•
•
Switching and Forwarding
Network Layer Part I
Routing & Forwarding:
Logical View of a Router
Switching and Forwarding
–
–
Generic Router Architecture
Forwarding Tables:
• Bridges/Layer 2 Switches; VLAN
• Routers and Layer 3 Switches
Forwarding in Layer 3 (Network Layer)
– Network Layer Functions
– Network
N
k Service
i Models:
M d l VC vs. Datagram
D
• ATM and IP Datagram Forwarding
– IP Addressing
• Network vs. host: address blocks, longest prefix matching
• Address allocation and DHCP
– IP Datagram Forwarding Model and ARP Protocol
– IP and ICMP Protocols, IP Fragmentation and Re-assembly
5
A
2
1
3
B
2
D
3
1
C
5
1
E
F
2
Readings: Textbook: Chapter 4: Section 4.1;
Csci 183/183W/232: Computer
Networks
Forwarding and Routing
Network Layer Part I
1
IP Addressing: Basics
– network part (high order
bits)
– host part (low order bits)
– router
router’ss typically have multiple interfaces
– host may have multiple interfaces
– IP addresses associated with each interface
223.1.1.1 = 11011111 00000001 00000001 00000001
1
3
“Classful” IP Addressing
class
77
A
0 network
B
10
C
110
D
1110
15
23
31
host
network
network
host
multicast address
Csci 183/183W/232: Computer
Networks
point-to-point
li k
link
223.1.7.1
223.1.8.0
223.1.3.27
223.1.3.1
223.1.3.2
Forwarding and Routing
Network Layer Part I
4
• Network portion of address is of arbitrary length
• Addresses allocated in contiguous blocks
– Number of addresses assigned always power of 2
192.0.0.0 to
223.255.255.255
• Address format: a.b.c.d/x
a b c d/x
224.0.0.0 to
239.255.255.255
– x is number of bits in network portion of address
network
part
• Disadvantage: 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
Forwarding and Routing
Network Layer Part I
223.1.7.0
CIDR: Classless InterDomain Routing
1.0.0.0 to
127.255.255.255
32 bits
Csci 183/183W/232: Computer
Networks
223.1.9.2
Classless Addressing: CIDR
128.0.0.0 to
191.255.255.255
host
223.1.1.4
223.1.1.3
– device interfaces with
same network part of IP 223.1.9.1
address
223.1.8.1
– can physically reach each 223.1.2.6
other without intervening
router
223.1.2.1
223.1.2.2
• Dot notation (for ease of human reading)
Forwarding and Routing
Network Layer Part I
223.1.1.1
• What’s a network ?
(from IP address perspective)
physical link
Csci 183/183W/232: Computer
Networks
multi-access
LAN
223.1.1.2
interface
• Interface: connection between host/router and
1
2
• Two-level hierarchy
• IP address: 32-bit identifier for host, router
1
Forwarding and Routing
Network Layer Part I
IP Addressing: Network vs. Host
• Globally unique (for “public” IP addresses)
223
Csci 183/183W/232: Computer
Networks
host
part
11001000 00010111 00010000 00000000
200.23.16.0/23
5
Csci 183/183W/232: Computer
Networks
Forwarding and Routing
Network Layer Part I
6
1
More Examples
Representation of Address Blocks
Three Address Blocks:
First IP address:
11001000 00010111 00010000 00000000
Network mask:
11111111 11111111 11111000 00000000
• “Human Readable” address format: a.b.c.d/x
– x is number of bits in network portion of address, the network
portion is also called the network prefix
•
machine representation of a network (addr block):
using a combination of
– first IP of address blocks of the network
– network mask ( x “1”’s followed by 32-x “0”’s
11001000 00010111 00010000 00000000
network mask:
11111111 11111111 11111110 00000000
Forwarding and Routing
Network Layer Part I
7
Consider a datagram network using 32-bit host addresses, suppose
a router has four links, numbered 0 through 3, and packets are to
be forwarded to the link interfaces as follows:
Destination Addr Range
11100001 00000000 00000000 00000000
through
11100001 00000000 11111111 11111111
1
11100001 00000001 00000000 00000000
through
11100001 11111111 11111111 11111111
2
O.W.
3
Forwarding and Routing
Network Layer Part I
8
Q: How does host get IP address?
Link Interface
0
Csci 183/183W/232: Computer
Networks
Use longest prefix matching!
IP Addresses: How to Get One?
Another Example
11100000 00000000 00000000 00000000
through
11100000 11111111 11111111 11111111
Example 2:
11001000 00010111 00011000 10101010
First IP address:
11001000 00010111 00011001 00000000
Last IP address:
11001000 00010111 00011111 11111111
what is the network prefix?
11001000 00010111 00011
first IP address of address block:
•
Example 1:
11001000 00010111 00010110 10100001
First IP address:
11001000 00010111 00011000 00000000
Last IP address:
11001000 00010111 00011000 11111111
what is the network prefix?
11001000 00010111 00011000
network w/ address block: 200.23.16.0/23
Csci 183/183W/232: Computer
Networks
Given an IP address, which
network (or address block)
does it belong to?
• “static” assigned: i.e., hard-coded in a file
– Wintel: control-panel->network->configuration->tcp/ip>properties
– UNIX: /etc/rc.config
• Dynamically assigned: using DHCP (Dynamic Host
Configuration Protocol)
– dynamically get address from a server
– “plug-and-play”
Provide the forwarding table – a table containing the network prefix and
the outgoing interface.
Csci 183/183W/232: Computer
Networks
Forwarding and Routing
Network Layer Part I
9
DHCP: Dynamic Host Configuration Protocol
Goal: allow host to dynamically obtain its IP address
from network DHCP server when it joins network
DHCP overview:
Csci 183/183W/232: Computer
Networks
Forwarding and Routing
Network Layer Part I
223.1.1.2
B
223.1.1.4
223.1.2.9
223 1 2 2
223.1.2.2
223.1.3.1
11
10
223.1.2.1
DHCP
server
223.1.1.1
223.1.1.3
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
Forwarding and Routing
Network Layer Part I
DHCP Client-Server Scenario
A
Can renew its lease on address in use
Allows reuse of addresses (only hold address while connected as “on”)
Support for mobile users who want to join network (more shortly)
–
–
–
–
Csci 183/183W/232: Computer
Networks
Csci 183/183W/232: Computer
Networks
223.1.3.27
E
223.1.3.2
Forwarding and Routing
Network Layer Part I
arriving
i i DHCP
client needs
address in this
network
12
2
DHCP Client-Server Scenario
DHCP server: 223.1.2.5
DHCP discover
IP Addresses: How to Get One? …
arriving
client
Q: How does a network get network part of IP
addr?
A: gets an allocated portion of its provider
ISP’s address space
src : 0.0.0.0, 68
dest.: 255.255.255.255,67
yiaddr: 0.0.0.0
transaction ID: 654
DHCP offer
src: 223.1.2.5, 67
dest: 255.255.255.255, 68
yiaddrr: 223.1.2.4
transaction ID: 654
Lifetime: 3600 secs
DHCP requestt
time
src: 0.0.0.0, 68
dest:: 255.255.255.255, 67
yiaddrr: 223.1.2.4
transaction ID: 655
Lifetime: 3600 secs
DHCP ACK
src: 223.1.2.5, 67
dest: 255.255.255.255, 68
yiaddrr: 223.1.2.4
transaction ID: 655
Lifetime: 3600 secs
Csci 183/183W/232: Computer
Networks
Forwarding and Routing
Network Layer Part I
13
ISP's block
11001000 00010111 00010000 00000000
200.23.16.0/20
Organization 0
Organization 1
Organization 2
...
11001000 00010111 00010000 00000000
11001000 00010111 00010010 00000000
11001000 00010111 00010100 00000000
…..
….
200.23.16.0/23
200.23.18.0/23
200.23.20.0/23
….
Organization 7
11001000 00010111 00011110 00000000
200.23.30.0/23
Csci 183/183W/232: Computer
Networks
Forwarding and Routing
Network Layer Part I
14
NAT: Network Address Translation
IP Addressing: the Last Word...
Q: How does an ISP get block of addresses?
A: ICANN: Internet Corporation for
Assigned Names and Numbers
rest of
Internet
local network
(e.g., home network)
10.0.0/24
10.0.0.4
– allocates addresses
– manages DNS
– assigns domain names, resolves disputes
10.0.0.1
10.0.0.2
138 76 29 7
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)
10.0.0.0/8 has been reserved for private networks!
Csci 183/183W/232: Computer
Networks
Forwarding and Routing
Network Layer Part I
15
NAT: Network Address Translation
• Motivation: local network uses just one IP address as
far as outside world 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).
Csci 183/183W/232: Computer
Networks
Forwarding and Routing
Network Layer Part I
17
Csci 183/183W/232: Computer
Networks
Forwarding and Routing
Network Layer Part I
16
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,
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
Csci 183/183W/232: Computer
Networks
Forwarding and Routing
Network Layer Part I
18
3
NAT: Network Address Translation
NAT translation table
WAN side addr
LAN side addr
2: NAT router
changes datagram
source addr from
10.0.0.1, 3345 to
138.76.29.7, 5001,
updates table
2
138.76.29.7, 5001 10.0.0.1, 3345
……
……
S: 10.0.0.1, 3345
D: 128.119.40.186, 80
1
S: 138.76.29.7, 5001
D: 128.119.40.186, 80
S: 128.119.40.186, 80
D: 10.0.0.1, 3345
3
10 0 0 1
10.0.0.1
• NAT is controversial:
– routers should only process up to layer 3
– violates end-to-end argument
10.0.0.2
• NAT possibility must be taken into account by app
designers, eg, P2P applications
4
10.0.0.3
4: NAT router
changes datagram
dest addr from
138.76.29.7, 5001 to 10.0.0.1, 3345
3: Reply arrives
dest. address:
138.76.29.7, 5001
Csci 183/183W/232: Computer
Networks
• 16-bit port-number field:
– 60,000 simultaneous connections with a single LAN-side
address!
10.0.0.4
138.76.29.7
S: 128.119.40.186, 80
D: 138.76.29.7, 5001
NAT: Network Address Translation
1: host 10.0.0.1
sends datagram to
128.119.40, 80
Forwarding and Routing
Network Layer Part I
19
IP Forwarding & IP/ICMP Protocol
– address shortage should instead be solved by IPv6
Csci 183/183W/232: Computer
Networks
Forwarding and Routing
Network Layer Part I
20
IP Service Model and Datagram Forwarding
• Connectionless (datagram-based)
– Each datagram carries source and destination
• Best-effort delivery (unreliable service)
Transport layer: TCP, UDP
Network
N
t
k
layer
–
–
–
–
IP protocol
•addressing conventions
•packet handling conventions
Routing protocols
•path selection
•RIP, OSPF, BGP
• Forwarding
F r rdin and
nd IP address
ddr ss
routing
table
– forwarding based on network id
ICMP protocol
•error reporting
•router “signaling”
• Delivers packet to the appropriate network
• Once on destination network, direct delivery using host id
• IP destination-based next-hop forwarding paradigm
Data Link layer (Ethernet, WiFi, PPP, …)
– Each host/router has IP forwarding table
• Entries like <network prefix, next-hop, output interface>
Physical Layer (SONET, …)
Csci 183/183W/232: Computer
Networks
Forwarding and Routing
Network Layer Part I
21
IP Datagram Format
IP protocol version
number
header length
(32-bit words)
“type” of data
max number
remaining hops
(decremented at
each router))
32 bits
ver head. type of
len service
length
fragment
16-bit identifier flgs
offset
time to upper
Internet
layer
live
checksum
upper layer protocol
to deliver payload to
how much overhead
with TCP?
• 20 bytes of TCP
• 20 bytes of IP
• = 40 bytes + app
layer overhead
Csci 183/183W/232: Computer
Networks
packets may be lost
packets can be delivered out of order
duplicate copies of a packet may be delivered
packets can be delayed for a long time
Forwarding and Routing
Network Layer Part I
22
IP Datagram Forwarding Model
for
fragmentation/
reassembly
forwarding table in A
Dest. Net. next router Nhops
E.g. timestamp,
record route
taken, specify
list of routers
to visit.
223.1.1
223.1.2
223.1.3
IP datagram:
misc source dest
fields IP addr IP addr
32 bit source IP address
data
(variable length,
typically a TCP
or UDP segment)
Forwarding and Routing
Network Layer Part I
total datagram
length (bytes)
32 bit destination IP address
Options (if any)
Csci 183/183W/232: Computer
Networks
data
• datagram remains
unchanged, as it travels
source to destination
• addr fields of interest
here
A
B
Csci 183/183W/232: Computer
Networks
1
2
2
223.1.1.1
223.1.1.2
223.1.1.4
223.1.2.1
223.1.2.9
223.1.2.2
223.1.1.3
223.1.3.1
23
223.1.1.4
223.1.1.4
Forwarding and Routing
Network Layer Part I
223.1.3.27
E
223.1.3.2
24
4
Forwarding Table Lookup
using Longest Prefix Matching
IP Forwarding Table
4 billion possible entries!
(in reality, far less, but can still have millions of “routes”)
Prefix Match
forwarding table entry format
destination network
(1st IP address , network mask )
next-hop (IP address)
11001000 00010111 00010000 00000000,
11111111 11111111 11111000 00000000
200.23.16.1
0
11001000 00010111 00011000 00000000,
11111111 11111111 11111111 00000000
- (direct)
1
11001000 00010111 00011001 00000000,
11111111 11111111 11111000 00000000
200.23.25.6
2
Next Hop
11001000 00010111 00010
11001000 00010111 00011000
11001000 00010111 00011
otherwise
link interface
Link Interface
200.23.16.1
200.23.25.6
128.30.0.1
0
1
2
3
Examples
otherwise
128.30.0.1
Csci 183/183W/232: Computer
Networks
25
forwarding table in A
Dest. Net. next router Nhops
Starting at A, send IP
datagram addressed to B:
• look up
p 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
A
B
– B and A are directly connected
223.1.1.4
223.1.1.4
1
2
2
Starting at A, given IP
datagram addressed to B:
223.1.2.1
frame source,
dest address
223.1.2.9
223.1.2.2
223.1.1.3
223.1.3.27
223.1.3.1
Forwarding and Routing
Network Layer Part I
• look up net. address of B, find B
on same net. as A
• link layer send datagram to B
inside link-layer frame
223.1.1.1
223.1.1.2
223.1.1.4
Csci 183/183W/232: Computer
Networks
26
IP Datagram Forwarding on Same LAN:
Interaction of IP and data link layers
IP Forwarding: Destination in Same Net
223.1.1
223.1.2
223.1.3
Which interface?
DA: 11001000 00010111 00011000 10101010
3
Forwarding and Routing
Network Layer Part I
misc
data
fields 223.1.1.1 223.1.1.3
Which interface?
DA: 11001000 00010111 00010110 10100001
E
223.1.3.2
A
223.1.1.1
223.1.2.1
223.1.1.2
223.1.1.4 223.1.2.9
B
223.1.1.3
datagram source, 223.1.3.1
dest address
A’s IP
addr
B’s MAC A’s MAC
addr
addr
B’s IP
addr
223.1.2.2
223.1.3.27
E
223.1.3.2
IP payload
datagram
frame
Csci 183/183W/232: Computer
Networks
Forwarding and Routing
Network Layer Part I
27
Csci 183/183W/232: Computer
Networks
– fixed for each adaptor, burned in the adapter ROM
– MAC address allocation administered by IEEE
• 1st bit: 0 unicast, 1 multicast.
• all 1’s : broadcast
32-bit IP address:
• network-layer address, logical
–
i.e., not bound to any physical device, can be re-assigned
• IP hierarchical address NOT portable
– depends on IP network to which an interface is attached
– when move to another IP network, IP address re-assigned
• MAC flat address ->> portability
• used to get IP packets to destination IP network
• MAC addressing operations on a LAN:
• IP network is “virtual,” actually packet delivery done by the
underlying physical networks
– Recall how IP datagram forwarding is performed
– can move LAN card from one LAN to another
–
–
–
–
–
each adaptor on the LAN “sees” all frames
accept a frame if dest. MAC address matches its own MAC address
accept all broadcast (MAC= all1’s) frames
accept all frames if set in “promiscuous” mode
can configure to accept certain multicast addresses (first bit = 1)
Csci 183/183W/232: Computer
Networks
Forwarding and Routing
Network Layer Part I
28
MAC vs. IP Addresses
MAC (Physical) Addresses -- Revisited
• used to get frames from one interface to another physicallyconnected interface (same physical network, i.e., p2p or LAN)
• 48 bit MAC address (for most LANs)
Forwarding and Routing
Network Layer Part I
29
– from source host to destination host, hop-by-hop via IP routers
– over each link, different link layer protocol used, with its own frame
headers, and source and destination MAC addresses
• Underlying physical networks do not understand IP protocol and
datagram format!
Csci 183/183W/232: Computer
Networks
Forwarding and Routing
Network Layer Part I
30
5
ARP: Address Resolution Protocol
ARP Protocol
Question: how to determine • Each IP node (host, router)
on LAN has ARP table
MAC address of B
• ARP Table: IP/MAC address
knowing B’s IP address?
mappings for some LAN
nodes
< IP address; MAC address;
timer>
– timer: time after which
address mapping will be
forgotten (typically 15
min)
Csci 183/183W/232: Computer
Networks
Forwarding and Routing
Network Layer Part I
31
ARP Messages
• A wants to send datagram
to B, and A knows B’s IP
address.
• A looks up B’s MAC address
in its ARP table
pp
B’s MAC address
• Suppose
is not in A’s ARP table.
• A broadcasts (why?) ARP
query packet, containing
B's IP address
– all machines on LAN
receive ARP query
Csci 183/183W/232: Computer
Networks
• B receives ARP packet,
replies to A with its (B's)
MAC address
– frame sent to A’s MAC
address (unicast)
A caches (saves) IP-to-MAC
address pair in its ARP table
until information becomes old
(times out)
– soft state: information
that times out (goes away)
unless refreshed
•
• ARP is “plug-and-play”:
– nodes create their ARP
tables without
intervention from net
administrator
Forwarding and Routing
Network Layer Part I
32
ARP Request & Response Processing
• The requester broadcasts ARP request
• The target node unicasts (why?) ARP reply to
requester
– With its physical address
– Adds the requester into its ARP table (why?)
• On receiving
g the response,
p
, requester
q
– updates its table, sets timer
• Other nodes upon receiving the ARP request
– Refresh the requester entry if already there
– No action otherwise (why?)
Hardware Address Type: e.g., Ethernet
Protocol address Type: e.g., IP
Operation: ARP request or ARP response
Csci 183/183W/232: Computer
Networks
Forwarding and Routing
Network Layer Part I
• Some questions to think about:
– Shall requester buffer IP datagram while performing ARP?
– What shall requester do if never receive any ARP response?
33
Csci 183/183W/232: Computer
Networks
Forwarding and Routing
Network Layer Part I
34
IP Forwarding: Destination in Diff. Net
ARP Operation Illustration
misc
data
fields 223.1.1.1 223.1.2.3
forwarding table in A
Dest. Net. next router Nhops
223.1.1
1
223.1.2
223.1.1.4
2
223.1.3
223.1.1.4
2
Starting at A, dest. E:
• look up network address of E
in forwarding table
• E on different network
A
223.1.1.1
– A,
A E not
n t di
directly
tl attached
tt h d
• 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…..
Csci 183/183W/232: Computer
Networks
Forwarding and Routing
Network Layer Part I
35
Csci 183/183W/232: Computer
Networks
B
223.1.1.2
223.1.1.4
223.1.1.3
223.1.3.1
Forwarding and Routing
Network Layer Part I
223.1.2.1
223.1.2.9
223.1.2.2
223.1.3.27
E
223.1.3.2
36
6
Forwarding to Another LAN:
IP Forwarding: Destination in Diff. Net …
forwarding table in router
misc
data
fields 223.1.1.1 223.1.2.3
Dest. Net router Nhops interface
223.1.1
223.1.2
223.1.3
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
t
k as router’s
t ’
interface 223.1.2.9
A
1
1
1
223.1.1.4
223.1.2.9
– router, E directly attached
B
walkthrough: send datagram from A to B via R
assume A knows B IP address
223.1.3.27
223.1.1.1
A
223.1.2.1
• 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!)
Csci 183/183W/232: Computer
Networks
-
Interaction of IP and Data Link Layer
223.1.1.2
223.1.1.4
223.1.2.9
223.1.2.2
223.1.1.3
223.1.3.27
223.1.3.1
•
•
•
223.1.3.2
Forwarding and Routing
Network Layer Part I
R
E
37
B
Two ARP tables in router R, one for each IP network (LAN)
In routing table at source host, find router 111.111.111.110
In ARP table at source, find MAC address E6-E9-00-17-BB-4B, etc
Csci 183/183W/232: Computer
Networks
Forwarding and Routing
Network Layer Part I
38
IP Datagram Format Again
A
R
IP protocol version
number
header length
(bytes)
“type” of data
B
max number
remaining hops
(decremented at
each router))
• A creates datagram with source A, destination B
• A uses ARP to get R’s MAC address for 111.111.111.110
• A creates link-layer frame with R's MAC address as dest,
frame contains A-to-B IP datagram
• A’s data link layer sends frame
• R’s data link layer receives frame
• R removes IP datagram from Ethernet frame, sees its
destined to B
• R uses ARP to get B’s physical layer address
• R creates frame containing A-to-B IP datagram sends to B
Csci 183/183W/232: Computer
Networks
Forwarding and Routing
Network Layer Part I
how much overhead
with TCP?
• 20 bytes of TCP
• 20 bytes of IP
• = 40 bytes + app
layer overhead
39
Fields in IP Datagram
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.
Forwarding and Routing
Network Layer Part I
• network links have MTU
(maximum transmission
unit) - largest possible data
gram.
– different link types,
different MTUs
• large IP datagram divided
(“f
(“fragmented”)
d”) within
i hi net
– one datagram becomes
several datagrams
– “reassembled” only at
final destination
– IP header bits used to
identify, order related
fragments
3-bit priority,e.g, delay, throughput, reliability bits, …
Forwarding and Routing
Network Layer Part I
Csci 183/183W/232: Computer
Networks
total datagram
length (bytes)
40
IP Fragmentation & Reassembly: Why
• Total length: including header (maximum 65535 bytes)
• Identification: all fragments of a packet have same
identification
• Flags: don’t fragment, more fragments
• Fragment offset: where in the original packet (count in 8
byte units)
• Time to live: maximum life time of a packet
• Protocol Type: e.g., ICMP, TCP, UDP etc
• IP Option: non-default processing, e.g., IP source routing
option, etc.
Csci 183/183W/232: Computer
Networks
length
fragment
16-bit identifier flgs
offset
time to upper
Internet
layer
live
checksum
upper layer protocol
to deliver payload to
• IP protocol version: current version is 4, IPv4, new: IPv6
• Header length: number of 32-bit words in the header
• Type of Service:
–
32 bits
ver head. type of
len service
41
Csci 183/183W/232: Computer
Networks
fragmentation:
in: one large datagram
out: 3 smaller datagrams
reassembly
Forwarding and Routing
Network Layer Part I
42
7
IP Fragmentation and Reassembly: Exp
IP Fragmentation & Reassembly: How
• An IP datagram is chopped by a router into smaller pieces if
– datagram size is greater than network MTU
– Don’t fragment option is not set
Example
• 4000 byte
datagram
• MTU = 1500 bytes
• Each datagram has unique datagram identification
– Generated by source hosts
– All fragments of a packet carry original datagram id
• All fragments except the last have more flag set
• Fragments of IP packet can be further fragmented by other
routers along the way to destination !
• Reassembly only done at destination host (why?)
length ID fragflag offset
=1500 =x
=1
=185
length ID fragflag offset
=1040 =x
=0
=370
– Use IP datagram id, fragment offset, fragment flags. Length
Forwarding and Routing
Network Layer Part I
43
Csci 183/183W/232: Computer
Networks
Csci 183/183W/232: Computer
Networks
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
Forwarding and Routing
Network Layer Part I
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
45
Forwarding and Routing
Network Layer Part I
44
ICMP Message Transport & Usage
ICMP: Internet Control Message Protocol
• used by hosts, routers,
gateways 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
One large datagram becomes
several smaller datagrams
length ID fragflag offset
=1500 =x
=1
=0
– Fragment offset and Length fields are modified appropriately
Csci 183/183W/232: Computer
Networks
length ID fragflag offset
=4000 =x
=0
=0
• ICMP messages carried in IP datagrams
• Treated like any other datagrams
– But no error message sent if ICMP message causes error
• Message sent to the source
– 8 bytes of the original header included
• ICMP Usage
U
(non-error,
(
i f
informational):
ti
l) E
Examples
l
– Testing reachability: ICMP echo request/reply
• ping
– Tracing route to a destination: Time-to-live field
• traceroute
– Path MTU discovery
• Don’t fragment bit
–
IP direct (for hosts only): inform hosts of better routes
Csci 183/183W/232: Computer
Networks
Forwarding and Routing
Network Layer Part I
46
8