ECN and Limited Transmit ECN can inform of congestion without dropping the packet, thus allowing TCP with small cwnd to be aware of congestion while avoiding timeouts. ECN requires the co-operation of the network, while Limited Transmit does not. Can be simultaneously used.
Definitions Long: High delay –Longer RTT. Thin: Low bandwidth –The delay*bw product can be small, what influence does it have on cwnd ? Example: wireless network. Counter examples: –Satellite (Long Fat Networks) –Wireless LAN (Short Fat Networks) More information can be found at: – http://www.cas.mcmaster.ca/~wmfarmer/SE-4C03- 02/projects/student_work/mcmahoj.htmlhttp://www.cas.mcmaster.ca/~wmfarmer/SE-4C03- 02/projects/student_work/mcmahoj.html –RFC 2757.
BER: Bit Error Rate Higher error rate than a wired link Possible solutions: 1.At the link layer: DLC that ensures FIFO and reliability (as learned in the previous course) 2.No DLC at the link layer: PEP: Performance Enhancing Proxies (Indirect- TCP) 3.Lower MTU
DLC and duplicate retransmission TCP RTO Link layer timeout Redundant transmission
PEP: Performance Enhancing Proxies. Instead of End-to-End TCP: User -> Intermediate Node (IN) -> Wire-line Internet. Application TCP MAC Link TCP MAC Link IP Link IP Application TCP Link IP IN internet
PEP: Pros –Shield the wire Internet from the wireless connection and vice-versa. –TCP for the wireless link can be enhanced, due to the knowledge we have on that specific network. Different cwnd and RTO Since the RTT of the single wireless link is smaller than the whole route RTT, faster local recovery is achieved. –More freedom for the wireless part; Doesn’t have to be TCP compatible.
PEP: Cons –TCP end-to-end Semantic is broken –IN is usually a single point of failure. –Movement of the client can cause IN change; Since we are referring to W-LTN and not W-LAN, This has much less probability. –Overloading of the IN: much problematic in LFN than LTN. The IN holds 2 stacks of TCP connections. –Handles disconnections poorly –Possible BS buffer overloading. Higher buffer increases hand-off latency when changing cells.
Indirect TCP Split TCP connection into 2 TCPs BS acts as a proxy and relays all data FH sends a packet –BS acknowledges this packet and forwards the packet to MH Packet is lost on wireless link –BS can notices faster due to lower RTT and retransmit packet Fixed Host (FH) Base Station (BS) Mobile Host (MH) wireless TCP Internet regular TCP
TCP-Aware Link Layer Protocol FHMHBS wireless physical link network transport application physical link network transport application physical link network transport application retrans TCP connection Retains local recovery of Split connection approach and link level retransmission schemes Snoop protocol, by Berkley: not in the course material