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Independent Submission M. Thornburgh
Request for Comments: 7425 Adobe
Category: Informational December 2014
ISSN: 2070-1721
Adobe's RTMFP Profile for Flash Communication
Abstract
This memo describes how to use Adobe's Secure Real-Time Media Flow
Protocol (RTMFP) to transport the video, audio, and data messages of
Adobe Flash platform communications. Aspects of this application
profile include cryptographic methods and data formats, flow metadata
formats, and protocol details for client-server and peer-to-peer
communication.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7425.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.
This document may not be modified, and derivative works of it may not
be created, except to format it for publication as an RFC or to
translate it into languages other than English.
Thornburgh Informational [Page 1]
RFC 7425 Adobe RTMFP for Flash Communication December 2014
Table of Contents
1. Introduction ....................................................3
2. Terminology .....................................................4
3. Common Syntax Elements ..........................................4
4. Cryptography Profile ............................................5
4.1. Default Session Key ........................................5
4.2. Diffie-Hellman Groups ......................................6
4.3. Certificates ...............................................6
4.3.1. Format ..............................................6
4.3.2. Fingerprint .........................................7
4.3.3. Options .............................................7
4.3.3.1. Hostname ...................................8
4.3.3.2. Accepts Ancillary Data .....................8
4.3.3.3. Extra Randomness ...........................8
4.3.3.4. Supported Ephemeral Diffie-Hellman Group ...9
4.3.3.5. Static Diffie-Hellman Public Key ...........9
4.3.4. Authenticity .......................................10
4.3.5. Signing and Verifying Messages .....................10
4.3.5.1. Options ...................................11
4.3.5.1.1. Simple Password ................11
4.3.6. Glare Resolution ...................................13
4.3.7. Session Override ...................................13
4.4. Endpoint Discriminators ...................................13
4.4.1. Format .............................................14
4.4.2. Options ............................................14
4.4.2.1. Required Hostname .........................15
4.4.2.2. Ancillary Data ............................15
4.4.2.3. Fingerprint ...............................16
4.4.3. Certificate Selection ..............................16
4.4.4. Canonical Endpoint Discriminator ...................17
4.5. Session Keying Components .................................18
4.5.1. Format .............................................19
4.5.2. Options ............................................19
4.5.2.1. Ephemeral Diffie-Hellman Public Key .......20
4.5.2.2. Extra Randomness ..........................20
4.5.2.3. Diffie-Hellman Group Select ...............21
4.5.2.4. HMAC Negotiation ..........................21
4.5.2.5. Session Sequence Number Negotiation .......22
4.6. Session Key Computation ...................................23
4.6.1. Public Key Selection ...............................23
4.6.1.1. Initiator and Responder Ephemeral .........23
4.6.1.2. Initiator Ephemeral and Responder Static ..23
4.6.1.3. Initiator Static and Responder Ephemeral ..24
4.6.1.4. Initiator and Responder Static ............24
4.6.2. Diffie-Hellman Shared Secret .......................24
4.6.3. Packet Encrypt/Decrypt Keys ........................25
4.6.4. Packet HMAC Send/Receive Keys ......................25
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RFC 7425 Adobe RTMFP for Flash Communication December 2014
4.6.5. Session Nonces .....................................26
4.6.6. Session Sequence Number ............................26
4.7. Packet Encryption .........................................27
4.7.1. Cipher .............................................27
4.7.2. Format .............................................27
4.7.3. Verification .......................................29
4.7.3.1. Simple Checksum ...........................30
4.7.3.2. HMAC ......................................30
4.7.3.3. Session Sequence Number ...................31
5. Flash Communication ............................................31
5.1. RTMP Messages .............................................31
5.1.1. Flow Metadata ......................................32
5.1.2. Message Mapping ....................................34
5.2. Flow Synchronization ......................................35
5.3. Client-to-Server Connection ...............................36
5.3.1. Connecting .........................................36
5.3.2. Server-to-Client Return Control Flow ...............37
5.3.3. setPeerInfo Command ................................37
5.3.4. Set Keepalive Timers Command .......................39
5.3.5. Additional Flows for Streams .......................40
5.3.5.1. To Server .................................40
5.3.5.2. From Server ...............................40
5.3.5.3. Closing Stream Flows ......................41
5.3.6. Closing the Connection .............................41
5.3.7. Example ............................................42
5.4. Direct Peer-to-Peer Streams ...............................43
5.4.1. Connecting .........................................43
5.4.2. Return Flows for Stream ............................43
5.4.3. Closing the Connection .............................44
6. IANA Considerations ............................................44
6.1. RTMFP URI Scheme Registration .............................44
7. Security Considerations ........................................46
8. References .....................................................47
8.1. Normative References ......................................47
8.2. Informative References ....................................49
Acknowledgements ..................................................49
Author's Address ..................................................49
1. Introduction
Adobe's Secure Real-Time Media Flow Protocol (RTMFP) [RFC7016] is a
general-purpose transport service for real-time media and bulk data
in IP networks, and it is suited to client-server and peer-to-peer
(P2P) communication. RTMFP provides a generalized framework for
securing its communications according to the needs of its
application.
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RFC 7425 Adobe RTMFP for Flash Communication December 2014
The Flash platform comprises the Flash runtime (including Flash
Player) from Adobe Systems Incorporated, communication servers such
as Adobe Media Server, and interoperable clients and servers provided
by other parties.
Real-time streaming network communication for the Flash platform of
video, audio, and data typically uses Adobe's Real-Time Messaging
Protocol (RTMP) [RTMP] messages. RTMP messages were originally
designed to be transported over RTMP Chunk Stream in TCP [RTMP];
however, other transports (such as the one described in this memo)
are possible.
This memo specifies the syntax and semantics for transporting RTMP
messages over RTMFP, and it extends Flash communication semantics to
include direct P2P communication. This memo further specifies a
concrete Cryptography Profile for RTMFP tailored to the application
and cryptographic needs of Flash platform client-server and P2P
communications.
These protocols and profiles were developed by Adobe Systems
Incorporated and are not the product of an IETF activity.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
"HMAC" means the Keyed-Hash Message Authentication Code (HMAC)
algorithm [RFC2104].
"HMAC-SHA256" means HMAC using the SHA-256 Secure Hash Algorithm
[SHA256] [RFC6234].
"HMAC-SHA256(K, M)" means the calculation of the HMAC-SHA256 of
message M using key K.
3. Common Syntax Elements
Definitions of types and structures in this specification use
traditional text diagrams paired with procedural descriptions using a
C-like syntax. The C-like procedural descriptions SHALL be construed
as definitive.
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RFC 7425 Adobe RTMFP for Flash Communication December 2014
Structures are packed to take only as many bytes as explicitly
indicated. There is no 32-bit alignment constraint, and fields are
not padded for alignment unless explicitly indicated or described.
Text diagrams may include a bit ruler across the top; this is a
convenience for counting bits in individual fields and does not
necessarily imply field alignment on a multiple of the ruler width.
Unless specified otherwise, reserved fields SHOULD be set to 0 by a
sender and MUST be ignored by a receiver.
The procedural syntax of this specification defines correct and
error-free encoded inputs to a parser. The procedural syntax does
not describe a fully featured parser, including error detection and
handling. Implementations MUST include means to identify error
circumstances, including truncations causing elementary or composed
types not to fit inside containing structures, fields, or elements.
Unless specified otherwise, an error circumstance SHALL abort the
parsing and processing of an element and its enclosing elements.
This memo uses the elementary and composed types described in
Section 2.1 of RFC 7016. The definitions of that section are
incorporated by reference as though fully set forth here.
4. Cryptography Profile
RTMFP defines a general security framework but delegates specifics,
such as packet encryption ciphers and key agreement algorithms, to an
application-defined Cryptography Profile.
This section defines the RTMFP Cryptography Profile for Flash
platform communication.
4.1. Default Session Key
RTMFP uses a Default Session Key and associated default cipher
configuration during session startup handshaking, where session-
specific keys and ciphers are negotiated.
The default cipher is the Advanced Encryption Standard [AES] with
128-bit keys operating in Cipher Block Chaining [CBC] mode, as
described in Section 4.7.1. The Default Session Key is the 16 bytes
of the string "Adobe Systems 02" encoded in UTF-8 [RFC3629]:
Hex: 41 64 6F 62 65 20 53 79 73 74 65 6D 73 20 30 32
The Default Session Key uses checksum mode for packet verification
and does not use session sequence numbers (Section 4.7.3).
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RFC 7425 Adobe RTMFP for Flash Communication December 2014
4.2. Diffie-Hellman Groups
Implementations conforming to this profile MUST support Diffie-
Hellman [DH] modular exponentiation (MODP) group 2 (1024 bits) as
defined in [RFC7296], and SHOULD support Diffie-Hellman MODP group 5
(1536 bits) and group 14 (2048 bits) as defined in [RFC3526].
Implementations MAY support additional groups.
4.3. Certificates
This section defines the certificate format for this Cryptography
Profile, and the mapping to the abstract properties and semantics for
RTMFP endpoint identities.
4.3.1. Format
A certificate in this profile is encoded as a sequence of zero or
more RTMFP Options and Markers (Section 2.1.3 of RFC 7016). The
first marker (if any) in the certificate separates the canonical
section of the certificate from the remainder. Some options are
ignored if they occur outside of the canonical section (that is,
after the first marker).
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RFC 7425 Adobe RTMFP for Flash Communication December 2014
+~~~/~~~/~~~+ +~~~/~~~/~~~+~~~~~+~~~/~~~/~~~+ +~~~/~~~/~~~+
| L \ T \ V |...| L \ T \ V | 0 | L \ T \ V |...| L \ T \ V |
+~~~/~~~/~~~+ +~~~/~~~/~~~+~~~~~+~~~/~~~/~~~+ +~~~/~~~/~~~+
^ ^ ^ ^ ^
| Zero or more non-empty | | | Zero or more Options |
| Options | | +------ or Markers -------+
| | |
+--- Canonical Section ---+ +---- First Marker
(if present)
struct certificate_t
{
canonicalStart = remainder();
canonicalEnd = remainder();
markerFound = false;
while(remainder() > 0)
{
option_t option :variable*8;
if(0 == option.length)
markerFound = true;
else if(!markerFound)
canonicalEnd = remainder();
};
canonicalSectionLength = canonicalStart - canonicalEnd;
} :variable*8;
4.3.2. Fingerprint
A certificate's fingerprint is the SHA-256 hash [SHA256] of the
canonical section of the certificate (that is, the hash of the first
canonicalSectionLength bytes of the certificate).
The certificate's fingerprint is also called the "peer ID".
4.3.3. Options
This section lists options that can appear in a certificate. The
following option type codes are defined:
0x00: Hostname (must be in canonical section) (Section 4.3.3.1)
0x0a: Accepts Ancillary Data (must be in canonical section)
(Section 4.3.3.2)
0x0e: Extra Randomness (Section 4.3.3.3)
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RFC 7425 Adobe RTMFP for Flash Communication December 2014
0x15: Supported Ephemeral Diffie-Hellman Group (must be in
canonical section) (Section 4.3.3.4)
0x1d: Static Diffie-Hellman Public Key (must be in canonical
section) (Section 4.3.3.5)
An implementation MUST ignore a certificate option type that is not
understood.
4.3.3.1. Hostname
This option gives an optional hostname for the endpoint. This option
MUST be ignored if is not in the canonical section. This option MUST
NOT occur more than once in a certificate.
+-------------/-+-------------/-+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
| length \ | 0x00 \ | hostname |
+-------------/-+-------------/-+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~/
struct hostnameCertOptionValue_t
{
uint8_t hostname[remainder()];
} :remainder()*8;
4.3.3.2. Accepts Ancillary Data
This option indicates that the endpoint will accept an Endpoint
Discriminator encoding an Ancillary Data option (Section 4.4.2.2).
This option MUST be ignored if it is not in the canonical section.
+-------------/-+-------------/-+
| length \ | 0x0a \ |
+-------------/-+-------------/-+
4.3.3.3. Extra Randomness
This option can be used to add extra entropy or randomness to a
certificate that doesn't have any other cryptographic pseudorandom
members (such as a public key). This option is typically used so
that endpoints using ephemeral Diffie-Hellman keying can have a
unique certificate fingerprint.
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RFC 7425 Adobe RTMFP for Flash Communication December 2014
+-------------/-+-------------/-+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
| length \ | 0x0e \ | extra randomness |
+-------------/-+-------------/-+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~/
struct extraRandomnessCertOptionValue_t
{
uint_t extraRandomness[remainder()];
} :remainder()*8;
4.3.3.4. Supported Ephemeral Diffie-Hellman Group
This option specifies a Diffie-Hellman group ID that is supported for
ephemeral keying. This option MUST be ignored if it is not in the
canonical section. This option may occur more than once in the
certificate; each instance indicates an additional group that is
supported for key agreement.
+-------------/-+-------------/-+-------------/-+
| length \ | 0x15 \ | group ID \ |
+-------------/-+-------------/-+-------------/-+
struct ephemeralDHGroupCertOptionValue_t
{
vlu_t groupID :variable*8;
} :variable*8;
The presence of this option means that the certificate uses ephemeral
Diffie-Hellman public keys only. The certificate MUST NOT contain a
Static Diffie-Hellman public key (Section 4.3.3.5).
4.3.3.5. Static Diffie-Hellman Public Key
This option specifies a Diffie-Hellman group ID and static public key
in that group. This option MUST be ignored if it is not in the
canonical section. This option MAY occur more than once in the
certificate; however, this option SHOULD NOT occur more than once for
each group ID. The behavior for specifying more than one public key
per group ID is not defined.
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RFC 7425 Adobe RTMFP for Flash Communication December 2014
+-------------/-+-------------/-+-------------/-+
| length \ | 0x1d \ | group ID \ |
+-------------/-+-------------/-+-------------/-+
+------------------------------------------------------------------+
| Diffie-Hellman Public Key |
+------------------------------------------------------------------/
struct staticDHPublicKeyCertOptionValue_t
{
vlu_t groupID :variable*8;
uintn_t publicKey :remainder()*8; // network byte order
} :remainder()*8;
The presence of this option means that the certificate uses static
Diffie-Hellman public keys only. The certificate MUST NOT contain
any Supported Ephemeral Diffie-Hellman Group options
(Section 4.3.3.4).
4.3.4. Authenticity
This profile does not use a public key infrastructure, nor are there
signing keys present in certificates. Therefore, any properly
encoded certificate is considered authentic according to Section 3.2
of RFC 7016.
A certificate containing a static public key can only be used
successfully for session communication if the holder of the
certificate actually holds the private key associated with the public
key. Authenticity of an identity and its peer ID (Section 4.3.2)
having a certificate containing a static public key is implied by
successful encrypted communication with the associated endpoint
(Section 4.6).
See Section 7 for further discussion of security issues related to
identities.
4.3.5. Signing and Verifying Messages
RTMFP Initiator Initial Keying and Responder Initial Keying messages
have a field for the sender's digital signature of the keying
parameters (Sections 2.3.7 and 2.3.8 of RFC 7016). In this profile,
the signature field of those messages is encoded as a sequence of
zero or more RTMFP Options.
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RFC 7425 Adobe RTMFP for Flash Communication December 2014
+~~~/~~~/~~~~~~~+ +~~~/~~~/~~~~~~~+
| L \ T \ V |...............| L \ T \ V |
+~~~/~~~/~~~~~~~+ +~~~/~~~/~~~~~~~+
^ ^
+------------- Zero or more Options ----------+
struct initialKeyingSignature_t
{
while(remainder() > 0)
option_t option :variable*8;
} :remainder()*8;
If a signer has no signature options to send, it MAY encode a
signature as a UTF-8 capital "X" (hex 58) or as empty. A verifier
MUST interpret a malformed signature field or a signature field
consisting only of a UTF-8 capital "X" as though it was empty.
If a verifier does not require a signature, it SHALL consider any
signature field (including an empty or malformed one) to be valid. A
verifier MAY require a signature comprising one or more non-empty
options that are valid according to their respective types.
This profile does not use a public key infrastructure, nor are there
signing keys present in certificates. Section 4.3.5.1.1 defines a
simple ID/password credential system.
4.3.5.1. Options
This section lists options that can appear in an RTMFP Initial Keying
signature field. The following option type code is defined:
0x1d: Simple Password (Section 4.3.5.1.1)
Future or derived profiles may define additional signature field
options and semantics; therefore, a verifier SHOULD ignore option
types that are not understood.
4.3.5.1.1. Simple Password
This option encodes a password identifier (such as a user name, or an
application-specific or implementation-specific selector) and an HMAC
over the signed parameters using the identified password as the HMAC
key. This option can occur more than once (for example, to allow
interoperation between a current and a previous version of an
implementation using implementation-specific passwords).
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RFC 7425 Adobe RTMFP for Flash Communication December 2014
To support the versioning use case, a verifier SHOULD ignore a Simple
Password option encoding an unrecognized password identifier. A
verifier SHOULD treat the entire signature as invalid if any Simple
Password option encodes a recognized password identifier with an
invalid password HMAC.
0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7
+-------------/-+-------------/-+
| length \ | 0x1d \ |
+-------------/-+-------------/-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
| |
+ - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
| |
+ - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
| hmacSHA256 |
+ - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
| |
+ - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
| |
+ - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
| |
+ - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
| passwordID |
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~/
struct simplePasswordSignatureOptionValue_t
{
uint8_t hmacSHA256[32];
uint8_t passwordID[remainder()];
} :remainder()*8;
hmacSHA256: HMAC-SHA256(K, M), where K is the password associated
with passwordID, and M is the signed parameters.
passwordID: The identifier (such as a user name) for the password
used as the HMAC key.
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RFC 7425 Adobe RTMFP for Flash Communication December 2014
4.3.6. Glare Resolution
Glare occurs when two endpoints initiate a session each to the other
concurrently.
Compare the near end's certificate to the far end's with a binary
lexicographic comparison, one byte at a time, up to the length of the
shorter certificate. At the first corresponding byte from each
certificate that is different, the certificate having the differing
byte (treated as an unsigned 8-bit integer) with the lower value is
ordered before the other certificate. If the certificates are not
the same length and they are identical up to the length of the
shorter certificate, then the shorter certificate is ordered before
the longer.
The near end prevails as the Initiator in case of glare if its
certificate is ordered before, or is identical to, the certificate of
the far end. Otherwise, the near end's certificate is ordered after
the far end's certificate, and the near end assumes the role of
Responder.
4.3.7. Session Override
A new incoming session overrides an existing session only if the
certificate for the new session is identical to the certificate for
the existing session.
4.4. Endpoint Discriminators
This section describes the Endpoint Discriminator (EPD) (Section 3.2
of RFC 7016) format and semantics for this Cryptography Profile, and
the mapping to RTMFP's abstract certificate and identity selection
semantics.
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RFC 7425 Adobe RTMFP for Flash Communication December 2014
4.4.1. Format
An EPD in this profile is encoded as a sequence of zero or more RTMFP
Options.
+~~~/~~~/~~~~~~~+ +~~~/~~~/~~~~~~~+
| L \ T \ V |...............| L \ T \ V |
+~~~/~~~/~~~~~~~+ +~~~/~~~/~~~~~~~+
^ ^
+------------- Zero or more Options ----------+
struct endpointDiscriminator_t
{
while(remainder() > 0)
option_t option :variable*8;
} :remainder()*8;
4.4.2. Options
This section lists options that can appear in an EPD. The following
option type codes are defined:
0x00: Required Hostname (Section 4.4.2.1)
0x0a: Ancillary Data (Section 4.4.2.2)
0x0f: Fingerprint (Section 4.4.2.3)
The use of these options for selecting certificates is described in
Section 4.4.3.
An implementation MUST ignore EPD option types that are not
understood.
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4.4.2.1. Required Hostname
This option indicates the hostname to match against the certificate's
Hostname option (Section 4.3.3.1).
+-------------/-+-------------/-+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
| length \ | 0x00 \ | hostname |
+-------------/-+-------------/-+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~/
struct hostnameEPDOptionValue_t
{
uint8_t hostname[remainder()];
} :remainder()*8;
This option MUST NOT occur more than once in an EPD.
4.4.2.2. Ancillary Data
In this profile, this option indicates the server Uniform Resource
Identifier (URI) [RFC3986] encoded in UTF-8 to which a client is
connecting on this session, for example,
"rtmfp://server.example.com/app/instance".
+-------------/-+-------------/-+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
| length \ | 0x0a \ | ancillary data |
+-------------/-+-------------/-+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~/
struct ancillaryDataEPDOptionValue_t
{
uint8_t ancillaryData[remainder()];
} :remainder()*8;
This option MUST NOT occur more than once in an EPD.
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RFC 7425 Adobe RTMFP for Flash Communication December 2014
4.4.2.3. Fingerprint
This option indicates the 256-bit (32-byte) fingerprint
(Section 4.3.2) of a certificate.
0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7
+-------------/-+-------------/-+
| length \ | 0x0f \ |
+-------------/-+-------------/-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
| |
+ - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
| |
+ - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
| fingerprint |
+ - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
| |
+ - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
| |
+ - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
| |
+ - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
struct fingerprintEPDOptionValue_t
{
uint8_t fingerprint[32];
} :256;
This option MUST NOT occur more than once in an EPD.
4.4.3. Certificate Selection
This section describes the REQUIRED method of determining whether an
EPD selects a certificate.
An EPD MUST contain at least one of Fingerprint, Required Hostname,
or Ancillary Data options to select any certificate.
A Fingerprint EPD option selects or rejects a certificate no matter
what other options are present.
Without a Fingerprint option, a Required Hostname EPD option, if
present, REQUIRES an identical Hostname option in the certificate.
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RFC 7425 Adobe RTMFP for Flash Communication December 2014
Without a Fingerprint option, an Ancillary Data EPD option, if
present, REQUIRES that the certificate has an Accepts Ancillary Data
option.
if EPD contains a Fingerprint option:
if certificate.fingerprint == option.fingerprint:
certificate is selected. stop.
else:
certificate is not selected. stop.
else:
if EPD contains a Required Hostname option:
if certificate contains a Hostname option:
if certificate.hostname != option.hostname:
certificate is not selected. stop.
else:
certificate is not selected. stop.
if EPD contains an Ancillary Data option:
if certificate doesn't have an Accepts Ancillary Data option:
certificate is not selected. stop.
else if EPD does not contain a Required Hostname option:
certificate is not selected. stop.
certificate is selected. stop.
Figure 1: Algorithm to Test Whether an EPD Selects a Certificate
4.4.4. Canonical Endpoint Discriminator
In this profile, a Canonical Endpoint Discriminator (Section 3.2 of
RFC 7016) contains only a Fingerprint option (Section 4.4.2.3) and no
other options. The option length and type code MUST be encoded as
1-byte VLUs, even though VLU encoding allows those fields to be
encoded in an arbitrary number of bytes. That is, the Canonical
Endpoint Discriminator MUST be exactly 34 bytes long, with a length
field of 0x21 encoded as one byte, a type code of 0x0f encoded as one
byte, and 32 bytes of fingerprint.
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0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x21 | 0x0f |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
| |
+ - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
| |
+ - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
| fingerprint |
+ - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
| |
+ - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
| |
+ - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
| |
+ - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
struct canonicalEndpointDiscriminator_t
{
uint8_t length = 0x21;
uint8_t type = 0x0f;
uint8_t fingerprint[32];
} :272;
4.5. Session Keying Components
This section describes the format of the Session Key Initiator
Component of the Initiator Initial Keying RTMFP chunk and the Session
Key Responder Component of the Responder Initial Keying RTMFP chunk
(Sections 2.3.7 and 2.3.8 of RFC 7016). The Initiator and Responder
Session Keying Components have the same format.
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4.5.1. Format
A Session Keying Component in this profile is encoded as a sequence
of zero or more RTMFP Options.
+~~~/~~~/~~~~~~~+ +~~~/~~~/~~~~~~~+
| L \ T \ V |...............| L \ T \ V |
+~~~/~~~/~~~~~~~+ +~~~/~~~/~~~~~~~+
^ ^
+------------- Zero or more Options ----------+
struct sessionKeyingComponent_t
{
while(remainder() > 0)
option_t option :variable*8;
} :remainder()*8;
4.5.2. Options
This section lists options that can appear in a Session Keying
Component. The following option type codes are defined:
0x0d: Ephemeral Diffie-Hellman Public Key (Section 4.5.2.1)
0x0e: Extra Randomness (Section 4.5.2.2)
0x1d: Diffie-Hellman Group Select (Section 4.5.2.3)
0x1a: HMAC Negotiation (Section 4.5.2.4)
0x1e: Session Sequence Number Negotiation (Section 4.5.2.5)
An implementation MUST ignore a session keying component option type
that is not understood.
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4.5.2.1. Ephemeral Diffie-Hellman Public Key
This option specifies a Diffie-Hellman group ID and public key in
that group. This option MUST NOT be sent if the sender's certificate
has a static Diffie-Hellman public key. This option MUST be sent if
the sender's certificate does not have a static Diffie-Hellman public
key. This option MUST NOT be sent more than once.
+-------------/-+-------------/-+-------------/-+
| length \ | 0x0d \ | group ID \ |
+-------------/-+-------------/-+-------------/-+
+------------------------------------------------------------------+
| Diffie-Hellman Public Key |
+------------------------------------------------------------------/
struct ephemeralDHPublicKeyKeyingOptionValue_t
{
vlu_t groupID :variable*8;
uintn_t publicKey :remainder()*8; // network byte order
} :remainder()*8;
4.5.2.2. Extra Randomness
This option can be used to add extra entropy or randomness to a
keying component, particularly when the sender uses a static public
key. When used for that purpose, the extra randomness SHOULD be
cryptographically strong pseudorandom bytes not less than 16 bytes
(for cryptographically significant entropy) and not more than 64
bytes (the length of a SHA-256 input block) in length. The extra
randomness serves as a salt when computing the session keys
(Section 4.6).
+-------------/-+-------------/-+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
| length \ | 0x0e \ | extra randomness |
+-------------/-+-------------/-+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~/
struct extraRandomnessKeyingOptionValue_t
{
uint_t extraRandomness[remainder()];
} :remainder()*8;
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4.5.2.3. Diffie-Hellman Group Select
This option is sent by the Initiator to specify which Diffie-Hellman
group to use for key agreement. The Initiator MUST send this option
when it advertises a static Diffie-Hellman public key in its
certificate and MUST NOT send this option if it sends an ephemeral
Diffie-Hellman public key. This option MUST NOT be sent more than
once.
+-------------/-+-------------/-+-------------/-+
| length \ | 0x1d \ | group ID \ |
+-------------/-+-------------/-+-------------/-+
struct staticDHGroupSelectKeyingOptionValue_t
{
vlu_t groupID :variable*8;
} :variable*8;
4.5.2.4. HMAC Negotiation
This option is used to negotiate sending and receiving of an HMAC
field for packet verification.
|0 1 2 3 4 5 6 7|
+-------------/-+-------------/-+-+-+-+-+-+-+-+-+-------------/-+
| \ | \ | |S|S|R| \ |
| length / | 0x1a / | rsv |N|O|E| hmacLength / |
| \ | \ | |D|R|Q| \ |
+-------------/-+-------------/-+-+-+-+-+-+-+-+-+-------------/-+
struct hmacNegotiationKeyingOptionValue_t
{
uintn_t reserved :5; // rsv
bool_t willSendAlways :1; // SND
bool_t willSendOnRequest :1; // SOR
bool_t request :1; // REQ
vlu_t hmacLength :variable*8;
} :variable*8;
willSendAlways: If set, the sender will send an HMAC on packets in
this session.
willSendOnRequest: If set, the sender will send an HMAC on packets
in this session if the other end sets the request flag in its HMAC
Negotiation.
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request: If set, the sender would very much like the receiver to
send an HMAC on its packets. If the other end doesn't send an
HMAC on its packets, the session can fail.
hmacLength: If the sender negotiates to send an HMAC on its packets,
the HMAC field will be this many bytes long. This value MUST be
between 4 and 32 inclusive, or 0 if and only if willSendAlways and
willSendOnRequest are clear.
The handshake operational semantics for this option are described in
Section 4.6.4.
4.5.2.5. Session Sequence Number Negotiation
This option is used to negotiate sending and receiving of the Session
Sequence Number field for packet verification.
|0 1 2 3 4 5 6 7|
+-------------/-+-------------/-+-+-+-+-+-+-+-+-+
| \ | \ | |S|S|R|
| length / | 0x1e / | rsv |N|O|E|
| \ | \ | |D|R|Q|
+-------------/-+-------------/-+-+-+-+-+-+-+-+-+
struct sseqNegotiationKeyingOptionValue_t
{
uintn_t reserved :5; // rsv
bool_t willSendAlways :1; // SND
bool_t willSendOnRequest :1; // SOR
bool_t request :1; // REQ
} :8;
willSendAlways: If set, the sender will send a session sequence
number in packets in this session.
willSendOnRequest: If set, the sender will send a session sequence
number in packets in this session if the other end sets the
request flag in its Session Sequence Number Negotiation.
request: If set, the sender would very much like the receiver to
send a session sequence number in its packets. If the other end
doesn't send a session sequence number in its packets, the session
can fail.
The handshake operational semantics for this option are described in
Section 4.6.6.
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4.6. Session Key Computation
This section describes how to compute the cryptographic keys and
other settings for packet encryption and verification.
The Session Key Near Component (SKNC) means the keying component sent
by the near end of the session; that is, it is the Session Key
Initiator Component at the Initiator and the Session Key Responder
Component at the Responder.
The Session Key Far Component (SKFC) means the keying component sent
by the far end of the session; that is, it is the Session Key
Responder Component at the Initiator and the Session Key Initiator
Component at the Responder.
4.6.1. Public Key Selection
This section enumerates the public key selection methods for all
possible combinations of static or ephemeral public key modes for
each endpoint according to their certificate options (Section 4.3.3).
4.6.1.1. Initiator and Responder Ephemeral
The Initiator and Responder list one or more Supported Ephemeral
Diffie-Hellman Group options (Section 4.3.3.4) in their certificates.
The Initiator sends exactly one Ephemeral Diffie-Hellman Public Key
option (Section 4.5.2.1) in its Session Key Initiator Component,
which selects one group from among those supported by the Responder
and Initiator. Responder sends exactly one Ephemeral Diffie-Hellman
Public Key option in its Session Key Responder Component, in the same
group as indicated by the Initiator.
4.6.1.2. Initiator Ephemeral and Responder Static
The Responder lists one or more Static Diffie-Hellman Public Key
options (Section 4.3.3.5) in its certificate. The Initiator lists
one or more Supported Ephemeral Diffie-Hellman Group options in its
certificate. The Initiator sends exactly one Ephemeral Diffie-
Hellman Public Key option in its Session Key Initiator Component,
which selects one group from among those supported by the Responder
and Initiator and the corresponding public key for the Responder.
Responder uses its public key from the indicated group, and sends
only an Extra Randomness option (Section 4.5.2.2) in its Session Key
Responder Component to salt the session keys.
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4.6.1.3. Initiator Static and Responder Ephemeral
The Responder lists one or more Supported Ephemeral Diffie-Hellman
Group options in its certificate. The Initiator lists one or more
Static Diffie-Hellman Public Key options in its certificate. The
Initiator sends exactly one Diffie-Hellman Group Select option
(Section 4.5.2.3) in its Session Key Initiator Component, which
selects one group from among those supported by the Responder and
Initiator and the corresponding public key for the Initiator, plus an
Extra Randomness option to salt the session keys. The Responder
sends an Ephemeral Diffie-Hellman Public Key option in its Session
Key Responder Component in the same group as indicated by the
Initiator.
4.6.1.4. Initiator and Responder Static
The Initiator and Responder each list one or more Static Diffie-
Hellman Public Key options in their certificates. The Initiator
sends exactly one Diffie-Hellman Group Select option in its Session
Key Initiator Component, which selects one group and corresponding
public keys from among those supported by the Responder and
Initiator, and an Extra Randomness option to salt the session keys.
The Responder sends an Extra Randomness option in its Session Key
Responder Component to add its own salt to the session keys.
4.6.2. Diffie-Hellman Shared Secret
To be acceptable, a Diffie-Hellman public key MUST have all of the
following properties:
o Be at least 16777216 (2^24);
o Be at most the group's prime modulus minus 16777216;
o Have at least 16 "1" bits;
o Have at least 16 "0" bits, not including leading zeros.
An endpoint MUST NOT complete to an S_OPEN session with a far
endpoint using a public key that is not acceptable according to these
criteria.
Once the group and corresponding public key of the far end is
determined, the far end's public key and the near end's private key
are combined according to Diffie-Hellman [DH] to compute the Diffie-
Hellman Shared Secret, an integer.
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In the following sections, DH_SECRET means the Diffie-Hellman Shared
Secret encoded as a byte-aligned unsigned integer in network byte
order with no leading zero bytes. For example, if the shared secret
is 4886718345, DH_SECRET would be the five bytes:
Hex: 01 23 45 67 89
4.6.3. Packet Encrypt/Decrypt Keys
Packets are encrypted using a symmetric cipher, such as the Advanced
Encryption Standard [AES]. Distinct keys are used for sending and
receiving packets. Each end's sending (encrypt) key is the other
end's receiving (decrypt) key.
The raw keys computed in this section for encryption and decryption
are transformed in a manner specific to the cipher with which they
are to be used. In this profile, AES-128 is the only currently
defined cipher. For this cipher, the first 128 bits (16 bytes) of
the 256-bit output of the calculation are taken to be the AES-128
key.
Set ENCRYPT_KEY = HMAC-SHA256(DH_SECRET, HMAC-SHA256(SKFC, SKNC));
Set DECRYPT_KEY = HMAC-SHA256(DH_SECRET, HMAC-SHA256(SKNC, SKFC));
The full 256 bits of ENCRYPT_KEY and DECRYPT_KEY are used in the
computations in the following sections.
4.6.4. Packet HMAC Send/Receive Keys
Packets can be verified that they were not corrupted or modified by
appending an HMAC to the packet. Whether to use an HMAC or a simple
checksum is determined during the initial keying phase using the HMAC
Negotiation option (Section 4.5.2.4). Distinct HMAC keys are used
for sending and receiving packets. Each end's sending key is the
other end's receiving key, and vice versa.
Set HMAC_SEND_KEY = HMAC_SHA256(DH_SECRET, ENCRYPT_KEY);
Set HMAC_RECV_KEY = HMAC_SHA256(DH_SECRET, DECRYPT_KEY);
If an endpoint sets the willSendAlways flag in its HMAC Negotiation
option, then it MUST send an HMAC on packets it sends with this
session key.
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If an endpoint's willSendAlways flag is clear but its
willSendOnRequest flag is set, then it MUST send an HMAC on packets
it sends with this session key if and only if the other endpoint's
request flag is set.
If a sending endpoint's willSendAlways and willSendOnRequest flags
are clear, then the receiving endpoint SHOULD reject that keying
component if the receiving endpoint is configured to require the
sending endpoint to send HMAC.
If HMAC is negotiated to be used, the corresponding hmacLength MUST
be between 4 and 32 inclusive.
If HMAC is negotiated not to be used, a simple checksum is used for
packet verification.
The Default Session Key uses the simple checksum and does not use
HMAC.
4.6.5. Session Nonces
Session nonces are per-session, cryptographically strong secret
values known only to the two endpoints of the session. They can be
used for application-layer cryptographic challenges (such as signing
or password verification). These nonces are a convenience being pre-
shared and pre-agreed-upon in a secure manner during the initial
keying handshake.
Each end's near nonce is the other end's far nonce, and vice versa.
Set NEAR_NONCE = HMAC_SHA256(DH_SECRET, SKNC);
Set FAR_NONCE = HMAC_SHA256(DH_SECRET, SKFC);
4.6.6. Session Sequence Number
Duplicate packets can be detected and rejected by using an optional
session sequence number inside the encrypted packets. The session
sequence number is a monotonically increasing unbounded integer and
does not wrap. Session sequence numbers SHOULD start at zero and
SHOULD increment by one for each packet sent using that session key.
Implementations MUST handle session sequence numbers with no less
than 64 bits of range.
If an endpoint's willSendAlways flag in its Session Sequence Number
Negotiation option (Section 4.5.2.5) is set, then it MUST send a
session sequence number in packets it sends with this session key.
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If an endpoint's willSendAlways flag is clear but its
willSendOnRequest flag is set, then it MUST send a session sequence
number on packets it sends with this session key if and only if the
other endpoint's request flag is set.
If a sending endpoint's willSendAlways and willSendOnRequest flags
are clear, then the receiving endpoint SHOULD reject that keying
component if the receiving endpoint is configured to require the
sending endpoint to send session sequence numbers.
The Default Session Key does not use session sequence numbers.
4.7. Packet Encryption
This section describes the concrete syntax and operational semantics
of RTMFP packet encryption for this Cryptography Profile.
4.7.1. Cipher
This profile defines AES-128 [AES] in CBC [CBC] mode as the only
cipher. Extensions to this profile can specify and negotiate
additional ciphers and modes by defining certificate and keying
component options and associated semantics.
For AES-128-CBC, the initialization vector (IV) for each packet is 16
zero bytes. The IV is not included in the packet.
4.7.2. Format
The Encrypted Packet is the encryptedPacket field of an RTMFP
Multiplex packet (Section 2.2.2 of RFC 7016); that is, the portion of
the Multiplex packet following the scrambled session ID. The
Encrypted Packet has the following format:
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+----------------+ +----------------+~~~~~~~~~~~~~~~~~~~~~~~+
| CBC Block 1 | ... | CBC Block N | truncatedHMAC |
+----------------+ +----------------+~~~~~~~~~~~~~~~~~~~~~~~+
^ ^ ^
| Zero or more AES-128 chained | hmacLength bytes long |
+-------- cipher blocks -----------+--- (may be zero) ---+
struct flashProfileEncryptedPacket_t
{
if(HMAC is being used)
hmacLength = negotiated length;
else
hmacLength = 0;
struct
{
iv[16 bytes] = { 0 };
blockCount = 0;
while((remainder() > hmacLength) && (remainder() >= 16))
{
uint8_t cbcBlock[16];
blockCount++;
}
} chainedCipherBlocks :variable*16*8;
if(HMAC is being used)
{
if(remainder() == hmacLength)
uint8_t truncatedHMAC[hmacLength];
else
packetVerificationFailed();
}
else if(remainder() > 0)
packetVerificationFailed();
} :encryptedPacket.length*8;
cbcBlock: The next AES-128-CBC block.
chainedCipherBlocks: The concatenation of every cipher block in the
packet (over which the HMAC is computed).
truncatedHMAC: If HMAC was negotiated to be used (Section 4.5.2.4),
this field is set to the first negotiated hmacLength bytes of the
HMAC of the chainedCipherBlocks.
The plaintext data before encryption or after decryption has the
following format:
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0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7
+~~~~~~~~~~~~~/~+
| SSEQ (opt.) \ |
+~~~~~~~~~~~~~/~+
+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+
| Checksum (opt.) |
+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
| Plain RTMFP Packet |
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~/
struct flashProfilePlainPacket_t
{
if(session sequence numbers being used)
vlu_t sessionSequenceNumber :variable*8; // SSEQ
if(HMAC not being used)
uint16_t checksum;
packet_t plainRTMFPPacket :variable*8;
} :chainedCipherBlocks.blockCount*16*8;
sessionSequenceNumber: If session sequence numbers were negotiated
to be used (Section 4.6.6), this field is present and is the VLU
session sequence number of this packet.
checksum: If HMAC was not negotiated to be used, this field is
present and is the simple checksum (Section 4.7.3.1) of the
remaining bytes of this structure.
plainRTMFPPacket: The (plain, unencrypted) RTMFP Packet
(Section 2.2.4 of RFC 7016) plus any necessary padding.
When assembling this structure and prior to calculating the checksum
(if present), if the structure's total length is not an integer
multiple of 16 bytes (the AES cipher block size), pad the end of
plainRTMFPPacket with as many bytes having a value of 0xff as are
needed to bring the structure's total length to an integer multiple
of 16 bytes. The receiver's RTMFP Packet parser (Section 2.2.4 of
RFC 7016) will consume this padding.
4.7.3. Verification
In RTMFP, the Cryptography Profile is responsible for packet
verification. In this profile, packets are verified with an HMAC or
a simple checksum, depending on the configuration of the endpoints,
and optionally verified against replay or duplication using session
sequence numbers. The simple checksum is inside the encrypted
packet, so it becomes essentially a 16-bit cryptographic checksum.
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4.7.3.1. Simple Checksum
The simple checksum is the 16-bit ones' complement of the 16-bit
ones' complement sum of all 16-bit (2 bytes in network byte order)
words to be checked. If there are an odd number of bytes to be
checked, then for purposes of this checksum, treat the last byte as
the lower 8 bits of a 16-bit word whose upper 8 bits are 0. This is
also known as the "Internet Checksum" [RFC1071].
When present, the checksum is calculated over all bytes of the
plaintext packet starting after the checksum field through the end of
the plain packet. It cannot be calculated until the plain packet is
padded, if necessary, to bring its length to an integer multiple of
16 bytes (the AES cipher block size). The session sequence number
field, if present, and the checksum field itself are not included in
the checksum.
On receiving a packet being verified with a checksum: calculate the
checksum over all the bytes of the plaintext packet following the
checksum field and compare the checksum to the value in the checksum
field. If they match, the packet is verified; if they do not match,
the packet is corrupt and MUST be discarded as though it was never
received.
4.7.3.2. HMAC
When present, the HMAC field is the last hmacLength bytes of the
packet and is calculated over all of the encrypted cipher blocks of
the packet preceding the HMAC field. The value of the HMAC field is
the first hmacLength bytes of the HMAC-SHA256 of the checked data,
using the computed HMAC keys (Section 4.6.4) and negotiated
hmacLength (Section 4.5.2.4). Note each endpoint independently
specifies the length of the HMAC it will send via its hmacLength
field.
When an endpoint has negotiated to send an HMAC, it encrypts the data
blocks, computes the HMAC over the encrypted data blocks using its
HMAC_SEND_KEY, and appends the first hmacLength bytes of that hash
after the final encrypted data block.
When an endpoint has negotiated to receive an HMAC, the endpoint
computes the HMAC over the encrypted data blocks using its
HMAC_RECV_KEY and then compares the first receive hmacLength bytes of
the computed HMAC to the HMAC field in the packet. If they are
identical, the packet is verified; if they are not identical, the
packet is corrupt and MUST be discarded as though it was never
received.
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HMAC and simple checksum verification are mutually exclusive.
4.7.3.3. Session Sequence Number
Session sequence numbers are used to detect and reject a packet that
was duplicated in the network or replayed by an attacker and to
ensure the first chained cipher block of every packet is unique, in
lieu of a full-block initialization vector. Sequence numbers start
at zero, increase by one for each packet sent in the session, do not
wrap, and do not repeat.
When session sequence numbers are negotiated to be used, the receiver
MUST allow for packets to be reordered in the network by up to at
least 32 sequence numbers; note, however, that reordering by more
than three packets can trigger loss detection and retransmission by
negative acknowledgement, just as with TCP, and is therefore not
likely to occur in the real Internet.
[RFC4302], [RFC4303], and [RFC6479] describe Anti-Replay Window
methods that can be employed to detect duplicate sequence numbers.
Other methods are possible.
Any packet received having a session sequence number that was already
seen in that session, either directly or by being less than the
lowest sequence number in the Anti-Replay Window, is a duplicate and
MUST be discarded as though never received.
5. Flash Communication
The Flash platform uses RTMP [RTMP] messages for media streaming and
communication. This section describes how to transport RTMP messages
over RTMFP flows and additional messages and semantics unique to this
transport.
5.1. RTMP Messages
An RTMP message comprises a virtual header and a payload. The
virtual header comprises a Message Type, a Payload Length, a
Timestamp, and a Stream ID. The format of the payload is dependent
on the type of message.
An RTMP message is mapped onto a lower transport layer, such as RTMP
Chunk Stream [RTMP] or RTMFP. RTMP messages were initially designed
along with, and for transport on, RTMP Chunk Stream. This design
constrains the possible values of RTMP message header fields. In
particular:
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Message Type is 8 bits wide, and is therefore constrained to
values from 0 to 255 inclusive;
Payload Length is 24 bits wide, so messages can be at most
16777215 bytes long;
Timestamp is 32 bits wide, so timestamps range from 0 to
4294967295 and wrap around;
Stream ID is 24 bits wide, and is therefore constrained to values
from 0 to 16777215 inclusive.
RTMP Chunk Stream Protocol Control messages (message types 1, 2, 3,
5, and 6) are not used when transporting RTMP messages in RTMFP
flows. Messages of those types SHOULD NOT be sent and MUST be
ignored.
5.1.1. Flow Metadata
All messages in RTMFP are transported in flows. In this profile, an
RTMFP flow for RTMP messages carries the messages for exactly one
RTMP Stream ID. Multiple flows can carry messages for the same
Stream ID; for example, the video and audio messages of a stream
could be sent on separate flows, allowing the audio to be given
higher transmission priority.
The User Metadata for flows in this profile begins with a distinct
signature to distinguish among different kinds of flows. The User
Metadata for a flow used for RTMP messages begins with the two-
character signature "TC".
The Stream ID is encoded in the flow's User Metadata so that it
doesn't need to be sent with each message.
The sender can have a priori knowledge about the kind of media it
intends to send on a flow and its intended use and can give the
receiver a hint as to whether messages should be delivered as soon as
possible or in their original queuing order. For example, the sender
might be sending real-time, delay-sensitive audio messages on a flow,
and hint that the receiver should take delivery of the messages on
that flow as soon as they arrive in the network, to reduce the end-
to-end latency of the audio.
The receiver can choose to take delivery of messages on flows as soon
as they arrive in the network or in the messages' original queuing
order. A receiver that chooses to take delivery of messages as soon
as they arrive in the network MUST be prepared for the messages to
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arrive out-of-order. For example, a receiver may choose not to
render a newly received audio message having a timestamp earlier than
the most recently rendered audio timestamp.
The sender can choose to abandon a message that it has queued in a
flow before the message has been delivered to the receiver. For
example, the sender may abandon a real-time, delay-sensitive audio
message that has not been delivered within one second, to avoid
spending transmission resources on stale media that is no longer
relevant.
Note: A gap will cause a delay at the receiver of at least one round-
trip time if the receiver is taking delivery of messages in original
queuing order.
0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+~~~~~~~~~~~~~/~+
| | | |S|r|R| \ |
| 0x54 'T' | 0x43 'C' | rsv |I|s|X| streamID / |
| | | |D|v|I| \ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+~~~~~~~~~~~~~/~+
struct RTMPMetadata_t
{
uint8_t signature[2] == { 'T', 'C' };
uintn_t reserved1 :5; // rsv
bool_t streamIDPresent :1; // SID
uintn_t reserved2 :1; // rsv
uintn_t receiveIntent :1; // RXI
// 0: original queuing order, 1: network arrival order
if(streamIDPresent)
vlu_t streamID :variable*8;
} :variable*8;
signature: Metadata signature for RTMP message flows, being the two
UTF-8 coded characters "TC".
streamIDPresent: A boolean flag indicating whether the streamID
field is present. In this profile, this flag MUST be set.
receiveIntent: A hint by the sender as to the best order in which to
take delivery of messages from the flow. A value of zero
indicates a hint that the flow's messages should be received in
the order they were originally queued by the sender (that is, in
ascending sequence number order); a value of one indicates a hint
that the flow's messages should be received in the order they
arrive in the network, even if there are sequence number gaps or
reordering. Network arrival order is typically hinted for live,
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RFC 7425 Adobe RTMFP for Flash Communication December 2014
delay-sensitive flows, such as for audio media. To take delivery
of a message as soon as it arrives in the network: receive it from
the receiving flow's RECV_BUFFER as soon as it becomes complete
(Section 3.6.3.3 of RFC 7016), and remove it from the RECV_BUFFER.
Section 3.6.3.3 of RFC 7016 describes how to take delivery of
messages in original queuing order.
streamID: If the streamIDPresent flag is set, this field is present
and is the RTMP stream ID to which the messages in this flow
belong. In this profile, this field MUST be present.
A receiver SHOULD reject an RTMP message flow if its streamIDPresent
flag is clear. This profile doesn't define a stream mapping for this
case.
Derived or composed profiles can define additional flow types and
corresponding metadata signatures. A receiver SHOULD reject a flow
having an unrecognized metadata signature.
5.1.2. Message Mapping
This section describes the format of an RTMP message (Section 5.1) in
an RTMFP flow.
0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| messageType |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| messagePayload |
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~/
struct RTMPMessage_t
{
uint8_t messageType;
uint32_t timestamp;
uint8_t messagePayload[remainder()];
} :flowMessageLength*8;
messageType: The RTMP Message Type;
timestamp: The RTMP Timestamp, in network byte order;
messagePayload: The payload of the RTMP message;
payload length: The RTMP message payload length is inferred from the
length of the RTMFP message;
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RFC 7425 Adobe RTMFP for Flash Communication December 2014
Stream ID: The Stream ID for this message is taken from the metadata
of the flow on which this message was received.
5.2. Flow Synchronization
RTMFP flows are independent and have no inter-flow ordering
guarantee. RTMP was designed for transport over a single, reliable,
strictly ordered byte stream. Some RTMP message semantics take
advantage of this ordering; for example, a Stream EOF User Control
event must not be processed until after all media messages for the
corresponding stream have been received. Flow Synchronization
messages provide a barrier to align message delivery across flows
when required by RTMP semantics.
A Flow Synchronization message is coded as a User Control event
message (Type 4) having Event Type 34. Message timestamps are
ignored and MAY be set to 0.
0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| eventType = 34 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| syncID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
struct flowSyncUserControlMessagePayload_t
{
uint16_t eventType = 34;
uint32_t syncID;
uint32_t count;
} :10*8;
eventType: The RTMP User Control Message Event Type. Flow
Synchronization messages have type 34 (0x22);
syncID: The identifier for this barrier;
count: The number of flows being synchronized by syncID. This field
MUST be at least 1 and SHOULD be at least 2.
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RFC 7425 Adobe RTMFP for Flash Communication December 2014
On receipt of a Flow Synchronization message, a receiver SHOULD
suspend receipt of further messages on that flow until count Flow
Synchronization messages (including this one) with the same syncID
have been received on flows in the same flow association tree.
Example: Consider flows F1 and F2 in the same NetConnection carrying
messages M, and let Sync(syncID,count) denote a Flow Synchronization
message.
| |
F1: M1 M2 M4 Sync(8,2) | Sync(13,2).....| M7
| |
F2: M3 Sync(8,2).......| M5 Sync(13,2) | M6
| |
Barrier 8 Barrier 13
Figure 2: Example Flow Synchronization Barriers
Flow Synchronization messages form a delivery barrier to impart at
least a partial message ordering across flows. In this example,
message M5 comes after M1..4 and before M6..7; however, M3 could be
delivered before or after any of M1, M2, or M4, and M6 could come
before or after M7.
Flow Synchronization can cause a priority inversion; therefore, it
SHOULD NOT be used except when necessary to preserve RTMP ordering
semantics.
5.3. Client-to-Server Connection
The client connects to a server. The connection comprises one main
control flow in each direction from client to server and from server
to client for NetConnection messages, and zero or more flows in each
direction for NetStream media messages. NetStream flows may come and
go naturally over time according to media transport needs. An
exception on a NetConnection control sending flow indicates the
closure by the other end of the NetConnection and all associated
NetStreams.
The client MUST NOT use the same client certificate for more than one
server connection; that is, a client's peer ID MUST NOT be reused.
5.3.1. Connecting
The client desires a connection to a server having an RTMFP URI, for
example, "rtmfp://server.example.com/app/instance". The client
gathers one or more initial candidate addresses for the server named
in the URI (for example, by using the Domain Name System (DNS)
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RFC 7425 Adobe RTMFP for Flash Communication December 2014
[RFC1035]). The client creates an EPD having an Ancillary Data
option (Section 4.4.2.2) encoding the URI. The client initiates an
RTMFP session to the one or more candidate addresses using the EPD.
When the session transitions to the S_OPEN state, the client opens a
new flow in that session for Stream ID 0 and Receive Intent 0
"original queuing order". This is the client's NetConnection main
control flow. The client sends an RTMP "connect" command on the flow
and waits for a response or exception.
5.3.2. Server-to-Client Return Control Flow
The server, on accepting the client's NetConnection control flow, and
receiving and accepting the "connect" command, opens one or more
return flows to the client having Stream ID 0 and associated to the
control flow from the client. Flows for Stream ID 0 are the server's
NetConnection control flows. The server sends a "_result" or
"_error" transaction response for the client's connect command.
When the client receives the first return flow from the server for
Stream ID 0 and associated to the client's NetConnection control
flow, the client assumes that flow is the canonical return
NetConnection control flow from the server, to which all new client-
to-server flows should be associated.
On receipt of a "_result" transaction response on Stream ID 0 for the
client's connect command, the connection is up.
The client MAY open additional return control flows to the server on
Stream ID 0, associated to the server's canonical NetConnection
control flow.
5.3.3. setPeerInfo Command
The "setPeerInfo" command is sent by the client to the server over
the NetConnection control flow to inform the server of candidate
socket addresses through which the client might be reachable. This
list SHOULD include all directly connected interface addresses and
proxy addresses except as provided below. The list MAY be empty.
The list need not include the address of the server, even if the
server is to act as an introducer for the client. The list SHOULD
NOT include link-local or loopback addresses.
This command is sent as a regular RTMP NetConnection command; that
is, as an RTMP Type 20 Command Message or an RTMP Type 17 Command
Extended Message on Stream ID 0. A Type 20 Command Message SHOULD be
used if the object encoding negotiated during the "connect" and
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RFC 7425 Adobe RTMFP for Flash Communication December 2014
"_result" handshake is AMF0 [AMF0], and a Type 17 Command Extended
Message SHOULD be used if the negotiated object encoding is AMF3
[AMF3].
Note: A Type 20 Command Message payload is a sequence of AMF objects
encoded in AMF0.
Note: A Type 17 Command Extended Message payload begins with a format
selector byte, followed by a sequence of objects in a format-specific
encoding. At the time of writing, only format 0 is defined;
therefore, the format selector byte MUST be 0. Format 0 is a
sequence of AMF objects, each encoded in AMF0 by default; AMF3
encoding for an object can be selected by prefixing it with an
"avmplus-object-marker" (0x11) as defined in [AMF0].
To complete the RTMFP NetConnection handshake, an RTMFP client MUST
send a setPeerInfo command to the server after receiving a successful
response to the "connect" command.
(
"setPeerInfo", // AMF String, command name
0.0, // AMF Number, transaction ID
NULL, // AMF Null, no command object
... // zero or more AMF Strings, each an address
)
Each listed socket address includes an IPv4 or IPv6 address in
presentation format and a UDP port number in decimal, separated by a
colon. Since the IPv6 address presentation format uses colons, IPv6
addresses are enclosed in square brackets [RFC3986].
(
"setPeerInfo",
0.0,
NULL,
"192.0.2.129:50001",
"[2001:db8:1::2]:50002"
)
Figure 3: Example setPeerInfo Command
A server SHOULD assume that the client is behind a Network Address
Translator (NAT) if and only if the observed far endpoint address of
the session for the flow on which this command was received does not
appear in the setPeerInfo address list.
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RFC 7425 Adobe RTMFP for Flash Communication December 2014
5.3.4. Set Keepalive Timers Command
The server can advise the client to set or change the client's
session keepalive timer periods for its connection to the server and
for its P2P connections. The server MAY choose keepalive periods
based on static configuration, application- or deployment-specific
circumstances, whether the client appears to be behind a NAT, or for
any other reason.
The Set Keepalive Timers command is sent by the server to the client
on Stream ID 0 as a User Control event message (Type 4) having Event
Type 41. Message timestamps are ignored and MAY be set to 0.
0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| eventType = 41 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| serverKeepalivePeriodMsec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| peerKeepalivePeriodMsec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
struct setKeepaliveUserControlMessagePayload_t
{
uint16_t eventType = 41;
uint32_t serverKeepalivePeriodMsec;
uint32_t peerKeepalivePeriodMsec;
} :10*8;
eventType: The RTMP User Control Message Event Type. Set Keepalive
Timers messages have type 41 (0x29);
serverKeepalivePeriodMsec: The keepalive period, in milliseconds,
that the client is advised to set on its RTMFP session with the
server;
peerKeepalivePeriodMsec: The keepalive period, in milliseconds, that
the client is advised to use on its RTMFP sessions with any peer
that is not the server.
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RFC 7425 Adobe RTMFP for Flash Communication December 2014
The client MUST define minimum values for these keepalive periods,
below which it will not set them, regardless of the values in this
message. The minimum keepalive timer periods SHOULD be at least five
seconds. The client MAY define maximum values for these keepalive
periods, above which it will not set them.
On receipt of this message from the server, a client SHOULD set its
RTMFP server and peer keepalive timer periods to the indicated values
subject to the client's minimum and maximum values. The server MAY
send this message more than once, particularly if conditions that it
uses to determine the timer periods change.
5.3.5. Additional Flows for Streams
The client or server opens additional flows to the other side to
carry messages for any stream. Additional flows are associated to
the canonical NetConnection control flow from the other side.
Client Server
------>--C2S-Control-Flow------------------------->--+
|
+--<------------------------S2C-Control-Flow---<--+
| |
| <------------------------S2C-Stream-Flow-1--<--+
| : |
| <------------------------S2C-Stream-Flow-M--<--+
|
+-->--C2S-Stream-Flow-1------------------------>
| :
+-->--C2S-Stream-Flow-N------------------------>
Figure 4: Schematic Flow Association Tree for a NetConnection
5.3.5.1. To Server
Additional flows from the client to the server for stream messages
are opened with the Stream ID for that stream and associated in
return to the server's canonical NetConnection control flow.
The client MAY create as many flows as desired for any Stream ID
(including Stream ID 0) at any time.
5.3.5.2. From Server
Additional flows from the server to the client for stream messages
are opened with the Stream ID for that stream, and associated in
return to the client's NetConnection control flow.
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RFC 7425 Adobe RTMFP for Flash Communication December 2014
The server MAY create as many flows as desired for any Stream ID
(including Stream ID 0) at any time.
5.3.5.3. Closing Stream Flows
Either end MAY close a sending flow that is not for Stream ID 0 at
any time with no semantic meaning for the stream.
At any time, either end MAY reject a receiving flow that is not one
of the other end's NetConnection control flows. No flow exception
codes are defined by this profile, so the receiving end SHOULD use
exception code 0 when rejecting the flow. The sending end, on
notification of any exception for a stream flow, SHOULD NOT open a
new flow to take the rejected flow's place for transport of messages
for that stream. If an end rejects any flow for a stream, it SHOULD
reject all the flows for that stream, otherwise Flow Synchronization
messages (Section 5.2) that were in flight could be discarded and
some flows might become or remain stuck in a suspended state.
5.3.6. Closing the Connection
The client or server can signal an orderly close of the connection by
closing its NetConnection control sending flows and all stream
sending flows. The other end, on receiving a close/complete
notification for the canonical NetConnection control receiving flow,
closes its sending flows. When both ends observe all receiving flows
have closed and completed, the connection has cleanly terminated.
Either end can abruptly terminate the connection by rejecting the
NetConnection control receiving flows or by closing the underlying
RTMFP session. On notification of any exception on a NetConnection
control sending flow, the end seeing the exception knows the other
end has terminated abruptly, and can immediately close all sending
and receiving flows for that connection.
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5.3.7. Example
Client Server
|IHello (EPD:anc=URI) |
-+- |------------------------>|
| | |
| | RHello (RCert:anc)|
RTMFP | |<------------------------|
Session| | |
Hand- | |IIKeying |
shake | |------------------------>|
| | |
| | RIKeying|
-+- |<------------------------|
| |
-+- |"connect" command |
(Str.ID=0)|-CFlow-0---------------->|
| | |
| | "_result" response|
RTMP | |<----------------SFlow-0-|(Str.ID=0,
Connect| | | Assoc=CFlow-0)
Hand- | |"setPeerInfo" command |
shake | |-CFlow-0---------------->|
-+- | |
|"createStream" command |
-+- |-CFlow-0---------------->|
| | |
| | "_result" (str.ID=5)|
| |<----------------SFlow-0-|
| | |
| |"play" command |
(Str.ID=5,|-CFlow-1---------------->|
Assoc=SFlow-0)| |
| | StreamBegin User Control|
| |<----------------SFlow-1-|(Str.ID=5,
| | | Assoc=CFlow-0)
| | (RTMP stream events) |
Streaming | |<----------------SFlow-1-|
| | |
| | Audio Data |
| |<----------------SFlow-2-|(Str.ID=5,
| | | Assoc=CFlow-0)
| | Video Data |
| |<----------------SFlow-3-|(Str.ID=5,
| | : | Assoc=CFlow-0)
| : |
Figure 5: Example NetConnection Message Exchange
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5.4. Direct Peer-to-Peer Streams
Clients can connect directly to other clients for P2P streaming and
data exchange. A client MAY have multiple separate P2P NetStreams
with a peer in one RTMFP session, each a separate logical connection.
P2P NetStreams are unidirectional, initiated by a subscriber (the
side issuing the "play" command) to a publisher. The subscribing
peer has a control flow to the publisher. The publisher has zero or
more return flows to the subscriber associated to the subscriber's
control flow, for the stream media and data.
5.4.1. Connecting
A client desires to subscribe directly to a stream being published in
P2P mode by a publishing peer. The client learns the peer ID of the
publisher and the stream name through application-specific means.
If the client does not already have an RTMFP session with that peer
ID, it initiates a new session, creating an EPD containing a
Fingerprint option (Section 4.4.2.3) for the publisher's peer ID and
using the server session's DESTADDR as the initial candidate address
for the session to the peer. The server acts as an Introducer
(Section 3.5.1.6 of RFC 7016), using forward and redirect messages to
help the client and the peer establish a session.
When an S_OPEN session exists to the desired peer, the client creates
a new independent flow to that peer. The flow MUST have a non-zero
Stream ID. The client sends an RTMP "play" command over the flow,
giving the name of the desired stream at the publisher. This flow is
the subscriber's control flow.
5.4.2. Return Flows for Stream
The publisher, on accepting a new flow not indicating a return
association with any of its sending flows and having a non-zero
Stream ID, receives and processes the "play" command. If and when
the request is acceptable to the publisher, it opens one or more
return flows to the subscribing peer, associated to the subscriber's
control flow and having the same Stream ID. The publisher sends a
StreamBegin User Control message, appropriate RTMP status events, and
the stream media over the one or more return flows.
The subscriber uses the return association of the media flows to the
subscriber control flow to determine the stream to which the media
belongs.
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RFC 7425 Adobe RTMFP for Flash Communication December 2014
The publisher MAY open any number of media flows for the stream and
close them at any time. The opening and closing of media flows has
no semantic meaning for the stream, except that the opening of at
least one flow and the reception of at least one media message or a
StreamBegin User Control message indicates that the publisher is
publishing the requested stream to the subscriber.
Subscriber Publisher
------>--Subscriber-Control-Flow------------------>--+
|
<------------------Publisher-Stream-Flow-1--<--+
: |
<------------------Publisher-Stream-Flow-N--<--+
Figure 6: Schematic Flow Association Tree for a P2P Direct Connection
5.4.3. Closing the Connection
Either end can close the stream by closing or rejecting the
subscriber's control flow. The publisher SHOULD close and unpublish
to the subscriber on receipt of a close/complete of the control flow.
The subscriber SHOULD consider the stream closed on notification of
any exception on the control flow.
6. IANA Considerations
This memo specifies option type code values for Certificate fields
(Section 4.3.3), Endpoint Discriminator fields (Section 4.4.2), and
Session Keying Component fields (Section 4.5.2). It also specifies a
flow metadata signature (Section 5.1.1). The type code values and
signatures for this profile are assigned and maintained by Adobe, and
therefore require no action from IANA.
6.1. RTMFP URI Scheme Registration
This memo describes use of an RTMFP URI scheme (Section 4.4.2.2,
Section 5.3.1, Figure 5). Per this section, the "rtmfp" URI scheme
has been registered by IANA.
The syntax and semantics of this URI scheme are described using the
Augmented Backus-Naur Form (ABNF) [RFC5234] rules from RFC 3986.
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RFC 7425 Adobe RTMFP for Flash Communication December 2014
URI scheme name: rtmfp
Status: provisional
URI scheme syntax:
rtmfp-uri-scheme = "rtmfp:"
/ "rtmfp://" host [ ":" port ] path-abempty
URI scheme semantics: The first form is used in the APIs of some
implementations to indicate instantiation of an RTMFP client
according to this memo, but without connecting to a server. Such
an instantiation might be used for pure peer-to-peer
communication.
The second form provides location information for the server to
which to connect and optional additional information to pass to
the server. The only operation for this URI form is to connect to
a server (initial candidate address(es) for which are named by
host and port) according to Section 5.3. The UDP port for initial
candidate addresses, if not specified, is 1935. If the host is a
reg-name, the initial candidate address set SHOULD comprise all
IPv4 and IPv6 addresses to which reg-name resolves. The semantics
of path-abempty are specific to the server. Connections are made
using RTMFP as specified by this memo.
Encoding considerations: The path-abempty component represents
textual data consisting of characters from the Universal Character
Set. This component SHOULD be encoded according to Section 2.5 of
RFC 3986.
Applications/protocols that use this URI scheme name: The Flash
runtime (including Flash Player) from Adobe Systems Incorporated,
communication servers such as Adobe Media Server, and
interoperable clients and servers provided by other parties, using
RTMFP according to this memo.
Interoperability considerations: This scheme requires use of RTMFP
as defined by RFC 7016 in the manner described by this memo.
Security considerations: See Security Considerations (Section 7) in
this memo.
Contact: Michael Thornburgh, Adobe Systems Incorporated,
<mthornbu@adobe.com>.
Author/Change controller: Michael Thornburgh, Adobe Systems
Incorporated, <mthornbu@adobe.com>.
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RFC 7425 Adobe RTMFP for Flash Communication December 2014
References:
Thornburgh, M., "Adobe's Secure Real-Time Media Flow Protocol",
RFC 7016, November 2013.
This memo.
7. Security Considerations
Section 4 details the cryptographic aspects of this profile.
This profile does not define or use a Public Key Infrastructure
(PKI). Clients SHOULD use static Diffie-Hellman keys in their
certificates (Section 4.3.3.5). Clients MUST create a new
certificate with a distinct fingerprint for each new NetConnection
(Section 5.3). These constraints make client identities ephemeral
but unable to be forged. A man-in-the-middle cannot successfully
interpose itself in a connection to a target client addressed by its
fingerprint/peer ID if the target client uses a static Diffie-Hellman
public key.
Servers can have long-lived RTMFP instances, so they SHOULD use
ephemeral Diffie-Hellman public keys for forward secrecy. This
allows server peer IDs to be forged; however, clients do not connect
to servers by peer ID, so this is irrelevant.
When a client connects to a server, the client will accept the
response of any endpoint claiming to be "a server". It is assumed
that an attacker that can passively observe traffic on a network
segment can also inject its own packets with any source or
destination and any payload. An attacker can trick a client into
connecting to a rogue server or man-in-the-middle, either by
observing Initiator Hello packets from the client and responding
earliest with a matching Responder Hello or by using tricks such as
DNS spoofing or poisoning to direct a client to connect directly to
the rogue. A TCP-based transport would be vulnerable to similar
attacks. Since there is no PKI, this profile gives no guarantee that
the client has actually connected to the desired server, versus a
rogue or man-in-the-middle. In circumstances where assurance is
required that the connection is directly to the desired server, the
client can use the Session Nonces (Section 4.6.5) to challenge the
server, for example, over a different channel having acceptable
security properties (such as an HTTPS) to transitively establish the
server's identity and verify that the end-to-end communication is
private and authentic.
When session sequence numbers (Section 4.7.3.3) are not used, it is
possible for an attacker to use traffic analysis techniques and
record encrypted packets containing the start of a new flow, and
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RFC 7425 Adobe RTMFP for Flash Communication December 2014
later to replay those packets after the flow has closed, which can
look to the receiver like a brand new flow. In circumstances where
this can be detrimental, session sequence numbers SHOULD be used.
Replay of packets for existing flows is not detrimental as the
receiver detects and discards duplicate flow sequence numbers, and
flow sequence numbers do not wrap or otherwise repeat.
Packet encryption uses CBC with the same (null) initialization vector
for each packet. This can reveal to an observer whether two packets
contain identical plaintext. However, the maximum-length RTMFP
common header and User Data or Data Acknowledgement header, including
flow sequence number, always fit within the first 16-byte cipher
block, so each initial cipher block for most packets will already be
unique even if timestamps are suppressed. Sending identical messages
in a flow uses unique flow sequence numbers, so cipher blocks will be
unique in this case. Keepalive pings and retransmission of lost data
can result in identical cipher blocks; however, traffic analysis can
also reveal likely keepalives or retransmissions, and retransmission
only occurs as a result of observable network loss, so this is
usually irrelevant. In circumstances where any identical cipher
block is unacceptable, session sequence numbers SHOULD be used as
they guarantee each initial cipher block will be unique.
Packet verification can use a 16-bit simple checksum
(Section 4.7.3.1). The checksum is inside the encrypted packet, so
for external packet modifications the checksum is equivalent to a
16-bit cryptographic digest. In circumstances where this is
insufficient, HMAC verification (Section 4.7.3.2) SHOULD be used.
8. References
8.1. Normative References
[AES] National Institute of Standards and Technology, "Advanced
Encryption Standard (AES)", FIPS PUB 197, November 2001,
<http://csrc.nist.gov/publications/fips/fips197/
fips-197.pdf>.
[AMF0] Adobe Systems Incorporated, "Action Message Format -- AMF
0", December 2007, <http://www.adobe.com/go/spec_amf0>.
[AMF3] Adobe Systems Incorporated, "Action Message Format -- AMF
3", January 2013, <http://www.adobe.com/go/spec_amf3>.
[CBC] Dworkin, M., "Recommendation for Block Cipher Modes of
Operation", NIST Special Publication 800-38A, December
2001, <http://csrc.nist.gov/publications/nistpubs/800-38a/
sp800-38a.pdf>.
Thornburgh Informational [Page 47]
RFC 7425 Adobe RTMFP for Flash Communication December 2014
[DH] Diffie, W. and M. Hellman, "New Directions in
Cryptography", IEEE Transactions on Information Theory, V.
IT-22, n. 6, June 1977.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February
1997, <http://www.rfc-editor.org/info/rfc2104>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3526] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
Diffie-Hellman groups for Internet Key Exchange (IKE)",
RFC 3526, May 2003,
<http://www.rfc-editor.org/info/rfc3526>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003,
<http://www.rfc-editor.org/info/rfc3629>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005,
<http://www.rfc-editor.org/info/rfc3986>.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.
[RFC6234] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234, May 2011,
<http://www.rfc-editor.org/info/rfc6234>.
[RFC7016] Thornburgh, M., "Adobe's Secure Real-Time Media Flow
Protocol", RFC 7016, November 2013,
<http://www.rfc-editor.org/info/rfc7016>.
[RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
Kivinen, "Internet Key Exchange Protocol Version 2
(IKEv2)", STD 79, RFC 7296, October 2014,
<http://www.rfc-editor.org/info/rfc7296>.
[RTMP] Adobe Systems Incorporated, "Real-Time Messaging Protocol
(RTMP) specification", December 2012,
<http://www.adobe.com/go/spec_rtmp>.
Thornburgh Informational [Page 48]
RFC 7425 Adobe RTMFP for Flash Communication December 2014
[SHA256] National Institute of Standards and Technology, "Secure
Hash Standard", FIPS PUB 180-4, March 2012,
<http://csrc.nist.gov/publications/fips/fips180-4/
fips-180-4.pdf>.
8.2. Informative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987,
<http://www.rfc-editor.org/info/rfc1035>.
[RFC1071] Braden, R., Borman, D., Partridge, C., and W. Plummer,
"Computing the Internet checksum", RFC 1071, September
1988, <http://www.rfc-editor.org/info/rfc1071>.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302, December
2005, <http://www.rfc-editor.org/info/rfc4302>.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
4303, December 2005,
<http://www.rfc-editor.org/info/rfc4303>.
[RFC6479] Zhang, X. and T. Tsou, "IPsec Anti-Replay Algorithm
without Bit Shifting", RFC 6479, January 2012,
<http://www.rfc-editor.org/info/rfc6479>.
Acknowledgements
Special thanks go to Glenn Eguchi, Matthew Kaufman, and Adam Lane for
their contributions to the design of this profile.
Thanks to Philipp Hancke, Kevin Igoe, Paul Kyzivat, and Milos
Trboljevac for their detailed reviews of this memo.
Author's Address
Michael C. Thornburgh
Adobe Systems Incorporated
345 Park Avenue
San Jose, CA 95110-2704
United States
Phone: +1 408 536 6000
EMail: mthornbu@adobe.com
URI: http://www.adobe.com/
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