Internet Networking Spring 2003 Tutorial 12 Limited Transmit RFC 3042 Long Thin Networks RFC 2757

Limited Transmit RFC 3042

The problem cwnd can be smaller than 4 Due to a large number of packet loss, less than 3 dup acks will arrive This causes a timeout rather than a Fast Retransmit.

TCP modification The sender will send a new segment for each dup-Ack it receives. The amount of data is limited to cwnd+2 cwnd does NOT change due to a dup-ack.

Example cwnd=3 Congestion avoidance ---- Fast retransmit Sstresh = 3/2 = 2 Cwnd = sstresh+3=2+3= X ACK(3) ACK(8)

Example #2 cwnd= cwnd= ACK(3) ACK(6)

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.

(Wireless) Long Thin Networks RFC 2757

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: – 02/projects/student_work/mcmahoj.htmlhttp:// 02/projects/student_work/mcmahoj.html –RFC 2757.

Wireless network

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

Indirect TCP

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

PEP: retransmission TCP RTO TCP2 RTO ACK

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