Lecture 3#1#1 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  limited length: (Ethernet) specifies maximum cable length (need small a)  large “collision domain” (can collide with many stations)  limited number of stations: (token ring) have token passing delays at each station

Lecture 3#3#3 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

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?  What happens to a?

Lecture 3#6#6 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

Lecture 3#7#7 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#8#8 Backbone Bridge

Lecture 3#9#9 Interconnection Without Backbone  Not recommended for two reasons: - single point of failure at Computer Science hub - all traffic between EE and SE must pass over CS segment

Lecture 3#10 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#11 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#12 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

Lecture 3#13 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 m bridge notes that C is on port 1 m frame ignored on upper LAN m frame received by D

Lecture 3#14 Bridge Learning: example  D generates reply to C, sends 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 C 1

Lecture 3#15 What will happen with loops? Incorrect learning A B A, 1 2 2

Lecture 3#16 What will happen with loops? Frame looping A C C,??

Lecture 3#17 What will happen with loops? Frame looping A B B,2 B,1

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, desirable to have redundant, alternative paths from source to dest  with multiple paths, cycles result - bridges may multiply and forward frame forever  solution: organize bridges in a spanning tree by disabling subset of interfaces Disabled

Lecture 3#25 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 N ote: redundant paths are good, active redundant paths are bad (they cause loops)

Lecture 3#26 How to construct a spanning tree?  Bridges run a distributed spanning tree algorithm m Select what ports (and bridges) should actively forward frames  Standardized in IEEE specification

Lecture 3#27 Overview of STP We make a series of simplifications:  Build a ST of bridges (in fact, need to span LAN segments!)  Assume that we are given a root bridge So we solve in order: 1. How to find 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 each bridge has a unique identifier  Each bridge remembers smallest ID seen so far (my_root_ID)  Periodically, send my_root_ID to all neighbors (“flooding”)  When receiving ID, update if necessary  Is that enough?!

Lecture 3#29 2. Compute ST Given a root Idea: each node finds its shortest paths 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 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 :  Mark neighbor with least distance as “parent”

Lecture 3#31 Why does this work?  Suppose all nodes start with distance , and suppose that updates are sent every time unit. C D B E F G A 0       

Lecture 3#32 Why does this work?  Suppose all nodes start with distance , and suppose that updates are sent every time unit. C D B E F G A  1 1  

Lecture 3#33 Why does this work?  Suppose all nodes start with distance , and suppose that updates are sent every time unit. C D B E F G A  2

Lecture 3#34 Why does this work?  Suppose all nodes start with distance , and suppose that updates are sent every time unit. C D B E F G A

Lecture 3#35 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! (later...)

Lecture 3#36 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...) Implementation: Bridges listen to all distance announcement on each port. Mark port as “designated port” iff best on that port’s LAN

Lecture 3#37 Spanning Tree Concepts: Path Cost  A cost associated with each port on each bridge (“weight” of the segment) m default is 1  The cost associated with transmission onto the LAN connected to the port m Can be manually or automatically assigned m Can be used to alter the path to the root bridge

Lecture 3#38 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#39 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#40 Example Spanning Tree B3 B5 B7 B2 B1 B4 B6 Root B4B5B6 B8 B1 Spanning Tree: root port B3 B7 B2 B8

Lecture 3#41 Spanning Tree Concepts: Designated Port  Each LAN has a single designated port  This is the port reporting minimum cost path to the root bridge for the LAN  Only designated and root ports remain active!

Lecture 3#42 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#43 Spanning Tree Requirements  Each bridge has a unique identifier  A broadcast address for bridges on a LAN  A unique port identifier for all ports on all bridges m Bridge id + port number

Lecture 3#44 Spanning Tree Algorithm: Implementation Keep pumping a single message: (my root ID, my cost to root, my ID) BPDU: Bridge Protocol Data Unit Update var’s when receiving: l My_root_ID: smallest seen so far l My_cost_to_root: smallest received to my_root_ID + link cost l Break ties by ID That’s enough!

Lecture 3#45 Spanning Tree Algorithm: Select Designated Bridges  Bridges send BPDU frames to its attached LANs m sender port ID m bridge and port ID of the bridge the sending bridge considers root m root path cost for the sending bridge  3. Best bridge wins, and it knows it (and winning port) m (lowest ID/cost/priority)

Lecture 3#46 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 To adjust to changes B3 B5 B7 B2 B1 B6 B4 Root B8 Designated Bridge ro ot p or t

Lecture 3#47 Spanning Tree Protocol: Execution B3 B5 B7 B2 B1 B6 B4 B8 (B1,root=B1, dist=0) (B4, root=B1, dist=1) (B6, Root=B1dist=1)

Lecture 3#48 Bridges vs. Routers  both store-and-forward devices m routers: network layer devices (examine network layer headers) m bridges are Link Layer devices  routers maintain routing tables, implement routing algorithms  bridges maintain filtering tables, implement filtering, learning and spanning tree algorithms

Lecture 3#49 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#50 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)

Lecture 3#51 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, but no collisions!

Lecture 3#52 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#53 Ethernet Switches (more) Dedicated Shared

Lecture 3#54 Optional: Wireless LAN and PPP

Lecture 3#55 IEEE Wireless LAN  wireless LANs: untethered (often mobile) networking  IEEE 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#56 Ad Hoc Networks  Ad hoc network: IEEE 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#57 IEEE MAC Protocol: CSMA/CA CSMA: sender - if sense channel idle for DISF sec. then transmit entire frame (no collision detection) -if sense channel busy then binary backoff CSMA receiver: if received OK return ACK after SIFS Why?

Lecture 3#58 IEEE MAC Protocol CSMA Protocol: others  NAV: Network Allocation Vector  frame has transmission time field  others (hearing data) defer access for NAV time units

Lecture 3#59 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#60 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#61 Collision Avoidance: RTS-CTS exchange  RTS and CTS short: m collisions less likely, of shorter duration m end result similar to collision detection  IEEE allows: m CSMA m CSMA/CA: reservations m polling from AP

Lecture 3#62 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#63 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 carry any bit pattern in the data field  error detection (no correction) m No error management (ack) required  connection liveness: detect, signal link failure to network layer  network layer address negotiation: endpoint can learn/configure each other’s network address

Lecture 3#64 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!!! Note the difference with Reason: different error rates

Lecture 3#65 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)

Lecture 3#66 PPP Data Frame  info: upper layer data being carried  check: cyclic redundancy check (CRC) for error detection

Lecture 3#67 Byte Stuffing  “data transparency” requirement: data field must be allowed to include flag pattern m Q: is received data or flag?  Sender: adds (“stuffs”) extra byte before each or data byte  Receiver: m Receive discard the byte, Next byte is data m Receive : flag byte

Lecture 3#68 Byte Stuffing flag byte pattern in data to send flag byte pattern plus stuffed byte in transmitted data

Lecture 3#69 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 m for IP: carry IP Control Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address

Lecture 3#70 Data Link: Summary  principles behind data link layer services: m error detection, correction m sharing a broadcast channel: multiple access m link layer addressing, ARP  various link layer technologies m Ethernet m hubs, bridges, switches m IEEE LANs m PPP  Chapter 5 Kurose and Ross

Lecture 3#71 Configuration Messages: BPDU