.\" $OpenBSD: ipcomp.4,v 1.5 2001/08/02 18:37:33 mpech Exp $ .\" .\" Copyright (c) 2001 Jean-Jacques Bernard-Gundol .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. All advertising materials mentioning features or use of this software .\" must display the following acknowledgement: .\" This product includes software developed by Jean-Jacques Bernard-Gundol .\" 4. The name of the author may not be used to endorse or promote products .\" derived from this software without specific prior written permission .\" .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. .\" .Dd June 27, 2001 .Dt IPCOMP 4 .Os .Sh NAME .Nm IPComp .Nd IP Payload Compression Protocol .Sh NOTE .Tn IPComp is enabled with the following .Xr sysctl 3 variable in .Pa /etc/sysctl.conf : .Bl -tag -width xxxxxxxxxxxxxxxxxxxxx .It net.inet.ipcomp.enable .El .Pp .Sh DESCRIPTION .Tn IPComp is a protocol used to reduce the size of IP datagrams. It can be used to enhance the communication performance between a pair of hosts/gateways, especially on slow links, by compressing the datagrams, provided the communicating entities have enough computation power. .Pp This protocol is especially useful when encryption or authentication is applied to IP datagrams using the .Tn IPsec protocol (see .Xr ipsec 4 for more information about IPsec). Encrypting information is increasing its entropy to a point where compression to a lower layer becomes completely useless (i.e PPP Compression Control Protocol). IPcomp is applied at the network layer before other encryption operations are applied (except encryption protocols applied at a higher layer such as .Xr ssh 1 or .Xr ssl 8 ). .Pp Just like for the other IPsec protocols, IPComp needs some parameters for each connection, specifying how the compression should be done between the entities. The parameters are collected in a structure called an IPComp Association or IPCA. The parameters stored in an IPCA are the destination address and the Compression Parameter Index (CPI). An IPCA is the pendant of the SA (Security Association) for IPsec. .Pp Currently, IPCA can be created using the .Xr ipsecadm 8 tool. Using .Xr ipsecadm 8 it is also possible to create IPComp flows and SA/IPCA bundles. Such a bundle is used to create a combination of IPsec and IPComp flows (thus enabling compression in an IPsec protocol). .Pp The compression is done on the data following the IP header and an IPComp header is inserted between the compressed data and the IP header. In the case of IPv6, there are extension headers which cannot be compressed since they are modified by the router along the way to the destination. These extension headers are hop-by-hop, routing, and fragmentation. .Pp When doing compression, it is possible that the uncompressed data is smaller in size than the compressed data. To avoid this behaviour, a non expansion policy is used in IPComp. If the data payload is smaller than a given threshold, it will not be compressed. No IPComp header will be inserted. .Pp IPComp uses the same policy framework as IPsec. However unlike IPsec, only one policy is available for IPComp: .Bl -tag -width IPSEC_LEVEL_USE .It IPSEC_LEVEL_USE Use IPComp for sending packets but still accept packets which are not compressed. .El .Sh DIAGNOSTICS .Xr netstat 1 can be used to obtain some statistics about IPComp usage, using the .Fl p flag. Just like for IPsec, using the .Fl r flag, .Xr netstat 1 displays information about IPComp flows. .Sh SEE ALSO .Xr enc 4 , .Xr inet 4 , .Xr ip 4 , .Xr ipsec 4 , .Xr netintro 4 , .Xr ipsecadm 8 .Sh HISTORY The .Nm protocol first appeared in .Ox 3.0 . .Sh AUTHORS Support for the .Nm protocol was written by Jean-Jacques Bernard-Gundol .