Lecture 5

1. Lecture 5 Advance Topics in Networking

2. Overview Introduction to Multi-hop Ad hoc Networks Bluetooth Piconets and Scatternets Mobile IP TCP for Wireless Indirect TCP Snooping TCP Mobile TCP Transaction oriented TCP

3. Introduction (Multihop Ad hoc networks) Mobile Multi-hop Ad Hoc Networks are collections of mobile nodes connected together over a wireless medium. These nodes can freely and dynamically self-organize into arbitrary and temporary, “ad-hoc” network topologies, allowing people and devices to seamlessly internetwork in areas with no pre-existing communication infrastructure, (e.g., disaster recovery environments).

4. Introduction (Single-hop Ad hoc Networks) Multi-hop ad hoc networking is not a new concept having been around for over twenty years, mainly exploited to design tactical networks. Recently, emerging wireless networking technologies for consumer electronics are pushing ad hoc networking outside the military domain. The simplest ad hoc network is a peer-to-peer network formed by a set of stations within the range of each other that dynamically configure themselves to set up a temporary single-hop ad hoc network.

5. Introduction (Bluetooth Piconets) Bluetooth piconet is the most widespread example of single-hop ad hoc networks. 802.11 WLANs can also be implemented according to this paradigm, thus enabling laptops’ communications without the need of an access point. Single-hop ad hoc networks just interconnect devices that are within the same transmission range. This limitation can be overcome by exploiting the multi-hop ad hoc paradigm.

6. Introduction (MANETs) In this new networking paradigm, the users' devices must cooperatively provide the functionalities that are usually provided by the network infrastructure. Nearby nodes can communicate directly by exploiting a single-hop wireless technology (e.g., Bluetooth, 802.11, etc.), while devices that are not directly connected communicate by forwarding their traffic via a sequence of intermediate devices. Generally, these users’ devices are mobile, therefore these networks are often referred to as Mobile Adhoc NETworks (MANETs).

7. Introduction (MANETs) In Being completely self organizing, MANETs are attractive for specialized scenarios like disaster recovery, vehicle-to-vehicle communications, and home networking. Unfortunately, nowadays they have a very limited penetration as a network technology for mass-market deployment. To turn mobile ad hoc networks in a commodity, we should move to a more pragmatic scenario in which multi-hop ad hoc networks are used as a flexible and “low cost” extension of Internet.

8. Introduction (Mesh Networks) Indeed, a new class of networks is emerging from this view: The mesh networks Unlike MANETs, where no infrastructure exists and every node is mobile, in a mesh network there is a set of nodes, the mesh routers, which are stationary and form a wireless multi-hop ad hoc backbone.

9. Introduction Some of the routers are attached to the Internet, and provide connectivity to the whole mesh network. Mesh routers are not users’ devices but they represent the infrastructure of a mesh. Routing protocols running on mesh routers allow the backbone to be self configuring, self healing, and easy to set up. Client nodes connect to the closest mesh router, and use the wireless ad hoc backbone to access the Internet.

10. Introduction (Opportunistic Networking) Mesh networks are moving multi-hop ad hoc networks from emergency-disaster-relief and battlefield scenarios to the main networking market. While mesh networks represent a short-term direction for the evolution of MANETs, opportunistic networking constitutes a long-term direction for the evolution of the ad hoc networking concept. The bottom line of this paradigm is providing end-to-end communication support also to very dynamic ad hoc networks, in which users disconnection is a feature rather than an exception.

11. Introduction (Opportunistic Networking) Nodes can be temporarily disconnected and/or the networks can be partitioned, and the mobility of nodes create the communication opportunities. The main idea is thus to opportunistically exploit, for data delivery, nodes’ mobility and contacts with other nodes/networks. In opportunistic networks the communication is still multi-hop, with intermediate nodes acting as routers but, in this case, forwarding is not necessarily “on-the-fly”.

12. Introduction Intermediate nodes store the messages when no forwarding opportunity exists (e.g., no other nodes are in the transmission range, or neighbors are not suitable for communication), and exploit any contact opportunity with other mobile devices to forward the data toward the destination. In this view, the existence of a simultaneous path between sender and receiver is not mandatory (as in traditional MANET) to communicate.

13. Introduction This networking paradigm is well suited for a world of pervasive devices equipped with various wireless networking technologies (802.11 family, Bluetooth, ZigBee, etc.) which are frequently out of range from a global network but are in the range of other networked devices, and sometime cross areas where some type of connectivity is available (e.g. Wi-Fi hotspots).

14. Introduction Among multi-hop ad hoc networks, wireless sensor networks have a special role. A sensor network is composed by a large number of small sensor nodes, which are typically densely (and possibly randomly) deployed inside the area in which a phenomenon is being monitored. Wireless multi-hop ad hoc networking techniques constitute the basis for sensor networks, too.

15. Introduction However, the special constraints imposed by the unique characteristics of sensing devices, and by the application requirements, make the solutions designed for multi-hop wireless networks (generally) not suitable for sensor networks. First of all, power management is a “pervasive” issue in the overall design of a sensor network. Sensor networks utilize on-board batteries with limited energy that cannot be replenished in most application scenarios.

16. Introduction Furthermore, sensor networks produce a shift in the networking paradigm from a node-centric to a data-centric view. The aim of a sensor network is to collect information about events occurring in the sensor field rather than supporting the communications between users’ devices. Multi-hop ad hoc network technologies have big potentialities for innovative applications of great impact on our everyday life.

17. Introduction (Research direction) However, after almost a decade of research, ad hoc networking technologies are rarely used and have not yet affected our way of using wireless networks. It is believed that this is due to a wrong approach in the research, which was dominated by simulation modeling and theoretical analyses with only few attempts to build network prototypes to understand how well MANETs work in reality.

18. Introduction (Research direction) In the last few years, this stimulated a new community of researchers combining theoretical research on ad hoc networking with experiences/measurements obtained by implementing ad hoc network prototypes.

19. Bluetooth Piconets and Scatternets

20. Piconets and Scatternets Piconet Basic unit of Bluetooth networking Master and one to seven slave devices Master determines channel and phase Scatternet Device in one piconet may exist as master or slave in another piconet Allows many devices to share same area Makes efficient use of bandwidth

21. Wireless Network Configurations

22. Network Topology Piconet = set of Bluetooth nodes synchronized to a master node The piconet hopping sequence is derived from the master Scatternet = set of piconet Master-Slaves can switch roles A node can only be master of one piconet. Why? Piconet 1 Piconet 2 Slave Master Master Scatternet

23. Frequency Hopping Total bandwidth divided into 1MHz physical channels FH occurs by jumping from one channel to another in pseudorandom sequence Hopping sequence shared with all devices on piconet Piconet access: Bluetooth devices use time division duplex (TDD) Access technique is TDMA FH-TDD-TDMA

24. Scatternets Each piconet has one master and up to 7 slaves Master determines hopping sequence, slaves have to synchronize Participation in a piconet = synchronization to hopping sequence Communication between piconets = devices jumping back and forth between the piconets piconets

25. Mobile IP

26. Motivation for Mobile IP Routing based on IP destination address, network prefix (e.g. 129.13.42) determines physical subnet change of physical subnet implies change of IP address to have a topological correct address (standard IP) or needs special entries in the routing tables Specific routes to end-systems change of all routing table entries to forward packets to the right destination does not scale with the number of mobile hosts and frequent changes in the location, security problems Changing the IP-address adjust the host IP address depending on the current location almost impossible to find a mobile system, DNS updates take too much time TCP connections break, security problems

27. Mobile IP Requirements Transparency mobile end-systems keep their IP address continuation of communication after interruption of link possible point of connection to the fixed network can be changed Compatibility support of the same layer 2 protocols as IP no changes to current end-systems and routers required mobile end-systems can communicate with fixed systems Security authentication of all registration messages Efficiency and scalability only little additional messages to the mobile system required (connection typically via a low bandwidth radio link) world-wide support of a large number of mobile systems in the whole Internet

28. Terminology Mobile Node (MN) system (node) that can change the point of connection to the network without changing its IP address Home Agent (HA) system in the home network of the MN, typically a router registers the location of the MN, tunnels IP datagrams to the COA Foreign Agent (FA) system in the current foreign network of the MN, typically a router forwards the tunneled datagrams to the MN, typically also the default router for the MN Care-of Address (COA) address of the current tunnel end-point for the MN (at FA or MN) actual location of the MN from an IP point of view can be chosen, e.g., via DHCP Correspondent Node (CN) communication partner

29. Example network HA MN Internet router home network mobile end-system (physical home network for the MN) FA foreign network router (current physical network for the MN) CN end-system router

30. Data transfer to the mobile HA 2 MN Internet home network 3 receiver foreign network FA 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 1 CN sender

31. Data transfer from the mobile HA 1 MN Internet home network sender FA foreignnetwork 1. Sender sends to the IP address of the receiver as usual, FA works as default router CN receiver

32. TCP for Wireless Networks

33. Motivation 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 Routers discard packets as soon as the buffers are full TCP recognizes congestion only indirectly via missing acknowledgements Retransmissions unwise, they would only contribute to the congestion and make it even worse Slow-start algorithm as reaction

34. TCP Slow Start 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

35. TCP Fast Retransmit/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)

36. Influences of mobility on TCP 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

37. Indirect TCP (1) Indirect TCP or I-TCP segments the connection 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 mobile host wired Internet access point (foreign agent) standard TCP “wireless” TCP

38. I-TCP socket and state migration access point2 socket migration and state transfer Internet access point1 mobile host

39. Indirect TCP (2) 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 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

40. Snooping TCP (1) 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 (+min. MH change) correspondent host local retransmission foreign agent „wired“ Internet snooping of ACKs buffering of data mobile host end-to-end TCP connection

41. Snooping TCP (2) 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 Advantages: Maintain end-to-end semantics No change to correspondent node No major state transfer during handover Problems Snooping TCP does not isolate the wireless link well May need change to MH to handle NACKs Snooping might be useless depending on encryption schemes

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

43. Mobile TCP

44. Transaction oriented TCP TCP phases connection setup, data transmission, connection release using 3-way-handshake needs 3 packets for setup and 3 for release, respectively thus, even short messages need a minimum of 7 packets! Transaction oriented TCP RFC1644, T-TCP, describes a TCP version to avoid this overhead connection setup, data transfer and connection release can be combined thus, only 2 or 3 packets are needed Advantage efficiency Disadvantage requires changed TCP mobility no longer transparent

45. References Multi-hop Ad hoc Networks from Theory to Reality Editors: Marco Conti (Inst for Informatics and Telematics, Pisa Italy) ; Jon Crowcroft and Andrea Passarella (Univ. of Cambridge) https://www.novapublishers.com/catalog/product_info.php?products_id=5556

46. Q&A • ?

47. Assignment Write Note on Fast TCP What factors can degrade the performance of Fast TCP? What are the problems of using Fast TCP over Multi-hop Ad hoc networks? How can Fast TCP be implemented in practical? Write note on text colored in Green in this slides

48. The End Questions?