Chapter 4 - Internetworking

1
Chapter 4 - Internetworking
2
A Simple Internetwork
(routergateway)
3
Protocol Layers
4
IP Service Model

• Packet Delivery Model
• Best Effort
• Global Addressing Scheme
• IP Addresses

5
Packet Delivery Model

• Connectionless (datagram-based)
• Best-effort delivery (unreliable service)
• packets are lost
• packets are delivered out of order
• duplicate copies of a packet are delivered
• packets can be delayed for a long time

6
Datagram format

• Version (4) currently 4
• Hlen (4) number of 32-bit words in header
• TOS (8) type of service (not widely used QoS)
• Length (16) number of bytes in this datagram
• Ident (16) used by fragmentation
• Flags/Offset (16) used by fragmentation
• TTL (8) number of hops this datagram has
  travelled
• Protocol (8) demux key (TCP6, UDP17)
• Checksum (16) of the header only
• DestAddr SrcAddr (32)

7
Fragmentation and Reassembly

• Each network has some MTU
• Strategy
• fragment when necessary (MTU lt Datagram)
• try to avoid fragmentation at source host
• refragmentation is possible
• fragments are self-contained datagrams
• use CS-PDU (not cells) for ATM
• delay reassembly until destination host
• do not recover from lost fragments

8
Example
9
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10
Global Addresses

• Properties
• globally unique
• hierarchical network host
• Format
• Dot notation
• 10.3.2.4
• 128.96.33.81
• 192.12.69.77

7
24
0
network
host
Class A
14
16
1
0
network
host
Class B
21
8
1
1
0
network
host
Class C
11
Datagram Forwarding
Strategy every datagram contains destination's
address if directly connected to destination
network, then forward to host if not directly
connected to destination network, then forward to
some router forwarding table maps network number
into next hop each host has a default router each
router maintains a forwarding table
12
Example (router R2)
Network Number 1 2 3 4
Next Hop R3 R1 interface 1 interface 0
13
Example
Network 1 (Ethernet)
Hn Host
Rn Router
Network Number 1 2 3 4
Next Hop R3 R1 interface 1 interface 0
H7
H8
R3
H1
H2
H3
Network 4
(point-to-point)
Network 2 (Ethernet)
R1
Interface 0
R2
Interface 1
H4
Network 3 (Token Ring)
H5
H6
14
Address Translation

• Map IP addresses into physical addresses
• destination host
• next hop router
• Techniques
• encode physical address in host part of IP
  address
• table-based
• ARP
• table of IP to physical address bindings
• broadcast request if IP address not in table
• target machine responds with its physical address
• table entries are discarded if not refreshed

15

• Request format
• HardwareType type of physical network (e.g.,
  Ethernet)
• ProtocolType type of higher layer protocol
  (e.g., IP)
• HLEN PLEN length of physical and protocol
  addresses
• Operation request or response
• Source/Target Physical/Protocol addresses

16
Notes

• table entries timeout in about 10 minutes
• update table with source when you are the target
• update table if already have an entry
• do not refresh table entries upon reference

17
DHCP
Unicast to server
DHCP
DHCP
Other networks
relay
server
Broadcast
Host
18
DHCP Packet Format
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Internet Control Message Protocol

• Echo (ping)
• Redirect (from router to source host)
• Destination unreachable (protocol, port, or host)
• TTL exceeded (so datagrams don't cycle forever)
• Checksum failed
• Reassembly failed
• Cannot fragment

22
Routing
When everything's coming your way, you're in the
wrong lane
23
Routing

• Forwarding versus Routing
• forwarding to select an output port based on
  destination address and routing table
• routing process by which routing table is built
• Network as a Graph

24
Routing

• Problem Find the lowest cost path between any
  two nodes
• Factors
• Static topology, SOL delays
• Dynamic load, congestion
• Scalable

25
Distance Vector (RIP)

• Each node maintains a set of triples
• (Destination, Cost, NextHop)
• Each node sends updates to (and receives updates
  from) its directly connected neighbors
• periodically (on the order of several seconds)
• whenever its table changes (called triggered
  update)
• Used by Routing Information Protocol (RIP)

26
Routing

• Each update is a list of pairs
• (Destination, Cost)
• Update local table if receive a better route
• smaller cost
• came from next-hop
• Refresh existing routes delete if they time out

27
Example
Routing table at node B
Destination A C D E F G
Cost 1 1 inf inf inf inf
Next Hop A C
C A A
A
28
Example

• Routing table at node B

B
C
A
D
E
F
G
Destination A C D E F G
Cost 1 1 2 2 2 3
Next Hop A C C A A A
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Routing loops

• Example 1
• F detects that link to G has failed
• F sets distance to G to infinity and sends update
  to A
• A sets distance to G to infinity since it uses F
  to reach G
• A receives periodic update from C with 2-hop path
  to G
• A sets distance to G to 3 and sends update to F
• F decides it can reach G in 4 hops via A

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Example 2

• Link from A to E fails
• A advertises distance of infinity to E
• B and C advertise a distance of 2 to E
• B decides it can reach E in 3 hops through C
  advertises this to A
• A decides it can reach E in 4 hops through B
  advertises this to C
• C decides that it can reach E in 5 hops......

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Heuristics to break routing loops

• set infinity to 16
• split horizon
• Dont send routes learned from particular
  neighbor to that neighbor (B doesnt send route
  E,2,A to A)
• split horizon with poison reverse
• Include negative information (B sends route E,?
  to A)
• Only work with routing loop with simple loop,
  more complex techniques later

32
Example 2 with poison

• Link from A to E fails
• A advertises distance of infinity to E
• B and C advertise a distance of 2 to E
• B decides it can reach E in 3 hops through C
  advertises this to A, sends ? to C,
• since C was using B to get to E, C advertises a
  distance of ? to E
• Since B was using C to get to E, B advertises a
  distance of ? to E

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RIP Network
C advertizes to router A and D, cost 0 to 2,3,
cost 1 to 5,6,1 and cost 2 to 4
34
RIP Packet format
35
Link State (OSPF)

• Strategy Send to all nodes (not just neighbors)
  information about directly connected links (not
  entire routing table).
• Link State Packet (LSP)
• id of the node that created the LSP
• cost of link to each directly connected neighbor
• sequence number (SEQNO)
• time-to-live (TTL) for this packet
• Example Open Shortest Path First Protocol (OSPF)

36
Reliable Flooding

• store most recent LSP from each node
• forward LSP to all nodes but one that sent it
• generate new LSP periodically increment SEQNO
• start SEQNO at 0 when reboot
• decrement TTL of each stored LSP discard when
  TTL0

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Reliable Flooding
38
Route Calculation (Dijkstras)
39
Route Calculation (in practice)

• Forward search algorithm
• Each switch maintains two lists
• Tentative and Confirmed
• Each list contains a set of triples
• (Destination, Cost, NextHop)

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• 1. Initialized Confirmed with entry for me cost
  0.
• 2. For the node just added to Confirmed (call it
  Next) select its LSP.
• 3. For each Neighbor of Next, calculate the Cost
  to reach this Neighbor as the sum of the cost
  from me to Next and from Next to Neighbor.
• 3.1. If Neighbor is currently in neither
  Confirmed or Tentative, add (Neighbor, Cost,
  NextHop) to Tentative, where NextHop is the
  direction to reach Next.
• 3.2 If Neighbor is currently in Tentative and
  Cost is less that current cost for Neighbor, then
  replace current entry with (Neighbor, Cost,
  NextHop), where NextHop is the direction to reach
  Next.
• 4. If Tentative is empty, stop. Otherwise, pick
  entry from Tentative with the lowest cost, move
  it to Confirmed, and return to step 2.

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Step 1. 2. 3. 4.
Confirmed (D,0,-) (D,0,-) (D,0,-) (C,2,C) (D,0
,-) (C,2,C)
Tentative (B,11,B) (C,2,C) (B,11,B) (B,5,C) (
A,12,C)
Step 5. 6. 7.
Confirmed (D,0,-) (C,2,C) (B,5,C) (D,0,-) (C,2,C)
(B,5,C) (D,0,-) (C,2,C) (B,5,C) (A,10,C)
Tentative (A,12,C) (A,10,C)
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OSPF Header
43
OSPF link-state Advertizement
44
Link State vs distance-vector

• Distance vector (original ARPANET)
• Node talks to direct neighbors
• Sends everything it has learned
• Link State (Later ARPANET)
• Node talks to everyone
• Only sends what it knows for sure

45
Original ARPANET metric

• Practical measurements of cost
• measured number of packets enqueued on each link
• took neither latency or bandwidth into
  consideration

46
New ARPANET Metric

• Used link bandwidth and latency
• used delay rather than queue length for load

47
New ARPANET metric

• stamp each incoming packet with its arrival time
  (AT)
• record departure time (DT)
• when link-level ACK arrives, compute
• Delay (DT - AT) Transmit Latency
• if timeout, reset DT to departure time for
  retransmission
• link cost average delay over some time period

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Problems with New metric

• under low load, static factors dominated cost
  worked OK
• under high load, congested links had very high
  costs packets oscillated between congested and
  idle links
• range of costs too large preferred path of 126
  lightly loaded 56Kbps links to a 1-hop 9.6Kbps
  path

49
Revised ARPANET metric

• replaced delay measurement with link utilization
• compressed dynamic range
• highly loaded link never has a cost more than 3
  times its idle cost
• most expensive link only 7 times the cost of the
  least expensive
• high-speed satellite link more attractive than
  low-speed terrestrial link
• cost is a function of link utilization only at
  moderate to high loads.

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Cost Function
51
Routing Characteristics

• Vern Paxson used traceroute to study 40,000
  routes
• Probability of encountering serious route failure
  1/30 with problem lasting 30 seconds
• 2/3 of routes persist for days or weeks
• 1/3 of route use different path in each
  direction.
• Routes becoming less predictable

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Routing for Mobile Hosts
53
Global Internet
54
Scalability Issues

• IP hides hosts in address hierarchy, but...
• Inefficient use of address space
• class C network with 2 hosts (2/255 0.78
  efficient)
• class B network with 256 hosts (256/65535 0.39
  efficient)
• Too many networks
• today's Internet has tens of thousands of
  networks
• routing tables do not scale
• route propagation protocols do not scale

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Internet Structure

• Recent Past
• Today

56
Subnetting

• Add another level to address/routing hierarchy
  subnet
• Subnet masks define variable partition of host
  part of class A and B addresses
• Subnets visible only within site

57
Subnet Example
Forwarding table at router R1
Subnet Number 128.96.34.0 128.96.34.128 128.96.33.
0
Subnet Mask 255.255.255.128 255.255.255.128 255.25
5.255.0
Next Hop interface 0 interface 1 R2
58
Forwarding Algorithm

• D destination IP address
• for each entry lt SubnetNum, SubnetMask, NextHopgt
• D1 SubnetMask D
• if D1 SubnetNum
• if NextHop is an interface
• deliver datagram directly to destination
• else
• deliver datagram to NextHop (a router)

59
Notes

• Would use a default router if nothing matches
• Not necessary for all ones in subnet mask to be
  contiguous
• Can put multiple subnets on one physical network
• Subnets not visible from the rest of the Internet

60
Numbers

• www.icann.org Internet Corporation for Assigned
  Names and Numbers
• www.arin.net is our authority and has more
  details
• Names and numbers have been privitized. The US
  government used to allocate them

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The big picture
62
Supernetting

• Assign block of contiguous network numbers to
  near-by networks
• Called CIDR Classless Inter-Domain Routing
• Represent blocks with a single pair
• ltfirst_network_address, countgt
• Restrict block sizes to powers of 2
• Use a bit mask (CIDR mask) to identify block size
• All routers must understand CIDR addressing

63
Route Propagation

• Idea Impose a second hierarchy on the network
  that limits what routers talk to each other. (The
  first hierarchy is the address hierarchy that
  governs how packets are forwarded.)
• Autonomous System (AS)
• corresponds to an administrative domain
• examples University, company, backbone network
• assign each AS a 16-bit number
• Two-level route propagation hierarchy
• interior gateway protocol (each AS selects its
  own)
• exterior gateway protocol (Internet-wide standard)

64
Popular Interior Gateway Protocols

• RIP Route Information Protocol
• developed at Berkeley
• distributed with Unix
• distance-vector algorithm- neighbors
• based on hop-count
• OSPF Open Shortest Path First
• recent Internet standard
• uses link-state algorithm-bcast
• supports load balancing
• supports authentication

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EGP Exterior Gateway Protocol

• Overview
• designed for tree-structured Internet
• concerned with reachability, not optimal routes
• Protocol messages
• neighbor acquisition one router requests that
  another be its peer peers exchange reachability
  information
• neighbor reachability one router periodically
  tests to see if the other router is still
  reachable exchange HELLO/ACK messages uses a
  k-out-of-n rule
• routing updates peers periodically exchange
  their routing tables (distance-vector)

66
EGP Example
Exterior Neighbor (Other system)
N1
Source Net N1
G1
G2
G1
1 N2
N3
N2
G3
G2
N4
1 N3
G3
G5
G4
1 N4
N6
N5
2 N5
2 N6
67
BGP-4 Border Gateway Protocol

• Assumes the Internet is an arbitrarily
  interconnected set of AS's (Autonomous Systems).
  Define local traffic as traffic that originates
  at or terminates on nodes within an AS, and
  transit traffic as traffic that passes through an
  AS, we can classify AS's into three types
• Stub AS an AS that has only a single connection
  to one other AS.
• Multihomed AS an AS that has connections to more
  than one other AS, but refuses to carry transit
  traffic.
• Transit AS an AS that has connections to more
  than one other AS, and is designed to carry both
  transit and local traffic.

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Autonomous System (AS)

• Each AS has
• One or more border routers
• One BGP speaker that advertises
• local networks
• other reachable networks (transit AS only)
• gives path information
• Still pass information about every network

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BGP Example
128.96
Customer P
192.4.153
(AS 4)
Regional Provider A
AS 2
Customer Q
192.4.32
(AS 5)
192.4.3
"Backbone" Network
AS 1
Regional Provider B
Customer R
192.12.69
(AS 6)
AS 3
Customer S
192.4.54
(AS 7)
192.4.23
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BGP Example

• Speaker for AS 2 advertises reachability to P and
  Q
• Network 128.96, 192.4.153, 192.4.32, and 192.4.3,
  can be reached directly from AS 2.
• Speaker for backbone network then advertises
• Networks 128.96, 192.4.153, 192.4.32, and 192.4.3
  can be reached along the path ltAS 1, AS 2gt.
• Speaker can also cancel previously advertised
  paths

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Domain Divided into areas
Area 3
Area 1
Area 0
R9
R7
R3
R8
R1
R4
R2
Area 2
R5
R6
72
Routing Basics

• Minimize the size of routing tables
• Create Autonomous routing systems
• Simplify routing
• hierarchical routing
• Optimize within the Autonomous system

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Classless interdomain routing (CIDR)

• Give multiple class C addresses instead of class
  B
• BGP would require a table entry for each of these
• CIDR aggregates adjacent networks
• network number consists of a base and length

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Next Generation IP (IPv6)
75
Major Features

• 128-bit addresses (1500/square foot of the
  earths surface)
• Multicast
• Real-time service
• Authentication and security
• Autoconfiguration
• End-to-end fragmentation
• Protocol extensions

76
IPv6 Addresses

• Classless addressing/routing (similar to CIDR)
• Notation xxxxxxxx (x 16-bit hex number)
• contiguous 0s are compressed 47CDA4560124
• IPv6 compatible IPv4 address 128.42.1.87
• Address assignment
• provider-based (cant change provider easily)
• geographic

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Prefix 0000 0000 0000 0001 0000 001 0000 010 0000
011 0000 1 0001 001 010 011 100 101 110 1110 1111
0 1111 10 1111 110 1111 1110 0 1111 1110 10 1111
1110 11 1111 1111
Use Reserved Unassigned Reserved for NSAP
Allocation Reserved for IPX Allocation Unassigned
Unassigned Unassigned Unassigned Provider-Based
Unicast Address IPV4-like Unassigned Reserved for
Geographic-Based Unicast Addresses
Unassigned Unassigned Unassigned Unassigned Unass
igned Unassigned Unassigned Link Local Use
Addresses no global uniqueness Site Local Use
Addresses no global uniqueness Multicast
Addresses
Roaming
78
IPv6 Header

• 40-byte base header
• Extension headers (fixed order, mostly fixed
  length)
• fragmentation
• source routing
• authentication and security
• other options

79
Transition

• Gradual Transition with IPV4 and IPV6
• Dual Stack - (both supported on some nodes)
• Tunneling
• When v6 passes through v4 network
• Encapsulate v6 inside v4 packet with a v6 router
  as a destination
• destination router then sends v6 packet
• loose QoS and other desirable features in v4
  segment

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Tunneling
B Z
IPV4
IPV4
B Z
B Z
B
IPV6D IPV4Z
B
IPV6C IPV4Y
IPV6B
IPV6A
81
IPv6 Sockets programming

• New address family AF_INET6
• New address data type in6_addr
• New address structure sockaddr_in6

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in6_addr

• struct in6_addr
• uint8_t s6_addr16

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sockaddr_in6

• struct sockaddr_in6
• uint8_t sin6_len
• sa_family_t sin6_family
• in_port_t sin6_port
• uint32_t sin6_flowinfo
• struct in6_addr sin6_addr

84
Dual Server

• In the future it will be important to create
  servers that handle both IPv4 and IPv6.
• The work is handled by the O.S. (which contains
  protocol stacks for both v4 and v6)
• automatic creation of IPv6 address from an IPv4
  client (IPv4-mapped IPv6 address).

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IPv6 server
IPv4-mapped IPv6 address
TCP
Datalink
86
IPv6 Clients

• If an IPv6 client specifies an IPv4 address for
  the server, the kernel detects and talks IPv4 to
  the server.
• DNS support for IPv6 addresses can make
  everything work.
• gethostbyname() returns an IPv4 mapped IPv6
  address for hosts that only support IPv4.

87
IPv6 - IPv4 Programming

• The kernel does the work, we can assume we are
  talking IPv6 to everyone!
• In case we really want to know, there are some
  macros that determine the type of an IPv6
  address.
• We can find out if we are talking to an IPv4
  client or server by checking whether the address
  is an IPv4 mapped address.

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Internet Multicast
89
Overview

• IPv4
• class D addresses
• demonstrated with Mbone (uses tunneling)
• Place least significant 23 bits of IP number in
  last 23 bits of ETH/FDDI address
• MSB on in Ethernet indicates multicast
• Integral part of IPv6
• problem is making it scale

90
Link-State Multicast

• Each host on a LAN periodically announces the
  groups it belongs to (IGMP).
• Augment update message (LSP) to include set of
  groups that have members on a particular LAN.
• Each router uses Dijkstra's algorithm to compute
  shortest-path spanning tree for each source/group
  pair.
• Each router caches tree for currently active
  source/group pairs.

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Example
92
Distance-Vector Multicast

• Reverse Path Broadcast (RPB)
• Each router already knows that shortest path to
  destination S goes through router N.
• When receive multicast packet from S, forward on
  all outgoing links (except the one on which the
  packet arrived), iff packet arrived from N.
• Eliminate duplicate broadcast packets by only
  letting parent for LAN (relative to S) forward
• shortest path to S (learn via distance vector)
• smallest address to break ties

93
Reverse Path Multicast (RPM)

• Goal Prune networks that have no hosts in group
  G
• Step 1 Determine of LAN is a leaf with no
  members in G
• leaf if parent is only router on the LAN
• determine if any hosts are members of G using
  IGMP
• Step 2 Propagate no members of G here
  information
• augment ltDestination, Costgt update sent to
  neighbors with set of groups for which this
  network is interested in receiving multicast
  packets.
• only happens with multicast address becomes
  active.

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PIM
95
RP
G
RP
G
G
R3
R2
R4
RP
G
G
R1
R5
G
Host