Network Layer - there is more than one network

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Network Layer- there is more than one network

• ???

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A Simple Internetwork
Network 1 (Ethernet)
H7
R3
H8
H2
H3
H1
Network 4
(point-to-point)
Network 2 (Ethernet)
R1
R2
H4
Network 3 (FDDI)
H5
H6
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A Simple Internetwork with Protocol Layers
H1
H8
TCP
TCP
R1
R2
R3
IP
IP
IP
IP
IP
ETH
FDDI
ETH
FDDI
PPP
PPP
ETH
ETH
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Datagram Delivery of IP Networks

• Datagram A type of packet that happens to be
  sent in a connectionless manner over a network.
• Each datagram carries enough information to let
  the network forward the packet to its correct
  destination.
• No need for any advance setup.
• Best effort
• Unreliable.
• Run over anything

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IP Packet Format
0
4
8
16
19
31
TOS
Length
V
ersion
HLen
Ident
Flags
Offset
TTL
Protocol
Checksum
SourceAddr
DestinationAddr
Pad
Options (variable)
(variable)
Data
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Fragmentation and Reassembly
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Header Fields Used in IP Fragmentation
Start of header
Ident
x
Offset
0
1
(b)
Start of header
Rest of header
Ident
x
Offset
0
0
512 data bytes
(a)
Rest of header
1400 data bytes
Start of header
Ident
x
Offset
512
1
Rest of header
512 data bytes
Start of header
Ident
x
Offset
1024
0
Rest of header
376 data bytes
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Addressing

• Ethernet address is flat.
• IP address is hierarchical.

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24
(a)
Network
Host
0
14
16
(b)
Network
Host
1
0
21
8
(c)
Network
Host
1
1
0
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Datagram Forwarding in IP
For a router if (NetworkNum of destination
NetworkNum of one of my interface) then
deliver packet to destination over that
interface else if (NetworkNum of destination
is in my forwarding table) then deliver
packet to NextHop router else deliver
packet to default router For a host if
(NetworkNum of destination my NetworkNum) then
deliver packet to destination directly else
deliver packet to default router
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Address Translation

• Mapping an IP address into a physical address
• ARP (Address Resolution Protocol)
• To enable each host on a network to build up a
  table of mappings between IP address and
  link-level address.
• The entries are timed out every 15 minutes.

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Routing
Goal determine good path (sequence of routers)
thru network from source to dest.

• Graph abstraction for routing algorithms
• graph nodes are routers
• graph edges are physical links
• link cost delay, cost, or congestion level

• good path
• typically means minimum cost path
• other defs possible

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Routing Algorithm Classification

• 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

• Static or dynamic?
• Static
• routes change slowly over time
• Dynamic
• routes change more quickly
• periodic update
• in response to link cost changes

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A Link-State Routing Algorithm

• Dijkstras 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 routing table for that node
• iterative after k iterations, know least cost
  path to k dest.s

• Notation
• c(i,j) link cost from node i to j. cost infinite
  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, that is next v
• N set of nodes whose least cost path
  definitively known

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Dijsktras Algorithm
1 Initialization 2 N A 3 for all
nodes v 4 if v adjacent to A 5 then
D(v) c(A,v) 6 else D(v) infty 7 8
Loop 9 find w not in N such that D(w) is a
minimum 10 add w to N 11 update D(v) for
all v adjacent to w and not in N 12 D(v)
min( D(v), D(w) c(w,v) ) 13 / new cost
to v is either old cost to v or known 14
shortest path cost to w plus cost from w to v /
15 until all nodes in N
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Dijkstras algorithm example
D(B),p(B) 2,A 2,A 2,A
D(D),p(D) 1,A
Step 0 1 2 3 4 5
D(C),p(C) 5,A 4,D 3,E 3,E
D(E),p(E) infinity 2,D
start N A AD ADE ADEB ADEBC ADEBCF
D(F),p(F) infinity infinity 4,E 4,E 4,E
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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

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Distance Table example
loop!
loop!
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Distance table gives routing table
Outgoing link to use, cost
A B C D
A,1 D,5 D,4 D,2
destination
Routing table
Distance table
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Metrics

• The simplest way counting the number of hops
• Other approaches
• Measuring the queue length
• Using delay as metric
• Delay (departTime arrivalTime) Trans.Time
  Prop.Time
• Smoothing the variation of the metric over time.
• A highly loaded link never shows a cost of more
  than three times its cost when idle.
• The most expensive link is only seven times the
  cost of the least expensive.

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Subnetting
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Interdomain Routing

• Autonomous system
• Border router
• BGP (Border Gateway Protocol)

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Internet inter-AS routing BGP

• BGP (Border Gateway Protocol) the de facto
  standard
• Path Vector protocol
• similar to Distance Vector protocol
• each Border Gateway broadcast to neighbors
  (peers) entire path (I.e, sequence of ASs) to
  destination
• E.g., Gateway X may send its path to dest. Z
• Path (X,Z) X,Y1,Y2,Y3,,Z

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ICMP Features

• ICMP Used by IP to send error and control
  messages
• ICMP uses IP to send its messages
• ICMP does not report errors on ICMP messages.
• ICMP message are not required on datagram
  checksum errors. (Some implementations still do)

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ICMP Message Format
8-bit type of message 8-bit error code 16-bit checksum
(contents depends on type and code) (contents depends on type and code) (contents depends on type and code)
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ICMP Message Types

• Type description
• 0 echo reply (ping)
• 3 destination unreachable
• source quench
• redirect
• 8 echo request (ping)
• 9 route advertisement
• 10 router discovery
• 11 TTL expired
• bad IP header
• timestamp request
• timestamp reply
• information request
• information reply
• address mask request
• address mask reply

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ICMP Messages

• Source Quench Please slow down! I just dropped
  one of your datagrams.
• Time Exceeded Time to live field in one of your
  packets became zero. or Reassembly timer expired
  at the destination.
• Fragmentation Required Datagram was longer than
  MTU and No Fragment bit was set.
• Address Mask Request/Reply What is the subnet
  mask on this net? Replied by Address mask agent
  

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Destination Unreachable
code Meaning
0 1 2 3 4 5 6 7 8 9 10 11 12 network unreachable host unreachable protocol unreachable port unreachable fragmentation needed but dont fragment bit set source route failed destination network unknown destination host unknown source host isolated destination network administratively prohibited destination host administratively prohibited network unreachable for TOS host unreachable for TOS
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IP version 6- IP Next Generation (IPng)
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Related RFCs
RFC Title Date
1752 The recommendation for the IP Next Generation Protocol Jan 1995
1809 Using the flow label in IPv6 Jun 1995
1881 IPv6 address allocation management Dec 1995
1883 IPv6 specification Dec 1995
1884 IPv6 addressing architecture Dec 1995
1885 ICMPv6 for the IPv6 specification Dec 1995
1886 DNS extensions to support IPv6 Dec 1995
1887 An architecture for IPv6 unicast address allocation Dec 1995
1897 IPv6 testing address allocation Jan 1996
1924 A compact representation of Ipv6 addresses Apr 1996
1933 Transition mechanisms for IPv6 hosts and routers Apr 1996
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IPng Considerations

• Addressing
• the two level structure of the IP address is
  convenient but wasteful of address spaces.
• it is general practice to assign a unique network
  number to an IP network whether or not it is
  actually connected to the Internet.
• networks are proliferating rapidly.
• growth of TCP/IP usage in new areas will result
  in a rapid growth in the demand for unique IP
  addresses.
• Performance
• reduced number of fields in the IPv6 packet
  header
• fixed length of header extension header
• disabling fragmentation

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IPng Considerations

• Network service
• should be possible to associate packets with
  particular services classes
• flow label
• Addressing flexibility
• anycast delivered to just one of a set of nodes
• multicast improved scalability by a scope filed
• Security capabilities
• providing authentication and privacy

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IPv6 Packet Header
0
4
12
16
24
31
V
ersion
T
rafficClass
FlowLabel
PayloadLen
NextHeader
HopLimit
SourceAddress
DestinationAddress
Next header/data
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Priority Field

• One bit for two classes
• congestion controlled traffic
• non-congestion controlled traffic
• Three bits for eight priorities in each class

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Flow Label

• a flow is a sequence of packets that
• are generated from a single application instance
  from the sources point of view
• share attributes which affect how they are
  handled from the routers point of view
• Rules for flow label
• hosts or routers that do not support the flow
  label field must
• set the field to zero when originating a packet
• pass the filed unchanged when forwarding a packet
• ignore the filed when receiving a packet
• all the packets with the same non-zero flow label
  must have the same destination address, source
  address, priority, hop-by-hop options header
  contents and routing header contents.
• the source assigns a flow label to a flow.

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Addressing

• 128 bit address space
• Address notation 47cd12344422ac0200221234a4
  560123

125

m

n

o

p
p
o
n
m
3
SubscriberID
ProviderID
RegistryID
010
InterfaceID
SubnetID
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Address Prefix Assignment
Prefix Use Prefix Use
0000 0000 Reserved 101 Unassigned
0000 0001 Unassigned 110 Unassigned
0000 001 Reserved for NSAP 1110 Unassigned
0000 010 Reserved for IPX 11110 Unassigned
0000 011 Unassigned 1111 0 Unassigned
0000 1 Unassigned 1111 10 Unassigned
0001 Unassigned 1111 110 Unassigned
001 Unassigned 1111 1110 0 Unassigned
010 Unicast address 1111 1110 10 Link local use
011 Unassigned 1111 1110 11 Site local use
100 Unassigned 1111 1111 Multicast
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Anycast

• Delivery method
• unicast one-to-one
• multicast one-to-many
• broadcast one-to-all
• anycast one-to-one-of-many
• Anycast address an address assigned to multiple
  hosts

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Multicast Technology

• Requires group communication
• one-to-many or many-to-many
• dynamic mgmt of group subscription
• Big problem with unicast technology bandwidth
  waste with multiple data flows

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Why Multicast?

• Typical applications
• Multimedia conference (video, audio, digital
  whiteboard)
• Resource discovery (e.g., auto-topology)
• War simulation
• Commercial apps (e.g., transactions, news
  distribution)
• Routing protocols (e.g., both EIGRP and OSPF use
  multicast to send updates to neighbors)
• Games (e.g., distributed arcades)
• Physics apps

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Principles of Multicasting

• Special IP addresses are used to identify
  multicast groups
• Hosts notify multicast routers about the
  multicast groups to which they (want to) belong
• Multicast groups are managed by the routers using
  multicast routing protocols

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Multicast Addresses

• Multicast packets are identified by Class D IP
  addresses
• Global range from 224.0.0.0 to 239.255.255.255
• Reserved 224.0.0.0-224.0.0.255
• Internet-wide addresses 224.0.1.0-238.255.255.255
  
• Local addresses 239.0.0.0-239.255.255.255
• Some special addresses
• 224.0.0.1 all multicast systems on a subnet
• 224.0.0.2 all multicast routers on a subnet

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Multicast Groups

• IGMP - Internet Group Management Protocol (RFC
  1112)
• defines how hosts tell routers which groups they
  are part of available on Unix, PCs, Mac
• routers query directly connected hosts sending an
  IGMP query to 224.0.0.1 (i.e., all multicast
  systems)
• when there is more than one multicast router on a
  LAN, only one has to send the query
• access lists can be applied to restrict the
  multicast groups hosts can receive
• hosts send a single reply per group per LAN

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Multicast Routing Protocols

• Forwarding decisions different from unicast
  protocols
• multicast protocols use the source address
  instead of the destination address to make their
  forwarding decisions
• Two main protocols
• DVMRP (Distance Vector Multicast Routing
  Protocol)
• Distance vector (RIP-like) algorithm
• Static configuration based on tunnels between
  DVMRP routers (often Sun workstations)
• Not easily scalable (e.g., max hops lt 32)
• PIM (Protocol Independent Multicast)
• Typically installed on routers
• Independent from the unicast routing protocols
  used by the router
• Scalable (Dense-mode Sparse-mode)

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Flooding

• When a router receives a packet that is addressed
  to a multicast group, it determines whether this
  is the first time the router has seen this
  particular packet.
• If so, it forwards the packet on all the
  interfaces except the one on which it arrived.
• Simple to implement.
• Does not scale well because of the large number
  of duplicate messages.

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RPF

• Reverse Path Forwarding (RPF)
• A router accepts a multicast packet from a source
  if and only if the packet has been received over
  the interface used to send unicast packets to the
  source
• If the RPF test succeeds, the packet is forwarded
  to all the interfaces included in the routers
  OIF (Outgoing Interface List)
• If the RPF test fails, the multicast packet is
  discarded
• A packet should never be re-sent over the RPF
  interface (to avoid loops)

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DVMRP

• DVMRP constructs source-rooted multicast delivery
  trees using RPF algorithm.
• Basic operations follow.
• The first datagram for any (source, group) pair
  is forwarded across the entire internetwork.
• The leaf routers transmit prune messages back
  toward the source if there are no group members
  on their directly attached leaf subnetworks.
• Periodically the prune state times out, and the
  next datagram for the (source, group) pair is
  forwarded across the entire internetworks.
• Implements a graft operation for quickly
  establishing a new branch.
• if a router that previously sent a prune message
  for a (source, group) pair discovers new group
  members on a leaf network, it sends a graft
  message to the groups previous-hop router.
• When an upstream router receives a graft
  message, it cancels the previously received prune
  message. Graft messages may cascade back towards
  the source to establish the branch to the
  multicast tree.