Chapter 4 Network Layer Chapter goals Overview r

Chapter 4: Network Layer Chapter goals: Overview: r understand principles r network layer services behind network layer services: m m routing (path selection) dealing with scale how a router works advanced topics: IPv 6, mobility r instantiation and implementation in the Internet r routing principles: path selection r hierarchical routing r IP r Internet routing protocols m m intra-domain inter-domain r what’s inside a router? r IPv 6 r mobility Network Layer 1

Chapter 4 roadmap 4. 1 Introduction and Network Service Models 4. 2 Routing Principles 4. 3 Hierarchical Routing 4. 4 The Internet (IP) Protocol 4. 5 Routing in the Internet 4. 6 What’s Inside a Router 4. 7 IPv 6 4. 8 Multicast Routing Network Layer 2

Network layer functions r transport packet from sending to receiving hosts r network layer protocols in every host, router three important functions: r path determination: route taken by packets from source to dest. Routing algorithms r forwarding: move packets from router’s input to appropriate router output r call setup: some network architectures require router call setup along path before data flows application transport network data link physical network data link physical network data link physical application transport network data link physical Network Layer 3

Network service model service abstraction Q: What service model for “channel” transporting packets from sender to receiver? r guaranteed bandwidth? r preservation of inter-packet timing (no jitter)? r loss-free delivery? r in-order delivery? r congestion feedback to sender? The most important abstraction provided by network layer: ? ? ? virtual circuit or datagram? Network Layer 4

Virtual circuits “source-to-dest path behaves much like telephone circuit” m m performance-wise network actions along source-to-dest path r call setup, teardown for each call before data can flow r each packet carries VC identifier (not destination host ID) r every router on source-dest path maintains “state” for each passing connection m transport-layer connection only involved two end systems r link, router resources (bandwidth, buffers) may be allocated to VC m to get circuit-like perf. Network Layer 5

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

Datagram networks: the Internet model 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 1. Send data physical application transport 2. Receive data network data link physical Network Layer 7

Evolution of ATM-Based B-ISDN ATM – Asynchronous Transfer Mode B-ISDN – Broadband Integrated Services Digital Networks r ISDN failed because m It had low transmission rates to be able to support the new emerging applications m Did not support integration of services over the same channel (at the physical/link levels) r New technology has emerged m Optical networks – low error rates m High-speed switching Network Layer 8

New services and Traffic r A number of new services needed to be supported – Video, voice, data, streaming r These have different traffic characteristics m Peak rate (PCR) m Mean (sustainable) Rate (SCR) m Minimum Rate (MCR) m Burst Size (MBS) r and different Quality of Service Requirements m End-to-end delay (CTD) m Delay jitter (SDV) m Error rate (CER) m Routing accuracy (CMR) Network Layer 9

Evolution of B-ISDN (cont. ) r Traditional networks have been designed and optimized for a single application (e. g. , voice, video, data, telegraph) r A large number of services have emerged, e. g. , HDTV, video conferencing, medical imaging, distant learning, video on demand, electronic commerce, etc. r It is more economical and cost effective to serve all these applications by one network r This trend is facilitated by the evolution in the semiconductor, optical technologies, and the shifting transport functions to network periphery, which reduced cost of services Network Layer 10

Evolution of B-ISDN (cont. ) r The (narrowband) Integrated Services Digital Network (ISDN) was one step in this direction: integrated voice & data services r Problems: r - limited maximum bandwidth (2 Mbits/sec max) r - based on circuit switching (64 Kbits/sec) advances in data compression are not directly supported by (N)ISDN switches Network Layer 11

Range of Services for B-ISDN Network Layer 12

Transfer Modes r A transfer mode is a technique which is used in a telecommunication network covering aspects related to transmission, multiplexing and switching r A transfer mode should provide flexibility & adaptability to varying bit rates Since B-ISDN required flexibility, but at the same time must employ network wide lightweight protocols, modes near the middle of the spectrum were a good compromise – Hence ATM. Network Layer 13

Operational Characteristics r No error protection inside the network (handled by higher layers) r No flow control on a link-by-link basis r Connection-oriented mode: m Quality of Service (QOS) guarantees m Lightweight routing decisions r Reduced header functionality (mainly routing): fast processing & high throughputs r The information field is relatively small: m high degree of pipelining (emulation of cut-through) m small delay & delay jitter Network Layer 14

ATM Service Categories r Real time m Constant bit rate (CBR) m Real time variable bit rate (rt-VBR) r Non-real time m Non-real time variable bit rate (nrt-VBR) m Available bit rate (ABR) m Unspecified bit rate (UBR) Network Layer 15

Real Time Services r Amount of delay r Variation of delay (jitter) Network Layer 16

CBR r Fixed data rate continuously available r Tight upper bound on delay r Uncompressed audio and video m Video conferencing m Interactive audio m A/V distribution and retrieval Network Layer 17

rt-VBR r Time sensitive application m Tightly constrained delay and delay variation r rt-VBR applications transmit at a rate that varies with time r e. g. compressed video m Produces varying sized image frames m Original (uncompressed) frame rate constant m So compressed data rate varies r Can statistically multiplex connections Network Layer 18

nrt-VBR r May be able to characterize expected traffic flow r Improve Qo. S in loss and delay r End system specifies: m Peak cell rate m Sustainable or average rate m Measure of how bursty traffic is r e. g. Airline reservations, banking transactions Network Layer 19

UBR r May be additional capacity over and above that used by CBR and VBR traffic m Not all resources dedicated m Bursty nature of VBR r For application that can tolerate some cell loss or variable delays m e. g. TCP based traffic r Cells forwarded on FIFO basis r Best effort service Network Layer 20

ABR r Application specifies peak cell rate (PCR) and minimum cell rate (MCR) r Resources allocated to give at least MCR r Spare capacity shared among all ABR sources r e. g. LAN interconnection Network Layer 21

ATM Bit Rate Services Network Layer 22

Network layer service models: Network Architecture Internet Service Model Guarantees ? Congestion Bandwidth Loss Order Timing feedback best effort none ATM CBR ATM VBR ATM ABR ATM UBR constant rate guaranteed minimum none no no no yes yes yes no no (inferred via loss) no congestion yes no no r Internet model being extended: Intserv, Diffserv m Chapter 6 Network Layer 23

Datagram or VC network: why? Internet r data exchange among ATM r evolved from telephony computers r human conversation: m “elastic” service, no strict m strict timing, reliability timing requirements r “smart” end systems m need for guaranteed (computers) service m can adapt, perform r “dumb” end systems control, error recovery m telephones m simple inside network, m complexity inside complexity at “edge” network r many link types m different characteristics m uniform service difficult Network Layer 24

Chapter 4 roadmap 4. 1 Introduction and Network Service Models 4. 2 Routing Principles m Link state routing m Distance vector routing 4. 3 Hierarchical Routing 4. 4 The Internet (IP) Protocol 4. 5 Routing in the Internet 4. 6 What’s Inside a Router 4. 7 IPv 6 4. 8 Multicast Routing Network Layer 25

Routing protocol 5 Goal: determine “good” path (sequence of routers) thru network from source to dest. Graph abstraction for routing algorithms: r graph nodes are routers r graph edges are physical links m link cost: delay, $ cost, or congestion level 2 A B 2 1 D 3 C 3 1 5 F 1 E 2 r “good” path: m typically means minimum cost path m other def’s possible Network Layer 26

Routing Algorithm classification Global or decentralized information? Global: r all routers have complete topology, link cost info r “link state” algorithms Decentralized: r router knows physicallyconnected neighbors, link costs to neighbors r iterative process of computation, exchange of info with neighbors r “distance vector” algorithms 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 Network Layer 27

A Link-State Routing Algorithm Dijkstra’s algorithm r net topology, link costs known to all nodes m accomplished via “link state broadcast” m all nodes have same info r computes least cost paths from one node (‘source”) to all other nodes m gives routing table for that node r iterative: after k iterations, know least cost path to k dest. ’s Notation: r c(i, j): link cost from node i to j. cost infinite if not direct neighbors r D(v): current value of cost of path from source to dest. V r p(v): predecessor node along path from source to v, that is next v r N: set of nodes whose least cost path definitively known Network Layer 28

Dijsktra’s Algorithm 1 Initialization: 2 N = {A} 3 for all nodes v 4 if v adjacent to A 5 then D(v) = c(A, v) 6 else D(v) = infinity 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N: 12 D(v) = min( D(v), D(w) + c(w, v) ) 13 /* new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v */ 15 until all nodes in N Network Layer 29

Dijkstra’s algorithm: example Step 0 1 2 3 4 5 start N A AD ADEBCF D(B), p(B) D(C), p(C) D(D), p(D) D(E), p(E) D(F), p(F) 2, A 1, A 5, A infinity 2, A 4, D 2, D infinity 2, A 3, E 4, E 5 2 A B 2 1 D 3 C 3 1 5 F 1 E 2 Network Layer 30

Dijkstra’s algorithm, discussion Algorithm complexity: n nodes r each iteration: need to check all nodes, w, not in N r n*(n+1)/2 comparisons: O(n**2) r more efficient implementations possible: O(nlogn) Oscillations possible: r e. g. , link cost = amount of carried traffic D 1 1 0 A 0 0 C e 1+e e initially B 1 2+e A 0 D 1+e 1 B 0 0 C … recompute routing 0 D 1 A 0 0 C 2+e B 1+e … recompute 2+e A 0 D 1+e 1 B e 0 C … recompute Network Layer 31

Distance Vector Routing Algorithm iterative: r continues until no nodes exchange info. r self-terminating: no “signal” to stop asynchronous: r nodes need not exchange info/iterate in lock step! distributed: r each node communicates only with directly-attached neighbors Distance Table data structure r each node has its own r row for each possible destination r column for each directly- attached neighbor to node r example: in node X, for dest. Y via neighbor Z: X D (Y, Z) distance from X to = Y, via Z as next hop Z = c(X, Z) + minw{D (Y, w)} Network Layer 32

Distance Table: example A E D (C, D) D (A, D) E C E cost to destination via D () A B D A 1 14 5 B 7 8 5 C 6 9 4 D 4 11 2 2 8 1 E B E 2 D D = c(E, D) + minw {D (C, w)} = 2+2 = 4 D = c(E, D) + minw {D (A, w)} = 2+3 = 5 loop! destination 7 1 B D (A, B) = c(E, B) + minw{D (A, w)} = 8+6 = 14 loop! Network Layer 33

Distance table gives routing table E cost to destination via Outgoing link to use, cost B D A 1 14 5 A A, 1 B 7 8 5 B D, 5 C 6 9 4 C D, 4 D 4 11 2 D D, 4 Distance table destination A destination D () Routing table Network Layer 34

Distance Vector Routing: overview Iterative, asynchronous: each local iteration caused by: r local link cost change r message from neighbor: its least cost path change from neighbor Distributed: r each node notifies neighbors only when its least cost path to any destination changes m neighbors then notify their neighbors if necessary Each node: wait for (change in local link cost of msg from neighbor) recompute distance table if least cost path to any dest has changed, notify neighbors Network Layer 35

Distance Vector Algorithm: At all nodes, X: 1 Initialization: 2 for all adjacent nodes v: 3 DX(*, v) = infinity /* the * operator means "for all rows" */ X 4 D (v, v) = c(X, v) 5 for all destinations, y X 6 send min D (y, w) to each neighbor /* w over all X's neighbors */ w Network Layer 36

Distance Vector Algorithm (cont. ): 8 loop 9 wait (until I see a link cost change to neighbor V 10 or until I receive update from neighbor V) 11 12 if (c(X, V) changes by d) 13 /* change cost to all dest's via neighbor v by d */ 14 /* note: d could be positive or negative */ 15 for all destinations y: DX(y, V) = DX(y, V) + d 16 17 else if (update received from V wrt destination Y) 18 /* shortest path from V to some Y has changed */ 19 /* V has sent a new value for its minw DV(Y, w) */ 20 /* call this received new value is "newval" */ 21 for the single destination y: DX(Y, V) = c(X, V) + newval 22 23 if we have a new minw DX(Y, w)for any destination Y 24 send new value of min w DX(Y, w) to all neighbors 25 Network Layer 26 forever 37

Distance Vector Algorithm: example X 2 Y 7 1 Z Network Layer 38

Distance Vector Algorithm: example X 2 Y 7 1 Z Z X D (Y, Z) = c(X, Z) + minw{D (Y, w)} = 7+1 = 8 Y X D (Z, Y) = c(X, Y) + minw {D (Z, w)} = 2+1 = 3 Network Layer 39

Distance Vector: link cost changes Link cost changes: r node detects local link cost change r updates distance table (line 15) r if cost change in least cost path, notify neighbors (lines 23, 24) “good news travels fast” 1 X 4 Y 1 50 Z algorithm terminates Network Layer 40

Distance Vector: link cost changes Link cost changes: r good news travels fast r bad news travels slow - “count to infinity” problem! 60 X 4 Y 1 50 Z algorithm continues on! Network Layer 41

Distance Vector: poisoned reverse If Z routes through Y to get to X : r Z tells Y its (Z’s) distance to X is infinite (so Y won’t route to X via Z) r will this completely solve count to infinity problem? 60 X 4 Y 50 1 Z algorithm terminates Network Layer 42

Example r Consider the following network. Let the delay distance vector at nodes A, I, H and K be as given below, and let their respective measured distance to node J be 6, 9, 13 and 5. What will the routing table at node J look like after the next information exchange? Put the answer in the box below. Network Layer 43

Comparison of LS and DV algorithms Message complexity r LS: with n nodes, E links, O(n. E) msgs sent each r DV: exchange between neighbors only m convergence time varies Speed of Convergence r LS: O(n 2) algorithm requires O(n. E) msgs m may have oscillations r DV: convergence time varies m may be routing loops m count-to-infinity problem Robustness: what happens if router malfunctions? LS: m m node can advertise incorrect link cost each node computes only its own table DV: m m DV node can advertise incorrect path cost each node’s table used by others • error propagate thru network Network Layer 44

Chapter 4 roadmap 4. 1 Introduction and Network Service Models 4. 2 Routing Principles 4. 3 Hierarchical Routing 4. 4 The Internet (IP) Protocol m 4. 4. 1 IPv 4 addressing m 4. 4. 2 Moving a datagram from source to destination m 4. 4. 3 Datagram format m 4. 4. 4 IP fragmentation m 4. 4. 5 ICMP: Internet Control Message Protocol m 4. 4. 6 DHCP: Dynamic Host Configuration Protocol m 4. 4. 7 NAT: Network Address Translation 4. 5 Routing in the Internet 4. 6 What’s Inside a Router 4. 7 IPv 6 4. 8 Multicast Routing Network Layer 45

The Internet Network layer Host, router network layer functions: Transport layer: TCP, UDP Network layer IP protocol • addressing conventions • datagram format • packet handling conventions Routing protocols • path selection • RIP, OSPF, BGP forwarding table ICMP protocol • error reporting • router “signaling” Link layer physical layer Network Layer 46

IP Addressing: introduction r IP address: 32 -bit identifier for host, router interface: connection between host/router and physical link m m m router’s typically have multiple interfaces host may have multiple interfaces IP addresses associated with each interface 223. 1. 1. 1 223. 1. 1. 2 223. 1. 1. 4 223. 1. 1. 3 223. 1. 2. 1 223. 1. 2. 9 223. 1. 3. 27 223. 1. 2. 2 223. 1. 1. 1 = 11011111 00000001 223 1 1 Network Layer 1 47

IP Addressing r IP address: m network part (high order bits) m host part (low order bits) r What’s a network ? (from IP address perspective) m device interfaces with same network part of IP address m can physically reach other without intervening router 223. 1. 1. 1 223. 1. 1. 2 223. 1. 1. 4 223. 1. 1. 3 223. 1. 2. 1 223. 1. 2. 9 223. 1. 3. 27 223. 1. 2. 2 LAN 223. 1. 3. 2 network consisting of 3 IP networks (for IP addresses starting with 223, first 24 bits are network address) Network Layer 48

IP Addressing How to find the networks? r Detach each interface from router, host r create “islands of isolated networks 223. 1. 1. 2 223. 1. 1. 1 223. 1. 1. 4 223. 1. 1. 3 223. 1. 9. 2 223. 1. 7. 0 223. 1. 9. 1 223. 1. 7. 1 223. 1. 8. 0 223. 1. 2. 6 Interconnected system consisting of six networks 223. 1. 2. 1 223. 1. 3. 27 223. 1. 2. 2 223. 1. 3. 2 Network Layer 49

IP Addresses given notion of “network”, let’s re-examine IP addresses: “class-full” addressing: class A 0 network B 10 C 110 D 1110 1. 0. 0. 0 to 127. 255 host network 128. 0. 0. 0 to 191. 255 host network multicast address host 192. 0. 0. 0 to 223. 255 224. 0. 0. 0 to 239. 255 32 bits Network Layer 50

IP addressing: CIDR r Classful addressing: m m inefficient use of address space, address space exhaustion e. g. , class B net allocated enough addresses for 65 K hosts, even if only 2 K hosts in that network r CIDR: Classless Inter. Domain Routing m m network portion of address of arbitrary length address format: a. b. c. d/x, where x is # bits in network portion of address network part host part 11001000 00010111 00010000 200. 23. 16. 0/23 Network Layer 51

IP addresses: how to get one? Q: How does host get IP address? r hard-coded by system admin in a file m Wintel: control-panel->network->configuration>tcp/ip->properties m UNIX: /etc/rc. config r DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server m “plug-and-play” (more shortly) Network Layer 52

IP addresses: how to get one? Q: How does network get network 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 53

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 54

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 55

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 56

Getting a datagram from source to dest. forwarding table in A Dest. Net. next router Nhops 223. 1. 1 223. 1. 2 223. 1. 3 IP datagram: misc source dest fields IP addr data A r datagram remains unchanged, as it travels source to destination r addr fields of interest here B 223. 1. 1. 4 1 2 2 223. 1. 1. 1 223. 1. 1. 2 223. 1. 1. 4 223. 1. 1. 3 223. 1. 2. 9 223. 1. 3. 27 223. 1. 2. 2 E 223. 1. 3. 2 Network Layer 57

Getting a datagram from source to dest. forwarding table in A misc data fields 223. 1. 1. 1 223. 1. 1. 3 Dest. Net. next router Nhops 223. 1. 1 223. 1. 2 223. 1. 3 Starting at A, send IP datagram addressed to B: r look up net. address of B in forwarding table r find B is on same net. as A r link layer will send datagram directly to B inside link-layer frame m B and A are directly connected A B 223. 1. 1. 4 1 2 2 223. 1. 1. 1 223. 1. 1. 2 223. 1. 1. 4 223. 1. 1. 3 223. 1. 2. 9 223. 1. 3. 27 223. 1. 2. 2 E 223. 1. 3. 2 Network Layer 58

Getting a datagram from source to dest. forwarding table in A misc data fields 223. 1. 1. 1 223. 1. 2. 3 Dest. Net. next router Nhops 223. 1. 1 223. 1. 2 223. 1. 3 Starting at A, dest. E: r look up network address of E r r r in forwarding table E on different network m A, E not directly attached routing table: next hop router to E is 223. 1. 1. 4 link layer sends datagram to router 223. 1. 1. 4 inside linklayer frame datagram arrives at 223. 1. 1. 4 continued…. . A B 223. 1. 1. 4 1 2 2 223. 1. 1. 1 223. 1. 1. 2 223. 1. 1. 4 223. 1. 1. 3 223. 1. 2. 9 223. 1. 3. 27 223. 1. 2. 2 E 223. 1. 3. 2 Network Layer 59

Getting a datagram from source to dest. misc data fields 223. 1. 1. 1 223. 1. 2. 3 Arriving at 223. 1. 4, destined for 223. 1. 2. 2 r look up network address of E in router’s forwarding table r E on same network as router’s interface 223. 1. 2. 9 m router, E directly attached r link layer sends datagram to 223. 1. 2. 2 inside link-layer frame via interface 223. 1. 2. 9 r datagram arrives at 223. 1. 2. 2!!! (hooray!) forwarding table in router Dest. Net router Nhops interface 223. 1. 1 223. 1. 2 223. 1. 3 A B - 1 1 1 223. 1. 1. 4 223. 1. 2. 9 223. 1. 3. 27 223. 1. 1. 1 223. 1. 1. 2 223. 1. 1. 4 223. 1. 1. 3 223. 1. 2. 9 223. 1. 3. 27 223. 1. 2. 2 E 223. 1. 3. 2 Network Layer 60

IP datagram format IP protocol version number header length (bytes) “type” of data max number remaining hops (decremented at each router) upper layer protocol to deliver payload to how much overhead with TCP? r 20 bytes of TCP r 20 bytes of IP r = 40 bytes + app layer overhead 32 bits ver head. type of len service length fragment 16 -bit identifier flgs offset upper time to Internet layer live checksum total datagram length (bytes) for fragmentation/ reassembly 32 bit source IP address 32 bit destination IP address Options (if any) data (variable length, typically a TCP or UDP segment) E. g. timestamp, record route taken, specify list of routers to visit. Network Layer 61

IP Fragmentation & 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 Network Layer 62

IP Fragmentation and Reassembly Example r 4000 byte datagram r MTU = 1500 bytes length ID fragflag offset =4000 =x =0 =0 One large datagram becomes several smaller datagrams length ID fragflag offset =1500 =x =1 =0 length ID fragflag offset =1500 =x =1 =1480 length ID fragflag offset =1040 =x =0 =2960 Network Layer 63

ICMP: Internet Control Message Protocol r used by hosts, routers, gateways to communication 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 0 3 3 3 4 Code 0 0 1 2 3 6 7 0 8 9 10 11 12 0 0 0 description echo reply (ping) dest. network unreachable dest host unreachable dest protocol unreachable dest port unreachable dest network unknown dest host unknown source quench (congestion control - not used) echo request (ping) route advertisement router discovery TTL expired bad IP header Network Layer 64

ICMP Message Formats Network Layer 65

Route Redirection Network Layer 66

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 67

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 68

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 69

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 70

NAT: Network Address Translation r Motivation: local network uses just one IP address as far as outside word is concerned: m no need to be allocated range of addresses from ISP: - just one IP address is used 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 71

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 72

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, 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 73

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, e. g. , P 2 P applications m address IPv 6 shortage should instead be solved by Network Layer 74

Chapter 4 roadmap 4. 1 Introduction and Network Service Models 4. 2 Routing Principles 4. 3 Hierarchical Routing 4. 4 The Internet (IP) Protocol 4. 5 Routing in the Internet m 4. 5. 1 Intra-AS routing: RIP and OSPF m 4. 5. 2 Inter-AS routing: BGP 4. 6 What’s Inside a Router? 4. 7 IPv 6 4. 8 Multicast Routing Network Layer 75

Internet as a Single Network Layer 76

Autonomous Systems r This “one network” was replaced with several “autonomous systems” (AS). Network Layer 77

Autonomous System (AS) AS 100 r Set of routers & networks under the same administration r Group of routers exchanging information via a common routing protocol r Interior routing protocols are used to provide internal connectivity Network Layer 78

Autonomous System (AS) … r Each AS is identified by a 16 -bit AS number. r E. g. has 1 autonomous system: r ASNumber: 31983 QUEENSU-KINGSTON 2004 -02 -04 19 Division Street IT Services, Dupuis Hall r ASName: r Reg. Date: r Address: Network Layer 79

Routing in the Internet r So, the Global Internet consists of Autonomous Systems (AS) interconnected with each other: m m m Stub AS: small corporation: one connection to other AS’s Multihomed AS: large corporation (no transit): multiple connections to other AS’s Transit AS: provider, hooking many AS’s together r Two-level routing: m Intra-AS: administrator responsible for choice of routing algorithm within network m Inter-AS: unique standard for inter-AS routing: BGP Network Layer 80

Internet AS Hierarchy Inter-AS border (exterior gateway) routers Intra-AS interior (gateway) routers Network Layer 81

Intra-AS Routing r Also known as Interior Gateway Protocols (IGP) r Most common Intra-AS routing protocols: m RIP: Routing Information Protocol m OSPF: Open Shortest Path First m IGRP: Interior Gateway Routing Protocol (Cisco proprietary) Network Layer 82

RIP ( Routing Information Protocol) r Distance vector algorithm r Included in BSD-UNIX Distribution in 1982 r Distance metric: # of hops (max = 15 hops) m Can you guess why? r Distance vectors: exchanged among neighbors every 30 sec via Response Message (also called advertisement) r Each advertisement: list of up to 25 destination nets within AS Network Layer 83

RIP: Example z w A x D B y C Destination Network w y z x …. Next Router Num. of hops to dest. …. . . A B B -- 2 2 7 1 Routing table in D Network Layer 84

RIP: Example Dest w x z …. Next C … w hops 4. . . A Advertisement from A to D z x Destination Network w y z x …. D B C y Next Router Num. of hops to dest. …. . . A B B A -- Routing table in D 2 2 7 5 1 Network Layer 85

RIP: Link Failure and Recovery If no advertisement heard after 180 sec --> neighbor/link declared dead m routes via neighbor invalidated m new advertisements sent to neighbors m neighbors in turn send out new advertisements (if tables changed) m link failure info quickly propagates to entire net m poison reverse used to prevent ping-pong loops (infinite distance = 16 hops) Network Layer 86

RIP Table processing r RIP routing tables managed by application-level process called route-d (daemon) r advertisements sent in UDP packets, periodically repeated routed Transprt (UDP) network (IP) link physical Transprt (UDP) forwarding table network (IP) link physical Network Layer 87

RIP Table example (continued) Router: giroflee. eurocom. fr Destination ----------127. 0. 0. 1 192. 168. 2. 193. 55. 114. 192. 168. 3. 224. 0. 0. 0 default Gateway Flags Ref Use Interface ---------- --------127. 0. 0. 1 UH 0 26492 lo 0 192. 168. 2. 5 U 2 13 fa 0 193. 55. 114. 6 U 3 58503 le 0 192. 168. 3. 5 U 2 25 qaa 0 193. 55. 114. 6 U 3 0 le 0 193. 55. 114. 129 UG 0 143454 r Three attached class C networks (LANs) r Router only knows routes to attached LANs r Default router used to “go up” r Route multicast address: 224. 0. 0. 0 r Loopback interface (for debugging) Network Layer 88

OSPF (Open Shortest Path First) r “open”: publicly available r Uses Link State algorithm m LS packet dissemination m Topology map at each node m Route computation using Dijkstra’s algorithm r OSPF advertisement carries one entry per neighbor router r Advertisements disseminated to entire AS (via flooding) m Carried in OSPF messages directly over IP (rather than TCP or UDP Network Layer 89

OSPF “advanced” features (not in RIP) r Security: all OSPF messages authenticated (to r r prevent malicious intrusion) Multiple same-cost paths allowed (only one path in RIP) For each link, multiple cost metrics for different TOS (e. g. , satellite link cost set “low” for best effort; high for real time) Integrated uni- and multicast support: m Multicast OSPF (MOSPF) uses same topology data base as OSPF Hierarchical OSPF in large domains. Network Layer 90

Hierarchical OSPF Network Layer 91

Hierarchical OSPF r Two-level hierarchy: local area, backbone. m Link-state advertisements only in area m each nodes has detailed area topology; only know direction (shortest path) to nets in other areas. r Area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers. r Backbone routers: run OSPF routing limited to backbone. r Boundary routers: connect to other AS’s. Network Layer 92

Inter-AS routing in the Internet: BGP Network Layer 93

Internet inter-AS routing: BGP r BGP (Border Gateway Protocol): the de facto standard r Path Vector protocol: m similar to Distance Vector protocol m each Border Gateway broadcast to neighbors (peers) entire path (i. e. , sequence of AS’s) to destination m BGP routes to networks (ASs), not individual hosts m E. g. , Gateway X may send its path to dest. Z: Path (X, Z) = X, Y 1, Y 2, Y 3, …, Z Network Layer 94

Internet inter-AS routing: BGP Suppose: gateway X send its path to peer gateway W r W may or may not select path offered by X m cost, policy (don’t route via competitors AS), loop prevention reasons. r If W selects path advertised by X, then: Path (W, Z) = w, Path (X, Z) r Note: X can control incoming traffic by controlling it route advertisements to peers: m e. g. , don’t want to route traffic to Z -> don’t advertise any routes to Z Network Layer 95

BGP: controlling who routes to you r A, B, C are provider networks r X, W, Y are customer (of provider networks) r X is dual-homed: attached to two networks m. X does not want to route from B via X to C m. . so X will not advertise to B a route to C Network Layer 96

BGP: controlling who routes to you r A advertises to B the path AW r B advertises to X the path BAW r Should B advertise to C the path BAW? m No way! B gets no “revenue” for routing CBAW since neither W nor C are B’s customers m B wants to force C to route to w via A m B wants to route only to/from its customers! Network Layer 97

BGP operation Q: What does a BGP router do? r Receiving and filtering route advertisements from directly attached neighbor(s). r Route selection. m To route to destination X, which path )of several advertised) will be taken? r Sending route advertisements to neighbors. Network Layer 98

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 Network Layer 99

BGP Routing Information Exchange r Update Message includes: m AS_Path: List of identifiers: {AS 7, AS 1} m Next_Hop: The IP address of D m NLRI: A list of all of the subnetworks in AS 1 and AS 7 AS 5 AS 1 B C A G Update Message D I H J F E AS 7 Network Layer 100

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 Network Layer 101

Chapter 4 roadmap 4. 1 Introduction and Network Service Models 4. 2 Routing Principles 4. 3 Hierarchical Routing 4. 4 The Internet (IP) Protocol 4. 5 Routing in the Internet 4. 6 What’s Inside a Router? 4. 7 IPv 6 4. 8 Multicast Routing Network Layer 102

Router Architecture Overview Two key router functions: r run routing algorithms/protocol (RIP, OSPF, BGP) r switching datagrams from incoming to outgoing link Network Layer 103

Input Port Functions Physical layer: bit-level reception Data link layer: e. g. , Ethernet see chapter 5 Decentralized switching: r given datagram dest. , lookup output port using routing table in input port memory r goal: complete input port processing at ‘line speed’ r queuing: if datagrams arrive faster than forwarding rate into switch fabric Network Layer 104

Input Port Queuing r Fabric slower that input ports combined -> queueing may occur at input queues r Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward r queueing delay and loss due to input buffer overflow! Network Layer 105

Three types of switching fabrics Network Layer 106

Switching Via Memory First generation routers: r packet copied by system’s (single) CPU r speed limited by memory bandwidth (2 bus crossings per datagram) Input Port Memory Output Port System Bus Modern routers: r input port processor performs lookup, copy into memory r Cisco Catalyst 8500 Network Layer 107

Switching Via a Bus r datagram from input port memory to output port memory via a shared bus r bus contention: switching speed limited by bus bandwidth r 1 Gbps bus, Cisco 1900: sufficient speed for access and enterprise routers (not regional or backbone) Network Layer 108

Switching Via An Interconnection Network r overcomes bus bandwidth limitations r Banyan networks, other interconnection nets initially developed to connect processors in multiprocessor r Advanced design: fragmenting datagram into fixed length cells, switch cells through the fabric. r Cisco 12000: switches Gbps through the interconnection network Network Layer 109

Output Ports r Buffering required when datagrams arrive from fabric faster than the transmission rate r Scheduling discipline chooses among queued datagrams for transmission Network Layer 110

Output port queueing r buffering when arrival rate via switch exceeds output line speed r queueing (delay) and loss due to output port buffer overflow! Network Layer 111

Chapter 4 roadmap 4. 1 Introduction and Network Service Models 4. 2 Routing Principles 4. 3 Hierarchical Routing 4. 4 The Internet (IP) Protocol 4. 5 Routing in the Internet 4. 6 What’s Inside a Router? 4. 7 IPv 6 Network Layer 112

IPv 6 r Initial motivation: 32 -bit address space completely allocated by 2008. r Additional motivation: m header format helps speed processing/forwarding m header changes to facilitate Qo. S m new “anycast” address: route to “best” of several replicated servers r IPv 6 datagram format: m fixed-length 40 byte header m no fragmentation allowed Network Layer 113

IPv 6 Header (Cont) Priority: identify priority among datagrams in flow Flow Label: identify datagrams in same “flow. ” (concept of “flow” – 5 -tuple – Src, dst IP, Src, dst Port, Protocol). Next header: identify upper layer protocol for data Network Layer 114

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 115

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 Two proposed approaches: m Dual Stack: some routers with dual stack (v 6, v 4) can “translate” between formats m Tunneling: IPv 6 carried as payload in IPv 4 datagram among IPv 4 routers Network Layer 116

Dual Stack Approach A B C D E F IPv 6 IPv 4 IPv 6 Flow: X Src: A Dest: F Flow: ? ? Src: A Dest: F data B-to-C: IPv 4 B-to-C: IPv 6 A-to-B: IPv 6 Network Layer 117

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 118

Chapter 4 roadmap 4. 1 Introduction and Network Service Models 4. 2 Routing Principles 4. 3 Hierarchical Routing 4. 4 The Internet (IP) Protocol 4. 5 Routing in the Internet 4. 6 What’s Inside a Router? 4. 7 IPv 6 4. 8 Multicast Routing Network Layer 119

Multicast: one sender to many receivers r Multicast: act of sending datagram to multiple receivers with single “transmit” operation m analogy: one teacher to many students r Question: how to achieve multicast Multicast via unicast r source sends N unicast datagrams, one addressed to each of N receivers routers forward unicast datagrams multicast receiver (red) not a multicast receiver (red) Network Layer 120

Multicast: one sender to many receivers r Multicast: act of sending datagram to multiple receivers with single “transmit” operation m analogy: one teacher to many students r Question: how to achieve multicast Network multicast r Router actively Multicast routers (red) duplicate and forward multicast datagrams participate in multicast, making copies of packets as needed and forwarding towards multicast receivers Network Layer 121

Multicast: one sender to many receivers r Multicast: act of sending datagram to multiple receivers with single “transmit” operation m analogy: one teacher to many students r Question: how to achieve multicast Application-layer multicast r end systems involved in multicast copy and forward unicast datagrams among themselves Network Layer 122

Internet Multicast Service Model 128. 59. 16. 12 128. 119. 40. 186 multicast group 226. 17. 30. 197 128. 34. 108. 63 128. 34. 108. 60 multicast group concept: use of indirection m hosts addresses IP datagram to multicast group m routers forward multicast datagrams to hosts that have “joined” that multicast group Network Layer 123

Multicast groups q class D Internet addresses reserved for multicast: q host group semantics: anyone can “join” (receive) multicast group o anyone can send to multicast group o no network-layer identification to hosts of members q needed: infrastructure to deliver mcast-addressed datagrams to all hosts that have joined that multicast group o Network Layer 124

Joining a mcast group: two-step process r local: host informs local mcast router of desire to join group: IGMP (Internet Group Management Protocol) r wide area: local router interacts with other routers to receive mcast datagram flow m many protocols (e. g. , DVMRP, MOSPF, PIM) IGMP wide-area multicast routing IGMP Network Layer 125

IGMP: Internet Group Management Protocol r host: sends IGMP report when application joins mcast group m IP_ADD_MEMBERSHIP socket option m host need not explicitly “unjoin” group when leaving r router: sends IGMP query at regular intervals m host belonging to a mcast group must reply to query report Network Layer 126

IGMP version 1 r router: Host Membership Query msg broadcast on LAN to all hosts r host: Host Membership Report msg to indicate group membership m m randomized delay before responding implicit leave via no reply to Query IGMP v 2: additions include r group-specific Query r Leave Group msg m m m last host replying to Query can send explicit Leave Group msg router performs groupspecific query to see if any hosts left in group RFC 2236 IGMP v 3: under development as Internet draft r RFC 1112 Network Layer 127

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 only look at basic approaches. Specific protocols adopting these approaches (DVMRP and PIM) are in the text.

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