Preservation of CRC integrity upon intentional data alteration during message transmission

In complex networks, data frames may be routed through different systems having different frame or addressing requirements. When the frames are transferred or bridged between such systems, known changes may have to be made in the frame contents to accommodate these requirements. To maintain the integrity of error checking provided by Cyclical Redundancy Checking (CRC) techniques, a system receiving a frame uses an improved Frame Check Sequence value modification technique which modifies a Frame Check Sequence (FCS) field value only as a function of the known or planned changes to be made in the frame. If unplanned changes (that is, errors) are introduced at the receiving system, application of standard CRC error checking techniques at the next system to receive the frame will indicate those errors.

FIELD OF THE INVENTION 
The present invention relates to data communications systems and more 
particularly to a method of maintaining the integrity of cyclical 
redundancy checking (CRC) error protection where data is intentionally 
changed at one or more intermediate points on the data path between a 
source and a destination. 
BACKGROUND OF THE INVENTION 
A data communications network can be thought of as a collection of separate 
data processing systems which may be connected to form a data path between 
a first end user at one system and a second end user at another system. 
Data to be transmitted through a network is almost always organized in 
discrete units called frames. A frame normally has recognizable bit 
patterns at the beginning and end. These bit patterns, commonly called 
delimiters, are used by a receiving system to identify the start and the 
end of each frame. A typical frame also has a header which includes the 
address of the system to which the data is to be sent (the destination 
system) and the address of the system from which the data is sent (the 
source system). The frame can also include routing information for setting 
up paths through the network between the source and destination systems. 
To enable a destination system to determine whether errors have been 
introduced into the data during transmission through the network, each 
frame may include a frame check sequence or FCS field. The original value 
of the FCS field is established at the source system by applying a 
predetermined mathematical algorithm to selected fields in the frame. The 
results of the algorithm are written into the FCS field. When the frame 
arrives at the destination system, that system applies the same algorithm 
to the same fields and compares the results with the value of the FCS 
field contained in the received frame. If the FCS value generated by the 
destination system and the FCS field value in the received frame are the 
same, it is assumed the data has been received without error. 
In a complex network, different systems may use different formats for a 
data frame. When data is transferred into a system having a different 
frame format, the contents of the fields used in calculation of the 
original FCS field value may intentionally be changed by the receiving 
system. 
Although the actual data within the frame may not change during frame 
format conversion operations, any system which subsequently receives a 
converted frame will be unable to perform normal error checking using the 
frame FCS field value as described above. This is because the system 
receiving the converted frame will generate its own FCS field value using 
the same algorithm and the same fields as were used by the source system 
in establishing an original FCS field value. If the contents of those 
fields have been changed (even intentionally) at some point along the data 
path, the receiving system can be expected to generate an FCS field value 
different from the original FCS field value calculated by the source 
system. The mismatch between calculated and received FCS field values 
would falsely indicate an error in the actual data in the received frame. 
It has been suggested that this problem can be avoided by having each 
intermediate system on a data path recalculate the FCS field value once 
the format changes have been made. The recalculated FCS field value would 
be inserted in the frame before the frame is passed on to the next system. 
Although this approach permits a greater degree of error checking than the 
approach originally described, it still has inadequacies. In implementing 
the "FCS recalculation" approach, each receiving system necessarily 
performs at least three sequential operations involving a data frame. 
First, the receiving system checks the incoming data frame for errors by 
calculating its own FCS field value and comparing it to the FCS field 
value in the received frame. Second, the receiving system changes certain 
fields in the frame to convert the frame to the new format. Third, the 
receiving system recalculates the FCS field value based upon the changed 
frame. 
This sequential three step process leaves the data unprotected against 
errors introduced after the first step is completed but before the third 
step is performed. If an error is introduced into the data during this 
time period, the FCS field value calculated during the third step will not 
in any way reflect or indicate that error since the presumption is that 
FCS field value calculations are being performed on error-free data. 
Assuming no further errors are introduced, error checking operations 
performed at subsequent receiving systems will falsely indicate that the 
data contained in the frame is free of errors. 
It has also been suggested that each frame being transferred into a system 
using a different frame format be encapsulated or enveloped within the 
frame structure appropriate for the receiving system. An FCS field value 
would be generated for the encapsulating frame. One drawback to this 
approach is that the encapsulation is necessary only if the data 
originates in a system having a different frame format. The receiving 
system must be able to treat incoming frames differently, depending on 
where these frames come from. This requirement adds to the complexity of 
the data transmission. 
SUMMARY OF THE INVENTION 
The present invention is a method of establishing an FCS field value in a 
data frame which permits detection of errors introduced into the data at 
intermediate systems. The method preserves the error checking value of the 
FCS field while avoiding the complexity of the encapsulating technique 
mentioned above. 
According to the present method, an FCS field value is calculated at a 
source system or station for the data by applying a predetermined cyclical 
redundancy checking algorithm to selected fields in each frame. At each 
intermediate station on a path between the source station and the 
destination station, the field value is modified to reflect planned or 
intentional changes to be made by that station in selected fields of the 
frame. If unintended changes (that is, errors) are introduced into the 
selected fields, those changes are not reflected in the modified FCS field 
value since the modification is based solely on intended or planned 
changes in the data. Therefore, when the next station on the path checks 
the integrity of the frame by calculating its own FCS field value based on 
selected fields in the frame and comparing its FCS value to the received 
FCS value, the difference in the two values will indicate the presence of 
an error or errors in the data.

TECHNICAL DESCRIPTION 
FIG. 1 discloses a data communications network consisting of a number of 
local area networks 10, 12, 14 and 16 which may be interconnected through 
an intermediate higher speed or backbone network 18. It should be noted 
that the term network is used both to refer to the individual local area 
networks, backbone network and the overall combination of these individual 
networks; i.e., the data communications networks. Which network is being 
referred to will be clear from the context in which the word is used. 
There are different kinds of local area networks. For purposes of 
illustration, it will be assumed that the local area networks 10, 12, 14 
and 16 are token-ring networks of a particular type. In this type of 
token-ring network, devices such as terminals, peripherals, cluster 
controllers and computers are connected to a single continuous ring or 
shared data transmission media. Different types of media, such as cable or 
twisted-pair copper wire, may be used for the ring. Optical fibers may 
also be used as ring media where high bandwidth or substantial immunity 
against electromagnetic interference or signal attenuation is required. 
In a specific type of token-ring network, a circulating electronic token 
passes sequentially from one node or device to the next around the ring, 
allowing each node an opportunity to transmit data. A node having data to 
transmit can "capture" the token and "expand" that token into a frame 
which is circulated around the ring. Each node receives the frame but only 
the node or nodes to which it is addressed make use of the frame data. 
When the frame returns to the original source node, that node must remove 
the frame from the ring and issue a token to allow other nodes in the ring 
the opportunity to transmit. 
A limited number of devices or nodes can be supported on a single local 
area network of this type. To expand the number of supported devices or 
nodes, individual local area networks can be interconnected through the 
high speed backbone network 18. The physical and logical connections 
between two networks are referred to as the bridge between the two 
networks. 
One example of a high speed or backbone network is an FDDI (Fiber 
Distributed Data Interface) network which uses optical fibers as a 
transmission medium and which supports data rates several times greater 
than the data rates that are employed in other typical local area 
networks. An FDDI network can also be characterized as a token-passing 
network. One of the significant differences between an FDDI network and 
the token-ring network described above is that an FDDI station or node 
originating a frame sends out a token at the end of that frame. That is, 
the originating node does not wait for the frame to return before issuing 
the token. FDDI networks are described in draft standards issued by an 
ANSI (American National Standards Institute) X3T9.5 committee. The details 
of the standard are not, however, critical to an understanding of the 
present invention. In fact, the present invention is not limited to 
particular kinds of networks. 
Token ring local area networks and FDDI networks employ different frame 
formats. FIG. 2 depicts a frame format employed in a token-ring network 
complying with current requirements set forth by Committee 802.5 of the 
IEEE (Institute of Electrical & Electronics Engineers) organization. The 
frame consists of a number of fields which can be classified as starting 
frame sequence fields, header fields, data fields, a frame check sequence 
or FCS field and ending frame sequence fields. The starting frame sequence 
fields include an 8-bit SD or starting delimiter field, which is a 
recognizable bit pattern identifying the start of the data frame. The AC 
or access control field is also an 8-bit field having several distinct 
subfields used for different functions. The subfields establish the 
priority of a token, distinguish between a frame and a token (which is 
actually an abbreviated frame), control whether certain tokens or frames 
remain on the ring and set the priority to be accorded the next token to 
be issued. 
The header fields include an 8-bit FC or frame control field which 
designates a frame either as a MAC (Medium Access Control) or a LLC 
(Logical Link Control) frame. MAC frames are used for control purposes 
while LLC frames transport user data. The header also has a 48-bit DA or 
destination address field containing the address of the user to which the 
data is to be sent and a 48-bit SA or source address field containing the 
address of the source user. The starting frame sequence fields and header 
fields of the frame are sometimes collectively referred to as a physical 
header. 
The frame also includes data fields, including an RI or routing information 
field containing data used to establish a path through the network. The RI 
or routing information is one of two variable length fields in the frame. 
The other variable length field is the INFO field, which contains the 
actual data being transmitted from a source to a destination. The data 
fields are followed by a 32-bit frame check sequence or FCS field which is 
used for error checking purposes. The manner in which the FCS field value 
is calculated is discussed in greater detail below. It should be noted, 
however, that the FCS field value is determined by the data contained in 
the message (header and data) portion of the frame. Each frame ends with 
ending frame sequence fields consisting of an 8-bit ending delimiter and 
an 8-bit frame status field. The ending delimiter is, of course, 
recognized by the receiving system as an end-of-frame indicator. Different 
bits in the frame status field serve different functions. One bit, 
initially set to 0 by the originating node is set to 1 by any intermediate 
node which detects an error, other bits, also originally 0's, are set to 1 
by any station which recognizes its own address and which copies the frame 
into its receive buffer. 
The frame format employed in an FDDI network meeting ANSI X3T9.5 standards 
is similar in many respects to the token-ring frame format. Referring to 
FIG. 3, an FDDI frame includes a starting delimiter, a message, a frame 
check sequence and an ending frame sequence. Unlike a token-ring frame, 
however, an FDDI frame is preceded by a variable length preamble at least 
64 bits long. Also, while the FDDI header includes a frame control field 
along with destination and source address fields, the frame control field 
for the FDDI frame has a different structure than the frame control field 
for a token-ring frame notwithstanding each field is 8 bits long. As noted 
above, the first two bits of a token-ring frame control field identify the 
frame as a LLC or MAC frame. Bits 3-5 of a token-ring frame control field 
are reserved (and thus are always set to zeros) while bits 6-8 are 
indicative of the priority to be given the frame at its destination. In an 
FDDI system, the first bit of the frame control field establishes whether 
the FDDI frame is an asynchronous frame. The second bit establishes 
whether the FDDI frame includes a 48-bit address or not. Bits 3-4 serve 
the same function as bits 1-2 of the token-ring frame control field in 
identifying whether the frame is an LLC or a MAC frame. In the FDDI frame 
control field, bit 5 is reserved while bits 6-8 establish the destination 
system priority level. 
Because an LLC or MAC frame is identified by different bits in frame 
control fields used for token-ring and FDDI frames, respectively, the 
content of the frame control field is necessarily different depending 
whether the frame is being routed through a token-ring network or an FDDI 
network. Because the frame control field is part of the message on which 
FCS field values are based, the differences in the frame control field 
must be reflected in any error checking process. 
FIG. 5 is a flow chart of one type of prior art process used to alter the 
FCS field when planned or intentional changes are made to the frame 
message during bridging from one type of network to another. 
After the frame is received (operation 60) in the network, standard error 
checking operations are performed. These operations, identified as block 
62, require that the receiving system calculate an FCS field value FCS' by 
applying a standard cyclical redundancy checking or CRC algorithm to the 
message portion of the received frame. The receiving system must determine 
(operation 64) whether a computed value FCS' is consistent with the FCS 
field value found in the received frame. A lack of consistency between the 
calculated value FCS' and the received value FCS indicates an error in the 
received message. Normal error handling protocols are followed (block 66) 
when an error is detected. If operation 64 indicated that FCS' was 
consistent with the received FCS field value, however, the receiving 
station would then process the received frame by changing the message 
(operation 68). The changes in the message may involve nothing more than 
substituting the frame control field required by the receiving system for 
the frame control field in the received frame. The receiving system would 
then calculate (operation 70) a new FCS field value FCS" by applying the 
standard CRC algorithm to the changed message. The frame could then be 
transmitted through the receiving system (operation 72) with the newly 
calculated FCS field value FCS". 
As noted earlier, there is a time interval during the processing of the 
frame in which the message contents are effectively unprotected. That time 
interval occurs between the completion of operation 64 and the calculation 
of FCS" in operation 70. If errors are introduced into the message during 
this time interval, FCS" is calculated using the erroneous message. The 
next system to receive the frame will not, however, be able to detect the 
error since both the FCS field calculated at the next system and the FCS 
field in the frame received at that system will be based on the same 
erroneous message. 
To eliminate the periods of time during which data is unprotected, the 
process illustrated in FIG. 6 can be employed. Certain steps in the 
process to be described are identical to corresponding steps in the prior 
art process. For example, the system must obviously receive a frame 
(operation 24) before any further processing can take place. The system 
will preferably also check for errors in the incoming frame by applying 
the standard CRC algorithm to the message portion of the frame to derive a 
calculated FCS field value FCS' (operation 26). The system will also check 
for consistency between the calculated FCS field value and the received 
FCS field value in operation 28 to invoke error handling protocols 
(operation 30) if an inconsistency is detected. At this point, the present 
method becomes quite different from the prior art method. 
The present method does not totally recalculate the frame check sequence 
field value but instead modifies the existing FCS value as a function of 
the planned or intentional message changes. To accomplish this, the 
receiving system creates a "dummy" message in an operation 32. This dummy 
message contains the same number of bits as the message portion of the 
frame (the header fields plus the data fields) but represents only the 
planned changes to be made in the received message. In most instances, one 
or two bits in the frame control field may be changed in the message when 
a frame is bridged from a token-ring network to a FDDI network or vice 
versa. Specifically, a frame control field for an LLC token-ring frame, 
which is the only type of frame that is commonly bridged to another 
network, will take the form FC=01000YYY where YYY represents the frame 
priority. A frame control field for the same LLC frame in an FDDI network 
will take the form FC=01010YYY. Since the priority of the frame does not 
change during bridging, it can be seen that only one bit in the frame 
control field must necessarily be changed when the frame is transferred 
from a token-ring to an FDDI network. Specifically, the fourth bit in the 
frame control field must be changed from 0 to 1. If the frame is being 
transferred from the FDDI network to a token-ring network, the same bit 
must be set from 1 to 0. 
The binary form of the "dummy" message can be established through simple 
binary or modulo-2 arithmetic. To provide a specific example, if a frame 
being transferred from a token-ring network to an FDDI network had a 
message area (header plus data) 256 bits long, in a simple case, the only 
difference between the message in the token-ring and the FDDI networks 
would be in fourth bit of the frame control field. The fourth bit would be 
converted from a 0 in the token-ring frame to a 1 in the FDDI frame. For 
this type of frame conversion, the dummy message would also be 256 bits 
long. In the final form of a message to be transferred in an FDDI network, 
the fourth bit of the frame control field would be a 1. In a token-ring 
network, the same bit would be a zero. Because of the specific operations 
performed by the standard CRC algorithm, this single bit change can only 
be accomplished by creating a dummy message in which the first thirty-two 
bits are the 1's complement of the actual modulo 2 difference between the 
FDDI frame and the corresponding token-ring frame. That is, bits 1-3 and 
5-32 are set to 1 in the dummy while bit 4 is set to zero. Assuming no 
changes are to be made in the message except in the frame control field, 
the remaining bits of the dummy message would be set to 0. 
In accordance with the present invention, an FCS field modifier is 
calculated in an operation 34 by applying the standard CRC algorithm 
(operation 34) to the 256 bit dummy message and (to comply with IEEE 
standards) by taking the 1's complement (operation 35) of the result. The 
result of operation 35 is an FCS(m) value. In an operation 36, a modified 
FCS value FCS" is then calculated by adding FCS(m) to the FCS value in the 
received frame, using modulo 2 arithmetic operations. 
While use of a standard CRC algorithm is envisioned, non-standard 
algorithms can be employed provided any algorithm used is a linear 
mathematical algorithm. A linear mathematical algorithm is one that 
satisfies the equation f(A+B)=f(A)+f(B) where f is indicative of a 
mathematical function while A and B are data. 
When the modified FCS field value FCS" has been calculated and the message 
has been changed (operation 38) as planned, the converted frame can be 
transmitted (operation 40) to the next system on the data path. Since the 
FCS field value is modified during conversion only as a function of 
planned changes, any errors introduced into the message are not factored 
into the modified FCS value. When the frame is received at the next system 
on the data path, the standard CRC error checking operation should reveal 
an inconsistency between the FCS field value calculated at that system and 
the modified FCS value received with the frame. 
A standard or conventional CRC algorithm is briefly described with 
reference to FIG. 7. The number of bits in the message portion of the 
frame (the header plus data) must be determined in an operation 42 in 
order to calculate an FCS field value. A Variable B is then set equal to 
the product of X.sup.k and a polynomial X.sup.31 +X.sup.30 +X.sup.29 + . . 
. +X+1 where the number of terms in the polynomial is equal to the number 
of bits in the FCS field. A second variable C is calculated (operation 46) 
as the remainder resulting from modulo 2 division of variable B by a 
standard Generating Polynomial. 
According to existing IEEE standards, the Generating Polynomial takes the 
form X.sup.32 +x.sup.26 +X.sup.23 +X.sup.22 +X.sup.16 +X.sup.12 +X.sup.11 
+X.sup.10 +X.sup.8 +X.sup.7 +X.sup.5 +X.sup.4 +X.sup.2 +X+1. A fourth 
variable D is computed in operation 48 as the product of X.sup.m and the 
binary representation of the frame message treated as a polynomial where m 
is the length of the frame check sequence field in bits. A variable E is 
calculated in operation 50 as the remainder resulting from a modulo 2 
division of variable D by the Generating Polynomial described above. A 
variable G is set equal to the modulo 2 sum of the computed variables C 
and E in operation 52 and the frame check sequence field value is set to 
the 1's complement of the variable G in an operation 54. 
The process described above is used on the orginal data message at the 
source station. At each intermediate station at which the message is to be 
changed, normally by changing the frame control field, the FIG. 7 process 
is applied only to the dummy message representing the planned changes to 
be made in the message. 
The preceding discussion has assumed that the invention may be used 
beneficially in situations where a receiving system performs frame format 
conversions. The invention may also be used in any other situation in 
which planned changes are to be made in a frame at a receiving station. 
For example, in a so-called frame relay system, each system receiving a 
frame may strip its own address from one of the fields of the frame and 
may substitute the address of the next system which is to receive the 
frame. The changes in the address field are known, of course, to the 
system doing the address conversion. The present invention may be used to 
maintain continuous CRC error protection in such a frame relay system. 
While there has been described what is considered to be a preferred 
embodiment of the invention, variations and modifications in that 
embodiment will occur to those skilled in the art once they become aware 
of the basic concepts of the invention. Therefore, it is intended that the 
appended claims shall be construed to include not only the preferred 
embodiment but all such variations and modifications as fall within the 
true spirit and scope of the invention.