A SIP Usage for RELOADCisco170 West Tasman DriveMS: SJC-21/2San JoseCA95134USA+1 408 421-9990fluffy@cisco.comSkypePalo AltoCAUSAbbl@lowekamp.netRTFM, Inc.2064 Edgewood DrivePalo AltoCA94303USA+1 650 678 2350ekr@rtfm.comColumbia University1214 Amsterdam AvenueNew YorkNYUSAsalman@cs.columbia.eduColumbia University1214 Amsterdam AvenueNew YorkNYUSAhgs@cs.columbia.eduHAW HamburgBerliner Tor 7Hamburg20099Germanyt.schmidt@haw-hamburg.de
RAI
P2PSIPThis document defines a SIP Usage for REsource LOcation And Discovery
(RELOAD). The SIP Usage provides the functionality of a SIP proxy or
registrar in a fully-distributed system and includes a lookup service
for Address of Records (AORs) stored in the overlay. It also defines
Globally Routable User Agent URIs (GRUUs) that allow the registrations
to map an AOR to a specific node reachable through the overlay. After
such initial contact of a peer, the RELOAD AppAttach method is used to
establish a direct connection between nodes through which SIP messages
are exchanged.REsource LOcation And Discovery (RELOAD) specifies a peer-to-peer (P2P) signaling
protocol for the general use on the Internet. This document defines a
SIP Usage of RELOAD that allows SIP user
agents (UAs) to establish peer-to-peer SIP (or SIPS) sessions without
the requirement for permanent proxy or registration servers, e.g., a
fully distributed telephony service. This service transparently supports
SIP addressing including telephone numbers. In such a network, the
RELOAD overlay itself performs the registration and rendezvous functions
ordinarily associated with such servers.The SIP Usage involves two basic functions.SIP UAs can use the RELOAD data storage
functionality to store a mapping from their address-of-record (AOR)
to their Node-ID in the overlay, and to retrieve the Node-ID of
other UAs.Once a SIP UA has identified the Node-ID
for an AOR it wishes to call, it can use the RELOAD message routing
system to set up a direct connection for exchanging SIP
messages.Mappings are stored in the SipRegistration Resource Record defined in
this document. All operations required to perform a SIP registration or
rendezvous are standard RELOAD protocol methods.For example, Bob registers his AOR, "bob@dht.example.com", for his
Node-ID "1234". When Alice wants to call Bob, she queries the overlay
for "bob@dht.example.com" and receives Node-ID "1234" in return. She
then uses the overlay routing to establish a direct connection with Bob
and can directly transmit a standard SIP INVITE. In detail, this works
along the following steps.Bob, operating Node-ID "1234", stores a mapping from his AOR to
his Node-ID in the overlay by applying a Store request for
"bob@dht.example.com -> 1234".Alice, operating Node-ID "5678", decides to call Bob. She
retrieves Node-ID "1234" by performing a Fetch request on
"bob@dht.example.com".Alice uses the overlay to route an AppAttach message to Bob's
peer (ID "1234"). Bob responds with his own AppAttach and they set
up a direct connection, as shown in . Note that mutual Interactive
Connectivity Establishment (ICE) checks are invoked automatically
from AppAttach message exchange.It is important to note that here the only role of RELOAD is to set
up the direct SIP connection between Alice and Bob. As soon as the ICE
checks complete and the connection is established, ordinary SIP or SIPS
is used. In particular, the establishment of the media channel for a
phone call happens via the usual SIP mechanisms, and RELOAD is not
involved. Media never traverses the overlay. After the successful
exchange of SIP messages, call peers run ICE connectivity checks for
media.In addition to mappings from AORs to Node-IDs, the SIP Usage also
allows mappings from AORs to other AORs. This enables an indirection
useful for call forwarding. For instance, if Bob wants his phone calls
temporarily forwarded to Charlie, he can store the mapping
"bob@dht.example.com -> charlie@dht.example.com". When Alice wants to
call Bob, she retrieves this mapping and can then fetch Charlie's AOR to
retrieve his Node-ID. These mechanisms are described in .Alternatively, Globally Routable User Agent URIs (GRUUs) can be used for directly accessing peers. They
are handled via a separate mechanism, as described in .The SIP Usage for RELOAD addresses a fully distributed deployment of
session-based services among overlay peers. This RELOAD usage may be
relevant in a variety of environments, including a highly regulated
environment of a "single provider" that admits parties using AORs with
domains from controlled namespace(s) only, or an open, multi-party
infrastructure that liberally allows a registration and rendezvous for
various or any domain namespace. It is noteworthy in this context that -
in contrast to regular SIP - domain names play no role in routing to a
proxy server. Once connectivity to an overlay is given, any name
registration can be technically processed.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 RFC 2119.We use the terminology and definitions from Concepts and Terminology for Peer to
Peer SIP and the RELOAD Base
Protocol extensively in this document.In addition, term definitions from SIP
apply to this memo. The term AOR is the SIP "Address of Record" used to
identify a user in SIP. For example, alice@example.com could be the AOR
for Alice. For the purposes of this specification, an AOR is considered
not to include the scheme (e.g. sip:) as the AOR needs to match the
rfc822Name in the X509v3 certificates . It
is worth noting that SIP and SIPS are distinguished in P2PSIP by the
Application-ID.In ordinary SIP, a UA registers the user's AOR and its network
location with a registrar. In RELOAD, this registrar function is
provided by the overlay as a whole. To register its location, a RELOAD
peer stores a SipRegistration Resource Record under its own AOR using
the SIP-REGISTRATION Kind, which is formally defined in . Note that the registration lifetime
known from the regular SIP REGISTER method is inherited from the
lifetime attribute of the basic RELOAD StoredData structure (see
Section 7 in ).A RELOAD overlay MAY restrict the storage of AORs. Namespaces
(i.e., the right hand side of the AOR) that are supported for
registration and lookup can be configured for each RELOAD deployment
as described in .As a simple example, consider Alice with AOR
"alice@dht.example.org" at Node-ID "1234". She might store the mapping
"alice@dht.example.org -> 1234" telling anyone who wants to call
her to contact node "1234".RELOAD peers can store two kinds of SIP mappings,from an AOR to a destination list (a single Node-ID is just a
trivial destination list), orfrom an AOR to another AOR.The meaning of the first kind of mapping is "in order to contact
me, form a connection with this peer." The meaning of the second kind
of mapping is "in order to contact me, dereference this AOR". The
latter allows for forwarding. For instance, if Alice wants her calls
to be forwarded to her secretary, Sam, she might insert the following
mapping "alice@dht.example.org -> sam@dht.example.org".This section defines the SipRegistration Resource Record as
follows:The contents of the SipRegistration Resource Record are:the type of the registrationthe length of the rest of the PDUthe registration dataIf the registration is of type "sip_registration_uri", then the
contents are an opaque string containing the AOR.If the registration is of type "sip_registration_route", then
the contents are an opaque string containing the registrant's
contact preferences and a destination list for the peer.The callee expresses its capabilities within the contact
preferences as specified in . It encodes
a media feature set comprised of its capabilities as a contact
predicate, i.e., a string of feature parameters that appear as part of
the Contact header field. Feature parameters are derived from the
media feature set syntax of (see also
) as described in .This encoding covers all SIP User Agent capabilities, as defined in
and registered in the SIP feature tag
registration tree. In particular, a callee can indicate that it
prefers contact via a particular SIP scheme - SIP or SIPS - by using
one of the following contact_prefs attribute:RELOAD explicitly supports multiple registrations for a single AOR.
The registrations are stored in a Dictionary with Node-IDs as the
dictionary keys. Consider, for instance, the case where Alice has two
peers:her desk phone (1234)her cell phone (5678)Alice might store the following in the overlay at resource
"alice@dht.example.com".A SipRegistration of type "sip_registration_route" with
dictionary key "1234" and value "1234".A SipRegistration of type "sip_registration_route" with
dictionary key "5678" and value "5678".Note that this structure explicitly allows one Node-ID to forward
to another Node-ID. For instance, Alice could set calls to her desk
phone to ring at her cell phone by storing a SipRegistration of type
"sip_registration_route" with dictionary key "1234" and value
"5678".In order to prevent hijacking or other misuse, registrations are
subject to access control rules. Two kinds of restrictions apply:A Store is permitted only for AORs with domain names that fall
into the namespaces supported by the RELOAD overlay instance.Storing requests are performed according to the USER-NODE-MATCH
access control policy of RELOAD.Before issuing a Store request to the overlay, any peer SHOULD
verify that the AOR of the request is a valid Resource Name with
respect to its domain name and the namespaces defined in the overlay
configuration document (see ).Before a Store is permitted, the storing peer MUST check that:The AOR of the request is a valid Resource Name with respect to
the namespaces defined in the overlay configuration document.The certificate contains a username that is a SIP AOR which
hashes to the Resource-ID it is being stored at.The certificate contains a Node-ID that is the same as the
dictionary key it is being stored at.If any of these checks fail, the request MUST be rejected with an
Error_Forbidden error.Note that these rules permit Alice to forward calls to Bob without
his permission. However, they do not permit Alice to forward Bob's
calls to her. See for additional
descriptions.The use of a SIP-enabled overlay MAY be restricted to users with
AORs from specific domains. When deploying an overlay service,
providers can decide about these use case scenarios by defining a set
of namespaces for admissible domain names. This section extends the
overlay configuration document by defining new elements for patterns
that describe a corresponding domain name syntax.A RELOAD overlay can be configured to accept store requests for any
AOR, or to apply domain name restrictions. To apply restrictions, the
overlay configuration document needs to contain a
<domain-restrictions> element. The <domain-restrictions>
element serves as a container for zero to multiple <pattern>
sub-elements. A <pattern> element MAY be present if the "enable"
attribute of its parent element is set to true. Each <pattern>
element defines a pattern for constructing admissible resource names.
It is of type xsd:string and interpreted as a regular expression
according to "POSIX Extended Regular Expression" (see the
specifications in ). Encoding of the
domain name complies to the restricted ASCII character set without
character escaping as defined in Section 19.1 of .Inclusion of a <domain-restrictions> element in an overlay
configuration document is OPTIONAL. If the element is not included,
the default behavior is to accept any AOR. If the element is included
and the “enable” attribute is not set or set to false, the
overlay MUST only accept AORs that match the domain name of the
overlay. If the element is included and the “enable”
attribute is set to true, the overlay MUST only accept AORs that match
patterns specified in the <domain-restrictions> element.In this example, any AOR will be accepted that is either of the
form <user>@dht.example.com, or ends with the domain
"my.example".The Relax NG Grammar for the AOR Domain Restriction reads:A RELOAD user, member of an overlay, who wishes to call another
user with given AOR SHALL proceed in the following way.If the AOR is a GRUU for this overlay,
the callee can be contacted directly as described in .If the domain part
of the AOR matches a domain pattern configured in the overlay, the
user can continue to resolve the AOR in this overlay. The user MAY
choose to query the DNS service records to search for additional
support of this domain name.If the domain
part of the AOR is not supported in the current overlay, the user
might query the DNS (or other discovery services at hand) to
search for an alternative overlay that services the AOR under
request. Alternatively, standard SIP procedures for contacting the
callee might be used.If all of the above contact
attempts fail, the call fails.The procedures described above likewise apply when nodes are
simultaneously connected to several overlays.A RELOAD user that has discovered a route to an AOR in the current
overlay SHALL execute the following steps.Perform a Fetch for Kind SIP-REGISTRATION at the Resource-ID
corresponding to the AOR. This Fetch SHOULD NOT indicate any
dictionary keys, so that it will fetch all the stored values.If any of the results of the Fetch are non-GRUU AORs, then
repeat step 1 for that AOR.Once only GRUUs and destination lists remain, the peer removes
duplicate destination lists and GRUUs from the list and initiates
SIP or SIPS connections to the appropriate peers as described in
the following sections. If there are also external AORs, the peer
follows the appropriate procedure for contacting them as well.Once the peer has translated the AOR into a set of destination
lists, it then uses the overlay to route AppAttach messages to each of
those peers. The "application" field MUST be either 5060 to indicate
SIP or 5061 for using SIPS. If certificate-based authentication is in
use, the responding peer MUST present a certificate with a Node-ID
matching the terminal entry in the destination list. Otherwise, the
connection MUST NOT be used and MUST be closed. Note that it is
possible that the peers already have a RELOAD connection mutually
established. This MUST NOT be used for SIP messages unless it is a SIP
connection. A previously established SIP connection MAY be used for a
new call.Once the AppAttach succeeds, the peer sends plain or (D)TLS
encrypted SIP messages over the connection as in normal SIP. A caller
MAY choose to contact the callee using SIP or SIPS, but SHOULD follow
a preference indicated by the callee in its contact_prefs attribute
(see ). A callee MAY choose to
listen on both SIP and SIPS ports and accept calls from either SIP
scheme, or select a single one. However, a callee that decides to
accept SIPS calls, only, SHOULD indicate its choice by setting the
corresponding attribute in its contact_prefs. It is noteworthy that
according to all overlay links are
built on (D)TLS secured transport. While hop-wise encrypted paths do
not prevent the use of plain SIP, SIPS requires protection of all
links that may include client links (if present) and endpoint
certificates.SIP messages carry the SIP URIs of actual overlay endpoints (e.g.,
"sip:alice@dht.example.com") in the Via and Contact headers, while the
communication continues via the RELOAD connection. However, a UA can
redirect its communication path by setting an alternate Contact header
field like in ordinary SIP.In many cases, RELOAD connections will traverse NATs and Firewalls
that maintain states established from ICE negotiations. It is the responsibility of the
Peers to provide sufficiently frequent traffic to keep NAT and
Firewall states present and the connection alive. Keepalives are a
mandatory component of ICE (see Section 10 of ) and no further operations are required.
Applications that want to assure maintenance of sessions individually
need to follow regular SIP means. Accordingly, a SIP Peer MAY apply
keep-alive techniques in agreement with its transport binding as
defined in Section 3.5 of .Globally Routable User Agent URIs (GRUUs) have been designed to allow direct routing to a
specific UA instance without the need for dereferencing by a
domain-specific SIP proxy function. The concept is transferred to RELOAD
overlays as follows. GRUUs in RELOAD are constructed by embedding a
base64-encoded destination list in the "gr" URI parameter of the GRUU.
The base64 encoding is done with the alphabet specified in table 1 of
with the exception that ~ is used in
place of =.GRUUs do not require to store data in the Overlay Instance. Rather
when a peer needs to route a message to a GRUU in the same P2P overlay,
it simply uses the destination list and connects to that peer. Because a
GRUU contains a destination list, it can have the same contents as a
destination list stored elsewhere in the resource dictionary.Anonymous GRUUs are constructed
analogously, but require either that the enrollment server issues a
different Node-ID for each anonymous GRUU required, or that a
destination list be used that includes a peer that compresses the
destination list to stop the Node-ID from being revealed.This section defines the SIP-REGISTRATION Kind.SIP-REGISTRATIONThe Resource Name for the SIP-REGISTRATION
Kind-ID is the AOR of the user as specified in Section 2. The data
stored is a SipRegistration, which can contain either another URI or
a destination list to the peer which is acting for the user.The data model for the SIP-REGISTRATION
Kind-ID is dictionary. The dictionary key is the Node-ID of the
storing peer. This allows each peer (presumably corresponding to a
single device) to store a single route mapping.USER-NODE-MATCH. Note that this matches
the SIP AOR against the rfc822Name in the X509v3 certificate. The
rfc822Name does not include the scheme so that the "sip:" prefix
needs to be removed from the SIP AOR before matching. Escaped
characters ('%' encoding) in the SIP AOR also need to be decoded
prior to matching (see ).Data stored under the SIP-REGISTRATION Kind is of type
SipRegistration. This comes in two varieties: a URI which the user can be reached at.a destination list which can be used to reach the user's
peer.This Usage for RELOAD does not define new protocol elements or
operations. Hence no new threats arrive from message exchanges in
RELOAD.This document introduces an AOR domain restriction function that
must be surveyed by the storing peer. A misconfigured or malicious
peer could cause frequent rejects of illegitimate storing requests.
However, domain name control relies on a lightweight pattern matching
and can be processed prior to validating certificates. Hence no extra
burden is introduced for RELOAD peers beyond loads already present in
the base protocol.Because SIP includes a forking capability (the ability to
retarget to multiple recipients), fork bombs (i.e., attacks using
SIP forking to amplify the effect on the intended victims) are a
potential DoS concern. However, in the SIP usage of RELOAD, fork
bombs are a much lower concern than in a conventional SIP Proxy
infrastructure, because the calling party is involved in each
retargeting event. It can therefore directly measure the number of
forks and throttle at some reasonable number.Another potential DoS attack is for the owner of an attractive
AOR to retarget all calls to some victim. This attack is common to
SIP and difficult to ameliorate without requiring the target of a
SIP registration to authorize all stores. The overhead of that
requirement would be excessive and in addition there are good use
cases for retargeting to a peer without its explicit
cooperation.A RELOAD overlay and enrollment service that liberally accept
registrations for AORs of domain names unrelated to the overlay
instance and without further authorisation, eventually store
presence state for misused AORs. An attacker could hijack names,
register a bogus presence and attract calls dedicated to a victim
that resides within or outside the Overlay Instance.A hijacking of AORs can be mitigated by restricting the name
spaces admissible in the Overlay Instance, or by additional
verification actions of the enrollment service. To prevent an
(exclusive) routing to a bogus registration, a caller can in
addition query the DNS (or other discovery services at hand) to
search for an alternative presence of the callee in another overlay
or a normal SIP infrastructure.All RELOAD SIP registration data is visible to all nodes in the
overlay. Location privacy can be gained from using anonymous GRUUs.
Methods of providing anonymity or deploying pseudonyms exist, but
are beyond the scope of this document.IANA shall register the following code point in the "RELOAD Data
Kind-ID" Registry (cf., ) to represent
the SIP-REGISTRATION Kind, as described in . [NOTE TO IANA/RFC-EDITOR: Please
replace RFC-AAAA with the RFC number for this specification in the
following list.]This document registers the following URI for the config XML
namespace in the IETF XML registry defined in urn:ietf:params:xml:ns:p2p:config-base:sipThe IESGN/A, the requested URI is an XML namespaceThis document was generated in parts from initial drafts and
discussions in the early specification phase of the P2PSIP base
protocol. Significant contributions (in alphabetical order) were from
David A. Bryan, James Deverick, Marcin Matuszewski, Jonathan Rosenberg,
and Marcia Zangrilli, which is gratefully acknowledged.Additional thanks go to all those who helped with ideas, discussions,
and reviews, in particular (in alphabetical order) Roland Bless, Michael
Chen, Alissa Cooper, Marc Petit-Huguenin, Brian Rosen, Meral
Shirazipour, and Matthias Waehlisch.IEEE Standard for Information Technology - Portable Operating
System Interface (POSIX) - Part 2: Shell and Utilities (Vol.
1)In traditional SIP, the mechanism of a third party registration
(i.e., an assistant acting for a boss, changing users register a
role-based AOR, ...) is defined in Section 10.2 of . This is a REGISTER which uses the URI of the
third-party in its From header and cannot be translated directly into a
P2PSIP registration, because only the owner of the certificate can store
a SIP-REGISTRATION in a RELOAD overlay.A way to implement third party registration is by using the extended
access control mechanism USER-CHAIN-ACL defined in . Creating a new Kind
"SIP-3P-REGISTRATION" that is ruled by USER-CHAIN-ACL allows the owner
of the certificate to delegate the right for registration to individual
third parties. In this way, original SIP functionality can be regained
without weakening the security control of RELOAD.Added subsection on keepaliveUpdated referencesAdded the handling of SIPSSpecified use of Posix regular expressions in configuration
documentAdded IANA registration for namespaceEditorial polishingUpdated and extended referencesCleared open issuesClarified use cases after WG discussionAdded configuration document extensions for configurable domain
namesSpecified format of contact_prefsClarified routing to AORsExtended security sectionAdded Appendix on Third Party RegistrationAdded IANA code pointsEditorial polishingUpdated and extended referencesAdded Open Issue