Internet Info 1994 March/Internet Info CD-ROM (Walnut Creek) (March 1994).iso/inet/ien/ien-100

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INDRA Note 754
IEN 100
May 3rd 1979










                Comparison of the DIN FTP and the NI FTP

                             C. J. Bennett
                            P. L. Higginson










               ABSTRACT: This note assesses the  DIN  FTP
               proposed   for  AUTODIN  II  according  to
               design aims  developed  in  IEN  99.   The
               facilities  available  in  the DIN FTP are
               compared with those in the NI FTP.

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     The FTP proposed by BBN for  AUTODIN  II  is  conceptually  closely
related  to  NI  FTP.   Many  of its basic concepts are derived from it,
including the fundamental notions  of  the  conceptual  file  store  and
parameter  negotiation.   Many  of the attributes used are extensions or
restrictions of  NI  FTP  attributes,  although  the  coding  is  rather
different.   Thus the approach taken in this note is to view the DIN FTP
as an alternative approach to realising the same design aims.   The  two
FTPs  are  therefore  compared  according  to  the  design  requirements
developed in IEN 99.  Additionally, where the FTPs differ  significantly
in  the  facilities  offered  to the user, these facilities are compared
directly.

DIN FTP AND THE NI FTP DESIGN REQUIREMENTS

(i) Transport service independence.  The DIN  FTP  makes  the  following
assumptions about the underlying transport service:

     (a) The transport service will  provide  a  synchronised  sequenced
     stream  of octets between the two ends.  This is the same as the NI
     FTP.

     (b) All recoverable errors are handled by  the  transport  service.
     (This  is by implication, as the point is not explicitly discussed.
     This is certainly the level of service provided by  TCP).   As  the
     DIN  FTP  incorporates  all  the recovery mechanisms of the NI FTP,
     this defect can be easily remedied, but as the specification stands
     it  cannot  be  used  above  X25  or  similar services, which use a
     transport service RESET involving loss of data.

     (c) The identity of the host of the connection  initiator  will  be
     made  available  by the transport service (as all references to the
     'Controlling Host ID' and 'Active Host ID' are by citation).   This
     is  of course a reasonable assumption but not a necessary one.  For
     example, the UCL NI FTP  project  is  investigating  NI  FTP  above
     mapped  concatenated  virtual  calls.   In this situation, the only
     addressing information which could be  supplied  from  a  transport
     service is the address of the last transport protocol convertor.

     (d) Any implementation  using  the  UserID  parameter  assumes  the
     transport  service will perform authentication actions at all sites
     involved in the transfer.  This assumption  is  only  true  of  the
     Secure  TCP; hence implementations using this parameter can only be
     used on AUTODIN II.  We will return to the issue of access  control
     below.

(ii) Supportable applications.  The DIN FTP has removed several  options
of the NI FTP which are useful in this regard.  These include:

     (a) The ability  to  mix  codes  in  a  file.   There  are  several
     applications where this is useful, such as job output and graphics.
     An example of  more  immediate  interest  is  mixed  FAX  and  text
     messages.   This  restriction  is  arbitrary  and  could  easily be

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     removed from the DIN FTP (though the FileDataType parameter must be
     redefined to be bit-coded).

     (b) Knowledge of the output device type.  This prevents the DIN FTP
     from being used in spooling applications.

     (c) The distinction between selectors and  modifiers.   This  is  a
     property of the conceptual filestore.  Its use is not very apparent
     in the NI FTP as such, but it enables other protocols such  as  Job
     Transfer   and  File  Management  protocols  to  be  built  easily.
     (Consider the difference between 'Select File with FileName X'  and
     'Modify  (selected)  FileName  to X'.) The topic of other protocols
     will be returned to later.

     (d) The ability to readily trace  file  transfers  in  system  logs
     through the Monitor Message parameter.  This is particularly useful
     for error tracing and for following up user complaints.

(iii) Operating system independence.  The DIN FTP  has  removed  several
attributes  which  will  make  it  difficult  to  implement  it  on some
operating  systems.   In  particular  we  note  the  following  NI   FTP
attributes  omitted  in  the  DIN  FTP  which may cause problems in this
regard:

     (a) Maximum Transfer Size.  This refers to the amount of data  that
     will actually be transferred; the Maximum File Size is the size the
     resultant file may not exceed.  The two are conceptually different,
     and  may  have different effects.  On some systems (eg IBM's OS360,
     GEC 4080) the Maximum File Size, reserved at  file  creation  time,
     sets  a  limit  for  all  eternity,  and must cover all anticipated
     appends.  The DIN FTP's MaximumFileSize  parameter  is  in  fact  a
     MaximumTransferSize parameter.

     (b) Filename Password.  Files may legitimately be accessed (even on
     TENEX)  by  giving  an  appropriate  key  without  the  user  being
     recognised as the owner of that file.  Some IBM systems can require
     a   legitimately   logged   in   user   to  provide  an  additional
     file-specific password before access is granted to a file.

     (c) Account Password.  Usernames and Accountnames are  conceptually
     orthogonal.   In  systems  where  a  'username'  corresponds  to  a
     superdirectory shared  between  many  different  users,  these  may
     distinguished  by  accounts  with  a  separate  level  of  password
     protection.  An  example  of  this  kind  of  double  lock  is  the
     superuser mode on UCL's UNIX.


The issue here is not one of meeting  the  peculiarities  of  particular
operating  systems.  The rationale behind such features of the NI FTP is
that they represent different fundamental concepts, and that even though
some  systems  may  not find the distinctions important, it was foreseen
that  some  implementations  and  applications  would  find  them   very

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necessary.   Nevertheless,  it should be stressed that the NI FTP design
group had experience of a wide range of operating systems, most of which
are  commonly  used in the UK.  The attributes introduced were all found
to be either immediately necessary, or represented areas in which it was
felt "escape routes" should be provided.

(iv) Extendability.  There are two aspects to this question.  One is the
provision  of  "escape  routes",  mentioned  above.   The  NI  FTP  spec
identifies several areas where such escape routes are  needed,  notably:
Access  Control (via the 'Account Password', which, as we saw above, can
be used to cover secondary levels of protection); File Description  (via
'Kinship');  Data coding (via the 'Private Codes' selector); and special
processing (via 'Special  Options').   With  the  exception  of  Private
Codes,  these  are omitted from the DIN FTP.  The DIN FTP does provide a
'HostHardwareTypes' escape parameter; this extends a  feature  which  is
not supported by NI FTP for reasons discussed below.

The second aspect is protocol extension.  As discussed above, the NI FTP
readily  extends  to  new  attributes during the negotiation phase.  The
inclusion of new data transfer management commands  (level  1)  is  more
difficult  than  with  DIN  FTP, but the memoryless transfer model means
that very few such commands should really be needed.  The mechanisms for
negotiating  sets  of  protocol extensions (with option sets, relational
operators and so on) are very similar in both protocols.  (A minor point
is  that  the  NI  FTP  has attempted to bit-encode option selectors and
operators.  This  allows,  for  example,  a  natural  extension  of  the
protocol to include LT and GT operators.)

FACILITIES COMPARISON

(i) The three party transfer model.  The approach taken by the  DIN  FTP
is  not  actually  incompatible  with  the  NI  FTP, and one could quite
reasonably implement an NI FTP (or a whole  family  of  implementations)
which used a private Controller/Active protocol, with extensions such as
'SourceFile', 'SourceUser', etc, and necessary additional commands  such
as 'Disconnect'.

(ii) Access Control.  We have presented above our reasons for  believing
that  the DIN FTP is inadequate as a general-purpose FTP in this regard.
The (very strong) access control available on AUTODIN  II  is  transport
service specific, and implementations which support it are not portable.
Use of this facility will tend to create a  "closed  community"  of  FTP
implementations.  The network-independent attributes provided for access
control will not always be sufficient.  The three party model used  also
allows  the  Active site to inspect the access control parameters issued
by the controller for the Passive, which could lead to security problems
when  authentication  is  an  important  issue.   (Obviously this cannot
happen in a two-party model).

Access control is not really an FTP issue.  Different systems can choose
different  methods:  source  host  lists, recall addresses, and password
protection  are  three.   An  FTP  should  have  hooks   for   providing

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information  to  systems where the access control is password based, but
this is simply a channel for contending with one type of access control.
The  actual requirements for access control are implementation-specific.
The argument that single-host ownership of files is a  major  impediment
to  distributed  file  sharing is undeniable, but we feel that it is not
the job of an FTP to solve this problem.  As and when  solutions  become
available  they should be exploited by FTP implementations, but the best
that the protocol specification can do  is  to  recognise  the  existing
state of chaos.

(iii) The DIN FTP provides formal grades of implementation.  The NI  FTP
is  more  flexible,  as it allows specific implementations to suit their
own  needs  and  facilities,  and  it  is  only  recommended  that   all
implementations  support a defined set of defaults, while 'core' DIN FTP
attributes are all required.  Thus a very specific NI FTP implementation
can be built for a specialised device which does not support unnecessary
default values.  (We at UCL intend to exploit this feature of NI FTP  in
our FAX project).

At a more philosophical level, this  reflects  the  design  environments
from which the FTPs emerged - the NI FTP is being offered as a basis for
standardisation for any  group.   These  cannot  be  controlled  by  any
central  or  regulating  body;  the  DIN FTP assumes the more closed and
controlled environment of AUTODIN II.

(iv) The Data Transfer Protocol.  We are  unconvinced  that  the  formal
separation of a data transfer protocol is a useful extension.  As far as
the FTP is concerned, it is little different from  the  NI  FTP  records
during the data transfer phase, but complicates the formatting of NI FTP
commands considerably.  An NI FTP SFT, as generated by the current TENEX
version,  may  occupy  78 octets complete with headers; with the DTP the
equivalent SFT requires 28 sets of headers instead of  2,  and  occupies
131  octets.   (With  data  segments,  the  NI FTP is more efficient for
records of less than 126 bytes, and the DTP becomes more efficient  than
NI  FTP  for  records  exceeding  189  octets,  though the difference is
marginal).   Where  efficiency  may  become   important   is   in   data
compression.   The  NI  FTP  breakeven  point  for compression is 3 data
bytes; for the DIN FTP it is between 5 and 7  bytes  (depending  on  how
many control headers are involved).

However, the basic point at issue is whether the DTP is the right  place
to provide hooks for more sophisticated protocols.  We contend that most
such protocols (job  entry,  FAX,  graphics,  mail  etc)  will  rely  on
primitives  manipulating  whole  files.   Hence  the  point of departure
should be the conceptual file store and the attributes  and  negotiation
mechanisms  used  to  control  a  file  within  it.   Once this has been
accepted, protocols performing other file-based functions can use  these
common descriptions and mechanisms.

(v) Partial File Transfers.  This is a useful facility available only in
the  DIN  FTP.   The  associated abilities to describe file structure as
indexed, variable record etc, and to nominate the keylength field length

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are  also  useful.  The fact that the NI FTP is restricted to sequential
files is recognised to be a major limitation.  In the NI FTP  model,  an
extension  to  handle  partial file transfer could negotiate the pair of
transfer delimiters during the negotiation phase, and transfer a  single
sequential  section  of  the  file  during the transfer phase.  Thus the
feature is supportable by extension,  but  requires  the  definition  of
several new parameters.

(vi) Multiple  file  transfers.   Both  protocols  can  readily  support
multiple  file  transfers, though the details vary.  The NI FTP requires
the passive side to be reinitialised each time, which  protects  against
parameter  interdependencies  persisting  between  transfers  (see (vii)
below).  Thus the facility becomes a problem for  the  designer  of  the
user  interface.  The same remark, of course, apples to multiple partial
transfers.

(vii) Parameter Negotiation.  This is much more formal in DIN  FTP  than
NI FTP.  In particular, parameters are explicitly grouped into different
commands (Login, TransactionDescription and  TransferDescription).   The
reasons for preserving this distinction beyond the user interface rather
than issuing a single  SFT,  which  allows  an  operating  system  total
freedom  to  make  its  own weird assumptions about the relationships of
parameters, are unclear.

The whole problem of interactions between parameters is potentially very
complex,  and  it  was  for this reason that the NI FTP design precludes
multiple attempts at SFT after a rejection.  The parameter settings left
from  the  last  transfer,  or  negotiation  attempt, may have undesired
consequences on the next.

The DIN FTP also allows an  attribute  to  be  specified  several  times
within   a   command,  which  the  NI  FTP  prohibits  (except  for  one
experimental version).  The specification  uses  multiple  citations  of
SourceFileName  as  the example for this.  Since it is not intended that
this ability be used to set ranges of restrictions (eg ByteSize ge 8 and
le 36 but ne 22), this case seems to be the only possible use outside of
statistics collection.

(viii) File Management Primitives.  The  NI  FTP  group  has  taken  the
attitude  that  file  management  is  the  proper  function  of  another
(compatible) protocol providing the complete  range  of  facilities  and
using  the same conceptual filestore structure.  Hence the NI FTP's file
manipulation, as expressed by the Mode of Access  attribute,  is  solely
related  to  copying  files.   The  access  modes  of  NI  FTP  are  all
supportable by the DIN FTP, with the exception  of  'destructive  read'.
DIN  FTP  provides two file management primitives: Delete and ListNames.
Delete is, of course, easy  to  implement.   ListNames  is  a  directory
interrogation  facility,  and is primarily useful for obtaining lists of
filenames for subsequent  multiple  file  transfer.   This  is  of  more
limited  use  with  the NI FTP's two-party model than with the DIN FTP's
three party model, and can only be used to full advantage where  grouped
file specification is possible.

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(ix) Foreground and background mode.  In practise, we can see  very  few
differences  between  them, as all three parties must be involved in the
entire negotiation phase.   Essentially,  the  difference  is  that  the
Controller/Active  and  Active/Passive  connections  do  not  have to be
simultaneously open but can be opened as needed (eg the  controller  may
have  to  answer  a LoginRequest from a passive in background mode) that
is,  in  background  mode  all  three  parties  retain  records  of  the
transaction  specification.   This  is  a consequence of the three-party
model; in the NI FTP it is a user interface issue and an  implementation
may be either foreground or background.

(x) Host Type.  There are two ways this type of parameter can  be  used.
It  is  clearly  useful for a user interface to offer shorthand profiles
for various parameter settings.   The  DIN  FTP  takes  the  alternative
approach  in  which  the  parameter  allows  transfers between identical
systems in efficient ways, but these are not defined within the DIN  FTP
specification.   We  do  not feel this is desirable, and we question the
implied assumption that it is not necessary.  Noncommunicating  families
of  FTPs can build up local interpretations for parameters of this sort.
When, at a later date, it  becomes  possible  for  two  such  groups  to
interconnect,   the   local   interpretations   will   be  found  to  be
incompatible.  This facility again reflects the fact that the DIN FTP is
developed  for  the AUTODIN II environment; in our opinion, this type of
facility should be implemented in  the  user  interface  via  a  profile
mechanism in a more open environment.

(xi) Format Effectors.  All additions made by the DIN FTP to the NI  FTP
list can be adequately described within that list (with the ANSI Fortran
Format setting).  The DIN FTP restriction that bits 1, 2  and  4  cannot
interact  is unnecessary (For example, it is perfectly reasonable to mix
imbedded  Horizontal  Tabs  with  the  reqiuirement  that  End-of-Record
implies  end  of  line).   We doubt whether the ability to change format
settings during the transmission of data is a capability which  will  be
widely  used.   In  any  case,  we  do  not think that there should be a
mandatory TextFormatter between records, and feel that  the  restriction
that  DTP  segments should only contain whole lines is unnecessary.  The
Tabstops attribute may be a  useful  idea.   As  is  currently  defined,
however,  it  requires  the  receiver  to  keep  table  space for it; we
recommend a fixed tab interval that can be changed as an  option.   This
approach  could  easily be supported by the NI FTP.  We note that the NI
FTP's earlier breakeven  point  for  compression  makes  compression  an
attractive approach to this problem.

(xii) Rollback.  Extending Rollback to allow the sender to  request  the
receiver   to  rollback  is  probably  a  good  idea.   However,  it  is
potentially more difficult for the receiver to roll back than it is  for
the  sender as his internal checkpoints may not correspond at all to the
FTP's  CheckPoints  (which  are  likely  to   correspond   to   internal
checkpoints of the sender).  Hence he may have to keep track of at least
a window's worth  of  FTP  checkpoints,  whether  acknowledged  or  not.
Additionally,  he  must  be  able  to  retrieve  the  stored data.  Some
acknowledgement strategies (eg Ack a full window  only)  will  sometimes

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require the receiver to retain checkpoint information for even longer to
avoid  races  between   Rollback   from   the   Donor   and   CheckPoint
Acknowledgement from the Recipient.

(xiii) CheckPoints.  A 16  bit  checkpoint  field  seems  rather  large;
allowing   the   checkpoints   to   be   any  arbitrary  numbers  forces
implementations to keep tables of unacknowledged checkpoints, which  may
not be necessary with NI FTP.

(xiv) Statistics Parameters.  Defining  statistics  parameters  for  the
active  side  is  a  reasonable  consequence  of  the three party model.
Standard statistics for the passive side is not so obviously useful;  it
seems  to  imply a model whereby statistics are gathered by some central
FTP Monitoring Centre, especially as these are required parameters.   If
true,  this  is  again  a  reflection  of the more closed environment of
AUTODIN II.

(xv) Binary Mapping.  This is a problem area with  both  protocols,  and
for much the same reasons.  These are:

     (a) For files where the byte size is less than eight, and bytes are
     being transferred in packed mode, it is not always possible to tell
     exactly how many bytes a  subrecord  contains.   The  DIN  FTP  has
     proposed using a flag pattern as a solution to this problem.

     (b) One of the reasons for transferring bytes in  packed  mode  for
     byte  sizes  of  other  than  eight  bits  is  to allow a 'natural'
     transfer for systems which do not use eight bit  bytes  internally.
     This  is  largely  negated by the fact that the subrecord header is
     fixed to be an eight bit byte.

     (c) It has recently become clear  that  specifying  the  conceptual
     transmission  order  of the bits has introduced a 'handedness' into
     the protocol.  The DIN FTP, which follows the 1822 convention  that
     the  high  bit is transmitted first, can be regarded as lefthanded,
     while the NI FTP, which makes the opposite choice (following  X25),
     can   be   regarded  as  righthanded.   An  implementation  of  one
     handedness wishing to send aligned data to one  of  the  other  (or
     even  across a hardware interface of the other handedness, relative
     to the internal word) must  first  permute  all  octets  holding  a
     larger  bytesized byte.  (Where the transfer is in packed mode, the
     action is even more complex.)

The NI FTP suffers from all three problems; the DIN FTP  has  adopted  a
solution to the first.  The NI FTP group is studying this area as one in
urgent need of revision.

SUMMARY

     The DIN FTP reflects in many ways assumptions about the environment
provided  by  AUTODIN II.  It is TCP-dependent to a small extent (and in
its Access Control it is Secure TCP dependent).  Its set  of  attributes

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and  choice  of  commands limit the range of applications it can be used
for and the range of operating systems it can be easily implemented  on.
Several  attributes  and  the  formality  of  the  implementation levels
reflect a situation where implementation is  under  a  stronger  central
influence  than  the NI FTP.  All these factors militate against its use
in the more open, uncontrolled  environment  of  the  worldwide  network
research community.

     The two protocols are very closely related.   Even  some  seemingly
major  differences can be resolved by regarding DIN FTP specification as
describing one form of NI FTP implementation (for  instance,  the  three
party  model, multiple file transfers, and foreground/background modes).
Both are significant advances on other FTPs which are currently in  use.
The DIN FTP offers several extensions to the facilities available in the
NI FTP, and some of these (most notably,  partial  file  transfers)  are
real   advances   on  it.   However,  where  the  two  offer  comparable
facilities, we feel the NI FTP is usually to be preferred.  The DIN  FTP
tends  to be more restrictive in the use of its facilities, and where it
is more free than the NI FTP, it tends to be rather  expensive  for  the
implementor  to  support in terms of the gains achieved.  Extensions can
easily be made to the NI FTP to include the real improvements  which  do
exist in the DIN FTP.