IPv4

Datagram

The basic unit of data transferred in the network layer.

Header fields:

• Version (4 bits)
  IP version. Set to 4 for IPv4.
• IHL (4 bits)
  Internet Header Length. Header size in 32-bit words. Minimum 5 = 20 bytes.
• DSCP/ToS (8 bits)
  Quality of service classification.
• Total Length (16 bits)
  Full packet size in bytes, header included.
• Identification (16 bits)
  Groups fragments belonging to the same original datagram.
• Flags (3 bits)
  Reserved, Don’t Fragment (DF), More Fragments (MF).
• Fragment Offset (13 bits)
  Position of this fragment within the original datagram, in 8-byte units.
• TTL (8 bits)
  Decremented by each router. Packet dropped when it reaches $0$.
• Protocol (8 bits)
  Transport-layer protocol: 6 = TCP, 17 = UDP.
• Header Checksum (16 bits)
  Error detection for the header only. Verified and recomputed at each hop because TTL changes with each hop.
• Source IP Address (32 bits)
  Sender’s IP address.
• Destination IP Address (32 bits)
  Receiver’s IP address.
• Options (0–40 bytes)
  Present only if IHL > 5. Rarely used in practice.

Maximum size of a IPv4 packet is $65{,}535$ bytes. Payload carries the transport-layer segment (TCP or UDP). As most equipment cannot handle very big packets, usually the maximum transmission unit is capped at 1500 bytes. In that case, the IPv4 payload’s maximum size is 1480 bytes.

Network and Host Parts

An IPv4 address is divided into a network part and a host part.

• Network part
  Identifies the network.
• Host part
  Identifies the device within the network.

Hosts sharing the same network part belong to the same subnet and communicate without a router. The boundary between parts is set by the subnet mask.

Network size is determined by the number of bits in the host part. More host bits means more addressable hosts but fewer subnets from the same block.

IP Address Allocation

• Host assignment
  Manually configured, or dynamically via DHCP.
• Network assignment
  ISP allocates an address block. ISPs receive blocks from a RIR (e.g. APNIC). RIRs receive blocks from ICANN.

Dynamic Host Configuration Protocol

Aka. DHCP. Dynamically assigns IP addresses to hosts. Enables automatic configuration, address reuse, and host mobility.

Uses the DORA exchange:

• Discover
  Client broadcasts to locate available DHCP servers.
• Offer
  Server responds with an available IP address and lease terms.
• Request
  Client broadcasts a request for the offered address.
• Ack
  Server confirms the lease. Client configures the address.

Packet Forwarding

Moving a packet toward its destination, hop by hop.

• Same network
  Direct delivery to the destination host.
• Different network
  Forward to the next-hop router.

Routers use forwarding tables keyed on destination network prefix. The datagram is unchanged except for mutable fields (TTL, checksum).

Address Resolution Protocol

Aka. ARP. Maps IP addresses to MAC addresses within a local network. The sender broadcasts a query. The target host replies with its MAC address.

ARP cannot operate beyond the local network because broadcasts do not cross routers.

Longest Prefix Match

Aka. LPM. A router selects the most specific (longest) matching prefix in its routing table when forwarding a packet.

• Software implementation
  Tree search. Slow but space-efficient.
• Hardware implementation
  TCAM. Fast but expensive and power-hungry.

Fragmentation

In IPv4, any router can fragment a packet that exceeds a link’s MTU. Reassembly happens only at the destination.

The Identification, Flags, and Fragment Offset header fields coordinate fragment reassembly. The MF flag marks all fragments except the last. Fragment Offset gives the fragment’s position in the original datagram.

Limitations

• Address exhaustion
  32-bit addresses cap the pool at ~$4.3$ billion. Insufficient for the number of connected devices.
• Routing table growth
  Flat addressing caused global BGP tables to exceed 1 million entries. TCAM storage is expensive, power-hungry, and physically limited.
• LPM cost
  Longest prefix match over large tables is costly even with TCAM.
• Convergence delay
  Topology changes propagate slowly when routing tables are large.