A Multi-Site Data Collection Facility

Preface:

This RFC reproduces most of a working document
prepared during the design and implementation of the protocols for the TIP-TENEX integrated system for
handling TIP accounting. Bernie Cosell (BBN-TIP)
and Bob Thomas (BBN-TENEX) have contributed to
various aspects of this work. The system has been
partially operational for about a month on selected
hosts. We feel that the techniques described here
have wide applicability beyond TIP accounting.

Section I

Protocols for a Multi-site Data Collection Facility

Introduction

The development of computer networks has provided the

groundwork for distributed computation: one in which a job or task

is comprised of components from various computer systems. In a

single computer system, the unavailability or malfunction of any of

the job components (e.g. program, file, device, etc.) usually

necessitates job termination. With computer networks, it becomes

feasible to duplicate certain job components which previously had no

basis for duplication. (In a single system, it does not matter how

many times a process that performs a certain function is duplicated;

a system crash makes all unavailable). It is such resource

duplication that enables us to utilize the network to achieve high

reliability and load leveling. In order to realize the potential of

resource duplication, it is necessary to have protocols which

provide for the orderly use of these resources. In this document,

we first discuss in general terms a problem of protocol definition

for interacting with a multiply defined resource (server). The

problem deals with providing a highly reliable data collection

facility, by supporting it at many sites throughout the network. In

the second section of this document, we describe in detail a

particular implementation of the protocol which handles the problem

of utilizing multiple data collector processes for collecting

accounting data generated by the network TIPs. This example also

illustrates the specialization of hosts to perform parts of a

computation they are best equipped to handle. The large network

hosts (TENEX systems) perform the accounting function for the small

network access TiPs.

The situation to be discussed is the following: a data

generating process needs to use a data collection service which is

duplicately provided by processes on a number of network machines.

A request to a server involves sending the data to be collected.

An Initial Approach

The data generator could proceed by selecting a particular

server and sending its request to that server. It might also take

the attitude that if the message reaches the destination host (the

communication subsystem will indicate this) the message will be

properly processed to completion. Failure of the request Message

would then lead to selecting another server, until the request

succeeds or all servers have been tried.

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Such a simple strategy is a poor one. It makes sense to

require that the servicing process send a positive acknowledgement

to the requesting process. If nothing else, the reply indicates

that the server process itself is still functioning. Waiting for

such a reply also implies that there is a strategy for selecting

another server if the reply is not forthcoming. Herein lies a

problem. If the expected reply is timed out, and then a new request

is sent to another server, we run the risk of receiving the

(delayed) original acknowledgement at a later time. This could

result in having the data entered into the collection system twice

(data duplication). If the request is re-transmitted to the same

server only, we face the possibility of not being able to access a

collector (data loss). In addition, for load leveling purposes, we

may wish to send new requests to some (or all) servers. We can then

use their reply (or lack of reply) as an indicator of load on that

particular instance of the service. Doing this without data

duplication requires more than a simple request and acknowledgement

protocol*.

Extension of the Protocol

The general protocol developed to handle multiple collection

servers involves having the data generator send the data request to

some (or all) data collectors. Those willing to handle the request

reply with an "I've got it" message. They then await further

notification before finalizing the processing of the data. The data

generator sends a "go ahead" message to one of the replying

collectors, and a "discard" message to all other replying

collectors. The "go ahead" message is the signal to process the

data (i.e. collect permanently), while the "discard" message

indicates that the data is being collected elsewhere and should not

be retained.

The question now arises as to whether or not the collector

process should acknowledge receipt of the "go ahead" message with a

reply of its own, and then should the generator process acknowledge

this acknowledgement, etc. We would like to send as few messages as

possible to achieve reliable communication. Therefore, when a state

--------------------

* If the servers are independent of each other to the extent that if

two or more servers all act on the same request, the end result is

the same as having a single server act on the request, then a simple

request/acknowledgement protocol is adequate. Such may be the case,

for example, if we subject the totality of collected data (i.e. all

data collected by all collectors for a certain period) to a

duplicate detection scan. If we could store enough context in each

entry to be able to determine duplicates, then having two or more

servers act on the data would be functionally equivalent to

processing by a single server.

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is reached for which further acknowledgements lead to a previously

visited state, or when the cost of further acknowledgements outweigh

the increase in reliability they bring, further acknowledgements

become unnecessary.

The initial question was should the collector process

acknowledge the "go ahead" message? Assume for the moment that it

should not send such an acknowledgement. The data generator could

verify, through the communication subsystem, the transmission of the

"go ahead" message to the host of the collector. If this message

did not arrive correctly, the generator has the option of

re-transmitting it or sending a "go ahead" to another collector

which has acknowledged receipt of the data. Either strategy

involves no risk of duplication. If the "go ahead" message arrives

correctly, and a collector acknowledgement to the "go ahead" message

is not required, then we incur a vulnerability to (collector host)

system crash from the time the "go ahead" message is accepted by the

host until the time the data is totally processed. Call the data

processing time P. Once the data generator has selected a

particular collector (on the basis of receiving its "I've got it"

message), we also incur a vulnerability to malfunction of this

collector process. The vulnerable period is from the time the

collector sends its "i've got it" message until the time the data is

processed. This amounts to two network transit times (2N) plus IMP

and host overhead for message delivery (0) plus data processing time

(P). [Total time=2N+P+O]. A malfunction (crash) in this period can

cause the loss of data. There is no potential for duplication.

Now, assume that the data collector process must acknowledge

the "go ahead" message. The question then arises as to when such an

acknowledgement should be sent. The reasonable choices are either

immediately before final processing of the data (i.c. before the

data is permanently recorded) or immediately after final processing.

It can be argued that unless another acknowledgement is required (by

the generator to the collector) to this acknowledgement BEFORE the

actual data update, then the best time for the collector to

acknowledge the "go ahead" is after final processing. This is so

because receiving the acknowledgement conveys more information if it

is sent after processing, while not receiving it (timeout), in

either case, leaves us in an unknown state with respect to the data

update. Depending on the relative speeds of various network and

system components, the data may or may not be permanently entered.

Therefore if we interpret the timeout as a signal to have the data

processed at another site, we run the risk of duplication of data.

To avoid data duplication, the timeout strategy must only involve

re-sending the "go ahead" message to the same collector. This will

only help if the lack of reply is due to a lost network message.

Our vulnerability intervals to system and process malfunction remain

as before.

It is our conjecture (to be analyzed further) that any further

acknowledgements to these acknowledgements will have virtually no

effect on reducing the period of vulnerability outlined above. As

such, the protocol with the fewest messages required is superior.

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Data Dependent Aspects of the Protocol

As discussed above, a main issue is which process should be the

last to respond (send an acknowledgement). If the data generator

sends the last message (i.e. "go ahead"), we can only check on its

correct arrival at the destination host. We must "take on faith"

the ability of the collector to correctly complete the transaction.

This strategy is geared toward avoiding data duplication. If on the

other hand, the protocol specifies that the collector is to send the

last message, with the timeout of such a message causing the data

generator to use another collector, then the protocol is geared

toward the best efforts of recording the data somewhere, at the

expense of possible duplication.

Thus, the nature of the problem will dictate which of the

protocols is appropriate for a given situation. The next section

deals in the specifics of an implement;tion of a data collection

protocol to handle the problem of collecting TIP accounting data by

using the TENEX systems for running the collection server processes.

It is shown how the general protocol is optimized for the accounting

data collection.

Section II

Protocol for TIP-TENEX Accounting Server Information Exchange

Overview of the Facility

When a user initially requests service from a TIP, the TIP will

perform a broadcast ICP to find an available RSEXEC which maintains

an authentication data base. The user must then complete s login

sequence in order to authenticate himself. If he is successful the

RSEXEC will transmit his unique ID code to the TIP. Failure will

cause the RSEXEC to close the connection and the TIP to hang up on

the user. After the user is authenticated, the TIP will accumulate

accounting data for the user session. The data includes a count of

messages sent on behalf of the user, and the connect time for the

user. From time to time the TIP will transmit intermediate

accounting data to Accounting Server (ACTSER) processes scattered

throughout the network. These accounting servers will maintain

files containing intermediate raw accounting data. The raw

accounting data will periodically be collected and sorted to produce

an accounting data base. Providing a number of accounting servers

reduces the possibility of being unable to find a repository for the

intermediate data, which otherwise would be lost due to buffering

limitations in the TiPs. The multitude of accounting servers can

also serve to reduce the load on the individual hosts providing this

facility.

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The rest of this document details the protocol that has been

developed to ensure delivery of TIP accounting data to one of the

available accounting servers for storage in the intermediate

accounting files.

Adapting the Protocol

The TIP to Accounting Server data exchange uses a protocol that

allows the TIP to select for data transmission one, some, or all

server hosts either sequentially or in parallel, yet insures that

the data that becomes part of the accounting file does not contain

duplicate information. The protocol also minimizes the amount of

data buffering that must be done by the limited capacity TiPs. The

protocol is applicable to a wide class of data collection problems

which use a number of data generators and collectors. The following

describes how the protocol works for TIP accounting.

Each TIP is responsible for maintaining in its memory the cells

indicating the connect time and the number of messages sent for each

of its current users. These cells are incremented by the TIP for

every quantum of connect time and message sent, as the case may be.

This is the data generation phase. Periodically, the TIP will scan

all its active counters, and along with each user ID code, pack the

accumulated data into one network message (i.e. less than 8K bits).

The TIP then transmits this data to a set of Accounting Server

processes residing throughout the network. The data transfer is

over a specially designated host-host link. The accounting servers

utilize the raw network message facility of TENEX 1.32 in order to

directly access that link. When an ACTSER receives a data message

from a TIP, it buffers the data and replies by returning the entire

message to the originating TIP. The TIP responds with a positive

acknowledgement ("go ahead") to the first ACTSER which returns the

data, and responds with a negative acknowledgement ("discard") to

all subsequent ACTSER data return messages for this series of

transfers. If the TIP does not receive a reply from any ACTSER, it

accumulates new data (i.e. the TIP has all the while been

incrementing its local counters to reflect the increased connect

time and message count; the current values will comprise new data

transfers) and sends the new data to the Accounting Server

processes. When an ACTSER receives a positive acknowledgement from

a TIP (i.e. "go ahead"), it appends the appropriate parts of the

buffered data to the locally maintained accounting information file.

On receiving a negative acknowledgement from the TIP (i.e.

"discard"), the ACTSER discards the data buffered for this TIP. In

-addition, when the TIP responds with a "go ahead" to the first

ACTSER which has accepted the data (acknowledged by returning the

data along with the "I've got it"), the TIP decrements the connect

time and message counters for each user by the amount indicated in

the data returned by the ACTSER. This data will already be

accounted for in the intermediate accounting files.

As an aid in determining which ACTSER replies are to current

requests, and which are tardy replies to old requests, the TIP

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maintains a sequence number indicator, and appends this number to

each data message sent to an ACTSER. On receiving a reply from an

ACTSER, the TIP merely checks the returned sequence number to see if

this is the first reply to the current set of TIP requests. If the

returned sequence number is the same as the current sequence number,

then this is the first reply; a positive acknowledgement is sent

off, the counters are decremented by the returned data, and the

sequence number is incremented. If the returned sequence number is

not the same as the current one (i.e. not the one we are now

seeking a reply for) then a negative acknowledgement is sent to the

replying ACTSER. After a positive acknowledgement to an ACTSER (and

the implied incrementing of the sequence number), the TIP can wait

for more information to accumulate, and then start transmitting

again using the new sequence number.

Further Clarification of the Protocol

There are a number of points concerning the protocol that

should be noted.

1 The data generator (TIP) can send different (i.e. updated

versions) data to different data collectors (accounting servers) as

part of the same logical transmission sequence. This is possible

because the TIP does not account for the data sent until it receives

the acknowledgement of the data echo. This strategy relieves the

TIP of any buffering in conjunction with re-transmission of data

which hasn't been acknowledged.

2 A new data request to an accounting server from a TIP will

also serve as a negative acknowledgement concerning any data already

buffered by the ACTSER for that TIP, but not yet acknowledged. The

old data will be discarded, and the new data will be buffered and

echoed as an acknowledgement. This allows the TIP the option of not

sending a negative acknowledgement when it is not convenient to do

so, without having to remember that it must be sent at a later time.

There is one exception to this convention. If the new data message

has the same sequence number as the old buffered message, then the

new data must be discarded, and the old data kept and re-echoed.

This is to prevent a slow acknowledgement to the old data from being

accepted by the TIP, after the TIP has already sent the new data to

the slow host. This caveat can be avoided if the TIP does not

resend to a non-responding server within the time period that a

message could possibly be stuck in the network, but could still be

delivered. Ignoring this situation may result in some accounting

data being counted twice. Because of the rule to keep old data when

confronted with matching sequence numbers, on restarting after a

crash, the TIP should send a "discard" message to all servers in

order to clear any data which has been buffered for it prior to the

crash. An alternative to this would be for the TIP to initialize

its sequence number from a varying source such as time of day.

3 The accounting server similarly need not acknowledge receipt

of data (by echoing) if it finds itself otherwise occupied. This

will mean that the ACTSER is not buffering the data, and hence is

not a candidate for entering the data into the file. However, the

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TIP may try this ACTSER at a later time (even with the same data),

with no ill effects.

4 Because of 2 and 3 above, the protocol is robust with respect

to lost or garbled transmissions of TIP data requests and accounting

server echo replies. That is, in the event of loss of such a

message, a re-transmission will occur as the normal procedure.

5 There is no synchronization problem with respect to the

sequence number used for duplicate detection, since this number is

maintained only at the TIP site. The accounting server merely

echoes the sequence number it has received as part of the data.

6 There are, however, some constraints on the size of the

sequence number field. It must be large enough so that ALL traces

of the previous use of a given sequence number are totally reMoved

from the network before the number is re-used by the TIP. The

sequence number is modulo the size of the largest number represented

by the number of bits allocated, and is cyclic. Problems generally

arise when a host proceeds from a service interruption while it was

holding on to a reply. If during the service interruption, we have

cycled through our sequence numbers exactly N times (where N is any

integer), this VERY tardy reply could be mistaken for a reply to the

new data, which has the same sequence number (i.e. N revolutions of

sequence numbers later). By utilizing a sufficiently large sequence

number field (16 bits), and by allowing sufficient time between

instances of sending new data, we can effectively reduce the

probability of such an error to zero.

7 Since the data involved in this problem is the source of

accounting information, care must be taken to avoid duplicate

entries. This must be done at the expense of potentially losing

data in certain instances. Other than the obvious TIP malfunction,

there are two known ways of losing data. One is the situation where

no accounting server responds to a TIP for an extended period of

time causing the TIP counters to overflow (highly unlikely if there

are sufficient Accounting Servers). In this case, the TIP can hold

the counters at their maximum value until a server comes up, thereby

keeping the lost accounting data at its minimum. The other

situation results from adapting the protocol to our insistence on no

duplicate data in the incremental files. We are vulnerable to data

loss with no recourse from the time the server receives the "go

ahead" to update the file with the buffered data (i.e. positive

acknowledgement) until the time the update is completed and the file

is closed. An accounting server crash during this period will cause

that accounting data to be lost. In our initial implementation, we

have slightly extended this period of vulnerability in order to save

the TIP from having to buffer the acknowledged data for a short

period of time. By updating TIP counters from the returned data in

parallel with sending the "go ahead" acknowledgement, we relieve the

TIP of the burden of buffering this data until the Request for Next

Message (RFNM) from the accounting server IMP is received. This

adds slightly to our period of vulnerability to malfunction, moving

the beginning of the period from the point when the ACTSER host

receives the "go ahead", back to the point when the TIP sends off

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the "go ahead" (i.e. a period of one network transit time plus some

IMP processing time). However, loss of data in this period is

detectable through the Host Dead or Incomplete Transmission return

in place of the RFNM. We intend to record such occurrences with the

Network Control Center. If this data loss becomes intolerable, the

TIP program will be modified to await the RFNM for the positive

acknowledgement before updating its counters. In such a case, if

the RFNM does not come, the TIP can discard the buffered data and

re-transmit new data to other servers.

8 There is adequate protection against the entry of forged data

into the intermediate accounting files. This is primarily due to

the system enforced limited access to Host-Imp messages and

Host-Host links. In addition, messages received on such designated

limited access links can be easily verified as coming from a TIP.

The IMP subnet appends the signature (address) of the sending host

to all of its messages, so there can be no forging. The Accounting

Server is in a position to check if the source of the message is in

fact a TIP data generator.

Current Parameters of the Protocol

In the initial implementation, the TIP sends its accumulated

accounting data about once every half hour. If it gets no positive

acknowledgement, it tries to send with greater frequency (about

every 5 minutes) until it finally succeeds. It can then return to

the normal waiting period. (A TIP user logout introduces an

exception to this behavior. In order to re-use the TIP port and its

associated counters as soon as possible, a user terminating his TIP

session causes the accounting data to be sent immediately).

initially, our implementation calls for each TIP to remember a

"favored" accounting server. At the wait period expiration, the TIP

will try to deposit the data at its "favored" site. If successful

within a short timeout period, this site remains the favored site,

and the wait interval is reset. If unsuccessful within the short

timeout, the data can be sent to all servers*. The one replying

first will update its file with the data and also become the

"favored" server for this TIP. With these parameters, a host would

have to undergo a proceedable service interruption of more than a

year in order for the potential sequence number problem outlined in

(6) above to occur.

Concluding Remarks

When the implementation is complete, we will have a general

data accumulation and collection system which can be used to gather

a wide variety of information. The protocol as outlined is geared

to gathering data which is either independent of the previously

accumulated data items (e.g. recording names), or data which

adheres to a commutative relationship (e.g. counting). This is a

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consequence of the policy of retransmission of different versions of

the data to different potential collectors (to relieve TIP buffering

problems).

In the specified version of the protocol, care was taken to

avoid duplicate data entries, at the cost of possibly losing some

data through collector malfunction. Data collection problems which

require avoiding such loss (at the cost of possible duplication of

some data items) can easily be accommodated with a slight adjustment

to the protocol. Collected data which does not adhere to the

commutative relationship indicated above, can also be handled by

utilizing more buffer space at the data generator sites.

The sequence number can be incremented for this new set of data

messages, and the new data can also be sent to the slow host. In

this way we won't be giving the tardy response from the old favored

host unfair advantage in determining which server can respond most

quickly. If there is no reply to this series of messages, the TIP

can continue to resend the new data. However, the sequence number

should not be incremented, since no reply was received, and since

indiscriminate incrementing of the sequence number increases the

chance of recycling during the lifetime of a message.

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