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Network Working Group J. Robinson
Request for Comments: 935 BBN
January 1985
RELIABLE LINK LAYER PROTOCOLS
Status of This Memo
This RFC discusses protocols proposed recently in RFCs 914 and 916,
and suggests a proposed protocol that could meet the same needs
addressed in those memos. The stated need is reliable communication
between two programs over a full-duplex, point-to-point communication
link, and in particular the RFCs address the need for such
communication over an asynchronous link at relatively low speeds.
The suggested protocol uses the methods of existing national and
international data link layer standards. This RFC suggests a
proposed protocol for the ARPA-Internet community, and requests
discussion and suggestions for improvements. Distribution of this
memo is unlimited.
Introduction
This RFC is motivated by recent RFCs 914 and 916, which propose new
standards for protocols that transfer serial data reliably over
asynchronous communication lines. In this note, I summarize
widely-used standards that have been in existence for some time that
might be appropriate for this environment. I hope that the existing
standards will be found to meet the needs the new proposals seek to
address.
In both the US and international standards areas, there are two major
categories of serial data communication standards for the link layer.
These categories are character-oriented and bit-oriented. The first
is the older class; it is standardized in the US standard ANSI
X3.28-1976 (which superseded the original version of 1971), and in
the ISO standards IS 1745, IS 2111, IS 2628 and IS 2629. Although
frequently used in synchronous environments, wherein the name binary
synchronous (or bisynch) is used, these standards use the term "basic
mode" to describe their procedures. The latter class is standardized
in the US standard ADCCP (Advanced Data Communication Control
Procedures), ANSI X3.66- 1979, and in the ISO standard HDLC
(High-level Data Link Control procedures), in IS 3309, IS 4335 and IS
7809.
Other international standards, draft standards and vendor standards
follow the ADCCP/HDLC procedures. Among these are SDLC (IBM), X.25
LAPB (CCITT), IEEE 802.2/ISO 8802.2 LLC (LAN Logical Link Control)
and ISDN LAPD (CCITT). Many vendors have built equipment which meets
Robinson [Page 1]
RFC 935 January 1985 Reliable Link Layer Protocols the character-oriented standards, in both synchronous and asynchronous environments, including all the major US mainframe manufacturers. The only other serial link layer protocol known to the author in as wide use as these is DEC's DDCMP (Digital Data Communications Message Protocol). This protocol uses a character count instead of framing characters, but is in other respects a character-oriented protocol. The next sections of this note will compare the three protocols above on several bases, paying particular attention to the characteristics that make particular aspects of the protocol appropriate to the low-speed, asynchronous, serial environment. Frame Structure All serial protocols divide the data to be transmitted into units known as frames. A frame is typically one to several hundred characters in length. The frame structure is the major difference used above to divide the protocols into three classes. Character-Oriented Framing Character-oriented protocols use two techniques for defining a frame. First, it is necessary to determine where characters start and stop. The technique used for this purpose is to transmit a number of unique characters prior to the start of a frame. The character generally used for this is the SYN character. Note that this is not required when using asynchronous transmission. Since each character is itself framed by start and stop bits, there is never a question of where characters begin and end. The main technique for structuring a frame is the use of special framing characters to delineate the start and end of a frame, and to delineate portions of the frame (such as header and text). Some uses of character-oriented protocols require that these characters never appear in the header or text of the frame, while others allow "transparent" transmission. Transparency is obtained by preceding each framing character by a unique control character, typically DLE. In this way, all characters may be sent as header or text, except for DLE. In order to allow DLE to be sent in the header or text, the DLE is doubled. Robinson [Page 2]
RFC 935 January 1985 Reliable Link Layer Protocols Bit-Oriented Framing Bit-oriented protocols also use a unique character (technically, it is just an arbitrary bit-string) for frame delineation, which is the FLAG. This character provides frame synchronization. All bits between two occurrences of FLAGs constitute a frame. The FLAG is a 0 bit, followed by six 1 bits, followed by another 0 bit. In order that the FLAG character not appear mistakenly in the data of the message, the sender inserts (and the receiver removes) an extra 0 bit after any five successive 1 bits in the data stream. Because this insertion of bits ("stuffing") results in arbitrary frame bit-lengths, bit-oriented protocols are generally useful only in synchronous transmission environments. Although it has never been attempted, however, one could imagine an asynchronous environment where each FLAG character that appears in the data is translated into a two- character sequence that avoids FLAGs, and at least one other character is similarly translated. For example, one could frame data with FLAGS, and send DLE-F to mean FLAG and DLE-DLE to mean DLE when these characters occur within the frame. Note that bit-oriented procedures do not require that the number of bits between FLAGs be an exact number of 8-bit characters, in distinction to character-oriented protocols and DDCMP. The necessity for character-alignment may be imposed at higher layers, as it is, for example, in X.25 Network Layer. Frame Structure in DDCMP DDCMP uses a third approach to frame structure. Like character-oriented protocols, it uses a SYN character to achieve character synchronization prior to starting a frame, but one cannot dispense with this over asynchronous lines (see below). Contained within the fixed-length header portion of the frame is a count field, which reports how many characters are contained in the variable-length text portion. Since no framing characters are required at all, transparency is not a problem. However, because the count must be received properly for the sender and receiver to stay in frame synchronization, the header is protected with a separate error control checksum, which is typically two characters long (see below). Also, once a header error has been detected, the count field must be assumed to be invalid, and so there must be a unique character sequence that introduces the next header in order that the receiver can regain synchronization with the sender. Therefore, the SYN characters preceding a frame are required even on asynch lines. Robinson [Page 3]
RFC 935 January 1985 Reliable Link Layer Protocols Error Detection Several types of error control may be used, and the various protocols above are similar. Most synchronous uses require a cyclic redundancy check sequence be attached to each frame. This is a 16-bit sequence which can be easily generated and checked in hardware using a shift register, and can be somewhat more tediously done in software with about 5-6 instructions per character sent or received, and a 256 by 16-bit lookup table. DDCMP and Bit-oriented protocols all require use of such a sequence. As noted above, DDCMP uses a check sequence twice, once for the header and once for the data. In some environments, weaker checks are used on character-oriented links. These take two forms. If the the number of significant bits per character is only 7, then the parity bit can be set to achieve either odd or even parity. ANSI standard X3.16-1976 specifies that odd parity should be used on synchronous links and even parity on asynchronous links. The second type of check is "longitudinal parity", wherein one character is added to the frame so that the number of 1 bits in each bit position summed over all the characters of the message and the check character is even. In other words, the exclusive-or of all the characters is 0. Character parity and longitudinal parity may be used together. Note also that most character-oriented control messages, such as those that poll, select, and acknowledge, are sent with only parity for error control. Sequence Control All these protocol provide reliable transmission by sequencing the frames and providing positive and (in some cases) negative acknowledgments. Senders can ask the receiver for status if a reply is late. In character-oriented protocols, frames are implicitly numbered (typically) and only one may be outstanding at a time. Acknowledgments are explicitly numbered. One variant allows each block (frame) to be explicitly numbered as well; in this case up to 7 may be outstanding. In bit-oriented protocols, frames are explicitly numbered and up to 7 may be outstanding at a time. Optional control field extension allows for up to 127 outstanding. An alternate procedure that has been defined for use both in the ISDN LAPD environment and in IEEE Robinson [Page 4]
RFC 935 January 1985 Reliable Link Layer Protocols 802 LAN environments uses, in effect, a one-bit sequence number and one outstanding frame. Also, unsequenced, unacknowledged information frames can be used when frames need not be sent reliably. In DDCMP, the frames are explicitly numbered and up to 255 may be outstanding. Addressing All of these protocols allow for addressing stations on a multipoint link separately. In LAN environments, both a sending and receiving address are required, whereas multipoint environments use a single address and assume one master station communicating with multiple addressed slave stations. In bit-oriented protocols, the address provides extra information in that frames can be categorized as commands or responses; in this sense, the address provides another control bit per frame. However, it is possible to operate without needing this distinction. Addresses are typically one character long; bit-oriented protocols allow for extension of this field to arbitrary length. Character-oriented protocols use two-character (controller and terminal) addresses. For point-point operation, the address is clearly superfluous (except to distinguish commands and replies in bit-oriented protocols); one might imagine dispensing with it. The Asynchronous Environment Which of these protocols is best for the asynchronous environment? This depends on the definition of "best", of course. One means of judging is to compare the amount of overhead that each protocol would add to each frame sent. We will examine the overhead costs in two groups: framing/transparency/error checking, and addressing/control. The two groups of functions are independent of each other, even though the protocols mentioned above use specific combinations of techniques from these two groups. Also, hardware available on minicomputer-class and larger machines today supports the first group of functions completely for these standard protocols; this fact should allow for far greater performance from the gateway machine. Robinson [Page 5]
RFC 935 January 1985 Reliable Link Layer Protocols To the extent that such hardware becomes available for personal computers, it can also be used there to reduce the protocol processing overhead. Here's a breakdown of framing costs in characters. RATP is also included for comparison. Protocol Frame Check Transp. Total F+C char-or. 4 2 2 8 6 bit-or. 1 2 2 5 3 DDCMP 4 4 0 8 8 RATP 2 3 0 5 5 The transparency column indicates the anticipated cost in inserted characters to achieve transparency across a 256-byte frame. The figure for bit-oriented protocols is a pessimistic guess, because I don't know the exact answer; it is between 0 and 8 characters, with the worst case occurring when the data is all one bits. For character-oriented protocols, we would expect on average one DLE character in a 256-byte frame; the worst case overhead (256 DLEs) is 256 bytes. Because transparency is so much a function of the user data, and because we have ignored the cost of loss of frame synchronization in the counting protocols (DDCMP and RATP), I argue that we should base the comparison on the frame and checksum costs only. For these two columns, character-oriented framing costs one more character per frame than RATP. This, plus its wide use and hardware chip support, create a strong case for its use in preference to RATP for framing. Bit-oriented framing, as noted previously, is appropriate only on synchronous links. The character oriented variant I postulated above would have the same costs, but as it is not a standard, it is not proposed here. So we now have constructed the following frame format: DLE STX <control and data ...> DLE ETX CRC CRC One objection to using character-oriented protocols as opposed to character-count protocols is that it is necessary to examine every character as it arrives. I respond to this objection as follows: 1. Under some circumstances, such as when a header has been hit with an error, it will be necessary for the receiver to look at every character anyway. 2. The environment for this protocol is a 1200 baud link; thus Robinson [Page 6]
RFC 935 January 1985 Reliable Link Layer Protocols 120 characters per second need to be examined at a maximum. Even on a relatively slow personal computer, this should not present a problem. We now turn our attention to the content and format of the control information preceding each link frame. There are three components to this cost, control, address, and acknowledgment. The address field allows multipoint configurations and is superfluous for the point-to-point environment proposed, but it is present in the public standards and we restrict ourselves to those. Acknowledgments are shown if they are required explicitly by the protocol. A "0" indicates that the acknowledgments may be included in the control information for traffic in the opposite direction, and only need be sent explicitly when no reverse traffic is present (and thus are assumed to take no required overhead). Only character-oriented protocols have required acknowledgments. Cont. Addr. Ack Total char-or. 0 3 2 5 bit-or. 1 1 0 2 DDCMP 3 1 0 4 RATP 1 0 0 1 Again, the bit-oriented procedures provide the lowest overhead among the public standards, but in this case there is no conflict in using them in the asynchronous environment. In fact, even if all the other aspects of RATP were to be adopted, I believe the control field codings of the bit- oriented procedures represent a more efficient use of the channel, are widely implemented, and allow for addition of more functions later if desired. As stated above, there are several protocols in the bit-oriented family. I would recommend use of LAPB, since this is the most widely known of the family. For those not familiar with bit-oriented control procedures, I have included a quick summary of these procedures in Appendix A. Refer to the standards listed at the end of this note for more detail. Robinson [Page 7]
RFC 935 January 1985 Reliable Link Layer Protocols RATP Compared to Public Protocols As can be seen from the above tables, RATP does not represent a significant savings compared to other widely-used protocols. Given frame lengths of 13 bytes, which appears to be the minimum for Thinwire II (RFC 914), 8 characters' overhead using the public standards represents 61% versus 46% for RATP's 6 characters. On a 1200 baud line, the bandwidth available assuming only such short frames is thus 74 versus 82 characters per second, respectively. Since 1/13 of these are actually user data, the typing rates supported by these protocols using TCP/IP are pretty low, like 5.6 versus 6.3 characters per second. Clearly a bigger cost is still found in the 12 characters overhead in Thinwire II (or 40 for TCP/IP with no compression). The costs improve dramatically when the number of user characters per frame increases. Thus, file transfer, or even line-blocked typing, should perform adequately. As frame size grows, the cost of the extra 2 characters per frame to use standard protocols rapidly drops to a few percent or less. RATP does allow one optimization which cannot be achieved in the standard protocols - the use of a one-character format that reduces the per-frame overhead to 3 characters (or 4 if a 16-bit CRC is used). However, in the scenario wherein single-character messages make sense, a user typing characters (with no higher layer protocols), the extra overhead is probably not a problem since the link is still lightly enough loaded that the extra overhead is still a small percentage of the available bandwidth. Also, allowing multiple frames in flight helps reduce the bottleneck caused by having one frame at a time outstanding. On Check Sequences Both RFCs 914 and 916 propose to use relatively simple check sequences, which can be easily computed in a general-purpose processor. In one case, this is an additive check and in the other it is an exclusive-or (or parity) check. Although the additive check is slightly more powerful than the exclusive-or, both are relatively weak compared to CRC techniques. Since the intended network-layer protocol (IP) provides for similar checks on its header, and the transport layer (TCP) checksums its header and data, one might question whether the protection should also be provided at the link layer at all, or if it should, then are these checks good enough? Providing for recovery at the TCP layer Robinson [Page 8]
RFC 935 January 1985 Reliable Link Layer Protocols leads to slow recovery times, so this approach will probably yield too poor a level of service for noisy links. More importantly, the link layer control field needs a certain degree of protection to prevent needless loss or duplication of frames in the face of line errors. A CRC check, in combination with the additive checks provided by IP and TCP, yield an error-protection that is greater than that afforded by either check by itself. This is because the two techniques address fundamentally different characteristics of the possible errors. The degree of increase is substantial compared to that of two additive checks. That is, if two additive checks are cascaded, there are many types of two-bit failures that will pass both the link layer and TCP/IP checking. Although I don't wish to include a detailed error analysis in this note, I would support the use of a CRC type of error check because of the far greater level of protection it affords. As I pointed out, the cost per character is roughly 5-6 instructions, assuming the use of a 256 by 16-bit lookup table. Again, at 120 characters per second, the increased cost is not deemed to be too great. Moreover, use of a standard CRC allows for the possibility that the serial line chip's own CRC generation and checking hardware may be used. Although such chips may not be present in the PCs in the environment envisioned, they are likely to be available in the gateway machine to which the PCs talk. Data Compression: An Aside I find the proposed methods of data compression of RFC 914 particularly interesting. I see these as independent of the choice of the underlying link layer protocol, as it is in RFC 914. I am aware of no such character-oriented compression that is in common use in the communication world. The only techniques that come close are in statistical multiplexing devices, which sometimes also include an adaptive Huffman-coding to reduce link bandwidth. Since the Thinwire II approach can recognize much longer repeated sequences than a Huffman code, I expect that the potential savings are correspondingly greater. I would like to see a version of the Thinwire protocols which allows for the template idea, but which keeps it independent of the higher layer protocols in use. One way to achieve this is to allow templates to be encoded and exchanged between the communicating parties when they start up, and perhaps adaptively as conditions warrant. I would anticipate that this sort of approach might well Robinson [Page 9]
RFC 935 January 1985 Reliable Link Layer Protocols have widespread applicability beyond the TCP/IP environment addressed in RFC 914. The most important gain for this environment is removal of the apparent exorbitant overhead that IP and TCP seem to present for use over slow links. Summary The link layer protocol I would advocate for asynchronous, dialup communication between PCs would use transparent, character-oriented framing, a 16-bit CRC error check, and the control procedures of LAPB. The CRC should be either CRC-16 or the CCITT CRC used in X.25, with the latter probably being preferred; modern link chips tend to support both of these if they support either. Evolution of integrated circuits that directly implement all of the public standards will dramatically drop the cost and raise the reliability of new implementations of standard protocols. Chip manufacturers have little motivation to address standards that are not widely used. Overhead for the suggested protocol is 8 characters per frame. Up to 7 frames may be outstanding at a time in either direction of transfer. Choice of an appropriate maximum frame size is application-dependent, and would certainly be influenced by the quality of the physical connection; however, I believe that IP datagrams are acceptable frames for dialup 1200 baud service. Non-standard modifications that would save a little link overhead would be to dispense with the one-character address field, and to use the RATP count-oriented frame structure. These are not recommended, because they depart from common practice and yield modest improvements at best. Postscript Those familiar with the early history of the Telenet Public Data Network should recognize that this proposal is essentially the same as the original link layer protocol specification for that network, circa 1976, except that the control procedures used at that time, known as LAP, have now been superseded by the more powerful and efficient LAPB, and their access links, as all X.25 access links, are synchronous rather than asynchronous. I did not set out to achieve this result, but just note it in passing. My personal view of where the world of personal computer access to data networks is heading is that X.25 will rapidly become the protocol of choice. One already sees third-party (for IBM PC) and Robinson [Page 10]
RFC 935 January 1985 Reliable Link Layer Protocols vendor (for Wang PC) implementations of X.25. CCITT is circulating a proposal for accessing an X.25 data network using a dial-up X.25 connection, as recommendation X.32. Thus, I feel that the type of communication proposed in this RFC and RFCs 914 and 916 will be used for a relatively short period of time. The future holds, I believe, LAN or X.25/X.32 data link layer and access, X.25 and/or ISO IP network layer, and TCP and/or ISO TP4 transport layer. References RFC 914, "Thinwire Protocol", Farber, Delp and Conte, 1984. RFC 916, "Reliable Asynchronous Transfer Protocol", Finn, 1984. "Technical Aspects of Data Communication", McNamara, Digital Press, 1977. ANSI X3.4-1968, "American National Standard Code for Information Interchange (ASCII)", American National Standards Institute, Inc., 1968. ANSI X3.16-1976, "American National Standard Character Structure and Character Parity Sense for Serial-by-Bit Data Communication in the American National Standard Code for Information Interchange", American National Standards Institute, Inc., 1976. ANSI X3.28-1976, "American National Standard Procedures for the Use of the Communication Control Characters of American National Standard Code for Information Interchange in Specified Data Communication Links", American National Standards Institute, Inc., 1976. ANSI X3.66-1979, "American National Standard for Advanced Data Communication Procedures (ADCCP)", American National Standards Institute, Inc., 1979. International Standard 1745, "Information Processing - Basic mode control procedures for data communication systems", International Organization for Standardization (ISO), 1975. International Standard 2111, "Data Communication - Basic mode control procedures - Code independent information transfer", ISO, 1973. International Standard 2628, "Basic mode control procedures - Complements", ISO, 1973. International Standard 2629, "Basic mode control procedures - Conversational information message transfer", ISO, 1973. Robinson [Page 11]
RFC 935 January 1985 Reliable Link Layer Protocols International Standard 3309, "Data Communication - High-level data link control procedures - Frame structure", ISO, 1982. International Standard 4335, "Data Communication - High-level data link control procedures - Consolidation of elements of procedures", ISO, 1982. International Standard 7809, "Data Communication - High-level data link control procedures - Consolidation of classes of procedures", ISO, 1984. Recommendation X.25, "Interface between Data Terminal Equipment (DTE) and Data Circuit Terminating Equipment (DCE) for Terminals Operating in the Packet Mode and Connected to Public Data Networks by Dedicated Circuit", The International Telegraph and Telephone Consultative Committee (CCITT), 1980 (to be revised, 1984). Recommendation Q.920, "ISDN User-network Interface Data Link Layer - General Aspects", CCITT, 1984 (draft E). Recommendation Q.921, "ISDN User-network Interface Data Link Layer Specification", CCITT, 1984 (draft E). Draft International Standard 8802.2, "Local Area Network Standards, Logical Link Control", ISO, 1984 (draft). Draft Proposed Addendum to DIS 8802.2, "Single Frame Service", IEEE, 1984 (Eighth Draft). Robinson [Page 12]
RFC 935 January 1985 Reliable Link Layer Protocols Appendix A - Bit-Oriented Control Field Contents There are three control field formats. The primary one is used for data frames (called "information frames" in the standards), and is coded as follows: 8 7 6 5 4 3 2 1 <- bit number, 1 sent first 0 (signifies data frame) S S S send seq , bit 2 low-order P/F poll/final bit, for recovery R R R receive seq (ACK) Acknowledgments are cumulative. Recovery is typically to back up and continue from the lost frame. Use of the poll/final bit is beyond the scope of this note. Acknowledgments may also be sent in supervisory frames, coded as follows: 8 7 6 5 4 3 2 1 <- bit number, 1 sent first 0 1 (signifies supervisory frame) T T frame type (see below) P/F poll/final bit, for recovery R R R receive seq (ACK) Up to four frame types are possible; only two are required. The first is called RR, for "receive ready", and indicates acknowledgment and that the receiver is prepared to process more frames. The other, RNR for "receive not ready", is used for flow control of the sender. If flow control is not necessary, I suppose even this frame could be dispensed with. The other supervisory frames, "reject" and "selective reject", are varieties of negative acknowledgement that ask for retransmission of all and one (respectively) of previously transmitted frames. Positive acknowledgment and retransmission are the only really necessary procedures, however. The third frame format is called Unnumbered. Many possible functions are specified in the various standards for these frames, including initializing the link, reset sequence numbers, etc. The basic format is: 8 7 6 5 4 3 2 1 <- bit number, 1 sent first 1 1 (signifies unnumbered frame) T T T T T frame type P/F poll/final bit, for recovery Robinson [Page 13]