Overview: IP

• addresses: network and host part
• header
• fragmentation and reassembly
• IPv6
• mobile IP
• ICMP
• DHCP
• SLIP
• PPP

Network and Host parts

• Class-based addressing
• Subnetting
• Classless addressing (Classless Inter-Domain Routing, CIDR)

Class-based addressing

• Class A addresses start with a 0 bit, followed by 7 bits of network ID and 24 bits of host ID
• Class B addresses start with 10, followed by 14 bits of network ID and 16 bits of host ID
• Class C addresses start with 110, followed by 21 bits of network ID and 8 bits of host ID
• Problems:
  • Class B is too large (uses too many bits)
  • Class C is too small for a lot of organizations (e.g. ISP, company)
  • don't ever want to renumber, so sign up for largest possible block
  • soon (1994?) "ran out" of class B addresses

Subnetting

• split big network into smaller networks
• use a network mask to identify the network part of the address
• e.g. 255.255.255.192 identifies a subnetwork with 64 host addresses
• from outside network, send to network (identified by class)
• within network, use network mask to find subnetwork/host parts

CIDR

• use network masks to route also from outside of network
• no longer need classes
• a single large network can include smaller networks which can include smaller networks (flexible hierarchy)

IP Header

IP Header

• version number is always 4 for this version of IP (IPv4)
• header length is 5 (20 bytes) unless there are options, in which case it is larger
• type of service is not used extensively in IPv4
• packet length helps us deal with padding (e.g. ethernet)
• time to live is decremented by each router, routers discard packets with TTL = 0
• protocol is like ethernet protocol type:
  • 1 for ICMP
  • 6 for TCP
  • 17 for UDP

IP Header Checksum

• 16-bit internet checksum
• covers the header, including any options
• error detection for payload must be done by higher-level protocol
• sending: checksum value set to zero before checksum computation
• receiving: checksum of entire header must be 0xffff
• because TTL (and possibly options) change at each router, checksum must be recomputed at each hop
• potential for router software errors

IP fragmentation

• IP packets cannot be larger than Maximum Transmission Unit (MTU) of underlying network
• many different networks in Internet
• router may be given packet of size N to send over network with MTU < N
• put "unique" identifier in each packet
• fragmentation may also be done by sender

IP reassembly

• ID identifies packet
• offset * 8 identifies fragment within packet
• flags identify end of packet (last fragment)

• packets, fragments may be delivered out of order
• use TTL to discard packets with lost fragments

IPv6

• design initiated in response to exhaustion of class B addresses
• 128-bit addresses
• better autoconfiguration
• better security (encryption, authentication)
• better type of service
• simpler to process a packet
• no header checksum
• "extension headers" instead of options, higher-level protocols

ICMP

• Internet Control Message Protocol
• ping
• try to inform sender when discarding a packet
• (never send in response to an ICMP packet)
• unreliable, but useful

Mobile IP

• post office forwarding address
• I get a guest IP address from the network I'm on
• forwarding agent on my network sends all my packets to the guest address
• I can either
  • send packets with my home IP address (probably dropped for security reasons)
  • send packets with my guest IP address (source address authentication won't work)

DHCP

• Dynamic Host Configuration Protocol
• "plug and play"
• broadcast request for IP address
• assigned IP address
• e.g. ISPs

PPP

• "lower layer" for IP (and other protocols)
• has IP address negotiation
• can use multiple serial lines
• 2-byte CRC
• complex, useful

SLIP

• "lower layer" for IP only
• marker character END, replaced in packet by ESC END
• escape character ESC, replaced in packet by ESC ESC
• END starts and ends packet
• IP+TCP headers are predictable: compression increases rate of useful data