[RFCs/IDs] [Plain Text]
EXPERIMENTAL
Network Working Group David Cheriton
Request for Comments: 1045 Stanford University
February 1988
VMTP: VERSATILE MESSAGE TRANSACTION PROTOCOL
Protocol Specification
STATUS OF THIS MEMO
This RFC describes a protocol proposed as a standard for the Internet
community. Comments are encouraged. Distribution of this document is
unlimited.
OVERVIEW
This memo specifies the Versatile Message Transaction Protocol (VMTP)
[Version 0.7 of 19-Feb-88], a transport protocol specifically designed
to support the transaction model of communication, as exemplified by
remote procedure call (RPC). The full function of VMTP, including
support for security, real-time, asynchronous message exchanges,
streaming, multicast and idempotency, provides a rich selection to the
VMTP user level. Subsettability allows the VMTP module for particular
clients and servers to be specialized and simplified to the services
actually required. Examples of such simple clients and servers include
PROM network bootload programs, network boot servers, data sensors and
simple controllers, to mention but a few examples.
RFC 1045 VMTP February 1988
Table of Contents
1. Introduction 1
1.1. Motivation 2
1.1.1. Poor RPC Performance 2
1.1.2. Weak Naming 3
1.1.3. Function Poor 3
1.2. Relation to Other Protocols 4
1.3. Document Overview 5
2. Protocol Overview 6
2.1. Entities, Processes and Principals 7
2.2. Entity Domains 9
2.3. Message Transactions 10
2.4. Request and Response Messages 11
2.5. Reliability 12
2.5.1. Transaction Identifiers 13
2.5.2. Checksum 14
2.5.3. Request and Response Acknowledgment 14
2.5.4. Retransmissions 15
2.5.5. Timeouts 15
2.5.6. Rate Control 18
2.6. Security 19
2.7. Multicast 21
2.8. Real-time Communication 22
2.9. Forwarded Message Transactions 24
2.10. VMTP Management 25
2.11. Streamed Message Transactions 25
2.12. Fault-Tolerant Applications 28
2.13. Packet Groups 29
2.14. Runs of Packet Groups 31
2.15. Byte Order 32
2.16. Minimal VMTP Implementation 33
2.17. Message vs. Procedural Request Handling 33
2.18. Bibliography 34
3. VMTP Packet Formats 37
3.1. Entity Identifier Format 37
3.2. Packet Fields 38
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3.3. Request Packet 45
3.4. Response Packet 47
4. Client Protocol Operation 49
4.1. Client State Record Fields 49
4.2. Client Protocol States 51
4.3. State Transition Diagrams 51
4.4. User Interface 52
4.5. Event Processing 53
4.6. Client User-invoked Events 54
4.6.1. Send 54
4.6.2. GetResponse 56
4.7. Packet Arrival 56
4.7.1. Response 58
4.8. Management Operations 61
4.8.1. HandleNoCSR 62
4.9. Timeouts 64
5. Server Protocol Operation 66
5.1. Remote Client State Record Fields 66
5.2. Remote Client Protocol States 66
5.3. State Transition Diagrams 67
5.4. User Interface 69
5.5. Event Processing 70
5.6. Server User-invoked Events 71
5.6.1. Receive 71
5.6.2. Respond 72
5.6.3. Forward 73
5.6.4. Other Functions 74
5.7. Request Packet Arrival 74
5.8. Management Operations 78
5.8.1. HandleRequestNoCSR 79
5.9. Timeouts 82
6. Concluding Remarks 84
I. Standard VMTP Response Codes 85
II. VMTP RPC Presentation Protocol 87
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II.1. Request Code Management 87
III. VMTP Management Procedures 89
III.1. Entity Group Management 100
III.2. VMTP Management Digital Signatures 101
IV. VMTP Entity Identifier Domains 102
IV.1. Domain 1 102
IV.2. Domain 3 104
IV.3. Other Domains 105
IV.4. Decentralized Entity Identifier Allocation 105
V. Authentication Domains 107
V.1. Authentication Domain 1 107
V.2. Other Authentication Domains 107
VI. IP Implementation 108
VII. Implementation Notes 109
VII.1. Mapping Data Structures 109
VII.2. Client Data Structures 111
VII.3. Server Data Structures 111
VII.4. Packet Group transmission 112
VII.5. VMTP Management Module 113
VII.6. Timeout Handling 114
VII.7. Timeout Values 114
VII.8. Packet Reception 115
VII.9. Streaming 116
VII.10. Implementation Experience 117
VIII. UNIX 4.3 BSD Kernel Interface for VMTP 118
Index 120
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List of Figures
Figure 1-1: Relation to Other Protocols 4
Figure 3-1: Request Packet Format 45
Figure 3-2: Response Packet Format 47
Figure 4-1: Client State Transitions 52
Figure 5-1: Remote Client State Transitions 68
Figure III-1: Authenticator Format 92
Figure VII-1: Mapping Client Identifier to CSR 109
Figure VII-2: Mapping Server Identifiers 110
Figure VII-3: Mapping Group Identifiers 111
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1. Introduction
The Versatile Message Transaction Protocol (VMTP) is a transport
protocol designed to support remote procedure call (RPC) and general
transaction-oriented communication. By transaction-oriented
communication, we mean that:
- Communication is request-response: A client sends a request
for a service to a server, the request is processed, and the
server responds. For example, a client may ask for the next
page of a file as the service. The transaction is terminated
by the server responding with the next page.
- A transaction is initiated as part of sending a request to a
server and terminated by the server responding. There are no
separate operations for setting up or terminating associations
between clients and servers at the transport level.
- The server is free to discard communication state about a
client between transactions without causing incorrect behavior
or failures.
The term message transaction (or transaction) is used in the reminder of
this document for a request-response exchange in the sense described
above.
VMTP handles the error detection, retransmission, duplicate suppression
and, optionally, security required for transport-level end-to-end
reliability.
The protocol is designed to provide a range of behaviors within the
transaction model, including:
- Minimal two packet exchanges for short, simple transactions.
- Streaming of multi-packet requests and responses for efficient
data transfer.
- Datagram and multicast communication as an extension of the
transaction model.
Example Uses:
- Page-level file access - VMTP is intended as the transport
level for file access, allowing simple, efficient operation on
a local network. In particular, VMTP is appropriate for use
by diskless workstations accessing shared network file
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servers.
- Distributed programming - VMTP is intended to provide an
efficient transport level protocol for remote procedure call
implementations, distributed object-oriented systems plus
message-based systems that conform to the request-response
model.
- Multicast communication with groups of servers to: locate a
specific object within the group, update a replicated object,
synchronize the commitment of a distributed transaction, etc.
- Distributed real-time control with prioritized message
handling, including datagrams, multicast and asynchronous
calls.
The protocol is designed to operate on top of a simple unreliable
datagram service, such as is provided by IP.
1.1. Motivation
VMTP was designed to address three categories of deficiencies with
existing transport protocols in the Internet architecture. We use TCP
as the key current transport protocol for comparison.
1.1.1. Poor RPC Performance
First, current protocols provide poor performance for remote procedure
call (RPC) and network file access. This is attributable to three key
causes:
- TCP requires excessive packets for RPC, especially for
isolated calls. In particular, connection setup and clear
generates extra packets over that needed for VMTP to support
RPC.
- TCP is difficult to implement, speaking purely from the
empirical experience over the last 10 years. VMTP was
designed concurrently with its implementation, with focus on
making it easy to implement and providing sensible subsets of
its functionality.
- TCP handles packet loss due to overruns poorly. We claim that
overruns are the key source of packet loss in a
high-performance RPC environment and, with the increasing
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performance of networks, will continue to be the key source.
(Older machines and network interfaces cannot keep up with new
machines and network interfaces. Also, low-end network
interfaces for high-speed networks have limited receive
buffering.)
VMTP is designed for ease of implementation and efficient RPC. In
addition, it provides selective retransmission with rate-based flow
control, thus addressing all of the above issues.
1.1.2. Weak Naming
Second, current protocols provide inadequate naming of transport-level
endpoints because the names are based on IP addresses. For example, a
TCP endpoint is named by an Internet address and port identifier.
Unfortunately, this makes the endpoint tied to a particular host
interface, not specifically the process-level state associated with the
transport-level endpoint. In particular, this form of naming causes
problems for process migration, mobile hosts and multi-homed hosts.
VMTP provides host-address independent names, thereby solving the above
mentioned problems.
In addition, TCP provides no security and reliability guarantees on the
dynamically allocated names. In particular, other than well-known
ports, (host-addr, port-id)-tuples can change meaning on reboot
following a crash. VMTP provides large identifiers with guarantee of
stability, meaning that either the identifier never changes in meaning
or else remains invalid for a significant time before becoming valid
again.
1.1.3. Function Poor
TCP does not support multicast, real-time datagrams or security. In
fact, it only supports pair-wise, long-term, streamed reliable
interchanges. Yet, multicast is of growing importance and is being
developed for the Internet (see RFC 966 and 988). Also, a datagram
facility with the same naming, transmission and reception facilities as
the normal transport level is a powerful asset for real-time and
parallel applications. Finally, security is a basic requirement in an
increasing number of environments. We note that security is natural to
implement at the transport level to provide end-to-end security (as
opposed to (inter)network level security). Without security at the
transport level, a transport level protocol cannot guarantee the
standard transport level service definition in the presence of an
intruder. In particular, the intruder can interject packets or modify
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packets while updating the checksum, making mockery out of the
transport-level claim of "reliable delivery".
In contrast, VMTP provides multicast, real-time datagrams and security,
addressing precisely these weaknesses.
In general, VMTP is designed with the next generation of communication
systems in mind. These communication systems are characterized as
follows. RPC, page-level file access and other request-response
behavior dominates. In addition, the communication substrate, both
local and wide-area, provides high data rates, low error rates and
relatively low delay. Finally, intelligent, high-performance network
interfaces are common and in fact required to achieve performance that
approximates the network capability. However, VMTP is also designed to
function acceptably with existing networks and network interfaces.
1.2. Relation to Other Protocols
VMTP is a transport protocol that fits into the layered Internet
protocol environment. Figure 1-1 illustrates the place of VMTP in the
protocol hierarchy.
+-----------+ +----+ +-----------------+ +------+
|File Access| |Time| |Program Execution| |Naming|... Application
+-----------+ +----+ +-----------------+ +------+ Layer
| | | | |
+-----------+-----------+-------------+------+
|
+------------------+
| RPC Presentation | Presentation
+------------------+ Layer
|
+------+ +--------+
| TCP | | VMTP | Transport
+------+ +--------+ Layer
| |
+-----------------------------------+
| Internet Protocol & ICMP | Internetwork
+-----------------------------------+ Layer
Figure 1-1: Relation to Other Protocols
The RPC presentation level is not currently defined in the Internet
suite of protocols. Appendix II defines a proposed RPC presentation
level for use with VMTP and assumed for the definition of the VMTP
management procedures. There is also a need for the definition of the
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Application layer protocols listed above.
If internetwork services are not required, VMTP can be used without the
IP layer, layered directly on top of the network or data link layers.
1.3. Document Overview
The next chapter gives an overview of the protocol, covering naming,
message structure, reliability, flow control, streaming, real-time,
security, byte-ordering and management. Chapter 3 describes the VMTP
packet formats. Chapter 4 describes the client VMTP protocol operation
in terms of pseudo-code for event handling. Chapter 5 describes the
server VMTP protocol operation in terms of pseudo-code for event
handling. Chapter 6 summarizes the state of the protocol, some
remaining issues and expected directions for the future. Appendix I
lists some standard Response codes. Appendix II describes the RPC
presentation protocol proposed for VMTP and used with the VMTP
management procedures. Appendix III lists the VMTP management
procedures. Appendix IV proposes initial approaches for handling entity
identification for VMTP. Appendix V proposes initial authentication
domains for VMTP. Appendix VI provides some details for implementing
VMTP on top of IP. Appendix VII provides some suggestions on host
implementation of VMTP, focusing on data structures and support
functions. Appendix VIII describes a proposed program interface for
UNIX 4.3 BSD and its descendants and related systems.
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2. Protocol Overview
VMTP provides an efficient, reliable, optionally secure transport
service in the message transaction or request-response model with the
following features:
- Host address-independent naming with provision for multiple
forms of names for endpoints as well as associated (security)
principals. (See Sections 2.1, 2.2, 3.1 and Appendix IV.)
- Multi-packet request and response messages, with a maximum
size of 4 megaoctets per message. (Sections 2.3 and 2.14.)
- Selective retransmission. (Section 2.13.) and rate-based flow
control to reduce overrun and the cost of overruns. (Section
2.5.6.)
- Secure message transactions with provision for a variety of
encryption schemes. (Section 2.6.)
- Multicast message transactions with multiple response messages
per request message. (Section 2.7.)
- Support for real-time communication with idempotent message
transactions with minimal server overhead and state (Section
2.5.3), datagram request message transactions with no
response, optional header-only checksum, priority processing
of transactions, conditional delivery and preemptive handling
of requests (Section 2.8)
- Forwarded message transactions as an optimization for certain
forms of nested remote procedure calls or message
transactions. (Section 2.9.)
- Multiple outstanding (asynchronous) message transactions per
client. (Section 2.11.)
- An integrated management module, defined with a remote
procedure call interface on top of VMTP providing a variety of
communication services (Section 2.10.)
- Simple subset implementation for simple clients and simple
servers. (Section 2.16.)
This chapter provides an overview of the protocol as introduction to the
basic ideas and as preparation for the subsequent chapters that describe
the packet formats and event processing procedures in detail.
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In overview, VMTP provides transport communication between network-
visible entities via message transactions. A message transaction
consists of a request message sent by the client, or requestor, to a
group of server entities followed by zero or more response messages to
the client, at most one from each server entity. A message is
structured as a message control portion and a segment data portion. A
message is transmitted as one or more packet groups. A packet group is
one or more packets (up to a maximum of 32 packets) grouped by the
protocol for acknowledgment, sequencing, selective retransmission and
rate control.
Entities and VMTP operations are managed using a VMTP management
mechanism that is accessed through a procedural interface (RPC)
implemented on top of VMTP. In particular, information about a remote
entity is obtained and maintained using the Probe VMTP management
operation. Also, acknowledgment information and requests for
retransmission are sent as notify requests to the management module.
(In the following description, reference to an "acknowledgment" of a
request or a response refers to a management-level notify operation that
is acknowledging the request or response.)
2.1. Entities, Processes and Principals
VMTP defines and uses three main types of identifiers: entity
identifiers, process identifiers and principal identifiers, each 64-bits
in length. Communication takes place between network-visible entities,
typically mapping to, or representing, a message port or procedure
invocation. Thus, entities are the VMTP communication endpoints. The
process associated with each entity designates the agent behind the
communication activity for purposes of resource allocation and
management. For example, when a lock is requested on a file, the lock
is associated with the process, not the requesting entity, allowing a
process to use multiple entity identifiers to perform operations without
lock conflict between these entities. The principal associated with an
entity specifies the permissions, security and accounting designation
associated with the entity. The process and principal identifiers are
included in VMTP solely to make these values available to VMTP users
with the security and efficiency provided by VMTP. Only the entity
identifiers are actively used by the protocol.
Entity identifiers are required to have three properties;
Uniqueness Each entity identifier is uniquely defined at any given
time. (An entity identifier may be reused over time.)
Stability An entity identifier does not change between valid
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meanings without suitable provision for removing
references to the entity identifier. Certain entity
identifiers are strictly stable, (i.e. never changing
meaning), typically being administratively assigned
(although they need not be bound to a valid entity at
all times), often called well-known identifiers. All
other entity identifiers are required to be T-stable,
not change meaning without having remained invalid for
at least a time interval T.
Host address independent
An entity identifier is unique independent of the host
address of its current host. Moreover, an entity
identifier is not tied to a single Internet host
address. An entity can migrate between hosts, reside on
a mobile host that changes Internet addresses or reside
on a multi-homed host. It is up to the VMTP
implementation to determine and maintain up to date the
host addresses of entities with which it is
communicating.
The stability of entity identifiers guarantees that an entity identifier
represents the same logical communication entity and principal (in the
security sense) over the time that it is valid. For example, if an
entity identifier is authenticated as having the privileges of a given
user account, it continues to have those privileges as long as it is
continuously valid (unless some explicit notice is provided otherwise).
Thus, a file server need not fully authenticate the entity on every file
access request. With T-stable identifiers, periodically checking the
validity of an entity identifier with period less than T seconds detects
a change in entity identifier validity.
A group of entities can form an entity group, which is a set of zero or
more entities identified by a single entity identifier. For example,
one can have a single entity identifier that identifies the group of
name servers. An entity identifier representing an entity group is
drawn from the same name space as entity identifiers. However, single
entity identifiers are flagged as such by a bit in the entity
identifier, indicating that the identifier is known to identify at most
one entity. In addition to the group bit, each entity identifier
includes other standard type flags. One flag indicates whether the
identifier is an alias for an entity in another domain (See Section 2.2
below.). Another flag indicates, for an entity group identifier,
whether the identifier is a restricted group or not. A restricted group
is one in which an entity can be added only by another entity with group
management authorization. With an unrestricted group, an entity is
allowed to add itself. If an entity identifier does not represent a
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group, a type bit indicates whether the entity uses big-endian or
little-endian data representation (corresponding to Motorola 680X0 and
VAX byte orders, respectively). Further specification of the format of
entity identifiers is contained in Section 3.1 and Appendix IV.
An entity identifier identifies a Client, a Server or a group of
Servers <1>. A Client is always identified by a T-stable identifier. A
server or group of servers may be identified by a a T-stable identifier
(group or single entity) or by strictly stable (statically assigned)
entity group identifier. The same T-stable identifier can be used to
identify a Client and Server simultaneously as long as both are
logically associated with the same entity. The state required for
reliable, secure communication between entities is maintained in client
state records (CSRs), which include the entity identifier of the Client,
its principal, its current or next transaction identifier and so on.
2.2. Entity Domains
An entity domain is an administration or an administration mechanism
that guarantees the three required entity identifier properties of
uniqueness, stability and host address independence for the entities it
administers. That is, entity identifiers are only guaranteed to be
unique and stable within one entity domain. For example, the set of all
Internet hosts may function as one domain. Independently, the set of
hosts local to one autonomous network may function as a separate domain.
Each entity domain is identified by an entity domain identifier, Domain.
Only entities within the same domain may communicate directly via VMTP.
However, hosts and entities may participate in multiple entity domains
simultaneously, possibly with different entity identifiers. For
example, a file server may participate in multiple entity domains in
order to provide file service to each domain. Each entity domain
specifies the algorithms for allocation, interpretation and mapping of
entity identifiers.
Domains are necessary because it does not appear feasible to specify one
universal VMTP entity identification administration that covers all
entities for all time. Domains limit the number of entities that need
to be managed to maintain the uniqueness and stability of the entity
_______________
<1> Terms such as Client, Server, Request, Response, etc. are
capitalized in this document when they refer to their specific meaning
in VMTP.
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name space. Domains can also serve to separate entities of different
security levels. For instance, allocation of a unclassified entity
identifier cannot conflict with secret level entity identifiers because
the former is interpreted only in the unclassified domain, which is
disjoint from the secret domain.
It is intended that there be a small number of domains. In particular,
there should be one (or a few) domains per installation "type", rather
than per installation. For example, the Internet is expected to use one
domain per security level, resulting in at most 8 different domains.
Cluster-based internetwork architectures, those with a local cluster
protocol distinct from the wide-area protocol, may use one domain for
local use and one for wide-area use.
Additional details on the specification of specific domains is provided
in Appendix IV.
2.3. Message Transactions
The message transaction is the unit of interaction between a Client that
initiates the transaction and one or more Servers. A message
transaction starts with a request message generated by a client. At
the service interface, a server becomes involved with a transaction by
receiving and accepting the request. A server terminates its
involvement with a transaction by sending a response message. In a
group message transaction, the server entity designated by the client
corresponds to a group of entities. In this case, each server in the
group receives a copy of the request. In the client's view, the
transaction is terminated when it receives the response message or, in
the case of a group message transaction, when it receives the last
response message. Because it is normally impractical to determine when
the last response message has been received. the current transaction is
terminated by VMTP when the next transaction is initiated.
Within an entity domain, a transaction is uniquely identified by the
tuple (Client, Transaction, ForwardCount). where Transaction is a
32-bit number and ForwardCount is a 4-bit value. A Client uses
monotonically increasing Transaction identifiers for new message
transactions. Normally, the next higher transaction number, modulo
2**32, is used for the next message transaction, although there are
cases in which it skips a small range of Transaction identifiers. (See
the description of the STI control flag.) The ForwardCount is used when
a message transaction is forwarded and is zero otherwise.
A Client generates a stream of message transactions with increasing
transaction identifiers, directed at a diversity of Servers. We say a
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Client has a transaction outstanding if it has invoked a message
transaction, but has not received the last Response (or possibly any
Response). Normally, a Client has only one transaction outstanding at a
time. However, VMTP allows a Client to have multiple message
transactions outstanding simultaneously, supporting streamed,
asynchronous remote procedure call invocations. In addition, VMTP
supports nested calls where, for example, procedure A calls procedure B
which calls procedure C, each on a separate host with different client
entity identifiers for each call but identified with the same process
and principal.
2.4. Request and Response Messages
A message transaction consists of a request message and one or more
Response messages. A message is structured as message control block
(MCB) and segment data, passed as parameters, as suggested below.
+-----------------------+
| Message Control Block |
+-----------------------+
+-----------------------------------+
| segment data |
+-----------------------------------+
In the request message, the MCB specifies control information about the
request plus an optional data segment. The MCB has the following
format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ ServerEntityId (8 octets) +
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | RequestCode |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ CoresidentEntity (8 octets) +
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
> User Data (12 octets) <
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MsgDelivery |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SegmentSize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The ServerEntityId is the entity to which the Request MCB is to be sent
(or was sent, in the case of reception). The Flags indicate various
options in the request and response handling as well as whether the
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CoresidentEntity, MsgDelivery and SegmentSize fields are in use. The
RequestCode field specifies the type of Request. It is analogous to a
packet type field of the Ethernet, acting as a switch for higher-level
protocols. The CoresidentEntity field, if used, designates a subgroup
of the ServerEntityId group to which the Request should be routed,
namely those members that are co-resident with the specified entity (or
entity group). The primary intended use is to specify the manager for a
particular service that is co-resident with a particular entity, using
the well-known entity group identifier for the service manager in the
ServerEntityId field and the identifier for the entity in the
CoresidentEntity field. The next 12 octets are user- or
application-specified.
The MsgDelivery field is optionally used by the RPC or user level to
specify the portions of the segment data to transmit and on reception,
the portions received. It provides the client and server with
(optional) access to, and responsibility for, a simple selective
transmission and reception facility. For example, a client may request
retransmission of just those portions of the segment that it failed to
receive as part of the original Response. The primary intended use is
to support highly efficient multi-packet reading from a file server.
Exploiting user-level selective retransmission using the MsgDelivery
field, the file server VMTP module need not save multi-packet Responses
for retransmission. Retransmissions, when needed, are instead handled
directly from the file server buffers.
The SegmentSize field indicates the size of the data segment, if
present. The CoresidentEntity, MsgDelivery and SegmentSize fields are
usable as additional user data if they are not otherwise used.
The Flags field provides a simple mechanism for the user level to
communicate its use of VMTP options with the VMTP module as well as for
VMTP modules to communicate this use among themselves. The use of these
options is generally fixed for each remote procedure so that an RPC
mechanism using VMTP can treat the Flags as an integral part of the
RequestCode field for the purpose of demultiplexing to the correct stub.
A Response message control block follows the same format except the
Response is sent from the Server to the Client and there is no
Coresident Entity field (and thus 20 octets of user data).
2.5. Reliability
VMTP provides reliable, sequenced transfer of request and response
messages as well as several variants, such as unreliable datagram
requests. The reliability mechanisms include: transaction identifiers,
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checksums, positive acknowledgment of messages and timeout and
retransmission of lost packets.
2.5.1. Transaction Identifiers
Each message transaction is uniquely identified by the pair (Client,
Transaction). (We defer discussion of the ForwardCount field to Section
2.9.) The 32-bit transaction identifier is initialized to a random
value when the Client entity is created or allocated its entity
identifier. The transaction identifier is incremented at the end of
each message transaction. All Responses with the same specified
(Client, Transaction) pair are associated with this Request.
The transaction identifier is used for duplicate suppression at the
Server. A Server maintains a state record for each Client for which it
is processing a Request, identified by (Client, Transaction). A Request
with the same (Client, Transaction) pair is discarded as a duplicate.
(The ForwardCount field must also be equal.) Normally, this record is
retained for some period after the Response is sent, allowing the Server
to filter out subsequent duplicates of this Request. When a Request
arrives and the Server does not have a state record for the sending
Client, the Server takes one of three actions:
1. The Server may send a Probe request, a simple query
operation, to the VMTP management module associated with the
requesting Client to determine the Client's current
Transaction identifier (and other information), initialize a
new state record from this information, and then process the
Request as above.
2. The Server may reason that the Request must be a new request
because it does not have a state record for this Client if it
keeps these state records for the maximum packet lifetime of
packets in the network (plus the maximum VMTP retransmission
time) and it has not been rebooted within this time period.
That is, if the Request is not new either the Request would
have exceeded the maximum packet lifetime or else the Server
would have a state record for the Client.
3. The Server may know that the Request is idempotent or can be
safely redone so it need not care whether the Request is a
duplicate or not. For example, a request for the current
time can be responded to with the current time without being
concerned whether the Request is a duplicate. The Response
is discarded at the Client if it is no longer of interest.
Cheriton [page 13]
RFC 1045 VMTP February 1988
2.5.2. Checksum
Each VMTP packet contains a checksum to allow the receiver to detect
corrupted packets independent of lower level checks. The checksum field
is 32 bits, providing greater protection than the standard 16-bit IP
checksum (in combination with an improved checksum algorithm). The
large packets, high packet rates and general network characteristics
expected in the future warrant a stronger checksum mechanism.
The checksum normally covers both the VMTP header and the segment data.
Optionally (for real-time applications), the checksum may apply only to
the packet header, as indicated by the HCO control bit being set in the
header. The checksum field is placed at the end of the packet to allow
it to be calculated as part of a software copy or as part of a hardware
transmission or reception packet processing pipeline, as expected in the
next generation of network interfaces. Note that the number of header
and data octets is an integral multiple of 8 because VMTP requires that
the segment data be padded to be a multiple of 64 bits. The checksum
field is appended after the padding, if any. The actual algorithm is
described in Section 3.2.
A zero checksum field indicates that no checksum was transmitted with
the packet. VMTP may be used without a checksum only when there is a
host-to-host error detection mechanism and the VMTP security facility is
not being used. For example, one could rely on the Ethernet CRC if
communication is restricted to hosts on the same Ethernet and the
network interfaces are considered sufficiently reliable.
2.5.3. Request and Response Acknowledgment
VMTP assumes an unreliable datagram network and internetwork interface.
To guarantee delivery of Requests and Response, VMTP uses positive
acknowledgments, retransmissions and timeouts.
A Request is normally acknowledged by receipt of a Response associated
with the Request, i.e. with the same (Client, Transaction). With
streamed message transactions, it may also be acknowledged by a
subsequent Response that acknowledges previous Requests in addition to
the transaction it explicitly identifies. A Response may be explicitly
acknowledged by a NotifyVmtpServer operation requested of the manager
for the Server. In the case of streaming, this is a cumulative
acknowledgment, acknowledging all Responses with a lower transaction
identifier as well.) In addition, with non-streamed communication, a
subsequent Request from the same Client acknowledges Responses to all
previous message transactions (at least in the sense that either the
client received a Response or is no longer interested in Responses to
Cheriton [page 14]
RFC 1045 VMTP February 1988
those earlier message transactions). Finally, a client response timeout
(at the server) acknowledges a Response at least in the sense that the
server need not be prepared to retransmit the Response subsequently.
Note that there is no end-to-end guarantee of the Response being
received by the client at the application level.
2.5.4. Retransmissions
In general, a Request or Response is retransmitted periodically until
acknowledged as above, up to some maximum number of retransmissions.
VMTP uses parameters RequestRetries(Server) and ResponseRetries(Client)
that indicate the number of retransmissions for the server and client
respectively before giving up. We suggest the value 5 be used for both
parameters based on our experience with VMTP and Internet packet loss.
Smaller values (such as 3) could be used in low loss environments in
which fast detection of failed hosts or communication channels is
required. Larger values should be used in high loss environments where
transport-level persistence is important.
In a low loss environment, a retransmission only includes the MCB and
not the segment data of the Request or Response, resulting in a single
(short) packet on retransmission. The intended recipient of the
retransmission can request selective retransmission of all or part of
the segment data as necessary. The selective retransmission mechanism
is described in Section 2.13.
If a Response is specified as idempotent, the Response is neither
retransmitted nor stored for retransmission. Instead, the Client must
retransmit the Request to effectively get the Response retransmitted.
The server VMTP module responds to retransmissions of the Request by
passing the Request on to the server again to have it regenerate the
Response (by redoing the operation), rather than saving a copy of the
Response. Only Request packets for the last transaction from this
client are passed on in this fashion; older Request packets from this
client are discarded as delayed duplicates. If a Response is not
idempotent, the VMTP module must ensure it has a copy of the Response
for retransmission either by making a copy of the Response (either
physically or copy-on-write) or by preventing the Server from continuing
until the Response is acknowledged.
2.5.5. Timeouts
There is one client timer for each Client with an outstanding
transaction. Similarly, there is one server timer for each Client
transaction that is "active" at the server, i.e. there is a transaction
Cheriton [page 15]
RFC 1045 VMTP February 1988
record for a Request from the Client.
When the client transmits a new Request (without streaming), the client
timer is set to roughly the time expected for the Response to be
returned. On timeout, the Request is retransmitted with the APG
(Acknowledge Packet Group) bit set. The timeout is reset to the
expected roundtrip time to the Server because an acknowledgment should
be returned immediately unless a Response has been sent. The Request
may also be retransmitted in response to receipt of a VMTP management
operation indicating that selected portions of the Request message
segment need to be retransmitted. With streaming, the timeout applies
to the oldest outstanding message transaction in the run of outstanding
message transactions. Without streaming, there is one message
transaction in the run, reducing to the previous situation. After the
first packet of a Response is received, the Client resets the timeout to
be the time expected before the next packet in the Response packet group
is received, assuming it is a multi-packet Response. If not, the timer
is stopped. Finally, the client timer is used to timeout waiting for
second and subsequent Responses to a multicast Request.
The client timer is set at different times to four different values:
TC1(Server) The expected time required to receive a Response from
the Server. Set on initial Request transmission plus
after its management module receives a NotifyVmtpClient
operation, acknowledging the Request.
TC2(Server) The estimated round trip delay between the client and
the server. Set when retransmitting after receiving no
Response for TC1(Server) time and retransmitting the
Request with the APG bit set.
TC3(Server) The estimated maximum expected interpacket time for
multi-packet Responses from the Server. Set when
waiting for subsequent Response packets within a packet
group before timing out.
TC4 The time to wait for additional Responses to a group
Request after the first Response is received. This is
specified by the user level.
These values are selected as follows. TC1 can be set to TC2 plus a
constant, reflecting the time within which most servers respond to most
requests. For example, various measurements of VMTP usage at Stanford
indicate that 90 percent of the servers respond in less than 200
milliseconds. Setting TC1 to TC2 + 200 means that most Requests receive
a Response before timing out and also that overhead for retransmission
Cheriton [page 16]
RFC 1045 VMTP February 1988
for long running transactions is insignificant. A sophisticated
implementation may make the estimation of TC1 further specific to the
Server.
TC2 may be estimated by measuring the time from when a Probe request is
sent to the Server to when a response is received. TC2 can also be
measured as the time between the transmission of a Request with the APG
bit set to receipt of a management operation acknowledging receipt of
the Request.
When the Server is an entity group, TC1 and TC2 should be the largest of
the values for the members of the group that are expected to respond.
This information may be determined by probing the group on first use
(and using the values for the last responses to arrive). Alternatively,
one can resort to default values.
TC3 is set initially to 10 times the transmission time for the maximum
transmission unit (MTU) to be used for the Response. A sophisticated
implementation may record TC3 per Server and refine the estimate based
on measurements of actual interpacket gaps. However, a tighter estimate
of TC3 only improves the reaction time when a packet is lost in a packet
group, at some cost in unnecessary retransmissions when the estimate
becomes overly tight.
The server timer, one per active Client, takes on the following values:
TS1(Client) The estimated maximum expected interpacket time. Set
when waiting for subsequent Request packets within a
packet group before timing out.
TS2(Client) The time to wait to hear from a client before
terminating the server processing of a Request. This
limits the time spent processing orphan calls, as well
as limiting how out of date the server's record of the
Client state can be. In particular, TS2 should be
significantly less than the minimum time within which it
is reasonable to reuse a transaction identifier.
TS3(Client) Estimated roundtrip time to the Client,
TS4(Client) The time to wait after sending a Response (or last
hearing from a client) before discarding the state
associated with the Request which allows it to filter
duplicate Request packets and regenerate the Response.
TS5(Client) The time to wait for an acknowledgment after sending a
Response before retransmitting the Response, or giving
Cheriton [page 17]
RFC 1045 VMTP February 1988
up (after some number of retransmissions).
TS1 is set the same as TC3.
The suggested value for TS2 is TC1 + 3*TC2 for this server, giving the
Client time to timeout waiting for a Response and retransmit 3 Request
packets, asking for acknowledgments.
TS3 is estimated the same as TC1 except that refinements to the estimate
use measurements of the Response-to-acknowledgment times.
In the general case, TS4 is set large enough so that a Client issuing a
series of closely-spaced Requests to the same Server reuses the same
state record at the Server end and thus does not incur the overhead of
recreating this state. (The Server can recreate the state for a Client
by performing a Probe on the Client to get the needed information.) It
should also be set low enough so that the transaction identifier cannot
wrap around and so that the Server does not run out of CSR's. We
suggest a value in the range of 500 milliseconds. However, if the
Server accepts non-idempotent Requests from this Client without doing a
Probe on the Client, the TS4 value for this CSR is set to at least 4
times the maximum packet lifetime.
TS5 is TS3 plus the expected time for transmission and reception of the
Response. We suggest that the latter be calculated as 3 times the
transmission time for the Response data, allowing time for reception,
processing and transmission of an acknowledgment at the Client end. A
sophisticated implementation may refine this estimate further over time
by timing acknowledgments to Responses.
2.5.6. Rate Control
VMTP is designed to deal with the present and future problem of packet
overruns. We expect overruns to be the major cause of dropped packets
in the future. A client is expected to estimate and adjust the
interpacket gap times so as to not overrun a server or intermediate
nodes. The selective retransmission mechanism allows the server to
indicate that it is being overrun (or some intermediate point is being
overrun). For example, if the server requests retransmission of every
Kth block, the client should assume overrun is taking place and increase
the interpacket gap times. The client passes the server an indication
of the interpacket gap desired for a response. The client may have to
increase the interval because packets are being dropped by an
intermediate gateway or bridge, even though it can handle a higher rate.
A conservative policy is to increase the interpacket gap whenever a
packet is lost as part of a multi-packet packet group.
Cheriton [page 18]
RFC 1045 VMTP February 1988
The provision of selective retransmission allows the rate of the client
and the server to "push up" against the maximum rate (and thus lose
packets) without significant penalty. That is, every time that packet
transmission exceeds the rate of the channel or receiver, the recovery
cost to retransmit the dropped packets is generally far less than
retransmitting from the first dropped packet.
The interpacket gap is expressed in 1/32nd's of the MTU packet
transmission time. The minimum interpacket gap is 0 and the maximum gap
that can be described in the protocol is 8 packet times. This places a
limit on the slowest receivers that can be efficiently used on a
network, at least those handling multi-packet Requests and Responses.
This scheme also limits the granularity of adjustment. However, the
granularity is relative to the speed of the network, as opposed to an
absolute time. For entities on different networks of significantly
different speed, we assume the interconnecting gateways can buffer
packets to compensate<2>. With different network speeds and intermediary
nodes subject to packet loss, a node must adjust the interpacket gap
based on packet loss. The interpacket gap parameter may be of limited
use.
2.6. Security
VMTP provides an (optional) secure mode that protects against the usual
security threats of peeking, impostoring, message tampering and replays.
Secure VMTP must be used to guarantee any of the transport-level
reliability properties unless it is guaranteed that there are no
intruders or agents that can modify packets and update the packet
checksums. That is, non-secure VMTP provides no guarantees in the
presence of an intelligent intruder.
The design closely follows that described by Birrell [1]. Authenticated
information about a remote entity, including an encryption/decryption
key, is obtained and maintained using a VMTP management operation, the
authenticated Probe operation, which is executed as a non-secure VMTP
message transaction. If a server receives a secure Request for which
the server has no entity state, it sends a Probe request to the VMTP
_______________
<2> Gateways must also employ techniques to preserve or intelligently
modify (if appropriate) the interpacket gaps. In particular, they must
be sure not to arbitrarily remove interpacket gaps as a result of their
forwarding of packets.
Cheriton [page 19]
RFC 1045 VMTP February 1988
management module of the client, "challenging" it to provide an
authenticator that both authenticates the client as being associated
with a particular principal as well as providing a key for
encryption/decryption. The principal can include a real and effective
principal, as used in UNIX <3>. Namely, the real principal is the
principal on whose behalf the Request is being performed whereas the
effective principal is the principal of the module invoking the request
or remote procedure call.
Peeking is prevented by encrypting every Request and Response packet
with a working Key that is shared between Client and Server.
Impostoring and replays are detected by comparing the Transaction
identifier with that stored in the corresponding entity state record
(which is created and updated by VMTP as needed). Message tampering is
detected by encryption of the packet including the Checksum field. An
intruder cannot update the checksum after modifying the packet without
knowing the Key. The cost of fully encrypting a packet is close to the
cost of generating a cryptographic checksum (and of course, encryption
is needed in the general case), so there is no explicit provision for
cryptographic checksum without packet encryption.
A Client determines the Principal of the Server and acquires an
authenticator for this Server and Principal using a higher level
protocol. The Server cannot decrypt the authenticator or the Request
packets unless it is in fact the Principal expected by the Client.
An encrypted VMTP packet is flagged by the EPG bit in the VMTP packet
header. Thus, encrypted packets are easily detected and demultiplexed
from unencrypted packets. An encrypted VMTP packet is entirely
encrypted except for the Client, Version, Domain, Length and Packet
Flags fields at the beginning of the packet. Client identifiers can be
assigned, changed and used to have no real meaning to an intruder or to
only communicate public information (such as the host Internet address).
They are otherwise just a random means of identification and
demultiplexing and do not therefore divulge any sensitive information.
Further secure measures must be taken at the network or data link levels
if this information or traffic behavior is considered sensitive.
VMTP provides multiple authentication domains as well as an encryption
qualifier to accommodate different encryption algorithms and their
_______________
<3> Principal group membership must be obtained, if needed, by a
higher level protocol.
Cheriton [page 20]
RFC 1045 VMTP February 1988
corresponding security/performance trade-offs. (See Appendix V.) A
separate key distribution and authentication protocol is required to
handle generation and distribution of authenticators and keys. This
protocol can be implemented on top of VMTP and can closely follow the
Birrell design as well.
Security is optional in the sense that messages may be secure or
non-secure, even between consecutive message transactions from the same
client. It is also optional in that VMTP clients and servers are not
required to implement secure VMTP (although they are required to respond
intelligently to attempts to use secure VMTP). At worst, a Client may
fail to communicate with a Server if the Server insists on secure
communication and the Client does not implement security or vice versa.
However, a failure to communicate in this case is necessary from a
security standpoint.
2.7. Multicast
The Server entity identifier in a message transaction can identify an
entity group, in which case the Request is multicast to every Entity in
this group (on a best-efforts basis). The Request is retransmitted
until at least one Response is received (or an error timeout occurs)
unless it is a datagram Request. The Client can receive multiple
Responses to the Request.
The VMTP service interface does not directly provide reliable multicast
because it is expensive to provide, rarely needed by applications, and
can be implemented by applications using the multiple Response feature.
However, the protocol itself is adequate for reliable multicast using
positive acknowledgments. In particular, a sophisticated Client
implementation could maintain a list of members for each entity group of
interest and retransmit the Request until acknowledged by all members.
No modifications are required to the Server implementations.
VMTP supports a simple form of subgroup addressing. If the CRE bit is
set in a Request, the Request is delivered to the subgroup of entities
in the Server group that are co-resident with one or more entities in
the group (or individual entity) identified by the CoresidentEntity
field of the Request. This is commonly used to send to the manager
entity for a particular entity, where Server specifies the group of such
managers. Co-resident means "using the same VMTP module", and logically
on the same network host. In particular, a Probe request can be sent to
the particular VMTP management module for an entity by specifying the
VMTP management group as the Server and the entity in question as the
CoResidentEntity.
Cheriton [page 21]
RFC 1045 VMTP February 1988
As an experimental aspect of the protocol, VMTP supports the Server
sending a group Response which is sent to the Client as well as members
of the destination group of Servers to which the original Request was
sent. The MDG bit indicates whether the Client is a member of this
group, allowing the Server module to determine whether separately
addressed packet groups are required to send the Response to both the
Client and the Server group. Normally, a Server accepts a group
Response only if it has received the Request and not yet responded to
the Client. Also, the Server must explicitly indicate it wants to
accept group Responses. Logically, this facility is analogous to
responding to a mail message sent to a distribution list by sending a
copy of the Response to the distribution list.
2.8. Real-time Communication
VMTP provides three forms of support for real-time communication, in
addition to its standard facilities, which make it applicable to a wide
range of real-time applications. First, a priority is transmitted in
each Request and Response which governs the priority of its handling.
The priority levels are intended to correspond roughly to:
- urgent/emergency.
- important
- normal
- background.
with additional gradations for each level. The interpretation and
implementation of these priority levels is otherwise host-specific, e.g.
the assignment to host processing priorities.
Second, datagram Requests allow the Client to send a datagram to another
entity or entity group using the VMTP naming, transmission and delivery
mechanism, but without blocking, retransmissions or acknowledgment.
(The client can still request acknowledgment using the APG bit although
the Server does not expect missing portions of a multi-packet datagram
Request to be retransmitted even if some are not received.) A datagram
Request in non-streamed mode supersedes all previous Requests from the
same Client. A datagram Request in stream mode is queued (if necessary)
after previous datagram Requests on the same stream. (See Section
2.11.)
Finally, VMTP provides several control bit flags to modify the handling
of Requests and Responses for real-time requirements. First, the
Cheriton [page 22]
RFC 1045 VMTP February 1988
conditional message delivery (CMD) flag causes a Request to be discarded
if the recipient is not waiting for it when it arrives, similarly for
the Response. This option allows a client to send a Request that is
contingent on the server being able to process it immediately. The
header checksum only (HCO) flag indicates that the checksum has been
calculated only on the VMTP header and not on the data segment.
Applications such as voice and video can avoid the overhead of
calculating the checksum on data whose utility is insensitive to typical
bit errors without losing protection on the header information.
Finally, the No Retransmission (NRT) flag indicates that the recipient
of a message should not ask for retransmission if part of the message is
missing but rather either use what was received or discard it.
None of these facilities introduce new protocol states. In fact, the
total processing overhead in the normal case is a bit flag test for CMD,
HCO or NRT plus assignment of priority on packet transmission and
reception. (In fact, CMD and NRT are not tested in the normal case.)
The additional code complexity is minimal. We feel that the overhead
for providing these real-time facilities is minimal and that these
facilities are both important and adequate for a wide class of real-time
applications.
Several of the normal facilities of VMTP appear useful for real-time
applications. First, multicast is useful for distributed, replicated
(fault-tolerant) real-time applications, allowing efficient state query
and update for (for example) sensors and control state. Second, the DGM
or idempotent flag for Responses has some real-time benefits, namely: a
Request is redone to get the latest values when the Response is lost,
rather than just returning the old values. The desirability of this
behavior is illustrated by considering a request for the current time of
day. An idempotent handling of this request gives better accuracy in
returning the current time in the case that a retransmission is
necessary. Finally, the request-response semantics (in the absence of
streaming) of each new Request from a Client terminating the previous
message transactions from that Client, if any, provides the "most recent
is most important" handling of processing that most real-time
applications require.
In general, a key design goal of VMTP was provide an efficient
general-purpose transport protocol with the features required for
real-time communication. Further experience is required to determine
whether this goal has been achieved.
Cheriton [page 23]
RFC 1045 VMTP February 1988
2.9. Forwarded Message Transactions
A Server may invoke another Server to handle a Request. It is fairly
common for the invocation of the second Server to be the last action
performed by the first Server as part of handling the Request. For
example, the original Server may function primarily to select a process
to handle the Request. Also, the Server may simply check the
authorization on the Request. Describing this situation in the context
of RPC, a nested remote procedure call may be the last action in the
remote procedure and the return parameters are exactly those of the
nested call. (This situation is analogous to tail recursion.)
As an optimization to support this case, VMTP provides a Forward
operation that allows the server to send the nested Request to the other
server and have this other server respond directly to the Client.
If the message transaction being forwarded was not multicast, not secure
or the two Servers are the same principal and the ForwardCount of the
Request is less than the maximum forward count of 15, the Forward
operation is implemented by the Server sending a Request onto the next
Server with the forwarded Request identified by the same Client and
Transaction as the original Request and a ForwardCount one greater than
the Request received from the Client. In this case, the new Server
responds directly to the Client. A forwarded Request is illustrated in
the following figure.
+---------+ Request +----------+
| Client +---------------->| Server 1 |
+---------+ +----------+
^ |
| | forwarded Request
| V
| Response +----------+
+----------------------| Server 2 |
+----------+
If the message transaction does not meet the above requirements, the
Server's VMTP module issues a nested call and simply maps the returned
Response to a Response to original Request without further Server-level
processing. In this case, the only optimization over a user-level
nested call is one fewer VMTP service operation; the VMTP module handles
the return to the invoking call directly. The Server may also use this
form of forwarding when the Request is part of a stream of message
transactions. Otherwise, it must wait until the forwarded message
transaction completes before proceeding with the subsequent message
transactions in the stream.
Cheriton [page 24]
RFC 1045 VMTP February 1988
Implementation of the user-level Forward operation is optional,
depending on whether the server modules require this facility. Handling
an incoming forwarded Request is a minor modification of handling a
normal incoming Request. In particular, it is only necessary to examine
the ForwardCount field when the Transaction of the Request matches that
of the last message transaction received from the Client. Thus, the
additional complexity in the VMTP module for the required forwarding
support is minimal; the complexity is concentrated in providing a highly
optimized user-level Forward primitive, and that is optional.
2.10. VMTP Management
VMTP management includes operations for creating, deleting, modifying
and querying VMTP entities and entity groups. VMTP management is
logically implemented by a VMTP management server module that is invoked
using a message transaction addressed to the Server, VMTP_MANAGER_GROUP,
a well-known group entity identifier, in conjunction with Coresident
Entity mechanism introduced in Section 2.7. A particular Request may
address the local module, the module managing a particular entity, the
set of modules managing those entities contained in a specific group or
all management modules, as appropriate.
The VMTP management procedures are specified in Appendix III.
2.11. Streamed Message Transactions
Streamed message transactions refer to two or more message transactions
initiated by a Client before it receives the response to the first
message transaction, with each transaction being processed and responded
to in order but asynchronous relative to the initiation of the
transactions. A Client streams messages transactions, and thereby has
multiple message transactions outstanding, by sending them as part of a
single run of message transactions. A run of message transactions is a
sequence of message transactions with the same Client and Server and
consecutive Transaction identifiers, with all but the first and last
Requests and Responses flagged with the NSR (Not Start Run) and NER
(Not End Run) control bits. (Conversely, the first Request and
Response does not have the NSR set and the last Request and Response
does not have the NER bit set.) The message transactions in a run use
Cheriton [page 25]
RFC 1045 VMTP February 1988
consecutive transaction identifiers (except if the STI bit <4> is used
in one, in which case the transaction identifier for the next message
transaction is 256 greater, rather than 1).
The Client retains a record for each outstanding transaction until it
gets a Response or is timed out in error. The record provides the
information required to retransmit the Request. On retransmission
timeout, the client retransmits the last Request for which it has not
received a Response the same as is done with non-streamed communication.
(I.e. there need be only one timeout for all the outstanding message
transactions associated with a single client.)
The consecutive transaction identifiers within a run of message
transactions are used as sequence numbers for error control. The Server
handles each message transaction in the sequence specified by its
transaction identifier. When it receives a message transaction that is
not marked as the beginning of a run, it checks that it previously
received a message transaction with the predecessor transaction
identifier, either 1 less than the current one or 256 less if the
previous one had the STI bit set. If not, the Server sends a
NotifyVmtpClient operation to the Client's manager indicating either:
(1) the first message transaction was not fully received, or else (2) it
has no record of the last one received. If the NRT control flag is set,
it does not await nor expect retransmission but proceeds with handling
this Request. This flag is used primarily when datagram Requests are
used as part of a stream of message transactions. If NRT was not
specified, the Client must retransmit from the first message transaction
not fully received (either at all or in part) before the Server can
proceed with handling this run of Requests or else restart the run of
message transactions.
The Client expects to receive the Responses in a consecutive sequence,
using the Transaction identifier to detect missing Responses. Thus, the
Server must return Responses in sequence except possibly for some gaps,
as follows. The Server can specify in the PGcount field in a Response,
the number of consecutively previous Responses that this Response
_______________
<4> The STI bit is used by the Client to effectively allocate 255
transaction identifiers for use by the Server in returning a large
Response or stream of Responses.
Cheriton [page 26]
RFC 1045 VMTP February 1988
corresponds to, up to a maximum of 255 previous Responses <5>. Thus,
for example, a Response with Transaction identifier 46 and PGcount 3
represents Responses 43, 44, 45 and 46. This facility allows the Server
to eliminate sending Responses to Requests that require no Response,
effectively batching the Responses into one. It also allows the Server
to effectively maintain strictly consecutive sequencing when the Client
has skipped 256 Transaction identifiers using the STI bit and the Server
does not have that many Responses to return.
If the Client receives a Response that is not consecutive, it
retransmits the Request(s) for which the Response(s) is/are missing
(unless, of course, the corresponding Requests were sent as datagrams).
The Client should wait at the end of a run of message transactions for
the last one to complete.
When a Server receives a Request with the NSR bit clear and a higher
transaction identifier than it currently has for the Client, it
terminates all processing and discards Responses associated with the
previous Requests. Thus, a stream of message transactions is
effectively aborted by starting a new run, even if the Server was in the
middle of handling the previous run.
Using a mixture of datagram and normal Requests as part of a stream of
message transactions, particularly with the use of the NRT bit, can lead
to complex behavior under packet loss. It is recommended that a run of
message transactions be all of one type to avoid problems, i.e. all
normal or all datagrams. Finally, when a Server forwards a Request that
is part of a run, it must suspend further processing of the subsequent
Requests until the forwarded Request has been handled, to preserve order
of processing. The simplest handling of this situation is to use a real
nested call when forwarding with streamed message transactions.
Flow control of streamed message transactions relies on rate control at
the Client plus receipt (or non-receipt) of management notify operations
indicating the presence of overrunning. A Client must reduce the number
of outstanding message transactions at the Server when it receives a
NotifyVmtpServer operation with the MSGTRANS_OVERFLOW ResponseCode. The
transact parameter indicates the last packet group that was accepted.
_______________
<5> PGcount actually corresponds to packet groups which are described
in Section 2.13. This (simplified) description is accurate when there
is one Request or Response per packet group.
Cheriton [page 27]
RFC 1045 VMTP February 1988
The implementation of multiple outstanding message transactions requires
the ability to record, timeout and buffer multiple outstanding message
transactions at the Client end as well as the Server end. However, this
facility is optional for both the Client and the Server. Client systems
with heavy-weight processes and high network access cost are most likely
to benefit from this facility. Servers that serve a wide variety of
client machines should implement streaming to accommodate these types of
clients.
2.12. Fault-Tolerant Applications
One approach to fault-tolerant systems is to maintain a log of all
messages sent at each node and replay the messages at a node when the
node fails, after restarting it from the last checkpoint <6>. As an
experimental facility, VMTP provides a Receive Sequence Number field in
the NotifyVmtpClient and NotifyVmtpServer operations as well as the Next
Receive Sequence (NRS) flag in the Response packet to allow a sender to
log a receive sequence number with each message sent, allowing the
packets to be replayed at a recovering node in the same sequence as they
were originally received, thereby recovering to the same state as
before.
Basically, each sending node maintains a receive sequence number for
each receiving node. On sending a Request to a node, it presume that
the receive sequence number is one greater than the one it has recorded
for that node. If not, the receiving node sends a notify operation
indicating the receive sequence number assigned the Request. The NRS in
the Response confirms that the Request message was the next receive
sequence number, so the sender can detect if it failed to receive the
notify operation in the previous case. With Responses, the packets are
ordered by the Transaction identifier except for multicast message
transactions, in which there may be multiple Responses with the same
identification. In this case, NotifyVmtpServer operations are used to
provide receive sequence numbers.
This experimental extension of the protocol is focused on support for
fault-tolerant real-time distributed systems required in various
critical applications. It may be removed or extended, depending on
further investigations.
_______________
<6> The sender-based logging is being investigated by Willy Zwaenepoel
of Rice University.
Cheriton [page 28]
RFC 1045 VMTP February 1988
2.13. Packet Groups
A message (whether Request or Response) is sent as one or more packet
groups. A packet group is one or more packets, each containing the same
transaction identification and message control block. Each packet is
formatted as below with the message control block logically embedded in
the VMTP header.
+------------------------------------++---------------------+
| VMTP Header || |
+------------+-----------------------|| segment data |
|VMTP Control| Message Control Block || |
+------------+-----------------------++---------------------+
The some fields of the VMTP control portion of the packet and data
segment portion can differ between packets within the same packet group.
The segment data portion of a packet group represents up to 16
kilooctets of the segment specified in the message control block. The
portion contained in each packet is indicated by the PacketDelivery
field contained in the VMTP header. The PacketDelivery field as a bit
mask has a similar interpretation to the MsgDelivery field in that each
bit corresponds to a segment data block of 512 octets. The
PacketDelivery field limits a packet group to 16 kilooctets and a
maximum of 32 VMTP packets (with a minimum of 1 packet). Data can be
sent in fewer packets by sending multiple data blocks per packet. We
require that the underlying datagram service support delivery of (at
minimum) the basic 580 octet VMTP packet <7>. To illustrate the use of
the PacketDelivery field, consider for example the Ethernet which has a
MTU of 1536 octets. so one would send 2 512-octet segment data blocks
per packet. (In fact, if a third block is last in the segment and less
than 512 octets and fits in the packet without making it too big, an
Ethernet packet could contain three data blocks. Thus, an Ethernet
packet group for a segment of size 0x1D00 octets (14.5 blocks) and
MsgDelivery 0x000074FF consists of 6 packets indicated as follows <8>.
_______________
<7> Note that with a 20 octet IP header, a VMTP packet is 600
octets. We propose the convention that any host implementing VMTP
implicitly agrees to accept IP/VMTP packets of at least 600 octets.
<8> We use the C notation 0xHHHH to represent a hexadecimal number.
Cheriton [page 29]
RFC 1045 VMTP February 1988
Packet
Delivery 1 1 1 1 1 1 1 1 0 0 1 0 1 0 1 0 0 0 0 0 0 . . .
0000 0400 0800 0C00 1000 1400 1800 1C00
+----+----+----+----+----+----+----+-+
Segment |....|....|....|....|....|....|....|.|
+----+----+----+----+----+----+----+-+
: : : : : : : / / :
v v v v v v v /| v
+----+----+----+----+ +----+ +---+
Packets | 1 | 2 | 3 | 4 | | 5 | | 6 |
+----+----+----+----+ +----+ +---+
Each '.' is 256 octets of data. The PacketDelivery masks for the 6
packets are: 0x00000003, 0x0000000C, 0x00000030, 0x000000C0, 0x00001400
and 0x00006000, indicating the segment blocks contained in each of the
packets. (Note that the delivery bits are in little endian order.)
A packet group is sent as a single "blast" of packets with no explicit
flow control. However, the sender should estimate and transmit at a
rate of packet transmission to avoid congesting the network or
overwhelming the receiver, as described in Section 2.5.6. Packets in a
packet group can be sent in any order with no change in semantics.
When the first packet of a packet group is received (assuming the Server
does not decide to discard the packet group), the Server saves a copy of
the VMTP packet header, indicates it is currently receiving a packet
group, initializes a "current delivery mask" (indicating the data in the
segment received so far) to 0, accepts this packet (updating the current
delivery mask) and sets the timer for the packet group. Subsequent
packets in the packet group update the current delivery mask.
Reception of a packet group is terminated when either the current
delivery mask indicates that all the packets in the packet group have
been received or the packet group reception timer expires (set to TC3 or
TS1). If the packet group reception timer expires, if the NRT bit is
set in the Control flags then the packet group is discarded if not
complete unless MDM is set. In this case, the MsgDelivery field in the
message control block is set to indicate the segment data blocks
actually received and the message control block and segment data
received is delivered to application level.
If NRT is not set and not all data blocks have been received, a
NotifyVmtpClient (if a Request) or NotifyVmtpServer (if a Response) is
sent back with a PacketDelivery field indicating the blocks received.
The source of the packet group is then expected to retransmit the
missing blocks. If not all blocks of a Request are received after
RequestAckRetries(Client) retransmissions, the Request is discarded and
Cheriton [page 30]
RFC 1045 VMTP February 1988
a NotifyVmtpClient operation with an error response code is sent to the
client's manager unless MDM is set. With a Response, there are
ResponseAckRetries(Server) retransmissions and then, if MDM is not set,
the requesting entity is returned the message control block with an
indication of the amount of segment data received extending contiguously
from the start of the segment. E.g. if the sender sent 6 512-octet
blocks and only the first two and the last two arrived, the receiver
would be told that 1024 octets were received. The ResponseCode field is
set to BAD_REPLY_SEGMENT. (Note that VMTP is only able to indicate the
specific segment blocks received if MDM is set.)
The parameters RequestAckRetries(Client) and ResponseAckRetries(Server)
could be set on a per-client and per-server basis in a sophisticated
implementation based on knowledge of packet loss.
If the APG flag is set, a NotifyVmtpClient or NotifyVmtpServer
operation is sent back at the end of the packet group reception,
depending on whether it is a Request or a Response.
At minimum, a Server should check that each packet in the packet group
contains the same Client, Server, Transaction identifier and SegmentSize
fields. It is a protocol error for any field other than the Checksum,
packet group control flags, Length and PacketDelivery in the VMTP header
to differ between any two packets in one packet group. A packet group
containing a protocol error of this nature should be discarded.
Notify operations should be sent (or invoked) in the manager whenever
there is a problem with a unicast packet. i.e. negative acknowledgments
are always sent in this case. In the case of problems with multicast
packets, the default is to send nothing in response to an error
condition unless there is some clear reason why no other node can
respond positively. For example, the packet might be a Probe for an
entity that is known to have been recently existing on the receiving
host but now invalid and could not have migrated. In this case, the
receiving host responds to the Probe indicating the entity is
nonexistent, knowing that no other host can respond to the Probe. For
packets and packet groups that are received and processed without
problems, a Notify operation is invoked only if the APG bit is set.
2.14. Runs of Packet Groups
A run of packet groups is a sequence of packet groups, all Request
packets or all Response packets, with the same Client and consecutive
transaction identifiers, all but the first and last packets flagged with
the NSR (Not Start Run) and NER (Not End Run) control bits. When each
packet group in the run corresponds to a single Request or Response, it
Cheriton [page 31]
RFC 1045 VMTP February 1988
is identical to a run of message transactions. (See Section 2.11)
However, a Request message or a Response message may consists of up to
256 packet groups within a run, for a maximum of 4 megaoctets of segment
data. A message that is continued in the next packet group in the run
is flagged in the current packet group by the CMG flag. Otherwise, the
next packet group in the run (if any) is treated as a separate Request
or Response.
Normally, each Request and Response message is sent as a single packet
group and each run consists of a single packet group. In this case
neither NSR or NER are set. For multi-packet group messages, the
PacketDelivery mask in the i-th packet group of a message corresponds to
the portion of the segment offset by i-1 times 16 kilooctets,
designating the the first packet group to have i = 1.
2.15. Byte Order
For purposes of transmission and reception, the MCB is treated as
consisting of 8 32-bit fields and the segment is a sequence of bytes.
VMTP transmits the MCB in big-endian order, performing byte-swapping, if
necessary, before transmission. A little-endian host must byte-swap the
MCB on reception. (The data segment is transmitted as a sequence of
bytes with no reordering.) The byte order of the sender of a message is
indicated by the LEE bit in the entity identifier for the sender, the
Client field if a Request and the Server field if a Response. The
sender and receiver of a message are required to agree in some higher
level protocol (such as an RPC presentation protocol) on who does
further swapping of the MCB and data segment if required by the types of
the data actually being transmitted. For example, the segment data may
contain a record with 8-bit, 16-bit and 32-bit fields, so additional
transformation is required to move the segment from a host of one byte
order to another.
VMTP to date has used a higher-level presentation protocol in which
segment data is sent in the native order of the sending host and
byte-swapped as necessary by the receiving host. This approach
minimizes the byte-swapping overhead between machines of common byte
order (including when the communication is transparently local to one
host), avoids a strong bias in the protocol to one byte-order, and
allows for the sending entity to be sending to a group of hosts with
different byte orders. (Note that the byte-swap overhead for the MCB is
minimal.) The presentation-level overhead is minimal because most
common operations, such as file access operations, have parameters that
fit the MCB and data segment data types exactly.
Cheriton [page 32]
RFC 1045 VMTP February 1988
2.16. Minimal VMTP Implementation
A minimal VMTP client needs to be able to send a Request packet group
and receive a Response packet group as well as accept and respond to
Requests sent to its management module, including Probe and NotifyClient
operations. It may also require the ability to invoke Probe and Notify
operations to locate a Server and acknowledge responses. (the latter
only if it is involved in transactions that are not idempotent or
datagram message transactions. However, a simple sensor, for example,
can transmit VMTP datagram Requests indicating its current state with
even less mechanism.) The minimal client thus requires very little code
and is suitable as a basis for (e.g.) a network boot loader.
A minimal VMTP server implements idempotent, non-encrypted message
transactions, possibly with no segment data support. It should use an
entity state record for each Request but need only retain it while
processing the Request. Without segment data larger than a packet,
there is no need for any timers, buffering (outside of immediate request
processing) or queuing. In particular, it needs only as many records as
message transactions it handles simultaneously (e.g. 1). The entity
state record is required to recognize and respond to Request
retransmissions during request processing.
The minimal server need only receive Requests and and be able to send
Response packets. It need have only a minimal management module
supporting Probe operations. (Support for the NotifyVmtpClient
operation is only required if it does not respond immediately to a
Request.) Thus the VMTP support for say a time server, sensor, or
actuator can be extremely simple. Note that the server need never issue
a Probe operation if it uses the host address of the Request for the
Response and does not require the Client information returned by the
Probe operation. The minimal server should also support reception of
forwarded Requests.
2.17. Message vs. Procedural Request Handling
A request-response protocol can be used to implement two forms of
semantics on reception. With procedural handling of a Request, a
Request is handled by a process associated with the Server that
effectively takes on the identity of the calling process, treating the
Request message as invoking a procedure, and relinquishing its
association to the calling process on return. VMTP supports multiple
nested calls spanning multiple machines. In this case, the distributed
call stack that results is associated with a single process from the
standpoint of authentication and resource management, using the
ProcessId field supported by VMTP. The entity identifiers effectively
Cheriton [page 33]
RFC 1045 VMTP February 1988
link these call frames together. That is, the Client field in a Request
is effectively the return link to the previous call frame.
With message handling of a Request, a Request message is queued for a
server process. The server process dequeues, reads, processes and
responds to the Request message, executing as a separate process.
Subsequent Requests to the same server are queued until the server asks
to receive the next Request.
Procedural semantics have the advantage of allowing each Request (up to
the resource limits of the Server) to execute concurrently at the
Server, with Request-specific synchronization. Message semantics have
the advantage that Requests are serialized at the Server and that the
request processing logically executes with the priority, protection and
independent execution of a separate process. Note that procedural and
message handling of a request appear no differently to the client
invoking the message transaction, except possibly for differences in
performance.
We view the two Request handling approaches as appropriate under
different circumstances. VMTP supports both models.
2.18. Bibliography
The basic protocol is similar to that used in the original form of the V
kernel [3, 4] as well as the transport protocol of Birrell and
Nelson's [2] remote procedure call mechanism. An earlier version of the
protocol was described in SIGCOMM'86 [6]. The rate-based flow control
is similar to the techniques of Netblt [9]. The support for idempotency
draws, in part, on the favorable experience with idempotency in the V
distributed system. Its use was originally inspired by the Woodstock
File Server [11]. The multicast support draws on the multicast
facilities in V [5] and is designed to work with, and is now implemented
using, the multicast extensions to the Internet [8] described in RFC 966
and 988. The secure version of the protocol is similar to that
described by Birrell [1] for secure RPC. The use of runs of packet
groups is similar to Fletcher and Watson's delta-T protocol [10]. The
use of "management" operations implemented using VMTP in place of
specialized packet types is viewed as part of a general strategy of
using recursion to simplify protocol architectures [7].
Finally, this protocol was designed, in part, to respond to the
requirements identified by Braden in RFC 955. We believe that VMTP
satisfies the requirements stated in RFC 955.
Cheriton [page 34]
RFC 1045 VMTP February 1988
[1] A.D. Birrell, "Secure Communication using Remote Procedure
Calls", ACM. Trans. on Computer Systems 3(1), February, 1985.
[2] A. Birrell and B. Nelson, "Implementing Remote Procedure Calls",
ACM Trans. on Computer Systems 2(1), February, 1984.
[3] D.R. Cheriton and W. Zwaenepoel, "The Distributed V Kernel and its
Performance for Diskless Workstations", In Proceedings of the 9th
Symposium on Operating System Principles, ACM, 1983.
[4] D.R. Cheriton, "The V Kernel: A Software Base for Distributed
Systems", IEEE Software 1(2), April, 1984.
[5] D.R. Cheriton and W. Zwaenepoel, "Distributed Process Groups in
the V Kernel", ACM Trans. on Computer Systems 3(2), May, 1985.
[6] D.R. Cheriton, "VMTP: A Transport Protocol for the Next
Generation of Communication Systems", In Proceedings of
SIGCOMM'86, ACM, Aug 5-7, 1986.
[7] D.R. Cheriton, "Exploiting Recursion to Simplify an RPC
Communication Architecture", in preparation, 1988.
[8] D.R. Cheriton and S.E. Deering, "Host Groups: A Multicast
Extension for Datagram Internetworks", In 9th Data Communication
Symposium, IEEE Computer Society and ACM SIGCOMM, September, 1985.
[9] D.D. Clark and M. Lambert and L. Zhang, "NETBLT: A Bulk Data
Transfer Protocol", Technical Report RFC 969, Defense Advanced
Research Projects Agency, 1985.
[10] J.G. Fletcher and R.W. Watson, "Mechanism for a Reliable Timer-
based Protocol", Computer Networks 2:271-290, 1978.
Cheriton [page 35]
RFC 1045 VMTP February 1988
[11] D. Swinehart and G. McDaniel and D. Boggs, "WFS: A Simple File
System for a Distributed Environment", In Proc. 7th Symp.
Operating Systems Principles, 1979.
Cheriton [page 36]
RFC 1045 VMTP February 1988
3. VMTP Packet Formats
VMTP uses 2 basic packet formats corresponding to Request packets and
Response packets. These packet formats are identical in most of the
fields to simplify the implementation.
We first describe the entity identifier format and the packet fields
that are used in general, followed by a detailed description of each of
the packet formats. These fields are described below in detail. The
individual packet formats are described in the following subsections.
The reader and VMTP implementor may wish to refer to Chapters 4 and 5
for a description of VMTP event handling and only refer to this detailed
description as needed.
3.1. Entity Identifier Format
The 64-bit non-group entity identifiers have the following substructure.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| |L|R|
|A|0|E|E| Domain-specific structure
|E| |E|S|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Domain-specific structure |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The field meanings are as follows:
RAE Remote Alias Entity - the entity identifier identifies
an entity that is acting as an alias for some entity
outside this entity domain. This bit is used by
higher-level protocols. For instance, servers may take
extra security and protection measures with aliases.
GRP Group - 0, for non-group entity identifiers.
LEE Little-Endian Entity - the entity transmits data in
little-endian (VAX) order.
RES Reserved - must be 0.
The 64-bit entity group identifiers have the following substructure.
Cheriton [page 37]
RFC 1045 VMTP February 1988
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| |U|R|
|A|1|G|E| Domain-specific structure
|E| |P|S|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Domain-specific structure |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The field meanings are as follows:
RAE Remote Alias Entity - same as for non-group entity
identifier.
GRP Group - 1, for entity group identifiers.
UGP Unrestricted Group - no restrictions are placed on
joining this group. I.e. any entity can join limited
only by implementation resources.
RES Reserved - must be 0.
The all-zero entity identifier is reserved and guaranteed to be
unallocated in all domains. In addition, a domain may reserve part of
the entity identifier space for statically allocated identifiers.
However, this is domain-specific.
Description of currently defined entity identifier domains is provided
in Appendix IV.
3.2. Packet Fields
Client 64-bit identifier for the client entity associated with
this packet. The structure, allocation and binding of
this identifier is specific to the specified Domain. An
entity identifier always includes 4 types bits as
specified in Section 3.1.
Version The 3-bit identifier specifying the version of the
protocol. Current version is version 0.
Domain The 13-bit identifier specifying the naming and
administration domain for the client and server named in
the packet.
Cheriton [page 38]
RFC 1045 VMTP February 1988
Packet Flags: 3 bits. (The normal case has none of the flags set.)
HCO Header checksum only - checksum has only been calculated
on the header. This is used in some real-time
applications where the strict correctness of the data is
not needed.
EPG Encrypted packet group - part of a secure message
transaction.
MPG Multicast packet group - packet was multicast on
transmission.
Length A 13-bit field that specifies the number of 32-bit words
in the segment data portion of the packet (if any),
excluding the checksum field. (Every VMTP packet is
required to be a multiple of 64 bits, possibly by
padding out the segment data.) The minimum legal Length
is 0, the maximum length is 4096 and it must be an even
number.
Control Flags: 9 bits. (The normal case has none of the flags set.)
NRS Next Receive Sequence - the associated Request message
(in a Response) or previous Response (if a Request) was
received consecutive with the last Request from this
entity. That is, there was no interfering messages
received.
APG Acknowledge Packet Group - Acknowledge packet group on
receipt. If a Request, send back a Request to the
client's manager providing an update on the state of the
transaction as soon as the request packet group is
received, independent of the response being available.
If a Response, send an update to the server's manager as
soon as possible after response packet group is received
providing an update on the state of the transaction at
the client
NSR Not Start Run - 1 if this packet is not part of the
first packet group of a run of packet groups.
NER Not End Run - 1 if this packet is not part of the last
packet group of a run of packet groups.
NRT No Retransmission - do not ask for retransmissions of
this packet group if not all received within timeout
Cheriton [page 39]
RFC 1045 VMTP February 1988
period, just deliver or discard.
MDG Member of Destination Group - this packet is sent to a
group and the client is a member of this group.
CMG Continued Message - the message (Request or Response) is
continued in the next packet group. The next packet
group has to be part of the same run of packet groups.
STI Skip Transaction Identifiers - the next transaction
identifier that the Client plans to use is the current
transaction plus 256, if part of the same run and at
least this big if not. In a Request, this authorizes
the Server to send back up to 256 packet groups
containing the Response.
DRT Delay Response Transmission - set by request sender if
multiple responses are expected (as indicated by the MRD
flag in the RequestCode) and it may be overrun by
multiple responses. The responder(s) should then
introduce a short random delay in sending the Response
to minimize the danger of overrunning the Client. This
is normally only used for responding to multicast
Requests where the Client may be receiving a large
number of Responses, as indicated by the MRD flag in the
Request flags. Otherwise, the Response is sent
immediately.
RetransmitCount:
3 bits - the ordinal number of transmissions of this
packet group prior to this one, modulo 8. This field is
used in estimation of roundtrip times. This count may
wrap around during a message transaction. However, it
should be sufficient to match acknowledgments and
responses with a particular transmission.
ForwardCount: 4 bits indicating the number of times this Request has
been forwarded. The original Request is always sent
with a ForwardCount of 0.
Interpacket Gap: 8 bits.
Indicates the recommended time to use between subsequent
packet transmissions within a multi-packet packet group
transmission. The Interpacket Gap time is in 1/32nd of
a network packet transmission time for a packet of size
MTU for the node. (Thus, the maximum gap time is 8
packet times.)
Cheriton [page 40]
RFC 1045 VMTP February 1988
PGcount: 8 bits
The number of packet groups that this packet group
represents in addition to that specified by the
Transaction field. This is used in acknowledging
multiple packet groups in streamed communication.
Priority 4-bit identifier for priority for the processing of this
request both on transmission and reception. The
interpretation is:
1100 urgent/emergency
1000 important
0000 normal
0100 background
Viewing the higher-order bit as a sign bit (with 1
meaning negative), low values are high priority and high
values are low priority. The low-order 2 bits indicate
additional (lower) gradations for each level.
Function Code: 1 bit - types of VMTP packets. If the low-order bit of
the function code is 0, the packet is sent to the
Server, else it is sent to the Client.
0 Request
1 Response
Transaction: 32 bits:
Identifier for this message transaction.
PacketDelivery: 32 bits:
Delivery indicates the segment blocks contained in this
packet. Each bit corresponds to one 512-octet block of
segment data. A 1 bit in the i-th bit position
(counting the LSB as 0) indicates the presence of the
i-th segment block.
Server: 64 bits
Entity identifier for the server or server group
associated with this transaction. This is the receiver
when a Request packet and the sender when a Response
packet.
Cheriton [page 41]
RFC 1045 VMTP February 1988
Code: 32 bits The Request Code and Response Code, set either at the
user level or VMTP level depending on use and packet
type. Both the Request and Response codes include 8
high-order bits from the following set of control bits:
CMD Conditional Message Delivery - only deliver the request
or response if the receiving entity is waiting for it at
the time of delivery, otherwise drop the message.
DGM DataGram Message - indicates that the message is being
sent as a datagram. If a Request message, do not wait
for reply, or retransmit. If a Response message, treat
this message transaction as idempotent.
MDM Message Delivery Mask - indicates that the MsgDelivery
field is being used. Otherwise, the MsgDelivery field
is available for general use.
SDA Segment Data Appended - segment data is appended to the
message control block, with the total size of the
segment specified by the SegmentSize field. Otherwise,
the segment data is null and the SegmentSize field is
not used by VMTP and available for user- or RPC-level
uses.
CRE CoResident Entity - indicates that the CoResidentEntity
field in the message should be interpreted by VMTP.
Otherwise, this field is available for additional user
data.
MRD Multiple Responses Desired - multiple Responses are
desired to to this Request if it is multicast.
Otherwise, the VMTP module can discard subsequent
Responses after the first Response.
PIC Public Interface Code - Values for Code with this bit
set are reserved for definition by the VMTP
specification and other standard protocols defined on
top of VMTP.
RES Reserved for future use. Must be 0.
CoResidentEntity
64-bit Identifier for an entity or group of entities
with which the Server entity or entities must be
co-resident, i.e. route only to entities (identified by
Server) on the same host(s) as that specified by
Cheriton [page 42]
RFC 1045 VMTP February 1988
CoResidentEntity, Only meaningful if CRE is set in the
Code field.
User Data 12 octets Space in the header for the VMTP user to
specify user-specific control and data.
MsgDelivery: 32 bits
The segment blocks being transmitted (in total) in this
packet group following the conventions for the
PacketDelivery field. This field is ignored by the
protocol and treated as an additional user data field if
MDM is 0. On transmission, the user level sets the
MsgDelivery to indicate those portions of the segment to
be transmitted. On receipt, the MsgDelivery field is
modified by the VMTP module to indicate the segment data
blocks that were actually received before the message
control block is passed to the user or RPC level. In
particular, the kernel does not discard the packet gro