Computer Networks · free preview
The Internet Protocol
The Internet Protocol glues heterogeneous networks into one logical internetwork by promising almost nothing, its best-effort datagram service may lose, duplicate, corrupt or reorder packets and therefore runs over any link technology that exists or might be invented, its header carries a hop-count time to live, a protocol demultiplexing key, a header checksum and full source and destination addresses, fragmentation and reassembly carry oversized datagrams across networks with small maximum transmission units using a shared identifier and byte offsets, addresses are hierarchical with a network part and a host part so routers can forward by prefix, subnet masks and classless interdomain routing reclaim the waste of the old classful system and aggregate routes, and the address resolution protocol and the dynamic host configuration protocol fill in the last-mile details of link-layer translation and host configuration.
1. The Narrow Waist
The Narrow Waist
2. The Datagram Header
The Datagram Header
3. Fragmentation and Reassembly
Fragmentation and Reassembly
4. Addresses That Aggregate
Addresses That Aggregate
5. The best-effort bargain
Why did IP's designers choose a service model that may lose, duplicate, and reorder packets?
6. Time to live
What failure does the time-to-live field protect against, given that routers decrement it by one per hop and discard the packet at zero?
7. Reading the fragments
A 1400-byte payload must cross a link whose maximum transmission unit leaves 512 bytes per fragment. What offsets and more-fragments flags do the fragments carry?
8. Why hierarchy scales
Why do hierarchical network-plus-host addresses scale better than flat addresses for forwarding?
9. Subnets and supernets
A campus with thirty physical networks wants to appear as one network to the outside. Which mechanisms make that work, and in which direction does each operate?
10. The ARP exchange
A host must deliver a datagram to another host on the same Ethernet but knows only its IP address. How does the mapping get resolved?
11. The demultiplexing key
Which IP header field names the higher-level protocol that should receive the payload, with 6 for the stream protocol and 17 for the datagram protocol?
12. Offset units
In IP fragmentation, the offset field positions each fragment within the original datagram in units of how many bytes?
13. Reassembly site
Fragments of an IP datagram are reassembled at which node: the fragmenting router, the next router, or the destination host?
14. Cache lifetime
Roughly how many minutes does a host keep an address resolution mapping in its cache before timing it out?
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