> For the complete documentation index, see [llms.txt](https://sliu583.gitbook.io/blog/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://sliu583.gitbook.io/blog/networking/index/computer-networking-a-top-down-approach/stanford-cs144/chapter-1/1.3-the-ip-service-model.md).

# 1.3 The IP Service Model

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* IP 
  * Datagram service: packet routed according to the header 
    * Each router contains a forwarding table (where to send the packets next), use destination address to index in the table to forward 
  * Unreliable
  * Best effort: won't drop datagrams arbitrarily, only drop if necessary 
    * E.x. congestion (drop), or duplicated by mistakes 
  * Connectionless 
    * Each datagram individually, independently 
* Why so simple? 
  * Simple, dumb, minimal: faster, more streamlined and lower cost to build and maintain
  * The end-to-end principle: where possible, implement features in the end hosts 
  * Allows a variety of reliable (or unreliable) services to be built on top 
    * E.x. some times there's no need to re-transmit (video services) 
  * Works over any link layer: makes very few assumptions about the link layer below 

#### IP Service Model (Details)

1. **Tries to prevent packets looping forever** 
   1. Likely to happen when forwarding table is changing
   2. Hop-count field (TTL field): decremented by every router it passes through, if reaches 0, then IP concludes that it must be stuck in the loop, delete the datagram
2. **Will fragment packets if they are too long**
   1. Ethernet: carry shorter than 1500 bytes packet  
   2. IP provides header field to help the router fragment the datagram into two self-contained IP datagrams 
3. **Uses a header checksum to reduce the chances of delivering datagram to wrong destination** 
4. **Allows for new versions of IP** 
   1. Currently IPv4 with 32 bit addresses 
   2. And IPv6 with 128 bit addresses 
5. **Allows for new options to be added to header** 

#### IPv4 Datagram 

* Protocol ID: what's inside the data field. Allows the destination end host to demultiplex arriving packets, sending them the correct code to process the packet 
  * E.x. 6: TCP segment --> TCP code --> parse the segment 
  * Total 140 values (representing different transport protocol) 
* Total packet length: up to 64k bytes 
* TTL: prevent looping. Decrement by router. If 0, router drops the packet. 
* Packet ID, flags, fragment offset: help router to fragment 
* Type of service: hint to routers of how important this packet is 
* Header length: how big the header is 
* Checksum: whole header, just in case the header is corrupted, we are not likely to deliver a packet to the wrong destination by mistake 

![](/files/-MgXAQ5oouyx-WSyTvFN)

#### Summary 

* Use IP every time we send and receive datagrams 
* IP provides deliberately simple service:
  * Datagram
  * Unreliable 
  * Best-effort 
  * Connectionless