Shared Congestion Management

					Course Review
Outline

• Networks: A top down view (for a change).
• Other topics.
  • Security
  • QoS
  • Multicast
• Questions?




                Final Review: 12/10/2001      2
    Protocol Stacks
      The Open Systems Interconnection (OSI) Model.

7   Application                                         Application

6 Presentation                                          Presentation

5    Session                                              Session

4   Transport                                            Transport

3    Network                                Network       Network

2    Data link     Data link                Data link    Data link

1    Physical      Physical                 Physical     Physical




                        Final Review: 12/10/2001                    3
    Browsing the Web


7 Web Browser                                         Web Server

6 Presentation                                        Presentation

5    Session                                            Session

4

3
2
1
    Transport
     Network
     Data link
     Physical
                 Data link
                 Physical
                             ?            Network
                                          Data link
                                          Physical
                                                       Transport
                                                        Network
                                                       Data link
                                                       Physical


      Client                                            Server
                      Final Review: 12/10/2001                    4
HTTP Request Example

GET / HTTP/1.1
Accept: */*
Accept-Language: en-us
Accept-Encoding: gzip, deflate
User-Agent: Mozilla/4.0 (compatible; MSIE 5.5;
  Windows NT 5.0)
Host: www.seshan.org
Connection: Keep-Alive


                   Final Review: 12/10/2001      5
HTTP Response Example
HTTP/1.1 200 OK
Date: Tue, 27 Mar 2001 03:49:38 GMT
Server: Apache/1.3.14 (Unix) (Red-Hat/Linux) mod_ssl/2.7.1
  OpenSSL/0.9.5a DAV/1.0.2 PHP/4.0.1pl2 mod_perl/1.24
Last-Modified: Mon, 29 Jan 2001 17:54:18 GMT
ETag: "7a11f-10ed-3a75ae4a"
Accept-Ranges: bytes
Content-Length: 4333
Keep-Alive: timeout=15, max=100
Connection: Keep-Alive
Content-Type: text/html
…..
                      Final Review: 12/10/2001          6
Single Transfer Example

                                              Client              Server
                               0 RTT                              SYN
 Client opens TCP connection                       SYN
                               1 RTT
                                                                  ACK
 Client sends HTTP request for HTML                DAT
                                                   ACK                   Server reads from disk
                                                                  DAT
                                                                   FIN
                               2 RTT
                                                                  ACK
 Client parses HTML
                                                   FIN
 Client opens TCP connection
                                                   ACK            SYN

                                                   SYN
                               3 RTT                              ACK
 Client sends HTTP request for image               DAT
                                                                         Server reads from disk
                                                   ACK
                               4 RTT
                                                   DAT
 Image begins to arrive



                                       Final Review: 12/10/2001                                   7
Persistent Connection Example

                                            Client               Server

                              0 RTT
                                                                 DAT
Client sends HTTP request for HTML
                                                 ACK                   Server reads from disk
                                                                 DAT
                              1 RTT                              ACK

Client parses HTML                                               DAT
Client sends HTTP request for image
                                                                       Server reads from disk
                                                 ACK             DAT

                              2 RTT
Image begins to arrive




                                      Final Review: 12/10/2001                                  8
DNS Server

                                                     Other DNS servers
• A DNS server is responsible for
  maintaining the name-address
  mapping in a specific domain.
   • E.g. cs.cmu.edu
• The network manager can add,
  remove, or change mappings.                           cs.cmu.edu
• Computers can send requests to the
  server to translate a name into an
  address.
   • But how do you find the server?
   • Recursively contact the parent in
     the hierarchical name space             hawaii.cs.cmu.edu
• Caching is used to speed up the
  lookup of frequently used names.                           128.17.4.174

                          Final Review: 12/10/2001                          9
Typical Exchange
• Sender creates and                    • Receiver creates and
  initializes a socket.                   initializes a socket.
• Sender issues an open                 • Receiver listens on the
  connection command.                     socket for a connection
   • Specifies destination IP and         request.
     application port addresses              • Can sometimes restrict the
   • Sender blocks while                       type of connection
     connection is established          • If receiver accepts the
• If the connection                       connection and the
  succeeds, data exchange                 connection succeeds, data
  can start.                              exchange can start.
   • Lots of things can go wrong:            • Communication typically
     wrong addresses, receiver                 uses a different socket
     or network down.

                          Final Review: 12/10/2001                       10
    Browsing the Web


7 Web Browser                                         Web Server

6 Presentation                                        Presentation

5    Session                                            Session

4   Transport                                          Transport




                             ?
3    Network                              Network       Network

2    Data link   Data link                Data link    Data link

1    Physical    Physical                 Physical     Physical


      Client                                            Server
                      Final Review: 12/10/2001                 11
Connection management
          Sender                  Receiver
                         syn                         Establish
                                            Open      Initial
                                                    Sequence
                   syn/ack                           Numbers


                           ack
                                            Data
              fin

                         ack
                            fin             Close

   Time              ack
                      Final Review: 12/10/2001                   12
Reliability
  • Checksum guarantees end-end data integrity.
  • Sequence numbers detect packet sequencing problems:
     • duplicate: ignore
     • reordered: reorder or drop
     • lost: retransmit
  • Lost packets detected by sender.
     • uses time out to detect lack of acknowledgment
     • requires reliable roundtrip time estimate
  • Retransmission requires that sender keeps copy of the
    data until ACK is received.
     • performance issue



                        Final Review: 12/10/2001            13
When to Send a Packet?
• End-to-end flow control.
  • avoid buffer overflow on receiver
  • receiver advertizes a window size
• Congestion control.
  • estimates amount of data that can be in network
  • implemented using the congestion window, slow start,
    and fast retransmit/recovery mechanisms
• Efficiency considerations.
  • try to send large packets (if possible)
     • more efficient in the network and on end points
  • piggybacking of acks

                         Final Review: 12/10/2001        14
  Window Size
  versus Throughput

Sender




Receiver
                          Time


                           Window Size
           Throughput =
                          Roundtrip Time

                      Final Review: 12/10/2001   15
TCP Congestion Avoidance
• Congestion avoidance limits how fast TCP can send
  data.
   • Implemented using a congestion window that limits how
     much data can be in the network
       • independent from flow control window
   • transmission is limited by minimum of the two windows
   • window grows in response to acknowledgement
• Packet loss is seen as sign of congestion.
   • multiplicative decrease of the congestion window
   • have to cut back fast since cost of congestion is high
• How do you detect when more bandwidth becomes
  available?
   • gradually increment congestion window (probing)
   • results in oscillation around congestion window size!
                         Final Review: 12/10/2001             16
TCP Saw Tooth Behavior

Congestion
 Window                                      Timeouts
                                             may still
                                               occur




                 Slowstart         Fast                  Time
       Initial
     Slowstart    to pace       Retransmit
                  packets      and Recovery


                  Final Review: 12/10/2001                      17
    Browsing the Web


7 Web Browser                                         Web Server

6 Presentation                                        Presentation

5    Session                                            Session

4   Transport                                          Transport

3    Network                              Network       Network

2
1
     Data link
     Physical
                 Data link
                 Physical    ?            Data link
                                          Physical
                                                       Data link
                                                       Physical


      Client                                            Server
                      Final Review: 12/10/2001                 18
     Hop-by-Hop Packet
     Forwarding in the Internet


                               Packets               Mixed
       Host   Ethernet                                         Host
                             over SONET             Ethernet




7
..
3

2
1


                         Final Review: 12/10/2001                     19
Addressing in IP v4 (Basic)
• Each host has an Internet address.
• Addresses are hierarchical.
   • address contains hint about location
• Address space is divided in three classes of point-to-
  point addresses, multicast addresses, and some
  special addresses.
                type     network                   host
          A      1            7                     24
          B      2           14                     16
          C      3           21                      8
          D      4          (multicast)             28
                     Example: 128.2.209.19

                        Final Review: 12/10/2001          20
  Routing based on
  Network Identifier
  Net ID Next
  Net ID Next
  Net ID Next
                          Host                         Host
  Net ID Next                             AN 3
Forwarding Table

                                                       AN 4    Host
    Host         AN 2            ISP 1



                        ISP 2                  ISP 3



    Host         AN 1                                   AN 5    Host

      Net.Host

                            Final Review: 12/10/2001                   21
Problems with Simple Address
Structure

  • Running out of addresses.
     • Especially true for mid-sized networks
  • Routing tables are becoming too big.
     • 100 of thousands of entries
  • Temporary solution: classless inter-domain
    routing.
     • Use address space more efficiently by relaxing the
       strict address structure,
        • length of network address is variable
        • generalization of subnetting idea
     • have internet service providers hand out blocks of
       addresses to their customers


                       Final Review: 12/10/2001             22
Route Lookup with CIDR

• Problem: with CIDR there
  can be multiple matches
  when looking up an
  address.                                      10110110       hosts
   • Can for example happen
     when a customer switches                   10110110 010     hosts
     ISPs but keeps addresses
• Solution: lookup is based
                                                10110110 010 0100011
  on longest prefix match.
   • If there are multiple matches
     in the lookup, the longest
     match (longest netmask)
     wins

                          Final Review: 12/10/2001                       23
What Does Routing Do?
                                              Other routers

• Routing protocol specifies how            Routing protocol
  routers jointly collect
  information about the network.                 Routing
                                                 Protocol
   • Routing protocols must be
      standardized
• Routing algorithm uses                        Routing
  network information to select                Algorithms
  appropriate routes and to set
  up the routing table.
• The data forwarding engine
  performs route lookup in the                          Route
  routing table.                                       Lookup
   • through which interface
      should a packet be
      forwarded?
                          Final Review: 12/10/2001              24
Dijkstra’s Algorithm
(Link State)

                5, E                          3, F
                5, E                          3, F

                 B           5                    D      3
            1              3
 6, B   A       3                                   2        F
 6, B
            2            4
                C                                 E      2
                               4
                6, E                              2, F
                6, E
                6, E
                       Final Review: 12/10/2001                  25
Distance Vector Routing
Example
                    A     - A/3 A/3 A/3
                    C     - C/1 C/1 C/1
                    D     - D/4 C/2 C/2

                          3      B       4
B   - B/3 B/3 B/3                                  A   - - B/7 C/5
                               1
C   - C/9 B/4 B/4   A                        D     B   - B/4 C/2 C/2
D   - - B/7 B/5                                    C   - C/1 C/1 C/1
                         9               1
                                 C
                    A     - A/9 B/4 B/4
                    B     - B/1 B/1 B/1
                    D     - D/1 D/1 D/1
                        Final Review: 12/10/2001                  26
Hierarchical Routing
 • Two level routing based on intra-domain and inter-domain
   routing to improve scalability.
 • Matches the structure of the address space.
 • Driven in part by business/management concerns.
    • Local network information is kept internal
    • Agreements with specific service providers at boundaries
        Host                     Host



                                                           Host
Host



         Host                                       Host

                         Final Review: 12/10/2001                 27
    Browsing the Web


7 Web Browser                                         Web Server

6 Presentation                                        Presentation

5    Session                                            Session

4   Transport                                          Transport

3    Network                              Network       Network

2    Data link   Data link                Data link    Data link

1    Physical    Physical
                             ?            Physical     Physical


      Client                                            Server
                      Final Review: 12/10/2001                 28
  Datalink in the Backbone

         • Routers are connected by point-point links or by
           (datalink layer) switched clouds.
         • Point-point links typically based on SONET.
            • E.g. Packets over SONET
         • Switched clouds often uses virtual connection
           datalink technologies.
            • E.g., ATM, frame relay
                 Router       Switched Cloud             Point-Point
                                                            link
PCs at
Work                                                                   PC at
                                                                       Home




                              Final Review: 12/10/2001                  29
802.3 Ethernet
  Broadcast technology                     host    host     host   host

   host   host   host     host
                                                      Hub

  • Carrier-sense multiple access with collision
    detection (CSMA/CD).
  • 10Mbps cable rate.
  • Maximum diameter 2.5km.
  • Minimum frame = 64 bytes.
  • Thick or thin coax; 10Base-T unshielded twisted
    pair in star configuration using hub.

                        Final Review: 12/10/2001                      30
Ethernet Switches

  • Bridges make it possible to increase LAN
    capacity.
     • Packets are no longer broadcasted - they are
       only forwarded on selected links
     • Adds a switching flavor to the broadcast LAN
  • Ethernet switch is a special case of a bridge: each
    bridge port is connected to a single host.
     • Simplifies the protocol and hardware used
       (only two stations on the link)
     • Can make the link full duplex (really simple
       protocol!)
     • Can have different port speeds
                     Final Review: 12/10/2001             31
Framing
• A link layer function, defining which bits have
  which function.
• Minimal functionality: mark off units of
  transmission.
• Some techniques:
   •   frame delimiter characters with character stuffing
   •   frame delimiter codes with bit stuffing
   •   out of band delimiters (e.g. FDDI control symbols)
   •   synchronous transmission (e.g. SONET)


                        Final Review: 12/10/2001            32
    Browsing the Web


7 Web Browser                                         Web Server

6 Presentation                                        Presentation

5    Session                                            Session

4   Transport                                          Transport

3    Network                              Network       Network

2    Data link   Data link                Data link    Data link

1    Physical    Physical                 Physical     Physical
                             ?
      Client                                            Server
                      Final Review: 12/10/2001                 33
The Frequency Domain

  • A (periodic) signal can be viewed as a sum of
    sine waves of different strengths.
  • Every signal has an equivalent representation in
    the frequency domain.
     • What frequencies are present and what is their
       strength
     • Similar to radio and TV signals




                                          Amplitude

          Time
                                                      Frequency
                       Final Review: 12/10/2001                   34
Wireless: Good News Bad News

  • Great technology: no wires to install, convenient
    mobility, ..
  • High attenuation limits distances.
     • Wave propagates out as a sphere
     • Signal strength reduces quickly (1/distance)2
  • High noise due to interference from other
    transmitters.
     • Use MAC and other rules to limit interference
     • Aggressive encoding techniques to make signal
       less sensitive to noise
  • Other effects: multipath fading, security, ..
  • Ether has limited bandwidth.
     • Try to maximize its use
                     Final Review: 12/10/2001           35
TCP Problems Over Noisy Links

• Wireless links are inherently error-prone
  • Fades, interference, attenuation
  • Errors often happen in bursts
• TCP cannot distinguish between corruption and
  congestion
  • TCP unnecessarily reduces window, resulting in low
    throughput and high latency
• Burst losses often result in timeouts
• Sender retransmission is the only option
  • Inefficient use of bandwidth

                      Final Review: 12/10/2001           36
Proposed Solutions

• End-to-end protocols
  • Selective ACKs, Explicit loss notification
• Split-connection protocols
  • Separate connections for wired path and
    wireless hop
• Reliable link-layer protocols
  • Error-correcting codes
  • Local retransmission


                   Final Review: 12/10/2001      37
    Browsing the Web


7 Web Browser                                             Web Server

6 Presentation                                            Presentation

5    Session                                                Session

4   Transport                                              Transport

3    Network                                  Network       Network

2    Data link       Data link                Data link    Data link

1    Physical        Physical                 Physical     Physical


      Client                                                Server
                 Everything Cleared Up!
                          Final Review: 12/10/2001                 38
Security Threats
  • Impersonation.
     • Pretend to be another user with the intent of getting
       access to information or services
  • Secrecy.
     • Get access to the contents of packets
  • Message integrity.
     • Change a message unbeknownst to the sender or
       receiver
  • Repudiation
     • Denying to have sent a message
  • Denial of service.
     • Flooding the system so users with legitimate needs
       cannot get service
  • Range of other threats: password guessing, exploiting
    programming bugs, …
                         Final Review: 12/10/2001              39
Encryption
                ciphertext = E(plaintext, k)
               plaintext = D(ciphertext, k’)
  •   Private key (symmetric, e.g. DES)
       • the two parties share a common private key k
  •   Public key (asymmetric, e.g. RSA)
       • derive two keys, kprivate and kpublic
       • kprivate is kept private by its owner
       • kpublic is published
  •   Tradeoffs between private and public key
      cryptography.
       • Key management, speed
  •   Challenge: key management.
                      Final Review: 12/10/2001          40
Example Applications
• Kerberos.
  • Support security in corporate environment
  • Based on key distribution center that knows all the
    entities
     • Know = share secret
• Secure socket layer (SSH).
  • Support secure channels in open internet environment
  • Based on certificates and certification authorities
  • Provides privacy, but trust is limited
• Pretty good privacy (PGP).
  • Provides privacy, authentication, repudiation in internet
    environment
  • Key management based on a “web of trust”
                       Final Review: 12/10/2001            41
How to Provide QoS?

 • Admission control limits number of users.
     • You cannot provide guarantees if there are too many
       users sharing the same set of resources (bandwidth)
     • For example, telephone networks - busy tone
     • This implies that your request for service can be rejected
 • Traffic enforcement limits how much traffic users can inject
   based on predefined limits.
     • Make sure user respects the traffic contract
     • Data outside of contract can be dropped (before entering
       the network!) or can be sent at a lower priority
 • Scheduling support in the routers guarantee that users get
   their share of the bandwidth.
     • Again based on pre-negotiated bounds
 • Signaling protocol gives routers the information they need to
   provide QoS.
     • E.g. RSVP          Final Review: 12/10/2001                  42
Qos Summary




              Final Review: 12/10/2001   43
IETF QoS Models

  • Integrated services: diverse QoS at the micro-flow level.
     • Range of QoS: best effort, controlled load, guaranteed
     • Specific end-to-end service defined for each class
     • Requires end-to-end support, e.g. edge and core routers
     • Concern about complexity, cost, marketing/charging
  • Differentiated services: QoS at the aggregate flow level.
     • Defines range of “forwarding behaviors”, but services
        are defined by the providers
     • Pushes most complexity to the edge of the network –
        fast core routers work only with small number of traffic
        classes
  • Based on the same building blocks.


                        Final Review: 12/10/2001                   44
Multimedia Challenges

• TCP/UDP/IP suite provides best-effort, no
  guarantees on expectation or variance of packet
  delay

• Streaming applications delay of 5 to 10 seconds is
  typical and has been acceptable, but performance
  deteriorate if links are congested (transoceanic)

• Real-Time Interactive requirements on delay and
  its jitter have been satisfied by over-provisioning
  (providing plenty of bandwidth), what will happen
  when the load increases?...
                    Final Review: 12/10/2001        45
Multicast – Efficient Data
Distribution
       Src                                 Src




                Final Review: 12/10/2001         46
IP Multicast Architecture

      Service model                              Hosts


    Host-to-router protocol
           (IGMP)
                                                 Routers



Multicast routing protocols
         (various)



                      Final Review: 12/10/2001             47
Multicast Routing

• Basic objective – build distribution tree for
  multicast packets
• Core based protocols
  • Examples: CBT, PIM-SM
• Flood and prune
  • Examples: DVMRP, PIM-DM
• Link-state multicast protocols
  • Example: MOSPF

                  Final Review: 12/10/2001        48
Shared vs. Source-based Trees

• Source-based trees
  • Separate shortest path tree for each sender
  • DVMRP, MOSPF, PIM-DM, PIM-SM
• Shared trees
  • Single tree shared by all members
  • Data flows on same tree regardless of sender
  • CBT, PIM-SM



                  Final Review: 12/10/2001         49
Questions?




             Final Review: 12/10/2001   50

				
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