CS 352 Internet Technology The Internet Protocol

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CS 352Internet TechnologyThe Internet Protocol

• Fall 2003
• Dept. of Computer Science
• Rutgers University

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BGP Border Gateway Protocol

• BGP uses TCP as its transport protocol, on port
  179. On connection start, BGP peers exchange
  complete copies of their routing tables, which
  can be quite large. However, only changes
  (deltas) are then exchanged, which makes long
  running BGP sessions more efficient than shorter
  ones.
• Four Basic messages
• Open Establishes BGP session (uses TCP port
  179)
• Notification Report unusual conditions
• UpdateInform neighbor of new routes that become
  activeInform neighbor of old routes that become
  inactive
• Keepalive Inform neighbor that connection is
  still viable

http//www.antc.uoregon.edu/route-views/
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OPEN Message

• During session establishment, two BGP speakers
  exchange their
• AS numbers
• BGP identifiers (usually one of the routers IP
  addresses)
• A BGP speaker has option to refuse a session
• Select the value of the hold timer
• maximum time to wait to hear something from
  other end before assuming session is down.
• authentication information (optional)

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NOTIFICATION and KEEPALIVE Messages

• NOTIFICATION
• Indicates an error
• terminates the TCP session
• gives receiver an indication of why BGP session
  terminated
• Examples header errors, hold timer expiry, bad
  peer AS, bad BGP identifier, malformed attribute
  list, missing required attribute, AS routing
  loop, etc.
• KEEPALIVE
• protocol requires some data to be sent
  periodically. If no UPDATE to send within the
  specified time period, then send KEEPALIVE
  message to assure partner that connection still
  alive

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UPDATE Message

• used to either advertise and/or withdraw prefixes
• path attributes list of attributes that pertain
  to ALL the prefixes in the Reachability Info field

Withdrawn routes length (2 octets)
FORMAT
Withdrawn routes (variable length)
Total path attributes length (2 octets)
Path Attributes (variable length)
Reachability Information (variable length)
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Advertising a prefix

• When a router advertises a prefix to one of its
  BGP neighbors
• information is valid until first router
  explicitly advertises that the information is no
  longer valid
• BGP does not require routing information to be
  refreshed
• if node A advertises a path for a prefix to node
  B, then node B can be sure node A is using that
  pathitself to reach the destination.

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PATH ATTRIBUTES

• ORIGIN(TYPE CODE1)
• Who originated the announcement? Where was a
  prefix injected into BGP?
• Manually configured, directly connected, by other
  intra-routing protocols
• AS-PATH (TYPE CODE 2)
• a list of ASs through which the announcement for
  a prefix has passed
• each AS prepends its AS to the AS-PATH
  attribute when forwarding an announcement
• useful to detect and prevent loops
• AS length can be used to select among routes
  unless a LOCAL PREF attribute overrides

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Attribute Local Preference (type code 5)

• Used to indicate preference among multiple paths
  for the same prefix anywhere in the internet.
• The higher the value the more it is preferred
• Default value is 100
• Local to the AS
• Often used to select a specific exit point for a
  particular destination
• Used when AS path lengths are same

BGP table at AS4
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Use of local pref
Local-pref500?
12.0.0.0/8
12.0.0.0/8
OC3
T1
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Attribute Multi-Exit Discriminator (MED) (code4)

• when ASs interconnected via 2 or more links
• AS path length are same
• AS announcing prefix sets MED
• enables AS2 to indicate its preference (lower MED
  is better)
• AS receiving prefix uses MED to select link
• a way to specify how close a prefix is to the
  link it is announced on

AS1
Link B
Link A
MED500
MED100
AS2
AS3
AS4
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Use of MED
10.0.0.0/8 MED 50 172.16.0.0/12 MED 100
10.0.0.0/8 MED 100 172.16.0.0/12 MED 50
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DHCP

• DHCP Dynamic Host Configuration Protocol
• DHCP features
• Dynamic IP address allocation
• IP addresses can be leased for a certain time
• Useful where there are a limited number of IP
  addresses
• Useful for temporary connections (testing,
  laptops, mobile networks)
• Make administrating networks easier

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Why DHCP

• Bad old days Every host assigned an IP address
  manually
• Do normal users want to deal with this task?
• What if I move my computer to a new network?
• DHCP allows hosts to receive IP addresses (and
  more) during boot
• IP addresses dont have to be manually configured
  into hosts
• Centralization of host to IP address mapping task
• Hosts viewed as a uniform pool, not specific IP
  addresses

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DHCP

• DHCP has two components
• A protocol for delivering bootstrap information
  from the server to the clients
• An algorithm for dynamically assigning addresses
  to clients

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Address Allocation Modes

• DHCP supports three modes of allocation
• Automatic allocation Server assigns a permanent
  address to a host
• Dynamic allocation Server assigns a host an IP
  address with a finite lease
• Manual allocation Server assigns host an IP
  address chosen by the network administrator

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DHCP Packets (contd)
0 7 8
15 16
23 24 31
Request/Reply
Hardware type
Hardware address length in bytes
Hop count
Transaction ID
Number of seconds
Flags
Client IP address
Your IP address
Server IP address
Gateway IP address
Client hardware address (16 bytes)
Server hostname (64 bytes)
Boot filename (128 bytes)
Options (312 bytes)
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DHCP Packet Fields

• Flags One flag currently defined
• Broadcast (bit 0) Clients can request that all
  DHCP server messages be broadcast to it
• Options
• All DHCP packets must use the DHCP message type
  option, which defines the type of DHCP message
  being sent
• DHCPDISCOVER
• DHCPOFFER
• DHCPREQUEST
• DHCPACK
• DHCPNAK
• etc.

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DHCP Message types

• DHCP message types
• DHCP Discover Client broadcasts to locate a
  server
• DHCP Offer Server responds with proposal of
  parameters
• DHCP Request Client broadcasts its choice of
  server. All other servers are implicitly
  declined.
• DHCP ACK Selected server responds to client with
  address
• DHCP NAK Selected server rejects the clients
  request
• DHCP Decline Client declines servers parameters
• DHCP Release Client releases its assigned address

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DHCP Protocol
Server 1
Server 2
Client
DHCPDISCOVER
DHCPDISCOVER
DHCPOFFER
DHCPOFFER
Collects replies Selects server 2
DHCPREQUEST
DHCPREQUEST
DHCPACK
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DHCP Protocol

• DHCP client broadcasts a DHCP Discover message
• Client may specify preference of a lease and/or
  IP address
• Many servers may respond with offers
• Client chooses one server from them
• Client broadcasts DHCP request with id of chosen
  server
• Selected server sends DHCP ACK or NAK
• Client begins using offered IP address once it
  receives ACK
• If the client finds a problem, it sends a DHCP
  Decline message to the server and starts over
  again
• Client may choose to release the address before
  lease expires by sending a DHCP Release message
  to the server

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DHCP Relay Agents

• Similar to BOOTP Relay Agents
• DHCP relay agents allow DHCP servers to handle
  requests from other subnets

DHCP Relay Agent
Client
IP Gateway Router
IP Gateway Router
DHCP Server
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NAT Network Address Translation

• Every host needs an IP address
• IPv4 address space is limited
• It is expensive to get an IP address for every
  device that may be connected to the internet
• May not want to allow outsiders access as all
• E.g. security concerns
• IETF has set aside private IP address for use
  within a network but can be translated into a
  fixed public address by a special router
• NAT box

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Private IP addreses

• 10.0.0.0 - 10.255.255.255 (10/8 prefix)
• 172.16.0.0 - 172.31.255.255 (172.16/12 prefix)
• 192.168.0.0 - 192.168.255.255 (192.168/16 prefix)
  
• These addresses can be assigned to any of the
  machines within a network but will be translated
  to a public address by the NAT router
• Ports are used to distinguish among multiple
  addresses that need to be mapped from one public
  address to multiple private addresses

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NAT

• NAT idea map internal address space to single
  public address expanded port space
• NAT router maintains mappings between a public
  address port and a private address and port
• Keep a table of internal addresses/ports and
  external hosts/ports contacted from the internals
  

10.0.0.1/ 1000
128.6.8.4/8001
128.6.8.4/8002
10.0.0.2/ 1000
10.0.0.3/ 5001
128.6.8.4/8004
Private LAN side
Public WAN side
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Disadvantages

• It is a hack
• Works only with TCP/UPD port connections
• NAT has to understand all higher layer
  application protocols to correctly map the port
• Every packet needs to be remapped
• Must rewrite layer 3 and 4 headers for every
  packet
• What happens if a service sends its IP address
  in the data of the packet? (E.g. FTP)

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Mobile-IP
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Mobile users

• Explosion in usage of hand helds
• Anytime, anywhere wireless services
• Some connectivity everywhere
• Many-time, many-where (Infostations)
• Users can be connected when moving
• Users can be connect and disconnect to different
  networks

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Mobility vs connectivity

• New research problems
• Continuous connectivity for a mobile host
• Seamless movement between networks
• Mobile systems
• Move from place to place while being wireless
• Move from place to place by plugging-in at
  different attachment points
• Why maintain connectivity?
• Avoid restarting applications/networks

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IP address problem

• Internet hosts/interfaces are identified by IP
  address
• Domain name service translates host name to IP
  address
• IP address identifies host/interface and locates
  its network
• Mixes naming and location
• Moving to another network requires different
  network address
• But this would change the hosts identity
• How can we still reach that host?

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Basic idea
Home Agent
Foreign Agent
MH Mobile Host
CH correspondent HOST
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Basic idea

• Mobile hosts attaches to foreign network and
  obtains guest address
• Via DHCP
• Via Foreign agent
• Registration with local agent
• LA has list of all foreign hosts visiting the
  network

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Routing for mobile hosts
MH mobile host
Foreign network
Home network
CH correspondent host
How to direct packets to moving hosts
transparently?
Home network
Foreign network
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Use ARP

• A designated router proxy-ARPs for mobile host

I have MH1
H4
Who has MH1? Know? mh1_at_h4
MH1
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Basic Mobile IP to mobile hosts
MH mobile host CH correspondent host HA
home agent FA foreign agent
(Well see later that FA is not necessary or even
desirable)
Foreign network
Home network
HA
FA

• MH registers new care-of address (FA) with HA
• HA tunnels packets to FA
• FA decapsulates packets and delivers them to MH

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IP-in-IP (Packet encapsulation)
Packet from CH to MH
Source address address of CH Destination
address home IP address of MH Payload
Home agent intercepts above packet and tunnels it
Source address address of HA Destination
address care-of address of MH Source address
address of CH Destination address home IP
address of MH Original payload
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When mobile host moves again
Foreign network 1
Home network
FA 1
HA
Foreign network 2
FA 2
CH

• MH registers new address (FA 2) with HA FA 1
• HA tunnels packets to FA 2, which delivers them
  to MH
• Packets in flight can be forwarded from FA 1 to
  FA 2

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Basic Mobile IP - from mobile hosts
Mobile hosts also send packets
Foreign network
Home network
FA
HA

• Mobile host uses its home IP address as source
  address
• Lower latency as MH can send packets directly to
  CH
• Still transparent to correspondent host
• This is called a triangle route or a dog-leg
  route

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Problems with Foreign Agents

• Assumption of support from foreign networks
• A foreign agent exists in all networks you visit
• The foreign agent is robust and up and running
• The foreign agent is trustworthy

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Solution

• Mobile host is responsible for itself
• (With help from infrastructure in its home
  network)
• Mobile host decapsulates packets
• Mobile host sends its own packets
• Co-located FA on MH

Foreign network
Home network

• MH must acquire its own IP address in foreign
  network
• This address is its new care-of address
• Mobile IP spec allows for this option

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Obtaining a foreign IP address

• How to get a new IP address?
• DHCP
• Dynamic IP address binding like some dial-up
  services

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Problems with ingress/egress filtering
Home network
Foreign network

• Mobile host uses its home IP address as source
  address
• Security-conscious boundary routers will drop
  this packet
• An egress router will see a packet with source
  address that does not belonging to its network

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Solution bi-directional tunnel
Home network
Foreign network

• Provide choice of safe route through home agent
  both ways
• Use encapsulation in both directions (MH? CH and
  CH ? MH)

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Solution yet more flexibility

• Use current care-of address and send packet
  directly
• This is regular IP!
• This is not mobility but portability

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Do we need Mobile IP

• When do we really need this
• Mobile clients have short lived sessions
• Reconnect on move
• Most mobile users are in private net
• Mobile servers?