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This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
To view the entire list of Internet-Drafts Shadow Directories, see http://www.ietf.org/shadow.html.
This Internet-Draft will expire on May 21, 2001.
Copyright (C) The Internet Society (2000). All Rights Reserved.
The local Message Bus (Mbus) is a simple message-oriented coordination infrastructure for group communication within groups of co-located application entities. The Message Bus comprises three logically distinct parts: a message transport infrastructure, a structured message hierarchy, and a general purpose addressing scheme. This document deals with message addressing, transport, and security issues and defines the message syntax for the Mbus. It does not define application oriented semantics and procedures for using the message bus. Application specific command sets and procedures for applications using the Mbus are expected to be defined in follow-up documents.
This document is a product of the Multiparty Multimedia Session Control (MMUSIC) working group of the Internet Engineering Task Force. Comments are solicited and should be addressed to the working group's mailing list at confctrl@isi.edu and/or the authors.
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The implementation of multiparty multimedia conferencing systems is one example where a simple coordination infrastructure can be useful: In a variety of conferencing scenarios, a local communication channel can provide conference-related information exchange between co- located but otherwise independent application entities, for example those taking part in application sessions that belong to the same conference. In loosely coupled conferences such a mechanism allows for coordination of applications entities to e.g. implement synchronization between media streams or to configure entities without user interaction. It can also be used to implement tightly coupled conferences enabling a conference controller to enforce conference wide control within a end system.
Three components constitute the message bus: the low level message passing mechanisms, a command syntax and naming hierarchy, and the addressing scheme.
The purpose of this document is to define the characteristics of the lower level Mbus message passing mechanism which is common to all Mbus implementations. This includes the specification of
In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in RFC 2119[1] and indicate requirement levels for compliant Mbus implementations.
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Local coordination involves a widely varying number of entities: some messages (such as membership information, floor control notifications, dissemination of conference state changes, etc.) may need to be destined for all local application entities. Messages may also be targeted at a certain application class (e.g. all whiteboards or all audio tools) or agent type (e.g. all user interfaces rather than all media engines). Or there may be any (application- or message- specific) subgrouping defining the intended recipients, e.g. messages related to media synchronization. Finally, there will be messages that are directed to a single entity, for example, specific configuration settings that a conference controller sends to a application entity or query-response exchanges between any local server and its clients.
The Mbus concept as presented here satisfies these different communication models by defining different message transport mechanisms (defined in Transport) and by providing a flexible addressing scheme (defined in Addressing).
Furthermore, Mbus messages exchanged between application entities may have different reliability requirements (which are typically derived from their semantics). Some messages will have a rather informational character conveying ephemeral state information (which is refreshed/updated periodically), such as the volume meter level of an audio receiver entity to be displayed by its user interface agent. Certain Mbus messages (such as queries for parameters or queries to local servers) may require a response from the peer(s) thereby providing an explicit acknowledgment at the semantic level on top of the Mbus. Other messages will modify the application or conference state and hence it is crucial that they do not get lost. The latter type of message has to be delivered reliably to the recipient, whereas message of the first type do not require reliability mechanisms at the Mbus transport layer. For messages confirmed at the application layer it is up to the discretion of the application whether or not to use a reliable transport underneath.
In some cases, application entities will want to tailor the degree of reliability to their needs, others will want to rely on the underlying transport to ensure delivery of the messages -- and this may be different for each Mbus message. The Mbus message passing mechanism specified in this document provides a maximum of flexibility by providing reliable transmission achieved through transport-layer acknowledgments (in case of point-to-point communications only) as well as unreliable message passing (for unicast, local multicast, and local broadcast). We address this topic in Addressing.
Finally, accidental or malicious disturbance of Mbus communications through messages originated by applications from other users needs to be prevented. Accidental reception of Mbus messages from other users may occur if either two users share the same host for conferencing or are using end systems spread across the same network link: in either case, the used Mbus multicast address and the port number may be identical leading to reception of the other party's Mbus messages in addition to a user's own ones. Malicious disturbance may happen because of applications multicasting (e.g. at a global scope) or unicasting Mbus messages. To eliminate the possibility of receiving unwanted Mbus messages, the Mbus protocol contains message digests for authentication. Furthermore, the Mbus allows for encryption to ensure privacy and thus enable using the Mbus for local key distribution and other functions potentially sensitive to eavesdropping. This document defines the framework for configuring Mbus applications with regard to security parameters in Mbus Configuration.
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This section contains some definitions of common ABNF [14] syntax elements that are later referenced by other definitions in this document:
base64 = base64_terminal /
( 1*(4base64_CHAR) [base64_terminal] )
base64_char = UPALPHA / LOALPHA / DIGIT / "+" / "/"
;; Case-sensitive
base64_terminal = (2base64_char "==") / (3base64_char "=")
UPALPHA = %x41-5A ;; Uppercase: A-Z
LOALPHA = %x61-7A ;; Lowercase: a-z
ALPHA = %x41-5A / %x61-7A ; A-Z / a-z
CHAR = %x01-7F
; any 7-bit US-ASCII character,
excluding NUL
CR = %x0D
; carriage return
CRLF = CR LF
; Internet standard newline
DIGIT = %x30-39
; 0-9
DQUOTE = %x22
; " (Double Quote)
HTAB = %x09
; horizontal tab
LF = %x0A
; linefeed
LWSP = *(WSP / CRLF WSP)
; linear white space (past newline)
SP = %x20
; space
WSP = SP / HTAB
; white space
Taken from RFC 2234 [14] and RFC 2554 [15].
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A Mbus message comprises a header and a body. The header is used to indicate how and where a message should be delivered, the body provides information and commands to the destination entity. The following information is included in the header:
The header is followed by the message body which contains zero or more commands to be delivered to the destination entity. The syntax for a complete message is given in Message Syntax.The MsgDigest is a Base64-encoded (see RFC1521[6]) calculated hash value of the entire message (starting from the ProtocolID field) as described in Mbus Security and Mbus Configuration.
A fixed ProtocolID field identifies the version of the message bus protocol used. The protocol defined in this document is "mbus/1.0" (case-sensitive).
A sequence number (SeqNum) is contained in each message. The first message sent by a source SHOULD have SeqNum equal to zero, and it MUST increment by one for each message sent by that source. A single sequence number is used for all messages from a source, irrespective of the intended recipients and the reliability mode selected. SeqNums are decimal numbers in ASCII representation.
The TimeStamp field is also contained in each message and SHOULD contain a decimal number representing the time at message construction in milliseconds since 00:00:00, UTC, January 1, 1970.
A MessageType field indicates the kind of message being sent. The value "R" indicates that the message is to be transmitted reliably and MUST be acknowledged by the recipient, "U" indicates an unreliable message which MUST NOT be acknowledged.
The SrcAddr field identifies the sender of a message. This MUST be a complete address, with all address elements specified. The addressing scheme is described in Addressing.
The DestAddr field identifies the intended recipient(s) of the message. This field MAY contain wildcards by omitting address element and hence address any number (including zero) of application entities. The addressing scheme is described in Addressing.
The AckList field comprises a list of SeqNums for which this message is an acknowledgment. See Reliability for details.
If multiple commands are contained within the same Mbus message payload, they MUST to be delivered to the Mbus application in the same sequence in which they appear in the message payload.
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Each entity on the message bus SHOULD respond to messages sent to one (or more) addresses. Addresses are sequences of address elements that are tag/value pairs. The tag and the value are separated by a colon and tag/value pairs are separated by whitespace, like this:
(tag:value tag:value ...)
The formal ABNF syntax definition for Mbus addresses and their elements is as follows:
mbus_address = "(" *address_element ")"
address_element = *WSP address_tag ":" address_value *WSP
address_tag = 1*32(ALPHA)
address_value = 1*64(%x21-7F)
; any 7-bit US-ASCII character
; excluding white space
; and control characters
Note that this and other ABNF definitions in this document use the core rules defined in Common Formal Syntax Rules.
Each entity has a fixed sequence of address elements constituting its address and MUST only process messages sent to addresses that either match all elements or consist of a subset of its own address elements. Each element value in this subset must match the correspoding value of the receiver's address element value. The order of address elements in an address sequence is not relevant. For example, an entity with an address of:
(conf:test media:audio module:engine app:rat id:4711-1@134.102.218.45)will process messages sent to
(media:audio module:engine)and
(module:engine)but must ignore messages sent to
(conf:test media:audio module:engine app:rat id:123-4@134.102.218.45 foo:bar)and
(foo:bar)
A message that should be processed by all entities requires an empty set of address elements.
Each Mbus entity MUST provide one mandatory address element that allows to identify the entity. The element name is "id" and the value MUST be be composed of the following components:
id-element = "id:" id-value id-value = entity-id "@" host-id entity-id = 1*10DIGIT "-" 1*5DIGIT host-id = (IPv4address / IPv6address)Please refer to [3] for productions of IPv4address and IPv6address.
An example for an id element:
id:4711-99@134.102.218.45
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All messages MUST use the UTF-8 character encoding. Note that US ASCII is a subset of UTF-8 and requires no additional encoding, and that a message encoded with UTF-8 will not contain zero bytes.
Each Message MAY be encrypted using a secret key algorithm as defined in Mbus Security.
A message starts with the header. The first field in the header is the message digest calculated using a keyed hash algorithm as described in Mbus Security followed by CRLF. The other fields in the header are separated by white space characters, and followed by CRLF. The format of the header is as follows:
msg_header = MsgDigest CRLF "mbus/1.0" 1*WSP SeqNum 1*WSP TimeStamp 1*WSP MessageType 1*WSP SrcAddr 1*WSP DestAddr 1*WSP AckList
The header fields are explained in Message Format. Here are the ABNF syntax definitions for the header fields:
MsgDigest = base64
SeqNum = 1*10DIGIT
TimeStamp = 1*10DIGIT
MessageType = "R" / "U"
ScrAddr = mbus_address
DestAddr = mbus_address
AckList = "(" *(1*DIGIT)) ")"
See Addressing for a
definition of "mbus_address".
The syntax definition of a complete message is as follows:
mbus_message = msg_header *1(CRLF msg_payload) msg_payload = mbus_command *(CRLF mbus_command)The definition of production rules for Mbus command is given below.
The header is followed by zero, or more, commands to be delivered to the application(s) indicated by the DestAddr field. Each message comprises a command followed by a list of zero, or more, parameters, and is followed by a newline.
command ( parameter parameter ... )
Syntactically, the command name MUST be a `symbol' as defined in the following table. The parameters MAY be any data type drawn from the following table:
val = Integer / Float / String / List / Symbol / Data
Integer = *1"-" 1*DIGIT
Float = *1"-" 1*DIGIT "." 1*DIGIT
String = DQUOTE *CHAR DQUOTE
; see below for escape characters
List = "(" *(val *(1*WSP val)) ")"
Symbol = ALPHA *(ALPHA / DIGIT / "_" / "-" / ".")
Data = "<" *base64 ">"
Boolean values are encoded as an integer, with the value of zero representing false, and non-zero representing true.
String parameters in the payload MUST be enclosed in the double quote (") character. Within strings, the escape character is the backslash (\), and the following escape sequences are defined:
+----------------+-----------+ |Escape Sequence | Meaning | +----------------+-----------+ | \\ | \ | | \" | " | | \n | newline | +----------------+-----------+
List parameters do not have to be homogeneous lists, i.e. they can contain parameters of varying types.
Opaque data is represented as Base64-encoded (see RFC1521[6]) character strings surrounded by "< " and "> "
The ABNF syntax definition for Mbus commands is as follows:
mbus_command = command_name arglist
command_name = ALPHA *(ALPHA / DIGIT / "_" / ".")
arglist = "(" *(*WSP parameter *WSP) ")"
parameter = Integer / Float / String / List
Symbol / Data
Command names SHOULD be constructed using hierarchical names to group conceptually related commands under a common hierarchy. The delimiter between names in the hierarchy is "." (dot). Applications profiles MUST NOT define commands starting with "mbus.".
The Mbus addressing scheme defined in Addressing provides for specifying incomplete addresses by omitting certain elements of an address element list, enabling entities to send commands to a group of Mbus entities. Therefore all command names SHOULD be unambiguous in a way that it is possible to interpret or ignore them without considering the message's address.
A set of commands within a certain hierarchy that MUST be understood by every entity is defined in Messages.
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All messages are transmitted as UDP messages with two ways of sending messages being possible:
Note that "unicast", "multicast" and "broadcast" mean IP-Unicast, IP-Multicast and IP-Broadcast respectively. It is possible to send a Mbus message that is addressed to a single entity using IP-multicast.
Since messages are transmitted in UDP datagrams, a maximum size of 64 KBytes MUST NOT be exceeded. It is RECOMMENDED that applications using a non host-local scope do not exceed a message size of the network's MTU.
Local multicast (host-local or link-local, see Mbus configuration), uses a scope relative multicast address of the administratively scoped multicast space as described in RFC 2365[11]. This scope relativ multicast address has yet to be assigned (see IANA Considerations). Implementations MUST NOT use scopes larger than the link-local scope.
There will also be a fixed, registered port number that all Mbus entities MUST use. Until the address and port number are assigned the values given in IANA Considerations SHOULD be used. In situations where multicast is not available, broadcast MAY be used instead. In these cases an IP broadcast address for the connected network SHOULD be used for sending. Broadcast MUST NOT be used in situations where multicast is available and supported by all systems participating in an Mbus session.
If a single application system is distributed across several co-located hosts, link local scope SHOULD be used for multicasting Mbus messages that potentially have recipients on the other hosts. The Mbus protocol is not intended (and hence deliberately not designed) for communication between hosts not on the same link.
Directed unicast (via UDP) to the port of a specific application is an alternative transport class. Directed unicast is an OPTIONAL optimization and MAY be used by Mbus implementations for delivering messages addressed at a single application entity only -- the address of which the Mbus implementation has learned from other message exchanges before. Note that the DestAddr field of such messages MUST still be filled in properly. Every Mbus entity SHOULD use a unique endpoint address for every message it sends to the Mbus multicast group or to individual receiving entities. A unique endpoint address is a tuple consisting of the entity's IP address and a port number, where the port number is different from the standard Mbus port number (yet to be assigned, see IANA Considerations).
Messages MUST only be sent via unicast if the Mbus target address is unique and if the sending entity can verify that the receiving entity uses a unique endpoint address. The latter can be verified by considering the last message received from that entity. (Note that several Mbus entities, say within the same process, may share a common transport address; in this case, the contents of the destination address field is used to further dispatch the message. Given the definition of "unique endpoint address" above the use of a shared endpoint address and a dispatcher still allows other Mbus entities to send unicast messages to one of the entities that share the endpoint address. So this can be considered an implementation detail.)
Messages with an empty target address list MUST always be sent to all Mbus entities (via multicast if available).
The following algorithm can be used by sending entities to determine whether a Mbus address is unique considering the current set of Mbus entities:
let ta=the target address;
iterate through the set of all
currently known Mbus addresses {
let ti=the address in each iteration;
count the addresses for which
the predicate isSubsetOf(ta,ti) yields true;
}
If the count of matching addresses is exactly 1 the address is unique. The following algorithm can be used for the predicate isSubsetOf, that checks whether the second message matches the first according to the rules specified in Addressing. (A match means that a receiving entity that uses the second Mbus address must also process received messages with the first address as a target address.)
isSubsetOf(addr a1,a2) yields true, iff every address element of a1 is contained in a2's address element list
An address element is contained in an address element list if the list contains an element that is equal to the first address element. An address element is considered equal to another address element if it provides the same values for both of the two address element fields (key and value).
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While most messages are expected to be sent using unreliable transport, it may be necessary to deliver some messages reliably. Reliability can be selected on a per message basis by means of the MessageType field. Reliable delivery is supported for messages with a single recipient only; i.e., all components of the DestAddr field have to be specified. An entity can thus only send reliable messages to known addresses, i.e. it can only send reliable messages to entities that have announced their existence on the Mbus (e.g. by means of mbus.hello() messages (mbus.hello)). A sending entity MUST NOT send a message reliably if the target address is not unique. (See Transport for the specification of an algorithm to determine whether an address is unique.) A receiving entity MUST only process and acknowledge a reliable message if the destination address exactly matches its own source address (the destination address MUST NOT be a subset of the source address).
Disallowing reliable message delivery for messages sent to multi- ple destinations is motivated by simplicity of the implementation as well as the protocol. Although ACK implosions are not really an issue and losses are exptected to be rare, achieving reliability for such messages would require full knowledge of the membership for each subgroup which is deemed too much effort. The desired effect can be achieved by application layers by sending individual reliable messages to each fully qualified destination address, if the membership information for the Mbus session is available.
Each message is tagged with a message sequence number. If the MessageType is "R", the sender expects an acknowledgment from the recipient within a short period of time. If the acknowledgment is not received within this interval, the sender SHOULD retransmit the message (with the same message sequence number), increase the timeout, and restart the timer. Messages MUST be retransmitted a small number of times (see below) before the recipient is considered to have failed. If the message is not delivered successfully, the sending application is notified. In this case, it is up to this application to determine the specific actions (if any) to be taken.
Reliable messages are acknowledged by adding their SeqNum to the AckList field of a message sent to the originator of the reliable message. Multiple acknowledgments MAY be sent in a single message. It is possible to either piggy-back the AckList onto another message sent to the same destination, or to send a dedicated acknowledgment message, with no commands in the message payload part.
The precise procedures are as follows:
- Sender:
- A sender A of a reliable message M to receiver B SHOULD transmit the message via IP-multicast or via IP-unicast, keep a copy of M, initialize a retransmission counter N to '1', and start a retransmission timer T (initialized to T_r). If an acknowledgment is received from B, timer T MUST be cancelled and the copy of M is discarded. If T expires, the message M SHOULD be retransmitted, the counter N SHOULD be incremented by one, and the timer SHOULD be restarted (set to N*T_r). If N exceeds the retransmission threshold N_r, the transmission is assumed to have failed, further retransmission attempts MUST NOT be undertaken, the copy of M SHOULD be discarded, and the sending application SHOULD be notified.
- Receiver:
- A receiver B of a reliable message from a sender A SHOULD acknowledge reception of the message within a time period T_c < T_r. This MAY be done by means of a dedicated acknowledgment message or by piggy-backing the acknowledgment on another message addressed only to A.
- Receiver optimization:
- In a simple implementation, B may choose to immediately send a dedicated acknowledgment message. However, for efficiency, it could add the SeqNum of the received message to a sender-specific list of acknowledgments; if the added SeqNum is the first acknowledgment in the list, B SHOULD start an acknowledgment timer TA (initialized to T_c). When the timer expires, B SHOULD create a dedicated acknowledgment message and send it to A. If B is to transmit another Mbus message addressed only to A, it should piggy-back the acknowledgments onto this message and cancel TA. In either case, B should store a copy of the acknowledgment list as a single entry in the per- sender copy list, keep this entry for a period T_k, and empty the acknowledgment list. In case any of the messages kept in an entry of the copy list is received again from A, the entire acknowledgment list stored in this entry is scheduled for (re-)transmission following the above rules.
- Constants and Algorithms:
- The following constants and algorithms SHOULD be used by implementations:
T_r=100ms
N_r=3
T_c=70ms
T_k=((N_r)*(N_r+1)/2)*T_r
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Before Mbus entities can communicate with one another, they need to mutually find out about their existence. After this bootstrap procedure that each Mbus entity goes through all other entities listening to the same Mbus know about the newcomer and the newcomer has learned about all the other entities. Furthermore entities need to be able to to notice the failure (or leaving) of other entities.
Any Mbus entity MUST announce its presence (on the Mbus) after starting up. This is to be done repeatedly throughout its lifetime to address the issues of startup sequence: Entities should always become aware of other entities independent of the order of starting.
Each Mbus entity MUST maintain the number of Mbus session members and continously update this number according to any observed changes. The mechanisms of how the existence and the leaving of other entities can be detected are dedicated Mbus messages for entity awareness: mbus.hello (mbus.hello) and mbus.bye (mbus.bye). Each Mbus protocol implementation MUST periodically send mbus.hello messages that are used by other entities to monitor the existence of that entity. If an entity has not received mbus.hello messages for a certain time (see Calculating the Timeout for Hello Messages) from an entity the respective entity is considered to have left the Mbus and MUST be excluded from the set of currently known entities. Upon the reception of a mbus.bye messages the respective entity is considered to have left the Mbus as well and MUST be excluded from the set of currently known entities immediately.
Each Mbus entity MUST send hello messages after startup to the Mbus. After transmission of the hello message, it shall start a timer after the expiration of which the next hello message is to be transmitted. Transmission of hello messages MUST NOT be stopped unless the entity detaches from the Mbus. The interval for sending hello messages is depending on the current number of entities in a Mbus group and can thus change dynamically in order to avoid congestion due to many entities sending hello messages at a constant high rate.
Hello Message Transmission Interval specifies the calculation of hello message intervals that MUST be used by protocol implementations. Using the values that are calculated for obtaining the current hello message timer, the timeout for received hello messages is calculated in Calculating the Timeout for Hello Messages. Messages specifies the command synopsis for the corresponding Mbus messages.
Since Mbus sessions may vary in size concerning the number of entities care must be taken to allow the Mbus protocol to scale well over different numbers of entities automatically. The average rate at which hello messages are received would increase linearly to the number of entities in a session if the sending interval was set to a fixed value. Given a interval of 1 second this would mean that an entity taking part in an Mbus session with n entities would receive n hello messages per second. Assuming all entities resided on one host this would lead to n*n messages that have to be processed per second -- which is obviously not a viable solution for larger groups. It is therefore necessary to deploy dynamically adapted hello message intervals taking varying numbers of entities into account. In the following we specify an algorithm that MUST be used by implementations to calculate the interval for hello messages considering the observed number of Mbus entities.
The algorithm features the following characteristics:
The following names for values are used in the calculation specified below (all time values in milliseconds):
- hello_p:
- The last time a hello message has been sent by a Mbus entity.
- hello_now:
- The current time
- hello_d:
- The deterministic calculated interval between hello messages.
- hello_e:
- The effective (randomized) interval between hello messages.
- hello_n:
- The time for the next scheduled transmission of a hello message.
- entities_p:
- The numbers of entities at the time hello_n has been last recomputed.
- entities:
- The number of currently known entities.
The interval between hello messages MUST be calculated as follows:
The number of currently known entities is multiplied by c_hello_factor, yielding the interval between hello messages in milliseconds. This is the deterministic calculated interval, denominated hello_d. The minimum value for hello_d is c_hello_min. Thus hello_d=max(c_hello_min,c_hello_factor * entities). Awareness of other Entities provides a specification of how to obtain the number of currently known entities. Constants provides values for the constants c_hello_factor and c_hello_min.
The effective interval hello_e that is to be used by individual entities is calculated by multiplying hello_d with a randomly chosen number between c_hello_dither_min and c_hello_dither_max (see Constants).
hello_n, the time for the next hello message in milliseconds is set to hello_e + hello_now.
Upon joining a session a protocol implementation sets hello_p, hello_now to 0 and entities, entities_p to 1 (the current Mbus entity itself) and then calculates the time for the next hello message as specified in Calculating the Interval for Hello Messages. The next hello message is scheduled for transmission at hello_n.
When the existence of a new entity is observed by a protocol implementation the number of currently known entities is updated. No further action concerning the calculation of the hello message interval is required. The reconsideration of the timer interval takes place when the current timer for the next hello message expires (see Expiration of hello timers).
Upon realizing that an entity has left the Mbus the number of currently known entities is updated and the following algorithm should be used to reconsider the timer interval for hello messages:
When the hello message timer expires, the protocol implementation MUST perform the following operations:
The hello interval hello_e is computed as specified in Calculating the Interval for Hello Messages.
If
- hello_e + hello_p is less than or equal to hello_now, a hello message is transmitted. hello_p is set to hello_now, hello_e is calculated again as specified in Calculating the Interval for Hello Messages and hello_n is set to hello_e + hello_now.
- else if hello_e + hello_p is greater than hello_now, hello_n is set to hello_e + hello_p. A new timer for the next hello message is started to expire at hello_n. No hello message is transmitted.
entities_p is set to entities.
Whenever an Mbus entity has not heard for a time span of c_hello_dead*(hello_d*c_hello_dither_max) milliseconds from another Mbus entity it may consider this entity to have failed (or have quit silently). The number of the currently known entities MUST be updated accordingly. Note that no need for any further action is necessarily implied from this observation.
Calculating the Interval for Hello Messages specifies how to obtain hello_d. Constants defines values for the constants c_hello_dead and c_hello_dither_max.
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This section defines some basic application independent messages that MUST be understood by all implementations. This specification does not contain application specific messages which are to be defined outside of the basic Mbus protocol specification.
An Mbus entity should be able to indicate that it is waiting for a certain event to happen (similar to a P() operation on a semaphore but without creating external state somewhere). In conjunction with this, an Mbus entity should be capable of indicating to another entity that this condition is now satisfied (similar to a semaphore's V() operation).
An appropriate commend set to implement the aforementioned concepts is presented in the following sections.
Syntax:
mbus.hello(parameters...)Parameters: see below
HELLO messages MUST be sent unreliably to all Mbus entities.
Each Mbus entity learns about other Mbus entities by observing their HELLO messages and tracking the sender address of each message and can thus calculate the current number of entities.
HELLO messages MUST be sent periodically in dynamically calculated intervals as specified in Awareness of other Entities.
Upon startup the first HELLO message MUST be sent after a delay hello_delay, where hello_delay be a randomly chosen number between 0 and c_hello_min (see Constants).
Syntax:
Parameters: - none -
An Mbus entity that is about to terminate (or "detach" from the Mbus) SHOULD announce this by transmitting a BYE message.
The BYE message MUST be sent unreliably to all entities.
Syntax:
Parameters: - none -
mbus.ping can be used to solicit other entities to signal their existence by replying with a mbus.hello message. Each protocol implementation MUST understand mbus.ping and reply with a mbus.hello message. The reply hello message MUST be delayed for hello_delay milliseconds, where hello_delay be a randomly chosen number between 0 and c_hello_min (see Constants).
As specified in mbus.hello hello messages MUST be sent unreliably to all Mbus entities. This is also the case for replies to ping messages. An entity that replies to mbus.ping with mbus.hello should stop any outstanding timers for hello messages after sending the hello message and schedule a new timer event for the subsequent hello message. (Note that using the variables and the algorithms of Calculating the Interval for Hello Messages this can be achieved by setting hello_p to hello_now.)
mbus.ping allows a new entity to quickly check for other entities without having to wait for the regular individual hello messages. By specifying a target address the new entity can restrict the solicitation for hello messages to a subset of entities it is interested in.
Syntax:
mbus.quit()Parameters: - none -
The QUIT message is used to request other entities to terminate themselves (and detach from the Mbus). Whether this request is honoured by receiving entities or not is up to the discretion of the application.
The QUIT message can be multicast or sent reliably via unicast to a single Mbus entity or a group of entities.
Syntax:
mbus.waiting(condition)Parameters:
symbol condition
The condition parameter is used to indicate that the entity transmitting this message is waiting for a particular event to occur.
The WAITING messages may be broadcast to all Mbus entities, multicast to an arbitrary subgroup, or unicast to a particular peer. Transmission of the WAITING message MUST be unreliable and hence has to be repeated at an application-defined interval (until the condition is satisfied).
If an application wants to indicate that it is waiting for several conditions to be met, several WAITING messages are sent (possibly included in the same Mbus payload). Note that HELLO and WAITING messages may also be transmitted in a single Mbus payload.
Syntax:
mbus.go(condition)Parameters:
symbol condition
This parameter specifies which condition is met.
The GO message is sent by an Mbus entity to "unblock" another Mbus entity -- the latter of which has indicated that it is waiting for a certain condition to be met. Only a single condition can be specified per GO message. If several conditions are satisfied simultaneously multiple GO messages MAY be combined in a single Mbus payload.
The GO message MUST be sent reliably via unicast to the Mbus entity to unblock.
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The following values for timers and counters mentioned in this document SHOULD be used by implementations:
+-------------------+------------------------+--------------+ |Timer / Counter | Value | Unit | +-------------------+------------------------+--------------+ |c_hello_factor | 200 | - | |c_hello_min | 1000 | milliseconds | |c_hello_dither_min | 0.9 | - | |c_hello_dither_max | 1.1 | - | |c_hello_dead | 5 | - | +-------------------+------------------------+--------------+
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In order to prevent accidental or malicious disturbance of Mbus communications through messages originated by applications from other users, message authentication is deployed (Message Authentication). For each message a digest is calculated based on the value of a shared secret key value. Receivers of messages can check if the sender belongs to the same Mbus security domain by re-calculating the digest and comparing it to the received value. Only if both values are equal the messages must be processed further. In order to allow different simultaneous Mbus sessions at a given scope and to compensate defective implementations of host local multicast ([20]) message authentication MUST be provided by conforming implementations.
Privacy of Mbus message transport can be achieved by optionally using symmetric encryption methods (Encryption). Each message can be encrypted using an additional shared secret key and a symmetric encryption algorithm. Encryption is OPTIONAL for applications, i.e. it is allowed to configure an Mbus domain not to use encryption. But conforming implementations MUST provide the possibility to use message encryption (see below).
Message authentication and encryption can be parameterized by certain values, e.g. by the algorithms to apply or by the keys to use. These parameters (amongst others) are defined in an Mbus configuration entity that is accessible to all Mbus entities that participate in an Mbus session. In order to achieve interoperability conforming implementations SHOULD consider the given Mbus configuration entity. Mbus Configuration defines the mandatory and optional parameters as well as storage procedures for different platforms. Only in cases where none of the options for configuration entities mentioned in Mbus Configuration is applicable alternative methods of configuring Mbus protocol entities MAY be deployed.
Either MD5 [16] or SHA-1 [17] SHOULD be used for message authentication codes (MACs). An implementation MAY provide SHA-1, whereas MD5 MUST be implemented. To generate keyed hash values the algorithm described in RFC2104[4] MUST be applied with hash values truncated to 96 bits (12 bytes). The resulting hash values MUST be Base64 encoded (16 characters). The HMAC algorithm works with both, MD5 and SHA-1.
HMAC values, regardless of the algorithm, MUST therefore always consist of 16 Base64-encoded characters.
Hash keys MUST have a length of 96 bit (12 bytes), that are 16 Base64-encoded characters.
Either DES, 3DES (triple DES) or IDEA SHOULD be used for encryption. Encryption MAY be neglected for applications, e.g. in situations where license regulations, export or encryption laws would be offended otherwise. However, the implementation of DES is RECOMMENDED as a baseline. DES implementations MUST use the DES Cipher Block Chaining (CBC) mode. For algorithms requiring en/decryption data to be padded to certain boundaries octets with a value of 0 SHOULD be used for padding characters. The padding characters MUST be appended after calculating the message digest when encoding and MUST be erased before recalculating the message digest when decoding. IDEA uses 128-bit keys (24 Base64-encoded characters). DES keys (56 bits) MUST be encoded as 8 octets as described in RFC1423[13], resulting in 12 Base64-encoded characters.
The mandatory subset of algorithms that MUST be provided by implementations is DES and MD5.
See Mbus Configuration for a specification of notations for Base64-strings.
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An implementation MUST be configurable by the following parameters:
The upper parameters are mandatory and MUST be present in every Mbus configuration entity.Configuration version
The version number of the given configuration entity. Version numbers allow implementations to check if they can process the entries of a given configuration entity. Version number are integer values. The version number for the version specified here is 1.
Encryption key
The secret key used for message encryption.
Hash key
The hash key used for message authentication.
Scope
The Internet scope to be used for sent messages.
The following parameters are optional. When they are present they MUST be honoured but when they are not present implementations SHOULD fall back to the predefined default values (as defined in Transport):
Address
The non-standard multicast/broadcast address to use for message transport.
Port
The non-standard port number to use for message transport.
Two distinct facilities for parameter storage are considered: For Unix-like systems a configuration file SHOULD be used and for Windows-95/98/NT/2000 systems a set of registry entries is defined that SHOULD be used.
The syntax of the values for the respective parameter entries remains the same for both configuration facilities. The following defines a set of ABNF (see RFC2234[14]) productions that are later referenced for the definitions for the configuration file syntax and registry entries:
algo-id = "NOENCR" / "DES" / "3DES" / "IDEA" /
"HMAC-MD5-96" / "HMAC-SHA1-96"
scope = "HOSTLOCAL" / "LINKLOCAL"
key = base64
version_number = 1*10DIGIT
key_value = "(" algo-id "," key ")"
address = IPv4address / IPv6address
port = 1*5DIGIT
Given the definition above, a key entry MUST be specified using this notation:
"("algo-id","base64string")"
algo-id is one of the character strings specified above. For algo-id==``NOENCR'' the other fields are ignored. The delimiting commas MUST always be present though.
A Base64 string consists of the characters defined in the Base64 char-set (see RFC1521[6]) including all eventual padding characters, i.e. the length of Base64-string is always a multiple of 4.
The scope parameter is used to configure an IP-Multicast scope and may be set to either "HOSTLOCAL" or "LINKLOCAL". Implementations SHOULD choose an appropriate IP-Multicast scope depending on the value of this parameter and construct an effective IP-Address using the Mbus scope relative address (see IANA Considerations). Implementations MUST NOT use scopes larger than link-local scope. Note that the IPv4 local scope is 239.255.0.0/16. Host-local scope in an IPv4 environment MUST be implemented by using a local scope address and an IP-Multicast-TTL of zero. Link-local scope in an IPv4 environment MUST be implemented by using a local scope address and an IP-Multicast-TTL of one. For IPv6 the link-local prefix is FF02, the node-local prefix is FF01.
The version_number parameter specifies a version number for the used configuration entity.
The file name for a Mbus configuration file is ".mbus" in the user's home-directory. If an environment variable called MBUS is defined implementations SHOULD interpret the value of this variable as a fully qualified file name that is to be used for the configuration file. Implementations MUST ensure that this file has appropriate file permissions that prevent other users to read or write it. The file MUST exist before a conference is initiated. Its contents MUST be UTF-8 encoded and MUST be structured as follows:
mbus-file = mbus-topic LF *(entry LF) mbus-topic = "[MBUS]" entry = 1*(version_info / hashkey_info / encryptionkey_info / scope_info / port_info / address_info) version_info = "CONFIG_VERSION=" version_number hashkey_info = "HASHKEY=" key_value encrkey_info = "ENCRYPTIONKEY=" key_value scope_info = "SCOPE=" scope port_info = "PORT=" port address_info = "ADDRESS=" addressPlease refer to [3] for productions of IPv4address.
The following entries are defined: CONFIG_VERSION, HASHKEY, ENCRYPTIONKEY, SCOPE, PORT, ADDRESS.
The entries CONFIG_VERSION, HASHKEY and ENCRYPTIONKEY are mandatory, they MUST be present in every Mbus configuration file. The order of entries is not significant.
An example Mbus configuration file:
[MBUS] CONFIG_VERSION=1 HASHKEY=(HMAC-MD5-96,MTIzMTU2MTg5MTEy) ENCRYPTIONKEY=(DES,MTIzMTU2MQ==) SCOPE=HOSTLOCAL ADDRESS=224.255.222.239 PORT=47000
For systems lacking the concept of a user's home-directory as a place for configuration files the suggested database for configuration settings (e.g. the Windows9x-, Windows NT-, Windows 2000-registry) SHOULD be used. The hierarchy for Mbus related registry entries is as follows:
HKEY_CURRENT_USER\Software\Mbone Applications\Mbus
The entries in this hierarchy section are:
+---------------+--------+----------------+ |Name | Type | ABNF production| +---------------+--------+----------------| |CONFIG_VERSION | DWORD | version_number | |HASHKEY | String | key_value | |ENCRYPTIONKEY | String | key_value | |SCOPE | String | scope | |ADDRESS | String | address | |PORT | DWORD | port | +---------------+--------+----------------+
The same syntax for key values as for the file based configuration facility MUST be used.
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The Mbus security mechanisms are specified in Security Model.
It should be noted that the Mbus transport specification defines a mandatory baseline set of algorithms that have to be supported by implementations. This baseline set does not neccessarily provide the best security due to the cryptographic weaknesses of the individual algorithms. For example, it has been stated in [4] that MD5 had been shown to be vulnerable to collision search attacks (although this was believed not to compromise the use of MD5 within HMAC generation). However, SHA-1 is usually considered to be the cryptographically stronger function ([18]).
Similar remarks can be made on the encryption functions. The base specification requires DES, an algorithm that has shown to be vulnerable to brute-force attacks ([18], [19]).
We do not consider the well-known weaknesses of the mentioned algorithms a problem:
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The IANA is requested to assign a port number and a scope relative multicast address. For the time being the tentative IPv4 multicast address 239.255.255.247 and the port number 47000 (decimal) SHOULD be used.
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| [1] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, BCP 14, March 1997. |
| [2] | Braden, R., "Requirements for Internet Hosts -- Communication Layers", RFC 1122, October 1989. |
| [3] | Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 2373, July 1998. |
| [4] | Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, February 1997. |
| 5 | Crocker, D.H., "STANDARD FOR THE FORMAT OF ARPA INTERNET TEXT MESSAGES", August 1982. |
| [6] | Borenstein, N. and N. Freed, "MIME (Multipurpose Internet Mail Extensions) Part One: Mechanisms for Specifying and Describing the Format of Internet Message Bodies", RFC 1521, September 1993. |
| [7] | Handley, M., Crowcroft, J., Bormann, C. and J. Ott, "The Internet Multimedia Conferencing Architecture", Internet Draft draft-ietf-mmusic-confarch-03.txt, July 2000. |
| [8] | Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobsen, "RTP: A Transport Protocol for Real-Time Applications", RFC 1889, January 1996. |
| [9] | Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg, "SIP: Session Initiation Protocol", RFC 2543, March 1999. |
| [10] | Handley, M. and V. Jacobsen, "SDP: Session Description Protocol", RFC 2327, April 1998. |
| [11] | Meyer, D., "Administratively Scoped IP Multicast", RFC 2365, July 1998. |
| [12] | Ott, J., Perkins, C. and D. Kutscher, "Requirements for Local Conference Control", Internet Draft draft-ietf-mmusic-mbus-req-00.txt, December 1999. |
| [13] | Balenson, D., "Privacy Enhancement for Internet Electronic Mail: Part III: Algorithms, Modes, and Identifiers", RFC 1423, February 1993. |
| [14] | Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, November 1997. |
| [15] | Myers, J., "SMTP Service Extension for Authentication", RFC 2554, March 1999. |
| [16] | Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April 1992. |
| [17] | U.S. DEPARTMENT OF COMMERCE/National Institute of Standards and Technology, "Secure Hash Standard", FIPS PUB 180-1, April 1995. |
| [18] | Schneier, B., "Applied Cryptography", Edition 2, Publisher John Wiley & Sons, Inc., 1996. |
| [19] | distributed.net, "Project DES", WWW http://www.distributed.net/des/, 1999. |
| [20] | Microsoft, "BUG: Winsock Sends IP Packets with TTL 0", WWW http://support.microsoft.com/support/kb/articles/Q138/2/68.asp, March 1999. |
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| Joerg Ott | |
| TZI, Universitaet Bremen | |
| Bibliothekstr. 1 | |
| Bremen 28359 | |
| Germany | |
| Phone: | +49.421.201-7028 |
| Fax: | +49.421.218-7000 |
| EMail: | jo@tzi.uni-bremen.de |
| Colin Perkins | |
| USC Information Sciences Institute | |
| 4350 N. Fairfax Drive #620 | |
| Arlington VA 22203 | |
| USA | |
| EMail: | csp@isi.edu |
| Dirk Kutscher | |
| TZI, Universitaet Bremen | |
| Bibliothekstr. 1 | |
| Bremen 28359 | |
| Germany | |
| Phone: | +49.421.218-7595 |
| Fax: | +49.421.218-7000 |
| EMail: | dku@tzi.uni-bremen.de |
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