quartz/content/notes/19-network-layer-data-plane.md

title	aliases	tags	sr-due	sr-interval	sr-ease
19-network-layer-data-plane		cosc203 lecture	2022-10-21	3	250

• transport layer messages are divided into segments
• passed to network layer
  • sender: encapsulates segments into IP packets, passes to link layer
  • reciever: deliver segments to tranport layer protocol
  • sender and reciever are connected by multple routers
• routers
  • examine header fields in all IP packets passing through it
  • moves IP packers from input ports to output ports to transfer packets along end-end path
• network layer protocols
  • IP
  • internet control message protocol (ICMP)
    • use for troubleshooting type things

Forwarding

forwarding

• move packets from a routers input link to appropriate output link
• local, per-router function
• function in data plane

routing

• determine route taken by packets from source to destination
  • network-wide logic
  • function in control plane
  • chooses shortest path through routers

network layer service model

best effort

• try its best to deliver packet
• does not guarantee successful delivery
• transport layer provides reliability
• no big deal if packet is lost
• does not guarantee timing or order of delivery
  • each packet sent individually, possibly through different paths
• does not guarantee bandwidth available
  • network layer uses packet switching which unlike socket switching does not reserve resources

Routers

destination-based forwarding: foward based only on destination IP address

input port queueing: if datagrams arrive faster than fowarding rate into switch fabric

IP addressing

• unreliable host-to-host communication protocol
  • datagram formatting, IP addressing, packet switching

• ver: IP protocol versin number
• head len: header length in bytes
• type of service: diffserv(0:5) or ECN (6:7)
• length: total datagram length (bytes)
• time to live: remaining max hops (decremented at each router)
• upper layer: which protocol use at transport layer
• 16-bit id, flgs, fragment offset: fragmentation/reassembly
• header checksum
• source IP
• destination IP
• options: e.g., timestamp, record route taken

overhead:

• 20 bytes TCP
• 20 bytes IP
• = 40 bytes + app layer overhead for TCP+IP

interface: connection between host/router and physical link

• router have multiple interfaces
• host has one or two interfaces :e.g., ethernet and wireless

IPv4 address: 32-bit id associated with each host or router interface

• two main components
  • network ID
    • addresses in the same network have the same network ID
  • host ID
• 5 classes
  • 

subnetting

• partitioning an IP network into multiple smaller network segments
  • designate some high-order bits from host part as subnet ID
  • 
• netmask
  • a 32-bit number with all 1s for network part and all 0s for host part

• use bitwise and to find network part from address and mask

fragmentation

• network links have an MTU (maximum transmission unit)
  • different linktypes have different MTUs
• large UP datagrams are fragmented at routers
  • one datagram become several
  • reassemble only at destination
  • IP header bits are used to identify fragments

header

• identifier
  • same for all fragments
• flag: 3 bits
  • 1st not used
  • 2nd - do fragment
  • 3rd - more fragment (0 for the last fragment)
• offset
  • offset of the fragment in the packets data field (units of 8 bytes)

what needs to change in the header of fragment

• flag
• offset
• length
• ttl
• checksum

special IPv4 Adresses and NAT

special addresses

• 127.0.0.1 localhost/loopback
• 10.0.0.0/8 private use
• 192.168.0.0/16 private use

NAT (network address translation): all devices in local network share just one IPv4 address

• allow multiple devices to share the same adresses
• help preserve the limited amount of IP address
• translate from private IP to public IP
• 

IPv6

• inital motivation: IPv4 mostly used up
• additional motivation
  • speed prcessing/forwarding: 40 byte fixed length header
  • enable different network-layer treatment of "flows"
  • better support for host mobility
  • better support for security

datagram format

adressses

• 128 bits
• hexadecimal colon notation
  • 12AB:0000:0000:CD30:0000:0000:0000:0000
• adress compression
  • consecutive sections of seros are replaced with ::
  • can be used only once per address
  • e.g., 12AB:0000:0000:CD30::

CIDR

• classless interdomain routing
• method for allocating IP addresses and IP routing
• notation
  • based of variable length subnet mask (VLSM)
  • address format: IP address/x, where x is number of bits from left to right as network part
  • 

extension headers: differencees:

transition from v4 to v6

• cant change all router at once
• will need mixed v4 and v6 routers

tunneling

• v6 datagram is carried as the payload in v4 datagram among v4 routers ("packet with a packet")
• used alot with 4G/5G
•