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Lecture 9

CSE 123 Lecture Notes - Lecture 9: Ipv4, Packet Loss, Checksum

Computer Science and Engineering
Course Code
CSE 123
Alex Snoeren

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So what does IP do?
- own addressing scheme (for routers to forward packets)
- fragmentation
E.g: FDDI’s max packet is 4500 bytes while Ethernet is 1500 bytes!
- “some” error detection (why not ALL?)
- something might go wrong in the LAST hop
- Routers only forward to NEXT hop
- does not worry about packet loss, duplication, reassembling, or retransmitting packets!
IP Packet Header (20 bytes min.)
Header Length (HL) - how MUCH of the packet is the header?
- ONLY allow HL to be a multiple of word (4 bytes)
Length - how long is the WHOLE packet in bytes?
- includes header
- limits total packet to 64K
- most link layers cannot accommodate an MTU of 64K!
IP Checksum
- validates content of header ONLY
- re-calculated at EACH hop!
- ensures correct destination receives packet
- why not CRC?
- we know how to “incrementally” modify the checksum in a very straightforward manner
- if it doesn’t pass, just DROP the packet!
- which transport protocol is the data using? (what’s UP?)
- there are MULTIPLE transport protocols!
- how should a host interpret the data
- which transport layer handed it down to IP?
TCP = 6, UDP = 17
- how should this packet be “treated”?
- “handle with care”
- up to router to “respect” this request!
TTL (Time-to-Live)
- get there before this “blows” up!
- how many MORE routers can this packet pass through?
- each one decrements the TTL field
- router discards packet when TTL = 0
- Internet is designed to now grow MORE THAN 255 hops!
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