Lecture 3#1#1 Chapter 3 Hubs, Bridges and Switches.
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Lecture 3#1#1 Chapter 3 Hubs, Bridges and Switches
Lecture 3#2#2 Interconnecting LANs Q: Why not just one big LAN? Limited amount of supportable traffic: on single LAN, all stations must share bandwidth Ethernet: limited length: 802.3 specifies maximum cable length Ethernet: large “collision domain” (can collide with many stations) Token Ring: token passing delay per station: 802.5 limits number of stations per LAN:
Lecture 3#3#3 Hubs Physical Layer devices: essentially repeaters operating at bit levels: repeat bits received on one interface to all other interfaces Hubs can be arranged in a hierarchy (or multi-tier design), with backbone hub at its top
Lecture 3#4#4 Hubs (more) Each connected LAN referred to as LAN segment Hubs do not isolate collision domains: node may collide with any node residing at any segment in LAN Hub Advantages: m simple, inexpensive device m Multi-tier provides graceful degradation: portions of the LAN continue to operate if one hub malfunctions m extends maximum distance between node pairs (100m per Hub)
Lecture 3#5#5 Hub limitations single collision domain results in no increase in max throughput m multi-tier throughput same as single segment throughput individual LAN restrictions pose limits on number of nodes in same collision domain and on total allowed geographical coverage cannot connect different Ethernet types (e.g., 10BaseT and 100baseT) Why?
Lecture 3#6#6 Bridges Link Layer devices: forward Ethernet frames selectively: m learn where each station is located m examine the header of each frame m forward on the proper link (if known) if dest. and source on same link, drop frame WHY? m if not known where dest. is, broadcast frame except on originating link, of course
Lecture 3#7#7 Bridges Bridge isolates collision domains m buffers frame m then forwards it, if needed, using CSMA/CD A broadcast frame is forwarded on all interfaces (except the incoming one) m thus broadcast frames propagate across brodges A set of segments connected by bridges and hubs is called a broadcast domain
Lecture 3#8#8 Bridges (more) Bridge advantages: m Isolates collision domains resulting in higher total max throughput, and does not limit the number of nodes nor geographical coverage m Can connect different type Ethernet since it is a store and forward device m Transparent: no need for any change to hosts LAN adapters
Lecture 3#10 Interconnection Without Backbone Not recommended for two reasons: - single point of failure at Computer Science hub - all traffic between EE and SE must path over CS segment
Lecture 3#11 Bridges: frame filtering, forwarding bridges filter packets m same-LAN -segment frames not forwarded onto other LAN segments forwarding: m how to know on which LAN segment to forward frame?
Lecture 3#12 Bridge Filtering bridges learn which hosts can be reached through which interfaces: maintain filtering tables m when frame received, bridge “learns” location of sender: incoming LAN segment m records sender location in filtering table filtering table entry: m (Node LAN Address, Bridge Interface, Time Stamp) m stale entries in Filtering Table dropped (TTL can be 60 minutes)
Lecture 3#13 Bridge Operation pseudocode Init: set filtering table to void Case: frame arrives on port P, src MAC , dest MAC /* Table Update stage */ if not listed, add mapping P with expiration time else update expiration time /* if listing fits */ /* Frame Forwarding stage */ look up in filtering table: listing P Q /* if listed */ if not listed, forward on all ports except P /* “flood */ if = P, drop the frame /*frame already there*/ /* WHY ? */ Otherwise, forward the frame on port Q
Lecture 3#14 Bridge Learning: example Suppose C sends frame to D and D replies back with frame to C C sends frame, bridge has no info about D, so floods to both LANs 2 and 3 m bridge notes that C is on port 1 m frame ignored on upper LAN m frame received by D
Lecture 3#15 Bridge Learning: example D generates reply to C, sends it m bridge sees frame from D m bridge notes that D is on interface 2 m bridge knows C on interface 1, so selectively forwards frame out via interface 1 only C 1
Lecture 3#16 What will happen with loops? Incorrect learning A B 1 1 2 2 A, 1 2 2
Lecture 3#17 What will happen with loops? Frame looping A C 1 1 2 2 C,?? … …
Lecture 3#18 Loop-free: tree A B C A message from A will mark A’s location
Lecture 3#19 Loop-free: tree A B C A message from A will mark A’s location A:
Lecture 3#20 Loop-free: tree A B C A: A message from A will mark A’s location
Lecture 3#21 Loop-free: tree A B C A: A: A: A: A message from A will mark A’s location
Lecture 3#22 Loop-free: tree A B C A: A: A: A: A message from A will mark A’s location
Lecture 3#23 Loop-free: tree A B C A: A: A: So a message to A will go by marks… A message from A will mark A’s location
Lecture 3#24 Bridges-Spanning Tree for increased reliability, it is desirable to have redundant, alternative paths from source to dest this causes cycles - bridges may multiply and forward frame forever solution: organize bridges in a spanning tree and disable all ports not aligned with the tree Disabled
Lecture 3#25 Introducing Spanning Tree Objective: Find tree spanning all LAN segments m each bridge transmits on a single port m each LAN transmits on a single bridge Bridges run the Spanning Tree Protocol m Use a distributed algorithm m Result: select what ports (and bridges) should actively forward frames, and which should accept frames m bridges communicate using special configuration messages (BPDUs) to perform this selection BPDU = Bridge Protocol Data Unit STP standardized in IEEE 802.1 D
Lecture 3#26 Method Bridges communicate using special configuration messages (BPDUs) BPDU = Bridge Protocol Data Unit Each bridge sends periodically a BPDU to all its neighbors BPDU contains: m ID of bridge sender views as root (my_root_ID) m known distance to that root senders own bridge ID
Lecture 3#27 Overview of STP We solve in order: 1. How to agree on a root bridge? 2. How to compute a ST of bridges? 3. How to compute a ST LAN segments?
Lecture 3#28 1. Choosing a root bridge Assume m each bridge has a unique identifier (ID) m within a bridge each port has a unique ID Each bridge remembers smallest bridge ID seen so far (= my_root_ID) m including own ID Periodically, send my_root_ID to all neighbors (“flooding”) (included in BPDU) When receiving ID, update if necessary Qn: Is that enough?!
Lecture 3#29 2. Compute ST given a root Idea: each node finds its shortest path to the root shortest paths tree Output: At each node, parent pointer (and distance) How: Bellman-Ford algorithm
Lecture 3#30 Distributed Bellman-Ford Assumption: There is a unique root node s m this was done in Step 1 Idea: Each node, periodically, tells all its neighbors what is its distance from s But how can they tell? s : easy. dist s = 0 always! Another node v : Bridge calls the neighbor with least distance to root - its “parent”
Suppose all nodes start with distance , and suppose that updates are sent every time unit. 1 1 0 0 0 0 Lecture 3#31 Why does this work? C D B E F G A 0 1 1 1 1 3 2 2 B sees same distance from A and E; A chosen since has smaller ID ID=17 ID=21 2 2
Lecture 3#32 Bellman-Ford: properties Works for any non-negative link weights w(u,v) : Works when the system operates asynchronously. Works regardless of the initial distances!
Lecture 3#33 3. ST of LAN segments Assumption: given a ST of the bridges Idea: Each segment has at least one bridge attached. Only one of them should forward packets! m Choose bridge closest to root. Break ties by bridge ID (and then by port ID on that bridge if needed) Implementation: Bridges listen to all distance announcements on each port. Bridge A marks a port as a “designated port” iff A is the best bridge for that port’s LAN, i.e: m Its distance D to root is the shorter than all distances received on this port m Of all bridges with distance D reported on this port it has lowest ID m of several ports to same LAN on same bridge take low ID
Lecture 3#34 Spanning Tree Concepts: Path Cost A cost is associated with each segment m = “weight” of the segment m = cost associated with transmission on the LAN segment connected to the port m bridge associates the weight with relevant port m default segment weight is 1 m Can be manually or automatically assigned m Can be used to alter the path to the root bridge Path cost is the sum of the component segment weights
Lecture 3#35 Spanning Tree Concepts: Root Port Each non-root bridge has a Root port: The port on the path towards the root bridge m = parent pointer The root port is part of the lowest cost path towards the root bridge If port costs are equal on a bridge, the port with the lowest ID becomes root port
Lecture 3#36 Example Spanning Tree B3 B5 B7 B2 B1 B6 B4 Protocol operation: 1.Pick a root 2.Each bridge picks a root port B8
Lecture 3#37 Example Spanning Tree B3 B5 B7 B2 B1 B4 B6 Root B4B5B6 B8 B1 Spanning Tree: root port B3 B7 B2 B8
Lecture 3#38 ST Concepts: Designated Port Each LAN has a single designated bridge m all other bridges on LAN know which one it is m all tfc of LAN towards root goes thru that bridge This is the bridge reporting minimum cost path to the root bridge for the LAN m ties broken by choosing lowest ID Only designated & root ports remain active in a bridge. Bridge uses: m designated ports to send downstream frames m root ports to send upstream frames (toward root) m Bridge with no designated port becomes inactive
Lecture 3#39 Example Spanning Tree B3 B5 B7 B2 B1 B6 B4 Root B8 B2B4B5B7 B8 B1 Forwarding Tree: Designated Bridge root port Note: B3, B6 forward nothing
Lecture 3#40 STP Requirements Each bridge has a unique identifier A multicast address for bridges on a LAN A unique port identifier for all ports on all bridges m Bridge id + port number
Lecture 3#41 Forwarding/Blocking State 1. Only root and designated ports are active for data forwarding l Other ports are in the blocking state: no forwarding! l If bridge has no designated port, no forwarding at all block root port too. 2. All ports send BPDU messages l including blocked ones l To adjust to changes
Lecture 3#43 Bridges vs. Routers both are store-and-forward devices m routers: network layer devices (examine network layer headers) m bridges are link layer devices routers have routing tables, use routing algorithms, designed for Wide Area addressing bridges have filtering tables, use filtering, learning & spanning tree algorithms, designed for local area
Lecture 3#44 Routers vs. Bridges Bridges + and - + Bridge operation is simpler, requiring less processing - Topologies are restricted with bridges: a spanning tree must be built to avoid cycles - Bridges do not offer protection from broadcast storms (endless broadcasting by a host will be forwarded by a bridge)
Lecture 3#45 Routers vs. Bridges Routers + and - + arbitrary topologies can be supported, cycling is limited by TTL counters (and good routing protocols) + provide firewall protection against broadcast storms - require IP address configuration (not plug and play) - require higher processing bridges do well in small (few hundred hosts) while routers used in large networks (thousands of hosts) and in Internet core
Lecture 3#46 Ethernet Switches layer 2 (frame) forwarding, filtering using LAN addresses Switching: A-to-B and A’- to-B’ simultaneously, no collisions large number of interfaces often: individual hosts, star-connected into switch m Ethernet w. no collisions! m = Switched Ethernet
Lecture 3#47 Ethernet Switches cut-through switching: frame forwarded from input to output port without awaiting for assembly of entire frame m slight reduction in latency combinations of shared/dedicated, 10/100/1000 Mbps interfaces
Lecture 3#49 Data Link: Summary principles behind data link layer services: m error detection, optional: error correction m sharing a broadcast channel: multiple access m link layer addressing, ARP various link layer technologies m Ethernet m hubs, bridges (STP), switches m IEEE 802.11 LANs m PPP Chapter 5 Kurose and Ross
Lecture 3#52 IEEE 802.11 Wireless LAN wireless LANs: mobile networking IEEE 802.11 standard: m MAC protocol m unlicensed frequency spectrum: 900Mhz, 2.4Ghz Basic Service Set (BSS) (a.k.a. “cell”) contains: m wireless hosts m access point (AP): base station BSS’s combined to form distribution system (DS)
Lecture 3#53 Ad Hoc Networks Ad hoc network: IEEE 802.11 stations can dynamically form network without AP Applications: m “laptop” meeting in conference room, car m interconnection of “personal” devices m battlefield IETF MANET (Mobile Ad hoc Networks) working group
Lecture 3#54 IEEE 802.11 MAC Protocol: CSMA/CA 802.11 CSMA: sender - if sense channel idle for DIFS sec. then transmit entire frame (no collision detection) -if sense channel busy then binary backoff 802.11 CSMA receiver: if received OK return ACK after SIFS Why Needed?
Lecture 3#55 IEEE 802.11 MAC Protocol 802.11 CSMA Protocol: others NAV: Network Allocation Vector 802.11 frame has transmission time field others (hearing data) defer access for NAV time units
Lecture 3#56 Hidden Terminal effect hidden terminals: A, C cannot hear each other m obstacles, signal attenuation m collisions at B goal: avoid collisions at B CSMA/CA: CSMA with Collision Avoidance
Lecture 3#57 Collision Avoidance: RTS-CTS exchange CSMA/CA: explicit channel reservation m sender: send short RTS: Request To Send m receiver: reply with short CTS: Clear To Send CTS reserves channel for sender, notifying (possibly hidden) stations avoid hidden station collisions
Lecture 3#58 Collision Avoidance: RTS-CTS exchange RTS and CTS short: m collisions less likely, of shorter duration m end result similar to collision detection IEEE 802.11 allows: m CSMA m CSMA/CA: reservations m polling from AP
Lecture 3#59 Point to Point Data Link Control one sender, one receiver, one link: easier than broadcast link: m no Media Access Control m no need for explicit MAC addressing m e.g., dialup link, ISDN line popular point-to-point DLC protocols: m PPP (point-to-point protocol) m HDLC: High level data link control (Data link used to be considered “high layer” in protocol stack!)
Lecture 3#60 PPP Design Requirements [RFC 1557] packet framing: encapsulation of network-layer datagram in data link frame m carry network layer data of any network layer protocol (not just IP) at same time m ability to demultiplex upwards bit transparency: must be able to carry any bit pattern in the data field error detection (no correction) connection livenes: detect, signal link failure to network layer network layer address negotiation: endpoint can learn/configure each other’s network address
Lecture 3#61 PPP non-requirements no error correction/recovery no flow control out of order delivery OK no need to support multipoint links (e.g., polling) Error recovery, flow control, data re-ordering all relegated to Transport layer!!!
Lecture 3#62 PPP Data Frame (1) Flag: delimiter (framing) Address: does nothing (only one option) Control: does nothing; in the future possible multiple control fields Protocol: upper layer protocol to which frame delivered (eg, PPP-LCP, PPP-NCP, IP, IPCP, etc)
Lecture 3#63 PPP Data Frame (2) info: upper layer data being carried check: cyclic redundancy check (CRC) for error detection
Lecture 3#64 Byte Stuffing “data transparency” requirement: data field must be allowed to include flag pattern m Q: is received data or flag? Byte Stuffing procedure Sender: adds (“stuffs”) extra byte before each or data byte Receiver: m when receive 01111101 discard the byte, Next byte is data, regardless of value m Receive 01111110: flag byte
Lecture 3#65 Byte Stuffing flag byte pattern in data to send flag byte pattern plus stuffed byte in transmitted data
Lecture 3#66 PPP Data Control Protocols Before exchanging network- layer data, data link peers must PPP-LCP: configure PPP link (max. frame length, authentication) learn/configure network layer information m for IP: carry IP Control Protocol (PPP-IPCP) msgs (protocol field: 8021) to configure/learn IP address