Download Network Layer

Network Layer: a. Forwarding
Goals:
 understand principles
behind network layer
services:



forwarding
routing (path selection)
dealing with scale
 instantiation and
implementation in the
Internet
Overview:
 network layer services
 IP addresses & their usage
 NAT
 IP header
 IP fragmentation
 ICMP
 IPv6
Ch. 4: Network Layer - Forwarding
#1
Network Layer objectives
 Transport packet from source to
move packets from source to
destination through routers
 Routing

prepare info (table) that
enables finding a path for
every packet/ data stream
 Call setup (VC only, see later)


find path for a data session
before data transfer starts
keep record of it in routers
“Control plane”

“Data plane”
dest.
o Net layer in all hosts, routers
Basic functions:
 Forwarding
Ch. 4: Network Layer - Forwarding
#2
Interplay between routing and forwarding
routing algorithm
Build routing tables
local routing table
header value output link
0100
0101
0111
1001
Routing
3
2
2
1
value in arriving
packet’s header
0111
Forwarding
1
3 2
Move packets from
input link to output link
Ch. 4: Network Layer Forwarding
4-3
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:
? ?
?
virtual circuit
or
datagram?
Ch. 4: Network Layer - Forwarding
#5
Virtual circuits: signaling protocols
 Principle
 prepare a path (= VC) before moving data
 each direction is a separate path
 Signaling
 used to set up, maintain, teardown VC
 used in ATM, frame-relay, X.25
 not used in today’s Internet
• but Cisco’s MPLS builds a VC service over IP
application
transport 5. Data flow begins
network 4. Call connected
data link 1. Initiate call
physical
path recorded
6. Receive data application
3. Accept call
2. incoming call
transport
network
data link
physical
more path details
Ch. 4: Network Layer - Forwarding
#6
Virtual Circuit: call setup
 Path preparation + resource allocation:

Call setup message flows from source to
destination.
• path recorded at this time

Path determination (routing):
• Source based or network based.

Msg may indicate required resources:
• BW, latency, buffer, etc.
A
router can either:
• accept (and commit required resources) or reject

Path accepted if all routers accept.
Ch. 4: Network Layer - Forwarding
#7
Virtual Circuit: Identifiers
 Forward call-setup pass:

each router allocates an ID for the VC
• intended for incoming (I/C) packets of the VC
• records it + the preceding &following node of path
 Backward call-setup pass:
 each router tells predecessor its ID for the VC
• will use this ID on outgoing (O/G) packets of this VC
lists in the I/C port’s fwding table the I/C VC-ID
and the corresponding O/G port+O/G ID
 Runtime: when receiving a packet with an ID :




find, in the I/C port’s forwarding table, the I/C ID’s record
read from it the outgoing port & the O/G ID
send packet on the required port with new ID .
Ch. 4: Network Layer - Forwarding
#8
VC : identifiers preparation
 Example: call setup stage
BW=1Mb
2
1
BW=1Mb
1
2
BW=1Mb
In
In
port
port
VC
VC id
id
in
in
Out
Out
port
port
VC
VC id
id
out
out
In
In
port
port
VC id
in
Out
port
VC id
out
11
38
2
22
11
22
2
xx
Ch. 4: Network Layer - Forwarding
#9
VC : identifiers usage
 Example: runtime stage
VCid=38
2
1
VCid=22
2
1
VCid=xx
In
In
port
port
VC
VC id
id
in
in
Out
Out
port
port
VC
VC id
id
out
out
In
In
port
port
VC id
in
Out
port
VC id
out
11
38
2
22
11
22
2
xx
Ch. 4: Network Layer - Forwarding
#10
Datagram networks: Internet model
 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
application
transport
network
data link 1. Send data
physical
application
transport
network
2. Receive data
data link
physical
Ch. 4: Network Layer - Forwarding
#14
ATM: overview
 Asynchronous Transfer Mode
 Fixed packets size: called cells
 53 bytes = 5 header + 48 data
 All virtual-circuit based
 Types of virtual circuits
“virtual circuits” aggregated into “virtual paths”
 Permanent or switched

 Architecture is QoS-focused
 Service Quality types: CBR, VBR, ABR, UBR
 Access traffic policing
 Typical tool: leaky-bucket access control
Ch. 4: Network Layer - Forwarding
#15
Network Layer Quality of Service
Network
Architecture
Internet
Service
Model
Guarantees ?
Congestion
Bandwidth Loss Order Timing feedback
best effort none
ATM
CBR
ATM
VBR
ATM
ABR
ATM
UBR
constant
rate
guaranteed
rate
guaranteed
minimum
none
no
no
no
yes
yes
yes
yes
yes
yes
no
yes
no
no (inferred
via loss/delay)
no
congestion
no
congestion
yes
no
yes
no
no
 Internet model is being extended: Intserv, Diffserv

multimedia networking
ATM: Asynchronous Transfer Mode; CBR: Constant Bit Rate; V: Variable; A: available; U: Unspecified
Ch. 4: Network Layer - Forwarding
#16
Datagram or VC network: why?
Internet (Datagram)
 data exchange among hosts
(mostly) “elastic” service,
no strict timing req.
 “smart” end systems
 can adapt, perform
control, error recovery
 simple inside network,
complexity at “edge”
 many link types
 different characteristics
 uniform service difficult

 Datagram benefit:
 Simplicity
ATM (VC)
 evolved from telephony

but supports also data
 human conversation:
strict timing &reliability
requirements
 svc guaranteed needed
 “dumb” end systems
 telephones
 complexity inside
network

 VC Benefits:
 Fast forwarding
 Traffic Engineering.
 In order delivery
Ch. 4: Network Layer - Forwarding
#17
IP Addressing Scheme
 We need an address to uniquely identify
each destination
 Routing scalability requires flexibility in
aggregation of destination addresses
we should be able to aggregate a set of
destinations as a single routing unit
 necessary for routing table scalability

 Preview: the unit of routing in the Internet
is a network - the destinations in the routing
protocols and tables are networks
Ch. 4: Network Layer - Forwarding
#19
IP Addressing: introduction
 IP address: 32-bit
identifier for host or
router interface
 interface: connection
between host/router
and physical link



223.1.1.1
223.1.2.1
223.1.1.2
223.1.1.4
223.1.1.3
223.1.2.9
223.1.3.27
223.1.2.2
router’s typically have
223.1.3.2
223.1.3.1
multiple interfaces
a host has typically a
single interface
IP addresses
associated with
223.1.1.1 = 11011111 00000001 00000001 00000001
interface, not host, or
223
1
1
1
router
Ch. 4: Network Layer - Forwarding
#20
IP Addressing
 IP address is divided
into two parts:

network prefix
• K high order bits

host number
223.1.1.1
223.1.2.1
223.1.1.2
223.1.1.4
223.1.1.3
• remaining low order
bits
 This partitioning of
the address depends
on the context
network in which we
see this NIC

networks are nested
inside each other
223.1.2.9
223.1.3.27
223.1.2.2
LAN
223.1.3.1
223.1.3.2
Qn: What is the router’s IP
address in the drawing we see?
Ch. 4: Network Layer - Forwarding
#21
What is a network in IP view?
IP network terminology:
 a Subnet is:


a set of devices that
can physically reach
each other without
intervening router(s)
e.g. a LAN
 a Network is:
 a subnet , or:
 the union of several
subnets that are
interconnected by links
223.1.1.1
223.1.2.1
223.1.1.2
223.1.1.4
223.1.1.3
223.1.2.9
223.1.3.27
223.1.2.2
LAN
223.1.3.1
223.1.3.2
three subnets (LANs) 223.1.1.*,
223.1.2.*, 223.1.3.*, together they
form a larger network with prefix
223.1 (16 bits) (OR MORE bits?)
Ch. 4: Network Layer - Forwarding
#22
IP Address Structure (CIDR method)
 the network prefix consists of the K most significant bits of the
address
 in some cases it is called the subnet prefix (see subnets below)
 the host number = the other (32-K) bits
 the size K of the network prefix differs and must be specified in
each case. Two methods used for this:
 network mask has all 1‘s in the prefix part and all 0’s elsewhere
 short notation is
/K (also called the CIDR notation)
Exercise 1
a)
b)
c)
write the following IP address in dotted decimal notation
specify corresponding netwk mask (binary and dotted decimal)
show network prefix & host # parts of that address (binary)
11001000 00010111 00010001 10110101 /23
see solutions at end of chapter
Ch. 4: Network Layer Forwarding
4-23
Special Types of IP Address
 network broadcast address : host # = 11...1



means: all the hosts in the network specified in address prefix
used only as destination address of packets
if dest. address = 11… 1 (32 1’s), broadcast on sender’s subnet
 network address : host # = 0 (all zeros)
 means: the whole network (used only in routing tables)
 therefore the IP address of a host/router can not
have host number = 0 or = “all ones”
Exercise 2
1.
2.
3.
4.
5.
write the network address of the network from Exercise 1
write the broadcast address for that network
how many IP host addresses are possible in that network?
write host & network address with /K notation
write the first and last host address on that network
Ch. 4: Network Layer Forwarding
4-24
Subnets
Example
Network 223.1.0.0 / 21
Recipe
 To determine the
subnets of a network,
detach each interface
from its host or
router, creating
islands of isolated
networks. Each
isolated network is
a subnet.
Ch. 4: Network Layer Forwarding
Divide network into subnets and
give an address to each subnet
4-25
Solution of Example
Stage 1
Stage 2
Network 223.1.0.0 / 21
Subnet 223.1.1.0 / 24
Subnet 223.1.2.0 / 24
223.1.1.1
223.1.2.1
223.1.1.2
223.1.2.9
223.1.1.4
223.1.3.27
223.1.1.3
223.1.2.2
223.1.3.1
223.1.3.2
Subnet 223.1.3.0 / 24
Subnets: /24
Ch. 4: Network Layer Forwarding
4-26
Subnets
Whole network: 223.1.0.0/20
223.1.1.2
Subnet 223.1.1.0/24
o How many subnets?
o Write an address for
223.1.1.1
223.1.1.3
each subnet,
223.1.9.2
including /K
Subnet 223.1.9.0/24
o Write an address for
the whole network,
223.1.9.1
including /K
223.1.8.1
223.1.2.6
223.1.7.2
Subnet 223.1.7.0/24
223.1.8.2
Subnet 223.1.8.0/24
223.1.2.2
223.1.2.1
Ch. 4: Network Layer Forwarding
223.1.1.4
Subnet 223.1.2.0/24
223.1.3.1
223.1.7.1
223.1.3.27
223.1.3.2
Subnet 223.1.3.0/24
4-27
IP Addresses
given notion of “network”, let’s re-examine IP addresses:
“classful” addressing:
(does not need mask or /K indicator)
class
A
0 network
B
10
C
110
D
1110
1.0.0.0 to
127.255.255.255
host
network
128.0.0.0 to
191.255.255.255
host
network
host
192.0.0.0 to
223.255.255.255
224.0.0.0 to
239.255.255.255
multicast address (*)
32 bits
(*) this range used as multicast also in CIDR method
Ch. 4: Network Layer - Forwarding
#28
IP addressing: CIDR
 classful addressing:


inefficient use of address space, address space exhaustion
e.g., class B net allocated enough addresses for 65K hosts,
even if only 2K hosts in that network
 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 of address
Requires inclusion of mask or “/K” in routing table
network
part
host
part
11001000 00010111 00010000 00000000
200.23.16.0/23
Ch. 4: Network Layer - Forwarding
#29
IP addresses: how to get one?
Hosts (host number):
 hard-coded by system admin in a file

Can see in IPConfig
 DHCP: Dynamic Host Configuration Protocol:
dynamically get address: “plug-and-play”
 host broadcasts “DHCP discover” msg
 DHCP server responds with “DHCP offer” msg
 host requests IP address: “DHCP request” msg
 DHCP server sends address: “DHCP ack” msg
 this is the common practice in LAN (why?)
 in home access: same procedure using PPP protocol
Ch. 4: Network Layer - Forwarding
#30
IP addresses: how to get one?
Network (network prefix+mask):
 get allocated portion of ISP’s address space:
ISP's block
11001000 00010111 00010000 00000000
200.23.16.0/20
Organization 0
11001000 00010111 00010000 00000000
200.23.16.0/23
Organization 1
11001000 00010111 00010010 00000000
200.23.18.0/23
Organization 2
...
11001000 00010111 00010100 00000000
…..
….
200.23.20.0/23
….
Organization 7
11001000 00010111 00011110 00000000
200.23.30.0/23
Ch. 4: Network Layer - Forwarding
#31
IP addresses: how to get one?
ISP
 Gets a block of addresses from ICANN:
A: ICANN: Internet Corporation for Assigned
Names and Numbers
 allocates addresses
 manages DNS
 assigns domain names, resolves disputes
 allocates codes for the various protocols
Ch. 4: Network Layer - Forwarding
#32
Hierarchical addressing: route aggregation
Hierarchical addressing allows efficient advertisement of routing
information:
Organization 0
200.23.16.0/23
Organization 1
200.23.18.0/23
Organization 2
200.23.20.0/23
Organization 7
.
.
.
.
.
.
Fly-By-Night-ISP
“Send me anything
with addresses
beginning
200.23.16.0/20”
Internet
200.23.30.0/23
ISPs-R-Us
“Send me anything
with addresses
beginning
199.31.0.0/16”
Ch. 4: Network Layer - Forwarding
#33
Hierarchical addressing: more specific
routes
ISPs-R-Us has a more specific route to Organization 1
Organization 0
200.23.16.0/23
Organization 2
200.23.20.0/23
Organization 7
.
.
.
.
.
.
Fly-By-Night-ISP
“Send me anything
with addresses
beginning
200.23.16.0/20”
Internet
200.23.30.0/23
ISPs-R-Us
Organization 1
200.23.18.0/23
“Send me anything
with addresses
beginning 199.31.0.0/16
or 200.23.18.0/23”
Ch. 4: Network Layer - Forwarding
#34
Routing table
Destination Address Range
4 billion
possible entries (*)
Link Interface
11001000 00010111 00010000 00000000
through
11001000 00010111 00010111 11111111
0
11001000 00010111 00011000 00000000
through
11001000 00010111 00011000 11111111
1
11001000 00010111 00011001 00000000
through
11001000 00010111 00011111 11111111
2
otherwise
3
(*) true for IPv4; in IPv6 MUCH more
Ch. 4: Network Layer - Forwarding
#35
Longest prefix matching
Network
11001000
11001000
11001000
00000000
/K
00010111
00010111
00010111
00000000
Network
200.23.16.0 /21
200.23.24.0 /24
200.23.20.0 /24
otherwise
00010000 00000000
00011000 00000000
00010100 00000000
00000000 00000000
Link Interface
0
1
2
3
Link Interface
/21
/24
/24
/0
0
1
2
3
Routing table
Examples:
Which interface will be used by this router for following dest addresses?
(a)
DA: 11001000 00010111 00010110 10100001
(b)
DA: 11001000 00010111 00010100 10101010
(c)
DA: 11001000 00010111 00011100 10111110
(d)
DA: 11001000
Network Layer
00010111
00011000
11101010
4-36
Getting a datagram from source to dest.
routing table in R
Dest. Net. next router Nhops
IP datagram:
misc source dest
fields IP addr IP addr
 datagram remains
223.1.1
223.1.2
223.1.3
data
unchanged (*), as it
travels source to
destination
 addr fields of interest
here
 Main field :
dest. IP addr
A
223.1.5.2
223.1.5.2
R
223.1.1.4
1
2
2
223.1.5.1
223.1.1.1
223.1.2.1
B
223.1.1.2
223.1.5.2
223.1.1.3
223.1.3.1
223.1.2.9
S
223.1.3.27
223.1.2.2
E
223.1.3.2
(*) almost
Ch. 4: Network Layer - Forwarding
#37
Getting a datagram from source to dest.
misc
data
fields 223.1.1.1 223.1.1.3
Starting at A, given IP
datagram addressed to B:
 A looks up its /K(*) in IPConfig
 Compares first K bits in dest
address with those in its own
 find B is on same net. as A
 same prefix  sane subnet
 link layer will send datagram
directly to B in link-layer frame
 using ARP table/protocol
 B and A are directly
connected
(*) in the form of subnet mask
A’s ARP Table:
223.1.1.3 => 
223.1.1.4 => 
Etc.
A’s IPConfig:
IP Addr: 223.1.1.1
Subnet /K = 24 (*)
Dflt Gtwy: 223.1.1.4
A
223.1.1.4
223.1.1.1
R
223.1.5.1
223.1.2.1
B
223.1.1.2
223.1.5.2
223.1.1.3
223.1.3.1
223.1.2.9
S
223.1.3.27
223.1.2.2
E
223.1.3.2
(*) subnet mask = 225.225.225.0
Ch. 4: Network Layer - Forwarding
#38
Getting a datagram from source to dest.
misc
data
fields 223.1.1.1 223.1.2.2
Starting at A, dest. E:
 look up network address of E
Routing Table
Dest. Net.
223.1.1.0 /24
223.1.2.0 /24 223.1.5.2
223.1.3.0 /24 223.1.5.2
a
 E on different network
A sees this by comparing
/K prefixes of A and E
routing table: next hop
router to E is 223.1.5.2
link layer sends datagram to
router 223.1.5.2 inside linklayer frame
datagram arrives at 223.1.5.2
cont. on next slide..





Next router Port
A
223.1.1.4
223.1.1.1
Hops
a
b
b
1
2
2
R
b 223.1.5.1
223.1.2.1
B
223.1.1.2
223.1.5.2
a
223.1.1.3
223.1.3.1
223.1.2.9
S b
223.1.3.27
c
223.1.2.2
E
223.1.3.2
Ch. 4: Network Layer - Forwarding
#39
Getting a datagram from source to dest.
misc
data
fields 223.1.1.1 223.1.2.2
Arrived at 223.1.5,2,
continuing to 223.1.2.2
 look up network address of E
 E on subnet directly attached
to router’s interface b
 link layer sends datagram to
223.1.2.2 inside link-layer
frame via I/F b (223.1.2.9)
 datagram arrives at
223.1.2.2!!! (hooray!)
 Qn: What table consulted
here?
Dest. Net.
Next router Port
223.1.1.0 /24 223.1.5.1
223.1.2.0 /24
223.1.3.0 /24
a
A
223.1.1.4
223.1.1.1
Hops
a
b
c
2
1
1
R
b 223.1.5.1
223.1.2.1
B
223.1.1.2
223.1.5.2
a
223.1.1.3
223.1.3.1
223.1.2.9
S b
223.1.3.27
c
223.1.2.2
E
223.1.3.2
Ch. 4: Network Layer - Forwarding
#40
Network Address Translation (NAT): Outline
 A local network uses just one public IP address as far as outside
world is concerned
 Each device on the local network is assigned a private IP address
rest of
Internet
local network
(e.g., home network)
192.168.1.0/24
192.168.1.1
192.168.1.2
192.168.1.3
138.76.29.7
192.168.1.4
All datagrams leaving local
network have same single source
NAT IP address: 138.76.29.7,
different source port numbers
Datagrams with source or
destination in this network
have 192.168.1/24 address for
source /destination (as usual)
Ch. 4: Network Layer - Forwarding
#41
NAT: Implementation
NAT router must:
for outgoing datagrams:
 replace (source IP address, port #) of every outgoing
datagram by (NAT IP address, new port #)
. . . remote clients/servers will respond using (NAT
IP address, new port #) as destination addr.
 remember (in NAT translation table) every (source
IP address, port #) to (NAT IP address, new port #)
translation pair
for incoming datagrams:
 replace (NAT IP address, new port #) in dest fields
of every incoming datagram with corresponding
(source IP address, port #) stored in NAT table
Ch. 4: Network Layer - Forwarding
#42
NAT: Network Address Translation
NAT translation table
WAN side addr
LAN side addr
1: host 192.168.1.2
2: NAT router
sends datagram to
changes datagram
138.76.29.7, 5001 192.168.1.2, 3345
128.119.40.186, 80
source addr from
……
……
192.168.1.2, 3345 to
138.76.29.7, 5001,
S: 192.168.1.2, 3345
updates table
D: 128.119.40.186, 80
2
S: 138.76.29.7, 5001
D: 128.119.40.186, 80
138.76.29.7
S: 128.119.40.186, 80
D: 138.76.29.7, 5001
3: Reply arrives
dest. address:
138.76.29.7, 5001
3
192.168.1.2
1
192.168.1.1
192.168.1.3
S: 128.119.40.186, 80
D: 192.168.1.2, 3345
4
192.168.1.4
4: NAT router
changes datagram
dest addr from
138.76.29.7, 5001 to 192.168.1.2, 3345
Ch. 4: Network Layer - Forwarding
#43
NAT: Advantages
 No need to be allocated range of addresses
from ISP: - just one public IP address is
used for all devices
16-bit port-number field allows 60,000
simultaneous connections with a single LAN-side
address !
 can change ISP without changing addresses of
devices in local network
 can change addresses of devices in local network
without notifying outside world

 Devices inside local net not explicitly
addressable, visible by outside world (a
security plus)
Ch. 4: Network Layer - Forwarding
#44
NAT: Drawbacks
 If both hosts are behind distinct NATs,
they will have difficulty establishing
connection
 NAT is controversial:
 violates
layer modularity principle:
routers should process up to only layer 3
 causes problem for some application protocols:
• if application writes an explicit IP address within the
L5 header, the peer application will get a useless
internal IP address as an argument
 proper address shortage solution : IPv6 !
Ch. 4: Network Layer - Forwarding
#45
IP datagram format
IP protocol version
number
header length
(bytes)
“type” of data
max number
remaining hops
(decremented at
each router)
upper layer protocol
to deliver payload to
32 bits
head. type of
length
len service
fragment
16-bit identifier flgs
offset
time to upper
Internet
layer
live
checksum
ver
total datagram
length (bytes)
for
fragmentation/
reassembly
32 bit source IP address
32 bit destination IP address
Options (if any)
data
(variable length,
typically a TCP
or UDP segment)
E.g. timestamp,
record route
taken, specify
list of routers
to visit.
Ch. 4: Network Layer - Forwarding
#46
IPv6
 Initial motivation: 32-bit address space soon
to be completely allocated.
 Additional motivation:

IPv6 header format helps speed processing
 IPv6 datagram format:
 16-byte
(128 bit) IP address
 fixed-length 40 byte header
• no options allowed inside the header
• each option gets its own header after the main IP header
 fragmentation
discouraged
• allowed only using an options header
Network Layer
4-47 ‫אפקה‬
"‫תשע‬
Transition From IPv4 To IPv6
 Not all routers can be upgraded simultaneously
 How will the network operate with mixed IPv4 & IPv6 routers?
 Tunneling: IPv6 datagrams are carried as payload in IPv4
datagrams when travelling through IPv4 routers
 source and destination network are IPv6, but need to transit
an existing IPv4 network
 How is tunneling done?
 gateway router in source network takes the IPv6 datagram as
payload and encapsulates it into an IPv4 datagram
• i.e. adds an IPv4 header in front of it

the IPv4 destination is the gateway router of the destination
network, which removes the IPv4 header and routes by IPv6
 Gateway router must support IPv4, IPv6 and tunneling
Network Layer
4-48 ‫אפקה‬
"‫תשע‬
Tunneling
Logical view:
A
B
IPv6
Physical view:
A
IPv4
IPv4
IPv6
Flow: X
Src: A
Dest: F
data
A-to-B:
IPv6
F
E
IPv6
IPv6
Network Layer
tunnel
IPv6
B
C
D
IPv4
IPv4
IPv4
IPv4
E
F
IPv6
IPv6
Src:B
Dest: E
Src:B
Dest: E
Flow: X
Src: A
Dest: F
Flow: X
Src: A
Dest: F
data
data
B-to-C:
IPv6 inside
IPv4
IPv6
B-to-C:
IPv6 inside
IPv4
Flow: X
Src: A
Dest: F
data
E-to-F:
IPv6
4-49 ‫אפקה‬
"‫תשע‬
Usage of Tunneling
 Tunneling is used to move a packet between
similar networks A, B through a network C
that is unable to understand its L3 header
 Possible reasons:
1.
2.
3.
C uses a different protocol (e.g. IPv6 vs IPv4)
A wants to encipher the data and the header
(VPN application)
All networks use same protocol, but the
destination node is currently at a foreign network
(Mobile IP application)
Network Layer
4-50 ‫אפקה‬
"‫תשע‬
IPv6 Header (Cont)
Priority: identify priority among datagrams in flow
Flow Label: identify datagrams in same “flow.”
(concept of“flow” not well defined).
Next header: identify upper layer protocol for data
Ch. 4: Network Layer - Forwarding
4-51
Transition From IPv4 To IPv6
 Not all routers can be upgraded simultaneous
no “flag days”
 How will the network operate with mixed IPv4 and
IPv6 routers?

 Tunneling: IPv6 carried as payload in IPv4
datagram among IPv4 routers
Ch. 4: Network Layer - Forwarding
4-52
IPv6 status report
 Operating systems –
 wide support – early 2000
 Windows (2000, XP, Vista), BSD, Linux, Apple
 Networking infrastructure
 Cisco
 Deployment
 Slow
 Penetration
 Host - minor (less than 1%)
 Used in 2008 in China Olympic games
 Motivation: CIDR & NAT
Ch. 4: Network Layer - Forwarding
#53
Extra
Ch. 4: Network Layer - Forwarding
#54
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
Ch. 4: Network Layer - Forwarding
4-55
IP Fragmentation and Reassembly
Example
 4000 byte
datagram
 MTU = 1500 bytes
1480 bytes in
data field
offset =
1480/8
length ID fragflag offset
=4000 =x
=0
=0
One large datagram becomes
several smaller datagrams
length ID fragflag offset
=1500 =x
=1
=0
length ID fragflag offset
=1500 =x
=1
=185
length ID fragflag offset
=1040 =x
=0
=370
Ch. 4: Network Layer - Forwarding
4-56
ICMP: Internet Control Message Protocol
 used by hosts & routers to
communicate network-level
information
 error reporting:
unreachable host, network,
port, protocol
 echo request/reply (used
by ping)
 network-layer “above” IP:
 ICMP msgs carried in IP
datagrams
 ICMP message: type, code plus
first 8 bytes of IP datagram
causing error
Type
0
3
3
3
3
3
3
4
Code
0
0
1
2
3
6
7
0
8
9
10
11
12
0
0
0
0
0
description
echo reply (ping)
dest. network unreachable
dest host unreachable
dest protocol unreachable
dest port unreachable
dest network unknown
dest host unknown
source quench (congestion
control - not used)
echo request (ping)
route advertisement
router discovery
TTL expired
bad IP header
Ch. 4: Network Layer - Forwarding
4-57
Traceroute and ICMP
 Source sends series of
UDP segments to dest



First has TTL =1
Second has TTL=2, etc.
Unlikely port number
 When nth datagram arrives
to nth router:



Router discards datagram
And sends to source an
ICMP message (type 11,
code 0)
Message includes name of
router& IP address
 When ICMP message
arrives, source calculates
RTT
 Traceroute does this 3
times
Stopping criterion
 UDP segment eventually
arrives at destination host
 Destination returns ICMP
“host unreachable” packet
(type 3, code 3)
 When source gets this
ICMP, stops.
Ch. 4: Network Layer - Forwarding
4-58
Exercise 1 Answers
11001000 00010111 00010001 10110101
128 64 32 16 8 4 2 1
27 26 25 24 23 22 21 20
/23
128 64 32 16 8 4 2 1
27 26 25 24 23 22 21 20
Ans 1: 11001000 00010111 00010001 10110101 =200.23.17.181
128+64+8= 200
16+7= 23
16+1= 17
128+32+16+5= 181
Ans 2: 11111111 11111111 11111110 00000000 = 255.255.254.0
255-1 = 254
Ans 3: 11001000 00010111 00010001 10110101
NETWORK
HOST
Ch. 4: Network Layer Forwarding
4-59
Exercise 2 Answers
11001000 00010111 00010001 10110101
/23
NETWORK
Ans 1: 11001000 00010111 00010000 00000000 =
200.23.16.0
Ans 2: 11001000 00010111 00010001 11111111 =
200.23.17.255
9
Ans 3: 2 -2 = 510 hosts
Ans 4: network: 200.23.16.0/23
host:
200.23.17.181/23
Ans 5: first host address: 200.23.16.1/23
last host address: 200.23.17.254/23
Ch. 4: Network Layer Forwarding
4-60