Requirements for an Internet Standard Point-to-Point Protocol
RFC 1547

RFC 1547          Point-to-Point Protocol Requirements     December 1993

3.2 Flow Control

   Flow control (such as XON/XOFF) is not required.  Any implementation
   of the ISPPP is expected to be capable of receiving packets at the
   full rate possible for the particular data link and physical layers
   used in the implementation.  If higher layers cannot receive packets
   at the full rate possible, it is up to those layers to discard
   packets or invoke flow control procedures.  As discussed above, end-
   to-end flow control is the responsibility of the transport layer.
   Including flow control within a point-to-point protocol often causes
   violation of the simplicity requirement.

3.3 Sequencing

   Sequencing of packets is not required.  The ISPPP need provide no
   more service than the IP protocol, an unreliable datagram service
   which is free to reorder packets.  In fact, it is specifically
   allowed to reorder packets based upon some type-of-service criteria
   implemented in higher-level protocols.

3.4 Backward Compatibility

   There is no requirement for the ISPPP to provide backward
   compatibility with any other point-to-point protocol.  First, there
   are no official Internet Standards with which backward compatibility
   must be maintained.  Second, attempting to maintain backward
   compatibility may lead to needless restrictions on the new protocol.
   However, there is no need for the designers of the ISPPP to go out of
   their way to inhibit backward compatibility.

3.5 Multi-Point Links

   There is no requirement for supporting multi-point links.  Many
   features which are required are only valid between two peers.  These
   links are sufficiently rare that the benefits of supporting them are
   outweighed by the added complexity their support would introduce into
   the ISPPP.

      Historical Note: The original rationale also stated: "Furthermore,
      it is unlikely that many new types of multi-point links will be
      introduced in the foreseeable future."  Since this was written,
      considerable effort has been expended in new multi-point links,
      including Switched Multimegabit Data Service, Frame Relay, and
      Asynchronous Transfer Mode.  However, it is clear that these are
      considerably more complex than ISPPP.

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3.6 Half-Duplex or Simplex Links

   Support for half-duplex or simplex links is not required.  These
   types of links are not in common use in the current Internet.  Half-
   duplex links require some method of turning the line around.  The
   ISPPP need not have an explicit mechanism for handling line turn-
   around.  Such support might possibly be added in the future via the
   required extension mechanism.

3.7  7-bit Asynchronous RS-232 Links

   The use of asynchronous RS-232 need not support 7-bit links.  8-bit
   links are predominant in the Internet environment and supporting 7-
   bit links introduces unnecessary complexity.

4.  Prior Work On PPP Protocols

   This section reviews a number of existing point-to-point and data
   link layer protocols and points out which of our requirements are not
   satisfied.

4.1 Internet Protocols

4.1.1 RFC 891 - DCN Local-Network Protocols, Appendix A

   In Appendix A of RFC 891, "DCN Local-Network Protocols" [4], D.L.
   Mills describes the data link layer packet formats used by the
   Fuzzball system for asynchronous, character-oriented synchronous,
   DDCMP, HDLC, ARPANET 1822, X.25 LAPB and ethernet links.  These
   protocols meet the stated requirements for simplicity, transparency,
   packet framing and efficiency, but fall short of many of the others.
   Most of these protocols assume the use of the IP protocol, and do not
   include any type of protocol demultiplexing field.  No error
   detection mechanism is provided except when necessary to comply with
   another standard such as ethernet.  RFC 891 does not mention the MTU
   used for any of these links.  Other requirements such as loopback
   detection and misconfiguration detection are not discussed.  Finally,
   no option negotiation scheme is defined; without a protocol
   demultiplexing field it would be difficult or impossible to include
   one.

4.1.2 RFC 914 - Thinwire Protocols

   RFC 914, "Thinwire Protocols" [5], discusses the use of low speed
   links in the Internet.  This document places its main emphasis on
   decreasing round-trip delay and increasing link efficiency with the
   help of header compression (vs. data compression) techniques.  Three
   "Thinwire" protocols are discussed, Thinwire I, Thinwire II and

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   Thinwire III.  The latter two protocols require the use of a reliable
   data link layer protocol; one such protocol, "SLIP" (not to be
   confused with Rick Adams' SLIP), is proposed in Appendix D of the
   RFC.  As proposed, "SLIP" does not meet many of the stated
   requirements.  Although not terribly complex, as a reliable, error
   detecting and correcting protocol, it is not "simple".  The 32 octet
   packet size makes it inefficient for large or uncompressed packets,
   requiring complex fragmentation and reassembly.  The use of other
   than asynchronous links is not mentioned.  The entire reliable link
   layer would be redundant over LAPB links.  There is no mechanism for
   option negotiation or future extensibility.

4.1.3 RFC 916 - Reliable Asynchronous Transfer Protocol

   RFC 916 [6] presents RATP, the Reliable Asynchronous Transfer
   Protocol.  RATP provides error detection and correction, sequencing
   and flow control across a point-to-point connection.  It is directed
   towards full duplex RS-232 links although it is useful for other
   point-to-point links.  Although the author claims that RATP is not as
   complex as some other protocols, it is far from simple.  RATP solves
   many of the problems which we have labeled non-requirements and fails
   to solve many of our stated requirements.  Specifically, RATP does
   not support option negotiation and has no mechanism for future
   extensibility.  Since RFC 916 was published, no consensus has emerged
   advocating RATP.  For these reasons RATP is not recommended as the
   ISPPP.

4.1.4 RFC 935 - Reliable Link Layer Protocols

   RFC 935 [7] is a rebuttal to the protocols proposed in RFCs 914 and
   916.  J. Robinson discusses existing and widely-used national and
   international standards which meet the needs addressed by the two
   prior RFCs.  The standards reviewed include character-oriented
   asynchronous and synchronous (bisynch) protocols and bit-oriented
   synchronous protocols.  RFC 935 does not present any higher level
   issues such as option negotiation or extensibility.

4.1.5 RFC 1009 - Requirements for Internet Gateways

   Section 3 of RFC 1009, "Constituent Network Interfaces" [8], briefly
   discusses requirements for transmission of IP datagrams over a number
   of types of point-to-point links including X.25 LAPB, HDLC framed
   synchronous links, Xerox Synchronous Point-to-Point synchronous lines
   and the MIT Serial Line Framing Protocol for asynchronous lines.  RFC
   1009 merely mentions these as reasonable candidates and does not go
   into depth on any of them.  All are discussed further in this
   document.

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4.1.6 RFC 1055 - Serial Line IP

   Rick Adams' Serial Line IP (SLIP) protocol [9] has become something
   of a de facto standard due to the popularity of the 4.2 and 4.3BSD
   UNIX operating systems.  SLIP is easily added to 4.2 systems and is
   included with 4.3.  Many other TCP/IP implementation have added SLIP
   implementations in order to be compatible.  Yet SLIP is not a real
   standard; the protocol was only recently published in RFC form.
   Before RFC 1055 it was specified in the SLIP source code.  SLIP does
   not meet most of the requirements set forth above.  SLIP certainly
   meets the requirement for simplicity, and also meets the requirements
   for transparency and bandwidth efficiency.  But SLIP only provides
   for sending IP packets over asynchronous serial lines.  Since it
   provides no higher level protocol field for demultiplexing, SLIP
   cannot support multiple concurrent higher level protocols.  Providing
   only a framing protocol, SLIP would be entirely redundant when used
   with a LAPB synchronous link.  SLIP includes absolutely no mechanism
   for error detection, not even parity.  Again due to its lack of a
   protocol type field, SLIP does not support any type of option
   negotiation or extensibility.

4.2 International Protocols

4.2.1 ISO 3309 - HDLC Frame Structure

   ISO 3309 [10], the HDLC frame structure, is a simple data link layer
   protocol which provides framing of packets transmitted over bit-
   oriented synchronous links.  Special flag sequences mark the
   beginning and end of frames and bit stuffing allows data containing
   flag characters to be transmitted.  A 16-bit Frame Check Sequence
   provides error detection.

   By itself, the HDLC frame structure does not meet most of the
   requirements.  HDLC does not provide protocol multiplexing, standard
   MTUs, fault detection or option negotiation.  There is no mechanism
   for future extensibility.

   Given the HDLC frame structure's wide acceptance and simplicity, it
   may be an ideal building block for the ISPPP.

4.2.2 ISO 6256 - HDLC Balanced Class of Procedures

   ISO 6256, the HDLC Balanced Class of Procedures, specifies a data
   link layer protocol which provides error correction, sequencing and
   flow control.  ISO 6256 builds on ISO 3309 and ISO 4335, HDLC
   Elements of Procedures.

   As far as meeting our requirements is concerned, ISO 6256 does not

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   provide any more utility than does ISO 3309.  The capabilities that
   are provided are all considered unnecessary and overly complex.

4.2.3 CCITT X.25 and X.25 LAPB

   CCITT recommendation X.25 [11] describes a network layer protocol
   providing error-free, sequenced, flow controlled virtual circuits.
   X.25 includes a data link layer, X.25 LAPB, which uses ISO 3309, 4335
   and 6256.  Neither X.25 LAPB or full LAPB meet any more of our
   requirements than the ISO protocols.

4.2.4 CCITT I.441 LAPD

   CCITT I.441 LAPD [12] defines the Link Access Procedure on the ISDN
   D-Channel.  The data link layer of LAPD is very similar to that of
   LAPB and fails to meet the same requirements.

4.3 Other Protocols

4.3.1 Cisco Systems point-to-point protocols

   The Cisco Systems gateway supports both asynchronous links using SLIP
   and synchronous links using either simple HDLC framing, X.25 LAPB or
   full X.25.  The HDLC framing procedure includes a four byte header.
   The first octet (address) is either 0x0F (unicast intent) or 0x8F
   (multicast intent).  The second octet (control byte) is left zero and
   is not checked on reception.  The third and fourth octets contain a
   standard 16 bit Ethernet protocol type code.

   A "keepalive" or "beaconing" protocol is used to ensure the two-way
   connectivity of the serial line.  Each end of the link periodically
   sends two 32 bit sequence numbers to the other side.  One sequence
   number is the local side's sequence number, the other is the sequence
   number received from the other side.  Hearing the local sequence
   number from the other side indicates that the link is working in both
   directions.

   The keepalive protocol is extensible.  One extension is used to
   default IP addresses on serial lines of systems without non-volatile
   memory.  A request for address is sent to the remote side.  The
   remote side responds with its own IP address and a subnet mask.  When
   the querying side receives the reply, it checks if the host portion
   of the remote address is either 1 or 2.  If so, the opposite address
   is chosen for the local address.  If not, the protocol cannot be used
   and we must rely on other address resolution means.  This protocol
   assumes that each serial link uses one subnet or network number.

   LAPB assuming IP is another possible encapsulation.  A multi-protocol

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   extension of LAPB (multi-LAPB) includes a 16 bit Ethernet type code
   after the address and control bytes and in front of the actual
   protocol data.  DDN X.25 and Commercial X.25 encapsulations are also
   supported.  Multiple protocols are supported by making protocol
   dependent CALL REQUEST's.

4.3.2 MIT PC/IP framing protocol

   The MIT PC/IP framing protocol [13] provides a mechanism for the
   transmission of IP datagrams over asynchronous links.  The low-level
   protocol (LLP) sublayer provides encapsulation while the local net
   protocol provides multiplexing of IP datagrams and IP address request
   packets.  The protocol only allows host-to-gateway connections.
   Host-to-gateway flow control is provided by requiring the host to
   transmit request packets to the gateway until an acknowledgment is
   received.  Rudimentary IP address negotiation requires the host to
   ascertain its IP address from the gateway.

   The protocol does not implement error detection, connection status
   determination, fault detection or option negotiation.  Only
   asynchronous links are supported.

4.3.3 Proteon p4200 point-to-point protocol

   The Proteon p4200 multi-protocol router supports transmission of
   packets over bit-oriented synchronous links with a wide range of
   speeds (zero to 2 Mb/sec).  The p4200 point-to-point protocol
   encapsulates packets inside HDLC frames but does not use the HDLC
   address or control fields; these two octets are instead used for a
   16-bit type field.  The p4200 does use the HDLC frame check sequence
   trailer.  Protocol type numbers are ad hoc and do not correspond to
   any existing standard.  A simple liveness protocol detects dead
   connections.

   Although the Proteon protocol does meet many of our requirements, it
   does not meet our requirements for option negotiation.

4.3.4 Ungermann Bass point-to-point protocol

   The Ungermann Bass router supports synchronous links using simple
   HDLC framing.  Neither the HDLC address or control field are used, IP
   datagrams are placed immediately after the HDLC flag.

   The U-B protocol does not meet any of our requirements for fault
   detection or option negotiation.  No mechanism for future
   extensibility is currently defined.

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4.3.5 Wellfleet point-to-point protocol

   The Wellfleet router supports synchronous links using simple HDLC
   framing.  The HDLC framing procedure uses the HDLC address and places
   the Unnumbered Information (UI) command in all frames.  A simple
   header following the UI command provides a two octet type field using
   the same values as Ethernet.

   The Wellfleet protocol does not meet any of our requirements for
   fault detection or option negotiation.  No mechanism for future
   extensibility is currently defined, although one could be added.

4.3.6 XNS Synchronous Point-to-Point Protocol

   The Xerox Network Systems Synchronous Point-to-Point protocol (XNS
   PPP) [14] was designed to address most of the same issues that an
   ISPPP must address.  In particular, it addresses the issues of
   simplicity, transparency, efficiency, packet framing, protocol
   multiplexing, error detection, standard MTUs, symmetry, switched and
   non-switched media, connection status, network address negotiation
   and future extensibility.  However, the XNS SPPP does not meet our
   requirements for multiple data link layer protocols, fault detection
   and data compression negotiation.  Although protocol multiplexing is
   provided, the packet type field has only 8 bits which is too few.

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References

   [1]  Postel, J., "Internet Protocol", STD 5, RFC 791, USC/Information
        Sciences Institute, September 1981.

   [2]  Postel, J., "User Datagram Protocol", STD 6, RFC768, USC/Information
        Sciences Institute, August 1980.

   [3]  Electronic Industries Association, EIA Standard RS-232-C,
        "Interface Between Data Terminal Equipment and Data
        Communications Equipment Employing Serial Binary Data
        Interchange", August 1969.

   [4]  Mills, D. L., "DCN Local-Network Protocols", STD 44, RFC 891,
        University of Delaware, December 1983.

   [5]  Farber, David J., Delp, Gary S., and Conte, Thomas M., "A
        Thinwire Protocol for Connecting Personal Computers to the
        Internet", RFC 914, University of Delaware, September 1984.

   [6]  Finn, G., "Reliable Asynchronous Transfer Protocol (RATP)",
        RFC 916, USC/Information Sciences Institute, October 1984.

   [7]  Robinson, J., "Reliable Link Layer Protocols", RFC 935, BBN,
        January 1985.

   [8]  Braden, R., and J. Postel, "Requirements for Internet
        Gateways", STD 4, RFC1009, USC/Information Sciences Institute,
        June 1987.

   [9]  Romkey, J., "A Nonstandard for the Transmission of IP Datagrams
        Over Serial Lines: SLIP", STD 47, RFC 1055, June 1988.  STD
        4, RFC 1009, June 1987.

   [10] ISO International Standard (IS) 3309, "Data Communications -
        High-level Data Link Control Procedures - Frame Structure",
        1979.

   [11] CCITT Recommendation X.25, "Interface Between Data Terminal
        Equipment (DTE) and Data Circuit Terminating Equipment (DCE)
        for Terminals Operating in the Packet Mode on Public Data
        Networks", Vol. VIII, Fascicle VIII.2, Rec. X.25.

   [12] CCITT Recommendation Q.921 "ISDN User-Network Interface Data
        Link Layer Specification".

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   [13] Romkey, J.L., "PC/IP Programmer's Manual", Massachussetts
        Institute of Technology Laboratory for Computer Science,
        January 1986.

   [14] Xerox Corporation, "Synchronous Point-to-Point Protocol", Xerox
        System Integration Standard, Stamford, Connecticut, XSIS
        158412, December 1984.

   [15] "Digital Data Communications Message Protocol", Digital
        Equipment Corporation.

Security Consideration

   Security issues are not discussed in this memo.

Chair's Address

   The working group can be contacted via the current chair:

      Fred Baker
      Advanced Computer Communications
      315 Bollay Drive
      Santa Barbara, California  93117

      EMail: fbaker@acc.com

Author's Address

   Questions about this memo can also be directed to:

      Drew Perkins
      4015 Holiday Park Drive
      Murrysville, PA  15668

      EMail: perkins+@cmu.edu

Editor's Address

   Typographic revision and historical notes by:

      William Allen Simpson
      1384 Fontaine
      Madison Heights, Michigan  48071

      EMail: Bill.Simpson@um.cc.umich.edu

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