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ALTO                                                             L. Deng
Internet-Draft                                                   W. Chen
Intended status: Informational                                     Q. Yi
Expires: August 17, 2014                                    China Mobile
                                                                Y. Zhang
                                             Chinese Academy of Sciences
                                                       February 13, 2014


        Considerations for ALTO with network-deployed P2P caches
                    draft-deng-alto-p2pcache-03.txt

Abstract

   Uploading from peers located in a public WLAN hotspot has been
   reported to severely impact other current users' experiences, and
   raised caution from the operator's side who are willing to
   increasingly participate in building public WLAN facilities to
   offload the explosive mobile data traffic from cellular networks.
   Cooperation between the network operator and the P2P service
   providers in form of intra-domain P2P caches is expected to be an
   effective mechanism to solve the problem.  This draft introduces
   considerations on ALTO deployment in terms of P2P caches and
   discusses potential extensions to the ALTO protocol to standardize
   this mutual cooperation.

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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   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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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 August 17, 2014.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Forwarding Cache for Wired subscribers  . . . . . . . . .   5
     4.2.  Bidirectional Cache for WLAN subscribers  . . . . . . . .   6
     4.3.  Generalized cache architecture for intra-ISP networks . .   7
   5.  Considerations for ALTO deployment with P2P Caches  . . . . .   8
     5.1.  Forwarding Cache: vertical separator from outsiders . . .   9
     5.2.  Bidirectional Cache: horizontal division within insiders    9
   6.  Open issues . . . . . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  Question: selective caching . . . . . . . . . . . . . . .   9
     6.2.  Discussion: cooperative caching and ALTO extension  . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   P2P applications like file sharing and multimedia streaming are so
   popular that lots of P2P technologies have been increasingly utilized
   throughout the world.  The goal of Application-Layer Traffic
   Optimization (ALTO) [I-D.ietf-alto-protocol] 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.

   Meanwhile, since wireless accesses to Internet have become pervasive
   with widely deployed WLANs, more and more people access Internet
   services via WLAN and the amount of P2P traffic in WLAN is
   explosively growing.  In addition to a huge number of individually
   setup WLANs at homes, there has been an increasing trend for the
   government, organizations, and even traditional network operators to
   set up publicly accessible WLAN facilities.  Even though the service



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   may be free of charge, to use the resources effectively in a fair
   way, and to avoid congestion for the purpose of service availability
   are vital for these public WLANs.

   However, recent statistics reveal that P2P traffic accounts for 80%
   in part of China Mobile's WLANs, and traffic congestion at APs
   (access points) frequently occurs because of P2P applications.  P2P
   traffic in WLANs not only causes problems on their own delivery
   quality, but also degrades the performance of other Internet
   applications in WLANs.

2.  Terminology

   ALTO: application layer traffic optimization.  For ALTO protocol,
   please refer to [I-D.ietf-alto-protocol].

   AP: a wireless access point (or WAP) is a device that allows wireless
   devices to connect to a wired network using WLAN.  The AP usually
   connects to a AC as a standalone device, but it can also be an
   integral component of the router itself.

   AC: a wireless access controller (or WAC) is the network entity that
   provides wire access via APs to the network infrastructure in the
   data plane, control plane, management plane, or a combination
   therein.

   DCP: Distributed coordination function (or DCF) is the fundamental
   MAC technique of the IEEE 802.11 based WLAN standard.  DCF employs a
   CSMA/CA with binary exponential backoff algorithm.

   Forwarding Cache: is a traditional content cache, which caches
   content flows from outside its coverage and serves subsequent
   requestors under its coverage for the content.

   Reverse Cache: is a special content cache proposed for WLAN peers,
   which caches content flows from inside its coverage and serves
   subsequent requestor from outside its coverage for the content on
   behalf of the peers within its coverage.

   Bidirectional Cache: is a combination of a forwarding cache and a
   reverse cache.

   Transparent Cache: is a content cache deployed by network operators
   for third party SPs (e.g. P2P streaming services), which participates
   in the overlay's service provision implicitly via request
   redirection.





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   Cooperative Cache: is a content cache deployed by network operators
   in cooperation with specific content delivering SPs (e.g. P2P
   streaming services), which participates in the overlay's service
   provision explicitly.

3.  Motivation

   On one hand, it is well accepted that compared to fixed networks,
   mobile networks have some special characters, including small link
   bandwidth, high cost, limited radio frequency resource, and terminal
   battery.  Therefore, it is recommended by [I-D.ietf-alto-deployments]
   that 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.

   On the other hand, efforts have been put on using forwarding caches
   to optimize traffic pattern in such scenarios, which demonstrates
   great improvement in user experience and considerable traffic
   reduction at interworking points.  What's more, owing to the
   characteristics of the DCF model in 802.11, there is an constant
   unfairness between uplink and downlink traffic in competing wireless
   channel resources, which leads to downlink congestion in WLAN
   resultant from P2P traffic (which constitutes the major part of
   uploading traffic).  In particular, There is basic indication under
   the current DCF model, that in order to achieve fairness among mobile
   stations, in the long run, each stations has a equal chance of
   getting the wireless slot given they all have a sustainable long
   traffic to send.  In other words, there is an implicit unfairness
   between uplink and downlink traffic under the BSS model, where the AP
   holds the throat of all the downlink traffic and competing with the
   uplink traffic from potentially a large group of other user mobile
   devices.  However, traditional P2P cache (as a forward cache) cannot
   help here, since it does nothing to stop a WLAN peer from uploading.
   Hence, bidirectional caches are proposed, which contains a reverse
   cache as well as a forward cache can be deployed at the AC (Access
   Controller).  The reverse cache can provide uploading service instead
   of the WLAN peers under the AC's coverage.  As a result, the uplink
   bandwidth consumption at each AP can be reduced and the uplink
   congestion can be alleviated effectively.  Simulation results in
   file-sharing scenarios show that, employing cache instead of WLAN
   peers accelerates file transfer by 42% while improving the throughput
   of other Internet applications under the same AP by 28%.

   With deployed P2P caches on the internal network, especially at a
   position as low as the AC-level, it could be sub-optimal to simply



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   use the accessing network type as the divider for different PIDs and
   assign sufficient high cost within the wireless PID to prefer
   accessing remote peers over local peers blindly.

   To this end, this draft discusses the optimal ALTO deployment
   recommendations for a P2P application in terms of a wireless
   accessing network with network-deployed application-agnostic caches.

   In summary, the goal here is to illuminate applications through ALTO
   about these existing network capabilities to make full use of them in
   achieving application performance improvement and network cost
   reduction.

4.  Architecture

4.1.  Forwarding Cache for Wired subscribers

   Fig. 1 illustrates the proposed architecture of a traditional uni-
   directional P2P cache (or Forwarding Cache for short) system for
   wired subscribers, deployed mainly for the purpose of reducing
   interworking P2P traffic.  Forwarding Caches are assumed to be
   deployed at the interworking gateways to maximize their coverage for
   local subscribers.  In transparent mode, they buffer downloading
   content from outside ISP networks, intercept the upcoming outgoing
   P2P requests from local subscribers and serve them with cached
   content instead.  In cooperative mode, they register to the tracker
   as a super peer and are under the regulation of the tracker's private
   protocol.























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      +--------------+                +------+
      | ISP 1 network+----------------+Peer 1|
      +-----+--------+                +------+
            |
   +--------+------------------------------------------------------+
   |                                               ISP 2 network   |
   |  +----------------+                +------------------+       |
   |  |Interworking GW |----------------| Forwarding Cache |       |
   |  +-----+----------+                +------------------+       |
   |        |                                                      |
   +--------+------------------------------------------------------+
            +---------------------------+
            |                           |
      +-----+-+                      +--+---+
      |Peer 2 |                      |Peer 3|
      +-------+                      +------+


    Figure 1: Architecture of Forwarding Cache at interworking gateway

4.2.  Bidirectional Cache for WLAN subscribers

   Fig. 2 illustrates the proposed architecture of a bidirectional cache
   system in WLAN.  In a WLAN, all AP will connect to a device named AC,
   and the AC can be seen as the gateway to Internet of the WLAN.  For
   most settings, both the traffic flowing into the WLAN and the traffic
   flowing out of the WLAN pass through the AC, hence Bidirectional
   Caches are assumed to be deployed at AC to exploit the traffic
   locality.  Besides the normal functions of a Forwarding Cache, A
   Bidirectional Cache in transparent mode buffers uploading content
   from inside the WLAN network, intercepts the upcoming outgoing P2P
   responses from local WLAN subscribers and serve the correspondent
   requester (be it another local WLAN subscriber or an outsider) with
   cached content instead.  In cooperative mode, they register to the
   tracker as a super peer and are under the regulation of the tracker's
   private protocol.















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      +--------------+                +------+
      | ISP 1 network+----------------+Peer 1|
      +-----+--------+                +------+
            |
   +--------+------------------------------------------------------+
   |        |                                      ISP 2 network   |
   |  +----------------+                +------------------+       |
   |  |Interworking GW |----------------| Forwarding Cache |       |
   |  +-----+----------+                +------------------+       |
   |        |                                                      |
   |        |                                                      |
   |  +-----+------+                +---------------------+        |
   |  |    AC      +----------------+ Bidirectional Cache |        |
   |  +-----+------+                +---------------------+        |
   |        |                                                      |
   |        +-------------------------------+                      |
   |   +----+------+                  +-----+-----+                |
   |   |   AP_1    |     . . . .      |   AP_n    |                |
   |   +----+------+                  +-----+-----+                |
   |        |                               |                      |
   +--------+-------------------------------+----------------------+
            |                               |
         +--+----------+                    |
         |             |                    |
      +--+--+       +--+--+              +--+--+
      |Peer2|       |Peer3|              |Peer4|
      +-----+       +-----+              +-----+




           Figure 2: Architecture of Bidirectional Cache in WLAN

4.3.  Generalized cache architecture for intra-ISP networks

   Fig. 3 generalized the overall architecture of the potential P2P
   cache deployments inside an ISP with various access network types.
   As it shows, P2P caches may be deployed at various levels, including:
   the interworking gateway linking with other ISPs, internal access
   network gateways linking with different types of accessing networks
   (e.g. WLAN, cellular and wired), and even within an accessing network
   at the entries of individual WLAN sub-networks.  Moreover, depending
   on the network context and the operator's policy, each cache can be a
   Forwarding Cache or a Bidirectional Cache.







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      +--------------+                +------+
      | ISP 1 network+----------------+Peer 1|
      +-----+--------+                +------+
      |
   +--------+------------------------------------------------------+
   |        |                                      ISP 2 network   |
   |  +---------+                                                  |
   |  |L1 Cache |                                                  |
   |  +-----+---+                                                  |
   |        +--------------------+----------------------+          |
   |        |                    |                      |          |
   | +------+------+      +------+-------+       +------+-------+  |
   | | AN1         |      | AN2          |       | AN3          |  |
   | | +---------+ |      | +----------+ |       |              |  |
   | | |L2 Cache | |      | |L2 Cache  | |       |              |  |
   | | +---------+ |      | +----------+ |       |              |  |
   | +------+------+      +------+-------+       +------+-------+  |
   |        |                                           |          |
   |        +--------------------+                      |          |
   |        |                    |                      |          |
   | +------+------+      +------+-------+       +------+-------+  |
   | | SUB-AN11    |      | SUB-AN12     |       | SUB-AN31     |  |
   | | +---------+ |      |              |       |              |  |
   | | |L3 Cache | |      |              |       |              |  |
   | | +---------+ |      |              |       |              |  |
   | +------+------+      +------+-------+       +------+-------+  |
   |        |                    |                      |          |
   +--------+--------------------+----------------------+----------+
            |                    |                      |
        +---+---+            +---+---+                  |
        |       |            |       |                  |
     +--+--+ +--+--+      +--+--+ +--+--+            +--+--+
     |Peer2| |Peer3|      |Peer4| |Peer5|            |Peer6|
     +-----+ +-----+      +-----+ +-----+            +-----+



          Figure 3: Generalized Architecture of intra-ISP Caches

5.  Considerations for ALTO deployment with P2P Caches

   For wired network operator, forwarding caching effectively localizes
   the downloading P2P traffic within the sub-net under its coverage
   resulting in reduction of network cost for cross-boundary peer
   selection, whereas reverse caching blocks the uploading traffic
   outside a wireless sub-net leading to elimination of network cost for
   wireless uploading peer selection.  In other words, caching between
   pairs of endpoints changes the traffic cost along the way.



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   Therefore, it is expected that by cooperation between the network
   operator and the P2P SP in building up various caching system and
   sharing information through ALTO protocol about these facilities
   brings benefits to both party.

   In order to do that, it is proposed to use locations of caches as
   dividers of different PIDs to guide intra-ISP network abstraction and
   mark costs among them according to the location and type of relevant
   caches.  Otherwise, if we stick to using network types as dividers
   for PIDs, there is no chance that a cost map would ever grasp this
   kind of information about node pairs within the same PID.

5.1.  Forwarding Cache: vertical separator from outsiders

   It is reasonable to use Forwarding Caches as separators for different
   PIDs, since it accelerates P2P traffic in a particular direction,
   indicating varied costs among these adjacent partitions.  For
   instance as shown in Fig.3, assuming the L2 Cache in AN1 of ISP 2
   network is a Forwarding Cache, the downloading traffic from other
   peers outside to AN1 but within the same ISP2 can be buffered once
   and served AN1 network subsequently.  The cost from AN2 or AN3 to AN1
   is reduced as result, but not vise visa.  In other words, the ISP 2
   network should be sub-divided into {AN1} and {AN2, AN3}, and the
   incoming P2P cost for {AN1} is reduced, for the sake of the L2
   Forwarding Cache located at the entry of AN1.

5.2.  Bidirectional Cache: horizontal division within insiders

   Since Bidirectional Cache are deployed in wireless accessing networks
   to further reduce the outgoing and local-in-local-out traffic costs
   in both directions, it seems straightforward to join the adjacent
   partitions together and modify the cost between insiders to zero.
   However, there is hidden layering within the Bidirectional Cache
   coverage, as the blocking of uploading traffic only works for traffic
   traverse pass the Bidirectional Cache.  If the cache is located too
   high as to be outside a local routing subnet, the local traffic flows
   within the subnet cannot benefit from the Bidirectional Cache.

6.  Open issues

6.1.  Question: selective caching

   As there is both CAPEX and OPEX expenditures for dedicated P2P Cache
   devices, it may be cost-efficient for caches to make buffering/
   serving decisions based on the popularity of the specific content.
   How to expose this application-relevant information to ALTO under
   such context is an open issue.




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6.2.  Discussion: cooperative caching and ALTO extension

   Luckily, in the cooperative-mode, a cache is playing as a normal peer
   under the tracker, and the latter can make the "right" decision in
   choosing in favor of the former under the guidance of the ALTO
   response while the tracker itself would take care of the content
   availability problem.  If the cache doesn't have the content in
   question, it would no appear in the peer list handed in to ALTO
   server by the tracker.

   In this case, the ALTO server can collect the information about
   caching sub-system in the network, identify those "caching" peers in
   the peer list of an cost request from an ALTO client, and arrange the
   returned rank list accordingly.  For example, a simple candidate-
   ranking policy for a cost query to a WLAN peer, could be caching
   peers at the beginning, then inside wired peers, and lastly outside
   wired peers.

   Moreover, the P2P SP and WLAN network operator may benefit even more
   by group popular files according to peers' geographic location or
   access types, and adapt its internal caching scheduling decisions
   about which files to be cached on which spot.  In order to do that,
   it would be helpful that the ALTO server could provide to the client
   with the requesting peer's subscription types (i.e. wired/WLAN/
   cellular/...) as well as geographic locations.  Specific definitions
   are specified separately in [I-D.draft-deng-alto-p2p-ext]

7.  Security Considerations

   TBA

8.  IANA Considerations

   None.

9.  References

9.1.  Normative References

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

   [RFC2234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 2234, November 1997.







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9.2.  Informative References

   [I-D.draft-deng-alto-p2p-ext]
              Deng, L. and H. Song, "End Point Properties for Peer
              Selection", draft-deng-alto-p2p-ext-00 (work in progress),
              February 2014.

   [I-D.ietf-alto-deployments]
              Stimerling, M., Kiesel, S., Previdi, S., and M. Scharf,
              "ALTO Deployment Considerations", draft-ietf-alto-
              deployments-08 (work in progress), October 2013.

   [I-D.ietf-alto-protocol]
              Alimi, R., Penno, R., and Y. Yang, "ALTO Protocol", draft-
              ietf-alto-protocol-25 (work in progress), January 2014.

Authors' Addresses

   Lingli Deng
   China Mobile

   Email: denglingli@chinamobile.com


   Wei Chen
   China Mobile

   Email: chenwei@chinamobile.com


   Qiuchao Yi
   China Mobile

   Email: yiqiuchao@chinamobile.com


   Yan Zhang
   Chinese Academy of Sciences

   Email: zhangy@hpnl.ac.cn











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