RFC 4975
The Message Session Relay Protocol (MSRP)
Network Working Group B. Campbell, Ed. Request for Comments: 4975 Estacado Systems Category: Standards Track R. Mahy, Ed. Plantronics C. Jennings, Ed. Cisco Systems, Inc. September 2007 The Message Session Relay Protocol (MSRP) Status of This Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.Abstract
This document describes the Message Session Relay Protocol, a protocol for transmitting a series of related instant messages in the context of a session. Message sessions are treated like any other media stream when set up via a rendezvous or session creation protocol such as the Session Initiation Protocol.
Table of Contents
1. Introduction ....................................................4 2. Conventions .....................................................5 3. Applicability of MSRP ...........................................5 4. Protocol Overview ...............................................6 5. Key Concepts ....................................................9 5.1. MSRP Framing and Message Chunking ..........................9 5.2. MSRP Addressing ...........................................10 5.3. MSRP Transaction and Report Model .........................11 5.4. MSRP Connection Model .....................................12 6. MSRP URIs ......................................................14 6.1. MSRP URI Comparison .......................................15 6.2. Resolving MSRP Host Device ................................16 7. Method-Specific Behavior .......................................17 7.1. Constructing Requests .....................................17 7.1.1. Sending SEND Requests ..............................18 7.1.2. Sending REPORT Requests ............................21 7.1.3. Generating Success Reports .........................22 7.1.4. Generating Failure Reports .........................23 7.2. Constructing Responses ....................................24 7.3. Receiving Requests ........................................25 7.3.1. Receiving SEND Requests ............................25 7.3.2. Receiving REPORT Requests ..........................27 8. Using MSRP with SIP and SDP ....................................27 8.1. SDP Connection and Media-Lines ............................28 8.2. URI Negotiations ..........................................29 8.3. Path Attributes with Multiple URIs ........................30 8.4. Updated SDP Offers ........................................31 8.5. Connection Negotiation ....................................31 8.6. Content Type Negotiation ..................................32 8.7. Example SDP Exchange ......................................34 8.8. MSRP User Experience with SIP .............................35 8.9. SDP Direction Attribute and MSRP ..........................35 9. Formal Syntax ..................................................36 10. Response Code Descriptions ....................................38 10.1. 200 ......................................................38 10.2. 400 ......................................................38 10.3. 403 ......................................................38 10.4. 408 ......................................................39 10.5. 413 ......................................................39 10.6. 415 ......................................................39 10.7. 423 ......................................................39 10.8. 481 ......................................................39 10.9. 501 ......................................................39 10.10. 506 .....................................................40
11. Examples ......................................................40 11.1. Basic IM Session .........................................40 11.2. Message with XHTML Content ...............................42 11.3. Chunked Message ..........................................43 11.4. Chunked Message with Message/CPIM Payload ................43 11.5. System Message ...........................................44 11.6. Positive Report ..........................................44 11.7. Forked IM ................................................45 12. Extensibility .................................................48 13. CPIM Compatibility ............................................48 14. Security Considerations .......................................49 14.1. Secrecy of the MSRP URI ..................................50 14.2. Transport Level Protection ...............................50 14.3. S/MIME ...................................................51 14.4. Using TLS in Peer-to-Peer Mode ...........................52 14.5. Other Security Concerns ..................................53 15. IANA Considerations ...........................................55 15.1. MSRP Method Names ........................................55 15.2. MSRP Header Fields .......................................55 15.3. MSRP Status Codes ........................................56 15.4. MSRP Port ................................................56 15.5. URI Schema ...............................................56 15.5.1. MSRP Scheme .......................................56 15.5.2. MSRPS Scheme ......................................57 15.6. SDP Transport Protocol ...................................57 15.7. SDP Attribute Names ......................................58 15.7.1. Accept Types ......................................58 15.7.2. Wrapped Types .....................................58 15.7.3. Max Size ..........................................58 15.7.4. Path ..............................................58 16. Contributors and Acknowledgments ..............................59 17. References ....................................................59 17.1. Normative References .....................................59 17.2. Informative References ...................................60
1. Introduction
A series of related instant messages between two or more parties can be viewed as part of a "message session", that is, a conversational exchange of messages with a definite beginning and end. This is in contrast to individual messages each sent independently. Messaging schemes that track only individual messages can be described as "page-mode" messaging, whereas messaging that is part of a "session" with a definite start and end is called "session-mode" messaging. Page-mode messaging is enabled in SIP via the SIP [4] MESSAGE method [22]. Session-mode messaging has a number of benefits over page-mode messaging, however, such as explicit rendezvous, tighter integration with other media-types, direct client-to-client operation, and brokered privacy and security. This document defines a session-oriented instant message transport protocol called the Message Session Relay Protocol (MSRP), whose sessions can be negotiated with an offer or answer [3] using the Session Description Protocol (SDP) [2]. The exchange is carried by some signaling protocol, such as SIP [4]. This allows a communication user agent to offer a messaging session as one of the possible media-types in a session. For instance, Alice may want to communicate with Bob. Alice doesn't know at the moment whether Bob has his phone or his IM client handy, but she's willing to use either. She sends an invitation to a session to the address of record she has for Bob, sip:bob@example.com. Her invitation offers both voice and an IM session. The SIP services at example.com forward the invitation to Bob at his currently registered clients. Bob accepts the invitation at his IM client, and they begin a threaded chat conversation. When a user uses an Instant Messaging (IM) URL, RFC 3861 [32] defines how DNS can be used to map this to a particular protocol to establish the session such as SIP. SIP can use an offer/answer model to transport the MSRP URIs for the media in SDP. This document defines how the offer/answer exchange works to establish MSRP connections and how messages are sent across the MSRP, but it does not deal with the issues of mapping an IM URL to a session establishment protocol. This session model allows message sessions to be integrated into advanced communications applications with little to no additional protocol development. For example, during the above chat session, Bob decides Alice really needs to be talking to Carol. Bob can transfer [21] Alice to Carol, introducing them into their own messaging session. Messaging sessions can then be easily integrated into call-center and dispatch environments using third-party call control [20] and conferencing [19] applications.
This document specifies MSRP behavior only for peer-to-peer sessions, that is, sessions crossing only a single hop. MSRP relay devices [23] (referred to herein as "relays") are specified in a separate document. An endpoint that implements this specification, but not the relay specification, will be unable to introduce relays into the message path, but will still be able to interoperate with peers that do use relays.2. Conventions
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 [5]. This document consistently refers to a "message" as a complete unit of MIME or text content. In some cases, a message is split and delivered in more than one MSRP request. Each of these portions of the complete message is called a "chunk".3. Applicability of MSRP
MSRP is not designed for use as a standalone protocol. MSRP MUST be used only in the context of a rendezvous mechanism meeting the following requirements: o The rendezvous mechanism MUST provide both MSRP URIs associated with an MSRP session to each of the participating endpoints. The rendezvous mechanism MUST implement mechanisms to protect the confidentiality of these URIs -- they MUST NOT be made available to an untrusted third party or be easily discoverable. o The rendezvous mechanism MUST provide mechanisms for the negotiation of any supported MSRP extensions that are not backwards compatible. o The rendezvous mechanism MUST be able to natively transport im: URIs or automatically translate im: URIs [27] into the addressing identifiers of the rendezvous protocol. To use a rendezvous mechanism with MSRP, an RFC MUST be prepared that describes how it exchanges MSRP URIs and meets these requirements listed here. This document provides such a description for the use of MSRP in the context of SIP and SDP. SIP meets these requirements for a rendezvous mechanism. The MSRP URIs are exchanged using SDP in an offer/answer exchange via SIP.
The exchanged SDP can also be used to negotiate MSRP extensions. This SDP can be secured using any of the mechanisms available in SIP, including using the sips mechanism to ensure transport security across intermediaries and Secure/Multipurpose Internet Mail Extensions (S/MIME) for end-to-end protection of the SDP body. SIP can carry arbitrary URIs (including im: URIs) in the Request-URI, and procedures are available to map im: URIs to sip: or sips: URIs. It is expected that initial deployments of MSRP will use SIP as its rendezvous mechanism.4. Protocol Overview
MSRP is a text-based, connection-oriented protocol for exchanging arbitrary (binary) MIME [8] content, especially instant messages. This section is a non-normative overview of how MSRP works and how it is used with SIP. MSRP sessions are typically arranged using SIP the same way a session of audio or video media is set up. One SIP user agent (Alice) sends the other (Bob) a SIP invitation containing an offered session- description that includes a session of MSRP. The receiving SIP user agent can accept the invitation and include an answer session- description that acknowledges the choice of media. Alice's session description contains an MSRP URI that describes where she is willing to receive MSRP requests from Bob, and vice versa. (Note: Some lines in the examples are removed for clarity and brevity.) Alice sends to Bob: INVITE sip:bob@biloxi.example.com SIP/2.0 To: <sip:bob@biloxi.example.com> From: <sip:alice@atlanta.example.com>;tag=786 Call-ID: 3413an89KU Content-Type: application/sdp c=IN IP4 atlanta.example.com m=message 7654 TCP/MSRP * a=accept-types:text/plain a=path:msrp://atlanta.example.com:7654/jshA7weztas;tcp
Bob sends to Alice: SIP/2.0 200 OK To: <sip:bob@biloxi.example.com>;tag=087js From: <sip:alice@atlanta.example.com>;tag=786 Call-ID: 3413an89KU Content-Type: application/sdp c=IN IP4 biloxi.example.com m=message 12763 TCP/MSRP * a=accept-types:text/plain a=path:msrp://biloxi.example.com:12763/kjhd37s2s20w2a;tcp Alice sends to Bob: ACK sip:bob@biloxi SIP/2.0 To: <sip:bob@biloxi.example.com>;tag=087js From: <sip:alice@atlanta.example.com>;tag=786 Call-ID: 3413an89KU Figure 1: Session Setup MSRP defines two request types, or methods. SEND requests are used to deliver a complete message or a chunk (a portion of a complete message), while REPORT requests report on the status of a previously sent message, or a range of bytes inside a message. When Alice receives Bob's answer, she checks to see if she has an existing connection to Bob. If not, she opens a new connection to Bob using the URI he provided in the SDP. Alice then delivers a SEND request to Bob with her initial message, and Bob replies indicating that Alice's request was received successfully.
MSRP a786hjs2 SEND To-Path: msrp://biloxi.example.com:12763/kjhd37s2s20w2a;tcp From-Path: msrp://atlanta.example.com:7654/jshA7weztas;tcp Message-ID: 87652491 Byte-Range: 1-25/25 Content-Type: text/plain Hey Bob, are you there? -------a786hjs2$ MSRP a786hjs2 200 OK To-Path: msrp://atlanta.example.com:7654/jshA7weztas;tcp From-Path: msrp://biloxi.example.com:12763/kjhd37s2s20w2a;tcp -------a786hjs2$ Figure 2: Example MSRP Exchange Alice's request begins with the MSRP start line, which contains a transaction identifier that is also used for request framing. Next she includes the path of URIs to the destination in the To-Path header field, and her own URI in the From-Path header field. In this typical case, there is just one "hop", so there is only one URI in each path header field. She also includes a message ID, which she can use to correlate status reports with the original message. Next she puts the actual content. Finally, she closes the request with an end-line of seven hyphens, the transaction identifier, and a "$" to indicate that this request contains the end of a complete message. If Alice wants to deliver a very large message, she can split the message into chunks and deliver each chunk in a separate SEND request. The message ID corresponds to the whole message, so the receiver can also use it to reassemble the message and tell which chunks belong with which message. Chunking is described in more detail in Section 5.1. The Byte-Range header field identifies the portion of the message carried in this chunk and the total size of the message. Alice can also specify what type of reporting she would like in response to her request. If Alice requests positive acknowledgments, Bob sends a REPORT request to Alice confirming the delivery of her complete message. This is especially useful if Alice sent a series of SEND requests containing chunks of a single message. More on requesting types of reports and errors is described in Section 5.3. Alice and Bob choose their MSRP URIs in such a way that it is difficult to guess the exact URI. Alice and Bob can reject requests to URIs they are not expecting to service and can correlate the specific URI with the probable sender. Alice and Bob can also use
TLS [1] to provide channel security over this hop. To receive MSRP requests over a TLS protected connection, Alice or Bob could advertise URIs with the "msrps" scheme instead of "msrp". MSRP is designed with the expectation that MSRP can carry URIs for nodes on the far side of relays. For this reason, a URI with the "msrps" scheme makes no assertion about the security properties of other hops, just the next hop. The user agent knows the URI for each hop, so it can verify that each URI has the desired security properties. MSRP URIs are discussed in more detail in Section 6. An adjacent pair of busy MSRP nodes (for example, two relays) can easily have several sessions, and exchange traffic for several simultaneous users. The nodes can use existing connections to carry new traffic with the same destination host, port, transport protocol, and scheme. MSRP nodes can keep track of how many sessions are using a particular connection and close these connections when no sessions have used them for some period of time. Connection management is discussed in more detail in Section 5.4.5. Key Concepts
5.1. MSRP Framing and Message Chunking
Messages sent using MSRP can be very large and can be delivered in several SEND requests, where each SEND request contains one chunk of the overall message. Long chunks may be interrupted in mid- transmission to ensure fairness across shared transport connections. To support this, MSRP uses a boundary-based framing mechanism. The start line of an MSRP request contains a unique identifier that is also used to indicate the end of the request. Included at the end of the end-line, there is a flag that indicates whether this is the last chunk of data for this message or whether the message will be continued in a subsequent chunk. There is also a Byte-Range header field in the request that indicates the overall position of this chunk inside the complete message. For example, the following snippet of two SEND requests demonstrates a message that contains the text "abcdEFGH" being sent as two chunks.
MSRP dkei38sd SEND Message-ID: 4564dpWd Byte-Range: 1-*/8 Content-Type: text/plain abcd -------dkei38sd+ MSRP dkei38ia SEND Message-ID: 4564dpWd Byte-Range: 5-8/8 Content-Type: text/plain EFGH -------dkei38ia$ Figure 3: Breaking a Message into Chunks This chunking mechanism allows a sender to interrupt a chunk part of the way through sending it. The ability to interrupt messages allows multiple sessions to share a TCP connection, and for large messages to be sent efficiently while not blocking other messages that share the same connection, or even the same MSRP session. Any chunk that is larger than 2048 octets MUST be interruptible. While MSRP would be simpler to implement if each MSRP session used its own TCP connection, there are compelling reasons to conserve connections. For example, the TCP peer may be a relay device that connects to many other peers. Such a device will scale better if each peer does not create a large number of connections. (Note that in the above example, the initial chunk was interruptible for the sake of example, even though its size is well below the limit for which interruptibility would be required.) The chunking mechanism only applies to the SEND method, as it is the only method used to transfer message content.5.2. MSRP Addressing
MSRP entities are addressed using URIs. The MSRP URI schemes are defined in Section 6. The syntax of the To-Path and From-Path header fields each allows for a list of URIs. This was done to allow the protocol to work with relays, which are defined in a separate document, to provide a complete path to the end recipient. When two MSRP nodes communicate directly, they need only one URI in the To- Path list and one URI in the From-Path list.
5.3. MSRP Transaction and Report Model
A sender sends MSRP requests to a receiver. The receiver MUST quickly accept or reject the request. If the receiver initially accepted the request, it still may then do things that take significant time to succeed or fail. For example, if the receiver is an MSRP to Extensible Messaging and Presence Protocol (XMPP) [30] gateway, it may forward the message over XMPP. The XMPP side may later indicate that the request did not work. At this point, the MSRP receiver may need to indicate that the request did not succeed. There are two important concepts here: first, the hop-by-hop delivery of the request may succeed or fail; second, the end result of the request may or may not be successfully processed. The first type of status is referred to as "transaction status" and may be returned in response to a request. The second type of status is referred to as "delivery status" and may be returned in a REPORT transaction. The original sender of a request can indicate if they wish to receive reports for requests that fail, and can independently indicate if they wish to receive reports for requests that succeed. A receiver only sends a success REPORT if it knows that the request was successfully delivered, and the sender requested a success report. A receiver only sends a failure REPORT if the request failed to be delivered and the sender requested failure reports. This document describes the behavior of MSRP endpoints. MSRP relays will introduce additional conditions that indicate a failure REPORT should be sent, such as the failure to receive a positive response from the next hop. Two header fields control the sender's desire to receive reports. The Success-Report header field can have a value of "yes" or "no" and the Failure-Report header field can have a value of "yes", "no", or "partial". The combinations of reporting are needed to meet the various scenarios of currently deployed IM systems. Success-Report might be "no" in many public systems to reduce load, but might be "yes" in certain enterprise systems, such as systems used for securities trading. A Failure-Report value of "no" is useful for sending system messages such as "the system is going down in 5 minutes" without causing a response explosion to the sender. A Failure-Report of "yes" is used by many systems that wish to notify the user if the message failed. A Failure-Report of "partial" is a way to report errors other than timeouts. Timeout error reporting requires the sending hop to run a timer and the receiving hop to send an
acknowledgment to stop the timer. Some systems don't want the
overhead of doing this. "Partial" allows them to choose not to do
so, but still allows error responses to be sent in many cases.
The term "partial" denotes that the hop-by-hop acknowledgment
mechanism that would be required with a Failure-Report value of
"yes" is not invoked. Thus, each device uses only "part" of the
set of error detection tools available to them. This allows a
compromise between no reporting of failures at all, and reporting
every possible failure. For example, with "partial", a sending
device does not have to keep transaction state around waiting for
a positive acknowledgment. But it still allows devices to report
other types of errors. The receiving device could still report a
policy violation such as an unacceptable content-type, or an ICMP
error trying to connect to a downstream device.
5.4. MSRP Connection Model
When an MSRP endpoint wishes to send a request to a peer identified
by an MSRP URI, it first needs a transport connection, with the
appropriate security properties, to the host specified in the URI.
If the sender already has such a connection, that is, one associated
with the same host, port, and URI scheme, then it SHOULD reuse that
connection.
When a new MSRP session is created, the initiating endpoint MUST act
as the "active" endpoint, meaning that it is responsible for opening
the transport connection to the answerer, if a new connection is
required. However, this requirement MAY be weakened if standardized
mechanisms for negotiating the connection direction become available
and are implemented by both parties to the connection.
Likewise, the active endpoint MUST immediately issue a SEND request.
This initial SEND request MAY have a body if the sender has content
to send, or it MAY have no body at all.
The first SEND request serves to bind a connection to an MSRP
session from the perspective of the passive endpoint. If the
connection is not authenticated with TLS, and the active endpoint
did not send an immediate request, the passive endpoint would have
no way to determine who had connected, and would not be able to
safely send any requests towards the active party until after the
active party sends its first request.
When an element needs to form a new connection, it looks at the URI
to decide on the type of connection (TLS, TCP, etc.) then connects to
the host indicated by the URI, following the URI resolution rules in
Section 6.2. Connections using the "msrps" scheme MUST use TLS. The
SubjectAltName in the received certificate MUST match the hostname part of the URI and the certificate MUST be valid according to RFC 3280 [16], including having a date that is valid and being signed by an acceptable certification authority. At this point, the device that initiated the connection can assume that this connection is with the correct host. The rules on certificate name matching and CA signing MAY be relaxed when using TLS peer-to-peer. In this case, a mechanism to ensure that the peer used a correct certificate MUST be used. See Section 14.4 for details. If the connection used mutual TLS authentication, and the TLS client presented a valid certificate, then the element accepting the connection can verify the identity of the connecting device by comparing the hostname part of the target URI in the SDP provided by the peer device against the SubjectAltName in the client certificate. When mutual TLS authentication is not used, the listening device MUST wait until it receives a request on the connection, at which time it infers the identity of the connecting device from the associated session description. When the first request arrives, its To-Path header field should contain a URI that the listening element provided in the SDP for a session. The element that accepted the connection looks up the URI in the received request, and determines which session it matches. If a match exists, the node MUST assume that the host that formed the connection is the host to which this URI was given. If no match exists, the node MUST reject the request with a 481 response. The node MUST also check to make sure the session is not already in use on another connection. If the session is already in use, it MUST reject the request with a 506 response. If it were legal to have multiple connections associated with the same session, a security problem would exist. If the initial SEND request is not protected, an eavesdropper might learn the URI, and use it to insert messages into the session via a different connection. If a connection fails for any reason, then an MSRP endpoint MUST consider any sessions associated with the connection as also having failed. When either endpoint notices such a failure, it MAY attempt to re-create any such sessions. If it chooses to do so, it MUST use a new SDP exchange, for example, in a SIP re-INVITE. If a replacement session is successfully created, endpoints MAY attempt to resend any content for which delivery on the original session could not be confirmed. If it does this, the Message-ID values for the
resent messages MUST match those used in the initial attempts. If the receiving endpoint receives more than one message with the same Message-ID, it SHOULD assume that the messages are duplicates. The specific action that an endpoint takes when it receives a duplicate message is a matter of local policy, except that it SHOULD NOT present the duplicate messages to the user without warning of the duplication. Note that acknowledgments as needed based on the Failure-Report and Success-Report settings are still necessary even for requests containing duplicate content. When endpoints create a new session in this fashion, the chunks for a given logical message MAY be split across the sessions. However, endpoints SHOULD NOT split chunks between sessions under non-failure circumstances. If an endpoint attempts to re-create a failed session in this manner, it MUST NOT assume that the MSRP URIs in the SDP will be the same as the old ones. A connection SHOULD NOT be closed while there are sessions associated with it.6. MSRP URIs
URIs using the "msrp" and "msrps" schemes are used to identify a session of instant messages at a particular MSRP device, as well as to identify an MSRP relay in general. This document describes the former usage; the latter usage is described in the MSRP relay specification [23]. MSRP URIs that identify sessions are ephemeral; an MSRP device will use a different MSRP URI for each distinct session. An MSRP URI that identifies a session has no meaning outside the scope of that session. An MSRP URI follows a subset of the URI syntax in Appendix A of RFC 3986 [10], with a scheme of "msrp" or "msrps". The syntax is described in Section 9. MSRP URIs are primarily expected to be generated and exchanged between systems, and are not intended for "human consumption". Therefore, they are encoded entirely in US-ASCII. The constructions for "authority", "userinfo", and "unreserved" are detailed in RFC 3986 [10]. URIs designating MSRP over TCP MUST include the "tcp" transport parameter.
Since this document only specifies MSRP over TCP, all MSRP URIs
herein use the "tcp" transport parameter. Documents that provide
bindings on other transports should define respective parameters
for those transports.
The MSRP URI authority field identifies a participant in a particular
MSRP session. If the authority field contains a numeric IP address,
it MUST also contain a port. The session-id part identifies a
particular session of the participant. The absence of the session-id
part indicates a reference to an MSRP host device, but does not refer
to a particular session at that device. A particular value of
session-id is only meaningful in the context of the associated
authority; thus, the authority component can be thought of as
identifying the "authority" governing a namespace for the session-id.
A scheme of "msrps" indicates that the underlying connection MUST be
protected with TLS.
MSRP has an IANA-registered recommended port defined in Section 15.4.
This value is not a default, as the URI negotiation process described
herein will always include explicit port numbers. However, the URIs
SHOULD be configured so that the recommended port is used whenever
appropriate. This makes life easier for network administrators who
need to manage firewall policy for MSRP.
The authority component will typically not contain a userinfo
component, but MAY do so to indicate a user account for which the
session is valid. Note that this is not the same thing as
identifying the session itself. A userinfo part MUST NOT contain
password information.
The following is an example of a typical MSRP URI:
msrp://host.example.com:8493/asfd34;tcp
6.1. MSRP URI Comparison
In the context of the MSRP protocol, MSRP URI comparisons MUST be
performed according to the following rules:
1. The scheme MUST match. Scheme comparison is case insensitive.
2. If the authority component contains an explicit IP address and/or
port, these are compared for address and port equivalence.
Percent-encoding normalization [10] applies; that is, if any
percent-encoded nonreserved characters exist in the authority
component, they must be decoded prior to comparison. Userinfo
parts are not considered for URI comparison. Otherwise, the
authority component is compared as a case-insensitive character
string.
3. If the port exists explicitly in either URI, then it MUST match
exactly. A URI with an explicit port is never equivalent to
another with no port specified.
4. The session-id part is compared as case sensitive. A URI without
a session-id part is never equivalent to one that includes one.
5. URIs with different "transport" parameters never match. Two URIs
that are identical except for transport are not equivalent. The
transport parameter is case insensitive.
Path normalization [10] is not relevant for MSRP URIs.
6.2. Resolving MSRP Host Device
An MSRP host device is identified by the authority component of an
MSRP URI.
If the authority component contains a numeric IP address and port,
they MUST be used as listed.
If the authority component contains a host name and a port, the
connecting device MUST determine a host address by doing an A or AAAA
DNS query and use the port as listed.
If a connection attempt fails, the device SHOULD attempt to connect
to the addresses returned in any additional A or AAAA records, in the
order the records were presented.
This process assumes that the connection port is always known
prior to resolution. This is always true for the MSRP URI uses
described in this document, that is, URIs exchanged in the SDP
offer and answer. The introduction of relays creates situations
where this is not the case. For example, when a user configures
her client to use a relay, it is desirable that the relay's MSRP
URI is easy to remember and communicate to humans. Often this
type of MSRP will omit the port number. Therefore, the relay
specification [23] describes additional steps to resolve the port
number.
MSRP devices MAY use other methods for discovering other such
devices, when appropriate. For example, MSRP endpoints may use other
mechanisms to discover relays, which are beyond the scope of this
document.
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