CSS432 End-to-End Protocols Textbook Ch5.1 – 5.2 Prof. Athirai Irissappane http://courses.washington.edu/css432/athirai/ athirai@uw.edu CSS432: End-to-End Protocols

Outline Transport Layer Protocols Simple Demultiplexer (UDP) Communication between applications running in the end nodes hence end-to-end protocols How to convert host-to-host packet delivery service into a process-to-process communication channel Simple Demultiplexer (UDP) Reliable Byte Stream (TCP) CSS432: Internetworking

End-to-End Protocols Common properties that a transport protocol can be expected to provide by Applications Guarantees message delivery Delivers messages in the same order they were sent Delivers at most one copy of each message Supports arbitrarily large messages Supports synchronization between the sender and the receiver Allows the receiver to apply flow control to the sender Supports multiple application processes on each host CSS432: Internetworking

End-to-End Protocols Typical limitations of underlying network (Best effort Delivery) Drop messages Reorder messages Deliver duplicate copies of a given message Limit messages to some finite size Deliver messages after an arbitrarily long delay Challenge for Transport Protocols Develop algorithms that turn the above limitations into the high level of service required by the application Request a retransmission of M3 and M5 M5, and M3 M5, M5, M3 M5 M2, M1, M4 M5, M4, M3, M2, M1 M5, M2, M1 M4 M4, M3 M3

Simple Demultiplexer (UDP) Extends host-to-host delivery service of the underlying network into a process-to-process communication service Adds a level of demultiplexing which allows multiple application processes on each host to share the network Allows checksum Unreliable and Unordered Datagram Service No flow control: preventing senders from overrunning the capacity of the receivers (messages are discarded if the receiving buffers are full) No reliable delivery UDP mostly use for unidirectional communication such a broadcasting CSS432: End-to-End Protocols

Simple Demultiplexor (UDP) Identify each process by port id associated to the sender and receiver Endpoints identified by ports servers have well-known ports see /etc/services file on Unix Header format Optional checksum Calculated using psuedo header + UDP header + data Pseudoheader: IP header (protocol number) + source IP + destination IP, UDP length field SrcPort DstPort Checksum Length Data 16 31 A TCP connection consists of two endpoints, and each endpoint consists of an IP address and a port number.  Therefore, when a client user connects to a server computer, an established connection can be thought of as the 4-tuple of (server IP, server port, client IP, client port).  Usually three of the four are readily known -- client machine uses its own IP address and when connecting to a remote service, the server machine's IP address and service port number are required.   What is not immediately evident is that when a connection is established that the client side of the connection uses a port number.  Unless a client program explicitly requests a specific port number, the port number used is an ephemeral port number.  Ephemeral ports are temporary ports assigned by a machine's IP stack, and are assigned from a designated range of ports for this purpose.  When the connection terminates, the ephemeral port is available for reuse,  CSS432: End-to-End Protocols

UDP: Simple Demultiplexer socket() bind() recv() Port: 13579 13579 socket() bind() recv() Port:12345 socket() sendto() int sd; sd = socket(AF_INET, SOCK_DGRAM, 0); struct sockaddr_in server; server.sin_family =AF_INET; server.sin_addr.s_addr = htonl( INADDR_ANY) server.sin_port = htons( 12345 ); bind( sd, (struct sockaddr *)&server, sizeof( server ) ); recv( sd, buf, sizeof( buf ), 0 ); struct hostent *hp, *gethostbyname( ); Server.sin_family = AF_INET; hp = gethostbyname( ); bcopy( hp->h_addr, &( server.sin_addr.s_addr ), sizeof( hp->h_length) ); sendto( sd, buf, sizeof( buf ), 0, (struct sockaddr *)&server, Packets demultiplexed UDP M5, M4, M3, M2, M1 CSS432: End-to-End Protocols

CSS432: End-to-End Protocols Common end-to-end services guarantee message delivery deliver messages in FIFO order deliver at most one copy of each message support arbitrarily large messages support synchronization allow the receiver to flow control the sender support multiple application processes on each host P1 P2 P3 P4 M5, M4, M3, M2, M1 Network m5, m4, m3, m2, m1 CSS432: End-to-End Protocols

CSS432: End-to-End Protocols TCP Overview Connection Oriented It guarantees that all sent packets will reach the destination in the correct order Use of ACK packets, re-transmission, time out Full duplex: bi-directional, send and receive at each end Flow control: keep sender from overrunning receiver Congestion control: keep sender from overrunning network CSS432: End-to-End Protocols

TCP (Reliable Byte Stream) Byte-oriented protocol, sender writes bytes into a TCP connection and the receiver reads bytes out of the TCP connection. Byte Stream: Not individual bytes source application buffers enough bytes from the sending process to fill a reasonably sized packet (segment) and sends to destination Destination empties the contents of the segment into a receive buffer, and the receiving process reads from this buffer at its leisure. Application process W rite bytes TCP Send buffer Segment T ransmit segments Read Receive buffer … CSS432: End-to-End Protocols

Sockets (Code Example) int sd = socket(AF_INET, SOCK_STREAM, 0); int sd, newSd; sd = socket(AF_INET, SOCK_STREAM, 0); socket() connect() write() socket() connect() write() socket() bind() listen() accept() sockaddr_in server; bzero( (char *)&server, sizeof( server ) ); server.sin_family =AF_INET; server.sin_addr.s_addr = htonl( INADDR_ANY ) server.sin_port = htons( 12345 ); bind( sd, (sockaddr *)&server, sizeof( server ) ); struct hostent *host = gethostbyname( arg[0] ); sockaddr_in server; bzero( (char *)&server, sizeof( server ) ); server.sin_family = AF_INET; server.s_addr = inet_addr( inet_ntoa( *(struct in_addr*)*host->h_addr_list ) ); server.sin_port = htons( 12345 ); read() read() listen( sd, 5 ); connect( sd, (sockaddr *)&server, sizeof( server ) ); sockaddr_in client; socklen_t len=sizeof(client); while( true ) { newSd = accept(sd, (sockaddr *)&client, &len); write( sd, buf1, sizeof( buf ) ); write( sd, buf2, sizeof( buf ) ); if ( fork( ) == 0 ) { close( sd ); read( newSd, buf1, sizeof( buf1 ) ); read( newSd, buf2, sizeof( buf2 ) ); } close( newSd ); buf2, buf1 buf2, buf1 close( newsd); exit( 0 ); CSS432: End-to-End Protocols

Data Link Versus Transport Data Link layer transfers data between two adjacent nodes (a single point-to-point physical link) whereas transport layer provides communication between processes running in different hosts need explicit connection establishment and termination Single physical point-to-point link fixed RTT, TCP connections Potentially different RTT (Round Trip Time) as they connect different hosts anywhere on the internet TCP connection between nodes in same room or across network (different RTT) need adaptive timeout mechanism for re-transmissions Point-to-point link, packets received in FIFO order. In TCP, they can be re-ordered as they cross internet, e.g., long delay in network, re-transmission, etc need to be prepared for arrival of very old packets Packets slightly out of order can be corrected using SeqNum of sliding window protocol How late a packet can be? need to set MSL (Maximum Segment Lifetime) CSS432: End-to-End Protocols

Data Link Versus Transport Hosts connected to point-to-point are engineered to support the link. Hosts at both ends have similar resources. If windowsize = bandwidth*RTT, sender and receiver likely to have window size buffer But for a TCP connection, resources dedicated to the TCP connection such as buffer space, etc, can vary, especially if one of the host supports multiple TCP connections need to accommodate different node capacity (flow control) Potentially different network capacity. In a directly connected point-to-point link, the bandwidth of the link is known and transmitter cannot send faster than the bandwidth and not possible to congest the network. In TCP, what links will be traversed is not known before hand, and multiple sources can traverse via the same link. need to be prepared for network congestion CSS432: End-to-End Protocols

Segment Format (TCP Header) Each TCP connection identified with 4-tuple: (SrcPort, SrcIPAddr, DsrPort, DstIPAddr) Sliding window + flow control Acknowledgment, SequenceNum, AdvertisedWindow Flags SYN, FIN, RESET, PUSH, URG, ACK SYN (Synchronize): Establishing a connection FIN (Finish): terminating a connection RESET: Confused and Terminating PUSH: Section 5.2.7 URG: Sending urgent data ACK: Validating acknowledgment field SequenceNum is incremented in all cases other than ACK. Sender Data (SequenceNum) Acknowledgment + AdvertisedWindow Receiver CSS432: End-to-End Protocols

Segment Format (TCP Header) The SrcPort and DstPort fields identify the source and destination ports, respectively. The Acknowledgment, SequenceNum, and AdvertisedWindow fields are all involved in TCP’s sliding window algorithm. Because TCP is a byte-oriented protocol, each byte of data has a sequence number; the SequenceNum field contains the sequence number for the first byte of data carried in that segment. The Acknowledgment and AdvertisedWindow fields carry information about the flow of data going in the other direction. CSS432: End-to-End Protocols

Segment Format (TCP Header) The 6-bit Flags field is used to relay control information between TCP peers. The possible flags include SYN, FIN, RESET, PUSH, URG, and ACK. The SYN and FIN flags are used when establishing and terminating a TCP connection, respectively. The ACK flag is set any time the Acknowledgment field is valid, implying that the receiver should pay attention to it. The URG flag signifies that this segment contains urgent data. When this flag is set, the UrgPtr field indicates where the nonurgent data contained in this segment begins. The urgent data is contained at the front of the segment body, up to and including a value of UrgPtr bytes into the segment. The PUSH flag allow the sender to tell TCP it should (send) flush all bytes collected to its peer and also notify it to the receiving side. Does not buffer at sender/receiver Finally, the RESET flag signifies that the receiver has become confused, it received a segment it did not expect to receive—and so wants to abort the connection. Finally, the Checksum field is used in exactly the same way as for UDP—it is computed over the TCP header, the TCP data, and the pseudoheader, which is made up of the source address, destination address, and length fields from the IP header. CSS432: End-to-End Protocols

TCP Connection Establishment and Termination Tree-Way Handshake Client Initiate a connection to a server by sending segment with seq=x Set a timer and retransmit the request upon an expiration Server Acknowledge the client request with ack=++x Initiate a reverse connection with its own start sequence num seq=y Acknowledge the server request with ack=++y (next seq num expected) X and y chosen at random Segment from earlier incarnation of same connection can interfere with a later incarnation of the connection Active participant (client) Passive participant (server) Flag=SYN, SequenceNum = x SYN + ACK, SequenceNum = y , ACK, Acknowledgment = + 1 Acknowledgment = CSS432: End-to-End Protocols

State Transition Diagram CLOSED LISTEN SYN_RCVD SYN_SENT ESTABLISHED CLOSE_WAIT LAST_ACK CLOSING TIME_WAIT FIN_WAIT_2 FIN_WAIT_1 Passive open Close Send/ SYN SYN/SYN + ACK SYN + ACK/ACK ACK /FIN FIN/ACK ACK + FIN/ACK Timeout after two segment lifetimes (2 * MSL) Active open /SYN Open Active open client connect( ) Passive open server listen( ) Close Active close client or server First close( ) Both side can be active Passive close close( ) in response to the first close( ) CSS432: End-to-End Protocols

State Transition Diagram States involved in opening and closing a TCP connection Anything between is hidden (ESTABLISHED) Each box represents the state of one end of TCP connection All connections start with CLOSED state As connection progresses, it moves from state to state Each arc represents the event/action. Two kinds of events: (1) a segment arrives from peer (2) local operation on TCP One/both sides can close connection. If one side alone closes, then it cannot send segments but can receive them Each arc is labeled using event/action. i.e., When event happens at a given state, it moves to the next state and takes the action CSS432: End-to-End Protocols

State Transition Diagram CLOSED Active open /SYN Opening a connection: CLOSED to LISTEN: Server invokes passive open on TCP waits for conn req. CLOSED to SYN_SENT: Client invokes an active open, moves to SYN_SENT state and SYN segment sent to server. LISTEN to SYN_RCVD: Server receives the SYN segment from client, moves to SYN_RCVD state, sends SYN+ACK to client SYN_SENT to ESTABLISHED: Client receives the SYN+ACK from server, moves to ESTABLISHED state, sends an ACK back to server SYN_RCVD to ESTABLISHED: Server receives ACK from client and moves to ESTABLISHED Passive open Close Close LISTEN SYN/SYN + ACK Send/ SYN SYN/SYN + ACK SYN_RCVD SYN_SENT ACK SYN + ACK/ACK Close /FIN ESTABLISHED Close /FIN FIN/ACK FIN_WAIT_1 CLOSE_WAIT FIN/ACK ACK Close /FIN ACK + FIN/ACK FIN_WAIT_2 CLOSING LAST_ACK Timeout after two ACK ACK segment lifetimes (2 * MSL) FIN/ACK CSS432: End-to-End Protocols TIME_WAIT CLOSED

State Transition Diagram This Side can close connection first Other Side can close connection first Both close the connection at the same time CLOSED LISTEN SYN_RCVD SYN_SENT ESTABLISHED CLOSE_WAIT LAST_ACK CLOSING TIME_WAIT FIN_WAIT_2 FIN_WAIT_1 Passive open Close Send/ SYN SYN/SYN + ACK SYN + ACK/ACK ACK /FIN FIN/ACK ACK + FIN/ACK Timeout after two segment lifetimes (2 * MSL) Active open /SYN Closing a connection: ESTABLISHED TO FIN_WAIT_1: Server sends termination request FIN. Waits for ACK from client ESTABLISHED TO CLOSE_WAIT: Client receives FIN, sends ACK to server, waits for its own local FIN CLOSE_WAIT TO LAST_ACK: Client sends own FIN to server waits for ACK FIN_WAIT_1 TO FIN_WAIT_2: ACK received from client, wait for FIN from client FIN_WAIT_2 TO TIME_WAIT: FIN received from client. Server sends ACK to Client, waits for enough time until client receives ACK LAST_ACK TO CLOSED: Client receives ACK from server, moves to CLOSED TIME_WAIT TO CLOSED: Server waits 2*MSL, moves to CLOSED MSL: maximum amount of time segment is in internet (e.g. 120 s) Maximum segment lifetime of a segment after which it is discarder even if it arrives at the destination

State Transition Diagram CLOSED LISTEN SYN_RCVD SYN_SENT ESTABLISHED CLOSE_WAIT LAST_ACK CLOSING TIME_WAIT FIN_WAIT_2 FIN_WAIT_1 Passive open Close Send/ SYN SYN/SYN + ACK SYN + ACK/ACK ACK /FIN FIN/ACK ACK + FIN/ACK Timeout after two segment lifetimes (2 * MSL) Active open /SYN Closing a connection: ESTABLISHED TO FIN_WAIT_1: Server, Client send termination requests FIN. FIN_WAIT_1 TO CLOSING: Server/client receive FIN from each other and send ACK, wait for own ACK CLOSING TO TIME_WAIT: Server/Client receive ACK for the FIN they sent, wait for enough time until other peer receives ACK they sent TIME_WAIT TO CLOSED: Peer waits 2*MSL, move to CLOSED FIN_WAIT_1 TO TIME_WAIT: Server/client receive FIN and ACK for the FIN that they sent simultaneously CSS432: End-to-End Protocols

State Transition Diagram In what condition can the state transit from FIN_WAIT_1 to TIME_WAIT? What is the purpose of the TIME_WAIT state? TCP is given a chance to resend the final ACK. Client sends FIN, Server receives FIN ACK sent by server can be delayed, Client times out ( 1 MSL) Client resends FIN, it can also be delayed (1 MSL) If no TIME_WAIT, new TCP connection can get the delayed FIN and close connection Server received FIN, ACK from client, server sent ACK to client. ACK may or may not have reached the client. Client can re-transmit the FIN Request to the server and it can be delayed in the network. It takes 2*MSL (timeout for ACK + timeout for retransmitted FIN). If no TIME_WAIT, When another connection is opened (between the same IP and ports), if this retransmitted FIN request reaches the server of the new connection, unwanted termination occurs. Host A has sent a FIN segment to host B, and has moved from ESTABLISHED to FIN WAIT 1. Host A then receives a segment from B that contains both the ACK of this FIN, and also B’s own FIN segment. This could happen if the application on host B closed its end of the connection immediately when the host A’s FIN segment arrived, and was thus able to send its own FIN along with the ACK. Normally, because the host B application must be scheduled to run before it can close the connection and thus have the FIN sent, the ACK is sent before the FIN. While “delayed ACKs” are a standard part of TCP, traditionally only ACKs of DATA, not FIN, are delayed. See RFC 813 for further details. CSS432: End-to-End Protocols

CSS432: End-to-End Protocols Timing Chart Client Server ( connect( ) ) SYN_SENT LISTEN ( listen( ) ) SYN seq=x SYN_RCVD Establishment SYN seq=y, ACK=x + 1 ESTABLISHED ACK=y + 1 ESTABLISHED ( write( ) ) seq=x+1 ACK=y + 1 ( read( ) ) Transfer Data ACK x + 2 ( close( ) ) FIN_WAIT_1 FIN seq=x+2 ACK=y + 1 CLOSE_WAIT ACK x + 3 Termination FIN_WAIT_2 FIN seq = y + 1 LAST_ACK( close( ) ) TIME_WAIT ACK=y + 2 Peek such a flow with tcpdump in assignment 3. CSS432: End-to-End Protocols

Sliding Window Revisited TCP’s variant of the sliding window algorithm, which serves several purposes: (1) it guarantees the reliable delivery of data, (2) it ensures that data is delivered in order, and (3) it enforces flow control between the sender and the receiver CSS432: End-to-End Protocols

Sliding Window (Reliable & Ordered Delivery) Sending application LastByteWritten TCP LastByteSent LastByteAcked Receiving application LastByteRead LastByteRcvd NextByteExpected Sending side LastByteAcked ≤ LastByteSent Receiver cannot ack the byte not sent LastByteSent ≤ LastByteWritten Cannot send a byte that has not been written to the send buffer buffer bytes between [LastByteAcked, LastByteWritten] Receiving side LastByteRead < NextByteExpected Byte cannot be read by receiver until received NextByteExpected ≤ LastByteRcvd+1 NextByteExpected points to the start of first gap when data arrive out of order buffer bytes between [LastByteRead, LastByteRcvd] LastByteWritten: written into the sender buffer but not transmitted LastByteReceived: received from sender LastByteRead: byte read by the receiver, after receiving from the sender

Sliding Window (Reliable & Ordered Delivery) Receiving side LastByteRead Data that has been received and also application has read it from the TCP buffer NextByteExpected Data that has not been received and expected as the next byte LastByteRcvd Data that has been received and in the receiver TCP buffer LastByteRead < NextByteExpected Bytes which are expected can’t be read as they have not yet reached the receiver The next expected byte points to the byte right after the last byte received if data is received in order, therefore NextByteExpected = LastByteRcvd+1 If due to some reason, data has arrived out of order, NextByteExpected will point to the first gap in the data NextByteExpected ≤ LastByteRcvd+1 buffer bytes between [LastByteRead, LastByteRcvd] LastByteWritten: written into the sender buffer but not transmitted LastByteReceived: received from sender LastByteRead: byte read by the receiver, after receiving from the sender

CSS432: End-to-End Protocols Flow Control Keep sender from overrunning receiver MaxSendBuffer, MaxRcvBuffer for sender, receiver Window is amount of data that can be sent without waiting for ACK Receiver advertises the sender a window <= MaxRcvBuffer To avoid overflowing receiver buffer, TCP on receiver sider must keep, LastByteRcvd − LastByteRead ≤ MaxRcvBuffer AdvertisedWindow of receiver: Amount of free space in receive buffer = MaxRcvBuffer − ((NextByteExpected − 1) − LastByteRead) If rate of reading = rate of receiving, Advertised Window = MaxRcvBuffer If rate of reading slower, LastByteRcvd increases and Advertised Window shrinks to 0 LastByteRcvd − LastByteRead ≤ MaxRcvBuffer MaxRcvBuffer – (LastByteRcvd − LastByteRead )>=0 MaxRcvBuffer – LastByteRcvd + LastByteRead >=0 - 1 AdvertisedWindow >=0 NextByteExpected − 1 <= LastByteRcvd Replace LastByteRcvd by NextByteExpected-1 in (1), equation is valid Since a smaller number will increase the AdvertisedWindow value and it will remain >=0 CSS432: End-to-End Protocols

CSS432: End-to-End Protocols Flow Control Sender should adhere to receiver’s advertised window LastByteSent − LastByteAcked ≤ AdvertisedWindow Sender computes an effective window that limits how much data it can send to receiver EffectiveWindow = AdvertisedWindow − (LastByteSent − LastByteAcked) Local application process must not overflow the send buffer LastByteWritten − LastByteAcked ≤ MaxSendBuffer If the sending process tries to write y bytes to TCP, but (LastByteWritten − LastByteAcked) + y > MaxSendBuffer then TCP blocks the sending process and does not allow it to generate more data. If no unordered data, NextByteExpected = LatestByteRcvd-1 CSS432: End-to-End Protocols

CSS432: End-to-End Protocols Flow Control Sending application Receiving application TCP TCP LastByteWritten LastByteRead y LastByteAcked LastByteSent NextByteExpected LastByteRcvd LastByteSent – LastByteAcked ≤ AdvertisedWindow LastByteRcvd – LastByteRead ≤ MaxRcvbuffer EffectiveWindow = AdvertisedWindow – (LastByteSent – LastByteAcked) AdvertisedWindow = MaxRcvBuffer – (NextByteExpected -1 – LastByteRead) LastByteWritten – LastByteAcked ≤ MaxsendBuffer Send ACK with an advertise window in response to arriving data segments as long as all the preceding bytes have also arrived and until the advertised window reaches 0. (ACK returned at the first time when it reaches 0) block sender if (LastByteWritten - LastByteAcked) + y > MaxSenderBuffer CSS432: End-to-End Protocols

Flow Control with A Slower Receiver Sending application Receiving application y y Read slow. TCP TCP LastByteWritten LastByteRead LastByteAcked LastByteSent LastByteRcvd NextByteExpected LastByteRcvd – LastByteRead ≤ MaxRcvbuffer LastByteSent – LastByteAcked ≤ AdvertisedWindow < 0 AdvertisedWindow = MaxRcvBuffer – (NextByteExpected – LastByteRead) EffectiveWindow = AdvertisedWindow – (LastByteSent – LastByteAcked) Read slow, LastByteRcvd inreases, MaxRcvBuffer fills up such that receiver cannot receive no more messages and LastByteRcvd stops increasing; NextByteExpected No more send, no more ack, thus it stays In the same value LastByteWritten – LastByteAcked ≤ MaxsendBuffer block sender since (LastByteWritten - LastByteAcked) + y > MaxSenderBuffer CSS432: End-to-End Protocols

CSS432: End-to-End Protocols Flow Control The sender won’t send any more data. The receiver won’t initiate to send any advertised window. Then, how can the sender find out when the receiver can receive more data? CSS432: End-to-End Protocols

CSS432: End-to-End Protocols Reviews UDP TCP: three-way handshake and state transition Exercises in Chapter 5 Ex. 5, 14, 22, and 39 (TCP state transition) CSS432: End-to-End Protocols

Supplementary CSS432: Internetworking