Internet  Protocols  Control Protocols ( IPCP)

 The  internet  protocols  control protocols  establishes  configures and terminates the TCP/ IP network  protocols  layer in a PPP  connection. The IPCP  packet    format  inside  a PPP frame  appears in  PPP frames with PCP packets  are subject  to PPP  frame  structure  modification negotiated during  the PPP connection  establishing stage.  IPCP fields  including ID, length  and IPCP data  are all similar to the corresponding  LCP.

                                                                  figure IPCP packet encapsulated in  PPP  frame

IPCP packets  include an 8 bit code to distinguish among  different  IPCP messages. Table  3 lists the legal values for he code  field and their  meanings. These codes are analogous to the ones used by LCP. But  do not be misled by the similarity. Although IPCP  appears similar to LCP it has nothing to do  with LCP responsibilities IPCP in only  for  establishing TCP/ IP network  layer  specific  option .

A PPP  endpoint  can send  the first  IPCP  configure request  packet  after a PPP connection reaches the network  state. If a peer respond with an LCP  protocols  reject, TCP/ IP transport isn't  available. Otherwise negotiations continue in a manner similar to LCP  and ICP may  or may not succeed in configuring TCP/IP. PPP endpoints can also continue  to negotiate  other networking  protocols  regardless of the status  of TCP/ IP.
IPCP packets may be  categorized  into three  types again like LCP. These  are configuration termination and errors. There is  one error code reject. An IPCP code reject  references an earlier IPCP configure request  message but otherwise  operates  identically  to the LCP  conditions. The followings  section describe the  remaining  IPCP types.

A PPP  endpoint  sends  an IPCP configure request packets with the  desired TCP/IP options  to its peer and a wait  a reply. If  the reply  is an IPCP  configure reject  or configure nak  this PPP  endpoint  must send  another configure request message  with  options  modifications. This IPCP  iterative procedure is identical to both PPP  endpoints  must  send an IPCP  configure request to  its peer since  TCP/ IP options  are independent for each  direction of  a PPP  connections.

IPCP packets carry their own distinct set  of TCP / IP  options  in the TPCP packet data  filed. The IPCP  configure  all carry these  options  encoding uses the  same  format  as LCP  packets IPCP  negotiates  all TCP/ IP  options simultaneously.  Currently the only  standard TCP/IP options  are for setting  IP  addresses and IP  compression.

When a IPCP configure request  sender  includes the IP  compression protocol option,  the sender  is requesting to receive TCP/IP  packets in compressed form. However the IP address options is the desired local address of the configure request sender. IP addresses a a 32 bits  number for a PPP connection endpoint.

IPXCP is used to enable configure and disable IPX protocols  modules at each end of the link . although multiple versions  of the NCP are available. IPCXP is the most  common  and is  overtaking the  other IPX- NCPs in popularity.