[Openswan dev] Proposal for dealing with ICMP black holes for IPsec

Ken Bantoft ken at xelerance.com
Sun Jul 4 15:37:36 CEST 2004

On Sat, 3 Jul 2004, Herbert Xu wrote:

> Hi:
> This is a proposal of a method of detecting the path MTU that can be
> applied by IPsec keying managers (e.g., openswan/racoon).
> My idea is to use the information available in TCP MSS clamping to
> help IPsec gateways.  This can be done by having the gateways
> establish a short-lived TCP connection with each other for the
> purposes of determining the MTU.  Obviously this TCP connection will
> need to be authenticated.  If the connection cannot be established
> (e.g., if an attacker is preventing the gateways from doing so), then
> we can fall back to the existing MTU discovery mechanisms.
> If however this connection succeeds, then we can deduce the path MTU
> from its MSS settings.  We can then apply that MTU to the path taken
> by the IPsec SAs.  This makes any ICMP black holes between the gateways
> invisible to the end users as long as the information provided by
> TCP MSS clamping is accurate.

Interesting idea - tcp/500 connection (or maybe it should be port 4500, to 
deal with NAT boxes.  I assume you want to do this as early as possible, 
eg: Phase 1, however until we have a PH2 we can't do a secure TCP, only 

> The advantage of this over the approach taken by KLIPS (the FreeSWAN
> kernel implementation for IPsec) is that we do not transmit fragments
> where possible which is the motivation behind PMTU discovery.  The
> advantage of this over the current implementation is that this will
> work correctly in environments where there are local ICMP black holes 
> that are known through TCP MSS clamping.
> Comments?

I like it it in concept - I'd be interested to see how much of a hack is 
needed to implement it in reality.  Should definatly use a VendorID to 
state whether or not each side supports it, so we don't open TCP 
connections and let them timeout/die/be hijacked.

Ken Bantoft			VP Business Development
ken at xelerance.com		Xelerance Corporation
sip://toronto.xelerance.com	http://www.xelerance.com

More information about the Dev mailing list