4.1: Network Layer - The Data Plane Flashcards
how do routing and forwarding differ?
network-layer functions: •forwarding: move packets from router’s input to appropriate router output •routing: determine route taken by packets from source to destination • routing algorithms
analogy: car journey forwarding: process of getting through single roundabout routing: process of planning trip from source to destination
what is the data plane vs control plane?
Data plane local, per-router function determines how datagram arriving on router input port is forwarded to router output port forwarding function
Control plane network-wide logic determines how datagram is routed among routers along end-end path from source host to destination host two control-plane approaches: • traditional routing algorithms: implemented in routers • software-defined networking (SDN): implemented in (remote) servers
how does a router know where to forward?
using the header field values of the incoming datagram
draw the input port
yeet
what are destination-based and generalised forwarding? which is traditional?
destination-based forwarding: forward based
only on destination IP address (traditional)
• generalised forwarding: forward based on
any set of header field values
how can we divide ip ranges in a forwarding table?
longest prefix matching
when looking for forwarding table entry for given
destination address, use longest address prefix
that matches destination address.
describe and draw the 3 switching fabrics
advantages?
memory, bus, crossbar
memory:
used in first gen computer
switching is under direct control of the CPU
packet is copied to systems memory
speed is limited by memory bandwidth - datagram has to cross bus twice
bus:
datagram from input port memory to output port memory via a shared bus
switching speed still limited by bus bandwidth
crossbar:
overcomes bus bandwidth limitations
uses networks initially designed to connect processors in a muliprocessor
fragments datagram into fixed length cells, switching them all through the fabric
how do we prevent input buffer overflow in switches>? and output?
• Head-of-the-Line (HOL) blocking: queued datagram at
front of queue prevents others in queue from moving
forward
what is a scheduling discipline? why needed?
buffering required when datagrams arrive from fabric faster than the
transmission rate
scheduling discipline chooses among queued datagrams for transmission
Datagram (packets) can be lost
due to congestion, lack of buffers
Priority scheduling – who gets best
performance, network neutrality
• buffering when arrival rate via switch exceeds output line
speed
• queueing (delay) and loss due to output port buffer overflow!
how can priority be classified in priority scheduling?
multiple classes, with different priorities • class may depend on marking or other header info, e.g. IP source/dest, port numbers, etc
queues for different classes
how does RR scheduling work?
Round Robin (RR) scheduling: • multiple classes • cyclically scan class queues, sending one complete packet from each class (if available) NOTE THE DIFFERENCE, WE HAVE DIFFERENT CLASSES
what is WFQ scheduling?
wieghted fair queeuing a generalised Round Robin where each class gets weighted amount of service in each cycle depending on their requiremnets
what is the role of the network layer?
routing protocols, IP protocol, ICMP
describe IP fragmentation. how might a 4000 byte datagram be split with an MTU of 1500?
network links have MTU (max.transfer size) - largest possible link-level frame • different link types, different MTUs • large IP datagram divided (“fragmented”) within net • one datagram becomes several datagrams • “reassembled” only at final destination • IP header bits used to identify, order related fragments
recall that new packets need 20 byte ip header, so
1500 (20 bytes for header)
1500
1040
how big is ip header?
20 bytes
what is an ip address?
a 32bit identifer for a host, router interface
there is an IP address associated with each interface
what is a subnet?
• device interfaces with same subnet part of IP address • can physically reach each other without intervening router
the high order bits of an ip address are the subnet
remember that the individual router-router path can be a subnet
what is CIDR? How many addresses in a /23?
Classless InterDomain Routing
• subnet portion of address of arbitrary length
•address format: a.b.c.d/x, where x is # bits in
subnet portion of addres
so
11001000 00010111 00010000 00000000
200.23.16.0/23
shows that the first 23 bits are the subnet
How many addresses in a /23?
2^9-2 (first address is network ID, last address is broadcast)
how are ip addresses obtained? why do this?
DCHP -Dynamic Host Configuration Protocol
dynamically get address from a server
can renew its lease on address in use
• allows reuse of addresses (only hold address while connected/“on”)
• support for mobile users who want to join network (more shortly)
• host broadcasts “DHCP discover” msg [optional]
• DHCP server responds with “DHCP offer” msg [optional]
• host requests IP address: “DHCP request”
msg
• DHCP server sends address: “DHCP ack”
msg
what can DHCP do other than provide IPs?
DHCP can return more than just allocated IP address on subnet:
• address of first-hop router for client
• name and IP address of DNS server (remember our earlier discussion)
• network mask (indicating network versus host portion of address)
how does a network get a subnet part of IP address?
allocated a portion from ISPs address space e.g.
Organisation 0 11001000 00010111 00010000 00000000 200.23.16.0/23
Organisation 1 11001000 00010111 00010010 00000000 200.23.18.0/23
Organisation 2 11001000 00010111 00010100 00000000 200.23.20.0/23
how does an ISP get addresses?
from regional registrars
how does NAT work? what must a NAT router do to datagrams?
all datagrams leaving local network have same single source NAT IP address: 138.76.29.7, different source port numbers
NAT maps local machine’s port nos to virtual ip addresses
outgoing: replace source ip and portno to the nat ip and new port (remote hosts will respond using the nat ip and port so we have to..)
remember in the NAT table, the mappings
incoming: replace nat ip nat port with port table’s mapping
datagrams with source or
destination in this network
have 10.0.0/24 address for
source, destination
what’s the motivation behind NAT?
range of addresses not needed from ISP: just one IP address for all
devices
can change addresses of devices in local network without notifying
outside world
can change ISP without changing addresses of devices in local
network
devices inside local net not explicitly addressable, visible by outside
world (a security plus)
why is NAT controversial?
• routers should only process up to layer 3
• address shortage should be solved by IPv6
• violates end-to-end argument ( application-specific features reside in the communicating end nodes of the network, rather than in intermediary nodes)
• NAT possibility must be taken into account by app
designers, e.g., P2P applications
• NAT traversal: what if client wants to connect to server
behind NAT?
motivations for IPv6? (3)
running out of 32 bit IPv4 addresses
the header format helps to speed processing
the header also facilitates QoS
how does IPv6 differ from v4? (4)
has a priority number in header
has a flow label, allows identification of datagrams in the same “flow” (concept of“flow” not well defined).
next header field identifies the upper layer protocol for the data
checksum is removed to reduce processing time
how can an ip4 router process an IPv6 packet?
not all routers can be upgraded simultaneously
•no
“flag days”
•how will network operate with mixed IPv4 and
IPv6 routers?
• tunnelling: IPv6 datagram carried as payload in IPv4 datagram
among IPv4 routers
