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Network Layer 4-
Chapter 4
Network Layer
Computer Networking: A Top Down Approach Featuring the Internet, 3 rd^ edition. Jim Kurose, Keith Ross Addison-Wesley, July
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Network Layer 4-
Chapter 4: Network Layer
Chapter goals:
understand principles behind network layer
services:
routing (path selection)
dealing with scale
how a router works
advanced topics: IPv6, mobility
instantiation and implementation in the
Internet
Network Layer 4-
Chapter 4: Network Layer
4. 1 Introduction
4.2 Virtual circuit and
datagram networks
4.3 What’s inside a
router
4.4 IP: Internet
Protocol
Datagram format IPv4 addressing ICMP IPv
4.5 Routing algorithms
Link state Distance Vector Hierarchical routing
4.6 Routing in the
Internet
RIP
OSPF
BGP
4.7 Broadcast and
multicast routing
Network Layer 4-
Network layer
transport segment from
sending to receiving host
on sending side
encapsulates segments
into datagrams
on rcving side, delivers
segments to transport
layer
network layer protocols
inevery host, router
Router examines header
fields in all IP datagrams
passing through it
network data link physical
network data link physical
network data link physical
network data link physical
network data link physical
network data link physical
network data link physical
network data link physical
application transport network data link physical
application transport network data link physical
Network Layer 4-
Connection setup
3 rd^ important function insome network
architectures:
ATM, frame relay, X.
Before datagrams flow, two hosts and
intervening routers establish virtual
connection
Routers get involved
Network and transport layer cnctn service:
Network: between two hosts
Transport: between two processes
Network Layer 4-
Network service model
Q: Whatservice model for “channel” transporting
datagrams from sender to rcvr?
Example services for
individual datagrams:
guaranteed delivery
Guaranteed delivery
with less than 40 msec
delay
Example services for a
flow of datagrams:
In-order datagram
delivery
Guaranteed minimum
bandwidth to flow
Restrictions on
changes in inter-
packet spacing
Network Layer 4-
Network layer service models:
Network Architecture
Internet
ATM
ATM
ATM
ATM
Service Model
best effort
CBR
VBR
ABR
UBR
Bandwidth
none
constant rate guaranteed rate guaranteed minimum none
Loss
no
yes
yes
no
no
Order
no
yes
yes
yes
yes
Timing
no
yes
yes
no
no
Congestion feedback
no (inferred via loss) no congestion no congestion yes
no
Guarantees?
Network Layer 4-
Chapter 4: Network Layer
4. 1 Introduction
4.2 Virtual circuit and
datagram networks
4.3 What’s inside a
router
4.4 IP: Internet
Protocol
Datagram format IPv4 addressing ICMP IPv
4.5 Routing algorithms
Link state Distance Vector Hierarchical routing
4.6 Routing in the
Internet
RIP
OSPF
BGP
4.7 Broadcast and
multicast routing
Network Layer 4-
VC implementation
A VC consists of:
1. Path from source to destination
2. VC numbers, one number for each link along
path
3. Entries in forwarding tables in routers along
path
Packet belonging to VC carries a VC
number.
VC number must be changed on each link.
New VC number comes from forwarding table
Network Layer 4-
Forwarding table
12 22 32 1 2 3
VC number
interface number
Incoming interface Incoming VC # Outgoing interface Outgoing VC #
1 12 2 22 2 63 1 18 3 7 2 17 1 97 3 87 … … … …
Forwarding table in
northwest router:
Routers maintain connection state information!
Network Layer 4-
Virtual circuits: signaling protocols
used to setup, maintain teardown VC
used in ATM, frame-relay, X.
not used in today’s Internet
application transport network data link physical
application transport network data link physical
- Initiate call (^) 2. incoming call
- Call connected 3. Accept call
- Data flow begins 6. Receive data
Network Layer 4-
Datagram networks
no call setup at network layer
routers: no state about end-to-end connections
no network-level concept of “connection”
packets forwarded using destination host address
packets between same source-dest pair may take different paths
application transport network data link physical
application transport network data link physical
- Send data (^) 2. Receive data
Network Layer 4-
Datagram or VC network: why?
Internet
data exchange among computers “elastic” service, no strict timing req.
“smart” end systems (computers) can adapt, perform control, error recovery simple inside network, complexity at “edge”
many link types
different characteristics uniform service difficult
ATM
evolved from telephony human conversation: strict timing, reliability requirements need for guaranteed service “dumb” end systems telephones complexity inside network
Network Layer 4-
Chapter 4: Network Layer
4. 1 Introduction
4.2 Virtual circuit and
datagram networks
4.3 What’s inside a
router
4.4 IP: Internet
Protocol
Datagram format IPv4 addressing ICMP IPv
4.5 Routing algorithms
Link state Distance Vector Hierarchical routing
4.6 Routing in the
Internet
RIP
OSPF
BGP
4.7 Broadcast and
multicast routing
Network Layer 4-
Router Architecture Overview
Two key router functions:
run routing algorithms/protocol (RIP, OSPF, BGP)
forwarding datagrams from incoming to outgoing link
Network Layer 4-
Input Port Functions
Decentralized switching : given datagram dest., lookup output port using forwarding table in input port memory goal: complete input port processing at ‘line speed’ queuing: if datagrams arrive faster than forwarding rate into switch fabric
Physical layer: bit-level reception
Data link layer: e.g., Ethernet see chapter 5
Network Layer 4-
Switching Via a Bus
datagram from input port memory
to output port memory via a shared
bus
bus contention: switching speed
limited by bus bandwidth
1 Gbps bus, Cisco 1900: sufficient
speed for access and enterprise
routers (not regional or backbone)
Network Layer 4-
Switching Via An Interconnection
Network
overcome bus bandwidth limitations
Banyan networks, other interconnection nets
initially developed to connect processors in
multiprocessor
Advanced design: fragmenting datagram into fixed
length cells, switch cells through the fabric.
Cisco 12000: switches Gbps through the
interconnection network
Network Layer 4-
Output Ports
Buffering required when datagrams arrive from
fabric faster than the transmission rate
Scheduling discipline chooses among queued
datagrams for transmission
Network Layer 4-
Output port queueing
buffering when arrival rate via switch exceeds
output line speed
queueing (delay) and loss due to output port buffer overflow!
Network Layer 4-
The Internet Network layer
forwarding table
Host, router network layer functions:
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
Network Layer 4-
Chapter 4: Network Layer
4. 1 Introduction
4.2 Virtual circuit and
datagram networks
4.3 What’s inside a
router
4.4 IP: Internet
Protocol
Datagram format IPv4 addressing ICMP IPv
4.5 Routing algorithms
Link state Distance Vector Hierarchical routing
4.6 Routing in the
Internet
RIP
OSPF
BGP
4.7 Broadcast and
multicast routing
Network Layer 4-
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.
how much overhead with TCP?
20 bytes of TCP
20 bytes of IP
= 40 bytes + app layer overhead
Network Layer 4-
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
Network Layer 4-
IP Addressing: introduction
IP address: 32-bit
identifier for host,
routerinterface
interface: connection
between host/router
and physical link
router’s typically have multiple interfaces host may have multiple interfaces IP addresses associated with each interface
223.1.1.
223.1.1.
223.1.1.
223.1.1.4 223.1.2.
223.1.2.
223.1.2.
223.1.3.1 223.1.3.
223.1.3.
223.1.1.1 = 11011111 00000001 00000001 00000001
223 1 1 1
Network Layer 4-
Subnets
IP address:
subnet part (high order bits) host part (low order bits)
What’s a subnet?
device interfaces with same subnet part of IP address can physically reach each other without intervening router
223.1.1.
223.1.1.
223.1.1.
223.1.1.4 223.1.2.
223.1.2.
223.1.2.
223.1.3.1 223.1.3.
223.1.3.
network consisting of 3 subnets
LAN
Network Layer 4-
Subnets
223.1.1.0/24 (^) 223.1.2.0/
223.1.3.0/
Recipe
To determine the
subnets, detach each
interface from its
host or router,
creating islands of
isolated networks.
Each isolated network
is called a subnet.
Subnet mask: /
Network Layer 4-
Subnets
How many? 223.1.1.
223.1.1.
223.1.1.
223.1.2.1 223.1.2.
223.1.2.
223.1.3.1 223.1.3.
223.1.3.
223.1.1.
223.1.7.
223.1.7. 223.1.8.1 223.1.8.
223.1.9.
223.1.9.
