Wireless Networks and Mobile Computing (COMP 5304)

1. Mobile Transport Layer Mohammad Reza Pasandideh mpasandi@connect.carleton.ca Carleton University, Department of SCE , Jan 2009 Wireless Networks and Mobile Computing (COMP 5304)

2. Content Introduction OSI Model vs. TCP/IP Model Transport Layer Challenges TCP/IP Protocol Overview Slow Start & congestion Avoidance TCP fast retransmit/fast recovery TCP sender congestion control TCP Protocol in Wireless Medium TCP Improvements TCP Last-HOP Enhancements Possible Adaptations Split Connection (Indirect TCP) Snooping TCP Explicit Loss Notification (ELN) Mobile TCP Forced Fast retransmit/fast recovery

3. Content Link Level Retransmission Transmission/time-out freezing Selective retransmission Transaction oriented TCP Comparison of different approaches TCP Improvements Cross Layer solutions Seamless Service Continuity SCTP Protocol Multi-homing SHIM6 Host Identity Protocol Conclusion Questions References

4. Introduction • Optimization of transport layer protocols and their performance has big impact on data transmission • Optimization and performance improvement of transport layer protocols, specially TCP is among highly interesting topics among academic researches. • Several techniques have been proposed and implemented. • Traditional TCP is not suitable for wireless medium. • Joint layer optimization gains more benefits. (Cross Layer Design) • Transport Layer survivability is important issue for a reliable network . • SCTP , SHIM6 and HIP are key solutions for a redundant network.

5. OSI Model vs. TCP/IP Model • Transport Layer defines three main standard transport protocols: • TCP implements a reliable data-stream protocol (connection oriented) • UDP implements an unreliable data-stream (connectionless) • SCTP implements some new features • -Both connection oriented and connectionless • -Multi-homing

6. Transport Layer Challenges • TCP is optimized for wired networks • Assumption • Packet loss is mostly caused by congestions • Congestion Control and Avoidance • drastic reduction of packet rate and slow start • larger timeout intervals for retransmissions • Assumption is not valid for wireless networks • Much higher packet error rates • packet losses during handovers and disconnections • Bundled packet losses • Necessary reaction: • fast error recovery • no reduction of packet rate • Result in case of un-adapted TCP for wireless networks: • Data rates far below possible data rate • Slow error recovery(high response times)

7. TCP/IP Protocol Overview • Provides a reliable, in-order abstraction • Recover lost packets and detect/drop duplicates • Detect and drop corrupted packets • Full-duplex: bi-directional data flow in same connection • Flow and congestion control • Flow control: sender will not overwhelm receiver • Congestion control: sender will not overwhelm the network • Sliding window flow control • Send and receive buffers • Congestion control done via adaptive flow control window size • TCP/TP Model defined by IETF in RFC 1122 • TCP tries to trade off efficiency and being a “Good Network Citizen” • Good Network Citizen: Judged by performance, reliability, availability[1] • Vast majority of bytes transferred on Internet today are TCP based: • Web, Mail, Peer-to-peer (Napster, Gnutella, FreeNet, KaZaa)

8. TCP protocol Connection Establishment (3-way handshake) 1) Client sends SYN with initial sequence number ( ISN = X) 2) Server responds with its own SYN w/seq number Y and ACK of client ISN with X+1 (next expected byte) 3) Client ACKs server's ISN with Y+1

9. TCP/IP, Slow Start • Sender may not only overrun receiver, • but may also overrun intermediate routers: • No way to explicitly know router buffer occupancy, • so infer congestion from packet losses. • Assumption :losses is due to congestion, • i.e. intermediate routers have no available buffers • Sender maintains a congestion window: • Max CW size is un-acknowledged data outstanding. • Successive ACKs from receiver cause CW to grow. • How CW grows based on which of 2 phases: • Slow-start: initial phase, CW exponentially grows • Continue up to threshold • Congestion avoidance: CW steady grows

10. Slow Start & congestion Avoidance • TCP slow-start algorithm • Sender calculates a congestion window for a receiver and a congestion threshold (i.e. 65535) • Start with a congestion CW=1 Segment • Exponential increase of the CW up to the congestion threshold. • Linear increase of the CW after threshold (Congestion avoidance) • Missing acknowledgement causes the reduction of the congestion threshold to one half of the current CW • CW =1 segment • Repeat

11. Slow Start & congestion Avoidance • TCP slow-start and Congestion control algorithm • Sender calculates a congestion window for a receiver and a congestion threshold (i.e. 65535) • Start with a congestion CW=1 • Exponential increase of the CW up to the congestion threshold. • Linear increase of the CW after threshold (Congestion avoidance) • Missing acknowledgement causes the reduction of the congestion threshold to one half of the current CW (Additive increase, Multiplicative decrease) • Set CW =1 segment & Repeat

12. TCP fast retransmit/fast recovery • TCP fast retransmit/fast recovery • TCP sends an acknowledgement only after receiving a packet. • If a sender receives triple 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. CW and congestion threshold is set to one half of the current CW, followed by congestion avoidance .(Do not use slow-start)

13. TCP sender congestion control

14. TCP Protocol in Wireless Medium • TCP on wireless works inefficiently! • On wire links, TCP assumes losses if congestion happens. • On wireless links, TCP assumes congestion if packets are dropped • Typically wrong in wireless networks, here we often have packet loss due to transmission errors. • Furthermore, mobility (i.e. handoff) 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. • However, TCP cannot be changed fundamentally due to the large base of installation in the fixed network. • TCP for mobility has to remain compatible. 14

15. TCP Improvements It is difficult to create a “one size fits all” TCP for last-hop wireless networks enhancements upon application area. B. Sardar and D. Saha, “A survey of TCP Enhancements for last-hop wireless networks”, IEEE Comm. Surveys and Tutorials, 2006.

16. TCP Last-HOP Enhancements • Classification of available solutions for last-hop wireless enhancement B. Sardar and D. Saha, “A survey of TCP Enhancements for last-hop wireless networks”, IEEE Comm. Surveys and Tutorials, 2006.

17. Possible Adaptations • Where in the path from sender to receiver to solve problem? •  Split connection • One TCP connection from wired end to base, another from base to wireless end • Problem solved locally • But solved at TCP layer (more semantics) •  End-to-end • Sender and receiver together addresses problem • They address congestion loss • They should also address medium loss •  Link layer addresses problem • Problem occurred in the link and thus should be solved there.

18. Split Connection (Indirect TCP) • Break a single TCP connection from wired end to wireless end into: • TCP connection from wired end to base station • TCP connection from base station to wireless end • The wireless connection performs all the retransmissions in response to wireless losses. • Two TCP stacks in the base station. • Hosts in fixed part of network do not notice the characteristics of the wireless part.

19. Split Connection (Indirect TCP) • 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. • 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 foreign agent

20. Snooping TCP • “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

21. Snooping TCP (Cont’) • 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. • Problems • Snooping TCP does not isolate the wireless link as good as I-TCP • Snooping might be useless depending on encryption schemes

22. Explicit Loss Notification (ELN) • This is an end-to-end approach. • Adds an Explicit Loss Notification (ELN) option to TCP acknowledgements. • When packet dropped over wireless, Subsequent acknowledgements indicate non-congestion related loss occurred. • Upon receiving this information with duplicate acknowledgements, the sender may perform retransmissions without invoking the associated congestion-control procedures. • Potential Problems with End-to-End • Un-necessary duplicate acknowledgements sent all the way to source • Un-necessary retransmissions from source to destination

23. Mobile TCP- Performance Management • Special handling of lengthy and/or frequent disconnections • M-TCP splits as I-TCP does • The mobile host must discover an interface switch or a handoff and notify the correspondent host. Then both endpoints are aware that subsequent observed losses of the data in transit can be attributed to mobility rather than congestion. • The sender marks the data that it is in its retransmit queue. A lost packet in this range triggers retransmission, but not congestion control mechanism. • Upon an interface switch, the sender resets the congestion window, slow start as though it were initiating a new connection. • Advantages • maintains semantics, supports disconnection, no buffer forwarding • Disadvantages • loss on wireless link propagated into fixed network • adapted TCP on wireless link • Not a good idea for heavy traffic.

24. Mobile TCP (Cont’) • Movement detection & notification • When IP endpoints at MH changes, the changing its address is trapped by the kernel device control code. • All the sockets that use the affected interface are notified, and a switch detection routine is invoked. • MH notifies the CH via an option field in the next MTCP packet. • If operating on top of IPv6, MTCP can make use of IPv6 CH notification mechanism instead of using a special option. • Connection management • MTCP preserves end-to-end semantics by dynamically changing end-point address for ongoing connections. • Disadvantage • Malicious host to “hijack” an ongoing TCP connection • Authorization mechanism is suggested

25. Forced 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 • Further mix of IP and TCP, no transparent approach

26. Link Level Retransmission • Handle the problem at the link level, that is where the loss occurred. Local retransmission instead of end-to-end retransmission • Link-level timer much smaller (~20ms) • TCP timers larger (multiples of 500 ms) • Depends on end-to-end delay • Take advantage of the smaller link-level timer; it can be used to retransmit several times before the TCP timer goes off. • Such as HSDPA • Potential problems: • “Incompatible” timers cause retransmission by both parties. • Competing retransmissions by the link and transport layers • often lead to significant performance degradation. • Packets may reach TCP receiver out-of-order.

27. Transmission/time-out freezing • Mobile hosts can be disconnected for a longer time • No packet exchange possible, e.g., in a tunnel, disconnection due to overloaded cells or mix 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 now not assume a congested link • MAC layer signals again if reconnected • Advantage • Scheme is independent of data • Disadvantage • TCP on mobile host has to be changed, mechanism depends on MAC layer

28. 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 • RFC2018 allows for acknowledgements of single packets, not only acknowledgements of in-sequence packet streams without gaps • sender can now retransmit only the missing packets • Advantage • much higher efficiency • Disadvantage • more complex software in a receiver, more buffer needed at the receiver

29. Transaction oriented TCP • TCP phases • Connection setup, data transmission, connection release • Using 3-way-handshake needs 3 packets for setup and release, respectively • 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 • Only 2 or 3 packets are needed • Advantage • Efficiency • Disadvantage • Requires changed TCP • Mobility not longer transparent

30. Comparison of different approaches for a “mobile” TCP

31. TCP Improvements • Initial research work • Indirect TCP, Snoop TCP, M-TCP, T/TCP, SACK, Transmission/time-out freezing, … • TCP over 2.5/3G wireless networks • Learn to live with • Data rates: 64 kbit/s up, 115-384 kbit/s down; asymmetry: 3-6, but also up to 1000 (broadcast systems), periodic allocation/release of channels • High latency, high jitter, packet loss • Fine tuning today’s TCP, Suggestions • Large (initial) sending windows, large maximum transfer unit, selective acknowledgement, explicit congestion notification, time stamp, no header compression • Already in use • i-mode running over FOMA • WAP 2.0 (“TCP with wireless profile”) • max. TCP BandWidth • Max. Segment Size • Round Trip Time • loss probability

32. TCP Improvements, Cross Layer Designs • Cross-Layer approaches violate the traditional layer architecture • -> Creation of new interfaces • -> Merging of adjacent layers • -> Sharing of variables and parameters among multiple layers • “Although layered architectures have served well for wired networks, they are not suitable for wireless networks” • Cross-layer design advantages • Exploits the interactions between layers • Promotes adaptability at all layers based • on information exchange between layers • Cross-layer design disadvantages • Hard to characterize the interactions • between protocols at different layers • Joint optimization across layers may • lead to complex algorithms • Potential to destroy modularity

33. Cross Layer solutions for TCP improvement • Freeze TCP • To implement Freeze-TCP, the network stack needs to be aware of mobility (at least to some extent). In essence, some cross-layer (layers of the protocol stack) efforts and information exchange are needed. [3] • TCP_CL for IEEE 802.11 networks • By making slight modifications to the legacy IEEE 802.11 MAC and TCP • protocols, TCP-CL provides a significant improvement in the performance of TCP in multi-hop wireless environments. [4] • TCP enhancement scheme for OFDM networks • Link recovery scheme combined with Freeze-TCP based on sub-carrier reallocation and AMC can improve the performance of TCP greatly in OFDM networks. [5]

34. Seamless Service Continuity • The 3GPP defines seamless service continuity as a transition between two wireless networks without user intervention and with minimal service disruptions (i.e., packet loss, etc.). • This transitioning between wireless networks is what is known as handover. • Requirements for a mobility enabled transport protocol • The transport connection continues while changing the associated IP address (Multi-homing) • The transport connection automatically switch over to a new IP address when the used path fails. • IP addresses can be added to or removed from the transport connection without affecting the established association.

35. SCTP Protocol • SCTP (Stream Control Transport Protocol) was developed to provide higher reliability for signaling transport over the Internet. • Key feature of SCTP is the multi-homing capability allowing immediate switch-over to a alternative path when anything fails. • Automatic switch-over when a link breaks is exactly the function needed for hand-over. • Dynamic establishment of new associations (adding new IP addresses - ADDIP) allows unlimited mobility.

36. Multi-homing • Objectives • Redundancy • Load sharing • Performance • Policy constraints • How • A host or a site has several network connections through: • - Several internet service providers (ISPs) • - Multiple network interfaces • - Multiple internet address

37. Multi-homing Types • Site Level Multi-homing • Host Level Multi-homing

38. Multi-homing Problem & Solution • Problem • Network connection dynamically changes • Dynamic changes of the IP address will disturb the live sessions of the transport-layer protocols. • Transport Layer survivability is not supported • Solutionshould satisfy the following: • During the dynamic changes, the established sessions should not be disturbed. • Sessions at transport layer should not be aware of routes changes at the IP layer • Shim6: Site Multi-homing by IPv6 Intermediation • HIP: Host Identity Protocol

39. SHIM6 Site Multi-homing by IPv6 Intermediation (SHIM6) • Shim6 is a protocol being developed in the Internet Engineering Task Force (IETF) • Provides a host-based solution to allow IPv6 sites to multi-home for load sharing, redundancy, and fault tolerance. • Shim6 uses the initial IP address as an endpoint identity at the transport layer, and multiple locators at the IP layer to perform multi-homing. • The protocol includes components for securely binding locator sets together to prevent address hijacking

40. Host Identity Protocol (HIP) • Host Identity Protocol (HIP) • Transport protocols are bound to IP addresses and disconnect when an address changes. • HIP defines a new global Internet name space. • HIP uses a defined public key at the transport • layer as an endpoint identity, and multiple • locators at the IP layer to perform multi-homing. • With HIP, the transport layer operates on Host • Identities instead of using IP addresses as • endpoint names. • At the same time, the network layer uses IP • addresses.

41. Conclusion • Traditional TCP is not optimized for wireless environment • Improvement is required on transport layer protocols, mainly TCP to achieve higher performance. • End-to-End, Split connection and Link layer solutions investigated. • Versus layers joint optimization gains more benefits. • Cross Layer Design exploit dependency between stack layers to achieve some performance gains • SCTP is a general purpose reliable transport protocol for the Internet, for real-time and non real-time traffic. • Multi-homing is achieved by Shim6 and Hip.

42. Questions • 1) Why TCP on wireless works inefficiently? • On wireless links, TCP assumes congestion if packets are dropped due to transmission errors. Furthermore, mobility (i.e. handoff) itself can cause packet loss. For example if 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. • 2) List three methods of TCP optimization solutions. • 1)Indirect TCP 2)Snooping TCP 3) Mobile-TCP • 4) Fast retransmit/ fast recovery 5) Transmission/ time-out freezing • 6) Selective retransmission 7)Transaction oriented TCP • 3) What is multi-homing? Explain its two famous solutions. • Multi-homing is a technique to increase the reliability of the Internet connection for an IP network. Multi-homing is generally used to eliminate network connectivity as a potential single point of failure. Multi-homing can be in site or host level. • Multi-homing solutions: • -Site Multi-homing by IPv6 Intermediation (Shim6)uses the initial IP address as an endpoint identity at the transport layer, and multiple locators at the IP layer to perform multi-homing. • -HIP(Host Identity Protocol)uses a defined public key at the transport layer as an endpoint identity, and multiple locators at the IP layer to perform multi-homing.

43. References [1] Douligeris,N. and N. Serpanos, D., “Network Security: Current Status and Future Directions”, John Wiley and Sons, 2007 [2] http://www.jochenschiller.de/ Schiller,J.,“Mobile Communications Chapter 9:Mobile Transport Layer”, [3] Sardar, B. and Saha, D.,“A survey of TCP enhancements for last-hop wireless networks“, IEEE Communications Surveys & Tutorials, Vol. 8, No. 3, 3rd Quarter 2006 [4] Cheng, R. and Lin, H.“ A cross-layer design for TCP end-to-end performance improvement in multi-hop wireless networks “, Computer Communications 31 ,3145–3152, 2008 [5] Jun, W., Xi, F., Quan,C., Min,J., Ping,Z.,“ A cross-layer wireless TCP enhancement scheme in OFDM network “, TENCON, 2006