P2PSIP Working Group D. Bryan
Internet-Draft College of William and Mary and
Intended status: Informational SIPeerior Technologies
Expires: May 18, 2008 P. Matthews
Avaya
E. Shim
Locus Telecommunications
D. Willis
Unaffiliated
November 15, 2007
Concepts and Terminology for Peer to Peer SIP
draft-ietf-p2psip-concepts-01
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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 and message routing functions are
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 being addressed by the P2PSIP working
group. As this document matures, it is expected to define the
general framework for P2PSIP.
Table of Contents
1. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. High Level Description . . . . . . . . . . . . . . . . . . . . 4
2.1. Services . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Clients . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Protocol . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.4. Relationship of Peer and Client Protocols . . . . . . . . 6
2.5. Relationship Between P2PSIP and SIP . . . . . . . . . . . 6
2.6. Relationship Between P2PSIP and Other AoR
Dereferencing Approaches . . . . . . . . . . . . . . . . . 6
2.7. NAT Issues . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Reference Model . . . . . . . . . . . . . . . . . . . . . . . 7
4. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 9
5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.1. The Distributed Database Function . . . . . . . . . . . . 13
5.2. Using the Distributed Database Function . . . . . . . . . 15
5.3. NAT Traversal . . . . . . . . . . . . . . . . . . . . . . 18
5.4. Locating and Joining an Overlay . . . . . . . . . . . . . 20
5.5. Possible Client Behavior . . . . . . . . . . . . . . . . . 21
5.6. Interacting with non-P2PSIP entities . . . . . . . . . . . 22
5.7. Architecture . . . . . . . . . . . . . . . . . . . . . . . 23
6. Additional Questions . . . . . . . . . . . . . . . . . . . . . 24
6.1. Selecting between Multiple Peers offering the Same
Service . . . . . . . . . . . . . . . . . . . . . . . . . 24
6.2. Visibility of Messages to Intermediate Peers . . . . . . . 24
6.3. Using C/S SIP and P2PSIP Simultaneously in a Single UA . . 25
6.4. Clients, Peers, and Services . . . . . . . . . . . . . . . 25
6.5. Relationships of Domains to Overlays . . . . . . . . . . . 25
7. Security Considerations . . . . . . . . . . . . . . . . . . . 25
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8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
9. Changes in This Version . . . . . . . . . . . . . . . . . . . 26
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 26
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
11.1. Normative References . . . . . . . . . . . . . . . . . . . 27
11.2. Informative References . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29
Intellectual Property and Copyright Statements . . . . . . . . . . 30
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1. Background
One of the fundamental problems in multimedia communication between
Internet nodes is that of discovering the host at which a given user
can be reached. In the Session Initiation Protocol (SIP) [RFC3261]
this problem is expressed as the problem of mapping an Address of
Record (AoR) for a user into one or more Contact URIs [RFC3986]. The
AoR is a name for the user that is independent of the host or hosts
where the user can be contacted, while a Contact URI indicates the
host where the user can be contacted.
In the common SIP-using architectures that we refer to as
"Conventional SIP" or "Client/Server SIP", there is a relatively
fixed hierarchy of SIP routing proxies and SIP user agents. To
deliver a SIP INVITE to the host or hosts at which the user can be
contacted, a SIP UA follows the procedures specified in [RFC3263] to
determine the IP address of a SIP proxy, and then sends the INVITE to
that proxy. The proxy will then, in turn, deliver the SIP INVITE to
the hosts where the user can be contacted.
This document gives a high-level description of an alternative
solution to this problem. In this alternative solution, the
relatively fixed hierarchy of Client/Server SIP is replaced by a
peer-to-peer overlay network. In this peer-to-peer overlay network,
the various AoR to Contact URI mappings are not centralized at proxy/
registrar nodes but are instead distributed amongst the peers in the
overlay.
The details of this alternative solution are currently being worked
out in the P2PSIP working group. This document describes the basic
concepts of such a peer-to-peer overlay, and lists the open questions
that still need to be resolved. As the work proceeds, it is expected
that this document will develop into a high-level architecture
document for the solution.
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 mechanism for mapping names to overlay
locations. This provides for the mapping of Addresses of Record
(AoRs) to Contact URIs, thereby providing the "location server"
function of [RFC3261]. A P2PSIP Overlay also provides a transport
function by which SIP messages can be transported between any two
nodes in the overlay.
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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 function. This provides a location function
within each overlay that is an alternative to the location functions
described in [RFC3263]. However, the model of [RFC3263] is used
between overlays.
2.1. Services
The nature of peer-to-peer computing is that each peer offers
services to other peers to allow the overlay to collectively provide
larger functions. In P2PSIP, peers offer storage and transport
services to allow the distributed database function and distributed
transport function 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, and perhaps what sort of capacity each peer has for
delivering each listed service.
2.2. Clients
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.
2.3. Protocol
Peers in an overlay need to speak some protocol between themselves to
maintain the overlay and to store and retrieve data. Until a better
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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.
2.4. Relationship of Peer and Client Protocols
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.
2.5. Relationship Between P2PSIP and SIP
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. However, there is no
hard requirement that every P2PSIP node (peer or client) be coupled
to a SIP entity, and some proposed architectures include peer nodes
that have no SIP function whatsoever.
2.6. Relationship Between P2PSIP and Other AoR Dereferencing Approaches
As noted above, the fundamental task of P2PSIP is turning an AoR into
a Contact. This task might be approached using zeroconf techniques
such as multicast DNS and DNS Service Discovery (as in Apple's
Bonjour protocol), link-local multicast name resolution [RFC4795],
and dynamic DNS [RFC2136].
These alternatives were discussed in the P2PSIP Working Group, and
not pursued as a general solution for a number of reasons related to
scalability, the ability to work in a disconnected state, partition
recovery, and so on. However, there does seem to be some continuing
interest in the possibility of using DNS-SD and mDNS for
bootstrapping of P2PSIP overlays.
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2.7. NAT Issues
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
distributed transport function that allows a peer to send a SIP
message to any other peer in the overlay - without this function two
peers in different IP 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, one P2PSIP Client, and an ordinary SIP UA. It
illustrates a typical P2PSIP overlay but does not limit other
compositions or variations; for example, Proxy Peer P might also talk
to a ordinary SIP proxy as well. The figure is not intended to cover
all possible architecture variations in this document.
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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 | Peer | /Client\
| | T | Q | |___C__|
| UA | N | |
| Peer | A +-------+
| D | T #
| | N #
+------+ A # P2PSIP
# T # Peer
# N +-------+ +-------+ # Protocol
# A | | | | #
#########T####| Proxy |########| Redir |#######
N | Peer | | Peer |
A | P | | R |
T +-------+ +-------+
| /
| SIP /
\__/ / /
/\ / ______________/ SIP
/ \/ /
/ UA \/
/______\
SIP UA A
Figure: P2PSIP Overlay Reference Model
Here, the large perimeter depicted by "#" represents a stylized view
of the 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, a peer "P"
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which is coupled with a SIP proxy, an ordinary peer "Q", 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
functions. 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: A capability contributed by a peer to an overlay or
to the members of an overlay. 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. Services might include routing of requests,
storing of routing data, storing of other data, STUN discovery,
STUN relay, and many other things. This model posits a
requirement for a service locator function, possibly including
supporting information such as the capacity of a peer to provide a
specific service or descriptions of the policies under which a
peer will provide that service. We currently expect that we will
need to be able to search for available service providers within
each overlay. We think we might need to be able to make searches
based on network locality or path minimalization.
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.
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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.
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 Responsible Peer The Peer that is responsible for storing the
Resource Record for a Resource. In the literature, the term "Root
Peer" is also used for this concept.
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.
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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.
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 Function
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
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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
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
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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
database algorithm. However, no decision has been taken so far.
5.2. Using the Distributed Database Function
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.
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Peer X Peer Z Peer Y
| | |
| | Put(U@Y) |
| |<---------------|
| | Put-Resp(OK) |
| |--------------->|
| | |
| Get(U) | |
|---------------->| |
| Get-Resp(U@Y)| |
|<----------------| |
| INVITE(To:U) | |
|--------------------------------->|
| | |
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 made 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 approach, 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. Through 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 approach, 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 a
[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 or more of its associated clients
instead, thus boosting its effective storage capacity.
In a third alternative, a client acts almost the same as a peer,
except that it does not store any resource records. In this
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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".
Another way to look at this distinction is that a client is simply a
peer that is not currently offering some or all services to the
overlay, possibly due to a run-time decision about available
resources such as bandwidth or storage capacity. With this approach,
the distinction between client and peer becomes much less distinct,
and probably eliminates the requirement to have two distinct terms
for the roles. Rather, we might speak in terms such as "high-
function" vs "low-function" peers. This approach would also seem to
eliminate the requirement for a distinct P2PSIP Client Protocol.
It has also been proposed that the client role could be fulfilled by
conventional SIP UAs served by a peer that is also acting as a proxy/
registrar. While this might fulfill the requirement, the authors
contend that such as device is a "SIP UA", not a "P2PSIP Client" as
defined in this document, and that exclusively using SIP UAs in this
role eliminates the need for P2PSIP Clients and P2PSIP Client
Protocol from the architecture.
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.
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5.7. Architecture
There has been much debate in the group over what an appropriate
architecture for P2PSIP should be. Currently, the group is
investigating architectures that involve a P2P layer that is distinct
from the applications that run on the overlay.
__________________________
| |
| SIP, other apps... |
| ___________________|
| | P2P Layer |
|______|___________________|
| Transport Layer |
|__________________________|
The P2P layer implements the Peer Protocol (and the Client Protocol,
if such a protocol exists). Applications access this P2P layer for
various overlay-related services. Applications are also free to
bypass this layer and access the existing transport layer protocols
(e.g., TCP, UDP, etc.) directly.
A notable feature of this architecture is that it envisions the use
of protocols other than SIP in the overlay. Though the working group
is primarily focused on the use of SIP in peer-to-peer overlays, this
architecture envisions a future in which other protocols can play a
role.
The group initially considered another architecture. In this
alternative architecture, the Peer Protocol was defined as an
extension to SIP. That is, that the necessary operations for forming
and maintaining the overlay and for storing and retrieving resource
records in the distributed database were defined as extensions to
SIP. Each peer in the overlay was viewed as a SIP proxy that would
forward the overlay maintenance and distributed database query
messages (expressed in SIP) on behalf of other peers.
[I-D.bryan-p2psip-dsip] presents a detailed design, and
[I-D.zangrilli-p2psip-whysip] argues for this general approach.
This architecture was eventually rejected by the working group for
the following reasons:
o The architecture was totally focused on SIP, and made it difficult
to use other protocols in the overlay.
o In SIP, proxies are assumed to be trusted parties. Relying on the
peers to route the message as proxies exposes the SIP messages to
attacks from untrusted proxies that SIP's design does not
anticipate. A design that does not allow the peers to modify the
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SIP message and ideally prevents them from reading it is
preferable.
o SIP was seen as a "heavy-weight" protocol for this task. SIP uses
a text-based encoding which is very flexible, but leads to both
large messages and slow processing times at proxies. This was
seen to be a poor match for P2PSIP, where a distributed database
lookup operation requires O(log N) peers to receive, process and
forward the message.
More discussion on this alternate approach and why it was rejected
can be found on the P2PSIP mailing list in a thread that started on
20 March 2007.
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.
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
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compromised in some way?
6.3. Using C/S SIP and P2PSIP Simultaneously in a Single UA
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?
6.4. Clients, Peers, and Services
1. Do all peers providing routing, storage, and all other services,
or do only some peers provide certain services?
2. What services, if any, must all peers provide?
3. Do we need clients as a discrete class, or do SIP UAs and/or low-
function peers completely satisfy the requirements?
4. How we can we describe the capacity of a peer for delivering a
given service?
6.5. Relationships of Domains to Overlays
1. Can there be names from more than one domain in a single overlay?
2. Can there be names from one domain in more than a single overlay?
If so, how do we route Client/Server SIP requests to the right
overlay?
3. Can the domain of an AoR be in more than one overlay?
4. Should we have a "default overlay" to search for peers in many
domains?
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
thoroughly with security topics.
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One critical security issue that will need to be addressed is
providing for the privacy and integrity of SIP messages being routed
by peer nodes, when those peer nodes might well be hostile. This is
a departure from Client/Server SIP, where the proxies are generally
operated by enterprises or service providers with whom the users of
SIP UAs have a trust relationship.
8. IANA Considerations
This document presently raises no IANA considerations.
9. Changes in This Version
1. Revised "Open Questions" to reflect current discussion.
2. Resolved conflict between "services provided by overlay" and
"named services provided by peers" by calling all overlay-level
operations "functions". Thus, we would now speak of an overlay
providing a "distributed transport function".
3. Resolved open issue "Does P2PSIP provide a distributed location
function or an alternative mechanism to RFC 3263? The answer
seems to be both, but what is the relationship between these?" by
documenting that each overlay provides an alternative to
[RFC3263] within that overlay, but that [RFC3263] is used in the
conventional manner between overlays.
4. Revised abstract to include SIP message routing within the scope.
5. Added brief mention of peer's capacity for services offered in
overview section on distributed database.
6. Revised definition of P2PSIP Service.
7. Revised abstract and high level discussion.
8. Added discussion of proposed peer models and relationship to SIP
UAs.
9. Revised reference model diagram to clarify client behavior.
10. Acknowledgements
This document draws heavily from the contributions of many
participants in the P2PSIP Mailing List but the authors are
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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
[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.
[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-p2psip-reload]
Bryan, D., "REsource LOcation And Discovery (RELOAD)",
draft-bryan-p2psip-reload-01 (work in progress),
July 2007.
[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., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for (NAT) (STUN)",
draft-ietf-behave-rfc3489bis-12 (work in progress),
November 2007.
[I-D.ietf-mmusic-ice]
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Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols",
draft-ietf-mmusic-ice-19 (work in progress), October 2007.
[I-D.ietf-sipping-nat-scenarios]
Boulton, C., "Best Current Practices for NAT Traversal for
SIP", draft-ietf-sipping-nat-scenarios-07 (work in
progress), July 2007.
[I-D.marocco-p2psip-xpp-pcan]
Marocco, E. and E. Ivov, "XPP Extensions for Implementing
a Passive P2PSIP Overlay Network based on the CAN
Distributed Hash Table", draft-marocco-p2psip-xpp-pcan-00
(work in progress), June 2007.
[I-D.matthews-p2psip-hip-hop]
Cooper, E., "A Distributed Transport Function in P2PSIP
using HIP for Multi-Hop Overlay Routing",
draft-matthews-p2psip-hip-hop-00 (work in progress),
June 2007.
[I-D.zangrilli-p2psip-whysip]
Zangrilli, M. and B. Lowekamp, "Why SIP should be used for
encoding the P2PSIP Peer Protocol.",
draft-zangrilli-p2psip-whysip-00 (work in progress),
March 2007.
[RFC2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, April 1997.
[RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral
Self-Address Fixing (UNSAF) Across Network Address
Translation", RFC 3424, November 2002.
[RFC4485] Rosenberg, J. and H. Schulzrinne, "Guidelines for Authors
of Extensions to the Session Initiation Protocol (SIP)",
RFC 4485, May 2006.
[RFC4795] Aboba, B., Thaler, D., and L. Esibov, "Link-local
Multicast Name Resolution (LLMNR)", RFC 4795,
January 2007.
[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).
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Copy available at http://mice.cs.columbia.edu/
getTechreport.php?techreportID=388/
Authors' Addresses
David A. Bryan
College of William and Mary and SIPeerior Technologies
3000 Easter Circle
Williamsburg, Virginia 23188
USA
Phone: +1 757 565 0101
Email: bryan@sipeerior.com
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
Unaffiliated
3100 Independence Pkwy #311-164
Plano, Texas 75075
USA
Phone: unlisted
Email: dean.willis@softarmor.com
Bryan, et al. Expires May 18, 2008 [Page 29]
Internet-Draft P2PSIP Concepts and Terminology November 2007
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Bryan, et al. Expires May 18, 2008 [Page 30]
