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Internetworking With TCP/IP Application Layer Telnet Gopher NFS FTP X Win TFTP SMTP SNMP REXEC DNS RPC Transport Layer TCP UDP Network Layer ICMP IP IGMP ARP RARP Parviz Kermani NYU:Poly Link Interface Ethernet, IEEE 802.3, Token Ring, X.25, SNA, FDDI,. Chapter 4: Network Layer & Routing

Chapter 4 Network Layer A note on the use of these ppt slides: We re making these slides freely available to all (faculty, students, readers). They re in PowerPoint form so you see the animations; and can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following: If you use these slides (e.g., in a class) that you mention their source (after all, we d like people to use our book!) If you post any slides on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material. Thanks and enjoy! JFK/KWR Computer Networking: A Top Down Approach 5 th edition. Jim Kurose, Keith Ross Addison-Wesley, April 2009. All material copyright 1996-2011 J.F Kurose and K.W. Ross, All Rights Reserved Network Layer 4-2

Legends Back to previous foil Page contains animation End of animation Note: The original of these foils were provided by the authors. There are additions/deletions made by me, Parviz Kermani. Network Layer 4-3

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 4-4

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 4-5

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 4-6

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. analogy: routing: process of planning trip from source to dest forwarding: process of getting through single interchange routing algorithms Network Layer 4-7

Interplay between routing and forwarding routing algorithm local forwarding table header value output link 0100 0101 0111 1001 3 2 2 1 routing algorithm determines end-end-path through network forwarding table determines local forwarding at this router value in arriving packet s header 0111 1 3 2 Network Layer 4-8

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 4-9

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 4-10

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 4-11

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 4-13

Virtual circuits source-to-dest path behaves much like telephone circuit performance-wise network actions along source-to-dest path call setup, teardown for each call before data can flow each packet carries VC identifier (not destination host address) every router on source-dest path maintains state for each passing connection link, router resources (bandwidth, buffers) may be allocated to VC (dedicated resources = predictable service) Network Layer 4-14

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 4-15

VC forwarding table 12 22 32 forwarding table in northwest router: VC number interface number 1 2 3 Incoming interface Incoming VC # Outgoing interface Outgoing VC # 1 12 3 22 2 63 1 18 3 7 2 17 1 97 3 87 VC routers maintain connection state information! Network Layer 4-16

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 4-17

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 4-18

Datagram forwarding table routing algorithm local forwarding table dest address output link address-range 1 address-range 2 address-range 3 address-range 4 3 2 2 1 4 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 1 3 2 Network Layer 4-19

Datagram forwarding table Destination Address Range 11001000 00010111 00010000 00000000 through 11001000 00010111 00010111 11111111 Link Interface 0 11001000 00010111 00011000 00000000 through 11001000 00010111 00011000 11111111 1 11001000 00010111 00011001 00000000 through 11001000 00010111 00011111 11111111 otherwise 2 3 Q: but what happens if ranges don t divide up so nicely? Network Layer 4-20

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 11001000 00010111 00010*** ********* 11001000 00010111 00011000 ********* 11001000 00010111 00011*** ********* otherwise DA: 11001000 00010111 00010110 10100001 DA: 11001000 00010111 00011000 10101010 Link interface 0 1 2 3 which interface? which interface? Network Layer 4-21

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 4-22

Router architecture overview two key router functions: run routing algorithms/protocol (RIP, OSPF, BGP) forwarding datagrams from incoming to outgoing link switching fabric router input ports routing processor router output ports Network Layer 4-24

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 4-25

Switching fabrics transfer packet from input buffer to appropriate output buffer switching rate: rate at which packets can be transfer 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 4-26

Switching via memory first generation routers: traditional computers with switching under direct control of CPU packet copied to system s memory speed limited by memory bandwidth (2 bus crossings per datagram) input port (e.g., Ethernet) memory output port (e.g., Ethernet) system bus Network Layer 4-27

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 32 Gbps bus, Cisco 5600: sufficient speed for access and enterprise routers bus Network Layer 4-28

Switching via interconnection network overcome bus bandwidth limitations banyan networks, crossbar, other interconnection nets initially developed to connect processors in multiprocessor advanced design: fragmenting datagram into fixed length cells, switch cells through the fabric. Cisco 12000: switches 60 Gbps through the interconnection network crossbar Network Layer 4-29

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 4-30

Output port queueing switch fabric switch fabric at t, packets more from input to output one packet time later buffering when arrival rate via switch exceeds output line speed queueing (delay) and loss due to output port buffer overflow! Network Layer 4-31

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 4-32

Input port queuing fabric slower than input ports combined -> queueing may occur at input queues queueing 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 4-33

The Internet network layer host, router network layer functions: transport layer: TCP, UDP network layer routing protocols path selection RIP, OSPF, BGP forwarding table IP protocol addressing conventions datagram format packet handling conventions ICMP protocol error reporting router signaling link layer physical layer Network Layer 4-35

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 32 bits type of service 16-bit identifier time to upper live layer 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 4-36

IP Datagram Fragmentation 3 bits of flags Don t fragment bit 0= may fragment 1= don t fragment More bit 1= more fragments to come 0= last fragment Spare bit 16-bit identifier flgs fragment offset Network Layer 4-37

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 4-38

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 4-39

IP Datagram & Reassembly Example 1 /8 Network Layer 4-40

Successive fragmentations Initial datagram: Don t fragment=0 in all datagrams IP HDR 1024 Data octets More bit (M) = 0 Offset (OS) =0 First fragmentation IP HDR 512 Data octets IP HDR 512 Data octets More bit (M) = 1 Offset (OS) =0 More bit (M) = 0 Offset (OS) =64 (=512/8) Second fragmentation IP HDR 256 Data octets IP HDR 256 Data octets IP HDR 256 Data octets IP HDR 256 Data octets M = 1 OS =0 M = 1 OS =(0+256)/8=32 M = 1 OS =(512+0)/8=64 M = 0 OS =(512+256)/8=96 Network Layer 4-41

Fragmentation Process Create n fragment datagrams so that length of each will meet network limitations. Copy IP header to each Divide data equally, along 8-octet boundaries. The last segment can have any length. Calculate and insert fragment offsets for each fragment other than the first. The offset value is length/8 Set a more bit for each fragment, except the last. Transport fragments independently through Internet. Note: If the Don t Fragment" bit is 1 and fragmentation is needed, then the datagram is discarded Network Layer 4-42

Datagram Reassembly Occurs only in destination host, after fragments have transited the Internet. Reassembly process: When first fragment arrives, create buffer, start reassembly timer. Note: The total length is not known. The length field is only for then current datagram Insert data from each fragment in proper buffer position, based on offset (may be received out of order). Continue until entire datagram is reassembled. Discard entire datagram if Reassembly timer expires. Fragment error is detected. Network Layer 4-43

Fragmentation: Last Word Fragmentation/reassembly puts additional load on routers Limit the TCP & UDP segments to a relatively small size All data link protocols supported by IP supposed to have MTU of at least 576 bytes Fragmentation eliminated by using an MSS of 536 bytes 20 bytes of TCP segment header 20 bytes of IP datagram header Most TCP segments for bulk data transfer (e.g. HTTP) are 512-536 bytes long Network Layer 4-44

IP addressing: introduction IP address: 32-bit identifier for host, router interface interface: connection between host/router and physical link router s typically have multiple interfaces host typically has one interface IP addresses associated with each interface 223.1.1.1 223.1.2.1 223.1.1.2 223.1.1.4 223.1.2.9 223.1.1.3 223.1.3.1 223.1.3.27 223.1.2.2 223.1.3.2 223.1.1.1 = 11011111 00000001 00000001 00000001 223 1 1 1 Network Layer 4-46

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 223.1.1.1 223.1.1.2 223.1.2.1 223.1.1.4 223.1.2.9 223.1.1.3 223.1.3.1 223.1.2.2 223.1.3.27 subnet 223.1.3.2 network consisting of 3 subnets Network Layer 4-47

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 223.1.1.0/24 223.1.2.0/24 223.1.1.1 223.1.1.2 223.1.2.1 223.1.1.4 223.1.2.9 223.1.1.3 223.1.3.1 223.1.2.2 223.1.3.27 subnet 223.1.3.2 223.1.3.0/24 subnet mask: /24 Network Layer 4-48

Subnets 223.1.1.2 how many? 223.1.1.1 223.1.1.4 223.1.1.3 223.1.9.2 223.1.7.0 223.1.9.1 223.1.8.1 223.1.8.0 223.1.7.1 223.1.2.6 223.1.3.27 223.1.2.1 223.1.2.2 223.1.3.1 223.1.3.2 Network Layer 4-49

(Classical) IP Address Structure An IP address is broken in two parts Network address Host address Network host The division between network and host is determined by the size of network Network Layer 4-50

IP Addresses given classful notion addressing: of network, let s re-examine IP addresses: class A B C D 0 network host 10 network host 110 network host 1110 multicast address 32 bits 1.0.0.0 to 127.255.255.255 128.0.0.0 to 191.255.255.255 192.0.0.0 to 223.255.255.255 224.0.0.0 to 239.255.255.255 Network Layer 4-51

IP Addresses IP Classful Addresses: Class A addresses begin with 0xxx, or 1 to 126 Class B addresses begin with 10xx, or 128 to 191 Class C addresses begin with 110x, or 192 to 223 Class D addresses begin with 1110, or 224 to 239 Multicast Class E addresses begin with 1111, or 240 to 254 Experimental Network Layer 4-52

Classful Addressing Number of elements in each class Class Number of classes Number of local addresses A 0xxx 128 16,777,216 B 10xx 16,384 65,534 C 110x 2,097,152 254 Network Layer 4-53

Classful Addressing Classful addressing: inefficient use of address space, address space exhaustion e.g., class B net allocated enough addresses for 65K hosts, even if only 2K hosts in that network No longer formally part of IP addressing architecture Network Layer 4-54

IP addressing: CIDR CIDR: Classless InterDomain Routing Adopted by IETF in 1993 Network (subnet) portion of address of arbitrary length subnet portion of address of arbitrary length address format: a.b.c.d/x, where x is # bits in network (subnet) portion of address To support 2000 hosts, a block of 2048 addresses of the form a.b.c.d/21 assigned 11 bits needed to store 2048 (2 11 =2048) In practice the 11 bit rightmost addressing could be further divided (subnetting, more on this later) network part host part 11001000 00010111 00010000 00000000 200.23.16.0/23 Network Layer 4-55

Subnet Mask For routing traffic, subnet mask is used to extract the network (subnet) portion of an IP address A string of 32 bits Bits corresponding to network (and subnet) part set to 1 (one) Bits corresponding to host part set to 0 Ex: Addr = 9. 2. 225. 65/24 = 00001001.00000010.11100001.01000001 Mask = 11111111.11111111.11111111.00000000 = 255. 255. 255. 0 Network Layer 4-56

Subnet Mask: How To Use? Logical AND the mask and the IP address EX: Addr = 9. 2. 225. 65/24 = 00001001.00000010.11100001.01000001 Mask = 11111111.11111111.11111111.00000000 N/ADR= 00001001.00000010.11100001.00000000 = 9. 2. 225. 0 Network Layer 4-57

Subnetting To minimize waste in classful addressing, IP subnetting is used The host bits of a classful address is further divided into subnet bits and host bits Note: Only the classful Host bits should be subnetted Classful Network bits should stay intact Network Layer 4-58

Subnetting Example Class B 162.150.0.0/16 16 subnets 162.150.xxxx0000.0/20 Mask: 11111111.11111111.11110000.00000000 255. 255. 240. 0 Example network 162.150.128.0/20 162.150.144.0/20 Network Layer 4-59

Private IP Addresses An IP address is universally unique Need for private IP address At home (NAT discussion later) Experiments Private networks Private IP addresses Class A: 10.0.0.0-10.255.255.255 Class B: 172.16.0.0-172.31.255.255 Class C: 192.168.0.0-192.168.255.255 Note: Private IP addresses are not publically routable. Network Layer 4-60

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 4-61

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 4-62

DHCP client-server scenario 223.1.1.0/24 223.1.1.1 DHCP server 223.1.2.1 223.1.1.2 223.1.1.4 223.1.2.9 223.1.2.2 223.1.1.3 223.1.3.27 arriving DHCP client needs address in this network 223.1.2.0/24 223.1.3.1 223.1.3.2 223.1.3.0/24 Network Layer 4-63

DHCP client-server scenario DHCP server: 223.1.2.5 DHCP discover src : 0.0.0.0, 68 dest.: 255.255.255.255,67 yiaddr: 0.0.0.0 transaction ID: 654 arriving client DHCP request DHCP offer src: 0.0.0.0, 68 dest:: 255.255.255.255, 67 yiaddrr: 223.1.2.4 transaction ID: 655 lifetime: 3600 secs src: 223.1.2.5, 67 dest: 255.255.255.255, 68 yiaddrr: 223.1.2.4 transaction ID: 654 lifetime: 3600 secs DHCP ACK src: 223.1.2.5, 67 dest: 255.255.255.255, 68 yiaddrr: 223.1.2.4 transaction ID: 655 lifetime: 3600 secs Network Layer 4-64

DHCP: more than IP addresses DHCP can return more than just allocated IP address on subnet: 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 4-65

DHCP: Broadcast or Unicast DHCPDISCOVER is broadcast over the subnet attached to the DHCP client (UDP broadcast to 255.255.255.255) If DHCP server does not reside on that subnet, the gateway need to direct the discover to the remote server (UDP unicast) The gateway router should be configured to do so. Otherwise DHCPDISCOVER fails Network Layer 4-66

DHCP: Broadcast or Unicast DHCPDISCOVER is broadcast over the subnet attached to the DHCP client (UDP broadcast to 255.255.255.255) If DHCP server does not reside on that subnet, The gateway needs to direct the discover to the remote server (UDP unicast) The gateway router should be configured to do so. Otherwise DHCPDISCOVER fails Network Layer 4-67

DHCP: example DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP UDP IP Eth Phy DHCP DHCP UDP IP Eth Phy 168.1.1.1 router with DHCP server built into router Connecting laptop needs its IP address, addr of first-hop router, addr of DNS server: use DHCP DHCP request encapsulated in UDP, encapsulated in IP, encapsulated in 802.1 Ethernet Ethernet frame broadcast (dest: FFFFFFFFFFFF) on LAN, received at router running DHCP server Ethernet demuxed to IP demuxed, UDP demuxed to DHCP Network Layer 4-68

DHCP: example DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP UDP IP Eth Phy DHCP UDP IP Eth Phy router with DHCP server built into router DHCP server formulates DHCP ACK containing client s IP address, IP address of first-hop router for client, name & IP address of DNS server encapsulation of DHCP server, frame forwarded to client, demuxing up to DHCP at client client now knows its IP address, name and IP address of DSN server, IP address of its first-hop router Network Layer 4-69

DHCP: Wireshark output (home LAN) Message type: Boot Request (1) Hardware type: Ethernet Hardware address length: 6 Hops: 0 request Transaction ID: 0x6b3a11b7 Seconds elapsed: 0 Bootp flags: 0x0000 (Unicast) Client IP address: 0.0.0.0 (0.0.0.0) Your (client) IP address: 0.0.0.0 (0.0.0.0) Next server IP address: 0.0.0.0 (0.0.0.0) Relay agent IP address: 0.0.0.0 (0.0.0.0) Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a) Server host name not given Boot file name not given Magic cookie: (OK) Option: (t=53,l=1) DHCP Message Type = DHCP Request Option: (61) Client identifier Length: 7; Value: 010016D323688A; Hardware type: Ethernet Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a) Option: (t=50,l=4) Requested IP Address = 192.168.1.101 Option: (t=12,l=5) Host Name = "nomad" Option: (55) Parameter Request List Length: 11; Value: 010F03062C2E2F1F21F92B 1 = Subnet Mask; 15 = Domain Name 3 = Router; 6 = Domain Name Server 44 = NetBIOS over TCP/IP Name Server Message type: Boot Reply (2) Hardware type: Ethernet reply Hardware address length: 6 Hops: 0 Transaction ID: 0x6b3a11b7 Seconds elapsed: 0 Bootp flags: 0x0000 (Unicast) Client IP address: 192.168.1.101 (192.168.1.101) Your (client) IP address: 0.0.0.0 (0.0.0.0) Next server IP address: 192.168.1.1 (192.168.1.1) Relay agent IP address: 0.0.0.0 (0.0.0.0) Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a) Server host name not given Boot file name not given Magic cookie: (OK) Option: (t=53,l=1) DHCP Message Type = DHCP ACK Option: (t=54,l=4) Server Identifier = 192.168.1.1 Option: (t=1,l=4) Subnet Mask = 255.255.255.0 Option: (t=3,l=4) Router = 192.168.1.1 Option: (6) Domain Name Server Length: 12; Value: 445747E2445749F244574092; IP Address: 68.87.71.226; IP Address: 68.87.73.242; IP Address: 68.87.64.146 Option: (t=15,l=20) Domain Name = "hsd1.ma.comcast.net." Network Layer 4-70

IP addresses: how to get one? Q: how does network get subnet part of IP address? A: gets allocated portion of its provider ISP s address space ISP's block 10101000 00010111 00010000 00000000 168.23.16.0/20 Organization 0 10101000 00010111 00010000 00000000 168.23.16.0/23 Organization 1 10101000 00010111 00010010 00000000 168.23.18.0/23 Organization 2 10101000 00010111 00010100 00000000 168.23.20.0/23....... Organization 7 10101000 00010111 00011110 00000000 168.23.30.0/23 Network Layer 4-71

Hierarchical addressing: route aggregation hierarchical addressing allows efficient advertisement of routing information: Organization 0 168.23.16.0/23 Organization 1 168.23.18.0/23 Organization 2 168.23.20.0/23 Organization 7.... Fly-By-Night-ISP Send me anything with addresses beginning 168.23.16.0/20 Internet 168.23.30.0/23 ISPs-R-Us Send me anything with addresses beginning 128.31.0.0/16 Network Layer 4-72

Hierarchical addressing: more specific routes ISPs-R-Us has a more specific route to Organization 1 Organization 0 168.23.16.0/23 Organization 2 168.23.20.0/23 Organization 7 168.23.30.0/23 Organization 1 168.23.18.0/23.... Fly-By-Night-ISP ISPs-R-Us Send me anything with addresses beginning 168.23.16.0/20 Send me anything with addresses beginning 128.31.0.0/16 (255.255.0.0) or 168.23.18.0/23 (255.255.252.0) Internet Longest prefix matching determines the route Network Layer 4-73

Hierarchical addressing: Network Addresses and Masks Net address Mask (Binary) Mask (Decimal) Net address Mask (Binary) Mask (Decimal) Net address Mask (Binary) Mask (Decimal) Net address Mask (Binary) Mask (Decimal) 168. 23. 16. 0/23 11111111.11111111.11111110.00000000 255. 255. 254. 0 168. 23. 16. 0/20 11111111.11111111.11110000.00000000 255. 255. 240. 0 128. 31. 0. 0/16 11111111.11111111.00000000.00000000 255. 255. 0. 0 168. 23. 18. 0/23 11111111.11111111.11111110.00000000 255. 255. 254. 0 Network Layer 4-74

IP addressing: the last word... Q: how does an ISP get block of addresses? A: ICANN: Internet Corporation for Assigned Names and Numbers http://www.icann.org/ allocates addresses manages DNS assigns domain names, resolves disputes Network Layer 4-75

NAT: network address translation rest of Internet 138.76.29.7 10.0.0.4 local network (e.g., home network) 10.0.0/24 10.0.0.1 10.0.0.2 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 4-76

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 4-77

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 4-78

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 138.76.29.7, 5001 10.0.0.1, 3345 S: 138.76.29.7, 5001 D: 128.119.40.186, 80 10.0.0.4 S: 10.0.0.1, 3345 D: 128.119.40.186, 80 1 1: host 10.0.0.1 sends datagram to 128.119.40.186, 80 10.0.0.1 10.0.0.2 138.76.29.7 S: 128.119.40.186, 80 D: 138.76.29.7, 5001 3 3: reply arrives dest. address: 138.76.29.7, 5001 S: 128.119.40.186, 80 D: 10.0.0.1, 3345 4 4: NAT router changes datagram dest addr from 138.76.29.7, 5001 to 10.0.0.1, 3345 10.0.0.3 Network Layer 4-79

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 4-80

NAT traversal problem client wants to connect to server with address 10.0.0.1 server address 10.0.0.1 local to LAN (client can t use it as destination addr) only one externally visible NATed address: 138.76.29.7 solution1: statically configure NAT to forward incoming connection requests at given port to server e.g., (123.76.29.7, port 2500) always forwarded to 10.0.0.1 port 25000 client? 138.76.29.7 NAT router 10.0.0.4 10.0.0.1 Network Layer 4-81

NAT traversal problem solution 2: Universal Plug and Play (UPnP) Internet Gateway Device (IGD) Protocol. Allows NATed host to: learn public IP address (138.76.29.7) add/remove port mappings (with lease times) i.e., automate static NAT port map configuration NAT router IGD 10.0.0.1 Network Layer 4-82

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 138.76.29.7 NAT router 10.0.0.1 Network Layer 4-83

ICMP: Internet Control Message Protocol In contrast to a single network, in an internet, no special hardware can assist in reporting and resolving problems. In an internet, software assists in reporting problems When things go wrong, ICMP comes to help ICMP Allows routers to send error messages or control messages to other routers/hosts ICMP provides communication between the IP software not application software All hosts and routers must be able to generate ICMP messages and process the ICMP messages they receive. An arbitrary machine can send an ICMP message to any other machine. ICMP is an error reporting mechanism. It does not fully specify the action to be taken for each possible error. ICMP is also used to help identify network problems (e.g. ping) Network Layer 4-85

ICMP (Cont.) ICMP reports problems to the original source. It cannot be used to inform intermediate routers about problems. Why? A datagram only contains source and destination address, not the intermediate nodes on a path Network Layer 4-86

ICMP Message Delivery ICMP message requires two levels of encapsulation ICMP utilizes IP, but is considered to be at same level in protocol stack ICMP messages are carried in IP datagram with ordinary IP headers with the Protocol field set to 1 IP Header Protocol=1 ICMP Message Network Layer 4-87

Types and Format of Error Messages ICMP messages originate from a router or a host depending on type of error condition ICMP Error Messages Message Destination Unreachable Time Exceeded Parameter Problem Source Quench Redirect Description A datagram cannot reach its destination host, utility, or application The time-to-live has expired at a router, or the Fragment Reassembly Time has expired at a destination host. There is a bad parameter in the IP header A router or destination is congested. (It is recommended that systems should not send Quench messages) A host has routed a datagram to the wrong local router Network Layer 4-88

When Not To Send ICMP ICMP is used to send error messages when a network is under stress Care should be taken that the ICMP traffic does not flood the network (making the situation worse!) ICMP must not report problems caused by Routing or delivering ICMP messages Broadcast or multicast datagrams Datagram fragments other than the first Messages whose source address do not identify a unique host E.g., source IP addresses such as 127.0.0.1 or 0.0.0.0 Network Layer 4-89

ICMP Message Format TYPE: CODE: CHECKSUM: identifies the message further information about the message type only covers the ICMP message In addition, ICMP messages that report errors always include the header and first 64 data bit of the datagram causing the problem. Network Layer 4-90

ICMP: internet control message protocol used by hosts & routers to communicate networklevel information error reporting: unreachable host, network, port, protocol echo request/reply (used by ping) network-layer above IP: ICMP msgs carried in IP datagrams ICMP message: type, code plus first 8 bytes of IP datagram causing error Type Code description 0 0 echo reply (ping) 3 0 dest. network unreachable 3 1 dest host unreachable 3 2 dest protocol unreachable 3 3 dest port unreachable 3 6 dest network unknown 3 7 dest host unknown 4 0 source quench (congestion control - not used) 8 0 echo request (ping) 9 0 route advertisement 10 0 router discovery 11 0 TTL expired 12 0 bad IP header Network Layer 4-91

traceroute and ICMP source sends series of UDP segments to dest first set has TTL =1 second set has TTL=2, etc. unlikely port number when nth set of datagrams arrives to nth router: router discards datagrams and sends source ICMP messages (type 11, code 0) ICMP messages includes name of router & IP address when ICMP messages arrives, source records RTTs stopping criteria: UDP segment eventually arrives at destination host destination returns ICMP port unreachable message (type 3, code 3) source stops 3 probes 3 probes 3 probes Network Layer 4-92

ICMP Example: Echo Request/Reply Used by processors to test whether a destination is alive and reachable Example: ping TYPE=8 (Request) or 0 (Reply) IDENTIFER/SEQUENCE NUMBER: used by sender to match replies to request DATA: Optional further matching information (returned by the sender) Network Layer 4-93

ICMP Example: Destination Unreachable Sent by a router to the source when it cannot forward or deliver an IP datagram Code Network Layer 4-94

ICMP Example: Redirect (Change Route) Used by router to notify host to change its routing table There are more than one routers on the network The host sends a datagram to a wrong router (resulting in a longer route) The router however forwards the datagram to the correct router and notifies the host The host should send the subsequent traffic to the shorter route. ICMP Redirect can be used to reduce manual network administration Hosts always use a default router The default router redirects the requests to optimal routers Hosts update their tables dynamically Network Layer 4-95

ICMP Example: Redirect (Cont.) Format Code Network Layer 4-96

ICMP Example: Destination Unreachable & Path MTU Discovery To transfer bulk data (e.g. file transfer), it is always better to send large datagrams IP and TCP headers are at least 40 bytes The MTU for each medium is different Use of a small, conservative datagram size wasteful (e.g. 576 bytes) A simple Path Discovery procedure to determine the biggest datagram size, the Path MTU Size The Don t Fragment flag in IP header is set to 1 The Path MTU size is set for the local interface If the datagram is too large for some routers, the router sends back an ICMP Destination Unreachable with code=4 Sending host reduces the datagram size and tries again Network Layer 4-97

Viewing ICMP Activities Use command netstat -s to view network activities Example: on Windows NT C>netstat -s IP Statistics Packets Received = 21111 Received Header Errors = 194 Received Address Errors = 1063 Datagrams Forwarded = 0 Unknown Protocols Received = 0 Received Packets Discarded = 0 Received Packets Delivered = 20208 Output Requests = 11559 Routing Discards = 0 Discarded Output Packets = 0 Output Packet No Route = 0 Reassembly Required = 0 Reassembly Successful = 0 Reassembly Failures = 0 Datagrams Successfully Fragmented = 0 Datagrams Failing Fragmentation = 0 Fragments Created = 0 Network Layer 4-98

Viewing ICMP Activities (Cont.) Example: Cont. ICMP Statistics Received Sent Messages 23 23 Errors 0 0 Destination Unreachable 0 0 Time Exceeded 0 0 Parameter Problems 0 0 Source Quenchs 0 0 Redirects 0 0 Echos 0 23 Echo Replies 23 0 Timestamps 0 0 Timestamp Replies 0 0 Address Masks 0 0 Address Mask Replies 0 0 Network Layer 4-99

IPv6: motivation initial motivation: 32-bit address space soon to be completely allocated. additional motivation: header format helps speed processing/forwarding header changes to facilitate QoS IPv6 datagram format: fixed-length 40 byte header no fragmentation allowed Network Layer 4-101

IPv6 datagram format 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 ver pri flow label payload len next hdr hop limit source address (128 bits) destination address (128 bits) data 32 bits Network Layer 4-102

Other changes from IPv4 checksum: removed entirely to reduce processing time at each hop options: allowed, but outside of header, indicated by Next Header field ICMPv6: new version of ICMP additional message types, e.g. Packet Too Big multicast group management functions Network Layer 4-103

Transition from IPv4 to IPv6 not all routers can be upgraded simultaneously no flag days how will network operate with mixed IPv4 and IPv6 routers? tunneling: IPv6 datagram carried as payload in IPv4 datagram among IPv4 routers IPv4 header fields IPv4 source, dest addr IPv6 header fields IPv6 source dest addr UDP/TCP payload IPv4 payload IPv6 datagram IPv4 datagram Network Layer 4-104

Tunneling logical view: A IPv6 B IPv6 IPv4 tunnel connecting IPv6 routers E IPv6 F IPv6 physical view: A B C D E F IPv6 IPv6 IPv4 IPv4 IPv6 IPv6 Network Layer 4-105

Tunneling logical view: A IPv6 B IPv6 IPv4 tunnel connecting IPv6 routers E IPv6 F IPv6 physical view: A B C D E F IPv6 IPv6 IPv4 IPv4 IPv6 IPv6 flow: X src: A dest: F data src:b dest: E Flow: X Src: A Dest: F src:b dest: E Flow: X Src: A Dest: F flow: X src: A dest: F data data data A-to-B: IPv6 B-to-C: IPv6 inside IPv4 B-to-C: IPv6 inside IPv4 E-to-F: IPv6 Network Layer 4-106

Interplay between routing, forwarding routing algorithm local forwarding table dest address output link address-range 1 address-range 2 address-range 3 address-range 4 3 2 2 1 routing algorithm determines end-end-path through network forwarding table determines local forwarding at this router IP destination address in arriving packet s header 1 3 2 Network Layer 4-108

Host Routing Table Intranet (Direct) Routing: Source and destination on the same (sub)network (LAN) From: 130.15.12.131/24 To: 130.15.12.22/24 Subnet mask: 255.255.255.0 The source and destination addresses are ANDed with the Mask to extract the network and subnet portion: 130.15.12.0 Both on the same subnet The datagram must be wrapped in a frame and transmitted directly to its destination on the same LAN The ARP table is checked to provide the physical address for the destination IP address If not there, ARP protocol is used to create one Network Layer 4-109

Datagram Encapsulation: Ethernet Ethernet frame for the previous example Link Hdr IP Hdr Dest IP=130.15.12.22 Dest Enet= Enet address of 130.15.12.22 Network Layer 4-110

Datagram Encapsulation: Ethernet IEEE 802.2/802.3 (RFC 1042) and Ethernet (RFC 894) Encapsulations Network Layer 4-111

Getting a datagram from source to destination Mask=255.255.255.0 forwarding table in A Dest. Net. next router Nhops IP datagram: 223.1.1 1 223.1.2 223.1.1.4 2 223.1.3 223.1.1.4 2 misc fields source IP addr dest IP addr data A 223.1.1.1 datagram remains unchanged, as it travels source to destination addr fields of interest here B 223.1.2.1 223.1.1.2 223.1.1.4 223.1.2.9 223.1.1.3 223.1.3.27 223.1.2.2 E 223.1.3.1 223.1.3.2 Network Layer 4-112

Getting a datagram from source to dest. misc fields 223.1.1.1 223.1.1.3 data Mask=255.255.255.0 Starting at A, send IP datagram addressed to B: look up net. address of B in forwarding table find B is on same net. as A link layer will send datagram directly to B inside link-layer frame B and A are directly connected A B forwarding table in A Dest. Net. next router Nhops 223.1.1 1 223.1.2 223.1.1.4 2 223.1.3 223.1.1.4 2 223.1.1.1 223.1.2.1 223.1.1.2 223.1.1.4 223.1.2.9 223.1.1.3 223.1.3.27 223.1.2.2 E 223.1.3.1 223.1.3.2 Network Layer 4-113

Getting a datagram from source to dest. misc fields Mask=255.255.255.0 223.1.1.1 223.1.2.2 data Starting at A, send IP datagram addressed to E: look up net. address of E in forwarding table find E is not on same net. as A The datagram is forwarded to the next router 223.1.1.4 A B forwarding table in A Dest. Net. next router Nhops 223.1.1 1 223.1.2 223.1.1.4 2 223.1.3 223.1.1.4 2 223.1.1.1 223.1.2.1 223.1.1.2 223.1.1.4 223.1.2.9 223.1.1.3 223.1.3.27 223.1.2.2 E 223.1.3.1 223.1.3.2 Network Layer 4-114

Host Routing Table Internet (non-direct) routing From: 130.15.12.131 To: 192.45.89.5 Subnet mask: 255.255.255.0 Destination is not on the same LAN (network) Consult the routing table Destination is not on the same LAN (network) If a destination is not on the local network, the only way to leave the local net is via a (the) router Each host contains a routing table to route datagrams to foreign hosts The default entry always points to a router: Forward any non-local datagrams to the default router Destination address 0.0.0.0 is used to mean default in routing tables Network Layer 4-115

Host Routing Table Example of two routers on a LAN Interface 128.121.54.2 leads to a small LAN A look at the routing table at host tigger Network Layer 4-116

Host Routing Table First destination 127.0.01 is the loopback address For clients/servers within the same node The default used for any destination not explicitly listed Datagrams to any systems on subnet 128.121.54.0 should be forwarded to router 128.121.50.2 The last entry declares any destination on subnet 128.121.50.0 is routed via 128.121.50.145, the node itself Flags indicate whether the route is up (U) and whether the next hop is a host (H) or a gateway (G) Network Layer 4-117

Rules for Routing Table Lookups The routing table entry can be An individual host A subnet A network A supernet Default General rule: The entry chosen should be based on the most precise match to the destination IP address First search for a complete IP address match If not, search for destination subnet match If not, search for destination network match If not, search for a routing prefix entry match If not, the default route is used Network Layer 4-118

Routing Example: Non-direct From: bsdi (140.252.13.35) To: ftp.uu.net (192.48.96.9) Subnet mask: 255.255.255.0 Network Layer 4-119

Getting a datagram from source to dest. misc fields 223.1.1.1 223.1.2.2 data Starting at A, dest. E: look up network address of E in forwarding table E on different network 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 link-layer frame datagram arrives at 223.1.1.4 continued.. A B forwarding table in A Dest. Net. next router Nhops 223.1.1 1 223.1.2 223.1.1.4 2 223.1.3 223.1.1.4 2 223.1.1.1 223.1.2.1 223.1.1.2 223.1.1.4 223.1.2.9 223.1.1.3 223.1.3.1 223.1.3.27 223.1.2.2 223.1.3.2 E Network Layer 4-120

Getting a datagram from source to dest. misc fields 223.1.1.1 223.1.2.2 data Dest. Net router Nhops interface 223.1.1-1 223.1.1.4 223.1.2-1 223.1.2.9 Arriving at 223.1.4, destined for 223.1.2.2 look up network address of E in router s forwarding table E on same network as router s interface 223.1.2.9 router, E directly attached link layer sends datagram to 223.1.2.2 inside link-layer frame via interface 223.1.2.9 datagram arrives at 223.1.2.2!!! (hooray!) 223.1.3-1 223.1.3.27 A B forwarding table in router 223.1.1.1 223.1.2.1 223.1.1.2 223.1.1.4 223.1.2.9 223.1.1.3 223.1.3.1 223.1.3.27 223.1.2.2 223.1.3.2 E Network Layer 4-121

Routing Algorithms Routing Algorithm Goal: determine good path (sequence of routers) through network from source to dest. Graph abstraction for routing algorithms: graph nodes are routers graph edges are physical links link cost: delay, $ cost, or congestion level A 1 2 5 B D 2 3 1 3 good path: C E 1 5 2 F typically means minimum cost path other def s possible Network Layer 4-122

Graph abstraction 5 graph: G = (N,E) u 1 2 v x 2 3 1 3 w y 1 5 2 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 4-123

Graph abstraction: costs u 1 2 5 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 4-124

Routing Algorithm and internet: Graph Construction Need to have graph abstraction of an internet General rule: All nodes in a subnet are fully connected Only routers are important Internet only worries about routing between routers (and not within subnets) Procedure: Ignore all non-router hosts Remove all connections (links) Fully connect all routers on the same subnet Network Layer 4-125

Graph abstraction of an internet Router Non-router host Network Layer 4-126

Graph abstraction of an internet Router Non-router host Step 1: ignore non-router hosts Network Layer 4-127

Graph abstraction of an internet Router Non-router host Step 2: remove all connections Network Layer 4-128