QUIZ: Longest Matching Prefix

Transcription

1 QUIZ: Longest Matching Prefix A router has the following routing table: /24 Send out on interface Z /24 Send out on interface A /22 Send out on interface B /22 Send out on interface C /16 Send out on interface D /8 Send out on interface E A packet comes with the a destination address of Which interface will the pkt. go to?

2 Extra-credit

3 QUIZ: IP What is the address exhaustion in IPv4? What is the main impediment to widespread adoption of IPv6? List at least 3 improvements that were made in IPv6 over IPv4.

4 Improvements in IPv6 over IPv4 Longer addresses (128 bits vs. 32) Simpler header (7 fields vs. 13) Better support for options (extension headers) Native support for security: Authentication Privacy QOS (Quality of Service) Diff.Serv. Field defines traffic class Flow Label field supports VCs

5 Chapter 6 The Transport Layer = L4 Messages Segments Packets Frames Bits / Bytes

6 L2-L3-L4 encapsulation Transport layer sends segments in packets (in frames) Segment Frame trailer Segment

7 Interactions L3-L4-L5 Transport layer offers connectionless (UDP) and connectionoriented (TCP) service to applications

8 L4 vs. L3 L3 is hop-by-hop, operating on the source host, destination host, and on all the routers in the network. L4 is end-to-end, operating only on the source host and destination host! The users have no control over the network, esp. when there are multiple networks in between L3 does not retransmit pkts. (it just sends error notifications via ICMP), i.e. it is unreliable L4 provides end-to-end reliability, i.e. it provides reliable service to L5 Yes, L4 also provides unreliable service (UDP), but that has very little functionality.

9 Simplified Transport Service Primitives (offered by L4 to L5) Applications (L5) invoke these primitives to implement connectionoriented transport service. Scenario: one server and multiple clients: Each client calls CONNECT, SEND, RECEIVE, DISCONNECT Server calls LISTEN, RECEIVE, SEND, DISCONNECT Segment

10 State diagram for simple connection-oriented service Dashed lines show server state sequence Solid lines show client state sequence Transitions in italics are due to segment arrivals.

11 Real-life Transport Service Primitives Berkeley Sockets Very widely used primitives, started with TCP on UNIX A socket is a transport endpoint, analogous to a hardware socket Like simple set plus SOCKET, BIND, and ACCEPT Always executed in this order by a server Unlike the simplified model, here LISTEN is not blocking! Same as DISCONNECT! Returns file handle which can be used for regular R/W Symmetrical, i.e. both client and server have to release their side

12 Real-life Transport Service Primitives Berkeley Sockets

13 Real-life Transport Service Primitives Client side Berkeley Sockets No need to BIND, since address Same as DISCONNECT! Returns file handle which can be used for regular R/W Symmetrical, i.e. both client and server have to release their side

14 Real-life Transport Service Primitives Berkeley Sockets

15 SKIP: Remainder of Sec (Internet File Server) Sec. 6.2 Sec. 6.3

16 6.4 UDP RFC 768 User Datagram Protocol Connectionless service! Encapsulation and little else No flow ctrl., congestion ctrl., or retransmissions Typical app.: Client-server w/short requests and replies (e.g. DNS) If error, client times out and sends request again! The UDP header.

17 6.5 TCP Initially defined in RFC 793 Transmission Control Protocol, followed by many subsequent RFCs there is even a guide: RFC 4614! Provides connection-oriented service, with: flow ctrl. congestion ctrl. retransmissions reordering Typical app.: Client-server with large replies (e.g. FTP)

18 TCP implementation The TCP entity can be a library function, a user process, or part of the kernel (OS). Main functions: Break the App stream into TCP payloads of a given maximum size (Usually 1460 Bytes Why?). Reorder the incoming segments (they may be out of order). Reassembly the App stream. Retransmit lost segments. Flow ctrl. Congestion ctrl.

19 The TCP Service Model The Application interacts with the TCP entity through the sockets API sockets for short. A TCP socket is uniquely identified by the pair [TCP port, IP addr.]. The TCP ports in the range are known as well-known ports (RFC 1700), e.g.: A pair of sockets uniquely identifies a TCP connection.

20 The TCP Service Model All TCP connections are: Point-to-point: no Mcast/Bcast Full duplex: both client and server can send data over the same conn. Byte stream: the TCP entity keeps track of individual Bytes! Byte stream example: (a) Four 512-byte segments sent as separate IP datagrams. (b) The 2048 bytes of data delivered to the application in a single READ CALL.

21 The TCP Segment Header At least the 20-byte IPv4 hdr. goes here (maybe IP options as well). of the first data byte in this segment. Next byte expected Ignored if ACK=0 6 bits unused Must be zero! If zero, it means Don t send me anything for now. Ignored if URG=0 not necessarily multiple of 4 byte. Same as IP hdr. length (takes into account options) If SYN=1, the sequence number is the (random) initial seq.# (ISN) and the first data Byte is ISN+1.

22 SYN = 1 is used for connection request and reply FIN = 1 sender has no more data to send (but can accept data, i.e. asymmetric connection release) Segments with SYN = 1 or FIN = 1 carry no data, but they still consume one unit of seq.# space (so they can be ACK-ed unambiguously). RST = 1 refuse attempt to open connection URG (and the Urgent pointer): Points to the byte after the end of the urgent data. (It s up to App to figure out where the urgent data started!) Is a positive offset from the seq.# of the current segment. Data sent by App may be sent immediately or buffered by TCP and sent later (why? Nagle s algorithm to avoid tinygrams ) Some Apps. want data to be sent immediately (e.g. telnet short, interactive messages) PSH

23 Conn. establishment: The 3-way handshake Client: CONNECT Server: LISTEN Server: ACCEPT 6-31 Client: SEND DATA 3-way handshake: (a) TCP connection establishment in the normal case. (b) Call collision: only 1 connection is established!

24 6-31 Note that the sequence numbers: Are different between the two directions. Are incremented by one even when no data has been sent yet

25 MSS (Maximum Segment Size) If the MSS option is not used during connection establishment, MSS defaults to 556 byte (536 payload). All Internet hosts are required to accept TCP segments of at least 556 Bytes Contrary to popular belief, MSS is not negotiated, but simply announced by each side. The communicating hosts will use the smallest of the two values announced. MSSes can be different in the two directions of a connection!

26 Connection release Asymmetric, i.e. a connection can be half-open To release on both sides, two FIN and two ACK are needed (a.k.a. four-way handshake).

27 Connection release It is also possible to terminate the connection by a 3-way handshake, when A sends a FIN and B replies with a FIN & ACK (combines 2 steps into one), then host A replies with an ACK:

28 Connection release If FIN is not ACK-ed within a certain time limit, the connection is released unilaterally. The other side times out later because it does not hear any more responses. The 2-army problem (pp ) proves that no synchronization algorithm can be 100% safe:

29 Connection mgmt. FSM Can you tell which path is the client and which the server?

30 Sliding window Can you tell which path is the client and which the server?

31 READ the rest of 6.5.8, including The silly widow syndrome

32 SKIP the rest of Ch.6, starting with This is the end of the material for our class Review for final next Wednesday make sure you understand all quizzes and text examples!