RFC 4975
The Message Session Relay Protocol (MSRP)
9. Formal Syntax
MSRP is a text protocol that uses the UTF-8 [14] transformation format. The following syntax specification uses the augmented Backus-Naur Form (BNF) as described in RFC 4234 [6]. msrp-req-or-resp = msrp-request / msrp-response msrp-request = req-start headers [content-stuff] end-line msrp-response = resp-start headers end-line req-start = pMSRP SP transact-id SP method CRLF resp-start = pMSRP SP transact-id SP status-code [SP comment] CRLF comment = utf8text pMSRP = %x4D.53.52.50 ; MSRP in caps transact-id = ident method = mSEND / mREPORT / other-method mSEND = %x53.45.4e.44 ; SEND in caps mREPORT = %x52.45.50.4f.52.54; REPORT in caps other-method = 1*UPALPHA status-code = 3DIGIT ; any code defined in this document ; or an extension document MSRP-URI = msrp-scheme "://" authority ["/" session-id] ";" transport *( ";" URI-parameter) ; authority as defined in RFC3986 msrp-scheme = "msrp" / "msrps" session-id = 1*( unreserved / "+" / "=" / "/" ) ; unreserved as defined in RFC3986 transport = "tcp" / 1*ALPHANUM URI-parameter = token ["=" token] headers = To-Path CRLF From-Path CRLF 1*( header CRLF )
header = Message-ID
/ Success-Report
/ Failure-Report
/ Byte-Range
/ Status
/ ext-header
To-Path = "To-Path:" SP MSRP-URI *( SP MSRP-URI )
From-Path = "From-Path:" SP MSRP-URI *( SP MSRP-URI )
Message-ID = "Message-ID:" SP ident
Success-Report = "Success-Report:" SP ("yes" / "no" )
Failure-Report = "Failure-Report:" SP ("yes" / "no" / "partial" )
Byte-Range = "Byte-Range:" SP range-start "-" range-end "/" total
range-start = 1*DIGIT
range-end = 1*DIGIT / "*"
total = 1*DIGIT / "*"
Status = "Status:" SP namespace SP status-code [SP comment]
namespace = 3(DIGIT); "000" for all codes defined in this document.
ident = ALPHANUM 3*31ident-char
ident-char = ALPHANUM / "." / "-" / "+" / "%" / "="
content-stuff = *(Other-Mime-header CRLF)
Content-Type 2CRLF data CRLF
Content-Type = "Content-Type:" SP media-type
media-type = type "/" subtype *( ";" gen-param )
type = token
subtype = token
gen-param = pname [ "=" pval ]
pname = token
pval = token / quoted-string
token = 1*(%x21 / %x23-27 / %x2A-2B / %x2D-2E
/ %x30-39 / %x41-5A / %x5E-7E)
; token is compared case-insensitive
quoted-string = DQUOTE *(qdtext / qd-esc) DQUOTE
qdtext = SP / HTAB / %x21 / %x23-5B / %x5D-7E
/ UTF8-NONASCII
qd-esc = (BACKSLASH BACKSLASH) / (BACKSLASH DQUOTE)
BACKSLASH = "\"
UPALPHA = %x41-5A
ALPHANUM = ALPHA / DIGIT
Other-Mime-header = (Content-ID / Content-Description / Content-Disposition / mime-extension-field) ; Content-ID, and Content-Description are defined in RFC2045. ; Content-Disposition is defined in RFC2183 ; MIME-extension-field indicates additional MIME extension ; header fields as described in RFC2045 data = *OCTET end-line = "-------" transact-id continuation-flag CRLF continuation-flag = "+" / "$" / "#" ext-header = hname ":" SP hval CRLF hname = ALPHA *token hval = utf8text utf8text = *(HTAB / %x20-7E / UTF8-NONASCII) UTF8-NONASCII = %xC0-DF 1UTF8-CONT / %xE0-EF 2UTF8-CONT / %xF0-F7 3UTF8-CONT / %xF8-Fb 4UTF8-CONT / %xFC-FD 5UTF8-CONT UTF8-CONT = %x80-BF Figure 11: MSRP ABNF10. Response Code Descriptions
This section summarizes the semantics of various response codes that may be used in MSRP transaction responses. These codes may also be used in the Status header field in REPORT requests.10.1. 200
The 200 response code indicates a successful transaction.10.2. 400
A 400 response indicates that a request was unintelligible. The sender may retry the request after correcting the error.10.3. 403
A 403 response indicates that the attempted action is not allowed. The sender should not try the request again.
10.4. 408
A 408 response indicates that a downstream transaction did not complete in the allotted time. It is never sent by any elements described in this specification. However, 408 is used in the MSRP relay extension; therefore, MSRP endpoints may receive it. An endpoint MUST treat a 408 response in the same manner as it would treat a local timeout.10.5. 413
A 413 response indicates that the receiver wishes the sender to stop sending the particular message. Typically, a 413 is sent in response to a chunk of an undesired message. If a message sender receives a 413 in a response, or in a REPORT request, it MUST NOT send any further chunks in the message, that is, any further chunks with the same Message-ID value. If the sender receives the 413 while in the process of sending a chunk, and the chunk is interruptible, the sender MUST interrupt it.10.6. 415
A 415 response indicates that the SEND request contained a media type that is not understood by the receiver. The sender should not send any further messages with the same content-type for the duration of the session.10.7. 423
A 423 response indicates that one of the requested parameters is out of bounds. It is used by the relay extensions to this document.10.8. 481
A 481 response indicates that the indicated session does not exist. The sender should terminate the session.10.9. 501
A 501 response indicates that the recipient does not understand the request method. The 501 response code exists to allow some degree of method extensibility. It is not intended as a license to ignore methods defined in this document; rather, it is a mechanism to report lack of support of extension methods.
10.10. 506
A 506 response indicates that a request arrived on a session that is already bound to another network connection. The sender should cease sending messages for that session on this connection.11. Examples
11.1. Basic IM Session
This section shows an example flow for the most common scenario. The example assumes SIP is used to transport the SDP exchange. Details of the SIP messages and SIP proxy infrastructure are omitted for the sake of brevity. In the example, assume that the offerer is sip:alice@example.com and the answerer is sip:bob@example.com. Alice Bob | | | | |(1) (SIP) INVITE | |----------------------->| |(2) (SIP) 200 OK | |<-----------------------| |(3) (SIP) ACK | |----------------------->| |(4) (MSRP) SEND | |----------------------->| |(5) (MSRP) 200 OK | |<-----------------------| |(6) (MSRP) SEND | |<-----------------------| |(7) (MSRP) 200 OK | |----------------------->| |(8) (SIP) BYE | |----------------------->| |(9) (SIP) 200 OK | |<-----------------------| | | | | Figure 12: Basic IM Session Example
1. Alice constructs a local URI of
msrp://alicepc.example.com:7777/iau39soe2843z;tcp .
Alice->Bob (SIP): INVITE sip:bob@example.com
v=0
o=alice 2890844557 2890844559 IN IP4 alicepc.example.com
s= -
c=IN IP4 alicepc.example.com
t=0 0
m=message 7777 TCP/MSRP *
a=accept-types:text/plain
a=path:msrp://alicepc.example.com:7777/iau39soe2843z;tcp
2. Bob listens on port 8888, and sends the following response:
Bob->Alice (SIP): 200 OK
v=0
o=bob 2890844612 2890844616 IN IP4 bob.example.com
s= -
c=IN IP4 bob.example.com
t=0 0
m=message 8888 TCP/MSRP *
a=accept-types:text/plain
a=path:msrp://bob.example.com:8888/9di4eae923wzd;tcp
3. Alice->Bob (SIP): ACK sip:bob@example.com
4. (Alice opens connection to Bob.) Alice->Bob (MSRP):
MSRP d93kswow SEND
To-Path: msrp://bob.example.com:8888/9di4eae923wzd;tcp
From-Path: msrp://alicepc.example.com:7777/iau39soe2843z;tcp
Message-ID: 12339sdqwer
Byte-Range: 1-16/16
Content-Type: text/plain
Hi, I'm Alice!
-------d93kswow$
5. Bob->Alice (MSRP):
MSRP d93kswow 200 OK
To-Path: msrp://alicepc.example.com:7777/iau39soe2843z;tcp
From-Path: msrp://bob.example.com:8888/9di4eae923wzd;tcp
-------d93kswow$
6. Bob->Alice (MSRP):
MSRP dkei38sd SEND
To-Path: msrp://alicepc.example.com:7777/iau39soe2843z;tcp
From-Path: msrp://bob.example.com:8888/9di4eae923wzd;tcp
Message-ID: 456s9wlk3
Byte-Range: 1-21/21
Content-Type: text/plain
Hi, Alice! I'm Bob!
-------dkei38sd$
7. Alice->Bob (MSRP):
MSRP dkei38sd 200 OK
To-Path: msrp://bob.example.com:8888/9di4eae923wzd;tcp
From-Path: msrp://alicepc.example.com:7777/iau39soe2843z;tcp
-------dkei38sd$
8. Alice->Bob (SIP): BYE sip:bob@example.com
Alice invalidates local session state.
9. Bob invalidates local state for the session.
Bob->Alice (SIP): 200 OK
11.2. Message with XHTML Content
MSRP dsdfoe38sd SEND
To-Path: msrp://alice.example.com:7777/iau39soe2843z;tcp
From-Path: msrp://bob.example.com:8888/9di4eae923wzd;tcp
Message-ID: 456so39s
Byte-Range: 1-374/374
Content-Type: application/xhtml+xml
<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "_http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd_"> <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> <head> <title>FY2005 Results</title> </head> <body> <p>See the results at <a href="http://example.org/">example.org</a>.</p> </body> </html> -------dsdfoe38sd$ Figure 13: Example Message with XHTML11.3. Chunked Message
For an example of a chunked message, see the example in Section 5.1.11.4. Chunked Message with Message/CPIM Payload
This example shows a chunked message containing a CPIM message that wraps a text/plain payload. It is worth noting that MSRP considers the complete CPIM message before chunking the message; thus, the CPIM headers are included in only the first chunk. The MSRP Content-Type and Byte-Range headers, present in both chunks, refer to the whole CPIM message. MSRP d93kswow SEND To-Path: msrp://bobpc.example.com:8888/9di4eae923wzd;tcp From-Path: msrp://alicepc.example.com:7654/iau39soe2843z;tcp Message-ID: 12339sdqwer Byte-Range: 1-137/148 Content-Type: message/cpim To: Bob <sip:bob@example.com> From: Alice <sip:alice@example.com> DateTime: 2006-05-15T15:02:31-03:00 Content-Type: text/plain ABCD -------d93kswow+ Figure 14: First Chunk
Alice sends the second and last chunk. MSRP op2nc9a SEND To-Path: msrp://bobpc.example.com:8888/9di4eae923wzd;tcp From-Path: msrp://alicepc.example.com:7654/iau39soe2843z;tcp Message-ID: 12339sdqwer Byte-Range: 138-148/148 Content-Type: message/cpim 1234567890 -------op2nc9a$ Figure 15: Second Chunk11.5. System Message
Sysadmin->Alice (MSRP): MSRP d93kswow SEND To-Path: msrp://alicepc.example.com:8888/9di4eae923wzd;tcp From-Path: msrp://example.com:7777/iau39soe2843z;tcp Message-ID: 12339sdqwer Byte-Range: 1-38/38 Failure-Report: no Success-Report: no Content-Type: text/plain This conference will end in 5 minutes -------d93kswow$11.6. Positive Report
Alice->Bob (MSRP): MSRP d93kswow SEND To-Path: msrp://bob.example.com:8888/9di4eae923wzd;tcp From-Path: msrp://alicepc.example.com:7777/iau39soe2843z;tcp Message-ID: 12339sdqwer Byte-Range: 1-106/106 Success-Report: yes Failure-Report: no Content-Type: text/html
<html><body> <p>Here is that important link... <a href="http://www.example.com/foobar">foobar</a> </p> </body></html> -------d93kswow$ Figure 16: Initial SEND Request Bob->Alice (MSRP): MSRP dkei38sd REPORT To-Path: msrp://alicepc.example.com:7777/iau39soe2843z;tcp From-Path: msrp://bob.example.com:8888/9di4eae923wzd;tcp Message-ID: 12339sdqwer Byte-Range: 1-106/106 Status: 000 200 OK -------dkei38sd$ Figure 17: Success Report11.7. Forked IM
Traditional IM systems generally do a poor job of handling multiple simultaneous IM clients online for the same person. While some do a better job than many existing systems, handling of multiple clients is fairly crude. This becomes a much more significant issue when always-on mobile devices are available, but it is desirable to use them only if another IM client is not available. Using SIP makes rendezvous decisions explicit, deterministic, and very flexible. In contrast, "page-mode" IM systems use implicit implementation-specific decisions that IM clients cannot influence. With SIP session-mode messaging, rendezvous decisions can be under control of the client in a predictable, interoperable way for any host that implements callee capabilities [31]. As a result, rendezvous policy is managed consistently for each address of record. The following example shows Juliet with several IM clients where she can be reached. Each of these has a unique SIP contact and MSRP session. The example takes advantage of SIP's capability to "fork" an invitation to several contacts in parallel, in sequence, or in combination. Juliet has registered from her chamber, the balcony, her PDA, and as a last resort, you can leave a message with her nurse. Juliet's contacts are listed below. The q-values express relative preference (q=1.0 is the highest preference).
When Romeo opens his IM program, he selects Juliet and types the message "art thou hither?" (instead of "you there?"). His client sends a SIP invitation to sip:juliet@thecapulets.example.com. The proxy there tries first the balcony and the chamber simultaneously. A client is running on each of those systems, both of which set up early sessions of MSRP with Romeo's client. The client automatically sends the message over MSRP to the two MSRP URIs involved. After a delay of a several seconds with no reply or activity from Juliet, the proxy cancels the invitation at her first two contacts, and forwards the invitation on to Juliet's PDA. Since her father is talking to her about her wedding, she selects "Do Not Disturb" on her PDA, which sends a "Busy Here" response. The proxy then tries the nurse, who answers and tells Romeo what is going on.
Romeo Juliet's Juliet/ Juliet/ Juliet/ Nurse Proxy balcony chamber PDA | | | | | | |--INVITE--->| | | | | | |--INVITE--->| | | | | |<----180----| | | | |<----180----| | | | | |---PRACK---------------->| | | | |<----200-----------------| | | | |<===Early MSRP Session==>| art thou hither? | | | | | | | | | |--INVITE---------------->| | | | |<----180-----------------| | | |<----180----| | | | | |---PRACK----------------------------->| | | |<----200------------------------------| | | |<========Early MSRP Session==========>| art thou hither? | | | | | | | | | | | | | | | .... Time Passes .... | | | | | | | | | | | | | | | | |--CANCEL--->| | | | | |<---200-----| | | | | |<---487-----| | | | | |----ACK---->| | | | | |--CANCEL---------------->| | | | |<---200------------------| | | | |<---487------------------| | | | |----ACK----------------->| | | | |--INVITE---------------------------->| romeo wants | | | | | to IM w/ you | |<---486 Busy Here--------------------| | | |----ACK----------------------------->| | | | | | | | | |--INVITE---------------------------------------->| | |<---200 OK---------------------------------------| |<--200 OK---| | | | | |---ACK------------------------------------------------------->| |<================MSRP Session================================>| | | | | | | | Hi Romeo, Juliet is | | with her father now | | can I take a message?| | | | Tell her to go to confession tomorrow.... | Figure 18: Forking Example
12. Extensibility
MSRP was designed to be only minimally extensible. New MSRP methods, header fields, and status codes can be defined in standards-track RFCs. MSRP does not contain a version number or any negotiation mechanism to require or discover new features. If an extension is specified in the future that requires negotiation, the specification will need to describe how the extension is to be negotiated in the encapsulating signaling protocol. If a non-interoperable update or extension occurs in the future, it will be treated as a new protocol, and MUST describe how its use will be signaled. In order to allow extension header fields without breaking interoperability, if an MSRP device receives a request or response containing a header field that it does not understand, it MUST ignore the header field and process the request or response as if the header field was not present. If an MSRP device receives a request with an unknown method, it MUST return a 501 response. MSRP was designed to use lists of URIs instead of a single URI in the To-Path and From-Path header fields in anticipation of relay or gateway functionality being added. In addition, "msrp" and "msrps" URIs can contain parameters that are extensible.13. CPIM Compatibility
MSRP sessions may go to a gateway to other Common Profile for Instant Messaging (CPIM) [27] compatible protocols. If this occurs, the gateway MUST maintain session state, and MUST translate between the MSRP session semantics and CPIM semantics, which do not include a concept of sessions. Furthermore, when one endpoint of the session is a CPIM gateway, instant messages SHOULD be wrapped in "message/cpim" [12] bodies. Such a gateway MUST include "message/cpim" as the first entry in its SDP accept-types attribute. MSRP endpoints sending instant messages to a peer that has included "message/cpim" as the first entry in the accept-types attribute SHOULD encapsulate all instant message bodies in "message/ cpim" wrappers. All MSRP endpoints MUST support the message/cpim type, and SHOULD support the S/MIME[7] features of that format. If a message is to be wrapped in a message/cpim envelope, the wrapping MUST be done prior to breaking the message into chunks, if needed.
All MSRP endpoints MUST recognize the From, To, DateTime, and Require header fields as defined in RFC 3862. Such applications SHOULD recognize the CC header field, and MAY recognize the Subject header field. Any MSRP application that recognizes any message/cpim header field MUST understand the NS (name space) header field. All message/cpim body parts sent by an MSRP endpoint MUST include the From and To header fields. If the message/cpim body part is protected using S/MIME, then it MUST also include the DateTime header field. The NS, To, and CC header fields may occur multiple times. Other header fields defined in RFC 3862 MUST NOT occur more than once in a given message/cpim body part in an MSRP message. The Require header field MAY include multiple values. The NS header field MAY occur zero or more times, depending on how many name spaces are being referenced. Extension header fields MAY occur more than once, depending on the definition of such header fields. Using message/cpim envelopes is also useful if an MSRP device wishes to send a message on behalf of some other identity. The device may add a message/cpim envelope with the appropriate From header field value.14. Security Considerations
Instant messaging systems are used to exchange a variety of sensitive information ranging from personal conversations, to corporate confidential information, to account numbers and other financial trading information. IM is used by individuals, corporations, and governments for communicating important information. IM systems need to provide the properties of integrity and confidentiality for the exchanged information, and the knowledge that you are communicating with the correct party, and they need to allow the possibility of anonymous communication. MSRP pushes many of the hard problems to SIP when SIP sets up the session, but some of the problems remain. Spam and Denial of Service (DoS) attacks are also very relevant to IM systems. MSRP needs to provide confidentiality and integrity for the messages it transfers. It also needs to provide assurances that the connected host is the host that it meant to connect to and that the connection has not been hijacked.
14.1. Secrecy of the MSRP URI
When an endpoint sends an MSRP URI to its peer in a rendezvous protocol, that URI is effectively a secret shared between the peers. If an attacker learns or guesses the URI prior to the completion of session setup, it may be able to impersonate one of the peers. Assuming the URI exchange in the rendezvous protocol is sufficiently protected, it is critical that the URI remain difficult to "guess" via brute force methods. Most components of the URI, such as the scheme and the authority components, are common knowledge. The secrecy is entirely provided by the session-id component. Therefore, when an MSRP device generates an MSRP URI to be used in the initiation of an MSRP session, the session-id component MUST contain at least 80 bits of randomness.14.2. Transport Level Protection
When using only TCP connections, MSRP security is fairly weak. If host A is contacting host B, B passes its hostname and a secret to A using a rendezvous protocol. Although MSRP requires the use of a rendezvous protocol with the ability to protect this exchange, there is no guarantee that the protection will be used all the time. If such protection is not used, anyone can see this secret. Host A then connects to the provided hostname and passes the secret in the clear across the connection to B. Host A assumes that it is talking to B based on where it sent the SYN packet and then delivers the secret in plain text across the connections. Host B assumes it is talking to A because the host on the other end of the connection delivered the secret. An attacker that could ACK the SYN packet could insert itself as a man-in-the-middle in the connection. When using TLS connections, the security is significantly improved. We assume that the host accepting the connection has a certificate from a well-known certification authority. Furthermore, we assume that the signaling to set up the session is protected by the rendezvous protocol. In this case, when host A contacts host B, the secret is passed through a confidential channel to A. A connects with TLS to B. B presents a valid certificate, so A knows it really is connected to B. A then delivers the secret provided by B, so that B can verify it is connected to A. In this case, a rogue SIP Proxy can see the secret in the SIP signaling traffic and could potentially insert itself as a man-in-the-middle. Realistically, using TLS with certificates from well-known certification authorities is difficult for peer-to-peer connections, as the types of hosts that end clients use for sending instant
messages are unlikely to have long-term stable IP addresses or DNS names that the certificates can bind to. In addition, the cost of server certificates from well-known certification authorities is currently expensive enough to discourage their use for each client. Using TLS in a peer-to-peer mode without well-known certificates is discussed in Section 14.4. TLS becomes much more practical when some form of relay is introduced. Clients can then form TLS connections to relays, which are much more likely to have TLS certificates. While this specification does not address such relays, they are described by a companion document [23]. That document makes extensive use of TLS to protect traffic between clients and relays, and between one relay and another. TLS is used to authenticate devices and to provide integrity and confidentiality for the header fields being transported. MSRP elements MUST implement TLS and MUST also implement the TLS ClientExtendedHello extended hello information for server name indication as described in [11]. A TLS cipher-suite of TLS_RSA_WITH_AES_128_CBC_SHA [13] MUST be supported (other cipher- suites MAY also be supported).14.3. S/MIME
The only strong security for non-TLS connections is achieved using S/MIME. Since MSRP carries arbitrary MIME content, it can trivially carry S/MIME protected messages as well. All MSRP implementations MUST support the multipart/signed media-type even if they do not support S/MIME. Since SIP can carry a session key, S/MIME messages in the context of a session could also be protected using a key-wrapped shared secret [28] provided in the session setup. MSRP can carry unencoded binary payloads. Therefore, MIME bodies MUST be transferred with a transfer encoding of binary. If a message is both signed and encrypted, it SHOULD be signed first, then encrypted. If S/MIME is supported, SHA-1, SHA-256, RSA, and AES-128 MUST be supported. For RSA, implementations MUST support key sizes of at least 1024 bits and SHOULD support key sizes of 2048 bits or more. This does not actually require the endpoint to have certificates from a well-known certification authority. When MSRP is used with SIP, the Identity [17] and Certificates [25] mechanisms provide S/MIME- based delivery of a secret between A and B. No SIP intermediary except the explicitly trusted authentication service (one per user) can see the secret. The S/MIME encryption of the SDP can also be used by SIP to exchange keying material that can be used in MSRP.
The MSRP session can then use S/MIME with this keying material to sign and encrypt messages sent over MSRP. The connection can still be hijacked since the secret is sent in clear text to the other end of the TCP connection, but the consequences are mitigated if all the MSRP content is signed and encrypted with S/MIME. Although out of scope for this document, the SIP negotiation of an MSRP session can negotiate symmetric keying material to be used with S/MIME for integrity and privacy.14.4. Using TLS in Peer-to-Peer Mode
TLS can be used with a self-signed certificate as long as there is a mechanism for both sides to ascertain that the other side used the correct certificate. When used with SDP and SIP, the correct certificate can be verified by passing a fingerprint of the certificate in the SDP and ensuring that the SDP has suitable integrity protection. When SIP is used to transport the SDP, the integrity can be provided by the SIP Identity mechanism [17]. The rest of this section describes the details of this approach. If self-signed certificates are used, the content of the subjectAltName attribute inside the certificate MAY use the URI of the user. In SIP, this URI of the user is the User's Address of Record (AOR). This is useful for debugging purposes only and is not required to bind the certificate to one of the communication endpoints. Unlike normal TLS operations in this protocol, when doing peer-to-peer TLS, the subjectAltName is not an important component of the certificate verification. If the endpoint is also able to make anonymous sessions, a distinct, unique certificate MUST be used for this purpose. For a client that works with multiple users, each user SHOULD have its own certificate. Because the generation of public/private key pairs is relatively expensive, endpoints are not required to generate certificates for each session. A certificate fingerprint is the output of a one-way hash function computed over the Distinguished Encoding Rules (DER) form of the certificate. The endpoint MUST use the certificate fingerprint attribute as specified in [18] and MUST include this in the SDP. The certificate presented during the TLS handshake needs to match the fingerprint exchanged via the SDP, and if the fingerprint does not match the hashed certificate then the endpoint MUST tear down the media session immediately. When using SIP, the integrity of the fingerprint can be ensured through the SIP Identity mechanism [17]. When a client wishes to use SIP to set up a secure MSRP session with another endpoint, it sends an SDP offer in a SIP message to the other endpoint. This offer includes, as part of the SDP payload, the fingerprint of the
certificate that the endpoint wants to use. The SIP message containing the offer is sent to the offerer's SIP proxy, which will add an Identity header according to the procedures outlined in [17]. When the far endpoint receives the SIP message, it can verify the identity of the sender using the Identity header. Since the Identity header is a digital signature across several SIP headers, in addition to the body or bodies of the SIP message, the receiver can also be certain that the message has not been tampered with after the digital signature was added to the SIP message. An example of SDP with a fingerprint attribute is shown in the following figure. Note the fingerprint is shown spread over two lines due to formatting consideration but should all be on one line. c=IN IP4 atlanta.example.com m=message 7654 TCP/TLS/MSRP * a=accept-types:text/plain a=path:msrps://atlanta.example.com:7654/jshA7weso3ks;tcp a=fingerprint:SHA-1 \ 4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB Figure 19: SDP with Fingerprint Attribute14.5. Other Security Concerns
MSRP cannot be used as an amplifier for DoS attacks, but it can be used to form a distributed attack to consume TCP connection resources on servers. The attacker, Mallory, sends a SIP INVITE with no offer to Alice. Alice returns a 200 with an offer and Mallory returns an answer with SDP indicating that his MSRP address is the address of Tom. Since Alice sent the offer, Alice will initiate a connection to Tom using up resources on Tom's server. Given the huge number of IM clients, and the relatively few TCP connections that most servers support, this is a fairly straightforward attack. SIP is attempting to address issues in dealing with spam. The spam issue is probably best dealt with at the SIP level when an MSRP session is initiated and not at the MSRP level. If a sender chooses to employ S/MIME to protect a message, all S/MIME operations apply to the complete message, prior to any breaking of the message into chunks. The signaling will have set up the session to or from some specific URIs that will often have "im:" or "sip:" URI schemes. When the signaling has been set up to a specific end user, and S/MIME is implemented, then the client needs to verify that the name in the SubjectAltName of the certificate contains an entry that matches the
URI that was used for the other end in the signaling. There are some cases, such as IM conferencing, where the S/MIME certificate name and the signaled identity will not match. In these cases, the client should ensure that the user is informed that the message came from the user identified in the certificate and does not assume that the message came from the party they signaled. In some cases, a sending device may need to attribute a message to some other identity, and may use different identities for different messages in the same session. For example, a conference server may send messages on behalf of multiple users on the same session. Rather than add additional header fields to MSRP for this purpose, MSRP relies on the message/cpim format for this purpose. The sender may envelop such a message in a message/cpim body, and place the actual sender identity in the From field. The trustworthiness of such an attribution is affected by the security properties of the session in the same way that the trustworthiness of the identity of the actual peer is affected, with the additional issue of determining whether the recipient trusts the sender to assert the identity. This approach can result in nesting of message/cpim envelopes. For example, a message originates from a CPIM gateway, and is then forwarded by a conference server onto a new session. Both the gateway and the conference server introduce envelopes. In this case, the recipient client SHOULD indicate the chain of identity assertions to the user, rather than allow the user to assume that either the gateway or the conference server originated the message. It is possible that a recipient might receive messages that are attributed to the same sender via different MSRP sessions. For example, Alice might be in a conversation with Bob via an MSRP session over a TLS protected channel. Alice might then receive a different message from Bob over a different session, perhaps with a conference server that asserts Bob's identity in a message/cpim envelope signed by the server. MSRP does not prohibit multiple simultaneous sessions between the same pair of identities. Nor does it prohibit an endpoint sending a message on behalf of another identity, such as may be the case for a conference server. The recipient's endpoint should determine its level of trust of the authenticity of the sender independently for each session. The fact that an endpoint trusts the authenticity of the sender on any given session should not affect the level of trust it assigns for apparently the same sender on a different session.
When MSRP clients form or acquire a certificate, they SHOULD ensure that the subjectAltName has a GeneralName entry of type uniformResourceIdentifier for each URI corresponding to this client and should always include an "im:" URI. It is fine if the certificate contains other URIs such as "sip:" or "xmpp:" URIs. MSRP implementors should be aware of a potential attack on MSRP devices that involves placing very large values in the byte-range header field, potentially causing the device to allocate very large memory buffers to hold the message. Implementations SHOULD apply some degree of sanity checking on byte-range values before allocating such buffers.
(page 55 continued on part 4)
