IP Routing Table (Linux)
Dest
Gateway
144.16.64.0
0.0.0.0
127.0.0.0
0.0.0.0
0.0.0.0
144.16.64.1
Genmask
Flags
Intfc
255.255.224.0
U
eth0
255.0.0.0
U
lo
0.0.0.0
UG
eth0
The routing table mask
IP Routing Tables

In Linux, a Gateway entry of 0.0.0.0 indicates
that the destination is directly connected

Also, a Genmask of 0.0.0.0 and a destination of
0.0.0.0 indicate a default entry
– Any “dest addr” & 0.0.0.0 = 0.0.0.0
– So any dest addr matches this entry
– Hence “default entry”
IP Routing Tables

The “Destination” column in the routing table is
better called network prefix

Stores the pattern against which a match of
packet D.A & genmask
is attempted

Possible for
packet D.A & genmask
to match multiple entries in col network prefix?
IP Routing Tables

Longest prefix match

Most specific match determines how the packet
will be handled

Presence of the H flag …
network prefix is 32 bits long
Most specific match possible
Genmask = 255.255.255.255
Routing Table Masks

The routing table was that of a machine whose
interface address is 144.16.68.2 and subnet
mask is 255.255.224.0

1st row: R.T.mask = subnet mask

2nd and 3rd rows: R.T.mask not equal to subnet
mask
Broadcast addresses

What is the limited broadcast address that
machine X will use?
– 255.255.255.255

What is the subnet-directed broadcast address
that machine X will use?
– 144.16.95.255
Link layer
Number of hosts accessible
via the link
One
(Serial links)
More than one
(Ethernet, FDDI, …)
Ethernet
Data (IP, ARP, RARP …)
Dest addr (6 bytes) Source addr (6 bytes) Type
10 Mb/s, 100 Mb/s, 1 Gb/s
CRC
Ethernet
IP
RARP
ARP
D.A.
S.A.
Type
CRC
IEEE 802.3
The type field is part of the additional 8-byte field
IP
RARP
ARP
D.A.
S.A.
CRC
Length
8 bytes
Ethernet/802.3 addresses

48-bit addresses

24 out of the 48 bits are obtained from IEEE
– IEEE: Official global authority for LAN addresses

These 24 bits: Organizationally Unique Identifier
(OUI)

Radia Perlman, “Interconnections”
Ethernet/802.3 addresses

1 bit out of the 24 OUI bits: Group/Individual bit
– 1: Address refers to a logical group of stations (ex:
multicast Ethernet addresses)
– 0: Address refers to a particular station

Another bit out of the 24 bits: Local/global bit
– 0: The address is globally unique
– 1: The address is locally assigned, no guarantee of
global uniqueness
Framing

How to distinguish between successive
frames?

Where does a frame start and where does
it end?
– Special characters as delimiters?
– Anything else?
Ethernet frame
Header
Data
Trailer
Medium
In Ethernet, gaps between frames act as delimiters…inter-frame gap
How would a station detect the beginning of the “gap”?
Drop in power level in the medium
Frame Length

Fixed or variable length?
– Ethernet frames are of variable length
– Minimum: 64 bytes (including 14 bytes of header and
4 bytes of CRC)
– Maximum: 1518 bytes
– Maximum Transmission Unit (MTU): 1500 bytes

How can we know the length of a frame?
Length

If the beginning and end of the frame can be
identified (delimiters), then the frame can be
parsed properly
Data
From the beginning of the frame: first 6 bytes -- D.A.,
next 6 bytes -- S.A. etc;
From the end of the frame: last 4 bytes -- CRC
Error Detection

How to handle errors on the link?
– Error detection
– CRC: Cyclic Redundancy Check

“Reliable” link or “unreliable” link
– Error detection is the first step
– If reliable link: sender retransmits
Reliability & Flow Control
– ARQ (Automatic Repeat Request)
– Acks / Nacks must be sent back to the sender
– Ethernet: no reliability

Flow Control
– Receiver stops a fast sender
– IEEE 802.3 defines a PAUSE frame for this
CRC

Cyclic Redundancy Check Code

Simple parity check codes
– Odd and even parity; modulo-2 sum

Horizontal and vertical parity check code
Row parity bit
Column parity bit
CRC

Can generalize the horizontal and vertical check
code to a general parity check code
– Parity bits are calculated on subsets of the data bits

In a CRC, the string of data bits and the code
bits are viewed as polynomials
– Data string = 1101, poly = (D^3 + D^2 + 1)
CRC

New entity: generator polynomial

L-bit CRC

Generator polynomial g(D) : of degree L in the
variable D

s(D): data polynomial
CRC

To get the CRC polynomial c(D)
– Obtain s(D)*(D^L)
– Divide s(D)*(D^L) by g(D)
– c(D) = Remainder when s(D)*(D^L) is divided by g(D)

“Data Networks” – Bertsekas and Gallager
– Chap 2
ARP
“Protocol address”
“Hardware address”
Ex: IP address
Ex: Ethernet address
RFC 826
ARP
ARP
S.A.
D.A.
Type
CRC
Sender h/w
Sender
proto
Target h/w
Target
proto
ARP header

Hardware type (2 octets)
– Value =1 for Ethernet

Protocol type (2 octets)
– Value = 0x0800 for IP

Hardware address size in bytes (1 octet)
– Value = 6 for Ethernet

Protocol address size in bytes (1 octet)
– Value = 4 for IP
ARP header

Opcode
– ARP request
– ARP reply
– RARP request
– RARP reply
ARP cache

Before issuing an ARP request, a machine
always checks its ARP cache to see if the desired
hardware address is present

If no such address mapping is found, then the
ARP request is issued

Timeout for cache entries

arp –a shows all the ARP cache entries