.\" $OpenBSD: ipcomp.4,v 1.17 2022/12/23 07:16:55 jmc Exp $
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.\" Copyright (c) 2001 Jean-Jacques Bernard-Gundol <jj@wabbitt.org>
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.Dd $Mdocdate: December 23 2022 $
.Dt IPCOMP 4
.Os
.Sh NAME
.Nm ipcomp
.Nd IP Payload Compression Protocol
.Sh DESCRIPTION
IPComp is enabled with the following
.Xr sysctl 2
variable in
.Pa /etc/sysctl.conf :
.Bl -tag -width xxxxxxxxxxxxxxxxxxxxx -offset indent
.It net.inet.ipcomp.enable
.El
.Pp
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 computational
power.
.Pp
This protocol is especially useful when encryption or authentication
is applied to IP datagrams using the 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 (e.g., the
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 (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 ipsecctl 8
tool.
Using
.Xr ipsecctl 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:
.Pp
.Dl $ netstat -s -p ipcomp
.Sh SEE ALSO
.Xr enc 4 ,
.Xr inet 4 ,
.Xr ip 4 ,
.Xr ipsec 4 ,
.Xr netintro 4 ,
.Xr ipsecctl 8 ,
.Xr sysctl 8
.Sh HISTORY
The
.Nm
protocol first appeared in
.Ox 3.0 .
.Sh AUTHORS
Support for the
.Nm
protocol was written by
.An Jean-Jacques Bernard-Gundol Aq Mt jj@wabbitt.org .