CSC 401 Data and Computer Communications Networks Network Layer Overview, Router Design, IP Sec 4.1. 4.2 and 4.3 Prof. Lina Battestilli Fall 2017
Chapter 4: Network Layer, Data Plane chapter goals: understand principles behind network layer services, focusing on data plane: network layer service models forwarding versus routing how a router works generalized forwarding instantiation, implementation in the Internet
Chapter 4 Outline Network Layer: Data Plane 4.1 Overview of Network layer data plane control plane 4.2 What s inside a router 4.3 Internet Protocol (IP) 4.4 Generalized Forward and SDN
The 5 Layer Internet Model Source End-Host Destination End-Host Application Application Transport Router Router Transport Network Network Network Network Link Link Link Link Physical Physical Physical Physical
IP is the thin waist We must use the Internet Protocol (IP) Application Transport Network Link/Physical http smtp ssh ftp TCP UDP RTP IP Ethernet WiFi DSL 3G
Network-layer functions 1. Forwarding: move packets from router s input ports to appropriate output ports 2. Routing : determine route taken by packets from source to destination - routing algorithms analogy: routing: process of planning trip from source to destination forwarding: process of getting through single interchange
Network layer: Data Plane, Control Plane Data plane local, per-router function determines how datagram arriving on router input port is forwarded to router output port forwarding function values in arriving packet header 0111 3 1 2 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 14
Per-router Control Plane Individual routing algorithm components in each and every router interact in the control plane Routing Algorithm control plane data plane values in arriving packet header 0111 3 1 2 15
Logically centralized Control Plane A distinct (typically remote) controller interacts with local control agents (CAs). The network is software defined Remote Controller CA control plane data plane values in arriving packet header CA CA CA CA 0111 3 1 2 Software Defined Networking (SDN) toward open source in networking. 16
Network Layer Services What services do we need from the network layer for delivering datagrams from sender to receiver? For individual datagrams: Guaranteed delivery Guaranteed delivery with bounded delay (e.g 40msec) For a flow of datagrams: in-order datagram delivery Guaranteed minimum bandwidth per flow Guaranteed maximum jitter (restrictions on changes in inter-packet spacing) Security Services 17
Network Layer Service models: Example from ATM (defined late 90s) Network Architecture Service Model Bandwidth Guarantees? Loss Order Timing Congestion feedback Internet ATM ATM ATM best effort CBR VBR ABR none constant rate guaranteed rate guaranteed minimum no yes yes no no yes yes yes no yes yes no no (inferred via loss) no congestion no congestion yes
Connection and Connectionless service Virtual-Circuit Network: provides network-layer connection service Datagram Network: provides network-layer connectionless service Analogous to TCP/UDP connection-oriented/connectionless transport-layer services, but: service: host-to-host no choice: network provides one or the other (to date) implementation: in network core
Virtual Circuit (VC) Network connection source-to-dest path behaves much like telephone circuit performance-wise network actions along source-to-dest path call setup, teardown for each call before data can flow each packet carries VC identifier (not destination host address) every router on path maintains state for each passing connection link, router resources (bandwidth, buffers) may be allocated to VC (dedicated resources = predictable service)
Datagram Networks connectionless No call setup at network layer routers: no state about end-to-end connections no network-level concept of connection packets forwarded using destination host address application transport network data link physical 1. send datagrams 2. receive datagrams application transport network data link physical
Chapter 4 Outline Network Layer: Data Plane 4.1 Overview of Network layer data plane control plane 4.2 What s inside a router 4.3 Internet Protocol (IP) 4.4 Generalized Forward and SDN
Router architecture overview High-level view of generic router architecture. Key router functions: 1. run routing algorithms/protocol (RIP, OSPF, BGP) 2. forward datagrams from incoming to outgoing link msec, sec timescale forwarding tables computed, pushed to input ports e.g. PCI routing processor routing, management control plane (software) forwarding data plane (hardware) high-seed switching fabric Must be done fast! 10Gbps link 64byte datagrams ~ 50ns timescale router input ports router output ports Roundabout Analogy Where do bottlenecks occur?
24 Example of High-End Routers Juniper MX2020 ISP Edge Router Cisco 12000 Series Service Providers 960 10Gbps ports Overall: 80 Tbps 16 slots, 1.28 Tbps 10 slots, 800 Gbps 16slots, 320 Gbps etc.
Input port functions Physical Layer: bit-level reception Link layer: e.g., Ethernet line termination link layer protocol (receive) lookup, forwarding queueing switch fabric Decentralized switching: using header field values, lookup output port using forwarding table in input port memory ( match plus action ) destination-based forwarding: forward based only on destination IP address (traditional) generalized forwarding: forward based on any set of header field values goal: complete input port processing at line speed queuing: if datagrams arrive faster than forwarding rate into switch fabric
Datagram Forwarding Table routing algorithm local forwarding table dest address output link address-range 1 address-range 2 address-range 3 address-range 4 3 2 2 1 IP address is 32 bits 4 billion IP addresses, so rather than list individual destination address list range of addresses (aggregate table entries) Match plus Action prevalent in network devices IP destination address in arriving packet s header 1 3 2
Destination-based forwarding Destination Address Range 11001000 00010111 00010000 00000000 through 11001000 00010111 00010111 11111111 11001000 00010111 00011000 00000000 through 11001000 00010111 00011000 11111111 11001000 00010111 00011001 00000000 through 11001000 00010111 00011111 11111111 otherwise Decimal 200.23.16.0 to 200.23.23.255 200.23.24.0 to 200.23.24.255 200.23.25.0 to 200.23.31.255 otherwise Link Interface 0 1 2 3 Q: but what happens if ranges don t divide up so nicely?
Longest prefix matching longest prefix matching when looking for forwarding table entry for given destination address, use longest address prefix that matches destination address. Destination Address Range 11001000 00010111 00010*** ********* 11001000 00010111 00011000 ********* 11001000 00010111 00011*** ********* otherwise examples: forwarding table DA: 11001000 00010111 00010110 10100001 DA: 11001000 00010111 00011000 10101010 Link interface 0 1 2 3 which interface? which interface?
29 Longest prefix matching must be fast! we ll see why longest prefix matching is used shortly, when we study IP addressing Often performed using Ternary Content Addressable Memories (TCAMs) content addressable: give the address to TCAM: retrieve address in one clock cycle, regardless of table size Cisco Catalyst: can up ~1M routing table entries in TCAM
Switching fabrics transfer packet from input buffer to appropriate output buffer switching rate: rate at which packets can be transfer from inputs to outputs often measured as multiple of input/output line rate N inputs: switching rate N times line rate desirable three types of switching fabrics memory memory bus crossbar
Switching via memory first generation routers: traditional computers with switching under direct control of CPU packet copied to system s memory speed limited by memory bandwidth (2 bus crossings per packet) More recent routers: Lookup, storing to memory done by the input line card Like shared-memory multiprocessors Cisco Catalyst 8500 (1Gbps, yr 2016) input port (e.g., Ethernet) memory output port (e.g., Ethernet) system bus Note: two packets can NOT be forwarded at the same time (even if they have diff destination ports)
Switching via a bus datagram from input port memory to output port memory via a shared bus bus contention: switching speed limited by bus bandwidth Sufficient speed for access and enterprise routers(32 Gbps bus, Cisco 5600 in year 2016) bus
Switching via interconnection network Modern routers use more sophisticated interconnection networks (such as those for interconnecting processors ) Crossbar, banyan networks, etc. Advanced Design: fragmenting datagram into fixed length cells, switch cells through the fabric. Cisco 12000: switches 60 Gbps through the interconnection network crossbar banyan
Input port queuing fabric slower than input ports combined -> queueing may occur at input queues queueing delay and loss due to input buffer overflow! Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward switch fabric switch fabric output port contention: only one red datagram can be transferred. lower red packet is blocked one packet time later: green packet experiences HOL blocking Might be advantageous to drop some packets! (AQM/RED)
Output ports switch fabric datagram buffer queueing link layer protocol (send) line termination buffering required when datagrams arrive from fabric faster than the transmission rate scheduling discipline chooses among queued datagrams for transmission
Output port queueing switch fabric switch fabric at t, packets more from input to output buffering when arrival rate via switching fabric exceeds the output line speed queueing (delay) and loss due to output port buffer overflow! packet scheduler at output port (FCFS, WFQ) used in QoS Which packet to drop? (AQM RED) one packet time later So how much buffer space should a router have? What to consider?
How much buffering? Rule of thumb [RFC 3439 ] [Villamizar 1994]: average buffering equal to avg. RTT times link capacity C RTT*C e.g., avg. RTT=250ms, C = 10 Gpbs link: 2.5 Gbit buffer Recent recommendation [Appenzeller 2004] with N flows, buffering equal to RTT * C N If N is large, the decrease of buffers size can become quite significant
Scheduling A line/queue has formed, who should go next? 38
Scheduling Policy: FIFO FIFO (first in first out) scheduling: send in order of arrival to queue discard policy: if packet arrives to full queue: who to discard? tail drop: drop arriving packet priority: drop/remove on priority basis random: drop/remove randomly Real world example? packet arrivals queue (waiting area) link (server) packet departures
Scheduling policies: priority priority scheduling: send highest priority queued packet multiple classes, with different priorities class may depend on marking or other header info, e.g. IP source/dest, port numbers, etc. arrivals arrivals packet in service classify high priority queue (waiting area) 2 1 3 low priority queue (waiting area) 4 link (server) 5 1 3 2 4 5 departures Real world example? departures 1 3 2 4 5
Scheduling policies: RR Round Robin (RR) scheduling: multiple classes cyclically scan class queues, sending one complete packet from each class (if available) arrivals 2 1 3 4 5 Real world example? packet in service 1 3 2 4 5 departures 1 3 3 4 5
Scheduling policies: WFQ Weighted Fair Queuing (WFQ): generalized Round Robin each class gets weighted amount of service in each cycle
Chapter 4 Outline Network Layer: Data Plane 4.1 Overview of Network layer data plane control plane 4.2 What s inside a router 4.3 Internet Protocol (IP) datagram format, IPv4 addressing, DHCP, IPv6, NAT 4.4 Generalized Forward and SDN
The Internet network layer host, router network layer functions: transport layer: TCP, UDP network layer routing protocols path selection RIP, OSPF, BGP forwarding table IP protocol addressing conventions datagram format packet handling conventions ICMP protocol error reporting router signaling link layer physical layer
IP datagram format IP protocol version number header length (bytes) type of data max number remaining hops (decremented at each router) upper layer protocol to deliver payload to how much overhead? 20 bytes of TCP 20 bytes of IP = 40 bytes + app layer overhead ver head. len 16-bit identifier time to live type of service upper layer 32 bits flgs length fragment offset header checksum 32 bit source IP address 32 bit destination IP address options (if any) data (variable length, typically a TCP or UDP segment) total datagram length (bytes) used for fragmentation/ reassembly e.g. timestamp, record route taken, specify list of routers to visit.
IP fragmentation, reassembly network links have MTU (max transmission unit) - 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 reassembly fragmentation: in: one large datagram out: 3 smaller datagrams
example: IP fragmentation, reassembly 4000 byte datagram 3980 bytes of payload MTU = 1500 bytes length =4000 ID =x fragflag =0 offset =0 1480 bytes of payload one large datagram becomes several smaller datagrams length =1500 ID =x fragflag =1 offset =0 Offset is specified in units of 8 bytes chunks offset = 1480/8 length =1500 length =1040 ID =x ID =x fragflag =1 fragflag =0 offset =185 offset =370 Does fragmentation have a cost?
IP addressing: introduction IP address: 32-bit identifier for host, router interface interface: connection between host/router and physical link router s typically have multiple interfaces host typically has one or two interfaces (e.g., wired Ethernet, wireless 802.11) IP addresses associated with each interface 223.1.1.1 223.1.1.3 dotted-decimal notation 223.1.2.1 223.1.1.4 223.1.2.9 223.1.3.27 223.1.2.2 223.1.3.1 223.1.3.2 223.1.1.1 = 11011111 00000001 00000001 00000001 223 1 1 1 Each interface must have a unique IP address
IP addressing: introduction Q: how are interfaces actually connected? A: we ll learn about that in chapter 6 and 7 223.1.1.2 223.1.1.1 223.1.1.4 223.1.2.9 223.1.2.1 A: wired Ethernet interfaces connected by Ethernet switches 223.1.1.3 223.1.3.27 223.1.3.1 223.1.2.2 223.1.3.2 For now: don t need to worry about how one interface is connected to another (with no intervening router) A: wireless WiFi interfaces connected by WiFi base station
Subnets no router IP address: subnet part - high order bits host part - low order bits netmask specifies subnet and host part bits. What s a subnet? device interfaces with same subnet part of IP address can communicate with each other without intervening router 223.1.1.1 223.1.1.2 223.1.2.1 223.1.1.4 223.1.2.9 223.1.1.3 223.1.3.1 223.1.2.2 223.1.3.27 subnet 223.1.3.2 network consisting of 3 subnets
Subnets recipe to determine the subnets, detach each interface from its host or router, creating islands of isolated networks each isolated network is called a subnet 223.1.1.0/24 223.1.2.0/24 223.1.1.1 223.1.1.2 223.1.2.1 223.1.1.4 223.1.2.9 223.1.1.3 223.1.3.1 223.1.2.2 223.1.3.27 subnet 223.1.3.2 223.1.3.0/24 Let s look at the the interfaces on my laptop subnet mask: /24
Subnets how many? 223.1.1.1 223.1.1.2 223.1.1.0/24 223.1.1.4 223.1.1.3 223.1.9.2 223.1.7.0 223.1.9.0/24 223.1.7.0/24 223.1.9.1 223.1.8.1 223.1.8.0 223.1.7.1 223.1.2.0/24 223.1.2.6 223.1.8.0/24 223.1.2.2 223.1.3.1 223.1.3.27 223.1.3.0/24 223.1.3.2
IP addressing: CIDR CIDR: 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 address subnet part host part 11001000 00010111 00010000 00000000 200.23.16.0/23 Broadcast Address 255.255.255.255 all hosts in a subnet get the those datagrams
References Some of the slides are identical or derived from 1. Slides for the 7 th edition of the book Kurose & Ross, Computer Networking: A Top-Down Approach, 2. Computer Networking, Nick McKeown and Philip Levis, 2014 Stanford University