Subject: [Ipsec] Rekeying of child SA in IKEv2

From: Jyothi <[email-masked]>
Date: Wed, 07 Jul 2004 10:00:10 +0530

Hi all,

In draft-ietf-ipsec-ikev2-14.txt, section 2.8 Rekeying, it contains the 
following information:

To allow for minimal IPsec implementations, the ability to rekey SAs
    without restarting the entire IKE_SA is optional. An implementation
    MAY refuse all CREATE_CHILD_SA requests within an IKE_SA. If an SA
    has expired or is about to expire and rekeying attempts using the
    mechanisms described here fail, an implementation MUST close the
    IKE_SA and any associated CHILD_SAs and then MAY start new ones.
    Implementations SHOULD support in place rekeying of SAs, since doing
    so offers better performance and is likely to reduce the number of
    packets lost during the transition.

May I know the reason behind this. Why the CREATE_CHAILD_SA exchange made 
as optional.

There may be some issues regarding this.

Suppose, I configured PFS in IPSEC as DH MODP2048, and DH group in IKE as 
MODP1536.

In IKE_SA_INIT exchange, only one KE payload is negotiated for shared 
secret used in IKE (MODP1536) to generate the key material.

May I know in detail how can I use PFS configured in IPSEC by not using 
CREATE_CHILD_SA exchange??

My understanding is that if we configure the PFS in IPSEC, we create IKE 
SAs using IKE_SA_INIT exchange and we create CHILD SAs using 
CREATE_CHILD_SA exchange.

I think, what I understood might be wrong. Please clarify.

Many thanks in advance,
Jyothi

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From: Tero Kivinen <[email-masked]>
Date: Thu, 8 Jul 2004 20:28:42 +0300

> May I know the reason behind this. Why the CREATE_CHAILD_SA exchange made 

To allow very tiny implementations. 

> Suppose, I configured PFS in IPSEC as DH MODP2048, and DH group in IKE as 

You cannot use such setup for such clients. 

> secret used in IKE (MODP1536) to generate the key material.

Note, that SAi2, TSi and TSr are not optional in the IKE_AUTH
exchange, so for that kind of setup you are creating one extra pair of
IPsec SAs in all cases. If the other end implementation only supports
exactly one SA (very small garage opener device), it will not allow
you to create second SA. 

> CREATE_CHILD_SA exchange??

You cannot. Why would you want to use that kind of setup. If you want
to have security from the 2048-bit group use it for the IKE SA also. 

> CREATE_CHILD_SA exchange.

No, you always create one IPsec SA along with the IKE_SA_INIT /
IKE_AUTH exchanges, thus for that kind of setups you create 2 sets of
IPsec SAs. 
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