Network Layer (2-89-90) 4-2 Chapter 4: Network Layer Chapter
goals: understanding principles behind network layer services:
routing (path selection) dealing with scale how a router works
advanced topics: IPv6, mobility instantiation and implementation in
the Internet Overview: network layer services routing principles:
path selection hierarchical routing IP Internet routing protocols
reliable transfer intra-domain inter-domain whats inside a router?
IPv6 mobility
Slide 3
Network Layer (2-89-90) 4-3 Chapter 4 Outline 4.1 Introduction
and Network Service Models 4.2 Routing Principles 4.3 Hierarchical
Routing 4.4 Routing in the Internet 4.5 The Internet (IP) Protocol
4.6 Whats Inside a Router 4.7 IPv6 4.8 Multicast Routing 4.9
Mobility
Slide 4
Network Layer (2-89-90) 4-4 Network Layer Functions transport
packet from sending to receiving hosts network layer protocols in
every host, router three important functions: path determination:
route taken by packets from source to dest. (Routing Algorithms)
forwarding: move packets from routers input to appropriate router
output call setup: some network architectures require router call
setup along path before data flows application transport network
data link physical network data link physical application transport
network data link physical
Slide 5
Network Layer (2-89-90) 4-5 Network Service Model Q: What
service model for channel transporting packets from sender to
receiver? Services guaranteed bandwidth? preservation of
inter-packet timing (no jitter)? loss-free delivery? in-order
delivery? congestion feedback to sender? virtual circuit or
datagram? The most important abstraction provided by network
layer:
Slide 6
Network Layer (2-89-90) 4-6 Virtual circuits call setup,
teardown for each call before data can flow each packet carries VC
identifier (not destination host ID) every router on
source-destination path maintains state for each passing connection
transport-layer connection only involved two end systems Link and
router resources (bandwidth, buffers) may be allocated to VC
(dedicated resources = predictable service) to get circuit-like
performance. source-to-destination path behaves much like telephone
circuit performance-wise network actions along
source-to-destination path
Slide 7
Network Layer (2-89-90) 4-7 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
destination address) VC number can be changed on each link. New VC
number comes from forwarding table
Slide 8
Network Layer (2-89-90) 4-8 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 router a Routers maintain connection state
information! a
Slide 9
Network Layer (2-89-90) 4-9 Virtual Circuits: Signaling
Protocols used to setup, maintain teardown VC used in ATM,
frame-relay, X.25 not used in todays Internet 1. Initiate call 2.
Incoming call 3. Accept call 4. Call connected 5. Data flow begins
6. Receive data application transport network data link physical
application transport network data link physical
Slide 10
Network Layer (2-89-90) 4-10 Datagram networks: the Internet
model 1. Send Data 2. Receive Data application transport network
data link physical application transport network data link physical
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 packets between same
source-destination pair may take different paths
Slide 11
Network Layer (2-89-90) 4-11 Network Layer Service Models:
Internet model being extended: Integrated services, Differentiated
Services Chapter 6 Network Architecture Internet ATM Service Model
best effort CBR VBR ABR UBR Bandwidth none constant rate guaranteed
rate guaranteed minimum none Loss no yes no Order no yes Timing no
yes no Congestion feedback no (inferred via loss) no congestion no
congestion yes no Guarantees ? CBR: Constant bit rate VBR: Variable
bit rate ABR: Available bit rate UBR: Unspecified bit rate
Slide 12
Network Layer (2-89-90) 4-12 QoS Factors Timing Connection
Establishment Delay End-To-End Delay Connection Establishment
Failure Probability Throughput or Bandwidth Guarantee Ordering
Preservation Congestion Indication (Control) Bit-Error rate or
Packet-Loss Rate Control Protection Priority Resilience (Return
Back to Normal Operation).
Slide 13
Network Layer (2-89-90) 4-13 Service Clases Guaranteed Quality
of Service Predictive Quality of Service Best Effort Quality of
Service
Slide 14
Network Layer (2-89-90) 4-14 Guaranteed QoS Specified through
QoS parameter values deterministic statistical Single value -
average (threshold, target) Pair of values - interval Triple of
values max., mean, min.
Slide 15
Network Layer (2-89-90) 4-15 Predictable Service Parameter
bounds based on history, that is, past network behavior. Parameter
values are measured, and certain statistical analyses may be
carried out
Slide 16
Network Layer (2-89-90) 4-16 Best Effort Services No guarantees
of quality, no QoS parameter values UDP/IP Partial guarantees, some
QoS parameter values are given. TCP/IP
Slide 17
Network Layer (2-89-90) 4-17 Datagram or VC Network: why?
Internet (Datagram) data exchange among computers elastic service,
no strict timing req. smart end systems (computers) can adapt,
perform control, error recovery simple inside network, complexity
at edge many link types different characteristics uniform service
is difficult ATM (Virtual Circuit) evolved from telephony human
conversation: strict timing, reliability requirements need for
guaranteed service dumb end systems telephones complexity inside
network
Slide 18
Network Layer (2-89-90) 4-18 Buffering in IP routers Buffer
size l Space for bursts of packets l Latency Router Internet Router
Network Interface Network Interface Dropping packets When?
What?
Slide 19
Network Layer (2-89-90) 4-19 FIFO Queueing in the Router (Drop
Tail) Single queue maintained Network Interface Network
Interface
Slide 20
Network Layer (2-89-90) 4-20 FIFO Queueing in the Router (Drop
Tail) Single queue maintained Dequeue from head Network Interface
Network Interface
Slide 21
Network Layer (2-89-90) 4-21 FIFO Queueing in the Router (Drop
Tail) Single queue maintained Dequeue from head Enqueue at tail
Network Interface Network Interface
Slide 22
Network Layer (2-89-90) 4-22 FIFO Queueing in the Router (Drop
Tail) Single queue maintained Dequeue from head Enqueue at tail
When full Network Interface Network Interface
Slide 23
Network Layer (2-89-90) 4-23 FIFO Queueing in the Router (Drop
Tail) Single queue maintained Dequeue from head Enqueue at tail
When full drop arriving packet (drop-tail) Network Interface
Network Interface
Slide 24
Network Layer (2-89-90) 4-24 Slow Feedback from Drop Tail
Feedback comes when buffer is completely full even though the
buffer has been filling for a while Plus, the filling buffer is
increasing RTT and the variance in the RTT Might be better to give
early feedback Get one or two flows to slow down, not all of them
Get these flows to slow down before it is too late
Slide 25
Network Layer (2-89-90) 4-25 Queue Management Performance
Degradation in current TCP Congestion Control Multiple packet loss
Low link utilization Congestion collapse The role of the router
(i.e., network) Control congestion effectively with a network
Allocate bandwidth fairly
Slide 26
Network Layer (2-89-90) 4-26 Active Queue Management Goals:
Better congestion notification for responsive flows (i.e. TCP)
Maintain shorter queues Fairness in drops (proportional)
Slide 27
Network Layer (2-89-90) 4-27 Random Early Detection (RED)
Invented by Sally Floyd and Van Jacobson in the early 1990s,
differs from the DECbit in two major ways Notification is implicit
just drop the packet (TCP will timeout) could make explicit by
marking the packet Early random drop rather than wait for queue to
become full, drop each arriving packet with some drop probability
whenever the queue length exceeds some drop level
Slide 28
Network Layer (2-89-90) 4-28 Random Early Detection (RED).
Basic idea of RED Router notices that the queue is getting
build-up. Randomly drops or marks arriving packets (before queue
gets full). Packet drop signals a congestion to the source. Packet
drop probability Drop probability increases as queue length
increases If buffer is below some level, dont drop anything
otherwise, set drop probability as function of queue
Slide 29
Network Layer (2-89-90) 4-29 RED Details Compute average queue
length (Geometric Moving Average) 0 < < 1 (usually 0.002)
SampleLen is queue length each time a packet arrives. SampleLen
MinTh MaxTh
Slide 30
Network Layer (2-89-90) 4-30 RED Details. On arrival of a
packet: calculate AvgLen if AvgLen MaxTh then drop arriving
packet
Slide 31
Network Layer (2-89-90) 4-31 RED Details.. Computing
probability P P maxP 1 minTh maxTh AvgLen
Slide 32
Network Layer (2-89-90) 4-32 RED Detail Weighted Running
Average Queue Length Time Max Queue Size Max Threshold Min
Threshold Forced drop Probabilistic drops No drops Drop probability
Average Queue Length
Slide 33
Network Layer (2-89-90) 4-33 Properties of RED Drops packets
before queue is full In the hope of reducing the rates of some
flows Drops packet in proportion to each flows rate High-rate flows
have more packets and, hence, a higher chance of being selected
Drops are spaced out in time Which should help desynchronize the
TCP senders Tolerant of burstiness in the traffic By basing the
decisions on average queue length
Slide 34
Network Layer (2-89-90) 4-34 34 Tuning RED MaxP is typically
set to 0.02, meaning that when the average queue size is halfway
between the two thresholds, the gateway drops roughly one out of
100 packets. If traffic is bursty, then MinThreshold should be
sufficiently large to allow link utilization to be maintained at an
acceptably high level. Difference between two thresholds should be
larger than the typical increase in the calculated average queue
length in one RTT; setting MaxThreshold to twice MinThreshold is
reasonable for traffic on todays Internet.
Slide 35
Network Layer (2-89-90) 4-35 Problems With RED Hard to get the
tunable parameters just right How early to start dropping packets?
What slope for the increase in drop probability? What time scale
for averaging the queue length? Sometimes RED helps but sometimes
not If the parameters arent set right, RED doesnt help And it is
hard to know how to set the parameters RED is implemented in
practice But, often not used due to the challenges of tuning right
Many variations With cute names like Blue and FRED
Slide 36
Network Layer (2-89-90) 4-36 Explicit Congestion Notification
Early dropping of packets Good: gives early feedback Bad: has to
drop the packet to give the feedback Explicit Congestion
Notification Router marks the packet with an ECN bit and sending
host interprets as a sign of congestion Surmounting the challenges
Must be supported by the end hosts and the routers Requires two
bits in the IP header (one for the ECN mark, and one to indicate
the ECN capability) Solution: borrow two of the Type-Of-Service
bits in the IPv4 packet header
Slide 37
Network Layer (2-89-90) 4-37 Chapter 4 Outline 4.1 Introduction
and Network Service Models 4.2 Routing Principles Distance vector
routing Link state routing 4.3 Hierarchical Routing 4.4 Routing in
the Internet 4.5 The Internet (IP) Protocol 4.6 Whats Inside a
Router 4.7 IPv6 4.8 Multicast Routing 4.9 Mobility
Slide 38
Network Layer (2-89-90) 4-38 CS244a Handout 5 The Problem A B
How does R choose a next-hop on the path towards host B? R
Slide 39
Network Layer (2-89-90) 4-39 1 2 3 128.9.16.14 dest. IP addr.
in arriving packets header routing algorithm local forwarding table
dest. net. addr. Output port 65/8 128.9/16 128.9.16/20 128.9.19/24
32213221 Interplay between routing, forwarding
Slide 40
Network Layer (2-89-90) 4-40 Graph: G = (N,E) 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) } Graph abstraction
Remark: Graph abstraction is useful in other network contexts
Example: P2P, where N is set of peers and E is set of TCP
connections z 2 2 1 3 1 1 2 5 3 5 y w v u x
Slide 41
Network Layer (2-89-90) 4-41 Graph abstraction: costs z 2 2 1 3
1 1 2 5 3 5 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 ) Question: Whats the least-cost path
between u and z ? Routing algorithm: algorithm that finds
least-cost path 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 y w v u x
Slide 42
Network Layer (2-89-90) 4-42 Routing Graph abstraction for
routing algorithms: graph nodes are routers graph edges are
physical links link cost: Delay (Make high speed links attractive,
but closeness counts), $ cost, Inverse of bandwidth, Path
utilization (congestion level & queue length), Stability (Is
path up or down?) Goal: determine good path (sequence of routers)
thru network from source to dest. good path: typically means
minimum cost path other definitions possible Routing protocol
Abstract model of a network A F D C E B 1 1 1 2 2 2 5 3 5 3 4
3
Slide 43
Network Layer (2-89-90) 4-43 CS244a Handout 5 Technique 1: Nave
Approach Advantages: Simple. Every destination in the network is
reachable. Disadvantages: Some routers receive a packet multiple
times. Packets can go round in loops forever. Inefficient. Flood!:
Routers forward packets to all ports except the input port. R
Slide 44
Network Layer (2-89-90) 4-44 CS244a Handout 5 Spanning Trees
Objective: Find the lowest cost route from each of (R 1, , R 7 ) to
R 8. 114 2 4 2 23 2 3 R1R1 R2R2 R3R3 R4R4 R5R5 R6R6 R7R7 R8R8 B
A
Slide 45
Network Layer (2-89-90) 4-45 CS244a Handout 5 A Spanning Tree
The solution is a spanning tree with R 8 as the root of the tree.
Tree: There are no loops. Spanning: All nodes included. Well see
two algorithms that build spanning trees automatically: The
distributed Bellman-Ford algorithm ( Distance Vector ). Dijkstras
shortest path first algorithm ( Link State ). 114 2 4 2 23 2 3 R1R1
R3R3 R5R5 R7R7 R8R8
Slide 46
Network Layer (2-89-90) 4-46 Routing protocol requirements
Minimizing route table spec: Node memory related issue Minimizing
control message: Overhead in bandwidth Robustness Retain its
correctness in dynamic situation. Should be free of loops, black
holes. Using optimal paths (optimality) Choosing the best path ( in
terms of some metrics) Stability: Free of oscillations Fairness
Should take the complete topology while computing the path
Efficiency: Convergence time Correctness
Slide 47
Network Layer (2-89-90) 4-47 Design Choices - 1 Centralized
versus Distributed routing Centralized: One node collects
information (node has complete topology and link cost) and then
installs the routing information in all nodes (Link state
algorithm). Distributed: All nodes co-operate to form the rooting
table. Source based versus hop by hop Source routing: data packet
contains the hop list. Hop by hop: Each hop takes decision based on
its routing table about the next hop (Distance vector
algorithm)
Slide 48
Network Layer (2-89-90) 4-48 Design Choices - 2 Stochastic
versus deterministic Stochastic: Routing table contains multiple
path information. Next hop is chosen randomly. Advantage: load
distribution. Deterministic: always follow same path. Single versus
multiple path Router can use multiple paths for a single
destination Dynamic versus Static Dynamic: Routing dependent on the
current network state routes update more quickly periodic update in
response to link cost changes Static: Routes update slowly over
time.
Slide 49
Network Layer (2-89-90) 4-49 Assumptions About Router Router
knows address of each neighbor. Router can communicate the
information with its neighbors. Router tells its neighbors its best
idea of distance to every other router in the network. Router
receives these distance vectors from its neighbors. Router updates
its notion of best path to each destination, and the next hop for
this destination.
Slide 50
Network Layer (2-89-90) 4-50 Distance Table Inside Router
Distance Table data structure row for each possible destination.
column for each directly-attached neighbor router. example: in
router x, for dest. y via neighbor z. This table is made based on
exchanged information about distance metric and calculation. D () y
y y y z1764z1764 z 14 5 9 2 z 5 8 4 11 x cost to dest. via
destination Distance table in X x z y z z y y y z,1 z,5 z,4 z,4
Distance Vector=Routing table
Slide 51
Network Layer (2-89-90) 4-51 Routers Information Exchange
Routers exchange information periodically of known: distance metric
(costs) routing table (distance vector) Exchange timing: whenever a
link fails Whenever a routing table entry changes.
Slide 52
Network Layer (2-89-90) 4-52 Distance Vector Routing Algorithm
Iterative: continues until no nodes exchange info.
self-terminating: no signal to stop Asynchronous: nodes need not
exchange info/iterate in lock step! distributed: each node
communicates only with directly-attached neighbors Distance Table
data structure each node has its own: row for each possible
destination column for each directly- attached neighbor to node
example: in node X, for dest. Y via neighbor Z: D X (Y,Z) distance
from X to Y, via Z as next hop D (Y,Z) X c(X,Z) + min {D (Y,w)} Z w
=
Slide 53
Network Layer (2-89-90) 4-53 Distance Table: example B w = 2 D
C B A E 1 7 1 8 2 source ABCDABCD A1764A1764 B 14 8 9 11 D5542D5542
c(E,B) + min {D (A,w)} = 8 + 6 = 14 E D (A,B)= A E B D (A,C) B D C
c(E,B) Es neighbor Bs neighbor neighbor: j destination: i D(i, j) E
Distance table:
Network Layer (2-89-90) 4-152 IP addresses: how to get one? Q:
How does host get IP address? IP addr. is configures into host by
admin. in a file Wintel:
control-panel->network->configuration-
>tcp/ip->properties UNIX: /etc/rc.config DHCP: Dynamic Host
Configuration Protocol (RFC2131) : dynamically get address from as
server (RFC2131) : plug-and-play
Slide 153
Network Layer (2-89-90) 4-153 IP addressing: ICANN Q: How does
an ISP get block of addresses? A: ICANN: Internet Corporation for
Assigned Names and Numbers allocates addresses manages DNS assigns
domain names, resolves disputes
Slide 154
Network Layer (2-89-90) 4-154 DHCPDHCP: Dynamic Host
Configuration Protocol DHCP 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. Support for
mobile users who want to join network. DHCP overview: host
broadcasts DHCP discover msg DHCP server responds with DHCP offer
msg host requests IP address: DHCP request msg DHCP server sends
address: DHCP ack msg
Slide 155
Network Layer (2-89-90) 4-155 DHCP client-server scenario
223.1.1.1 223.1.1.2 223.1.1.3 223.1.1.4 223.1.2.9 223.1.2.2
223.1.2.1 223.1.3.2 223.1.3.1 223.1.3.27 DHCP server arriving DHCP
client needs address in this network A B E
Network Layer (2-89-90) 4-157 Chapter 4 Outline 4.1
Introduction and Network Service Models 4.2 Routing Principles 4.3
Hierarchical Routing 4.4 Routing in the Internet 4.5 The Internet
(IP) Protocol 4.6 Whats Inside a Router? 4.7 IPv6 4.8 Multicast
Routing 4.9 Mobility
Slide 158
Network Layer (2-89-90) 4-158 Router Architecture Overview Two
key router functions: run routing algorithms/protocol (RIP, OSPF,
BGP) switching datagrams from incoming to outgoing link
Slide 159
Network Layer (2-89-90) 4-159 Input Port Functions - function:
lookup output port using routing table in input port memory - goal:
complete input port processing at line speed - Queuing (buffering):
if datagrams arrive faster than forwarding rate into switch fabric
Physical layer: bit-level reception Data link layer: e.g., Ethernet
see chapter 5
Slide 160
Network Layer (2-89-90) 4-160 Input Port Queuing (Buffering)
Fabric slower than input ports combined -> queuing may occur at
input queues Head-of-the-Line (HOL) blocking: queued datagram at
front of queue prevents others in queue from moving forward queuing
delay and loss due to input buffer overflow!
Slide 161
Network Layer (2-89-90) 4-161 Three Types of Switching
Fabrics
Slide 162
Network Layer (2-89-90) 4-162 Switching Via Memory First
generation routers: packet copied by systems (single) CPU speed
limited by memory bandwidth (2 bus crossings per datagram) Input
PortOutput Port Memory System Bus Modern routers: input port
processor performs lookup, copy into memory Cisco Catalyst
8500
Slide 163
Network Layer (2-89-90) 4-163 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 1 Gbps bus,
Cisco 1900: sufficient speed for access and enterprise routers (not
regional or backbone)
Slide 164
Network Layer (2-89-90) 4-164 Switching Via An Interconnection
Network overcome bus bandwidth limitations Banyan networks, 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 Gbps through the interconnection network
Slide 165
Network Layer (2-89-90) 4-165 Output Ports Buffering required
when datagrams arrive from fabric faster than the transmission rate
Scheduling discipline chooses among queued datagrams for
transmission queuing (delay) and loss due to output port buffer
overflow!
Slide 166
Network Layer (2-89-90) 4-166 How much buffering? RFC 3439 rule
of thumb: average buffering = typical RTT X link capacity C e.g.,
RTT=250 msec and C = 10 Gps link: Buffer=2.5 Gbit. Recent
recommendation: with N flows, buffering = RTT C. N
Slide 167
Network Layer (2-89-90) 4-167 Chapter 4 Outline 4.1
Introduction and Network Service Models 4.2 Routing Principles 4.3
Hierarchical Routing 4.4 Routing in the Internet 4.5 The Internet
(IP) Protocol 4.6 Whats Inside a Router? 4.7 IPv6 4.8 Multicast
Routing 4.9 Mobility
Slide 168
Network Layer (2-89-90) 4-168 IPv6 Initial motivation: 32-bit
address space completely allocated by 2008. Additional motivation:
header format helps speed processing/forwarding header changes to
facilitate QoS new anycast address: route to best of several
replicated servers IPv6 datagram format: fixed-length 40 byte
header no fragmentation allowed
Slide 169
Network Layer (2-89-90) 4-169 IPv6 Header Priority: identify
priority among datagrams in flow Flow Label: identify datagrams in
same flow. (concept offlow not well defined). Next header: identify
upper layer protocol for data
Slide 170
Network Layer (2-89-90) 4-170 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
Slide 171
Network Layer (2-89-90) 4-171 Transition From IPv4 To IPv6 Not
all routers can be upgraded simultaneous no flag days How will the
network operate with mixed IPv4 and IPv6 routers? Two proposed
approaches: Dual Stack: some routers with dual stack (v6, v4) can
translate between formats Tunneling: IPv6 carried as payload in
IPv4 datagram among IPv4 routers
Slide 172
Network Layer (2-89-90) 4-172 Dual Stack Approach IPv6 IPv4
Flow: X Src: A Dest: F data Flow: ?? Src: A Dest: F data Src:A
Dest: F data A-to-B: IPv6 Src:A Dest: F data B-to-C: IPv4 B-to-C:
IPv4 B-to-C: IPv6 ABCDE F
Network Layer (2-89-90) 4-174 Tunneling-Physical View Physical
view: IPv6 IPv4 Flow: X Src: A Dest: F data Flow: X Src: A Dest: F
data Flow: X Src: A Dest: F data Src:B Dest: E Flow: X Src: A Dest:
F data Src:B Dest: E A-to-B: IPv6 E-to-F: IPv6 B-to-C: IPv6 inside
IPv4 B-to-C: IPv6 inside IPv4 ABCDE F IPv6 tunnel Logical view: ABE
F
Slide 175
Network Layer (2-89-90) 4-175 Chapter 4 Outline 4.1
Introduction and Network Service Models 4.2 Routing Principles 4.3
Hierarchical Routing 4.4 Routing in the Internet 4.5 The Internet
(IP) Protocol 4.6 Whats Inside a Router? 4.7 IPv6 4.8 Multicast
Routing 4.9 Mobility
Slide 176
Network Layer (2-89-90) 4-176 In unicast routing, the router
forwards the received packet through only one of its interfaces.
Unicast
Slide 177
Network Layer (2-89-90) 4-177 In multicast routing, the router
may forward the received packet through several of its interfaces.
Multicast
Slide 178
Network Layer (2-89-90) 4-178 Multicast: act of sending
datagram to multiple receivers with single transmit operation
analogy: one teacher to many students How to achieve multicast:
Multiple unicasts Application-Layer Multicast Network multicast
Multicast: one sender to many receivers
Slide 179
Network Layer (2-89-90) 4-179 Multicasts Multiple unicast
(One-to-All unicast) Using an underlying unicast network layer
Duplicated at the sender transport layer Application-layer
multicast Involving the receivers in the replication and forwarding
of data. Explicit multicast Network player supports multicast Data
is replicated at the network router
Slide 180
Network Layer (2-89-90) 4-180 1- Multiple Unicasts Source sends
3 unicast datagrams, One addressed to each of 3 receivers
Source
Slide 181
Network Layer (2-89-90) 4-181 2- Application-layer Multicast
End systems involved in multicast copy and forward unicast
datagrams among themselves example: p2p file sharing
Slide 182
Network Layer (2-89-90) 4-182 3- Network Multicast Router
actively participate in multicast, making copies of packets as
needed and forwarding towards multicast receivers Multicast routers
duplicate and forward multicast datagrams
Slide 183
Network Layer (2-89-90) 4-183 Multicast Uses Bulk data transfer
(software updates, mailing list distribution, stock updates)
Streamed Continuous Media (audio/visual conferences) Shared
Application Data (shared whiteboard) Interactive Gaming or
Simulations (very intensive)
Slide 184
Network Layer (2-89-90) 4-184 Internet Multicast Service Model
Multicast group concept: use of indirection Sending host (source)
addresses IP datagram to multicast group. Routers forward multicast
datagrams to hosts that have joined that multicast group multicast
group 226.17.30.197 Source
Slide 185
Network Layer (2-89-90) 4-185 Multicast Groups Class D Internet
addresses reserved for multicast: Host group semantics: anyone can
join (receive) multicast group anyone can send to multicast group
no network-layer identification to hosts of members Needed:
infrastructure to deliver multicast-addressed datagrams to all
hosts that have joined that multicast group. 1110Multicast Group
Address (ID) -24 bits
Slide 186
Network Layer (2-89-90) 4-186 Joining a Multicast Group:
Two-Step Process Local: host informs local multicast router of
desire to join a group: IGMP (Internet Group Management Protocol)
Wide Area: local router interacts with other routers to receive
multicast datagram flow: many protocols (e.g., DVMRP, MOSPF, PIM)
multicast group IGMP DVMRP or MOSPF or PIM IGMP :router with
attached group member
Slide 187
Network Layer (2-89-90) 4-187 IGMP: Internet Group Management
Protocol Host: sends IGMP report when application joins multicast
group. IP_ADD_MEMBERSHIP socket option host need not explicitly
unjoin group when leaving Router: sends IGMP query at regular
intervals host belonging to a multicast group must reply to query
IGMP Report IGMP Query
Slide 188
Network Layer (2-89-90) 4-188 IGMP IGMP-v1 router: Host
Membership Query Message broadcast on LAN to all hosts host: Host
Membership Report message to indicate group membership randomized
delay before responding implicit leave via no reply to Query RFC
1112 IGMP-v2: additions include group-specific Query Leave Group
message last host replying to Query can send explicit Leave Group
message router performs group- specific query to see if any hosts
left in group RFC 2236 IGMP-v3: under development as Internet
draft
Slide 189
Network Layer (2-89-90) 4-189 IGMP v2 Message Types IGMP
Message TypeSent ByPurpose Membership query: General RouterQuery
multicast groups joined by attached hosts Membership query:
Specific RouterQuery if specific multicast group joined by attached
hosts Membership reportHostReport host wants to join or is joined
to given multicast group Leave GroupHostReport leaving given
multicast group Report Query
Slide 190
Network Layer (2-89-90) 4-190 Computer Networks IGMP Message
Format
Slide 191
Network Layer (2-89-90) 4-191 Multicast Routing Objectives
Every member receives EXACTLY ONE copy of the packet Non-members
receive nothing No loops in route Optimal path from source to each
destination. Terminology Spanning Tree: Source is the root, group
members are the leaves. Shortest Path Spanning Tree: Each path from
root to a leaf is the shortest according to some metric
Slide 192
Network Layer (2-89-90) 4-192 Multicast Connections Goal: find
a tree (or trees) connecting routers having local multicast group
members tree: not all paths between routers used Group-shared tree:
same tree used by all group members Source-based tree: different
tree from each sender to receivers Group- Shared tree
Slide 193
Network Layer (2-89-90) 4-193 Tree Approaches Source-based
trees 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 , : sources
Slide 194
Network Layer (2-89-90) 4-194 Shortest Path Tree Source-Based
Multicast forwarding tree: tree of shortest path routes from source
to all receivers Dijkstras algorithm. R1 R2 R3 R4 R5 R6 R7 2 1 6 3
4 5 i :router with attached group member link used for forwarding,
i indicates order link added by algorithm Source
Slide 195
Network Layer (2-89-90) 4-195 Reverse Path Forwarding
Source-Based Rely on routers knowledge of unicast shortest path
from it to sender Each router has simple forwarding behavior: if
(multicast datagram received on incoming link on shortest path back
to center) then flood datagram onto all outgoing links else ignore
datagram
Slide 196
Network Layer (2-89-90) 4-196 Reverse Path Forwarding: Example
Result is a source-specific reverse Shortest Path Tree May be a bad
choice with asymmetric links. R1 R2 R3 R4 R5 R6 R7 :router with
attached group member datagram will not be Forwarded datagram will
be forwarded Source
Slide 197
Network Layer (2-89-90) 4-197 Reverse Path Forwarding: Pruning
R1 R2 R3 R4 R5 R6 R7 :router with attached group member prune
message links with multicast forwarding Source P P P Forwarding
tree contains sub-trees with no multicast group members No need to
forward datagrams down sub-tree prune messages sent upstream by
router with no downstream group members
Slide 198
Network Layer (2-89-90) 4-198 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
Shared-Tree: Steiner Tree
Slide 199
Network Layer (2-89-90) 4-199 Center-Based Trees Single
delivery tree shared by all One router identified as center of tree
to join: edge router sends unicast join-message addressed to center
router join-message processed by intermediate routers and forwarded
towards center join-message either hits existing tree branch for
this center, or arrives at center path taken by join-message
becomes new branch of tree for this router
Slide 200
Network Layer (2-89-90) 4-200 Center-Based Trees Suppose R6
chosen as center: R1 R2 R3 R4 R5 R6 R7 :router with attached group
member path order in which join messages generated Source P 1 3 2
1
Network Layer (2-89-90) 4-202 DVMRP - 1 DVMRP: distance vector
multicast routing protocol, RFC1075 flood and prune: reverse path
forwarding, source- based tree RPF tree based on DVMRPs own routing
tables constructed by communicating DVMRP routers no assumptions
about underlying unicast initial datagram to multicast group
flooded everywhere via RPF routers not wanting group: send upstream
prune messages.
Slide 203
Network Layer (2-89-90) 4-203 DVMRP - 1 Soft state: DVMRP
router periodically (1 min.) forgets branches are pruned: multicast
data again flows down unpruned branch downstream router: reprune or
else continue to receive data Routers can quickly redraft to tree
following IGMP join at leaf Odds and ends commonly implemented in
commercial routers Mbone routing done using DVMRP.
Slide 204
Network Layer (2-89-90) 4-204 Tunneling Q: How to connect
islands of multicast routers in a sea of unicast routers? multicast
datagram encapsulated inside normal (non- multicast-addressed)
datagram. normal IP datagram sent thru tunnel via regular IP
unicast to receiving multicast router. receiving multicast router
unencapsulates to get multicast datagram physical topology logical
topology
Slide 205
Network Layer (2-89-90) 4-205 PIM: Protocol Independent
Multicast Not dependent on any specific underlying unicast routing
algorithm (works with all) Two different multicast distribution
scenarios: 1-Dense group members densely packed, in close
proximity. bandwidth more plentiful 2-Sparse: # networks with group
members small wrt # interconnected networks group members widely
dispersed bandwidth not plentiful
Slide 206
Network Layer (2-89-90) 4-206 Consequences of Sparse-Dense
Dichotomy: Dense: group membership by routers assumed until routers
explicitly prune data-driven construction on multicast tree (e.g.,
RPF) bandwidth and non- group-router processing profligate Sparse :
no membership until routers explicitly join receiver- driven
construction of multicast tree (e.g., center-based) bandwidth and
non-group- router processing conservative
Slide 207
Network Layer (2-89-90) 4-207 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
Slide 208
Network Layer (2-89-90) 4-208 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 R1 R2 R3 R4 R5 R6 R7
P join rendezvous point all data multicast from rendezvous
point
Slide 209
Network Layer (2-89-90) 4-209 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! R1 R2 R3 R4 R5 R6 R7 P join
rendezvous point all data multicast from rendezvous point
Slide 210
Network Layer (2-89-90) 4-210 Chapter 4 roadmap 4.1
Introduction and Network Service Models 4.2 Routing Principles 4.3
Hierarchical Routing 4.4 Routing in the Internet 4.5 The Internet
(IP) Protocol 4.6 Whats Inside a Router? 4.7 IPv6 4.8 Multicast
Routing 4.9 Mobility
Slide 211
Network Layer (2-89-90) 4-211 What is mobility? spectrum of
mobility, from the network perspective: no mobility high mobility
mobile user, using same access point mobile user, passing through
multiple access point while maintaining ongoing connections ( like
cell phone) mobile user, connecting/ disconnecting from network
using DHCP.
Slide 212
Network Layer (2-89-90) 4-212 Mobility: Vocabulary home
network: permanent home of mobile (e.g., 128.119.40/24) Permanent
address: address in home network, can always be used to reach
mobile e.g., 128.119.40.186 home agent: entity that will perform
mobility functions on behalf of mobile, when mobile is remote wide
area network correspondent
Slide 213
Network Layer (2-89-90) 4-213 Mobility: more vocabulary
Care-of-address: address in visited network. (e.g., 79,129.13.2)
wide area network visited network: network in which mobile
currently resides (e.g., 79.129.13/24) Permanent address: remains
constant ( e.g., 128.119.40.186) home agent: entity in visited
network that performs mobility functions on behalf of mobile.
correspondent: wants to communicate with mobile
Slide 214
Network Layer (2-89-90) 4-214 How do you contact a mobile
friend: search all phone books? call her parents? expect her to let
you know where he/she is? I wonder where Ali moved to? Consider
friend frequently changing addresses, how do you find her?
Slide 215
Network Layer (2-89-90) 4-215 Mobility: approaches Let routing
handle it: routers advertise permanent address of
mobile-nodes-in-residence via usual routing table exchange. routing
tables indicate where each mobile located no changes to end-systems
Let end-systems handle it: indirect routing: communication from
correspondent to mobile goes through home agent, then forwarded to
remote direct routing: correspondent gets foreign address of
mobile, sends directly to mobile
Slide 216
Network Layer (2-89-90) 4-216 Mobility: approaches Let routing
handle it: routers advertise permanent address of
mobile-nodes-in-residence via usual routing table exchange. routing
tables indicate where each mobile located no changes to end-systems
let end-systems handle it: indirect routing: communication from
correspondent to mobile goes through home agent, then forwarded to
remote direct routing: correspondent gets foreign address of
mobile, sends directly to mobile not scalable to millions of
mobiles
Slide 217
Network Layer (2-89-90) 4-217 Mobility: registration End
result: Foreign agent knows about mobile Home agent knows location
of mobile wide area network home network visited network 1 mobile
contacts foreign agent on entering visited network 2 foreign agent
contacts home agent home: this mobile is resident in my
network
Slide 218
Network Layer (2-89-90) 4-218 Mobility via Indirect Routing
wide area network home network visited network 3 2 4 1
correspondent addresses packets using home address of mobile home
agent intercepts packets, forwards to foreign agent foreign agent
receives packets, forwards to mobile mobile replies directly to
correspondent
Slide 219
Network Layer (2-89-90) 4-219 Indirect Routing: comments Mobile
uses two addresses: permanent address: used by correspondent (hence
mobile location is transparent to correspondent) care-of-address:
used by home agent to forward datagrams to mobile foreign agent
functions may be done by mobile itself triangle routing:
correspondent-home-network- mobile inefficient when correspondent,
mobile are in same network
Slide 220
Network Layer (2-89-90) 4-220 Forwarding datagrams to remote
mobile Permanent address: 128.119.40.186 Care-of address:
79.129.13.2 dest: 128.119.40.186 packet sent by correspondent dest:
79.129.13.2 dest: 128.119.40.186 packet sent by home agent to
foreign agent: a packet within a packet dest: 128.119.40.186
foreign-agent-to-mobile packet
Slide 221
Network Layer (2-89-90) 4-221 Indirect Routing: moving between
networks suppose mobile user moves to another network registers
with new foreign agent new foreign agent registers with home agent
home agent update care-of-address for mobile packets continue to be
forwarded to mobile (but with new care-of-address) Mobility,
changing foreign networks transparent: on going connections can be
maintained!
Slide 222
Network Layer (2-89-90) 4-222 Mobility via Direct Routing wide
area network home network visited network 4 2 4 1 correspondent
requests, receives foreign address of mobile correspondent forwards
to foreign agent foreign agent receives packets, forwards to mobile
mobile replies directly to correspondent 3
Slide 223
Network Layer (2-89-90) 4-223 Mobility via Direct Routing:
comments overcome triangle routing problem non-transparent to
correspondent: correspondent must get care-of-address from home
agent What happens if mobile changes networks?
Slide 224
Network Layer (2-89-90) 4-224 Mobile IP RFC 3220 has many
features weve seen: home agents, foreign agents, foreign-agent
registration, care-of-addresses, encapsulation
(packet-within-a-packet) three components to standard: agent
discovery registration with home agent indirect routing of
datagrams
Slide 225
Network Layer (2-89-90) 4-225 Mobile IP: agent discovery agent
advertisement: foreign/home agents advertise service by
broadcasting ICMP messages (typefield = 9) R bit: registration
required H,F bits: home and/or foreign agent
Network Layer (2-89-90) 4-227 Network Layer: summary What weve
covered: network layer services routing principles: link state and
distance vector hierarchical routing IP Internet routing protocols
RIP, OSPF, BGP whats inside a router? IPv6 mobility
