Networking II: The Link and Network Layers

1
Networking IIThe Link and Network Layers
2
Announcements

• Prelim II will be Thursday, November 20th, in
  class
• Homework 5 available later today, November 4th
• Vote today

3
Review OSI Levels

• Physical Layer
• electrical details of bits on the wire
• Data Link Layer
• sending frames of bits and error detection
• Network Layer
• routing packets to the destination
• Transport Layer
• reliable transmission of messages,
  disassembly/assembly, ordering, retransmission of
  lost packets
• Session Layer
• really part of transport, typically Not
  implemented
• Presentation Layer
• data representation in the message
• Application
• high-level protocols (mail, ftp, etc.)

4
Review OSI Levels
Node A
Application
Node B
Application
Presentation
Presentation
Session
Session
Transport
Transport
Network
Network
Data Link
Data Link
Physical
Physical
Network
5
What is purpose of this layer?

• Invoke Physical Layer
• Physically encode bits on the wire
• Link pipe to send information
• E.g. point to point or broadcast
• Can be built out of
• Twisted pair, coaxial cable, optical fiber, radio
  waves, etc
• Links should only be able to send data
• Could corrupt, lose, reorder, duplicate, (fail in
  other ways)

6
Broadcast Networks Details
ID1 (ignore)
ID4 (ignore)
ID2 (receive)

• Delivery When you broadcast a packet, how does a
  receiver know who it is for? (packet goes to
  everyone!)
• Put header on front of packet Destination
  Packet
• Everyone gets packet, discards if not the target
• In Ethernet, this check is done in hardware
• No OS interrupt if not for particular destination
• This is layering were going to build complex
  network protocols by layering on top of the
  packet

7
Point-to-point networks

• Why have a shared broadcast medium? Why not
  simplify and only have point-to-point links
  routers/switches?
• Didnt used to be cost-effective
• Now, easy to make high-speed switches and routers
  that can forward packets from a sender to a
  receiver.
• Point-to-point network a network in which every
  physical wire is connected to only two computers
• Switch a bridge that transforms a shared-bus
  configuration into a point-to-point network.
• Router a device that acts as a junction between
  two networks to transfer data packets among them.

8
Point-to-Point Networks Discussion

• Advantages
• Higher link performance
• Can drive point-to-point link faster than
  broadcast link since less capacitance/less echoes
  (from impedance mismatches)
• Greater aggregate bandwidth than broadcast link
• Can have multiple senders at once
• Can add capacity incrementally
• Add more links/switches to get more capacity
• Better fault tolerance (as in the Internet)
• Lower Latency
• No arbitration to send, although need buffer in
  the switch
• Disadvantages
• More expensive than having everyone share
  broadcast link
• However, technology costs now much cheaper
• Examples
• ATM (asynchronous transfer mode)
• The first commercial point-to-point LAN
• Inspiration taken from telephone network
• Switched Ethernet
• Same packet format and signaling as broadcast
  Ethernet, but only two machines on each ethernet.

9
How to connect routers/machines?

• WAN/Router Connections
• Commercial
• T1 (1.5 Mbps), T3 (44 Mbps)
• OC1 (51 Mbps), OC3 (155 Mbps)
• ISDN (64 Kbps)
• Frame Relay (1-100 Mbps, usually 1.5 Mbps)
• ATM (some Gbps)
• To your home
• DSL
• Cable
• Local Area
• Ethernet IEEE 802.3 (10 Mbps, 100 Mbps, 1 Gbps)
• Wireless IEEE 802.11 b/g/a (11 Mbps, 22 Mbps, 54
  Mbps)

10
Link level Issues

• Encoding map bits to analog signals
• Framing Group bits into frames (packets)
• Arbitration multiple senders, one resource
• Addressing multiple receivers, one wire

11
Encoding

• Map 1s and 0s to electric signals
• Simple scheme Non-Return to Zero (NRZ)
• 0 low voltage, 1 high voltage
• Problems
• How to tell an error? When jammed? When is bus
  idle?
• When to sample? Clock recovery is difficult.
• Idea Recover clock using encoding transitions

1 0 1 1
0
12
Manchester Encoding

• Used by Ethernet
• Idea Map 0 to low-to-high transition, 1 to
  high-to-low
• Plusses can detect dead-link, can recover clock
• Bad reduce bandwidth, i.e. bit rate ½ baud
  rate
• If wire can do X transition per second?

13
Framing

• Why send packets?
• Error control
• How do you know when to stop reading?
• Sentinel approach send start and end sequence
• For example, if sentinel is 11111
• 11111 00101001111100 11111 10101001 11111 010011
  11111
• What if sentinel appears in the data?
• map sentinel to something else, receiver maps it
  back
• Bit stuffing

14
Example HDLC

• High-Level Data Link Control (HLDC)
• Data link layer protocol developed by the ISO
• Same sentinel for begin and end 0111 1110
• packet format
• Bit stuffing
• Sender If 5 1s then insert a 0
• Receiver if 5 1s followed by a 0, remove 0
• Else read next bit
• Packet size now depends on the contents

0111 1110 header data CRC
0111 1110
0111 1110 0111 1101 0
0111 1101 0 0111 1110
15
Broadcast Network Arbitration

• Arbitration Act of negotiating use of shared
  medium
• What if two senders try to broadcast at same
  time?
• Concurrent activity but cant use shared memory
  to coordinate!
• Aloha network (70s) packet radio within Hawaii
• Blind broadcast, with checksum at end of packet.
  If received correctly (not garbled), send back
  an acknowledgement. If not received correctly,
  discard.
• Need checksum anyway in case airplane flies
  overhead
• Sender waits for a while, and if doesnt get an
  acknowledgement, re-transmits.
• If two senders try to send at same time, both get
  garbled, both simply re-send later.
• Problem Stability what if load increases?
• More collisions ? less gets through ?more resent
  ? more load ? More collisions
• Unfortunately some sender may have started in
  clear, get scrambled without finishing

16
Arbitration

• One medium, multiple senders
• What did we do for CPU, memory, readers/writers?
• New Problem No centralized control
• Approaches
• TDMA Time Division Multiple Access
• Divide time into slots, round robin among senders
• If you exceed the capacity ? do not admit more
  (busy signal)
• FDMA Frequency Division Multiple Access (AMPS)
• Divide spectrum into channels, give each sender a
  channel
• If no more channels available, give a busy signal
• Good for continuous streams fixed delay,
  constant data rate
• Bad for bursty Internet traffic idle slots

17
Ethernet

• Developed in 1976, Metcalfe and Boggs at Xerox
• Uses CSMA/CD
• Carrier Sense Multiple Access with Collision
  Detection
• Easy way to connect LANs

Metcalfes Ethernet sketch
18
CSMA/CD

• Carrier Sense
• Listen before you speak
• Multiple Access
• Multiple hosts can access the network
• Collision Detection
• Can make out if someone else started speaking

Older Ethernet Frame
19
CSMA
Wait until carrier free
20
CSMA/CD
Garbled signals
If the sender detects a collision, it will stop
and then retry! What is the problem?
21
CSMA/CD
Packet?
Sense Carrier
Detect Collision
Send
Discard Packet
Jam channel bCalcBackoff() wait(b) attempts
22
Ethernets CSMA/CD (more)

• Jam Signal make sure all other transmitters are
  aware of collision 48 bits
• Exponential Backoff
• Goal adapt retransmission attempts to estimated
  current load
• heavy load random wait will be longer
• Adaptive and Random
• First time, pick random wait time with some
  initial mean. If collide again, pick random value
  from bigger mean wait time. Etc.
• Randomness is important to decouple colliding
  senders
• Scheme figures out how many people are trying to
  send!
• Example
• first collision choose K from 0,1 delay is K
  x 512 bit transmission times
• after second collision choose K from 0,1,2,3
• after ten or more collisions, choose K from
  0,1,2,3,4,,1023

23
Packet Size

• If packets are too small, the collision goes
  unnoticed
• Limit packet size
• Limit network diameter
• Use CRC to check frame integrity
• truncated packets are filtered out

24
Ethernet Problems

• What if there is a malicious user?
• Might not use exponential backoff
• Might listen promiscuously to packets
• Integrating Fast and Gigabit Ethernet

25
Addressing ARP
128.84.96.89
128.84.96.90
128.84.96.91
What is the physical address of the host named
128.84.96.89
Im at 1a342c9adecc

• ARP is used to discover physical addresses
• ARP Address Resolution Protocol

26
Addressing RARP
???
128.84.96.90 RARP Server
128.84.96.91
I just got here. My physical address is
1a342c9adecc. Whats my name ?
Your name is 128.84.96.89

• RARP is used to discover virtual addresses
• RARP Reverse Address Resolution Protocol

27
Repeaters and Bridges

• Both connect LAN segments
• Usually do not originate data
• Repeaters (Hubs) physical layer devices
• forward packets on all LAN segments
• Useful for increasing range
• Increases contention
• Bridges link layer devices
• Forward packets only if meant on that segment
• Isolates congestion
• More expensive

28
Backbone Bridge
29
Summary

• Data Link Layer
• layer two of the seven-layer OSI model
• Layer two of the five-layer TCP/IP reference
  model as well.
• Responds to service requests from the network
  layer and issues service requests to the physical
  layer.
• Broadcast vs Point-to-point
• Point-to-point is often higher performance, but
  traditionally higher cost as well
• Switched Ethernet is common now
• Data Link Layer Issues
• Encoding map bits to analog signals
• Manchester encoding
• Framing Group bits into frames (packets)
• Bit stuffing
• Arbitration multiple senders, one resource
• Ethernet uses CSMA/CD (carrier sense multiple
  access/collision detection)
• Addressing multiple receivers, one wire
• ARP (address resolution protocol)

30
The Network Layer
31
Review OSI Levels

• Physical Layer
• electrical details of bits on the wire
• Data Link Layer
• sending frames of bits and error detection
• Network Layer
• routing packets to the destination
• Transport Layer
• reliable transmission of messages,
  disassembly/assembly, ordering, retransmission of
  lost packets
• Session Layer
• really part of transport, typically Not
  implemented
• Presentation Layer
• data representation in the message
• Application
• high-level protocols (mail, ftp, etc.)

32
Review OSI Levels
Node A
Application
Node B
Application
Presentation
Presentation
Session
Session
Transport
Transport
Network
Network
Data Link
Data Link
Physical
Physical
Network
33
Review OSI Levels
Node A
Application
Node B
Application
Presentation
Presentation
Session
Session
Transport
Transport
Router
Network
Network
Network
Data Link
Data Link
Data Link
Physical
Physical
Physical
Network
34
Purpose of Network layer

• Given a packet, send it across the network to
  destination
• 2 key issues
• Portability
• connect different technologies
• Scalability
• To the Internet scale

35
What does it involve?

• Two important functions
• routing determine path from source to dest.
• forwarding move packets from routers input to
  output

T3
T1 T3
Sts-1
T1
36
Network service model
?
?

• Q What service model for channel transporting
  packets from sender to receiver?
• guaranteed bandwidth?
• preservation of inter-packet timing (no jitter)?
• loss-free delivery?
• in-order delivery?
• congestion feedback to sender?

?
The most important abstraction provided by
network layer
service abstraction
virtual circuit or datagram?
Which things can be faked at the transport
layer?
37
Two connection models

• Connectionless (or datagram)
• each packet contains enough information that
  routers can decide how to get it to its final
  destination
• Connection-oriented (or virtual circuit)
• first set up a connection between two nodes
• label it (called a virtual circuit identifier
  (VCI))
• all packets carry label

1
A
38
Virtual circuits signaling protocols

• used to setup, maintain teardown VC
• setup gives opportunity to reserve resources
• used in ATM (Asynchronous Transfer Mode),
  frame-relay, X.25 (or OSI protocol suite)
• not used in todays Internet

6. Receive data
5. Data flow begins
4. Call connected
3. Accept call
1. Initiate call
2. incoming call
39
Virtual circuit switching

• Forming a circuit
• send a connection request from A to B.
• Contains VCI address of B
• VCI is the Virtual Circuit Identifier
• rule VCI must be unique on the link its used on
• switch creates an entry mapping input messages
  with VCI to output port
• switch picks a new VCI unique between it and next
  switch

40
Virtual circuit forwarding

• For each VCI switch has a table which maps input
  link to output link and gives the new VCI to use
• if as messages come into switch 1 on link 2 and
  go out on link 3 then the table will be

(Input link,VCI) (output link, new VCI) (1,
2) (?, ?) (1, 5) (?, ?)
Switch 1
2
Switch 2
1
5
2
1
Switch 3
2
1
41
Virtual Circuits Discussion

• Plusses easy to associate resources with VC
• Easy to provide QoS guarantees (bandwidth, delay)
• Very little state in packet
• Minuses
• Not good in case of crashes
• Requires explicit connect and teardown phases
• What if teardown does not get to all routers?
• What if one switch crashes?
• Will have to teardown and rebuild route

42
Datagram networks

• no call setup at network layer
• routers no state about end-to-end connections
• no network-level concept of connection
• packets typically routed using destination host
  ID
• packets between same source-dest pair may take
  different paths
• Best effort data corruption, packet drops, route
  loops

1. Send data
2. Receive data
43
Datagrams Forwarding

• How does packet get to the destination?
• switch creates a forwarding table, mapping
  destinations to output port (ignores input ports)
• when a packet with a destination address in the
  table arrives, it pushes it out on the
  appropriate output port
• when a packet with a destination address not in
  the table arrives, it must find out more routing
  information (next problem)

44
Datagrams

• Plusses
• No round trip connection setup time
• No explicit route teardown
• No resource reservation ? each flow could get max
  bandwidth
• Easily handles switch failures routes around it
• Minuses
• Difficult to provide resource guarantees
• Higher per packet overhead
• Internet uses datagrams IP (Internet Protocol)

45
Datagrams Forwarding

• How to build forwarding tables?
• Manually enter it
• What if nodes crashed
• What about scale?
• The graph-theoretic routing problem
• Given a graph, with vertices (switches), edges
  (links), and edge costs (cost of sending on that
  link)
• Find the least cost path between any two nodes
• Path cost ? (cost of edges in path)

46
Simple Routing Algorithm

• Choose a central node
• All nodes send their (nbr, cost) information to
  this node
• Central node uses info to learn entire topology
  of the network
• It then computes shortest paths between all pairs
  of nodes
• Using All Pair Shortest Path Algorithm
• Sends the new matrix to every node
• Nice, simple, elegant!
• What is the problem?
• Scalability centralization hurts scalability
• Central node is crushed with traffic

47
Link State Routing

• Basic idea
• Every node propagates its (nbr, cost) information
• This information at all nodes is enough to
  construct topology
• Can use a graph algorithm to find the shortest
  routes
• Mechanisms required
• Reliable flooding of link information
• Method to calculate shortest route (Dijkstras
  algorithm)
• Example link state update packet
• node id, (nbr, cost) list, seq. no., ttl
• Seq. no. to identify latest updates, ttl
  specifies when to stop msg.

48
Reliable flooding

• receive(pkt)
• If already have a copy of LSP from pkt.ID
• or if pkts sequence number lt copys
• discard pkt
• else
• decrement pkt.TTL
• replace copy with pkt
• forward pkt to all links besides the
• one that we received it on
• done every 10 minutes or so
• gen_LSP()
• increment nodes sequence by one
• recompute cost vector
• send created LSP to all neighbors

49
Discussion Link-State Routing

• Plusses
• Simple, determines the optimal route most of the
  time
• Used by OSPF (Open Shortest Path First)
• Minuses
• Might have oscillations
• Avoid using load as cost metric, reduce herding
  effect

1
1e
0
2e
0
0
0
0
e
0
1
1e
1
1
e
recompute
recompute Least loaded gt Most loaded
Initially start with almost equal routes
everyone goes with least loaded
50
Is our routing algo scalable?

• Route table size grows with size of network
• Because our address structure is flat!
• Solution have a hierarchical structure
• Used by OSPF
• Divide the network into areas, each area has
  unique number
• Nodes carry their area number in the address 1.A,
  2.B, etc.
• Nodes know complete topology in their area
• Area border routers (ABR) know how to get to any
  other area

51
Hierarchical Addressing
Zone 2
0
1
S1
1
0
2
S2
2
3
1
0
2
Zone 3
52
IP has 2-layer addressing

• Each IP address is 32 bits
• Network part which network the host is on?
• Host part identifies the host.
• All hosts on same network have the same network
  part
• 3 classes of addresses A, B and C

18.26.0.1
host
network
32-bits
1 0 net host
110 net host
2 14 16 bits
3 21 8 bits
53
IP addressing

• The different classes
• Problems inefficient, address space exhaustion
• cornell.edu is a class B network (can address 64K
  hosts)
• mit.edu is a class A network (can address 4M
  hosts)

class
1.0.0.0 to 127.255.255.255
A
network
0
host
128.0.0.0 to 191.255.255.255
Unicast
B
192.0.0.0 to 223.255.255.255
C
224.0.0.0 to 239.255.255.255
D
Multicast
240.0.0.0 to 255.255.255.255
reserved
E
Reserved
1111
54
IP addressing CIDR

• Classless InterDomain Routing
• network portion of address of arbitrary length
• address format a.b.c.d/x, where x is bits in
  network portion
• Examples
• Class A /8
• Class B /16
• Class C /24

55
Internet Protocol Datagram
IP protocol version Number
32 bits
total datagram length (bytes)
type of service
head. len
header length
ver
length
for fragmentation/ reassembly
fragment offset
type of data
flgs
16-bit identifier
max number remaining hops (decremented at each
router)
upper layer
time to live
Internet checksum
32 bit source IP address
32 bit destination IP address
upper layer protocol to deliver payload to
E.g. timestamp, record route taken, pecify list
of routers to visit.
Options (if any)
data (variable length, typically a TCP or UDP
segment)
56
Datagram Portability

• IP Goal To create one logical network from
  multiple physical networks
• All intermediate routers should understand IP
• IP header information sufficient to carry the
  packet to destination
• Goal Run over anything!
• Problem
• Physical networks have different MTUs (maximum
  transfer units)
• max. transmission unit 1500 for Ethernet, 48
  for ATM
• Solution 1
• Fit everything in the MTU (!)

57
IP Fragmentation Reassembly

• Solution 2 (the one used)
• If packet size gt MTU of network, then fragment
  into pieces
• Each fragment is less than MTU size
• Each has IP headers frag bit set frag id
  offset
• Packets may get refragmented on the way to
  destination
• Reassembly only done at the destination
• What is a good initial packet size?

reassembly
fragmentation in one large datagram out 3
smaller datagrams
58
Summary

• Virtual Circuit
• Plusses easy to associate resources with VC
• Easy to provide QoS guarantees (bandwidth, delay)
• Very little state in packet
• Minuses
• Not good in case of crashes
• Datagrams
• Plusses
• Easily handles switch failures routes around it
• No round trip connection setup time
• No explicit route teardown
• No resource reservation ? each flow could get max
  bandwidth
• Minuses
• Difficult to provide resource guarantees
• Higher per packet overhead
• Forwarding
• Link-state routing OSPF
• Hierarchical addressing IP and OSPF