Mobile Network and Transport Layers

Mobile Network and Transport Layers

Contents:

1.     Mobile IP

2.     Dynamic Host Configuration Protocol

3.     Mobile Ad Hoc Routing Protocols

4.     Multicast routing

5.     TCP over Wireless Networks

6.     Indirect TCP

7.     Snooping TCP

8.     Mobile TCP

9.     Fast Retransmit / Fast Recovery

10.                        Transmission/Timeout Freezing-Selective Retransmission

11.                        Transaction Oriented TCP- TCP over 2.5 / 3G wireless Networks

Pre requisite Discussion :

In this unit we discuss what is cellular systems and how the frequency and channels are allocated. Medium access control tells how to reduce traffic in the network and we discuss about frequency , time, space and code division multiple access.

1. Mobile IP:

Concept:

Goals For Mobile IP

The Internet is the network for global data communication with hundreds of millions of users. The reason is quite simple:

You will not receive a single packet as soon as you leave your home network, ie., The network your computer is configured for, and reconnect your computer (wireless or wired) at another place. A host sends an IP packet with the header containing a destination address besides other fields.

The destination address not only determines the receiver of the packet, but also the physical subnet of the receiver.

Routers in the Internet now look at the destination addresses of incoming pack-ets and forward them according to internal look-up tables.

To avoid an explosion of routing tables, only prefixes are stored and further optimizations are applied.

Otherwise a router would have to store the addressed of all computers in the Internet which is obviously not feasible.

As long as the receiver can be reached within its physical subnet it gets the packets as soon as it moves outside the subnet, no packet will reach it anymore.

Thus a host needs a so called topologically correct address.

Solutions:

Assigning the computer a new topologically correct IP address.

So moving to a new location would also mean assigning a new address.

Now the problem is that nobody knows of this new address.

It is almost impossible to find a (mobile) host in the Internet which has just changed its address. Especially the Domain Name System (DNS) needs some time before it update its internal tables necessary for the mapping of a logical name to an IP address.

This approach does not work if the mobile node moves quite often. Furthermore the Internet and DNS have not been built for frequent updates.

Significance:

∑     IP packet Delivery

∑     Path Delivery

2.               DYNAMICHOST CONFIGURATION PROTOCOL

Concept:

The Dynamic Host Protocol (DHCP,RFC 2131) mainly used

TO simply the installation

Maintenance of networked computers

∑     If a new computer is connected to a network DHCP can provide it with all the necessary Information for full system integration into the network e.g:-Addresses of DNS server and Default router Subnet Mask Domain name IP address

∑     Providing an IP address makes DHCP very attractive for mobile IP as well source of care-of address.

Basic DHCP configuration

DHCP clients send a request to a server to which the server responds.

A client sends requests using MAC broadcasts to reach all devices in the LAN.

A DHCP relay might be needed to forward requests across inter-working units to a DHCP server.

Client Initialization

The client broadcasts a DHCP DISCOVER into subnet. There may be relay to forwards this broadcast.

Two servers receive this broadcast and determine the configuration they offer to the client. Servers reply to the client  s request with DHCPOFFER and offer a list of configuration parameters.

The client can now choose one of the configurations offered.

The client in turn replies to the servers accepting one of the configurations and rejecting the others using DHCP REQUEST.

If a server receives a DHCP REQUEST with a rejection it can free the reserved configuration for other possible clients.

The server with the configuration accepted by the client now confirms the configuration with DHCP ACK.This completes the initialization phase.

If a client leaves the subnet it should release the configuration received by the server using

DHCP RELEASE.

Now the server can free the context stored for the client and offer the configuration again.

The configuration a client from a server is only leased for a certain amount of time it has to be reconfirmed from time to time.

Otherwise the server will be free the configuration. This timeout of configuration helps in the case of crashed nodes or nodes moved away without realizing the context.

DHCP Features

DHCP supporting the acquisition of care-of-address for mobile nodes

A DHCP server should located in the subnet of the access point of the mobile note.

DHCP relay should provide forwarding of the Messages.

RFC 3118 specifies authentication for DHCP messages which id needed to protect mobile nodes from malicious DHCP servers.

Significance:

Provides a protocol for the countries where the calls and IPs are dynamically allotted.

3. Mobile Adhoc Routing:

Concept:

In wireless networks using an infrastructure cells have been defines. within a cell the bse station can reach all mobile nodes.

In -hoc networks each node must be able to forward data for other nodes. At a certain time t1 the network topology consists of five nodes N1 to N5.

Nodes are connected depending upon the current transmission characteristics between them. In this network N4 can receive N1 over a good link.

But N1 receive N4 via a weak link.

Links do not necessarily have the same characteristics in both directions.

Reason:

Different antenna characteristics or transmit power. N1 cannot receive N2 at all

N2 receives a signal from N1. At a certain time t2 the network topology consists of five nodes N1 to N5.This situation can change quite fast N1 cannot receive N4 any longer

N4 receives N1 only via a weak link.

But N1 has as asymmetric but bi-directional link to N2 that did not exist before.

The Fundamental differences between wired networks and ad-hoc networks related to routing.

Asymmetric Links

Node A receives a signal from node B.

But this does not tell anything about the quality of the connection in reverse. Node B might

Receive nothing

Have a weak link

Even have a better link than the reverse direction.

Routing information collected for one direction is of almost no use for the other direction.

Redundant Links

Wired networks too have a redundant links to survive link failures.

There is only some redundancy in wired networks which additionally are controlled by a network administrator.

In ad-hoc networks nobody controls redundancy so there might be many redundant links up to the extreme of a completely meshed topology.

Routing algorithms for wired networks can handle some redundancy but a high redundancy can cause a large computational overhead for routing table updates.

Signifiacance:

Gives a protocol for configuring the mobiles.

4. Multicast Routing:

Concept:

Multicast IP Routing protocols are used to distribute data (for example, audio/video streaming broadcasts) to multiple recipients. Using multicast, a source can send a single copy of data to a single multicast address, which is then distributed to an entire group of recipients.

A multicast group identifies a set of recipients that are interested in a particular data stream, and is represented by an IP address from a well-defined range. Data sent to this IP address is forwarded to all members of the multicast group.

Routers between the source and recipients duplicate data packets and forward multiple copies wherever the path to recipients diverges. Group membership information is used to calculate the best routers at which to duplicate the packets in the data stream to optimize the use of the network.

A source host sends data to a multicast group by simply setting the destination IP address of the datagram to be the multicast group address. Any host can become a source and send data to a multicast group. Sources do not need to register in any way before they can begin sending data to a group, and do not need to be members of the group themselves.

There are many different multicast protocols and modes of operation, each optimized for a particular scenario. Many of these are still at an early stage of standardization. However, they all operate in the same general way, as follows.

∑     A Multicast Group Membership Discovery protocol is used by receiving hosts to advertise their group membership to a local multicast router, enabling them to join and leave multicast groups. The main Multicast Group Membership Discovery protocols are Internet Group Management Protocol (IGMP) for IPv4 and Multicast Listener Discovery (MLD) for IPv6.

∑     A Multicast Routing Protocol is used to communicate between multicast routers and enables them to calculate the multicast distribution tree of receiving hosts. Protocol Independent Multicast (PIM) is the most important Multicast Routing Protocol.

The multicast distribution tree of receiving hosts holds the route to every recipient that has joined the multicast group, and is optimized so that

∑     multicast traffic does not reach networks that do not have any such recipients (unless the network is a transit network on the way to other recipients)

∑     duplicate copies of packets are kept to a minimum.

Significance:

This one provides a protocol for sending the call/data from one mobile station to several other stations.

5. TCP over Wireless networks:

Concept:

Slow Start

TCP  s reaction to a missing acknowledgement is quite drastic but it is necessary to get rid of congestion quickly.

The behavior 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). The sender sends one packet and waits for acknowledgement.

If this acknowledgement arrives the sender increases the congestion window by one now sending two packets (congestion window=2)

After arrival of the two corresponding acknowledgements now the congestion window equals 4. This scheme doubles the congestion window every time the acknowledgements come back which takes one Round Trip Time (RIT).This is called the exponential growth of the congestion window in the slow start mechanism.

It is too dangerous to double the congestion window each time because the steps might become too large. The exponential growth stops at the Congestion Threshold.

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 because of continuous acknowledgements for the same packet.

In either case the sender sets the congestion threshold to half of the current congestion window. The congestion window itself is set to one segment and the sender starts sending a single segment.

The exponential growth starts once more up to the new congestion threshold then the window grows in Linear fashion.

Significance:

Can be used in wireless communications too.

6. Indirect TCP or I-TCP Concept:

Indirect TCP Advantages and Disadvantages

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 with the foreign agent and forwarding to a new foreign agent

High trust at foreign agent; end-to-end encryption impossible

Significance:

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 a foreign agent and a mobile host Therefore, a very fast retransmission of packets is possible, the short delay on the mobile hop is known

7. Snooping TCP

Concepts:

The foreign agent buffers all packets with Destination mobile host.

Additionally         snoops  the packet flow in both directions to recognize acknowledgements.

The reason for buffering the packets toward the mobile node is to enable the foreign agent to perform a local transmission in case of packet loss on the wireless link.

The foreign agent buffers every packet until it receives an acknowledgement from the mobile host.

If the foreign agent does not receive an acknowledgement from the mobile host within a certain amount of time either the packet or the acknowledgement has been lost.

Alternatively the foreign agent could receive a duplicate ACK which also shows the loss of a packet.

Now the foreign agent

·        Retransmits the packet directly from the buer.

·        Performing a much faster retransmission completed to the correspondent host.

·        The time out for acknowledgements can be much shorter because it reflects only the delay of one hop plus processing time.

·        To remain transparent the foreign agent must not acknowledge data to the correspondent host.

·         

·        The correspondent host believe that the mobile host had received the data would violate the end-to end semantic in case of a foreign agent failure.

·         

·        The foreign agent can filter the duplicate acknowledgements to avoid unnecessary retransmissions of data from the correspondent host.

If the foreign agent now crashes the time-out of the correspondent host still works and triggers a retransmission.

The foreign agent may discard duplicates of packets already retransmitted locally and acknowledged by the mobile host.

Significance:

This avoids unnecessary traffic on the wireless link.

8.               Mobile TCP Concept:

Special handling of lengthy and/or frequent disconnections

M-TCP splits as I-TCP does unmodified TCP fixed network to supervisory host (SH)

optimized TCP SH to MH Supervisory host no caching, no retransmission monitors all packets, if disconnection detected l set sender window size to 0

l sender automatically goes into persistent mode q old or new SH reopen the window Disadvantages

loss on wireless link propagated into fixed network adapted TCP on wireless link

Significance:

maintains semantics, supports disconnection, no buffer forwardin

9.               Fast Retransmit / Fast Recovery

Concept:

Reduction of the congestion threshold

A sender receiving continuous acknowledgements for the same packets. This informs the sender of two things. One is that the receiver got all packets up to the acknowledged packet in sequence.

Fast Retransmit

The other reason for activating slow start is a time-out due to a missing acknowledgement. TCP using Fast Retransmit / Fast Recovery interprets this congestion in the

network and activates the slow start mechanism.

10. Transmission / Timeout Freezing-Selective Retransmission

Concepts:

Mobile hosts can be disconnected for a longer time

no packet exchange possible, e.g., in a tunnel, disconnection due to overloaded cells or mux. with higher priority traffic TCP disconnects after time-out completely

TCP freezing

MAC layer is often able to detect interruption in advance MAC can inform TCP layer of upcoming loss of connection TCP stops sending, but does not assume a congested link MAC layer signals again if reconnected

Disadvantage

TCP on mobile host has to be changed, mechanism depends on MAC layer

Significance:

Scheme is independent of data

11. Transaction Oriented TCP- TCP over 2.5 / 3G wireless Networks

Concepts:

Fine tuning today  s TCP Learn to live with

_ Data rates: 64 kbit/s upstream, 384 kbit/s downstream (UMTS release99); asymmetry: 3-6, but also up to 1000 (broadcast systems), periodic allocation/release of channels

_ High latency, high jitter, packet loss Suggestions

_ Large (initial) window size, large maximum transfer unit, selective acknowledgement, explicit congestion notification, timestamps, no header compression

Already in use _ i-mode in Japan

_ WAP 2.0 (  TCP with wireless profile  ) Transport layer

_ Local retransmissions and acknowledgements Additionally on the application layer

_ Content filtering, compression, picture downscaling _ E.g., Internet/WAP gateways

_ Web service gateways?

Big problem: breaks end-to-end semantics _ Disables use of IP security!

More issues

RFC 3150 (slow links)

_ Recommends header compression, no timestamp RFC 3155 (links with errors)

_ States that explicit congestion notification cannot be used In contrast to 2.5G/3G recommendations!

Significance:

Used to differentiate the 2.8G/3G over the TCP

Applications:

o   Fine tuned for Mobile usage over the TCP

o Almost all the Devices Makes use of this layer for sending the data o Used for the Mobile devices that are designed these days either2.5/3G