P2PSIP WG                                                     V. Pascual
Internet-Draft                                   Pompeu Fabra University
Intended status: Informational                            M. Matuszewski
Expires: August 27, 2008                                           Nokia
                                                                 E. Shim
                                                Locus Telecommunications
                                                                H. Zheng
                                                                 Y. Song
                                                     Huawei Technologies
                                                       February 24, 2008

                             P2PSIP Clients

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   This document describes why and when some devices would better be a
   Client rather than a Peer.  The purpose of this document is to
   facilitate the discussion and understanding about the Client node

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Definition of P2PSIP Client  . . . . . . . . . . . . . . . . .  3
     3.1.  Differences between a Client device and a device with
           SIP UA connected to a SIP Proxy on a Peer  . . . . . . . .  4
   4.  When a device should be a Client . . . . . . . . . . . . . . .  5
   5.  What functions a client can contribute in P2P layer  . . . . .  7
     5.1.  Storage function by a Client . . . . . . . . . . . . . . .  8
     5.2.  P2P Relay function by a client . . . . . . . . . . . . . .  9
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  9
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
   Intellectual Property and Copyright Statements . . . . . . . . . . 13

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1.  Introduction

   The P2PSIP Client node type was proposed and introduced quite a while
   ago.  Many drafts mention the concept of Clients [3] [4] [5] [6] [7]
   [8].  Nevertheless, there is some confusion about its concept or
   definition or the benefit of having it yet in the group.  The
   document elaborates on the concept of the role of Clients to
   facilitate the discussion and understanding about the Client node

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   The other concepts used in this document are compatible with RFC3261
   [2] and the concept draft [3]

3.  Definition of P2PSIP Client

   Typical DHT-based P2P overlay networks (hereafter overlay) need the
   following fundamental functions.
      - Bootstrapping (Letting a new node find some of the existing
      nodes in the overlay and facilitating the new node to join the
      - Overlay maintenance (nodes in the overlay maintain information
      about other nodes and a routing table)
      - Routing (nodes in the overlay route messages to other nodes in
      the overlay; the messages may be for joining the overlay, storing
      data(PUT), searching data (GET), and so on.)
      - Storage (nodes store resource (user) records that contain
      information about resources and users, for example, the location
      information of a resource.  Each user and resource has assigned an
      identifier that is used to locate user's or resource's records in
      the overlay.  The user's and resource's records stored in a
      particular node have assigned identifiers picked from the same
      identifier space as the node identifier(s).)

   Among these four functions, the Peers must support at least the
   overlay maintenance, routing, and storage functions.  Whether a
   device provides any other function in addition to these three
   functions is irrelevant to being a Peer or not.

   Then what is a Client?  It is a device that uses the overlay services
   but does not perform overlay maintenance and routing.  In this sense,

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   a possible analogy is that a Peer is a routing node and a Client a
   non-routing node.  Again whether a device provides any other services
   is irrelevant to being a Client.  A client uses the overlay services
   through its associated peer; it can be associated with more than one
   peer simultaneously to enhance redundancy.

   Being a Client or a Peer is a matter in the overlay layer.  It is
   independent of what a device does in a different layer or different
   context.  For example, a device being a SIP UA or Proxy is completely
   independent of being a Client or a Peer.  As a Peer may not be
   coupled with any SIP entity, a Client may not be coupled with any SIP

   Declaring a device as a Peer or a Client makes the role of the device
   significantly different from the overlay perspective.  A device must
   define its role in the overlay layer, i.e., whether to be a Peer or a
   Client, explicitly and this should be recognized by other nodes
   clearly.  Otherwise, the DHT or the overlay operation may be messed

3.1.  Differences between a Client device and a device with SIP UA
      connected to a SIP Proxy on a Peer

   First of all, please note that SIP UA and Client belong to completely
   different layers.  SIP UA is an entity in the SIP 'layer' and Client
   is a node type in the overlay 'layer'.  One device may work only as a
   SIP UA entity whereas another may work only as a Client device that
   do not support SIP protocol.

   A device running a SIP UA entity may be associated with a SIP Proxy
   running on a Peer.  This device is quite similar to a Client device
   in that sense that it uses the service of the overlay without being a
   Peer.  But there are a number of differences.

   First, the device with just SIP UA entity is not aware of the overlay
   or supports the Peer or Client protocol.  The Client device is aware
   of the overlay and contains implementation of the Client Protocol.
   It may contain the implementation of the Peer Protocol as well (in
   particular, if the Client Protocol is a subset of the Peer Protocol).
   Then a Client device may turn into a Peer if its situation changes.
   For example, a device may choose to be a Client because its network
   connection is wireless or unstable.  When its network connection
   becomes wired or stable, it can become a Peer.  Even if the Client
   Protocol is something quite different from the Peer Protocol, it is
   anticipated that the two Protocols are going to be often packaged
   together so that a device may choose to be a Client or a Peer
   depending on its hardware resource or network condition, or uptime.

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   Second, if we allow only SIP UA on a device that is not a Peer take
   advantage of the overlay, applications running over different
   application protocols in the same device will not be able to use the
   P2PSIP overlay.  Let say conference announcements are made as data in
   the overlay.  Since the interaction between SIP UA and Proxy is not
   for generic data search, a conferencing application running on a
   device with just SIP UA entity cannot get conference announcements.
   There is no such limitation with a Client device.  Any application on
   a Client device can use the overlay.  When someone says we don't need
   Clients because we can have Peers with SIP Proxy and SIP UA can
   connect to the SIP Proxies, she/he is thinking about only what a SIP
   UA can do via SIP Proxy.  Clients can enable more than that and
   increase the value of the P2PSIP overlay significantly.

   Third, when a device with SIP UA is connected to a Proxy on a Peer,
   the SIP messages from/to the SIP UA should go through the SIP Proxy
   on the Peer.  This may not be something the user of the device likes
   since the SIP Proxy on a not-so-trusted Peer may modify the SIP
   messages.  On the other hand, a SIP application running in a device
   that acts as a Client, may use the overlay just for search of data
   (eg.  Location of the callee) and communicates with the SIP UA at the
   callee's device directly for session establishment.  The callee
   location data is generated by the callee and may be digitally signed
   easily by the callee device.  So getting just the location data
   through an associated Peer has much less security threat than passing
   INVITE messages through a SIP Proxy on a Peer.

   Fourth, devices with just SIP UA need Peers with SIP Proxy entity.
   In another word, many if not most of the Peers must be coupled with a
   SIP Proxy entity if UA-Proxy relation is the only way non-Peer device
   can use the overlay for session establishment.  If the Client type is
   allowed, there is no such need.

4.  When a device should be a Client

   In general, it is better for the overlay when more 'qualified'
   devices become Peers rather than Clients.  So it is an issue whether
   a device should become a Client or a Peer.

   If the main service of the particular P2P overlay network is to share
   the bandwidth and storage (eg. for file sharing) and thus the
   resource abundance is more important than search/lookup speed,
   definitely all of the devices should become peers whenever possible.

   On the other hand, in real-time applications like P2PSIP, the search
   performance would be at least as important as the storage capacity.
   Concerning the search performance, the smaller number of Peers is the

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   faster the lookup can potentially be.  Therefore a larger number of
   Peers is not necessarily beneficial.  Of course the decision about
   what devices should become Peers must be made carefully.  We have to
   remember that the smaller number of Peers is, the bigger impact of a
   particular peer on the P2P overlay network performance can be.  There
   are situations where it is better for a device to be a Client rather
   than a Peer.

   First, some devices have unstable network connection or they churn
   frequently.  If a Peer churns frequently, it generates overhead to
   its neighbors for resynchronization of the routing tables and
   transfer of resource (user) records.  If the device does abrupt
   churning, the data stored in the device become unreachable.  To cover
   this, the overlay should increase the number of replicas.

   Second, a device can be behind a very strict firewall/NAT that makes
   it almost impossible for the device to operate as a Peer.  If a
   device is behind the symmetric NAT, then it is very hard for it to
   setup direct connections with its neighbors.  In this situation the
   device would have to use relay servers when routing messages to its
   neighbors.  If many devices of this kind become peers, then the
   overlay need many relays and the efficiency of the overlay becomes
   very low.

   Third, a device may have insufficient resources to support Peer
   operation.  Low-end mobile devices may have a little memory, a slow
   CPU and may not support fast packet radio interfaces.

   Forth, a device does not support the routing algorithm used by the
   overlay.  This is mentioned in [9].  The current direction of the
   P2PSIP protocol design is to support pluggable DHT and it is likely
   that overlays have choices of DHT algorithms to use.  And a device
   may not support the particular DHT algorithm used by the overlay the
   device wants to join.  Then such a device will have to be a Client
   even if it has hardware resources and network conditions good enough
   to be a Peer.  This situation would not happen often with devices
   which can download and install software easily.  But small devices
   with embedded software may be put into in this situation.

   There are also other reasons why a device should not become a peer:

   Even if it is possible to traverse NATs and firewalls, every
   additional connection that has to be maintained with other peers in
   the overlay will cause the battery of battery powered devices to
   drain faster.  If a device acts as a Client, it needs to maintain
   only one connection with the overlay (a connection with a peer).
   Besides the power consumption is impacted by maintenance of the
   routing state and by routing incoming messages.  As measurements

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   shown these operations have huge impact on how long a battery-powered
   device can stay online without recharging.  Nor the device user, nor
   the overlay network operator benefits by enforcing the mobile devices
   to be Peers and their battery to drain quickly.  Such policy will
   push away mobile device users from the overlay.

   Operating on battery does not mean necessarily that the device should
   be or wants to be a Client.  For example, mobile devices in an
   emergency situation running in an ad-hoc fashion might have to be
   Peers since there is no other kind of devices to be Peers.  Besides
   the battery consumption caused by maintenance of the routing state
   and by routing incoming messages can be reasonable in small overlays.

   In mobile communication using P2PSIP, the consumptions of rare mobile
   bandwidth in the access networks due to P2PSIP traffic will be
   significantly reduced if mobile devices become more Clients and less
   P2PSIP peers as possible (assuming there are enough non-mobile
   devices being Peers).  Besides we have to remember that bandwidth is
   expensive.  If someone has to pay for every packet her/his device
   exchanges, probably having the device be a Peer makes no economical
   sense to the her/him especially that the overlay maintenance and
   routing of incoming messages are not directly linked to the service

   A device may not meet the requirements of Peers for the particular
   overlay desired by the overlay operator.  For example, the OpenDHT is
   built with nodes on PlanetLab.  Any devices that use the OpenDHT
   become a kind of Client.  This was the choice of the people that
   designed and built the OpenDHT.  Probably they needed access and
   control to the nodes to participate in the DHT that is not available
   in general devices.  A similar thing may happen that an overlay
   operator wants only certain devices to be Peers due to various
   reasons such as high security requirements.  For example, only
   devices that were authenticated using offline means may be allowed be
   a Peer or only devices that have been a Client for a long time
   without any bad behavior may be allowed to be a Peer or a devices
   that have enough fast network interface and uptime can become a peer.

5.  What functions a client can contribute in P2P layer

   Clients get services from the overlay network, however, clients can
   also contribute varialbe useful functions to the overlay.  For
   example, a client can provide STUN server function to help establish
   connections between other peers, and also other useful functions.  We
   focus on several functions that a client can contribute to the
   overlay in P2P layer here.

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5.1.  Storage function by a Client

   In DHT, a Peer is responsible for storing data (resource (user)
   records) of a certain range of Resource IDs.  For example, in Pastry/
   Bamboo/OpenDHT, data (resource (user) records) are assigned to a Peer
   whose Node ID is closest to the Resource IDs than any other Peers.

   It is possible for a Peer to delegate actual storage of the data to
   another node and just keep a pointer (location information) to the
   data in a different node.  For example, a Peer may have a remote
   storage device and uses it as actual storage for the data the Peer is
   responsible for.  Such a remote storage device may be another Peer, a
   Client, or a node not involved in the overlay at all.  In this
   situation, any lookup message for the data will be routed to the Peer
   responsible for storing the data (Responsible Peer) and it is the
   responsibility of the Responsible Peer to reply to the lookup message
   with the data or a pointer to a node where the data is located.  This
   is how a typical DHT operates.  It should be transparent to other
   Peers whether a Peer uses its local memory or disk or remote memory
   or disk to store the data it is responsible for.

   Whether to allow a Client to be a remote storage for a Peer (or
   multiple Peers) does not affect the overlay operation significantly
   because it is a local arrangement between the respective Peer(s) and
   the Client.  However it affects the design of the protocol used
   between Peer and Client.

   The reason for a device to become a Client instead of a Peer must not
   depend on the decision whether a Client may serve as a remote storage
   for a Peer or not.

   Many of the reasons a device to become a Client rather than become a
   Peer make the device unsuitable to provide the storage service.  For
   example, if a device is online only intermittently, storing data in
   such a device does not bring much benefit.  However there is a case a
   Client may be able to store data for its associated Peers.  It is
   when the device became a Client because it did not support the
   overlay's DHT algorithm while it met other requirements for a Peer.

   In a sense, it is better for the overlay if some of its Clients can
   provide the storage service since the overlay's overall storage
   capacity increases.  We need to look into two aspects:
      - How much complexity is introduced to allow Clients to provide
      the storage service?

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      - Is the benefit of having Clients provide the storage service
      large enough?

   More thought will be given to these questions.

5.2.  P2P Relay function by a client

   In some scenarios like for real time applications, a larger number of
   Peers is not necessarily beneficial, therefore there is probability
   that some capable devices act as clients in the overlay.  These
   devices have the ability to relay messages for other devices.

   A node behind strict firewall/NAT may need a relay when it wants to
   communicate with others.  Generally, it can route messages with the
   overlay routing, however, for more efficiency, it can use a "Relay"
   with public address to relay requests or responses when it knows the
   contact address of the other party.  All capable peers or clients can
   act as Relays.  The choice of Relays must be well designed for some
   criterions, e.g. proximity.  The best choice of Relay for a peer or
   client may be a peer or a client.  Not in all cases can peer Relays
   be more efficient than client Relays.

   Clients that provide relay function do not need to understand the DHT
   algorithm of the overlay.  It only provides relay service to a
   certain peer or client that needs its help for communication

   We only focus on the relay function for P2P layer communication here,
   e.g., P2P requests or responses.  All relay functions beyond those
   are out of scope.

6.  Acknowledgments

   Some of the idea described in the draft came from the discussion in
   the P2PSIP mailing list.  Thanks to Henning Schulzrinne,Henry
   Sinreich, and Lichun Li

7.  Security Considerations

   Clients (providing no storage service) are free riders.  If not many
   devices qualifed to be a Peer do not volunterr to be a Peer, the
   P2PSIP network may not work well.  It is a concern for P2P networks
   in general.

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8.  IANA Considerations

   There are no IANA considerations associated to this memo.

9.  References

9.1.  Normative References

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

   [2]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
        Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
        Session Initiation Protocol", RFC 3261, June 2002.

9.2.  Informative References

   [3]  Bryan, D., Matthew, Shim, E., and D. Willis, "Concepts and
        Terminology for Peer to Peer SIP", Internet Draft draft-
        ietf=p2psip-concepts-00, June 2007.

   [4]  Bryan, D., Baset, S., Matuszewski, M., and Sinreich, "P2PSIP
        Protocol Framework and Requirements", Internet
        Draft draft-bryan-p2psip-requirements-00, June 2007.

   [5]  Jennings, C., Lowekamp, B., Rescorla, E., and Rosenberg,
        "REsource LOcation And Discovery (RELOAD)", Internet
        Draft draft-bryan-p2psip-reload-02, November 2007.

   [6]  Baset, S., Schulzrinne, H., and M. Matuszweski, "Peer-to-Peer
        Protocol (P2PP)", Internet Draft draft-baset-p2psip-p2pp-01,
        November 2007.

   [7]  Song, Y., Jiang, X., Zheng, H., and H. Deng, "P2PSIP Client
        Protocol", Internet Draft draft-zheng-p2psip-client-protocol-01,
        February 2008.

   [8]  Jiang, X., Zheng, H., Macian, C., and V. Pascual, "Service
        Extensible P2P Peer Protocol", Internet
        Draft draft-jiang-p2psip-sep-01, February 2008.

   [9]  Li, L Ch. and Y. Wang, "Different types of nodes in P2PSIP",
        Internet Draft draft-li-p2psip-node-types-00, November 2007.

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Authors' Addresses

   Victor Pascual
   Pompeu Fabra University
   Barcelona, Passeig de la Circumval.lacio  8 08003

   Phone: +34-93-5421561
   Fax:   +34-93-5422517
   Email: victor.pascuala@upf.edu

   Marchin Matuszewski
   P.O.Box 407
   NOKIA GROUP, FIN  00045

   Phone: unlisted
   Email: marcin.matsuszewski@nokia.com

   Eunsoo Shim
   Locus Telecommunications
   111 Sylvan Avenue
   Englewood Cliffs, New Jersey  07632

   Phone: unlisted
   Email: eunsooshim@gmail.com

   Hewen Zheng
   Huawei Technologies
   Baixia Road No. 91
   Nanjing, Jiangsu Province  210001

   Phone: +86-25-84565467
   Fax:   +86-25-84565354
   Email: hwzheng@huawei.com

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   Song Yongchao
   Huawei Technologies
   Baixia Road No. 91
   Nanjing, Jiangsu Province  210001

   Phone: +86-25-84565081
   Fax:   +86-25-84565070
   Email: melodysong@huawei.com

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