Applicability of Reliable Server Pooling for SCTP-Based Endpoint Mobility
Simula Research Laboratory, Network Systems Group
Martin Linges vei 17
1364 Fornebu
Akershus
Norway
+47-6782-8200
+47-6782-8201
dreibh@simula.no
http://www.iem.uni-due.de/~dreibh/
University of Duisburg-Essen, Institute for Experimental Mathematics
Ellernstraße 29
45326 Essen
Nordrhein-Westfalen
Germany
+49-201-1837637
+49-201-1837673
jp@iem.uni-due.de.de
Internet-Draft
This document describes a novel mobility concept based on a combination
of SCTP with Dynamic Address Reconfiguration extension and Reliable
Server Pooling (RSerPool).
An increasing amount of Internet devices is getting mobile. Therefore, there
is a growing demand for software solutions allowing for a seamless handover of
communication sessions between multiple networks, e.g. to allow for a laptop
or PDA to use a fast Ethernet connection when available, hand over to a WLAN when
moving and hand over again to UMTS when the WLAN becomes unreachable - without
interrupting the running communication sessions.
Mobility handling is a deficiency of the common IP-based networks. Most of the
available solutions are based on the network layer. The disadvantage of such
solutions is that fundamental changes in the network infrastructure are needed.
Therefore, we propose a new solution based on the upper layers to overcome these
disadvantages. In this document, we present our mobility solution based on the
SCTP protocol with Dynamic Address Reconfiguration extension and
Reliable Server Pooling (RSerPool).
In the concept of Mobile IP
every node must register to a Home-Agent (HA) in
its own home network. Then, the nodes are reachable under their home
addresses managed by the HA. When a node leaves its home network, it must
also register at a Foreign Agent (FA) in the new network. After that, a
tunnel is established between the HA and the FA. Any traffic to the mobile
node is then tunnelled by its HA to the FA and forwarded by the FA to the
node itself. Clearly, the detour of all traffic via HA and FA is
inefficient and results in an increased transmission delay.
Mobile IPv6 is an extension of Mobile IP.
In Mobile IPv6, the FA is not needed. The packets will be
tunnelled from the HA to the Gateway Router in the foreign network, which
forwards the packets to the endpoint. The inefficiency due to the detour of
traffic as described for Mobile IP remains.
Using the SCTP protocol (see
together with its Dynamic Address Reconfiguration extension
(Add-IP, see ),
it is possible for a mobile endpoint to inform its
peer on address changes. That is, when a moving mobile client gets in the
vicinity of an additional radio station, it sends an "ASCONF Add Address
Request" to tell its peer that it is now reachable under an additional
network-layer address. After that, the peer endpoint can use this additional
address for a new SCTP path.
When the first radio station becomes unreachable, the node can send an "ASCONF
Delete Address Request" to the peer endpoint. After that, the peer removes the
corresponding SCTP path to the unusable network-layer address.
The following two cases for handovers are possible:
Make-before-Break: An additional SCTP path can be used before the original
path becomes unusable. This case is trivial, since there is a continuous
connectivity.
Break-before-Make: The original SCTP path becomes unusable before a new
SCTP path can be used. For the case that only one endpoint performs a
handover procedure at the same time, the mobile endpoint can always use
Add-IP to communicate its new address to its peer endpoint. However, when
both endpoints perform a handover simultaneously, no endpoint is able to
tell its corresponding peer the new address.
Using SCTP with Add-IP and Mobile IP/Mobile IPv6, the ASCONF messages
will be sent to the home address of the peer node. That is, even when
both nodes are mobile, each endpoint is able to reach its peer endpoint
using the corresponding home address. However, this solution still
requires the full Mobile IP/Mobile IPv6 infrastructure.
Using RSerPool (see
,
, , ,
, , ,
at least one node registers as a Pool Element (PE) at an ENRP server
under a Pool Handle (PH) known to both endpoints.
Upon handover, it is simply necessary for the PE endpoint to re-register,
i.e. to update its registration with its new address. The other endpoint
can - in the role of a Pool User (PU) - ask an ENRP server for its peer
node's new addresses. After the new address is known, it is able to create
a new SCTP path and continue the communication.
The usage of RSerPool to provide support for mobile endpoints provides
the following advantages:
Simplicity: No Mobile IP/Mobile IPv6 infrastructure is needed.
In particular, it is not necessary that the providers of used
networks (e.g. public WLAN access points, UMTS providers, etc.)
provide any support for the mobility solution.
Efficiency: No tunnelling of traffic is necessary.
Applicability: All major SCTP implementations already support
the Dynamic Address Reconfiguration extension. It is only
necessary to provide support for RSerPool, e.g. in the form of a
userspace library, which is much easier to deploy than
kernel extensions.
Flexibility: RSerPool provides a complete session layer. That is,
providing applications on top of RSerPool makes the support for
high availability simple.
A more detailed description of our approach for endpoint mobility, as well
as a performance analysis using a prototype implementation,
can be found in our paper .
The RSerPool reference implementation RSPLIB can be found at
. It supports the functionalities
defined by
,
,
,
and
as well as the options
,
and
.
An introduction to this implementation is provided in
.
A large-scale and realistic Internet testbed platform with support for the multi-homing feature of the underlying SCTP protocol is NorNet. A description of NorNet is provided in , some further information can be found on the project website .
Security considerations for RSerPool systems are described by
.
This document introduces no additional considerations for IANA.