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Internet Networking - 1- 2001. Hae-Kwang Kim Sejong University Internet Addresses A 0 7 bits netid 24 bits hostid 0 .0.0.0 to 127 .255.255.255 B 1 0 14 bits netid 16 bits hostid 128 .0.0.0 to 191 .255.255.255 C 1 1 0 21 bits netid 8 bits hostid

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2001 hae kwang kim sejong university

Internet Networking

- 1-

2001.

Hae-Kwang Kim

Sejong University

internet addresses
Internet Addresses

A

0

7 bits netid

24 bits hostid

0.0.0.0to 127.255.255.255

B

1 0

14 bits netid

16 bits hostid

128.0.0.0to 191.255.255.255

C

1 1 0

21 bits netid

8 bits hostid

192.0.0.0to 223.255.255.255

D

1 1 1 0

28 bits multicast groupid

224.0.0.0to 239.255.255.255

E

1 1 1 1

28 bits reserved

240.0.0.0to 255.255.255.255

encapsulation
Encapsulation

user data

Appl. header

user data

TCP header

Application data

IP header

TCP header

Application data

Ethernet header

IP header

TCP header

Application data

Ethernet tailer

header info for demultiplexing
Header info for demultiplexing
  • 8bit protocol field in IP header
    • 1: ICMP, 2: IGMP, 6: TCP, 17: UDP
  • 16bit port number for TCP/UDP header
    • identify applications
  • 16bit frame type fled in Ethernet header
    • identify IP, ARP, RARP
demultiplexing
Demultiplexing

User process

User process

User process

User process

TCP

UDP

ICMP

IGMP

IP

ARP

RARP

Ethernet drive

An Ethernet frame

client server model
Client-Server Model
  • Concurrent Server (in general, TCP server)

1. Wait for a client request to arrive

2. Start a new server to handle this client’s request

new process, task, thread

3. the new server handles this client’s request

4. When complete, the new server terminates

  • Iterative server (in general, UDP server)
    • no multiple concurrent clients
port numbers
Port numbers
  • Application identification
  • Well known port numbers
    • FTP server: 21
    • Telnet server: 23
    • TFTP server: 69
  • IANA (Internet Assigned Numbers Authority)
    • between 1 to 1023
    • Unix specific services 256:1023
    • Telnet vs. Rlogin
  • Client don’t care port numbers
    • unique on the host
    • ephemeral ports (1024 - 5000)
      • * solaris 2.2: start at 32768
port number repository on unix
Port number repository on Unix
  • File
    • /etc/services
  • grep telnet /etc/services
    • telnet 23/tcp
  • grep domain /etc/services

* reserved ports: 1-1023

    • only used by process with superuser privilege
internet standard organization
Internet standard Organization
  • ISOC
  • IAB
    • 15 members
    • final editorial and technical review board
  • IETF
    • develop standard specifications
  • IRTF
    • long-term research projects
slide10
RFCs
  • Official standards and information purposes
  • RFC index
    • replacement or update by a newer RFC
  • Important RFCs
    • The assigned Numbers RFC (1340)
    • Internet Official Protocol Standards RFC (1600)
      • state of standardization: standard, draft standard, proposed standard, experimental, informational, historic
      • requirement level: required, recommended, elective, limited user or not recommended
    • Host Requirements RFC (1122/1123)
      • link, network, transport, application layers
    • Router requirements RFC (1009)
standard simple services
Standard simple services
  • Why Odd numbers for port number?
    • NCP used pair of odd-even connections
  • echo (port number 7)
  • discard (port number 9)
  • daytime (port number 13)
  • chargen (port number 19)
  • time (port number 37)
tcp ip implementations
TCP/IP implementations
  • UC at Berkely
  • API
    • sockets (“Berkely Sockets”)
    • TLI (Transport Layer Interface) - AT&T
link layer
Link-Layer
  • Conveys
    • IP datagrams
    • ARP/RARP requests/replies
  • many types of networking hardware
    • token ring, FDDI, RS-232
    • Ethernet
    • serial interfaces (SLIP and PPP)
    • loopback driver
ethernet and ieee802
Ethernet and IEEE802
  • 1982 by DEC, Intel and Xerox
  • CSMA/CD (Carrier Sense, Multiple Access with Collision Detection)
    • 10 Mbits/sec
    • 48-bit addresses
  • IEEE 802
    • 802.3 (CSMA/CD), 802.4(token bus), 802.5 (token ring), 802.2 (LLC)
    • different frame format from Ethernet
host requirements rfc for ethernet 10mbits sec
Host Requirements RFC for Ethernet 10Mbits/sec
  • Send and receive packets using RFC 894 (Ethernet encapsulation)
  • Receive RFC 1042 (IEEE 802) packets intermixed with RFC 894 packets
  • Send packets using RFC 1042 encapsulation
encapsulation 802 2 802 3 rfc 1042
Encapsulation (802.2/802.3) RFC 1042

Dest. address

Source address

length

802.3 MAC

DSAP AA

SSAP AA

Cntr 03

802.2 LLC

Org code 00

Type

data

CRC

802.2 SNAP

Type

IP datagram

Type

ARP request/reply

PAD

Type

RARP request/reply

PAD

encapsulation ethernet rfc 894
Encapsulation (Ethernet) RFC 894

Dest. address

Source address

length

802.3 MAC

Type

data

CRC

Type

IP datagram

Type

ARP request/reply

PAD

Type

RARP request/reply

PAD

ieee 802 vs ethernet
IEEE 802 vs Ethernet
  • 802.3 allows 16-bit addresses
    • hardware address
  • ARP/RARP
    • map between 32-bit IP address / 48-bit address
  • non of the 802 length values is the same as the Ethernet type values
  • data size
    • 802: 38-1492 bytes
    • Ethernet: 46-1500 bytes
trailer encapsulation 893
Trailer encapsulation (893)
  • Rearrange the order of the fields in the IP datagram
    • variable-length fields (IP header and the TCP header) were moved to the end, right before CRC
    • data portion of the frame to be mapped to a hardware page, saving a memory-to-memory copy when the data is copied in the kernel
    • TCP data that is a multiple of 512 bytes in size can be moved by just manipulating the kernel’s page tables
    • Two hosts negotiated the use of trailer encapsulation using an extension of ARP
    • Different Ethernet frame type values are defined for these frames
    • Deprecated
slip serial line ip rfc 1055
SLIP: Serial Line IP (RFC 1055)
  • Simple form of encapsulation for IP datagrams
    • Connecting Home systems to Internet
    • RS-232, high-speed modems
  • SLIP framing rules
    • IP datagram is started and terminated by the special character END (0xc0)
    • If a byte of the IP datagram equals the END character,
      • 2 byte sequence 0xdb (SLIP ESC character), 0xdc is transmitted instead
    • If a byte of the IP datagram equals the SLIP ESC character, the 2-byte sequence 0xdb, 0xdd is transmitted instead
difficiency of slip
Difficiency of SLIP
  • Each end must know the other’s IP address
  • No type field
  • No checksum
    • upper layers provide some form of CRC
    • always a checksum for the IP header, TCP header and TCP data
    • newer modems can detect and correct corrupted frames
  • popular as the speed and reliability of modems increase
compressed slip
Compressed SLIP
  • SLIP is
    • slow (19200 bits/sec below)
    • used for interactive traffic (Telnet, Rlogin)
    • many small TCP packets
    • To carry on3 byte of data, a 20-byte IP header and a 20-byte TCP header
  • CSLIP (Newer version): RFC 1144
    • 3 or head 5 bytes er
    • maintains the stae of up to 16 tCP connections on each end of the CSLIP link
    • some of the fields in the two headers for a given conection normally don’t change
    • Of the fileds that do change, most change by a small positive amount
ppp point to point protocol
PPP: Point-to-Point Protocol
  • Two kind of links
    • an asynchronous link with 8 bits of data and no parity
    • bit-oriented synchronous links
  • Link Control Protocol
    • establish, configure and test the data-link connection
    • each end negotiate various options
  • Family of network control protocols (NCPs)
    • specific to different network layer protocols (RFCs for IP, OSI network layer, DECnet and AppleTalk)
    • IP NCP: allows each end to specify if it can perform header compression)
ppp encapsulation rfc 1548
PPP encapsulation (RFC 1548)

flag 7E

Address FF

Control 03

802.3 MAC

protocol

information

CRC

flag 7E

Protocol 0021

IP datagram

Protocol c021

Link control data

Protocol 8021

Network control data

escaping for flag code 0x7e
Escaping for flag code, 0x7e
  • Synchronous link
    • done by hardware using bit stuffing
  • Asynchronous link
    • 0x7d is used for escape character
    • when 0x7d appears in a PPP frame, the character has had its sixth bit complemented
      • 0x7e is transmitted by 0x7d, 0x5e
      • 0x7d is transmitted by 0x7d, 0x5d
      • for ASCII control character, the sixth bit is turned on, for example, 0x01 is transmitted by 0x7d, 0x21
      • it’s possible touse the link control protocol to specify which, if any, of these 32 values must be escaped
slip enhancement
SLIP- enhancement
  • Using Link control protocol
    • negotiate to omit the constant address and control fields and to reduce the protocol field form 2 bytes to 1 byte.
  • PPP overhead 3 bytes
    • 1 byte for the protocol field and 2 bytes for the CRC
    • SLIP 2 bytes
  • Using IP network control protocol,
    • negotiate to use Van Jacobson header compression
advantage of ppp over slip
Advantage of PPP over SLIP
  • Support for multiple protocols on a single serial line
  • CRC on every frame
  • Dynamic negotiation of the IP address for each end (using the IP network control protocol)
  • TCP/IP header compression
  • a link control protocol for negotiating many data-link options
  • The price
    • 3 bytes of additional overhead per frame
    • a few frames of negotiation when the link is established
    • more complex implementation
loopback interface
Loopback interface
  • Allows a client and server on the same host to communicate with each other using TCP/IP
  • The class A network ID 127 is reserved for the loopback interface
    • IP address of 127.0.0.1 to this interface (local host)
    • An IP datagram sent to the loopback interface must not appear on any network
  • No short circuiting some of the transport layer logic and all of the network layer logic
    • complete processing of the data in the transport layer and network layer
    • seems inefficient, simplifies the design considering the loopback interface appears as just another link layer
processing of ip datagrams by loopback interface
Processing of IP datagrams by loopback interface

IP input function

IP output function

Destination IP address equal broadcast address or multicast address?

Place on IP input queue

Place on IP input queue

yes

no

yes

Loop back driver

Destination IP address equal interface IP address?

no

ARP

Demultiplex based on Ethernet frame type

send

recieve

mtu maximum transmission unit
MTU (Maximum transmission unit)
  • Ethernet (1500), IEEE 802 (1492)
  • If IP datagram is larger than the MTU
    • fragmentation
  • Path MTU
    • smallest MTU of any data link that packets traverse between the two hosts
      • depends on route being used at any time
      • path MTU need not be the same in the two directions
serial line throughput calculations
Serial Line Throughput Calculations
  • Line speed: 9600 bits/sec, 8 bits/byte, 1 start and 1 stop bits
    • line speed is 960 bytes/sec
    • transferring a 1024-byte packet takes 1066ms
    • with SLIP for an interactive application, along with an FTP that sends or receives 1024-byte packets, should wait on average 533ms to send interactive packets
    • type-of-service queueing: place interactive traffic ahead of bulk data traffic
  • an interactive response time longer than 100-200 ms is bad
    • round-trip time for a packet to be sent and response be returned ( normally a character echo)
serial line throughput calculations32
Serial Line Throughput Calculations
  • Reducing MTU of the SLIPO link to 256
    • 133ms wait: half reducing
    • not perfect but good for bulk data transfer
  • Assuming 5-byte CSLIP header, 256 bytes of data
    • 98.1% of the line to data and 1.9% to headers
    • reducing MTU below 256 reduces the maximum throughput for bulk data transfers
  • MTU is a value that IP queries the link layer for
    • must include the normal TCP and IP headers
    • This is how IP makes its framentation decision
      • IP knows nothing about the header compression that CSLIP performs
serial line throughput calculations33
Serial Line Throughput Calculations
  • When only interactive traffic is being exchanged
    • 1 byte of data in each direction (assuming 5-byte compressed headers) takes around 12.5 ms for the round trip at 9600 bits/sec
    • compressing the headers from 40 bytes to 5 bytes reduces the round-trip time for the 1 byte of data from 85 to 12.5ms
  • For newer error correcting, compressing modems, difficult to calculate
    • the number of bytes sent over the network reduced
    • error correction may increase the amount of time to transfer these bytes
ip rfc 791
IP (RFC 791)
  • TCP, UDP, ICMP, IGMP dta gets transmitted as IP datagrams
  • an unreliable, delivery service
    • Simple error handling algorithm
      • throw away the datagram and send an ICMP message back to the source
      • any required reliability should be provided by TCP
  • connectionless datagram
    • out of order delivery
    • each datagram may follow different route
ip header
IP Header

32 (LSB)

0 (MSB)

4-bit version

4-bit header length

8-bit type of service (TOS)

16-bit total length (in bytes)

16-bit identification

3-bit flags

13-bit fragment offset

8-bit protocol

16-bit jeader checksum

8-bit time to live (TTL)

32-bit source IP address

32-bit destination IP address

Options (if any)

data

ip header36
IP header
  • Big endian (Network byte order)
    • Most signficant byte: first transmission
  • TOS: Minimize delay, Maximize throughput, Maximize reliability, Minimize monetary cost: only one bit can be turned on
    • not supported by most TCP/IP implementations
    • new routing protocols OSPF and IS-IS are based on this field
    • SLIP drivers provide type-of-service queueing, allowing interactive traffic to be handled before bulk data
      • it looks the protocol field to see if it’s a TCP segment and then checks the source and destination TCP port number to see if it’s for interactive service
ip header37
IP header
  • Big endian (Network byte order)
    • Most signficant byte: first transmission
  • TOS: Minimize delay, Maximize throughput, Maximize reliability, Minimize monetary cost: only one bit can be turned on
    • not supported by most TCP/IP implementations
    • new routing protocols OSPF and IS-IS are based on this field
    • SLIP drivers provide type-of-service queueing, allowing interactive traffic to be handled before bulk data
      • it looks the protocol field to see if it’s a TCP segment and then checks the source and destination TCP port number to see if it’s for interactive service
recommended values for type of service field
Recommended values for type-of-service field
  • Telnet and Rlogin: minimum delay
  • FTP: maximum through put
  • SNMP: maximum reliability
  • NNTP: minimize monetary cost
  • ICMP: no setting
ip header39
IP Header
  • Maximum size of IP datagram: 65535 bytes
    • most data link layer fragment this
  • a host is not required to receive a datagram larger than 576 bytes
  • With UDP, numerous applications (RIP, TFTP, BOOTP,DNS, SNMP) limit to 512 bytes of user data
  • Most implementations (especially NFS allow for just over 8192-byte IP datagrams)
  • Some data links pad small frames to be a minimum length (Ethernet: 46 bytes)
    • total length enable to guess about what portion of Ethernet frame actually is IP datagram when the IP datagram is smaller than 46 bytes
ip header40
IP Header
  • Identification
    • uniquely identifies each datgram sent by a host
    • increments by one each time a datagram is sent
    • used for fragmentation and reassembly with flags and fragmentation offset
  • TTL
    • upper limit on the number of routers through which a datagram passes
    • decremented by on by every router
    • when reaches to 0, the datagram is thrown away and the sender is notified with ICMP message
header checksum
Header Checksum
  • Same checksum for ICMP, IGMP, UDP, TCP, IP
  • Checksum computing
    • the checksum = 0
    • 16-bit one’s complement sum of the header
    • receiver verifies all one-bit checksum
  • IP discards the datagram, no error-message
  • a router often changes only the TTL filed
    • incrementally upodate the checksum without recalculating
options variable length list of optional information
Options (variable-length list of optional information)
  • Security and handling restrictions
  • record rout
  • timestamp
  • loose source routing
  • strict source routing
  • always ends on a 32-bit boundary
    • IP header is always a multiple of 32 bits
ip routing
IP routing
  • When the destination is directed connected to the host or on a shared network
    • the IP datagram is sent directly to the destination
  • Otherwise
    • the host sends the datagram to a default router which will deliver the datagram to its destination
    • the host can be itself a router
  • A host embedding a router never forward datagram unless it is configured to to so
  • the IP layer has a routing table in memory that it searches each time it receives a datagram to send
  • When IP layer receives a datagram, if it contains its address or broadcasting address, it is sent to the protocol module in the protocol field, else the datagram is forwarded if configured to act as a router
entry of routing table
Entry of routing table
  • Destination address
    • complete host address (non-zero hostid) or network address (hostid 0 depending on the flag
  • IP address of a next-hop router or of a directly connected network
  • Flags
    • if destination address is host address or network address
    • if next-hop router is real next-hop router or a directly connected interface
  • Specification of which network interface the datagram should be passed to for transmission
  • Assumption
    • the next-hop router is closer to the destination than the sending host and the next-hop router is directly connected to the sending host
ip routing action
IP routing action

1. Search the complete destination IP address (networkid and hostid) in the routing table (RT)

    • if found, send the packet to the indicated next-hop router or to the directly connected interface, point-to-point links

2. Search the destination network IP address (networkid) in the RT

    • if found, send the packet to the indicated next-hop router or to the directly connected interface
    • all the hosts on the destination network can be handled
    • must take into accout a possible subnet mask

3. Search the routing table for an entry labeled “default”, send the packet to the indicated next-hop router

  • if non-of these is successful, undeliverable message “host unreachable”, “network unreachable” ICMP message to the sending application
  • Default routes, along with the ICMP redirect message sent by a next-hop router, when forwarding fails
ip routing example from bsdi to sun
IP routing example: from bsdi to sun

Destination network = 140.252.13.0

bsdi

sun

.13.15

.13.33

Ethernet IP = 140.252.13

IP hdr

Link hdr

Destination IP = 140.252.13.33

Destination Ethernet of 140.252.13.33

slide47

Link hdr

IP hdr

bsdi

Ethernet IP = 140.252.1

.1.183

Next hop = 140.252.1.4 (default)

netb

modem

SLIP

IP hdr

Destination IP = 192.48.96.9

modem

.1.29

Next hop = 140.252.1.183 (default)

bsdi

sun

.13.15

.13.33

Ethernet IP = 140.252.13

IP hdr

Link hdr

Destination IP = 192.48.96.9

Destination Ethernet of 140.252.13.33

subnet addressing
Subnet addressing
  • Host ID portion is divided into a subnet ID and a host ID (too many hostids for a network)
  • local system administrator decide to subnet or not
  • Class B IP address example

Netid=140.252

8-bit hostid

8-bit subnetid

  • Allows 254 subnets, with 254 hosts per subnet
  • Subnetting hides the details of internal network organization
  • reduces the size of the Internet’s routing tables
    • only one routing table for all the subnetworks
slide49

.57.0

192.68.189.0

.82.0

R57

.52.0

.53.0

.54.0

.55.0

.58.0

.60.0

R192

R82

R52

R53

R54

R55

R58

R60

KP

.51.0

.81.0

140.252.104.1 Internet

aix

GATE

solaris

.1.4

.1.0

.1.92

.1.32

.1.11

.1.183

R2

R3

gem

R4

R6

R7

R8

R10

netb

.3.54

.2.0

.3.0

.4.0

.6.0

.7.0

.8.0

.9.0

.10.0

.11.0

.1.29

.13.65

.13.66

slip

bsd

svr

sun

R12

.13.35

.13.36

.13.0

.12.0

subnet mask
Subnet mask
  • When host bootstraps
    • ip address, subnet mask is configured; 0xffffff00 = 255.255.255.0
  • given its own IP Address and its subnet mask, a host know if a datagram is destined for
    • a host on its own subnet
    • a host on a different subnet on its own network
    • a host on a different network

1111111111111111 (networkid)

00000000 (hostid)

11111111 (subnetid)

subnet mask example
Subnet mask example
  • Assumption
    • Host address is 140.252.1.1 (class B)
    • subnet mask is 255.255.255.0
  • Which network?
    • destination IP address is 140.252.4.5
    • destination IP address is 140.252.1.22
    • destination IP address is 192.43.235.6
a subnet example variable length subnets
A subnet example (variable length subnets)

140.252.104.1

gateway

.4

Ethernet subnet 140.252.1

140.252.1.29

SLIP subnet 140.252.13.64

SLIP

bsdi

sun

sun

.35

.66

.35

.33

.34

Ethernet subnet 140.252.13.32

a subnet example variable length subnets54
A subnet example (variable length subnets)

1111111111111111 (networkid)

00000 (hostid)

11111111 111 (subnetid)

Subnet mask = 0xffffffe0 = 255.255.255.224

ifconfig command
Ifconfig command
  • Configure or query a network interfacer for use by TCP/IP
  • normally run at bootstrap time to configure each interface on a host
  • for SLIP links, ifconfig should run everytime the link is brought up or down

sun % /user/etc/ifconfig -a

le0: flags = 63<UP,BROADCAST, NOTRALIERS, RUNNING>

inet 140.252.13.33 netmask ffffffe0 broadcast 140.252.13.63

s10: flags = 1051<UP,POINTTOPOINT,, RUNNING, LINK0>

inet 140.252.1.29 --> 140.252.1.83 netmask ffffff00

lo0: flags = 49 <UP, LOOPBACK,, RUNNING>

inet 127.0.0.1 netmask ff000000

netstat command
Netstat command
  • Provides informatin about the interfaces on a system
  • - i flag: prints the interface information
  • - n flag: IP addresses instead of hostnames
  • sun % netstat -in
ip futures
IP futures
  • Short of IP addresses
  • flat routing structure
    • one routing table entry for each network
      • CIDR (Classless Interdomain routing)
  • IPng (IPv6)
    • 64 bit address, etc.
arp address resolution protocol rfc 826
ARP (Address Resolution Protocol) RFC 826
  • Address resolution
    • A mapping between the two different forms of addresses
  • ARP
    • 32 bit IP address -> 48 bit Ethernet
  • RARP
    • 48-bit Ethernet address -> 32-bit IP address
arp procedure
ARP Procedure
  • ARP is intended for broadcast networks
  • ARP sends an broadcast Ethernet frame called an ARP request containing the IP address of the destination host
  • The host corresponding the IP address replies its IP and hardware address with ARP reply frame
arp cache
ARP cache
  • Maintenance of an ARP cache on each host
    • the recent mapping fro IP addresses to hardware addresses
    • normal expiration time of an entry in the cache is 20 minutes
  • arp command

bsdi % arp -a

sun (140.252.13.33) at 8:0:20:3:f6:42

arp packet format
ARP packet format

Ehternet (6) destinaton addr

Ehternet source addr (6)

Frame type (2)

Hard type (2)

Proto type (2)

Hard size (1)

Proto size (1)

Op (2)

Sender Ehternet addr (6)

sender IP addr (4)

target Ehternet addr (6)

target IP addr (4)

arp examples
ARP examples

bsdi % arp -a

bsdi % telnet svr4.discard

Trying 140.252.13.34…

connected to svr4.

Escape character is ‘^]’.

^]

telnet> quit

connection closed

arp examples63
ARP examples

Sun % tcpdump -e

1. 0.0 0:0:c0:6f:2d:40 ff:ff:ff:ff:ff:ff arp 60:

arp who-has svr4 tell bsdi

2. 0.002174 (0.0022) 0:0:c0:c2:9b:26 0:0:c0:6f:2d:40 arp 60:

arp reply svr4 is-at 0:0:c0:c2:9b:26:

3. 0.002831 (0.007) 0:0:c0:6f:2d:40 0:0:c0:c2:9b:26 ip 60

bsdi.1030 > svr4.discard: S 596459521: 596459521(0)

win 4096 <mss 1024> [tos 0x10]

4. 0.007834 (0.0050) 0:0:c0:c2:9b:26 0:0:c0:6f:2d:40 ip 60

svr4.discard > bsdi.1030 : S 3562228255: 3562228255(0)

ack 596459521 win 4096 <mss 1024>

5. 0.009615 (0.0018) 0:0:c0:6f:2d:40 0:0:c0:c2:9b:26 ip 60

bsdi.1030 > svr4.discard: .ack 1 win 4096 [tos 0x10]

arp request to no n existent host
ARP request to no n-existent host

bsdi % date; telnet 140.252.13.36;date

Sat Jan 30 06:46:33 MST 1993

Trying 140.252.13.36…

telnet: Unable to connect to remote host: connection timed out

Sat Jan 30 06:47:49 MST 1993

bsdi % arp -a

? (140.252.13.36) at (incomplete)

arp examples65
ARP examples

Sun % tcpdump

1 0.0 arp who-has 140.252.13.36 tell bsdi

2 5.509069 (5.5091) arp who-has 140.252.13.36 tell bsdi

3 29.509745 (24.0007) arp who-has 140.252.13.36 tell bsdi

  • ARP Timeout (Berkely-derived implementations)
    • 20 minutes for a completed entry
    • 3 minutes for an incomplete entry
proxy arp
Proxy ARP
  • Lets a router answer ARP requests on one of its networks for a host on another of its networks
  • promiscuous ARP (ARP hack)
    • hide two physical networks
    • hide a gourp of hosts with older implementations of TCP/IP on a separate physical cable
gratuitous arp
Gratuitous ARP
  • When a host sends an ARP request looking for its own IP address
    • occur when bootstrap time
  • bsdi bootstrap, tcpdump on sun

1. 0.0 0:0:c0:6f:2d:40 ff:ff:ff:ff:ff:ff arp 60:

arp who-has 40.252.13.35 tell 140.252.13.35

  • gratutious ARP provides
    • a host determine if another host is already configured with the sampe IP address
      • if reply is received, print on the console, “duplicate PI address sent from Ethernet address: a:b:c:d:e: f”
    • if the host sending the gratutious ARP has just changed its hardware address, causes other host to change an entry in its cache
  • When ARP request is received, the host updates its entry with the hardware address
backup server
Backup server
  • Issue a gratutious ARP request with t
    • he backup’s hardware address
    • the failed server’s IP address
arp command
Arp command
  • -a: display all the entries in the cache
  • -d: delete an entry
  • -s: adding an entry with host name and ethernet address
    • permanent, no timeout
    • with keyword pub, ARP agent for the host
    • when the ethernet address is its own: proxy ARP
slide70

gemini

ARP request for 140.252.1.29

Ethernet IP = 140.252.1

.1.183

netb

ARP reply

modem

SLIP

modem

.65

.66

.1.29

sun

slip

bsdi

.33

.35

Ethernet IP = 140.252.13

Gemini % arp -a

netb (140.252.1.183) at 0:30:ad:3:6a:80

sun (140.252.1.29) at 0:30:ad:3:6a:80