University of Berne Institute of Computer Science and Applied Mathematics – IAM/RVS

1. University of Berne Institute of Computer Science and Applied Mathematics – IAM/RVS TCP Issues in Mobile IP Networks Ruy de OliveiraDecember 05, 2001

2. Topics addressed • Brief review on TCP algorithm • Challenges for TCP under mobile environment • Main proposed approaches for cellular net. • Requirements on mobile multi-hop networks • Some proposals for multi-hop environment • Conclusions and outlooks

3. TCP review • TCP has been designed to work on wired networks • Negligible medium loss (low BER) • Under loss it starts probing the net at lower rate by shrinking its congestion window (CWND) • Slow Start (timeout) exponential back off (RTO) • Congestion Avoidance • Fast retransmit and recover (3 dacks) • Receiver window (RW) limits the maximum rate of the sender • Upon receiving a RW set to zero, sender enters into “persist mode”

4. TCP under mobile environment • In mobile networks, pck losses refer to: • Congestion within wired network • Non-negligible wireless losses (high BER) • Disconnection (Handover, fading, etc) • As TCP does not discriminate such losses, it can waste bandwidth by dropping its CWND when • A pck loss occurs in the wireless link • A fast handover takes place towards a cell with enough bandwidth • Serial timeouts

5. Dealing with TCP in mobile IP • The main techniques used to get over TCP behavior in mobile networks include: • To split the e2e connection into two, namely wired and wireless connection • To push the sender into “persist mode” during handover by either the • Base Station (BS) (or another intermediate node) or • Mobile Host (MH) (by predicting imminent disconnect.) • To improve local wireless retransmission • To speed up the TCP recovery after a handover

6. TCP approaches for cellular network • I-TCP • Snoop • M-TCP • Delayed duplicate acks (dacks) • EBSN • WTCP • Freeze-TCP • TCP-probing • Fast retransmit

7. Indirect-TCP (I-TCP) • It splits the e2e connection into two parts: • The wireless connection can even use another transport protocol that suits wireless medium • During handover pcks from FH are cached at old BS to be transferred to the new one • It’s backward compatible with fixed network

8. Indirect TCP operation

9. I-TCP drawbacks • Maintains no e2e TCP semantics • BS acknowledges (ACK) pcks to the sender • It requires cooperation of application layer to provide reliability • The BS can run out of buffer • High processing at BS • Latency to transfer state information can be prohibitive

10. Snoop Protocol • Changes are restricted to BS and optionally to MH as well • E2e TCP semantics is preserved • A (snoop) layer is added to the routing code at BS which keep track of pcks in both directions • Pcks meant to MH are buffered at BS and, if needed, retransmitted in the wireless link • It’s robust in dealing with multiple pck losses in a single transmission window

11. Snoop Protocol functioning

12. Snoop Protocol drawbacks • Recovery from handover can be slow due to considerable state information to be handed over • Under long disconnection, sender times out • Encrypted traffic cannot be handled

13. M-TCP Protocol • Also splits the connection into two • Unlike I-TCP, it maintains e2e TCP semantics • Under long disconnection pushes the sender into “persist mode” • It avoids frequent transferring of state information during handover • It’s appropriated for environment with high cells switching

14. M-TCP Protocol operation

15. M-TCP Protocol disadvantages • When sender transmit occasionally only, it will time out as the SH-agent does not send last ACK • Some retransmission overhead • High processing at SH • Considerable complexity • Encryption is not possible • Reliability issues

16. TCP approaches cellular network • I-TCP • Snoop • M-TCP • Delayed duplicate acks • EBSN • WTCP • Freeze-TCP • TCP-probing • Fast retransmit

17. Approaches comparison

18. Mobile multi-hop (Ad hoc) networks • Mobile multi-hop = mobile Ad hoc = Manet • This wireless framework is “wired infrastructure” independent • Each node is both end-user and router • It’s appropriate for environment where wired network cannot be used or is not desired

19. TCP challenges in manet networks • All those met in Cellular networks (1-hop) • Environment under high route failures • Frequent routing changes • Partitions • Multi-path routing needs to be considered • Power saving awareness is extremely necessary • CWND may not represents actual available BW (route dependent)

20. Manet scheme

21. Approaches for TCP within manets To • lead the sender into “persist mode” or a similar one • fix the RTO under route failure • make use of feedback information • rely on cooperation from network and link layers • improve link protocol recovery strategy

22. Some proposals • TCP-F • ELFN-based approach • Fixed RTO • ATCP

23. TCP-F • Based on feedback scheme • Sender to distinguish route failure from net. cong. • Sender enters snooze state when receives RFN • It resumes transmission when receives a RRN • Lack of RFN or RRN makes it performs like std TCP

24. ELFN-based approach • Employs the concept of Explicit Link Failure Notification (ELFN) techniques • Via ELFN sender is told about link and route failures • ELFN carried by routing protocol itself (piggy-back) • Upon receiving an ELFN TCP disables cong. control • Instead it enters a “stand-by” mode  timers frozen • Starts probing the network • Retransmission resumes at “full rate” • Routing protocol (DSR) staled cache problem degrades performance significantly

25. Fixed RTO • The exponential back off algorithm is disable so the sender retransmits at regular intervals • A 2nd RTO happening, indicates route loss • The scheme was evaluated for two on-demand (AODV, DSR) and one proactive (ADV) routing algorithms • On-demand ones performed well • Proactive didn’t experience improvement with this approach • This approach is only feasible for wireless networks

26. ATCP • Std TCP is not modified  Interoperability • Defines ad hoc layer to work between layers 3 and 4 • ECN and ICMP “Dest. Unreach.” signaling are used • ECN  congestion • ICMP  router failure (partition or re-computation) • ATCP spoofs TCP to obtain the following behavior: • High error  Simply retransmit pck from TCP buffer • Route update delay  Stop/resume with new CWND • Transient partition  idem • Multi-path routing  invoke CC • ICMP messages might not reach the sender

27. ATCP • Based on network feedback atcp puts sender into: • Persist mode • Congestion control mode • Retransmit mode

28. ATCP State transition at sender

29. AD Hoc approaches

30. Conclusions and outlooks • Any TCP improvements need consider interoperab. • Power saving awareness is essential • Cooperation among protocol layers seems to be unavoidable • Further investigation on CWND on resuming • BS tends to be part of encryption scheme • Ad hoc networks (multi-hop) • TCP performance is Highly dependent on routing pr. • Geographical-based location protocol seems to be useful • Link layer strategies to play a key role (high BER) • Longer periods of disconnection is highly likely