Mumbai University > Computer Engineering > Sem6 > Mobile Communication and Computing

Marks: 5M

Year: Dec 2015

• Since mobility is an important feature of Mobile IP, it is very important to track where the user’s cell has moved.

• Tracking here means to locate the MN’s foreign agent (FA). One method to find it is by the method called as Mobile Advertisement.

Agent Advertisement

• Over here the HA and FA periodically advertise their presence by using special advertisements messages.

• Usually, the FA periodically broadcasts the IRDP message in its own network to let the visited MN know the FA is here and what services the FA provides (Agent Advertisement). Thus, the MN knows which network it belongs to.

• In case the MN does not receive this message, it can request the service by sending a solicitation message to inform the FA directly (Agent Solicitation).

• These advertisements can be seen as beacon broadcast to the subnets.

• Here we use ICMP messages according to RFC 1256. The agent advertisement packet is show alongside.

• In the figure the upper half is the ICMP packet while the lower half is the extension needed for mobility

• The IP destination address (not in fig.) can be set to 224.0.0.1 (which is the standard address for multicast) or 255.255.255.255 (broadcast address)

• Some of the fields used are as follows:

  • Type → set to 9
  • Code → 0 if agent routes traffic from non-mobile nodes as well or else 16
  • Check sum → The 16-bit one's complement of the one's complement sum of the ICMP /IRDP message
  • #addresses → The number of router addresses advertised in this message
  • Addr. Size → The number of 32-bit words of information per each router address
  • Lifetime → The maximum number of seconds that the router addresses may be considered valid.
  • Router Address [i=1,2,3..] → The sending router's IP address on the ith interface from which this message is sent.
  • Prefernce level [i=1,2,…] →The preferability of each Router Address[i]

• Till now what was done is standard ICMP procedure. After this we move to the extra extension created i.e. mobility extension. The fields in it are:

  • Type → 16 (Mobility advertisement extension)
  • Length → 6+ 4*(number of addresses)
  • Sequence number → The count of Agent Advertisement messages sent since the agent was initialized.
  • Registration lifetime → maximum lifetime in seconds a node can request during registration.