20-03-2012, 11:32 AM
Mobile and Wireless Networking
Lecture-12.pdf (Size: 1.63 MB / Downloads: 41)
Transport protocols typically designed for
q Fixed end-systems
q Fixed, wired networks
Research activities
q Performance
q Congestion control
q Efficient retransmissions
TCP congestion control
q packet loss in fixed networks typically due to (temporary) overload
situations
q router have to discard packets as soon as the buffers are full
q TCP recognizes congestion only indirect via missing
acknowledgements, retransmissions unwise, they would only
contribute to the congestion and make it even worse
q slow-start algorithm as reaction
Influences of mobility on TCP-mechanisms
TCP assumes congestion if packets are dropped
l typically wrong in wireless networks, here we often have packet loss
due to transmission errors
l furthermore, mobility itself can cause packet loss, if e.g. a mobile node
moves from one access point (e.g. foreign agent in Mobile IP) to
another while there are still packets in transit and forwarding is not
possible
Additional problem:
l it takes a long time to increase the congestion window if the latency of
the wireless link is high
The performance of an unchanged TCP degrades severely
l but TCP cannot be changed due to the large base of installation in the
fixed network (end-to-end protocol)
l therefore TCP for mobility has to remain compatible
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 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