| P2PSIP Working Group | D. Bryan |
| Internet-Draft | Cogent Force, LLC |
| Intended status: Informational | P. Matthews |
| Expires: October 23, 2016 | Alcatel-Lucent |
| E. Shim | |
| Samsung Electronics Co., Ltd. | |
| D. Willis | |
| Softarmor Systems | |
| S. Dawkins | |
| Huawei (USA) | |
| April 21, 2016 |
Concepts and Terminology for Peer to Peer SIP
draft-ietf-p2psip-concepts-09
This document defines concepts and terminology for the use of the Session Initiation Protocol in a peer-to-peer environment where the traditional proxy-registrar and message routing functions are replaced by a distributed mechanism. These mechanisms may 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 problems addressed by the P2PSIP working group and the RELOAD protocol and SIP usage document defined by the working group.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 23, 2016.
Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
One of the fundamental problems in multimedia communication between Internet nodes is the rendezvous problem, or discovering the host at which a given user can be reached. In the Session Initiation Protocol (SIP) [RFC3261] this problem is expressed as the problem of mapping an Address of Record (AoR) for a user into one or more Contact URIs [RFC3986]. The AoR is a name for the user that is independent of the host or hosts where the user can be contacted, while a Contact URI indicates the host where the user can be contacted.
In the common SIP-using architectures that we refer to as "Conventional SIP" or "Client/Server SIP", there is a relatively fixed hierarchy of SIP routing proxies and SIP user agents. To deliver a SIP INVITE to the host or hosts at which the user can be contacted, a SIP UA follows the procedures specified in [RFC3263] to determine the IP address of a SIP proxy, and then sends the INVITE to that proxy. The proxy will then, in turn, deliver the SIP INVITE to the hosts where the user can be contacted.
This document gives a high-level description of an alternative solution to this problem. In this alternative solution, the relatively fixed hierarchy of Client/Server SIP is replaced by a peer-to-peer overlay network. In this peer-to-peer overlay network, the various AoR to Contact URI mappings are not centralized at proxy/registrar nodes but are instead distributed amongst the peers in the overlay.
The details of this alternative solution are specified by the RELOAD protocol [RFC6940], which defines a mechanism to distribute using a Distributed Hash Table (DHT) and specifies the wire protocol, security, and authentication mechanisms needed to convey this information. This DHT protocol was designed specifically with the purpose of enabling a distributed SIP registrar in mind. While designing the protocol other applications were considered, and when possible design decisions were made that allow RELOAD to be used in other instances where a DHT is desirable, but only when making such decisions did not add undue complexity to the RELOAD protocol. The RELOAD sip draft [I-D.ietf-p2psip-sip] specifies how RELOAD is used with the SIP protocol to enable a distributed, server-less SIP solution.
A P2PSIP Overlay is a collection of nodes organized in a peer-to-peer fashion for the purpose of enabling real-time communication using the Session Initiation Protocol (SIP). Collectively, the nodes in the overlay provide a distributed mechanism for mapping names to overlay locations. This provides for the mapping of Addresses of Record (AoRs) to Contact URIs, thereby providing the "location server" function of [RFC3261]. An Overlay also provides a transport function by which SIP messages can be transported between any two nodes in the overlay.
A P2PSIP Overlay consists of one or more nodes called Peers. The nodes in the overlay collectively run a distributed database algorithm. This distributed database algorithm allows data to be stored on nodes and retrieved in an efficient manner. It may also ensure that a copy of a data item is stored on more than one node, so that the loss of a node does not result in the loss of the data item to the overlay.
One use of this distributed database is to store the information required to provide the mapping between AoRs and Contact URIs for the distributed location function. This provides a location function within each overlay that is an alternative to the location functions described in [RFC3263]. However, the model of [RFC3263] is used between overlays.
The nature of peer-to-peer computing is that each peer offers services to other peers to allow the overlay to collectively provide larger functions. In P2PSIP, peers offer both distributed storage and distributed message routing services, allowing these functions to be implemented across the overlay. Additionally, the RELOAD protocol offers a simplistic discovery mechanism specific to the TURN [RFC5766] protocol used for NAT traversal. Individual peers may also offer other services as an enhancement to P2PSIP functionality (for example to support voicemail) or to support other applications beyond SIP. To support these additional services, peers may need to store additional information in the overlay. [RFC7374] describes the mechanism used in P2PSIP for resource discovery.
An overlay may or may not also include one or more nodes called clients. Clients are supported in the RELOAD protocol as peers that have not joined the overlay, and therefore do not route messages or store information. Clients access the services of the RELOAD protocol by connecting to a peer which performs operations on the behalf of the client. Note that in RELOAD there is no distinct client protocol. Instead, a client connects using the same protocol, but never joins the overlay as a peer. For more information, see [RFC6940].
A special peer may also be a member of the P2PSIP overlay and may present the functionality of one or all of a SIP registrar, proxy or redirect server to conventional SIP devices (i.e., unmodified SIP UA or client). In this way, existing, unmodified SIP clients may connect to the P2PSIP network. Note that in the context of P2PSIP, the unmodified SIP client is also sometimes referred to as a client. These unmodified SIP devices do not speak the RELOAD protocol, and this is a distinct concept from the notion of client discussed in the previous paragraph.
The RELOAD protocol defined by the P2PSIP working group implements a DHT primarily for use by server-less, peer-to-peer SIP deployments. However, the RELOAD protocol could be used for other applications as well. As such, a "P2PSIP" deployment is generally assumed to be a use of RELOAD to implement distributed SIP, but it is possible that RELOAD is used as a mechanism to distribute other applications, completely unrelated to SIP.
Since P2PSIP is about peer-to-peer networks for real-time communication, it is expected that most peers and clients will be coupled with SIP entities (although RELOAD may be used for other applications than P2PSIP). For example, one peer might be coupled with a SIP UA, another might be coupled with a SIP proxy, while a third might be coupled with a SIP-to-PSTN gateway. For such nodes, the peer or client portion of the node is logically distinct from the SIP entity portion. However, there is no hard requirement that every P2PSIP node (peer or client) be coupled to a SIP entity. As an example, additional peers could be placed in the overlay to provide additional storage or redundancy for the RELOAD overlay, but might not have any direct SIP capabilities.
As noted above, the fundamental task of P2PSIP is turning an AoR into a Contact. This task might be approached using zero configuration techniques such as multicast DNS and DNS Service Discovery [RFC6762][RFC6763], link-local multicast name resolution [RFC4795], and dynamic DNS [RFC2136].
These alternatives were discussed in the P2PSIP Working Group, and not pursued as a general solution for a number of reasons related to scalability, the ability to work in a disconnected state, partition recovery, and so on. However, there does seem to be some continuing interest in the possibility of using DNS-SD and mDNS for bootstrapping of P2PSIP overlays.
Network Address Translators (NATs) are impediments to establishing and maintaining peer-to-peer networks, since NATs hinder direct communication between nodes. Some peer-to-peer network architectures avoid this problem by insisting that all nodes exist in the same address space. However, RELOAD provides capabilities that allow nodes to be located in multiple address spaces interconnected by NATs, to allow RELOAD messages to traverse NATs, and to assist in transmitting application-level messages (for example SIP messages) across NATs.
The following diagram shows a P2PSIP Overlay consisting of a number of Peers, one Client, and an ordinary SIP UA. It illustrates a typical P2PSIP overlay but does not limit other compositions or variations; for example, Proxy Peer P might also talk to a ordinary SIP proxy as well. The figure is not intended to cover all possible architecture variations, but simply to show a deployment with many common P2PSIP elements.
--->PSTN
+------+ N +------+ +---------+ /
| | A | | | Gateway |-/
| UA |####T#####| UA |#####| Peer |########
| Peer | N | Peer | | G | # RELOAD
| E | A | F | +---------+ # P2PSIP
| | T | | # Protocol
+------+ N +------+ # |
# A # |
NATNATNATNAT # |
# # | \__/
NATNATNATNAT +-------+ v / \
# N | |#####/ UA \
+------+ A P2PSIP Overlay | Peer | /Client\
| | T | Q | |___C__|
| UA | N | |
| Peer | A +-------+
| D | T #
| | N #
+------+ A # RELOAD
# T # P2PSIP
# N +-------+ +-------+ # Protocol
# A | | | | #
#########T####| Proxy |########| Redir |#######
N | Peer | | Peer |
A | P | | R |
T +-------+ +-------+
| /
| SIP /
\__/ / /
/\ / ______________/ SIP
/ \/ /
/ UA \/
/______\
SIP UA A
Figure: P2PSIP Overlay Reference Model
Here, the large perimeter depicted by "#" represents a stylized view of the Overlay (the actual connections could be a mesh, a ring, or some other structure). Around the periphery of the Overlay rectangle, we have a number of Peers. Each peer is labeled with its coupled SIP entity -- for example, "Proxy Peer P" means that peer P which is coupled with a SIP proxy. In some cases, a peer or client might be coupled with two or more SIP entities. In this diagram we have a PSTN gateway coupled with peer "G", three peers ("D", "E" and "F") which are each coupled with a UA, a peer "P" which is coupled with a SIP proxy, an ordinary peer "Q" with no SIP capabilities, and one peer "R" which is coupled with a SIP Redirector. Note that because these are all Peers, each is responsible for storing Resource Records and transporting messages around the Overlay.
To the left, two of the peers ("D" and "E") are behind network address translators (NATs). These peers are included in the P2PSIP overlay and thus participate in storing resource records and routing messages, despite being behind the NATs.
On the right side, we have a client "C", which uses the RELOAD Protocol to communicate with Proxy Peer "Q". The Client “C” uses RELOAD to obtain information from the overlay, but has not inserted itself into the overlay, and therefore does not participate in routing messages or storing information.
Below the Overlay, we have a conventional SIP UA "A" which is not part of the Overlay, either directly as a peer or indirectly as a client. It does not speak the RELOAD P2PSIP protocol, and is not participating in the overlay as either a Peer nor Client. Instead, it uses SIP to interact with the Overlay via an adapter peer or peers which communicate with the overlay using RELOAD.
Both the SIP proxy coupled with peer "P" and the SIP redirector coupled with peer "R" can serve as adapters between ordinary SIP devices and the Overlay. Each accepts standard SIP requests and resolves the next-hop by using the P2PSIP protocol to interact with the routing knowledge of the 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 Overlay, or from the P2PSIP overlay to an ordinary SIP device.
The PSTN Gateway at peer "G" provides a similar sort of adaptation to and from the public switched telephone network (PSTN).
This section defines a number of concepts that are key to understanding the P2PSIP work.
A P2PSIP Overlay functions as a distributed database. The database serves as a way to store information about Resources. A piece of information, called a Resource Record, can be stored by and retrieved from the database using a key associated with the Resource Record called its Resource-ID. Each Resource must have a unique Resource-ID. In addition to uniquely identifying the Resource, the Resource-ID is also used by the distributed database algorithm to determine the peer or peers that store the Resource Record in the overlay.
Users are humans that can use the overlay to do things like making and receiving calls. Information stored in the resource record associated with a user can include things like the full name of the user and the location of the UAs that the user is using (the users SIP AoR). Full details of how this is implemented using RELOAD are provided in [I-D.ietf-p2psip-sip]
Before information about a user can be stored in the overlay, a user needs a User Name. The User Name is a human-friendly identifier that uniquely identifies the user within the overlay. In RELOAD, users are issued certificates, which in the case of centrally signed certificates, identify the User Name as well as a certain number of Resource-IDs where the user may store their information. For more information, see [RFC6940].
The P2PSIP suite of protocols also standardizes information about how to locate services. Services represent actions that a peer (and perhaps a client) can do to benefit other peers and clients in the overlay. Information that might be stored in the resource record associated with a service might include the peers (and perhaps clients) offering the service. Service discovery for P2PSIP is defined in [RFC7374].
Each service has a human-friendly Service Name that uniquely identifies the service. Like User Names, the Service Name is not a resource-id, rather the resource-id is derived from the service name using some function defined by the distributed database algorithm used by the overlay.
A class of algorithms known as Distributed Hash Tables are one way to implement the Distributed Database. The RELOAD protocol is extensible and allows many different DHTs to be implemented, but specifies a mandatory to implement DHT in the form of a modified Chord DHT. For more information, see [Chord]
While there are a number of ways the distributed database described in the previous section can be used to establish multimedia sessions using SIP, the basic mechanism defined in the RELOAD protocol and SIP usage is summarized below. This is a very simplistic overview. For more detailed information, please see the RELOAD protocol document.
Contact information for a user is stored in the resource record for that user. Assume that a user is using a device, here called peer A, which serves as the contact point for this user. The user adds contact information to this resource record, as authorized by the RELOAD certificate mechanism. The resource record itself is stored with peer Z in the network, where peer Z is chosen by the particular distributed database algorithm in use by the overlay.
When the SIP entity coupled with peer B has an INVITE message addressed to this user, it retrieves the resource record from peer Z. It then extracts the contact information for the various peers that are a contact point for the user, including peer A, and uses the overlay to establish a connection to peer A, including any appropriate NAT traversal (the details of which are not shown).
Note that RELOAD is used only to establish the connection. Once the connection is established, messages between the peers are sent using ordinary SIP.
This exchange is illustrated in the following figure. The notation "Store(U@A)" is used to show the distributed database operation of updating the resource record for user U with the contract A, and "Fetch(U)" illustrates the distributed database operation of retrieving the resource record for user U. Note that the messages between the peers A, B and Z may actually travel via intermediate peers (not shown) as part of the distributed lookup process or so as to traverse intervening NATs.
Peer B Peer Z Peer A
| | |
| | Store(U@Y)|
| |<------------------|
| |Store-Resp(OK) |
| |------------------>|
| | |
|Fetch(U) | |
|------------------->| |
| Fetch-Resp(U@Y)| |
|<-------------------| |
| | |
(RELOAD IS USED TO ESTABLISH CONNECTION)
| | |
| SIP INVITE(To:U) | |
|--------------------------------------->|
| | |
NAT Traversal in P2PSIP using RELOAD treats all peers as equal and establishes a partial mesh of connections between them. Messages from one peer to another are routed along the edges in the mesh of connections until they reach their destination. To make the routing efficient and to avoid the use of standard Internet routing protocols, the partial mesh is organized in a structured manner. If the structure is based on any one of a number of common DHT algorithms, then the maximum number of hops between any two peers is log N, where N is the number of peers in the overlay. Existing connections, along with the ICE NAT traversal techniques [RFC5245], are used to establish new connections between peers, and also to allow the applications running on peers to establish a connection to communicate with one another.
Before a peer can attempt to join a P2PSIP overlay, it must first obtain a Node-ID, configuration information, and optionally a set of credentials. The Node-ID is an identifier that will uniquely identify the peer within the overlay, while the credentials show that the peer is allowed to join the overlay.
The P2PSIP WG does not impose a particular mechanism for how the peer-ID and the credentials are obtained, but the RELOAD protocol does specify the format for the configuration information, and specifies how this information may be obtained, along with credentials and a Node-ID, from an offline enrollment server.
Once the configuration information is obtained, RELOAD specifies a mechanism whereby a peer may obtain a multicast-bootstrap address in the configuration file, and can broadcast to this address to attempt to locate a bootstrap peer. Additionally, the peer may store previous peers it has seen and attempt to use these as bootstrap peers, or may obtain an address for a bootstrap peer by some other mechanism. For more information, see the RELOAD protocol.
The job of the bootstrap peer is simple: refer the joining peer to a peer (called the "admitting peer") that will help the joining peer join the network. The choice of admitting peer will often depend on the joining node - for example, the admitting peer may be a peer that will become a neighbor of the joining peer in the overlay. It is possible that the bootstrap peer might also serve as the admitting peer.
The admitting peer will help the joining peer learn about other peers in the overlay and establish connections to them as appropriate. The admitting peer and/or the other peers in the overlay will also do whatever else is required to help the joining peer become a fully-functional peer. The details of how this is done will depend on the distributed database algorithm used by the overlay.
At various stages in this process, the joining peer may be asked to present its credentials to show that it is authorized to join the overlay. Similarly, the various peers contacted may be asked to present their credentials so the joining peer can verify that it is really joining the overlay it wants to.
As mentioned above, in RELOAD, from the perspective of the protocol, clients are simply peers that do not store information, do not route messages, and which have not inserted themselves into the overlay. The same protocol is used for the actual message exchanged. Note that while the protocol is the same, the client need not implement all the capabilities of a peer. If, for example, it never routes messages, it will not need to be capable of processing such messages, or understanding a DHT.
For SIP devices, another way to realize this functionality is for a Peer to behave as a [RFC3261] proxy/registrar. SIP devices then use standard SIP mechanisms to add, update, and remove registrations and to send SIP messages to peers and other clients. The authors here refer to these devices simply as a "SIP UA", not a "P2PSIP Client", to distinguish it from the concept described above.
The architecture adopted by RELOAD to implement P2PSIP is shown below. An application, for example SIP (or another application using RELOAD) uses RELOAD to locate other peers and (optionally) to establish connections to those peers, potentially across NATs. Messages may still be exchanged directly between the peers. The overall block diagram for the architecture is as follows:
__________________________
| |
| SIP, other apps... |
| ___________________|
| | RELOAD Layer |
|______|___________________|
| Transport Layer |
|__________________________|
This specification is an overview of existing specifications and does not introduce any security considerations on its own. Please refer to the security considerations of the respective specifications, particularly the RELOAD protocol specification ([RFC6940]) for further details.
This document has no actions for IANA.