SIPPING Working Group D. Willis, Ed.
Internet-Draft Cisco Systems
Intended status: Experimental D. Bryan
Expires: February 19, 2007 P2PSIP.org and William and Mary
Department of Computer
Science
P. Matthews
Avaya
E. Shim
Panasonic Digital Networking
Laboratory
August 18, 2006
Concepts and Terminology for Peer to Peer SIP
draft-willis-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
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.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [1].
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Table of Contents
1. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1. What Kinds of P2PSIP Overlay Peers and Clients Might
Exist? . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2. Reference Model . . . . . . . . . . . . . . . . . . . . . 10
3.3. Conceptual Outline of Operations . . . . . . . . . . . . . 12
3.3.1. Enrolling and Inserting an P2PSIP Overlay Peer . . . . 12
3.3.2. More on The Difference Between a Peer, Client, and
User Agent . . . . . . . . . . . . . . . . . . . . . . 12
3.3.3. Enrolling a User and Inserting a P2PSIP Overlay
User Agent . . . . . . . . . . . . . . . . . . . . . . 13
3.3.4. Placing a Call from a P2PSIP Overlay Client UA to
a P2PSIP Overlay Client UA . . . . . . . . . . . . . . 14
3.3.5. Bootstrapping . . . . . . . . . . . . . . . . . . . . 14
4. Questions . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.1. Definition of P2PSIP Overlay Peer Enrollment . . . . . . . 15
4.2. PP2PSIP Overlay Peer Protocol . . . . . . . . . . . . . . 15
4.3. P2PSIP Overlay Client Protocol . . . . . . . . . . . . . . 15
4.4. Universal Routing: . . . . . . . . . . . . . . . . . . . . 15
4.5. How To Find Media Relays? . . . . . . . . . . . . . . . . 15
4.6. How Do We Find Gateways? . . . . . . . . . . . . . . . . . 16
4.7. NAT Traversal . . . . . . . . . . . . . . . . . . . . . . 16
4.8. Peer-Adjacency Through NATs . . . . . . . . . . . . . . . 16
4.9. Cryptotransparency . . . . . . . . . . . . . . . . . . . . 16
4.10. Record Formats . . . . . . . . . . . . . . . . . . . . . . 16
4.11. Peer and Client Enrollment Protocols . . . . . . . . . . . 16
4.12. Peer and User Credentials . . . . . . . . . . . . . . . . 16
4.13. Bootstrapping . . . . . . . . . . . . . . . . . . . . . . 17
4.14. Credential Recovery . . . . . . . . . . . . . . . . . . . 17
4.15. Overlapping Domains . . . . . . . . . . . . . . . . . . . 17
4.16. Hybrid Domains . . . . . . . . . . . . . . . . . . . . . . 17
4.17. Admissions Control . . . . . . . . . . . . . . . . . . . . 17
4.18. Users versus Resources . . . . . . . . . . . . . . . . . . 18
5. Security Considerations . . . . . . . . . . . . . . . . . . . 18
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
8.1. Normative References . . . . . . . . . . . . . . . . . . . 18
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8.2. Informative References . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
Intellectual Property and Copyright Statements . . . . . . . . . . 21
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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) [2].
The Session Initiation Protocol (SIP) [3] 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) [4]. SIP's location process [5] assumes that host part of the
URI indicates either the target SIP user agent (UA), or a proxy that
has knowledge of how to to reach the target UA, presumably gained
through SIP's registration process.
This approach, referred to herein as "Conventional SIP" or "Client/
Server SIP", assumes a relatively fixed hierarchy of SIP routing
proxies (servers) and SIP user agents (clients). The routing proxies
are typically resolvable using conventional Internet mechanisms with
static IP addresses and associated DNS entries. This structure may
not be ideal in all cases, including specifically ad-hoc, serverless,
and reduced-administration scenarios. As an alternative, several
authors [7] [8] [9] [10] have proposed using peer-to-peer (P2P) [11]
approaches to solving the correspondent discovery problem.
This document offers a consolidation of the more important terms and
concepts from several of the above documents, presented in the
context of a reference model for peer-to-peer SIP (P2PSIP). It is
intended that this document serve as a starting point for describing
the work needed to resolve a number of open questions such that an
IETF working group could be chartered to do the work needed to
resolve these questions and present a standard solution. The authors
believe that this goal is roughly consistent with that of a Protocol
Model as defined in [12].
2. Definitions
Defined terms include:
Overlay Network: An overlay network is a computer network which is
built on top of another network. Nodes in the overlay can be
thought of as being connected by virtual or logical links, each of
which corresponds to a path, perhaps through many physical links,
in the underlying network. For example, many peer-to-peer
networks are overlay networks because they run on top of the
Internet. Dial-up Internet is an overlay upon the telephone
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network. <http://en.wikipedia.org/wiki/P2P_overlay>
P2P Network: A peer-to-peer (or P2P) computer network is a network
that relies primarily on the computing power and bandwidth of the
participants in the network rather than concentrating it in a
relatively low number of servers. P2P networks are typically used
for connecting nodes via largely ad hoc connections. Such
networks are useful for many purposes. Sharing content files (see
file sharing [15]) containing audio, video, data or anything in
digital format is very common, and realtime data, such as
telephony traffic, is also passed using P2P technology.
<http://en.wikipedia.org/wiki/Peer-to-peer>. A P2P Network may
also be called a "P2P Overlay" or "P2P Overlay Network" or "P2P
Network Overlay" , since its organization is not at the physical
layer, but is instead "on top of" an existing Internet Protocol
network.
P2PSIP: A communications protocol related to the Session Initiation
Protocol (sip) [3] that extends SIP by using peer-to-peer
techniques for resolving the targets of SIP requests.
P2PSIP Overlay: A P2PSIP Overlay is an association, collection, or
federation of nodes that provides SIP registration, SIP request
routing, and similar functions using a P2P organization, as
defined by "P2P Network" above. Short form: overlay.
P2PSIP Overlay Identifier: Information that identifies a specific
P2PSIP Overlay. This is presumed in the general case to be scoped
to a name within the DNS, but other scopes may apply, particularly
in ad-hoc environments. Short forms: overlay identifier, overlay
ID.
P2PSIP Overlay Peer: A node participating in a P2PSIP Overlay that
provides storage and routing services (fully participates in the
P2P routing) to other nodes in that P2PSIP Overlay. Each P2PSIP
Overlay Peer is presumed to have a unique identifier within the
P2PSIP Overlay. Each P2PSIP Overlay Peer may or may not be
coupled to one or more P2PSIP Overlay User Agents. Within the
P2PSIP Overlay, the peer is capable of performing several
different operations, including: joining and leaving the overlay,
routing requests within the overlay, storing information on behalf
of the overlay, putting information into the overlay, and getting
information from the overlay. Short forms: overlay peer,
supernode, P2PSIP peer, peer.
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P2PSIP Overlay Peer Key: Information (perhaps a string, number or
URI) that uniquely identifies each P2PSIP Overlay Peer within a
given P2PSIP Overlay. These keys are completely independent of
identifier of any user of a user agent coupled to a peer. Short
forms: P2PSIP peer key, overlay peer key, peer key.
P2PSIP Overlay Client: A node participating in a P2PSIP Overlay that
provides neither routing nor route storage and retrieval functions
to that P2PSIP Overlay. The P2PSIP Overlay Client interacts with
the P2PSIP Overlay only to request the insertion of routing
information (put in a Contact), request the retrieval of routing
information (get a Contact), or to request the routing of a
message to elsewhere in the P2PSIP Overlay. Unlike the P2PSIP
Overlay Peer, the client is presumed not to have a unique
identifier or "key" within the overlay. A P2PSIP Overlay Client
may be coupled to one or more P2PSIP Overlay User Agents. Short
forms: overlay client, P2PSIP client, client.
P2PSIP Overlay User Agent: A SIP user agent that is coupled to a
P2PSIP Overlay Peer or P2PSIP Overlay Client, such that the peer
or client can assist the UA with registration (storage of a route
to users of the UA) and/or routing of requests using the P2PSIP
Overlay. A P2PSIP Overlay SUer Agent differs from a conventional
SIP user agent in that it is coupled directly to a P2PSIP Overlay
Peer or P2PSIP Overlay Client, and can therefore directly interact
with a P2PSIP Overlay, which a conventional SIP UA cannot do.
P2PSIP Overlay User Agents are presumed to have no distinguished
name or identifier. Short forms: overlay UA, P2PSIP UA.
P2PSIP Overlay User: An addressable user endpoint, entity, service,
or function within a P2PSIP Overlay. Examples include but are not
limited to humans, automata, bridges, mixers, media relays,
gateways, and media storage. Short forms: overlay user, P2PSIP
user.
P2PSIP Overlay User Identifier: A distinguished name that identifies
a specific P2PSIP Overlay User within a given P2PSIP Overlay.
This is presumed to be a URI scoped to the P2PSIP Overlay
Identifier. This is presumably the same as a SIP Address of
Record, or AOR Short forms: overlay user identifier, P2PSIP user
identifier, P2PSIP user-ID, P2PSIP AOR.
P2PSIP Overlay User Record: A block of data, stored in the data
mechanism of the P2PSIP Overlay, that includes information
relevant to a specific user. We presume that there may be
multiple types of user records. Some may describe routes to a
client at which the user is presumed reachable (a "user routing
record", like a SIP "Contact:"). Others might store presence
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information. The types, usages, and formats of user records are a
question for future study.
P2PSIP Overlay Peer Protocol: The protocol spoken between P2P
Overlay peers to share information and organize the P2P Overlay
Network. Short form: peer protocol.
P2PSIP Overlay Client Protocol: The protocol spoken between P2P
Overlay Clients and the P2P Overlay Peer they use to place into or
retrieve information from the P2P Overlay Network. This is
presumably a functional subset of the P2P Overlay Peer Protocol,
but may differ in syntax and protocol implementation (i.e., may
not be syntactically related). Note that the precise relationship
between the P2PSIP Overlay Peer Protocol and the P2PSIP Overlay
Client Protocol remains an open question and is expected to be a
principle topic of the detailed design work. Short form: client
protocol.
P2PSIP Overlay Neighbors: The set of P2P Overlay Peers that either a
P2P Overlay Peer or P2P Overlay Client know of directly and can
reach without further lookups. Short form: neighbor
P2PSIP Overlay Bootstrap Server: A network node used by P2PSIP
Overlay Peers or Clients who are attempting to enter to locate an
entry into the P2P Overlay Network. It may return an entry point
(address of a Peer) to the P2PSIP Overlay or act as one itself.
This should be a quasi-stable and well known host, located using a
configuration or discovered via , DNS, broadcast, or other
mechanism. Example: a P2PSIP Overlay Peer that reboots and has no
knowledge of other peers uses a P2PSIP Overlay Bootstrap Server to
find other peers in the P2P Overlay Network and establish P2PSIP
Overlay Peer Insertion. (Note: An overlay peer might or might not
provide this functionality). Short forms: bootstrap server, boot
server.
P2PSIP Overlay Peer Insertion: The act of inserting a P2PSIP Overlay
Peer into the current routing structure (presumably a distributed
database or hash table) of a P2PSIP Overlay. In general, this
routing structure maps the peer's P2PSIP Overlay Peer Key to the
IP address or DNS name of the peer. During insertion, the peer
discovers its P2PSIP Overlay neighbors. Following insertion, the
peer will be able to store user records (such as routing
information), query other peers for user records, and pass
requests to route messages to other peers. Short form: peer
insertion.
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P2PSIP Overlay User Record Insertion: The act of inserting a record
for a P2PSIP Overlay User into the data maintained by the P2PSIP
Overlay Peers. Following insertion, the data stored at one or
more peers will contain a record (such as a P2PSIP Overlay User
Routing Record), keyed at least in part by a P2PSIP Overlay User
Identifier. Short form: User record insertion.
P2PSIP Overlay User Routing Record: A P2PSIP overlay user record
that provides a routing vector that points to a P2PSIP UA where
the user can presumably be reached. This is analogous to the
combination of a SIP [3] "Contact:" and a "Path:" [6]. The P2PSIP
equivalent of a SIP registration process would be the insertion of
an overlay user routing record into the overlay. Short form: user
route record or user route.
P2PSIP Overlay Peer Enrollment: The initial one-time process a
P2PSIP Overlay Peer follows to obtain an identifier and
credentials, if any, within a P2PSIP Overlay. This is not
performed each time the peer comes online, only the first time
they do so, or following a loss of identifier or credentials by
the peer. Short form: peer enrollment.
P2PSIP Overlay User Enrollment: The initial one-time process a
P2PSIP Overlay User follows to obtain a unique identifier within a
P2PSIP Overlay. This is not performed each time the resource
comes online, only the first time they do so, or following a loss
of identifier or credentials by the client. Short form: user
enrollment.
3. Discussion
3.1. What Kinds of P2PSIP Overlay Peers and Clients Might Exist?
In general, P2PSIP nodes might have the same sorts of duties and
profiles as traditional client-server SIP nodes. This includes but
is not limited to:
1. User Agent: a phone, voice mail server, bridge, or other device
that initiates or terminates session requests.
2. Media relay: a peer or client capable of relaying RTP sessions,
as described in [13]
3. Gateway: a peer or client that converts from P2PSIP to some other
protocol, such as PSTN.
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4. Redirector or Location Server: a peer or client that, given a SIP
(or other SIP request) to a P2PSIP overlay resource identifier,
returns the route to a resource in a 302 or 305 response.
5. Registrar: A peer or client that processes SIP REGISTER requests,
either storing or retrieving the contact information to/from the
routing data of the P2PSIP Overlay.
6. Proxy: A peer or client that accepts SIP requests, resolves the
next hop or hops using the routing information of the P2PSIP
Overlay, and passes the request on towards the next hop.
3.2. Reference Model
The following diagram depicts a reference or "boxes and arrows" model
showing several of the above peer and client types, along with two
conventional SIP user agents.
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--->PSTN
-----------/
| Gateway |
________ ----------| Peer |-------------
| | | | | |Client Protocol
| UA |\ | ----------- | GET/PUT
| Peer |\\|N|----- | |
|______| \|A| | | \__/
|T| ---- | v / \
Peer Protocol/ |---- / UA \
/ P2PSIP Overlay | /Client\
|N|/ | |______|
-------- /|A|------ Route Data | ^
| |//|T| ____|___ | |
| UA |/ | | | |
| Peer | | UA | | |
-------- | Peer | | |
|______| | |
| | |SIP
Peer Protocol | --------- --------- | |
| | | | | | |
----| Proxy |----- --| Redir |----| |
| Peer | | Peer | | |
| | | | |
--------- ---------- |
/ \ /
/ \ /
\__/ / SIP SIP \ \__/ /
/\ / \ /\ /
/ \/ \/ \/
/ UA \ / UA \
/______\ /______\
SIP UA A SIP UA B
Figure: Reference Model
Here, the large rectangle represents the P2PSIP Overlay and its
associated routing data. Around the periphery of the P2PSIP Overlay
rectangle, we have several P2PSIP Overlay Peer nodes -- a PSTN
gateway, a user-agent, a proxy, and a redirector. To the left we
have two UA peers are behind network address translator. On the
right side, we have a P2PSIP Overlay "UA client", which uses the
P2PSIP Overlay Client Protocol to interact with the P2PSIP Overlay.
Below, we have conventional SIP UA "A", which has used SIP to
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interact with the Proxy Peer and establish a dialog with the UA peer,
and SIP UA "B" which has been redirected by the Redirect Peer and set
up a dialog with the UA Client. Note that the Proxy Peer and the UA
Peer interact using the P2PSIP Overlay Peer Protocol, which is also
presumably used on the keepalive links between the UA Peers and their
neighbors.
Both the "Proxy Peer" and "Redir Peer" serve as adapters between
ordinary SIP devices and the the P2PSIP Overlay. Each accepts 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).
The Gateway Peer provides a similar sort of adaptation to and from
the public switched telephone network (PSTN). However, there is a
subtle distinction. The gateway function itself can be viewed as a
"user" or within the P2PSIP overlay, and is addressed using a P2PSIP
Overlay User Identifier. This gateway functionality could also be
located in a P2PSIP Overlay Client, or even in a traditional SIP UA
that is reached via P2P (using a P2P proxy or redirector) or
conventional SIP mechanisms.
3.3. Conceptual Outline of Operations
3.3.1. Enrolling and Inserting an P2PSIP Overlay Peer
Peers are the full-function "routing and storage" nodes of a P2PSIP
Overlay. When a new peer is first created, it must enroll in the
P2PSIP Overlay. We currently have no defined mechanism for this (do
we need one?), but we know that once the process is complete, the new
peer will have at least a P2PSIP Overlay Peer Key and a set of
credentials.
After enrollment, each time the peer connects to the overlay, it must
insert itself. We don't have a defined protocol and process for
this, and assume we need one. Presumably the inserting peer connects
to one or more existing peers (possibly with the aid of a bootstrap
server) presents its credentials, and ends up connected to the
overlay, with some knowledge of neighbors (successor, precursor,
finger tables, or whatever the distribution mechanism defines) and is
able to store data on behalf of and route requests to other nodes in
the P2PSIP overlay.
3.3.2. More on The Difference Between a Peer, Client, and User Agent
P2PSIP Overlay Peers directly interact with and contain the routing
and storage fabric of the overlay. P2PSIP Overlay Clients just use
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the routing and storage facilities provided by the peers. The peers
speak the P2PSIP Overlay Peer Protocol, which presumably has a full
range of expressivity for the routing and storage facilities of the
overlay. Clients speak the P2PSIP Overlay Client protocol, which is
presumably a subset of the peer protocol, and is limited to storage
insertion (put), storage retrieval (get), and message routing (send).
Some peers and some clients may be coupled to SIP user agents, making
them P2PSIP Overlay User Agents capable of both sending and receiving
conventional SIP messages (as per a SIP UA) using conventional SIP
resolution procedures and of using the resolution facilities provided
by the overlay
The mix and configuration of peers, clients, and P2PSIP UAs is
expected to vary depending on the deployment scenario. For example,
an ad-hoc scenario might deploy nothing but P2PSIP Overlay Peers,
each of which is coupled to a P2PSIP User Agent, using a broadcast or
multicast bootstrap mechanism. Another common scenario, the "self
organizing proxy farm", might consist of P2PSIP Overlay Peers, each
of which is coupled to a SIP proxy/registrar function.
3.3.3. Enrolling a User and Inserting a P2PSIP Overlay User Agent
Clients and Peers are the "end points" or "user agents" of a P2PSIP
Overlay. Users are the named entities that participate in a P2PSIP
overlay using a client.
To get started, users must be enrolled in the overlay. We do not
have a process or protocol for this, nor are we certain we need a
standardized mechanism. We presume that after enrollment, the user
has a distinguished name within the overlay (example:
sip:bob@example.com) and a set of credentials useful for
authenticating its usage of the distinguished name. One possible
mechanism for these credentials would be an x.509 certificate. It
might also be possible to use a PGP key, a password, or some other
mechanism. Presumably following enrollment, the user is also
equipped with the information needed to connect to the overlay, such
as the address of a bootstrap server. Whether this startup
information is delivered as a part of enrollment of through some
separate configuration process remains an open question.
Once a user is enrolled, the user may exercise a P2PSIP Overlay User
Agent to insert into the P2PSIP Overlay. We currently have no
protocol for this, and need one. The P2PSIP UA exercises either a
P2PSIP Overlay Peer or P2PSIP Overlay Client to execute the
"registration" function and insert a route for the user into the
P2PSIP overlay. This function is described as a "PUT" request, and
results in the storage of an authenticated route-set for the user in
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the P2PSIP overlay, such that the terminus of the route is the URI of
the user at the P2PSIP UA. This is analogous to "registration" in a
classic SIP environment, and might even be doable using the SIP
REGISTER method. Presumably, the P2PSIP UA connects to a peer or
client and uses the user's credentials to authenticate a route-set
(Contact: plus Path:) to itself, and the peer or client stores the
route-set into the overlay, using a key derived from the user's
identity.
3.3.4. Placing a Call from a P2PSIP Overlay Client UA to a P2PSIP
Overlay Client UA
Assume we have two users, Alice and Bob, who have successfully
enrolled and inserted themselves into a P2PSIP Overlay using clients.
Alice wants to call Bob, and enters Bob's user identity (AOR) into
her client. Alice's client does not have current knowledge of a
route-set to Bob's client(s), so it needs to discover one. Alice's
Client then does a GET operation on Bob's identity, using a
previously-discovered peer, or using whatever procedures are needed
(such as RFC 3263, a bootstrap server, etc.) to find a peer. Alice's
client send the GET request to the selected peer.
The peer transforms the requested identity into a key, presumably by
hashing it, and determines the peer ID at which the resource (a
route-set) is most likely stored. The first peer then asks the
second peer to return the desired resource. The second peer may
return the desired resource, return a pointer to another peer, or
pass the request recursively on to another peer for resolution.
Eventually, some peer returns a resource containing a route-set for
Bob, and the first peer returns this to Alice's client.
Alice then sends a SIP INVITE with a request-URI equal to Bob's user
identity, and populated with a route from the returned route-set.
Bob's client returns a SIP response, and we proceed with SIP as
usual.
3.3.5. Bootstrapping
If a client or peer is just starting up and has no knowledge of how
to reach the other nodes of the overlay, it may exercise a bootstrap
server to find one. Presumably it discovers the bootstrap server by
some mechanism such as a DNS lookup, multicast, broadcast or
configuration, then queries the bootstrap server and receives an
address for a peer or set of peers that can be used to reach the
overlay. Ideally, these target peers will be selected from a
relatively large pool of peers that are currently online and
reachable.
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After discovering the address of a peer, the behavior of the starting
node will vary depending on whether it is intending to be a peer or a
client. If it is intending to be a peer, it goes into the P2PSIP
Overlay Peer Insertion process, at the conclusion of which it is
actively participating in the target overlay as a peer and is capable
of routing requests and storing records on behalf of the P2PSIP
overlay. If it is intending to be a client, it does not bother with
insertion, but merely contacts the discovered peer in order to use
the overlay.
In the typical case, the peer or client coming up is also a P2PSIP
Overlay User Agent with one or more associated P2PSIP Overlay User
Identifiers. The next step then is to insert a P2PSIP Overlay User
Routing Record (a Contact:) into the P2PSIP Overlay.
4. Questions
4.1. Definition of P2PSIP Overlay Peer Enrollment
The definition for P2PSIP Overlay Peer Enrollment in the above
section doesn't sound quite right. It predates a decision made to
split off the UA from the Peer and Client nodes. What would a better
definition look like?
4.2. PP2PSIP Overlay Peer Protocol
This may or may not be SIP. What should it be? Alternatives include
SIP, OpenDHT, or something else. Do we need to define a new
protocol?
4.3. P2PSIP Overlay Client Protocol
This may or may not be SIP. What should it be? It defines only GET/
PUT operations, which could be done using SIP REGISTER transactions.
4.4. Universal Routing:
Do all P2PSIP Overlay Peers route requests? How about clients?
4.5. How To Find Media Relays?
This needs to be net-path efficient. Is this possible? Is it enough
just to construct a key with a "relay" identifier? What sorts of
access controls do we need on media relays?
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4.6. How Do We Find Gateways?
This needs to be not only netpath efficient, but also embodies
elements of the TRIP and SPEERMINT problems.
4.7. NAT Traversal
Some peers or clients may be isolated by NATs from other peers or
clients. How do we structure persistent keepalive connections to
them? Is it enough to maintain links to left and right neighbors and
construct dual routes?
4.8. Peer-Adjacency Through NATs
We assume that some or even many peers will be behind NATs, and
therefore reached through peer-to-peer routing. How do we keep alive
the NAT-crossing peer bindings? Is some variant of "outbound" [14]
usable?
4.9. Cryptotransparency
When forwarding requests, are the bodies of the requests visible to
peers? If so, this creates substantial security problems that the
deployers of conventional SIP have been willing to mostly ignore.
Can we make peers cryptotransparent (like HTTP proxies) when security
is requested?
4.10. Record Formats
Clearly we need user routing records stored into the p2PSIP overlay.
Do we need other sorts of record? If so, what? How do we
differentiate between or classify records? Do we end up with many
records per user per client, or do we aggregate the per-client or
per-user view using something like XML?
4.11. Peer and Client Enrollment Protocols
We know that we need to enroll peer and client nodes into a P2PSIP
Overlay. Do we define a protocol or process for this, assume it will
happen externally, or just provide an existence-proof argument?
4.12. Peer and User Credentials
We believe we need some sort of credentials for authenticating peers
and users of each P2PSIP Overlay. What should we use for these
credentials? Certificates? PGP keys? Passwords? If certificates,
should these be signed by a CA associated with the overlay, or can
self-signed certificates work in some or all cases? Do we need to
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specify a standard credential format, or should we allow different
implementations to use different credential formats?
4.13. Bootstrapping
We know that sometimes peers or clients will start up without
knowledge of how to find a peer for insertion. Do we need to define
a bootstrap mechanism or mechanisms? Do we need to define supporting
protocols?
4.14. Credential Recovery
One reader suggested that we extend the definition of P2PSIP Overlay
Peer Enrollment to cover the case where a previously-inserted peer
has lost its credentials (through, perhaps, being moved to a
different host) and wishes to recover them without necessarily
creating a new P2PSIP Overlay Peer Key. The editors are inclined to
believe that this is an operational issue, not a matter of
definition, but would like to seek a broader consensus before
concluding the topic. A similar question applies to user enrollment.
4.15. Overlapping Domains
If the P2PSIP Overlay User Identifier is not scoped to a single DNS
domain, this would appear to allow nodes from two or more apparent
domains to share a single P2PSIP Overlay. What, if anything, do we
need to say about this mode of operation?
4.16. Hybrid Domains
It appears possible to have some hosts within a domain using
conventional SIP and some using P2PSIP. This potentially raises a
number of questions: 1) What should happen if we want to run a P2PSIP
overlay in an existing SIP domain? 2) Do the existing redir/proxy
servers need to be coupled with a peer layer? 3) When would an
overlay peer want to discover them as opposed to looking in the
overlay? 4) Is better not to run conventional SIP with overlay SIP?
5) When conventional and P2PSIP are run together, shall the existing
redir servers keep their local databases or switch to the overlay
storage.
4.17. Admissions Control
What do we need to say about admissions control with respect to the
enrollment of peers and users? Do we need to discuss per-call
admissions control in a P2P environment?
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4.18. Users versus Resources
This model presumes that all addressable elements, aka "users", are
unique. Are their other classes of resources that need some sort of
class-addressable identifier that does not refer to a unique user?
5. Security Considerations
This specification probably has all sorts of security requirements
that we just haven't gotten to.
6. IANA Considerations
This document raises no IANA considerations. Yet.
7. 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 Henning Schulzrinne and Cullen
Jennings, both of whom spent many long pain-filled hours on the phone
with us.
8. References
8.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
Resource Locators (URL)", RFC 1738, December 1994.
[3] 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.
[4] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[5] Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol
(SIP): Locating SIP Servers", RFC 3263, June 2002.
[6] Willis, D. and B. Hoeneisen, "Session Initiation Protocol (SIP)
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Extension Header Field for Registering Non-Adjacent Contacts",
RFC 3327, December 2002.
8.2. Informative References
[7] Bryan, D., "A P2P Approach to SIP Registration and Resource
Location", draft-bryan-sipping-p2p-02 (work in progress),
March 2006.
[8] Shim, E., "An Architecture for Peer-to-Peer Session Initiation
Protocol (P2P SIP)", draft-shim-sipping-p2p-arch-00 (work in
progress), March 2006.
[9] Sinnreich, H. and A. Johnston, "SIP, P2P, and Internet
Communications", draft-johnston-sipping-p2p-ipcom-02 (work in
progress), March 2006.
[10] Matthews, P., "Industrial-Strength P2P SIP",
draft-matthews-sipping-p2p-industrial-strength-00 (work in
progress), February 2005.
[11] 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.
[12] Rescorla, E. and IAB, "Writing Protocol Models", RFC 4101,
June 2005.
[13] Rosenberg, J., "Obtaining Relay Addresses from Simple Traversal
of UDP Through NAT (STUN)", draft-ietf-behave-turn-00 (work in
progress), March 2006.
[14] Jennings, C. and R. Mahy, "Managing Client Initiated
Connections in the Session Initiation Protocol (SIP)",
draft-ietf-sip-outbound-03 (work in progress), March 2006.
URIs
[15] <http://en.wikipedia.org/wiki/File_sharing>
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Authors' Addresses
Dean Willis (editor)
Cisco Systems
3100 Independence Pkwy #311-164
Plano, Texas 75075
USA
Phone: unlisted
Email: dean.willis@softarmor.com
David Bryan
P2PSIP.org and William and Mary Department of Computer Science
P.O. Box 6741
Williamsburg, Virginia 23188
USA
Phone: unlisted
Email: bryan@ethernot.org
Philip Matthews
Avaya
100 Innovation Drive
Ottawa, Ontario K2K 3G7
Canada
Phone: +1 613 592 4343 x224
Email: philip_matthews@magma.ca
Eunsoo Shim
Panasonic Digital Networking Laboratory
Two Research Way, 3rd Floor
Princeton, New Jersey 08540
USA
Phone: unlisted
Email: eunsoo@research.panasonic.com
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