Unit IV Mobile IP and Transport Layer
Mobile IP
Mobile IP?
Mobile IP or MIP is an Internet Engineering Task Force (IETF) RFC 2002, De-Facto standard communication protocol. It is created by extending Internet Protocol, IP.
The Mobile IP allows mobile device users to move from one network to another while maintaining the same permanent IP address.
The concept and role of Mobile IP are very important in the field of mobile computing technology.
The mobile IP makes the communication flawless and ensures that the communication will occur without the user's sessions or connections being dropped.
Mobile IP is based on IP, so it is scalable for the Internet. Any media that supports IP can also support Mobile IP.
Introduction to Mobile IP Technology
In IP networks, when a device is within its home network, the routing is based on the static IP addresses. The device within a network is connected through normal IP routing by the IP address assigned on the network. It is the same as how a postal letter is delivered to the fixed address on the envelope. The problem occurs when a device goes away from its home network and is no longer reachable using normal IP routing. In this condition, the active sessions of the device are terminated. The idea of Mobile IP was introduced to resolve this issue. It facilitates users to keep the same IP address while going to a different network or a different wireless operator without being communication disrupted or without sessions or connections being dropped.
The mobility function of the Mobile IP is performed on the network layer rather than the physical layer.
The architecture of Mobile IP Technology
The components of the Mobile IP and the relationship among them are specified in the following image:
This is the architecture of Mobile IP technology. It consists of the following components:
Mobile Node (MN)
Home Agent (HA)
Foreign Agent (FA)
Home Network (HN)
Foreign Network (FN)
Corresponding Node (CN)
Care of Address (COA)
Mobile Node
The Mobile Node is a device or a user or a router that can frequently change their network positions without changing its original IP address. Examples of mobile nodes are cell phone, personal digital assistant (PDA), laptop, etc. whose software enables network roaming capabilities.
Home Agent
The Home Agent is a router on the home network. It serves as the anchor point for communication with the Mobile Node.
Foreign Agent
The Foreign Agent is a router that provides several services such as tunneling data-grams whenever a mobile node visits a foreign network. It is responsible for delivering packets from the Home Agent to the Mobile Node.
Home Network
The home network is the base station network to which the mobile node originally belongs to.
Foreign Network
Any network other than the home network or the networks on which mobile nodes have a registered IP is called a foreign network.
Corresponding Node
The partner nodes which are used for communication with mobile nodes are called corresponding nodes.
Care of Address
The Care of Address or COA is used to define the mobile node's current position or user. It is used to deliver data packets through the process of tunneling.
Working of Mobile IP
The working of Mobile IP can be described in 3 phases:
Agent Discovery
In the Agent Discovery phase, the mobile nodes discover their Foreign and Home Agents. The Home Agent and Foreign Agent advertise their services on the network using the ICMP Router Discovery Protocol (IRDP).
Registration
The registration phase is responsible for informing the current location of the home agent and foreign agent for the correct forwarding of packets.
Tunneling
This phase is used to establish a virtual connection as a pipe for moving the data packets between a tunnel entry and a tunnel endpoint.
Applications of Mobile IP
The mobile IP technology is used in many applications where the sudden changes in network connectivity and IP address can cause problems. It was designed to support seamless and continuous Internet connectivity.
It is used in many wired and wireless environments where users have to carry their mobile devices across multiple LAN subnets.
Although Mobile IP is not required within cellular systems such as 3G, it is often used in 3G systems to provide seamless IP mobility between different packet data serving node (PDSN) domains
IP packet delivery :
Mobile node (MN):
1. A mobile node is an end-system or router that can change its point of attachment
to the internet using mobile IP.
2. The MN keeps its IP address and can continuously communicate with any other
system in the internet as long as link-layer connectivity is given.
3. Mobile nodes are not necessarily small devices such as laptops with antennas or
mobile phones; a router onboard an aircraft can be a powerful mobile node.
Correspondent node (CN):
At least one partner is needed for communication. In the following the CN
represents this partner for the MN. The CN can be a fixed or mobile node.
Mobile node (MN):
1. A mobile node is an end-system or router that can change its point of attachment
to the internet using mobile IP.
2. The MN keeps its IP address and can continuously communicate with any other
system in the internet as long as link-layer connectivity is given.
3. Mobile nodes are not necessarily small devices such as laptops with antennas or
mobile phones; a router onboard an aircraft can be a powerful mobile node.
Correspondent node (CN):
At least one partner is needed for communication. In the following the CN
represents this partner for the MN. The CN can be a fixed or mobile node.
1. A correspondent node CN wants to send an IP packet to the MN.
2. One of the requirements of mobile IP was to support hiding the mobility of the MN.
3. CN does not need to know anything about the MN9s current location and sends the packet as usual to the IP address of MN (step 1). This means that CN sends an IP packet with MN as a destination address and CN as a source address. The internet, not having information on the current location of MN, routes the packet to the router responsible for the home network of MN. This is done using the standard routing mechanisms of the internet
4. The HA now intercepts the packet, knowing that MN is currently not in its home network.
5. The packet is not forwarded into the subnet as usual, but encapsulated and tunnelled to the COA.
6. A new header is put in front of the old IP header showing the COA as new destination and HA as source of the encapsulated packet (step 2).
7. The foreign agent now decapsulates the packet, i.e., removes the additional header, and forwards the original packet with CN as source and MN as destination to the MN (step 3).
8. Again, for the MN mobility is not visible. It receives the packet with the same sender and receiver address as it would have done in the home network.
9. The MN sends the packet as usual with its own fixed IP address as source and CN9s address as destination (step 4).
10. The router with the FA acts as default router and forwards the packet in the same way as it would do for any other node in the foreign network. As long as CN is a fixed node the remainder is in the fixed internet as usual. If CN were also a mobile node residing in a foreign network, the same mechanisms as described in steps 1 through 3 would apply now in the other direction.
Agent Discovery :
One initial problem of an MN after moving is how to find a foreign agent. How does the MN discover that it has moved? For this purpose, mobile IP describes two methods: agent advertisement and agent solicitation, which are in fact router discovery methods plus extensions.
Here foreign agents and home agents advertise their presence periodically using special agent advertisement messages. ➢ These advertisement messages can be seen as a beacon broadcast into the subnet. Routers in the fixed network implementing this mechanisms also advertise their routing service periodically to the attached links. The agent advertisement packet according to RFC 1256 with the extension for mobility is shown in Figure.
The upper part represents the ICMP packet while the lower part is the extension needed for mobility.
1. The TTL field of the IP packet is set to 1 for all advertisements to avoid forwarding them.
2. The IP destination address according to standard router advertisements can be either set to 224.0.0.1, which is the multicast address for all systems on a link (Deering, 1989), or to the broadcast address 255.255.255.255.
3. The fields in the ICMP part are defined as follows. The type is set to 9, the code can be 0, if the agent also routes traffic from non-mobile nodes, or 16, if it does not route anything other than mobile traffic.
4. Foreign agents are at least required to forward packets from the mobile node.
5. The number of addresses advertised with this packet is in #addresses while the addresses themselves follow as shown.
6. Lifetime denotes the length of time this advertisement is valid.
7. Preference levels for each address help a node to choose the router that is the most eager one to get a new node.
8. The difference compared with standard ICMP advertisements is what happens after the router addresses.
9. This extension for mobility has the following fields defined: type is set to 16, length depends on the number of COAs provided with the message and equals 6 + 4*(number of addresses).
10. An agent shows the total number of advertisements sent since initialization in the sequence number.
11. By the registration lifetime, the agent can specify the maximum lifetime in seconds a node can request during registration.
12. The following bits specify the characteristics of an agent in detail.
◼ The R bit (registration) shows, if a registration with this agent is required even when using a colocated COA at the MN.
◼ If the agent is currently too busy to accept new registrations it can set the B bit.
◼ The following two bits denote if the agent offers services as a home agent (H) or foreign agent (F) on the link where the advertisement has been sent.
◼ Bits M and G specify the method of encapsulation used for the tunnel.
◼ While IP-in-IP encapsulation is the mandatory standard, M can specify minimal encapsulation and G generic routing encapsulation.
◼ In the first version of mobile IP (RFC 2002) the V bit specified the use of header compression according to RFC 1144 (Jacobson,1990). Now the field r at the same bit position is set to zero and must be ignored.
◼ The new field T indicates that reverse tunneling is supported by the FA.
◼ The following fields contain the COAs advertised.
◼ A foreign agent setting the F bit must advertise at least one COA.
A mobile node in a subnet can now receive agent advertisements from either its home agent or a foreign agent. This is one way for the MN to discover its location.
Agent solicitation :
1. If no agent advertisements are present or the inter-arrival time is too high, and an MN has not received a COA by other means, e.g., DHCP, the mobile node must send agent solicitations.
2. Care must be taken to ensure that these solicitation messages do not flood the network, but basically an MN can search for an FA endlessly sending out solicitation messages.
3. Typically, a mobile node can send out three solicitations, one per second, as soon as it enters a new network.
4. It should be noted that in highly dynamic wireless networks with moving MNs and probably with applications requiring continuous packet streams even one second intervals between solicitation messages might be too long.
5. Before an MN even gets a new address many packets will be lost without additional mechanisms.
6. If a node does not receive an answer to its solicitations it must decrease the rate of solicitations exponentially to avoid flooding the network until it reaches a maximum interval between solicitations (typically one minute).
7. Discovering a new agent can be done anytime, not just if the MN is not connected to one. 8. Consider the case that an MN is looking for a better connection while still sending via the old path.
9. This is the case while moving through several cells of different wireless networks. After these steps of advertisements or solicitations the MN can now receive a COA, either one for an FA or a co-located COA.
10. The MN knows its location (home network or foreign network) and the capabilities of the agent (if needed).
Tunnelling and Encapsulation:
A tunnel establishes a virtual pipe for data packets between a tunnel entry and a tunnel endpoint. Packets entering a tunnel are forwarded inside the tunnel and leave the tunnel unchanged. Tunneling, i.e., sending a packet through a tunnel, is achieved by using encapsulation. Encapsulation is the mechanism of taking a packet consisting of packet header and data and putting it into the data part of a new packet. The reverse operation, taking a packet out of the data part of another packet, is called decapsulation.
Encapsulation and decapsulation are the operations typically performed when apacket is transferred from a higher protocol layer to a lower layer or from a lower to a higher layer respectively.
Here these functions are used within the same layer. This mechanism is shown in Figure 4 and describes exactly what the HA at the tunnel entry does. Fig 4 : IP Encapsulation The HA takes the original packet with the MN as destination, puts it into the data part of a new packet and sets the new IP header in such a way that the packet is routed to the COA.
The new header is also called the outer header for obvious reasons.
Additionally, there is an inner header which can be identical to the original header as this is the case for IP-in-IP encapsulation, or the inner header can be computed during encapsulation.
Route Optimization in Mobile IP:
The route optimization adds a conceptual data structure, the binding cache, to the correspondent node. The binding cache contains 2. bindings for mobile node’s home address and its current care-of-address. Every time the home agent receives a IP datagram that is destined to a mobile node currently away from the home network, it sends a binding update to the correspondent node to update the information in the correspondent node’s binding cache. After this the correspondent node can directly tunnel packets to the mobile node.
Process of Mobile IP The mobile IP process has following three main phases, which are:
1. Agent Discovery During the agent discovery phase the HA and FA advertise their services on the network by using the ICMP router discovery protocol (IROP). Mobile IP defines two methods: agent advertisement and agent solicitation which are in fact router discovery methods plus extensions. o Agent advertisement: For the first method, FA and HA advertise their presence periodically using special agent advertisement messages. These messages advertisement can be seen as a beacon broadcast into the subnet. For this advertisement internet control message protocol (ICMP) messages according to RFC 1256, are used with some mobility extensions. o Agent solicitation: If no agent advertisements are present or the inter arrival time is too high, and an MN has not received a COA, the mobile node must send agent solicitations. These solicitations are again bases on RFC 1256 for router solicitations.
2. Registration The main purpose of the registration is to inform the home agent of the current location for correct forwarding of packets.
Registration can be done in two ways depending on the location of the COA. o If the COA is at the FA, the MN sends its registration request containing the COA to the FA which is forwarding the request to the HA. The HA now set up a mobility binding containing the mobile node's home IP address and the current COA. Additionally, the mobility biding contains the lifetime of the registration which is negotiated during the registration process. Registration expires automatically after the lifetime and is deleted; so a mobile node should register before expiration. After setting up the mobility binding, the HA send a reply message back to the FA which forwards it to the MN. o If the COA is co-located, registration can be very simpler. The mobile node may send the request directly to the HA and vice versa. This by the way is also the registration procedure for MNs returning to their home network.
3. Tunneling A tunnel is used to establish a virtual pipe for data packets between a tunnel entry and a tunnel endpoint. Packets which are entering in a tunnel are forwarded inside the tunnel and leave the tunnel unchanged. Tunneling, i.e., sending a packet through a tunnel is achieved with the help of encapsulation. Tunneling is also known as "port forwarding" is the transmission and data intended for use only within a private, usually corporate network through a public network.
Transport Layer
TRADITIONAL TCP Mechanisms that influence the efficiency of TCP in a mobile environment
• Congestion control
• Slow start
• Fast retransmit/fast recovery
• Implications on mobility
Congestion control
• TCP has been designed for fixed networks with fixed end-systems
• Hardware and software are mature enough to ensure reliability of data
• The probable reason for a packet loss in a fixed network is a temporary overload some point in the transmission path, i.e., a state of congestion at a node
• The packet buffers of a router are filled and the router cannot forward the packets fast enough • The only thing a router can do in this situation is to drop packets
• The sender notices the missing acknowledgement for the lost packet and assumes a packet loss due to congestion
• Retransmitting the missing packet and continuing at full sending rate would now be unwise, as this might only increase the congestion. Slow start
• The behavior TCP shows after the detection of congestion is called slow start
• The sender always calculates a congestion window for a receiver.
• The start size of the congestion window is one segment (TCP packet).
• This scheme doubles the congestion window every time the acknowledgements come back, which takes one round trip time (RTT) like 1, 2, 4, 8 etc.
• This is called the exponential growth of the congestion window in the slow start mechanism. • The exponential growth stops at the congestion threshold.
Slow start
• The behavior TCP shows after the detection of congestion is called slow start
• The sender always calculates a congestion window for a receiver.
• The start size of the congestion window is one segment (TCP packet).
• This scheme doubles the congestion window every time the acknowledgements come back, which takes one round trip time (RTT) like 1, 2, 4, 8 etc.
• This is called the exponential growth of the congestion window in the slow start mechanism. • The exponential growth stops at the congestion threshold.
As soon as the congestion window reaches the congestion threshold, further increase of the transmission rate is only linear by adding 1 to the congestion window each time the acknowledgements come back
• Linear increase continues until a time-out at the sender occurs due to a missing acknowledgement, or until the sender detects a gap in transmitted data o the sender sets the congestion threshold to half of the current congestion window o The congestion window itself is set to one segment Fast retransmit/fast recovery Fast Retransmit
• a receiver sends acknowledgements only if it receives any packets from the sender. • Receiving acknowledgements from a receiver also shows that the receiver continuously receives something from the sender.
• The gap in the packet stream is not due to severe congestion, but a simple packet loss due to a transmission error.
• The sender can now retransmit the missing packet(s) before the timer expires. • This behavior is called fast retransmit Fast Recovery
• The receipt of acknowledgements shows that there is no congestion to justify a slow start.
• The sender can continue with the current congestion window.
• The sender performs a fast recovery from the packet loss
• This mechanism can improve the efficiency of TCP dramatically Implications on mobility
• TCP concludes a congestion situation from a missing acknowledgement o typically wrong in wireless networks, here we often have packet loss due to transmission errors o mobility itself can cause packet loss, if e.g. a mobile node roams from one access point (e.g. foreign agent in Mobile IP) to another while there are still packets in transit to the wrong access point and forwarding is not possible • The performance of an unchanged TCP degrades severely o TCP cannot be changed fundamentally due to the large base of installation in the fixed network,
▪ TCP for mobility has to remain compatible o the basic TCP mechanisms keep the whole Internet together CLASSICAL TCP IMPROVEMENTS
• Indirect TCP (I-TCP)
• Snooping TCP • Mobile TCP
• Fast retransmit/fast recovery
• Transmission/time-out freezing
• Selective retransmission
• Transaction-oriented TCP
Indirect TCP (I-TCP)
I-TCP segments a TCP connection into a o fixed part - Standard TCP is used o wireless part - optimized TCP protocol
• splitting of the TCP connection at, e.g., the foreign agent into 2 TCP connections, no real end-to-end connection any longer
• hosts in the fixed part of the net do not notice the characteristics of the wireless part
Advantages
• no changes in the fixed network necessary, no changes for the hosts (TCP protocol) necessary, all current optimizations to TCP still work
• transmission errors on the wireless link do not propagate into the fixed network
• simple to control, mobile TCP is used only for one hop between, e.g., a foreign agent and mobile host
• therefore, a very fast retransmission of packets is possible, the short delay on the mobile hop is known
Disadvantages
• loss of end-to-end semantics, an acknowledgement to a sender does now not any longer mean that a receiver really got a packet, foreign agents might crash
• higher latency possible due to buffering of data within the foreign agent and forwarding to a new FA
Snooping TCP
• the foreign agent buffers all packets with destination mobile host and additionally ‘snoops’ the packet flow in both directions to recognize acknowledgements
• buffering enable the FA to perform a local retransmission in case of packet loss on the wireless link
• Transparent extension of TCP within the foreign agent
• buffering of packets sent to the mobile host
• lost packets on the wireless link (both directions!) will be retransmitted immediately by the mobile host or foreign agent, respectively (so called “local” retransmission)
• the foreign agent therefore “snoops” the packet flow and recognizes acknowledgements in both directions, it also filters ACKs
• changes of TCP only within the foreign agent
• Data transfer to the mobile host o FA buffers data until it receives ACK of the MH, FA detects packet loss via duplicated ACKs or time-out o fast retransmission possible, transparent for the fixed network
• Data transfer from the mobile host o FA detects packet loss on the wireless link via sequence numbers,
▪ FA answers directly with a NACK to the MH o MH can now retransmit data with only a very short delay
• Integration of the MAC layer o MAC layer often has similar mechanisms to those of TCP o thus, the MAC layer can already detect duplicated packets due to retransmissions and discard them
• Problems o snooping TCP does not isolate the wireless link as good as I-TCP o snooping might be useless depending on encryption schemes
Advantages
• The end-to-end TCP semantic is preserved
• The correspondent host does not need to be changed; most of the enhancements are in the foreign agent
• It does not need a handover of state as soon as the mobile host moves to another foreign agent. • It does not matter if the next foreign agent uses the enhancement or not
Disadvantages
• Snooping TCP does not isolate the behavior of the wireless link as well as ITCP
• Using negative acknowledgements between the foreign agent and the mobile host assumes additional mechanisms on the mobile host.
• All efforts for snooping and buffering data may be useless if certain encryption schemes are applied end-to- end between the correspondent host and mobile host
Mobile TCP
• Special handling of lengthy and/or frequent disconnections
• M-TCP splits as I-TCP does
o unmodified TCP fixed network to supervisory host (SH)
o optimized TCP SH to MH
• Supervisory host o no caching, no retransmission o monitors all packets, if disconnection detected ▪ set sender window size to 0
▪ sender automatically goes into persistent mode
o old or new SH reopen the window
• Advantages
o maintains semantics, supports disconnection, no buffer forwarding
• Disadvantages
o loss on wireless link propagated into fixed network o adapted TCP on wireless link
Fast retransmit / fast recovery
• Change of foreign agent often results in packet loss
o TCP reacts with slow-start although there is no congestion
• Forced fast retransmit o as soon as the mobile host has registered with a new foreign agent, the MH sends duplicated acknowledgements on purpose
o this forces the fast retransmit mode at the communication partners o additionally, the TCP on the MH is forced to continue sending with the actual window size and not to go into slow-start after registration
• Advantage
o simple changes result in significant higher performance
• Disadvantage
o further mix of IP and TCP, no transparent approach Transmission / time-out freezing
• Mobile hosts can be disconnected for a longer time o no packet exchange possible, e.g., in a tunnel, disconnection due to overloaded cells or mux. with higher priority traffic
o TCP disconnects after time-out completely
TCP freezing o MAC layer is often able to detect interruption in advance
o MAC can inform TCP layer of upcoming loss of connection o TCP stops sending, but does now not assume a congested link
o MAC layer signals again if reconnected
• Advantage
o scheme is independent of data
• Disadvantage
o TCP on mobile host has to be changed, mechanism depends on MAC layer
Selective retransmission
• TCP acknowledgements are often cumulative o ACK n acknowledges correct and in-sequence receipt of packets up to n
o if single packets are missing quite often a whole packet sequence beginning at the gap has to be retransmitted (go-back-n), thus wasting bandwidth
• Selective retransmission as one solution
o RFC2018 allows for acknowledgements of single packets, not only acknowledgements of in-sequence packet streams without gaps
o sender can now retransmit only the missing packets
• Advantage
o much higher efficiency
• Disadvantage
o more complex software in a receiver, more buffer needed at the receiver
Transaction-oriented TCP
• TCP phases o setup, data transmission, connection release o using 3-way-handshake needs 3 packets for setup and release, respectively o thus, even short messages need a minimum of 7 packets!
• Transaction oriented TCP o RFC1644, T-TCP, describes a TCP version to avoid this overhead
o connection setup, data transfer and connection release can be combined o thus, only 2 or 3 packets are needed
• Advantage : efficiency
• Disadvantage o requires changed TCP o mobility not longer transparent.
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