Chapter 4: network layer
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1 Chapter 4: network layer chapter goals: understand principles behind network layer services: network layer service models forwarding versus routing how a router works routing (path selection) broadcast, multicast instantiation, implementation in the Internet Network Layer 4-1
2 Network layer transport segment from sending to receiving host on sending side encapsulates segments into datagrams on receiving side, delivers segments to transport layer network layer protocols in every host, router router examines header fields in all IP datagrams passing through it application transport network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical application transport network data link physical Network Layer 4-2
3 Two key network-layer functions forwarding: move packets from router s input to appropriate router output routing: determine route taken by packets from source to dest. routing algorithms analogy: routing: process of planning trip from source to dest forwarding: process of getting through single interchange Network Layer 4-3
4 Interplay between routing and forwarding routing algorithm local forwarding table header value output link routing algorithm determines end-end-path through network forwarding table determines local forwarding at this router value in arriving packet s header Network Layer 4-4
5 Connection setup 3 rd important function in some network architectures: ATM, frame relay, X.25 before datagrams flow, two end hosts and intervening routers establish virtual connection routers get involved network vs transport layer connection service: network: between two hosts (may also involve intervening routers in case of VCs) transport: between two processes Network Layer 4-5
6 Network service model Q: What service model for channel transporting datagrams from sender to receiver? example services for individual datagrams: guaranteed delivery guaranteed delivery with less than 40 msec delay example services for a flow of datagrams: in-order datagram delivery guaranteed minimum bandwidth to flow restrictions on changes in inter-packet spacing Network Layer 4-6
7 Network layer service models: Network Architecture Service Model Bandwidth Guarantees? Loss Order Timing Congestion feedback Internet ATM ATM ATM ATM best effort CBR VBR ABR UBR none constant rate guaranteed rate guaranteed minimum none no yes yes no no no yes yes yes yes no yes yes no no no (inferred via loss) no congestion no congestion yes no Network Layer 4-7
8 Connection, connection-less service datagram network provides network-layer connectionless service virtual-circuit network provides network-layer connection service analogous to TCP/UDP connecton-oriented / connectionless transport-layer services, but: service: host-to-host no choice: network provides one or the other implementation: in network core Network Layer 4-8
9 VC implementation a VC consists of: 1. path from source to destination 2. VC numbers, one number for each link along path 3. entries in forwarding tables in routers along path packet belonging to VC carries VC number (rather than dest address) VC number can be changed on each link. new VC number comes from forwarding table Network Layer 4-9
10 VC forwarding table forwarding table in northwest router: VC number interface number Incoming interface Incoming VC # Outgoing interface Outgoing VC # VC routers maintain connection state information! Network Layer 4-10
11 Virtual circuits: signaling protocols used to setup, maintain teardown VC used in ATM, frame-relay, X.25 not used in today s Internet application transport network data link physical 5. data flow begins 6. receive data 4. call connected 3. accept call 1. initiate call 2. incoming call application transport network data link physical Network Layer 4-11
12 Datagram networks no call setup at network layer routers: no state about end-to-end connections no network-level concept of connection packets forwarded using destination host address application transport network data link physical 1. send datagrams 2. receive datagrams application transport network data link physical Network Layer 4-12
13 Datagram forwarding table routing algorithm local forwarding table dest address output link address-range 1 address-range 2 address-range 3 address-range billion IP addresses, so rather than list individual destination address list range of addresses (aggregate table entries) IP destination address in arriving packet s header Network Layer 4-13
14 Datagram forwarding table Destination Address Range through Link Interface through through otherwise 2 3 Q: but what happens if ranges don t divide up so nicely? Network Layer 4-14
15 Longest prefix matching longest prefix matching when looking for forwarding table entry for given destination address, use longest address prefix that matches destination address. examples: Destination Address Range *** ********* ********* *** ********* otherwise DA: DA: Link interface which interface? which interface? Network Layer 4-15
16 Datagram or VC network: why? Internet (datagram) data exchange among computers elastic service, no strict timing req. many link types different characteristics uniform service difficult smart end systems (computers) can adapt, perform control, error recovery simple inside network, complexity at edge ATM (VC) evolved from telephony human conversation: strict timing, reliability requirements need for guaranteed service dumb end systems telephones complexity inside network Network Layer 4-16
17 Chapter 4: outline 4.1 introduction 4.2 virtual circuit and datagram networks 4.3 what s inside a router 4.4 IP: Internet Protocol datagram format IPv4 addressing ICMP IPv6 4.5 routing algorithms link state distance vector hierarchical routing 4.6 routing in the Internet RIP OSPF BGP 4.7 broadcast and multicast routing Network Layer 4-17
18 Router architecture overview two key router functions: run routing algorithms/protocol (RIP, OSPF, BGP) forwarding datagrams from incoming to outgoing link forwarding tables computed, pushed to input ports routing processor routing, management control plane (software) forwarding data plane (hardware) high-seed switching fabric router input ports router output ports Network Layer 4-18
19 Input port functions line termination link layer protocol (receive) lookup, forwarding queueing switch fabric physical layer: bit-level reception data link layer: e.g., Ethernet see chapter 5 decentralized switching: given datagram dest., lookup output port using forwarding table in input port memory goal: complete input port processing at line speed queuing: if datagrams arrive faster than forwarding rate into switch fabric Network Layer 4-19
20 Switching fabrics transfer packet from input buffer to appropriate output buffer switching rate: rate at which packets can be transferred from inputs to outputs often measured as multiple of input/output line rate N inputs: switching rate N times line rate desirable three types of switching fabrics memory memory bus crossbar Network Layer 4-20
21 Switching via memory first generation routers: traditional computers with switching under direct control of CPU packet copied to system s memory CPU extracts dest address from packet s header, looks up output port in forwarding table, copies to output port speed limited by memory bandwidth (2 bus crossings per datagram) one packet at a time input port (e.g., Ethernet) memory output port (e.g., Ethernet) system bus Network Layer 4-21
22 Switching via a bus datagram from input port memory to output port memory via a shared bus bus contention: switching speed limited by bus bandwidth one packet a time 32 Gbps bus, Cisco 5600: sufficient speed for access and enterprise routers bus Network Layer 4-22
23 Switching via interconnection network forwards multiple packets in parallel banyan networks, crossbar, other interconnection nets initially developed to connect processors in multiprocessor When packet from port A needs to forwarded to port Y, controller closes cross point at intersection of two buses advanced design: fragmenting datagram into fixed length cells, switch cells through the fabric. A B C crossbar X Y Z Network Layer 4-23
24 Output ports switch fabric datagram buffer queueing link layer protocol (send) line termination buffering required when datagrams arrive from fabric faster than the transmission rate scheduling discipline chooses among queued datagrams for transmission Network Layer 4-24
25 Output port queueing switch fabric switch fabric at t, packets more from input to output one packet time later suppose R switch is N times faster than R line still have output buffering when multiple inputs send to same output queueing (delay) and loss due to output port buffer overflow! Network Layer 4-25
26 How much buffering? RFC 3439 rule of thumb: average buffering equal to typical RTT (say 250 msec) times link capacity C e.g., C = 10 Gpbs link: 2.5 Gbit buffer recent recommendation: with N flows, buffering equal to RTT. C N Network Layer 4-26
27 Input port queuing fabric slower than input ports combined queuing may occur at input queues queuing delay and loss due to input buffer overflow! Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward switch fabric switch fabric output port contention: only one red datagram can be transferred. lower red packet is blocked one packet time later: green packet experiences HOL blocking Network Layer 4-27
28 Chapter 4: outline 4.1 introduction 4.2 virtual circuit and datagram networks 4.3 what s inside a router 4.4 IP: Internet Protocol datagram format IPv4 addressing ICMP IPv6 4.5 routing algorithms link state distance vector hierarchical routing 4.6 routing in the Internet RIP OSPF BGP 4.7 broadcast and multicast routing Network Layer 4-28
29 The Internet network layer host, router network layer functions: transport layer: TCP, UDP network layer routing protocols path selection RIP, OSPF, BGP forwarding table link layer physical layer IP protocol addressing conventions datagram format packet handling conventions ICMP protocol error reporting router signaling Network Layer 4-29
30 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? 20 bytes of TCP 20 bytes of IP = 40 bytes + app layer overhead ver head. len type of service 16-bit identifier time to upper live layer 32 bits flgs length fragment offset header checksum 32 bit source IP address 32 bit destination IP address options (if any) data (variable length, typically a TCP or UDP segment) total datagram length (bytes) for fragmentation/ reassembly e.g. timestamp, record route taken, specify list of routers to visit. Network Layer 4-30
31 IP fragmentation, reassembly network links have MTU (max.transfer size) - largest possible link-level frame different link types, different MTUs large IP datagram divided ( fragmented ) within net one datagram becomes several datagrams reassembled only at final destination IP header bits used to identify, order related fragments reassembly fragmentation: in: one large datagram out: 3 smaller datagrams Network Layer 4-31
32 IP fragmentation, reassembly example: 4000 byte datagram MTU = 1500 bytes length =4000 ID =x fragflag =0 offset =0 one large datagram becomes several smaller datagrams 1480 bytes in data field length =1500 ID =x fragflag =1 offset =0 offset = 1480/8 length =1500 ID =x fragflag =1 offset =185 length =1040 ID =x fragflag =0 offset =370 Network Layer 4-32
33 Chapter 4: outline 4.1 introduction 4.2 virtual circuit and datagram networks 4.3 what s inside a router 4.4 IP: Internet Protocol datagram format IPv4 addressing ICMP IPv6 4.5 routing algorithms link state distance vector hierarchical routing 4.6 routing in the Internet RIP OSPF BGP 4.7 broadcast and multicast routing Network Layer 4-33
34 IP addressing: introduction IP address: 32-bit identifier for host, router interface interface: connection between host/router and physical link routers typically have multiple interfaces host typically has one active interface (e.g., wired Ethernet, wireless ) one IP address associated with each interface = Network Layer 4-34
35 IP addressing: introduction Q: how are interfaces actually connected? A: we ll learn about that in chapter 5, A: wired Ethernet interfaces connected by Ethernet switches For now: don t need to worry about how one interface is connected to another (with no intervening router) A: wireless WiFi interfaces connected by WiFi base station Network Layer 4-35
36 Subnets IP address: subnet part - high order bits host part - low order bits what s a subnet? device interfaces with same subnet part of IP address can physically reach each other without intervening router subnet network consisting of 3 subnets Network Layer 4-36
37 Subnets recipe to determine the subnets, detach each interface from its host or router, creating islands of isolated networks each isolated network is called a subnet / / subnet /24 subnet mask: /24 Network Layer 4-37
38 Subnets how many? Network Layer 4-38
39 IP addressing: CIDR CIDR: Classless InterDomain Routing subnet portion of address of arbitrary length address format: a.b.c.d/x, where x is # bits in subnet portion of address subnet part /23 host part Network Layer 4-39
40 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 /20 Organization /23 Organization /23 Organization / Organization /23 Network Layer 4-40
41 Hierarchical addressing: route aggregation hierarchical addressing allows efficient advertisement of routing information: Organization /23 Organization /23 Organization /23 Organization Fly-By-Night-ISP Send me anything with addresses beginning /20 Internet /23 ISPs-R-Us Send me anything with addresses beginning /16 Network Layer 4-41
42 Hierarchical addressing: more specific routes ISPs-R-Us has a more specific route to Organization 1 Organization /23 Organization /23 Organization /23 Organization / Fly-By-Night-ISP ISPs-R-Us Send me anything with addresses beginning /20 Send me anything with addresses beginning /16 or /23 Internet Network Layer 4-42
43 IP addressing: how to get a block? Q: how does an ISP get block of addresses? A: ICANN: Internet Corporation for Assigned Names and Numbers allocates addresses manages DNS assigns domain names, resolves disputes Network Layer 4-43
44 IP addresses: how to get one? Q: How does a host get IP address? hard-coded by system admin in a file Windows: control-panel->network->configuration- >tcp/ip->properties UNIX: /etc/rc.config DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server plug-and-play Network Layer 4-44
45 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/ on ) support for mobile users who want to join network (more shortly) DHCP overview: host broadcasts DHCP discover msg [optional] DHCP server responds with DHCP offer msg [optional] host requests IP address: DHCP request msg DHCP server sends address: DHCP ack msg Network Layer 4-45
46 DHCP client-server scenario / DHCP server arriving DHCP client needs address in this network / /24 Network Layer 4-46
47 DHCP client-server scenario DHCP server: DHCP discover src : , 68 dest.: ,67 yiaddr: transaction ID: 654 arriving client DHCP request DHCP offer src: , 68 dest:: , 67 yiaddrr: transaction ID: 655 lifetime: 3600 secs src: , 67 dest: , 68 yiaddrr: transaction ID: 654 lifetime: 3600 secs DHCP ACK src: , 67 dest: , 68 yiaddrr: transaction ID: 655 lifetime: 3600 secs Network Layer 4-47
48 DHCP: more than IP addresses DHCP returns: IP address address of first-hop router for client name and IP address of DNS sever network mask (indicating network versus host portion of address) Network Layer 4-48
49 NAT: network address translation rest of Internet local network (e.g., home network) / all datagrams leaving local network have same single source NAT IP address: ,different source port numbers datagrams with source or destination in this network have /24 address for source, destination (as usual) Network Layer 4-49
50 NAT: network address translation motivation: local network uses just one IP address as far as outside world is concerned: range of addresses not needed from ISP: just one IP address for all devices can change addresses of devices in local network without notifying outside world can change ISP without changing addresses of devices in local network devices inside local net not explicitly addressable, visible by outside world (a security plus) Network Layer 4-50
51 NAT: network address translation implementation: NAT router must: 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 remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair 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 4-51
52 NAT: network address translation 2: NAT router changes datagram source addr from , 3345 to , 5001, updates table 2 NAT translation table WAN side addr LAN side addr , , 3345 S: , 5001 D: , S: , 3345 D: , : host sends datagram to , S: , 80 D: , : reply arrives dest. address: , 5001 S: , 80 D: , : NAT router changes datagram dest addr from , 5001 to , Network Layer 4-52
53 NAT: network address translation 16-bit port-number field: 60,000 simultaneous connections with a single LAN-side address! NAT is controversial: routers should only process up to layer 3 violates end-to-end argument NAT possibility must be taken into account by app designers, e.g., P2P applications address shortage should instead be solved by IPv6 Network Layer 4-53
54 NAT traversal problem client wants to connect to server with address server address local to LAN (client can t use it as destination addr) only one externally visible NATed address: solution1: statically configure NAT to forward incoming connection requests at given port to server e.g., ( , port 25000) always forwarded to port client? NAT router Network Layer 4-54
55 NAT traversal problem solution 2: Universal Plug and Play (UPnP) Internet Gateway Device (IGD) Protocol. Allows NATed host to: learn public IP address ( ) add/remove port mappings (with lease times) NAT router IGD i.e., automate static NAT port map configuration Network Layer 4-55
56 NAT traversal problem solution 3: relaying (used in Skype) NATed client establishes connection to relay external client connects to relay relay bridges packets between to connections client 2. connection to relay initiated by client 3. relaying established 1. connection to relay initiated by NATed host NAT router Network Layer 4-56
57 Chapter 4: outline 4.1 introduction 4.2 virtual circuit and datagram networks 4.3 what s inside a router 4.4 IP: Internet Protocol datagram format IPv4 addressing ICMP IPv6 4.5 routing algorithms link state distance vector hierarchical routing 4.6 routing in the Internet RIP OSPF BGP 4.7 broadcast and multicast routing Network Layer 4-57
58 Interplay between routing, forwarding routing algorithm local forwarding table dest address output link address-range 1 address-range 2 address-range 3 address-range routing algorithm determines end-end-path through network forwarding table determines local forwarding at this router IP destination address in arriving packet s header Network Layer 4-58
59 Graph abstraction 5 graph: G = (N,E) u 1 2 v x w y z 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) } aside: graph abstraction is useful in other network contexts, e.g., P2P, where N is set of peers and E is set of TCP connections Network Layer 4-59
60 Graph abstraction: costs u c(x,x ) = cost of link (x,x ) v 3 e.g., c(w,z) = 5 w 5 cost could always be 1, or 2 1 z 3 inversely related to bandwidth, x y 2 or inversely related to 1 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 ) key question: what is the least-cost path between u and z? routing algorithm: algorithm that finds that least cost path Network Layer 4-60
61 Routing algorithm classification Q: global or decentralized information? global: all routers have complete topology, link cost info link state algorithms decentralized: router knows physicallyconnected neighbors, link costs to neighbors iterative process of computation, exchange of info with neighbors distance vector algorithms Q: static or dynamic? static: routes change slowly over time dynamic: routes change more quickly periodic update in response to link cost changes Network Layer 4-61
62 Chapter 4: outline 4.1 introduction 4.2 virtual circuit and datagram networks 4.3 what s inside a router 4.4 IP: Internet Protocol datagram format IPv4 addressing ICMP IPv6 4.5 routing algorithms link state distance vector hierarchical routing 4.6 routing in the Internet RIP OSPF BGP 4.7 broadcast and multicast routing Network Layer 4-62
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