The Transport Layer TCP Timeouts and Connection management

The Transport Layer: TCP Timeouts and Connection management CS 352, Lecture 11, Spring 2020 http: //www. cs. rutgers. edu/~sn 624/352 Srinivas Narayana 1

Course announcements • Project 2 will go online today • Quiz 4 will go online today • Due Tue 03/10 at 10 PM • Mid-term grades will be released this weekend • Papers in class on Wednesday

Review of concepts • TCP congestion control: need distributed, efficient, fair • signals (loss) and knobs (congestion window) • ACK clocking • Slow start • Additive increase and the slow start threshold • Multiplicative decrease • Triple duplicate ACKs and fast retransmit

TCP performs additive increase and multiplicative decrease of its congestion window. This is often termed AIMD results in the so-called TCP sawtooth.

Why AIMD? (x 1, x 2) • Converges to fairness (b. Dx 1+a. I, b. Dx 2+a. I) User 2: x 2 • Can also show it converges to efficiency fairness line (b. Dx 1, b. Dx 2) • Intuition: Increments to rate get smaller as fairness increases efficiency line User 1: x 1 5

Calculating the TCP timeout

TCP timeout (RTO) • Useful for reliable delivery and congestion control • How to pick the RTO value? • Too long: slow reaction to loss • Too short: premature unnecessary retransmissions • Intuition: somehow use the observed RTT (sample. RTT) • Can we just directly set the latest RTT as the RTO? • RTT can vary significantly! • Intermittent congestion, path changes, signal quality changes on wireless channel, etc.

Estimated RTT • Exponential weighted moving average (typical alpha = 1/8) Estimated. RTT = (1 - )*Estimated. RTT + *Sample. RTT (milliseconds) RTT: gaia. cs. umass. edu to fantasia. eurecom. fr sample. RTT Estimated. RTT

Timeout == estimated RTT + safety • Estimated RTT can have a large variance • Use a larger safety margin if larger variance

Timeout == estimated RTT + safety Dev. RTT = (1 - )*Dev. RTT + *|Sample. RTT-Estimated. RTT| (typically, = 0. 25) Timeout. Interval = Estimated. RTT + 4*Dev. RTT estimated RTT “safety margin”

Managing a single timer data rcvd from app: § create segment with seq # § seq # is byte-stream number of first data byte in segment § start timer if not already running • think of timer as for oldest unacked segment • expiration interval: Time. Out. Interval timeout: § retransmit segment that caused timeout § restart timer ack rcvd: § if acknowledges previously unacked segments • update what is known to be ACKed • restart timer if there are still unacked segments

Problem with sample. RTT calculation Sender Receiver 2 K SEQ= 0 Sender Timeout RTT? 2 K RTT? SEQ= 0 K AC =2 048

Retransmission ambiguity • If you retransmitted, how do you measure sample. RTT for it? • Measure RTT from original data segment? • Measure RTT from most recent (retransmitted) segment? • There could be an error in RTT estimate, since we can’t be sure • One solution • Never update RTT measurements based on acknowledgements from retransmitted packets • Problem: Sudden change in RTT, coupled with many retransmissions, can cause system to update RTT very late • Ex: Primary path failure leads to a slower secondary path 13

Karn’s algorithm • Use back-off as part of sample. RTT computation • Whenever packet loss, RTO is increased by a factor • Use this increased RTO as RTO estimate for the next segment (not from Estimated. RTT) • Only after an acknowledgment received for a successful transmission is the timer set to new RTT obtained from Estimated. RTT 14

Connection Management

Connection Management before exchanging data, sender/receiver “handshake”: • agree to establish connection • agree on connection parameters application connection state: ESTAB connection variables: seq # client-to-server-to-client rcv. Buffer size at server, client connection state: ESTAB connection Variables: seq # client-to-server-to-client rcv. Buffer size at server, client network Socket client. Socket = new. Socket("hostname", "port number"); Socket connection. Socket = welcome. Socket. accept(); 16

Agreeing to establish a connection 2 -way handshake: Let’s talk ESTAB choose x ESTAB OK ESTAB req_conn(x) acc_conn(x) ESTAB Q: will 2 -way handshake always work in network? • variable delays • retransmitted messages (e. g. req_conn(x)) due to message loss • message reordering • can’t “see” other side 17

2 -way handshake failure scenarios choose x req_conn(x) ESTAB retransmit req_conn(x) acc_conn(x) ESTAB req_conn(x) client terminates connection x completes acc_conn(x) data(x+1) retransmit data(x+1) server forgets x ESTAB half open connection! (no client!) client terminates connection x completes req_conn(x) data(x+1) accept data(x+1) server forgets x ESTAB accept data(x+1) 18

TCP 3 -way handshake client state LISTEN choose init seq num, x send TCP SYN msg SYNSENT server state LISTEN SYNbit=1, Seq=x SYNbit=1, Seq=y ACKbit=1; ACKnum=x+1 choose init seq num, y send TCP SYNACK SYN RCVD msg, acking SYN received SYNACK(x) ESTAB indicates server is live; send ACK for SYNACK; this segment may contain ACKbit=1, ACKnum=y+1 client-to-server data received ACK(y) indicates client is live ESTAB 19

TCP state machine (partially shown) closed socket connection. Socket = server. Socket. accept(); L SYN(x) SYNACK(seq=y, ACKnum=x+1) create new socket for communication back to client listen SYN(seq=x) SYN sent SYN rcvd ACK(ACKnum=y+1) socket client. Socket = socket. connect("hostname", "port number"); ESTAB SYNACK(seq=y, ACKnum=x+1) ACK(ACKnum=y+1) L 20

TCP: closing a connection § client, server each close their side of connection • send TCP segment with FIN bit = 1 • In general, TCP is full-duplex: both sides can send • But FIN is unidirectional: stop one side of the communication • respond to received FIN with ACK • on receiving FIN, ACK can be combined with own FIN § simultaneous FIN exchanges can be handled 21

TCP: closing a connection client state server state ESTAB client. Socket. close() FIN_WAIT_1 FIN_WAIT_2 can no longer send but can receive data FINbit=1, seq=x CLOSE_WAIT ACKbit=1; ACKnum=x+1 wait for server close FINbit=1, seq=y TIMED_WAIT timed wait for 2*max segment lifetime can still send data LAST_ACK can no longer send data ACKbit=1; ACKnum=y+1 CLOSED 22

TCP summary • Reliability • Ordering • Flow control • Congestion control • Timeout computation • Connection management, state machine