draft-ietf-p2psip-concepts-09.txt		rfc7890.txt
	P2PSIP Working Group D. Bryan		Internet Engineering Task Force (IETF) D. Bryan
	Internet-Draft Cogent Force, LLC		Request for Comments: 7890 Cogent Force, LLC
	Intended status: Informational P. Matthews		Category: Informational P. Matthews
	Expires: October 23, 2016 Alcatel-Lucent		ISSN: 2070-1721 Nokia
	E. Shim		E. Shim
	Samsung Electronics Co., Ltd.		Samsung Electronics Co., Ltd.
	D. Willis		D. Willis
	Softarmor Systems		Softarmor Systems
	S. Dawkins		S. Dawkins
	Huawei (USA)		Huawei (USA)
	April 21, 2016		June 2016
	Concepts and Terminology for Peer to Peer SIP		Concepts and Terminology for Peer-to-Peer SIP (P2PSIP)
	draft-ietf-p2psip-concepts-09
	Abstract		Abstract
	This document defines concepts and terminology for the use of the		This document defines concepts and terminology for using the Session
	Session Initiation Protocol in a peer-to-peer environment where the		Initiation Protocol in a peer-to-peer environment where the
	traditional proxy-registrar and message routing functions are		traditional proxy-registrar and message-routing functions are
	replaced by a distributed mechanism. These mechanisms may be		replaced by a distributed mechanism. These mechanisms may be
	implemented using a distributed hash table or other distributed data		implemented using a Distributed Hash Table or other distributed data
	mechanism with similar external properties. This document includes a		mechanism with similar external properties. This document includes a
	high-level view of the functional relationships between the network		high-level view of the functional relationships between the network
	elements defined herein, a conceptual model of operations, and an		elements defined herein, a conceptual model of operations, and an
	outline of the related problems addressed by the P2PSIP working group		outline of the related problems addressed by the P2PSIP working
	and the RELOAD protocol and SIP usage document defined by the working		group, the REsource LOcation And Discovery (RELOAD) protocol, and the
	group.		SIP usage document defined by the working group.
	Status of This Memo		Status of This Memo
	This Internet-Draft is submitted in full conformance with the		This document is not an Internet Standards Track specification; it is
	provisions of BCP 78 and BCP 79.		published for informational purposes.
	Internet-Drafts are working documents of the Internet Engineering
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	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		This document is a product of the Internet Engineering Task Force
	and may be updated, replaced, or obsoleted by other documents at any		(IETF). It represents the consensus of the IETF community. It has
	time. It is inappropriate to use Internet-Drafts as reference		received public review and has been approved for publication by the
	material or to cite them other than as "work in progress."		Internet Engineering Steering Group (IESG). Not all documents
			approved by the IESG are a candidate for any level of Internet
			Standard; see Section 2 of RFC 7841.
	This Internet-Draft will expire on October 23, 2016.		Information about the current status of this document, any errata,
			and how to provide feedback on it may be obtained at
			http://www.rfc-editor.org/info/rfc7890.
	Copyright Notice		Copyright Notice
	Copyright (c) 2016 IETF Trust and the persons identified as the		Copyright (c) 2016 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
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	to this document. Code Components extracted from this document must		to this document. Code Components extracted from this document must
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	described in the Simplified BSD License.		described in the Simplified BSD License.
	Table of Contents		Table of Contents
	1. Background . . . . . . . . . . . . . . . . . . . . . . . . . 2		1. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
	2. High-Level Description . . . . . . . . . . . . . . . . . . . 3		2. High-Level Description . . . . . . . . . . . . . . . . . . . 4
	2.1. Services . . . . . . . . . . . . . . . . . . . . . . . . 4		2.1. Services . . . . . . . . . . . . . . . . . . . . . . . . 4
	2.2. Clients . . . . . . . . . . . . . . . . . . . . . . . . . 4		2.2. Clients . . . . . . . . . . . . . . . . . . . . . . . . . 4
	2.3. Relationship Between P2PSIP and RELOAD . . . . . . . . . 5		2.3. Relationship between P2PSIP and RELOAD . . . . . . . . . 5
	2.4. Relationship Between P2PSIP and SIP . . . . . . . . . . . 5		2.4. Relationship between P2PSIP and SIP . . . . . . . . . . . 5
	2.5. Relationship Between P2PSIP and Other AoR Dereferencing		2.5. Relationship between P2PSIP and Other AoR-Dereferencing
	Approaches . . . . . . . . . . . . . . . . . . . . . . . 5		Approaches . . . . . . . . . . . . . . . . . . . . . . . 6
	2.6. NAT Issues . . . . . . . . . . . . . . . . . . . . . . . 6		2.6. NAT Issues . . . . . . . . . . . . . . . . . . . . . . . 6
	3. Reference Model . . . . . . . . . . . . . . . . . . . . . . . 6		3. Reference Model . . . . . . . . . . . . . . . . . . . . . . . 6
	4. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 8		4. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 8
	5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 12		5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 12
	5.1. The Distributed Database Function . . . . . . . . . . . . 12		5.1. The Distributed Database Function . . . . . . . . . . . . 12
	5.2. Using the Distributed Database Function . . . . . . . . . 13		5.2. Using the Distributed Database Function . . . . . . . . . 13
	5.3. NAT Traversal . . . . . . . . . . . . . . . . . . . . . . 14		5.3. NAT Traversal . . . . . . . . . . . . . . . . . . . . . . 14
	5.4. Locating and Joining an Overlay . . . . . . . . . . . . . 14		5.4. Locating and Joining an Overlay . . . . . . . . . . . . . 14
	5.5. Clients and Connecting Unmodified SIP Devices . . . . . . 15		5.5. Clients and Connecting Unmodified SIP Devices . . . . . . 15
	5.6. Architecture . . . . . . . . . . . . . . . . . . . . . . 16		5.6. Architecture . . . . . . . . . . . . . . . . . . . . . . 16
	6. Security Considerations . . . . . . . . . . . . . . . . . . . 16		6. Security Considerations . . . . . . . . . . . . . . . . . . . 16
	7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16		7. Informative References . . . . . . . . . . . . . . . . . . . 16
	8. Informative References . . . . . . . . . . . . . . . . . . . 16		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
	1. Background		1. Background
	One of the fundamental problems in multimedia communication between		One of the fundamental problems in multimedia communication between
	Internet nodes is the rendezvous problem, or discovering the host at		Internet nodes is the rendezvous problem, or discovering the host at
	which a given user can be reached. In the Session Initiation		which a given user can be reached. In the Session Initiation
	Protocol (SIP) [RFC3261] this problem is expressed as the problem of		Protocol (SIP) [RFC3261], this problem is expressed as the problem of
	mapping an Address of Record (AoR) for a user into one or more		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		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,		independent of the host or hosts where the user can be contacted,
	while a Contact URI indicates the host where the user can be		while a Contact URI indicates the host where the user can be
	contacted.		contacted.
	In the common SIP-using architectures that we refer to as		In the common SIP-using architectures that we refer to as
	"Conventional SIP" or "Client/Server SIP", there is a relatively		"Conventional SIP" or "Client/Server SIP", there is a relatively
	fixed hierarchy of SIP routing proxies and SIP user agents. To		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		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		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		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		that proxy. The proxy will then, in turn, deliver the SIP INVITE to
	the hosts where the user can be contacted.		the hosts where the user can be contacted.
	This document gives a high-level description of an alternative		This document gives a high-level description of an alternative
	solution to this problem. In this alternative solution, the		solution to this problem. In this alternative solution, the
	relatively fixed hierarchy of Client/Server SIP is replaced by a		relatively fixed hierarchy of Client/Server SIP is replaced by a
	peer-to-peer overlay network. In this peer-to-peer overlay network,		peer-to-peer overlay network. In this peer-to-peer overlay network,
	the various AoR to Contact URI mappings are not centralized at proxy/		the various mappings of AoRs to Contact URIs are not centralized at
	registrar nodes but are instead distributed amongst the peers in the		proxy/registrar nodes but are instead distributed amongst the peers
	overlay.		in the overlay.
	The details of this alternative solution are specified by the RELOAD		The details of this alternative solution are specified by the RELOAD
	protocol [RFC6940], which defines a mechanism to distribute using a		protocol [RFC6940], which defines a mechanism for distribution using
	Distributed Hash Table (DHT) and specifies the wire protocol,		a Distributed Hash Table (DHT) and specifies the wire protocol,
	security, and authentication mechanisms needed to convey this		security, and authentication mechanisms needed to convey this
	information. This DHT protocol was designed specifically with the		information. This DHT protocol was designed specifically with the
	purpose of enabling a distributed SIP registrar in mind. While		purpose of enabling a distributed SIP registrar in mind. While
	designing the protocol other applications were considered, and when		designing the protocol, other applications were considered, and then
	possible design decisions were made that allow RELOAD to be used in		design decisions were made that allow RELOAD to be used in other
	other instances where a DHT is desirable, but only when making such		instances where a DHT is desirable, but only when such decisions did
	decisions did not add undue complexity to the RELOAD protocol. The		not add undue complexity to the RELOAD protocol. The RELOAD SIP
	RELOAD sip draft [I-D.ietf-p2psip-sip] specifies how RELOAD is used		document [P2PSIP] specifies how RELOAD is used with the SIP protocol
	with the SIP protocol to enable a distributed, server-less SIP		to enable a distributed, server-less SIP solution.
	solution.
	2. High-Level Description		2. High-Level Description
	A P2PSIP Overlay is a collection of nodes organized in a peer-to-peer		A Peer-to-Peer SIP (P2PSIP) Overlay is a collection of nodes
	fashion for the purpose of enabling real-time communication using the		organized in a peer-to-peer fashion for the purpose of enabling real-
	Session Initiation Protocol (SIP). Collectively, the nodes in the		time communication using the Session Initiation Protocol (SIP).
	overlay provide a distributed mechanism for mapping names to overlay		Collectively, the nodes in the Overlay provide a distributed
	locations. This provides for the mapping of Addresses of Record		mechanism for mapping names to Overlay locations. This provides for
	(AoRs) to Contact URIs, thereby providing the "location server"		the mapping of Addresses of Record (AoRs) to Contact URIs, thereby
	function of [RFC3261]. An Overlay also provides a transport function		providing the "location server" function of [RFC3261]. An Overlay
	by which SIP messages can be transported between any two nodes in the		also provides a transport function by which SIP messages can be
	overlay.		transported between any two nodes in the Overlay.
	A P2PSIP Overlay consists of one or more nodes called Peers. The		A P2PSIP Overlay consists of one or more nodes called "Peers". The
	nodes in the overlay collectively run a distributed database		nodes in the Overlay collectively run a distributed database
	algorithm. This distributed database algorithm allows data to be		algorithm. This distributed database algorithm allows data to be
	stored on nodes and retrieved in an efficient manner. It may also		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		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		that the loss of a node does not result in the loss of the data item
	to the overlay.		to the Overlay.
	One use of this distributed database is to store the information		One use of this distributed database is to store the information
	required to provide the mapping between AoRs and Contact URIs for the		required to provide the mapping between AoRs and Contact URIs for the
	distributed location function. This provides a location function		distributed location function. This provides a location function
	within each overlay that is an alternative to the location functions		within each Overlay that is an alternative to the location functions
	described in [RFC3263]. However, the model of [RFC3263] is used		described in [RFC3263]. However, the model of [RFC3263] is used
	between overlays.		between Overlays.
	2.1. Services		2.1. Services
	The nature of peer-to-peer computing is that each peer offers		The nature of peer-to-peer computing is that each peer offers
	services to other peers to allow the overlay to collectively provide		services to other peers to allow the overlay to collectively provide
	larger functions. In P2PSIP, peers offer both distributed storage		larger functions. In P2PSIP, Peers offer both distributed storage
	and distributed message routing services, allowing these functions to		and distributed message-routing services, allowing these functions to
	be implemented across the overlay. Additionally, the RELOAD protocol		be implemented across the Overlay. Additionally, the RELOAD protocol
	offers a simplistic discovery mechanism specific to the TURN		offers a simplistic discovery mechanism specific to the Traversal
	[RFC5766] protocol used for NAT traversal. Individual peers may also		Using Relays around NAT (TURN) [RFC5766] protocol used for NAT
	offer other services as an enhancement to P2PSIP functionality (for		traversal. Individual Peers may also offer other services as an
	example to support voicemail) or to support other applications beyond		enhancement to P2PSIP functionality (for example, to support
	SIP. To support these additional services, peers may need to store		voicemail) or to support other applications beyond SIP. To support
	additional information in the overlay. [RFC7374] describes the		these additional services, Peers may need to store additional
	mechanism used in P2PSIP for resource discovery.		information in the Overlay. [RFC7374] describes the mechanism used
			in P2PSIP for resource discovery.
	2.2. Clients		2.2. Clients
	An overlay may or may not also include one or more nodes called		An Overlay may or may not also include one or more nodes called
	clients. Clients are supported in the RELOAD protocol as peers that		"Clients". Clients are supported in the RELOAD protocol as peers
	have not joined the overlay, and therefore do not route messages or		that have not joined the Overlay, and therefore do not route messages
	store information. Clients access the services of the RELOAD		or store information. Clients access the services of the RELOAD
	protocol by connecting to a peer which performs operations on the		protocol by connecting to a Peer that performs operations on the
	behalf of the client. Note that in RELOAD there is no distinct		behalf of the Client. Note that in RELOAD, there is no distinct
	client protocol. Instead, a client connects using the same protocol,		client protocol. Instead, a Client connects using the same protocol,
	but never joins the overlay as a peer. For more information, see		but never joins the Overlay as a Peer. For more information, see
	[RFC6940].		[RFC6940].
	A special peer may also be a member of the P2PSIP overlay and may		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		present the functionality of one or all of a SIP registrar, proxy, or
	redirect server to conventional SIP devices (i.e., unmodified SIP UA		redirect server to conventional SIP devices (i.e., unmodified SIP
	or client). In this way, existing, unmodified SIP clients may		user agent (UA) or client). In this way, existing, unmodified SIP
	connect to the P2PSIP network. Note that in the context of P2PSIP,		clients may connect to the P2PSIP network. Note that in the context
	the unmodified SIP client is also sometimes referred to as a client.		of P2PSIP, the unmodified SIP client is also sometimes referred to as
			a "client". These unmodified SIP devices do not speak the RELOAD
	These unmodified SIP devices do not speak the RELOAD protocol, and		protocol, and this is a distinct concept from the notion of "Client"
	this is a distinct concept from the notion of client discussed in the		discussed in the previous paragraph.
	previous paragraph.
	2.3. Relationship Between P2PSIP and RELOAD		2.3. Relationship between P2PSIP and RELOAD
	The RELOAD protocol defined by the P2PSIP working group implements a		The RELOAD protocol defined by the P2PSIP working group implements a
	DHT primarily for use by server-less, peer-to-peer SIP deployments.		DHT primarily for use by server-less, peer-to-peer SIP deployments.
	However, the RELOAD protocol could be used for other applications as		However, the RELOAD protocol could be used for other applications as
	well. As such, a "P2PSIP" deployment is generally assumed to be a		well. As such, a "P2PSIP" deployment is generally assumed to be a
	use of RELOAD to implement distributed SIP, but it is possible that		use of RELOAD to implement distributed SIP, but it is possible that
	RELOAD is used as a mechanism to distribute other applications,		RELOAD is used as a mechanism to distribute other applications,
	completely unrelated to SIP.		completely unrelated to SIP.
	2.4. Relationship Between P2PSIP and SIP		2.4. Relationship between P2PSIP and SIP
	Since P2PSIP is about peer-to-peer networks for real-time		Since P2PSIP is about peer-to-peer networks for real-time
	communication, it is expected that most peers and clients will be		communication, it is expected that most Peers and Clients will be
	coupled with SIP entities (although RELOAD may be used for other		coupled with SIP entities (although RELOAD may be used for other
	applications than P2PSIP). For example, one peer might be coupled		applications than P2PSIP). For example, one Peer might be coupled
	with a SIP UA, another might be coupled with a SIP proxy, while a		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,		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		the Peer or Client portion of the node is logically distinct from the
	SIP entity portion. However, there is no hard requirement that every		SIP entity portion. However, there is no hard requirement that every
	P2PSIP node (peer or client) be coupled to a SIP entity. As an		P2PSIP node (Peer or Client) be coupled to a SIP entity. As an
	example, additional peers could be placed in the overlay to provide		example, additional Peers could be placed in the Overlay to provide
	additional storage or redundancy for the RELOAD overlay, but might		additional storage or redundancy for the RELOAD Overlay, but might
	not have any direct SIP capabilities.		not have any direct SIP capabilities.
	2.5. Relationship Between P2PSIP and Other AoR Dereferencing Approaches		2.5. Relationship between P2PSIP and Other AoR-Dereferencing Approaches
	As noted above, the fundamental task of P2PSIP is turning an AoR into		As noted above, the fundamental task of P2PSIP is to turn an AoR into
	a Contact. This task might be approached using zero configuration		a Contact. This task might be approached using zero configuration
	techniques such as multicast DNS and DNS Service Discovery		techniques such as multicast DNS (mDNS) and DNS Service Discovery
	[RFC6762][RFC6763], link-local multicast name resolution [RFC4795],		(DNS-SD) [RFC6762] [RFC6763], Link-Local Multicast Name Resolution
	and dynamic DNS [RFC2136].		[RFC4795], and dynamic DNS [RFC2136].
	These alternatives were discussed in the P2PSIP Working Group, and		These alternatives were discussed in the P2PSIP working group, and
	not pursued as a general solution for a number of reasons related to		not pursued as a general solution for a number of reasons related to
	scalability, the ability to work in a disconnected state, partition		scalability, the ability to work in a disconnected state, partition
	recovery, and so on. However, there does seem to be some continuing		recovery, and so on. However, there does seem to be some continuing
	interest in the possibility of using DNS-SD and mDNS for		interest in the possibility of using mDNS and DNS-SD for the
	bootstrapping of P2PSIP overlays.		bootstrapping of P2PSIP overlays.
	2.6. NAT Issues		2.6. NAT Issues
	Network Address Translators (NATs) are impediments to establishing		Network Address Translators (NATs) are impediments to establishing
	and maintaining peer-to-peer networks, since NATs hinder direct		and maintaining peer-to-peer networks, since NATs hinder direct
	communication between nodes. Some peer-to-peer network architectures		communication between nodes. Some peer-to-peer network architectures
	avoid this problem by insisting that all nodes exist in the same		avoid this problem by insisting that all nodes exist in the same
	address space. However, RELOAD provides capabilities that allow		address space. However, RELOAD provides capabilities that allow
	nodes to be located in multiple address spaces interconnected by		nodes to be located in multiple address spaces interconnected by
	NATs, to allow RELOAD messages to traverse NATs, and to assist in		NATs, to allow RELOAD messages to traverse NATs, and to assist in
	transmitting application-level messages (for example SIP messages)		transmitting application-level messages (for example, SIP messages)
	across NATs.		across NATs.
	3. Reference Model		3. Reference Model
	The following diagram shows a P2PSIP Overlay consisting of a number		The following diagram shows a P2PSIP Overlay consisting of a number
	of Peers, one Client, and an ordinary SIP UA. It illustrates a		of Peers, one Client, and an ordinary SIP UA. It illustrates a
	typical P2PSIP overlay but does not limit other compositions or		typical P2PSIP Overlay but does not limit other compositions or
	variations; for example, Proxy Peer P might also talk to a ordinary		variations; for example, Proxy Peer P might also talk to an ordinary
	SIP proxy as well. The figure is not intended to cover all possible		SIP proxy as well. The figure is not intended to cover all possible
	architecture variations, but simply to show a deployment with many		architecture variations, but simply to show a deployment with many
	common P2PSIP elements.		common P2PSIP elements.
	--->PSTN		--->PSTN
	+------+ N +------+ +---------+ /		+------+ N +------+ +---------+ /
	| | A | | | Gateway |-/		| | A | | | Gateway |-/
	| UA |####T#####| UA |#####| Peer |########		| UA |####T#####| UA |#####| Peer |########
	| Peer | N | Peer | | G | # RELOAD		| Peer | N | Peer | | G | # RELOAD
	| E | A | F | +---------+ # P2PSIP		| E | A | F | +---------+ # P2PSIP
	skipping to change at page 7, line 41 ¶		skipping to change at page 7, line 41 ¶
	T +-------+ +-------+		T +-------+ +-------+
	| /		| /
	| SIP /		| SIP /
	\__/ / /		\__/ / /
	/\ / ______________/ SIP		/\ / ______________/ SIP
	/ \/ /		/ \/ /
	/ UA \/		/ UA \/
	/______\		/______\
	SIP UA A		SIP UA A
	Figure: P2PSIP Overlay Reference Model		Figure 1: P2PSIP Overlay Reference Model
	Here, the large perimeter depicted by "#" represents a stylized view		Here, the large perimeter depicted by "#" represents a stylized view
	of the Overlay (the actual connections could be a mesh, a ring, or		of the Overlay (the actual connections could be a mesh, a ring, or
	some other structure). Around the periphery of the Overlay		some other structure). Around the periphery of the Overlay
	rectangle, we have a number of Peers. Each peer is labeled with its		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		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		is coupled with a SIP proxy. In some cases, a Peer or Client might
	might be coupled with two or more SIP entities. In this diagram we		be coupled with two or more SIP entities. In this diagram, we have a
	have a PSTN gateway coupled with peer "G", three peers ("D", "E" and		Public Switched Telephone Network (PSTN) gateway coupled with Peer
	"F") which are each coupled with a UA, a peer "P" which is coupled		"G", three Peers ("D", "E", and "F") that are each coupled with a UA,
	with a SIP proxy, an ordinary peer "Q" with no SIP capabilities, and		a Peer "P" that is coupled with a SIP proxy, an ordinary Peer "Q"
	one peer "R" which is coupled with a SIP Redirector. Note that		with no SIP capabilities, and one Peer "R" that is coupled with a SIP
	because these are all Peers, each is responsible for storing Resource		redirector. Note that because these are all Peers, each is
	Records and transporting messages around the Overlay.		responsible for storing Resource Records and transporting messages
			around the Overlay.
	To the left, two of the peers ("D" and "E") are behind network		To the left, two of the Peers ("D" and "E") are behind network
	address translators (NATs). These peers are included in the P2PSIP		address translators (NATs). These Peers are included in the P2PSIP
	overlay and thus participate in storing resource records and routing		Overlay, and thus participate in storing resource records and routing
	messages, despite being behind the NATs.		messages, despite being behind the NATs.
	On the right side, we have a client "C", which uses the RELOAD		On the right side, we have a Client "C", which uses the RELOAD
	Protocol to communicate with Proxy Peer "Q". The Client "C" uses		Protocol to communicate with Proxy Peer "Q". The Client "C" uses
	RELOAD to obtain information from the overlay, but has not inserted		RELOAD to obtain information from the Overlay, but has not inserted
	itself into the overlay, and therefore does not participate in		itself into the Overlay, and therefore does not participate in
	routing messages or storing information.		routing messages or storing information.
	Below the Overlay, we have a conventional SIP UA "A" which is not		Below the Overlay, we have a conventional SIP UA "A" that is not part
	part of the Overlay, either directly as a peer or indirectly as a		of the Overlay, either directly as a Peer or indirectly as a Client.
	client. It does not speak the RELOAD P2PSIP protocol, and is not		It does not speak the RELOAD P2PSIP protocol and is not participating
	participating in the overlay as either a Peer nor Client. Instead,		in the Overlay as a Peer or a Client. Instead, it uses SIP to
	it uses SIP to interact with the Overlay via an adapter peer or peers		interact with the Overlay via an adapter Peer or Peers that
	which communicate with the overlay using RELOAD.		communicate with the Overlay using RELOAD.
	Both the SIP proxy coupled with peer "P" and the SIP redirector		Both the SIP proxy coupled with Peer "P" and the SIP redirector
	coupled with peer "R" can serve as adapters between ordinary SIP		coupled with Peer "R" can serve as adapters between ordinary SIP
	devices and the Overlay. Each accepts standard SIP requests and		devices and the Overlay. Each accepts standard SIP requests and
	resolves the next-hop by using the P2PSIP protocol to interact with		resolves the next hop by using the P2PSIP protocol to interact with
	the routing knowledge of the Overlay, then processes the SIP requests		the routing knowledge of the Overlay, and then processes the SIP
	as appropriate (proxying or redirecting towards the next-hop). Note		requests as appropriate (proxying or redirecting towards the next
	that proxy operation is bidirectional - the proxy may be forwarding a		hop). Note that proxy operation is bidirectional -- the proxy may be
	request from an ordinary SIP device to the Overlay, or from the		forwarding a request from an ordinary SIP device to the Overlay, or
	P2PSIP overlay to an ordinary SIP device.		from the P2PSIP Overlay to an ordinary SIP device.
	The PSTN Gateway at peer "G" provides a similar sort of adaptation to		The PSTN Gateway at Peer "G" provides a similar sort of adaptation to
	and from the public switched telephone network (PSTN).		and from the PSTN.
	4. Definitions		4. Definitions
	This section defines a number of concepts that are key to		This section defines a number of concepts that are key to
	understanding the P2PSIP work.		understanding the P2PSIP work.
	Overlay Network: An overlay network is a computer network which is		Overlay Network: An overlay network is a computer network that is
	built on top of another network. Nodes in the overlay can be		built on top of another network. Nodes in the overlay can be
	thought of as being connected by virtual or logical links, each of		thought of as being connected by virtual or logical links, each of
	which corresponds to a path, perhaps through many physical links,		which corresponds to a path, perhaps through many physical links,
	in the underlying network. For example, many peer-to-peer		in the underlying network. For example, many peer-to-peer
	networks are overlay networks because they run on top of the		networks are overlay networks because they run on top of the
	Internet. Dial-up Internet is an overlay upon the telephone		Internet. Dial-up Internet is an overlay upon the telephone
	network.		network.
	P2P Network: A peer-to-peer (or P2P) computer network is a network		P2P Network: A peer-to-peer (or P2P) computer network is a network
	that relies primarily on the computing power and bandwidth of the		that relies primarily on the computing power and bandwidth of the
	participants in the network rather than concentrating it in a		participants in the network rather than concentrating it in a
	relatively low number of servers. P2P networks are typically used		relatively low number of servers. P2P networks are typically used
	for connecting nodes via largely ad hoc connections. Such		for connecting nodes via largely ad hoc connections. Such
	networks are useful for many purposes. Sharing content files		networks are useful for many purposes. Sharing content files
	containing audio, video, data or anything in digital format is		containing audio, video, data, or anything in digital format is
	very common, and real-time data, such as telephony traffic, is		very common, and real-time data, such as telephony traffic, is
	also exchanged using P2P technology. A P2P Network may also be		also exchanged using P2P technology. A P2P Network may also be
	called a "P2P Overlay" or "P2P Overlay Network" or "P2P Network		called a "P2P Overlay", a "P2P Overlay Network", or a "P2P Network
	Overlay", since its organization is not at the physical layer, but		Overlay", since its organization is not at the physical layer, but
	is instead "on top of" an existing Internet Protocol network.		is instead "on top of" an existing Internet Protocol network.
	P2PSIP: A suite of communications protocols related to the Session		P2PSIP: A suite of communications protocols related to the Session
	Initiation Protocol (SIP) [RFC3261] that enable SIP to use peer-		Initiation Protocol (SIP) [RFC3261] that enable SIP to use peer-
	to-peer techniques for resolving the targets of SIP requests,		to-peer techniques for resolving the targets of SIP requests,
	providing SIP message transport, and providing other SIP-related		providing SIP message transport, and providing other SIP-related
	functions. At present, these protocols include [RFC6940],		functions. At present, these protocols include [RFC6940],
	[I-D.ietf-p2psip-sip], [I-D.ietf-p2psip-diagnostics], [RFC7374]		[RFC7363], [RFC7374], [RFC7851] and [P2PSIP].
	and [RFC7363].
	User: A human that interacts with the overlay through SIP UAs		User: A human that interacts with the Overlay through SIP UAs
	located on peers and clients (and perhaps other ways).		located on Peers and Clients (and perhaps in other ways).
	The following terms are defined here only within the scope of		The following terms are defined here only within the scope of P2PSIP.
	P2PSIP. These terms may have conflicting definitions in other		These terms may have conflicting definitions in other bodies of
	bodies of literature. Some earlier versions of this document		literature. Some draft versions of this document prefixed each term
	prefixed each term with "P2PSIP" to clarify the term's scope.		with "P2PSIP" to clarify the term's scope. This prefixing has been
	This prefixing has been eliminated from the text; however the		eliminated from the text; however, the scoping still applies.
	scoping still applies.
	Overlay Name: A human-friendly name that identifies a specific		Overlay Name: A human-friendly name that identifies a specific
	P2PSIP Overlay. This is in the format of (a portion of) a URI,		P2PSIP Overlay. This is in the format of (a portion of) a URI,
	but may or may not have a related record in the DNS.		but may or may not have a related record in the DNS.
	Peer: A node participating in a P2PSIP Overlay that provides storage		Peer: A node participating in a P2PSIP Overlay that provides storage
	and transport services to other nodes in that P2PSIP Overlay.		and transport services to other nodes in that P2PSIP Overlay.
	Each Peer has a unique identifier, known as a Peer-ID, within the		Each Peer has a unique identifier, known as a Peer-ID, within the
	Overlay. Each Peer may be coupled to one or more SIP entities.		Overlay. Each Peer may be coupled to one or more SIP entities.
	Within the Overlay, the peer is capable of performing several		Within the Overlay, the Peer is capable of performing several
	different operations, including: joining and leaving the overlay,		different operations, including: joining and leaving the Overlay,
	transporting SIP messages within the overlay, storing information		transporting SIP messages within the Overlay, storing information
	on behalf of the overlay, putting information into the overlay,		on behalf of the Overlay, putting information into the Overlay,
	and getting information from the overlay.		and getting information from the Overlay.
	Node-ID: Information that uniquely identifies each Node within a		Node-ID: Information that uniquely identifies each Node within a
	given Overlay. This value is not human-friendly -- in a DHT		given Overlay. This value is not human-friendly -- in a DHT
	approach, this is a numeric value in the hash space. These Node-		approach, this is a numeric value in the hash space. These Node-
	IDs are completely independent of the identifier of any user of a		IDs are completely independent of the identifier of any user of a
	user agent associated with a peer.		user agent associated with a Peer.
	Client: A node participating in a P2PSIP Overlay but that does not		Client: A node that participates in a P2PSIP Overlay but does not
	store information or forward messages. A client can also be		store information or forward messages. A Client can also be
	thought of as a peer that has not joined the overlay. Clients can		thought of as a peer that has not joined the Overlay. Clients can
	store and retrieve information from the overlay.		store and retrieve information from the Overlay.
	User Name: A human-friendly name for a user. This name must be		User Name: A human-friendly name for a user. This name must be
	unique within the overlay, but may be unique in a wider scope.		unique within the Overlay, but may be unique in a wider scope.
	User Names are formatted so that they can be used within a URI		User Names are formatted so that they can be used within a URI
	(likely a SIP URI), perhaps in combination with the Overlay Name.		(likely a SIP URI), perhaps in combination with the Overlay Name.
	Service: A capability contributed by a peer to an overlay or to the		Service: A capability contributed by a Peer to an Overlay or to the
	members of an overlay. Not all peers and clients will offer the		members of an Overlay. Not all Peers and Clients will offer the
	same set of services, and P2PSIP provides service discovery		same set of services, and P2PSIP provides service discovery
	mechanisms to locate services.		mechanisms to locate services.
	Service Name: A unique, human-friendly, name for a service.		Service Name: A unique, human-friendly name for a service.
	Resource: Anything about which information can be stored in the		Resource: Anything about which information can be stored in the
	overlay. Both Users and Services are examples of Resources.		Overlay. Both Users and Services are examples of Resources.
	Resource-ID: A non-human-friendly value that uniquely identifies a		Resource-ID: A non-human-friendly value that uniquely identifies a
	resource and which is used as a key for storing and retrieving		resource and that is used as a key for storing and retrieving data
	data about the resource. One way to generate a Resource-ID is by		about the resource. One way to generate a Resource-ID is by
	applying a mapping function to some other unique name (e.g., User		applying a mapping function to some other unique name (e.g., User
	Name or Service Name) for the resource. The Resource-ID is used		Name or Service Name) for the resource. The Resource-ID is used
	by the distributed database algorithm to determine the peer or		by the distributed database algorithm to determine the Peer or
	peers that are responsible for storing the data for the overlay.		Peers that are responsible for storing data for the Overlay.
	Resource Record: A block of data, stored using distributed database		Resource Record: A block of data, stored using the distributed
	mechanism of the Overlay, that includes information relevant to a		database mechanism of the Overlay, that includes information
	specific resource. We presume that there may be multiple types of		relevant to a specific resource. We presume that there may be
	resource records. Some may hold data about Users, and others may		multiple types of resource records. Some may hold data about
	hold data about Services, and the working group may define other		Users, and others may hold data about Services, and the working
	types. The types, usages, and formats of the records are a		group may define other types. The types, usages, and formats of
	question for future study.		the records are a question for future study.
	Responsible Peer The Peer that is responsible for storing the		Responsible Peer The Peer that is responsible for storing the
	Resource Record for a Resource. In the literature, the term "Root		Resource Record for a Resource. In the literature, the term "Root
	Peer" is also used for this concept.		Peer" is also used for this concept.
	Peer Protocol: The protocol spoken between P2PSIP Overlay peers to		Peer Protocol: The protocol spoken between P2PSIP Overlay Peers to
	share information and organize the P2PSIP Overlay Network. In		share information and organize the P2PSIP Overlay Network. In
	P2PSIP, this is implemented using the RELOAD [RFC6940] protocol.		P2PSIP, this is implemented using the RELOAD protocol [RFC6940].
	Client Protocol: The protocol spoken between Clients and Peers. In		Client Protocol: The protocol spoken between Clients and Peers. In
	P2PSIP and RELOAD, this is the same protocol syntactically as the		P2PSIP and RELOAD, this is syntactically the same protocol as the
	Peer Protocol. The only difference is that Clients are not		Peer Protocol. The only difference is that Clients are not
	routing messages or routing information, and have not (or can not)		routing messages or routing information, and have not (or cannot)
	insert themselves into the overlay.		insert themselves into the Overlay.
	Peer Protocol Connection / P2PSIP Client Protocol Connection:		Peer Protocol Connection / P2PSIP Client Protocol Connection:
	The TLS, DTLS, TCP, UDP or other transport layer protocol		The Transport Layer Security (TLS), Datagram Transport Layer
			Security (DTLS), TCP, UDP, or other transport-layer protocol
	connection over which the RELOAD Peer Protocol messages are		connection over which the RELOAD Peer Protocol messages are
	transported.		transported.
	Neighbors: The set of P2PSIP Peers that a Peer or Client know of		Neighbors: The set of P2PSIP Peers that a Peer or Client know of
	directly and can reach without further lookups.		directly and can reach without further lookups.
	Joining Peer: A node that is attempting to become a Peer in a		Joining Peer: A node that is attempting to become a Peer in a
	particular Overlay.		particular Overlay.
	Bootstrap Peer: A Peer in the Overlay that is the first point of		Bootstrap Peer: A Peer in the Overlay that is the first point of
	contact for a Joining Peer. It selects the peer that will serve		contact for a Joining Peer. It selects the Peer that will serve
	as the Admitting Peer and helps the joining peer contact the		as the Admitting Peer and helps the Joining Peer contact the
	admitting peer.		Admitting Peer.
	Admitting Peer: A Peer in the Overlay which helps the Joining Peer		Admitting Peer: A Peer in the Overlay that helps the Joining Peer
	join the Overlay. The choice of the admitting peer may depend on		join the Overlay. The choice of the Admitting Peer may depend on
	the joining peer (e.g., depend on the joining peer's Peer-ID).		the Joining Peer (e.g., depend on the Joining Peer's Peer-ID).
	For example, the admitting peer might be chosen as the peer which		For example, the Admitting Peer might be chosen as the Peer which
	is "closest" in the logical structure of the overlay to the future		is "closest" in the logical structure of the Overlay to the future
	position of the joining peer. The selection of the admitting peer		position of the Joining Peer. The selection of the Admitting Peer
	is typically done by the bootstrap peer. It is allowable for the		is typically done by the Bootstrap Peer. It is allowable for the
	bootstrap peer to select itself as the admitting peer.		Bootstrap Peer to select itself as the Admitting Peer.
	Bootstrap Server: A network node used by Joining Peers to locate a		Bootstrap Server: A network node used by Joining Peers to locate a
	Bootstrap Peer. A Bootstrap Server may act as a proxy for		Bootstrap Peer. A Bootstrap Server may act as a proxy for
	messages between the Joining Peer and the Bootstrap Peer. The		messages between the Joining Peer and the Bootstrap Peer. The
	Bootstrap Server itself is typically a stable host with a DNS name		Bootstrap Server itself is typically a stable host with a DNS name
	that is somehow communicated (for example, through configuration,		that is somehow communicated (for example, through configuration,
	specification on a web page, or using DHCP) to peers that want to		specification on a web page, or using DHCP) to Peers that want to
	join the overlay. A Bootstrap Server is NOT required to be a peer		join the Overlay. A Bootstrap Server is NOT required to be a Peer
	or client, though it may be if desired.		or Client, though it may be if desired.
	Peer Admission: The act of admitting a node (the "Joining Peer")		Peer Admission: The act of admitting a node (the "Joining Peer")
	into an Overlay as a Peer. After the admission process is over,		into an Overlay as a Peer. After the admission process is over,
	the joining peer is a fully-functional peer of the overlay.		the Joining Peer is a fully functional Peer of the Overlay.
	During the admission process, the joining peer may need to present		During the admission process, the Joining Peer may need to present
	credentials to prove that it has sufficient authority to join the		credentials to prove that it has sufficient authority to join the
	overlay.		Overlay.
	Resource Record Insertion: The act of inserting a P2PSIP Resource		Resource Record Insertion: The act of inserting a P2PSIP Resource
	Record into the distributed database. Following insertion, the		Record into the distributed database. Following insertion, the
	data will be stored at one or more peers. The data can be		data will be stored at one or more Peers. The data can be
	retrieved or updated using the Resource-ID as a key.		retrieved or updated using the Resource-ID as a key.
	5. Discussion		5. Discussion
	5.1. The Distributed Database Function		5.1. The Distributed Database Function
	A P2PSIP Overlay functions as a distributed database. The database		A P2PSIP Overlay functions as a distributed database. The database
	serves as a way to store information about Resources. A piece of		serves as a way to store information about Resources. A piece of
	information, called a Resource Record, can be stored by and retrieved		information, called a "Resource Record", can be stored by and
	from the database using a key associated with the Resource Record		retrieved from the database using a key associated with the Resource
	called its Resource-ID. Each Resource must have a unique Resource-		Record called its "Resource-ID". Each Resource must have a unique
	ID. In addition to uniquely identifying the Resource, the Resource-		Resource-ID. In addition to uniquely identifying the Resource, the
	ID is also used by the distributed database algorithm to determine		Resource-ID is also used by the distributed database algorithm to
	the peer or peers that store the Resource Record in the overlay.		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		Users are humans that can use the Overlay to do things like making
	and receiving calls. Information stored in the resource record		and receiving calls. Information stored in the resource record
	associated with a user can include things like the full name of the		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		user and the location of the UAs that the user is using (the user's
	SIP AoR). Full details of how this is implemented using RELOAD are		SIP AoR). Full details of how this is implemented using RELOAD are
	provided in [I-D.ietf-p2psip-sip]		provided in [P2PSIP].
	Before information about a user can be stored in the overlay, a user		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		needs a User Name. The User Name is a human-friendly identifier that
	uniquely identifies the user within the overlay. In RELOAD, users		uniquely identifies the user within the Overlay. In RELOAD, users
	are issued certificates, which in the case of centrally signed		are issued certificates, which in the case of centrally signed
	certificates, identify the User Name as well as a certain number of		certificates, identify the User Name as well as a certain number of
	Resource-IDs where the user may store their information. For more		Resource-IDs where the user may store their information. For more
	information, see [RFC6940].		information, see [RFC6940].
	The P2PSIP suite of protocols also standardizes information about how		The P2PSIP suite of protocols also standardizes information about how
	to locate services. Services represent actions that a peer (and		to locate services. Services represent actions that a Peer (and
	perhaps a client) can do to benefit other peers and clients in the		perhaps a Client) can do to benefit other Peers and Clients in the
	overlay. Information that might be stored in the resource record		Overlay. Information that might be stored in the resource record
	associated with a service might include the peers (and perhaps		associated with a service might include the Peers (and perhaps
	clients) offering the service. Service discovery for P2PSIP is		Clients) offering the service. Service discovery for P2PSIP is
	defined in [RFC7374].		defined in [RFC7374].
	Each service has a human-friendly Service Name that uniquely		Each service has a human-friendly Service Name that uniquely
	identifies the service. Like User Names, the Service Name is not a		identifies the service. Like User Names, the Service Name is not a
	resource-id, rather the resource-id is derived from the service name		Resource-ID, rather the Resource-ID is derived from the service name
	using some function defined by the distributed database algorithm		using some function defined by the distributed database algorithm
	used by the overlay.		used by the Overlay.
	A class of algorithms known as Distributed Hash Tables are one way to		A class of algorithms known as Distributed Hash Tables (DHTs) are one
	implement the Distributed Database. The RELOAD protocol is		way to implement the distributed database. The RELOAD protocol is
	extensible and allows many different DHTs to be implemented, but		extensible and allows many different DHTs to be implemented, but
	specifies a mandatory to implement DHT in the form of a modified		specifies a mandatory-to-implement DHT in the form of a modified
	Chord DHT. For more information, see [Chord]		Chord DHT. For more information, see [Chord].
	5.2. Using the Distributed Database Function		5.2. Using the Distributed Database Function
	While there are a number of ways the distributed database described		While there are a number of ways the distributed database described
	in the previous section can be used to establish multimedia sessions		in the previous section can be used to establish multimedia sessions
	using SIP, the basic mechanism defined in the RELOAD protocol and SIP		using SIP, the basic mechanism defined in the RELOAD protocol and SIP
	usage is summarized below. This is a very simplistic overview. For		usage is summarized below. This is a very simplistic overview. For
	more detailed information, please see the RELOAD protocol document.		more detailed information, please see the RELOAD protocol [RFC6940].
	Contact information for a user is stored in the resource record for		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,		that user. Assume that a user is using a device, here called "Peer
	which serves as the contact point for this user. The user adds		A", that serves as the contact point for this user. The user adds
	contact information to this resource record, as authorized by the		contact information to this resource record, as authorized by the
	RELOAD certificate mechanism. The resource record itself is stored		RELOAD certificate mechanism. The resource record itself is stored
	with peer Z in the network, where peer Z is chosen by the particular		with Peer Z in the network, where Peer Z is chosen by the particular
	distributed database algorithm in use by the overlay.		distributed database algorithm in use by the Overlay.
	When the SIP entity coupled with peer B has an INVITE message		When the SIP entity coupled with Peer B has an INVITE message
	addressed to this user, it retrieves the resource record from peer Z.		addressed to this user, it retrieves the resource record from Peer Z.
	It then extracts the contact information for the various peers that		It then extracts the contact information for the various Peers that
	are a contact point for the user, including peer A, and uses the		are a contact point for the user, including Peer A, and uses the
	overlay to establish a connection to peer A, including any		Overlay to establish a connection to Peer A, including any
	appropriate NAT traversal (the details of which are not shown).		appropriate NAT traversal (the details of which are not shown).
	Note that RELOAD is used only to establish the connection. Once the		Note that RELOAD is used only to establish the connection. Once the
	connection is established, messages between the peers are sent using		connection is established, messages between the Peers are sent using
	ordinary SIP.		ordinary SIP.
	This exchange is illustrated in the following figure. The notation		This exchange is illustrated in the following figure. The notation
	"Store(U@A)" is used to show the distributed database operation of		"Store(U@A)" is used to show the distributed database operation of
	updating the resource record for user U with the contract A, and		updating the resource record for user U with the contract A, and
	"Fetch(U)" illustrates the distributed database operation of		"Fetch(U)" illustrates the distributed database operation of
	retrieving the resource record for user U. Note that the messages		retrieving the resource record for user U. Note that the messages
	between the peers A, B and Z may actually travel via intermediate		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		Peers (not shown) as part of the distributed lookup process or so as
	to traverse intervening NATs.		to traverse intervening NATs.
	Peer B Peer Z Peer A		Peer B Peer Z Peer A
	| | |		| | |
	| | Store(U@Y)|		| | Store(U@A)|
	| |<------------------|		| |<------------------|
	| |Store-Resp(OK) |		| |Store-Resp(OK) |
	| |------------------>|		| |------------------>|
	| | |		| | |
	|Fetch(U) | |		|Fetch(U) | |
	|------------------->| |		|------------------->| |
	| Fetch-Resp(U@Y)| |		| Fetch-Resp(U@A)| |
	|<-------------------| |		|<-------------------| |
	| | |		| | |
	(RELOAD IS USED TO ESTABLISH CONNECTION)		(RELOAD IS USED TO ESTABLISH CONNECTION)
	| | |		| | |
	| SIP INVITE(To:U) | |		| SIP INVITE(To:U) | |
	|--------------------------------------->|		|--------------------------------------->|
	| | |		| | |
			Figure 2: SIP Exchange Using Distributed Database Function
	5.3. NAT Traversal		5.3. NAT Traversal
	NAT Traversal in P2PSIP using RELOAD treats all peers as equal and		NAT traversal in P2PSIP using RELOAD treats all Peers as equal and
	establishes a partial mesh of connections between them. Messages		establishes a partial mesh of connections between them. Messages
	from one peer to another are routed along the edges in the mesh of		from one Peer to another are routed along the edges in the mesh of
	connections until they reach their destination. To make the routing		connections until they reach their destination. To make the routing
	efficient and to avoid the use of standard Internet routing		efficient and to avoid the use of standard Internet routing
	protocols, the partial mesh is organized in a structured manner. If		protocols, the partial mesh is organized in a structured manner. If
	the structure is based on any one of a number of common DHT		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		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		log N, where N is the number of peers in the overlay. Existing
	connections, along with the ICE NAT traversal techniques [RFC5245],		connections, along with the Interactive Connectivity Establishment
	are used to establish new connections between peers, and also to		(ICE) NAT traversal techniques [RFC5245], are used to establish new
	allow the applications running on peers to establish a connection to		connections between Peers, and also to allow the applications running
	communicate with one another.		on Peers to establish a connection to communicate with one another.
	5.4. Locating and Joining an Overlay		5.4. Locating and Joining an Overlay
	Before a peer can attempt to join a P2PSIP overlay, it must first		Before a Peer can attempt to join a P2PSIP Overlay, it must first
	obtain a Node-ID, configuration information, and optionally a set of		obtain a Node-ID, configuration information, and optionally a set of
	credentials. The Node-ID is an identifier that will uniquely		credentials. The Node-ID is an identifier that uniquely identifies
	identify the peer within the overlay, while the credentials show that		the Peer within the Overlay, while the credentials show that the Peer
	the peer is allowed to join the overlay.		is allowed to join the Overlay.
	The P2PSIP WG does not impose a particular mechanism for how the		The P2PSIP WG does not impose a particular mechanism for how the
	peer-ID and the credentials are obtained, but the RELOAD protocol		Peer-ID and the credentials are obtained, but the RELOAD protocol
	does specify the format for the configuration information, and		does specify the format for the configuration information and how
	specifies how this information may be obtained, along with		this information may be obtained, along with credentials and a
	credentials and a Node-ID, from an offline enrollment server.		Node-ID, from an offline enrollment server.
	Once the configuration information is obtained, RELOAD specifies a		Once the configuration information is obtained, RELOAD specifies a
	mechanism whereby a peer may obtain a multicast-bootstrap address in		mechanism whereby a Peer may obtain a multicast-bootstrap address in
	the configuration file, and can broadcast to this address to attempt		the configuration file and broadcast to this address to attempt
	to locate a bootstrap peer. Additionally, the peer may store		locating a Bootstrap Peer. Additionally, the Peer may store previous
	previous peers it has seen and attempt to use these as bootstrap		Peers it has seen and attempt using these as Bootstrap Peers, or it
	peers, or may obtain an address for a bootstrap peer by some other		may obtain an address for a Bootstrap Peer by some other mechanism.
	mechanism. For more information, see the RELOAD protocol.		For more information, see the RELOAD protocol.
	The job of the bootstrap peer is simple: refer the joining peer to a		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		Peer (called the "Admitting Peer") that will help the Joining Peer
	join the network. The choice of admitting peer will often depend on		join the network. The choice of the Admitting Peer will often depend
	the joining node - for example, the admitting peer may be a peer that		on the Joining Peer -- for example, the Admitting Peer may be a Peer
	will become a neighbor of the joining peer in the overlay. It is		that will become a neighbor of the Joining Peer in the Overlay. It
	possible that the bootstrap peer might also serve as the admitting		is possible that the Bootstrap Peer might also serve as the Admitting
	peer.		Peer.
	The admitting peer will help the joining peer learn about other peers		The Admitting Peer will help the Joining Peer learn about other Peers
	in the overlay and establish connections to them as appropriate. The		in the Overlay and establish connections to them as appropriate. The
	admitting peer and/or the other peers in the overlay will also do		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-		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		functional Peer. The details of how this is done will depend on the
	distributed database algorithm used by the overlay.		distributed database algorithm used by the Overlay.
	At various stages in this process, the joining peer may be asked to		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		present its credentials to show that it is authorized to join the
	overlay. Similarly, the various peers contacted may be asked to		Overlay. Similarly, the various Peers contacted may be asked to
	present their credentials so the joining peer can verify that it is		present their credentials so the Joining Peer can verify that it is
	really joining the overlay it wants to.		really joining the Overlay it wants to.
	5.5. Clients and Connecting Unmodified SIP Devices		5.5. Clients and Connecting Unmodified SIP Devices
	As mentioned above, in RELOAD, from the perspective of the protocol,		As mentioned above, in RELOAD, from the perspective of the protocol,
	clients are simply peers that do not store information, do not route		Clients are simply peers that do not store information, do not route
	messages, and which have not inserted themselves into the overlay.		messages, and have not inserted themselves into the Overlay. The
	The same protocol is used for the actual message exchanged. Note		same protocol is used for the actual message exchanged. Note that
	that while the protocol is the same, the client need not implement		while the protocol is the same, the Client need not implement all the
	all the capabilities of a peer. If, for example, it never routes		capabilities of a Peer. If, for example, it never routes messages,
	messages, it will not need to be capable of processing such messages,		it will not need to be capable of processing such messages or
	or understanding a DHT.		understanding a DHT.
	For SIP devices, another way to realize this functionality is for a		For SIP devices, another way to realize this functionality is for a
	Peer to behave as a [RFC3261] proxy/registrar. SIP devices then use		Peer to behave as a proxy/registrar as specified in [RFC3261]. SIP
	standard SIP mechanisms to add, update, and remove registrations and		devices then use standard SIP mechanisms to add, update, and remove
	to send SIP messages to peers and other clients. The authors here		registrations and to send SIP messages to Peers and other Clients.
	refer to these devices simply as a "SIP UA", not a "P2PSIP Client",		The authors here refer to these devices simply as a "SIP UA", not a
	to distinguish it from the concept described above.		"P2PSIP Client", to distinguish it from the concept described above.
	5.6. Architecture		5.6. Architecture
	The architecture adopted by RELOAD to implement P2PSIP is shown		The architecture adopted by RELOAD to implement P2PSIP is shown
	below. An application, for example SIP (or another application using		below. An application (for example, SIP or another application using
	RELOAD) uses RELOAD to locate other peers and (optionally) to		RELOAD) uses RELOAD to locate other Peers and (optionally) to
	establish connections to those peers, potentially across NATs.		establish connections to those Peers, potentially across NATs.
	Messages may still be exchanged directly between the peers. The		Messages may still be exchanged directly between the Peers. The
	overall block diagram for the architecture is as follows:		overall block diagram for the architecture is as follows:
	__________________________		__________________________
	| |		| |
	| SIP, other apps... |		| SIP, other apps... |
	| ___________________|		| ___________________|
	| | RELOAD Layer |		| | RELOAD Layer |
	|______|___________________|		|______|___________________|
	| Transport Layer |		| Transport Layer |
	|__________________________|		|__________________________|
			Figure 3: Architecture for Implementing P2PSIP
	6. Security Considerations		6. Security Considerations
	This specification is an overview of existing specifications and does		This specification is an overview of existing specifications and does
	not introduce any security considerations on its own. Please refer		not introduce any security considerations on its own. Please refer
	to the security considerations of the respective specifications,		to the security considerations of the respective specifications,
	particularly the RELOAD protocol specification ([RFC6940]) for		particularly the RELOAD protocol specification ([RFC6940]) for
	further details.		further details.
	7. IANA Considerations		7. Informative References
	This document has no actions for IANA.
	8. Informative References
	[Chord] Singh, K., Stoica, I., Morris, R., Karger, D., Kaashock,
	M., Dabek, F., and H. Balakrishman, "Chord: A scalable
	peer-to-peer lookup protocol for internet applications",
	IEEE/ACM Transactions on Neworking Volume 11 Issue 1, pp.
	17-32, Feb. 2003, August 2001.
	Copy available at http://pdos.csail.mit.edu/chord/papers/
	paper-ton.pdf
	[I-D.ietf-p2psip-diagnostics]		[Chord] Stoica, I., Morris, R., Liben-Nowell, D., Karger, D.,
	Song, H., Xingfeng, J., Even, R., Bryan, D., and Y. Sun,		Kaashoek, M., Dabek, F., and H. Balakrishnan, "Chord: A
	"P2P Overlay Diagnostics", draft-ietf-p2psip-		scalable peer-to-peer lookup protocol for internet
	diagnostics-22 (work in progress), March 2016.		applications", IEEE/ACM Transactions on Networking,
			Volume 11, Issue 1, pp. 17-32,
			DOI 10.1109/TNET.2002.808407, February 2003.
	[I-D.ietf-p2psip-sip]		[P2PSIP] Jennings, C., Lowekamp, B., Rescorla, E., Baset, S.,
	Jennings, C., Lowekamp, B., Rescorla, E., Baset, S.,
	Schulzrinne, H., and T. Schmidt, "A SIP Usage for RELOAD",		Schulzrinne, H., and T. Schmidt, "A SIP Usage for RELOAD",
	draft-ietf-p2psip-sip-20 (work in progress), April 2016.		Work in Progress, draft-ietf-p2psip-sip-21, April 2016.
	[RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,		[RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
	"Dynamic Updates in the Domain Name System (DNS UPDATE)",		"Dynamic Updates in the Domain Name System (DNS UPDATE)",
	RFC 2136, DOI 10.17487/RFC2136, April 1997,		RFC 2136, DOI 10.17487/RFC2136, April 1997,
	<http://www.rfc-editor.org/info/rfc2136>.		<http://www.rfc-editor.org/info/rfc2136>.
	[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,		[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
	A., Peterson, J., Sparks, R., Handley, M., and E.		A., Peterson, J., Sparks, R., Handley, M., and E.
	Schooler, "SIP: Session Initiation Protocol", RFC 3261,		Schooler, "SIP: Session Initiation Protocol", RFC 3261,
	DOI 10.17487/RFC3261, June 2002,		DOI 10.17487/RFC3261, June 2002,
	skipping to change at page 18, line 24 ¶		skipping to change at page 18, line 20 ¶
	[RFC7363] Maenpaa, J. and G. Camarillo, "Self-Tuning Distributed		[RFC7363] Maenpaa, J. and G. Camarillo, "Self-Tuning Distributed
	Hash Table (DHT) for REsource LOcation And Discovery		Hash Table (DHT) for REsource LOcation And Discovery
	(RELOAD)", RFC 7363, DOI 10.17487/RFC7363, September 2014,		(RELOAD)", RFC 7363, DOI 10.17487/RFC7363, September 2014,
	<http://www.rfc-editor.org/info/rfc7363>.		<http://www.rfc-editor.org/info/rfc7363>.
	[RFC7374] Maenpaa, J. and G. Camarillo, "Service Discovery Usage for		[RFC7374] Maenpaa, J. and G. Camarillo, "Service Discovery Usage for
	REsource LOcation And Discovery (RELOAD)", RFC 7374,		REsource LOcation And Discovery (RELOAD)", RFC 7374,
	DOI 10.17487/RFC7374, October 2014,		DOI 10.17487/RFC7374, October 2014,
	<http://www.rfc-editor.org/info/rfc7374>.		<http://www.rfc-editor.org/info/rfc7374>.
			[RFC7851] Song, H., Jiang, X., Even, R., Bryan, D., and Y. Sun,
			"Peer-to-Peer (P2P) Overlay Diagnostics", RFC 7851,
			DOI 10.17487/RFC7851, May 2016,
			<http://www.rfc-editor.org/info/rfc7851>.
	Authors' Addresses		Authors' Addresses
	David A. Bryan		David A. Bryan
	Cogent Force, LLC		Cogent Force, LLC
	Cedar Park, TX, Texas		Cedar Park, Texas
	USA		United States
	Email: dbryan@ethernot.org		Email: dbryan@ethernot.org
	Philip Matthews		Philip Matthews
	Alcatel-Lucent		Nokia
	600 March Road		600 March Road
	Ottawa, Ontario K2K 2E6		Ottawa, Ontario K2K 2E6
	Canada		Canada
	Phone: +1 613 784 3139		Phone: +1 613 784 3139
	Email: philip_matthews@magma.ca		Email: philip_matthews@magma.ca
	Eunsoo Shim		Eunsoo Shim
	Samsung Electronics Co., Ltd.		Samsung Electronics Co., Ltd.
	San 14, Nongseo-dong, Giheung-gu,		San 14, Nongseo-dong, Giheung-gu
	Yongin-si, Gyeonggi-do, 446-712		Yongin-si, Gyeonggi-do 446-712
	South Korea		South Korea
	Email: eunsooshim@gmail.com		Email: eunsooshim@gmail.com
	Dean Willis		Dean Willis
	Softarmor Systems		Softarmor Systems
	3100 Independence Pkwy #311-164		3100 Independence Pkwy #311-164
	Plano, Texas 75075		Plano, Texas 75075
	USA		United States
	Phone: +1 214 504 1987		Phone: +1 214 504 1987
	Email: dean.willis@softarmor.com		Email: dean.willis@softarmor.com
	Spencer Dawkins		Spencer Dawkins
	Huawei Technologies (USA)		Huawei Technologies (USA)
	Phone: +1 214 755 3870		Phone: +1 214 755 3870
	Email: spencerdawkins.ietf@gmail.com		Email: spencerdawkins.ietf@gmail.com
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