CS 457 Lecture 10 Internetworking and IP Fall 2011 The Network - - PowerPoint PPT Presentation

CS 457 – Lecture 10 Internetworking and IP

Fall 2011

The Network layer

Forwarding table

• Routing protocols
• path selection
• RIP, OSPF, BGP
• IP protocol
• addressing conventions
• datagram format
• packet handling conventions
• ICMP protocol
• error reporting
• router “signaling”
• Transport layer: TCP, UDP
• Link layer
• physical layer
• Network
• layer

IP datagram format

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

IP Fragmentation & Reassembly

• network links have MTU

(max.transfer size) - largest possible link-level frame. – different link types, different MTUs

• large IP datagram divided

(“fragmented”) within net – one datagram becomes several datagrams – “reassembled” only at final destination – IP header bits used to identify, order related fragments

• fragmentation:
• in: one large datagram
• out: 3 smaller datagrams
• reassembly

IP Fragmentation and Reassembly

ID =x Offset =0 Fragflag =0 Length =4000 ID =x Offset =0 Fragflag =1 Length =1500 ID =x Offset =185 Fragflag =1 Length =1500 ID =x Offset =370 Fragflag =0 Length =1040

• One large datagram becomes
• several smaller datagrams
• Example
• 4000 byte

datagram

• MTU = 1500

bytes

• 1480 bytes in

data field

• offset =
• 1480/8

Addressing

• Already have MAC layer addresses
• Ethernet provides addresses
• And so do other link layers
• But can’t use Ethernet address at IP
• Ethernet addresses are flat
• Ethernet address assignment is

not related to the network topology

• Ethernet is not the only link layer!

IP Address (IPv4)

• A unique 32-bit number

(i.e., 4B addresses)

• Identifies an interface

(on a host, on a router, …)

• Represented in dotted-quad notation

00001100 00100010 10011110 00000101

12 34 158 5

Scalability Challenge

• Suppose hosts had arbitrary addresses

– Then every router would need a lot of information – …to know how to direct packets toward the host

host host host

• LAN 1

... host host host LAN 2

• ...

router router router WAN WAN

1.2.3.4 5.6.7.8 2.4.6.8 1.2.3.5 5.6.7.9 2.4.6.9 1.2.3.4 1.2.3.5

forwarding table

Grouping Related Hosts

• The Internet is an “inter-network”

– Used to connect networks together, not hosts – Needs a way to address a network (i.e., group of hosts)

• host
• host
• host
• LAN 1
• ...
• host
• host
• host
• LAN 2
• ...
• router
• router
• router
• WAN
• WAN
• LAN = Local Area Network
• WAN = Wide Area Network

Hierarchical Addressing: IP Prefixes

• Divided into network & host portions

(left and right)

• 12.34.158.0/24 is a 24-bit prefix with 28

addresses

– More on Hierarchy (CIDR) in next lectures

00001100 00100010 10011110 00000101

Network (24 bits) Host (8 bits)

12 34 158 5

IP Address and 24-bit Subnet Mask

00001100 00100010 10011110 00000101

12 34 158 5

11111111 11111111 11111111 00000000

255 255 255

Address Mask

Scalability Improved

• Number related hosts from a common subnet

– 1.2.3.0/24 on the left LAN – 5.6.7.0/24 on the right LAN

host host host LAN 1 ... host host host LAN 2 ... router router router WAN WAN

1.2.3.4 1.2.3.7 1.2.3.156 5.6.7.8 5.6.7.9 5.6.7.212 1.2.3.0/24 5.6.7.0/24

• forwarding table

Easy to Add New Hosts

• No need to update the routers

– E.g., adding a new host 5.6.7.213 on the right – Doesn’t require adding a new forwarding entry

host host host LAN 1 ... host host host LAN 2 ... router router router WAN WAN

1.2.3.4 1.2.3.7 1.2.3.156 5.6.7.8 5.6.7.9 5.6.7.212 1.2.3.0/24 5.6.7.0/24

• forwarding table

host

5.6.7.213

Avoiding Manual Configuration

• Address Resolution Protocol (ARP)

– Learn mapping between IP address and MAC address

• Dynamic Host Configuration Protocol (DHCP)

– End host learns IP address, DNS servers, and gateway

host host DNS ... host host DNS ... router router

1.2.3.0/24 5.6.7.0/24

1.2.3.7 1.2.3.156

???

1.2.3.19

router

Key Ideas in ARP and DHCP

• Broadcasting: when in doubt, shout!

– Broadcast query to all hosts in the local-area-network – … when you don’t know how to identify the right one

• Caching: remember the past for a while

– Store the information you learn to reduce overhead – Remember your own address & other host’s addresses

• Soft state: eventually forget the past

– Associate a time-to-live field with the information – … and either refresh or discard the information – Key for robustness in the face of unpredictable change

Broadcasting

• Broadcasting: sending to everyone

– Special destination address: FF-FF-FF-FF-FF-FF – All adapters on the LAN receive the packet

• Delivering a broadcast packet

– Easy on a “shared media” – Like shouting in a room – everyone can hear you – E.g., Ethernet, wireless, and satellite links

MAC Address vs. IP Address

• MAC addresses

– Hard-coded in read-only memory when adaptor is built – Like a social security number – Flat name space of 48 bits (e.g., 00-0E-9B-6E-49-76) – Portable, and can stay the same as the host moves – Used to get packet between interfaces on same network

• IP addresses

– Configured, or learned dynamically – Like a postal mailing address – Hierarchical name space of 32 bits (e.g., 12.178.66.9) – Not portable, and depends on where the host is attached – Used to get a packet to destination IP subnet

What’s Next

• Read Chapter 1, 2, 3, and 4.1-4.3
• Next Lecture Topics from Chapter 4.2 and 4.3

– Routing

• Homework

– Due Thursday

• Project 2

– You should be working on Project 2!