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Versions: 00

P2PSIP Working Group                                           E. Cooper
Internet-Draft                                               A. Johnston
Intended status: Standards Track                             P. Matthews
Expires: December 18, 2007                                         Avaya
                                                           June 16, 2007


   A Distributed Transport Function in P2PSIP using HIP for Multi-Hop
                            Overlay Routing
                    draft-matthews-p2psip-hip-hop-00

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This document examines a P2PSIP architecture where the peer-to-peer
   (P2P) layer is separate from and lies below the SIP layer.  We
   discuss the functions of the P2P layer in such an architecture, and
   focus in on the Distributed Transport function - the function that
   allows a peer to exchange messages with any other peer in the
   overlay, even in the presence of NATs.  We list the features that the



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   Distributed Transport function needs to provide, and observe that the
   Host Identity Protocol (HIP) already provides a number of these
   features.  We then propose extensions to HIP that allow it to provide
   the missing features.  We discuss how a complete P2PSIP architecture
   can be built around HIP, and contrast this approach with other
   approaches for implementing a P2P layer.  Two of the advantages of
   HIP approach are that (a) most existing applications can run in an
   overlay without needing any changes and (b) peer mobility and NAT
   traversal are handled in a way that is transparent to most
   applications.

Terminology

   Descriptions of the basic concepts and terminology used in this
   document can be found in the P2PSIP Concepts and Terminology document
   [Concepts].



































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Distributed Database function  . . . . . . . . . . . . . .  5
     1.2.  Overlay Maintenance function . . . . . . . . . . . . . . .  6
     1.3.  Distributed Transport function . . . . . . . . . . . . . .  6
     1.4.  Realizing the Distributed Transport function with HIP  . .  8

   2.  Brief Introduction to HIP  . . . . . . . . . . . . . . . . . .  9

   3.  Brief Introduction to our HIP extensions . . . . . . . . . . . 12

   4.  What are the alternatives? . . . . . . . . . . . . . . . . . . 13

   5.  Details of our Proposal  . . . . . . . . . . . . . . . . . . . 14
     5.1.  Protocol Layering  . . . . . . . . . . . . . . . . . . . . 15
     5.2.  Peer IDs . . . . . . . . . . . . . . . . . . . . . . . . . 16
     5.3.  Signaling  . . . . . . . . . . . . . . . . . . . . . . . . 16
     5.4.  Sending Packets between Peers in the Overlay . . . . . . . 17
       5.4.1.  Routing Packets hop-by-hop through the Overlay . . . . 18
       5.4.2.  Sending packets directly to the destination peer . . . 18
     5.5.  Security . . . . . . . . . . . . . . . . . . . . . . . . . 21

   6.  One Possible Implementation  . . . . . . . . . . . . . . . . . 22

   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23

   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 23

   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 23

   10. Informative References . . . . . . . . . . . . . . . . . . . . 23

   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
   Intellectual Property and Copyright Statements . . . . . . . . . . 26
















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1.  Introduction

   Consider the architecture for a P2PSIP peer shown in Figure 1.


                 _________________________
                |           SIP           |  Other applications ...
                |_________________________|_________________________
                | . . . . . . . . . . . .   . . . . . . . . . . . . |
                |  Distributed Database :   :  Overlay Maintenance  |
      P2P Layer | . . . . . . . . . . . : . : . . . . . . . . . . . |
                |              Distributed Transport                |
                |___________________________________________________|


                                 Figure 1

   In this architecture, there is a P2P layer which is distinct from the
   SIP layer, and which provides services to the SIP layer and other
   applications.  This P2P layer is internally divided into three parts,
   each of which provides a distinct function to the upper layers.
   These three functions are:

   o  Distributed Database.  This allows peers to store and retrieve
      information.  The initial envisioned use of this database is to
      store Address-of-Record (AoR) to Contact mappings for users, but
      it seems likely that this database will be used to store other
      things as P2PSIP evolves.

   o  Overlay Maintenance.  This establishes and maintains peer
      connections, distributes overlay configuration information to
      peers, and does anything else required to maintain the overlay.

   o  Distributed Transport.  This allows a peer to send an arbitrary
      packet to any other peer in the overlay, even if the destination
      peer is behind one or more NATs.  This is the most basic function
      of the three, and is used by the other two functions.

   The SIP layer utilizes the functions provided by the P2P layer to set
   up multimedia sessions between peers.  SIP queries the Distributed
   Database function to resolve an AoR to one or more Contact addresses,
   and then uses the Distributed Transport function to deliver SIP
   messages to the remote peer(s).  Note that SIP and other applications
   can access the Distributed Transport function directly without going
   through the other two functions.

   It is important to note that we are proposing that the P2P layer
   provide these functions to all upper-layer protocols, not just SIP.



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   The authors strongly believe that people will want to run protocols
   other than SIP over P2PSIP overlays, and providing a solution that
   works only for SIP will just encourage people to run these other
   protocols over SIP - a solution which goes directly against
   [RFC4485].

   This architecture proposal is not new.  The initial suggestion to use
   this architecture for P2PSIP was made by us two years ago in [IPCom]
   and [Industrial], and has been explored by others in some detail in
   [P2P-Arch] and [P2PCommon].  The contribution of this document is not
   in suggesting the architecture, but in making a concrete suggestion
   for how to realize it that leverages a large body of existing work.

   This architecture stands in contrast to the dSIP architecture [dSIP],
   where there is not a distinct P2P layer, but instead the SIP and P2P
   layers are merged and the functions of the P2P layer are implemented
   using an extended version of SIP.


   In the following subsections, we examine in more detail the functions
   that the P2P layer provides in this architecture.

1.1.  Distributed Database function

   This function provides a way for upper-layer applications to provide
   and retrieve data that is actually stored by distributing the data
   out to the peers in the overlay.

   In particular, the Distributed Database function provides the
   following:

   o  Distribution of data across peers in the overlay in a way such
      that no one peer needs to store all the data (unless there is only
      one peer in the overlay);

   o  Replication and shuffling so that data is not lost if one or more
      peers leave the overlay;

   o  Security (to the extent possible) to verify the origin of the data
      and guard against malicious data modification by other peers;

   o  Put and Get operations to store and retrieve the data from the
      distributed database.

   There have already been some proposals for how the Distributed
   Database function might be realized.  For example, [P2PCommon]
   proposes Insert, Lookup, and Remove messages that implement these
   many of the above features.  We believe that these messages could be



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   easily modified to work with the Distributed Transport design
   described here.

1.2.  Overlay Maintenance function

   The Overlay Maintenance function provides the controls that causes
   the peers in the overlay to function together in a harmonious way.

   For example, the Overlay Maintenance function provides the following:

   o  Admission of peers to the overlay, including checking the
      credentials of peers to make sure they are authorized to join;

   o  Controlling the creation of connections in the overlay to ensure
      that the appropriate pattern of connections exists for efficient
      routing and lookup;

   o  Distributing information about the overlay that needs to be known
      by all (or a subset) of the peers.  This might include the name of
      the overlay, the values to use for adjustable parameters,
      encryption keys for data that all peers can read but nodes outside
      the overlay cannot, etc.  This information is likely given to a
      peer when it joins the overlay, but there may be ways an
      administrator can change certain values without having to break up
      the overlay and allowing it to re-form.

   This document does not propose an Overlay Maintenance protocol,
   leaving this to future work.  However, later in this document we
   describe the role of the Overlay Maintenance protocol in driving the
   routing feature of the Distributed Transport function.

1.3.  Distributed Transport function

   The Distributed Transport function provides a way to uniquely
   identify peers and to deliver messages to an arbitrary peer in the
   presence of NATs and mobile peers.

   The presence of NATs has a major influence on this function, since
   NATs often hinder two peers from exchanging data directly.  The
   proposed approach for solving this problem is to establish a partial
   mesh of connections between peers, and then allow data to be sent
   indirectly between peers by sending it along existing connections in
   the overlay .  To make this possible, there must be a way to identify
   a peer (a peer ID), a way to establish and maintain connections, and
   a way to add the destination peer ID to the packet.  In essence, the
   overlay forms a network, with peer IDs serving as addresses,
   connections serving as links, peers serving as routers, and the tag
   serving as a network layer header.



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   Having peer IDs also makes it possible to gracefully handle mobile
   peers.  If a peer changes its IP address, then this could be
   considered equivalent to the peer leaving the overlay and later
   rejoining with a new IP address, but it is better if this could be
   viewed as simply a change in the address used to contact the peer.

   Providing these functions at the P2P layer means that applications
   themselves do not need to worry about NAT traversal and mobility.
   This is a big advantage over competing approaches that require each
   application to handle these on their own.

   The approach mentioned above provides datagram delivery, but to be
   useful, the Distributed Transport function must also provide all the
   usual transport layer services that applications depend upon.  For
   example, the Distributed Transport function must provide services
   like TCP and TLS.  If these services are not provided, then the
   P2PSIP WG will have to redo a large collection of SIP-related
   standards that depend on these services being available.

   Thus the Distributed Transport function provides the following:

   o  Peer IDs: A unique identifier for each peer in the overlay.

   o  Network layer: The ability to deliver a message to an arbitrary
      peer in the overlay.  In our view, this involves adding a header
      to the message that specifies the destination peer ID and then
      routing that message along existing connections in the overlay to
      the destination peer.

   o  Signaling: The ability to add, maintain, and remove connections in
      the overlay.  The signaling procedures must work in the presence
      of NATs.

   o  Bootstrapping: The ability for a peer that is not currently a
      member of the overlay to locate and establish an initial
      connection to a peer in the overlay.

   o  Transport layer: The usual transport layer functions such as port
      numbers, reliable in-order delivery of messages (if desired), and
      the segmentation of user data into Path-MTU-sized chunks (if
      desired).  This also includes transport layer security (TLS and
      DTLS ) if desired.

   o  Mobility and Multihoming: The ability for a peer have multiple IP
      addresses and to change these addresses dynamically while
      remaining a member of the overlay.  In make-before-break scenarios
      (= adding new addresses before losing all the old addresses), this
      is seamless; in break-before-make scenarios, connections go down



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      and must be re-established but the peer remains part of the
      overlay.

   o  Security: Message integrity and sequencing to prevent outsiders
      and intermediate peers from corrupting or replaying messages;
      encryption to prevent the message body from being read by
      outsiders or intermediate peers; protection against DoS attacks
      from outsiders or (to the extent possible) from intermediate
      peers.

   The following figure shows a simple example of some of these
   concepts.


                                  Peer E
                                    O
                                 /  |  \
                    Peer D     O    |    O   Peer F
                             / | \  |     \
                   Peer C   O  |  \---     O   Peer G
                             \ |    |  \   /
                      Peer B   O    |    O   Peer H
                                 \  |  /
                                    O
                                  Peer A


                                 Figure 2

   Figure 2 shows a number of peers arranged in an overlay network.
   Each peer in the network is behind its own NAT.  Each peer has one or
   more connections to other peers in the overlay.  If peer H wants to
   send a packet to peer B, it could try to send the packet directly,
   but most likely the filtering property of B's NAT would prevent the
   packet from getting through.  So peer H has two options: (a) it can
   send the packet to peer A which then forwards it to peer B, or (b) it
   can set up a direct connection to peer B, using ICE-like signaling
   procedures [ICE], and then send the packet directly to B.

1.4.  Realizing the Distributed Transport function with HIP

   In this document, we propose to realize the Distributed Transport
   function with an extended version of the Host Identity Protocol (HIP)
   [HIP-Base] currently being developed in the HIP WG.  We describe how
   HIP currently provides a number of the Distributed Transport features
   listed in the previous section, and then describe how to extend HIP
   to provide the remaining features.  We contrast this approach of
   using HIP with the approach of producing a new protocol from scratch,



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   and conclude that HIP is such a good fit that any compelling new
   protocol would end up stealing many ideas from HIP.

   The current version of this document is not a fully fleshed-out
   proposal, but rather a high-level presentation of the big picture.
   In many cases, we only describe the key ideas behind a proposed HIP
   extension, or the key ideas on how a Distributed Transport feature
   can be realized using either existing or proposed HIP features.  We
   have taken this approach in part to keep the document short and
   readable, but mostly because in many cases we have not work out the
   details.  In addition, some of this work is perhaps best done in the
   HIP WG rather than the P2PSIP WG.  We expect that future revisions of
   this document and/or follow-on documents will provide more details.


2.  Brief Introduction to HIP

   In this section, we give a brief introduction to HIP and how it is
   used in our proposal.  This section is especially targeted at those
   who know little or nothing about HIP.  The goal is to give the reader
   a sense that HIP has a lot to offer P2PSIP.

   The Host Identity Protocol (HIP) is an alternative to the dual use of
   IP addresses as "locators" (routing labels) and "identifiers" (host
   identifiers).  In HIP, the transport layer is decoupled from the
   network layer by introducing an identifier for a host which is
   independent of the host's IP address(es).  Though this decoupling,
   the transport layer and the applications above it are mostly
   insulated from changes in IP addresses.  This host identifier concept
   of HIP is very similar to the peer ID concept of P2PSIP.

   In HIP, hosts are identified using two closely-related concepts:

   o  A Host Identity (HI), which is the public half of a public/private
      key pair; and

   o  A Host Identity Tag (HIT), which is a 128-bit SHA-1 hash of a Host
      Identity.

   An HI is the definitive identification for a host.  HIs are long-
   lived, but it is easy for a host to have multiple HIs, and it is
   possible for hosts to create HIs without needing to access a central
   server.

   The HIT is a compact (128-bit) shorthand for the HI with the
   following properties:





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   o  It uniquely identifies the host.  The HIT is large enough to make
      it extremely unlikely that two different HIs will generate the
      same HIT.

   o  It is self-certifying.  That is, given a HIT, it is
      computationally hard to find a Host Identity that matches the HIT.

   o  It looks like an IPv6 address.  The first 20 bits of a HIT are
      fixed, and the corresponding range of IPv6 addresses have been
      reserved for HITs.  Thus a HIT can be used anywhere an IPv6
      address can be used, while retaining the ability to distinguish a
      HIT from a regular IPv6 address.  This has huge advantages, both
      when extending protocols to work with HIP, and when adapting
      existing protocol implementations and APIs to work with HIP.

   In our proposal, the HIT serves as the peer ID that applications use
   to uniquely identify peers in the overlay.


   The HIP protocol itself is a signaling protocol for setting up,
   maintaining, and tearing down security associations between two
   hosts.  Associations are created using a four-packet exchange.  The
   first party is called the Initiator and the second party the
   Responder.  The four-packet design helps to make HIP DoS resilient.
   The protocol exchanges Diffie-Hellman keys in the 2nd and 3rd
   packets, and authenticates the parties in the 3rd and 4th packets.
   Once the association is established, HIP defines other procedures for
   maintaining this association, even in the case where one or both ends
   change their IP address.

   To allow the HIP association to traverse intervening NATs, HIP uses a
   variation of the ICE protocol [ICE]; see [NAT-Traversal-for-HIP].

   A HIP association is logically a connection between two hosts.  Once
   an association between two hosts is set up, HIP multiplexes all
   application-level protocols over the association.  This is done by
   running the standard Internet transport protocols over the
   association, and using port numbers for demultiplexing in the usual
   manner.


   In our proposal, HIP is used in two different ways: (a) the HIP
   signaling procedures are used as an important first step in setting
   up and maintaining a connection in the overlay, and (b) HIP is
   extended to act as an encapsulation protocol for carrying upper-layer
   application data hop-by-hop through the overlay.

   The HIP header is illustrated in Figure 3.



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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Next Header   | Header Length |0| Packet Type |  VER. | RES.|1|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Checksum             |           Controls            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                Sender's Host Identity Tag (HIT)               |
      |                                                               |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Receiver's Host Identity Tag (HIT)              |
      |                                                               |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      /                        HIP Parameters                         /
      /                                                               /
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                                 Figure 3

   The header contains the following fields:

   o  Next Header: Specifies the protocol that follow (lies above) HIP.

   o  Header Length: The total length of the header, including any
      optional HIP parameters.

   o  Packet Type: There are currently 8 different HIP packet types
      defined.

   o  Version.

   o  Checksum: A checksum over the header.

   o  Controls: A set of one-bit flags.  Only one is currently defined.

   o  Source (sender) and Destination (receiver) HITs.

   o  HIP Parameters.  Optional TLVs that carry additional information.
      A number of TLVs are currently defined.





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3.  Brief Introduction to our HIP extensions

   The previous section described the features that HIP offers today and
   how it provides some of the features of the Distributed Transport
   function.  In this section, we sketch the extensions to HIP required
   to provide the remaining features.  The goal of this section is to
   give a quick overview of these extensions; more details are provided
   later.

   The HIP extensions that we propose in this document are:

   o  Encapsulation of higher-level messages.  HIP as currently defined
      is a signaling protocol only.  To extend HIP to serve an
      encapsulation protocol for higher-layer messages to transport them
      hop-by-hop through the overlay, we exploit the fact that HIP
      header (Figure 3) already has a Next Protocol field.  Thus
      encapsulating higher-layer messages is simply a matter of defining
      a codepoint for a new Packet Type (which we call the Data packet)
      which is used to carry the higher-layer messages.

   o  Hop-by-hop routing through an overlay.  HIP as currently defined
      is a protocol for setting up a point-to-point security association
      between two hosts.  To extend it to provide multi-hop routing
      between peers in an overlay, we exploit the fact that the HIP
      header (Figure 3) contains the source and destination HITs (= peer
      IDs).  Thus doing multi-hop routing is simply a matter of defining
      how to forward a HIP message in the case when the peer receiving
      the message is not the destination peer listed in the header.

   o  Bootstrapping.  In [Bootstrap], the authors described procedures
      that allowed a joining peer to locate and establish a connection
      to an admitting peer in an overlay.  Those procedures were defined
      using SIP as the signaling protocol, but these procedures can also
      be realized using HIP as the signaling protocol.

   o  Efficient transport of SRTP.  HIP inserts an extra layer into the
      standard networking stack, and two layers when there is a NAT
      between the two peers.  For protocols like the Secure Real-Time
      Protocol (SRTP), these extra layers can cause problems.  We show
      how the usual protocol stack (e.g., SRTP / UDP / IP) can be used
      in these situations, while maintaining the NAT traversal, multi-
      homing, and mobility properties of our version of HIP.

   More details on these extensions and other aspects of our proposal
   can be found in Section 5.






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4.  What are the alternatives?

   Before we jump into the details of our proposal, it is worth
   considering what an alternative design for the Distributed Transport
   layer would look like if the P2PSIP group was not to use HIP but
   design one from scratch.  For the authors, it was a real breakthrough
   when we realized that any protocol we designed from scratch to be a
   Distributed Transport layer would likely re-invent much of HIP.

   To start with, consider the format of a peer ID.  In [dSIP] and
   elsewhere, it is proposed that P2PSIP use 160-bit peer IDs.  To use
   these peer IDs, [dSIP] and [Bootstrap] propose to extend the URI
   scheme of SIP to express peer IDs, perhaps by adding a "peerid="
   parameter to the URI.  There are two problems with this approach: (a)
   every application that wants to work in an overlay has to be extended
   to understand the new URI scheme, and (b) new procedures have to be
   defined to describe how an application resolves this URI.  A counter-
   argument might be made that many DHTs today are defined to work with
   160-bit hashes, but the authors believe that all the major DHTs today
   can be easily adapted to work with a 128-bit peer ID.

   By contrast, the approach of HIP is to make a peer ID (= HIT) look
   like an IPv6 address.  With this approach, in most cases the existing
   approaches for resolving a URI to an address continue to work if
   (behind the scenes) a peer ID is returned instead of an address.  As
   we show below, this means that most applications need no changes to
   work in an overlay.  From this, we conclude that the advantages of
   making a peer ID look like an IPv6 address are substantial, and any
   alternate proposal for P2PSIP would need strong reasons to take a
   different approach.

   Next, consider the design of the "network layer" header, and consider
   the fields that would be needed in this header if the P2PSIP WG was
   to design its own protocol header.  Most likely, these fields would
   include:

   o  Source and destination peer ID (required for routing around the
      overlay);

   o  Demux field for indicating the higher-layer protocol (required to
      determine the upper-layer protocol);

   o  Protocol version number;

   o  Packet type field (if the protocol also does signaling or other
      things);





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   o  Optional TLVs for extensibility;

   o  Some way to detect the end of the header when optional TLVs are
      present; for example, a header length field.

   Comparing with the HIP header in section, we see that only the Header
   Checksum and Controls fields might be eliminated, and even these
   fields can be easily argued for.  From this, we conclude that the HIP
   header extremely well-suited for the Distributed Transport layer.

   Next, consider the signaling protocol.  The basic functions of the
   HIP signaling protocol (setting up, maintaining, and tearing down
   connections, handling endpoint mobility, reporting errors, etc) are
   the same as a dedicated P2PSIP protocol would need.  Though it is
   possible, perhaps even likely, that a P2PSIP design team would make
   some different design choices, the resulting protocol would likely
   have all the same basic properties.

   Finally, consider the transport layer functions.  In HIP, these are
   performed by the existing transport layer protocols (TCP, UDP, TLS,
   etc) using the existing APIs (sockets, etc.), exploiting the fact
   that HITs look like IPv6 addresses.  In this way, little or no
   changes are required to existing applications.  This makes for a very
   compelling story in comparison with the alternatives of developing
   new APIs and/or new protocols.


5.  Details of our Proposal

   This section gives the details of our proposal for using an extended
   version of HIP for the Distributed Transport function of P2PSIP.

   While reading this proposal, there are a few facts that reader should
   keep in mind:

   o  The proposal does NOT require the underlying network to be IPv6.
      Though peer IDs look like IPv6 addresses at the application layer,
      the underlying network addresses can be IPv4, IPv6, or a mixture
      of the two.

   o  Only peers and bootstrap servers need to run the HIP-related
      protocols.  No changes are required on other nodes in the network
      (e.g., routers, client-server SIP nodes, or other nodes that
      interact with the overlay).

   The following sections give a high-level view of the proposal.  More
   details will be provided in subsequent versions and/or separate
   documents.



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5.1.  Protocol Layering

   Figure 4 shows the fundamental protocol layering in our proposal.


                 _________________________
                |           SIP           |  Other applications ...
                |_________________________|_________________________
                | . . . . . . . . . . . .   . . . . . . . . . . . . |
                |  Distributed Database :   :  Overlay Maintenance  |
                | . . . . . . . . . . . : . : . . . . . . . . . . . |
                |               TCPv6, UDPv6, TLS, etc.             |
                | . . . . . . . . . . . . . . . . . . . . . . . . . |
     P2P Layers |                     HIP or ESP                    |
                | . . . . . . . . . . . . . . . . . . . . . . . . . |
                |           UDPv4         :          UDPv6          |
                |_________________________:_________________________|
                |           IPv4          |          IPv6           |
                |_________________________|_________________________|


                                 Figure 4

   In Figure 4, the Distributed Transport box of Figure 1 has been
   replaced by three sub-layers.  The upper sub-layer is the existing
   Internet transport layer, consisting of protocols such as TCP, UDP,
   SCTP, DCP, etc along with extensions such as TLS and DTLS.  These are
   the v6 versions of these protocols, since HITs (peer IDs) look like
   IPv6 addresses.

   The middle sub-layer is the HIP/ESP layer.  HIP is used for signaling
   and for encapsulation of data packets in multi-hop scenarios, while
   ESP (Encapsulated Security Payload) [HIP-ESP] is used for
   encapsulation in single-hop scenarios -- we discuss this in more
   detail below.

   The lower sub-layer is a UDP encapsulation layer.  This layer is
   present because most NATs, firewalls, and other middleware boxes
   today do not understand HIP and will usually drop a packet if the
   protocol above the IP layer is not TCP or UDP.  Placing a UDP header
   between IP and HIP will allow HIP packets to traverse these boxes.
   This layer is used only when required.  Using ICE-like connectivity
   checks, HIP detects if packets without this encapsulation layer can
   make it through and eliminates this layer when it is not needed.

   This stack runs over either IPv4 or IPv6.  A peer can have both IPv4
   and IPv6 interfaces, and connections in the overlay can be a mixture
   of these two protocols.



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   NOTE: Readers concerned about how to implement Figure 4 may wish to
   jump ahead to Section 6 before reading further.

5.2.  Peer IDs

   Host Identities could be assigned to peers in at least two different
   ways.  One way is for peers to generate their own public/private key
   pairs.  Another way is to allocate them to peers, perhaps in
   conjunction with a set of credentials, using a centralized allocation
   system.  The pros and cons of these and other schemes requires
   further investigation.

   Once a Host Identity is allocated to a peer, the peer uses the
   standardized method to form its HIT [HIP-Base].

   A HIT is the typical way to identify a peer in the overlay.  Because
   a HIT fits in an IPv6 address, in many cases applications need not be
   aware that they are talking to a peer in an overlay, and many IPv6-
   ready applications can run in an overlay without changes.  Consider
   an application that uses the IPv6 form of the socket API, uses HITs
   to identify peers on the overlay, and uses IPv4 addresses (in IPv4-
   in-IPv6 format) and/or IPv6 addresses to identify nodes off the
   overlay.  In many situations, the application can freely mix these
   three formats internally, leaving the transport and HIP layers to
   sort out the differences.  The exceptions are cases where the
   application would otherwise do something like send a HIT to an IPv4-
   only node not on the overlay.

5.3.  Signaling

   In our proposal, there are two layers of signaling involved in
   establishing, maintaining, and terminating connections in the
   overlay.  The HIP layer is responsible for establishing, maintaining,
   and terminating HIP associations with other nodes.  The nodes may be
   peers in the overlay, or they may be ordinary nodes with which a HIP
   association is desired.  The Overlay Maintenance layer is responsible
   for admitting some of these HIP associations to the overlay, and for
   ensuring that the pattern of connections in the overlay follow the
   pattern required for the DHT or other protocol.  In this section, we
   discuss HIP signaling for overlays in more detail, and leave the
   discussion of Overlay Maintenance signaling to other documents.

   Establishing a new HIP association within an overlay falls into one
   of two cases: (a) the initiating peer is not currently in the overlay
   and is trying to establish its first connection to another peer in
   the overlay, and (b) the initiating peer is already in the overlay.
   The basic format of the signaling exchange is the same in both cases;
   the difference is in how the HIP signaling messages are routed



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   between the two peers.

   In case (a), procedures similar to those in [Bootstrap] are used.
   [Bootstrap] defines two mechanisms for a joining peer to locate an
   admitting peer: using a Bootstrap Server and using multicast.  HIP
   already a mechanism similar to the Bootstrap Server mechanism (the
   RVS mechanism) which is used to locate a single node -- in our
   proposal, this mechanism is extended to work with overlays.  The key
   idea is to identify the overlay either by name, or by assigning a HIT
   to the overlay itself.  In that way, the bootstrap peers can register
   with the Bootstrap Server using the overlay name or HIT, and the
   Bootstrap Server can route HIP I1 packets (= the first packet in the
   HIP signaling exchange) received from the joining peer to a bootstrap
   peer associated with the overlay.  For the multicast mechanism, a
   similar approach is used: a multicast I1 packet specifying the
   overlay to join is sent out by the joining peer, one or more
   bootstrap peers reply, and the joining peer selects one to continue
   the exchange with.

   In case (b), the signaling messages are delivered to the remote peer
   by routing them hop-by-hop through the overlay (section 4.4.1).  The
   initiating peer places the HIT of the remote peer into the I1 message
   and sends the I1 message to its direct neighbor which is closest to
   the remote peer, and the I1 message is then routed hop-by-hop to the
   remote peer.  In this way, the originator does not need have a priori
   knowledge of the remote peer's IP address, and the signaling messages
   can be delivered even if the remote peer is behind a NAT or firewall.

   At any time, a given peer may have some associations which are a part
   of one overlay, some associations which are part of other overlays,
   and some associations which are not part of any overlay (or
   equivalently, a part of a 2-node overlay only).  The question of
   whether a given HIP association can be simultaneously part of two
   different overlays is for further study.

5.4.  Sending Packets between Peers in the Overlay

   There are two ways to send a packet to another peer in the overlay:
   send it on a direct connection to the remote peer, or send it hop-by-
   hop through the overlay.  A peer typically uses hop-by-hop routing
   when it has only a small amount of data to transfer to the remote
   peer (for example, a Distributed Database update or a SIP INVITE
   transaction), and sets up a direct connection when it has a larger
   amount of data to transfer (for example, an RTP session).







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5.4.1.  Routing Packets hop-by-hop through the Overlay

   To route a packet hop-by-hop through the overlay, it must have a HIP
   header.  In this HIP header, the sender field gives the HIT of the
   peer sending the packet, and the receiver field gives the HIT of the
   peer to which the packet is destined -- this might be a peer that is
   multiple hops away.

   The HIP packet might be one of the existing packet types uses to set
   up and maintain HIP associations, or it might be a new packet type
   called Data that is used to encapsulate messages from higher-layer
   protocols and carry them hop-by-hop through the overlay.  This new
   packet type has the HIP header shown in Figure 3, a packet type of
   "Data" (codepoint is TBD), and the Next Protocol field in the header
   is used to indicate the encapsulated protocol.

   We then extend HIP with the concept of multi-hop routing.  When a HIP
   packet arrives at a peer, the packet is delivered to the HIP layer
   which checks if the destination HIT is a HIT that belongs to this
   peer.  If not, then the peer tries to forward the packet.  To do
   this, the peer must decide which of its (directly connected)
   neighboring peers to forward the packet to.  This is done by having
   the HIP layer consult a table, called the HFIB (HIP Forwarding
   Information Base), which plays a role similar to the FIB table used
   in IP forwarding by routers.  The calculation of the HFIB is done by
   the Overlay Maintenance layer and downloaded to the HIP layer.

   The Overlay Maintenance layer constructs the HFIB using the principle
   of greedy routing, where at each hop, packets are forwarded to the
   neighboring peer whose peer ID is the closest match to the
   destination peer ID.  This is the routing approach used in most DHT
   algorithms (Chord, Bamboo, Kademlia, etc).  The Overlay Maintenance
   layer makes this routing algorithm efficient by adding the
   appropriate connections to the overlay.  More discussion of this
   approach can be found in [NATs-and-Overlays].

   It is possible for given peer to be a member of multiple overlays.
   It is also possible for a peer to have HIP associations with nodes
   that are not part of an overlay.  In these case, a peer needs to know
   on which overlay (or otherwise) a given packet should be forwarded.
   One way to solve this problem is to include the overlay ID in a TLV
   in the HIP header.  This is an area of ongoing investigation.

5.4.2.  Sending packets directly to the destination peer

   The second way to send a packet to a peer in the overlay is to
   establish a direct connection to the remote peer, and then send the
   packets directly.



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   When sending a packet on a direct connection to the destination peer,
   the relatively large HIP header (40 bytes) can be replaced with
   something smaller.  In this document, we discuss two replacements:
   the first is currently defined for used with HIP, while the second is
   a proposed extension.

   The first alternative is shown in Figure 5.


                 _________________________
                |           SIP           |  Other applications ...
                |_________________________|_________________________
                | . . . . . . . . . . . .   . . . . . . . . . . . . |
                |  Distributed Database :   :  Overlay Maintenance  |
                | . . . . . . . . . . . : . : . . . . . . . . . . . |
                |               TCPv6, UDPv6, TLS, etc.             |
                | . . . . . . . . . . . . . . . . . . . . . . . . . |
     P2P Layers |                        ESP                        |
                | . . . . . . . . . . . . . . . . . . . . . . . . . |
                |           UDPv4         :          UDPv6          |
                |_________________________:_________________________|
                |           IPv4          |          IPv6           |
                |_________________________|_________________________|


                                 Figure 5

   Here HIP has been replaced with ESP (Encapsulated Security Payload)
   [HIP-ESP].  ESP serves two functions when used in this way:

   o  It provides optional encryption, optional message integrity, and
      protection against replay attacks.  The pros and cons of using ESP
      vs. TLS/DTLS for this purpose in P2PSIP overlays in an area of
      ongoing investigation.

   o  It provides a field, called the SPI (Security Parameter Index),
      which is used as a shorthand for the (source, destination) HIT
      pair.  In this way, the receiver can determine the source and
      destination HITs to associate with the packet.

   Following the lead of the HIP WG, this protocol stack is the default
   when sending a packet directly from the source to the destination.

   The advantage of the protocol stack in Figure 5 is that it provides a
   smaller header (8 bytes for ESP vs. 40 bytes for HIP), while
   maintaining the separation between the transport layer and the IP
   layer that allows the IP addresses to change without affecting the
   transport layer.  For most applications, this protocol stack



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   represents a good tradeoff between efficiency and flexibility.

   However, for some applications, the protocol stack in Figure 5 is
   inappropriate.  A good example is SRTP, where a small header is very
   important, where there has been a fair amount of work on compressing
   the header further [RFC3095], and where the security properties of
   ESP are unnecessary since these properties are already provided by
   the application protocol.

   For those applications, a second protocol stack is available, as
   shown in Figure 6.


                    _______________
                   |     SRTP     |  Other apps...
                   |______________|______________
        P2P Layer  |          UDP(v4/v6)         |
                   |_____________________________|
                   |           IP(v4/v6)         |
                   |_____________________________|


                                 Figure 6

   Here the default protocol layering on direct connections (shown in
   Figure 5) has been replaced with an alternative layering.  This is
   not a general-purpose layering; this layering must be explicitly
   negotiated by the two peers before it can be used and is available
   only for the specific combinations of (local HIT, local port, remote
   HIT, remote port, protocol=UDP) that have been negotiated.  For all
   other combinations, the two peers continue to use the layering of
   Figure 5.

   When a packet is received at a peer with this layering, the
   combination of (local IP address, local port, remote IP address,
   remote port, protocol=UDP) is used to map this packet to a specific
   (local HIT, local port, remote HIT, remote port, protocol=UDP)
   combination.  In this way, both the HIT pair and the destination
   application are identified.

   To negotiate this usage between two peers, one end (peer X) sends a
   HIP message to the other (peer Y) saying that it would like to
   negotiate an alternative protocol layering for a particular UDP port
   combination.  Peer X includes a set of ICE candidates to use for the
   alternative layering; in ICE terms this can be viewed as another
   media stream between the two peers (where the HIP association is the
   primary media stream).  If peer Y is agreeable, it replies with its
   own set of candidates, and the two peers then run connectivity checks



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   to select a valid pair.  Later, if one endpoint changes its IP
   address, the two peers negotiate a new valid candidate pair.

   Since the use of this alternative protocol layering requires extra
   HIP messaging between the two peers to establish and maintain the
   additional "media stream", its use is recommended only in situations
   where the alternate protocol layering is important.  In most
   situations, the default protocol layering of Figure 5 is quite
   sufficient.

   From a HIP protocol perspective, this mechanism can be viewed as an
   instance of a more general mechanism for negotiating alternative
   protocol layerings.  However, it is worthwhile noting that the
   details of doing a similar layering for TCP are significantly more
   complex.  Consider the case where peer Y changes IP address when peer
   X has a number of unacknowledged segments outstanding.  The sequence
   numbers of the new TCP connections must be related back to the old
   TCP connection to allow segments on the new connection to acknowledge
   segments on the old connection.  The details and even the
   desirability of supporting this is left for future study.

5.5.  Security

   Security in HIP can be divided into two areas.  The first is the
   security of the HIP protocol itself, the second is the security
   provided to the upper layers.

   For the first, HIP currently provides encryption, message integrity,
   and protection against replay and denial-of-service attacks against
   HIP itself.  We believe that these mechanisms extend in a straight-
   forward way to multi-hop message exchanges, though we have not yet
   investigated all the details.

   For the second, more investigation is needed to determine whether the
   security of application protocols should be provided by the HIP/ESP
   layer, provided at the transport layer by mechanisms such as TLS/
   DTLS, or provided at the application layer.  The answer will probably
   be application-dependent.  For SRTP, protection at the application
   layer seems appropriate.  For SIP, protection at the transport layer
   seems appropriate, since SIP is already defined to use TLS over TCP.

   For many applications, there is an interesting question of whether
   TLS or ESP is most appropriate.  ESP seems to provide security only
   on an overlay-hop-by-overlay-hop basis, while TLS provides end-to-end
   security even across multiple overlay hops.  ESP may be appropriate
   if the goal is to protect against outside attacks, while TLS may be
   more appropriate if the goal is to also protect against attacks from
   rogue peers.



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6.  One Possible Implementation

   Consider implementing this proposal on a device which is IPv4-only
   and has a networking stack built into the OS that you cannot change.
   One way to do this is shown in Figure 7.


                                  __________________
                                 | (S)RTP |   SIP   | Other apps
                                 |________|_________|
                                 ______________________________
                                |  Distrib DB  | Overlay Maint |
                                |______________|_______________|

                   Socket API (v6) ..............................
                                      _________________________
                                     |   TCPv6    |   UDPv6    |
                                     |____________|____________|
                                     |        HIP / ESP        |
                                     |_________________________|
                                                                  User
                                                                  Space
  Socket API (v4) ......................................................
                    ___________________________________________   Kernel
                   |        TCPv4        |         UDPv4       |  Space
                   |_____________________|_____________________|
                   |                    IPv4                   |
                   |___________________________________________|


                                 Figure 7

   In Figure 7, a standard IPv4 stack is built into the kernel and is
   accessed via the IPv4 version of the socket API.  The HIP/ESP layer,
   with a second copy of the TCP/UDP layer, is located in user space and
   is accessible via the IPv6 version of the socket API.  The HIP/ESP
   layer uses the socket-v4 interface into the kernel to send and
   receive packets.  (Note: the v6 and v4 versions of the TCP and UDP
   protocols differ only in how their checksums are computed).  The
   Distributed DB and Overlay Maintenance protocols live above the
   socket-v6 interface and uses that API to send and receive packets.
   Finally, the P2PSIP applications (SIP, (S)RTP, etc.) use the services
   of all the lower layers.  If there is just one process that
   participates in the P2PSIP overlay, then all the layers shown in user
   space could be bundled together in that process.

   Open-source code for many of the pieces in this diagram are available
   today (albeit without the HIP extensions described above).



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7.  IANA Considerations

   The present version of this document introduces no new IANA
   considerations.


8.  Security Considerations

   The present version of this document gives only a high-level
   description of the proposal.  A detailed security analysis will be
   provided in subsequent versions and/or related documents that
   describe the detailed mechanisms.


9.  Acknowledgments

   The authors thank Spencer Dawkins, Dean Willis, Kevin Chen, and Scott
   Hutchens for their helpful comments on this document.


10.  Informative References

   [Bootstrap]
              Cooper, E., Johnston, A., and P. Matthews, "Bootstrap
              Mechanisms for P2PSIP", Internet
              Draft draft-matthews-p2psip-bootstrap-mechanisms.

   [Concepts]
              Bryan, D., Matthews, P., Shim, E., and D. Willis,
              "Concepts and Terminology for Peer to Peer SIP", Internet
              Draft draft-willis-p2psip-concepts-04, March 2007.

   [HIP-Base]
              Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson,
              "Host Identity Protocol", Internet
              Draft draft-ietf-hip-base-08, June 2007.

   [HIP-ESP]  Jokela, P., Moskowitz, R., and P. Nikander, "Using ESP
              transport format with HIP", Internet
              Draft draft-ietf-hip-esp-06, June 2007.

   [ICE]      Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Methodology for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", Internet
              Draft draft-ietf-mmusic-ice.

   [IPCom]    Johnston, A., "SIP, P2P, and Internet Communications",
              Internet Draft draft-johnston-sipping-p2p-ipcom-00,



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              January 2005.

   [Industrial]
              Matthews, P. and B. Poustchi, "Industrial-Strength P2P
              SIP", Internet
              Draft draft-matthews-sipping-p2p-industrial-strength-00,
              February 2005.

   [NAT-Traversal-for-HIP]
              Komu, M., Schuetz, S., Stiemerling, M., and AG. Gurtov,
              "NAT Traversal for HIP", Internet
              Draft draft-ietf-hip-nat-traversal-02 (to appear),
              June 2007.

   [NATs-and-Overlays]
              Cooper, E. and P. Matthews, "The Effect of NATs on P2PSIP
              Overlay Architecture", Internet
              Draft draft-matthews-p2psip-nats-and-overlays,
              February 2007.

   [P2P-Arch]
              Shim, E., Narayanan, S., and G. Daley, "An Architecture
              for Peer-to-Peer Session Initiation Protocol (P2P SIP)",
              Internet Draft draft-shim-sipping-p2p-arch-00,
              February 2006.

   [P2PCommon]
              Baset, S. and H. Schulzrinne, "Peer-to-Peer Protocol
              (P2PP)", Internet Draft draft-baset-p2psip-p2pcommon-01
              (available at www.p2psip.org), February 2007.

   [RFC3095]  Bormann, C., Burmeister, C., Degermark, M., Fukushima, H.,
              Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le,
              K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K.,
              Wiebke, T., Yoshimura, T., and H. Zheng, "RObust Header
              Compression (ROHC): Framework and four profiles: RTP, UDP,
              ESP, and uncompressed", RFC 3095, July 2001.

   [RFC4485]  Rosenberg, J. and H. Schulzrinne, "Guidelines for Authors
              of Extensions to the Session Initiation Protocol (SIP)",
              RFC 4485, May 2006.

   [dSIP]     Bryan, D., Lowekamp, B., and C. Jennings, "dSIP: A P2P
              Approach to SIP Registration and Resource Location",
              Internet Draft draft-bryan-p2psip-dsip-00, February 2007.






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Authors' Addresses

   Eric Cooper
   Avaya
   1135 Innovation Drive
   Ottawa, Ontario  K2K 3G7
   Canada

   Phone: +1 613 592 4343 x228
   Email: ecooper@avaya.com


   Alan Johnston
   Avaya
   St. Louis, MO  63124
   USA

   Email: alan@sipstation.com


   Philip Matthews
   Avaya
   100 Innovation Drive
   Ottawa, Ontario  K2K 3G7
   Canada

   Phone: +1 613 592 4343 x224
   Email: philip_matthews@magma.ca























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Full Copyright Statement

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