RFC 1332                        PPP IPCP                        May 19923.2.  IP-Compression-Protocol   Description      This Configuration Option provides a way to negotiate the use of a      specific compression protocol.  By default, compression is not      enabled.   A summary of the IP-Compression-Protocol 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     |     IP-Compression-Protocol   |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |    Data ...   +-+-+-+-+   Type      2   Length      >= 4   IP-Compression-Protocol      The IP-Compression-Protocol field is two octets and indicates the      compression protocol desired.  Values for this field are always      the same as the PPP Data Link Layer Protocol field values for that      same compression protocol.      The most up-to-date values of the IP-Compression-Protocol field      are specified in the most recent "Assigned Numbers" RFC [6].      Current values are assigned as follows:         Value (in hex)          Protocol         002d                    Van Jacobson Compressed TCP/IP   Data      The Data field is zero or more octets and contains additional data      as determined by the particular compression protocol.McGregor                                                        [Page 6]RFC 1332                        PPP IPCP                        May 1992   Default      No compression protocol enabled.McGregor                                                        [Page 7]RFC 1332                        PPP IPCP                        May 19923.3.  IP-Address   Description      This Configuration Option provides a way to negotiate the IP      address to be used on the local end of the link.  It allows the      sender of the Configure-Request to state which IP-address is      desired, or to request that the peer provide the information.  The      peer can provide this information by NAKing the option, and      returning a valid IP-address.      If negotiation about the remote IP-address is required, and the      peer did not provide the option in its Configure-Request, the      option SHOULD be appended to a Configure-Nak.  The value of the      IP-address given must be acceptable as the remote IP-address, or      indicate a request that the peer provide the information.      By default, no IP address is assigned.   A summary of the IP-Address 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     |           IP-Address   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+           IP-Address (cont)       |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Type      3   Length      6   IP-Address      The four octet IP-Address is the desired local address of the      sender of a Configure-Request.  If all four octets are set to      zero, it indicates a request that the peer provide the IP-Address      information.   Default      No IP address is assigned.McGregor                                                        [Page 8]RFC 1332                        PPP IPCP                        May 19924.  Van Jacobson TCP/IP header compressionVan Jacobson TCP/IP header compression reduces the size of the TCP/IPheaders to as few as three bytes.  This can be a significant improvementon slow serial lines, particularly for interactive traffic.The IP-Compression-Protocol Configuration Option is used to indicate theability to receive compressed packets.  Each end of the link mustseparately request this option if bi-directional compression is desired.The PPP Protocol field is set to the following values when transmittingIP packets:   Value (in hex)   0021      Type IP.  The IP protocol is not TCP, or the packet is a             fragment, or cannot be compressed.   002d      Compressed TCP.  The TCP/IP headers are replaced by the             compressed header.   002f      Uncompressed TCP.  The IP protocol field is replaced by             the slot identifier.4.1.  Configuration Option Format   A summary of the IP-Compression-Protocol Configuration Option format   to negotiate Van Jacobson TCP/IP header compression 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     |     IP-Compression-Protocol   |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |  Max-Slot-Id  | Comp-Slot-Id  |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Type      2   Length      6McGregor                                                        [Page 9]RFC 1332                        PPP IPCP                        May 1992   IP-Compression-Protocol      002d (hex) for Van Jacobson Compressed TCP/IP headers.   Max-Slot-Id      The Max-Slot-Id field is one octet and indicates the maximum slot      identifier.  This is one less than the actual number of slots; the      slot identifier has values from zero to Max-Slot-Id.         Note: There may be implementations that have problems with only         one slot (Max-Slot-Id = 0).  See the discussion in reference         [3].  The example implementation in [3] will only work with 3         through 254 slots.   Comp-Slot-Id      The Comp-Slot-Id field is one octet and indicates whether the slot      identifier field may be compressed.         0  The slot identifier must not be compressed.  All compressed            TCP packets must set the C bit in every change mask, and            must include the slot identifier.         1  The slot identifer may be compressed.      The slot identifier must not be compressed if there is no ability      for the PPP link level to indicate an error in reception to the      decompression module.  Synchronization after errors depends on      receiving a packet with the slot identifier.  See the discussion      in reference [3].McGregor                                                       [Page 10]RFC 1332                        PPP IPCP                        May 1992A.  IPCP Recommended Options   The following Configurations Options are recommended:      IP-Compression-Protocol -- with at least 4 slots, usually 16      slots.      IP-Address -- only on dial-up lines.Security Considerations   Security issues are not discussed in this memo.References   [1]   Simpson, W., "The Point-to-Point Protocol", RFC 1331, May 1992.   [2]   Postel, J., "Internet Protocol", RFC 791, USC/Information         Sciences Institute, September 1981.   [3]   Jacobson, V., "Compressing TCP/IP Headers", RFC 1144, January         1990.   [4]   Postel, J., "The TCP Maximum Segment Size Option and Related         Topics", RFC 879, USC/Information Sciences Institute, November         1983.   [5]   Mogul, J., and S. Deering, "Path MTU Discovery", RFC 1191,         November 1990.   [6]   Reynolds, J., and J. Postel, "Assigned Numbers", RFC 1060,         USC/Information Sciences Institute, March 1990.   [7]   Perkins, D., and R. Hobby, "Point-to-Point Protocol (PPP)         initial configuration options", RFC 1172, August 1990.Acknowledgments   Some of the text in this document is taken from RFCs 1171 & 1172, by   Drew Perkins of Carnegie Mellon University, and by Russ Hobby of the   University of California at Davis.   Information leading to the expanded IP-Compression option provided by   Van Jacobson at SIGCOMM '90.McGregor                                                       [Page 11]RFC 1332                        PPP IPCP                        May 1992   Bill Simpson helped with the document formatting.Chair's Address   The working group can be contacted via the current chair:      Brian Lloyd      Lloyd & Associates      3420 Sudbury Road      Cameron Park, California 95682      Phone: (916) 676-1147      EMail: brian@ray.lloyd.comAuthor's Address   Questions about this memo can also be directed to:      Glenn McGregor      Merit Network, Inc.      1071 Beal Avenue      Ann Arbor, MI 48109-2103      Phone: (313) 763-1203      EMail: Glenn.McGregor@Merit.eduMcGregor                                                       [Page 12]