Lecture-8.txt

###Lecture 8

####Designing the IP Header
- Think of the IP as an interface
	- between the source and destination end-systems
	- between the source and network (routers)
- Designing an interface
	- what task(s) are we trying to accomplish
	- what information is needed to do it?
- Header reflects information needed for basic tasks

####What information do we need?
- Parse packet
	- IP version number (4 bits), packet length (16 bits)
- Carry packet to the destination
	- Destination's IP address (32 bits)
- Deal with problems along the way
	- loops: TTL (8 bits)
	- corruption: checksum (16 bits)
	- packet too large: fragmentation fields (32 bits)
- Accomodate evolution
	- version number (4 bits) (+ fields for special handling)
- Specify any special handling
	- ToS (8 bits)
	- Options (variable length)

#####Preventing Loops (TTL)
- Forwarding loops cause packets to cycle for a long time
	- Left unchecked would accumulate to consume all capacity
- Time-to-Live (TTL) Field (8 bits)
	- decremented at each hop, packet discarded if reaches 0
	- ...and "time exceeded" message is sent to the source

#####Header Corruption (Checksum)
- Checksum (16 bits)
	- Particular form of checksum **over packet header**
- If not correct, router discards packets
	- So it doesn't act on bogus information
- Checksum recalculated at every router
	- Why?
	- Why only header?

#####Fragmentation
- Every link has a "Maximum Transmission Unit" (MTU)
	- largest number of bits it can carry as one unit
- A router can split a packet into multiple "fragments" if the packet size exceeds the link's MTU
- Must reassemble to recover original packet

#####Special handling
- "Type of Service" (8 bits)
	- also called "Differentiated Services Code Point (DSCP)"
	- allow packets to be treated differently based on needs
		- e.g., indicate priority, congestion notification
	- has been redefined several times

#####Options
- Optional directives to the network
	- not used very often
	- 16 bits of metadata + option-specific data
- Examples of options
	- Record Route
	- Strict Source Route
	- Loose Source Route
	- Timestamp

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####Parse packet
- Header length (4 bits)
	- Number of 32-bit words in the header
	- Typically "5" (for a 20-byte IPv4 header)
	- Can be more when IP options are used

####What are these tasks? (at the destination end-system)
- Tell destination what to do with the received packet
	- Transport layer protocol (8 bits)
- Get responses to the packet back to the source
	- Source IP address (32 bits)

#####Telling End-Host how to handle packet
- Protocol (8 bits)
	- identifies the higher-level protocol
	- important for **de-multiplexing** at receiving host
- Most common examples
	- E.g., "6" for the Transmission Control Protocol (TCP)
	- E.g., "17" for the User Datagram Protocol (UDP)