CSE 30264

Computer Networks

Prof. Aaron StriegelDepartment of Computer Science & Engineering

University of Notre Dame

Lecture 11 – February 16, 2010

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Today’s Lecture

• Internetworking– Local Network– NAT– ICMP

• Routing– Routing in the WAN

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Physical

Data

Network

Transport

Application

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Internetworking

Outline Local Network Routing

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Routing Table – netscale01

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Mask result

Local or next hop?

Interface to pass to

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Datagram Forwarding • Strategy

– Always have dest address• In IP header

– Two choices• Local network (subnet)

– Pass it off directly• Not on my local network

– Pass to some router– Routing table

• Maps network to next hop• Routing entries• Default router How do I find that next hop?

Have IP, need Ethernet

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Address Translation • Layer 3 -> Layer 2 Translation

– IP -> Ethernet– Destination host– Next hop router

• Techniques– Do nothing

• Make Layer 2 part of Layer 3 address

– Table• Convert Layer 3 to Layer 2

• ARP – Address Resolution Protocol– Broadcast, observe, refresh

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Example – ARP Table

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ARP Key Concepts• Broadcast request

– Send to FF:FF:FF:FF:FF:FF– Who is 129.74.50.20?

• Says AA:BB:CC:DD:EE:FF of 129.74.153.157• Response

– Respond directly or broadcast– I am 01:34:57:AB:CD:EF, aka 129.74.50.20

• Interesting concepts– Broadcast locally only – not

beyond the router– Soft state

• Old entries periodically expire– Nosy neighbor

• Note ARP requests of others

Spring 2010

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ARP Details

• Request Format– HardwareType: type of physical network (e.g., Ethernet)– ProtocolType: type of higher layer protocol (e.g., IP)– HLEN: Length of physical (hardware) address– PLEN: Length of protocol addresses– Operation: request or response – Source/Target-Physical/Protocol addresses

• Notes– Table entries timeout in about 15 minutes– Update table with source when you are the target – Update table if already have an entry– Do not refresh table entries upon reference

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ARP Packet Format

TargetHardwareAddr (bytes 2 – 5) TargetProtocolAddr (bytes 0 – 3)

SourceProtocolAddr (bytes 2 - 3)

Hardware type = 1 ProtocolType = 0x0800

SourceHardwareAddr (bytes 4 – 5)TargetHardwareAddr (bytes 0 – 1)SourceProtocolAddr (bytes 0 – 1)

HLen = 48 PLen = 32 OperationSourceHardwareAddr (bytes 0 - 3)

0 8 16 31

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Observe the Packets

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Wireshark

www.wireshark.org

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Addressing

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• How do I get my initial IP address?• Static IP

• System Administrator• Private network space• Know IP, mask, subnet, gateway, DNS

• Dynamic IP BOOTP / DHCP• I don’t know my IP, please tell me• Two flavors

• Anybody can come in• Authenticated / MAC filtered

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DHCP

• Dynamic Host Configuration Protocol

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DHCP

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More on DHCP

• Uses UDP– Why not TCP?

• Nearly all sites use DHCP– Static server only

• Other notes– Lease time for IP address– Variety of option fields

• Thin clients

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NAT – Network Address Translation

• Limited public IP space– Share addresses

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I’ll give you one IP address via DHCP

Xfinity The largerInternet

???

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Enter the NAT• Translate private address to public

– Private network space• 10.*• 192.168.*

– Translate via table• Use connection tuple

– Src IP, Dst IP, Src Port, Dst Port

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67.56.43.2192.168.1.1

192.168.1.100

192.168.1.101

DHCP Server

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NAT Table

• Must originate from inside– Map internal tuple to external tuple

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67.56.43.2192.168.1.1

S: 192.168.1.100D: 25.78.200.3TCPSP: 9876DP: 80

S: 67.56.43.2D: 25.78.200.3TCPSP: 42778DP: 80

Mapping Table Ext: 67.56.43.2:42778 25.78.200.3:80 Int: 192.168.1.100:9876 25.78.200.3.80

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Mapping Table

• Maintain a list of all active connections– Map across the boundary– New connection

• Pick an unused external port• Issues

– Reach internal host from external (SSH)• DMZ – One host becomes a catch all

– Scalability– UDP– IPsec / VPN

Spring 2010

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Internet Control Message Protocol (ICMP)

• 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

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Routing

OutlineDistance VectorLink State

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Overview• Forwarding vs 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

• Problem: Find lowest cost path between two nodes• Factors

– static– dynamic

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Distance Vector• Each node maintains a set of triples

– (Destination, Cost, NextHop)• Directly connected neighbors exchange updates

– periodically (on the order of several seconds)– whenever table changes (called triggered update)

• 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

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Example

Destination Cost NextHop A 1 A

C 1 C D 2 C

E 2 A F 2 A G 3 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 t o 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

• 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; advertises this to A– A decides it can read E in 4 hops; advertises this to C– C decides that it can reach E in 5 hops…

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Loop-Breaking Heuristics

• Set infinity to 16• Split horizon• Split horizon with poison reverse

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Routing Information Protocol (RIP)

• Distributed along with BSD Unix• Straightforward implementation of DV• Updates sent every 30 seconds• Link costs constant at 1 (16 = infinity)

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Link State• 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

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Link State (cont)

• 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 LSP

• discard when TTL=0

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Link State (cont)

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Route Calculation• Dijkstra’s shortest-path algorithm• Let

– N denotes set of nodes in the graph– l (i, j) denotes non-negative cost (weight) for edge (i, j)– s denotes this node– M denotes the set of nodes incorporated so far– C(n) denotes cost of the path from s to node n

M = {s}for each n in N - {s}

C(n) = l(s, n)while (N != M)

M = M union {w} such that C(w) is the minimum for all w in (N - M)

for each n in (N - M)C(n) = MIN(C(n), C (w) + l(w, n ))

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OSPF

• Open Shortest Path First Protocol• Authentication• Additional hierarchy• Load balancing

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Metrics • Original ARPANET metric

– measures number of packets queued on each link– took neither latency or bandwidth into consideration

• 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

• Revised ARPANET metric– compressed dynamic range– replaced Delay with link utilization

• Practice– static metrics (e.g., 1/bandwidth)

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Routing for Mobile Hosts

• Mobile IP:– home agent, home address, foreign agent– triangle routing problem