ARP, Fragmentation and Reassembly

2024-08-26 10:12:02

Address Resolution Protocol

IP addresses are said to be logical, because they are defined in terms of logical topology of the routers and end systems.
The logical IP addresses need to be converted into specific physical addresses that identify the physical endpoints for the Ethernet sender and receiver
ARP: conversion between IP address and Physical address

Address Resolution Protocol (ARP)

How to map an IP address to a physical address? How to speed up? How fresh?
How to map:First, broadcast. Second, specific host reply.
How to speed up:cache the mac address

Fragmentation(分裂) and Reassembly

Each physical network imposes(强加) a certain packet size limitation on the packet to be carried, called maximum transmission unit MTU.
Router will devide packet into smaller pakcet according to the MTU.

RE: IP Packet Header

Identification, Flags, and Fragment Offset: used for fragmentation and reassembly
Identification is used to identify which packet a particular fragment belongs to.
Flag:contain tree

unsed bit:
don't fragment is set to one :it will force the router not to fragment the pakcet.
more fragment is set to one:tell the estination host that there are more fragments to follow.

The Fragment Offset field identidies the location of a fragment in the packet.
Fragment offset(分段偏移) is 13 bits; total length is 16 bits, what does it imply?

Example: Fragmenting a Packet

Packet is to be forwarded to a network with MTU of 576 bytes. The packet has an IP header of 20 bytes and a data part of 1484 bytes.
Maximum data length per fragment = 576 - 20 = 556 bytes.
Set maximum data length to 552 bytes to get multiple of 8.
All the value except the header checksum are the same as in original packet.

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