Packet Forwarding

---

• when a router receives a packet:
  • it reads the destination address in the packet
  • looks up a matching destination network IP address and prefix in its routing table
• if there is a match,
  • router will forward the packet out of one of its interfaces by encapsulating the packet in a new frame:
    • if the packet can be delivered to a directly connected network via an Ethernet interface,
      • the router uses ARP (IPv4) or Neighbor Discovery (IPv6) to determine the Data Link layer address of the destination interface
    • if the packet can be forwarded via a gateway over an Ethernet interface,
      • it inserts the next hop router’s MAC address as the destination address in a new frame
      • uses the MAC address of the outgoing interface as the source address
    • if the packet can be forwarded via a gateway over another type of interface (e.g., leased line or DSL),
      • the router encapsulates the packet in an appropriate frame type
    • if the destination address cannot be matched to a route entry,
      • the packet is either:
        • forwarded via the default route
        • dropped and the source host is notified that it was undeliverable

Hop Count

• if the packet is forwarded via a gateway,
  • this process is repeated at each router to deliver the packet through the internetwork
  • each router along the path counts as one hop

Example

• Host A takes one hop to communicate with LOCAL_SRV
  • via directly connected interface on the LAN router
  • switches do no count as hops
• Host B takes 9 hops to communicate with REMOTE_SRV
  • traffic routed via two ISP networks
  • observer the alternate routes that could be taken
  • do any have a lower hop count

Time To Live

• at each router, time to live (TTL) header field is decreased by at least 1
• could be greater if the router is congested
• TTL is nominally the number of seconds a packet can stay on the network before being discarded
• defined as a unit of time (seconds)
  • but in practice, it is interpreted as a maximum hop count
• when TTL is 0, the packet is discarded
  • prevents badly addressed packets from permanently circulating the network

Info

• In IPv6, the field is named Hop Limit to formalize the face that it is a counter not a timer