rfc1332.txt
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RFC 1332 PPP IPCP May 1992
3.2. 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.
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RFC 1332 PPP IPCP May 1992
3.3. 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 1992
4. Van Jacobson TCP/IP header compression
Van Jacobson TCP/IP header compression reduces the size of the TCP/IP
headers to as few as three bytes. This can be a significant improvement
on slow serial lines, particularly for interactive traffic.
The IP-Compression-Protocol Configuration Option is used to indicate the
ability to receive compressed packets. Each end of the link must
separately request this option if bi-directional compression is desired.
The PPP Protocol field is set to the following values when transmitting
IP 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
6
McGregor [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].
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RFC 1332 PPP IPCP May 1992
A. 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.com
Author'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.edu
McGregor [Page 12]
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