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UDP—User Datagram Protocol

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1 UDP—User Datagram Protocol
An unreliable, connectionless transport layer protocol UDP format. See 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 pseudoheader

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

3 Back to UDP—User Datagram Protocol
UDP pseudoheader Source IP Address Destination IP Address Protocol = UDP Length 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. Figure 8.17

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 TCP operations TCP delivers byte stream.See picture
TCP deals with old packets from old connections by several methods. See picture TCP uses sliding-window to implement reliable transfer of byte stream. See picture TCP uses advertised window for flow control. Adaptive timer: tout = tRTT+4dRTT , tRTT(new) =  tRTT(old) +(1-)n , dRTT(new)=dRTT(old) + (1-)(n-tRTT) 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. tRTT is mean round-trip-time, dRTT is average of deviation. TCP uses congestion window for congestion control. See picture

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

7 An old segment could not be distinguished from current ones
Host A Host 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 Question: How does TCP prevent old packets of old connections? 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!! Back to TCP operations Figure 8.23

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

9 Dynamics of TCP congestion window
Congestion occurs Congestion 20 avoidance 15 Congestion window Threshold 10 Slow start 5 Round-trip times Back to TCP operations Figure 7.63

10 TCP protocol TCP segment See Segment format
TCP pseudoheader. See pseudoheader TCP connection establishment. See establishment Client-server application See socket TCP Data transfer Sliding window with window sliding on byte basis Flow control and piggybacking See 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 termination TCP state transition diagram

11 TCP segment format Back to TCP protocol
Source Port Destination Port Sequence Number Acknowledgement Number U A P R S F Header Reserved R C S S Y I (Advertised) Window Size Length G K H T N N Checksum Urgent Pointer Options Padding Data 1.SYN: request to set a connection 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). Figure 8.20

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

13 Three-way handshake to set up connection
Back to TCP protocol Host A Host B Random initial SN 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. SYN, Seq_no = x SYN, Seq_no = y, ACK, Ack_no = x+1 Seq_no = x+1, ACK, Ack_no = y+1 Three-way handshake to set up connection Figure 8.22

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

15 TCP window flow control
Host A Host B t0 Seq_no = 1, Ack_no = 2000, Win = 2048, No Data t1 Seq_no = 2000, Ack_no = 1, Win = 1024, Data = t2 Seq_no = 3024, Ack_no = 1, Win = 1024, Data = t3 Seq_no = 1, Ack_no = 4048, Win = 512, Data = 1-128 t4 Seq_no = 4048, Ack_no = 129, Win = 1024, Data = Back to TCP protocol Figure 8.25

16 TCP graceful termination
Back to TCP protocol TCP graceful termination Host A Host B 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? FIN, seq = 5086 ACK = 5087 Data (150 bytes), seq. = 303, ACK = 5087 ACK = 453 No, Saying goodbye is hard to do. Famous blue-red armies problem. FIN, seq. =453, ACK = 5087 ACK = 454 Figure 8.27

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

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 232=4,294,967,296 bytes have been sent (maximum window size=231) T-1 line, (2328)/(1.544  106) = 6 hours T-3 line, (2328)/(45  106) = 12 minutes OC-48 line, (2328)/(2.4  109) = 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, 232232=264 is big enough right now.

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 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 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 RIP message format Command Version Zero Address Family Identifier Zero IP Address Metric . . . 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. Figure 8.32

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 Multicasting Source S sends packets to multicast group G1 G1 7 2 5 8 S
3 4 5 6 7 8 G2 Source S sends packets to multicast group G1

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 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 Example: Shortest Paths from S
G1 G3 1 2 3 4 5 6 7 8 G2 Spanning tree of shortest paths to node S and parent ports are shown in blue

28 Example: S sends a packet
G1 G1 1 7 2 3 2 4 2 3 4 2 1 1 5 2 5 3 3 G1 4 8 2 1 S 1 1 4 G1 3 5 4 2 2 4 1 3 6 3 2 1 1 3 4 G2 3 G3 G3 S sends a packet to node 1 Node 1 forwards to all ports, except parent port

29 Example: Hop 1 nodes broadcast
G1 G1 1 7 2 3 2 4 2 3 4 2 1 1 5 2 5 3 3 G1 4 8 2 1 S 1 4 1 G1 3 5 4 2 2 4 1 3 6 3 2 1 1 3 4 G2 3 G3 G3 Nodes 2, 3, 4, and 5 broadcast, except on parent ports All nodes, not only G1, receive packets

30 Example: Broadcast continues
G1 G1 1 7 2 3 2 4 2 3 4 2 1 1 5 2 5 3 3 G1 4 8 2 1 S 1 1 4 G1 3 5 4 2 2 4 1 3 6 3 2 1 1 3 4 G2 3 G3 G3 Truncated RPB (TRPB): Leaf routers do not broadcast if none of its attached hosts belong to packet’s multicast group

31 Internet Group Management Protocol (IGMP)
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 Multicast programming
2.1 Multicast addresses. 2.2 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 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

34 Simplified header format:
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 1038 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 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 Payload Length Next Header Hop Limit
IPv6 Header Format Version Traffic Class Flow Label Payload Length Next Header Hop Limit Source Address Destination Address 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

37 Payload Length Next Header Hop Limit
IPv6 Header Format Version Traffic Class Flow Label Payload Length Next Header Hop Limit Source Address Destination Address 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

38 IPv6 Addressing Address Categories Hexadecimal notation
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 Example

40 Address Types based on Prefixes
Binary prefix Types Percentage of address space Reserved 0.39 Unassigned ISO network addresses 0.78 IPX network addresses 0000 1 3.12 0001 6.25 001 12.5 010 Provider-based unicast addresses 011 100 Geographic-based unicast addresses 101 110 1110 1111 0 1.56 0.2 Link local use addresses 0.098 Site local use addresses Multicast addresses

41 Special Purpose Addresses
010 Registry ID Provider ID Subscriber ID Subnet ID Interface ID n bits m bits o bits p bits (125-m-n-o-p) bits 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

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

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 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 Deliver packets to mobile host through home agent and foreign agent
Foreign network Home network Foreign agent Mobile host 2 Home agent Internet 3 1 Correspondent host Figure 8.29

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