Internet DRAFT - draft-goswami-mobileip-analysis-v4
draft-goswami-mobileip-analysis-v4
INTERNET-DRAFT Subrata Goswami
Expires February 12, 2003 Independent Consultant
Sept 13, 2002
A Simple Analysis of Mobile IP v4
<draft-goswami-mobileip-analysis-v4-00.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC 2119].
Abstract
This document analyzes Mobile IP v4 from a deployment point of view. With
the popularity of wireless LAN technologies subnet roaming has become a
prominent issue. Some critical scalabilty issues for Home Agent
and Foreign Agents are pointed out.
1. Overview and Rationale
In the 802.11 wireless lan [WiFi] network an 802.11 client connects to an
802.11 Access Point (AP) at the link level.Mobile-IPv4 (MIP4)[MIPv4] enables
a Mobile Node (MN), equiped with 802.11 access card, to roam from subnet to
subnet. At present MIP4 is the most mature and widely implemented standard
for network level mobility. Hence it makes sense to see what are the issues
that may limit its usefulness from a practical perspective.
2. Number of Tunnels
A hypothetical Mobile IP network is shown in the following figure. 9
subnets are shown in the figure. Each subnet has one Home Agent (HA)
and one Foreign Agent (FA). A Mobile Nodes (MN) would have
their home network in one of these subnetworks. The FA and HA could be
seperate entities or may be the same entity (i.e. have the same IP
address).
-------------------------------------
| [FA0]| [FA1]| [FA2]|
| [HA0]| [HA1]| [HA2]|
| | | |
|[MN11] | | |
|[MN21] | | |
-------------------------------------
| [FA3]| [FA4]| [FA5]|
| [HA3]| [HA4]| [HA5]|
| | | |
| | | |
| | | |
-------------------------------------
| [FA6]| [FA7]| [FA8]|
| [HA6]| [HA7]| [HA8]|
| | | |
| | | |
| | | |
-------------------------------------
[MNij] - Mobile Node j in Home Network i
[FAi] - Foreign Agent in network i
[HAi] - Home Agent in network i
Figure 1: Mobile IPv4 deployment over an address space of 9 sub-nets.
A tunnel needs to be created when a single MN roams into a foreign
subnet. If there are Sn subnets, then there can be Sn*(Sn-1) or
Sn*(Sn-1)*0.5 tunnels depending on when FA and HA are distinct or
the same entities respectively. Thus tunnel configuration and management
is an n^2 problem.
For the example in Figure 1, there would 9*8=72 or 36 tunnels. A large
organization that has an Class-A (or /8) address space can have 2^16
subnets of 254 nodes. That can potentially imply 2^16*(2^16-1)*0.5 = about
2 billion tunnels. Although, at any point of time only 2^16*254 = about
16 million of the tunnels would be active. For an organization with
Class-B (or /16) address space, those numbers would be 2^8*(2^8-1) and
2^8*254 or about 64,000 and 64,000 respectively.
3. Home Agent Throughput
The second scalability issue is the throughput of the HA. If the larget
subnet has Nn nodes, then the HA of that subnet may potentially have to
support Nn tunnels. Now if each node has bandwidht requirement of b bits
per second, then the HA needs to have a throughput of bNn. Thus there is
a linear relationship between an HA and the number of nodes in the subnet.
For example, in a 1022 node subnet with 1.0 Mbps per node demabd, the HA
needs to have a throughput of 1Gbps.
4. Foreign Agent Throughput
The third scalability issue is the throughput of the FA. If the average
number of nodes is An nodes per subnet, then there is the pathological case
of all hosts of the Sn subnets roaming into one subnet. In such a situation
the FA would have to have a throughput of bAnSn. To get an idea of what this
means, let us consider a Class B space with 254 node subnets. If each MN
demands 1Mpbs, then the FA would need to pass 254*255*1= about 64 Gbps.
Although, in most real situations only a fraction of the nodes would roam
into one subnet. In the pathologocal case mentioned above, the number of
tunnels that needs to be terminated by the FA is approximately the number of
nodes available in the adderss space.
If the pathological case needs to be supported,then it might be worhwhile to
have multiple FA's per subnet so that the load can be sahred. This in the
extreme case would result in co-located FA , which has severe implications in
the number of IP addresses used.
5. Mobile Node Issues
The primary impact on an MN in the pathological case mentioned above would be
inability to register (hence no service ) with an FA, if the Visitor Table
in FA is limited. Even if the Visitor Table supports 64,000 entries, the FA
would still need to add, delete, and search a large table.
6. Acknowledgments
All the RFC's, IDĘs, freely available 802.11 standards, and Linux web-sites.
7. References
[MIPv4] Perkins, C., "IP Mobility Support", RFC 2002, October 1996.
8. Author's Address
Subrata Goswami, Ph.D.
Independent Consultant
Newark, CA 94560
sgoswami@umich.edu
This document expires February 12, 2003.
