ALTO N. Schwan
Internet-Draft B. Roome
Intended status: Standards Track Alcatel-Lucent Bell Labs
Expires: June 3, 2012 December 1, 2011
ALTO Incremental Updates
draft-schwan-alto-incr-updates-00
Abstract
The goal of Application-Layer Traffic Optimization (ALTO) is to
bridge the gap between network and applications by provisioning
network related information. This allows applications to make
informed decisions, for example when selecting a target host from a
set of candidates.
Therefore an ALTO server provides network and cost maps to its
clients. This draft discusses options on how to provide incremental
updates for these maps, with the goal of reducing the amount of data
needed for transmitting the maps.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on June 3, 2012.
Copyright Notice
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document authors. All rights reserved.
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publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 5
3. Determine Client Map Version . . . . . . . . . . . . . . . . . 6
3.1. If-Modified-Since HTTP Header . . . . . . . . . . . . . . 6
3.2. Version-based incremental updates . . . . . . . . . . . . 7
3.2.1. CURRENT NETWORK MAP vtag . . . . . . . . . . . . . . . 8
3.2.2. Extensions to full cost-map response: . . . . . . . . 8
4. Incremental Update Options . . . . . . . . . . . . . . . . . . 10
4.1. Send entire map . . . . . . . . . . . . . . . . . . . . . 10
4.2. Patch map . . . . . . . . . . . . . . . . . . . . . . . . 10
4.3. Encode map . . . . . . . . . . . . . . . . . . . . . . . . 10
4.4. JSON patch . . . . . . . . . . . . . . . . . . . . . . . . 10
4.5. Send only changed values . . . . . . . . . . . . . . . . . 11
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
6. Security Considerations . . . . . . . . . . . . . . . . . . . 14
7. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 15
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
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1. Introduction
The goal of Application-Layer Traffic Optimization (ALTO) is to
bridge the gap between network and applications by provisioning
network related information. This allows applications to make
informed decisions, for example when selecting a target host from a
set of candidates. Typical applications are file sharing, real-time
communication and live streaming peer-to-peer networks [RFC5693] as
well as Content Distribution Networks
[I-D.jenkins-alto-cdn-use-cases].
The ALTO protocol [I-D.ietf-alto-protocol] is specified as a client-
server protocol based on the HyperText Transfer Protocol (HTTP) and
encoded in JavaScript Object Notation (JSON). An ALTO server
provides services that guide ALTO clients in their decisions. The
Endpoint Property Service allows ALTO clients to look up properties
of endpoints, for example its Netwok Location. The Endpoint Cost
Service allows ALTO server to rank endpoints amongst each other with
respect to numerical or ordinal costs. The Map Service and the Map
Filtering Service allows ALTO client to retrieve full or partial
Network Maps and the associated Cost Maps that are provisioned by an
ALTO server.
The ALTO Network Map contains groupings of endpoints as defined by
the ALTO server. By aggregating multiple endpoints that are close to
one another with respect to their network connectivity a greater
scalability is achieved. Each group of endpoints is associated to a
Network Location identifier called a PID, for example by a list of IP
prefixes that belong to the PID. The ALTO Server then indicates
preferences amongst the PIDs in the Cost Map by defining Path Costs
amongst sets of Netwok Locations.
The size of the Network and Cost Maps depend on the granularity of
the map an ALTO server provides for its clients. While some use
cases allow operators to configure their servers to support only a
small numbers of PIDs, other use cases are expected to require a much
greater accuracy in terms of network locations. In order to avoid
the transmission of the same information in each client request, a
mechanism that allows a server to send incremental updates, in
particular for large Network and Cost Maps, is needed.
The goal of this draft is to list and discuss the different options
that allow such incremental updates of Network and Cost Maps. We
divide the discussion in two larger parts. The first part lists
options that allow a server to anticipate the map a client has
without the transmission of the whole map. The second part then
disusses the options a server has to send incremental updates.
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Comments and discussions about this memo should be directed to the
ALTO working group: alto@ietf.org.
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2. Problem Statement
The ALTO protocol uses Network and Cost Maps to allow ALTO servers
the specification of its own aggregated network view. Essentially
the Network Map contains information on how the endpoints are grouped
together, which is typically done according to their proximity. The
Cost Map contains Path Costs between the network regions defined in
the Network Map. The size of these maps strongly depends on the
scenario an ALTO server is configured for by its operator. While in
some scenarios both maps might only comprise s small number of PIDs,
others need much greater accuracy. For large maps partial updates
might become necessary.
Both map types have slightly different characteristics. Network Maps
in general are expected to be smaller than Cost Maps. As an example,
a Network Map with 5,000 PIDs, each having 10 cidrs will result in a
map with the size of roughly 1.25 megabytes. A Cost Map in contrast
contains a m*n matrix for cost entries. Even for short PID names a
full cost map for 5,000 PIDs takes up to 417 megabytes. Network Maps
are also seen to be less dynamic than Cost Maps. This is due to the
fact that the topology an ALTO server sees changes slower than the
path costs of the network. Another characteristic is that changes to
the Network Map will impact the Cost Map, whereas vice versa this is
presumably not the case. A final discussion on whether partial
updates are useful for both map types is out of the scope of this
document.
The remainder of this document discusses options to allow partial
updates of Network and Cost Maps. Therefore two sections focus on
two seperate problems that need a solution. The first part
(Section 3) discusses how an ALTO client and an ALTO server can
synchronize their map state without transmitting the whole map. This
is needed to identify whether a partial update can be applied and
also to calculate the partial update itself. The second part
(Section 4) of the document discusses how partial updates can be
encoded and sent to the client.
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3. Determine Client Map Version
To allow a server sending incremental updates to a client it first
needs to know what version of the map a client already has. In this
section we discuss options that allow a server to do so without
having a client to send the whole map to the server.
3.1. If-Modified-Since HTTP Header
One possible option is to use the HTTP If-Modified-Since header in
the request if the client has previously contacted the ALTO service
and thus already has a version of the map. Therefore it caches both,
the map as well as the value of the Date header field of the HTTP
response that contained the map. A server can then use the value of
the If-Modified-Since header to compare if the clients current map is
still up-to-date.
The following figure illustrates a GET request for a Network Map
Information Resource. Additionally the client indicates the time
when it retrieved the Network Map the last time in the If-Modified-
Since header field.
GET /networkmap HTTP/1.1
Host: alto.example.com
Accept: application/alto-networkmap+json,application/alto-error+json
If-Modified-Since: Sat, 24 Dec 2011 19:43:31 GMT
Figure 1: If-Modified-Since HTTP Header
A server retrieving this request uses the timestamp provided by the
client to decide whether to send a full map, only partial updates of
the map or no map at all in case there were no changes.
In case the Network Map has not been modified since the time provided
by the client in the request, the server SHOULD reply with a 304 HTTP
response. The client can then cache the updated value of the Date
header field for future requests.
304 Not Modified
Date: Sun, 25 Dec 2011 09:33:31 GMT
Figure 2: HTTP Response 304
In case the server determines that the timestamp provided by the
client is out-of-date and cannot be reused for a partial update it
returns the full Network Map, as defined in the ALTO core protocol
specification. The following figure shows the mandatory Date header
field in the HTTP response that is used as a timestamp for the map.
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HTTP/1.1 200 OK
Content-Length: [TODO]
Content-Type: application/alto-networkmap+json
Date: Sun, 25 Dec 2011 09:33:31 GMT
{
"meta" : {},
"data" : {
"map-vtag" : "1266506139",
"map" : {
"PID1" : {
"ipv4" : [
"192.0.2.0/24",
"198.51.100.0/25"
]
},
"PID2" : {
"ipv4" : [
"198.51.100.128/25"
]
},
"PID3" : {
"ipv4" : [
"0.0.0.0/0"
],
"ipv6" : [
"::/0"
]
}
}
}
}
Figure 3: Full Network Map HTTP Response
In case the server determines that there was a change to the Network
Map the server MAY choose not to send the full map, but a partial
update only. Options for sending these partial updates are discussed
in Section 4.
3.2. Version-based incremental updates
With this approach, clients poll the ALTO server for changes. The
server provides hints as to the polling frequency. We propose two
different mechanisms in the ALTO message format, one for network-map
changes, the other for cost-map changes.
For network-map changes, add a new GET-mode request, "CURRENT NETWORK
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MAP VTAG." The response is short and simple: just the current map-
vtag and a hint about how often the network-map might change. Once a
client has the full network map, the client periodically sends that
CURRENT VTAG request to the server. If the map-vtag changes, the
client re-gets the network map. For cost-map changes, add two new
fields to the full cost-map response: a "cost-map-vtag" and a hint
about the how often the server updates the cost map.
Using these vtags both client and server can determine if it is
necessary to request or to send an updated map, a full map, or if the
current version is still up-to-date.
3.2.1. CURRENT NETWORK MAP vtag
This is a GET-mode request. The response is a simple json structure
with
o The current map-vtag for the network map.
o The average number of seconds between changes to the network map.
It needs a new media type, say application/alto-currentmapvtag+json
For example,
HTTP/1.1 200 OK
Content-Length: [TODO]
Content-Type: application/alto-currentmapvtag+json
{
"meta" : {},
"data" : {
"map-vtag" : "123456",
update-interval: 86400
}
}
Figure 4: CURRENT NETWORK MAP vtag
3.2.2. Extensions to full cost-map response:
Add two new fields to the costmap response, as in:
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object {
CostMode cost-mode;
CostType cost-type;
VersionTag map-vtag;
VersionTag cost-map-vtag; // Optional
JSONNumber update-interval; // Optional
CostMapData map;
} InfoResourceCostMap;
Figure 5: Extensions to full cost-map response
cost-map-vtag: A string that (together with the network map-vtag)
uniquely identifies this version of the cost map.
update-interval: Average time between cost-map updates, in seconds.
(A hint, not a guarantee). Perhaps required if cost-map-vtag is
present
These fields would only be in the full cost map response, not in a
filtered cost map response.
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4. Incremental Update Options
Once a server has decided to send a partial update only there are
several ways to do so. These section discusses these response
options.
4.1. Send entire map
One trivial option is always to send the entire map anyways. The
advantage of sending the whole map typically is that there is no
computational effort needed on the server side. Thus this can always
be a fallback in case the server is under load, or in case a partial
update appears to be not inefficient.
4.2. Patch map
A server that knows the version of the map a client currently has can
use this information to calculate the contextual diff to the newest
version of the map. This can also be done in a batch process for all
previous versions once a new map is loaded on the server to avoid a
per request calculation. The diff output can then be sent in a
response to the client, which in turn can use it to patch its version
of the map. By doing this the newest version of the map can be
recreated.
4.3. Encode map
One major goal of applying partial updates is to reduce transmission
time by reducing the amount of data which is to be transferred to the
client. This goal can be achieved by applying compression
techniques, such as gzip, to the message content, both for partial
updates as well as for entire maps.
HTTP supports this by the Content-Encoding entity header field. The
advantage of using compression is that there is no need to change the
underlying media-type of the reponse. Typically not all ALTO clients
will support this optimization from the beginning, thus the server
will need to store two representations of the maps: One which is
compressed and one uncompressed.
4.4. JSON patch
JSON Patch [I-D.pbryan-json-patch] defines a JSON document structure
that allows partial modifications to a JSON document and defines the
associated media type "application/json-patch". Therefore JSON Patch
is a suitable option for incremental udates of the Network and Cost
Maps. JSON patch supports add, remove and replace operations that
can be used in combination with JSON Pointers
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[I-D.pbryan-zyp-json-pointer] to modify values and arrays of the JSON
document members.
Typically JSON Patch is used in combination with the HTTP PATCH
method [RFC5789] to partially modify existing resources on a server.
As an ALTO client is not modifying a resource, but wants to be
updated if the resource has changed it needs to signal to the server
that it is able to receive and understand JSON Patch updates. This
can be done by including the media type "application/json-patch" in
the Accept header field of the HTTP request.
The following figure illustrates one example where the server decides
to send a partial update to the client using JSON Patch. The server
indicates this in the reponse Content-Type header. In the following
example the Network Map from the example in Figure 3 above has
changed. The map-vtag element has been incremented by 1, which
results in a replace operation for the respective element containing
the new value. Also two new subnets are added to the Network Map in
PID1 and PID2 by two add operations at the indexes 1 and 0 of the
ipv4 arrays.
HTTP/1.1 200 OK
Content-Length: [TODO]
Content-Type: application/json-patch
Date: Sun, 25 Dec 2011 09:33:31 GMT
{ "replace": "/data/map-vtag", "value": "1266506140" },
{ "add": "/data/map/PID1/ipv4/1", "value": "198.51.200.0/25" },
{ "add": "/data/map/PID2/ipv4/0", "value": "198.51.200.128/25" }
Figure 6: Partial update with JSON Patch
4.5. Send only changed values
Another option is to offer a dedicated ALTO service for partial
updates. A client that detemines that its current map is out-of-
date, for example by comparing cost-map-vtag values can then query
this service to retrieve the partial update.
This service can be implemented in a new POST-mode request, "GET
COST-MAP CHANGES". The POST data would have a cost-map-vtag from a
previous cost-map response. The response would be the same as for a
full costmap request, except that it only has costs that have changed
since the specified cost-map-vtag. The response would contain the
current cost-map-vtag and update-interval.
If the current cost-map-vtag is the same as the specified one -- that
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is, if there are no changes -- the "map" entry in the response has no
cost entries.
If the specified cost-map-vtag is invalid, or if it's sufficiently
old that the server no longer knows what changed since that version,
the server returns a complete cost map. Thus the response MUST have
costs that changed since the specified version, but MAY have other
costs as well.
A possible implementation strategy for this service is to maintain at
the server for each cost type an "instantaneous" cost map and the
last N "numbered versions" of those costs. For a full-map requests
without a version number, the server returns the most recent
numbered-version map. For a COST-MAP CHANGES request, the server
finds the client's version number in its stack of saved versions, and
returns the diffs between that version and the most recent numbered
version. If the server doesn't have the old version any more, the
server returns the full latest version.
Then periodically the server copies the instantaneous map to the
last-stable map, assigns it a new version number, shuffles the list
down, and discards the oldest version. The server could cache diffs
and discard old ones.
For filtered cost-map requests, the server could use either the
"instantaneous" cost map or the most recent "versioned" cost map.
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5. IANA Considerations
None.
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6. Security Considerations
To be done in later versions of this document.
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7. Conclusion
This document describes different options that can be applied to
support incremental updates of ALTO Network and Cost maps. In
particular it comprises option for client and server to synchronize
themselves about their current map state, and further includes
options on how to encode partial updates.
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8. References
[I-D.ietf-alto-protocol]
Alimi, R., Penno, R., and Y. Yang, "ALTO Protocol",
draft-ietf-alto-protocol-10 (work in progress),
October 2011.
[I-D.jenkins-alto-cdn-use-cases]
Niven-Jenkins, B., Watson, G., Bitar, N., Medved, J., and
S. Previdi, "Use Cases for ALTO within CDNs",
draft-jenkins-alto-cdn-use-cases-01 (work in progress),
June 2011.
[I-D.pbryan-json-patch]
Bryan, P., "JSON Patch", draft-pbryan-json-patch-02 (work
in progress), October 2011.
[I-D.pbryan-zyp-json-pointer]
Bryan, P. and K. Zyp, "JSON Pointer",
draft-pbryan-zyp-json-pointer-02 (work in progress),
October 2011.
[RFC5693] Seedorf, J. and E. Burger, "Application-Layer Traffic
Optimization (ALTO) Problem Statement", RFC 5693,
October 2009.
[RFC5789] Dusseault, L. and J. Snell, "PATCH Method for HTTP",
RFC 5789, March 2010.
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Appendix A. Acknowledgments
The authors would like to thank Vijay Gurbani for his valuable input
and excellent feedback to this document.
Nico Schwan is partially supported by the ENVISION project
(http://www.envision-project.org), a research project supported by
the European Commission under its 7th Framework Program (contract no.
248565). The views and conclusions contained herein are those of the
authors and should not be interpreted as necessarily representing the
official policies or endorsements, either expressed or implied, of
the ENVISION project or the European Commission.
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Authors' Addresses
Nico Schwan
Alcatel-Lucent Bell Labs
Lorenzstrasse 10
Stuttgart 70435
Germany
Email: nico.schwan@alcatel-lucent.com
URI: www.alcatel-lucent.com/bell-labs
Bill Roome
Alcatel-Lucent Bell Labs
Email: w.roome@alcatel-lucent.com
URI: www.alcatel-lucent.com/bell-labs
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