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Hubs, Bridges and Switches: 

Hubs, Bridges and Switches Lecture 3

Interconnecting LANs: 

Interconnecting LANs Q: Why not just one big LAN? Limited amount of supportable traffic: on single LAN, all stations must share bandwidth limited length: 802.3 (Ethernet) specifies maximum cable length large “collision domain” (can collide with many stations) limited number of stations: 802.5 (token ring) have token passing delays at each station


Hubs Physical Layer devices: essentially repeaters operating at bit levels: repeat received bits on one interface to all other interfaces Hubs can be arranged in a hierarchy (or multi-tier design), with backbone hub at its top

Hubs (more): 

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: simple, inexpensive device Multi-tier provides graceful degradation: portions of the LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m per Hub)

Hub limitations: 

Hub limitations single collision domain results in no increase in max throughput 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?


Bridges Link Layer devices: operate on Ethernet frames, examining frame header and selectively forwarding frame based on its destination Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment, bridge uses CSMA/CD to access segment and transmit

Bridges (more): 

Bridges (more) Bridge advantages: Isolates collision domains resulting in higher total max throughput, and does not limit the number of nodes nor geographical coverage Can connect different type Ethernet since it is a store and forward device Transparent: no need for any change to hosts LAN adapters

Backbone Bridge: 

Backbone Bridge

Interconnection Without Backbone: 

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

Bridges: frame filtering, forwarding: 

Bridges: frame filtering, forwarding bridges filter packets same-LAN -segment frames not forwarded onto other LAN segments forwarding: how to know on which LAN segment to forward frame?

Bridge Filtering: 

Bridge Filtering bridges learn which hosts can be reached through which interfaces: maintain filtering tables when frame received, bridge “learns” location of sender: incoming LAN segment records sender location in filtering table filtering table entry: (Node LAN Address, Bridge Interface, Time Stamp) stale entries in Filtering Table dropped (TTL can be 60 minutes)

Bridge Operation: 

Bridge Operation bridge procedure(in_MAC, in_port,out_MAC) Set filtering table (in_MAC) to in_port /*learning*/ lookup in filtering table (out_MAC) receive out_port if (out_port not valid) /* no entry found for destination */ then flood; /* forward on all but the interface on which the frame arrived*/ if (in_port = out_port) /*destination is on LAN on which frame was received */ then drop the frame Otherwise (out_port is valid) /*entry found for destination */ then forward the frame on interface indicate

Bridge Learning: example: 

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 bridge notes that C is on port 1 frame ignored on upper LAN frame received by D

Bridge Learning: example: 

Bridge Learning: example D generates reply to C, sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1, so selectively forwards frame out via interface 1 C 1

What will happen with loops? Incorrect learning: 

What will happen with loops? Incorrect learning

What will happen with loops? Frame looping: 

What will happen with loops? Frame looping A C 1 1 2 2 C,?? C,??

What will happen with loops? Frame looping: 

What will happen with loops? Frame looping A B 1 1 2 2 B,2 B,1

Introducing Spanning Tree: 

Introducing Spanning Tree Allow a path between every LAN without causing loops (loop-free environment) Bridges communicate with special configuration messages (BPDUs) Standardized by IEEE 802.1D Note: redundant paths are good, active redundant paths are bad (they cause loops)

Spanning Tree Requirements: 

Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all bridges MAC address Bridge id + port number

Spanning Tree Concepts: Root Bridge: 

Spanning Tree Concepts: Root Bridge The bridge with the lowest bridge ID value is elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the root bridge

Spanning Tree Concepts: Path Cost: 

Spanning Tree Concepts: Path Cost A cost associated with each port on each bridge default is 1 The cost associated with transmission onto the LAN connected to the port Can be manually or automatically assigned Can be used to alter the path to the root bridge

Spanning Tree Concepts: Root Port: 

Spanning Tree Concepts: Root Port The port on each bridge that is on the path towards the root bridge 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

Spanning Tree Concepts: Root Path Cost: 

Spanning Tree Concepts: Root Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost independently based on their view of the network

Spanning Tree Concepts: Designated Bridge: 

Spanning Tree Concepts: Designated Bridge Only one bridge on a LAN at one time is chosen the designated bridge This bridge provides the minimum cost path to the root bridge for the LAN Only the designated bridge passes frames towards the root bridge

Example Spanning Tree: 

Example Spanning Tree B3 B5 B7 B2 B1 B6 B4 Protocol operation: Picks a root For each LAN, picks a designated bridge that is closest to the root. All bridges on a LAN send packets towards the root via the designated bridge. B8

Example Spanning Tree: 

Example Spanning Tree B3 B5 B7 B2 B1 B6 B4 Root B8 B2 B4 B5 B7 B8 B1 Spanning Tree: Designated Bridge root port

Spanning Tree Algorithm: An Overview: 

Spanning Tree Algorithm: An Overview 1. Determine the root bridge among all bridges 2. Each bridge determines its root port The port in the direction of the root bridge 3. Determine the designated bridge on each LAN The bridge which accepts frames to forward towards the root bridge The frames are sent on the root port of the designated bridge

Spanning Tree Algorithm: Selecting Root Bridge: 

Spanning Tree Algorithm: Selecting Root Bridge Initially, each bridge considers itself to be the root bridge Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge considers root The root path cost for the sending bridge Best one wins (lowest root ID/cost/priority)

Spanning Tree Algorithm: Selecting Root Ports: 

Spanning Tree Algorithm: Selecting Root Ports Each bridge selects one of its ports which has the minimal cost to the root bridge In case of a tie, the lowest uplink (transmitter) bridge ID is used In case of another tie, the lowest port ID is used

Spanning Tree Algorithm: Select Designated Bridges: 

Spanning Tree Algorithm: Select Designated Bridges Initially, each bridge considers itself to be the designated bridge Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge considers root The root path cost for the sending bridge 3. Best one wins (lowest ID/cost/priority)

Forwarding/Blocking State: 

Forwarding/Blocking State Root and designated bridges will forward frames to and from their attached LANs All other ports are in the blocking state

Spanning Tree Protocol: Execution: 

Spanning Tree Protocol: Execution B3 B5 B7 B2 B1 B6 B4 B8 (B1,root=B1, dist=0) (B1,root=B1,dist=0) (B4, root=B1, dist=1) (B6, Root=B1dist=1)

Bridges vs. Routers: 

Bridges vs. Routers both store-and-forward devices routers: network layer devices (examine network layer headers) bridges are Link Layer devices routers maintain routing tables, implement routing algorithms bridges maintain filtering tables, implement filtering, learning and spanning tree algorithms

Routers vs. Bridges: 

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)

Routers vs. Bridges: 

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)

Ethernet Switches: 

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 Ethernet, but no collisions!

Ethernet Switches: 

Ethernet Switches cut-through switching: frame forwarded from input to output port without awaiting for assembly of entire frame slight reduction in latency combinations of shared/dedicated, 10/100/1000 Mbps interfaces

Ethernet Switches (more): 

Ethernet Switches (more) Dedicated Shared

Optional: Wireless LAN and PPP: 

Optional: Wireless LAN and PPP

IEEE 802.11 Wireless LAN: 

IEEE 802.11 Wireless LAN wireless LANs: untethered (often mobile) networking IEEE 802.11 standard: MAC protocol unlicensed frequency spectrum: 900Mhz, 2.4Ghz Basic Service Set (BSS) (a.k.a. “cell”) contains: wireless hosts access point (AP): base station BSS’s combined to form distribution system (DS)

Ad Hoc Networks: 

Ad Hoc Networks Ad hoc network: IEEE 802.11 stations can dynamically form network without AP Applications: “laptop” meeting in conference room, car interconnection of “personal” devices battlefield IETF MANET (Mobile Ad hoc Networks) working group

IEEE 802.11 MAC Protocol: CSMA/CA: 

IEEE 802.11 MAC Protocol: CSMA/CA 802.11 CSMA: sender - if sense channel idle for DISF 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?

IEEE 802.11 MAC Protocol: 

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

Hidden Terminal effect: 

Hidden Terminal effect hidden terminals: A, C cannot hear each other obstacles, signal attenuation collisions at B goal: avoid collisions at B CSMA/CA: CSMA with Collision Avoidance

Collision Avoidance: RTS-CTS exchange: 

Collision Avoidance: RTS-CTS exchange CSMA/CA: explicit channel reservation sender: send short RTS: request to send receiver: reply with short CTS: clear to send CTS reserves channel for sender, notifying (possibly hidden) stations avoid hidden station collisions

Collision Avoidance: RTS-CTS exchange: 

Collision Avoidance: RTS-CTS exchange RTS and CTS short: collisions less likely, of shorter duration end result similar to collision detection IEEE 802.11 allows: CSMA CSMA/CA: reservations polling from AP

Point to Point Data Link Control: 

Point to Point Data Link Control one sender, one receiver, one link: easier than broadcast link: no Media Access Control no need for explicit MAC addressing e.g., dialup link, ISDN line popular point-to-point DLC protocols: PPP (point-to-point protocol) HDLC: High level data link control (Data link used to be considered “high layer” in protocol stack!)

PPP Design Requirements [RFC 1557]: 

PPP Design Requirements [RFC 1557] packet framing: encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time ability to demultiplex upwards bit transparency: must 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

PPP non-requirements: 

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 higher layers!!!

PPP Data Frame: 

PPP Data Frame 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, IP, IPCP, etc)

PPP Data Frame: 

PPP Data Frame info: upper layer data being carried check: cyclic redundancy check (CRC) for error detection

Byte Stuffing: 

Byte Stuffing “data transparency” requirement: data field must be allowed to include flag pattern <01111110> Q: is received <01111110> data or flag? Sender: adds (“stuffs”) extra < 01111101> byte before each < 01111110> or <01111101> data byte Receiver: Receive 01111101 discard the byte, Next byte is data Receive 01111110: flag byte

Byte Stuffing: 

Byte Stuffing flag byte pattern in data to send flag byte pattern plus stuffed byte in transmitted data

PPP Data Control Protocol: 

PPP Data Control Protocol Before exchanging network-layer data, data link peers must configure PPP link (max. frame length, authentication) learn/configure network layer information for IP: carry IP Control Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address

Data Link: Summary: 

Data Link: Summary principles behind data link layer services: error detection, correction sharing a broadcast channel: multiple access link layer addressing, ARP various link layer technologies Ethernet hubs, bridges, switches IEEE 802.11 LANs PPP Chapter 5 Kurose and Ross

Configuration Messages: BPDU: 

Configuration Messages: BPDU

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