Networking Layer Slides originally prepared by Jim Kurose

Networking Layer Slides originally prepared by Jim Kurose and Keith Ross (for their textbook Computer Networking: A Top Down Approach 4 th edition) Adapted by Suman Banerjee for CS 640 Network Layer 1

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: m host broadcasts “DHCP discover” msg m DHCP server responds with “DHCP offer” msg m host requests IP address: “DHCP request” msg m DHCP server sends address: “DHCP ack” msg Network Layer 2

DHCP client-server scenario A B 223. 1. 1. 2 223. 1. 1. 4 223. 1. 1. 3 223. 1. 2. 1 DHCP server 223. 1. 1. 1 223. 1. 2. 9 223. 1. 3. 27 223. 1. 2. 2 223. 1. 3. 2 E arriving DHCP client needs address in this network Network Layer 3

DHCP client-server scenario DHCP server: 223. 1. 2. 5 DHCP discover src : 0. 0, 68 dest. : 255, 67 yiaddr: 0. 0 transaction ID: 654 arriving client DHCP offer src: 223. 1. 2. 5, 67 dest: 255, 68 yiaddrr: 223. 1. 2. 4 transaction ID: 654 Lifetime: 3600 secs DHCP request time src: 0. 0, 68 dest: : 255, 67 yiaddrr: 223. 1. 2. 4 transaction ID: 655 Lifetime: 3600 secs DHCP ACK src: 223. 1. 2. 5, 67 dest: 255, 68 yiaddrr: 223. 1. 2. 4 transaction ID: 655 Lifetime: 3600 secs Network Layer 4

More on DHCP r Servers often have 2 sets of IP address m Static pool (to configure say, desktop machines) and dynamic pool (to configure transient machines) r Servers often use the policy of “ping- before-offer” for IP address allocation m If a (rogue) machine responds to a ping for an IP address in the dynamic pool, that IP address gets abandoned by the server Network Layer 5

IP addresses: how to get one? Q: How does network get subnet part of IP addr? A: gets allocated portion of its provider ISP’s address space ISP's block 11001000 00010111 00010000 200. 23. 16. 0/20 Organization 1 Organization 2. . . 11001000 00010111 00010000 11001000 00010111 00010010 0000 11001000 00010111 00010100 0000 …. 200. 23. 16. 0/23 200. 23. 18. 0/23 200. 23. 20. 0/23 …. Organization 7 11001000 00010111 00011110 0000 200. 23. 30. 0/23 Network Layer 6

Hierarchical addressing: route aggregation Hierarchical addressing allows efficient advertisement of routing information: Organization 0 200. 23. 16. 0/23 Organization 1 200. 23. 18. 0/23 Organization 2 200. 23. 20. 0/23 Organization 7 . . . Fly-By-Night-ISP “Send me anything with addresses beginning 200. 23. 16. 0/20” Internet 200. 23. 30. 0/23 ISPs-R-Us “Send me anything with addresses beginning 199. 31. 0. 0/16” Network Layer 7

Hierarchical addressing: more specific routes ISPs-R-Us has a more specific route to Organization 1 Organization 0 200. 23. 16. 0/23 Organization 2 200. 23. 20. 0/23 Organization 7 . . . Fly-By-Night-ISP “Send me anything with addresses beginning 200. 23. 16. 0/20” Internet 200. 23. 30. 0/23 ISPs-R-Us Organization 1 200. 23. 18. 0/23 “Send me anything with addresses beginning 199. 31. 0. 0/16 or 200. 23. 18. 0/23” Network Layer 8

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 Network Layer 9

NAT: Network Address Translation rest of Internet local network (e. g. , home network) 10. 0. 0/24 10. 0. 0. 1 10. 0. 0. 2 138. 76. 29. 7 10. 0. 0. 3 All datagrams leaving local network have same single source NAT IP address: 138. 76. 29. 7, different source port numbers Datagrams with source or destination in this network have 10. 0. 0/24 address for source, destination (as usual) Network Layer 10

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). Network Layer 11

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 Network Layer 12

NAT: Network Address Translation 2: NAT router changes datagram source addr from 10. 0. 0. 1, 3345 to 138. 76. 29. 7, 5001, updates table 2 NAT translation table WAN side addr LAN side addr 1: host 10. 0. 0. 1 sends datagram to 128. 119. 40. 186, 80 138. 76. 29. 7, 5001 10. 0. 0. 1, 3345 …… …… S: 10. 0. 0. 1, 3345 D: 128. 119. 40. 186, 80 S: 138. 76. 29. 7, 5001 D: 128. 119. 40. 186, 80 138. 76. 29. 7 S: 128. 119. 40. 186, 80 D: 138. 76. 29. 7, 5001 3: Reply arrives dest. address: 138. 76. 29. 7, 5001 3 1 10. 0. 0. 4 S: 128. 119. 40. 186, 80 D: 10. 0. 0. 1, 3345 10. 0. 0. 2 4 10. 0. 0. 3 4: NAT router changes datagram dest addr from 138. 76. 29. 7, 5001 to 10. 0. 0. 1, 3345 Network Layer 13

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, P 2 P applications m address IPv 6 shortage should instead be solved by Network Layer 14

NAT traversal problem r client wants to connect to server with address 10. 0. 0. 1 m m server address 10. 0. 0. 1 local Client to LAN (client can’t use it as destination addr) 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 10. 0. 0. 1 ? 138. 76. 29. 7 10. 0. 0. 4 NAT router e. g. , (123. 76. 29. 7, port 2500) always forwarded to 10. 0. 0. 1 port 25000 Network Layer 15

NAT traversal problem r solution 2: Universal Plug and Play (UPn. P) Internet Gateway Device (IGD) Protocol. Allows NATted host to: v learn public IP address (138. 76. 29. 7) v add/remove port mappings (with lease times) 10. 0. 0. 1 IGD 10. 0. 0. 4 138. 76. 29. 7 NAT router i. e. , automate static NAT port map configuration Network Layer 16

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 2. connection to relay initiated by client Client 3. relaying established 1. connection to relay initiated by NATted host 138. 76. 29. 7 10. 0. 0. 1 NAT router Network Layer 17

IPv 6 r Initial motivation: 32 -bit address space soon to be completely allocated. r Additional motivation: m header format helps speed processing/forwarding m header changes to facilitate Qo. S IPv 6 datagram format: m fixed-length 40 byte header m no fragmentation allowed Network Layer 18

IPv 6 Header (Cont) Priority: identify priority among datagrams in flow Flow Label: identify datagrams in same “flow. ” (concept of“flow” not well defined). Next header: identify upper layer protocol for data Network Layer 19

Other Changes from IPv 4 r Checksum: removed entirely to reduce processing time at each hop r Options: allowed, but outside of header, indicated by “Next Header” field r ICMPv 6: new version of ICMP m additional message types, e. g. “Packet Too Big” m multicast group management functions Network Layer 20

Transition From IPv 4 To IPv 6 r Not all routers can be upgraded simultaneous m no “flag days” m How will the network operate with mixed IPv 4 and IPv 6 routers? r Tunneling: IPv 6 carried as payload in IPv 4 datagram among IPv 4 routers Network Layer 21

Tunneling Logical view: Physical view: E F IPv 6 IPv 6 A B IPv 6 tunnel IPv 4 Network Layer 22

Tunneling Logical view: Physical view: A B IPv 6 A B C IPv 6 IPv 4 Flow: X Src: A Dest: F data A-to-B: IPv 6 E F IPv 6 D E F IPv 4 IPv 6 tunnel Src: B Dest: E Flow: X Src: A Dest: F data B-to-C: IPv 6 inside IPv 4 Flow: X Src: A Dest: F data E-to-F: IPv 6 Network Layer 23

IP Multicast Network Layer 24

Broadcast Routing r deliver packets from source to all other nodes r source duplication is inefficient: duplicate creation/transmission R 1 duplicate R 2 R 3 R 1 R 4 source duplication R 3 R 4 in-network duplication r source duplication: how does source determine recipient addresses? Network Layer 25

In-network duplication r flooding: when node receives brdcst pckt, sends copy to all neighbors m Problems: cycles & broadcast storm r controlled flooding: node only brdcsts pkt if it hasn’t brdcst same packet before m Node keeps track of pckt ids already brdcsted m Or reverse path forwarding (RPF): only forward pckt if it arrived on shortest path between node and source r spanning tree m No redundant packets received by any node Network Layer 26

Spanning Tree r First construct a spanning tree r Nodes forward copies only along spanning tree A B c F A E B c D F G (a) Broadcast initiated at A E D G (b) Broadcast initiated at D Network Layer 27

Spanning Tree: Creation r Center node r Each node sends unicast join message to center node m Message forwarded until it arrives at a node already belonging to spanning tree A A 3 B c 4 F 1 2 E B c D F 5 E D G G (a) Stepwise construction of spanning tree (b) Constructed spanning tree Network Layer 28

Multicast Routing: Problem Statement r Goal: find a tree (or trees) connecting routers having local mcast group members m m m tree: not all paths between routers used source-based: different tree from each sender to rcvrs shared-tree: same tree used by all group members Shared tree Source-based trees

Approaches for building mcast trees Approaches: r source-based tree: one tree per source m shortest path trees m reverse path forwarding r group-shared tree: group uses one tree m minimal spanning (Steiner) m center-based trees …we first look at basic approaches, then specific protocols adopting these approaches

Shortest Path Tree r mcast forwarding tree: tree of shortest path routes from source to all receivers m Dijkstra’s algorithm S: source LEGEND R 1 1 2 R 4 R 2 3 R 3 router with attached group member 5 4 R 6 router with no attached group member R 5 6 R 7 i link used forwarding, i indicates order link added by algorithm

Reverse Path Forwarding q rely on router’s knowledge of unicast shortest path from it to sender q each router has simple forwarding behavior: if (mcast datagram received on incoming link on shortest path back to center) then flood datagram onto all outgoing links else ignore datagram

Reverse Path Forwarding: example S: source LEGEND R 1 R 4 router with attached group member R 2 R 5 R 3 R 6 R 7 router with no attached group member datagram will be forwarded datagram will not be forwarded • result is a source-specific reverse SPT – may be a bad choice with asymmetric links

Reverse Path Forwarding: pruning r forwarding tree contains subtrees with no mcast group members m no need to forward datagrams down subtree m “prune” msgs sent upstream by router with no downstream group members LEGEND S: source R 1 router with attached group member R 4 R 2 P R 5 R 3 R 6 P R 7 P router with no attached group member prune message links with multicast forwarding

Shared-Tree: Steiner Tree r Steiner Tree: minimum cost tree connecting all routers with attached group members r problem is NP-complete r excellent heuristics exists r not used in practice: m computational complexity m information about entire network needed m monolithic: rerun whenever a router needs to join/leave

Center-based trees r single delivery tree shared by all r one router identified as “center” of tree r to join: m edge router sends unicast join-msg addressed to center router m join-msg “processed” by intermediate routers and forwarded towards center m join-msg either hits existing tree branch for this center, or arrives at center m path taken by join-msg becomes new branch of tree for this router

Center-based trees: an example Suppose R 6 chosen as center: LEGEND R 1 3 R 2 router with attached group member R 4 2 R 5 R 3 1 R 6 R 7 1 router with no attached group member path order in which join messages generated

Internet Multicasting Routing: DVMRP r DVMRP: distance vector multicast routing protocol, RFC 1075 r flood and prune: reverse path forwarding, source-based tree m RPF tree based on DVMRP’s own routing tables constructed by communicating DVMRP routers m no assumptions about underlying unicast m initial datagram to mcast group flooded everywhere via RPF m routers not wanting group: send upstream prune msgs

DVMRP: continued… r soft state: DVMRP router periodically (1 min. ) “forgets” branches are pruned: m mcast data again flows down unpruned branch m downstream router: reprune or else continue to receive data r routers can quickly regraft to tree m following IGMP join at leaf r odds and ends m commonly implemented in commercial routers m Mbone routing done using DVMRP

Tunneling Q: How to connect “islands” of multicast routers in a “sea” of unicast routers? physical topology logical topology q mcast datagram encapsulated inside “normal” (non-multicast- addressed) datagram q normal IP datagram sent thru “tunnel” via regular IP unicast to receiving mcast router q receiving mcast router unencapsulates to get mcast datagram

PIM: Protocol Independent Multicast r not dependent on any specific underlying unicast routing algorithm (works with all) r two different multicast distribution scenarios : Dense: Sparse: q group members q # networks with group densely packed, in “close” proximity. q bandwidth more plentiful members small wrt # interconnected networks q group members “widely dispersed” q bandwidth not plentiful

Consequences of Sparse-Dense Dichotomy: Dense r group membership by Sparse: r no membership until routers assumed until routers explicitly prune r r data-driven construction on mcast tree (e. g. , RPF) r bandwidth and non-group r -router processing profligate routers explicitly join receiver- driven construction of mcast tree (e. g. , center-based) bandwidth and non-grouprouter processing conservative

PIM- Dense Mode flood-and-prune RPF, similar to DVMRP but q underlying unicast protocol provides RPF info for incoming datagram q less complicated (less efficient) downstream flood than DVMRP reduces reliance on underlying routing algorithm q has protocol mechanism for router to detect it is a leaf-node router

PIM - Sparse Mode r center-based approach r router sends join msg to rendezvous point (RP) m router can switch to source-specific tree increased performance: less concentration, shorter paths R 4 join intermediate routers update state and forward join r after joining via RP, m R 1 R 2 R 3 join R 5 join R 6 all data multicast from rendezvous point R 7 rendezvous point

PIM - Sparse Mode sender(s): r unicast data to RP, which distributes down RP-rooted tree r RP can extend mcast tree upstream to source r RP can send stop msg if no attached receivers m “no one is listening!” R 1 R 4 join R 2 R 3 join R 5 join R 6 all data multicast from rendezvous point R 7 rendezvous point

Chapter 4: summary r 4. 1 Introduction r 4. 2 Virtual circuit and datagram networks r 4. 3 What’s inside a router r 4. 4 IP: Internet Protocol m m Datagram format IPv 4 addressing ICMP IPv 6 r 4. 5 Routing algorithms m Link state m Distance Vector m Hierarchical routing r 4. 6 Routing in the Internet m m m RIP OSPF BGP r 4. 7 Broadcast and multicast routing Network Layer 46