Internet Experiment Note: 195
COMMENTS ON NBS TRANSPORT PROTOCOL PROPOSAL 
by Carl Sunshine
University of Southern California
Information Sciences Institute
1a The U.S. National Bureau of Standards is sponsoring the
development of protocols for several levels of computer network
architecture. Numerous documents have been produced, mostly by
Bolt Beranek and Newman, as part of this program. The purpose of
this note is to comment on the recent proposal for a tranpsort
protocol , concerning both the general specification
methodology and the particular mechanisms suggested for this
1b These comments stem from a moderately detailed reading of the
NBS transport protocol  and specification methodology 
documents, and a general familiarity with other work in the NBS
program. They necessarily focus on objections and problems
rather than compliments, although lack of comment should not be
taken as agreement since it has not been possible to completely
review these documents. My comments fall into four broad
categories: service specification, protocol specification,
architecture, and comparison with TCP. The final section should
be of special interest to readers who are familiar with U.S.
Department of Defense protocol development efforts but not with
the details of the NBS proposals.
2 SERVICE SPECIFICATION
2a One of my first points must be that the service specification
is very unsatisfactory. I understand there was a decision to be
less formal in this area than with the protocol itself, but I
think that decision should be reconsidered. In my view, there is
no reason that the layer as a whole cannot be viewed as a
"service machine" and services defined in essentially the same
way as for the protocol. The recent work by SDC on DoD IP and TCP
[3,4] provide strong support for this view. An alternative
approach based on sequencing expressions has been extensively
developed by Schindler and applied to a variety of OSI protocols
and services .
2b Even if the informal "time line" figures and text now in use
for service specifications are maintained, major deficiencies
must be corrected. The figures purport to show the relative
timing of service primitives within a given service. They do NOT
show the relative timing allowed for interaction primitives of
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different services (e.g. that T_DATA can only follow T_CONNECT).
They do NOT show the relative timing allowed for multiple
instances of the same service (e.g. several T_UNIT_DATA data
requests may be made, and their corresponding indiciations may
occur in a different order than the requests, but with normal
T_DATA, the indications must be in the same order as the
requests). They do NOT show that some services can be interrupted
or completed by other services (e.g. T_CONNECT service primtives
can be followed at various stages by T_DISCONNECT primitives, for
example Connect.Req -> Connect.Ind -> Disconnect.Req ->
Disconnect.Ind, which is not shown in any figure).
2c There is no indication in the service primitives of how they
are associated by the receiver with a particular connection. For
example, the DATA primitives have only the data as a parameter.
Apparently the connection ID is an implicit parameter of all
service primitives. This should be discussed explicitly. For
example, what happens when two users try to connect to each other
at the same time (using the same transport addresses)? Is this a
simultaneous T_CONNECT situation which is not shown, or is it
considered two different connections? What if two remote
entities issue T_CONNECT requests to the same third party
transport address? Are multiple connections to the same address
allowed? When a T_UNIT_DATA confirmation is returned to the
user, how is it to be associated with the proper initial request
(there may be several to different destinations, or even to the
same destination in progress)?
2d There is no explicit statement that the parameter values of a
request are related to those of the corresponding indication.
Presumably they are to be copied, but is this always true?
2e There are many questions about addressing and connection
establishment. Some have been mentioned two paragraphs above.
Another is that the T_CONNECT service definition specifies that a
request will be followed by an indication to the remote address,
but what if this address is invalid, or "no one is there"
somehow, so the indication cannot be delivered (for example,
there may be no process bound to the specified address on the
remote system, and it may not operate to create processes on
demand). Will this result in a T_DISCONNECT from the remote
entity (reason address unknown or unreachable)?
2f Specifying the to and from parameters when the service as a
whole is defined (e.g., page 28-29 for T_UNIT_DATA) seems
redundant with the specification of all parameters for each
service primitive (page 31), and is in fact misleading since it
implies that every primitive will carry all of these parameters.
2g There is no discussion or specification anywhere of flow
control which is mentioned as one element of the service provided
(page 15). In fact the event model used seems to allow an
unlimited number of primitives to be requested by the user or
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indicated by the entity. There is no mention of any primitives
explicitly intended to indicate cease/resume type flow control,
or any limit on event queue sizes that could be checked by either
party or block acceptance of primitives. Again in Section 6.3
describing an X.25 interface machine, the X.25 RR/RNR flow
control packets are accepted, but have no specified effect on the
using layer. The machine apparently contains an unlimited size
queue of data to be sent (snd_buf), there are no enablng
conditions of a flow control nature on user data sending
primitives, and no explicit flow control is passed from the X.25
layer up to the user.
2h The expedited data is specified to be "unsynchronized with
respect to data in the normal flow," meaning (a) that the service
does not tell the receiver where in the normal data stream the
expedited data was generated, and (b) it may be delivered AFTER a
subsequently sent normal data unit. I think point b is highly
undesirable, and point a is rather inconvenient since it forces
users to put synchroniztion markers into the normal data stream.
A more powerful service should be adopted in this area.
2i The time figures seem to provide minimal information beyond
the text, and in fact all correspond to a
request-followed-by-indication model. They could be omitted
entirely, or their size greatly reduced. If they are retained,
they should be placed in the document so they appear AFTER their
citations in the text (many appear before).
3 PROTOCOL SPECIFICATION
3a Most of the comments in this section apply to the extended
class protocol found in Section 5.2 of the specification.
3b Sequence numbers for normal data are discussed in Section
5.1.8, but sequence numbers for expedited data are never
discussed. Only in the section on header formats do we discover
that this field is 7 bits.
3c Section 5.1.12 states that the transport connection will be
disconnected if the underlying net connection is closed
"unexpectedly." I would think it is an important service feature
to protect users from network level difficulties, and to maintain
the transport level connection, at least in the extended class,
by opening another network connection if necessary. Indeed, the
main purpose of the transport layer is to mask network level
errors and provide reliable service--giving up in this case seems
exactly counter to this goal.
3d The explanation of the difference between two-way and
three-way connection establishment in Section 5.1.16 is nice.
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3e Section 5.1.17 essentially defines the default processing for
all unspecified conditions to be disconnection. It is important
to make this notion more rigorous, and to discuss it when
defining the basic machine model. It should be extended to
include processing of unspecified user requests as well as PDUs.
That is, the model should be extended to include definition of an
"else" transition that applies if no other transition is matched,
with its precise actions given as for other transitions. It is
not clear that the desired action is always disconnection--simply
ignoring or discarding certain inputs is often desired.
3f The data types defined in Section 5.2.1 are never used later.
For example, the type given for the "reason" field of the machine
on page 48 is "int_type" rather than the "reason_type" defined on
page 46. The types actually used in the bulk of the
specification (int, data, adr, and stat), on the other hand, are
not defined formally anywhere. The adr_type in particular seems
to be a catch-all for several types of addresses (e.g. transport
and network address fields are both specified as adr_type).
3g Sections 5.2.3, 5.2.7, and 5.2.8 have similar information and
purposes, and probably should appear together. There are far too
many items that are "primitives" and hence not defined formally.
For example, only two out of perhaps 25 predicates are defined.
3h The function of the "reference" numbers assigned to each side
of a connection is never explanined clearly. I am guessing that
they are intended to function as a sort of incarnation number,
and serve to prevent old packets between the same addresses from
being confused with newer ones. This is intended to allow
connections to always start with sequence number zero, since they
will have different reference numbers. Hence reference numbers
must not be reused too frequently, leading to the REF_WAIT state
whenever a connection is closed, and the timeout discussed on
page 66. As noted, if a host crashes, it must not start ANY
connections for the appropriate time period if it forgets the
reference numbers used before the crash. All of this should be
clarified, with a few references to the relevant literature. The
reference numbers also appear to serve an addressing function,
being used (implicitly?--see next) to associate inputs with the
correct state machine instance.
3i There is a major omission concerning how to associate incoming
packets and user commands with a particular connection. Section
5.1.1 states that reference number pairs serve to identify
connections, but this is not reflected in the formal spec in any
way. Technically, the interfaces should be extended to include
these fields, and the enabling conditions should say
([from N:PDU.dst_ref] = src_ref) for an arriving PDU, or
([from U:Src_ref]=src_ref) for a user request, or
([from S:Src_ref]=src_ref) for a timer indication
Interestingly, the X.25 Interface machine in Section 18.104.22.168.3
does include an explicit check of this type on the logical
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channel. However, this will not work for initial connection
requests where no machine has been esatblished yet--in this case,
a new one must be created. This is not modeled formally either.
3j There are problems with events not associated with any
specific connection. For example, some timer requests are made
before any reference numbers have been assigned, such as in
transition 9, page 82. How can the Current State item on page 82
refer to "The transport connection" when there is no tranport
level connection associated with this event yet (a specific
transport address will only be identified when the CR PDU arrives
as a subsequent event)? How is the timeout specified in
transition 11, page 84, associated with the correct protocol
machine, particularly if more than one N_Connect indication has
been received? Similarly, how is the N_Accept indication in
tranition 2, page 70, associated with the proper machine?
3k Transitions 12 (p. 85) and 71-2 (pp. 183-6) are applicable to
the state set "listening", but the first action (cancelling the
timer) is only relevant in the CALLED state, not the CLOSED
state. I have not had the time to check all transitions in
detail, but if this is indicative of sloppy treatment of state
classes, there may be many other bugs of this sort.
3l The 81 transitions are presented in a seemingly unstructured
and haphazard fashion. This makes it difficult to understand the
protocol, and nearly impossible to check its completeness (has
the possibility of every event in every state been considered?).
With the default connection termination suggested in Section
5.1.17, technically every specification is complete, but the
reader would have much greater confidence if the spec was
presented systematically by types of events, or by states, and
"error" inputs were explicitly listed, so a syntactic
completeness check could be performed.
3m It would be extremely helpful to provide some sort of index to
the numerous transitions, perhaps a conventional state transition
diagram with the transition numbers written on the arcs, or a
listing of all the transition line specs (with no procedures)
together. This would allow the reader to trace interesting
scenarios much more easily.
3n The use of overlapping enabling conditions seems dangerous.
For example, see pages 367 and 373 where an incoming DATA unit is
received from X.25. Combined with the lack of ordering of the
transitions noted above, it is difficult for the reader (and the
designer) to remember that implicit in transition 24 is the fact
that packets with bad sequence numbers have been "filtered out"
and handled in transition 18 (which has a higher priority).
Perhaps these transitions should be combined, or else the
enabling conditions expanded to be mutually exclusive so each
stands complete on its own. In the current format, it would be
all too easy to modify the specification in one place, forgetting
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that it has an effect on another transition that appears pages
3o The syntax chosen for transitions, listing the new state first
and the old state second seperated by a right-to-left arrow, is
at odds with every other example of transition specifications I
have seen. The conventional method is to write old -> new, left
to right. The authors should at least highlight this difference
and provide some justification for it in Section 3.3.3.
3p When default values are passed for parameters, those
parameters should not be specified explicitly (e.g. Subscript and
Datum parameters at the end of transition two, page 70). This
will simplify the specification. Otherwise, why define defaults?
4a Both basic class and extended class protocols are designed to
make use of a virtual circuit or connection oriented network
level service. This may make sense for basic class, but
certainly does not for extended class. Presumably, the reason
for selecting extended class mode of operation would be in the
situation where unreliable and/or datagram network service was
being used. In this case, the transport level goes through
significant effort to manage network level "connection" setup and
clearing, only to have the interface sublayer undo all of this
work for a datagram system! The design of extended class should
clearly be optimized for operation over datagram nets. This
would allow substantial simplification of both the transport
protocol (CALLED and CALLING states, and transitions 1-2, 9-11,
59-60, etc. would be eliminated) and the interface sublayer
(essentially null for datagram nets).
4b There seems to be little relation between basic class and
extended class protocols. In particular, basic class is in no
sense a "subset" of extended class so that an extended class
implementation could also communicate with basic class by
refraining from use of certain PDUs. While there is some
similarity in the interaction primitives of the two classes, it
seems that these are essentially two different protocols.
Acceptance of this fact would allow redesign of each protocol for
greater efficiency. In particular, the attempt to base extended
class on connection oriented network service could be abandoned
with great savings as discussed above.
4c The wisdom of defining two classes of protocol at all is open
to question. It clearly introduces the possibility of inability
to communicate between different "class" users. The extended
class protocol is motivated by a view of underlying nets as not
fully reliable, providing minimal services, a situation which
clearly applies "in general." Basic class is motivated by a view
of highly reliable end-to-end connections available at the
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network level. Even if some public networks are expected to
provide this service, the reality of user-to-user
interconnections will in most cases also involve private or local
networks, leading to serious questions about the validity of the
situations for which basic class is optimized.
4d Masking the datagram nature of the network level from the
extended class tranport protocol leads to another inefficiency.
The ARPA IP datagram protocol allows the user to supply a
datagram "identifier" which is used to reassemble fragments at
the destination. If the transport level is aware of this
feature, it can purposely use the same ID on retransmissions of
the same packet, thereby increasing chances of correct reassembly
at the destination (fragments of different retransmissions will
be merged). When this feature is hidden from the transport level
and the interface sublayer provides a new ID to each packet
(including retransmissions) from the transport layer (as
specified on p. 395), this is not possible.
4e A difficulty in designing the interaction among layers is
apparent in use of the datagram interaface sublayer defined in
Section 6.4. When the "connection" is closed at one end
(transactions 6,8,10 on pages 407,9,11), no network level
disconnect is sent to the remote partner. Hence the remote
interface layer machine remains in the OPEN state indefinitely,
even though the transport layer has closed its connection.
5 COMPARISON WITH TCP
5a The NBS extended class tranport protocol bears some
resemblance to the DoD TCP protocol. Because there is some
interest in the applicability of public standards to DoD, I will
try to highlight some of the differences.
5b The NBS protocol is designed to operate above connection
oriented network services. In DoD, datagram services
predominate. Hence the NBS protocol can be expected to be more
costly and less efficient in this architecture as discussed
above. In addition to the extra states and transitions for
network level connection management (see 4a above), the NBS
protocol will be less robust since it drops transport connections
when there are network problems (see 3c), and cannot support the
merging of retransmissions by the IP (see 4d).
5c The TCP uses carefully selected initial sequence numbers to
prevent confusion of packets from different incarnations of
connections. The NBS protocol uses reference numbers which also
perform an addressing function in a fashion not fully explained
(see 3h-3i). The NBS protocol allows a less reliable two way
handshake to establish connections in addition to the three way
handshake used in TCP.
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5d The addressing features are not sufficiently well-defined in
the NBS protocol to allow a clear comparison with TCP.
Consistent with its use of underlying connection services, only
the initial call request PDU includes transport addresses (format
and semantics unspecified), while subsequent PDUs carry only the
reference number or numbers (presumably--this is left
unspecified), and no network level addresses. In TCP all packets
carry transport level ports (16 bits). Certain ports are
assigned to "well-known" applications with TCP, for which there
seems no counterpart in the NBS protocol. The NBS protocol
preserves the boundaries between sent transport service data
units when they are delivered (i.e., provides record boundaries
to users), while the latest version (August 1981) of the TCP is
stream oriented and provides no record boundaries. Instead the
TCP provides a "Push" service feature to force the prompt
delivery of all data sent up to that point.
5e The unit of sequencing and flow control in TCP is 8-bit bytes,
while in the NBS protocol it is protocol data units (PDUs) whose
maximum size is defined at connection establishment. This
follows the stream oriented nature of TCP data transmission
versus the record oriented aspect of the NBS protocol. Sequence
and acknowledgement numbers are 32 bits in TCP, and 15 bits in
the NBS protocol.
5f The NBS protocol maintains a minimum level of activity by
sending Acks if necessary in either direction. Periodic
transmission, particularly when a zero window is opened, is only
suggested in TCP. Acks do not carry a sequence number in the NBS
protocol, seemingly allowing them to be received and processed
out of order more easily than with TCP. Acks can not be
"piggybacked" on data packets in the NBS protocol, seemingly
leading to more packets exchanged (see section 5.4.2 giving
format of TPDUs). Error packets have no sequence or
acknowledgement numbers in the NBS protocol, seemingly allowing
duplicates to be accepted.
5g A single unit of "expedited data" may be transmitted at one
time in the NBS protocol whose position and delay relative to the
normal data units is unknown (see 2h above). The latest TCP
(August 1981) provides an "Urgent" service feature which
indicates a point in the normal data stream at which "urgent" or
priority data has been sent. The urgent indication is presented
to the receiver at least as soon as the urgent data.
5h The features for terminating connections (both "graceful" and
immediate disconnect) in the NBS protocol appear to duplicate
those in TCP. The security, compartment, and precedence features
in the NBS protocol also seem to duplicate those in TCP although
the fields are smaller and their semantics are left undefined.
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 J. Burruss and others, Specification of the Transport
Protocol, Report No. ICST/HLNP-81-1, National Bureau of
Standards, February 1981.
 R. Tenney and T. Blumer, An Automated Formal Specification
Technique for Protocols, Report No. 4581, Bolt Beranek and
Newman, February 1981.
 G. A. Simon and M. M. Bernstein, DCEC Protocols
Standardization Program/ Protocol Specification Report,
TM-7038/204/00, System Development Corp., July 1981. (Also
see DARPA Internet Experiment Note No. 186.)
 M. M. Bernstein, DCEC Protocols Standardization Program/
Preliminary Transport Protocol Specification Report,
TM-7038/207/01, System Development Corp., August 1981.
 S. Schindler, U. Flasche, and D. Altenkrueger, "The OSA
Project: Formal Specification of the ISO Transport Service,"
Proc. Computer Networking Symposium, National Bureau of
Standards, December 1980.