22-05-2012, 01:30 PM
Mobile Communications -Mobile Transport Layer
[attachment=22705]
Influences of mobility on TCP-mechanisms
• TCP assumes congestion if packets are dropped
• typically wrong in wireless networks, here we often have packet loss due to transmission errors
• furthermore, mobilityitself 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
Early approach: Indirect TCP I
• 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
Early approach: Snooping TCP I
• “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
Snooping TCP II
• 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
[attachment=22705]
Influences of mobility on TCP-mechanisms
• TCP assumes congestion if packets are dropped
• typically wrong in wireless networks, here we often have packet loss due to transmission errors
• furthermore, mobilityitself 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
Early approach: Indirect TCP I
• 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
Early approach: Snooping TCP I
• “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
Snooping TCP II
• 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