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Lecture 12 - Network Services
Alejandro Saucedo - Comp2008 Lecture 11 FlashCard Set
23
Computer Networking
Undergraduate 2
05/16/2013

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Cards

Term
What are some ways to achieve host configuration?
Definition
  • Manually
  • Automatically
  • Autonomously
Term
What are some of the minimum information required for Host configuration?
Definition
  • IP address
  • Default router
  • Subnet Mask
  • DNS resolver
  • NTP
  • Domain name
  • ...
Term
What are some characteristics of manual host configuration?
Definition
  • Gives best level of control
  • Common method for servers
  • Does not scale well
  • Problematic if network environment changes
    • e.g. if network renumbering is required due to a change of ISP; using hard encoded addresses
  • Not consumer friendly
Term
What are the characteristics of automatic (with administration) host configuration?
Definition
  • Hosts can fetch settings using some protocol
    • Settings are administratively configured in servers that hosts can query
  • Provide a single point of administration
  • Can potentially enforce policy on hosts
  • Aids accountability (User logged in at time x)
Term
What are the main principles of Dynamic Host Configuration Protocol (DHCP)?
Definition
  • Supports stateful address assignments
    • Uses a pool of IP addresses allocated to the service
    • Allocates IP addresses to the clients on request
  • Can provide other configuration informaion (e.g. NTP time server, local domain name, etc)
Term
What does "stateful" for the DHCP mean?
Definition

The DHCP server has to keep state (information) on which devices were allocate which IPs

 

All addresses assigned with a lease timer

Term
What are the principles of DHCP address assignments?
Definition
  • Uses a 4-way protocol exchange
    • Client looks (broadcasts) for server on local subnet
    • Server responds with DHCP OFFER
      • This includes the offered IP address
    • Client confirms address with DHCP REQUEST
    • Server reconfirms with DHCP ACK
      • Includes lease timer and any other configuration information
Term
What is lease?
Definition
  • The time period when the DHCP assigned address is still valid
    • Tuning the lease timer is important
    • Typically set to 1 hour
    • When lease expires, client must renew; it may request same address again

 

Term
What are the principles of IPv6 Stateless Autoconfiguration?
Definition
  • IPv6 formed from the 64-bit network prefix sent in a router advertisement, with a 64-bit host part appended (based on MAC/Ethernet address, or 'random' for privacy addresses)
  • No server is required to hold the state of the address allocation to the internet
  • There is no lease
    • Instead the advertisement has a valid lifetime indicator
    • You can run an IPv6 network without DHCPv6
Term
What are the key advantages of Stateful (DHCP) assignment?
Definition
  • Managed service; administrative control
  • Improved accountability; (IP x logged in at time y)
  • Can give specific addresses to specific hosts
  • Can give different information to hosts in the same subnet
Term
What are the key advantages of Stateless (SLAAC) address autoconfiguration?
Definition
  • Doesn't require a DHCP server to be supported
  • Host 'fate shares' with its router
    • Gets prefix and default gateway from router
  • If a DHCP server goes down, there is no way to tell the hosts to get a new address
  • More efficient: can multicast Router Advertisements
Term
How are DNS and DHCP configured?
Definition
  • DNS is configured by zone files
    • Forward and reverse DNS information
    • Can be manually configured
    • Can be dynamic (Dynamic DNS from hosts)
  • Usually configure "friendly" names for DHCP clients
    • Pre-populate reverse DNS zone file
    • Gives meaningful answer to reverse lookup
Term
What is zero configuration networking?
Definition
  • Networks self-configuring without manual intervention
    • No administrative configured DHCP or DNS service
    • Rely on hosts to provide the service themselves
Term
When is zero configuration networking desirable?
Definition
  • Particularly in ad-hoc (possibly not externally connected) network subnets
  • Usually targeted to work within a signal local subnet
Term
What components does zero configuration networking need?
Definition
  • A method to obtain link-local IP addresses
  • Need a way to advertise and resolve host names
  • Need automatic discovery of network services
Term
What are some fundamentals of Zeroconf Addressing?
Definition
  • Need link-local addresses that can be used within a subnet
    • Hosts need to be able to generate these
    • Packets with these addresses are never forwarded by routers
  • IPv4
    • Use reserved range 169.254.0.0/16
    • May see this if IPv4 fails to contact a DHCP server when connecting network
  • IPv6
    • Use standard link-local addresses under fe80::/10
Term
What is multicast DNS?
Definition
  • A Zeroconf host name resolution service
  • Similar to unicast DNS, to resolve hostnames to IPs in small networks that don't include nameserver
  • DNS query/response sent using multicast on local subnet (instead than to DNS resolver)
    • If host has answer, it multicasts respond
  • Up to hosts to query and respond as required
Term
Show some common implementations of Multicast DNS
Definition
  • mDNS (Apple)
    • Uses .local domain
  • Link-local multicast name resolution (LLMNR) (MSFT)
    • Can use any domain
Term
What are some principles of mDNS?
Definition
  • Apple's local Zeroconf name resolution service
  • Hosts hold their own DNS records (A, AAAA, etc)
  • They join the mDNS multicast group
  • Clients send mDNS requests to multicast group
    • Host with matching information responds
  • No requirement on an administratively controlled DNS resolved, or off-site recursion
    • Restricted to local subnet - can't discover hosts in different subnets
Term
What are some service discovery approaches?
Definition
  • Well-known addresses
  • DNS SRV records
  • DNS-SD (used with mDNS)
Term
What are some characteristics on DNS SRV service discovery?
Definition
  • Lookup services by name in DNS
    • Uses a specific DNS SRV record type
    • Just another DNS record type A or AAAA
  • DNS entry of format:
    • _Service._protocol.Name TTL Class SRV Priority Weight Port Target
  • The response indicates the service available
Term
What are some characteristics on DNS-SD service discovery?
Definition
  • Apple's service discovery solution
    • Uses DNS SRV, TXT and PTR records
  • Hosts offering services use multicast to announce them, or respond multicast service discovery requests
Term
What is Universal Plug n Play (UPnP)
Definition
  • ISO standard, promoted by UPnP Forum
  • Defines addressing and service discovery
    • Clients join multicast groups to announce/discover services
    • Discovery gives URL from which XML service descriptions can be retreived
  • Control messages
    • Discovery includes actions that can be sent to a device from a control point
      • Uses Simple Object Access Protocol (SOAP)
    • Can control a home router (Adding NAT)
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