Network Layer

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Overview

• Sending side, chunks segments into datagrams
• Receiving side, delivers segments to transport layer
• Key functions:
  • Forwarding: move packets from router’s input to appropriate output router (getting packet through single interchange)
  • Routing: determine route taken by packets from source to destination (planning packet route from source to destination)
• Connection setup
  • Before sending datagrams, two hosts and intervening routers establish virtual connection
  • Distinction between network and transport:
    • network: between two hosts (may involve intervening routers)
    • transport: between two processes
• Services, per datagram
  • guaranteed delivery
  • guaranteed delivery with less than 40 msec delay
• Services for a flow of datagrams
  • in order delivery
  • guaranteed minimum bandwidth
  • restrictions on inter-packet spacing

Connection, connectionless service

• datagram network provides network layer connectionless service
• virtual circuit network provides network layer connection service
• analogous to TCP/UDP connection-oriented/connectionless transport layer services, but:
  • service: host-to-host
  • no choice: network provides one or the other
  • implementation: in network core

Virtual Circuit

• source-to-destination path behaves like a phone circuit
• call setup, teardown for each call before data can flow
• each packet carries VC identifier, not destination host address
• every router on source-to-destination path has state for each passing connection
• link, router resources (bandwidth and buffers) may be allocated to VC
  • dedicated resources gives predictable service

Datagram network

• no call setup at network layer
• no state about connections in routers
• packets forwarded using destination host address
• forwarding table has ranges of addresses instead of individual addresses
  • Longest prefix matching: looks for forwarding table entry for a destination address that has the longest matching address prefix

Router architecture

• Input, output ports
• Switching fabric
  • forwards data
  • Transfer packet from input buffer to output buffer
  • switching rate: rate at which packets can transfer from input to output
    • often measured as multiple of i/o line rate

    • \(n\)

      inputs: switching rate

      \(n\)

      times line rate is desirable

  • types: memory, bus, crossbar
• routing processor
  • Runs routing algorithms and protocols
• Has line termination, link layer protocol receiver, lookup forwarding and queueing

Input port functions

Decentralized switching

• given datagram destination, look up output port using forwarding table in input port memory (match plus action)
• goal: complete input port processing at “line speed”
• queueing: if datagrams arrive faster than can be outputted, queue them up

Switching factors

• transfer packet from input buffer to appropriate output buffer
• Switching via memory
  • Traditional computers with switching under direct control of CPU
  • packet copied to system’s memory
  • spreed limited by memory bandwidth (2 bus crossings per datagram)
• Switching via bus
  • datagram from input port memory to output port memory via shared bus
  • bus contention: switching speed limited by bandwidth
  • 32 Gbps: sufficient for access and enterprise routers
• Switching via interconnection network
  • Overcome bus bandwidth limitation
  • banyan networks, crossbar, other interconnection nets initially developed to connect processors in multiprocessors
  • fragments datagram into fixed cell lengths, switch cells through fabric
  • switches 60 Gbps through the interconnection network

Output port functions

• Same as input ports but in opposite order
• buffering required for datagrams that arrive faster from fabric than transmission rate
• scheduling discipline chooses among queued datagrams for transmission
• Avg buffering equal to typical RTT (e.g. 250 msec) times link capacity

  \(C\)

  • e.g.

    \(C\)

    = 10 Gbps, link is 2.5 Gbit buffer

  • Recent recommendation: with

    \(N\)

    flows, buffering is equal to

    \(\frac{RTT \cdot C}{\sqrt{N}}\)

Queueing

• input queueing happens when transmission rate of input is faster than fabric rate
• Head-of-the-line (HOL) blocking: queued datagram at front of queue prevents other in queue from moving forward

Internet Protocol

• IP has addressing conventions, datagram format, and packet handling conventions
• ICMP (Internet Control Message Protocol) is for error reporting and router signalling

Datagram format

32 bits

version
head len
type of service
length

16-bit identifier
flags
fragment offset

time to live
upper layer
header checksum

32 bit source IP address
32 bit destination IP address
options, if any
data (variable lenfth, typically a TCP or UDP segment)

• Time to live is the max number of remaining hops, decremented at each router
• Overhead: 20 bytes of TCP header + 20 bytes of IP header = 40 bytes, plus app layer overhead

Fragmentation, reassembly

• network links have maximum transfer size MTU: largest possible link level frame
  • different link types have different MTUs
• large IP datagram becomes divided (“fragmented”) within network
  • one datagram becomes several
  • reassembled only at final destination
  • IP header bits used to identify, order related fragments
• Have length, ID, frag flag, offset
  • fragments are specified in units of 8 byte offsets
  • fragflag = 0 implies that it is the last one, so when received, the datagram can be reassembled

IP Addressing

• 32 bit identifier for host, router interface
• interface: connection between router/host and physical link
  • routers typically have multiple interfaces
  • host typically has one or two interfaces (e.g. wired ethernet, wireless 802.11)
• IP addresses associated with each interface

Subnets

• A subnet includes device interfaces with same subnet part of IP address
  • can physically reach other without a router
• Address
  • Subnet part: high order bits
  • Host part: low order bits
• subnet mask: e.g. /24 at end of address means that the most significant 24 bits are the fixed subnet part, and the rest are the host part.
  • Class A: /8
  • Class B: /16
  • Class C: /24

CIDR

• Classless InterDomain Routing
• Subnet portion of address can be arbitrary length
• format: a.b.c.d/x, where x is the number of bits in the subnet portion
• All ones in the host part is used for broadcast
• All zeros is used for network identification

Getting an IP address

• hard-coded sometimes
• DHCP

DHCP

• Goal is to allow any computer to automatically get an IP address
• leases an address, can be renewed
• allows reuse of addresses (only holds while connected)
• host broadcasts DHCP discover message
• DHCP server responds with DHCP offer
• host requests IP address with DHCP request msg
• DHCP server replies with DHCP acknowledgement
• Port 67 reserved for DHCP server, Port 68 reserved for incoming DHCP client
• Server uses broadcast to reply because the client doesn’t have an address yet

• After receiving a reply from the server, the client knows the server’s address, and can reply directly to the server

• How servers pick IPs
  • network gets allocated a portion of its provider ISP’s address space
  • ISP gets block from ICANN: Internet Corporation for Assigned Names and Numbers

Hierarchical Routing

• “send me anything with addresses beginning (e.g.) 200.23.16.0/20”
• Gets progressively more specific

NAT: Network Address Translation

• All datagrams leaving local network have the same single source NAT IP address, assigned by ISP, different source port numbers
• Datagrams with source or destination in this network have 10.0.0/24 address for sources, destination
• Outside IP address can change without affecting local IP addresses and vice versa (e.g. changing an ISP)
• Router has NAT translation table from WAN side address to LAN side address
• Talking to computers on another local network:
  • manual configuration
  • IGD
  • both connect to relay

ICMP: Internet Control Message Protocol

• used by hosts and routers to communicate network-level information
• ICMP message: type, code, plus first 8 bytes of datagram causing error

Traceroute

• Send series of UDP segments to destination
  • First has TTL=1, second with TTL=2, etc
  • unlikely port number
• When nth set of datagrams arrives to nth router:
  • router discards datagrams
  • sends source ICMP message for TTL expired (Type 11, code 0)
  • ICMP messages include name of router and IP address
• When ICMP messages arrive, source records RTTs
• Stops when:
  • UDP segment eventually arrives at destination host
  • destination returns ICMP “port unreachable” (type 3, code 3), because it arrived, but can’t do anything with the port specified
  • source stops

IPv6

• 32-bit address space completely allocated
• In IPv6, there is a fixed 40 byte header, with no fragmentation allowed

Header

ver
pri
flow label

payload len
next hdr
hop limit

source address (128 bits)
destination address (128 bits)
data

• No checksum
• No options field, but is allowed outside of header, indicated by “next header” field
• ICMPv6
  • additional types like “packet too big”

Transition from IPv4

• not all routers can be upgraded simultaneously
• Tunneling: IPv6 datagram carried as a payload in IPv4 datagram among IPv4 routers

Routing algorithms

Graph abstraction

A graph \(G = (N, E)\). \(c(x, x’)\) is the cost of the link \((x, x’)\).

Question: what is the least cost path from the source to the destination?

• global:
  • all routers have complete info on topology
  • “link state” algorithms
• decentralized
  • router knows physically connected neighbours, link costs to neughbours
  • iterative process of computation, exchange of info with neighbours
  • “distance vector” algorithms
• static
  • routes change slowly over time
• dynamic
  • routes change more quickly
  • periodic update
  • in response to link cost changes

Link State

• e.g. Dijkstra’s algorithm

• \(c(x, x’)\)

  edge weights

• \(D(v)\)

  current value of cost path from source to dest

  \(v\)

• \(p(v)\)

  : predecessor

• \(N’\)

  set of routers who we know the least cost path for

Distance vector

Bellman-Ford equation (DP)
Let \(d_x(y) :=\) cost of least cost path from \(x\) to \(y\)
Then, \(d_x(y) = \min\{c(x,v) + d_v(y)\}\)

• \(D_x(y)\)

  is the estimate of least cost from

  \(x\)

  to

  \(y\)

• \(x\)

  maintains the distance vector

  \(D_x = [D_x(y): y \in N]\)

• For each node

  \(x\)

  it knows the distance to each of its neighbours

• From time to time, each node sends its own distance bector estimate to neghbours
• when

  \(x\)

  receives new DV estimate from neighbour, it updates its own DV using B-F equation:

  • \(D_x(y) \leftarrow \min_v\{x(x,v) + D_v(y)\} \quad \forall y \in N\)

• poisoned reverse:
  • if Z routes through Y to get to X:
    • X tells Y its distance to X is infinite
• message complexity
  • LS: with n nodes, E links,

    \(O(nE)\)

  • DV: exchange between neighbours only. Convergence time varies
• Speed of convergence
  • LS:

    \(O(n^2)\)

    , requires

    \(O(nE)\)

    messages. May have oscillations

  • DV: convergence time varies. May be routing loops, count-to-infinity problem
• Robustness
  • LS: node can advertise incorrect link cost, each node computes only its own table
  • DV: DV node can advertise incorrect path cost
  • each node’s table is used by others so the error propagates through the network

Hierarchical Routing

• aggregate routers into regions, “autonomous systems”
• routers in same AS run same routing protocol
  • intra-AS routing protocol
  • routers in different AS can run different intra-AS routing protocols
• gateway router:
  • at edge of its own AS
  • has link to router in another AS
• forwarding table configured by both intra- and inter-AS routing algorithms

Inter-AS tasks

• Router in AS1 receives datagram destined for outside AS1
• AS1 must learn which dests are reachable through AS2, and which through AS3
• AS1 must propagate this reachability info to all routers in AS1
• Use internal routing to find shortest path to exit nodes in AS1, add those to forwarding table
• At output nodes, the connecting nodes in other ASes give info to AS1 that they can both reach the final destination
  • Hot potato routing: pick closest one to forward to that will still get to the destination

Internet Intra-AS Routing

• AKA interior gateway protocols (IGP)

RIP: Routing Information Protocol

• included in BSD-UNIX since 1982
• distance vector algorithm
  • distance metric: number of hops (up to 15 max), each link has cost 1
  • DVs exchanged with neighbours every 30 seconds in response message (this is advertisement)
  • each advertisement: list of up to 25 destination subnets and the hops to get there
• Link failure
  • If no advertisement heard after 180s, neighbour/link declared dead
  • routes via neighbour invalidated
  • new advertisements sent to neighbours
  • neighbours in turn send out new advertisements
  • poison reverse used to stop infinite loops: max 16 hops
• RIP routing tables managed by application level process called routed
  • Advertisements sent as UDP packets

OSPF: Open Shortest Path First

• uses link state
  • topology map at each node, route computation using Dijkstra’s
• advertisement carries one entry per neighbour
  • broadcast link state every 30 minutes even if there is no change
• advertisements flooded to entire AS
  • carried in OSPF messages directly over IP, not TCP or UDP
• IS-IS routing protocol is nearly identical to OSPF
• Adds security: messages authenticated
• multiple same-cost paths allowed (compared to only one in RIP)
• for each link, multiple cost metrics for different types of service (e.g. satellite link cost set to “low” for best effort ToS, high for real time ToS)
• integrated uni- and multi-cast support: Multicast OSPF (MOSPF) uses same topology data base as OSPF
• hierarchical OSPF in large domains

BGP: Border Gateway Protocol

• the de facto inter-domain routing protocol
• BGP provides each AS as a means to:
  • eBGP: (external) obtain subnet reachability information form neighbour ASes
  • iBGP: (internal) propagate reachability info to all AS-internal routers
  • determine good routes to other networks based on reachability info and policy
• Allows subnet to advertise its existence on internet
• Messages, sent over TCP:
  • OPEN: opens TCP connection to peer and authenticates sender
  • UPDATE: advertises new path or withdraws old
  • KEEPALIVE: keeps connection open in apsence of UPDATEs, also ACKs OPEN request
  • NOTIFICATION: reports errors in previous msg, also used to close connections
• BGP session: two BGP peers echange BGP messages:
  • advertising paths to different destination network prefices (“path vector” protocol)
  • exchanged over semi permanent TCP connections
• Advertised prefix includes BGP attributes:
  • AS-PATH: contains ASes through which prefix advertisement has passed
  • NEXT-HOP: indicates internal-AS router to next-hop AS
• Route selection
  • local preference value attribute: policy decision
  • shortest AS-PATH
  • closest NEXT-HOP router
  • additional criteria
• Routing policies
  • inter-AS: admin wants control over how its traffic is routed, who routes through its network
  • intra-AS: single admin, so no policy decisions needed
• Scale
  • hierarchical routing saves table size, reduced update traffic
• Performance
  • intra-AS: can focus on performance
  • inter-AS: policy may dominate over performance

Broadcast Routing

• Deliver packets from source to all other nodes
• Source transmit

  \(n\)

  copies to the

  \(n\)

  destinations using unicast routing.

  \(n\)

  -way-unicast (source duplication)

  • Simple but inefficient
  • how does source determine recipient addresses?
    • need additional protocol mechanisms (e.g. broadcast membership.) Adds overhead
• Uncontrolled flooding
  • When node receives broadcast, it sends to all neighbours except from the source it came from
    • Problem: cycles lead to endless flooding
    • Problem: broadcast storm. When a node is connected to more than two other nodes, it will create and forward multiple copies of the broadcast, resulting in endless multiplication of packets
• Controlled flooding
  • Only broadcast if it hasn’t broadcast the same packet before
  • Sequence number controlled flooding: source puts its address and a sequence number into the packet, and sends that to all neighbours
  • Each node maintains a list of source address and sequence number of each broadcast packed it has received and forwarded
  • When a node receives a broadcast packet, it first checks whether the packed is in the list. If so, it is dropped. Otherwise, it is duplicated and forwarded and added to the list
• Reverse path forwarding (RPF)
  • only forward packet if it arrived on the shortest path between node and source
  • RPF does not require that a router knows the complete shortest path. Only needs to know the next neighbour on its unicast shortest path to the sender
• Spanning tree
  • No redundant packets received by any node
  • first, construct spanning tree
    • Each node sends unicast join message to centre node
    • message forwarded until it arrives at a node already in tree
  • nodes then forward or make copies along spanning tree