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Nov 02, 2004 CS573: Network Protocols and Standards
1
Subnetting, ICMP, NAT, BOOTP
Network Protocols and Standards
Autumn 2004-2005
Nov 02, 2004 CS573: Network Protocols and Standards 2
Subnet Routing Conventional routing table entry
(network address, next hop address) Network address format is predetermined for a given
class (e.g., first 16 bits for class B addresses!)
With subnetting, routing table entry becomes (subnet mask, network address, next hop
address) Then compare with network address field of
entries to find next hop address Subnet mask indicates the network address!
Nov 02, 2004 CS573: Network Protocols and Standards 3
Subnet Routing The use of mask generalizes the subnet routing algorithm to
handle all the special cases of the standard algorithm Routes to individual hosts Default route Routes to directly connected networks Routes to conventional networks (that do not use subnet
addressing) Merely combine the 32-bit mask field with the 32-bit IP
address Example: To install a route for:
Individual host (Mask of all 1’s, Host IP address) Default Route (Mask of all 0’s, network address all 0’s) Class B network address (Mask of two octets of 1’s and two of
0’s)
Nov 02, 2004 CS573: Network Protocols and Standards 4
Subnet Routing Algorithm
Extract destination IP (D) from datagram Compute IP address of destination network N If N matches any directly connected network address
Send datagram over that network (obviously encapsulated in a frame)
Else For each entry in the routing table, do N* = bitwise-AND of D and subnet mask If N* equals the network address field of the entry, then
route the datagram to the specified next hop
Nov 02, 2004 CS573: Network Protocols and Standards 5
Subnetting: Example Consider a corporate network assigned
a class C address P.Q.R.00000000 The company needs 5 subnets:
2 subnets of 16 hosts each 3 subnets with 32, 64, and 128 hosts
External routers reach the corporate via single routing table entry P.Q.R.0 network and 255.255.255.0 mask (if
any) What about internal routers?
Nov 02, 2004 CS573: Network Protocols and Standards 6
Subnetting: Example
S5
S4
S3
S2
S1
255.255.255.1 0000000P.Q.R.1 111 hhhh
255.255.255.1 0000000P.Q.R.1 110 hhhh
255.255.255.1 0000000P.Q.R.1 101 hhhh
255.255.255.1 0000000P.Q.R.1 100 hhhh
255.255.255.1 0000000P.Q.R.1 011 hhhh
255.255.255.1 0000000P.Q.R.1 010 hhhh
255.255.255.1 0000000P.Q.R.1 001 hhhh
P.Q.R.1 0000000
255.255.255.1 0000000P.Q.R.1 000 hhhh
255.255.255.11 000000P.Q.R.01 11 hhhh
255.255.255.11 000000P.Q.R.01 10 hhhh
255.255.255.11 000000P.Q.R.01 01 hhhhP.Q.R.01 000000
255.255.255.11 000000P.Q.R.01 00 hhhh
255.255.255.111 00000P.Q.R.001 1 hhhhP.Q.R.001 00000
255.255.255.111 00000P.Q.R.001 0 hhhh
P.Q.R.0001 0000255.255.255.1111 0000P.Q.R.0001 hhhh
P.Q.R.0000 0000255.255.255.1111 0000P.Q.R.0000 hhhh
IP addresses Network/Subnet addressSubnet Mask Subnet Name{{
Nov 02, 2004 CS573: Network Protocols and Standards 7
Subnetting: Example
S5
S4
S3
S2
S1
255.255.255.1 0000000
P.Q.R.1 0000000P.Q.R.1
hhhhhhh
255.255.255.11 000000
P.Q.R.01 hhhhhh
P.Q.R.01 000000
P.Q.R.001 00000255.255.255.1110
0000P.Q.R.001
hhhhh
P.Q.R.0001 0000255.255.255.1111 0000P.Q.R.0001 hhhh
P.Q.R.0000 0000255.255.255.1111 0000P.Q.R.0000 hhhh
IP addresses Network/Subnet addressSubnet Mask Subnet Name
Nov 02, 2004 CS573: Network Protocols and Standards 8
Subnetting: Routing Table
P5
P4
P3
P2
P1
255.255.255.1 0000000
P.Q.R.1000 0000
255.255.255.11 000000
P.Q.R.0100 0000
P.Q.R.0010 0000255.255.255.1110
0000
P.Q.R.0001 0000255.255.255.1111
0000
P.Q.R.0000 0000255.255.255.1111
0000
Network/Subnet addressSubnet Mask Next Hop/Port
Nov 02, 2004 CS573: Network Protocols and Standards 9
Subnetting: Routing Table
P.Q.R.0000 0000 / 28 P1
P.Q.R.0001 0000 / 28 P2
P.Q.R.0010 0000 / 27 P3
P.Q.R.0100 0000 / 26 P4
P.Q.R.1000 0000 / 25 P5
Network/Subnet address Next Hop/Port
Number after / indicates number of bits to look at!
Nov 02, 2004 CS573: Network Protocols and Standards 10
Subnetting: Routing TableSubnet S4 has 64 hosts. Can we make two subnets? 16+48?
255.255.255.11 000000P.Q.R.01 11 hhhh
255.255.255.11 000000P.Q.R.01 10 hhhh
255.255.255.11 000000P.Q.R.01 01 hhhh P.Q.R.0100 0000
255.255.255.11 000000P.Q.R.01 00 hhhh
Old mask Old subnet New mask
255.255.255.11 000000P.Q.R.01 hhhhhh
255.255.255.11 000000P.Q.R.01 hhhhhh
255.255.255.11 000000P.Q.R.01 hhhhhh
255.255.255.1111 0000P.Q.R.01 00 hhhh{
S4255.255.255.11 000000
P.Q.R.01 hhhhhh
P.Q.R.01 000000
S41255.255.255.1111 0000
P.Q.R.0100 hhhh
P.Q.R.0100 0000
S42255.255.255.11 000000
P.Q.R.01 hhhhhh
P.Q.R.01 000000
Nov 02, 2004 CS573: Network Protocols and Standards 11
Subnetting: Routing Table
What if an IP in S42 is received?It will match on the second entry!
What if an IP in S41 is received?It will match both entries!Which entry should be used?
USE LONGEST PREFIX MATCH
S41255.255.255.1111 0000
P.Q.R.0100 hhhh
P.Q.R.0100 0000
S42255.255.255.11 000000
P.Q.R.01 hhhhhh
P.Q.R.01 000000
Nov 02, 2004 CS573: Network Protocols and Standards 12
Subnetting: Routing TableWhere else longest prefix match can be used?
P345
P345
P345
P2
P1
255.255.255.1 0000000 P.Q.R.1000 0000
255.255.255.11 000000 P.Q.R.0100 0000
P.Q.R.0010 0000255.255.255.1110 0000
P.Q.R.0001 0000255.255.255.1111 0000
P.Q.R.0000 0000255.255.255.1111 0000
Network/Subnet addressSubnet Mask Next Hop/Port
Aggre
gate
P345
P2
P1
P.Q.R.0000 0000255.255.255.0000 0000
P.Q.R.0001 0000255.255.255.1111 0000
P.Q.R.0000 0000255.255.255.1111 0000
Network/Subnet addressSubnet Mask Next Hop/Port
Router
Router
S1S2
S5S4S3
Nov 02, 2004 CS573: Network Protocols and Standards 13
Supernet Addressing Use of many IP network addresses for a
single organization Example:
To conserve class B addresses, issue multiple class C address to the same organization
Issue: increase in the number of entries in the routing tables for routers outside the network
Solutions: Collapse a block of contiguous class C address into
the pair: (network address, count) where network address is the smallest number in the block
Nov 02, 2004 CS573: Network Protocols and Standards 14
Supernet Addressing It requires each block to be a power of 2
and uses bit mask to identify the size of the block
Example Dotted decimal 32-bit binary equivalent
Lowest: 234.170.168.0 11101010 10101010 10101000 00000000
Highest: 234.170.175.255 11101010 10101010 10101111 11111111
A block of 2048 addresses 32-bit mask is 11111111 11111111 11111000 00000000
Do we really need address classes when we have masks?
Answer: NO CIDR (Classless Inter Domain Routing)
Nov 02, 2004 CS573: Network Protocols and Standards 15
Supernet Addressing In the router, the entry consists of:
The lowest address and the 32-bit mask
A block of addresses can be subdivided, and separate route can be entered for each subdivision
When looking up a route, the routing software uses a longest-match paradigm to select a route
Nov 02, 2004 CS573: Network Protocols and Standards
16
ICMP: Internet Control Message Protocol
Network Protocols and Standards
Autumn 2004-2005
Nov 02, 2004 CS573: Network Protocols and Standards 17
ICMP Motivation Questions in Routing:
What if a router cannot route or deliver a datagram? What if a router experiences congestion? What if the TTL expires?
Router needs to inform the source to take action to avoid or correct the problem
ICMP – error reporting mechanism Can only report condition back to the original source Routers and hosts send error or control messages to
others Specified in RFC 792
Nov 02, 2004 CS573: Network Protocols and Standards 18
ICMP ICMP messages are encapsulated in IP datagrams, with
protocol type 1 In the data portion of the datagram, first byte indicates the
ICMP message type and the format for the rest of the message
Some ICMP packets have a code that further qualifies the type Most ICMP messages include the full IP header plus the first 8
bytes of the data portion of the datagram they refer to Helps sender identify the packet
To avoid explosion of ICMP messages No ICMP packets are generated to report errors on ICMP packets If an ICMP message is generated about a fragmented datagram,
it is generated only for the first fragment (fragment 0)
Nov 02, 2004 CS573: Network Protocols and Standards 19
Some ICMP Message Types
Type Field
ICMP Message Type
0 Echo Reply
3 Destination Unreachable
4 Source Quench
5 Redirect (change a route)
8 Echo Request
9 Router Advertisement
10 Router Solicitation
11 Time Exceeded for a Datagram
12 Parameter Problem on a Datagram
13 Timestamp Request
14 Timestamp Reply
17 Address Mask Request
18 Address Mask Reply
Reference:RFC 1700
Nov 02, 2004 CS573: Network Protocols and Standards 20
Echo Request/Reply Testing destination reachability and status
Echo Request Message Echo Reply Message
Command used to send ICMP echo request is, in most systems, called “ping”
Echo request may contain some data, which is returned unchanged in the reply
The ICMP Echo Request/Reply header also contains a sequence number and identifier, to aid the host in matching the request with the reply
Nov 02, 2004 CS573: Network Protocols and Standards 21
Echo Request/Reply
ICMP Echo Request or Reply Message Format
TYPE(0/8) CODE(0)
IDENTIFIER
CHECKSUM
SEQUENCE NUMBER
OPTIONAL DATA
… … …
Nov 02, 2004 CS573: Network Protocols and Standards 22
Destination Unreachable Reports of unreachable
destinations When a router can not forward or
deliver an IP datagram, it sends a “destination unreachable” message back to the original source
Code determines specific condition (see table)
Nov 02, 2004 CS573: Network Protocols and Standards 23
Destination Unreachable
ICMP Destination Unreachable Message Format
TYPE(3) CODE(0-12) CHECKSUM
UNUSED (MUST BE ZERO)
INTERNET HEADER+FIRST 8 BYTES OF DATA
… … …
Nov 02, 2004 CS573: Network Protocols and Standards 24
Destination Unreachable Codes
Code Value
Meaning
0 Network Unreachable
1 Host Unreachable
2 Protocol Unreachable
3 Port Unreachable
4 Fragmentation Needed and DF Set
5 Source Route Failed
6 Destination Network Unknown
7 Destination Host Unknown
8 Source Host Isolated
9 Communication with Destination Network Administratively Prohibited
10 Communication with Destination Host Administratively Prohibited
11 Network Unreachable for Type of Service
12 Host Unreachable for Type of Service
Nov 02, 2004 CS573: Network Protocols and Standards 25
ICMP Source Quench Congestion and datagram flow control
Report congestion to the original source Request to source to reduce current rate
Usually sent for each datagram discarded Can be sent by a host or a router Some routers may be more sophisticated
Monitor incoming traffic Quench sources that have the highest rates Avoid congestion by quenching before
datagrams are lost
Nov 02, 2004 CS573: Network Protocols and Standards 26
Source Quench
ICMP Source Quench Message Format
TYPE(4) CODE(0) CHECKSUM
UNUSED (MUST BE ZERO)
INTERNET HEADER+FIRST 8 BYTES OF DATA
… … …
Nov 02, 2004 CS573: Network Protocols and Standards 27
ICMP Redirect Message Host sends a datagram to router R1 to
be forwarded to a certain destination Router R1 looks at its routing table, and
finds the next router in the path as R2 If R2 is directly accessible to the sending
host, R1 generates an ICMP Redirect Message back to the sender. R1 also forwards the datagram to R2 normally
The purpose is to inform the host that there is a better route to that destination
Nov 02, 2004 CS573: Network Protocols and Standards 28
ICMP Redirect MessageTYPE(5) CODE(0-3) CHECKSUM
SUGGESTED ROUTER INTERNET ADDRESS
INTERNET HEADER+FIRST 8 BYTES OF DATA
… … …
Code Value
Meaning
0 Redirect datagrams for the Net (now obsolete)
1 Redirect datagrams for the Host
2 Redirect datagrams for the Type of Service and Net
3 Redirect datagrams for the Type of Service and Host
Nov 02, 2004 CS573: Network Protocols and Standards 29
ICMP Time ExceededTYPE(11) CODE(0/1) CHECKSUM
UNUSED (MUST BE ZERO)
INTERNET HEADER+FIRST 8 BYTES OF DATA
… … …
Code Value
Meaning
0 Time-to-live count Exceeded
1 Fragment reassembly time exceeded
A router sends this message whenever a datagram is discarded because theTTL field in the datagram has reached zero or because its reassembly timerExpired while waiting for fragments
Nov 02, 2004 CS573: Network Protocols and Standards 30
Address Mask Request/Reply Obtaining a subnet mask
ICMP address mask request message ICMP address mask reply message
Request Sent directly to the router (if known) Broadcast (if router unknown)
Response is unicast if the request contains a valid IP address; otherwise, it is a broadcast
Any host can respond (see RFC 950)
Nov 02, 2004 CS573: Network Protocols and Standards 31
Address Mask Request/Reply
ICMP address mask request or reply message format.Usually, hosts broadcast a request without knowing which specific router will respond.
TYPE(17/18) CODE(0)
IDENTIFIER
CHECKSUM
SEQUENCE NUMBER
ADDRESS MASK
Nov 02, 2004 CS573: Network Protocols and Standards 32
Router Advertisement/Solicitation Options for the host to learn the router
address(es) Manually enter entries
Not up to date and cumbersome Host listens to routing protocol messages
Protocols and their messages differ Complexity is introduced at the host
Use of ICMP messages as defined in RFC 1256 Routers periodically send an ICMP “router
Advertisement” – either broadcast or multicast Hosts may solicit such advertisements with a
Router Solicitation message
Nov 02, 2004 CS573: Network Protocols and Standards 33
Router Advertisement
TYPE(9) CODE(0) CHECKSUM
LIFETIME (SEC)
ROUTER ADDRESS [1]
NUM ADDRS ADDR ENTRYSZIE = 2
ROUTER ADDRESS [2]
PREFERENCE LEVEL [2]
PREFERENCE LEVEL [1]
… … …
Nov 02, 2004 CS573: Network Protocols and Standards 34
Router Solicitation
TYPE(10) CODE(0) CHECKSUM
RESERVED
– Default advertisement rate is once every 7-10 minutes– The router solicitation message causes the routers to
send their advertisements earlier– Lifetime of advertisements is typically 30 minutes
Nov 02, 2004 CS573: Network Protocols and Standards 35
Application: Traceroute Goal: Find the path a packet takes between two hosts Originator host sends a series of packets, starting with
TTL=1 and increasing the TTL for each packet The first router in the path will drop the TTL=1 packet
and send back an ICMP Time Exceeded Host learns who is the first hop
Second router in the path will drop the packet that originated with TTL=2 and send back an ICMP Time Exceeded
Third router will do the same upon receiving packet that originated with TTL=3
By collecting the ICMP responses, the host can figure out the path taken by the packet. Will this work?
Nov 02, 2004 CS573: Network Protocols and Standards 36
Application: Traceroute Current method described above requires 2N
messages for a N-hop path Will also give wrong results if path changes
ICMP Traceroute (RFC 1393) can do it in N+1 messages
Idea: Define a traceroute IP option Send an IP packet with this option set Every intermediate system handling this
packet will send back an ICMP traceroute to the source
Nov 02, 2004 CS573: Network Protocols and Standards 37
Application: TracerouteTracing route to nova.stanford.edu [171.64.90.123] over a maximum of 30 hops:
1 <10 ms <10 ms <10 ms shahalami.lums.edu.pk [203.128.0.1] 2 1938 ms 1890 ms 1860 ms 202.125.139.29 3 1515 ms 1875 ms 1938 ms 202.125.139.249 4 1812 ms 1672 ms 1578 ms 202.125.159.53 5 1969 ms 1672 ms 1953 ms 203.208.147.85 6 1437 ms 1641 ms 1594 ms p5-2.nycmny1-cr11.bbnplanet.net [4.25.14.41] 7 1593 ms 1688 ms 1719 ms p3-0.nycmny1-nbr1.bbnplanet.net [4.24.10.78] 8 1859 ms * 1687 ms so-6-0-0.chcgil2-br2.bbnplanet.net [4.24.4.17] 9 1610 ms 1718 ms 1625 ms so-1-0-0.dnvtco1-br2.bbnplanet.net [4.24.9.62] 10 1516 ms 1718 ms 2000 ms p15-0.snjpca1-br2.bbnplanet.net [4.0.6.225] 11 1922 ms 1844 ms 1562 ms p2-0.paix-bi3.bbnplanet.net [4.24.7.38] 12 1562 ms 1813 ms 1812 ms p2-0.paix-bi2.bbnplanet.net [4.0.3.174] 13 1828 ms 1625 ms 1688 ms p6-0.paloalto-nbr1.bbnplanet.net [4.0.6.97] 14 1844 ms 1734 ms 2016 ms p1-0.paloalto-cr1.bbnplanet.net [4.0.6.74] 15 2031 ms 1813 ms 1687 ms p1-0-0.paloalto-cr13.bbnplanet.net [4.0.2.222] 16 2109 ms 1985 ms 1937 ms sunet-gateway.stanford.edu [198.31.10.1] 17 * * * Request timed out. 18 * * * Request timed out. 19 2078 ms 2203 ms 2078 ms nova.Stanford.EDU [171.64.90.123]
Nov 02, 2004 CS573: Network Protocols and Standards 38
Application: Traceroutetraceroute to suraj.lums.edu.pk (203.128.0.6): 1-30 hops, 38 byte packets
1 quad-rtr.Stanford.EDU (171.64.90.1) 1.49 ms (ttl=64!) 1.25 ms (ttl=64!) 1.32 ms (ttl=64!) 2 default-gateway-2.Stanford.EDU (198.31.86.129) 2.27 ms 1.98 ms 2.82 ms 3 sunet-gateway.Stanford.EDU (198.31.86.1) 2.18 ms 1.18 ms 1.25 ms 4 g1.ba21.b003123-1.sfo01.atlas.cogentco.com (66.250.7.137) 3.27 ms 3.79 ms 3.04 ms 5 g1-1.core01.sfo01.atlas.cogentco.com (66.28.6.9) 4.23 ms 3.40 ms 3.18 ms 6 p5-0.core03.sfo01.atlas.cogentco.com (66.28.4.146) 3.44 ms 3.51 ms 5.33 ms 7 ds3.st-paix.ix.singtel.com (198.32.176.50) 10.9 ms (ttl=248!) 10.2 ms (ttl=248!) 12.1 ms (ttl=248!) 8 p6-1.plapx-cr1.ix.singtel.com (203.208.172.45) 12.9 ms 13.5 ms 13.6 ms 9 POS2-0.above-core1.ix.singtel.com (202.160.250.45) 14.6 ms 14.0 ms 13.3 ms10 203.208.154.94 (203.208.154.94) 63.7 ms 58.7 ms 57.6 ms11 203.208.154.97 (203.208.154.97) 78.1 ms (ttl=244!) 80.7 ms (ttl=244!) 82.0 ms (ttl=244!)12 203.208.154.102 (203.208.154.102) 79.9 ms 80.5 ms 78.3 ms13 203.208.147.86 (203.208.147.86) 373 ms (ttl=241!) 323 ms (ttl=241!) 310 ms (ttl=241!)14 202.125.159.46 (202.125.159.46) 326 ms (ttl=240!) 329 ms (ttl=240!) 328 ms (ttl=240!)15 202.125.139.250 (202.125.139.250) 328 ms (ttl=239!) 326 ms (ttl=239!) 326 ms (ttl=239!)16 202.125.139.30 (202.125.139.30) 2075 ms (ttl=238!) 2146 ms (ttl=238!) 2216 ms (ttl=238!)17 suraj.lums.edu.pk (203.128.0.6) 2395 ms (ttl=237!) 2294 ms (ttl=237!) 2209 ms (ttl=237!)
Nov 02, 2004 CS573: Network Protocols and Standards
39
Network Address Translation
Network Protocols and Standards
Autumn 2004-2005
Nov 02, 2004 CS573: Network Protocols and Standards 40
Private Networks Private networks have no “direct”
connection to the Internet Blocks of addresses have been reserved
for the private networks (RFC 1918) Blocks in different classes
10.0.0.0 – 10.255.255.255 (1 class A) 172.16.0.0 – 172.31.255.255 (16 class B) 192.168.0.0 – 192.168.255.255 (256 class
C)
Nov 02, 2004 CS573: Network Protocols and Standards 41
Purpose Machines in the
protected network can access the Internet normally
Packets coming from the protected network all appear to be coming from IP1
Addresses in the protected network are in the private range
Host 1
Host 2
Host N
ProtectedNetwork
Firewall
Internet
IP1 IP2
Nov 02, 2004 CS573: Network Protocols and Standards 42
Implementation Hosts inside the private network are configured
to use the firewall (IP2) as their gateway The firewall rewrites the IP datagram header for
the outbound packets, replacing the source IP with IP1
All packets “seem” to be coming from IP1 The destination IP in the packets received from
the Internet is IP1; it is rewritten replacing IP1 with the IP address of the internal destination
Problem: How to figure out what is the right destination in the private network?
Nov 02, 2004 CS573: Network Protocols and Standards 43
Demultiplexing Incoming Packets There is not enough information in the
IP header to demultiplex incoming packets
It is necessary to use information from the higher layers (transport layer)
Common transport layers: TCP and UDP Transport layer has the concept of port
which identifies which process in the host should finally get the packet
Nov 02, 2004 CS573: Network Protocols and Standards 44
Ports 16-bit numbers
identifying which process should get the packet
UDP and TCP ports exist in different spaces
Each packet carries two port numbers
The source port of the process which generated it in the source host
The destination port of the process which should get it at the destination
IP
TCP UDP
Telnet FTP
Nov 02, 2004 CS573: Network Protocols and Standards 45
Implementation (revisited) Upon receiving an outbound packet from a host in
the private network, the firewall: Rewrites the source IP with its own IP (IP1) Generates a local source port and rewrites the source
port in the packet as this port and makes a record of it Upon receiving an inbound packet from the
Internet, the firewall checks whether the destination port in the packet is in the list of local ports:
If not, the packet is dropped Can not initiate connections from outside!
If yes, the firewall knows where to send this packet
Nov 02, 2004 CS573: Network Protocols and Standards
46
Dynamic Addressing
Network Protocols and Standards
Autumn 2004-2005
Nov 02, 2004 CS573: Network Protocols and Standards 47
BOOTP Alternative to RARP
RARP operates at a low level, requesting the direct access to the network hardware
Difficult for an application programmer to build a server
RARP gives “only” the IP address
Nov 02, 2004 CS573: Network Protocols and Standards 48
BOOTP Devised to allow a machine to
obtain: Its IP address Address of a router Subnet mask to use Address of a name server
Can be implemented with an application program Uses UDP/IP for communication
Nov 02, 2004 CS573: Network Protocols and Standards 49
BOOTP Using IP to determine an IP address
Request from a client is broadcast on the local network using IP address all 1’s
Since the client does not know its IP address (yet!), the reply from the server must also be broadcast; otherwise
Using clients IP address would require use of ARP to map IP address to a hardware address, which in turn requires client to already know its IP address
Using client’s request to manually add an entry to its ARP cache – Not desirable
Nov 02, 2004 CS573: Network Protocols and Standards 50
BOOTP Reliability in communication is
based on UDP checksum Timeout and retransmissions
To minimize collisions among many clients, use random timeouts
Increase timeouts with each retransmission Starting with the interval 0-4 seconds Doubling interval each retransmission up to 60s
Nov 02, 2004 CS573: Network Protocols and Standards 51
BOOTP Message Format
OP HTYPE HLEN HOPS
Seconds UnusedTransaction ID
Client IP AddressYour IP Address
Server IP Address
Client Hardware Address (16 octets)Router IP Address
Boot File Name (128 octets)Server Hostname (64 octets)
Vendor-specific area (64 octets)
0 8 16 24 31 bits
Nov 02, 2004 CS573: Network Protocols and Standards 52
BOOTP Message Field OP
Specifies whether a request(1) or reply(2) HTYPE and HLEN
Hardware type and address length (For Ethernet, HTYPE is 1 and HLEN is 6)
HOPS Client passes 0 in this field; BOOTP server increments it if
the request is passed to another server across a router Transaction ID
Contains an integer that machines use to match requests with responses
Seconds Number of seconds since the client started to boot
Nov 02, 2004 CS573: Network Protocols and Standards 53
BOOTP Message Remaining fields in the message
To allow the greatest flexibility Clients fill in as much information as they
know; unknown fields are set to zero Example
If server IP or server hostname are non-zero, only the server with matching address/name will answer the request
If they are zero, any server that receives the request will reply
Nov 02, 2004 CS573: Network Protocols and Standards 54
BOOTP Message Format BOOTP can be used by a client that already
knows its IP address (e.g., to obtain boot file information)
A client that knows its IP address places it in the client IP address field; other clients set this field to zero
If the client’s IP address in the request message is zero, a server returns the client IP address in the “your IP address” field
Nov 02, 2004 CS573: Network Protocols and Standards 55
DHCP Dynamic Host Configuration Protocol RARP and BOOTP designed for relatively static
environment Each host a permanent network connection Manager creates a BOOTP configuration file specifying
BOOTP parameters for each host Manager configures server with mapping of host
identifier to IP address New Requirements
Portable computers Number of computers exceeds available IP host
addresses (although not all will be up and running at the same time)
Nov 02, 2004 CS573: Network Protocols and Standards 56
DHCP DHCP allows:
Manual configuration Automatic configuration Manager let DHCP server assign a
permanent address when a computer first attaches to the network
Dynamic configuration Loaning IP addresses for a limited time