1. Transport Layer3-1 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.

2. Transport Layer3-2 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

3. Transport Layer3-3 Why is TCP fair? Two competing sessions: r Additive increase gives slope of 1, as throughout increases r 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

4. Transport Layer3-4 Fairness (more) Fairness and UDP r Multimedia apps often do not use TCP m do not want rate throttled by congestion control r Instead use UDP: m pump audio/video at constant rate, tolerate packet loss r Research area: TCP friendly Fairness and parallel TCP connections r nothing prevents app from opening parallel connections between 2 hosts. r Web browsers do this r Example: link of rate R supporting 9 connections; m new app asks for 1 TCP, gets rate R/10 m new app asks for 11 TCPs, gets R/2 !

5. Network Layer4-5 Chapter 4 Network Layer A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following:  If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!)  If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material. Thanks and enjoy! JFK/KWR All material copyright 1996-2009 J.F Kurose and K.W. Ross, All Rights Reserved Computer Networking: A Top Down Approach 5 th edition. Jim Kurose, Keith Ross Addison-Wesley, April 2009.

6. Review: TCP r Reliable data transfer: acks r Pipelined protocol: in-flight packets r Cumulated acks (single timer) r Flow control (receiver window size) r Congestion control (congestion window size): AIMD r TCP's two phase operations: Slow Start + Congestion Control Network Layer4-6

7. Network Layer4-7 Chapter 4: Network Layer Chapter goals: r understand principles behind network layer services: m IP addresses (+ getting an IP address via DHCP) m Routing algorithms m Network of networks (BGP, dealing with scales) m ICMP m NAT (network address translation)

8. Network Layer4-8 Datagram networks r no call setup at network layer r routers: no state about end-to-end connections m no network-level concept of “connection” r packets forwarded using destination host address m 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

9. Network Layer4-9 The Internet Network layer forwarding table Host, router network layer functions: Routing protocols path selection RIP, OSPF, BGP IP protocol addressing conventions datagram format packet handling conventions ICMP protocol error reporting router “signaling” Transport layer: TCP, UDP Link layer physical layer Network layer

10. IP datagram format ver length 32 bits data (variable length, typically a TCP or UDP segment) 16-bit identifier header checksum time to live 32 bit source IP address IP protocol version number header length (bytes) max number remaining hops (decremented at each router) for fragmentation/ reassembly total datagram length (bytes) upper layer protocol to deliver payload to head. len type of service “type” of data flgs fragment offset upper layer 32 bit destination IP address Options (if any) E.g. timestamp, record route taken, specify list of routers to visit. how much overhead with TCP? r 20 bytes of TCP r 20 bytes of IP r = 40 bytes + app layer overhead TCP/UDP IP Ethernet Application

11. Network Layer4-11 IP Fragmentation and Reassembly r network links have MTU (max.transfer size) - largest possible link-level frame. m different link types, different MTUs r large IP datagram divided (“fragmented”) within net m one datagram becomes several datagrams m “reassembled” only at final destination m IP header bits used to identify, order related fragments fragmentation: in: one large datagram out: 3 smaller datagrams reassembly

12. Network Layer4-12 IP Addressing r IP address: 32-bit identifier for host, router interface r interface: connection between host/router and physical link m router’s typically have multiple interfaces m host typically has one interface m IP addresses associated with each interface 223.1.1.1 223.1.1.2 223.1.1.3 223.1.1.4 223.1.2.9 223.1.2.2 223.1.2.1 223.1.3.2 223.1.3.1 223.1.3.27 223.1.1.1 = 11011111 00000001 00000001 00000001 223 111 interface

13. Network Layer4-13 Subnets r IP address: m subnet part (high order bits) m host part (low order bits) r What’s a subnet ? m device interfaces with same subnet part of IP address m can physically reach each other without intervening router 223.1.1.1 223.1.1.2 223.1.1.3 223.1.1.4 223.1.2.9 223.1.2.2 223.1.2.1 223.1.3.2 223.1.3.1 223.1.3.27 network consisting of 3 subnets subnet

14. Network Layer4-14 IP addressing: CIDR CIDR: Classless InterDomain Routing m subnet portion of address of arbitrary length m address format: a.b.c.d/x, where x is # bits in subnet portion of address 11001000 00010111 00010000 00000000 subnet part host part 200.23.16.0/23

15. Network Layer4-15 IP addresses: how to get one? Q: How does a host get IP address? r hard-coded by system admin in a file m Windows: control-panel->network->configuration- >tcp/ip->properties m Linux (ubuntu): /etc/network/interface r DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server m “plug-and-play”

16. Network Layer4-16 DHCP: Dynamic Host Configuration Protocol Goal: allow host to dynamically obtain its IP address from network server when it joins network Can renew its lease on address in use Allows reuse of addresses (only hold address while connected an “on”) Support for mobile users who want to join network (more shortly) DHCP overview: 1. host broadcasts “DHCP discover” msg 2. DHCP server responds with “DHCP offer” msg 3. host requests IP address: “DHCP request” msg 4. DHCP server sends address: “DHCP ack” msg

17. Network Layer4-17 Graph abstraction of a network u y x wv z 2 2 1 3 1 1 2 5 3 5 c(x,x’) = cost of link (x,x’) - e.g., c(w,z) = 5 cost could always be 1, or inversely related to bandwidth, or inversely related to congestion Cost of path (x 1, x 2, x 3,…, x p ) = c(x 1,x 2 ) + c(x 2,x 3 ) + … + c(x p-1,x p ) Question: What’s the least-cost path between u and z ? Routing algorithm: algorithm that finds least-cost path Graph: G = (N,E) N = set of routers = { u, v, w, x, y, z } E = set of links ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) }

18. Network Layer4-18 Routing algorithms Global or decentralized information? Global: r all routers have complete topology, link cost info r “link state” algorithms (OSPF) Decentralized: r router knows physically-connected neighbors, link costs to neighbors r iterative process of computation, exchange of info with neighbors r “distance vector” algorithms (RIP) Static or dynamic? Static: r routes change slowly over time Dynamic: r routes change more quickly m periodic update m in response to link cost changes

19. Network Layer4-19 1 2 3 0111 value in arriving packet’s header routing algorithm local forwarding table header value output link 0100 0101 0111 1001 32213221 Interplay between routing, forwarding

20. Network Layer4-20 Hierarchical routing for scalability scale: with 200 million destinations: r can’t store all dest’s in routing tables! r routing table exchange would swamp links! administrative autonomy r internet = network of networks r each network admin may want to control routing in its own network Our routing study thus far - idealization r all routers identical r network “flat” … not true in practice

21. Network Layer4-21 Hierarchical addressing: route aggregation “Send me anything with addresses beginning 200.23.16.0/20” 200.23.16.0/23200.23.18.0/23200.23.30.0/23 Fly-By-Night-ISP Organization 0 Organization 7 Internet Organization 1 ISPs-R-Us “Send me anything with addresses beginning 199.31.0.0/16” 200.23.20.0/23 Organization 2...... Hierarchical addressing allows efficient advertisement of routing information:

22. Network Layer4-22 Hierarchical addressing: more specific routes ISPs-R-Us has a more specific route to Organization 1 “Send me anything with addresses beginning 200.23.16.0/20” 200.23.16.0/23200.23.18.0/23200.23.30.0/23 Fly-By-Night-ISP Organization 0 Organization 7 Internet Organization 1 ISPs-R-Us “Send me anything with addresses beginning 199.31.0.0/16 or 200.23.18.0/23” 200.23.20.0/23 Organization 2......

23. Network Layer4-23 Hierarchical routing for scalability r aggregate routers into regions, “autonomous systems” (AS) r routers in same AS run same routing protocol m “intra-AS” routing protocol m routers in different AS can run different intra- AS routing protocol Gateway router r Direct link to router in another AS

24. Network Layer4-24 3b 1d 3a 1c 2a AS3 AS1 AS2 1a 2c 2b 1b Intra-AS Routing algorithm Inter-AS Routing algorithm Forwarding table 3c Interconnected ASs r forwarding table configured by both intra- and inter-AS routing algorithm m intra-AS sets entries for internal dests m inter-AS & intra-AS sets entries for external dests

25. Network Layer4-25 Internet inter-AS routing: BGP r BGP (Border Gateway Protocol): the de facto standard r BGP provides each AS a means to: 1. Obtain subnet reachability information from neighboring ASs. 2. Propagate reachability information to all AS- internal routers. 3. Determine “good” routes to subnets based on reachability information and policy. r allows subnet to advertise its existence to rest of Internet: “I am here”

26. Network Layer4-26 BGP basics r pairs of routers (BGP peers) exchange routing info over TCP connections (called BGP sessions) m BGP sessions need not correspond to physical links. r when AS2 advertises prefix “ 200.23.16.0/23 ” to AS1: m AS2 promises it will forward datagrams towards that prefix. m AS2 can aggregate prefixes in its advertisement 3b 1d 3a 1c 2a AS3 AS1 AS2 1a 2c 2b 1b 3c eBGP session iBGP session

27. Network Layer4-27 Distributing reachability info r using eBGP session between 3a and 1c, AS3 sends prefix reachability info to AS1. m 1c can then use iBGP do distribute new prefix info to all routers in AS1 m 1b can then re-advertise new reachability info to AS2 over 1b-to-2a eBGP session r when router learns of new prefix, it creates entry for prefix in its forwarding table. 3b 1d 3a 1c 2a AS3 AS1 AS2 1a 2c 2b 1b 3c eBGP session iBGP session Any dest w/ IP addr AS1 should be routed to 1c Any dest w/ IP addr AS1 should be routed to 2a

28. Network Layer4-28 Path attributes & BGP routes r advertised prefix includes BGP attributes. m prefix + attributes = “route” r two important attributes: m AS-PATH: contains ASs through which prefix advertisement has passed: e.g, AS 67, AS 17 m NEXT-HOP: indicates specific internal-AS router to next-hop AS. (may be multiple links from current AS to next-hop-AS) r when gateway router receives route advertisement, uses local import policy to accept/decline.

29. Network Layer4-29 BGP route selection r router may learn about more than 1 route to some prefix. Router must select route. r elimination rules: 1. local preference value attribute: policy decision 2. shortest AS-PATH 3. closest NEXT-HOP router: hot potato routing 4. additional criteria

30. Network Layer4-30 Why different Intra- and Inter-AS routing ? Policy: r Inter-AS: admin wants control over how its traffic routed, who routes through its net. r Intra-AS: single admin, so no policy decisions needed Scale: r hierarchical routing saves table size, reduced update traffic Performance: r Intra-AS: can focus on performance r Inter-AS: policy may dominate over performance

31. Network Layer4-31 NAT: Network Address Translation r Motivation: local network uses just one IP address as far as outside world is concerned: m range of addresses not needed from ISP: just one IP address for all devices m can change addresses of devices in local network without notifying outside world m can change ISP without changing addresses of devices in local network m devices inside local net not explicitly addressable, visible by outside world (a security plus).

32. Network Layer4-32 NAT: Network Address Translation 10.0.0.1 10.0.0.2 10.0.0.3 10.0.0.4 138.76.29.7 local network (e.g., home network) 10.0.0/24 rest of Internet Datagrams with source or destination in this network have 10.0.0/24 address for source, destination (as usual) All datagrams leaving local network have same single source NAT IP address: 138.76.29.7, different source port numbers

33. Network Layer4-33 NAT: Network Address Translation 10.0.0.1 10.0.0.2 10.0.0.3 S: 10.0.0.1, 3345 D: 128.119.40.186, 80 1 10.0.0.4 138.76.29.7 1: host 10.0.0.1 sends datagram to 128.119.40.186, 80 NAT translation table WAN side addr LAN side addr 138.76.29.7, 5001 10.0.0.1, 3345 …… S: 128.119.40.186, 80 D: 10.0.0.1, 3345 4 S: 138.76.29.7, 5001 D: 128.119.40.186, 80 2 2: NAT router changes datagram source addr from 10.0.0.1, 3345 to 138.76.29.7, 5001, updates table S: 128.119.40.186, 80 D: 138.76.29.7, 5001 3 3: Reply arrives dest. address: 138.76.29.7, 5001 4: NAT router changes datagram dest addr from 138.76.29.7, 5001 to 10.0.0.1, 3345

34. Network Layer4-34 NAT: Network Address Translation r 16-bit port-number field: m 60,000 simultaneous connections with a single LAN-side address! r NAT is controversial: m routers should only process up to layer 3 m violates end-to-end argument NAT possibility must be taken into account by app designers, eg, P2P applications m address shortage should instead be solved by IPv6

35. Network Layer4-35 NAT traversal problem r client wants to connect to server with address 10.0.0.1 m server address 10.0.0.1 local to LAN (client can’t use it as destination addr) m only one externally visible NATted address: 138.76.29.7 r solution 1: statically configure NAT to forward incoming connection requests at given port to server m e.g., (123.76.29.7, port 2500) always forwarded to 10.0.0.1 port 25000 10.0.0.1 10.0.0.4 NAT router 138.76.29.7 Client ?

36. Network Layer4-36 NAT traversal problem r solution 2: Universal Plug and Play (UPnP) Internet Gateway Device (IGD) Protocol. Allows NATted host to:  learn public IP address (138.76.29.7)  add/remove port mappings (with lease times) i.e., automate static NAT port map configuration 10.0.0.1 10.0.0.4 NAT router 138.76.29.7 IGD

37. Network Layer4-37 NAT traversal problem r solution 3: relaying (used in Skype) m NATed client establishes connection to relay m External client connects to relay m relay bridges packets between to connections 138.76.29.7 Client 10.0.0.1 NAT router 1. connection to relay initiated by NATted host 2. connection to relay initiated by client 3. relaying established

38. Network Layer4-38

39. Network Layer4-39 BGP messages r BGP messages exchanged using TCP. r BGP messages: m OPEN: opens TCP connection to peer and authenticates sender m UPDATE: advertises new path (or withdraws old) m KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN request m NOTIFICATION: reports errors in previous msg; also used to close connection

40. Network Layer4-40 NAT: Network Address Translation Implementation: NAT router must: m outgoing datagrams: replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #)... remote clients/servers will respond using (NAT IP address, new port #) as destination addr. m remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair m incoming datagrams: replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table

41. Network Layer4-41 The Internet Network layer forwarding table Host, router network layer functions: Routing protocols path selection RIP, OSPF, BGP IP protocol addressing conventions datagram format packet handling conventions ICMP protocol error reporting router “signaling” Transport layer: TCP, UDP Link layer physical layer Network layer

42. Network Layer4-42 Traceroute and ICMP r Source sends series of UDP segments to dest m First has TTL =1 m Second has TTL=2, etc. m Unlikely port number r When nth datagram arrives to nth router: m Router discards datagram m And sends to source an ICMP message (type 11, code 0) m Message includes name of router& IP address r When ICMP message arrives, source calculates RTT r Traceroute does this 3 times Stopping criterion r UDP segment eventually arrives at destination host r Destination returns ICMP “host unreachable” packet (type 3, code 3) r When source gets this ICMP, stops.

43. Network Layer4-43 ICMP: Internet Control Message Protocol r used by hosts & routers to communicate network-level information m error reporting: unreachable host, network, port, protocol m echo request/reply (used by ping) r network-layer “above” IP: m ICMP msgs carried in IP datagrams r ICMP message: type, code plus first 8 bytes of IP datagram causing error Type Code description 0 0 echo reply (ping) 3 0 dest. network unreachable 3 1 dest host unreachable 3 2 dest protocol unreachable 3 3 dest port unreachable 3 6 dest network unknown 3 7 dest host unknown 4 0 source quench (congestion control - not used) 8 0 echo request (ping) 9 0 route advertisement 10 0 router discovery 11 0 TTL expired 12 0 bad IP header

44. Network Layer4-44 IP addressing: the last word... Q: How does an ISP get block of addresses? A: ICANN: Internet Corporation for Assigned Names and Numbers m allocates addresses m manages DNS m assigns domain names, resolves disputes