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					                                         Module 4
Mobile Network and Transport Layers:- Mobile IP- Goals, Requirements, IP packet delivery,
Advertisement and discovery. Registration, Tunneling and Encapsulation, Optimization, Reverse
Tunneling, IPv6, Dynamic Host configuring protocol, Ad hoc networks – Routing, DSDV, Dynamic
source routing. Hierarchical Algorithms.
Traditional TCP, Indirect TCP, Snooping TCP, Mobile TCP, Transmission.


Mobile IP
It is the extension of IP to support mobility.
Mobile IP adds mobility support to the internet layer protocol IP.It allows users to keep the same IP
address, stay connected, and maintain ongoing applications while roaming between IP networks.
Mobile IP is scalable for the Internet because it is based on IP—any media that can support IP can
support Mobile IP.
Goals,assumptions and requirements
The internet is the network of global data communication with millions of users.A host sends an IP
packet with the header containing the destination address with other fields.The destination address
determines the reciever of the packet as well as the subnet of the receiver.As long as the receiver is
within its subnet ,it gets the packet. As soon as it moves outside the subnet the packet wll not reach
it.ie,A host needs a topologically correct address .
Quick solution to this problem is
                     1. Assign new topologically correct address each time the host changes the
                        location.
Problems:
-Frequent updation in DNS.
Domain name system requires some time before it updates the internal tables.so it wii not wirk if
the host moves very frequently.ie DNS will become inconsistent.
Also, changing the IP address while still having TCP open means breaking the connection.ie TCP
connection cannot survive any address change.
                     2. Creation of specific routes to mobile nodes
       -Routing calculations and routing updates create additional overhead
Since the quick solutions will not work,mobile IP is developed.
Requirements
      Compatibility
A new standard like mobile IP cannot introduce changes for existing applications.
Mobile IP has to be integrated into the existing OS.
Should be compatible with all lower layer protocols.
Nodes enhanced with mobile IP should communicate with fixed end systems.
      Transparency
Mobility should be invisible to all higher layer protocols and applications.
Besides lower bandwidth and some interruptions in service ,higher layers should continue to work
even if the node has changed its location.
      Scalability
Enhancing IP to support mobility should not reduce the efficiency of the network. Also it should be
scalable to include a large number of nodes in the internet.
      Security
       Mobility causes many security problems. All messages related to the management of mobile
       IP should be authenticated.
The goal of Mobile IP is defined as “supporting end system mobility while maintaining
scalability,efficiency , and compatibility in all respects with existing applications and protocols.”

Components of a Mobile IP Network




Mobile IP has the following components,
      Mobile Node(MN)
The Mobile Node is a device such as a cell phone, personal digital assistant, or laptop whose
software enables network roaming capabilities. MN keeps its IP address and can continuously
communicate with any other system in the internet.
      Correspondent node(CN)
It is the communication partner at home. It can be mobile or fixed.
      Home network
It is the subnet the MN belongs to with respect to its IP address.No mobile IP support is needed at
Home network.
      Foreign network
The foreign network is the current network where MN visits.
• Home Agent(HA)
he Home Agent is a router on the home network serving as the anchor point for communication
with the Mobile Node; it tunnels packets from a device on the Internet, called a Correspondent
Node, to the roaming Mobile Node. (A tunnel is established between the Home Agent and a
reachable point for the Mobile Node in the foreign network.)
      Foreign Agent (FA)
       The Foreign Agent is a router that may function as the point of attachment for the Mobile
       Node when it roams to a foreign network, delivering packets from the Home Agent to the
       Mobile Node.
      Care of address(COA)
       The COA defines the current location of the MN from an IP point of view.All packets sent to
       the MN are delivered to the COA.( the termination point of the tunnel toward the Mobile
       Node when it is on a foreign network.) The Home Agent maintains an association between
       the home IP address of the Mobile Node and its care-of address.
Two types of COA.
   1. Foreign agent COA :The COA located at FA ie,the COA is an IP address of FA.FA is the
      tunnel endpoint.
   2. Co-located COA :Address assigned to MN at foreign network. MN is the tunnel endpoint.

       How a mobile IP works ?




1. Sender sends to the IP address of MN,HA intercepts packet (proxy ARP)
2. HA tunnels packet to COA, here FA,by encapsulation
3. FA forwards the packet to the MN
A correspondent node CN wants to sent an IP packet to the MN. CN does not know anything about
current location and sent an IP packet with MN as destination address and CN as source address.
The internet routes the packet to the home network of MN. The HA intercepts the packets and
understands that it is not in its HN. The packet is then encapsulated and tunneled to COA. A new
header is put in front of the old IP header showing COA as new destination and HA as source. The
foreign agent now de-capsulate the packet and forward the original packet to MN. The MN sends
the packet (if any) as usual with its own fixed IP address as source and CN as destination, if CN is
fixed. If CN is a mobile node all the steps will repeat.
The Mobile IP process has three main phases,
•Agent Discovery
A Mobile Node discovers its Foreign and Home Agents during agent discovery.
•Registration
The Mobile Node registers its current location with the Foreign Agent and Home Agent during
registration.


•Tunneling
A reciprocal tunnel is set up by the Home Agent to the care-of address (current location of the
Mobile Node on the foreign network) to route packets to the Mobile Node as it roams.
Agent Discovery
A Mobile Node discovers its Foreign and Home Agents during agent discovery.
Two methods are there
1.Agent advertisement
 The Home Agent and Foreign Agent advertise their services on the network by using special
    agent advertisement messages.(These messages are ICMP (Internet ControlMessage Protocol)
    packets with mobility extensions.)
 MN listens to these messages and detects, if it is in the home or a foreign network (standard case
for home network)
MN reads a COA from the FA advertisement messages.


2.Agent solicitation
Rather than waiting for agent advertisements, a Mobile Node can send out an agent solicitation.
This solicitation forces any agents on the link to immediately send an agent advertisement.Care
must be taken that solicitation messages should not flood the network.
       If a Mobile Node determines that it is connected to a foreign network, it acquires a care-of
address. Two types of care-of addresses exist:Foreign agent Care-of address and Colocated care-of
address.
Advertisement packet




Registration
The Mobile Node registers its current location with the Foreign Agent and Home Agent during
registration.(Inform HA about COA)
This can be done in two ways.
For Foreign agent COA,
 MN sends its registration request containing the COA to the FA which is forwarding to HA.
HA sets up a mobility binding which contains mobile node's home IP address and current
COA.Binding contains a registration life time and the registration expires after the time.
After setting up mobility binding,HA acknowledges via FA to MN.
For co located COA
Registration is simpler
 The MN sends request directly to HA and vice versa.
Registration request packet




Registration process




Tunneling and encapsulation
A reciprocal tunnel is set up by the Home Agent to the care-of address (current location of the
Mobile Node on the foreign network) to route packets to the Mobile Node as it roams.
A tunnel establishes a virtual pipe for data packets .Tunneling is acheived by using encapsulation.
Encapsulation is the process of taking a packet consisting of packet header and data and putting it
into the data part of another packet.The reverse operation is called decapsulation.



   IP in IP encapsulation




       Encapsulation of one packet into another as payload
       IP in IP is mandatory for Mobile IP.




Minimal encapsulation
Several fields in Ip in Ip encapsultion are redundant.
Avoids repetition of identical fields.The inner header is different for minimal.




Generic Encapsulation
Ip in Ip and minimal encapsulation techniques works only for IP .This generic method supports all
other network layer protocols.This allows the encapsulation of packets of one protocol into the
payload portion of a packet of another protocol suite.
The outer header is the standard headerwith HA as source address and COA as destination.




Optimizations
Triangular Routing is a major problem in non optimized mobile IP.
      sender sends all packets via HA to MN ie, even if the communicating partners are nearby,the
        packets have to travel more distance.
      higher latency and network load
―Solutions”
Inform CN about the current location of MN.Four additional messages are used for optimization.
      Binding request
Any node that wants to know the current location of an MN can send the binding request.
      Binding update
Sent by HA to CN to reveal the current location of MN.
      Binding acknowledgement
Sent by CN after accepting the update message.(if requested)
      Binding warning
If a node decapsulates a packet for an MN and it has changed its location,this node sends a binding
warning.The recepient then understand that a fresh binding request is needed.

Change of FA
packets on-the-fly during the change can be lost
new FA informs old FA to avoid packet loss, old FA now forwards remaining packets to new FA
this information also enables the old FA to release resources for the MN

Problems with Mobile IP
Security
authentication with FA problematic, for the FA typically belongs to another organization
 no protocol for key management and key distribution has been standardized in the Internet
 patent and export restrictions
Firewalls
 typically mobile IP cannot be used together with firewalls, special set-ups are needed (such as
reverse tunneling)
QoS
many new reservations in case of RSVP
 tunneling makes it hard to give a flow of packets a special
treatment needed for the QoS
Reverse Tunneling




A tunnel that starts at the mobile node's care-of address and terminates at the home agent.
1. MN sends to FA
2. FA tunnels packets to HA by encapsulation
    3. HA forwards the packet to the receiver (standard case
Mobile IP with reverse tunneling
Router accept often only ―topological correct― addresses (firewall!) a packet from the MN
encapsulated by the FA is now topological correct
 furthermore multicast and TTL problems solved (TTL in the home network correct, but MN is to
far away from the receiver)
Reverse tunneling does not solve problems with firewalls, the reverse tunnel can be abused to
circumvent security mechanisms (tunnel hijacking)
 optimization of data paths, i.e. packets will be forwarded through the tunnel via the HA to a sender
(double triangular routing)



Mobile IP and IPv6
Mobile IP was developed for IPv4, but IPv6 simplifies the protocols
   security is integrated and not an add-on, authentication of registration is included
   COA can be assigned via auto-configuration (DHCPv6 is one candidate), every node has
      address autoconfiguration
   no need for a separate FA, all routers perform router advertisement which can be used
      instead of the special agent advertisement
   MN can signal a sender directly the COA, sending via HA not needed in this case
      (automatic path optimization)
   "soft― hand-over, i.e. without packet loss, between two subnets is supported
              MN sends the new COA to its old router
              the old router encapsulates all incoming packets for the MN and
             forwards them to the new COA
              authentication is always granted.
IPv4 uses 32-bit addresses, and with the growth of the Internet, these have become a scarce and
valuable commodity. Organizations have gone to great lengths to deal with the shortage and high
cost of IPv4 addresses. The most visible change in IPv6 is that addresses balloon from 32-bits to
128-bits.

Features of IPV6
Address Space
Increase from 32-bit to 128-bit address space
Management
Stateless autoconfiguration means no more need to configure IP addresses for end systems, even via
DHCP
Performance
Predictable header sizes and 64-bit header alignment mean better performance from routers and
bridges/switches
Multicast/Multimedia
Built-in features for multicast groups, management, and new "anycast" groups
Mobile IP
Eliminate triangular routing and simplify deployment of mobile IP-based systems
Virtual Private Networks
Built-in support for ESP/AH encrypted/authenticated virtual private network protocols; built-in
support for QoS tagging


Difference Between IPv4 and IPv6

IPv4

    * Source and destination addresses are 32 bits (4 bytes) in length.
    * IPSec support is optional.
    * IPv4 header does not identify packet flow for QoS handling by routers.
    * Both routers and the sending host fragment packets.
    * Header includes a checksum.
    * Header includes options.
    * Address Resolution Protocol (ARP) uses broadcast ARP Request frames to resolve an IP
address to a link-layer address.
    * Internet Group Management Protocol (IGMP) manages membership in local subnet groups.
    * ICMP Router Discovery is used to determine the IPv4 address of the best default gateway, and
it is optional.
    * Broadcast addresses are used to send traffic to all nodes on a subnet.
    * Must be configured either manually or through DHCP.
    * Uses host address (A) resource records in Domain Name System (DNS) to map host names to
IPv4 addresses.
    * Uses pointer (PTR) resource records in the IN-ADDR.ARPA DNS domain to map IPv4
addresses to host names.
    * Must support a 576-byte packet size (possibly fragmented).
IPv6

  * Source and destination addresses are 128 bits (16 bytes) in length.
  * IPSec support is required.
  * IPv6 header contains Flow Label field, which identifies packet flow for QoS handling by
router.
  * Only the sending host fragments packets; routers do not.
  * Header does not include a checksum.
  * All optional data is moved to IPv6 extension headers.
  * Multicast Neighbor Solicitation messages resolve IP addresses to link-layer addresses.
  * Multicast Listener Discovery (MLD) messages manage membership in local subnet groups.
  * ICMPv6 Router Solicitation and Router Advertisement messages are used to determine the IP
address of the best default gateway, and they are required.
  * IPv6 uses a link-local scope all-nodes multicast address.
  * Does not require manual configuration or DHCP.
  * Uses host address (AAAA) resource records in DNS to map host names to IPv6 addresses.
  * Uses pointer (PTR) resource records in the IP6.ARPA DNS domain to map IPv6 addresses to
host names.
  * Must support a 1280-byte packet size (without fragmentation).

IPV6 header format




 * Version - This field contains the version of the IP used in the packet. It is of 4-bit in IP version 6.
 * Traffic class - This is an 8-bits field determining the packet priority. Priority values subdivide
      into ranges: traffic where the source provides congestion control and non-congestion control
      traffic.
 * Flow label - This 20 bits specifies the QoS management. Originally created for giving real-time
      applications special service, but currently unused.
  * Payload length - This 16 bits determines the payload length in bytes. When cleared to zero, the
      option is a "Jumbo payload" (hop-by-hop).
  * Next header - This 8-bits field specifies the next encapsulated protocol. The values are
      compatible with those specified for the IPv4 protocol field.
  * Hop limit - This is an 8-bits field newly introduced in IPv6. It replaces the time to live field of
      IPv4.
  * Source Address - This 128 bits field determines the logical address of the host that is sending
      the packet.
  * Destination Address - This 128 bits field determines the logical address of the host that is
      receiving the packet.
                  DHCP: Dynamic Host Configuration Protocol

    simplification of installation and maintenance of networked computers
    supplies systems with all necessary information, such as IP address, DNS server
     address, domain name, subnet mask, default router etc.
    enables automatic integration of systems into an Intranet or the Internet,
    can be used to acquire a COA for Mobile IP

Client/Server-Model
       the client sends via a MAC broadcast a request to the DHCP serve r (might
be via a DHCP relay)




   1. Client broadcast a DHCP Discover to subnet.
   2. Here,two servers recieve this message and determine the configuration they can offer to this
      client.(for eg,checking of available IP address and choosing one fot it)
   3. Servers reply with DHCP offer and offer a list of available configuration parameters.
   4. Client choose one of them.
   5. Client then send a DHCP request to the servers.(one accepting and one rejecting)
   6. The server which receives the rejected message now free the reserved configuration,The
      accepted one send an acknowledgement message DHCP ack as confirmation.thus
      initialization phase is completed.
   7. If a client leaves the subnet it should free the reserved configuration using DHCP release.
                           MANET (Mobile Ad-hoc Networking)
A mobile ad hoc network (MANET), sometimes called a mobile mesh network, is a self-
configuring network of mobile devices connected by wireless links.

Each device in a MANET is free to move independently in any direction, and will therefore change
its links to other devices frequently. Each must forward traffic unrelated to its own use, and
therefore be a router. The primary challenge in building a MANET is equipping each device to
continuously maintain the information required to properly route traffic.
Why Mobile Ad-Hoc Networks?

Instant Infrastructure – Unplanned meetings, Spontaneous communications It would take too long
for infrastructure to set up.Manet is a solution.No prior planning is needed.

Disaster Relief – Infrastructures typically breakdown in disasters :Hurricane, Flooding, Fire etc

Remote area – Impossible to reach, expensive for infrastructure to setup in remote areas.

Effectiveness
Mobile adhoc networks offer cheap solutions in some cases when very little information to be sent
Registration takes a long time & Overhead too high with existing applications.
Pro-active (table-driven) routing
This type of protocols maintains fresh lists of destinations and their routes by periodically
distributing routing tables throughout the network. The main disadvantages of such algorithms are:

  1. Respective amount of data for maintenance.
    2. Slow reaction on restructuring and failures.
Reactive (on-demand) routing
This type of protocols finds a route on demand by flooding the network with Route Request
packets. The main disadvantages of such algorithms are:

 1. High latency time in route finding.
  2. Excessive flooding can lead to network clogging.
MANETS and Mobile IP
Mobile IP and DHCP handle the connection of mobile devices to a fixed infrastructure ,manet
comprises mobile routers also.Mobile devices can be connected either direcly with an infrastructure
using mobile IP for mobility support and DHCP as a source of IP address.




Example adhoc network




Fundamental differences between wired and adhoc network related to routing
   1. Asymmetric links.

Node A recieves from node B.But reverse may not be true.In wired network link quality is almost
same in both directions.
2.Redundant links
Redundancy in wired link is managed by network admnistrator.But large redundancy in adhoc
creates additional overhead for routing table updates.
3.Interference.
Interference is more in adhoc networks.

4. Dynamic topology –
The greatest problem for routing arises from the highly dynamic topology.

Snapshot of topology may be valid for a very short period of time

Routing table must somehow reflect these frequent changes. A
routing algorithm must be adaptive. In fixed network, routing updates are about 30 sec, it is too
slow for ad-hoc.

In fixed networks, some require complete picture of the whole network. Failure in links are rare.

Observations concerning routing in adhoc networks.




Traditional routing algorithms

Distance Vector
    periodic exchange of messages with all physical neighbors that contain information about
       who can be reached at what distance
    selection of the shortest path if several paths available
Link State
    periodic notification of all routers about the current state of all physical links
    router get a complete picture of the network
Problems of traditional routing algorithms
Dynamic of the topology
    frequent changes of connections, connection quality, participants
    Limited performance of mobile systems
    Periodic routing table updates need energy, sleep modes difficult
    limited bandwidth further reduced due to routing info exchange
    links can be asymmetric, directional transmission quality
DSDV (Destination Sequenced Distance Vector)

Expansion of distance vector routing
    DSDV is a proactive routing protocol based on the classical Bellman-ford algorithm. The
       improvements made to the basic algorithm include freedom from loops in routing tables
    Every MN maintains a routing table in which all of the possible destinations within the
       network and the number of hops to each destination are recorded.
    Each entry is marked with a sequence number assigned by the dest node. The seq numbers
       enable the MN to distinguish stale routes from new ones, thereby avoiding the formation of
loops.
    Routing updates are periodically transmitted throughout the network to maintain table
       consistency.
Dynamic source routing(DSR)
Split routing into discovering a path and maintaining a path
Discover a path only if a path for sending packets to a certain destination is needed and no path is
currently available.
Maintaining a path only while the path is in use one has to make sure that it can be used
continuously
No periodic updates needed!
Path discovery
     Broadcast a packet with destination address and unique ID
     if a station receives a broadcast packet
        if the station is the receiver (i.e., has the correct destination address) then return
        the packet to the sender (path was collected in the packet)
        if the packet has already been received earlier (identified via ID) then discard
        the packet
        otherwise, append own address and broadcast packet
     sender receives packet with the current path (address list)
Optimizations
        limit broadcasting if maximum diameter of the network is known
        caching of address lists (i.e. paths) with help of passing packets
        stations can use the cached information for path discovery (own paths or paths for other
hosts)

Maintaining paths
after sending a packet wait for a layer 2 acknowledgement (if applicable)
listen into the medium to detect if other stations forward the packet (if possible)
request an explicit acknowledgement if a station encounters problems it can inform the sender of a
packet or look-up a new path locally
Heirarchical Ad hoc Routing
Routing like DSDV and DSR works only for smaller number of nodes.
For larger networks clustering of nodes and different routing algorithms within and outside the
clusters are needed.
If the nodes within a cluster changes ,only nodes of the cluster are to be informed.Nodes outside the
clusters are unaffected.Nodes of other cluster only needs to know how to reach a cluster,not
individual nodes.Clusters can be combined to form super clusters.One or more clusters can be act as
cluster heads,which manages all traffic to and from a cluster.These act as gateway for all nodes in
the cluster and all other cluster heads.Different routing protocols can be used inside and outside the
clusters.
Cluster head Gateway switching Routing
Based on DV(Distance Vector) routing.
Hierarchy reduce routing tables.
Zone Routing Protocol
Hybrid heirarchical routing.Each node using ZRP has a predefined zone with the node as center.
Proactive routing inside the zone,ondemand routing outside the zone.
Hierarchical Geographic-position-assisted adhoc routing
The geographical position of a mobile node can be used to improve the performance of routing
algorithms.
The global positioning system ( GPS) can be used for acquiring position information.
GeoCast allows messages to be sent to all nodes in a specific regions because a node’s
address is based on geographic information instead of logical numbers like IP.



Mobile Transport Layer
TCP is Internet’s connection-oriented transport service,offering reliable, inorder,byte-stream data-
transfer to applications.
 Traditional TCP (Transmission control protocol)
     used by several common apps/protocols
• Web/HTTP, file transfer/FTP, mail/SMTP,…
     state-full protocol
• com.-parties exchange info prior to data-transfer
• negotiation/set-up procedure (―handshaking‖)
– resource reservation, window parameters, seq.-numbers,…
     stream-oriented
• byte-streams (from apps) divided into distinct TCP-segments
     ―network friendly‖
• mechanisms to slow down senders transfer-rate
if network congested
     reliable, in-order delivery
• CRC, ACKs, retransmission,…(errors, loss,…)
• seq.-numbers (duplication control, in-order segments)

Transport protocols typically designed for
            Fixed end-systems
            Fixed, wired networks

TCP congestion control
          packet loss in fixed networks typically due to (temporary) overload situations
          router have to discard packets as soon as the buffers are full
          TCP recognizes congestion only indirect via missing acknowledgements,
             retransmissions unwise, they would only contribute to the congestion and make it
             even worse
      slow-start algorithm as reaction to congestion

TCP slow-start algorithm
       sender calculates a congestion window for a receiver
       start with a congestion window size equal to one segment
       exponential increase of the congestion window up to the congestion threshold, then linear
       increase
       missing acknowledgement causes the reduction of the congestion threshold to one half of
       the current congestion window
       congestion window starts again with one segment
TCP fast retransmit/fast recovery
      TCP sends an acknowledgement only after receiving a packet
      if a sender receives several acknowledgements for the same packet, this is due to a gap in
      received packets at the receiver
      however, the receiver got all packets up to the gap and is actually receiving packets
      therefore, packet loss is not due to congestion, continue with current congestion window (do
      not use slow-start)
Influences of mobility on TCP-mechanisms
TCP assumes congestion if packets are dropped
       typically wrong in wireless networks, here we often have packet loss due to transmission
       errors
       furthermore, 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
       however, TCP cannot be changed fundamentally due to the large base of installation in the
       fixed network, TCP for mobility has to remain compatible
       the basic TCP mechanisms keep the whole Internet together.

Indirect TCP




Indirect TCP or I-TCP segments the connection

       I-TCP splits a TCP connection into a fixed part and a wireless part at the base station.
                          Between the fixed computer and the access point, standard TCP is
                             used.
                          The access point terminates the standard TCP connection, acting as a
                             proxy. This means that the access point is now seen as the mobile host
                             for the fixed host and as the fixed host for the mobile host.
                          Between the access point and the mobile host, a special TCP, adapted
                             to wireless links, is used. However, even an unchanged TCP can
                             benefit from the much shorter round trip time, thus starting
                             retransmission much faster.
                          The correspondent host in the fixed network does not notice the
                             wireless link or the segmentation of the connection.

                             If the correspondent host sends a packet, the access point
                              acknowledges this packet. Then the access point tries to forward the
                              packet to the mobile host. If the mobile host receives the packet, it
                              acknowledges the packet. However, this ack is only used by the
                              access point.

                             If a packet is lost on the wireless link due to a transmission error, the
                              correspondent host would not notice it. However, the access point
                              tries to retransmit this packet locally to maintain reliable data
                              transport.

                              I-TCP socket and State Mitigation




Advantages:

            no changes to the TCP protocol for hosts connected to the wired Internet, millions of
             computers use (variants of) this protocol
          optimized TCP protocol for mobile hosts
          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
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 foreign agent
         
Snooping TCP




                    It works completely transparently and leaves the TCP end-to-end connection
                     intact.
                    It buffers data at the access point to perform fast local retransmission in case
                      of packet loss.
                     The foreign agent buffers all packets with destination mobile host and
                      additionally ―snoops‖ the packet flow in both directions to recognize
                      acknowledgements
                     The reason for buffering packets toward the mobile node is to enable the
                      foreign agent to perform a local retransmission in case of packet loss on the
                      wireless link.
                     The foreign agent buffers every packet until it receives an ack from the mobile
                      host.

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
        FA buffers data until it receives ACK of the MH, FA detects packet loss via duplicated
        ACKs or time-out
        fast retransmission possible, transparent for the fixed network
Data transfer from the mobile host
        FA detects packet loss on the wireless link via sequence numbers, FA answers directly with a
        NACK to the MH
        MH can now retransmit data with only a very short delay
Integration of the MAC layer
        MAC layer often has similar mechanisms to those of TCP
        thus, the MAC layer can already detect duplicated packets due to retransmissions and
        discard them .
        Advantages

      TCP’s end-to-end connection/semantics intact (no segmentation)
           • if FA crashes, standard TCP-connection still exist (CN <-> MN)
           • (only lacks benefits of snoop + local retrans.)
      . CN don’t need to be modified (TCP compatible !)
           • enhancement implemented in FA (some modific. in MN,…NACKs)
      . no ―state-handover‖ required between FAs (roaming)
           • packets buffered in FAold are not forwarded to FAnew
           • relies on CN timeout/retransmission (standard-TCP)
      . HO between snoop-TCP supported FA and non-supported FA
           • (…falls back on standard-TCP)

Disadvantages
          snooping TCP does not isolate the wireless link as good as I-TCP
          snooping might be useless depending on encryption schemes
TCP: Disconnections TCP: Disconnections

 common in mobile/wireless networks
       • Ex: MN temp. loose radio-connection to base shadowing/blocking, interference, bad
coverage,…
 TCP’s reaction on temp. disconnections: timeout -> retransmission
       • (implied that disconnect. cause packet-loss)
 Standard Standard-TCP TCP
       • retransmission timer (retrans.-interval) doubled for each unsuccessful retrans.-attempt (up
       to max. 1 min)
       • if connection returns, receiver may have to wait up to 1 min before receiving
      retransmission (+ slow-start triggered)
I-TCP TCP
      • FA must buffer more and more packets depending on duration of disconnection (-> buffer
      overflow)
Snooping TCP Snooping TCP
      MN unable to return ACKs (CN reacts according to standard-TCP)
      • if CN’s Cong.-window is large at time of disconnect (many outstanding unACKed-
      packets), FA’s buffers may overflow
Mobile TCP

Special handling of lengthy and/or frequent disconnections
prevent triggering of slow-start + reduction of Cong.-threshold
         • (on packet-loss due to temp. disconnections)
         • (+ preserve ―end-to-end‖ semantics and support FA-handovers)

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
               set sender window size to 0
               sender automatically goes into persistent mode
       old or new SH reopen the window
Advantages
            Maintains end to end semantics
            supports disconnection
            no buffer forwarding
Disadvantages
            loss on wireless link propagated into fixed network
            adapted TCP on wireless link

Fast retransmit/fast recovery

Change of foreign agent often results in packet loss
       TCP reacts with slow-start although there is no congestion
Forced fast retransmit
       as soon as the mobile host has registered with a new foreign agent, the MH sends duplicated
       acknowledgements on purpose
       this forces the fast retransmit mode at the communication partners
       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
       simple changes result in significant higher performance
Disadvantage
       only focus on problems regarding HO (not trans.-errors, disconnect,…)
       further mix of IP and TCP, no transparent approach

Transmission/time-out freezing

       focus on long periods of interruptions/disconnections (radio-channel)
       • Ex: delayed HOs, enter overcrowded cell, long period of
       interference, blocking/shadowing,…
       • (packet-loss + TCP-timeout/disconnect)
       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 now not assume a congested link
       MAC layer signals again if reconnected
Advantage
       scheme is independent of data ,improves TCP performance
Disadvantage
       TCP on mobile host has to be changed, mechanism depends on MAC layer,too much
       modifications needed .
Selective retransmission

TCP acknowledgements are often cumulative
        ACK n acknowledges correct and in-sequence receipt of packets up to n
        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
        more efficient retransmission schema
         single ACK for each packet (not ―bulks‖ of packets)
         if packet-loss, TCP only retransmits that ―selected‖ packet (…saving scarce bandwidth)
        Advantage
        much higher efficiency
Disadvantage
        more complex software in a receiver, more buffer needed at the receiver

Transaction oriented TCP

TCP phases
      connection setup, data transmission, connection release
      using 3-way-handshake needs 3 packets for setup and release, respectively
      thus, even short messages need a minimum of 7 packets!




Transaction oriented TCP

       T-TCP combines setup-/release msgs with actual data-transfer
       • reduce # msgs to min. 3
Advantage
      efficiency
Disadvantage
      requires changed TCP
      mobility not longer transparent

Comparison of different approaches for a “mobile” TCP

				
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