![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](https://image.slidesharecdn.com/4-180206150220/85/4-network-layer-54-320.jpg)
![IPv6: adoption
• US National Institutes of Standards estimate
[2013]:
– ~3% of industry IP routers
– ~11% of US gov’t routers
• Long (long!) time for deployment, use
– 20 years and counting!
– think of application‐level changes in last 20 years:
WWW, Facebook, …
– Why?](https://image.slidesharecdn.com/4-180206150220/85/4-network-layer-82-320.jpg)
![Distance vector algorithm
• Dx(y) = estimate of least cost from x to y
– x maintains distance vector Dx = [Dx(y): y є N ]
• node x:
– knows cost to each neighbor v: c(x,v)
– maintains its neighbors’ distance vectors. For each
neighbor v, x maintains
Dv = [Dv(y): y є N ]](https://image.slidesharecdn.com/4-180206150220/85/4-network-layer-96-320.jpg)
4. network layer
- 2. Learning outcomes 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
- 3. 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 Chapter 4: outline
- 4. 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 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 physicalnetwork data link physical
- 5. 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
- 6. 1 23 0111 value in arriving packet’s header routing algorithm local forwarding table header value output link 0100 0101 0111 1001 3 2 2 1 Interplay between routing and forwarding routing algorithm determines end‐end‐path through network forwarding table determines local forwarding at this router
- 7. Connection setup • 3rd 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
- 8. Network service model 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 Q: What service model for “channel” transporting datagrams from sender to receiver?
- 9. Service Model… • Guaranteed delivery. This service guarantees that the packet will eventually arrive at its destination. • Guaranteed delivery with bounded delay. This service not only guarantees delivery of the packet, but delivery within a specified host‐to‐ host delay bound (for example, within 100 msec). • In‐order packet delivery. This service guarantees that packets arrive at the destination in the order that they were sent. • Guaranteed minimal bandwidth. This network‐ layer service emulates the behavior of a transmission link of a specified bit rate (for example, 1 Mbps) between sending and receiving hosts.
- 13. Two types of Network Architecture • Connection‐Oriented and Connection‐Less Virtual Circuit Switching Example:ATM, X.25 Analogy: Telephone Datagram forwarding Example: IP networks Analogy: Postal service
- 14. 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
- 15. Virtual circuits • 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) “source‐to‐dest path behaves much like telephone circuit” – performance‐wise – network actions along source‐to‐dest path
- 16. VC.. • There are three identifiable phases in a virtual circuit: – VC setup: the sending transport layer contacts the network layer, specifies the receiver’s address, and waits for the network to set up the VC. – Data transfer. As shown in Figure 4.4, once the VC has been established, packetscan begin to flow along the VC. – VC teardown. This is initiated when the sender (or receiver) informs the network layer of its desire to terminate the VC.
- 17. 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
- 18. TCP/IP Protocol Suite 18 Sender Network Network Receiver R4 R5 R1 R2 R3 A connection-oriented packet-switched network Packets Virtual circuit Legend 4 3 2 1 4 3 2 1 4 3 2 1 4 3 2 1 4 3 2 1 A connection‐oriented packet switched network
- 19. VC forwarding table 12 22 32 1 2 3 VC number interface number Incoming interface Incoming VC # Outgoing interface Outgoing VC # 1 12 3 22 2 63 1 18 3 7 2 17 1 97 3 87 … … … … forwarding table in northwest router: VC routers maintain connection state information!
- 20. application transport network data link physical Virtual circuits: signaling protocols • used to setup, maintain teardown VC • used in ATM, frame‐relay, X.25 • not used in today’s Internet 1. initiate call 2. incoming call 3. accept call4. call connected 5. data flow begins 6. receive data application transport network data link physical
- 21. 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 1. send datagrams application transport network data link physical application transport network data link physical 2. receive datagrams
- 22. Sender Network Network ReceiverOut of orderR3 R4 R5 R1 R2 A connectionless packet-swtiched network 4 3 2 1 1 2 3 4 2 3 3 1 4 43 21 A connectionless packet‐switched network
- 23. 1 23 Datagram forwarding table IP destination address in arriving packet’s header 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)
- 24. Destination Address Range 11001000 00010111 00010000 00000000 through 11001000 00010111 00010111 11111111 11001000 00010111 00011000 00000000 through 11001000 00010111 00011000 11111111 11001000 00010111 00011001 00000000 through 11001000 00010111 00011111 11111111 otherwise Link Interface 0 1 2 3 Q: but what happens if ranges don’t divide up so nicely? Datagram forwarding table
- 25. Longest prefix matching Destination Address Range 11001000 00010111 00010*** ********* 11001000 00010111 00011000 ********* 11001000 00010111 00011*** ********* otherwise DA: 11001000 00010111 00011000 10101010 examples: DA: 11001000 00010111 00010110 10100001 which interface? which interface? when looking for forwarding table entry for given destination address, use longest address prefix that matches destination address. longest prefix matching Link interface 0 1 2 3
- 26. 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
- 28. Router architecture overview two key router functions: run routing algorithms/protocol (RIP, OSPF, BGP) forwarding datagrams from incoming to outgoing link high‐seed switching fabric routing processor router input ports router output ports forwarding data plane (hardware) routing, management control plane (software) forwarding tables computed, pushed to input ports
- 29. Four components of a router • Input ports. The input port performs several functions. – It performs the physical layer functionality (shown in light blue in Figure last slide) of terminating an incoming physical link to a router. – It performs the data link layer functionality (shown in dark blue) needed to interoperate with the data link layer functionality on the other side of the incoming link. – It also performs a lookup and forwarding function (shown in red) so that a datagram forwarded into the switching fabric of the router emerges at the appropriate output port. – Control packets (e.g., packets carrying routing protocol information such as RIP, OSPF or IGMP) are forwarded from the input port to the routing processor. In practice, multiple ports are often gathered together on a single line card within a router.
- 30. Four components of a router • Switching fabric. The switching fabric connects the router's input ports to its output ports. This switching fabric is completely contained with the router ‐ a network inside of a network router! • Output ports. An output port stores the datagrams that have been forwarded to it through the switching fabric, and then transmits the datagrams on the outgoing link. The output port thus performs the reverse data link and physical layer functionality as the input port. • Routing processor. The routing processor executes the routing protocols (e.g., the protocols we studied in section 4.4), maintains the routing tables, and performs network management functions (see chapter 8), within the router. Since we cover these topics elsewhere in this book, we defer discussion of these topics to elsewhere.
- 31. line termination link layer protocol (receive) lookup, forwarding queueing Input port functions decentralized switching: • given datagram dest., lookup output port using forwarding table in input port memory (“match plus action”) • goal: complete input port processing at ‘line speed’ • queuing: if datagrams arrive faster than forwarding rate into switch fabric physical layer: bit‐level reception data link layer: e.g., Ethernet see chapter 5 switch fabric
- 33. 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
- 36. Output ports buffering required when datagrams arrive from fabric faster than the transmission rate scheduling discipline chooses among queued datagrams for transmission line termination link layer protocol (send) switch fabric datagram buffer queueing Datagram (packets) can be lost due to congestion, lack of buffers Priority scheduling – who gets best performance, network neutrality
- 38. Output port queueing • buffering when arrival rate via switch exceeds output line speed • queueing (delay) and loss due to output port buffer overflow! at t, packets more from input to output one packet time later switch fabric switch fabric
- 39. 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 output port contention: only one red datagram can be transferred. lower red packet is blocked switch fabric one packet time later: green packet experiences HOL blocking switch fabric
- 41. The Internet network layer host, router network layer functions: forwarding table routing protocols • path selection • RIP, OSPF, BGP IP protocol • addressing conventions • datagram format • packet handling conventions ICMP protocol • error reporting • router “signaling” transport layer: TCP, UDP link layer physical layer network layer
- 42. ver length 32 bits data (variable length, typically a TCP or UDP segment) 16‐bit identifier header checksum time to live 32 bit source IP address head. len type of service flgs fragment offset upper layer 32 bit destination IP address options (if any) IP datagram format IP protocol version number header length (bytes) upper layer protocol to deliver payload to total datagram length (bytes) “type” of data for fragmentation/ reassemblymax number remaining hops (decremented at each router) e.g. timestamp, record route taken, specify list of routers to visit. how much overhead? 20 bytes of TCP 20 bytes of IP = 40 bytes + app layer overhead
- 43. IP Header • Data Unit Idnetifier (ID) • Sending host puts an identification number in each datagram • Total length: Length of user data plus header in octets • Data Offset ‐ Position of fragment in original datagram • In multiples of 64 bits (8 octets) • More flag • Indicates that this is not the last fragment • Datagrams can be fragmented/refragmented at any router • Datagrams are reassembled only at the destination host
- 45. 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 fragmentation: in: one large datagram out: 3 smaller datagrams reassembly … …
- 46. ID =x offset =0 fragflag =0 length =4000 ID =x offset =0 fragflag =1 length =1500 ID =x offset =185 fragflag =1 length =1500 ID =x offset =370 fragflag =0 length =1040 one large datagram becomes several smaller datagrams example: 4000 byte datagram MTU = 1500 bytes 1480 bytes in data field offset = 1480/8 IP fragmentation, reassembly
- 47. IP fragmentation, reassembly… • IP fragmentation plays an important role in gluing together the many disparate link‐layer technologies. • But fragmentation also has its costs. – First, it complicates routers and end systems, which need to be designed to accommodate datagram fragmentation and reassembly. – Second, fragmentation can be used to create lethal DoS attacks, whereby the attacker sends a series of bizarre and unexpected fragments. • Jolt2 attack, where the attacker sends a stream of small fragments to the target host, none of which has an offset of zero. • Another class sends overlapping IP fragments, that is, fragments whose offset values are set so that the fragments do not align properly
- 48. 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 or two interfaces (e.g., wired Ethernet, wireless 802.11) • IP addresses associated with each interface 223.1.1.1 223.1.1.2 223.1.1.3 223.1.1.4 223.1.2.9 223.1.2.2 223.1.2.1 223.1.3.2223.1.3.1 223.1.3.27 223.1.1.1 = 11011111 00000001 00000001 00000001 223 1 11
- 50. 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 network consisting of 3 subnets 223.1.1.1 223.1.1.3 223.1.1.4 223.1.2.9 223.1.3.2223.1.3.1 subnet 223.1.1.2 223.1.3.27 223.1.2.2 223.1.2.1
- 53. 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 11001000 00010111 00010000 00000000 subnet part host part 200.23.16.0/23
- 54. 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
- 56. DHCP server: 223.1.2.5 arriving client DHCP discover src : 0.0.0.0, 68 dest.: 255.255.255.255,67 yiaddr: 0.0.0.0 transaction ID: 654 DHCP offer src: 223.1.2.5, 67 dest: 255.255.255.255, 68 yiaddrr: 223.1.2.4 transaction ID: 654 lifetime: 3600 secs DHCP request src: 0.0.0.0, 68 dest:: 255.255.255.255, 67 yiaddrr: 223.1.2.4 transaction ID: 655 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 DHCP client‐server scenario Broadcast: is there a DHCP server out there? Broadcast: I’m a DHCP server! Here’s an IP address you can use Broadcast: OK. I’ll take that IP address! Broadcast: OK. You’ve got that IP address!
- 57. 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)
- 58. DHCP: example connecting laptop needs its IP address, addr of first‐hop router, addr of DNS server: use DHCP router with DHCP server built into router 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 168.1.1.1 DHCP UDP IP Eth Phy DHCP DHCP DHCP DHCP DHCP DHCP UDP IP Eth Phy DHCP DHCP DHCP DHCPDHCP
- 59. DHCP: example • DCP 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 router with DHCP server built into router DHCP DHCP DHCP DHCP DHCP UDP IP Eth Phy DHCP DHCP UDP IP Eth Phy DHCP DHCP DHCP DHCP client now knows its IP address, name and IP address of DSN server, IP address of its first-hop router
- 60. IP addresses: how to get one? Q: how does network get subnet part of IP addr? A: gets allocated portion of its provider ISP’s address space ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20 Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23 Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23 Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23 ... ….. …. …. Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23
- 61. Hierarchical addressing: route aggregation “Send me anything with addresses beginning 200.23.16.0/20” 200.23.16.0/23 200.23.18.0/23 200.23.30.0/23 Fly‐By‐Night‐ISP Organization 0 Organization 7 Internet Organization 1 ISPs‐R‐Us “Send me anything with addresses beginning 199.31.0.0/16” 200.23.20.0/23 Organization 2 . . . . . . hierarchical addressing allows efficient advertisement of routing information:
- 62. ISPs-R-Us has a more specific route to Organization 1 “Send me anything with addresses beginning 200.23.16.0/20” 200.23.16.0/23 200.23.18.0/23 200.23.30.0/23 Fly‐By‐Night‐ISP Organization 0 Organization 7 Internet Organization 1 ISPs‐R‐Us “Send me anything with addresses beginning 199.31.0.0/16 or 200.23.18.0/23” 200.23.20.0/23 Organization 2 . . . . . . Hierarchical addressing: more specific routes
- 63. 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
- 64. NAT: network address translation 10.0.0.1 10.0.0.2 10.0.0.3 10.0.0.4 138.76.29.7 local network (e.g., home network) 10.0.0/24 rest of Internet datagrams with source or destination in this network have 10.0.0/24 address for source, destination (as usual) all datagrams leaving local network have same single source NAT IP address: 138.76.29.7,different source port numbers
- 65. 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)
- 66. 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
- 67. 10.0.0.1 10.0.0.2 10.0.0.3 S: 10.0.0.1, 3345 D: 128.119.40.186, 80 1 10.0.0.4 138.76.29.7 1: host 10.0.0.1 sends datagram to 128.119.40.186, 80 NAT translation table WAN side addr LAN side addr 138.76.29.7, 5001 10.0.0.1, 3345 …… …… S: 128.119.40.186, 80 D: 10.0.0.1, 3345 4 S: 138.76.29.7, 5001 D: 128.119.40.186, 802 2: NAT router changes datagram source addr from 10.0.0.1, 3345 to 138.76.29.7, 5001, updates table S: 128.119.40.186, 80 D: 138.76.29.7, 5001 3 3: reply arrives dest. address: 138.76.29.7, 5001 4: NAT router changes datagram dest addr from 138.76.29.7, 5001 to 10.0.0.1, 3345 NAT: network address translation
- 68. 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
- 69. 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 10.0.0.1 10.0.0.4 NAT router 138.76.29.7 client ?
- 70. 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 10.0.0.1 NAT router IGD
- 71. 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 138.76.29.7 client 1. connection to relay initiated by NATed host 2. connection to relay initiated by client 3. relaying established NAT router 10.0.0.1
- 72. ICMP: internet control message protocol (RFC 792) • used by hosts & routers to communicate network‐ level 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
- 73. 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
- 74. 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 • 2128 addresses? = 340,282,366,920,938,463,463,374,607,431,770,000,000 = 340 billion billion billion billion addresses?
- 75. IPv6 datagram format priority: identify priority among datagrams in flow flow Label: identify datagrams in same “flow.” (concept of“flow” not well defined RFC 1752 and RFC 2460). next header: identify upper layer protocol for data data destination address (128 bits) source address (128 bits) payload len next hdr hop limit flow labelpriver 32 bits
- 76. 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
- 77. 77 • Dual stack hosts – Two TCP/IP stacks co‐exists on one host – Supporting IPv4 and IPv6 – Client uses whichever protocol it wishes IPv4 IPv6 www.abc.net ?? IPv4 TCP/UDP Application IPv6 Link Transition from IPv4 to IPv6
- 78. 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 source, dest addr IPv4 header fields IPv4 datagram IPv6 datagram IPv4 payload UDP/TCP payload IPv6 source dest addr IPv6 header fields
- 79. 79 • IPv6 tunnel over IPv4 IPv4 Network IPv6 IPv6 IPv6 Header Data IPv4 Header IPv6 Header Data IPv6 Header Data tunnel Transition from IPv4 to IPv6
- 80. Tunneling physical view: IPv4 IPv4 A B IPv6 IPv6 E IPv6 IPv6 FC D logical view: IPv4 tunnel connecting IPv6 routers E IPv6 IPv6 FA B IPv6 IPv6 ,
- 81. flow: X src: A dest: F data A‐to‐B: IPv6 Flow: X Src: A Dest: F data src:B dest: E B‐to‐C: IPv6 inside IPv4 E‐to‐F: IPv6 flow: X src: A dest: F data B‐to‐C: IPv6 inside IPv4 Flow: X Src: A Dest: F data src:B dest: E physical view: A B IPv6 IPv6 E IPv6 IPv6 FC D logical view: IPv4 tunnel connecting IPv6 routers E IPv6 IPv6 FA B IPv6 IPv6 Tunneling IPv4 IPv4
- 82. IPv6: adoption • US National Institutes of Standards estimate [2013]: – ~3% of industry IP routers – ~11% of US gov’t routers • Long (long!) time for deployment, use – 20 years and counting! – think of application‐level changes in last 20 years: WWW, Facebook, … – Why?
- 86. Graph abstraction: costs u yx wv z 2 2 1 3 1 1 2 5 3 5 c(x,x’) = cost of link (x,x’) e.g., c(w,z) = 5 cost could always be 1, or inversely related to bandwidth, or inversely related to congestion cost of path (x1, x2, x3,…, xp) = c(x1,x2) + c(x2,x3) + … + c(xp‐1,xp) key question: what is the least-cost path between u and z ? routing algorithm: algorithm that finds that least cost path
- 87. Routing algorithm classification Q: global or decentralized information? global: • all routers have complete topology, link cost info • “link state” algorithms decentralized: • router knows physically‐ connected 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 Q: Load‐sensitive or Load‐insensitive? Load‐sensitive: link costs vary dynamically based on the current level of congestion of the link load‐insensitive doesn’t reflect the current or past level of congestion of the link
- 88. A Link‐State Routing Algorithm Dijkstra’s algorithm • net topology, link costs known to all nodes – accomplished via “link state broadcast” – all nodes have same info • computes least cost paths from one node (‘source”) to all other nodes – gives forwarding table for that node • iterative: after k iterations, know least cost path to k dest.’s notation: • c(x,y): link cost from node x to y; = ∞ if not direct neighbors • D(v): current value of cost of path from source to dest. v • p(v): predecessor node along path from source to v • N': set of nodes whose least cost path definitively known
- 90. w3 4 v x u 5 3 7 4 y 8 z 2 7 9 Dijkstra’s algorithm: example Step N' D(v) p(v) 0 1 2 3 4 5 D(w) p(w) D(x) p(x) D(y) p(y) D(z) p(z) u ∞∞7,u 3,u 5,u uw ∞11,w6,w 5,u 14,x 11,w 6,wuwx uwxv 14,x 10,v uwxvy 12,y notes: construct shortest path tree by tracing predecessor nodes ties can exist (can be broken arbitrarily) uwxvyz
- 92. Dijkstra’s algorithm: example (2) u yx wv z resulting shortest-path tree from u: v x y w z (u,v) (u,x) (u,x) (u,x) (u,x) destination link resulting forwarding table in u:
- 93. Dijkstra’s algorithm, discussion algorithm complexity: n nodes each iteration: need to check all nodes, A, not in N n(n+1)/2 comparisons: O(n2) more efficient implementations possible: O(nlogn) oscillations possible: e.g., support link cost equals amount of carried traffic: A D C B 1 1+e e0 e 1 1 0 0 initially A D C B given these costs, find new routing…. resulting in new costs 2+e 0 00 1+e 1 A D C B given these costs, find new routing…. resulting in new costs 0 2+e 1+e1 0 0 A D C B given these costs, find new routing…. resulting in new costs 2+e 0 00 1+e 1
- 94. Distance vector algorithm Bellman‐Ford equation (dynamic programming) let dx(y) := cost of least‐cost path from x to y then dx(y) = min {c(x,v) + dv(y) }v cost to neighbor v min taken over all neighbors v of x cost from neighbor v to destination y
- 95. Bellman‐Ford example u yx wv z 2 2 1 3 1 1 2 5 3 5 clearly, dv(z) = 5, dx(z) = 3, dw(z) = 3 du(z) = min { c(u,v) + dv(z), c(u,x) + dx(z), c(u,w) + dw(z) } = min {2 + 5, 1 + 3, 5 + 3} = 4 node achieving minimum is next hop in shortest path, used in forwarding table B‐F equation says:
- 96. Distance vector algorithm • Dx(y) = estimate of least cost from x to y – x maintains distance vector Dx = [Dx(y): y є N ] • node x: – knows cost to each neighbor v: c(x,v) – maintains its neighbors’ distance vectors. For each neighbor v, x maintains Dv = [Dv(y): y є N ]
- 97. Distance vector algorithm key idea: from time‐to‐time, each node sends its own distance vector estimate to neighbors when x receives new DV estimate from neighbor, it updates its own DV using B‐F equation: Dx(y) ← minv{c(x,v) + Dv(y)} for each node y N under minor, natural conditions, the estimate Dx(y) converge to the actual least cost dx(y)
- 98. Distance vector algorithm iterative, asynchronous: each local iteration caused by: • local link cost change • DV update message from neighbor distributed: • each node notifies neighbors only when its DV changes – neighbors then notify their neighbors if necessary wait for (change in local link cost or msg from neighbor) recompute estimates if DV to any dest has changed, notify neighbors each node:
- 99. x y z x y z 0 2 7 ∞∞ ∞ ∞∞ ∞ from cost to fromfrom x y z x y z 0 x y z x y z ∞ ∞ ∞∞ ∞ cost to x y z x y z ∞∞ ∞ 7 1 0 cost to ∞ 2 0 1 ∞ ∞ ∞ 2 0 1 7 1 0 time x z 12 7 y node x table Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)} = min{2+0 , 7+1} = 2 Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)} = min{2+1 , 7+0} = 3 32 node y table node z table cost to from
- 100. x y z x y z 0 2 3 from cost to x y z x y z 0 2 7 from cost to x y z x y z 0 2 3 from cost to x y z x y z 0 2 3 from cost to x y z x y z 0 2 7 from cost to 2 0 1 7 1 0 2 0 1 3 1 0 2 0 1 3 1 0 2 0 1 3 1 0 2 0 1 3 1 0 time x y z x y z 0 2 7 ∞∞ ∞ ∞∞ ∞ from cost to fromfrom x y z x y z 0 x y z x y z ∞ ∞ ∞∞ ∞ cost to x y z x y z ∞∞ ∞ 7 1 0 cost to ∞ 2 0 1 ∞ ∞ ∞ 2 0 1 7 1 0 time x z 12 7 y node x table Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)} = min{2+0 , 7+1} = 2 Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)} = min{2+1 , 7+0} = 3 32 node y table node z table cost to from
- 101. Distance vector: link cost changes link cost changes: node detects local link cost change updates routing info, recalculates distance vector if DV changes, notify neighbors “good news travels fast” x z 14 50 y 1 t0 : y detects link‐cost change, updates its DV, informs its neighbors. t1 : z receives update from y, updates its table, computes new least cost to x , sends its neighbors its DV. t2 : y receives z’s update, updates its distance table. y’s least costs do not change, so y does not send a message to z.
- 102. Distance vector: link cost changes link cost changes: node detects local link cost change bad news travels slow - “count to infinity” problem! 44 iterations before algorithm stabilizes: see text x z 14 50 y 60 poisoned reverse: If Z routes throughY to get to X : Z tellsY its (Z’s) distance to X is infinite (soY won’t route to X via Z) will this completely solve count to infinity problem?
- 103. Comparison of LS and DV algorithms message complexity • LS: with n nodes, E links, O(nE) msgs sent • DV: exchange between neighbors only – convergence time varies speed of convergence • LS: O(n2) algorithm requires O(nE) msgs – may have oscillations • DV: convergence time varies – may be routing loops – count‐to‐infinity problem robustness: what happens if router malfunctions? LS: – node can advertise incorrect link cost – each node computes only its own table DV: – DV node can advertise incorrect path cost – each node’s table used by others • error propagate thru network
- 104. Hierarchical routing scale: with 600 million destinations: • can’t store all dest’s in routing tables! • routing table exchange would swamp links! administrative autonomy internet = network of networks each network admin may want to control routing in its own network our routing study thus far - idealization all routers identical network “flat” … not true in practice
- 105. Hierarchical routing • aggregate routers into regions, “autonomous systems” (AS) • routers in same AS run same routing protocol – “intra‐AS” routing protocol – routers in different AS can run different intra‐AS routing protocol gateway router: • at “edge” of its own AS • has link to router in another AS
- 106. 3b 1d 3a 1c 2a AS3 AS1 AS2 1a 2c 2b 1b Intra‐AS Routing algorithm Inter‐AS Routing algorithm Forwarding table 3c Interconnected ASes forwarding table configured by both intra‐ and inter‐AS routing algorithm intra‐AS sets entries for internal dests inter‐AS & intra‐AS sets entries for external dests
- 107. Inter‐AS tasks suppose router in AS1 receives datagram destined outside of AS1: router should forward packet to gateway router, but which one? AS1 must: 1. learn which dests are reachable through AS2, which through AS3 2. propagate this reachability info to all routers in AS1 job of inter‐AS routing! AS3 AS2 3b 3c 3a AS1 1c 1a 1d 1b 2a 2c 2b other networks other networks
- 108. Example: setting forwarding table in router 1d • suppose AS1 learns (via inter‐AS protocol) that subnet x reachable via AS3 (gateway 1c), but not via AS2 – inter‐AS protocol propagates reachability info to all internal routers • router 1d determines from intra‐AS routing info that its interface I is on the least cost path to 1c – installs forwarding table entry (x,I) AS3 AS2 3b 3c 3a AS1 1c 1a 1d 1b 2a 2c 2b other networks other networks x
- 109. Example: choosing among multiple ASes • now suppose AS1 learns from inter‐AS protocol that subnet x is reachable from AS3 and from AS2. • to configure forwarding table, router 1d must determine which gateway it should forward packets towards for dest x – this is also job of inter‐AS routing protocol! AS3 AS2 3b 3c 3a AS1 1c 1a 1d 1b 2a 2c 2b other networks other networks x ?
- 110. learn from inter‐AS protocol that subnet x is reachable via multiple gateways use routing info from intra‐AS protocol to determine costs of least‐cost paths to each of the gateways hot potato routing: choose the gateway that has the smallest least cost determine from forwarding table the interface I that leads to least‐cost gateway. Enter (x,I) in forwarding table Example: choosing among multiple ASes • now suppose AS1 learns from inter‐AS protocol that subnet x is reachable from AS3 and from AS2. • to configure forwarding table, router 1d must determine towards which gateway it should forward packets for dest x – this is also job of inter‐AS routing protocol! • hot potato routing: send packet towards closest of two routers.
- 111. Intra‐AS Routing also known as interior gateway protocols (IGP) most common intra‐AS routing protocols: RIP: Routing Information Protocol OSPF: Open Shortest Path First IGRP: Interior Gateway Routing Protocol (Cisco proprietary)
- 112. RIP ( Routing Information Protocol) • included in BSD‐UNIX distribution in 1982 • distance vector algorithm – distance metric: # hops (max = 15 hops), each link has cost 1 – DVs exchanged with neighbors every 30 sec in response message (aka advertisement) – each advertisement: list of up to 25 destination subnets (in IP addressing sense) DC BA u v w x y z subnet hops u 1 v 2 w 2 x 3 y 3 z 2 from router A to destination subnets:
- 113. RIP Messages • Request – A request message is sent by a router that has just come up or by a router that has some time‐out entries – A request can ask about specific entries or all entries • Response – A response can be either solicited or unsolicited (30s or when there is a change in the routing table)
- 114. RIP: example destination subnet next router # hops to dest w A 2 y B 2 z B 7 x ‐‐ 1 …. …. .... routing table in router D w x y z A C D B Desta Next hop No of hop Z C 4 W ‐ 1 x ‐ 1
- 115. w x y z A C D B destination subnet next router # hops to dest w A 2 y B 2 z B 7 x ‐‐ 1 …. …. .... routing table in router D A 5 dest next hops w ‐ 1 x ‐ 1 z C 4 …. … ... A‐to‐D advertisement RIP: example
- 116. RIP: link failure, recovery if no advertisement heard after 180 sec ‐‐> neighbor/link declared dead routes via neighbor invalidated new advertisements sent to neighbors neighbors in turn send out new advertisements (if tables changed) link failure info quickly (?) propagates to entire net poison reverse used to prevent ping‐pong loops (infinite distance = 16 hops)
- 119. OSPF (Open Shortest Path First) • “open”: publicly available • uses link state algorithm – LS packet dissemination – topology map at each node – route computation using Dijkstra’s algorithm • OSPF advertisement carries one entry per neighbor • advertisements flooded to entire AS – carried in OSPF messages directly over IP (rather than TCP or UDP • IS‐IS routing protocol: nearly identical to OSPF
- 120. OSPF “advanced” features (not in RIP) • security: all OSPF messages authenticated (to 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 ToS; high for real time ToS) • integrated uni‐ and multicast support: – Multicast OSPF (MOSPF) uses same topology data base as OSPF • hierarchical OSPF in large domains.
- 122. Hierarchical OSPF • two‐level hierarchy: local area, backbone. – link‐state advertisements only in area – each nodes has detailed area topology; only know direction (shortest path) to nets in other areas. • area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers. • backbone routers: run OSPF routing limited to backbone. • boundary routers: connect to other AS’s.
- 123. Internet inter‐AS routing: BGP • BGP (Border Gateway Protocol): the de facto inter‐domain routing protocol – “glue that holds the Internet together” • BGP provides each AS a means to: – eBGP: obtain subnet reachability information from neighboring ASs. – iBGP: propagate reachability information to all AS‐ internal routers. – determine “good” routes to other networks based on reachability information and policy. • allows subnet to advertise its existence to rest of Internet: “I exist and I am here”
- 124. BGP basics • when AS3 advertises a prefix to AS1: – AS3 promises it will forward datagrams towards that prefix – AS3 can aggregate prefixes in its advertisement AS3 AS2 3b 3c 3a AS1 1c 1a 1d 1b 2a 2c 2b other networks other networks BGP session: two BGP routers (“peers”) exchange BGP messages: advertising paths to different destination network prefixes (“path vector” protocol) exchanged over semi-permanentTCP connections BGP message
- 125. BGP basics: distributing path information using eBGP session between 3a and 1c, AS3 sends prefix reachability info to AS1. 1c can then use iBGP do distribute new prefix info to all routers in AS1 1b can then re‐advertise new reachability info to AS2 over 1b‐to‐2a eBGP session when router learns of new prefix, it creates entry for prefix in its forwarding table. AS3 AS2 3b 3a AS1 1c 1a 1d 1b 2a 2c 2b other networks other networks eBGP session iBGP session
- 126. Path attributes and BGP routes • advertised prefix includes BGP attributes – prefix + attributes = “route” • two important attributes: – AS‐PATH: contains ASs through which prefix advertisement has passed: e.g., AS 67, AS 17 – NEXT‐HOP: indicates specific internal‐AS router to next‐ hop AS. (may be multiple links from current AS to next‐ hop‐AS) • gateway router receiving route advertisement uses import policy to accept/decline – e.g., never route through AS x – policy‐based routing
- 127. BGP route selection router may learn about more than 1 route to destination AS, selects route based on: 1. local preference value attribute: policy decision 2. shortest AS‐PATH 3. closest NEXT‐HOP router: hot potato routing 4. additional criteria
- 128. BGP messages • BGP messages exchanged between peers over TCP connection • BGP messages: – OPEN: opens TCP connection to peer and authenticates sender – UPDATE: advertises new path (or withdraws old) – KEEPALIVE: keeps connection alive in absence of UPDATES; also ACKs OPEN request – NOTIFICATION: reports errors in previous msg; also used to close connection
- 129. Putting it Altogether: How Does an Entry Get Into a Router’s Forwarding Table? • Answer is complicated! • Ties together hierarchical routing (Section 4.5.3) with BGP (4.6.3) and OSPF (4.6.2). • Provides nice overview of BGP!
- 131. High‐level overview 1. Router becomes aware of prefix 2. Router determines output port for prefix 3. Router enters prefix‐port in forwarding table How does entry get in forwarding table?
- 132. Router becomes aware of prefix AS3 AS2 3b 3c 3a AS1 1c 1a 1d 1b 2a 2c 2b other networks other networks BGP message BGP message contains “routes” “route” is a prefix and attributes: AS‐PATH, NEXT‐HOP,… Example: route: Prefix:138.16.64/22 ; AS‐PATH: AS3 AS131 ; NEXT‐HOP: 201.44.13.125
- 133. Router may receive multiple routes AS3 AS2 3b 3c 3a AS1 1c 1a 1d 1b 2a 2c 2b other networks other networks BGP message Router may receive multiple routes for same prefix Has to select one route
- 134. Select best BGP route to prefix • Router selects route based on shortest AS‐ PATH Example: AS2 AS17 to 138.16.64/22 AS3 AS131 AS201 to 138.16.64/22 What if there is a tie? We’ll come back to that! select
- 135. Find best intra‐route to BGP route • Use selected route’s NEXT‐HOP attribute – Route’s NEXT‐HOP attribute is the IP address of the router interface that begins the AS PATH. • Example: AS‐PATH: AS2 AS17 ; NEXT‐HOP: 111.99.86.55 • Router uses OSPF to find shortest path from 1c to 111.99.86.55 AS3 AS2 3b 3c 3a AS1 1c 1a 1d 1b 2a 2c 2b other networks other networks 111.99.86.55
- 136. Router identifies port for route • Identifies port along the OSPF shortest path • Adds prefix‐port entry to its forwarding table: – (138.16.64/22 , port 4) AS3 AS2 3b 3c 3a AS1 1c 1a 1d 1b 2a 2c 2b other networks other networks router port 1 2 3 4
- 137. Hot Potato Routing • Suppose there two or more best inter‐routes. • Then choose route with closest NEXT‐HOP – Use OSPF to determine which gateway is closest – Q: From 1c, chose AS3 AS131 or AS2 AS17? – A: route AS3 AS201 since it is closer AS3 AS2 3b 3c 3a AS1 1c 1a 1d 1b 2a 2c 2b other networks other networks
- 138. How does entry get in forwarding table? Summary 1. Router becomes aware of prefix – via BGP route advertisements from other routers 2. Determine router output port for prefix – Use BGP route selection to find best inter‐AS route – Use OSPF to find best intra‐AS route leading to best inter‐AS route – Router identifies router port for that best route 3. Enter prefix‐port entry in forwarding table
- 139. BGP routing policy A,B,C are provider networks X,W,Y are customer (of provider networks) X is dual-homed: attached to two networks X does not want to route from B via X to C .. so X will not advertise to B a route to C A B C W X Y legend: customer network: provider network
- 140. BGP routing policy (2) A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C? No way! B gets no “revenue” for routing CBAW since neitherW nor C are B’s customers B wants to force C to route to w via A B wants to route only to/from its customers! A B C W X Y legend: customer network: provider network
- 141. Why different Intra‐, Inter‐AS routing ? policy: • inter‐AS: admin wants control over how its traffic routed, who routes through its net. • intra‐AS: single admin, so no policy decisions needed scale: • hierarchical routing saves table size, reduced update traffic performance: • intra‐AS: can focus on performance • inter‐AS: policy may dominate over performance
- 143. R1 R2 R3 R4 source duplication R1 R2 R3 R4 in‐network duplication duplicate creation/transmissionduplicate duplicate Broadcast routing deliver packets from source to all other nodes source duplication is inefficient: source duplication: how does source determine recipient addresses?
- 144. In‐network duplication • flooding: when node receives broadcast packet, sends copy to all neighbors – problems: cycles & broadcast storm • controlled flooding: node only broadcasts pkt if it hasn’t broadcast same packet before – node keeps track of packet ids already broadacsted – or reverse path forwarding (RPF): only forward packet if it arrived on shortest path between node and source • spanning tree: – no redundant packets received by any node
- 147. Multicast routing: problem statement goal: find a tree (or trees) connecting routers having local mcast group members tree: not all paths between routers used shared‐tree: same tree used by all group members shared tree source‐based trees group member not group member router with a group member router without group member legend source-based: different tree from each sender to rcvrs
- 148. Approaches for building mcast trees approaches: source‐based tree: one tree per source shortest path trees reverse path forwarding group‐shared tree: group uses one tree minimal spanning (Steiner) center‐based trees …we first look at basic approaches, then specific protocols adopting these approaches
- 150. Reverse path forwarding if (mcast datagram received on incoming link on shortest path back to center) then flood datagram onto all outgoing links else ignore datagram rely on router’s knowledge of unicast shortest path from it to sender each router has simple forwarding behavior:
- 151. Reverse path forwarding: example result is a source-specific reverse SPT may be a bad choice with asymmetric links router with attached group member router with no attached group member datagram will be forwarded LEGEND R1 R2 R3 R4 R5 R6 R7 s: source datagram will not be forwarded
- 152. Reverse path forwarding: pruning • forwarding tree contains subtrees with no mcast group members – no need to forward datagrams down subtree – “prune” msgs sent upstream by router with no downstream group members router with attached group member router with no attached group member prune message LEGEND links with multicast forwarding P R1 R2 R3 R4 R5 R6 R7 s: source P P
- 153. Shared‐tree: steiner tree steiner tree: minimum cost tree connecting all routers with attached group members problem is NP‐complete excellent heuristics exists not used in practice: computational complexity information about entire network needed monolithic: rerun whenever a router needs to join/leave
- 154. Center‐based trees • single delivery tree shared by all • one router identified as “center” of tree • to join: – edge router sends unicast join‐msg addressed to center router – join‐msg “processed” by intermediate routers and forwarded towards center – join‐msg either hits existing tree branch for this center, or arrives at center – path taken by join‐msg becomes new branch of tree for this router
- 156. Internet Multicasting Routing: DVMRP • DVMRP: distance vector multicast routing protocol, RFC1075 • flood and prune: reverse path forwarding, source‐based tree – RPF tree based on DVMRP’s own routing tables constructed by communicating DVMRP routers – no assumptions about underlying unicast – initial datagram to mcast group flooded everywhere via RPF – routers not wanting group: send upstream prune msgs
- 157. DVMRP: continued… • soft state: DVMRP router periodically (1 min.) “forgets” branches are pruned: – mcast data again flows down unpruned branch – downstream router: reprune or else continue to receive data • routers can quickly regraft to tree – following IGMP join at leaf • odds and ends – commonly implemented in commercial router
- 158. Tunneling Q: how to connect “islands” of multicast routers in a “sea” of unicast routers? mcast datagram encapsulated inside “normal” (non- multicast-addressed) datagram normal IP datagram sent thru “tunnel” via regular IP unicast to receiving mcast router (recall IPv6 inside IPv4 tunneling) receiving mcast router unencapsulates to get mcast datagram physical topology logical topology
- 159. PIM: Protocol Independent Multicast not dependent on any specific underlying unicast routing algorithm (works with all) two different multicast distribution scenarios : dense: group members densely packed, in “close” proximity. bandwidth more plentiful sparse: # networks with group members small wrt # interconnected networks group members “widely dispersed” bandwidth not plentiful
- 160. Consequences of sparse‐dense dichotomy: dense • group membership by routers assumed until routers explicitly prune • data‐driven construction on mcast tree (e.g., RPF) • bandwidth and non‐group‐ router processing profligate sparse: no membership until routers explicitly join receiver‐ driven construction of mcast tree (e.g., center‐ based) bandwidth and non‐group‐ router processing conservative
- 161. PIM‐ dense mode flood-and-prune RPF: similar to DVMRP but… underlying unicast protocol provides RPF info for incoming datagram less complicated (less efficient) downstream flood than DVMRP reduces reliance on underlying routing algorithm has protocol mechanism for router to detect it is a leaf-node router
- 162. PIM ‐ sparse mode • center‐based approach • router sends join msg to rendezvous point (RP) – intermediate routers update state and forward join • after joining via RP, router can switch to source‐specific tree – increased performance: less concentration, shorter paths all data multicast from rendezvous point rendezvous point join join join R1 R2 R3 R4 R5 R6 R7
- 163. PIM ‐ sparse mode sender(s): • unicast data to RP, which distributes down RP‐rooted tree • RP can extend mcast tree upstream to source • RP can send stop msg if no attached receivers – “no one is listening!” all data multicast from rendezvous point rendezvous point join join join R1 R2 R3 R4 R5 R6 R7
- 164. 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 Chapter 4: done! 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