introduction to networking - Ch.5 network Layer

Submitted by keithyau on Thu, 01/14/2010 - 15:37

Network Layer

Network Layer function
  1. Forwarding - move packets from router's input to appropriate router output
  2. Routing - determine route taken by packets from source to destination


Network vs Transport layer connection service

  1. network: between two hosts (may also involve intervening routers in case of VCs)
  2. Transport: between two processes
Virtual Circuits (Not Internet network)
  1. Like telephone calls
  2. performance-wise
  3. network actions along source-to-destination path
  4. Logical setup, not physically

Internet - datagram network
  1. data exchange among computers
  2. smart end systems - PC/servers
  3. many link types
  4. In Internet, Network layer is like this
Network mask (netmask)
  1. With flexible network/host part boundary, need to allow software in routers and hosts to identify where the division between the network number and the host number is.
  2. The mask is a 32-bit number where all bits in the network portion of the address are set to 1 and all bits in the host portion are set to 0
  3. E.g. 255.255.255.128=11111111.1111111.1111111.10000000 <-True
  4. E.g. 255.255.3.0=11111111.11111111.00000011.0 <-False, Since 1 is not contiguous
  5. subnetting is a procedure to multiple netmask(Above) by "AND gate" with IP address
  6. E.g. 192.168.1.123 AND 255.255.255.0 = 192.168.1.255
  7. Use to extract network-portion, divide networks
  8. Build Hierarchical addressing using subnetting
Looking up an IP address

For a given dest. IP address

  1. Search the routing table for the longest prefix match for the address.
  2. Extract the outgoing interface (or next-hop address.) from the routing table entry
  3. Send the packet to this outgoing interface (or next-hop addr. which may require recursive lookup(s))
  4. If no match was found, report that the destination is unreachable

CIDR (Classless InterDomain Routing)

  1. Reduce the shortage of Class B addresses
  2. Slow down the growth of routing table
How can your ISP - Internet Service Provider get block of addresses?
- Submit request to ICANN: Internet Corporation for Assigned Names and Numbers
NAT (Network Address Translation)
  1. Hide your network with private addresses
  2. One Location (E.g. an office) is just using one public IP address
  3. Have NAT translation table to map those IP addresses
  4. Same external IP address, different source port numbers


ICMP: Internet Control Message Protocol

  1. Used by hosts, routers, gateways to communicate network-level information
  2. echo request/reply
  3. Code:
Router
  1. Run routing protocols
  2. forwarding datagrams from incoming to outgoing link
Switch
Three types of switches
  1. Memory
  2. Bus
  3. Crossbar
Performance

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