Chapter 5: Network Layer

1
Chapter 5 Network Layer

• CS455/555

2
Network Layer Design Issues

• Services provided to the Transport Layer
  Connection-oriented vs Connectionless service
• Underlying network Internet (connectionless) and
  ATM ( Connection-oriented)
• Virtual circuits vs. datagrams

3
Routing Algorithms

• Objective Route packets from source to
  destination.
• At a given node, it decides which output line an
  incoming packet should be sent.
• Desirable properties Correctness, simplicity,
  robustness, stability, fairness, and optimality.
• Goals Maximize network throughput and minimize
  mean packet delay
• Optimality principle

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Routing algorithms (Cont.)

• Shortest path routing
• Graph Routers and nodes and links connecting the
  routers are the edges.
• Edges of the graph may represent a hop, the mean
  delay, the distance, cost, traffic flow, etc.
• Dijkstras shortest path algorithm To find the
  shortest path and distance from a source node to
  all other nodes in a graph. Initially, only the
  source node has a permanent label. In each
  iteration, one other node is assigned a permanent
  label.

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Routing algorithms (Contd.)

• Flooding Every incoming packet is sent out on
  every outgoing link.
• Of course, dont send a packet to a node from
  which it is received.
• Keep a count in the packet to limit the number of
  nodes it may visit during its life time to avoid
  infinite looping.
• Uses (i) When highly reliable communication is
  needed (ii) As a baseline for other algorithms to
  compare with in terms of shortest delay.

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Flow-based Routing

• Assuming that the capacity (Cij) and flow (Fij)
  along all the links in a network are known, path
  with a shortest delay can be determined.
• T 1/(mC-L) where C is capacity in bps, L is
  arrival of packets/sec, 1/m is the average number
  of bits per packet.
• By trying out different possible paths, minimal
  routing can be determined

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Routing algorithms (contd.)

• Distance vector routing Each router maintains a
  vector giving the best known distance to each
  destination and which outgoing link to use next.
• The vectors are periodically exchanged among the
  neighbors.
• The local vector is modified after receiving
  information from neighboring routers.
• Count-to-infinity problem Reacts rapidly to good
  news but leisurely to bad news.

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Routing algorithms

• Link State Routing Replaced distance vector
  routing.
• Steps Each router must
• (1) Discover its neighbors and learn their
  network addresses.
• (2) Measure the delay or cost to each of
  its neighbors.
• (3) Construct a packet telling all it has
  learned.
• (4) Send this packet to all other routers.
• (5) Compare the shortest path to every
  other router.

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Routing algorithms (Cont.)

• Hierarchical routing When network size os large,
  the routing tables also are large. One way to
  solve the problem is by dividing the network into
  regions.
• Routers within a region would know only about
  nodes in their region. There will be one or more
  nodes that are connected to other regions. Thus a
  node also has hierarchical table indicating which
  internal node it has to route a message for a
  given destination region.

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Routing algorithms (Cont.)

• Broadcast routing
• Multicast routing

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CONGESTION CONTROL ALGORITHMS

• When too many packets are present in a subnet or
  a part of it, performance degrades. This is
  called congestion.
• Causes of congestion
• (1) When a stream of input packets arrive to
  go on the same outgoing link, packets may be lost
  due to insufficient buffer size on that queue.
  But Nagle discovered that infinite amount of
  memory at routers may lead to worsening of
  congestion.
• (2) Slow processors (Mismatch is always a
  problem)
• (3) Congestion tends to feed upon itself and
  become worse.

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CONGESTION CONTROL ALGORITHMS (Contd.)

• Flow control vs. Congestion control Flow control
  deals with point-to-point traffic---fast sender
  and slow receiver.This usually achieved by a
  feedback from receiver to sender (e.g., ACKs in
  sliding window protocols). Congestion control is
  more global and is concerned about the ability of
  the subnet to carry out its tasks.

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CONGESTION CONTROL ALGORITHMS (Contd.)

• General principles of congestion control
  Approaches (1) Open-loop (2) Closed-loop
• Open loop solutions rely on good designs to make
  sure that congestion does not occur in the first
  place.
• Closed-loop solutions rely on feedback
  control---(1) Monitor subnet for any congestion.
  (2) Pass the congestion information to places
  where action can be taken. (3) Adjust the system
  operation to correct the problem.

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CONGESTION CONTROL ALGORITHMS (Contd.)

• What to monitor? of all packets discarded for
  lack of buffers, average queue lengths, the of
  packets that timeout and are retransmitted, the
  average packet delay, and the standard deviation
  of packet delay.
• Who to inform? (1) Sources can be informed via
  control messages---this further increases network
  load (2) A router puts a flag in all its
  outgoing packets of the impending congestion (3)
  Host or routers send explicit control packets to
  know about congestion
• Explicit feedback vs implicit feedback

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CONGESTION CONTROL ALGORITHMS (Contd.)

• Congestion prevention policies Open-loop
  systems
• (I) Data link layer retransmission policy,
  out-of-order caching policy, ACK policy, flow
  control policy
• (ii) Network layer VC vs. datagrams inside
  the subnet, packet queueing and service policy,
  packet discard policy, routing algorithms, packet
  lifetime management
• (iii) Transport layer retransmission
  policy, out-of-order caching policy, ACK policy,
  flow control policy, timeout determination

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CONGESTION CONTROL ALGORITHMS (Contd.)

• Traffic shaping Open loop method To force
  packets to be sent at more predictable rate and
  reducing the effect of bursty traffic.
• Traffic shaping vs. sliding window protocol
• Agreement between traffic carrier and user
• Traffic policing
• Leaky bucket and token bucket algorithms

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CONGESTION CONTROL ALGORITHMS (Contd.)

• Leaky bucket (1) A fixed capacity bucket (2) The
  output from the bucket is at a constant rate (3)
  When bucket overflows, information is lost.
• It can be thought of a single server queue with
  finite buffer and constant service time.
• Assuming constant packet (or cell sizes as in in
  ATM), it can be implemented as finite queue of
  buffers with one packet being serviced at each
  tick and put on the network.

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CONGESTION CONTROL ALGORITHMS (Contd.)

• Example of Leaky bucket algorithm A source
  generates data in terms of bursts 3 MB bursts
  lasting 2 msec once every 100 msec. The network
  offers a bandwidth of 60 MB/sec. Thus the leaky
  bucket can have an output rate of 50 MB/sec. The
  leaky bucket has a capacity of 4 MB. How does the
  output look like?
• Input 0-2 msec 1500 MB/sec 100-102 msec 1500
  MB/sec 200-202 msec 1500 MB/sec
• Output 0-50 msec 60 M/sec 100-150 msec 60
  MB/sec .

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CONGESTION CONTROL ALGORITHMS (Contd.)

• What should be the capacity of the leaky bucket
  to avoid loss? The burst cannot be held and hence
  there will be an overflow. How much is lost?
  During the burst, data inflow is at the rate of
  1.5 MB/msec and the outflow is at the rate of 0.6
  MB/msec. So accumulation is at the rate of 0.9
  MB/msec. So at the end of 2 msec, there will be
  an accumulation of 1.8 MB. This is the minimum
  leaky bucket capacity to avoid buffer overflow
  and hence data loss.

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CONGESTION CONTROL ALGORITHMS (Contd.)

• Token bucket algorithmLeaky bucket has a
  stringent outflow rate. A more flexible approach
  is the token bucket algorithm.Here, the overall
  rate is controlled rather than a fixed outflow
  rate all the time.
• Token bucket has a capacity indicating the
  maximum unused token that may be outstanding at
  any given instant.
• Tokens arrive into the bucket at a constant rate.
  
• Data can flow out of the bucket at a maximum rate
  limited by the network bandwidth as long as the
  bucket is not empty.

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CONGESTION CONTROL ALGORITHMS (Contd.)

• Token bucket example
• Bucket capacity 1 Mbytes
• Token arrival rate 2 Mbytes/sec
• Network capacity 10 Mbytes/sec
• Application produces 0.5 Mbyte burst every 250
  msec. For 3 seconds
• Initially, output can be at the rate of 10
  Mbytes/sec. But how long does this prevail? X
  seconds
• 1 2X 10X 8X 1 X 1/8 sec 125
  milliseconds. During this time, it can transmit
  1.25 Mbytes. But the burst size is not that much.
• So this will continue until the 0.5 Mbytes
  is sent. This takes 0.05 seconds or 50
  milliseconds
• Output 0-50 msec 10 Mbytes/sec
• 50-250 msec None
• By the time the next burst arrives, how many
  tokens will be accumulated? 2200/1000 0.4
  Mbytes of tokens
• How long with the next burst last? 0.4 2X
  10 X X 0.4/8 50 msec.
• Output 250-300msec 10 Mbytes/sec
• This will continue for 3 seconds.

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Congestion Control Alg. (Contd.)

• Flow Specifications
• Congestion control in virtual circuit
  subnets---admission control
• Choke packets---VC and datagrams when the
  utilization of an output line exceeds a
  threshold, a router sends a choke packet to the
  source host---reduce exponentially and increase
  in small increments
• Weighted Fair Queueing A queue is maintained for
  each source host at an output queue packets are
  selected in a round-robin fashion different
  weights for different sources
• Hop-by-hop Choke Packets---quick relief

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Congestion Control Algorithm

• Load shedding Wine (old packet is worth more)
  and milk (new packet is worth more) Low priority
  and high priority specified by the host
• Jitter control
• RSVPResource Reservation protocol for multicast
  routing

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INTERNETWORKING

• Deals with connecting subnets of different type
• Networks may differ in service offered,
  protoocls, addressing, multicasting, packet size,
  QoS, error handling, flow control, congestion
  control, security, parameters (e.g., timeouts),
  accounting
• Concatenated virtual circuits Several VCs are
  set up and connected
• Connectionless internetworking
• Tunneling (Multiprotocol router)
• Internetwork routing---interior gateway protocol
  and exterior gateway protocol
• Fragmentation---transparent, nontransparent
• Firewalls Two routers application gateway(s)

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The Network Layer in the Internet

• High-speed backbones, regional networks, local
  LANs
• IP or Internet protocol at the network layer
• Typically, when a message is fragmented along the
  way, the network layer at the host reassembles
  them.
• IP Protocol See Fig. 5-45 for IP header format
  Minimum 5 words (32-bit) maximum 15 words

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The Network Layer in the Internet (Cont.)

• Fields in the IP Header
• Version, lheader length, type of service (T/D/R),
  Total length, Identification (unique for each
  datagram), DF, MF, fragment offset, time to live,
  Protocol (transport level), header checksum,
  source address, destination address, options

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The Network Layer in the Internet (Cont.)

• IP Addresses Every host and router on the
  Internet has an IP address network number host
  number
• Class A, B, and C Depending on the environment
  Few networks/many hosts, medium Nw/medium hosts,
  many networks/few hosts
• Class D- Multicast address
• 4-byte (32 bit) addresses
• Each byte is separated from the other by a DOT
  (.).
• Hexadecimal address ? DOT notation (take two
  characters at a time and convert to a decimal
  number (0-255).

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The Network Layer in the Internet (Cont.)

• Dividing host address into ltsubnet, hostgt pair
  what the NIC gives is the network address.
• Each router has a table listing some with
  (network,0) entries and some with (this-network,
  host) IP addresses.
• With subnets, entries are (this-network,
  subnet,0) and (this-network, this-subnet, host),
  and (network,0)

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The Network Layer in the Internet (Cont.)

• Internet control protocols In addition to IP
• ICMP
• Ethernet boards (data link layer) have a 48-bit
  Ethernet address different from 32-bit IP
  addresses solution ARP
• ARP Address Resolution protocol (RFC 826) A
  source host broadcasts a message with IP address
  on its LAN. The receiving host responds back by
  broadcasting its Ethernet address.
• RARP Reverse Address Resolution Protocol Given
  an Ethernet address, what is the IP address?

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The Network Layer in the Internet (Cont.)

• IPv4 (current standard) IP is running out of
  addresses.
• IPv6 (1) Extend the address space to billions of
  hosts (2) Security (3) Reduce routing table sizes
  (4) QoS (5) Coexistence of old and new protocols
  (see page 437 for a complete list)

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The Network Layer in the Internet (Cont.)

• Improvements due to IPv6 (over IPv4)
• Address is 16 bytes rather than 4 bytes
• Simplified header (40-byte) version, priority,
  flow label, payload length, next header, hop
  limit, source/destination addresses
• Better specification of options
• Security Authentication and privacy
• Flow label for QoS is much larger than 2 bytes in
  IPv4
• Different prefixes for an IPv6 address meant
  different things (page 441)

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IPv4 vs. IPv6

• No checksum For better performance. Assumes that
  other layers will have their own.
• No fragmentation field The fragmentation
  responsibility is with the host and not with a
  router
• No IHL field Fixed length headers
• Extension headers (next header field) See pages
  443-446 for more details

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The Network layer in ATM Networks

• The ATM layer in the ATM reference model (page
  63) functions as a network layer end-to-end VC,
  switching, and routing connection-oriented.
• Virtual channel is the basic VC which connects a
  source with a destination
• Virtual path A group of VCs from one a given
  source to a given destination
• Does not provide any ACK
• Within a VC ordering is guaranteed but cells
  could be dropped

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The Network layer in ATM Networks (cont.)

• ATM cell 5-byte header 48-byte payload
• Interfaces ATM-ATM (NNI) User-Network (UNI)
• NNI VPIVCIPTICLPHEC
• UNI GFC (not used)VPIVCIPTIHEC
• AAL might use some of the 48-byte payload

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The Network layer in ATM Networks

• ATM Connection setup Permanent vs. switched VCs
  part of the control panel on top of ATM layer
• First setup a signaling VC and then use this
  channel to negotiate Several ltsetup, call
  proceedinggt pairs followed by a series of
  ltConnect, connect ACKgt uses a routing algorithm
  to determine the path
• Routing and switching Uses only VPI and not VCI
• VPI helps reduce the routing table length,
  speeding-up connection set up, and switching a
  route in case of a congestion
• Routing table example See Fig. 5-67

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The Network layer in ATM Networks

• Service categories CBR, VBR (RT and Non-RT), ABR
  (specifies minimum but could vary and may have
  loss rate), UBR
• Figure 5-70 summarizes the four categories
• Quality of service ContractTraffic to be
  offered, service agreed upon, and compliance
  requirements
• QoS parameters See Figure 5-71

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Traffic Shaping and Policing

• GCRA or generic-cell-rate algorithm Inputs T
  (the period) and L (leeway) where T1/PCR or
  peak-cell rate
• Congestion control (1) Admission control---to
  prevent congestion (2) Resource
  reservation---related to admission control (3)
  Rate-based congestion control A special RM cell
  is sent by the sender periodically to examine the
  status