Simtel MSDOS 1992 June

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Packet Driver Protocol G. L. Chesson Bell Laboratories Abstract These notes describe the packet driver link protocol that was supplied with the Seventh Edition of and is used by the UUCP program. General Information flow between a pair of machines may be regu- lated by first representing the data as sequence-numbered packets of data and then establishing conventions that govern the use of sequence numbers. The PK, or packet driver, protocol is a par- ticular instance of this type of flow-control discipline. The technique depends on the notion of a transmission window to determine upper and lower bounds for valid sequence numbers. The transmitter is allowed to retransmit packets having sequence numbers within the window until the receiver indicates that pack- ets have been correctly received. Positive acknowledgement from the receiver moves the window; negative acknowledgement or no acknowledgement causes retransmission. The receiver must ignore duplicate transmission, detect the various errors that may occur, and inform the transmitter when packets are correctly or in- correctly received. The following paragraphs describe the packet formats, message exchanges, and framing used by the protocol as coded in the UUCP program and the kernel. Although no attempt will be made here to present internal details of the algorithms that were used, the checksum routine is supplied for the benefit of other implementors. Packet Formats The protocol is defined in terms of message transmissions of 8-bit bytes. Each message in- cludes one control byte plus a data segment of zero or more in- formation bytes. The allowed data segment sizes range between 32 and 4096 as determined by the formula 32(2k) where k is a 3-bit number. The packet sequence numbers are likewise constrained to 3-bits; i.e. counting proceeds modulo-8. The control byte is partitioned into three fields as depicted below. bit 7 6 5 4 3 2 1 0 t t x x x y y y The t bits indicate a packet type and determine the interpreta- tion to be placed on the xxx and yyy fields. The various in- terpretations are as follows: tt interpretation 00 control packet 10 data packet 11 `short' data packet 01 alternate channel A data segment accompanies all non-control packets. Each transmitter is constrained to observe the maximum data segment size established during initial synchronization by the receiver that it sends to. Type 10 packets have maximal size data seg- ments. Type 11, or `short', packets have zero or more data bytes but less than the maximum. The first one or two bytes of the data segment of a short packet are `count' bytes that indicate the difference between the maximum size and the number of bytes in the short segment. If the difference is less than 127, one count byte is used. If the difference exceeds 127, then the low-order seven bits of the difference are put in the first data byte and the high-order bit is set as an indicator that the remaining bits of the difference are in the second byte. Type 01 packets are never used by UUCP and need not be discussed in de- tail here. The sequence number of a non-control packet is given by the xxx field. Control packets are not sequenced. The newest sequence number, excluding duplicate transmissions, accepted by a receiver is placed in the yyy field of non-control packets sent to the `other' receiver. There are no data bytes associated with a control packet, the xxx field is interpreted as a control mes- sage, and the yyy field is a value accompanying the control mes- sage. The control messages are listed below in decreasing prior- ity. That is, if several control messages are to be sent, the lower-numbered ones are sent first. xxx name yyy 1 CLOSE n/a 2 RJ last correctly received sequence number 3 SRJ sequence number to retransmit 4 RR last correctly received sequence number 5 INITC window size 6 INITB data segment size 7 INITA window size The CLOSE message indicates that the communications channel is to be shut down. The RJ, or reject, message indicates that the re- ceiver has detected an error and the sender should retransmit after using the yyy field to update the window. This mode of re- transmission is usually referred to as a `go-back-N' procedure. The SRJ, or selective reject, message carries with it the se- quence number of a particular packet to be retransmitted. The RR, or receiver ready, message indicates that the receiver has detected no errors; the yyy field updates the sender's window. The INITA/B/C messages are used to set window and data segment sizes. Segment sizes are calculated by the formula 32(2yyy) as mentioned above, and window sizes may range between 1 and 7. Measurements of the protocol running on communication links at rates up to 9600 baud showed that a window size of 2 is optimal given a packet size greater than 32 bytes. This means that the link bandwidth can be fully utilized by the software. For this reason the SRJ message is not as important as it might otherwise be. Therefore the implementations no longer generate or respond to SRJ messages. It is mentioned here for historical accuracy only, and one may assume that SRJ is no longer part of the proto- col. Message Exchanges Initialization Messages are ex- changed between four cooperating entities: two senders and two receivers. This means that the communication channel is thought of as two independent half-duplex data paths. For example the window and segment sizes need not be the same in each direction. Initial synchronization is accomplished with two 3-way handshakes: two each of INITA/INITB/INITC. Each sender transmits INITA messages repeatedly. When an INITA message is received, INITB is sent in return. When an INITB message is received and an INITB message has been sent, an INITC message is sent. The INITA and INITB messages carry with them the packet and window size that each receiver wants to use, and the senders are sup- posed to comply. When a receiver has seen all three INIT mes- sages, the channel is considered to be open. It is possible to design a protocol that starts up using fewer messages than the interlocked handshakes described above. The advantage of the more complicated design lies in its use as a research vehicle: the initial handshake sequence is completely symmetric, a handshake can be initiated by one side of the link while the con- nection is in use, and the software to do this can utilize code that would ordinarily be used only once at connection setup time. These properties were used in experiments with dynamically ad- justed parameters. That is attempts were made to adapt the win- dow and segment sizes to changes observed in traffic while a link was in use. Other experiments used the initial handshake in a different way for restarting the protocol without data loss after machine crashes. These experiments never worked well in the packet driver and basically provided the impetus for other proto- col designs. The result as far as UUCP is concerned is that ini- tial synchronization uses the two 3-way handshakes, and the INIT messages are ignored elsewhere. Data Transport After in- itial synchronization each receiver sets a modulo-8 incrementing counter R to 0; each sender sets a similar counter S to 1. The value of R is always the number of the most recent correctly re- ceived packet. The value of S is always the first sequence number in the output window. Let W denote window size. Note that the value of W may be different for each sender. A sender may transmit packets with sequence numbers in the range S to (S+W-1) mod-8. At any particular time a receiver expects arriv- ing packets to have numbers in the range (R+1) mod-8 to (R+W) mod-8. Packets must arrive in sequence number order are are only acknowledged in order. That is, the `next' packet a re- ceiver will acknowledge must have sequence number (R+1) mod-8. A receiver acknowledges receipt of data packets by arranging for the value of its R counter to be sent across the channel where it will be used to update an S counter. This is done in two ways. If data is flowing in both directions across a channel then each receiver's current R value is carried in the yyy field of non- control packets. Otherwise when there is no bidirectional data flow, each receiver's R value is transmitted across the link as the yyy field of an RR control packet. Error handling is up to the discretion of the receiver. It can ignore all errors in which case transmitter timeouts must provide for retransmission. The receiver may also generate RJ error control packets. The yyy field of an incoming RJ message replaces the S value of the local sender and constitutes a request for retransmission to start at that sequence number. The yyy field of an incoming SRJ message selects a particular packet for retransmission. The resemblance between the flow control procedure in the packet driver and that defined for X.25 is no accident. The packet driver protocol be- gan life as an attempt at cleaning up X.25. That is why, for ex- ample, control information is uniform in length (one byte), there is no RNR message (not needed), and there is but one timeout de- fined in the sender. Termination The CLOSE message is used to terminate communications. Software on either or both ends of the communication channel may initiate termination. In any case when one end wants to terminate it sends CLOSE messages until one is received from the other end or until a programmable limit on the number of CLOSE messages is reached. Receipt of a CLOSE message causes a CLOSE message to be sent. In the environ- ment it also causes the SIGPIPE or `broken pipe' signal to be sent to the local process using the communication channel. Framing The term framing is used to denote the technique by which the beginning and end of a message is detected in a byte stream; error control denotes the method by which transmission errors are detected. Strategies for framing and error control depend upon additional information being transmitted along with the control byte and data segment, and the choice of a particular strategy usually depends on characteristics of input/output dev- ices and transmission media. Several framing techniques are in used in support of PK protocol implementations, not all of which can be described in detail here. The technique used on asynchro- nous serial lines will be described. A six byte framing envelope is constructed using the control byte C of a packet and five oth- er bytes as depicted below. <DLE><k><c0><c1><C><x> The <DLE> symbol denotes the ASCII ctrl/P character. If the en- velope is to be followed by a data segment, <k> has the value log2(size)-4; i.e. 1 < k < 8. If k is 9, then the envelope represents a control packet. The <c0> and <c1> bytes are the low-order and high-order bytes respectively of a 16-bit checksum of the data segment, if there is one. For control packets <c1> is zero and <c0> is the same as the control byte C. The <x> byte is the exclusive-or of <k><c0><c1><C>. Error control is accom- plished by checking a received framing envelope for compliance with the definition, and comparing a checksum function of the data segment with <c0><c1>. This particular framing strategy as- sumes data segments are constant-sized: the `unused' bytes in a short packet are actually transmitted. This creates a certain amount of overhead which can be eliminated by a more complicated framing technique. The advantage of this strategy is that i/o devices can be programmed to take advantage of the constant-sized framing envelopes and data segments. The checksum calculation is displayed below as a C function. Note that the code is not truly portable because the definitions of and are not necessarily uniform across all machines that might support this language. This code assumes that and are 16 and 8- bits respectively. /* [Original document's version corrected to actual version] */ chksum(s,n) register char *s; register n; { register short sum; register unsigned short t; register short x; sum = -1; x = 0; do { if (sum<0) { sum <<= 1; sum++; } else sum <<= 1; t = sum; sum += (unsigned)*s++ & 0377; x += sum^n; if ((unsigned short)sum <= t) { sum ^= x; } } while (--n > 0); return(sum); }