1 / 69

TCP Over Wireless Ad-hoc Networks

TCP Over Wireless Ad-hoc Networks. Vinay Sridhara vsridhar@usc.edu Nagendra Subramanya nsubrama@usc.edu. Wired Vs Wireless.

josie
Télécharger la présentation

TCP Over Wireless Ad-hoc Networks

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. TCP Over Wireless Ad-hoc Networks

  2. Vinay Sridhara vsridhar@usc.edu Nagendra Subramanya nsubrama@usc.edu

  3. Wired Vs Wireless • In wired LANs, an address is equivalent to a physical location. This is implicitly assumed in the design of the wired LANs. In wireless, the addressable unit is the station. The station is a message destination, but not a fixed location. • Use a medium that has neither absolute nor readily observable boundaries outside of which stations with conformant physical transceivers are known to be unable to receive network frames

  4. Continued.. • They are unprotected from outside signals . • Have dynamic topologies. • They lack full connectivity and therefore the assumption that one station can hear every other station is invalid. • They have time varying and asymmetric propagation properties.

  5. 802.11 • Main purpose is to provide the MAC and physical layer specification for wireless. • Permits the operation of an IEEE 802.11 conformant device within a wireless local area network that may coexist with multiple overlapping IEEE 802.11 conformant devices. • Describes the requirements and procedures to provide privacy of user information being transferred over the wireless medium and authentication of IEEE 802.11 conformant devices.

  6. 802.11 in ISO/OSI Model.

  7. A few terminologies of 802.11 802.11 Components • STA – Any mobile, portable or stationary terminal. • BSS – Basic service set. • The BSS is an entity which consists of several mobile stations that can interact with each other. BSS1 STA1 STA2 STA1 BSS2 STA2

  8. Continued. • A BSS maybe comprised of only two mobile stations. • This is called a IBSS (Independent Basic Service Set). • This is the one that is called Ad – Hoc Networks. • A network composed solely of stations within mutual communication range of each other via the wireless medium • network is typically created in a spontaneous manner • The principal distinguishing char of an Ad Hoc network is its limited temporal and spatial extent

  9. Continued. • Instead of existing independently, multiple BSSs may form a network. • The architectural component used to interconnect BSSs is the Distribution system (DS). • The access points are the stations used to connect BSSs to DS.

  10. Continued. STA1 STA-C STA1’ • BSSs may generally overlap to provide continuous coverage in physical volume. • The BSSs could be physically disjoint. Logically there is no limit to the distance between BSSs. STA1 STA2 STA1’ STA2’

  11. Continued. • ESS - The DS and the BSSs allow the IEEE 802.11 to create a wireless network of arbitrary size and complexity. • Stations within an ESS may communicate and Mobile stations may move from one BSS to another transparently to LLC.

  12. Extended Service Set • The stations communicate to Access points which are part of a Distribution System • Access point serves the stations in a BSS The set of BSSs are called Extended Service Set (ESS)

  13. Communication with external world • Portal – Is the logical point at which the MSDUs enter and leave the IEEE 802.11 network. • This portal provides logical integration between the IEEE 802.11 architecture and the existing wired LANs • The stations act as both AP and portal when the DS is a wired network.

  14. The Big Picture

  15. Services • The services provided by the 802.11 are divided into two. • Those that are part of every STA, called Station services • Authentication • De-Authentication • Privacy • MSDU Delivery • The services provided by the DS are known as the Distribution system service. • Association • Disassociation • Distribution • Integration • Reassociation

  16. MAC layer Services • Services provided by the MAC layer • Security Services • Security is provided by the WEP (Wired equivalent privacy) • The security provided is limited to Station-to-Station exchange of data • The privacy service offered by the IEEE 802.11 WEP is the encryption of the MSDU • The security services provided are • Confidentiality • Authentication • Access control in conjunction with layer management.

  17. Continued. • Asynchronous data services • Provided by the following three primitives. • MA-UNITDATA.request – generated by the LLC when an MSDU is to transmitted to the peer LLC. The format is as show below. MA-UNITDATA.request { Source address, Destination address, Routing information, Data, Priority, Service class }

  18. MA-UNITDATA.indication – generated by the MAC sublayer entity to indicate to the LLC the arrival of a MSDU. MA-UNITDATA.indication { Source address, Destination address, Routing information, Data, Reception status, Priority, Service class }

  19. MA-UNITDATA-STATUS.indication - provides the LLC sublayer with status information of the corresponding MA-UNITDATA.request primitive. MA-UNITDATA-STATUS.indication { Source address, Destination address, Transmission status, Provided priority, Provided service class }

  20. MAC header • The four address fields in the MAC header are used to indicate the BSSID, Source address, destination address, transmitting station address and the receiving station address. • The frame control field has the following format

  21. Collision Avoidance - 802.11 • 802.11 uses a protocol scheme know as carrier-sense, multiple access, collision avoidance (CSMA/CA). • Avoidance scheme is used because it is difficult to detect collisions in the RF media. • Hence is interoperates with the physical layer by sampling the transmitted energy over the medium transmitting data. • The physical layer uses an algorithm for clear channel assessment (CCA) to determine if the channel is clear. • If the received signal is below a certain threshold the channel is declared clear.

  22. Station A Station B Station C Why Do We Need RTS/CTS ? • Used because of the Hidden terminal problem. • It occurs when there is a station in a service set that cannot detect the transmission of another station, and thus cannot detect that the media is busy

  23. What Does RTS/CTS Do? • Communications is established when one of the wireless nodes sends a short message RTS frame. • The message duration is known as the network allocation vector (NAV). This alerts to back off during the transmission time. • The receiving station issues a CTS frame which echoes the senders address and the NAV. • If the CTS frame is not received, it is assumed that a collision occurred and the RTS process starts over.

  24. Characteristics of Wireless Networks • Limited bandwidth • High latencies and bit-error rates • Mobility

  25. Difference between General wireless networks and Ad-Hoc Networks ? • Absence of a central base station • Frequent route re-computation • Network partitions • Multi-path routing algorithms

  26. What is an Ad-Hoc Network? • AD-HOC network is a collection of mobile nodes with wireless network infrastructure capable of organizing themselves into a temporary network without the aid of any centralized network manager. • According to the IETF definition, a mobile Ad Hoc network is an autonomous system of mobile routers (and associated hosts) connected by wireless links--the union of which form an arbitrary graph.

  27. SOURCE DESTINATION AD-HOC Networks

  28. Implications of using Normal TCP over Ad-Hoc Wireless N/W • TCP does not distinguish between congestion and packet loss due to transmission errors and route failures • This inability results in performance degradation in ad hoc networks

  29. Implications of using Normal TCP over Ad-Hoc Wireless N/W • Route re-computation takes a finite amount of time • During this time no packet can reach the destination through the existing route • Packets and ACKs may get queued and possibly dropped • In turn leads to timeouts at the source, which is misinterpreted as congestion

  30. Consequences • Source retransmits unACKed packets • Invokes congestion control • Enters slow start recovery • These are undesirable because? • Why retransmit when there is no route • Retransmission wastes power and bandwidth • Low throughput as a result of slow start recovery after route restoration (this is actually desirable. Why?)

  31. Why use TCP at all in such cases ? • For seamless portability to applications like file transfer, e-mail and browsers which use standard TCP

  32. Different approaches to improve the performance of TCP over Ad-Hoc Networks • Last hop wireless networks • I-TCP • Snoop TCP • M-TCP • Freeze-TCP • Ad hoc networks • TCP-F • ELFN • ATCP

  33. Indirect TCP (I-TCP) Connection between end points split into separate connections • No need of changes in TCP on wired hosts • Transmission errors on wireless link not propagated to wired network • Different optimal transport layer protocol between BS and MH • Loss of E2E semantics of TCP • Increased hand-off latency • Copying overhead

  34. Snoop TCP BS caches packets from sender Performs local re-transmissions • Preserves E2E TCP semantics • Only BS needs to be changed • Doesn’t isolate behavior of wireless link as good as I-TCP • MH may need to be modified to accept NACKs • Snooping and caching may fail if E2E encryption schemes are used

  35. M-TCP Same as I-TCP, but the BS is known as SH SH holds back ACK to the last byte Uses this ACK to freeze the sender when disconnection is detected • Maintains E2E semantics • Avoids useless re-transmissions and slow-starts • SH doesn’t buffer data • Needs a bandwidth manager to implement fair sharing over the wireless link

  36. Freeze-TCP The client freezes the sender when it detects possible hand-off or predicts a temporary disconnection • No need of base station • Only mobile client’s TCP needs to be changed • Can be used with encrypted data • MAC has to detect future interruptions • Freezing fails when encryption scheme uses timestamps • Re-transmission restarted with old CWND

  37. SOURCE DESTINATION X Network state after a while

  38. TCP-Feedback (TCP-F) K. Chandran, S. Raghunathan, S. Venkatesan and R. Prakash

  39. TCP-F: Introduction • ECN is not used. Why? • Instead uses Route Failure Notification (RFN) packet to inform source when route is disrupted • And Route Reestablishment Notification (RRN) packet informs the source when route is reestablished

  40. TCP-F: Protocol • Failure Point (intermediate node) detects the route disruption • Explicitly sends RFN to source and records the event • Each intermediate node that receives the RFN: • Invalidates the particular route • If it knows of an alternate route, that route is used for further communication and RFN is discarded • Else, RFN is propagated towards source

  41. TCP-F: Protocol (contd.) • On receiving RFN, source goes into “snooze” state

  42. TCP-F: Protocol (contd.) • Source when in “snooze state” • Stops sending further packets • All existing timers are marked invalid • Send window and other state variables (RTO, etc) are frozen • Starts a route failure timer whose timeout = worst case route reestablishment time. Why? • Stays in this state till it receives an RRN packet

  43. TCP-F: Protocol (contd.) • One of the intermediate nodes that had previously forwarded RFN learns about a new route • Sends an RRN to the source • Further RRNs received by this node for the same source-destination pair are discarded • Other nodes simply forward RRN towards the source • Source on receiving RRN • Changes to active state • Flushes out all unACKed packets in its current window

  44. TCP-F: Conclusion • Communication resumes at the same rate as before the route failure occurred • There is no unnecessary loss of throughput • Is this ok? • Use of an additional packet RRN. Is it really needed? • Simulation environment • Based on a simple one-hop network • Links failed/recovered according to an exponential model • Routing protocol was not simulated

  45. TCP-F: Conclusion • Overhead on routers • Detect route failures and reestablishments • Provide feedback to the source • Store the source id after forwarding an RFN so that it can send an RRN on finding a route • The paper does not discuss about multiple flows • Enhancements • Buffering at intermediate nodes

  46. TCP with Explicit Link Failure Notification (ELFN) G. Holland and N. Vaidya

  47. ELFN: Introduction • DSR complicates the situation – stale routes may be cached and propagated • Throughput is degraded • Turning of caching works only with single TCP connection • What if there are more sources?

  48. ELFN • Similar to TCP-F • “Expected throughput” – throughput of a static wireless network • ELFN is piggy-backed to DSR’s route failure message • ELFN has a payload similar to that of “host unreachable” ICMP message

  49. ELFN: Protocol • TCP sender receives ELFN • Disables congestion control mechanism, enters “stand-by” mode • Sends probes • On receiving an ACK for a probe, leaves “stand-by” mode

  50. ELFN: Variations • Time interval between probe packets • greater it is, slower is the route discovery • smaller it is, causes congestion • best would be to have it as a function of RTT

More Related