System Configuration: DHCP and Autoconfiguration Chapter 6
Slide 2
DHCP: Dynamic Host Configuration Protocol Goal: allow host to
dynamically obtain its IP address from network server when it joins
network can renew its lease on address in use allows reuse of
addresses (only hold address while connected/on) support for mobile
users who want to join network (more shortly) DHCP overview: host
broadcasts DHCP discover msg [optional] DHCP server responds with
DHCP offer msg [optional] host requests IP address: DHCP request
msg DHCP server sends address: DHCP ack msg
Slide 3
DHCP client-server scenario 223.1.1.0/24 223.1.2.0/24
223.1.3.0/24 223.1.1.1 223.1.1.3 223.1.1.4 223.1.2.9 223.1.3.2
223.1.3.1 223.1.1.2 223.1.3.27 223.1.2.2 223.1.2.1 DHCP server
arriving DHCP client needs address in this network
DHCP: more than IP addresses DHCP can return more than just
allocated IP address on subnet: address of first-hop router for
client name and IP address of DNS sever network mask (indicating
network versus host portion of address)
Slide 6
connecting laptop needs its IP address, addr of first-hop
router, addr of DNS server: use DHCP router with DHCP server built
into router DHCP request encapsulated in UDP, encapsulated in IP,
encapsulated in 802.1 Ethernet Ethernet frame broadcast (dest:
FFFFFFFFFFFF ) on LAN, received at router running DHCP server
Ethernet demuxed to IP demuxed, UDP demuxed to DHCP 168.1.1.1 DHCP
UDP IP Eth Phy DHCP UDP IP Eth Phy DHCP DHCP: example
Slide 7
DHCP server formulates DHCP ACK containing clients IP address,
IP address of first-hop router for client, name & IP address of
DNS server encapsulation of DHCP server, frame forwarded to client,
demuxing up to DHCP at client DHCP: example router with DHCP server
built into router DHCP UDP IP Eth Phy DHCP UDP IP Eth Phy DHCP
client now knows its IP address, name and IP address of DSN server,
IP address of its first-hop router
Slide 8
Highlights Address Pools Leases Lease duration: minutes or days
Clients request requested lease duration, a copy of the address it
is already using or last used, and other parameters Servers
response Based on clients information (including the requesting MAC
address) in addition to other exogenous information (e.g., the time
of day, the interface on which the request was received)
Slide 9
DHCP Key difference from old BOOTP Leases Address Allocation
Automatic Dynamic Manual
Slide 10
DHCP/BOOTP Message Format
Slide 11
Some options Common options include Pad (0) Subnet Mask (1)
Router Address (3) Domain Name Server (6) Domain Name (15)
Requested IP Address (50) Address Lease Time (51) DHCP Message Type
(53) Server Identifier (54) Parameter Request List (55) DHCP Error
Message (56) Lease Renewal Time (58) Lease Rebinding Time (59)
Client Identifier (61) Domain Search List (119) End (255).
Protocol Operation The IPv6 header is of fixed size (40 bytes)
and contains 128-bit source and destination addresses. The Next
Header field is used to indicate the presence and types of
additional extension headers that follow the IPv6 header, forming a
daisy chain of headers that may include special extensions or
processing directives. Application data follows the header chain,
usually immediately following a transport-layer header.
Slide 14
DHCP Message Types T: Lease Time T1: Renewal Time: start
renewal with same server after this time expires Typically T/2 T2:
Rebinding Time: renewal must be done with same server by T2. If
not, then rebinding of the address is needed which means doing it
from scratch while indicating that the client already has an IP
address (in the client addr) Typically 7T/8
Slide 15
Lease Timers: An Example (tcpipguide.com)
Slide 16
DHCP client state machine
Slide 17
IPv6 Address Lifecycle Tentative: can be used only for IPv6
Neighbor Discovery DAD: Duplicate Address Detection is performed in
this state Optimistic: It is like Tentative, but an address is
allocated for some limited use Deprecated: can be used for existing
connections, but not for new connections DAD: Duplicate Address
Detection
Slide 18
Duplicate Address Detection (DAD) send one or more ICMPv6
Neighbor Solicitation messages If a Neighbor Advertisement message
is received in response, DAD has failed, and the address being
checked is abandoned
Slide 19
DHCPv6
Slide 20
Location Information GEOCONF option(Geographical configuration)
Latitude, longitude, altitude GEOCONF_CIVIC option Country, city,
district etc. More complex as the categories vary by country and
characters in different languages have complex representations Why
are these options needed?
Slide 21
Mobility and Handoff Information DHCP options to convey
information about cellular configuration and handovers between
different networks Types of such options Information Services
available networks Command Services functions for controlling link
parameters Event Services notification of link status changes
Slide 22
DHCP Snooping Implemented in some switches which limits DHCP
traffic to those addresses that are in a list Two problems can be
potentially addressed Rogue DHCP server is limited in the damage it
can do Allocation limited to a set of MAC addresses
Slide 23
Stateless Address Autoconfiguration (SLAAC) Consider a network
that has a single link and suppose there are no DHCP servers
Single-Link or Link Local Addresses Automatic Private IP Addressing
Generate a random address (169.254.1.1 through 169.254.254.254
using the 16-bit subnet mask 255.255.0.0) Check for conflicts using
ACD (Address Conflict Detection)
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