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1 UDP—User Datagram Protocol An unreliable, connectionless transport layer protocol UDP format. See pictureSee picture Two additional functions beyond.

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Presentation on theme: "1 UDP—User Datagram Protocol An unreliable, connectionless transport layer protocol UDP format. See pictureSee picture Two additional functions beyond."— Presentation transcript:

1 1 UDP—User Datagram Protocol An unreliable, connectionless transport layer protocol UDP format. See pictureSee picture Two additional functions beyond IP: –Demultiplexing: deliver to different upper layer entities such as DNS, RTP, SNMP based on the destination port # in the header. i.e., UDP can support multiple applications in the same end systems. –(Optionally) check the integrity of entire UDP. (recall IP only checks the integrity of IP header.) If source does not want to compute checksum, fill checksum with all 0s. If compute checksum and the checksum happens to be 0s, then fill all 1s. UDP checksum computation is similar to IP checksum, with two more: –Add extra 0s to entire datagram if not multiple of 16 bits. –Add pseudoheader to the beginning of datagram. UDP pseudoheaderUDP pseudoheader

2 2 Source Port Destination Port UDP Length UDP Checksum Data Figure 8.16 UDP datagram Back to UDP—User Datagram Protocol

3 Protocol = 17 UDP Length Source IP Address Destination IP Address Figure 8.17 UDP pseudoheader 1.Pseudoheader is to ensure that the datagram has indeed reached the correct destination host and port. 2. The padding of 0s and pseudoheader is only for the computation of checksum and not be transmitted. Back to UDP—User Datagram Protocol

4 4 TCP—transmission control protocol TCP functionality –Provides connection-oriented, reliable, in-sequence, byte-stream service –Provides a logical full-duplex (two way) connection –Provides flow-control by advertised window. –Provides congestion control by congestion window. –Support multiple applications in the same end systems. TCP establishes connection by setting up variables that are used in two peer TCP entities. Most important variables are initial sequence numbers. TCP uses Selective Repeat ARQ. TCP terminates each direction of connection independently, allowing data to continue flowing in one direction after closing the other direction. TCP does not keep messages boundaries and treats data as byte stream. e.g, when source sends out two chunks of data with length 400 and 600 bytes, the receiver may receive data in chunks of 300, 400, and 300 bytes, or 100 and 900 bytes.

5 5 TCP operations 1.TCP delivers byte stream.See pictureSee picture 2.TCP deals with old packets from old connections by several methods. See pictureSee picture 3.TCP uses sliding-window to implement reliable transfer of byte stream. See pictureSee picture 4.TCP uses advertised window for flow control. 5.Adaptive timer: 1.t out = t RTT +4d RTT, 2.t RTT (new) =  t RTT (old) +(1-  )  n, d RTT (new)=  d RTT (old) + (1-  )(  n -t RTT ) 3.Where  n is the time from transmitting a segment until receiving its ACK. ,  are in 0 to 1 with  being 7/8 and  being ¼ typically. t RTT is mean round- trip-time, d RTT is average of deviation. 6.TCP uses congestion window for congestion control. See pictureSee picture

6 6 byte stream Send buffer segments Receive buffer byte stream Application ACKs Transmitter Receiver Figure 8.18 TCP byte stream

7 7 Host AHost B SYN, Seq_no = n SYN, Seq_no = n, ACK, Ack_no = n+1 Seq_no = n+1, ACK, Ack_no = n+1 Delayed segment with Seq_no = n+2 will be accepted Figure 8.23 Back to TCP operations Question: How does TCP prevent old packets of old connections? An old segment could not be distinguished from current ones –Using long (32 bit) sequence number –Random initial sequence number -- set a timer at the end of a connection to clear all lost packets from this connection. As a result, that an old packet from an old connection conflicts with packets in current connection is very low!!

8 8 Octets transmitted and ACKed R last highest-numbered octet not yet read by the application R next next expected octet R new highest numbered octet received correctly R last +W R -1 highest-numbered octet that can be accommodated in receive buffer Transmitter Receiver Receive Window S last S last +W S Send Window S recent R next... S last +W A -1 R last R last +W R +1 S last oldest unacknowledged octet S recent highest-numbered transmitted octet S last +W A -1 highest-numbered octet that can be transmitted S last +W S -1 highest-numbered octet that can be accepted from the application R new Figure 8.19 Back to TCP operations TCP uses Selective-Repeat ARQ Note: 1. R new highest bytes received correctly, which are out-of sequence bytes. 2. Advertised window W A : S recent – S last  W A =W R – ( R new – R last ) ……… Advertised window

9 9 Congestion window Round-trip times Slow start Congestion avoidance Congestion occurs Threshold Figure 7.63 Back to TCP operations Dynamics of TCP congestion window

10 10 TCP protocol TCP segment See Segment formatSee Segment format –TCP pseudoheader. See pseudoheaderSee pseudoheader TCP connection establishment. See establishmentSee establishment –Client-server application See socketSee socket TCP Data transfer –Sliding window with window sliding on byte basis –Flow control and piggybacking See flow controlSee flow control TCP connection termination –After receiving ACK for previous data, but no more data to send, the TCP will terminate the connection in its direction by issuing an FIN segment. Graceful terminationGraceful termination TCP state transition diagram

11 11 Source Port Destination Port Sequence Number Acknowledgement Number Checksum Urgent Pointer Options Padding U R G A C K P S H R S T S Y N F I N Header Length Reserved (Advertised) Window Size Data Figure 8.20 Back to TCP protocol TCP segment format 1.SYN: request to set a connection. 2. RST: tell the receiver to abort the connection. 3. FIN: tell receiver this is the final segment, no more data, i.e, close the connection in this direction. 4. ACK: tell the receiver (or sender) that the value is the field of acknowledgment number is valid. 5. PSH: tell the receiving TCP entity to pass the data to the application immediately. 6. URG: tell the receiver that the Urgent Pointer is valid. Urgent Pointer: this pointer added to the sequence number points to the last byte of the “Urgent Data”, (the data that needs immediately delivery).

12 Protocol = 6 TCP Segment Length Source IP Address Destination IP Address Figure 8.21 Back to TCP protocol TCP pseudoheader The padding of 0s and pseudoheader is only used in computation of checksum but not be transmitted, as in UDP checksum.

13 13 Host AHost B SYN, Seq_no = x SYN, Seq_no = y, ACK, Ack_no = x+1 Seq_no = x+1, ACK, Ack_no = y+1 Figure 8.22 Back to TCP protocol Three-way handshake to set up connection 1.Random initial SN 2.Initial SNs in two directions are different 3. Initial SNs for two connections are different. 4. It should be clear here that what setting up connection means: both A and B know that they will exchange data, and go into ready state to send and receive data. Most important is that they agree upon the initial SNs.

14 14 Host A (Client) Host B (Server) SYN, Seq_no = x SYN, Seq_no = y, ACK, Ack_no = x+1 Seq_no = x+1, ACK, Ack_no = y+1 socket bind listen accept (blocks) socket connect (blocks) connect returns accept returns read (blocks) write read (blocks) read returns write read (blocks) read returns request message reply message Figure 8.24 Back to TCP protocol

15 15 Host AHost B Seq_no = 2000, Ack_no = 1, Win = 1024, Data = Seq_no = 1, Ack_no = 4048, Win = 512, Data = Seq_no = 3024, Ack_no = 1, Win = 1024, Data = Seq_no = 4048, Ack_no = 129, Win = 1024, Data = t1t1 t2t2 t3t3 t4t4 Seq_no = 1, Ack_no = 2000, Win = 2048, No Data t0t0 Figure 8.25 Back to TCP protocol TCP window flow control

16 16 FIN, seq = 5086 ACK = 5087 Data (150 bytes), seq. = 303, ACK = 5087 FIN, seq. =453, ACK = 5087 ACK = 454 Host A Host B ACK = 453 Figure 8.27 Back to TCP protocol TCP graceful termination Question: is termination easier than establishment? Or to say, is it possible that a connection is closed when both of two parties confirm with each other? No, Saying goodbye is hard to do. Famous blue-red armies problem.

17 17 CLOSED LISTEN SYN_RCVD ESTABLISHED CLOSING TIME_WAIT SYN_SENT FIN_WAIT_1 CLOSE_WAIT LAST_ACK FIN_WAIT_2 active open,create TCB send SYN passive open, create TCB send SYN receive SYN, send SYN, ACK receive RST receiveACK receive SYN, ACK, send ACK applic. close, send FIN applic. close, send FIN receive FIN, send ACK receive FIN send ACK receive FIN, ACK send ACK receive ACK receive FIN send ACK receive ACK applic. close send FIN receive ACK applic. close or timeout, delete TCB 2MSL timeout delete TCB receive SYN, send ACK applic. close Figure 8.28 Thick lines: normal client states Dashed lines: normal server states Back to TCP protocol

18 18 Sequence number wraparound and timestamps Original TCP specification for MSL (Maximum Segment Lifetime) is 2 minutes. How long will it take to wrap around 32 bit sequence number when 2 32 =4,294,967,296 bytes have been sent (maximum window size=2 31 ) –T-1 line, (2 32  8)/(1.544  10 6 ) = 6 hours –T-3 line, (2 32  8)/(45  10 6 ) = 12 minutes –OC-48 line, (2 32  8)/(2.4  10 9 ) = 14 seconds !!! When sequence number wrap around, the wraparounded sequence number will confuse with previous sequence number. Solution: optional timestamp field (32 bits) in TCP header, thus, 2 32  2 32 =2 64 is big enough right now.

19 19 Internet routing protocols Autonomous system (AS) –A set of routers or networks technically administrated by a single organization. –No restriction that an AS must run a single routing protocol –Only requirement is that from outside, an AS presents a consistent picture of which ASs are reachable through it. Three types of ASs: –Stub AS: has only a single connection to outside. –Multihomed AS: has multiple connections to outside, but refuses to carry out transit traffic –Transit AS: multiple connections to outside and carry transit traffic. ASs need to be assigned globally unique AS number (ASN)

20 20 Classification of Internet routing protocols IGP (Interior Gateway Protocol): –For routers to communicate within an AS and relies on IP address to construct paths. –Provides a map of a county dealing with how to reach each building. –RIP (Routing Information Protocol): distance vector –OSPF (Open Shortest Path First): link state EGP (Exterior Gateway Protocol): –For routers to communicate among different ASs and relies on AS numbers to construct AS paths. –Provides a map of a country, connecting each county. –BGP (Border Gateway Protocol): (distance) path vector

21 21 RIP—Routing Information Protocol Distance vector On top of UDP with port #520 Metric is number of hops –Maximum number of hops is 15, 16 stands for infinity –Using split-horizon with poisoned reverse. –May speed up convergence by triggered updates. Routers exchange distance vector every 30 seconds –If a router does not receive distance vector from its neighbor X within 180 seconds, the link to X is considered broken and the router sets the cost to X is 16 (infinity). RIP-2 contains more information: subnet mask, next hop, routing domain, authentication, CIDR

22 22 Command Version Zero Address Family Identifier Zero IP Address Zero Metric Figure 8.32 RIP message format 1.Command: 1: request other routers to send routing information 2: a response containing its routing information 2. Version: 1 or 2 3. Up to 25 routing information message 3.1 Family identifier: only 2 for IP address 3.2 IP address: can be a host address or a network address 3.3 Metric: 1— indicates infinity Problems of RIP: not scalable, slow convergence, counting-to-infinity, therefore replaced By OSPF in 1979.

23 23 Internet multicast A packet is to be sent to multiple hosts with the same multicast address Class D multicast addresses: e.g., – all systems on a LAN – all routers on a LAN – all OSPF routers on a LAN – all designated OSPF routers on a LAN It is not efficient to implement multicast by unicast, i.e., the source sends a separate copy for every destination. Reverse-path broadcasting / multicasting, each packet is transmitted once per link IGMP (Internet Group Management Protocol): allow a user to join a multicast group and let routers collect multicast group membership information.

24 24 Multicasting S G1 G G2 G3 3 4 Source S sends packets to multicast group G1

25 25 Multicast Routing Multicast routing useful when a source wants to transmit its packets to several destinations simultaneously Relying on unicast routing by transmitting each copy of packet separately works, but can be very inefficient if number of destinations is large Typical applications is multi-party conferencing over the Internet Example: Multicast Backbone (MBONE) uses reverse path multicasting

26 26 Reverse-Path Broadcasting (RPB) Fact: Set of shortest paths to the source node S forms a tree that spans the network –Approach: Follow paths in reverse direction Assume each router knows current shortest path to S –Upon receipt of a multicast packet, router records the packet’s source address and the port it arrives on –If shortest path to source is through same port (“parent port”), router forwards the packet to all other ports –Else, drops the packet Loops are suppressed; each packet forwarded by a router exactly once Implicitly assume shortest path to source S is same as shortest path from source –If paths asymmetric, need to use link state info to compute shortest paths from S

27 27 Example: Shortest Paths from S Spanning tree of shortest paths to node S and parent ports are shown in blue S G1 G G2 G3 3 4

28 28 Example: S sends a packet S sends a packet to node 1 Node 1 forwards to all ports, except parent port S G1 G G2 G3 3 4

29 29 Example: Hop 1 nodes broadcast Nodes 2, 3, 4, and 5 broadcast, except on parent ports All nodes, not only G1, receive packets S G1 G G2 G3 3 4

30 30 Example: Broadcast continues Truncated RPB (TRPB): Leaf routers do not broadcast if none of its attached hosts belong to packet’s multicast group S G1 G G2 G3 3 4

31 31 Internet Group Management Protocol (IGMP) Internet Group Management Protocol: –Host can join a multicast group by sending an IGMP message to its router Each multicast router periodically sends an IGMP query message to check whether there are hosts belonging to multicast groups –Hosts respond with list of multicast groups they belong to –Hosts randomize response time; cancel response if other hosts reply with same membership Routers determine which multicast groups are associated with a certain port Routers only forward packets on ports that have hosts belonging to the multicast group

32 32 Multicast programming 2.1 Multicast addresses. – Levels of conformance. –0: no, 1: sending, 2: receiving 2.3 Sending Multicast Datagrams. –Open UDP socket, and send to multicast address –TTL 0 Restricted to the same host. 1 Restricted to the same subnet. <32 Restricted to the same site, organization or department. <64 Restricted to the same region. <128 Restricted to the same continent. <255 Unrestricted in scope. Global. 2.4 Receiving Multicast Datagrams. –Joining multicast group –Drop multicast group Mapping of IP Multicast Addresses to Ethernet/FDDI addresses.

33 33 Multicast functions int getsockopt(int s, int level, int optname, void* optval, int* optlen); int setsockopt(int s, int level, int optname, const void* optval, int optlen); setsockopt() getsockopt() IP_MULTICAST_LOOP yes yes IP_MULTICAST_TTL yes yes IP_MULTICAST_IF yes yes IP_ADD_MEMBERSHIP yes no IP_DROP_MEMBERSHIP yes no ther-formats/html_single/Multicast- HOWTO.html#ss2.1

34 34 IPv6 (IPng): IPv4 is very successful but the victim of its own success. Longer address field: –128 bits can support up to 3.4 x hosts Simplified header format: –Simpler format to speed up processing of each header –All fields are of fixed size –IPv4 vs IPv6 fields: Same: Version Dropped: Header length, ID/flags/frag offset, header checksum Replaced: –Datagram length by Payload length –Protocol type by Next header –TTL by Hop limit –TOS by traffic class New: Flow label

35 35 Other IPv6 Features Flexible support for options: more efficient and flexible options encoded in optional extension headers Flow label capability: “flow label” to identify a packet flow that requires a certain QoS Security: built-in authentication and confidentiality Large packets: supports payloads that are longer than 64 K bytes, called jumbo payloads. Fragmentation at source only: source should check the minimum MTU along the path No checksum field: removed to reduce packet processing time in a router

36 36 IPv6 Header Format Version field same size, same location Traffic class to support differentiated services Flow: sequence of packets from particular source to particular destination for which source requires special handling Version Traffic Class Flow Label Payload Length Next Header Hop Limit Source Address Destination Address

37 37 IPv6 Header Format Payload length: length of data excluding header, up to B Next header: type of extension header that follows basic header Hop limit: # hops packet can travel before being dropped by a router Version Traffic Class Flow Label Payload Length Next Header Hop Limit Source Address Destination Address

38 38 IPv6 Addressing Address Categories –Unicast: single network interface –Multicast: group of network interfaces, typically at different locations. Packet sent to all. –Anycast: group of network interfaces. Packet sent to only one interface in group, e.g. nearest. Hexadecimal notation –Groups of 16 bits represented by 4 hex digits –Separated by colons 4BF5:AA12:0216:FEBC:BA5F:039A:BE9A:2176 –Shortened forms: 4BF5:0000:0000:0000:BA5F:039A:000A:2176 To 4BF5:0:0:0:BA5F:39A:A:2176 To 4BF5::BA5F:39A:A:2176 –Mixed notation: ::FFFF:

39 39 Example

40 40 Address Types based on Prefixes Binary prefixTypesPercentage of address space 0000 Reserved Unassigned ISO network addresses IPX network addresses Unassigned Unassigned Unassigned Unassigned Provider-based unicast addresses Unassigned Geographic-based unicast addresses Unassigned Unassigned Unassigned Unassigned Unassigned Unassigned Unassigned Link local use addresses Site local use addresses Multicast addresses0.39

41 41 Special Purpose Addresses Provider-based Addresses: 010 prefix –Assigned by providers to their customers –Hierarchical structure promotes aggregation Registry ID: ARIN, RIPE, APNIC ISP Subscriber ID: subnet ID & interface ID Local Addresses: do not connect to global Internet –Link-local: for single link –Site-local: for single site –Designed to facilitate transition to connection to Internet 010 Registry ID Provider ID Subscriber ID Subnet ID Interface ID n bitsm bitso bitsp bits(125-m-n-o-p) bits

42 42 Special Purpose Addresses Unspecified Address: 0::0 –Used by source station to learn own address Loopback Address: ::1 IPv4-compatible addresses: 96 0’s + IPv4 –For tunneling by IPv6 routers connected to IPv4 networks –:: IP-mapped addresses: 80 0’s ’s + IPv4 –Denote IPv4 hosts & routers that do not support IPv6

43 43 Migration from IPv4 to IPv6 Gradual transition from IPv4 to IPv6 Dual IP stacks: routers run IPv4 & IPv6 –Type field used to direct packet to IP version IPv6 islands can tunnel across IPv4 networks –Encapsulate user packet insider IPv4 packet –Tunnel endpoint at source host, intermediate router, or destination host –Tunneling can be recursive

44 44 Migration from IPv4 to IPv6 Source Destination IPv6 network Link (b) Source Destination IPv6 network IPv4 network IPv6 network Tunnel Tunnel head-end Tunnel tail-end IPv6 header IPv4 header (a)

45 45 DHCP (Dynamic Host Configuration Protocol) A host broadcasts a DHCP discovery message in its physical network for an IP address. Server(s) reply with DHCP offer message The host selects one IP address and broadcasts a DHCP request message including the IP address The selected server allocates the IP address and sends back a DHCP ACK message with a lease time T, two thresholds T1 (=0.5T), T2(=0.875T) – when T1 expires, the host asks the server for extension. – If T2 expire, the host broadcasts DHCP request to any server on the network –If T expires, the host relinquishes the IP address and reapply from scratch.

46 46 Mobile IP Mobile host, home agent, foreign agent If mobile host is currently at the same network with HA (home agent), the packet to the mobile host will be broadcast to it. If mobile host moves to another network, the mobile host will register itself with FA (foreign agent) and gets a new care-of IP address. Then packet is sent to HA, which will forward to the FA and FA continues to forward to destination.

47 47 Home agent Foreign agent Home network Foreign network Internet Correspondent host Mobile host Figure 8.29 Deliver packets to mobile host through home agent and foreign agent


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