# where does the prime number taken in DH algorithm in IPSEC

I am studying & configuring IPSEC ikev1 and in between i am analysing the wireshark captures. I am using the linux kernel for TCP/IP stack and user-space i took ipsec-tools.

In the first two packet exchanges initiator is offering the proposals & responder will respond to the supported proposal.

Next two packet exchanges(packet 3[initiator] & 4[responder]) will perform DH key exchange.Here how the responder will came to know that which prime number the initiator uses & how it will be exchanged.

• Yeah i seen it in wireshark capture, In Transform IKE Attribute Type: Group Description 1024-bit MODP. There what it specifies? In this expression A=g(pow)a mod p. The g & p are generator and prime number respectively, how they choosen both same at initiator & responder. Mar 3 '17 at 10:22
• @Kumar2080: if an IKE implementation supports group 2, it is expected to know that, for that group, $g=2$, and $p$ is the specific value listed in section 6.2 of RFC2409. Mar 10 '17 at 4:14