Computer Networking Chapter 4 The Network Layer

4.1 Introduction

4.1.1 Forwarding and Routing

  • forwarding: move packets from router's input to appropriate router output
  • routing: determine route taken by packets from source to dest. Connection setup

  • 3rd important function in some network architectures: ATM, frame relay, MPLS
  • before datagrams flow, two end hosts and intervening routers establish virtual connection

4.1.2 Network Service Models

  • example services for individual datagrams
    • guaranteed delivery
    • guaranteed delivery with bounded delay like less than 40 msec delay
  • example services for a flow of datagrams
    • in-order datagram delivery
    • guaranteed minimum bandwidth to flow
    • guaranteed maximum jitter
    • restrictions on changes in inter-packet spacing
    • security services
  • best-effort service
    • no service at all
Figure 4.1 Internet, ATM CBR ... AND ATM ABR service models
Figure 4.1 Internet, ATM CBR ... AND ATM ABR service models
  • Constant bit rate (CBR) ATM network service
    • a cell's end-to-end delay, the variability in a cells end-to-end delay (jitter) and the fraction of cells are guaranteed
  • Available bit rate (ABR) ATM network service
    • a minimum cell transmission rate (MCR) is guaranteed to a connection using ABR service

4.2 Virtual Circuit and Datagram Network

  • datagram network provides network-layer connectionless service
  • virtual-circuit network provides network-layer connection service
  • analogous to TCP/UDP connection-oriented / connectionless transport-layer services, but:
    • service: host-to-host
    • no choice: network provides one or the other
    • implementation: in network core

4.2.1 Virtual-Circuit Networks

A VC consists of:

  • path from source to destination
  • VC numbers, one number for each link 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
  • entries in forwarding tables in routers along path

VC routers maintain connection state information

Three phases in a virtual circuit

  • VC setup

    • specifies the receiver's address
    • determines the path between sender and receiver
  • Data transfer

    Figure 4.2 Virtual-circuit setup
    Figure 4.2 Virtual-circuit setup
  • VC teardown

    • when the sender or receiver informs the network layer to terminate the VC

signaling messages: the message that the end systems send into the network to initiate or terminate a VC, and the messages passed between the routers to set up the VC

signaling protocols: the protocols used to exchange these messages

  • used to setup, maintain teardown VC
  • used in ATM, frame-relay, X.25
  • not used in today's Internet

4.2.2 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
Figure 4.3 Datagram network
Figure 4.3 Datagram network
  • each router has a forwarding table that maps destination addresses to link interface

    • destination address are 32 bits

    • 4 billion IP addresses, so rather than list individual destination address list range of addresses

    • So:Figure 4.4 Destination address range

      Figure 4.5 Destination address range prefix match
      Figure 4.5 Destination address range prefix match
    • when looking for forwarding table entry for given destination address, use longest address prefix that matches destination address.

4.3 What's Inside a Router?

Figure 4.6 Router architecture
Figure 4.6 Router architecture

two key router functions:

  • run routing algorithms / protocol (RIP. OSPF, BGP)
  • forwarding datagrams from incoming to outgoing link

four components:

  • Input ports
    • terminate an incoming physical link at a router
    • intcroperate with the link layer at the other side of the incoming link
  • Switching fabric
    • connects the router's input ports to its output ports
  • Output ports
    • be paired with the input port for that link on the same line card
  • Routing processor
    • executes the routing protocols

4.3.1 Input Processing

Figure 4.7 Input port processing
Figure 4.7 Input port processing
  • physical layer
    • bit-level reception
  • data link layer
    • Ethernet
  • decentralized switching
    • given datagram dest., look up output port using forwarding table in input port memory ("match plus action")
    • goal: compete input port processing at 'line speed'
    • queuing: if datagrams arrives faster than forwarding rate into switch fabric

4.3.2 Switching

  • 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

    Figure 4.8 Three switching techniques
    Figure 4.8 Three switching techniques
    • 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)
    • 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
    • switching via an 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

4.3.3 Output Processing

Figure 4.9 Output port processing
Figure 4.9 Output port processing
  • buffering required when datagrams arrive from fabric faster than the transmission rate
    • Datagram (packets) can be lost due to congestion, lack of buffers
  • scheduling discipline chooses among queued datagrams for transmission
    • Priority scheduling – who gets best performance, network neutrality

4.3.4 Where Does Queuing Occur?

  • Output port queueing

    • when arrival rate via switch exceeds output line speed
    • queueing (delay) and loss due to output port buffer overflow!
    • RFC 3439 rule of thumb: average buffering equal to "typical" RTT (250 msec) times link capacity C
      • e.g., C = 10 Gbps link: 2.5 Gbit buffer
    • recent recmomendation: with N flows, buffering equal to \(RTT*C/\sqrt{N}\)
    Figure 4.10 Output port queuing
    Figure 4.10 Output port queuing
  • 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
    Figure 4.11 HOL blocking at an input queued switch
    Figure 4.11 HOL blocking at an input queued switch

4.4 The Internet Protocol (IP): Forwarding and Addressing in the Internet

Figure 4.12 A look inside the Internet's network layer
Figure 4.12 A look inside the Internet's network layer

Three major components

  • IP protocol
  • routing component
  • ICMP protocol

4.4.1 Datagram Format

Figure 4.13 IPv4 datagram format
Figure 4.13 IPv4 datagram format
  • Version number

    • 4 bits
    • the IP protocol version of the datagram
  • Header length

    • 4 bits
    • a variable number of options
    • where in the datagram the data actually begin
    • most IP datagrams do not contain options
    • typical IP datagram has a 20-byte header
  • Type of service (TOS)

    • 8 bits
    • distinguish each other
    • e.g., FTP or IP
  • Datagram length

    • 16 bits
    • total length of the IP datagram (header plus data), measured in bytes
    • theoretical maximum size of the IP datagram is 65535 bytes
    • actually datagrams rarely lager than 1500 bytes
  • Identifier, flags, fragmentation offset

    • have to do with IP fragmentation
    • don't be allowed in IPv6
  • Time-to-live (TTL)

    • max number remaining hops (decremented at each router)
  • Protocol

    • upper layer protocol to deliver payload to
    • 6 - TCP
    • 17 - UDP

    • detect its errors

  • Source and destination IP addresses

  • Options

    • e.g. timestamp, record route taken, specify list or routers to visit
    • dropped in the IPv6
  • Data (payload)

    • variable length
    • typically a TCP or UDP segment

IP Datagram Fragmentation

  • network links have MTU (max transmission Unit) - 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
    Figure 4.14 IP fragmentation and reassembly
    Figure 4.14 IP fragmentation and reassembly
    Figure 4.15 IP fragments
    Figure 4.15 IP fragments

4.4.2 IPv4 Addressing

Interface: the boundary between the host and the physical link. An IP address is technically associated with an interface, rather than with the host or router containing that interface

32 bits long = 4 bytes

Figure 4.16 Interface addresses and subnets
Figure 4.16 Interface addresses and subnets

subnet: the network interconnecting three hosts interfaces and one router interface

subnet mask: 22.3.1.1.0/24 where the /24 notation, indicates that the leftmost 24 bits of the 32-bit quantity define the subnet address

  • subnet part: high order bits
  • host part: low order bits
Figure 4.17 Three subnets in Figure 4.16
Figure 4.17 Three subnets in Figure 4.16
Figure 4.18 Three routers interconnecting six subnets
Figure 4.18 Three routers interconnecting six subnets

Classless Interdomain Routing (CIDR)

  • subnet portion of address of arbitrary length

  • address format: a.b.c.d/x, where x is # bits in subnet portion of address

IP addresses: how to get one?

  1. Obtaining a Block of Addresses

    • ask its ISP to provide addresses from a large block of addresses

      Figure 4.19 ISP's addresses
      Figure 4.19 ISP's addresses
    • from ICANN (Internet Corporation for Assigned Names and Numbers)

  2. Obtaining a Host Address: the Dynamic Host Configuration Protocol

    • plug and play

    • allow a host to obtain an IP address automatically or been assigned to a temporary IP address

    • Four steps:

      1. DHCP server discovery
        • find a DHCP server to interact with through a DHCP discover message within a UDP packet to port 67
        • IP des: 255.255.255.255
        • IP sour: 0.0.0.0
        • pass the IP datagram to the link layer
      2. DHCP server offer(s)
        • respond with a DHCP offer message
        • IP broadcast address of 255.255.255.255
      3. DHCP request
        • choose one server and offer with a DHCP request message, echoing back the configuration parameters
      4. DHCP ACK
        • respond with a DHCP ACK message, confirming the requested parameters
      Figure 4.20 DHCP client-server interaction
      Figure 4.20 DHCP client-server interaction
  3. Network Address Translation (NAT)

    Figure 4.21 Network address translation
    Figure 4.21 Network address translation
    • 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)
    • 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

4.4.3 Internet Control Message Protocol (ICMP)

  • 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
Figure 4.22 ICMP message types
Figure 4.22 ICMP message types
  • 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

4.4.4 IPv6

IPv6 Datagram Format

Most important changes

  • expanded addressing capabilities, from 32 bits to 128 bits
    • anycast address: send datagram to any one of a group of hosts
  • a streamlined 40-byte header
  • flow labeling and priority
Figure 4.23 IPv6 datagram format
Figure 4.23 IPv6 datagram format

Fields:

  • version: 4-bit field identifies the IP version number, 6
  • traffic class: 8-bit filed like TOS in IPv4
  • flow label: 20-bit field identifies datagrams in same "flow"
  • payload length: 16-bit unsigned integer giving number of bytes following the fixed-length, 40-byte datagram header
  • next header: identify upper layer protocol for data
  • Hop limit: decremented by one by each router that forwards the datagram, if the hop limit count reaches zero, the datagram is discarded
  • Source and destination addresses: 128-bit address
  • Data

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

Figure 4.24 Tunneling
Figure 4.24 Tunneling

4.5 Routing Algorithms

Figure 4.25 Abstract graph model of a computer network
Figure 4.25 Abstract graph model of a computer network

key question: what is the least-cost path between u and z ?

routing algorithm: algorithm that finds that least cost path

A global routing algorithm

  • all routers have complete topology, link cost info
  • "link state" algorithms

A decentralized routing algorithm

  • router knows physically-connected neighbors, link costs to neighbors
  • iterative process of computation, exchange of info with neighbors
  • "distance vector" algorithms

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
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2
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4
5
6
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8
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Initialization:
N' = {u}
for all nodes v:
if v adjacent to u:
then D(v) = c(u, v)
else D(v) = ∞

LOOP
find w not in N' such that D(w) is a minimum
add w to N'
update D(v) for each neighbout v of w and not in N':
D(v) = min( D(v), D(w) + c(w, v) )
/* new cost to v is either old cost to v or known least path cost to w plus cost from w to v */
until N' = N

4.5.2 The Distance-Vector (DV) Routing Algorithm

Bellman-Ford equation (dynamic programming)

  • let dx(y) := cost of least-cost path from x to y

  • dx(y) = minv { c(x, v) + dv(y) }

  • 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 ]

Figure 4.26 Distance-Vector Algorithm
Figure 4.26 Distance-Vector Algorithm

Example:

Figure 4.27 an example of DV algorithm
Figure 4.27 an example of DV algorithm
  • node detects local link cost change
  • updates routing info, recalculates distance vector
  • if DV changes, notify neighbors
  • bad news travels slow - "count to infinity" problem!
  • good news travels fast
  • 44 iterations before algorithm stabilizes: see text

Distance-Vector Algorithm: Adding Poisoned Reverse

If Z routes through Y to get to X :

Z tells Y its (Z’s) distance to X is infinite (so Y won’t route to X via Z)

not completely solve count to infinity problem?

A comparison of LS and DV Routing Algorithm

  • 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

4.5.3 Hierarchical Routing

  • our routing study thus far-idealization, for: scale and administrative autonomy
  • 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 or its own AS
    • has link to router in another AS
  • 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

4.6 Routing in the Internet

  • 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)

4.6.1 Inter-AS Routing in the Internet: 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)
    Figure 4.28 Number of hops from source router A to various subnets
    Figure 4.28 Number of hops from source router A to various subnets
  • 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)
  • RIP routing tables managed by application-level process called route-d (daemon)

  • advertisements sent in UDP packets, periodically repeated

4.6.2 Inter-AS Routing in the Internet: OSPF

  • "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
  • 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
Figure 4.29 Hierarchical OSPF
Figure 4.29 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.

4.6.3 Inter-AS Routing: BGP (Broder Gateway Protocol)

BGP Basics

  • BGP provide each AS a means to:

    • eBGP: obtain subnet reachability information from neighboring routers
    • iBGP: propagate reachability information to all AS-internal routers
    • determine "good" routes to other networks based on reachability information and policy
  • BGP session: two BGP routers (peers) exchange BGP messages

    • advertising paths to different destination network prefixes ("path vector protocol")
    • exchanged over semi-permanent TCP connections
  • when AS3 advertises a prefix to AS1

    • AS3 promise it will forward datagrams towards that prefix
    • AS3 can aggregate prefixes in its advertisement
    Figure 4.30 eBGP and iBGP sessions
    Figure 4.30 eBGP and iBGP sessions
  • 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.

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
    • NEXT-HOP: indicates specific internal AS router to next-hop AS
  • gateway router receiving route advertisement uses import policy to accept / decline
    • policy-based routing

BGP Route Selection

router may learn about more than 1 route to destination AS, selects route based on:

  • local preference value attribute: policy decision
  • shortest AS-PATH
  • closest NEXT-HOP router: hot potato routing
  • additional criteria

Routing Policy

Figure 4.31 A Simple BGP scenario
Figure 4.31 A Simple BGP scenario
  • 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 neither W 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!

4.7 Broadcast and Multicast Routing

4.7.1 Broadcast Routing Algorithms

deliver packets from source to all other nodes, source duplication is inefficient

Figure 4.32 Source-duplication versus in-network duplication
Figure 4.32 Source-duplication versus in-network duplication
  • uncontrolled Flooding

    • when node receives broadcast packet, sends copy to all neighbors
    • problems: cycles & broadcast storm
  • Controlled Flooding

    • sequence-number-controlled flooding:

      • node only broadcasts pkt if it hasn’t broadcast same packet before
      • node keeps track of packet ids already broadacasted
    • reverse path forwarding (RPF):

      • only forward packet if it arrived on shortest path between node and source

        Figure 4.33 Reverse path forwarding
        Figure 4.33 Reverse path forwarding
  • Spanning-Tree Broadcast

    • no redundant packets received by any node

    • first construct a spanning tree

    • nodes then forward / make copies only along spanning tree

    • each node sends unicast join message to center node

      • message forwarded until it arrives at a node already belonging to spanning tree
      Figure 4.34 Center-based construction of a spanning tree
      Figure 4.34 Center-based construction of a spanning tree

4.7.2 Multicast

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
  • source-based: different tree from each sender to rcvrs
Figure 4.35 The multicast group: A datagram addressed to the group is delivered to all members of the multicast group
Figure 4.35 The multicast group: A datagram addressed to the group is delivered to all members of the multicast group

Internet Group Management Protocol (IGMP)

Figure 4.36 The two components of network-layer multicast in the Internet: IGMP and multicast routing protocols
Figure 4.36 The two components of network-layer multicast in the Internet: IGMP and multicast routing protocols
  • between a host and its directly attached router
  • three message type:
    • membership_query: from a router to all hosts
    • membership_report: used by host to respond membership_query
    • leave_group: optional
  • soft state
    • if state explicitly refreshed by membership_message, it is removed

Multicast Routing Algorithm

  • source-based tree: one tree per source

    • shortest path trees

      • mcast forwarding tree: tree of shortest path routes from source to all receivers
      • Dijkstra’s algorithm
    • reverse path forwarding

      • rely on router’s knowledge of unicast shortest path from it to sender

      • each router has simple forwarding behavior:

        1
        2
        3
        if (mcast datagram received on incoming link on shortest path back to center)
        then flood datagram onto all outgoing links
        else ignore datagram
      • 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
  • group-shared tree: group uses one tree

    • minimal spanning (Steiner)

      • minimum cost tree connecting all routers with attached group members
    • 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

Multicast Routing in the Internet

  • 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
    • soft state: VMRP 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
  • 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
        • 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:

        • # networks with group members small wrt # interconnected networks

        • group members “widely dispersed”

        • bandwidth not plentiful
        • no membership until routers explicitly join
        • receiver-driven construction of mcast tree (e.g., center-based)
        • bandwidth and non-group-router processing conservative

    • 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

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