network layer Flashcards
what is the network layer
transport seg from sender to receiver
- send side: encapsulates seg into datagram
- net layer protocols in every host, router
- router examines header field
2 key net layer principles
addressing, link layer independence
what is link layer independence
services provided does not depend on internal organization of the underlying datalink layers
what is addressing
each net layer entity is identified by net layer address
What are the two net layer planes
control
data
what control plane used for
compute and install routing tables on routers - needs to understand entire network that host is part of - network wide logic
what is data plane used for
local -per router - used by hosts and routers to create and process packets that contain user data - transform to transport packet
two key net layer functions
routing, forwarding
what is routing
determine route taken by packets using routing algos
what is forwarding
move packets from router inp to router out
describe the interplay between routinng and forwarding
routing algo determines end-end path through network, forwarding table determines local forwarding for this router
describe IP fragmentation and reassembly
network links have Max Transfer Size which is the largest possible link-level frame
large IP datagram fragmented within the net
- one DGram becomes several
-reassembled at final dest
- IP header bits identify and order fragments
name and explain 2 network layer service models
Datagram network - connectionless service
Virtual circuit - connection service
difference between DG network/virtual circuit vs UDP/TCP
service - host to host
no choice in which service is used - implemented in network core
What are some properties of datagram networks
- no call setup at net layer
- pkt contains src and dest host addresss plus data
- routers: no state about end-to-end connections (hop by hop forwarding)
- pkts forwarded using dest host address
what is IP address
32 bit identifier for host, router interface
what is an interface
connection between host/router and physical link - routers have multiple interface, hosts typically have one or two interfaces (e.g. ethernet, wifi 802.111)
what is a subnet
device interfaces with same same subnet part of the ip address - can physically reach each other without intervening router
how to change subnet sizes - explain what happens when you change the size of the host part of the IP address
increase host part - fewer subnets - more hosts in each subnet
decrease - smaller subnets - less hosts - more subnets
where do ISPs get addresses?
ICANN - internet corporation for assigned names and numbers - allocate addresses and manage DNS
motivation for IPv6
32 bit addresses run out
header format helps speed processing/forwarding
header changes to facilitate QoS
IPv6 header format
fixed length - 40byte
no fragmentation
128 bit address space
IPv6 header changes from IPv4
- priority
- flow label - identify DG in same flow
- next header - identify upper layer protocol for data
- no checksum
- options are outside of header - indicated by next header field
- ICMPv6
- additional message types - pkt too big
- multicast group management functions
what is tunneling?
IPv6 datagram carried as payload in IPv4 DG among IPv4 routers
What is created when aggregating routers into regions
Autonomous Systems
Do routers in the same AS run different routing protocols?
No, routers in same AS run the same routing prots - intra AS routing prot
what is a gateway router?
At edge of own AS - links to router in another AS
interconnected AS - what is the forwarding table configured by?
Intra - sets entries for internal dests
inter - sets entries for internal and external dests
Most common intra AS routing protocols
RIP - routing info prot
OSPF - Open shortest path first
IGRP - Interior gateway routing prot
OSPF properties
Link state algo
-uses Djikstra
-net topology at each node
-ls packet dissemation
ospf advertisement carries one entry per neighbor
advertisements flooded to entire AS - carried in OSPF messages over IP rather than TCP/UDP
Hierarchical OSPF - components
local area, backbone
- link state adverts only in area
- each nodes detailed area topology only know direction (shortest path) to nets in other areas
what are area border routers
summarize distances to nets in own area, advertise to other Area border routers
what are backbone routers
run ospf routing limited to backbone
what are boundary routers
connect to other AS’s
what is BGP
Border gateway prot - glue that holds internet together, allows subnet to advertise its existence to the rest of the internet
eBGP
obtain subnet reachability from neighboring AS’s
iBGP
propogates reachability to all AS internal routers
what does BGP provide to each AS
means to: eBGP, iBGP, determine good routes based on reachability info
why different Intra and inter AS routing?
Policy
-inter AS wants control over how its traffic is routed, who routes through its net
- intra AS: single admint so no policy decisions needed
Scale
-Hierarchical routing saves table size, reduced update traffic
performance
- intra - can only focus on performance
- inter - policy may dominate over performance
