ALTO Incremental Updates
draft-schwan-alto-incr-updates-01

ALTO                                                           N. Schwan
Internet-Draft                                                  W. Roome
Intended status: Standards Track                Alcatel-Lucent Bell Labs
Expires: September 13, 2012                               March 12, 2012

                        ALTO Incremental Updates
                   draft-schwan-alto-incr-updates-01

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.

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Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 13, 2012.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Problem Statement  . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Determine Client Map Version . . . . . . . . . . . . . . . . .  7
     3.1.  HTTP . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
       3.1.1.  If-Modified-Since HTTP Header  . . . . . . . . . . . .  7
       3.1.2.  If-None-Match HTTP Header  . . . . . . . . . . . . . .  9
     3.2.  Version-based incremental updates as ALTO extension  . . . 10
       3.2.1.  CURRENT NETWORK MAP vtag . . . . . . . . . . . . . . . 10
       3.2.2.  Extensions to full cost-map response:  . . . . . . . . 11
   4.  Incremental Update Options . . . . . . . . . . . . . . . . . . 12
     4.1.  Send entire map  . . . . . . . . . . . . . . . . . . . . . 12
     4.2.  Patch map  . . . . . . . . . . . . . . . . . . . . . . . . 12
     4.3.  Encode map . . . . . . . . . . . . . . . . . . . . . . . . 12
     4.4.  JSON patch . . . . . . . . . . . . . . . . . . . . . . . . 12
     4.5.  Send only changed values . . . . . . . . . . . . . . . . . 14
   5.  Partial Update Capability  . . . . . . . . . . . . . . . . . . 15
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 18
   8.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 19
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 21
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22

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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.  The
   draft is structured as follows: First it lists options that allow a
   server to validate the cached version of a map a client has.  Then it
   disusses the options a server has to send incremental updates.
   Finally it introduces an information resource capability that allows
   servers to advertise which ALTO services are able to handle partial

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   updates.

   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 n*n matrix for cost entries where n is the number of PIDs.
   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 check the version of a cached map a client already has.  In
   this section we discuss options for this validation.  We focus our
   discussion on approaches where the client polls the ALTO server for
   map changes and the server decides based on the client request.
   Therefore we first disuss HTTP built-in options and follow-up with a
   possible extension to ALTO network and cost maps themselves.

3.1.  HTTP

   HTTP [RFC2616] provides request-header fields to express conditional
   requests.  Typically these conditional requests are used by caches to
   decide whether a copy of a resource they have can be served to a
   requesting client directly or not.

   Note that it has to be checked if partial updates as reponse on HTTP
   conditional requests are conform with the HTTP specification.

3.1.1.  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 the server
   includes date and time when the map was last modified into the Last-
   Modified entity-header for a particular map, which is cached by the
   client together with the map and sends it in new requests for the
   same map in the If-Modified-Since header.

   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

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   response.  The client then caches the updated value of the Last-
   Modified entity-header for future requests.

       304 Not Modified
       Last-Modified: 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 Last-Modified entity-
   header in the HTTP response that is used as a timestamp for the map
   as well as the ETag header for a If-None-Match request, as explained
   in section Section 3.1.2.

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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
      ETag: "nm000012"

      {
        "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.

3.1.2.  If-None-Match HTTP Header

   A second HTTP based mechanism could employ ETags in combination with
   the If-None-Match header.  Here the server creates entity tags that
   identify the version of a map.  A client that caches a map includes
   this identifier in every future request.  The server can use this
   ETag to calculate the necessary partial update for both map versions.

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   The following example illustrates a client GET request which includes
   an ETag, identifying a network map.  The example assumes the client
   received the Network Map response in Figure 3.

    GET /networkmap HTTP/1.1
    Host: alto.example.com
    Accept: application/alto-networkmap+json,application/alto-error+json
    If-None-Match: "nm000012"

                    Figure 4: If-None-Match HTTP Header

3.2.  Version-based incremental updates as ALTO extension

   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
   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,

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      HTTP/1.1 200 OK
      Content-Length: [TODO]
      Content-Type: application/alto-currentmapvtag+json

      {
        "meta" : {},
        "data" : {
          "map-vtag" : "123456",
          update-interval: 86400
        }
      }

                    Figure 5: CURRENT NETWORK MAP vtag

3.2.2.  Extensions to full cost-map response:

   Add two new fields to the costmap response, as in:

      object {
        CostMode    cost-mode;
        CostType    cost-type;
        VersionTag  map-vtag;
        VersionTag  cost-map-vtag;     // Optional
        JSONNumber  update-interval;   // Optional
        CostMapData map;
      } InfoResourceCostMap;

              Figure 6: 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.  This 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 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.

   Although JSON Patch permits pointers to index individual array
   elements, that's potentially ambiguous.  "Add" and "delete" change
   the array indexes; do subsequent updates to that array refer to the
   original indexes or the revised indexes?  And even if that's well-
   specified, it's a potential source of error.  Furthermore, some
   clients may store the CIDRs in a PID as a set, rather than an array,
   so they don't keep the original index numbers.

   To avoid these problems, we recommend that when updating the CIDRs
   for a PID, the server replaces the full array value for that PID,
   rather than updating individual CIDRs by index.

   This may also simplify the server, because it just has to flag the
   PID as having changed; it doesn't have to remember the previous
   sequence of CIDRs.

   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 replace operations on 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" },
      { "replace": "/data/map/PID1/ipv4", "value":
              ["192.0.2.0/24", "198.51.200.0/25", "198.51.100.0/25"] },
      { "replace": "/data/map/PID2/ipv4", "value":
              ["198.51.100.128/25", "198.51.200.128/25"] }

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                 Figure 7: 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
   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.  Partial Update Capability

   An ALTO server needs to advertise its ability of providing
   incremental updates to a client.  One option of doing this is by
   including an ALTO information resource capability indicating the
   partial update option.  This can be done per ALTO service, which
   allows a fine grained control over which URIs handle partial
   requests.

   The following Information Resource Directory illustrates one example
   where network and costmaps are available with and without partial
   update option respectively.  This is expressed by the JSONBool
   capability element "partial-update".

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    GET /directory HTTP/1.1
    Host: alto.example.com
    Accept: application/alto-directory+json,application/alto-error+json

    HTTP/1.1 200 OK
    Content-Length: [TODO]
    Content-Type: application/alto-directory+json

    {
      "resources" : [
        {
          "uri" : "http://alto.example.com/serverinfo",
          "media-types" : [ "application/alto-serverinfo+json" ]
        }, {
          "uri" : "http://alto.example.com/networkmap",
          "media-types" : [ "application/alto-networkmap+json" ]
        }, {
          "uri" : "http://alto.example.com/costmap/rcost",
          "media-types" : [ "application/alto-costmap+json" ],
          "capabilities" : {
            "cost-modes" : [ "numerical" ],
            "cost-types" : [ "routingcost" ]
          }
        }, {
          "uri" : "http://alto.example.com/partialupdate/networkmap",
          "media-types" : [ "application/alto-networkmap+json" ]
          "capabilities" : {
            "partial-update" : true,
          }
        }, {
          "uri" : "http://alto.example.com/partialupdate/costmap/rcost",
          "media-types" : [ "application/alto-costmap+json" ],
          "capabilities" : {
             "partial-update" : true,
             "cost-modes" : [ "numerical" ],
             "cost-types" : [ "routingcost" ]
          }
        }
        }
      ]
    }

               Figure 8: IRD With Partial Update Capability

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6.  IANA Considerations

   None.

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7.  Security Considerations

   To be done in later versions of this document.

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8.  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.  Finally it proposes an new
   parial update capability for the Information Resource Directory.

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9.  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.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [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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