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Internet Transport Glenford Mapp Digital Technology Group (DTG)

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1 Internet Transport Glenford Mapp Digital Technology Group (DTG)

2 Myths about Internet Transport TCP/IP was always around All packet networks work using TCP/IP TCP/IP inherently superior to other protocols

3 TCP/IP was not dominant in late 70s and early 80s Most computer vendors were developing their own protocol suites Mainframe and mini-computers vendors –IBM - SNA Architecture –DEC - DECnet - See Ethernet Frame Types –Xerox - XNS

4 PC manufacturers Apple - Appletalk Novell - Netware Suite Microsoft Networking - SMB, NetBIOS and NetBEUI

5 Big Telecoms X.25 - Packet-based data communication –specified by the CCITT - part of the ITU –Virtual Circuit Technology(Telephone people understood it) –Connection oriented so there were definite phases of connection CONNECTION ESTABLISHED DATA TRANSFER CONNECTION TERMINATION

6 X.25 used as a data-connect technology Two main ways –Connect sites using X.25 –Link in terminals using X.25 Interface`between the X.25 concentrator/MUX and the terminal is`called X.3 –Mainframe in a building and you have hundreds/thousands of terminals using links to X.25 concentrators Credit card/ financial industry - big users

7 X.25 Connection represented by a Virtual Circuit Number, part of your packet. If you passed through an X.25 Switch, map incoming VCI with outgoing VCI. –X.25 gave rise to Frame Relay –Frame Relay influenced ATM X.25 Links still exist today –The idea that TCP/IP has obliterated everything before it is not true

8 But why did TCP/IP win? Fragmented Opposition –Networks were being used to sell hardware and software applications. They were not being used to connect different systems together –TCP/IP was designed to connect different systems together

9 Why did TCP/IP win It had the backing of the US military –Funded projects on Internet transport, etc. It was incorporated into Unix which was more or less free to academic institutions –Most academic institutions could afford a PDP- 11 running Unix - they got networking for free Academics ironed out the kinks It had a killer appl -

10 Was it the best transport around? The two most thought-out systems were –IBM SNA too proprietary –Xerox XNS adopted by many network vendors –Biggest adopter of XNS was Novell changed one or two things

11 Two Main comparisons The Suite as a whole –how well do the layers fit together do the upper and lower layers gel Head-to-head on the individual layers –Compare the same layers in different protocols

12 IP world in terms of OSI Physical Layer Data Link Layer Network Layer Transport Layer Session Layer Presentation Layer Application Layer Copper, Fibre Twisted par IPv4, IPv6 TCP, UDP Ethernet MAC Sockets RPC, CORBA Java

13 OSI Model and Netware Protocol Layers Physical Data Link Network Transport Session Presentation Application Ethernet IEEE Token Ring FDDI PPP Others IPX SPX Application NetBIOS emulator Netware Shell Netware Core Protocol NCP RPC Apps LU 6.2 Support IBM

14 Suite Comparison IP strong in the lower layers –Layers 3 and 4 very strong! Netware is strong throughout –Lots of work done through IPX Netware was built with applications in mind –IP Suite more undefined at the upper layers Netware wins it

15 IPv4 Header 32-bit source IP address 32-bit destination IP address TOS TOTAL LENGTHIHLV 16-bit IDENTIFIER Flags 13-bit Frag Offset TTL PROTO NO 16-bit header checksum Options (if any)

16 IPX Frame Structure Checksum Packet length Packet TypeTransport Control Destination Network (4 bytes) Destination Node (6 bytes) Destination socket Source Network(4 bytes) Source Node (6 bytes) Source Socket Data 0 15

17 IPX Frame Structure Checksum: Usually set to FFFF hex (i.e. disabled) because IPX relies on Ethernet/Token Ring's Cyclic Redundancy Check (CRC). Length: includes 30 byte-header + data Transport control(1 byte): Hop count (router-to-router) limit 16.

18 IPX Packet Type field: Identifies which higher level protocol receives the data –0 hello -1 Routing –2 Echo -3 Error –4 Netware 386 or SAP 5 SPX –17 - Netware 286

19 Addressing in IPX 12-byte address structure 4 byte – network address –Assigned by network administrator –0 = local network 6 byte – node number –Hardware LAN (Ethernet) Address –FFFF broadcast

20 Addressing in IPX Socket Number (2 bytes) –Identifies a given endpoint or higher-layer packet service. –NCP – 0x451 –SAP – 0x452 –Diagnostics – 0x456

21 IP vs IPX IP is small compared to IPX IPX does more than just networking –uniquely identifies endpoints as well as interfaces Really does IP/UDP - Unreliable datagram service

22 TCP header Options (if any) 16-bit source port no 16-bit destination port no 32-bit sequence number 32-bit acknowledge number 16-bit window size 16-bit urgent pointer THL FLAGS RESV 16-bit TCP checksum

23 TCP header 16-bit source and destination ports 32-bit sequence no - refers to bytes sent 32-bit acknowledge no - acknowledges bytes received THL - TCP header length Window size - the number of bytes that the sender can send to the receiver before waiting for an acknowledgement

24 SPX Frame Connection Control Flag Datastream Type Source Connection ID Destination Connection ID Sequence No Acknowledge No Allocation No Data: bytes 0 15

25 SPX frame Connection control field: –regulating flow of data. –Bit 4 - end of message –Bit5 - Attention bit, ignored by SPX –Bit 6 - Acknowledgement Requested –Bit 7 - Transport Conrol Data Stream Type: –Identifies data within the packet

26 SPX Frame contd Source and Destination IDs identify the connection on both sides Sequence number: no of packets transmitted Acknowledge number: next expected packet Allocation number –no. of packets sent but not yet acknowledged

27 Sequenced Packet Exchange II (SPX II) Introduced to provide improvements over SPX protocol in –window flow control (sending several packets before ack), –larger packet sizes (>576 bytes), improved negotiation of network options: safer method of closing connections Packet: added a 2-byte Negotiation size.

28 Comparisons TCP is much bigger than SPX TCP has to do de-multiplexing of packets to find the connection endpoints Endpoints are specified in IPX and the actual connection is specified in SPX packet SPX basically gives reliability but is built on the datagram service provided by IPX

29 The winner is Its a draw! –IPX/SPX = IP/UDP/TCP Probably the correct way to do it but it adds lots to the network layer –IP is a pure network layer, IPX is not –TCP is build directly on IP so more complicated than SPX

30 Challenges TCP/IP faced Congestion –Late 80s TCP/IP getting going huge blackouts begin to occur –TCP is not reacting to congestion Van Jacobson comes up with a algorithm called slow start

31 Handling Congestion Slow Start Algorithm TCP attempts to avoid causing congestion Slow Start implemented at the start of the connection The connection now has a congestion window; cwnd.

32 Slow Start contd At the start, cwnd is set to 1 and only one packet is sent If the segment is successfully acknowledged then cwnd is increased to 2 and so now two packets are sent, If these are successfully acknowledged, then 4 packets are sent, etc

33 Slow Start - Already In Data Phase Client Server DATA 1 (1024) ACK 1 WIN 4096 DATA 2 (1024) DATA 3 (1024) ACK 3 WIN 4096 DATA 4 (1024) DATA 5 (1024) DATA 6 (1024) DATA 7 (1024) ACK 7 WIN 4096 Already filling receive buffer cwnd = 1 cwnd = 2 cwnd = 4

34 Slow Start Contd We continue to double the number of packets sent until: –we reach the size of the receive buffer as in the last slide –we see packet loss very likely for large packet transfers going very long distances even though it starts slowly; slow start is in fact growing exponentially so its very aggressive for large window sizes

35 Slow Start - Packet Loss When we see packet loss we do the following: –Set the maximum size to aim for as half the current value of cwnd: ssthresh = cwnd/2 –Set cwnd back to 1 and repeat slow start –If we get above or equal to ssthresh; we increase cwnd by 1 for every successful transmission i.e linear instead of exponential

36 Reaction to Retransmission TCP uses a Go-back-n retransmission policy All packets starting from the first missing packet must be retransmitted –even if packets later in the sequence arrived OK on the first transmission, they must still be retransmitted

37 Problems with Standard TCP Didnt work well on satellite links or on distances with large RTT Main reason: Retransmission using the Go- back-n approach is too costly. The pipe contains a lot of packets and to have to retransmit all of them if say, the first packet gets corrupted, is too complicated

38 Selective Retransmission Introduced in RFC This defined two new TCP options SACK-permitted –indicates that selective acknowledgements are allowed SACK –Sender only retransmits packets not received

39 Present Issues: Problems with Slow Start Key assumption of TCP is that packet loss is due to congestion. –Clumsy indicator at best. –Dead wrong at worst. Better to let the network indicate congestion explicitly

40 Explicit Congestion Notification (ECN) With ECN, we use 2 bits in the IP header and 2 bits in TCP header to explicitly indicate to the sender and receiver that packets on this connection have experienced congestion So when there is congestion in the network IP routers set a bit in the IP header saying that this packet has been through a congested area

41 ECN contd When the packet reaches the receiver, the IP processing engine notes the congestion and sets the appropriate bit in the TCP header TCP receive engine sends a TCP ACK packet to the sender saying that congestion has been experienced on this connection Sender reduces sending rate and signals to the receiver that appropriate action has been taken

42 ECN Sender Router Receiver 1. Congestion Bit set (IP) 2. Congestion bit set (TCP) 3. Congestion ACKed (TCP)

43 Present Issues: Slow Start and Wireless Networks Key assumption of TCP is that packet loss is due to congestion –hence the slow start algorithm –true in wired networks with good link quality In wireless networks where there is handoff and channel fading; packet loss is very temporary –slow start represents drastic action which cuts the bandwidth of the connection

44 Slow Start and Wireless networks Must avoid TCP going into slow start on wireless networks Solution: –have normal TCP for the wired core network –different kind of TCP for wireless last-mile part –Proxy TCP server in the middle

45 TCP Proxy Sender Receiver TCP Proxy Wireless Network Local Wired Network

46 TCP Proxy –can buffer packets and retransmit packet locally when the mobile node has lost packets due to channel fading and handoff –Splits the connection into 2 connections –Big issue: do you try to maintain end-to-end semantics Yes, then sender sees what happening - slower response No, then the sender can presume things about the connection, e.g Round-Trip-Time and Window control that are not true

47 M-TCP Splits the connection in two but maintains the end-to-end semantics –Proxy does not perform caching/retransmission –Geared to handling long period of disconnection –Closes the window hence stops the sender when the receiver loses contact –prevents slow start when connection is re- established

48 I-TCP Also splits the connections Breaks end-to-end semantics Packets from the sender are acknowledged by the TCP Proxy and forwarded on a different connection mobile node. TCP proxy handles buffering and retransmission

49 Key Issues for the future New applications require a low-latency environment –Voice over IP –Networked games –Multimedia TCP is too heavyweight –most of these applications do not need the byte- stream paradigm

50 Network implications To engineer low latency, a lot of people are pushing for the development of a super-fast core: ATM, MPLS. Traditional routers replaced by very fast switches. All intelligence on routing and connections will be pushed to the edge

51 Transport Support for Low latency New Approach is to go back the Netware style, so we use UDP/IP as a data-carrying substrate and build our protocol on that –flexible –protocols can run in user-space –new low latency NIC card support memory- mapping in user-space

52 User-Space Transport Protocols Easier to test and implement Also avoids multiplexing and cross talk in the kernel Since the process and not the kernel implementing the protocol some issues: –Timers –Packet Handling

53 Timers Since the process is called to run periodically it cannot be too dependent on timers since they will be imprecise without hardware support –TCP is very timer-dependent –User-level TCP hasnt performed well needs lot of hardware timer support

54 Packet Handling Since the protocol is running in user-space when the process is finally called there may be lots of packets waiting to be processed; data from the remote side, ACKs or NACKs for data that you sent,etc. Dont have to treat them in FIFO order

55 Packet Handling Treat NACKs first, retransmit the packet –allows the other end to get on with it Treat ACKs next, frees up local buffers that you might need For data stream, treat retransmitted packets first Have a priority bit to indicate which packets you want treated first

56 A1 - Transport Developed at AT&T Laboratories- Cambridge Ltd User-space protocol developed to support multimedia applications –very flexible –supported QoS vectors Performance over 155 Mbits/s ATM link –111 Mbits/s (reliable), 130 Mbits/s Raw

57 Newer Transport Protocols (NTP) NTPs running directly on top of IP –Compete with both TCP and UDP Applications – streaming, low-latency –QoS, congestion issues Support for mobility and/or multi-homing Security – easier mechanisms to setup security Some are gathering a following

58 NTP Contd Datagram Congestion Control Protocol (DCCP) – Berkeley Institute 2003 –Driven by Media-Streaming Applications –Combines unreliable delivery with Congestion Control –Supports ECN and congestion negotiation on setup

59 NTP Contd The Stream Control Transport Protocol (SCTP) –Originally used as the transport protocol on the SS7 Signalling Network –Multi-streaming – One logical connection is used to support a number of streams not just one –Multi-homing support

60 SCTP contd Support for mobility –Uses a different mechanism than Mobile IP –Associate a set of local addresses with a set of remote addresses and you can add new IP addresses and delete old one as you move around Support for security –Verfication tag / cookie –Specifics IPSec for strong security if required

61 NTP contd Explicit Congestion Control Protocol (XCP) –Precise congestion signalling –XCP congestion header Sender uses header to request higher QoS –Routers know about XCP header and can modify fields based on the congestion they are seeing –Run different congestion algorithms –XCP-i – for inter-networking

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