CompSci 356: Computer Network Architectures. Lecture 8: Spanning Tree Algorithm and Basic Internetworking Ch & 3.2. Xiaowei Yang
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1 CompSci 356: Computer Network Architectures Lecture 8: Spanning Tree Algorithm and Basic Internetworking Ch & 3.2 Xiaowei Yang
2 Review Past lectures Single link networks Point-to-point, shared media Ethernet, token ring, wireless networks Encoding, framing, error detection, reliability Delay-bandwidth product, sliding window, exponential backoff, carrier sense collision detection, hidden/exposed terminals Packet switching: how to connect multiple links Connectionless: Datagram Connection-oriented: Virtual circuits Source routing Pros and cons Ethernet switches
3 Today Spanning Tree Algorithm Virtual LAN New topic: how to connect different types of networks E.g., how to connect an Ethernet and an ATM network
4 Learning Bridges Overall design goal: complete transparency Plug-and-play Self-configuring without hardware or software changes Bridges should not impact operations of existing LANs Three parts to learning bridges: (1) Forwarding of Frames (2) Learning of Addresses (3) Spanning Tree Algorithm
5 (1) Frame Forwarding Assume a MAC frame arrives on port x. Is MAC address of destination in forwarding table? Port A Port x Bridge 2 Port B Port C Found? Not found? Forward the frame on the appropriate port Flood the frame, i.e., send the frame on all ports except port x.
6 Danger of Loops Consider the two LANs that are connected by two bridges. Assume host A is transmitting a frame F with a broadcast address What is happening? Bridges A and B flood the frame to LAN 2. Bridge B sees F on LAN 2, and updates the port mapping of MAC_A, and copies the frame back to LAN 1 Bridge A does the same. The copying continues Where s the problem? What s the solution? F Bridge A F LAN 2 LAN 1 F F Bridge B F host A
7 Spanning Tree Algorithm A solution is the spanning tree algorithm that prevents loops in the topology By Radia Perlman at DEC LAN 2 Bridge 3 d Bridge 4 Bridge 1 LAN 5 Bridge 5 LAN 1 Bridge 2 LAN 3 LAN 4
8 Algorhyme (the spanning tree poem) I think that I shall never see A graph more lovely than a tree. A tree whose crucial property Is loop-free connectivity. A tree that must be sure to span So packets can reach every LAN. First, the root must be selected. By ID, it is elected. Least-cost paths from root are traced. In the tree, these paths are placed. A mesh is made by folks like me, Then bridges find a spanning tree. Radia Perlman
9 Graph theory on spanning tree For any connected graph consisting of nodes and edges connecting pairs of nodes, a spanning tree of edges maintains the connectivity of the graph but contains no loops n-node s graph, n - 1 edges on a spanning tree No redundancy
10 The protocol IEEE 802.1d has an algorithm that organizes the bridges as spanning tree in a dynamic environment Bridges exchange messages to configure the bridge (Configuration Bridge Protocol Data Unit, Configuration BPDUs) to build the tree Select ports they use to forward packets
11 Configuration BPDUs Destination MAC address Source MAC address Configuration Message protocol identifier version message type flags root ID Cost bridge ID port ID Set to 0 Set to 0 Set to 0 lowest bit is "topology change bit (TC bit) ID of root Cost of the path from the bridge sending this message to root bridge ID of bridge sending this message Time between BPDUs from the root (default: 1sec) message age maximum age hello time forward delay Time between recalculations of the spanning tree (default: 15 secs) ID of port from which message is sent time since root sent a message on which this message is based
12 What do the BPDUs do? Elect a single bridge as the root bridge Calculate the distance of the shortest path to the root bridge Each bridge can determine a root port, the port that gives the best path to the root Each LAN can determine a designated bridge, which is the bridge closest to the root. A LAN's designated bridge is the only bridge allowed to forward frames to and from the LAN for which it is the designated bridge. A LAN's designated port is the port that connects it to the designated bridge Select ports to be included in the spanning tree.
13 Terms Each bridge has a unique identifier: Bridge ID Bridge ID = {Priority : 2 bytes; Bridge MAC address: 6 bytes} Priority is configured Bridge MAC address is the lowest MAC addresses of all ports Each port within a bridge has a unique identifier (port ID) Root Bridge: The bridge with the lowest identifier is the root of the spanning tree Root Port: Each bridge has a root port which identifies the next hop from a bridge to the root
14 Terms Root Path Cost: For each bridge, the cost of the min-cost path to the root Assume it is measured in #hops to the root Designated Bridge, Designated Port: Single bridge on a LAN that is closest to the root for this LAN: If two bridges have the same cost, select the one with the highest priority; if they have the same priority, select based on the bridge ID If the min-cost bridge has two or more ports on the LAN, select the port with the lowest identifier
15 Spanning Tree Algorithm Each bridge is sending out BPDUs that contain the following information: root ID cost bridge ID port ID root bridge (what the sender thinks it is) root path cost for sending bridge Identifies sending bridge Identifies the sending port The transmission of BPDUs results in the distributed computation of a spanning tree The convergence of the algorithm is very fast
16 Ordering of Messages We define an ordering of BPDU messages (lexicographically) ID R1 C1 ID B1 M1 ID P1 ID R2 C2 ID B2 ID P2 M2 We say M1 advertises a better path than M2 ( M1<<M2 ) if (R1 < R2), Or (R1 == R2) and (C1 < C2), Or (R1 == R2) and (C1 == C2) and (B1 < B2), Or (R1 == R2) and (C1 == C2) and (B1 == B2) and (P1 < P2)
17 Initializing the Spanning Tree Protocol Initially, all bridges assume they are the root bridge. Each bridge B sends BPDUs of this form on its LANs from each port P: B 0 B P Each bridge looks at the BPDUs received on all its ports and its own transmitted BPDUs. Root bridge is the one with the smallest received root ID that has been received so far whenever a smaller ID arrives, the root is updated
18 Spanning Tree Protocol Each bridge B looks on all its ports for BPDUs that are better than its own BPDUs Suppose a bridge with BPDU: M1 receives a better BPDU: M2 R1 C1 B1 R2 C2 B2 P1 P2 Then it will update the BPDU to: R2 C2+1 B1 However, the new BPDU is not necessarily sent out On each bridge, the port where the best BPDU (via relation < ) was received is the root port of the bridge No need to send out updated BPDUs to root port P1
19 When to send a BPDU Say, B has generated a BPDU for each port x R Cost B x B will send this BPDU on port x only if its BPDU is better (via relation < ) than any BPDU that B received from port x. Port x In this case, B also assumes that it is the designated bridge for the LAN to which the port connects Port A Bridge B Port B Port C And port x is the designated port of that LAN
20 Selecting the Ports for the Spanning Tree Each bridge makes a local decision which of its ports are part of the spanning tree Now B can decide which ports are in the spanning tree: B s root port is part of the spanning tree All designated ports are part of the spanning tree All other ports are not part of the spanning tree B s ports that are in the spanning tree will forward packets (=forwarding state) B s ports that are not in the spanning tree will not forward packets (=blocking state)
21 Building the Spanning Tree Consider the network on the right. Assume that the bridges have calculated the designated ports (D) and the root ports (R) as indicated. Bridge3 LAN 2 D Bridge4 d LAN 5 Bridge5 R D R R What is the spanning tree? On each LAN, connect D ports to the R ports on this LAN Which bridge is the root bridge? Suppose a packet is originated in LAN 5. How is the packet flooded? D LAN 3 LAN 1 Bridge1 Bridge2 D R D LAN 4
22 Example Assume that all bridges send out their BPDU s once per second, and assume that all bridges send their BPDUs at the same time Bridge1 < Bridge2 < Bridge3 < Bridge4 < Bridge5 Assume that all bridges are turned on simultaneously at time T=0 sec. A LAN 1 B Brige2 LAN 2 Brige1 A B A Brige5 Brige3 B A B LAN 3 A LAN 4 Brige4 B
23 Example: BPDUs sent Bridge1 Bridge2 Bridge3 Bridge4 Bridge5 T=1sec
24 Example: BPDUs sent T=2sec Bridge1 Bridge2 Bridge3 Bridge4 Bridge5
25 Example: BPDUs sent T=3sec Bridge1 Bridge2 Bridge3 Bridge4 Bridge5
26 Example: BPDUs sent Bridge1 Bridge2 Bridge3 Bridge4 Bridge5 T=1sec Send: A: (B1,0,B1,A) B: (B1,0,B1,B) Recv: A: (B5,0,B5,A) (B2,0,B2,B) B: (B3,0,B3,B) (B4,0,B4,A) Send: A: (B2,0,B2,A) B: (B2,0,B2,B) Recv: A: B: (B1,0,B1,A) (B5,0,B5,A) Send: A:(B3,0,B3,A) B:(B3,0,B3,B) Recv: A: (B5,0,B5,B) (B4,0,B4,B) B: (B1,0,B1,B) (B4,0,B4,A) Send: A:(B4,0,B4,A) B:(B4,0,B4,B) Recv: A: (B3,0,B3,B) (B1,0,B1,B) B: (B3,0,B3,A) (B5,0,B5,B) Send: A:(B5,0,B5,A) B:(B5,0,B5,B) Recv: A: (B2,0,B2,B) (B1,0,B1,A) B: (B3,0,B3,A) (B4,0,B4,B)
27 Example: BPDU s sent Bridge1 Bridge2 Bridge3 Bridge4 Bridge5 T=2sec D-port: A,B Send: A: (B1,0,B1,A) B: (B1,0,B1,B) Recv: R-port: B D-port: A Send: A: (B1,1,B2,A) Recv: A: B: (B1,0,B1,A) R-port: B D-port: A Send: A: (B1,1,B3,A) Recv: A: (B1,1,B4,B) (B1,1,B5,B) B: (B1,0,B1,B) R-port: A D-port: B Send: B: (B1,1,B4,B) Recv: A: (B1,0,B1,B) B: (B1,1,B3,A) (B1,1,B5,B) R-port: A D-port: B Send: B: (B1,1,B5,B) Recv: A: (B1,0,B1,A) B: (B1,1,B3,A) (B1,1,B4,B)
28 Example: BPDU s sent Bridge 1 Bridge 2 Bridge 3 Bridge4 Bridge5 T=3sec D-port: A,B Send: A: (B1,0,B1,A) B: (B1,0,B1,B) Recv: R-port: B D-port: A Send: A: (B1,1,B2,A) Recv: A: B: (B1,0,B1,A) R-port: B D-port: A Send: A: (B1,1,B3,A) Recv: A: B: (B1,0,B1,B) R-port: A Blocked: B Recv: A: (B1,0,B1,B) B: (B1,1,B3,A) R-port: A Blocked: B Recv: A: (B1,0,B1,A) B: (B1,1,B3,A)
29 Root Port Example: the spanning tree Bridge1 Bridge2 Bridge3 Bridge4 Bridge5 Designated bridge Designated ports LAN 1 A B Bridge2 Bridge1 LAN 3 A B Bridge3 LAN 2 B A B A A Bridge5 LAN 4 Bridge4 B A packet is sent from LAN2
30 Example: the spanning tree Bridge1 Bridge2 Bridge3 Bridge4 Bridge5 Root Port B B A A Designated bridge LAN2,3 LAN1 LAN4 Designated ports A,B A A LAN 1 A B Bridge2 Bridge1 LAN 3 A B Bridge3 LAN 2 B A B A A Bridge5 LAN 4 Bridge4 B A packet is sent from LAN2
31 Limitations of bridges Scalability Broadcast packets reach every host! Security Every host can snoop Non-heterogeneity Can t connect ATM networks
32 Virtual LANs To address the scalability and security issues A bridge s port is configured to have a VLAN ID Each VLAN has a spanning tree A VLAN header is inserted to a packet Packets are flooded to ports with the same VLAN ID
33 VLAN U 5 U VLAN200 B1 VLAN100 B2 VLAN200
34 Today Spanning Tree Algorithm Virtual LAN New topic: how to connect different types of networks E.g., how to connect an Ethernet and an ATM network
35 Inter-networking Routers interface different networks Uniform addressing (IP) Routers send packets to their destination IP addresses
36 IP (Internet Protocol) is a Network Layer Protocol IP s current version is Version 4 (IPv4). It is specified in RFC 791. IPv6 is also deployed Internet Protocol TCP UDP Transport Layer ICMP IP IGMP Network Layer ARP Network Access Link Layer Media
37 IP: the thin waist of the hourglass IP is the waist of the hourglass of the Internet protocol architecture Multiple higher-layer protocols Multiple lower-layer protocols Only one protocol at the network layer. What is the advantage of this architecture? To avoid the N * M problem Applications HTTP FTP SMTP TCP UDP IP Data link layer protocols Physical layer technologies
38 Application protocol Routers look at a packet s IP header and link layer header Application Application protocol Application TCP TCP protocol TCP IP IP protocol IP IP protocol IP IP protocol IP Data Link Data Link Data Link Data Link Data Link Data Link Data Link Data Link Data Link Host Router Router Host
39 A simple network
40 Delivery service of IP is minimal IP Service Model IP provides an unreliable connectionless best effort service (also called: datagram service ). Unreliable: IP does not make an attempt to recover lost packets Connectionless: Each packet ( datagram ) is handled independently. IP is not aware that packets between hosts may be sent in a logical sequence Best effort: IP does not make guarantees on the service (no throughput guarantee, no delay guarantee, ) Consequences: Higher layer protocols have to deal with losses or with duplicate packets Packets may be delivered out-of-order
41 Basic IP router functions Things you need to understand to do lab2 Internet protocol IP header IP addressing IP forwarding Address resolution protocol Error reporting and control Internet Control Message Protocol
42 Fields of the IP header ToS (8-bit): specifies the type of differentiated services for a packet HLen (4-bit): the length of header in 32-bit words Length (16-bit): packet length in bytes, including the header bytes Fragmentation and reassembly
43 Fields of the IP Header Identification (16 bits): Unique identification of a datagram from a host. Incremented whenever a datagram is transmitted (in some OS) Flags (3 bits): First bit always set to 0 DF bit (Do not fragment) MF bit (More fragments) Will be explained laterà Fragmentation Fragment offset (13 bits)
44 Fields of the IP Header Time To Live (TTL) (1byte): Specifies the longest path before a datagram is dropped Role of TTL field: Ensure that a packet is eventually dropped when a routing loop occurs Used as follows: Sender sets the value (e.g., 64) Each router decrements the value by 1 When the value reaches 0, the datagram is dropped
45 Fields of the IP Header Protocol (1 byte): Specifies the higher-layer protocol. Used for demultiplexing to higher layers. 4 = IP-in-IP encapsulation 6 = TCP 17 = UDP 1 = ICMP 2 = IGMP IP Header checksum (2 bytes): A simple 16-bit long checksum which is computed for the header of the datagram Function?
46 Fields of the IP Header Options: Record Route: each router that processes the packet adds its IP address to the header. Timestamp: each router that processes the packet adds its IP address and time to the header. (loose) Source Routing: specifies a list of routers that must be traversed. (strict) Source Routing: specifies a list of the only routers that can be traversed. IP options increase routers processing overhead. IPv6 does not have the option field. Padding: Padding bytes are added to ensure that header ends on a 4-byte boundary
47 Summary Spanning Tree Algorithm Virtual LAN New topic: how to connect different types of networks E.g., how to connect an Ethernet and an ATM network Looking forward More about IP
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