Advanced Computer Networks CS716 Advanced Computer Networks By Dr. Amir Qayyum 1

Lecture No. 32

TCP Flow Control Issues Problem: app. delivers tiny pieces of data to TCP e.g. telnet in character mode Each piece sent as segment, returned as ACK Very inefficient Solutions Delay transmission to accumulate more data Nagle’s algorithm Send first piece Accumulate data until first piece ACK’d Send accumulated data and restart accumulation Not ideal for some traffic, e.g. mouse motion

TCP Flow Control Issues Problem: slow application reads data in tiny pieces Receiver advertises tiny window Sender fills tiny window Known as silly window syndrome Solution: due to Clark Advertise window opening only when MSS or ½ of buffer is available Sender delays sending until window is MSS or ½ of receiver’s buffer (estimated) Overridden by using PUSH

TCP Flow Control Math Send buffer size: MaxSendBuffer Receive buffer size: MaxRcvBuffer Receiving side LastByteRcvd - LastByteRead < = MaxRcvBuffer AdvertisedWindow = MaxRcvBuffer - (NextByteExpected - NextByteRead) Sending side LastByteSent - LastByteAcked < = AdvertisedWindow EffectiveWindow = AdvertisedWindow - (LastByteSent - LastByteAcked) LastByteWritten - LastByteAcked < = MaxSendBuffer block sender if (LastByteWritten - LastByteAcked) + y > MaxSenderBuffer Always send ACK in response to arriving data segment Persist when AdvertisedWindow = 0

TCP Bit Allocation Limitations Sequence numbers vs packet lifetime Assumed that IP packets live less than 60s Can we send 232 (4G) bytes in 60 seconds ? Only need a data rate of 573 Mbps! Less than an STS-12 line... (less than Gigabit Ethernet) Advertised window vs delay-bandwidth Only 16 bits (64kB) for advertised window For cross-country RTT of 100 milliseconds, adequate for a mere 5.24 Mbps!

Protection Against Wrap Around 32-bit SequenceNum Bandwidth Time Until Wrap Around T1 (1.5 Mbps) 6.4 hours Ethernet (10 Mbps) 57 minutes T3 (45 Mbps) 13 minutes FDDI (100 Mbps) 6 minutes STS-3 (155 Mbps) 4 minutes STS-12 (622 Mbps) 55 seconds STS-24 (1.2 Gbps) 28 seconds

Keeping the Pipe Full 16-bit AdvertisedWindow Bandwidth Delay x Bandwidth Product T1 (1.5 Mbps) 18KB Ethernet (10 Mbps) 122KB T3 (45 Mbps) 549KB FDDI (100 Mbps) 1.2MB STS-3 (155 Mbps) 1.8MB STS-12 (622 Mbps) 7.4MB STS-24 (1.2 Gbps) 14.8MB

Adaptive Retransmission Algorithm Original algorithm used only RTT estimate Theory: measure RTT for each segment + its ACK Estimate RTT Timeout is 2 × RTT to allow for variations

Adaptive Retransmission Algorithm Practice Use exponential moving average ( = 0.8 to 0.9) Estimate =  × estimate + (1 - ) measurement Measured RTT depends on  time

Adaptive Retransmission Algorithm Problem: it did not handle variations well Ambiguity for retransmitted packets: was ACK in response to first, second, etc. transmission ? Measured RTT time transmission retransmission RTT ? ? ?

Adaptive Retransmission (Original Algorithm) Measure SampleRTT for each segment/ACK pair Compute weighted average of RTT EstRTT = a × EstRTT + b × SampleRTT where a + b = 1 a between 0.8 and 0.9 b between 0.1 and 0.2 Set timeout based on EstRTT TimeOut = 2 × EstRTT

Karn/Partridge Algorithm Sender Receiver Sender Receiver Original transmission Original transmission TT TT ACK Retransmission SampleR SampleR Retransmission ACK Do not sample RTT when retransmitting Double timeout (RTT estimate) after each retransmission Still did not handle variations well Did not solve network congestion problems as desired

Jacobson/Karels Algorithm Estimate variance of RTT Calculate mean interpacket RTT deviation as an approximation of variance (Diff = RTT_estimate – measurement) RTT_estimate = a x RTT_estimate + (1 - a ) × measurement Using another exponential moving average Deviation = β x |RTT_estimate - measurement| + (1 - β) deviation β = 0.25, α = 0.875 in RTT_estimate

Jacobson/Karels Algorithm Use variance estimate as component of RTT estimate next_RTT (timeout) = RTT_estimate + 4 × deviation Protects against high jitter Algorithm only as good as clock granularity(500ms on Unix) Accurate timeout mechanism important to congestion control