In a multi-homed network, a mobile host may have one or more network inter-faces. The network interfaces may support the mobile host one or more public IP address or private IP address. If the IP address is a private one, an NAT box is resides in the network. Here we summarize five different situations of the network architecture:
Case 1 The mobile host has only one network interface and the edge router has n links. The host A connects to a single network realm X; the edge router of realm X has one or more path to link to Internet. The IP address assigned to host A is a public IP address.
Case 2 The mobile host A connects to a single network realm X; realm X has more
RSIP Client A
Figure 2.5: Redundant link in RSIP
than one edge router with several interface to link to Internet. The IP address assigned to host A is a private IP address. NAT function is performed in the edge router.
Case 3 The mobile host has only one network interface and the edge router has n links. The host A connects to a single network realm X; the edge router of realm X has one or more path to link to Internet, whereby one NAT box is resides in one link. The IP address assigned to host A is a private IP address.
Case 4 The mobile host A connects to several network realms; the IP addresses
assigned to host A are all public IP addresses. Each realm may contain several outbound links.
Case 5 The mobile host A connects to several network realms, the IP addresses assigned to host A may be public IP address or private IP address.
“Inside” Network
Figure 2.6: Multi-homed Network with NAT, case 1
To overcome the problems described above, the Internet Engineering Task Force (IETF) proposed two protocols to support terminal mobility among IP sub-nets, the Mobile IP protocol and the Stream Control Transmission Protocol (SCTP) [61]. However, both of them still contain several problems. In the mobile IP net-work, a mobile host sends a binding update message to perform a roaming op-eration when a mobile host migrates from one interface to another. If the home
“Inside” Network 1
Figure 2.7: Multi-homed Network with NAT, case 2
agent (HA) is unreachable at this time, the foreign agent (FA) cannot process this location update request. Packets send to mobile node (MN) cannot be forwarded to the newest location and the connection will be terminated.
Another two solutions are based on DNS: Round robin DNS and Dynamic DNS. Round robin DNS is usually used for balancing the load of geographically distributed Web servers, but can be used in a multi-homing environment. Dy-namic Domain Name System (DDNS) is a method of keeping a domain name linked to a changing IP address as not all computers use static IP addresses. An mobile host with DDNS supports will update it’s current IP addresses with the DNS server, which means other users just need to use DNS query to find out the current location of the mobile node.
“Inside” Network
Figure 2.8: Multi-homed Network with NAT, case 3
2.2.1 Mobile IP and mobile IPv6
In Mobile IP network [45], a mobile node (MN) gets a Home Address from its home agent (HA). When a mobile node handoffs to a foreign network, it gets a Care-of Address (CoA) from foreign agent (FA) and informs home agent (HA) its care-of address by sending a registration request message to the home agent.
The home agent maintains the binding between the care-of address and the home address of each mobile node. When a valid binding for a mobile node exists, the home agent will capture all the packets sent from correspondent nodes (CNs) to the mobile node’s home address and forward them by tunneling to the care-of ad-dress. In MIPv6 [25], the mobile node uses can inform correspondent nodes about its current location by using a binding-update message; the correspondent nodes will be able to send packets directly to mobile node’s care-of address, instead of sending packets through mobile node’s home address.
2.2.2 SCTP protocol
The Stream Control Transmission Protocol (SCTP) [61] is an IP-based end-to-end, connection oriented transport protocol developed by the Internet Engineering Task Force (IETF) Signaling Transport working group for the transport of signal-ing data. However, SCTP is a general purpose transport protocol which provides numerous advantages over user datagram protocol (UDP) and transmission con-trol protocol (TCP). For instance, SCTP combines the datagram orientation of UDP with the sequencing and reliability of TCP. Additionally, SCTP uses multi-stream, message-oriented routing in multi-homed environments. SCTP provides applications with enhanced performance, reliability, and control functions.
SCTP protocol overview
In SCTP, data is transmitted between endpoints through a connection referred to as an association. An association begins with an initiation of a four-way hand-shake between two endpoints and is maintained until all data has been success-fully transmitted and received. Within SCTP, user data and control messages are assembled into chunks. An SCTP packet contains a common header and zero or more chunks.
SCTP message streams
The term ”stream” is used in SCTP to refer to a sequence of user messages that are to be delivered to the upper-layer protocol in order with respect to other messages within the same stream [61]. SCTP multi-streaming logically divides user data into unidirectional streams with each stream having its own delivery mechanism.
All streams within a single association share the same congestion and flow con-trol parameters. Through multi-streaming, SCTP eliminates unnecessary blocking that often occurs in TCP transmission.
In TCP, user data is delivery in a single sequence of bytes which is strictly ordered. This delivery mode results in a major drawback known as ”head-of-the-line blocking (HOL),” where messages are not allowed to bypass each other.
Multi-streaming decouples data delivery and transmission, and in doing so pre-vents Head-of-Line blocking.
SCTP streams are effectively unidirectional channels, within which messages are usually transported in sequence, unless the user requests a message to be de-livered by an unordered service. The stream mechanism may reduce the effects of head-of-line blocking, especially in the case of a large number of small messages and a large number of stream. SCTP also provides a mechanism for unordered delivery service as UDP. User messages sent using this mechanism are delivered to the SCTP user as soon as they are received without any processing.
SCTP Multi-Homing
The SCTP supports multi-homed endpoints with more than one IP address. SCTP has a built in failure detection and recovery scheme, known as failover, which al-lows associations to dynamically send traffic to an alternative destination address when needed without losing the end-to-end association or requiring the applica-tion to intervene. This failover occurs after a threshold number of transmission timeouts to the primary destination address have occurred. SCTP also exploits this path redundancy in its retransmission policy.
2.2.3 IP round-robin and dynamic DNS
IP Round robin works on a rotating basis in that one server IP address is handed out, then moves to the back of the list; the next server IP address is handed out, and then it moves to the end of the list; and so on. Round robin DNS is usually used for balancing the load of geographically distributed Web servers. For example, a company has one domain name and three identical home pages residing on three servers with three different IP addresses. When one user accesses the home page it will be sent to the first IP address. The second user who accesses the home page will be sent to the next IP address, and the third user will be sent to the third IP address. In each case, once the IP address is given out, it goes to the end of the list. The fourth user, therefore, will be sent to the first IP address, and so forth. Although very easy to implement, round robin DNS has important drawbacks, such as those inherited from the DNS hierarchy itself and TTL times, which causes undesired address caching to be very difficult to manage. Moreover, its simplicity makes remote servers that go unpredictably down inconsistent in the DNS tables. However, this technique, together with other load balancing and clustering methods, can produce good solutions for some situations.
Dynamic Domain Name System (DDNS) [66] is a method of keeping a do-main name linked to a changing IP address as not all computers use static IP addresses. Typically, when a user connects to the Internet, the user’s ISP assigns an unused IP address from a pool of IP addresses, and this address is used only for the duration of that specific connection. This method of dynamically assign-ing addresses extends the usable pool of available IP addresses. A dynamic DNS service provider uses a special program that runs on the user’s computer,
contact-ing the DNS service each time the IP address provided by the ISP changes and subsequently updating the DNS database to reflect the change in IP address. In this way, even though a domain name’s IP address will change often, other users do not have to know the changed IP address in order to connect with the other computer.