Network Layer4-1 Summary: TCP Congestion Control When CongWin is below Threshold, sender in slow-start phase, window grows exponentially. When CongWin is above Threshold, sender is in congestion-avoidance phase, window grows linearly. When a triple duplicate ACK occurs, Threshold set to CongWin/2 and CongWin set to Threshold. When timeout occurs, Threshold set to CongWin/2 and CongWin is set to 1 MSS.

Network Layer4-2 TCP sender congestion control StateEventTCP Sender ActionCommentary Slow Start (SS) ACK receipt for previously unacked data CongWin = CongWin + MSS, If (CongWin > Threshold) set state to “Congestion Avoidance” Resulting in a doubling of CongWin every RTT Congestion Avoidance (CA) ACK receipt for previously unacked data CongWin = CongWin+MSS * (MSS/CongWin) Additive increase, resulting in increase of CongWin by 1 MSS every RTT SS or CALoss event detected by triple duplicate ACK Threshold = CongWin/2, CongWin = Threshold, Set state to “Congestion Avoidance” Fast recovery, implementing multiplicative decrease. CongWin will not drop below 1 MSS. SS or CATimeoutThreshold = CongWin/2, CongWin = 1 MSS, Set state to “Slow Start” Enter slow start SS or CADuplicate ACK Increment duplicate ACK count for segment being acked CongWin and Threshold not changed

Network Layer4-3 TCP Throughput What’s the average throughout of TCP as a function of window size and RTT? Ignore slow start Let W be the window size when loss occurs. When window is W, throughput is W/RTT Just after loss, window drops to W/2, throughput to W/2RTT. Average throughout:.75 W/RTT

Network Layer4-4 TCP Futures Example: 1500 byte segments, 100ms RTT, want 10 Gbps throughput Requires window size W = 83,333 in-flight segments Throughput in terms of loss rate: ➜ L = 2 · Wow New versions of TCP for high-speed needed!

Network Layer4-5 Fairness goal: if K TCP sessions share same bottleneck link of bandwidth R, each should have average rate of R/K TCP connection 1 bottleneck router capacity R TCP connection 2 TCP Fairness

Network Layer4-6 Why is TCP fair? Two competing sessions: Additive increase gives slope of 1, as throughput increases Multiplicative decrease decreases throughput proportionally R R equal bandwidth share Connection 1 throughput Connection 2 throughput congestion avoidance: additive increase loss: decrease window by factor of 2 congestion avoidance: additive increase loss: decrease window by factor of 2

Network Layer4-7 Fairness Fairness and UDP Multimedia apps often do not use TCP Do not want rate throttled by congestion control Instead use UDP: Pump audio/video at constant rate, tolerate packet loss Research area: TCP friendly Fairness and parallel TCP connections Nothing prevents app from opening parallel connections between 2 hosts. Web browsers do this Example: link of rate R supporting 9 connections; New app asks for 1 TCP, gets rate R/10 New app asks for 11 TCPs, gets R/2 !

Network Layer4-8 Chapter 4 Network Layer Computer Networking: A Top Down Approach Featuring the Internet, 3 rd edition. Jim Kurose, Keith Ross Addison-Wesley, July Modified by Jelena Mirkovic All material copyright J.F Kurose and K.W. Ross, All Rights Reserved

Network Layer4-9 Network layer Transport segment from sending to receiving host Sending side encapsulates segments into datagrams Receiving side delivers segments to transport layer Network layer protocols in every host, router Router examines header fields in all IP datagrams passing through it network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical application transport network data link physical application transport network data link physical

Network Layer4-10 Key Network-Layer Functions forwarding: move packets from router’s input to appropriate router output routing: determine route taken by packets from source to dest. Routing algorithms analogy: routing: process of planning trip from source to dest forwarding: process of getting through single interchange

Network Layer value in arriving packet’s header routing algorithm local forwarding table header value output link Interplay between routing and forwarding

Network Layer4-12 Connection setup 3 rd important function in some network architectures: ATM, frame relay, X.25 Before datagrams flow, two hosts and intervening routers establish virtual connection Routers get involved Network and transport layer service Network: between two hosts Transport: between two processes

Network Layer4-13 Network service model Q: What service model for “channel” transporting datagrams from sender to rcvr? Example services for individual datagrams: Guaranteed delivery Guaranteed delivery with less than 40 msec delay Example services for a flow of datagrams: In-order datagram delivery Guaranteed minimum bandwidth to flow Restrictions on changes in inter- packet spacing

Network Layer4-14 Network layer service models: Network Architecture Internet ATM Service Model best effort CBR VBR ABR UBR Bandwidth none constant rate guaranteed rate guaranteed minimum none Loss no yes no Order no yes Timing no yes no Congestion feedback no (inferred via loss) no congestion no congestion yes no Guarantees ?

Network Layer4-15 Network layer connection and connection-less service Datagram network provides network-layer connectionless service VC network provides network-layer connection service Analogous to the transport-layer services, but: Service: host-to-host Host has no choice: network provides one or the other Implementation: in the core

Network Layer4-16 Virtual circuits Call setup, teardown for each call before data can flow Each packet carries VC identifier (not destination host address) Every router on source-dest path maintains “state” for each passing connection Link, router resources (bandwidth, buffers) may be allocated to VC “source-to-dest path behaves much like telephone circuit” performance-wise network actions along source-to-dest path

Network Layer4-17 VC implementation A VC consists of: 1. Path from source to destination 2. VC numbers, one number for each link along path 3. Entries in forwarding tables in routers along path Packet belonging to VC carries a VC number. VC number must be changed on each link. New VC number comes from forwarding table

Network Layer4-18 Forwarding table VC number interface number Incoming interface Incoming VC # Outgoing interface Outgoing VC # … … Forwarding table in northwest router: Routers maintain connection state information!

Network Layer4-19 Virtual circuits: signaling protocols Used to setup, maintain teardown VC Used in ATM, frame-relay, X.25 Not used in today’s Internet application transport network data link physical application transport network data link physical 1. Initiate call 2. incoming call 3. Accept call 4. Call connected 5. Data flow begins 6. Receive data

Network Layer4-20 Datagram networks No call setup at network layer Routers: no state about end-to-end connections No network-level concept of “connection” Packets forwarded using destination host addres Packets between same source-dest pair may take different paths application transport network data link physical application transport network data link physical 1. Send data 2. Receive data

Network Layer4-21 Forwarding table Destination Address Range Link Interface through through through otherwise 3 4 billion possible entries

Network Layer4-22 Longest prefix matching Prefix Match Link Interface otherwise 3 DA: Examples DA: Which interface?

Network Layer4-23 Datagram or VC network Internet Data exchange among computers No strict timing req. “smart” end systems (computers), simple core many link types different characteristics uniform service difficult ATM Evolved from telephony Human conversation: strict timing, reliability requirements “dumb” end systems, complexity inside network