleneck link
Figure 6: Without Intra-Network ALTO Traffic Localization
The operator can guide the peers in such a situation to try first
local peers in the same network islands, avoiding or at least
lowering the effect on the bottleneck link, as shown in Figure 7.
,-------. +-----------+
,---. ,-' `-. | Peer 1 |
,-' `-. / ISP 1 #########|ALTO Client|
/ \ / Net 2 # \ +-----------+
/ ISP 1 \ | # | +-----------+
/ Net 1 \ \ #########| Peer 2 |
; ; \ ##########|ALTO Client|
| #~~~~~~~~~~~########,-' +-----------+
| ### | ^ `-------'
| | |
: ; |
\ / Bottleneck
\ /
\ /
`-. ,-'
`---'
Legend:
### peer "connections"
~~~ bottleneck link
Figure 7: With Intra-Network ALTO Traffic Localization
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TBD: describe how maps would look like.
2.4. Provisiong ALTO Maps
This section will describe how ALTO maps in the protocol can be
populated before using them.
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3. Deployment Considerations by ISPs
The Internet is a large network constituted of multiple networks
worldwide. Numerous of these networks are built by telecom operators
or network operators (named ISP in this memo), and these networks
provide network connectivity, such as cable networks, 3G and so on.
As well as some of networks are built by universities or big
organizations themselves, and these networks are used to provide
connectivity for research and work. The essence of Internet is its
connectivity and sharing capability. However, ISPs emphasize
network's manageability and controllability, because ISPs provide
public network access service for most person and families, they need
to manage, to control and to audit the traffic. Thus, it's important
for ISPs to understand the requirement of optimizing traffic, and how
to deploy ALTO service in these manageability and controllability
networks.
3.1. Requirement for Traffic Optimization by ISPs
All networks of ISPs are connected to each other through peering
points. From view of business mode, the inter-network settlement is
needed in traffic exchanging between these ISP's networks. The
current settlement can be costly. So to save these cost, the simple
and basic method is to decrease the traffic exchange across the
peering points and keep the traffic in own network area.
For some large ISPs, their whole network is layered. The upper layer
network includes one or several backbone networks, and the lower
layer network includes multiple access networks. These access
networks are connected to backbone networks, and the exchange traffic
with others through backbone network. In this kind of layered
network, the bandwidth of backbone network is important and may be
scarce. Traffic should be limited to the access networks, so to
decrease the usage of backbone as far as possible.
Compared to fixed networks, mobile networks have some special
characters, including small link bandwidth, high cost, limited radio
frequency resource, and terminal battery. In mobile network, the
usage of wireless link should be decreased as far as possible and be
high-efficient. For example, in the case of a P2P service, the
clients in the fixed network should decrease the data transport from
the clients in the mobile networks, as well as the clients in the
mobile networks should prefer the data transmission from the clients
in the fixed networks.
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3.2. Considerations for ISPs
3.2.1. Very small ISPs with simple Network Structure
For very small ISPs, the traffic optimizing problem they focus is
that how to decrease the traffic exchanging with other ISPs, because
of high settlement costs. To use the ALTO service to optimize
traffic, small ISPs can define two optimization areas: one is their
own network; the other is all outer networks connected with their
network. The cost map can be defined like this: the cost of link
between clients of inner ISP's networks is lower than from clients of
outer ISP's networks to clients of inner ISP's networks. So the
client of this ISP will prefer to require data from the clients in
the same ISP with high priority.
One example is given as below in Figure 8. ISP A is one small ISP,
only having one access network. In ALTO service deploying, we can
define ISP A to be one optimization area, named as PID1, and define
other networks to be the other optimization area, named as PID2. C1
is denoted as the link cost in inner ISP A. C2 is denoted as the link
cost from PID2 to PID1. We define the cost map as:
C1<C2
-----------
//// \\\\
// \\
// \\ /-----------\
| +---------+ | //// \\\\
| | ALTO | ISP A | C2 | Other Networks |
| | Service | PID 1 <----------- PID 2
| +---------+ C1 | | |
| | \\\\ ////
\\ // \-----------/
\\ //
\\\\ ////
-----------
Figure 8: ALTO deployment in small ISPs
3.2.2. Large ISPs with layered fixed Network Structure
For large ISPs with layered fixed network structure, the traffic
optimizing problems they focus will include that: using backbone
network by high-efficiency, adjusting traffic balance in different
access networks according to traffic conditions and management
policies, and considering settlement cost with other ISPs. So in
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ALTO service deploying to this kind of large ISP, first the
optimization area can be defined according to real network condition.
For example, each access network can be defined to be one
optimization area. Then cost can be defined according to the
optimizing requirement by ISPs. There is one example described below
and also shown in Figure 9.
In this example, ISP A has one backbone network and three access
networks, named as AN A, AN B, and AN C. A P2P application is used in
this example. For the traffic optimization, the first requirement is
to decrease the P2P traffic of backbone network in inner ISP A; and
the second requirement is to decrease the P2P traffic to outer ISPs.
Always, the second requirement is prior to the first one. Also, we
assume that the settlement rate with ISP B is lower than with other
ISPs. Then ISP A can deploy ALTO service to meet the need of traffic
optimization. We will give the detail example of ALTO service
definition and configuration according to requirements above.
In inner network of ISP A, we can define each access network to be
one optimization area, and assign one PID to every access network,
such as PID1, PID2, and PID 3. Because of different settlement with
different outer ISPs, we define ISP B to be one optimization area,
and assign PID 4 to it, as well as define all other networks to be
one optimization area and PID 5.
We assign cost names (C1, C2, C3, C4, C5, C6, C7) as the figure
below. C1 is denoted as the link cost in inner AN A, the same as C2
and C3. C4 is denoted as the link cost from PID 1 to PID 2, the same
as C5. C6 is denoted as the link cost from the ISP B to ISP A. C7 is
denoted as the link cost from other networks to ISP A.
According to discussion of the first requirement and the second
requirement above, the relationship of these costs will be defined
as: (C1, C2, C3) < (C4, C5) < (C6) < (C7)
This is one very simple example above, in which we do not consider
the different link type of access network. In deploying ALTO service
in real network, we must consider more real network conditions and
requirements. One real example is described in greater detail in
[I-D.lee-alto-chinatelecom-trial].
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+------------------------------------+ +----------------+
| ISP A +---------------+ | | |
| | Backbone | | C6 | ISP B |
| +--+ Network +---+ |<--------+ PID 4 |
| | +-------+-------+ | | | |
| | | | | | |
| | | | | +----------------+
| +---+--+ +--+---+ +-+----+ |
| |AN A | C4 |AN B | C5 |AN C | |
| |PID 1 +--->|PID 2 |<----+PID 3 | |
| |C1 | |C2 | |C3 | | +----------------+
| +---+--+ +---+--+ +-+----+ | | |
| | C7 | Other Networks |
| |<--------+ PID 5 |
| | | |
| | | |
+------------------------------------+ +----------------+
Figure 9: ALTO deployment in large ISPs with layered fixed network
structures
3.2.3. ISPs with Mobile Network
For ISPs with mobile network and fixed network, the traffic
optimizing problems they focus will be optimizing the mobile traffic,
except problems on last hop section. Wireless radio frequency
resource is scarce and costly in mobile network. The requirement of
traffic optimization in mobile network is mainly decreasing the usage
of radio resource. The ALTO service can be deployed to meet these
needs.
For example in one ISP A as below in Figure 10, there is one mobile
network is connected to backbone network. In this kind of network
structure, mobile network can be defined as one optimization area,
and assigned PID 1. We also define other PID and cost as figure
below.
To decrease the usage of wireless link, the relationship of these
costs will be defined to:
From view of mobile network:(C4 < C1). This means that, the clients
in mobile network requiring data resource from clients of the other
access networks is prior to clients of mobile network. This policy
can decrease the usage of wireless link and power consumption in
terminal.
From view of AN A:(C2 < C6, C5 = maximum cost). This means that, to
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other optimization area, requiring data from mobile network should be
avoided.
+-----------------------------------------------------------------+
| |
| ISP A +-------------+ |
| +--------+ ALTO +---------+ |
| | | Service | | |
| | +------+------+ | |
| | | | |
| | | | |
| | | | |
| +-------+-------+ | C6 +--------+------+ |
| | AN A |<-------------- AN B | |
| | PID 2 | C7 | | PID 3 | |
| | C2 -------------->| C3 | |
| +---------------+ | +---------------+ |
| ^ | | | ^ |
| | | | | | |
| | |C4 | | | |
| C5 | | | | | |
| | | +--------+---------+ | | |
| | +-->| Mobile Network |<---+ | |
| | | PID 1 | | |
| +------- | C1 |----------+ |
| +------------------+ |
+-----------------------------------------------------------------+
Figure 10: ALTO deployment in ISPs with mobile network
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4. Using ALTO for P2P
,-------.
,---. ,-' `-. +-----------+
,-' `-. / ISP 1 \ | Peer 1 |*****
/ \ / +-------------+ \ | | *
/ ISP X \ +=====>+ ALTO Server | )+-----------+ *
/ \ = \ +-------------+ / +-----------+ *
; +-----------+ : = \ / | Peer 2 | *
| | Tracker |<====+ `-. ,-' | |*****
| |ALTO Client|<====+ `-------' +-----------+ **
| +-----------+ | = ,-------. **
: * ; = ,-' `-. +-----------+ **
\ * / = / ISP 2 \ | Peer 3 | **
\ * / = / +-------------+ \ | |*****
\ * / +=====>| ALTO Server | )+-----------+ ***
`-. * ,-' \ +-------------+ / +-----------+ ***
`-*-' \ / | Peer 4 |*****
* `-. ,-' | | ****
* `-------' +-----------+ ****
* ****
* ****
***********************************************<******
Legend:
=== ALTO client protocol
*** Application protocol
Figure 11: Global tracker accessing ALTO server at various ISPs
Figure 11 depicts a tracker-based system, where the tracker embeds
the ALTO client. The tracker itself is hosted and operated by an
entity different than the ISP hosting and operating the ALTO server.
Initially, the tracker has to look-up the ALTO server in charge for
each peer where it receives a ALTO query for. Therefore, the ALTO
server has to discover the handling ALTO server, as described in
[I-D.ietf-alto-server-discovery]. However, the peers do not have any
way to query the server themselves. This setting allows to give the
peers a better selection of candidate peers for their operation at an
initial time, but does not consider peers learned through direct
peer-to-peer knowledge exchange. This is called peer exchange (PEX)
in bittorent, for instance.
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,-------. +-----------+
,---. ,-' `-. +==>| Peer 1 |*****
,-' `-. / ISP 1 \ = |ALTO Client| *
/ \ / +-------------+<=+ +-----------+ *
/ ISP X \ | + ALTO Server |<=+ +-----------+ *
/ \ \ +-------------+ /= | Peer 2 | *
; +---------+ : \ / +==>|ALTO Client|*****
| | Global | | `-. ,-' +-----------+ **
| | Tracker | | `-------' **
| +---------+ | ,-------. +-----------+ **
: * ; ,-' `-. +==>| Peer 3 | **
\ * / / ISP 2 \ = |ALTO Client|*****
\ * / / +-------------+<=+ +-----------+ ***
\ * / | | ALTO Server |<=+ +-----------+ ***
`-. * ,-' \ +-------------+ /= | Peer 4 |*****
`-*-' \ / +==>|ALTO Client| ****
* `-. ,-' +-----------+ ****
* `-------' ****
* ****
***********************************************<****
Legend:
=== ALTO client protocol
*** Application protocol
Figure 12: Global Tracker - Local ALTO Servers
The scenario in Figure 12 lets the peers directly communicate with
their ISP's ALTO server (i.e., ALTO client embedded in the peers),
giving thus the peers the most control on which information they
query for, as they can integrate information received from trackers
and through direct peer-to-peer knowledge exchange.
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,-------. +-----------+
,---. ,-' ISP 1 `-. ***>| Peer 1 |
,-' `-. /+-------------+\ * | |
/ \ / + Tracker |<** +-----------+
/ ISP X \ | +-----===-----+<** +-----------+
/ \ \ +-----===-----+ /* | Peer 2 |
; +---------+ : \+ ALTO Server |/ ***>| |
| | Global | | +-------------+ +-----------+
| | Tracker | | `-------'
| +---------+ | +-----------+
: ^ ; ,-------. | Peer 3 |
\ * / ,-' ISP 2 `-. ***>| |
\ * / /+-------------+\ * +-----------+
\ * / / + Tracker |<** +-----------+
`-. *,-' | +-----===-----+ | | Peer 4 |<*
`---* \ +-----===-----+ / | | *
* \+ ALTO Server |/ +-----------+ *
* +-------------+ *
* `-------' *
***********************************************
Legend:
=== ALTO client protocol
*** Application protocol
Figure 13: P4P approach with local tracker and local ALTO server
There are some attempts to let ISP's to deploy their own trackers, as
shown in Figure 13. In this case, the client has no chance to get
guidance from the ALTO server, other than talking to the ISP's
tracker. However, the peers would have still chance the contact
other trackers, deployed by entities other than the peer's ISP.
Figure 13 and Figure 11 ostensibly take peers the possibility to
directly query the ALTO server, if the communication with the ALTO
server is not permitted for any reason. However, considering the
plethora of different applications of ALTO, e.g., multiple tracker
and non-tracker based P2P systems and or applications searching for
relays, it seems to be beneficial for all participants to let the
peers directly query the ALTO server. The peers are also the single
point having all operational knowledge to decide whether to use the
ALTO guidance and how to use the ALTO guidance. This is a preference
for the scenario depicted in Figure Figure 12.
4.1. Using ALTO for Tracker-based Peer-to-Peer Applications
The scope of this section is the interaction of peer-to-peer
applications that use a centralized resource directory ("tracker"),
with the ALTO service. In this scenario, the resource consumer
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("peer") asks the resource directory for a list of candidate resource
providers, which can provide the desired resource.
For efficiency reasons (i.e., message size), usually only a subset of
all resource providers known to the resource directory will be
returned to the resource consumer. Some or all of these resource
providers, plus further resource providers learned by other means
such as direct communication between peers, will be contacted by the
resource consumer for accessing the resource. The purpose of ALTO is
giving guidance on this peer selection, which is supposed to yield
better-than-random results. The tracker response as well as the ALTO
guidance are most beneficial in the initial phase after the resource
consumer has decided to access a resource, as long as only few
resource providers are known. Later, when the resource consumer has
already exchanged some data with other peers and measured the
transmission speed, the relative importance of ALTO may dwindle.
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 15), 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 17), the resource directory has an
embedded ALTO client, which we will refer to as RDAC in this
document. After receiving a query for a given resource (F1) the
resource directory invokes the RDAC 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 14: 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 15: 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 16: Tracker-based P2P Application with ALTO client in tracker
Peer Tracker w. RDAC 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 17: Basic message sequence chart for third-party ALTO query
Note: the message sequences depicted in Figure 15 and Figure 17 may
occur both in the target-aware and the target-independent query mode
(c.f. [I-D.ietf-alto-reqs]). 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. ssume 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 the that respective peer.
4.2. Expectations of ALTO
This section hints to some recent experiments conducted with ALTO-
like deployments in Internet Service Provider (ISP) network's. NTT
performed tests with their HINT server implementation and dummy nodes
to gain insight on how an ALTO-like service influence a peer-to-peer
systems [I-D.kamei-p2p-experiments-japan]. The results of an early
experiment conducted in the Comcast network are documented
here[RFC5632]
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5. Using ALTO for CDNs
Section 2 discussed the placement and usage of ALTO for P2P systems,
but not beyond. This section discuss the usage of ALTO for Content
Delivery Networks (CDNs). CDNs are used to bring a service (e.g., a
web page, videos, etc) closer to the location of the user - where
close refers to shorten the distance between the client and the
server in the IP topology. CDNs use several techniques to decide
which server is closest to a client requesting a service. One common
way to do so, is relying on the DNS system, but there are many other
ways, see [RFC3568].
The general issue for CDNs, independent of DNS or HTTP Redirect based
approaches (see, for instance, [I-D.penno-alto-cdn]), is that the CDN
logic has to match the client's IP address with the closest CDN
cache. This matching is not trivial, for instance, in DNS based
approaches, where the IP address of the DNS original requester is
unknown (see [I-D.vandergaast-edns-client-ip] for a discussion of
this and a solution approach).
5.1. Request Routing using the Endpoint Cost Service
Alternatively, the Request Router may request the Endpoint service
from the ALTO client.
Specifically, the Request Router requests the Endpoint Cost Service
in order to rank/rate the content locations (i.e., IP addresses of
CDN nodes) based on their distance/cost (by default the Endpoint Cost
Service operates based on Routing Distance) from/to the user address.
Once the Request Router obtained from the ALTO Server the ranked list
of locations (for the specific user) it can incorporate this
information into its selection mechanisms in order to point the user
to the most appropriate location.
A Request Router that uses the Endpoint Cost Service may query the
ALTO Server for rankings of CDN Node IP addresses for each
interesting host and cache the results for later usage.
Maps Services and ECS deliver similar ALTO service by allowing the
CDN to optimize internal selection mechanisms. Both services deliver
similar level of security, confidentiality of layer-specific
information (i.e.: application and network) however, Maps and ECS
differ in the way the ALTO service is delivered and address a
different set of requirements in terms of topology information and
network operations.
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5.1.1. ALTO Topology Vs. Network Topology
The ALTO server builds a ALTO-specific network topology that
represents the network as it should be understood and utilized by the
application layer (the CDN). Besides the security requirements that
consist of not delivering any confidential or critical information
about the infrastructure, there are efficiency requirements in terms
of what visibility of the network, and which level of granularity, it
is required by the CDN and more in general by the application layer.
The ALTO server builds topology (for either Map and ECS services)
based on multiple sources that may include: routing protocols,
network policies, state and performance information, geo-location,
etc. In all cases, the ALTO topology will not contain any details
that would endanger the network integrity and security (e.g.: There
will be no leaking of OSPF/ISIS/BGP databases to ALTO clients).
5.1.2. Topology Computation and ECS Delivery
ECS allows the CDN not to have to implement any specific algorithm or
mechanism in order to retrieve, maintain and process network topology
information (of any kind). The complexity of the network topology
(computation, maintenance and distribution) is kept in the ALTO
server and ECS is delivered on demand. Thus ECS is used in order to
implement a lightweight integration of ALTO services in the CDN
layer. ECS implies an ALTO and CDN implementation with the necessary
scalability in order to cope with the amount of transactions that CDN
and ALTO server will have to handle (knowing that the CDN is able to
cache ALTO ECS results for further use).
The ALTO server delivering ECS may integrate various information
sources such as routing topology, policies, state and performance,
geo-location, etc, and deliver the ranking service to the CDN upon
request. The network topology information is controlled, managed by
the ALTO server and the CDN benefits from ranking services in order
to optimize application layer mechanisms used for content location
selection. This allows the ALTO server to enhance and modify the way
the topology information sources are used and combined without
requiring any update in the mechanisms the ECS is delivered and do
not require any update process between ALTO and the CDN.
5.1.3. Ranking Service
When a user request a given content, the CDN locates the content in
one or more caches and executes a selection algorithms in order to
redirect the user to the 'best' cache. In order to achieve that, the
CDN issues an ECS request with the endpoint address (IPv4/IPv6) of
the user (content requester) and the set of endpoint addresses of the
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content caches (content targets). The ALTO server, receives the
request and ranks the list of content targets addresses based on
their distance from the content requester. By default, according to
[I-D.ietf-alto-protocol], the distance represents the routing cost as
computed by the routing layer (OSPF, ISIS, BGP) and may take into
consideration other routing criteria such as MPLS-VPN (MP-BGP) and
MPLS-TE (RSVP), policy and state and performance information in
addition to other information sources (policy, geo-location, state
and performance).
Once the ALTO server computed the distance it replies with the ranked
list of content target addresses. The list being ranked by distance,
the CDN is capable of integrating the rankings into its selection
process (that will also incorporate other criteria) and redirect the
user accordingly.
5.1.4. Ranking and Network Events
ALTO server ranks addresses based on topology information it acquires
from the network. The different methods and algorithms through which
the ALTO server computes topology information and rankings is out of
the scope of this document. However, and in the case the rankings
are based on routing (IP/MPLS) topology, it is obvious that network
events may impact the ranking computation. The scope of the ECS
service delivered to a CDN is not to maintain the CDN aware of any
possible network topology changes since, due to redundancy of current
networks, most of the network events happening in the infrastructure
will have limited impact on the CDN. However, catastrophic events
such as main trunks failures or backbone partition will have to take
into account by the ALTO server so to redirect traffic away from the
failure impacted area.
5.1.5. Caching and Lifetime
Each reply sent back by the ALTO server to the ALTO client running in
the CDN has a validity in time so that the CDN can cache the results
in order to re-use it and hence reducing the number of transactions
between CDN and ALTO server. The ALTO server may indicate in the
reply message how long the content of the message is to be considered
reliable and insert a lifetime value that will be used by the CDN in
order to cache (and then flush or refresh) the entry.
An ALTO server implementation may want to keep state about ALTO
clients so to inform and signal to these clients when a major network
event happened so to clear the ALTO cache in the client. In a CDN/
ALTO interworking architecture where there's a few CDN component
interacting with the ALTO server there are no scalability issues in
maintaining state about clients in the ALTO server.
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5.1.6. Redirection
When ALTO server receives an ECS request, it may not have the most
appropriate topology information in order to accurately determine the
ranking. In such case, the ALTO server, may want to adopt the
following strategies:
o Reply with available information (best effort).
o Redirect the request to another ALTO server presumed to have
better topology information (redirection).
o Doing both (best effort and redirection). In this case, the reply
message contains both the rankings and the indication of another
ALTO server where more accurate rankings may be delivered.
The decision process that is used to determine if redirection is
necessary (and which mode to use) is out of the scope of this
document. As an example, an ALTO server may decide to redirect any
request having addresses that are located into a remote Autonomous
System. In such case the redirection message includes the ALTO
server to be used and that resides in the remote AS. Redirection
implies communication between ALTO servers so to be able to signal
their identity, location and type of visibility (AS number).
5.1.7. Groups and Costs
An automated ALTO implementation may use dynamic algorithms to
aggregate network topology. However, it is often desirable to have a
mechanism through which the network operator can control the level
and details of network aggregation based on a set of requirements and
constraints. IP/MPLS networks make use of a common mechanism to
aggregate and group prefixes that is called BGP Communities. BGP is
the protocol all SP networks use in order to exchange information
about their prefix reachability. BGP Community us an attribute used
to tag a prefix so to group prefixes based on mostly any criteria (as
an example, most SP networks originate BGP prefixes with communities
identifying the Point of Presence (PoP) where the prefix has been
originated).
The ALTO server may leverage the BGP information that is available in
the SP network layer and compute group of prefixes. By policy, the
ALTO server operator may decide an arbitrary cost to set between
groups. Alternatively, there are algorithms that allows a dynamic
computation of cost between groups.
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6. Advanced Features
6.1. Cascading ALTO Servers
The main assumptions of ALTO seems to be each ISP operates its own
ALTO server independently, irrespectively of the ISP's situation.
This may true for most envisioned deployments of ALTO but there are
certain deployments that may have different settings. Figure 18
shows such setting, were for example, a university network is
connected to two upstream providers. ISP2 if the national research
network and ISP1 is a commercial upstream provider to this university
network. The university, as well as ISP1, are operating their own
ALTO server. The ALTO clients, located on the peers will contact the
ALTO server located at the university.
+-----------+
| ISP1 |
| ALTO |
| Server |
+----------=+
,-------= ,------.
,-' =`-. ,-' `-.
/ Upstream= \ / Upstream \
( ISP1 = ) ( ISP2 )
\ = / \ /
`-. =,-' `-. ,-'
`---+---= `+------'
| = |
| =======================
|,-------------. | =
,-+ `-+ +-----------+
,' University `. |University |
( Network ) | ALTO |
`. =======================| Server |
`-= +-' +-----------+
=`+------------'|
= | |
+--------+-+ +-+--------+
| Peer1 | | PeerN |
+----------+ +----------+
Figure 18: Cascaded ALTO Server
In this setting all "destinations" useful for the peers within ISP2
are free-of-charge for the peers located in the university network
(i.e., they are preferred in the rating of the ALTO server).
However, all traffic that is not towards ISP2 will be handled by the
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ISP1 upstream provider. Therefore, the ALTO server at the university
has also to include the guidance given by the ISP1 ALTO server in its
replies to the ALTO clients. This can be called cascaded ALTO
servers.
6.2. ALTO for IPv4 and IPv6
TBD
6.3. Monitoring ALTO
In addition to providing configuration, an ISP providing ALTO may
want to deploy a monitoring infrastructure to assess the benefits of
ALTO and adjust its ALTO configuration according to the results of
the monitoring.
To construct an effective monitoring infrastructure, the ISP should
(1) define the performance metrics to be monitored; (2) and identify
and deploy data sources to collect data to compute the performance
metrics. We discuss both below.
[Editor's note: Is there a relationship to the IPPM working group at
the IETF?]
6.3.1. Monitoring Metrics Definition
o Inter-domain ALTO-Integrated Application Traffic (Network metric):
This metric includes total cross domain traffic generated by
applications that utilize ALTO guidance. This metric evaluates
the impacts of ALTO on the inbound and outbound traffic of a
domain.
o Total Inter-domain Traffic (Network metric): This is similar to
the preceding but focuses on all of the traffic, ALTO aware or
not. One possibility is that some of the reduction of interdomain
traffic by ALTO aware applications may (XXX missing words?). This
metric is always used with the preceding and the following
metrics.
o Intra-domain ALTO-Integrated Application Traffic (Network metric).
(XXX description missing)
o Network hop count (Network metric): This metric provides the
average number of hops that traffic traverses inside a domain.
ALTO may reduce not only traffic volume but also the hops. The
metric can also indirectly reflect some application performance
(e.g., latency).
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o Application download rate (Application metric): This metric
measures application performance directly. Download means inbound
traffic to one user. Global average means the average value of
all users' download rates in one or more domains.
o Application Client type audit(Application metric): this metric
gives the audit of client types in ALTO service. The current
types include fixed network client and mobile network client.
6.3.2. Monitoring Data Sources
The preceding metrics are derived from data sources. We identify
three data sources.
1. Application Log Server: Many application systems deploy Log
Servers to collect data.
2. P2P Clients: Some P2P applications may not have Log Servers.
When available, P2P client logs can provide data. This is for
P2P application
3. OAM: Many ISPs deploy OAM systems to monitor IP layer traffic.
An OAM provides traffic monitoring of every network device in its
management area. It provides data such as link physical
bandwidth and traffic volumes.
6.3.3. Monitoring Structure
As discussed in the preceding section, some data sources are from ISP
while some others are from application. When there is a
collaboration agreement between the ISP and an application, there can
be an integrated monitoring system as shown in the figure below. In
particular, an application developer may deploy Monitor Clients to
communicate with Monitor Server of the ISP to transmit raw data from
the Log Server or P2P clients of the application to the ISP.
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+------------------------------------------------+
| |
| New Entities +--------------------------------------+
| | Service Provider |
| | (P2P/CDN Operator etc)|
| +-----------+ | +-----------+ | |
| |ALTO Server|-------------|ALTO Client| | |
| +-----------+ | +-----------+ | |
| | | +----------+ |
| | | |Log Server| |
| | | +----------+ |
| +--------------+ | +--------------+ | +----------+ |
| |Monitor Server|----------|Monitor Client| | |P2P Client| |
| +--------------+ | +--------------+ | +----------+ |
| | | | |
| +--------|--------+ +--------------------------------------+
+-|--------|--------|----------------------------+
| | |
| | |
| +---+ |
| |OAM| |
| +---+ |
| ISP |
-----------------
Figure 19: Monitoring Structure
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7. Known Limitations of ALTO
This section describes some known limitations of ALTO in general or
specific mechanisms in ALTO.
7.1. Limitations of Map-based Approaches
The specification of the ALTO protocol [I-D.ietf-alto-protocol] uses,
amongst others mechanism, so-called network maps. The network map
approach uses Host Group Descriptors that group one or multiple
subnetworks (i.e., IP prefixes) to a single Host Group Descriptor. A
set of IP prefixes is called partition and the associated Host Group
Descriptor is called partition ID. The "costs" between the various
partition IDs is stored in a second map, the cost map. Map-based
approaches are chosen as they lower the signaling load on the server,
as the maps have only to be retrieved if they are changed.
The main assumption for map-based approaches is that the information
provided in these maps is static for a longer period of time, where
this period of time refers to days, but not hours or even minutes.
This assumption is fine, as long as the network operator does not
change any parameter, e.g., routing within the network and to the
upstream peers, IP address assignment stays stable (and thus the
mapping to the partitions). However, there are several cases where
this assumption is not valid, as:
1. ISPs reallocate IPv4 subnets from time to time;
2. ISPs reallocate IPv4 subnets on short notice;
3. IP prefix blocks may be assigned to a single DSLAM which serves a
variety of access networks.
For 1): ISPs reallocate IPv4 subnets within their infrastructure from
time to time, partly to ensure the efficient usage of IPv4 addresses
(a scarce resource), and partly to enable efficient route tables
within their network routers. The frequency of these "renumbering
events" depend on the growth in number of subscribers and the
availability of address space within the ISP. As a result, a
subscriber's household device could retain an IPv4 address for as
short as a few minutes, or for months at a time or even longer.
Some folks have suggested that ISPs providing ALTO services could
sub-divide their subscribers' devices into different IPv4 subnets
(or certain IPv4 address ranges) based on the purchased service
tier, as well as based on the location in the network topology.
The problem is that this sub-allocation of IPv4 subnets tends to
decrease the efficiency of IPv4 address allocation. A growing ISP
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that needs to maintain high efficiency of IPv4 address utilization
may be reluctant to jeopardize their future acquisition of IPv4
address space.
However, this is not an issue for map-based approaches if changes are
applied in the order of days.
For 2): ISPs can use techniques, such as ODAP (XXX) that allow the
reallocation of IP prefixes on very short notice, i.e., within
minutes. An IP prefix that has no IP address assignment to a host
anymore can be reallocate to areas where there is currently a high
demand for IP addresses.
For 3): In DSL-based access networks, IP prefixes are assigned to
DSLAMs which are the first IP-hop in the access-network between the
CPE and the Internet. The access-network between CPE and DSLAM
(called aggregation network) can have varying characteristics (and
thus associated costs), but still using the same IP prefix. For
instance one IP addresses IP11 out of a IP prefix IP1 can be assigned
to a VDSL (e.g., 2 MBit/s uplink) access-line while the subsequent IP
address IP12 is assigned to a slow ADSL line (e.g., 128 kbit/s
uplink). These IP addresses are assigned on a first come first
served basis, i.e., the a single IP address out of the same IP prefix
can change its associated costs quite fast. This may not be an issue
with respect to the used upstream provider (thus the cross ISP
traffic) but depending on the capacity of the aggregation-network
this may raise to an issue.
7.2. Limitiations of Non-Map-based Approaches
The specification of the ALTO protocol [I-D.ietf-alto-protocol] uses,
amongst others mechanism, a mechanism called Endpoint Cost Service.
ALTO clients can ask guidance for specific IP addresses to the ALTO
server. However, asking for IP addresses, asking with long lists of
IP addresses, and asking quite frequent may overload the ALTO server.
The server has to rank each received IP address which causes load at
the server. This may be amplified by the fact that not only a single
ALTO client is asking for guidance, but a larger number of them.
Caching of IP addresses at the ALTO client or the usage of the H12
approach [I-D.kiesel-alto-h12] in conjunction with caching may lower
the query load on the ALTO server.
7.3. General Challenges
An ALTO server stores information about preferences (e.g., a list of
preferred autonomous systems, IP ranges, etc) and ALTO clients can
retrieve these preferences. However, there are basically two
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different approaches on where the preferences are actually processed:
1. The ALTO server has a list of preferences and clients can
retrieve this list via the ALTO protocol. This preference list
can be partially updated by the server. The actual processing of
the data is done on the client and thus there is no data of the
client's operation revealed to the ALTO server .
2. The ALTO server has a list of preferences or preferences
calculated during runtime and the ALTO client is sending
information of its operation (e.g., a list of IP addresses) to
the server. The server is using this operational information to
determine its preferences and returns these preferences (e.g., a
sorted list of the IP addresses) back to the ALTO client.
Approach 1 (we call it H1) has the advantage (seen from the client)
that all operational information stays within the client and is not
revealed to the provider of the server. On the other hand, does
approach 1 require that the provider of the ALTO server, i.e., the
network operator, reveals information about its network structure
(e.g., AS numbers, IP ranges, topology information in general) to the
ALTO client.
Approach 2 (we call it H2) has the advantage (seen from the operator)
that all operational information stays with the ALTO server and is
not revealed to the ALTO client. On the other hand, does approach 2
require that the clients send their operational information to the
server.
Both approaches have their pros and cons and are extensively
discussed on the ALTO mailing list. But there is basically a
dilemma: Approach 1 is seen as the only working solution by peer-to-
peer software vendors and approach 2 is seen as the only working by
the network operators. But neither the software vendors nor the
operators seem to willing to change their position. However, there
is the need to get both sides on board, to come to a solution.
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8. Extensions to the ALTO Protocol
8.1. Host Group Descriptors
Host group descriptors are used in the ALTO client protocol to
describe the location of a host in the network topology. The ALTO
client protocol specification defines a basic set of host group
descriptor types, which have to be supported by all implementations,
and an extension procedure for adding new descriptor types . The
following list gives an overview on further host group descriptor
types that have been proposed in the past, or which are in use by
ALTO-related prototype implementations. This list is not intended as
normative text. Instead, the only purpose of the following list is
to document the descriptor types that have been proposed so far, and
to solicit further feedback and discussion:
o Autonomous System (AS) number
o Protocol-specific group identifiers, which expand to a set of IP
address ranges (CIDR) and/or AS numbers. In one specific solution
proposal, these are called Partition ID (PID).
8.2. Rating Criteria
Rating criteria are used in the ALTO client protocol to express
topology- or connectivity-related properties, which are evaluated in
order to generate the ALTO guidance. The ALTO client protocol
specification defines a basic set of rating criteria, which have to
be supported by all implementations, and an extension procedure for
adding new criteria . The following list gives an overview on
further rating criteria that have been proposed in the past, or which
are in use by ALTO-related prototype implementations. This list is
not intended as normative text. Instead, the only purpose of the
following list is to document the rating criteria that have been
proposed so far, and to solicit further feedback and discussion:
8.2.1. Distance-related Rating Criteria
o Relative topological distance: relative means that a larger
numerical value means greater distance, but it is up to the ALTO
service how to compute the values, and the ALTO client will not be
informed about the nature of the information. One way of
generating this kind of information MAY be counting AS hops, but
when querying this parameter, the ALTO client MUST NOT assume that
the numbers actually are AS hops.
o Absolute topological distance, expressed in the number of
traversed autonomous systems (AS).
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o Absolute topological distance, expressed in the number of router
hops (i.e., how much the TTL value of an IP packet will be
decreased during transit).
o Absolute physical distance, based on knowledge of the approximate
geolocation (continent, country) of an IP address.
8.2.2. Charging-related Rating Criteria
o Traffic volume caps, in case the Internet access of the resource
consumer is not charged by "flat rate". For each candidate
resource provider, the ALTO service could indicate the amount of
data that may be transferred from/to this resource provider until
a given point in time, and how much of this amount has already
been consumed. Furthermore, it would have to be indicated how
excess traffic would be handled (e.g., blocked, throttled, or
charged separately at an indicated price). The interaction of
several applications running on a host, out of which some use this
criterion while others don't, as well as the evaluation of this
criterion in resource directories, which issue ALTO queries on
behalf of other peers, are for further study.
8.2.3. Performance-related Rating Criteria
The following rating criteria are subject to the remarks below.
o The minimum achievable throughput between the resource consumer
and the candidate resource provider, which is considered useful by
the application (only in ALTO queries), or
o An arbitrary upper bound for the throughput from/to the candidate
resource provider (only in ALTO responses). This may be, but is
not necessarily the provisioned access bandwidth of the candidate
resource provider.
o The maximum round-trip time (RTT) between resource consumer and
the candidate resource provider, which is acceptable for the
application for useful communication with the candidate resource
provider (only in ALTO queries), or
o An arbitrary lower bound for the RTT between resource consumer and
the candidate resource provider (only in ALTO responses). This
may be, for example, based on measurements of the propagation
delay in a completely unloaded network.
The ALTO client MUST be aware, that with high probability, the actual
performance values differ significantly from these upper and lower
bounds. In particular, an ALTO client MUST NOT consider the "upper
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bound for throughput" parameter as a permission to send data at the
indicated rate without using congestion control mechanisms.
The discrepancies are due to various reasons, including, but not
limited to the facts that
o the ALTO service is not an admission control system
o the ALTO service may not know the instantaneous congestion status
of the network
o the ALTO service may not know all link bandwidths, i.e., where the
bottleneck really is, and there may be shared bottlenecks
o the ALTO service may not know whether the candidate peer itself is
overloaded
o the ALTO service may not know whether the candidate peer throttles
the bandwidth it devotes for the considered application
o the ALTO service may not know whether the candidate peer will
throttle the data it sends to us (e.g., because of some fairness
algorithm, such as tit-for-tat)
Because of these inaccuracies and the lack of complete, instantaneous
state information, which are inherent to the ALTO service, the
application must use other mechanisms (such as passive measurements
on actual data transmissions) to assess the currently achievable
throughput, and it MUST use appropriate congestion control mechanisms
in order to avoid a congestion collapse. Nevertheless, these rating
criteria may provide a useful shortcut for quickly excluding
candidate resource providers from such probing, if it is known in
advance that connectivity is in any case worse than what is
considered the minimum useful value by the respective application.
8.2.4. Inappropriate Rating Criteria
Rating criteria that SHOULD NOT be defined for and used by the ALTO
service include:
o Performance metrics that are closely related to the instantaneous
congestion status. The definition of alternate approaches for
congestion control is explicitly out of the scope of ALTO.
Instead, other appropriate means, such as using TCP based
transport, have to be used to avoid congestion.
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9. API between ALTO Client and Application
This sections gives some informational guidance on how the interface
between the actual application using the ALTO guidance and the ALTO
client can look like.
This is still TBD.
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10. Security Considerations
The ALTO protocol itself, as well as, the ALTO client and server
raise new security issues beyond the one mentioned in
[I-D.ietf-alto-protocol] and issues related to message transport over
the Internet. For instance, Denial of Service (DoS) is of interest
for the ALTO server and also for the ALTO client. A server can get
overloaded if too many TCP requests hit the server, or if the query
load of the server surpasses the maximum computing capacity. An ALTO
client can get overloaded if the responses from the sever are, either
intentionally or due to an implementation mistake, too large to be
handled by that particular client.
10.1. Information Leakage from the ALTO Server
The ALTO server will be provisioned with information about the owning
ISP's network and very likely also with information about neighboring
ISPs. This information (e.g., network topology, business relations,
etc) is consider to be confidential to the ISP and must not be
revealed.
The ALTO server will naturally reveal parts of that information in
small doses to peers, as the guidance given will depend on the above
mentioned information. This is seen beneficial for both parties,
i.e., the ISP's and the peer's. However, there is the chance that
one or multiple peers are querying an ALTO server with the goal to
gather information about network topology or any other data
considered confidential or at least sensitive. It is unclear whether
this is a real technical security risk or whether this is more a
perceived security risk.
10.2. ALTO Server Access
Depending on the use case of ALTO, several access restrictions to an
ALTO server may or may not apply. For an ALTO server that is solely
accessible by peers from the ISP network (as shown in Figure 12), for
instance, the source IP address can be used to grant only access from
that ISP network to the server. This will "limit" the number of
peers able to attack the server to the user's of the ISP (however,
including botnet computers).
On the other hand, if the ALTO server has to be accessible by parties
not located in the ISP's network (see Figure Figure 11), e.g., by a
third-party tracker or by a CDN system outside the ISP's network, the
access restrictions have to be more loose. In the extreme case,
i.e., no access restrictions, each and every host in the Internet can
access the ALTO server. This might no the intention of the ISP, as
the server is not only subject to more possible attacks, but also on
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the load imposed to the server, i.e., possibly more ALTO clients to
serve and thus more work load.
10.3. Faking ALTO Guidance
It has not yet been investigated how a faked or wrong ALTO guidance
by an ALTO server can impact the operation of the network and also
the peers.
Here is a list of examples how the ALTO guidance could be faked and
what possible consequences may arise:
Sorting An attacker could change to sorting order of the ALTO
guidance (given that the order is of importance, otherwise the
ranking mechanism is of interest), i.e., declaring peers located
outside the ISP as peers to be preferred. This will not pose a
big risk to the network or peers, as it would mimic the "regular"
peer operation without traffic localization, apart from the
communication/processing overhead for ALTO. However, it could
mean that ALTO is reaching the opposite goal of shuffling more
data across ISP boundaries, incurring more costs for the ISP.
Preference of a single peer A single IP address (thus a peer) could
be marked as to be preferred all over other peers. This peer can
be located within the local ISP or also in other parts of the
Internet (e.g., a web server). This could lead to the case that
quite a number of peers to trying to contact this IP address,
possibly causing a Denial of Service (DoS) attack.
This section is solely giving a first shot on security issues related
to ALTO deployments.
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11. Conclusion
This is the first version of the deployment considerations and for
sure the considerations are yet incomplete and imprecise.
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12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3568] Barbir, A., Cain, B., Nair, R., and O. Spatscheck, "Known
Content Network (CN) Request-Routing Mechanisms",
RFC 3568, July 2003.
12.2. Informative References
[I-D.ietf-alto-protocol]
Penno, R., Alimi, R., and Y. Yang, "ALTO Protocol",
draft-ietf-alto-protocol-10 (work in progress),
October 2011.
[I-D.ietf-alto-reqs]
Kiesel, S., Previdi, S., Stiemerling, M., Woundy, R., and
Y. Yang, "Application-Layer Traffic Optimization (ALTO)
Requirements", draft-ietf-alto-reqs-13 (work in progress),
January 2012.
[I-D.ietf-alto-server-discovery]
Kiesel, S., Stiemerling, M., Schwan, N., and M. Scharf,
"ALTO Server Discovery",
draft-ietf-alto-server-discovery-02 (work in progress),
September 2011.
[I-D.kamei-p2p-experiments-japan]
Kamei, S., Momose, T., Inoue, T., and T. Nishitani, "ALTO-
Like Activities and Experiments in P2P Network Experiment
Council", draft-kamei-p2p-experiments-japan-06 (work in
progress), September 2011.
[I-D.kiesel-alto-h12]
Kiesel, S. and M. Stiemerling, "ALTO H12",
draft-kiesel-alto-h12-02 (work in progress), March 2010.
[I-D.lee-alto-chinatelecom-trial]
Li, K. and G. Jian, "ALTO and DECADE service trial within
China Telecom", draft-lee-alto-chinatelecom-trial-03 (work
in progress), October 2011.
[I-D.penno-alto-cdn]
Penno, R., Medved, J., Alimi, R., Yang, R., and S.
Previdi, "ALTO and Content Delivery Networks",
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draft-penno-alto-cdn-03 (work in progress), March 2011.
[I-D.vandergaast-edns-client-ip]
Contavalli, C., Gaast, W., Leach, S., and D. Rodden,
"Client IP information in DNS requests",
draft-vandergaast-edns-client-ip-01 (work in progress),
May 2010.
[RFC5632] Griffiths, C., Livingood, J., Popkin, L., Woundy, R., and
Y. Yang, "Comcast's ISP Experiences in a Proactive Network
Provider Participation for P2P (P4P) Technical Trial",
RFC 5632, September 2009.
[RFC5693] Seedorf, J. and E. Burger, "Application-Layer Traffic
Optimization (ALTO) Problem Statement", RFC 5693,
October 2009.
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Appendix A. Contributors List and Acknowledgments
This memo is the result of contributions made by several people, such
as:
o Xianghue Sun, Lee Kai, and Richard Yang contributed Section 3 and
Section 6.3.
o Stefano Previdi contributed Section Section 5 on "Using ALTO for
CDNs".
Martin Stiemerling is partially supported by the COAST project
(COntent Aware Searching, retrieval and sTreaming,
http://www.coast-fp7.eu), a research project supported by the
European Commission under its 7th Framework Program (contract no.
248036). The views and conclusions contained herein are those of the
authors and should not be interpreted as necessarily representing the
official policies or endorsements, either expressed or implied, of
the COAST project or the European Commission.
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Authors' Addresses
Martin Stiemerling (editor)
NEC Laboratories Europe
Kurfuerstenanlage 36
Heidelberg 69115
Germany
Phone: +49 6221 4342 113
Fax: +49 6221 4342 155
Email: martin.stiemerling@neclab.eu
URI: http://ietf.stiemerling.org
Sebastian Kiesel (editor)
University of Stuttgart, Computing Center
Allmandring 30
Stuttgart 70550
Germany
Email: ietf-alto@skiesel.de
Stefano Previdi
Cisco Systems, Inc.
Via Del Serafico 200
Rome 00191
IT
Email: sprevidi@cisco.com
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