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Copyright © The Internet Society (2006).
This document defines concepts and terminology for use of the Session Initiation Protocol in a peer-to-peer environment where the traditional proxy-registrar function is replaced by a distributed mechanism that might be implemented using a distributed hash table or other distributed data mechanism with similar external properties. This document includes a high-level view of the functional relationships between the network elements defined herein, a conceptual model of operations, and an outline of the related open problems that might be addressed by an IETF working group.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in  (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).
3.1. What Kinds of P2PSIP Peers and Clients Might Exist?
3.2. Reference Model
3.3. Example Signalling Message Flows
3.3.1. P2PSIP Peer contacts P2PSIP Peer
3.3.2. P2PSIP Client contacts P2PSIP Peer
3.3.3. Conventional SIP Device using a Proxy Peer
3.3.4. Conventional SIP Device Using a Redirect Peer
3.4. Conceptual Outline of Operations
3.4.1. Enrolling and Inserting an P2PSIP Peer
3.4.2. More on The Difference Between a Peer, Client, and User Agent
3.4.3. Enrolling a User and Inserting a P2PSIP User Agent
4.1. PP2PSIP Peer Protocol
4.2. P2PSIP Client Protocol
4.3. How To Find Media Relays?
4.4. How Do We Find Gateways?
4.5. Peer-Adjacency Through NATs
4.7. Record Formats
4.8. Peer and Client Enrollment Protocols
4.9. Peer and User Credentials
4.11. Credential Recovery
4.12. Overlapping Domains
4.13. Hybrid Domains
4.14. Admissions Control
4.15. Users versus Resources
5. Security Considerations
6. IANA Considerations
8.1. Normative References
8.2. Informative References
§ Authors' Addresses
§ Intellectual Property and Copyright Statements
One of the fundamental problems in multimedia communications between Internet nodes is that of a discovering the IP address at which a given correspondent can be reached. Correspondents are frequently identified by distinguished names, perhaps represented in the form of a universal resource indicator (URI)  (Berners-Lee, T., Masinter, L., and M. McCahill, “Uniform Resource Locators (URL),” December 1994.).
The Session Initiation Protocol (SIP)  (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) commonly addresses this task assuming that the domain part of the URI indicates an internet host address or internet domain name using the Domain Name System (DNS)  (Mockapetris, P., “Domain names - concepts and facilities,” November 1987.). SIP's location process  (Rosenberg, J. and H. Schulzrinne, “Session Initiation Protocol (SIP): Locating SIP Servers,” June 2002.) assumes that host part of the URI indicates either the target SIP user agent (UA), or a proxy that has knowledge of how to to reach the target UA, presumably gained through SIP's registration process.
This approach, referred to herein as "Conventional SIP" or "Client/Server SIP", assumes a relatively fixed hierarchy of SIP routing proxies (servers) and SIP user agents (clients). The routing proxies are typically resolvable using conventional Internet mechanisms with static IP addresses and associated DNS entries. This structure may not be ideal in all cases, including specifically ad-hoc, serverless, and reduced-administration scenarios. As an alternative, several authors  (Bryan, D., “A P2P Approach to SIP Registration and Resource Location,” March 2006.)  (Shim, E., “An Architecture for Peer-to-Peer Session Initiation Protocol (P2P SIP),” March 2006.)  (Sinnreich, H. and A. Johnston, “SIP, P2P, and Internet Communications,” March 2006.)  (Matthews, P., “Industrial-Strength P2P SIP,” February 2005.) have proposed using peer-to-peer (P2P)  (Risson, J. and T. Moors, “Survey of Research towards Robust Peer-to-Peer Networks: Search Methods,” March 2006.) approaches to solving the correspondent discovery problem. The motivations for a P2P approach in SIP have been documented in  (Bryan, D., “Use Cases for Peer-to-Peer Session Initiation Protocol (P2P SIP),” December 2005.).
This document offers a consolidation of the more important terms and concepts from several of the above documents, presented in the context of a reference model for peer-to-peer SIP (P2PSIP). It is intended that this document serve as a starting point for describing the work needed to resolve a number of open questions such that an IETF working group could be chartered to do the work needed to resolve these questions and present a standard solution. The authors believe that this goal is roughly consistent with that of a Protocol Model as defined in  (Rescorla, E. and IAB, “Writing Protocol Models,” June 2005.).
We provide a list of terms used, as well as alternate forms that have been used for these in drafts or discussions. In general, the thought is to use the primary suggested form for clarity -- we have included the other forms for simplicity and to provide a "mapping" to existing drafts. Defined terms include:
- Overlay Network:
- An overlay network is a computer network which is built on top of another network. Nodes in the overlay can be thought of as being connected by virtual or logical links, each of which corresponds to a path, perhaps through many physical links, in the underlying network. For example, many peer-to-peer networks are overlay networks because they run on top of the Internet. Dial-up Internet is an overlay upon the telephone network. http://en.wikipedia.org/wiki/P2P_overlay
- P2P Network:
- A peer-to-peer (or P2P) computer network is a network that relies primarily on the computing power and bandwidth of the participants in the network rather than concentrating it in a relatively low number of servers. P2P networks are typically used for connecting nodes via largely ad hoc connections. Such networks are useful for many purposes. Sharing content files (see file sharing) containing audio, video, data or anything in digital format is very common, and realtime data, such as telephony traffic, is also passed using P2P technology. http://en.wikipedia.org/wiki/Peer-to-peer. A P2P Network may also be called a "P2P Overlay" or "P2P Overlay Network" or "P2P Network Overlay" , since its organization is not at the physical layer, but is instead "on top of" an existing Internet Protocol network.
- A communications protocol related to the Session Initiation Protocol (sip)  (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) that extends SIP by using peer-to-peer techniques for resolving the targets of SIP requests.
- P2PSIP Overlay:
- A P2PSIP Overlay is an association, collection, or federation of nodes that provides SIP registration, SIP request routing, and similar functions using a P2P organization, as defined by "P2P Network" above. Other forms: overlay.
- P2PSIP Peer:
- A node participating in a P2PSIP Overlay that provides storage and routing services (fully participates in the P2P routing) to other nodes in that P2PSIP Overlay. Each P2PSIP Peer is presumed to have a unique identifier within the P2PSIP Overlay. Each P2PSIP Peer may or may not be coupled to one or more P2PSIP User Agents. Within the P2PSIP Overlay, the peer is capable of performing several different operations, including: joining and leaving the overlay, routing requests within the overlay, storing information on behalf of the overlay, putting information into the overlay, and getting information from the overlay. Other forms: overlay peer or node, peer or node, superpeer or supernode (in systems with peers and clients), peer.
- P2PSIP Client:
- A node participating in a P2PSIP Overlay that provides neither routing nor route storage and retrieval functions to that P2PSIP Overlay. The P2PSIP Client interacts with the P2PSIP Overlay only to request the insertion of routing information (put in a Contact), request the retrieval of routing information (get a Contact), or to request the routing of a message to elsewhere in the P2PSIP Overlay. Unlike the P2PSIP Peer, the client is presumed not to have a unique identifier within the overlay. In cases where conventional SIP is used for the P2PSIP Client protocol, this entity could be identical to a standard SIP user agent. A P2PSIP Client may be coupled to one or more P2PSIP Overlay User Agents. A P2PSIP Client is a logical subset of a P2PSIP Peer; anything a P2PSIP Client can do, a P2PSIP Peer can do as well. Other forms: overlay client, client.
- P2PSIP Resource (User):
- An addressable user endpoint, entity, service, or function within a P2PSIP Overlay. Examples include but are not limited to humans, automata, bridges, mixers, media relays, gateways, and media storage. Other forms: resource (user).
- P2PSIP Overlay Identifier:
- Information that identifies a specific P2PSIP Overlay. All the P2PSIP Peers in a particular P2PSIP Overlay have the same P2PSIP Overlay Identifier. This is may be scoped to a name within the DNS, but other scopes may apply, particularly in ad-hoc environments. Short forms: overlay name, overlay identifier, overlay ID.
- P2PSIP Peer-ID:
- Information that uniquely identifies each P2PSIP Peer within a given P2PSIP Overlay. This value is not human-friendly -- in a DHT approach, this is a numeric value in the hash space. These Peer-IDs are completely independent of the identifier of any user of a user agent associated with a peer. Other forms: Node-ID
- P2PSIP Resource (User) Name:
- A distinguished, human readable name that identifies a specific P2PSIP Resource or User within a given P2PSIP Overlay. This is presumed to be a URI scoped to the P2PSIP Overlay Identifier. This is presumably the same or very similar to a SIP Address of Record, or AOR. Other forms: overlay resource (user) name, P2PSIP AOR.
- P2PSIP Resource-ID:
- A non-human friendly value that uniquely determines which P2PSIP Peer is responsible for storing information about this resource (user). In a DHT approach, this is a numeric value in the hash space resulting from hashing the P2PSIP Resource Name. Since Resource-ID is in the same space as the P2PSIP Peer-ID, it allows for a mapping between the values, used to map a P2PSIP Resource to the P2PSIP Peer that stores it. Other forms: P2PSIP User-ID.
- P2PSIP Resource (User) Record:
- A block of data, stored using the data mechanism of the P2PSIP Overlay, that includes information relevant to a specific resource. We presume that there may be multiple types of resource records. Some may describe routes to a client at which the user is presumed reachable (a "user routing record", like a SIP "Contact:"). Others might store presence information. The types, usages, and formats of user records are a question for future study.
- P2PSIP User Agent:
- A SIP user agent that is coupled with or incorporates a P2PSIP Peer or P2PSIP Client, such that the peer or client can assist the UA with registration (storage of a route to users of the UA) and/or routing of requests using the P2PSIP Overlay. A P2PSIP User Agent differs from a conventional SIP user agent in that it is coupled directly to a P2PSIP Peer or P2PSIP Client, and can therefore directly interact with a P2PSIP Overlay, which a conventional SIP UA cannot do. P2PSIP User Agents do not themselves have a distinguished name or identifier, although the P2PSIP User associated with it may, and if it is associated with a P2PSIP Peer, that peer may as well. Other forms: overlay UA, P2PSIP UA.
- P2PSIP Peer Protocol:
- The protocol spoken between P2PSIP Overlay peers to share information and organize the P2PSIP Overlay Network. Short form: peer protocol.
- P2PSIP Client Protocol:
- The protocol spoken between P2PSIP Clients and the P2PSIP Peer they use to store or retrieve information from the P2P Overlay. This is a functional subset of the P2P Peer Protocol, but may differ in syntax and protocol implementation (i.e., may not be syntactically related). Note that the precise relationship between the P2PSIP Peer Protocol and the P2PSIP Client Protocol (the same? subset?) remains an open question and is expected to be a principle topic of the detailed design work. This protocol may not exist (it may simply be conventional SIP) in some designs. Short form: client protocol.
- P2PSIP Overlay Neighbors:
- The set of P2PSIP Peers that either a P2PSIP Peer or P2PSIP Client know of directly and can reach without further lookups. Short form: neighbor
- P2PSIP Bootstrap Server:
- A network node used by P2PSIP Peers or Clients who are attempting to locate an entry into the P2PSIP Overlay Network. It may return an entry point (address of a Peer) to the P2PSIP Overlay or act as one itself. This should be a quasi-stable and well known host, located using a configuration or discovered via , DNS, broadcast, or other mechanism. This is a logical role, meaning it can be implemented as a P2PSIP Peer, as a standalone server, etc., but not every peer must provide this functionality. Example: a P2PSIP Peer that reboots and has no knowledge of other peers uses a P2PSIP Bootstrap Server to find other peers in the P2P Overlay Network and establish P2PSIP Peer Insertion. Other forms: bootstrap peer or node.
- P2PSIP Resource (User) Record:
- A P2PSIP overlay user record that provides a routing vector that points to a location where the resource can presumably be reached. This is analogous to the combination of a SIP  (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) "Contact:" and a "Path:"  (Willis, D. and B. Hoeneisen, “Session Initiation Protocol (SIP) Extension Header Field for Registering Non-Adjacent Contacts,” December 2002.). The P2PSIP equivalent of a SIP registration process would be the insertion of an P2PSIP Resource Record into the overlay. Other forms: resource (user) record, resource (user) registration.
- P2PSIP Peer Insertion:
- The act of inserting a P2PSIP Overlay Peer into the current routing structure (presumably a distributed database or hash table) of a P2PSIP Overlay. For example, the routing structure map the peer's IP address or DNS name to the peer's P2PSIP Peer-ID. During insertion, the peer discovers its P2PSIP Overlay neighbors. Following insertion, the peer will be able to store user records (such as routing information), query other peers for user records, and pass requests to route messages to other peers. Other forms: peer or node registration, peer or node join.
- P2PSIP Resource (User) Record Insertion:
- The act of inserting a record for a P2PSIP Resource (User) into the data maintained by the P2PSIP Peers. Following insertion, the data stored at one or more peers will contain a record (such as a P2PSIP Resource Routing Record), keyed at least in part by a P2PSIP User Identifier. Other forms: Resource registration, User record insertion.
- P2PSIP Peer Enrollment:
- The initial one-time process a P2PSIP Peer follows to obtain an identifier and credentials, if any, within a P2PSIP Overlay. This is not performed each time the peer comes online (contrast to P2PSIP Peer Insertion above), but only the first time they do so, or following a loss of identifier or credentials by the peer. Other forms: node enrollment, peer enrollment.
- P2PSIP Resource (User) Enrollment:
- The initial one-time process a P2PSIP Resource follows to obtain a unique identifier within a P2PSIP Overlay. This is not performed each time the resource comes online, only the first time they do so, or following a loss of identifier or credentials by the client (contrast to P2PSIP Resource Record Insertion). Other forms: user enrollment.
In general, P2PSIP nodes might have the same sorts of duties/logical roles as traditional client-server SIP nodes. This includes but is not limited to:
The following diagram depicts a reference or "boxes and arrows" model showing several of the above peer and client types, along with a conventional SIP user agent.
--->PSTN +------+ N +------+ +---------+ / | | A | | | Gateway |-/ | UA |####T#####| UA |#####| Peer |######## | Peer | N | Peer | | G | # Client Protocol | E | A | F | +---------+ # GET/PUT | | T | | # | +------+ N +------+ # | # A # | NATNATNATNAT # | # # | \__/ NATNATNATNAT +-------+ v / \ # N | |=====/ UA \ +------+ A P2PSIP Overlay | Proxy | /Client\ | | T | Peer | |___C__| | UA | N Route Data | Q | | Peer | A +-------+ | D | T P2PSIP Peer Protocol # | | N # +------+ A # # T # # N +-------+ +-------+ # # A | | | | # #########T####| Proxy |########| Redir |####### | Peer | | Peer | | P | | R | +-------+ +-------+ \__/ /\ / \ / UA \ /______\ SIP UA A
Figure: P2PSIP Overlay Reference Model
Here, the large perimeter depicted by "#" represents a stylized view of the P2PSIP Overlay and its associated routing data (the actual connections could be a mesh, ring, or some other structure). Around the periphery of the P2PSIP Overlay rectangle, we have a number of P2PSIP Peers -- a PSTN gateway peer "G", three user-agent peers "D", "E" and "F", two proxy peers "P" and "Q", and a redirector peer "R". Note that because these are all P2PSIP Peers, each is responsible for helping store some information of the P2PSIP Overlay.
To the left, two of the peers ("D" and "E") are behind network address translators. These peers are included in the P2PSIP overlay and thus participate in storing information, despite being behind the NATs.
Below the P2PSIP Overlay, we have a conventional SIP UA "A" which is not part of the P2PSIP Overlay, either directly as a peer or indirectly as a client. It speaks neither the P2PSIP Peer nor P2PSIP Client protocols. Instead, it uses pure (unmodified/extended) SIP to interact with with the P2PSIP Overlay.
On the right side, we have a P2PSIP UA client "C", which uses the P2PSIP Client Protocol depicted by "=" to communicate with Proxy Peer "Q". The P2PSIP Client protocol only allows for gets and puts to the overlay. Therefore, "C" does NOT participate directly in/store information in the overlay. In a solution where the P2PSIP Client Protocol is SIP, such as is proposed in  (Bryan, D., “A P2P Approach to SIP Registration and Resource Location,” March 2006.), there is no difference between UA client "C" and standard SIP UAs "A", and the special P2PSIP client protocol is not needed.
Note that in some scenarios, the P2PSIP Peers involved in the overlay might use a keepalive mechanism to ensure that messages to neighbors can pass through NATs. Presumably, these messages will be in the form of the P2PSIP Peer protocol.
Both the "Proxy Peers" and "Redirect Peers" can serve as adapters between ordinary SIP devices and the the P2PSIP Overlay. Each accepts standard SIP requests and resolves the next-hop by using the P2PSIP overlay Peer Protocol to interact with the routing knowledge of the P2PSIP Overlay, then processes the SIP requests as appropriate (proxying or redirecting towards the next-hop). Note that proxy operation is bidirectional - the proxy may be forwarding a request from an ordinary SIP device to the P2PSIP overlay, or from the P2PSIP overlay to an ordinary SIP device.
The Gateway Peer provides a similar sort of adaptation to and from the public switched telephone network (PSTN). However, there is a subtle distinction. The gateway function itself can be viewed as a "user" or within the P2PSIP overlay, and is addressed using a P2PSIP Overlay User Identifier. This gateway functionality could also be located in a P2PSIP Client, or even in a traditional SIP UA that is reached via P2P (using a P2P proxy or redirector) or conventional SIP mechanisms.
The functions of various types of peers (redirect, UA, proxy, gateway) are logical roles. There is no reason a particular implementation could not support one, several, or all of these functions in one entity. For clarity, we show each as a fully distinct entity.
The following show very high level examples of message flows for various interactions of devices within the reference model. In each case, we DO NOT show the flow of messages exchanged between P2PSIP peers to lookup information, since the exact nature of these flows and even who the messages would flow between will be a function of the particular data structure and protocol that is selected. We do however indicate when the lookups occur. This leads to the somewhat odd situation of a diagram having numbered flows to indicate ordering, but some numbers missing. This is regrettable, but we aren't sure how else to do this since we cannot currently know what the lookup flows will look like in the final P2PSIP Peer protocol.
In a solution where the P2PSIP Client Protocol is some protocol other than SIP, all of the following example flows are needed. In a design where unmodified SIP is used for the P2PSIP Client, Section Section 3.3.2 (P2PSIP Client contacts P2PSIP Peer) is not needed.
The following diagram shows P2PSIP UA Peer "E" placing a call to P2PSIP UA Peer "F". 1) UA Peer "E" first uses the P2PSIP Peer protocol to communicate among the peers and obtain the location of "F" (flow not shown as this will depend on the protocol designed). 2) "E" then establishes a session directly with "F" using a conventional SIP INVITE/200 OK mechanism.
2) SIP INVITE/200 OK /----------------\ / \ --->PSTN +------+ N +------+ +---------+ / | | A | | | Gateway |-/ | UA |####T#####| UA |#####| Peer |######## | Peer | N | Peer | | G | # Client Protocol | E | A | F | +---------+ # GET/PUT | | T | | # | +------+ N +------+ # | # A # | NATNATNATNAT # | # # | \__/ NATNATNATNAT +-------+ v / \ # N | |=====/ UA \ +------+ A P2PSIP Overlay | Proxy | /Client\ | | T | Peer | |___C__| | UA | N Route Data | Q | | Peer | A +-------+ | D | T P2PSIP Peer Protocol # | | N # +------+ A # # T # # N +-------+ +-------+ # # A | | | | # #########T####| Proxy |########| Redir |####### | Peer | | Peer | | P | | R | +-------+ +-------+
Figure: P2PSIP Peer to Peer
NOTE: In a design where unmodified SIP is used for the P2PSIP Client protocol, this case does not exist/is not needed. Sections Section 3.3.3 (Conventional SIP Device using a Proxy Peer) and Section 3.3.4 (Conventional SIP Device Using a Redirect Peer), covering conventional SIP access are all that are required.
The following diagram shows P2PSIP UA Client "C" placing a call to P2PSIP UA Peer "F". 1) "C" first sends a GET request using the P2P Client GET/PUT protocol to a Peer in the overlay, in this case "Q". 2) Some messages are exchanged among client "C" and the peers in the overlay to perform the lookup (flow not shown as this will depend on the protocol designed), and the address of "F" is passed back to "C" using the P2PSIP Client protocol. 3) "C" then establishes a session directly with "F" using a conventional SIP INVITE/200 OK mechanism.
3) SIP INVITE/200 OK /---------------------------------------------+ / | / --->PSTN | +------+ N +------+ +---------+ / | | | A | | | Gateway |-/ | | UA |####T#####| UA |#####| Peer |######## | | Peer | N | Peer | | G | # 1) Client Protocol | | E | A | F | +---------+ # GET | | | T | | # | | +------+ N +------+ # | | # A # | / NATNATNATNAT # | / # # | \__/ / NATNATNATNAT +-------+ v / \ / # N | |=====/ UA \ / +------+ A P2PSIP Overlay | Proxy | /Client\/ | | T | Peer | |___C__| | UA | N Route Data | Q | | Peer | A +-------+ | D | T P2PSIP Peer Protocol # | | N # +------+ A # # T # # N +-------+ +-------+ # # A | | | | # #########T####| Proxy |########| Redir |####### | Peer | | Peer | | P | | R | +-------+ +-------+
Figure: P2PSIP Client to Peer
The following diagram shows a conventional SIP device, SIP UA "A", establishing a dialog with UA Peer "F". 1) "A" sends a conventional SIP INVITE to Proxy Peer "P". 2) Proxy Peer "P" uses the P2PSIP Overlay Protocol to locate the target (flow not shown as this will depend on the protocol designed), in this case UA Peer "F". 3) "P" forwards the SIP request to the destination and proxies the response back to "A".
--->PSTN +------+ N +------+ +---------+ / | | A | | | Gateway |-/ | UA |####T#####| UA |#####| Peer |######## | Peer | N | Peer | | G | # Client Protocol | E | A | F | +---------+ # GET/PUT | | T | | # | +------+ N +------+ # | # A | # | NATNATNATNAT | # | # | # | \__/ NATNATNATNAT | +-------+ v / \ # N | | |=====/ UA \ +------+ A P2PSIP Overlay | Proxy | /Client\ | | T | | Peer | |___C__| | UA | N | | Q | | Peer | A | +-------+ | D | T |3) SIP INVITE/200 OK # | | N | # +------+ A | # # T | # # N +-------+ +-------+ # # A | | | | # #########T####| Proxy |########| Redir |####### | Peer | | Peer | | P | | R | +-------+ +-------+ / / \__/ / 1) SIP INVITE/200 OK /\ / / \/ / UA \ /______\ SIP UA A
Figure: Proxied SIP dialog from SIP UA to P2PSIP UA through Peer Proxy
The following diagram shows a second conventional SIP device, SIP UA "A" establishing a dialog with a P2PSIP Client UA "C". 1) "A" sends a conventional SIP INVITE to the Redirect Peer "R". 2) Redirect Peer "R" uses the P2PSIP Peer Protocol to locate the target (flow not shown as this will depend on the protocol designed), in this case P2PSIP Client UA "C". In contrast to the Proxy peer above, "R" returns the result of the lookup to "A" as a 302 Moved message, with a contact of "C" (the conventional SIP 302 mechanism), rather than proxying the request for "A". 3) The conventional SIP UA "A" device can then establish the dialog directly with UA Client "C", even though "A" has no awareness of the P2PSIP Overlay, or of the P2PSIP Client Protocol.
--->PSTN +------+ N +------+ +---------+ / | | A | | | Gateway |-/ | UA |####T#####| UA |#####| Peer |######## | Peer | N | Peer | | G | # Client Protocol | E | A | F | +---------+ # GET/PUT | | T | | # | +------+ N +------+ # | # A # | NATNATNATNAT # | # # | \__/ NATNATNATNAT +-------+ v / \ # N | |=====/ UA \ +------+ A P2PSIP Overlay | Proxy | /Client\ | | T | Peer | |___C__| | UA | N Route Data | Q | | | Peer | A +-------+ | | D | T P2PSIP Peer Protocol # | | | N # 3) SIP INVITE +------+ A # /200 OK # T # | # N +-------+ +-------+ # | # A | | | | # | #########T####| Proxy |########| Redir |####### | | Peer | | Peer | / | P | | R | / +-------+ +-------+ / \ / \ / 1) SIP INVITE \ \__/ / /302 Moved \ /\ / \ / \ / \/ UA \/ /______\ SIP UA A
Figure: Redirect from P2PSIP Overlay
Peers are the full-function "routing and storage" nodes of a P2PSIP Overlay. When a new peer is first created, it must enroll in the P2PSIP Overlay. We currently have no defined mechanism for this (should this group define one?), but we know that once the process is complete, the new peer will have at least a P2PSIP Peer-ID and optionally a set of credentials.
After enrollment, each time the peer connects to the overlay, it must insert itself. We don't have a defined protocol mechanism for this, and assume we need one. Presumably the inserting peer connects to one or more existing peers (possibly with the aid of a bootstrap server) presents its credentials, and after exchanging some messages with other P2PSIP Peers, ends up connected to the overlay. It will then have some knowledge of neighbors (successor, precursor, finger tables, or whatever the distribution mechanism defines) and is able to store data on behalf of and route requests to other nodes in the P2PSIP overlay.
P2PSIP Peers directly interact with and contain the routing and storage fabric of the overlay. P2PSIP Clients simply use the routing and storage facilities provided by the peers to get/put information. The peers speak the P2PSIP Peer Protocol, which presumably has a full range of expressivity for the routing and storage facilities of the overlay. Clients speak the P2PSIP Client protocol, which is presumably a subset of the peer protocol, and is limited to storage insertion (put), storage retrieval (get), and message routing (send). Some designs do not require a separate client protocol.
Some peers and some clients may be coupled to SIP user agents, making them P2PSIP User Agents capable of both sending and receiving conventional SIP messages (as per a SIP UA) using conventional SIP resolution procedures and of using the resolution facilities provided by the overlay.
The mix and configuration of peers, clients, and P2PSIP UAs is expected to vary depending on the deployment scenario. For example, an ad-hoc scenario might deploy nothing but P2PSIP Peers, each of which is coupled to a P2PSIP User Agent, using a broadcast or multicast bootstrap mechanism. Another common scenario, the "self organizing proxy farm", might consist of P2PSIP Peers, each of which is coupled to a SIP proxy/registrar function.
Some of the systems proposed that use SIP for the P2PSIP Client protocol essentially remove that protocol from their design. Standard SIP messages are sent to a proxy or redirect server which speaks the P2PSIP server protocol, eliminating the need for another protocol.
Clients and Peers are devices, the "end points" or "user agents" of a P2PSIP Overlay. Users are the named entities that participate in a P2PSIP overlay using a client.
To get started, users must be enrolled in the overlay. We do not have a process or protocol for this, nor are we certain we need a standardized mechanism. We presume that after enrollment, the user has a distinguished name within the overlay (example: sip:firstname.lastname@example.org) and a set of credentials useful for authenticating its usage of the distinguished name. One possible mechanism for these credentials would be an x.509 certificate. It might also be possible to use a PGP key, a password, or some other mechanism. Presumably following enrollment, the user is also equipped with the information needed to connect to the overlay, such as the address of a bootstrap server. Whether this startup information is delivered as a part of enrollment of through some separate configuration process remains an open question, and it is not clear it is within the scope of the proposed WG.
Once a user is enrolled, the user may exercise a P2PSIP User Agent to insert into the P2PSIP Overlay. We currently have no protocol mechanism for this, and need to define one. The P2PSIP UA exercises the associated P2PSIP Peer or P2PSIP Client to execute the "registration" function and insert a route for the user into the P2PSIP overlay. This function is described as a "PUT" request, and results in the storage of an authenticated route-set for the user in the P2PSIP overlay, such that the terminus of the route is the URI of the user at the P2PSIP UA. This is analogous to "registration" in a classic SIP environment, and one mechanism proposed is in fact to use the SIP REGISTER method. Presumably, the P2PSIP UA connects to a peer or client and uses the user's credentials to authenticate a route-set (Contact: plus Path:) to itself, and the peer or client stores the route-set into the overlay, using a key derived from the user's identity.
If a client or peer is just starting up and has no knowledge of how to reach the other nodes of the overlay, it may exercise a bootstrap server to find one. Presumably it discovers the bootstrap server by some mechanism such as a DNS lookup, multicast, broadcast or configuration, then queries the bootstrap server and receives an address for a peer or set of peers that can be used to reach the overlay. Ideally, these target peers will be selected from a relatively large pool of peers that are currently online and reachable.
After discovering the address of a peer, the behavior of the starting node will vary depending on whether it is intending to be a peer or a client. If it is intending to be a peer, it goes into the P2PSIP Peer Insertion process, at the conclusion of which it is actively participating in the target overlay as a peer and is capable of routing requests and storing records on behalf of the P2PSIP overlay. If it is intending to be a client, it does not bother with insertion, but merely contacts the discovered peer in order to use the overlay.
In the typical case, the peer or client coming up is also a P2PSIP User Agent with one or more associated P2PSIP Resource (User) Identifiers. The next step then is to insert a P2PSIP Resource Record (a Contact:) into the P2PSIP Overlay.
We may wish to have a mechanism in place where a particular bootstrap server can send a redirect response, offloading a heavily loaded server.
This may or may not be SIP. What should it be? Alternatives include SIP, a full IETF protocol based on OpenDHT, or something else. Do we need to define a new protocol? Will implementors want to implement a new protocol?
This may or may not be SIP. What should it be? It defines only GET/PUT operations, which could be done using SIP REGISTER transactions. Essentially disappears if we do select SIP.
This needs to be net-path efficient. Is this possible? Is it enough just to construct a key with a "relay" identifier? What sorts of access controls do we need on media relays?
This needs to be not only netpath efficient, but also embodies elements of the TRIP and SPEERMINT problems.
We assume that some or even many peers will be behind NATs, and therefore reached through peer-to-peer routing. How do we keep alive the NAT-crossing peer bindings? Is some variant of "outbound"  (Jennings, C. and R. Mahy, “Managing Client Initiated Connections in the Session Initiation Protocol (SIP),” June 2006.) usable?
When forwarding requests, are the bodies of the requests visible to peers? If so, this creates substantial security problems that the deployers of conventional SIP have been willing to mostly ignore. Can we make peers cryptotransparent (like HTTP proxies) when security is requested?
Clearly we need user routing records stored into the P2PSIP overlay. Do we need other sorts of record? If so, what? How do we differentiate between or classify records? Do we end up with many records per user per client, or do we aggregate the per-client or per-user view using something like XML?
We know that we need to enroll peer and client nodes into a P2PSIP Overlay. Do we define a protocol or process for this, assume it will happen externally, or just provide an existence-proof argument?
We believe we need some sort of credentials for authenticating peers and users of each P2PSIP Overlay. What should we use for these credentials? Certificates? PGP keys? Passwords? If certificates, should these be signed by a CA associated with the overlay, or can self-signed certificates work in some or all cases? Do we need to specify a standard credential format, or should we allow different implementations to use different credential formats?
We know that sometimes peers or clients will start up without knowledge of how to find a peer for insertion. Do we need to define a bootstrap mechanism or mechanisms? Do we need to define supporting protocols?
One reader suggested that we extend the definition of P2PSIP Peer Enrollment to cover the case where a previously-inserted peer has lost its credentials (through, perhaps, being moved to a different host) and wishes to recover them without necessarily creating a new P2PSIP Peer-ID. The editors are inclined to believe that this is an operational issue, not a matter of definition, but would like to seek a broader consensus before concluding the topic. A similar question applies to user enrollment.
If the P2PSIP Resource (User) Identifier is not scoped to a single DNS domain, this would appear to allow nodes from two or more apparent domains to share a single P2PSIP Overlay. What, if anything, do we need to say about this mode of operation?
It appears possible to have some hosts within a domain using conventional SIP and some using P2PSIP. This potentially raises a number of questions: 1) What should happen if we want to run a P2PSIP overlay in an existing SIP domain? 2) Do the existing redir/proxy servers need to be coupled with a peer layer? 3) When would an overlay peer want to discover them as opposed to looking in the overlay? 4) Is better not to run conventional SIP with P2PSIP? 5) When conventional and P2PSIP are run together, shall the existing redir servers keep their local databases or switch to the overlay storage.
What do we need to say about admissions control with respect to the enrollment of peers and users? Do we need to discuss per-call admissions control in a P2P environment?
This model presumes that all addressable elements, aka "users", are unique. Are their other classes of resources that need some sort of class-addressable identifier that does not refer to a unique user?
Building a P2PSIP system has many security considerations, many of which we have only begun to consider. We anticipate that the protocol documents describing the actual protocols will deal more thoroughly with security topics.
This document presently raises no IANA considerations.
This document draws heavily from the contributions of many participants in the P2PSIP Mailing List but the authors are especially grateful for the support of Spencer Dawkins, Cullen Jennings, and Henning Schulzrinne, all of whom spent time on phone calls about this document or provided text. Additionally, Spencer provided a large portion of the ASCII art contained in this document.
|||Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML).|
|||Berners-Lee, T., Masinter, L., and M. McCahill, “Uniform Resource Locators (URL),” RFC 1738, December 1994.|
|||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.|
|||Mockapetris, P., “Domain names - concepts and facilities,” STD 13, RFC 1034, November 1987.|
|||Rosenberg, J. and H. Schulzrinne, “Session Initiation Protocol (SIP): Locating SIP Servers,” RFC 3263, June 2002.|
|||Willis, D. and B. Hoeneisen, “Session Initiation Protocol (SIP) Extension Header Field for Registering Non-Adjacent Contacts,” RFC 3327, December 2002.|
|||Bryan, D., “A P2P Approach to SIP Registration and Resource Location,” draft-bryan-sipping-p2p-02 (work in progress), March 2006.|
|||Shim, E., “An Architecture for Peer-to-Peer Session Initiation Protocol (P2P SIP),” draft-shim-sipping-p2p-arch-00 (work in progress), March 2006.|
|||Sinnreich, H. and A. Johnston, “SIP, P2P, and Internet Communications,” draft-johnston-sipping-p2p-ipcom-02 (work in progress), March 2006.|
|||Matthews, P., “Industrial-Strength P2P SIP,” draft-matthews-sipping-p2p-industrial-strength-00 (work in progress), February 2005.|
|||Risson, J. and T. Moors, “Survey of Research towards Robust Peer-to-Peer Networks: Search Methods,” draft-irtf-p2prg-survey-search-00 (work in progress), March 2006.|
|||Bryan, D., “Use Cases for Peer-to-Peer Session Initiation Protocol (P2P SIP),” draft-bryan-sipping-p2p-usecases-00 (work in progress), December 2005.|
|||Rescorla, E. and IAB, “Writing Protocol Models,” RFC 4101, June 2005.|
|||Rosenberg, J., “Obtaining Relay Addresses from Simple Traversal Underneath NAT (STUN),” draft-ietf-behave-turn-02 (work in progress), October 2006.|
|||Jennings, C. and R. Mahy, “Managing Client Initiated Connections in the Session Initiation Protocol (SIP),” draft-ietf-sip-outbound-04 (work in progress), June 2006.|
|Dean Willis (editor)|
|3100 Independence Pkwy #311-164|
|Plano, Texas 75075|
|David A. Bryan|
|SIPeerior Technologies and William & Mary|
|3000 Easter Circle|
|Williamsburg, Virginia 23188|
|100 Innovation Drive|
|Ottawa, Ontario K2K 3G7|
|Phone:||+1 613 592 4343 x224|
|Panasonic Digital Networking Laboratory|
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|Princeton, New Jersey 08540|
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