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 m how a router works routing (path selection) dealing with scale r instantiation and implementation in the Internet (incl. advanced topics: IPv 6, multicast) m VC, datagram r what’s inside a router? r Addressing, forwarding, IP r routing principle: path selection m m hierarchical routing Internet routing protocols r (multicast routing) 4: Network Layer 4 a-1

Network layer r transport packet from sending to receiving hosts r network layer protocols in every host, router important functions r path determination: route taken by packets from source to dest. Routing algorithms r switching: move packets from router’s input to appropriate router output r call setup: (in some network architectures) along path before data flows r congestion control (in some network architectures) application transport network data link physical network data link physical network data link physical 4: Network Layer application transport network data link physical 4 a-2

Interplay between routing and forwarding r routing: process of planning trip from source to dest routing algorithm local forwarding table header value output link 0100 0101 0111 1001 r forwarding: process of getting through single interchange 3 2 2 1 value in arriving packet’s header 0111 1 3 2 4: Network Layer 4 a-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? 4: Network Layer 4 a-4

Virtual circuits: “source-to-dest path behaves almost like telephone circuit” r call setup, teardown for each call before data can flow m signaling protocols to setup, maintain teardown VC (ATM, frame-relay, X. 25; not in IP) r each packet carries VC identifier (not destination host) r every router maintains “state” for each passing connection r resources (bandwidth, buffers) may be allocated to VC 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 4: Network Layer 4 a-5

Forwarding table in a VC network VC number 22 12 1 Forwarding table in northwest router: Incoming interface 1 2 3 1 … 2 32 3 interface number Incoming VC # 12 63 7 97 … Outgoing interface 3 1 2 3 … Outgoing VC # 22 18 17 87 … Routers maintain connection state information! 4: Network Layer 4 a-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 typically routed using destination host ID 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 4: Network Layer 4 a-7

Forwarding table in a datagram network Destination Address Range 4 billion possible entries Link Interface 11001000 00010111 00010000 through 11001000 00010111 1111 0 11001000 00010111 00011000 0000 through 11001000 00010111 00011000 1111 1 11001000 00010111 00011001 0000 through 11001000 00010111 00011111 2 otherwise 3 4: Network Layer 4 a-8

Forwarding table in datagram NWs: in practice by masking: Longest prefix matching Prefix Match 11001000 00010111 00010 11001000 00010111 00011000 11001000 00010111 00011 otherwise Link Interface 0 1 2 3 Examples DA: 11001000 00010111 00010110 10100001 Which interface? DA: 11001000 00010111 00011000 1010 Which interface? 4: Network Layer 4 a-9

Datagram or VC network: why? e. g. Internet r data exchange among e. g. ATM r evolved from telephony r human conversation-like computers paradigm: 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 Like (older) telephones m simple inside network, complexity at “edge” r “clever”, fast routers: to efficiently pipeline r But: many traffic types receiving/forwarding, m different characteristics eliminating store&forward m uniform service difficult delays 4: Network Layer 4 a-10

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 extented: Intserv, Diffserv m (will study these later on) 4: Network Layer 4 a-11

Router Architecture Overview 4: Network Layer 4 a-12

Router Architecture Overview Two key router functions: r run routing algorithms/protocol r switching packets from incoming to outgoing link 4: Network Layer 4 a-13

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 4: Network Layer 4 a-14

Input Port Queuing r Fabric slower that input ports combined -> queueing may occur at input queues r Head-of-the-Line blocking: queued datagram at front of queue prevents others in queue from moving forward r queueing delay and loss due to input buffer overflow! 4: Network Layer 4 a-15

Three types of switching fabrics 4: Network Layer 4 a-16

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 4: Network Layer 4 a-17

Switching Via 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) 4: Network Layer 4 a-18

Switching Via An Interconnection Network r overcome 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 (ATMnetwork principle). r Cisco 12000: switches 60 Gbps through the interconnection network 4: Network Layer 4 a-19

Output Ports r Buffering required when datagrams arrive from fabric faster than the transmission rate r Scheduling discipline chooses among queued datagrams for transmission (cf. Qo. S guarantees, to be discussed in multimedia context) 4: Network Layer 4 a-20

Output port queueing r buffering when arrival rate via switch exceeeds ouput line speed r queueing (delay) and loss due to output port buffer overflow! 4: Network Layer 4 a-21

Roadmap Chapter goals: Overview: r understand principles r network layer services behind network layer m VC, datagram services: r what’s inside a router? m how a router works r Addressing, forwarding, m routing (path IP selection) r routing principle: path m dealing with scale selection r instantiation and m hierarchical routing Internet routing protocols m implementation in the Internet (incl. IPv 6, r (multicast routing) multicast) 4: Network Layer 4 a-22

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 routing table ICMP protocol • error reporting • router “signaling” Link layer physical layer 4: Network Layer 4 a-23

IPv 4 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 (www. iana. org: dynamic DB for numbers, constants, etc) 32 bits type of ver head. len service length fragment 16 -bit identifier flgs offset time to upper Internet layer live checksum 32 bit source IP address total datagram length (bytes) for fragmentation/ reassembly Why? 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. 4: Network Layer 4 a-24

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 typically has one interface 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 1 4: Network Layer 4 a-25

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 in their IP addresses 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 subnet 223. 1. 3. 2 network consisting of 3 subnets 4: Network Layer 4 a-26

Subnets Recipe r To determine the subnets, detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet. 223. 1. 1. 0/24 223. 1. 2. 0/24 223. 1. 3. 0/24 Subnet mask: /24 4: Network Layer 4 a-27

IP addressing: CIDR: Classless Inter. Domain 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 subnet part host part 11001000 00010111 00010000 200. 23. 16. 0/23 4: Network Layer 4 a-28

Internet hierarchical routing C. b a Host h 1 C b A. a Inter-AS routing between A and B A. c a d c b A Intra-AS routing within AS A scale: with 50 million destinations: r can’t store all dest’s in routing tables! r routing table exchange would swamp links! B. a a c B Host h 2 b Intra-AS routing within AS B r We’ll examine Internet routing algorithms and protocols shortly 4: Network Layer 4 a-29

IP addresses: how to get one? Host portion: r hard-coded by system admin in a file; or r DHCP: Dynamic Host Configuration Protocol: dynamically get address: 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 4: Network Layer 4 a-30

IP addresses: how to get one? Network portion: r get allocated portion of ISP’s address space: ISP's block 11001000 00010111 00010000 200. 23. 16. 0/20 Organization 0 11001000 00010111 00010000 200. 23. 16. 0/23 Organization 1 11001000 00010111 00010010 0000 200. 23. 18. 0/23 Organization 2. . . 11001000 00010111 00010100 0000 …. 200. 23. 20. 0/23 …. Organization 7 11001000 00010111 00011110 0000 200. 23. 30. 0/23 4: Network Layer 4 a-31

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 4: Network Layer 4 a-32

Well, it was not really the last word… 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) 4: Network Layer 4 a-33

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). 4: Network Layer 4 a-34

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 4: Network Layer 4 a-35

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. 1 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 4: Network Layer 4 a-36

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 4: Network Layer 4 a-37

NAT traversal problem r client want 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 (manual): 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 2500 4: Network Layer 4 a-38

NAT traversal problem r solution 2 (protocol) : Universal Plug and Play (UPn. P) Internet Gateway Device (IGD) Protocol. Allows NATted host to: v learn public IP address (138. 76. 29. 7) 138. 76. 29. 7 v enumerate existing port mappings v add/remove port mappings (with lease times) 10. 0. 0. 1 IGD 10. 0. 0. 4 NAT router i. e. , automate static NAT port map configuration 4: Network Layer 4 a-39

NAT traversal problem r solution 3 (application): relaying (used in Skype) m NATed server establishes connection to relay m External client connects to relay m relay bridges packets between two 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 4: Network Layer 4 a-40

Getting a datagram from source to dest. Routing/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 4: Network Layer 4 a-41

Getting a datagram from source to dest. 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, given IP datagram addressed to B: r look up net. address of B r find B is on same net. as A (B and A are directly connected) r link layer will send datagram directly to B (inside link-layer frame) 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 4: Network Layer 4 a-42

Getting a datagram from source to dest. 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 E on different network r routing table: next hop router to E is 223. 1. 1. 4 r link layer is asked to send datagram to router 223. 1. 1. 4 (inside link-layer frame) r datagram arrives at 223. 1. 1. 4 r 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 4: Network Layer 4 a-43

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 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!) Dest. next network 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 4: Network Layer 4 a-44

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 4: Network Layer 4 a-45

IP Fragmentation and Reassembly 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 =1500 length ID fragflag offset =1000 =x =0 =3000 4: Network Layer 4 a-46

IPv 6 r Initial motivation: prediction: 32 -bit address space completely allocated by approx. 2008. r Additional motivation: m header format helps speed processing/forwarding m header changes to facilitate provisioning of services that could guarantee timing, bandwidth m new “anycast” address: route to “best” of several replicated servers r IPv 6 datagram format (to speed-up pkt-processing): m fixed-length 40 byte header m no (intermediate) fragmentation allowed m no checksum 4: Network Layer 4 a-47

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: (e. g. extend header with info such as identify upper layer protocol for data) 4: Network Layer 4 a-48

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 n IPv 4 datagram among IPv 4 routers 4: Network Layer 4 a-49

Dual Stack Approach 4: Network Layer 4 a-50

Tunneling Logical view: Physical view: E F IPv 6 IPv 6 A B IPv 6 tunnel IPv 4 4: Network Layer 4 a-51

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 D-to-E: IPv 6 inside IPv 4 Flow: X Src: A Dest: F data E-to-F: IPv 6 4: Network Layer 4 a-52

ICMP: Internet Control Message Protocol r used by hosts, routers, gateways to communicate network-level information: m error reporting: m control: echo request/reply (used by ping), cong. Control (tentative) r ICMP message: type, code plus first 8 bytes of IP datagram causing error r network-layer-protocol “above” IP: m ICMP msgs carried in IP datagrams r What if an ICMP message gets lost? 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 4: Network Layer 4 a-53

Roadmap Chapter goals: Overview: r understand principles r network layer services behind network layer m VC, datagram services: r what’s inside a router? m how a router works r Addressing, forwarding, m routing (path IP selection) r NEXT: routing principle: m dealing with scale path selection r instantiation and m hierarchical routing Internet routing protocols m implementation in the Internet (incl. IPv 6, r (multicast routing) multicast) 4: Network Layer 4 a-54