Transcript Internet Protocol

Internet Protocol
ECS 152A
Xin Liu
Ref: slides by J. Kurose and K. Ross
Goals
• Principles of network layer services
• Internet Protocol
– Addressing
– Routing
– ICMP
Encapsulation
User
process
Overview
HTTP
User
process
TCP
SNMP
UDP
application
message
transport
segment
ICMP
IP
ARP
Hardware
interface
network
datagram
RARP
link
frame
Demultiplexing
Network layer functions
• transport packet
from sending to
receiving hosts
• network layer
protocols in every
host, router
application
transport
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
network
data link
physical
application
transport
network
data link
physical
Functions
• path determination: route taken by packets
from source to dest. Routing algorithms
• forwarding: move packets from router’s
input to appropriate router output
• call setup: some network architectures
require router call setup along path before
data flows (not Internet)
Network service model
Q: What service model for
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?
Virtual circuits
“source-to-dest path behaves much like
telephone circuit”
• call setup, teardown for each call before data can flow
• each packet carries VC identifier (not destination host
ID)
• every router on source-dest path maintains “state” for
each passing connection
• link, router resources (bandwidth, buffers) may be
allocated to VC
– to get circuit-like perf.
Virtual circuits: signaling protocols
• used to setup, maintain teardown VC
• used in ATM, frame-relay, X.25
• not used in today’s Internet
application
transport 5. Data flow begins
network 4. Call connected
data link 1. Initiate call
physical
6. Receive data application
3. Accept call
2. incoming call
transport
network
data link
physical
Datagram networks: the Internet model
• 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
application
– packets between same source-dest pair mayapplication
take
transport
different paths
transport
network
network
data link 1. Send data
2. Receive data
data link
physical
physical
VC vs. Datagram
• VC
– Guaranteed service
– Complexity
• Datagram
– Simple
– Best effort
Internet Protocol
• Functionality:
– Determine how to route packets from source to
destination
– Hide the details of the physical network
– Unreliable, connectionless, datagram delivery
• To be mentioned:
– Addressing
– Routing
– ICMP
Encapsulation
source
destination
original message
Application
Application
Transport
Transport
Network
Network
Link
Link
IP header
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
20 bytes overhead
32 bits
head. type of
length
ver
len service
fragment
16-bit identifier flgs
offset
upper
time to
header
layer
live
checksum
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.
IP Header
•
•
•
•
•
•
•
Version: 4
Header length: 4 bits, max 15x4=60 bytes
TOS: 0 for normal service,
Total length: 16 bits, max 65535 bytes
TTL: 32/64, decrease by one in each hop
Protocol field: TCP,UCP,ICMP,IGMP,etc.
Checksum: header only
IP Address
Class-based address:
class
7 bits
A
0 network
B
C
D
1.0.0.0 to
127.255.255.255
host
14 bits
10
network
128.0.0.0 to
191.255.255.255
host
21 bits
110
network
host
28 bits
1110
multicast address
32 bits
192.0.0.0 to
223.255.255.255
224.0.0.0 to
239.255.255.255
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 500 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
host
network
part
part
11001000 00010111 00010000 00000000
200.23.16.0/23
CIDR
• Network address: 200.23.16.0/23
– /23 : network mask
• More efficient use of address
– Consider a network with 500 hosts
– Classful address: a class B address, wasting over 64K
addresses
– CIDR: a network with /23
– One class B address can be used for 128 such networks using
CIDR
• Routing difficulty
– Classful: only need the IP address to determine the network add
– CIDR: also need network mask information to determine the
network address
– Longest match first
IP routing
• IP address: 32-bit
identifier for host,
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
– host may have multiple
interfaces
– IP addresses associated
223.1.1.1 = 11011111 00000001 00000001 00000001
with each interface
223
1
Network address: 223.1.1.0/24
1
1
IP Routing
• IP address:
– network part (high
order bits) is used
for routing
– host part (low order
bits) not used for
routing
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
network consisting of 3 IP networks
(for IP addresses starting with 223,
first 24 bits are network address)
Getting a datagram from source to
dest.
forwarding table in
Dest. Net. next router Nhops
A
223.1.1
223.1.2
223.1.3
Others
IP datagram:
misc source dest
fields IP addr IP addr
data
• datagram remains
unchanged, as it
travels source to
destination
• addr fields of interest
here
• Default router for all
other networks
A
223.1.1.4
223.1.1.4
223.1.1.4
1
2
2
x
223.1.1.1
223.1.2.1
B
223.1.1.2
223.1.1.4
223.1.2.9
223.1.2.2
223.1.1.3
223.1.3.1
223.1.3.27
223.1.3.2
E
Getting a datagram from source to
dest.
forwarding table in
Dest. Net. next router Nhops
A
misc
data
fields 223.1.1.1 223.1.1.3
Starting at A, send IP
datagram addressed to
B:
• look up net. address of B in
forwarding table
• find B is on same net. as A
• link layer will send
datagram directly to B
inside link-layer frame
– B and A are directly
connected
223.1.1
223.1.2
223.1.3
A
223.1.1.4
223.1.1.4
1
2
2
223.1.1.1
223.1.2.1
B
223.1.1.2
223.1.1.4
223.1.2.9
223.1.2.2
223.1.1.3
223.1.3.1
223.1.3.27
223.1.3.2
E
Getting a datagram from source to
dest.
forwarding table in
Dest. Net. next router Nhops
A
misc
data
fields 223.1.1.1 223.1.2.3
223.1.1
223.1.2
223.1.3
Starting at A, dest. E:
• look up network address of E in
forwarding table
• E on different network
– A, E not directly attached
• routing table: next hop router to
E is 223.1.1.4
• link layer sends datagram to
router 223.1.1.4 inside linklayer frame
• datagram arrives at 223.1.1.4
• continued…..
A
223.1.1.4
223.1.1.4
1
2
2
223.1.1.1
223.1.2.1
B
223.1.1.2
223.1.1.4
223.1.2.9
223.1.2.2
223.1.1.3
223.1.3.1
223.1.3.27
223.1.3.2
E
Getting a datagram from source to
dest.
misc
data
fields 223.1.1.1 223.1.2.3
Arriving at 223.1.4, destined for
223.1.2.2
• look up network address of E in
router’s forwarding table
• E on same network as router’s
interface 223.1.2.9
– router, E directly attached
• link layer sends datagram to
223.1.2.2 inside link-layer frame
via interface 223.1.2.9
• datagram arrives at 223.1.2.2!!!
(hooray!)
forwarding table in
Dest. Net router Nhops interface
router
223.1.1
223.1.2
223.1.3
A
-
1
1
1
223.1.1.4
223.1.2.9
223.1.3.27
223.1.1.1
223.1.2.1
B
223.1.1.2
223.1.1.4
223.1.2.9
223.1.2.2
223.1.1.3
223.1.3.1
223.1.3.27
223.1.3.2
E
CIDR Routing
network
part
host
part
11001000 00010111 00010000 00000000
200.23.16.0/23
network
part
host
part
11001000 00010111 00000000 00000000
200.23.0.0/17
CIDR routing: longest match first
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
Example
• 4000 byte
datagram
• MTU = 1500
bytes
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
=1480
length ID fragflag offset
=1040 =x
=0
=2960
IPv6
• Initial motivation: 32-bit address space
completely allocated by 2008.
• Additional motivation:
– header format helps speed processing/forwarding
– header changes to facilitate QoS
– new “anycast” address: route to “best” of several
replicated servers
• IPv6 datagram format:
– fixed-length 40 byte header
– no fragmentation allowed
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
Other Changes from IPv4
• Checksum: removed entirely to reduce
processing time at each hop
• Options: allowed, but outside of header,
indicated by “Next Header” field
• ICMPv6: new version of ICMP
– additional message types, e.g. “Packet Too
Big”
– multicast group management functions
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?
• Two proposed approaches:
– Dual Stack: some routers with dual stack (v6, v4) can
“translate” between formats
– Tunneling: IPv6 carried as payload in IPv4 datagram
among IPv4 routers
Dual Stack Approach
A
B
C
D
E
F
IPv6
IPv6
IPv4
IPv4
IPv6
IPv6
Flow: X
Src: A
Dest: F
Src:A
Dest: F
Src:A
Dest: F
Flow: ??
Src: A
Dest: F
data
data
data
data
B-to-C:
IPv4
B-to-C:
IPv4
B-to-C:
IPv6
A-to-B:
IPv6
Tunneling
Logical view:
Physical view:
A
B
IPv6
IPv6
A
B
C
IPv6
IPv6
IPv4
Flow: X
Src: A
Dest: F
data
A-to-B:
IPv6
E
F
IPv6
IPv6
D
E
F
IPv4
IPv6
IPv6
tunnel
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
B-to-C:
IPv6 inside
IPv4
Flow: X
Src: A
Dest: F
data
E-to-F:
IPv6
ICMP (Internet Control Message Protocol)
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
Query
x
Error
x
x
x
x
x
x
x
x
x
x
x
x
ICMP
IP datagram
IP header
8-bit type 8-bit code
ICMP message
16-bit checksum
Contents depends on type and code
Error message
• ICMP error message:
– ICMP header:
• type, code, checksum,
– ICMP message
• IP header plus first 8 bytes of IP datagram causing error
• To prevent broadcast storm: NOT generate ICMP in
response to
–
–
–
–
–
ICMP error message
Dest=IP broadcast address
Link layer broadcast
A fragment other than the first
Source address not defined as a single host
Ping
• Basic connectivity test
• uses ICMP eco request/reply messages
instead of UDP/TCP.
• Client/server paradigm
• Usually implemented in the kernel.
• “man ping”
Format
code(0)
type (0)
identifier
16-bit checksum
sequence no.
Optional data
Ping
bread% ping -s shannon.cs.ucdavis.edu
PING shannon.cs.ucdavis.edu: 56 data bytes
64 bytes from shannon.cs.ucdavis.edu (169.237.6.199): icmp_seq=0. time=0. ms
64 bytes from shannon.cs.ucdavis.edu (169.237.6.199): icmp_seq=1. time=0. ms
64 bytes from shannon.cs.ucdavis.edu (169.237.6.199): icmp_seq=2. time=0. ms
64 bytes from shannon.cs.ucdavis.edu (169.237.6.199): icmp_seq=3. time=0. ms
64 bytes from shannon.cs.ucdavis.edu (169.237.6.199): icmp_seq=4. time=0. ms
64 bytes from shannon.cs.ucdavis.edu (169.237.6.199): icmp_seq=5. time=0. ms
64 bytes from shannon.cs.ucdavis.edu (169.237.6.199): icmp_seq=6. time=0. ms
64 bytes from shannon.cs.ucdavis.edu (169.237.6.199): icmp_seq=7. time=0. ms
64 bytes from shannon.cs.ucdavis.edu (169.237.6.199): icmp_seq=8. time=0. ms
64 bytes from shannon.cs.ucdavis.edu (169.237.6.199): icmp_seq=9. time=0. ms
…
----shannon.cs.ucdavis.edu PING Statistics---30 packets transmitted, 30 packets received, 0% packet loss
round-trip (ms) min/avg/max = 0/0/0
Ping
bread% ping -s mark.ecn.purdue.edu
PING mark.ecn.purdue.edu: 56 data bytes
64 bytes from mark.ecn.purdue.edu (128.46.209.167): icmp_seq=0. time=66. ms
64 bytes from mark.ecn.purdue.edu (128.46.209.167): icmp_seq=1. time=64. ms
64 bytes from mark.ecn.purdue.edu (128.46.209.167): icmp_seq=3. time=64. ms
64 bytes from mark.ecn.purdue.edu (128.46.209.167): icmp_seq=4. time=65. ms
64 bytes from mark.ecn.purdue.edu (128.46.209.167): icmp_seq=5. time=64. ms
64 bytes from mark.ecn.purdue.edu (128.46.209.167): icmp_seq=8. time=65. ms
64 bytes from mark.ecn.purdue.edu (128.46.209.167): icmp_seq=10. time=65. ms
64 bytes from mark.ecn.purdue.edu (128.46.209.167): icmp_seq=11. time=65. ms
64 bytes from mark.ecn.purdue.edu (128.46.209.167): icmp_seq=12. time=65. ms
64 bytes from mark.ecn.purdue.edu (128.46.209.167): icmp_seq=15. time=64. ms
^C
----mark.ecn.purdue.edu PING Statistics---18 packets transmitted, 10 packets received, 44% packet loss
round-trip (ms) min/avg/max = 64/65/66
Traceroute
• By Van Jacobson
• See route that IP datagram follow
• Use ICMP and TTL
– A router gets an IP datagram with TTL 0/1, discards
the packet and sends back an ICMP to the source
“time exceeded”.
– Source sends UDP fragment with 1,2,3, TTL values
– IP packet contains an UDP with unused post #. dest.
Replies “port unreachable” ICMP message.
Traceroute
bread% traceroute ector.cs.purdue.edu
traceroute: Warning: Multiple interfaces found; using 169.237.6.16 @ qfe0
traceroute to ector.cs.purdue.edu (128.10.2.10), 30 hops max, 40 byte packets
1 169.237.5.254 (169.237.5.254) 0.594 ms 0.337 ms 0.298 ms
2 169.237.246.238 (169.237.246.238) 0.533 ms 0.479 ms 0.474 ms
3 128.120.2.49 (128.120.2.49) 0.547 ms 0.475 ms 0.475 ms
4 core0.ucdavis.edu (128.120.0.30) 0.616 ms 0.671 ms 0.642 ms
5 area0-area14p.ucdavis.edu (128.120.0.222) 0.570 ms 0.468 ms 0.821 ms
6 area14p-border20.ucdavis.edu (128.120.0.250) 1.149 ms 0.691 ms 3.132 ms
7 dc-oak-dc2--ucd-ge.cenic.net (137.164.24.225) 4.751 ms 2.434 ms 4.521 ms
8 dc-oak-dc1--oak-dc2-ge.cenic.net (137.164.22.36) 2.394 ms 4.217 ms 2.452 ms
9 dc-svl-dc1--oak-dc1-10ge.cenic.net (137.164.22.30) 201.245 ms 5.091 ms 183.393 ms
10 dc-sol-dc1--svl-dc1-pos.cenic.net (137.164.22.28) 13.421 ms 11.258 ms 11.155 ms
11 hpr-lax-hrp1--dc-lax-dc1-ge.cenic.net (137.164.22.13) 11.571 ms 14.390 ms 11.809 ms
12 abilene-LA--hpr-lax-gsr1-10ge.cenic.net (137.164.25.3) 13.431 ms 11.417 ms 11.289 ms
13 snvang-losang.abilene.ucaid.edu (198.32.8.95) 19.141 ms 20.516 ms 19.117 ms
14 kscyng-snvang.abilene.ucaid.edu (198.32.8.103) 54.300 ms 53.943 ms 53.998 ms
15 iplsng-kscyng.abilene.ucaid.edu (198.32.8.80) 64.783 ms 68.220 ms 63.659 ms
16 ul-abilene.indiana.gigapop.net (192.12.206.250) 63.567 ms 63.381 ms 63.025 ms
17 tel-210-m10-01-gp.tcom.purdue.edu (192.5.40.9) 65.017 ms * 64.982 ms
18 cs-2u01-c3550-01-242.tcom.purdue.edu (128.210.242.51) 65.527 ms 65.282 ms 65.083 ms
19 * ector.cs.purdue.edu (128.10.2.10) 65.528 ms *
NAT: Network Address Translation
rest of
Internet
local network
(e.g., home network)
10.0.0/24
10.0.0.4
10.0.0.1
10.0.0.2
138.76.29.7
10.0.0.3
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 10.0.0/24 address for
source, destination (as usual)
NAT: Network Address Translation
• Motivation: local network uses just one IP address as far as
outside word is concerned:
– no need to be allocated range of addresses from ISP: just one IP address is used for all devices
– can change addresses of devices in local network
without notifying outside world
– can change ISP without changing addresses of devices
in local network
– devices inside local net not explicitly addressable,
visible by outside world (a security plus).
NAT: Network Address Translation
Implementation: NAT router must:
– outgoing datagrams: replace (source IP address, port #)
of every outgoing datagram to (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
– 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
NAT: Network Address Translation
2: NAT router
changes datagram
source addr from
10.0.0.1, 3345 to
138.76.29.7, 5001,
updates table
2
NAT translation table
WAN side addr
LAN side addr
1: host 10.0.0.1
sends datagram to
128.119.40, 80
138.76.29.7, 5001 10.0.0.1, 3345
……
……
S: 10.0.0.1, 3345
D: 128.119.40.186, 80
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
1
10.0.0.4
S: 128.119.40.186, 80
D: 10.0.0.1, 3345
10.0.0.1
10.0.0.2
4
10.0.0.3
4: NAT router
changes datagram
dest addr from
138.76.29.7, 5001 to 10.0.0.1, 3345
NAT: Network Address Translation
• 16-bit port-number field:
– 60,000 simultaneous connections with a
single LAN-side address!
• NAT is controversial:
– routers should only process up to layer 3
– violates end-to-end argument
• NAT possibility must be taken into account by app
designers, eg, P2P applications
– address shortage should instead be solved by
IPv6