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Network Working Group Deepinder P. Sidhu
Request for Comments: 963 Iowa State University
November 1985
SOME PROBLEMS WITH THE SPECIFICATION OF THE
MILITARY STANDARD INTERNET PROTOCOL
STATUS OF THIS MEMO
The purpose of this RFC is to provide helpful information on the
Military Standard Internet Protocol (MIL-STD-1777) so that one can
obtain a reliable implementation of this protocol standard.
Distribution of this note is unlimited.
ABSTRACT
This paper points out several significant problems in the
specification of the Military Standard Internet Protocol
(MIL-STD-1777, dated August 1983 [MILS83a]). These results are based
on an initial investigation of this protocol standard. The problems
are: (1) a failure to reassemble fragmented messages completely; (2)
a missing state transition; (3) errors in testing for reassembly
completion; (4) errors in computing fragment sizes; (5) minor errors
in message reassembly; (6) incorrectly computed length for certain
datagrams. This note also proposes solutions to these problems.
1. Introduction
In recent years, much progress has been made in creating an
integrated set of tools for developing reliable communication
protocols. These tools provide assistance in the specification,
verification, implementation and testing of protocols. Several
protocols have been analyzed and developed using such tools.
Examples of automated verification and implementation of several real
world protocols are discussed in [BLUT82] [BLUT83] [SIDD83] [SIDD84].
We are currently working on the automatic implementation of the
Military Standard Internet Protocol (IP). This analysis will be
based on the published specification [MILS83a] of IP dated 12 August
1983.
While studying the MIL Standard IP specification, we have noticed
numerous errors in the specification of this protocol. One
consequence of these errors is that the protocol will never deliver
fragmented incoming datagrams; if this error is corrected, such
datagrams will be missing some data and their lengths will be
incorrectly reported. In addition, outgoing datagrams that are
divided into fragments will be missing some data. The proof of these
statements follows from the specification of IP [MILS83a] as
discussed below.
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2. Internet Protocol
The Internet Protocol (IP) is a network layer protocol in the DoD
protocol hierarchy which provides communication across interconnected
packet-switched networks in an internetwork environment. IP provides
a pure datagram service with no mechanism for reliability, flow
control, sequencing, etc. Instead, these features are provided by a
connection-oriented protocol, DoD Transmission Control Protocol (TCP)
[MILS83b], which is implemented in the layer above IP. TCP is
designed to operate successfully over channels that are inherently
unreliable, i.e., which can lose, damage, duplicate, and reorder
packets.
Over the years, DARPA has supported specifications of several
versions of IP; the last one appeared in [POSJ81]. A few years ago,
the Defense Communications Agency decided to standardize IP for use
in DoD networks. For this purpose, the DCA supported formal
specification of this protocol, following the design discussed in
[POSJ81] and the technique and organization defined in [SDC82]. A
detailed specification of this protocol, given in [MILS83a], has been
adopted as the DoD standard for the Internet Protocol.
The specification of IP state transitions is organized into decision
tables; the decision functions and action procedures are specified in
a subset of Ada[1], and may employ a set of machine-specific data
structures. Decision tables are supplied for the pairs <state name,
interface event> as follows: <inactive, send from upper layer>,
<inactive, receive from lower layer>, and <reassembling, receive from
lower layer>. To provide an error indication in the case that some
fragments of a datagram are received but some are missing, a decision
table is also supplied for the pair <reassembling, reassembly time
limit elapsed>. (The event names are English descriptions and not
the names employed by [MILS83a].)
3. Problems with MIL Standard IP
One of the major functions of IP is the fragmentation of datagrams
that cannot be transmitted over a subnetwork in one piece, and their
subsequent reassembly. The specification has several problems in
this area. One of the most significant is the failure to insert the
last fragment of an incoming datagram; this would cause datagrams to
be delivered to the upper-level protocol (ULP) with some data
missing. Another error in this area is that an incorrect value of the
data length for reassembled datagrams is passed to the ULP, with
unpredictable consequences.
As the specification [MILS83a] is now written, these errors are of
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Some Problems with MIL-STD IP
little consequence, since the test for reassembly completion will
always fail, with the result that reassembled datagrams would never
be delivered at all.
In addition, a missing row in one of the decision tables creates the
problem that network control (ICMP) messages that arrive in fragments
will never be processed. Among the other errors are the possibility
that a few bytes will be discarded from each fragment transmitted and
certain statements that will create run-time exceptions instead of
performing their intended functions.
A general problem with this specification is that the program
language and action table portions of the specification were clearly
not checked by any automatic syntax checking process. Variable and
procedure names are occasionally misspelled, and the syntax of the
action statements is often incorrect. We have enumerated some of
these problems below as a set of cautionary notes to implementors,
but we do not claim to have listed them all. In particular, syntax
errors are only discussed when they occur in conjunction with other
problems.
The following section discusses some of the serious errors that we
have discovered with the MIL standard IP [MIL83a] during our initial
study of this protocol. We also propose corrections to each of these
problems.
4. Detailed Discussion of the Problems
Problem 1: Failure to Insert Last Fragment
This problem occurs in the decision table corresponding to the
state reassembling and the input "receive from lower layer"
[MILS83a, sec 9.4.6.1.3]. The problem occurs in the following row
of this table:[2]
________________________________________________________
check- SNP TTL where a reass ICMP
sum params valid to frag done check-
valid? valid? ? ? ? ? sum?
__________________________________________________________________
YES YES YES ULP YES YES d reass_
delivery;
state :=
INACTIVE
__________________________________________________________________
The reass_done function, as will be seen below, returns YES if the
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fragment just received is the last fragment needed to assemble a
complete datagram and NO otherwise. The action procedure
reass_delivery simply delivers a completely reassembled datagram
to the upper-level protocol. It is the action procedure
reassemble that inserts an incoming fragment into the datagram
being assembled. Since this row does not call reassemble, the
result will be that every incoming fragmented datagram will be
delivered to the upper layer with one fragment missing. The
solution is to rewrite this row of the table as follows:
________________________________________________________
check- SNP TTL where a reass ICMP
sum params valid to frag done check-
valid? valid? ? ? ? ? sum?
__________________________________________________________________
YES YES YES ULP YES YES d reassemble;
reass_
delivery;
state :=
INACTIVE
__________________________________________________________________
Incidentally, the mnemonic value of the name of the reass_done
function is questionable, since at the moment this function is
called datagram reassembly cannot possibly have been completed. A
better name for this function might be last_fragment.
Problem 2: Missing State Transition
This problem is the omission of a row of the same decision table
[MILS83a, sec 9.4.6.1.3]. Incoming packets may be directed to an
upper-level protocol (ULP), or they may be network control
messages, which are marked ICMP (Internet Control Message
Protocol). When control messages have been completely assembled,
they are processed by an IP procedure called analyze. The
decision table contains the row
________________________________________________________
check- SNP TTL where a reass ICMP
sum params valid to frag done check-
valid? valid? ? ? ? ? sum?
__________________________________________________________________
YES YES YES ICMP YES NO d reassemble;
__________________________________________________________________
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but makes no provision for the case in which where_to returns
ICMP, a_frag returns YES, and reass_done returns YES. An
additional row should be inserted, which reads as follows:
________________________________________________________
check- SNP TTL where a reass ICMP
sum params valid to frag done check-
valid? valid? ? ? ? ? sum?
__________________________________________________________________
YES YES YES ICMP YES YES d reassemble;
analyze;
state :=
INACTIVE
__________________________________________________________________
Omitting this row means that incoming fragmented ICMP messages
will never be analyzed, since the state machine does not have any
action specified when the last fragment is received.
Problem 3: Errors in reass_done
The function reass_done, as can be seen from the above, determines
whether the incoming subnetwork packet contains the last fragment
needed to complete the reassembly of an IP datagram. In order to
understand the errors in this function, we must first understand
how it employs its data structures.
The reassembly of incoming fragments is accomplished by means of a
bit map maintained separately for each state machine. Since all
fragments are not necessarily the same length, each bit in the map
represents not a fragment, but a block, that is, a unit of eight
octets. Each fragment, with the possible exception of the "tail"
fragment (we shall define this term below), is an integral number
of consecutive blocks. Each fragment's offset from the beginning
of the datagram is given, in units of blocks, by a field in the
packet header of each incoming packet. The total length of each
fragment, including the fragment's header, is specified in the
header field total_length; this length is given in octets. The
length of the header is specified in the field header_length; this
length is given in words, that is, units of four octets.
In analyzing this subroutine, we must distinguish between the
"tail" fragment and the "last" fragment. We define the last
fragment as the one which is received last in time, that is, the
fragment that permits reassembly to be completed. The tail
fragment is the fragment that is spatially last, that is, the
fragment that is spatially located after any other fragment. The
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length and offset of the tail fragment make it possible to compute
the length of the entire datagram. This computation is actually
done in the action procedure reassembly, and the result is saved
in the state vector field total_data_length; if the tail fragment
has not been received, this value is assumed to be zero.
It is the task of the reass_done function [MILS83a, sec 9.4.6.2.6]
to determine whether the incoming fragment is the last fragment.
This determination is made as follows:
1) If the tail fragment has not been received previously and
the incoming fragment is not the tail fragment, then return NO.
2) Otherwise, if the tail fragment has not been received, but
the incoming fragment is the tail fragment, determine whether
all fragments spatially preceding the tail fragment have also
been received.
3) Otherwise, if the tail fragment has been received earlier,
determine whether the incoming fragment is the last one needed
to complete reassembly.
The evaluation of case (2) is accomplished by the following
statment:
if (state_vector.reassembly_map from 0 to
(((from_SNP.dtgm.total_length -
(from_SNP.dtgm.header_length * 4) + 7) / 8)
+ 7) / 8 is set)
then return YES;
The double occurrence of the subexpression " + 7 ) / 8" is
apparently a misprint. The function f(x) = (x + 7) / 8 will
convert x from octets to blocks, rounding any remainder upward.
There is no need for this function to be performed twice. The
second problem is that the fragment_offset field of the incoming
packet is ignored. The tail fragment specifies only its own
length, not the length of the entire datagram; to determine the
latter, the tail fragment's offset must be added to the tail
fragment's own length. The third problem hinges on the meaning of
the English "... from ... to ..." phrase. If this phrase has the
same meaning as the ".." range indication in Ada [ADA83, sec 3.6],
that is, includes both the upper and lower bounds, then it is
necessary to subtract 1 from the final expression.
The expression following the word to, above, should thus be
changed to read
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from_SNP.dtgm.fragment_offset +
((from_SNP.dtgm.total_length -
(from_SNP.dtgm.header_length * 4) + 7) / 8) - 1
Another serious problem with this routine occurs when evaluating
case (3). In this case, the relevant statement is
if (all reassembly map from 0 to
(state_vector.total_data_length + 7)/8 is set
then return YES
If the tail fragment was received earlier, the code asks, in
effect, whether all the bits in the reassembly map have been set.
This, however, will not be the case even if the incoming fragment
is the last fragment, since the routine reassembly, which actually
sets these bits, has not yet been called for this fragment. This
statement must therefore skip the bits corresponding to the
incoming fragment. In specifying the range to be tested,
allowance must be made for whether these bits fall at the
beginning of the bit map or in the middle (the case where they
fall at the end has already been tested). The statement must
therefore be changed to read
if from_SNP.dtgm.fragment_offset = 0 then
if (all reassembly map from
from_SNP.dtgm.fragment_offset +
((from_SNP.dtgm.total_length -
from_SNP.dtgm.header_length * 4) + 7) / 8
to ((state_vector.total_data_length + 7) / 8 - 1) is set)
then return YES;
else return NO;
end if;
else
if (all reassembly map from 0 to
(from_SNP.dtgm.fragment_offset - 1) is set)
and (all reassembly map from
from_SNP.dtgm.fragment_offset +
((from_SNP.dtgm.total_length -
from_SNP.dtgm.header_length * 4) + 7) / 8
to ((state_vector.total_data_length + 7) / 8 - 1) is set)
then return YES;
else return NO;
end if;
end if;
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Note that here again it is necessary to subtract 1 from the upper
bound.
Problem 4: Errors in fragment_and_send
The action procedure fragment_and_send [MILS83a, sec 9.4.6.3.7] is
used to break up datagrams that are too large to be sent through
the subnetwork as a single packet. The specification requires
[MILS83a sec 9.2.2, sec 9.4.6.3.7] each fragment, except possibly
the "tail" fragment, to contain a whole number of 8-octet groups
(called "blocks"); moreover, each fragment must begin at a block
boundary.
In the algorithm set forth in fragment_and_send, all fragments
except the tail fragment are set to the same size; the procedure
begins by calculating this size. This is done by the following
statement:
data_per_fragment := maximum subnet transmission unit
- (20 + number of bytes of option data);
Besides the failure to allow for header padding, which is
discussed in the next section, this statement makes the serious
error of not assuring that the result is an integral multiple of
the block size, i.e., a multiple of eight octets. The consequence
of this would be that as many as seven octets per fragment would
never be sent at all. To correct this problem, and to allow for
header padding, this statement must be changed to
data_per_fragment := (maximum subnet transmission unit
- (((20 + number of bytes of option data)+3)/4*4)/8*8;
Another problem in this procedure is the failure to provide for
the case in which the length of the data is an exact multiple of
eight. The procedure contains the statements
number_of fragments := (from_ULP.length +
(data_per_fragment - 1)) / data_per_fragment;
data_in_last_frag := from_ULP.length modulo data_per_fragment;
(Note that in our terminology we would rename data_in_last_frag as
data_in_tail_frag; notice, also, that the proper spelling of the
Ada operator is mod [ADA83, sec 4.5.5].)
If data_in_last_frag is zero, some serious difficulties arise.
One result might be that the datagram will be broken into one more
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fragment than necessary, with the tail fragment containing no data
bytes. The assignment of data into the tail fragment will succeed
even though it will now take the form
output_data [i..i-1] := input_data [j..j-1];
because Ada makes provision for so-called "null slices" [ADA83,
sec 4.1.2] and will treat this assignment as a no-op [ADA83, sec
5.2.1].
This does, however, cause the transmission of an unnecessary
packet, and also creates difficulties for the reassembly
procedure, which must now be prepared to handle empty packets, for
which not even one bit of the reassembly map should be set.
Moreover, as the procedure is now written, even this will not
occur. This is because the calculation of the number of fragments
is incorrect.
A numerical example will clarify this point. Suppose that the
total datagram length is 16 bytes and that the number of bytes per
fragment is to be 8. Then the above statements will compute
number_of_fragments = (16 + 7)/8 = 2 and data_in_last_frag = 16
mod 8 = 0. The result of the inconsistency between
number_of_fragments and data_in_last_frag will be that instead of
sending three fragments, of lengths 8, 8, and 0, the procedure
will send only two fragments, of lengths 8 and 0; the last eight
octets will never be sent.
To avoid these difficulties, the specification should add the
following statement, immediately after computing
data_in_last_frag:
if data_in_last_frag = 0 then
data_in_last_frag := data_per_fragment;
end if;
This procedure also contains several minor errors. In addition to
failures to account for packet header padding, which are
enumerated in the next section, there is a failure to convert the
header length from words (four octets) to octets in one statement.
This statement, which calculates the total length of the non-tail
fragments, is
to_SNP.dtgm.total_length := to_SNP.dtgm.header_length
+ data_per_fragment;
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Since header length is expressed in units of words, this
statement should read
to_SNP.dtgm.total_length := to_SNP.dtgm.header_length * 4
+ data_per_fragment;
This is apparently no more than a misprint, since the
corresponding calculation for the tail fragment is done correctly.
Problem 5: Errors in reassembly
The action procedure reassembly [MILS83a, sec 9.4.6.3.9], which is
referred to as reassemble elsewhere in the specification [MILS83a,
sec 9.4.6.1.2, sec 9.4.6.1.3], inserts an incoming fragment into a
datagram being reassembled. This procedure contains several
relatively minor errors.
In two places in this procedure, a range is written to contain one
more member than it ought to have. In the first, data from the
fragment is to be inserted into the datagram being reassembled:
state_vector.data [from_SNP.dtgm.fragment_offset*8 ..
from_SNP.dtgm.fragment_offset*8 + data_in_frag] :=
from_SNP.dtgm.data [0..data_in_frag-1];
In this statement, the slice on the left contains one more byte
than the slice on the right. This will cause a run-time exception
to be raised [ADA83, sec 5.2.1]. The statement should read
state_vector.data [from_SNP.dtgm.fragment_offset*8 ..
from_SNP.dtgm.fragment_offset*8 + data_in_frag - 1] :=
from_SNP.dtgm.data [0..data_in_frag-1];
A similar problem occurs in the computation of the range of bits
in the reassembly map that corresponds to the incoming fragment.
This statement begins
for j in (from_SNP.dtgm.fragment_offset) ..
((from_SNP.dtgm.fragment_offset +
data_in_frag + 7)/8) loop
Not only are the parentheses in this statement located incorrectly
(because the function f(x) = (x + 7) / 8 should be executed only
on the argument data_in_frag), but also this range contains one
extra member. The statement should read
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for j in (from_SNP.dtgm.fragment_offset) ..
(from_SNP.dtgm.fragment_offset +
(data_in_frag + 7)/8) - 1 loop
Note that if the statement is corrected in this manner it will
also handle the case of a zero-length fragment, mentioned above,
since the loop will not be executed even once [ADA83, sS 5.5].
Another minor problem occurs when this procedure attempts to save
the header of the leading fragment. The relevant statement is
state_vector.header := from_SNP.dtgm;
This statement attempts to transfer the entire incoming fragment
into a record that is big enough to contain only the header. The
result, in Ada, is not truncation, but a run-time exception
[ADA83, sec 5.2]. The correction should be something like
state_vector.header := from_SNP.dtgm.header;
This correction cannot be made without also defining the header
portion of the datagram as a subrecord in [MILS83a, sec 9.4.4.6];
such a definition would also necessitate changing many other
statements. For example, from_SNP.dtgm.fragment_offset would now
have to be written as from_SNP.dtgm.header.fragment_offset.
Another possible solution is to write the above statement as a
series of assignments for each field in the header, in the
following fashion:
state_vector.header.version :=
from_SNP.dtgm.version;
state_vector.header.header_length :=
from_SNP.dtgm.header_length;
state_vector.header.type_of_service :=
from_SNP.dtgm.type_of_service;
-- etc.
Note also that this procedure will fail if an incoming fragment,
other than the tail fragment, does not contain a multiple of eight
characters. Implementors must be careful to check for this in the
decision function SNP_params_valid [MILS83a, sec 9.4.6.2.7].
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Problem 6: Incorrect Data Length for Fragmented Datagrams
The procedure reassembled_delivery [MILS83a, sec 9.4.6.3.10] does
not deliver the proper data length to the upper-level protocol.
This is because the assignment is
to_ULP.length := state_vector.header.total_length
- state_vector.header.header_length * 4;
The fields in state_vector.header have been filled in by the
reassembly procedure, discussed above, by copying the header of
the leading fragment. The field total_length in this fragment,
however, refers only to this particular fragment, and not to the
entire datagram (this is not entirely clear from it definition in
[MILS83a, sec 9.3.4], but the fragment_and_send procedure
[MILS83a, sec 9.4.6.3.7] insures that this is the case).
The length of the entire datagram can only be computed from the
length and offset of the tail fragment. This computation is
actually done in the reassembly procedure [MILS83a, sec
9.4.6.3.9], and the result saved in state_vector.total_data_length
(see above). It is impossible, however, for reassembly to fill in
state_vector.header.total_length at this time, because
state_vector.header.header_length is filled in from the lead
fragment, which may not yet have been received.
Therefore, reassembled_delivery must replace the above statement
with
to_ULP.length := state_vector.total_data_length;
The consequence of leaving this error uncorrected is that the
upper-level protocol will be informed only of the delivery of as
many octets as there are in the lead fragment.
5. Implementation Difficulties of MIL Standard IP
In addition to the problems discussed above, there are several
features of the MIL standard IP specification [MILS83a] which lead to
difficulties for the implementor. These difficulties, while not
actually errors in the specification, take the form of assumptions
which are not explicitly stated, but of which implementors must be
aware.
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5.1 Header Padding
In several places, the specification makes a computation of the
length of a packet header without explicitly allowing for padding.
The padding is needed because the specification requires [MILS83a,
sec 9.3.14] that each header end on a 32-bit boundary.
One place this problem arises is in the need_to_frag decision
function [MILS83a, sec 9.4.6.2.5]. This function is used to
determine whether fragmentation is required for an outgoing
datagram. It consists of the single statement
if ((from_ULP.length + (number of bytes of option data)
+ 20) > maximum transmission unit of the local subnetwork
then return YES
else return NO;
end if;
(A minor syntax error results from not terminating the first
return statement with a semicolon [ADA83, sec 5.1, sec 5.3, sec
5.9].) In order to allow for padding, the expression for the
length of the outgoing datagram should be
(((from_ULP.length + (number of bytes of option data) + 20)
+ 3)/4 * 4)
Another place that this problem arises is in the action procedure
build_and_send [MILS83a, sec 9.4.6.3.2], which prepares
unfragmented datagrams for transmission. To compute the header
field header_length, which is expressed in words, i.e., units of
four octets [MILS83a, sec 9.3.2], this procedure contains the
statement
to_SNP.dtgm.header_length := 5 +
(number of bytes of option data)/4;
In order to allow for padding, this statement should read
to_SNP.dtgm.header_length :=
5 + ((number of bytes of option data)+3)/4;
The identical statement appears in the action procedure
fragment_and_send [MILS83a, sec 9.4.6.3.7], which prepares
datagram fragments for transmission, and requires the same
correction.
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The procedure fragment_and_send also has this problem in two other
places. In the first, the number of octets in each fragment is
computed by
data_per_fragment := maximum subnet transmission unit
- (20 + number of bytes of option data);
In order to allow for padding, this statement should read
data_per_fragment := maximum subnet transmission unit
- (((20 + number of bytes of option data)+3)/4*4);
(Actually, this statement must be changed to
data_per_fragment := (maximum subnet transmission unit
- (((20 + number of bytes of option data)+3)/4*4)/8*8;
in order to accomplish its intended purpose, for reasons which
have been discussed above.)
A similar problem occurs in the statement which computes the
header length for individual fragments:
to_SNP.dtgm.header_length := 5 +
(number of copy options octets/4);
To allow for padding, this should be changed to
to_SNP.dtgm.header_length := 5 +
(number of copy options octets+3/4);
Notice that all of these errors can also be corrected if the
English phrase "number of bytes of option data", and similar
phrases, are always understood to include any necessary padding.
5.2 Subnetworks with Small Transmission Sizes
When an outgoing datagram is too large to be transmitted as a
single packet, it must be fragmented. On certain subnetworks, the
possibility exists that the maximum number of bytes that may be
transmitted at a time is less than the size of an IP packet header
for a given datagram. In this case, the datagram cannot be sent,
even in fragmented form. Note that this does not necessarily mean
that the subnetwork cannot send any datagrams at all, since the
size of the header may be highly variable. When this problem
arises, it should be detected by IP. The proper place to detect
this situation is in the function can_frag.
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The can_frag decision function [MILS83a, sec 9.4.6.2.2] is used to
determine whether a particular outgoing datagram, which is too
long to be transmitted as a single fragment, is allowed to be
fragmented. In the current specification, this function consists
of the single statement
if (from_ULP.dont_fragment = TRUE)
then return NO
else return YES
end if;
(A minor syntax error is that the return statements should be
terminated by semicolons; see [ADA83, sec 5.1, sec 5.3, sec 5.9].)
If the above problem occurs, the procedure fragment_and_send will
obtain negative numbers for fragment sizes, with unpredictable
results. This should be prevented by assuring that the subnetwork
can send the datagram header and at least one block (eight octets)
of data. The can_frag function should be recoded as
if ((8 + ((number of bytes of option data)+3)/4*4 + 20)
> maximum transmission unit of the local subnetwork)
then return NO;
elsif (from_ULP.dont_fragment = TRUE)
then return NO
else return YES
end if;
This is similar to the logic of the function need_to_frag,
discussed above.
5.3 Subnetwork Interface
Provision is made for the subnetwork to report errors to IP
[MILS83a, sec 6.3.6.2], but no provision is made for the IP entity
to take any action when such errors occur.
In addition, the specification [MILS83a, sec 8.2.1.1] calls for
the subnetwork to accept type-of-service indicators (precedence,
reliability, delay, and throughput), which may be difficult to
implement on many local networks.
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5.4 ULP Errors
The IP specification [MILS83a, sec 9.4.6.3.6] states
The format of error reports to a ULP is implementation
dependent. However, included in the report should be a value
indicating the type of error, and some information to identify
the associated data or datagram.
The most natural way to provide the latter information would be to
return the datagram identifier to the upper-level protocol, since
this identifier is normally supplied by the sending ULP [MILS83a,
sec 9.3.5]. However, the to_ULP data structure makes no provision
for this information [MILS83a, sec 9.4.4.3], probably because this
information is irrelevant for datagrams received from the
subnetwork. Implementors may feel a need to add this field to the
to_ULP data structure.
5.5 Initialization of Data Structures
The decision function reass_done [MILS83a, sec 9.4.6.2.6] makes
the implicit assumption that data structures within each finite
state machine are initialized to zero when the machine is created.
In particular, this routine will not function properly unless
state_vector.reassembly_map and state_vector.total_data_length are
so initialized. Since this assumption is not stated explicitly,
implementors should be aware of it. There may be other
initialization assumptions that we have not discovered.
5.6 Locally Defined Types
The procedures error_to_source [MILS83a, sec 9.4.6.3.5] and
error_to_ULP [MILS83a, sec 9.4.6.3.6] define enumeration types in
comments. The former contains the comment
error_param : (PARAM_PROBLEM, EXPIRED_TTL, PROTOCOL_UNREACH);
and the latter
error_param : (PARAM_PROBLEM, CAN'T_FRAGMENT, NET_UNREACH,
PROTOCOL_UNREACH, PORT_UNREACH);
These enumerated values are used before they are encountered
[MILS83a, sec 9.4.6.1.1, sec 9.4.6.1.2, sec 9.4.6.1.3, et al.];
implementors will probably wish to define some error type
globally.
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5.7 Miscellaneous Difficulties
The specification contains many Ada syntax errors, some of which
have been shown above. We have only mentioned syntax errors
above, however, when they occurred in conjunction with other
problems. One of the main syntactic difficulties that we have not
mentioned is that the specification frequently creates unnamed
types, by declaring records within records; such declarations are
legal in Pascal, but not in Ada [ADA83, sec 3.7].
Another problem is that slice assignments frequently do not
contain the same number of elements on the left and right sides,
which will raise a run-time exception [ADA83, sec 5.2.1]. While
we have mentioned some of these, there are others which are not
enumerated above.
In particular, the procedure error_to_source [MILS83a, sec
9.4.6.3.5] contains the statement
to_SNP.dtgm.data [8..N+3] := from_SNP.dtgm.data [0..N-1];
We believe that N+3 is a misprint for N+8, but even so the left
side contains one more byte than the right. Implementors should
carefully check every slice assignment.
6. An Implementation of MIL Standard IP
In our discussion above, we have pointed out several serious problems
with the Military Standard IP [MILS83a] specification which must be
corrected to produce a running implementation conforming to this
standard. We have produced a running C implementation for the MIL
Standard IP, after problems discussed above were fixed in the IP
specification. An important feature of this implementation is that
it was generated semi-automatically from the IP specification with
the help of a protocol development system [BLUT82] [BLUT83] [SIDD83].
Since this implementation was derived directly from the IP
specification with the help of tools, it conforms to the IP standard
better that any handed-coded IP implementation can do.
The problems pointed out in this paper with the current specification
of the MIL Standard IP [MILS83a] are based on an initial
investigation of the protocol.
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Some Problems with MIL-STD IP
NOTES
[1] Ada is a registered trademark of the U.S. Government - Ada Joint
Program Office.
[2] d indicates a "don't care" condition.
ACKNOWLEDGEMENTS
The author extends his gratitude to Tom Blumer Michael Breslin, Bob
Pollack and Mark J. Vincenzes, for many helpful discussions. Thanks
are also due to B. Simon and M. Bernstein for bringing to author's
attention a specification of the DoD Internet Protocol during 1981-82
when a detailed study of this protocol began. The author is also
grateful to Jon Postel and Carl Sunshine for several informative
discussions about DoD IP/TCP during the last few years.
REFERENCES
[ADA83] Military Standard Ada(R) Programming Language, United
States Department of Defense, ANSI/MIL-STD-1815A-1983, 22
January 1983
[BLUT83] Blumer, T. P., and Sidhu, D. P., "Mechanical Verification
and Automatic Implementation of Communication Protocols,"
to appear in IEEE Trans. Softw. Eng.
[BLUT82] Blumer, T. P., and Tenney, R. L., "A Formal Specification
Technique and Implementation Method for Protocols,"
Computer Networks, Vol. 6, No. 3, July 1982, pp. 201-217.
[MILS83a] "Military Standard Internet Protocol," United States
Department of Defense, MIL-STD-1777, 12 August 1983.
[MILS83b] "Military Standard Transmission Control Protocol," United
States Department of Defense, MIL-STD-1778, 12 August 1983.
[POSJ81] Postel, J. (ed.), "DoD Standard Internet Protocol," Defense
Advanced Research Projects Agency, Information Processing
Techniques Office, RFC-791, September 1981.
[SDC82] DCEC Protocol Standardization Program: Protocol
Specification Report, System Development Corporation,
TM-7172/301/00, 29 March 1982
[SIDD83] Sidhu, D. P., and Blumer, T. P., "Verification of NBS Class
4 Transport Protocol," to appear in IEEE Trans. Comm.
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[SIDD84] Sidhu, D. P., and Blumer, T. P., "Some Problems with the
Specification of the Military Standard Transmission Control
Protocol," in Protocol Specification, Testing and
Verification IV, (ed.) Y. Yemini et al (1984).
Sidhu [Page 19]