Mobile Transport layer
- 1. Motivation TCP-mechanisms Classical approaches Indirect TCP Snooping TCP Mobile TCP PEPs in general Mobile Transport Layer Additional optimizations Fast retransmit/recovery Transmission freezing Selective retransmission Transaction oriented TCP
- 2. Motivation Transport layer responsible for Fixed end-end systems Fixed, wired networks Error recovery Flow control Congestion control Ensuring complete data transfer in TCP Efficient retransmissions
- 3. 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
- 4. 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 Linear increase Continues until a time-out at The sender occurs due to a missing acknowledgement, or until the sender detects a gap in transmitted Data because of Continuous acknowledgements for The same packet. missing acknowledgement causes the reduction of the congestion threshold to one half of the current congestion window congestion window starts again with one segment
- 6. TCP fast retransmit/fast recovery If a sender receives several acknowledgements for the same packet, this is due to a gap in received packets at the receiver or missing acknowledgements The sender can now retransmit the missing packet(s) before the timer expires. This behavior is called fast retransmit. It is an early enhancement for preventing slow-start to trigger on losses not caused by congestion. The receipt of acknowledgements shows that there is no congestion to justify a slow start. The sender can continue with the current congestion window. The sender performs a fast recovery from the packet loss. therefore, packet loss is not due to congestion, continue with current congestion window (do not use slow-start)
- 7. Influences of mobility on TCP-mechanisms Slow Start mechanism in fixed networks decreases the efficiency of TCP if used with mobile receivers or senders. Error rates on wireless links are orders of magnitude higher compared to fixed fiber or copper links. This makes compensation for packet loss by TCP quite difficult. Mobility itself can cause packet loss. There are many situations where a soft handover from one access point to another is not possible for a mobile end-system. Standard TCP reacts with slow start if acknowledgements are missing, which does not help in the case of transmission errors over wireless links and
- 9. Early approach: Indirect TCP 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 access point (foreign agent) „wired“ Internet „wireless“ TCP standard TCP
- 11. 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 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 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
- 13. 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 „wired“ Internet buffering of data end-to-end TCP connection local retransmission correspondent hostforeign agent mobile host snooping of ACKs
- 15. Snooping TCP 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
- 16. Advantages The end-to-end TCP semantic is preserved. Most of the enhancements are done in the foreign agent itself which keeps correspondent host unchanged. Handover of state is not required as soon as the mobile host moves to another foreign agent. Even though packets are present in the buffer, time out at the CH occurs and the packets are transmitted to the new COA. No problem arises if the new foreign agent uses the enhancement or not. If not, the approach automatically falls back to the standard solution.
- 17. Disadvantages Snooping TCP does not isolate the behavior of the wireless link as well as I-TCP. Transmission errors may propagate till CH. Using negative acknowledgements between the foreign agent and the mobile host assumes additional mechanisms on the mobile host. This approach is no longer transparent for arbitrary mobile hosts. Snooping and buffering data may be useless if certain encryption schemes are applied end-to-end between the correspondent host and mobile host. If encryption is used above the transport layer, (eg. SSL/TLS), snooping TCP can be used.
- 18. MOBILE TCP
- 19. Mobile TCP - Motivation Dropping packets due to a handover or higher bit error rates is not the only phenomenon of wireless links and mobility – the occurrence of lengthy and/or frequent disconnections is another problem. A TCP sender tries to retransmit data controlled by a retransmission timer that doubles with each unsuccessful retransmission attempt, up to a maximum of one minute What happens in the case of I-TCP if the mobile is disconnected? The proxy has to buffer more and more data, so the longer the period of disconnection, the more buffer is needed. The snooping approach also suffers from being disconnected. The mobile will not be able to send ACKs so, snooping cannot help in this situation.
- 20. Objective – M-TCP To prevent the sender window from shrinking if bit errors or disconnection but not congestion cause current problems. To improve overall throughput, to lower the delay, to maintain end-to-end semantics of TCP, and to provide a more efficient handover. Adapted to the problems arising from lengthy or frequent disconnections
- 21. Mobile TCP The M-TCP splits up the connection into two parts: An unmodified TCP is used on the Standard host- Supervisory Host section An optimized TCP is used on the Supervisory Host- Mobile Host section. The Supervisory Host (SH) adorns the same role as the proxy (Foreign Agent) in I-TCP. The SH is responsible for exchanging data to both the Standard host and the Mobile host. Here in this approach, we assume that the error bit rate is less as compared to other wireless links. So if any packet is lost, the retransmission has to occur from the original sender and not by the SH. (This also maintains the end-to-end TCP semantic)
- 22. Mobile TCP The SH monitors the ACKs (ACK means acknowledgement) being sent by the MH. If for a long period ACKs have not been received, then the SH assumes that the MH has been disconnected (maybe due to failure or moved out of range, etc...). If so the SH chokes the sender by setting its window size to 0. Because of this the sender goes into persistent mode i.e. the sender’s state will not change no matter how long the receiver is disconnected. This means that the sender will not try to retransmit the data. Now when the SH detects a connectivity established again with the MH (the old SH or new SH if handover), the window of the sender is restored to
- 23. Advantages: Maintains the TCP end-to-end semantics. (No failed packet retransmission is done by the SH .All job handled by original sender) Does not require the change in the sender’s TCP. If MH disconnected, it doesn’t waste time in useless transmissions and shrinks the window size to 0. No need to send old buffer data to new SH in case of handover (as in I-TCP).
- 24. Disadvantages: M-TCP assumes low bit error which is not always true. So, any packet loss due to bit-errors occurring, then its propagated to the sender. Modifications are required for the MH protocol software.
- 25. 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. 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 mux. 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 It offers a way to resume TCP connections even after long interruptions of the connection. It can be used together with encrypted data as it is independent of other TCP mechanisms such as sequence no or acknowledgements Disadvantage Lots of changes have to be made in software of MH, CH and FA.
- 27. 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
- 28. 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!
- 29. 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
- 30. Advantage efficiency Disadvantage requires changed TCP mobility not longer transparent
- 31. Comparison of different approaches for a “mobile” TCP Approach Mechanism Advantages Disadvantages Indirect TCP splits TCP connection into two connections isolation of wireless link, simple loss of TCP semantics, higher latency at handover Snooping TCP “snoops” data and acknowledgements, local retransmission transparent for end-to- end connection, MAC integration possible problematic with encryption, bad isolation of wireless link M-TCP splits TCP connection, chokes sender via window size Maintains end-to-end semantics, handles long term and frequent disconnections Bad isolation of wireless link, processing overhead due to bandwidth management Fast retransmit/ fast recovery avoids slow-start after roaming simple and efficient mixed layers, not transparent Transmission/ time-out freezing freezes TCP state at disconnect, resumes after reconnection independent of content or encryption, works for longer interrupts changes in TCP required, MAC dependant Selective retransmission retransmit only lost data very efficient slightly more complex receiver software, more buffer needed Transaction oriented TCP combine connection setup/release and data transmission Efficient for certain applications changes in TCP required, not transparent