Internet DRAFT - draft-madhavan-alto-caching
draft-madhavan-alto-caching
Internet Engineering Task Force S. Madhavan
Internet-Draft R. Nandiraju
Intended status: Informational Huawei Technologies
Expires: August 1, 2013 February 1, 2013
ALTO Caching
draft-madhavan-alto-caching-01
Abstract
The specification of the ALTO protocol uses map based approaches
assuming that the information provided is static for a longer period
of time. The purpose of this memo is to provide a mechanism where
ALTO clients cache the map information and the servers provide the
expiration time for invalidation.
Status of this Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3
3. Protocol Structure . . . . . . . . . . . . . . . . . . . . . . 3
4. Overall Operation . . . . . . . . . . . . . . . . . . . . . . 4
5. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 5
6. ALTO Types . . . . . . . . . . . . . . . . . . . . . . . . . . 5
6.1. EndpointAddrGroup . . . . . . . . . . . . . . . . . . . . 5
6.2. NetworkMapData . . . . . . . . . . . . . . . . . . . . . . 5
7. ALTO caching and Consistency . . . . . . . . . . . . . . . . . 5
7.1. Server behavior . . . . . . . . . . . . . . . . . . . . . 5
7.2. Client behavior . . . . . . . . . . . . . . . . . . . . . 6
7.3. Cache Consistency Approaches . . . . . . . . . . . . . . . 6
7.4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.4.1. Network Map with Expiry Example . . . . . . . . . . . 8
7.4.2. Filtered Network Map with Expiry Example . . . . . . . 9
8. Transport Considerations . . . . . . . . . . . . . . . . . . . 9
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
10. Security Considerations . . . . . . . . . . . . . . . . . . . 10
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1. Normative References . . . . . . . . . . . . . . . . . . . 10
11.2. Informative References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
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1. Introduction
The goal of Application-Layer Traffic Optimization (ALTO) is to
provide guidance to applications, which have to select one or several
hosts from a set of candidates, that are able to provide a desired
resource [RFC5693]. The requirements for ALTO are itemized in
I-D.ietf-alto-reqs. ALTO is realized by a client-server
protocol.ALTO clients send queries to ALTO servers, in order to
solicit guidance.
This memo defines a mechanism for ALTO clients to cache the map
information and for the servers to provide the expiration time for
invalidation at subset level or map level.
2. Problem Statement
The ALTO protocol uses HTTP for discovering available Information
resources at an ALTO server and retrieving information resources.
The protocol defines Map-based and Non-map based approaches for
retrieving information resources.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.
HTTP protocol already defines mechanisms for caching the content.
There are many limitations in using those mechanisms. HTTP based
caches store the responses only for HTTP GET requests. ALTO queries
can be GET or POST(in case of filtered map) requests. In the case of
POST requests, HTTP caching mechanism cannot be used because
responses for POST are generally not cached.
The validity of the information in the map need not be same for all
the elements in the map and there is currently no mechanism(in HTTP
also) to convey this information. This mechanism has to be
incorporated into the protocol itself.
3. Protocol Structure
The ALTO Protocol uses a simple extensible framework to convey
network information. In the general framework, the ALTO protocol
will convey properties on both Network Locations and the paths
between Network Locations.[I-D.ietf-alto-protocol].
The Map filtering and endpoint property services can be extended to
include the map level and subset level expiration time.
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.--------------------------------------------------------.
| |
|.--------..--------------------------------------------.|
|| || ALTO Info Services ||
|| || .-----------. .----------. .----------. ||
|| || | Map | | Endpoint | | Endpoint | ||
|| || | Filtering | | Property | | Cost | ||
|| || | | | | | | ||
|| || | | | | | | ||
|| || | Service | | Service | | Service | ||
|| Server || `-----------' `----------' `----------' ||
|| Info. || .-------------------------------------. ||
|| Service|| | Map Service | ||
|| || | .-------------. .--------------. | ||
|| || | | Network Map | | Cost Map | | ||
|| || | `-------------' `--------------' | ||
|| || `-------------------------------------' ||
|`--------'`--------------------------------------------'|
| |
`--------------------------------------------------------'
Figure 1: ALTO Protocol Structure
4. Overall Operation
The overall operation is based on the ALTO client caching the map
information. The ALTO server sends the map information providing
details of full or subset level expiration.
ALTO server can choose to use HTTP based caching mechanisms or ALTO
caching mechanism (defined in this document) based on the request.
A typical message flow would be:
ALTO Client ALTO Server
|-----ALTO Query--->| ALTO Query for complete map information
|<---ALTO Response--| ALTO Response with caching related information
| |
Subset level |------ALTO Query-->| ALTO Query for expired maps
validity expires |<-------200--------| ALTO response
| |
Figure 2: Message flow
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5. Definitions
ALTO cache: Any logical entity which caches the ALTO responses for
the purpose of reducing repeated requests by the alto client to the
alto server. The ALTO cache MAY be co-located with the ALTO client
or it can be a separate node
6. ALTO Types
This section details the format for particular data values used for
ALTO caching framework. Base types defined by I-D.ietf-alto-protocol
are used by this memo with some extensions listed below.
6.1. EndpointAddrGroup
EndpointAddrGroup JSON object is extended as below
object {
JSONString expires;
EndpointPrefix [AddressType]<0..*>;
...
} EndpointAddrGroup;
6.2. NetworkMapData
NetworkMapData JSON object is extended as below
object {
JSONString expires;
EndpointAddrGroup [pidname]<0..*>;
...
} NetworkMapData;
7. ALTO caching and Consistency
ALTO servers specify the expiration time of the complete map or the
subset level maps in the response. Expiration time format is an
absolute date and time with GMT as reference. ISPs MAY use any
mechanism to determine the expiration time. This is out of the scope
of this specification.
7.1. Server behavior
ALTO server MAY use HTTP based or ALTO caching mechanism to convey
the expiration time. When the request is HTTP GET,then the ALTO
server MAY use HTTP based caching mechanism. In the case of request
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being POST, ALTO server SHOULD use ALTO caching mechanism. To ensure
consistent behaviour and reduce implementation complexity it is
recommended that the mechanism defined by this memo is used for all
caching scenarios.
7.2. Client behavior
ALTO cache MUST be able to identify expiration of map level or subset
level information. On expiry, cache SHOULD trigger a new request to
the server to refresh the expiration.
7.3. Cache Consistency Approaches
ALTO caches are useful when cache consistency is maintained,that is,
cached copies should be updated when the originals change.There are
two different types of consistency approaches - weak consistency and
strong consistency. Weak consistency is an approach in which stale
documents may be returned to user , while the latter approach ensures
that stale documents are never returned to user.
ALTO caches SHOULD have strong invalidation mechanisms. In the case
of P2P, the stale data can result in the unoptimized peer selection
and resulting in selecting the longest PATH which will defeat the
purpose of ALTO. In the case of cdni(see [I-D.seedorf-i2aex-alto-
cdni-perpective] for details)) there can be service failures as
requests are redirected to wrong/invalid dCDN.
The below table shows comparison of different validation approaches.
Assume that interleave of requests and file modifications happens in
the order RRRMMMRRMRRRMMRM.Let R be the number of times a node views
a resource and RI be the number of intervals during which client
repeatedly asks for resource while resource is changed. It is
evident that notification based mechanisms takes few control messages
and ensures that there is no stale data returned.
The mechanism of notification is proposed in
[draft-madhavan-alto-subscription]
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+-------------------+-----------+--------------+--------------------+
| Messages | Polling | Notification | TTL based approach |
| | everytime | approach | |
| | approach | | |
+-------------------+-----------+--------------+--------------------+
| GET requests | 1 | 1 | 1 |
| If-Modified-Since | R-1 | 0 | TTLmissed - 1 |
| 304 response | R-RI | 0 | TTLmissed - |
| | | | TTLmissedreschange |
| Notification Msg | 0 | 1 Sub + RI * | 0 |
| | | Not | |
| File Transfers | RI | RI | RI - |
| | | | Stalehitintervals |
| Stale document | No | No | Yes |
+-------------------+-----------+--------------+--------------------+
Message contents for consistency approaches
Table 1: Cache consistency tradeoff
7.4. Examples
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7.4.1. Network Map with Expiry Example
GET /networkmap HTTP/1.1
Host: alto.example.com
Accept: application/alto-networkmap+json,application/alto-error+json
HTTP/1.1 200 OK
Content-Length: [TODO]
Content-Type: application/alto-networkmap+json
Cache-Control: no-cache
{
"meta" : {},
"data" : {
"map-vtag" : "1266506139",
"map" : {
"expires": "Wed, 14 Sep 2011 16:00:00 GMT",
"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"
]
}
}
}
}
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7.4.2. Filtered Network Map with Expiry Example
POST /networkmap/filtered HTTP/1.1
Host: custom.alto.example.com
Content-Length: [TODO]
Content-Type: application/alto-networkmapfilter+json
Accept: application/alto-networkmap+json,application/alto-error+json
{
"pids": [ "PID1", "PID2" ]
}
HTTP/1.1 200 OK
Content-Length: [TODO]
Content-Type: application/alto-networkmap+json
{
"meta" : {},
"data" : {
"map-vtag" : "1266506139",
"map" : {
"PID1" : {
"expires": "Wed, 14 Sep 2011 16:00:00 GMT",
"ipv4" : [
"192.0.2.0/24",
"198.51.100.0/24"
]
},
"PID2" : {
"expires": "Wed, 14 Dec 2011 16:00:00 GMT",
"ipv4": [
"198.51.100.128/24"
]
}
}
}
}
8. Transport Considerations
9. Acknowledgements
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10. Security Considerations
TBD
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
11.2. Informative References
[I-D.ietf-alto-deployments]
Stiemerling, M., Kiesel, S., and S. Previdi, "ALTO
Deployment Considerations", draft-ietf-alto-deployments-04
(work in progress), March 2012.
[I-D.ietf-alto-protocol]
Alimi, R., Penno, R., and Y. Yang, "ALTO Protocol",
draft-ietf-alto-protocol-11 (work in progress),
March 2012.
[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-16 (work in progress),
June 2012.
[I-D.seedorf-i2aex-alto-cdni-perpective]
Seedorf, J., "Infrastructure-to-application information
exposure from an ALTO-CDNI Perspective",
draft-seedorf-i2aex-alto-cdni-perpective-00 (work in
progress), March 2012.
[RFC5693] Seedorf, J. and E. Burger, "Application-Layer Traffic
Optimization (ALTO) Problem Statement", RFC 5693,
October 2009.
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Authors' Addresses
Sreekanth Madhavan
Huawei Technologies
Bangalore,
India
Phone:
Email: sreekanth.madhavan@huawei.com
Nandiraju Ravishankar
Huawei Technologies
Bangalore,
India
Phone:
Email: ravi.nandiraju@huawei.com
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