Network Working Group Robert Friend Internet Draft Stac Electronics W A Simpson DayDreamer expires in six months February 1996 PPP Stac LZS Compression Protocol draft-ietf-pppext-stacker-06.txt Status of this Memo This document is a submission to the Point-to-Point Protocol Working Group of the Internet Engineering Task Force (IETF). Comments should be submitted to the ietf-ppp@merit.edu mailing list. Distribution of this memo is unlimited. This document is an Internet-Draft. Internet Drafts are working documents of the Internet Engineering Task Force (IETF), its Areas, and its Working Groups. Note that other groups may also distribute working documents as Internet Drafts. Internet Drafts are draft documents valid for a maximum of six months, and may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material, or to cite them other than as a ``working draft'' or ``work in progress.'' To learn the current status of any Internet-Draft, please check the ``1id-abstracts.txt'' listing contained in the internet-drafts Shadow Directories on: ftp.is.co.za (Africa) nic.nordu.net (Europe) ds.internic.net (US East Coast) ftp.isi.edu (US West Coast) munnari.oz.au (Pacific Rim) Friend & Simpson expires in six months [Page i] DRAFT Stac LZS February 1996 Abstract The Point-to-Point Protocol (PPP) [1] provides a standard method for transporting multi-protocol datagrams over point-to-point links. The PPP Compression Control Protocol [2] provides a method to negotiate and utilize compression protocols over PPP encapsulated links. This document describes the use of the Stac LZS data compression algorithm, with single or multiple compression histories, for compressing PPP encapsulated packets. Friend & Simpson expires in six months [Page ii] DRAFT Stac LZS February 1996 1. Introduction Starting with a sliding window compression history, similar to LZ1 [3], Stac Electronics developed a new, enhanced compression algorithm identified as Stac LZS. The LZS algorithm is optimized to compress all file types as efficiently as possible. Even string matches as short as two octets are effectively compressed. The Stac LZS compression algorithm supports both single compression history communication and multiple compression history communication. A single compression history will require the minimum amount of memory to implement, but may not provide as much compression as a multiple history implementation. Often, many streams of information are interleaved over the same link. Each virtual link will transmit data that is independent of other virtual links. Using multiple compression histories can improve the compression ratio of a communication link by associating separate compression histories with separate virtual links of communication. 1.1. Licensing Source and object licenses are available on a non-discriminatory basis. Hardware implementations are also available. Contact Stac Electronics at the address and phone number listed with the author's address for further information. 1.2. Specification of Requirements In this document, several words are used to signify the requirements of the specification. These words are often capitalized. MUST This word, or the adjective "required", means that the definition is an absolute requirement of the specification. MUST NOT This phrase means that the definition is an absolute prohibition of the specification. SHOULD This word, or the adjective "recommended", means that there may exist valid reasons in particular circumstances to ignore this item, but the full implications MUST be understood and carefully weighed before choosing a different course. MAY This word, or the adjective "optional", means that this item is one of an allowed set of alternatives. An Friend & Simpson expires in six months [Page 1] DRAFT Stac LZS February 1996 implementation which does not include this option MUST be prepared to interoperate with another implementation which does include the option. 2. LZS Packets Before any LZS packets may be communicated, PPP must reach the Network-Layer Protocol phase. When the Compression Control Protocol (CCP) has reached the Opened state, and LZS is negotiated as the primary compression algorithm, exactly one Stac LZS datagram is encapsulated in the PPP Information field, where the PPP Protocol field indicates type hex 00FD (compressed datagram) or type hex 00FB (Individual link compressed datagram). Type hex 00FD is used when compression is negotiated over a single physical link or when compression is negotiated over a single bundle consisting of multiple physical links. Type hex 00FB is used when compression is negotiated separately over individual physical links to the same destination. For more information, please refer to PPP Compression Control Protocol. When CCP has not successfully reached the Opened state, or LZS is not the primary compression algorithm, exactly one LZS datagram is encapsulated in the PPP Information field, where the PPP Protocol field indicates type hex 4021 (Stac LZS). Note that in the latter case, use of LZS is terminated by the PPP LCP Protocol-Reject. The default format is used: a single history with no History Number field and no Check Value field (as if the negotiated history count were 1). The maximum length of the Stac LZS datagram transmitted over a PPP link is the same as the maximum length of the Information field of a PPP encapsulated packet. Prior to compression, the uncompressed data begins with the PPP Protocol ID Field. Protocol-Field-Compression MAY be used on this value, if has been successfully negotiated for the link. The PPP Protocol ID Field is followed by the original Information field. The length of the uncompressed data field is limited only by the allowed size of the compressed data field and the higher protocol layers. PPP Link Control Protocol packets MUST NOT be sent within Stac LZS packets. PPP Network Control Protocol packets MUST NOT be sent within Stac LZS packets. 2.1. Padding Friend & Simpson expires in six months [Page 2] DRAFT Stac LZS February 1996 The LZS Information field always ends with the last compressed data byte (also known as the ), which is used to disambiguate padding. This allows trailing bits as well as octets to be considered padding. 2.2 Zero Deletion/Insertion When the sender does not add Padding [1], any trailing zero octets are removed prior to transmission. A single trailing zero octet is appended upon receipt, after removal of any framing FCS. 2.3. Reliability and Sequencing When no Compression History is kept, the algorithm does not depend on a reliable link, and does not require that packets be delivered in sequence. However, per packet compression results in a lower compression ratio than it could be on a stream. Some reasons for resetting the history on a per packet basis include: - The link has a high error rate. - The resources of the transmitter or receiver limit the ability to maintain a compression history between packets. When more than 1 Compression History is negotiated, the packet sequence MUST be preserved within specific History Numbers. There is no sequence requirement between different History Numbers. When using one or more compression histories, the implementation MUST rely on either a lower layer reliable link protocol (RFC 1663), use a technique to keep the compressor and decompressor histories in synchronization, or both. To enable this feature, the implementation MAY rely on the Reset-Request and Reset-Ack messages of the Compression Control Protocol. The Data field of the CCP Reset-Request and Reset-Ack MUST contain the two octet History Number to be reset, most significant octet first. The transmitter MAY clear a Compression History at any time. The receiver is implicitly notified of this event, and the decompression history will automatically be affected. The transmitter MUST reset a history after a CCP Reset-Request for a given History Number. 2.4. Data Expansion The maximum expansion of Stac LZS is 12.5%. A Maximum Receive Unit (MRU) MAY be negotiated that is 12.5% Friend & Simpson expires in six months [Page 3] DRAFT Stac LZS February 1996 larger than the size of a normal packet. Then, packets can always be sent compressed regardless of expansion. When the expansion plus compression header exceeds the size of the peer's MRU for the link, the PPP packet MUST be sent without compression, in the original PPP packet form with the "native" PPP Protocol ID number. The transmitter MUST reset the affected history. If it is detected that most packets are expanding (for example, due to the use of already compressed data), then the transmitter SHOULD stop sending compressed packets, and reset the appropriate history. Data compression MAY be resumed on this data link later. 2.5. Packet Format A summary of the Stac LZS packet format is shown below. The fields are transmitted from left to right. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PPP Protocol | (History Number) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | (Check Value) | Compressed Data ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2.5.1. PPP Protocol The PPP Protocol field is described in the Point-to-Point Protocol Encapsulation [1]. When the Stac LZS compression protocol is successfully negotiated by the PPP Compression Control Protocol [2], the value is 00FD hex or 00FB hex as described in section 2. This value MAY be compressed when Protocol-Field-Compression is negotiated. 2.5.2. History Number The number of the compression history which was used, ranging from 2 to the negotiated History Count. By default a History Count of value 1 is supported and this field is not present. If the negotiated History Count is less than 2, this field is removed. There is no need for the field when no history is kept, or only a single history is kept. Friend & Simpson expires in six months [Page 4] DRAFT Stac LZS February 1996 If the negotiated History Count is 2 or more, but less than 256,this field is 1 octet. If 256 or more histories are negotiated, this field is 2 octets, most significant octet first. If multiple histories are used in one direction on a link, the history number field MUST be present on all packets in both directions, and sized according to the largest number of histories in either direction. 2.5.3. Check Value The check value field comprises 0, 1, or 2 octets. By default, no check is added to the packet (the field comprises 0 octets) 2.5.3.1. LCB A simple one octet Longitudinal Check Byte (LCB) MAY be used, after successful negotiation of the LCB option. The LCB is the Exclusive-OR of FF(hex) and each octet of the uncompressed datagram (prior to the compression operation). On receipt, the receiver computes the Exclusive-OR of FF(hex) and each octet of the decompressed packet. If this value does not match the received LCB, then a receive failure for that history has occurred. The receive failure is handled according to the history synchronization procedure in section 2.4.4. 2.5.3.2. CRC A two octet Cyclic Redundancy Check (CRC) MAY be used, instead of the LCB, after successful negotiation of the CRC option. The transmitter MUST initialize the CRC value to FFFF(hex) at the beginning of each packet. The CRC computation is based on the HDLC FCS-16 polynomial: x**16 + x**12 + x**5 + 1 The ones complement of the CRC is transmitted least significant octet first, which contains the coefficient of the highest term. On receipt, the receiver initializes the CRC to FFFF (hex), and computes the CRC based on the formula above for each octet of the decompressed packet. If the received CRC value does not match the transmitted CRC value, then a receive failure for that history has occurred. The receive failure is handled according to the history synchronization procedure in section 2.4.4. 2.5.3.3. Sequence Number A one octet Sequence Number MAY be used, instead of a LCB or Friend & Simpson expires in six months [Page 5] DRAFT Stac LZS February 1996 CRC, after successful negotiation of the Sequence Number option. The value of the sequence number field (the sequence number of the packet) MUST begin with "1" and increment modulo 256 on successive packets that contain data fields. This number is relative to the history number used. If the sequence number of the packet is any number other than (N+1) mod 256, where N is the sequence number of the last packet received for the same history, or an initial value of "1", a receive failure for that history has occurred. The receive failure MUST be handled according to the synchronization procedure in section 2.4.4. The sequence number MUST NOT be reset by the transmitter when a packet containing a Reset-Req is received. The decompressor MUST resynchronize its sequence number reference for the indicated history when a packet containing a Reset-Ack is received. 2.5.4. History Synchronization Procedure On receipt, if Sequence Number one (1) follows any other number than zero (0), or is otherwise out of sequence, or the LCB or CRC is invalid, a CCP Reset-Request MUST BE sent, containing the full two octet History Number (most significant octet first, and which is the value 1 when no History Number is present), with a CCP identifier. Upon receipt of the Reset-Request, the affected compression history is reset by the transmitter, and a Reset-Ack is sent. However, the Sequence Number (if implemented) is not reset. For each packet that generates a receive failure, the receiver MUST increment the Identifier and transmit a CCP Reset-Request. After transmitting the Reset-Request packet, the receiver MUST continue ignoring valid compressed packets for a particular history, until the correct CCP Reset-Ack Identifier (corresponding to the Reset-Request) for that History Number is received. 2.5.5. Compressed Data The data field MUST contain only one datagram in compressed form. The length of this field is always an integer number of octets. There MUST BE only one end marker per block of compressed data. The form of the data field is one block of compressed data as defined in 3.2 of X3.241-1994, and is repeated here for informational purposes ONLY. := [] := 0 | 1 Friend & Simpson expires in six months [Page 6] DRAFT Stac LZS February 1996 := (8-bit byte) := := 1 | (7-bit offset) 0 (11-bit offset) := 110000000 := 1 | 0 := 00 = 2 1111 0110 = 14 01 = 3 1111 0111 = 15 10 = 4 1111 1000 = 16 1100 = 5 1111 1001 = 17 1101 = 6 1111 1010 = 18 1110 = 7 1111 1011 = 19 1111 0000 = 8 1111 1100 = 20 1111 0001 = 9 1111 1101 = 21 1111 0010 = 10 1111 1110 = 22 1111 0011 = 11 1111 1111 0000 = 23 1111 0100 = 12 1111 1111 0001 = 24 1111 0101 = 13 ... 3. Sending Compressed Datagrams The reliable and efficient transport of datagrams on the data link depends on the following processes. 3.1. Transmitter Process When a network datagram is received, it is assigned to a particular history buffer and processed according to ANSI X3.241- 1994 to form compressed data. Prior to the compression operation, if a Reset-Request is outstanding for the history buffer to be used or if the negotiated history count for this data link is 0, the history buffer is cleared. Uncompressed data MUST be sent (in the original PPP packet form with the "native" PPP Protocol ID number) if compression causes enough expansion to cause the data compression datagram size to exceed the Information field's MRU. In this case, since the compressor has modified the history buffer before sending an uncompressed datagram, the history buffer MUST be cleared before the next datagram is processed. The output of the compression operation is placed in the information field of the datagram. If the sequence number field is present according the value of the check mode field, the sequence number counter for the applicable history number MUST be incremented and its value placed in the sequence number field. If Friend & Simpson expires in six months [Page 7] DRAFT Stac LZS February 1996 the LCB field is present according the value of the check mode field, the LCB value MUST be computed as specified in section 2.4.3.1. and the resultant value placed in the LCB field. If the CRC field is present according the value of the check mode field, the CRC value MUST be computed as specified in section 2.4.3.2. and the resultant value placed in the LCB field. Upon reception of a CCP Reset-Request packet, the transmitting compressor MUST be cleared to an initial state, which includes clearing the history buffer. In addition to the reset of the compressor, a CCP Reset-Ack packet MUST be transmitted. The data field of this packet MUST be filled with the corresponding history number, most significant octet first. 3.2. Receiver Process If a CCP Reset-Request packet is received, the local compression engine MUST be signaled that a Reset-Request has been received for the history number specified in the data field. If a CCP Reset- Ack packet is received, any outstanding receive failure for the specified history MUST be cleared. If no receive failure is outstanding, and the sequence number field is present, its value is checked. If a receive failure has occurred, it MUST be handled according to the history resynchronization mechanism described below, and the remainder of the datagram is discarded. If no receive failure is detected, the data is assigned to the indicated decompression history buffer and the compressed data block MUST be decompressed according to ANSI X3.241-1994. If the LCB or CRC fields are present on the received datagram, an LCB or CRC for the uncompressed data MUST be computed and checked against the received LCB or CRC according to sections 2.3.4.1. or 2.4.3.2., respectively. If a receive failure has occurred, it MUST be handled according to the History Resynchronization Mechanism described in section 3.4. If a CCP Reset-Ack packet is received, the receiving decompressor's corresponding history MAY be reset to an initial state. (However, due to the characteristics of the Stac LZS algorithm, a decompressor history reset is not required). After reset, any compressed or uncompressed data contained in the packet is processed. On the occurrence of a receive failure, an implementation MUST transmit a CCP Reset-Request packet with the data field containing the history number (most significant octet first) matching the history that had the failure. Once a receive failure has occurred, the data in any subsequent packets received for that history MUST be discarded until a CCP Reset-Ack packet containing a valid Identifier matching the Identifier that was sent with the Friend & Simpson expires in six months [Page 8] DRAFT Stac LZS February 1996 last CCP Reset-Request packet is received. It is the responsibility of the receiver to ensure the reliability of the Reset-Request/Ack mechanism. This may require the transmission of additional CCP Reset-Request packets before a CCP Reset-Ack packet is received. 3.3. History Maintenance The History Count field determines the number of history buffers to be maintained for the compression protocol. For example, each history buffer could represent a separate logical connection between the data compression peers. When maintaining a history, the peers MUST use link error detection and signaling to ensure that both the compressor and decompressor copies of each history buffer are always identical. Setting the History Count field to the value "0" indicates that the compression is to be on a connectionless basis. In this case, a single history buffer is used and MUST be cleared at the beginning of every datagram. When the History Count field is set to the value "1", a single history buffer is maintained by each of the data compression peers. (A single logical connection.) When the History Count field is set to a value greater than "1", separate history buffers, error detection states, and signaling states are maintained by the decompressing entity for each history. The compressing peer may transmit data on any number of separate histories, up to the value of the History Count field. 3.4. History Resynchronization Mechanism The Stac LZS protocol utilizes CCP Reset-Request/Reset-Ack mechanism in order to provide a mechanism for indicating a receiver failure in one direction of a compressed link without affecting traffic in the other direction. A receive failure is determined using the LCB, CRC, or sequence number mechanisms, according to the value of the check mode field. Reset-Requests and Reset-Acks are specific to the history number of the packet containing them. Reset-Request/Reset-Ack history synchronization signaling is provided to recover from a loss of synchronization between peers, especially in unreliable transport layers. As with all compression algorithms, the decompressor can not recover from dropped, erroneous, or mis-ordered datagrams, and will propagate errors catastrophically until both peers are reset to an initial Friend & Simpson expires in six months [Page 9] DRAFT Stac LZS February 1996 state. The Stac LZS protocol provides a means to detect these error conditions: LCB or CRC for erroneous datagrams, and sequence number for dropped or mis-ordered datagrams. There is a means for correcting a loss of synchronization: clear both the failing compression and decompression histories, and follow the transmitter and receiver processes in sections 3.1. and 3.2. 4. Configuration Option Format Description The CCP Stac LZS Configuration Option negotiates the use of Stac LZS on the link. By ultimate disagreement, no compression is used. All implementations must support the default values. A summary of the Stac LZS Configuration Option format is shown below. The fields are transmitted from left to right. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | History Count | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Check Mode | +-+-+-+-+-+-+-+-+ Type 17 Length 5 History Count The History Count field is two octets, most significant octet first, and specifies the maximum number of Compression Histories. The value 0 indicates that the implementation expects the peer to clear the Compression History at the beginning of every packet. The value 1 is the default value, and is used to indicate that only one history is maintained. Other valid values range from 2 to 65535. The peer is not Friend & Simpson expires in six months [Page 10] DRAFT Stac LZS February 1996 required to send as many histories as the implementation indicates that it can accept. However, it should be noted that resources are allocated in each peer to support the number of negotiated histories in this field. Check Mode The Check Mode field indicates support of LCB, CRC or Sequence checking, and other future extensions to this standard. This field comprises 2 sub-fields, and is considered to be bit-mapped. The lower 3 bits comprise 5 mutually exclusive values. The upper 5 bits are all "Reserved" bit locations must be set to "0" to allow for future backward-compatible extensions to this standard. Defined values: 0 None (default value) 1 LCB 2 CRC 3 Sequence Number (recommended) 4 Extended Mode 0 1 2 3 4 5 6 7 +-------+-------+----------+-----+-----+-----+-----+-----+ | LCB/CRC/Seq#/Extd | Res | Res | Res | Res | Res | +-------+-------+----------+-----+-----+-----+-----+-----+ 5. Definition of Extended Mode When Check Mode 4 (Extended Mode) is successfully negotiated, the packet format is different from the format described above. The Extended Mode format is described below. Extended Mode only supports a history count of 1. 5.1. Extended Mode Packet Format 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PPP Protocol |A|B|C|D| Coherency Count | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Compressed Data... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ PPP Protocol The PPP Protocol field is described in the Point-to-Point Protocol Encapsulation [1]. Friend & Simpson expires in six months [Page 11] DRAFT Stac LZS February 1996 When a compression protocol is successfully negotiated by the PPP Compression Control Protocol [2], the value is hex 00FD. This value MAY be compressed when Protocol-Field-Compression is negotiated. Bit A - PACKET_FLUSHED This bit indicates that the history buffer has just been flushed before this packet was generated. Thus, this packet can ALWAYS be decompressed because it is not based on any previous history. This bit is typically sent to inform the peer that it has flushed it's history buffer and that the peer can accept this packet and re-synchronize. Bit B This bit is not used with Stac LZS compression. Bit C - PACKET_COMPRESSED This bit is used to indicate that the packet is compressed. Bit D This bit is not used with Stac LZS compression. Coherency Count The coherency count is used to assure that the packets are sent in proper order and that no packet has been dropped. This count is always increased by 1 and NEVER decreases or goes back. When all bits are 1, the count returns to 0. The coherency count is 12 bits so the decompressor must handle the rollover case. Compressed Data The compressed data begins with the protocol field. For example, an IP packet may contain 0021 followed by an IP header. The compressor will first try to compress the 0021 protocol field and then move on to the IP header. 5.2. Extended Mode Synchronization Packets may be lost during transfer. If the decompressor maintained coherency count does not match the coherency count received in the compressed packet or if the decompressor detects that a received packet is corrupted, the decompressor drops the packet and sends a Friend & Simpson expires in six months [Page 12] DRAFT Stac LZS February 1996 CCP Reset-Request packet. The compressor on receiving this packet flushes the history buffer and sets the PACKET_FLUSHED bit in the next frame it sends. The decompressor on receiving a packet with its PACKET_FLUSHED bit set, flushes its history buffer and sets its coherency count to the one shipped by the compressor in that packet. Thus synchronization is achieved without a Reset-Ack packet. Security Considerations Security issues are not discussed in this memo. References [1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, Daydreamer, July 1994. [2] Rand, D., "The PPP Compression Control Protocol (CCP)", work in progress. [3] Lempel, A. and Ziv, J., "A Universal Algorithm for Sequential Data Compression", IEEE Transactions On Information Theory, Vol. IT-23, No. 3, May 1977. [4] Rand, D., "PPP Reliable Transmission", RFC 1663, Novell, July 1994. Friend & Simpson expires in six months [Page 13] DRAFT Stac LZS February 1996 Chair's Address The working group can be contacted via the current chair: Fred Baker Cisco Systems 519 Lado Drive Santa Barbara, California 93111 EMail: fred@cisco.com Author's Address Questions about this memo can also be directed to: Robert Friend Stac Electronics 5993 Avenida Encinas Carlsbad, CA 92008 (619)794-4542 Email: rfriend@stac.com William Allen Simpson Daydreamer Computer Systems Consulting Services 1384 Fontaine Madison Heights, Michigan 48071 Bill.Simpson@um.cc.umich.edu bsimpson@MorningStar.com (prefered) Friend & Simpson expires in six months [Page 14] DRAFT Stac LZS February 1996 Table of Contents 1. Introduction .......................................... 1 1.1 Licensing ....................................... 1 1.2 Specification of Requirements ................... 1 2. LZS Packets ........................................... 2 2.1 Padding ......................................... 2 2.2 Zero Deletion/Insertion ......................... 3 2.2 Reliabliity and Squencing ....................... 3 2.3 Data Expansion .................................. 3 2.5 Packet Format ................................... 4 2.5.1 PPP Protocol .................................... 4 2.5.2 History Number .................................. 4 2.5.3 Check Value ..................................... 5 2.5.3.1 LCB ........................................ 5 2.5.3.2 CRC ........................................ 5 2.5.3.3 Sequence Number ............................ 5 2.5.4 History Synchronization Procedure ............... 6 2.5.5 Compressed Data ................................. 6 3. Sending Compressed Datagrams .......................... 7 3.1 Transmitter Process ............................. 7 3.2 Receiver Process ................................ 8 3.3 History Maintenance ............................. 9 3.4 History Resynchronization Mechanism ............. 9 4. Configuration Option Format ........................... 10 5. Definition of Extended Mode ........................... 11 5.1 Extended Mode Packet Format ..................... 11 5.2 Extended Mode Synchronization ................... 12 SECURITY CONSIDERATIONS ...................................... 13 REFERENCES ................................................... 13 CHAIR'S ADDRESS ........................................... 14 AUTHORS' ADDRESS ............................................. 14