P2PSIP Working Group D. Bryan
Internet-Draft SIPeerior Technologies and
Intended status: Informational College of William and Mary
Expires: September 5, 2007 P. Matthews
Avaya
E. Shim
Locus Telecommunications
D. Willis
Cisco Systems
March 4, 2007
Concepts and Terminology for Peer to Peer SIP
draft-willis-p2psip-concepts-04
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Abstract
This document defines concepts and terminology for use of the Session
Initiation Protocol in a peer-to-peer environment where the
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traditional proxy-registrar function is replaced by a distributed
mechanism that might be implemented using a distributed hash table or
other distributed data mechanism with similar external properties.
This document includes a high-level view of the functional
relationships between the network elements defined herein, a
conceptual model of operations, and an outline of the related open
problems that might be addressed by an IETF working group.
Table of Contents
1. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. High Level Description . . . . . . . . . . . . . . . . . . . . 3
3. Reference Model . . . . . . . . . . . . . . . . . . . . . . . 5
4. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1. The Distributed Database . . . . . . . . . . . . . . . . . 11
5.2. Using the Distributed Database . . . . . . . . . . . . . . 13
5.3. NAT Traversal . . . . . . . . . . . . . . . . . . . . . . 16
5.4. Locating and Joining an Overlay . . . . . . . . . . . . . 18
5.5. Possible Client Behavior . . . . . . . . . . . . . . . . . 19
5.6. Interacting with non-P2PSIP entities . . . . . . . . . . . 20
6. Additional Questions . . . . . . . . . . . . . . . . . . . . . 20
6.1. Selecting between Multiple Peers offering the Same
Service . . . . . . . . . . . . . . . . . . . . . . . . . 21
6.2. Visibility of Messages to Intermediate Peers . . . . . . . 21
6.3. Hybrid Domains . . . . . . . . . . . . . . . . . . . . . . 21
7. Security Considerations . . . . . . . . . . . . . . . . . . . 21
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
9. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 22
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
11.1. Normative References . . . . . . . . . . . . . . . . . . . 22
11.2. Informative References . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
Intellectual Property and Copyright Statements . . . . . . . . . . 26
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1. Background
One of the fundamental problems in multimedia communications between
Internet nodes is that of a discovering the IP address at which a
given correspondent can be reached. Correspondents are frequently
identified by distinguished names, perhaps represented in the form of
a universal resource indicator (URI) [RFC3986].
The Session Initiation Protocol (SIP) [RFC3261] commonly addresses
this task assuming that the domain part of the URI indicates an
internet host address or internet domain name using the Domain Name
System (DNS) [RFC1034]. SIP's location process [RFC3263] assumes
that the host part of the URI indicates either the target SIP user
agent (UA), or a proxy that has knowledge of how to reach the target
UA, presumably gained through SIP's registration process.
This approach, referred to herein as "Conventional SIP" or "Client/
Server SIP", assumes a relatively fixed hierarchy of SIP routing
proxies (servers) and SIP user agents (clients). The routing proxies
are typically resolvable using conventional Internet mechanisms with
static IP addresses and associated DNS entries. This structure may
not be ideal in all cases, including specifically ad-hoc, serverless,
and reduced-administration scenarios. As an alternative, several
authors [I-D.bryan-p2psip-dsip] [I-D.shim-sipping-p2p-arch]
[I-D.johnston-sipping-p2p-ipcom]
[I-D.matthews-sipping-p2p-industrial-strength] have proposed using
peer-to-peer (P2P) [I-D.irtf-p2prg-survey-search] approaches to
solving the problem of locating the correspondent. The motivations
for a P2P approach in SIP have been documented in
[I-D.bryan-sipping-p2p-usecases].
This document offers a consolidation of the more important terms and
concepts from several of the above documents, presented in the
context of a reference model for peer-to-peer SIP (P2PSIP). It is
intended that this document serve as a starting point for describing
the work needed to resolve a number of open questions such that an
IETF working group can do the work needed to resolve these questions
and present a standard solution. The authors believe that this goal
is roughly consistent with that of a Protocol Model as defined in
[RFC4101].
2. High Level Description
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 implementation of the location service
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[RFC3261] for mapping Addresses of Record (AoRs) to Contact URIs.
They also provide a transport service by which SIP messages can be
transported between any two nodes in the overlay.
A P2PSIP Overlay consists of one or more nodes called P2PSIP Peers.
The peers in the overlay collectively run a distributed database
algorithm. This distributed database algorithm allows data to be
stored on peers and retrieved in an efficient manner. It may also
ensure that a copy of a data item is stored on more than one peer, so
that the loss of a peer 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 service.
The nature of peer-to-peer computing is that each peer offers
services to other peers to allow the overlay to collectively provide
larger services. In P2PSIP, each individual peer needs to offer a
storage service and a transport service to allow the distributed
database service and distributed transport service to be implemented.
It is expected that individual peers may also offer other services.
Some of these additional services (for example, a STUN server service
[I-D.ietf-behave-rfc3489bis]) may be required to allow the overlay to
form and operate, while others (for example, a voicemail service) may
be enhancements to the basic P2PSIP functionality.
To allow peers to offer these additional services, the distributed
database may need to store information about services. For example,
it may need to store information about which peers offer which
services.
An overlay may or may not also include one or more nodes called
P2PSIP Clients. The role of a client in the P2PSIP model is still
under discussion, with a number of suggestions for roles being put
forth, and some arguing that clients are not needed at all. However,
if they exist, then people agree that they will also be able to store
and retrieve information from the overlay. Section 5.5 discusses the
possible roles of a client in more detail.
Peers in an overlay need to speak some protocol between themselves to
maintain the overlay and to store and retrieve data. Until a better
name is found, this protocol has been dubbed the P2PSIP Peer
Protocol. The details of this protocol are still very much under
debate: some have suggested that the protocol should be SIP with some
extensions, while others have suggested that it should be an entirely
new protocol.
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If the P2PSIP model also contains clients, then a protocol is needed
for client - peer communication. Until a better name is found, this
protocol has been dubbed the P2PSIP Client Protocol. The details of
this protocol are also very much under debate. However, if the
client protocol exists, then it is agreed that it should be a logical
subset of the peer protocol. In other words, the syntax of the peer
and client protocols may be completely different, but any operation
supported by client protocol is also supported by the peer protocol.
This implies that clients cannot do anything that peers cannot also
do.
Since P2PSIP is about peer-to-peer networks for real-time
communication, it is expected that most (if not all) peers and
clients will be coupled with SIP entities. 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, we think of the peer or client portion of the node as
being distinct from the SIP entity portion.
Network Address Translators (NATs) are impediments to establishing
and maintaining peer-to-peer networks, since NATs hinder direct
communication between peers. Some peer-to-peer network architectures
avoid this problem by insisting that all peers exist in the same
address space. However, in the P2PSIP model, it has been agreed that
peers can live in multiple address spaces interconnected by NATs.
This implies that Peer Protocol connections must be able to traverse
NATs. It also means that the peers must collectively provide a
transport service that allows a peer to send a SIP message to any
other peer in the overlay - without this service two peers in
different address spaces might not be able to exchange SIP messages.
3. Reference Model
The following diagram shows a P2PSIP Overlay consisting of a number
of P2PSIP Peers and one P2PSIP Client. It also shows an ordinary SIP
UA.
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--->PSTN
+------+ N +------+ +---------+ /
| | A | | | Gateway |-/
| UA |####T#####| UA |#####| Peer |########
| Peer | N | Peer | | G | # P2PSIP
| E | A | F | +---------+ # Client
| | T | | # Protocol
+------+ N +------+ # |
# A # |
NATNATNATNAT # |
# # | \__/
NATNATNATNAT +-------+ v / \
# N | |=====/ UA \
+------+ A P2PSIP Overlay | Proxy | /Client\
| | T | Peer | |___C__|
| UA | N | Q |
| Peer | A +-------+
| D | T P2PSIP Peer Protocol #
| | N #
+------+ A #
# T #
# N +-------+ +-------+ #
# A | | | | #
#########T####| Proxy |########| Redir |#######
N | Peer | | Peer |
A | P | | R |
T +-------+ +-------+
\__/
/\
/ \
/ UA \
/______\
SIP UA A
Figure: P2PSIP Overlay Reference Model
Here, the large perimeter depicted by "#" represents a stylized view
of the P2PSIP Overlay (the actual connections could be a mesh, a
ring, or some other structure). Around the periphery of the P2PSIP
Overlay rectangle, we have a number of P2PSIP 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, two peers
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("P" and "Q") which are each coupled with a SIP proxy, and one peer
"R" which is coupled with a SIP Redirector. Note that because these
are all P2PSIP Peers, each is responsible for storing P2PSIP Resource
Records and transporting messages around the P2PSIP 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.
Below the P2PSIP Overlay, we have a conventional SIP UA "A" which is
not part of the P2PSIP Overlay, either directly as a peer or
indirectly as a client. It speaks neither the P2PSIP Peer nor P2PSIP
Client protocols. Instead, it uses SIP to interact with the P2PSIP
Overlay.
On the right side, we have a P2PSIP client "C", which uses the P2PSIP
Client Protocol depicted by "=" to communicate with Proxy Peer "Q".
The P2PSIP client "C" could communicate with a different peer, for
example peer "F", if it establishes a connection to "F" instead of or
in addition to "Q". The exact role that this client plays in the
network is still under discussion (see Section 5.5).
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 P2PSIP Overlay. Each accepts standard SIP requests
and resolves the next-hop by using the P2PSIP overlay Peer Protocol
to interact with the routing knowledge of the P2PSIP Overlay, then
processes the SIP requests as appropriate (proxying or redirecting
towards the next-hop). Note that proxy operation is bidirectional -
the proxy may be forwarding a request from an ordinary SIP device to
the P2PSIP overlay, or from the P2PSIP overlay to an ordinary SIP
device.
The PSTN Gateway at peer "G" provides a similar sort of adaptation to
and from the public switched telephone network (PSTN).
4. Definitions
This section defines a number of concepts that are key to
understanding the P2PSIP work.
Overlay Network: An overlay network is a computer network which is
built on top of another network. Nodes in the overlay can be
thought of as being connected by virtual or logical links, each of
which corresponds to a path, perhaps through many physical links,
in the underlying network. For example, many peer-to-peer
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networks are overlay networks because they run on top of the
Internet. Dial-up Internet is an overlay upon the telephone
network. <http://en.wikipedia.org/wiki/P2P_overlay>
P2P Network: A peer-to-peer (or P2P) computer network is a network
that relies primarily on the computing power and bandwidth of the
participants in the network rather than concentrating it in a
relatively low number of servers. P2P networks are typically used
for connecting nodes via largely ad hoc connections. Such
networks are useful for many purposes. Sharing content files (see
<http://en.wikipedia.org/wiki/File_sharing>) containing audio,
video, data or anything in digital format is very common, and
realtime data, such as telephony traffic, is also exchanged using
P2P technology. <http://en.wikipedia.org/wiki/Peer-to-peer>. A
P2P Network may also be called a "P2P Overlay" or "P2P Overlay
Network" or "P2P Network Overlay", since its organization is not
at the physical layer, but is instead "on top of" an existing
Internet Protocol network.
P2PSIP: A suite of communications protocols related to the Session
Initiation Protocol (SIP) [RFC3261] that enable SIP to use peer-
to-peer techniques for resolving the targets of SIP requests,
providing SIP message transport, and providing other SIP-related
services. The exact contents of this protocol suite are still
under discussion, but is likely to include the P2PSIP Peer
Protocol and may include a P2PSIP Client Protocol (see definitions
below).
P2PSIP Overlay: A P2PSIP Overlay is an association, collection, or
federation of nodes that provides SIP registration, SIP message
transport, and similar functions using a P2P organization, as
defined by "P2P Network" above.
P2PSIP Overlay Name: A human-friendly name that identifies a
specific 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.
P2PSIP Peer: A node participating in a P2PSIP Overlay that provides
storage and transport services to other nodes in that P2PSIP
Overlay. Each P2PSIP Peer has a unique identifier, known as a
Peer-ID, within the P2PSIP Overlay. Each P2PSIP Peer may be
coupled to one or more SIP entities. Within the P2PSIP Overlay,
the peer is capable of performing several different operations,
including: joining and leaving the overlay, transporting SIP
messages within the overlay, storing information on behalf of the
overlay, putting information into the overlay, and getting
information from the overlay.
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P2PSIP Peer-ID: Information that uniquely identifies each P2PSIP
Peer within a given P2PSIP Overlay. This value is not human-
friendly -- in a DHT approach, this is a numeric value in the hash
space. These Peer-IDs are completely independent of the
identifier of any user of a user agent associated with a peer.
(Note: This is often called a "Node-ID" in the P2P literature).
P2PSIP Client: A node participating in a P2PSIP Overlay that is less
capable than a P2PSIP Peer in some way. The role of a P2PSIP
Client is still under debate, with a number of competing
proposals, and some have suggested removing the concept entirely
(see the discussion on this later in the document). If clients
exist, then it has been agreed that they do have the ability to
add, modify, inspect, and delete information in the overlay. Note
that the term client does not imply that this node is a SIP UAC.
Some have suggested that the word 'client' be changed to something
else to avoid both this confusion and the implication of a client-
server relationship.
User: A human that interacts with the overlay through SIP UAs
located on peers and clients (and perhaps other ways).
P2PSIP 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.
User Names are formatted so that they can be used within a URI
(likely a SIP URI), perhaps in combination with the Overlay Name.
P2PSIP Service: Actions that a peer (and perhaps a client) can do
for the benefit of other peers and clients. Examples include
acting as a STUN server, and acting as a voicemail server. It is
expected that not all peers and clients will offer the same set of
services, so a means of finding peers (and perhaps clients) that
offer a particular service is required.
P2PSIP Service Name: A unique, human-friendly, name for a service.
P2PSIP Resource: Anything about which information can be stored in
the overlay. Both Users and Services are examples of Resources.
P2PSIP Resource-ID: A non-human-friendly value that uniquely
identifies a resource and which is used as a key for storing and
retrieving data about the resource. One way to generate a
Resource-ID is by applying a mapping function to some other unique
name (e.g., User Name or Service Name) for the resource. The
Resource-ID is used by the distributed database algorithm to
determine the peer or peers that are responsible for storing the
data for the overlay.
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P2PSIP Resource Record: A block of data, stored using distributed
database mechanism of the P2PSIP Overlay, that includes
information relevant to a specific resource. We presume that
there may be multiple types of resource records. Some may hold
data about Users, and others may hold data about Services, and the
working group may define other types. The types, usages, and
formats of the records are a question for future study.
P2PSIP Peer Protocol: The protocol spoken between P2PSIP Overlay
peers to share information and organize the P2PSIP Overlay
Network.
P2PSIP Client Protocol: The protocol spoken between P2PSIP Clients
and P2PSIP Peers. It is used to store and retrieve information
from the P2P Overlay. The nature of this protocol, and even its
existence, is under discussion. However, if it exists, it has
been agreed that the Client Protocol is a functional subset of the
P2P Peer Protocol, but may differ in syntax and protocol
implementation (i.e., may not be syntactically related).
P2PSIP Peer Protocol Connection / P2PSIP Client Protocol Connection:
The TCP, UDP or other transport layer protocol connection over
which the P2PSIP Peer Protocol (or respectively the Client
protocol) is transported.
P2PSIP Neighbors: The set of P2PSIP Peers that either a P2PSIP Peer
or P2PSIP Client know of directly and can reach without further
lookups.
P2PSIP Joining Peer: A node that is attempting to become a P2PSIP
Peer in a particular P2PSIP Overlay.
P2PSIP Bootstrap Peer: A P2PSIP Peer in the P2PSIP Overlay that is
the first point of contact for a P2PSIP Joining Peer. It selects
the peer that will serve as the P2PSIP Admitting Peer and helps
the joining peer contact the admitting peer.
P2PSIP Admitting Peer: A P2PSIP Peer in the P2PSIP Overlay which
helps the P2PSIP Joining Peer join the Overlay. The choice of the
admitting peer may depend on the joining peer (e.g., depend the
joining peer's P2PSIP Peer-ID). For example, the admitting peer
might be chosen as the peer which is "closest" in the logical
structure of the overlay to the future position of the joining
peer. The selection of the admitting peer is typically done by
the bootstrap peer. It is allowable for the bootstrap peer to
select itself as the admitting peer.
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P2PSIP Bootstrap Server: A network node used by P2PSIP Joining Peers
to locate a P2PSIP Bootstrap Peer. Typically, a P2PSIP Bootstrap
Server acts as a proxy for messages between the P2PSIP Joining
Peer and the P2PSIP Bootstrap Peer. The P2PSIP Bootstrap Server
itself is typically a stable host with a DNS name that is somehow
communicated (for example, through configuration) to peers that
want to join the overlay. A P2PSIP Bootstrap Server is NOT
required to be a peer or client, though it may be if desired.
P2PSIP Peer Admission: The act of admitting a node (the "P2PSIP
Joining Peer") into a P2PSIP Overlay as a P2PSIP Peer. After the
admission process is over, the joining peer is a fully-functional
peer of the overlay. During the admission process, the joining
peer may need to present credentials to prove that it has
sufficient authority to join the overlay.
P2PSIP Resource Record Insertion: The act of inserting a P2PSIP
Resource Record into the distributed database. Following
insertion, the data will be stored at one or more peers. The data
can be retrieved or updated using the P2PSIP Resource-ID as a key.
5. Discussion
5.1. The Distributed Database
A P2PSIP Overlay functions as a distributed database. The database
serves as a way to store information about things called 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.
It is expected that the P2PSIP working group will standardize the
way(s) certain types of resources are represented in the distributed
database.
One type of resource representation that the working group is
expected to standardize is information about users. 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 might include things like the full name of the user and the
location of the UAs that the user is using.
Before information about a user can be stored in the overlay, a user
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needs a User Name. The User Name is a human-friendly identifier that
uniquely identifies the user within the overlay. The User Name is
not a Resource-ID, rather the Resource-ID is derived from the User
Name using some mapping function (often a cryptographic hash
function) defined by the distributed database algorithm used by the
overlay.
The overlay may also require that the user have a set of credentials.
Credentials may be required to authenticate the user and/or to show
that the user is authorized to use the overlay.
Another type of resource representation that the working group is
expected to standardize is information about 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.
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.
It is expected that the working group will standardize at least one
service. For each standardized service, the working group will
likely specify the service name, the nature and format of the
information stored in the resource record associated with the
service, and the protocol used to access the service.
The overlay may require that the peer (or client) have a set of
credentials for a service. For example, credentials might be
required to show that the peer (or client) is authorized to offer the
service, or to show that the peer (or client) is a providing a
trustworthy implementation of the service.
It is expected that the P2PSIP WG will not standardize how a User
Name is obtained, nor how the credentials associated with a User Name
or a Service Name are obtained, but merely standardize at least one
acceptable format for each. To ensure interoperability, it is
expected that at least one of these formats will be specified as
"mandatory-to-implement".
A class of algorithms known as Distributed Hash Tables
<http://en.wikipedia.org/wiki/P2P_overlay> are one way to implement
the Distributed Database. In particular, both the Chord and Bamboo
algorithms have been suggested as good choices for the distributed
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database algorithm. However, no decision has been taken so far.
5.2. Using the Distributed Database
There are a number of ways the distributed database described in the
previous section might be used to establish multimedia sessions using
SIP. In this section, we give four possibilities as examples. It
seems likely that the working group will standardize at least one way
(not necessarily one of the four listed here), but no decisions have
been taken yet.
The first option is to store the contact information for a user in
the resource record for the user. A peer Y that is a contact point
for this user adds contact information to this resource record. The
resource record itself is stored with peer Z in the network, where
peer Z is chosen by the distributed database algorithm.
When the SIP entity coupled with peer X 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 Y, and forwards the
INVITE onward.
This exchange is illustrated in the following figure. The notation
"Put(U@Y)" is used to show the distributed database operation of
updating the resource record for user U with the contract Y, and
"Get(U)" illustrates the distributed database operation of retrieving
the resource record for user U. Note that the messages between the
peers X, Y 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 X Peer Z Peer Y
| | |
| | Put(U@Y) |
| |<---------------|
| | Put-Resp(OK) |
| |--------------->|
| | |
| Get(U) | |
|---------------->| |
| Get-Resp(U@Y)| |
|<----------------| |
| INVITE(To:U) | |
|--------------------------------->|
| | |
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The second option also involves storing the contact information for a
user in the resource record of the user. However, SIP entity at peer
X, rather than retrieving the resource record from peer Z, instead
forwards the INVITE message to the proxy at peer Z. The proxy at peer
Z then uses the information in the resource record and forwards the
INVITE onwards to the SIP entity at peer Y and the other contacts.
Peer X Peer Z Peer Y
| | |
| | Put(U@Y) |
| |<---------------|
| | Put-Resp(OK) |
| |--------------->|
| | |
| INVITE(To:U) | |
|-----------------| INVITE(To:U) |
| |--------------->|
| | |
The third option is for a single peer W to place its contact
information into the resource record for the user (stored with peer
Z). A peer Y that is a contact point for the user retrieves the
resource record from peer Z, extracts the contact information for
peer W, and then uses the standard SIP registration mechanism
[RFC3261] to register with peer W. When the SIP entity at peer X has
to forward an INVITE request, it retrieves the resource record and
extracts the contact information for W. It then forwards the INVITE
to the proxy at peer W, which proxies it onward to peer Y and the
other contacts.
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Peer X Peer Z Peer Y Peer W
| | | |
| | Put(U@W) | |
| |<---------------------------------|
| | Put-Resp(OK) | |
| |--------------------------------->|
| | | |
| | | |
| | | REGISTER(To:U) |
| | |---------------->|
| | | 200 |
| | |<----------------|
| | | |
| | | |
| Get(U) | | |
|---------------->| | |
| Get-Resp(U@W)| | |
|<----------------| | |
| INVITE(To:U) | | |
|--------------------------------------------------->|
| | | INVITE(To:U) |
| | |<----------------|
| | | |
The fourth option works as in option 3, with the exception that,
rather than X retrieving the resource record from Z, peer X forwards
the INVITE to a SIP proxy at Z, which proxies it onward to W and
hence to Y.
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Peer X Peer Z Peer Y Peer W
| | | |
| | Put(U@W) | |
| |<---------------------------------|
| | Put-Resp(OK) | |
| |--------------------------------->|
| | | |
| | | |
| | | REGISTER(To:U) |
| | |---------------->|
| | | 200 |
| | |<----------------|
| | | |
| | | |
| INVITE(To:U) | | |
|---------------->| INVITE(To:U) | |
| |--------------------------------->|
| | | INVITE(To:U) |
| | |<----------------|
| | | |
The pros and cons of option 1 and 3 are briefly discussed in
[Using-an-External-DHT].
5.3. NAT Traversal
Two approaches to NAT Traversal for P2PSIP Peer Protocol have been
suggested. The working group has not many any decision yet on the
approach that will be selected.
The first, the traditional approach adopted by most peer-to-peer
networks today, divides up the peers in the network into two groups:
those with public IP addresses and those without. The networks then
select a subset of the former group and elevate them to "super peer"
status, leaving the remaining peers as "ordinary peers". Since super
peers all have public IP addresses, there are no NAT problems when
communicating between them. The network then associates each
ordinary peer with (usually just one) super peer in a client-server
relationship. Once this is done, an ordinary peer X can communicate
with another ordinary peer Y by sending the message to X's super
peer, which forwards it to Y's super peer, which forwards it to Y.
The connection between an ordinary peer and its super peer is
initiated by the ordinary peer, which makes it easy to traverse any
intervening NATs. In this approach, the number of hops between two
peers is at most 3.
The second approach treats all peers as equal and establishes a
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partial mesh of connections between them. Messages from one peer to
another are then 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.
The first approach is significantly more efficient than the second in
overlays with large numbers of peers. However, the first approach
assumes there are a sufficient number of peers with public IP
addresses to serve as super peers. In some usage scenarios
envisioned for P2PSIP, this assumption does not hold. For example,
this approach fails completely in the case where every peer is behind
a distinct NAT.
The second approach, while less efficient in overlays with larger
numbers of peers, is efficient in smaller overlays and can be made to
work in many use cases where the first approach fails.
Both of these approaches assume a method of setting up Peer Protocol
connections between peers. Many such methods exist; the now expired
[I-D.iab-nat-traversal-considerations] is an attempt to give a fairly
comprehensive list along with a discussion of their pros and cons.
After a consideration of the various techniques, the P2PSIP working
group has decided to select the Unilateral Self-Address Fixing method
[RFC3424] of NAT Traversal, and in particular the ICE
[I-D.ietf-mmusic-ice] implementation of this approach.
The above discussion covers NAT traversal for Peer Protocol
connections. For Client Protocol connections, the approach depends
on the role adopted for clients and we defer the discussion on that
point until the role becomes clearer.
In addition to Peer Protocol and Client Protocol messages, a P2PSIP
Overlay must also provide a solution to the NAT Traversal problem for
SIP messages. If it does not, there is no reliable way for a peer
behind one NAT to send a SIP INVITE to a peer behind another NAT.
One way to solve this problem is to transport SIP messages along Peer
and Client Protocol connections: this could be done either by
encapsulating the SIP messages inside Peer and Client Protocol
messages or by multiplexing SIP with the Peer (resp.Client) Protocol
on a Peer (resp. Client) Protocol connection.
Finally, it should be noted that the NAT traversal problem for media
connections signaled using SIP is outside the scope of the P2PSIP
working group. As discussed in [I-D.ietf-sipping-nat-scenarios], the
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current recommendation is to use ICE.
5.4. Locating and Joining an Overlay
Before a peer can attempt to join a P2PSIP overlay, it must first
obtain a Peer-ID and optionally a set of credentials. The Peer-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 will not standardize how the peer-ID and the
credentials are obtained, but merely standardize at least one
acceptable format for each. To ensure interoperability, it is
expected that at least one of these formats will be specified as
"mandatory-to-implement".
Once a peer (the "joining peer") has a peer-ID and optionally a set
of credentials, it can attempt to join the overlay. To do this, it
needs to locate a bootstrap peer for the Overlay.
A bootstrap peer is a peer that serves as the first point of contact
for the joining peer. The joining peer uses a bootstrap mechanism to
locate a bootstrap peer. Locating a bootstrap peer might be done in
any one of a number of different ways:
o By remembering peers that were part of the overlay the last time
the peer was part of the overlay;
o Through a multicast discovery mechanism;
o Through manual configuration; or
o By contacting a P2PSIP Bootstrap Server, and using its help to
locate a bootstrap peer.
The joining peer might reasonably try each of the methods (and
perhaps others) in some order or in parallel until it succeeds in
finding a bootstrap peer.
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
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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 in 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.
5.5. Possible Client Behavior
As mentioned above, a number of people have proposed a second type of
P2PSIP entity, known as a "P2PSIP client". The question of whether
the concept of a "client" is needed and, if it is needed, its exact
nature, is still very much under debate. This section presents some
of the alternatives that have been suggested for the possible role of
a client.
In one alternative, a client interacts with the P2PSIP overlay
through an associated peer (or perhaps several such peers) using the
Client Protocol. The client does not run the distributed database
algorithm, does not store resource records, and is not involved in
routing messages to other peers or clients. Though interactions with
its associated peer, a client can insert, modify, examine, and remove
resource records. A client can also send SIP messages to its
associated peer for routing through the overlay. In this
alternative, a client is a node that wants to take advantage of the
overlay, but is unable or unwilling to contribute resources back to
the overlay.
One way to realize this alternative is for a peer to behave as
[RFC3261] proxy/registrar. Clients then use standard SIP mechanisms
to add, update, and remove registrations and to send SIP messages to
peers and other clients. If this is done, there is no need for a
separate Client Protocol and no need for P2PSIP to define a distinct
"P2PSIP Client" concept.
In a second alternative, a client behaves in a way similar to the way
described in first alternative, except that it does store resource
records. In essence, the client contributes its storage capacity to
its associated peer. A peer which needs to store a resource record
may elect to store this on one of its associated clients instead,
thus boosting its effective storage capacity.
In a third alternative, a client acts almost the same as a peer,
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except that it does not store any resource records. In this
alternative, a client has a "peer-ID" and joins the overlay in the
same way as a peer, perhaps establishing the same network of
connections that a peer would. Clients participate in the
distributed database algorithm, and can help in transporting messages
to other peers and clients. However, the distributed database
algorithm does not assign resource records to clients. The role of a
client in this model has been described as "a peer with bad memory".
In these three alternatives, clients can act as SIP entities and make
and receive calls on behalf of users. They can also use services
offered by peers, and depending on the details of how services are
defined, they may also be able to offer services to other clients and
peers.
5.6. Interacting with non-P2PSIP entities
It is possible for network nodes that are not peers or clients to
interact with a P2PSIP overlay. Such nodes would do this through
mechanisms not defined by the P2PSIP working group provided they can
find a peer or client that supports that mechanism and which will do
any related P2PSIP operations necessary. In this section, we briefly
describe two ways this might be done. (Note that these are just
examples and the descriptions here are not recommendations).
One example is a peer that also acts as a standard SIP proxy and
registrar. SIP UAs can interact with it using mechanisms defined in
[RFC3261]. The peer inserts registrations for users learned from
these UAs into the distributed database, and retrieves contact
information when proxying INVITE messages.
Another example is a peer that has a fully-qualified domain name
(FQDN) that matches the name of the overlay and acts as a SIP proxy
for calls coming into the overlay. A SIP INVITE addressed to
"user@overlay-name" arrives at the peer (using the mechanisms in
[RFC3263]) and this peer then looks up the user in the distributed
database and proxies the call onto it.
6. Additional Questions
This section lists some additional questions that the proposed P2PSIP
Working Group may need to consider in the process of defining the
Peer and Client protocols.
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6.1. Selecting between Multiple Peers offering the Same Service
If a P2PSIP network contains two or more peers that offer the same
service, then how does a peer or client that wishes to use that
service select the peer to use? This question comes up in a number
of contexts:
o When two or more peers are willing to serve as a STUN Relay, how
do we select a peer that is close in the netpath sense and is
otherwise appropriate for the call?
o When two or more peers are willing to serve as PSTN gateways, how
do we select an appropriate gateway for a call that is both
netpath efficient and provides good quality or inexpensive PSTN
routing?
It has been suggested that, at least initially, the working group
should restrict itself to defining a mechanism that can return a list
of peers offering a service and not define the mechanism for
selecting a peer from that list.
6.2. Visibility of Messages to Intermediate Peers
When transporting SIP messages through the overlay, are the headers
and/or bodies of the SIP messages visible to the peers that the
messages happen to pass through? If they are, what types of security
risks does this pose in the presence of peers that have been
compromised in some way?
6.3. Hybrid Domains
If a given UA is capable of operating in both P2PSIP and conventional
SIP modalities (especially simultaneously), is it possible for it to
use and respond to the same AOR using both conventional and P2PSIP?
An example of such a topology might be a UA that registers an AOR
(say, "sip:alice@example.com" conventionally with a registrar and
then inserts a resource record for that resource into a P2PSIP
topology, such that both conventional SIP users and P2PSIP users
(within the overlay or a federation thereof) would be able to contact
the user without necessarily traversing some sort of gateway. Is
this something that we want to make work?
7. Security Considerations
Building a P2PSIP system has many security considerations, many of
which we have only begun to consider. We anticipate that the
protocol documents describing the actual protocols will deal more
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thoroughly with security topics.
8. IANA Considerations
This document presently raises no IANA considerations.
9. Open Issues
Here are some open issues in this document:
1. The word "service" is being overloaded to refer both to what the
overlay provides (distributed location service, distributed
transport service) and what an individual peer or client
provides. How can we fix this?
2. Does P2PSIP provide a distributed location service or an
alternative mechanism to RFC 3263? The answer seems to be both,
but what is the relationship between these?
10. Acknowledgements
This document draws heavily from the contributions of many
participants in the P2PSIP Mailing List but the authors are
especially grateful for the support of Spencer Dawkins, Cullen
Jennings, and Henning Schulzrinne, all of whom spent time on phone
calls about this document or provided text. In addition, Spencer
contributed the Reference Model figure.
11. References
11.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3263] Rosenberg, J. and H. Schulzrinne, "Session Initiation
Protocol (SIP): Locating SIP Servers", RFC 3263,
June 2002.
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[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
11.2. Informative References
[I-D.bryan-p2psip-dsip]
Bryan, D., "dSIP: A P2P Approach to SIP Registration and
Resource Location", draft-bryan-p2psip-dsip-00 (work in
progress), February 2007.
[I-D.bryan-sipping-p2p-usecases]
Bryan, D., "Use Cases for Peer-to-Peer Session Initiation
Protocol (P2P SIP)", draft-bryan-sipping-p2p-usecases-00
(work in progress), December 2005.
[I-D.iab-nat-traversal-considerations]
Rosenberg, J., "Considerations for Selection of Techniques
for NAT Traversal",
draft-iab-nat-traversal-considerations-00 (work in
progress), October 2005.
[I-D.ietf-behave-rfc3489bis]
Rosenberg, J., "Simple Traversal Underneath Network
Address Translators (NAT) (STUN)",
draft-ietf-behave-rfc3489bis-05 (work in progress),
October 2006.
[I-D.ietf-mmusic-ice]
Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Methodology for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols",
draft-ietf-mmusic-ice-13 (work in progress), January 2007.
[I-D.ietf-sipping-nat-scenarios]
Boulton, C., "Best Current Practices for NAT Traversal for
SIP", draft-ietf-sipping-nat-scenarios-06 (work in
progress), March 2007.
[I-D.irtf-p2prg-survey-search]
Risson, J. and T. Moors, "Survey of Research towards
Robust Peer-to-Peer Networks: Search Methods",
draft-irtf-p2prg-survey-search-00 (work in progress),
March 2006.
[I-D.johnston-sipping-p2p-ipcom]
Sinnreich, H. and A. Johnston, "SIP, P2P, and Internet
Communications", draft-johnston-sipping-p2p-ipcom-02 (work
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in progress), March 2006.
[I-D.matthews-sipping-p2p-industrial-strength]
Matthews, P., "Industrial-Strength P2P SIP",
draft-matthews-sipping-p2p-industrial-strength-00 (work in
progress), February 2005.
[I-D.shim-sipping-p2p-arch]
Shim, E., "An Architecture for Peer-to-Peer Session
Initiation Protocol (P2P SIP)",
draft-shim-sipping-p2p-arch-00 (work in progress),
March 2006.
[RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral
Self-Address Fixing (UNSAF) Across Network Address
Translation", RFC 3424, November 2002.
[RFC4101] Rescorla, E. and IAB, "Writing Protocol Models", RFC 4101,
June 2005.
[Using-an-External-DHT]
Singh, K. and H. Schulzrinne, "Using an External DHT as a
SIP Location Service", Columbia University Computer
Science Dept. Tech Report 388).
Copy available at http://mice.cs.columbia.edu/
getTechreport.php?techreportID=388/
Authors' Addresses
David A. Bryan
SIPeerior Technologies and College of William and Mary
3000 Easter Circle
Williamsburg, Virginia 23188
USA
Phone: +1 757 565 0101
Email: bryan@sipeerior.com
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Philip Matthews
Avaya
1135 Innovation Drive
Ottawa, Ontario K2K 3G7
Canada
Phone: +1 613 592 4343 x224
Email: philip_matthews@magma.ca
Eunsoo Shim
Locus Telecommunications
111 Sylvan Avenue
Englewood Cliffs, New Jersey 07632
USA
Phone: unlisted
Email: eunsooshim@gmail.com
Dean Willis
Cisco Systems
3100 Independence Pkwy #311-164
Plano, Texas 75075
USA
Phone: unlisted
Email: dean.willis@softarmor.com
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