IP Payload Compression (IPComp) is a protocol that allows compressing the content of IP packets. It's defined in RFC 3173.

It may be combined with IPsec to provide compression before encryption (making up for some of the overhead introduced by IPsec itself). To use it both peers have to support it and it must be negotiated during IKE. strongSwan currently only supports DEFLATE as compression algorithm.

To enable IPComp in strongSwan set compress=yes in ipsec.conf or connections.<conn>.children.<child>.ipcomp=yes in swanctl.conf.


When debugging connections that use IPComp on Linux there are some things to be aware of.

For packets smaller than a certain threshold the kernel does not do any compression (e.g. default-sized pings for IPv4). To handle uncompressed packets in tunnel mode the kernel will implicitly create an IPIP tunnel besides the actual IPComp SAs. These IPIP states can be seen in ip xfrm state with proto 4. For IPv6 the kernel also creates such states - proto 41 - but since the compression threshold is relatively low and IPv6 packets are naturally larger even default-sized pings are compressed. These IPIP tunnels also have an impact on the firewall rules that are installed with leftfirewall=yes (some issues were fixed with 5.1.2 and for IPv6 with 5.4.0) or manually, as IPIP traffic has to be allowed explicitly in the INPUT chain.

Due to how the six states (IPsec, IPComp, IPIP, in- and outbound each) are used, the traffic counters (bytes, packets) seen in ip -s xfrm state might look a bit confusing at first:

  • For outbound traffic the IPIP state is not used, instead all traffic directly goes through the outbound IPComp and IPsec SAs. The kernel will simply not perform any compression for small packets (the byte and packet counters of the IPComp SA are still increased, though). The thresholds are statically defined in net/xfrm/xfrm_algo.c for each compression algorithm. For deflate it is 90 bytes.
  • For the inbound traffic, on the other hand, the traffic counters for the IPComp SAs are accurate and only count actually decompressed traffic. The other packets pass through the implicitly created IPIP tunnel, increasing its traffic counters.

This can be seen in the ikev2/compress test case (or for IPv6 in ipv6/rw-compress-ikev2). Host carol sends two pings to alice via gateway moon, but only one ping actually gets compressed.

So on carol the packet and byte counts for all six states are as follows:

State Bytes Packets Notes
ESP out 173 2 The larger ping request got highly compressed as it basically contains the same data repeated over and over
IPComp out 8296 2 Only one packet of these actually gets compressed
IPIP out 0 0 Not used
ESP in 172 2
IPComp in 8212 1 For the compressed ping reply
IPIP in 84 1 For the uncompressed ping reply

The IPComp/IPsec packet processing in the kernel may be visualized as follows:

                        len < threshold
            +---------+    no header     +---------+
outbound--->|IPComp SA|----------------->| ESP SA  |--->
            +----+----+                  +----+----+
                 |        /----------\        |
len >= threshold +------->| compress |--------+ IPComp header
                          \----------/              added

                          +---------+    +---------+
        <--------+--------|  IPIP   |<---| ESP SA  |<---inbound
                 |        +---------+    +----+----+
                 |        +---------+         |
                 +--------|IPComp SA|<--------+ IPComp header
                          +---------+               found