1

SCTP: Stream Control Transmission Protocol

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Table of Content Introduction Motivations for Developing SCTP SCTP Features SCTP Terminology SCTP Packets SCTP States SCTP Data Transmission SCTP and Multihoming SCTP Streams SCTP Extensions Other STCP Research Topics References and Resources

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Introduction

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SCTP is a reliable transport protocol op

erating on top of a potentially unreliable connectionless packet service such as IP.

offers acknowledged error-free non-duplicated transfer of datagrams.

a selective retransmission mechanism is applied to correct loss or corruption of data.

IPNetwork

IPv4/IPv6

Application

Link Layer

UDP TCP SCTP

Physical Layer

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Motivations for Developing SCTP

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Development by IETF SIGTRAN working group To create a new, IP based transport protocol For transport of signaling data over IP-based networks

Anticipation of carrier network convergence All-IP networks 3GPP networks based on IPv6 possible communication between signaling gateways (SGs), media ga

teway controllers (MGCs) and signaling end- and transfer-points (SEP, STPs)

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Problems for TCP Byte-Stream Oriented Provide strict ordering of information, which causes Head of

Line (HOL) Blocking problem No built-in support for multi-homed IP hosts Vulnerable for SYN flooding attacks (a kind of DoS attack u

sing TCP three-way handshake Problems for UDP

Unreliable data transfer No congestion/flow control (applications must handle it by t

hemselves)

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SCTP Features

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Provide an end-to-end reliable transmission service over IP networks

Support multiple streams: multiple streams per path Support multi-homed hosts

Multiple IP addresses per host More tolerant to network failures

Message-oriented: conserve message boundaries Unordered delivery: SCTP can deliver messages as ordered or u

nordered. Congestion Control: SCTP congestion control is similar to TCP.

Enable seamless introduction of SCTP into IP networks SCTP is rate adaptive similar to TCP

slow start, congestion avoidance, fast retransmit and fast recovery delayed Selective ACKs (SACKs) and duplicate SACKs

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Features Comparison

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SCTP Terminology

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Chunk A unit of information within an SCTP packet, consisting of a

chunk header and chunk-specific content. SCTP association

A protocol relationship between SCTP endpoints. It is composed of the two SCTP endpoints and protocol state

information including Verification Tags and the currently active set of Transmission Sequence Numbers (TSNs), etc.

An association can be uniquely identified by the transport addresses used by the endpoints in the association.

Two SCTP endpoints MUST NOT have more than one SCTP association between them at any given time.

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Path The route taken by the SCTP packets sent by one SCTP endp

oint to a specific destination transport address of its peer SCTP endpoint.

Primary Path The destination and source address that will be put into a pac

ket outbound to the peer endpoint by default. The above definition includes the source address since an im

plementation MAY wish to control the return path taken by reply chunks better and on which interface the packet is transmitted when the data sender is multi-homed.

Stream: A uni-directional logical channel established from one to ano

ther associated SCTP endpoint. All user messages are delivered in sequence except for those

submitted to the unordered delivery service.

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15

Application

SCTP

IPIP1IP2

s1 s2 s3

10.0.1.1:99910.0.2.1:999

Application

SCTP

IPIP2IP1

s1 s2 s3

10.0.2.2:88810.0.1.2:888

7:3

4:2

1:1

8:3

5:2

2:1

9:3

6:2

3:1

Association

TSN:SSN Data Chunk

Retransmission Path

Primary Path

buffer buffer

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Idle destination address An address that has not had user messages sent to it within

some length of time.

Inactive destination transport address An address which is considered inactive due to errors and

unavailable to transport user messages.

SCTP packet The unit of data delivery across the interface between SCTP

and the connectionless packet network (e.g., IP). An SCTP packet includes the common SCTP header,

possible SCTP control chunks, and user data encapsulated within SCTP DATA chunks.

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Transmission Sequence Number (TSN) A 32-bit sequence number used internally by SCTP. One TSN is attached to each chunk containing user data to

permit the receiving SCTP endpoint to acknowledge its receipt and detect duplicate deliveries.

Stream Sequence Number (SSN) A 16-bit sequence number used internally by SCTP to assure

delivery of the user messages within a given stream. One stream sequence number is attached to each user

message.

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Message Authentication Code (MAC) An integrity check mechanism based on cryptographic hash

functions using a secret key. It is used by an endpoint to validate the State Cookie

information that is returned from the peer in the COOKIE ECHO chunk.

Verification Tag A 32 bit unsigned integer that is randomly generated. It provides a key allowing a receiver to verify that the SCTP

packet belongs to the current association and is not an old or stale packet from a previous association.

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SCTP Packets

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An SCTP packet forms the payload of an IP packet. An SCTP packet is composed of a common header and

chunks. A chunk may contain either control information or user

data. Multiple chunks may be multiplexed into one SCTP

packet up to the Path-MTU size. Control chunks bundled before data chunks

21 An SCTP-Protocol Data Unit with several chunks

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SCTP Common Header Source and Destination Port

the same port concept as TCP and UDP Verification Tag

the receiver of this packet uses the Verification Tag to validate the sender of this SCTP packet.

Checksum each SCTP packet is protected by a 32 bit checksum (Adler-32

algorithm), which is thus more robust than the 16 bit checksum of TCP and UDP

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SCTP Chunks

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Payload Data (DATA) (0) Stream Identifier

Identifies the stream to which the following user data belongs. Stream Sequence Number

this value represents the stream sequence number of the following user data within the stream S.

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Initiation (INIT) Initiate Tag

This value MUST be placed into the Verification Tag field of every SCTP packet.

Advertised Receiver Window Credit (a_rwnd) This value represents the dedicated buffer space.

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Initiation Acknowledgement (INIT ACK) The parameter part of INIT ACK is formatted similarly to the

INIT chunk. It uses two extra variable parameters: The State Cookie and the Unrecognized Parameter:

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Selective Acknowledgement (SACK)

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Selective Acknowledgement (SACK) SACK is sent to the peer endpoint to acknowledge received D

ATA chunks and to inform the peer endpoint of gaps in the received subsequences of DATA chunks as represented by their TSNs.

Cumulative TSN Ack This parameter contains the TSN of the last DATA chunk received in s

equence before a gap. Gap Ack Block Start

Indicates the Start offset TSN for this Gap Ack Block. Gap Ack Block End

Indicates the End offset TSN for this Gap Ack Block. Gap Ack Blocks

TSNs >= (Cumulative TSN Ack + Gap Ack Block Start) and TSNs <= (Cumulative TSN Ack + Gap Ack Block End)

These TSNs are assumed to have been received correctly.

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Selective Acknowledgement (SACK) Duplicate TSN

Indicates the number of times a TSN was received in duplicate since the last SACK was sent.

Every time a receiver gets a duplicate TSN (before sending the SACK) it adds it to the list of duplicates. The duplicate count is re-initialized to zero after sending each SACK.

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Selective Acknowledgement (SACK)

TSN=17

miss

TSN=15

TSN=14

miss

TSN=12

TSN=11

TSN=10

Cumulative TSN Ack = 12

a_rwnd = 4660

Num of block=2 Num of dup=0

Block #1 start=2 Block #1 end=3

Block #2 start=5 Block #2 end=5

12+2 ~ 12+3

12+5 ~ 12+5

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Selective Acknowledgement (SACK)

TSN=16

TSN=15

TSN=14

TSN=13

TSN=13

TSN=12

miss

TSN=10

Cumulative TSN Ack = 10

a_rwnd = 4660

Num of block=1 Num of dup=1

Block #1 start=2 Block #1 end=6

Duplicate TSN 13

10+2 ~ 10+6

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Heartbeat Request (HEARTBEAT) Sent by endpoint and to probe the reachability of a particular des

tination transport address.

Heartbeat Acknowledgement (HEARTBEAT ACK)

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Abort Association (ABORT) It is sent to the peer of an association to close the association. If an endpoint receives an ABORT with a format error or for an

association that doesn’t exist, it MUST silently discard it. DATA chunks MUST NOT be bundled with ABORT.

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Shutdown Association (SHUTDOWN)

An endpoint in an association MUST use this chunk to initiate a graceful close of the association with its peer.

Shutdown Acknowledgement (SHUTDOWN ACK)

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Operation Error (ERROR) An endpoint sends this chunk to its peer endpoint to notify it of

certain error conditions.

Error causes are defined as variable-length parameters

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Operation Error (ERROR) Each error cause may carry its own set of parameters. The error

causes that have been defined are

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Cookie Echo (COOKIE ECHO)

It is used only during the initiation of an association It must precede any DATA chunk sent within the association,

but may be bundled with one or more DATA chunks in the same packet.

Cookie Acknowledgement (COOKIE ACK)

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Shutdown Complete (SHUTDOWN COMPLETE) Be sent to acknowledge the receipt of the SHUTDOWN ACK

chunk at the completion of the shutdown process.

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SCTP States

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Association establishment and shutdown. SCTP uses a cookie mechanism in a four-way handshake to establish an association.

The shutdown process is a three-way handshake.

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Normal Association Establishment

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1. Node A generates an INIT chunk and sends it to Node B. Node A starts the INIT timer.

2. If Node B wishes to accept the association, it generates an INIT_ACK chunk that includes a cookie. It then sends the INIT ACK chunk, along with a cookie, back to Node A.

The endpoint that initiates the association will be referred to as “Node A”; the peer endpoint that receives the association establishment requests will be referred to as “Node B”.

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3. Node A receives the INIT ACK chunk and stops INIT timer. Node A generates a COOKIE ECHO chunk, which is then sent to Node B. Node A starts the cookie timer. DATA chunks may be also be bundled in this packet.

4. Node B checks the validity of the cookie. Following validation it sends a COOKIE ACK back to Node A.

5. Node A receives the COOKIE ACK and enters into the next phase of data transmission.

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Association Termination

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1. Node A sends a SHOTDOWN chunk to Node B and start the shutdown timer.

2. Node B acknowledges the receipt of the SHUTDOWN chunk through the generation of SHUTDOWN ACK chunk, which is sent to Node A.

3. Node A receives the SHUTDOWN ACK and responds by stopping the SHUTDOWN timer. Then, Node A generates a SHUTDOWN COMPLETE chunk and sends the chunk to Node B.

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SCTP Data Transmission

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General Concepts Detection of loss and duplication of data chunks is enabled by nu

mbering all data chunks in the sender with the so-called Transmission Sequence Number (TSN).

The acknowledgements sent from the receiver to the sender are based on TSNs.

Retransmissions are timer-controlled. The timer duration is derived from continuous measurements of the round trip delay (RTT).

Whenever such a retransmission timer expires, (and congestion control allows transmissions) all non-acknowledged data chunks are retransmitted and the timer is started again doubling its initial duration (like in TCP).

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General Concepts When the receiver detects one or more gaps in the sequence of d

ata chunks, each received SCTP packet is acknowledged by sending a Selective Acknowledgement (SACK) which reports all gaps.

Whenever the sender receives four consecutive SACKs reporting the same data chunk missing, this data chunk is immediately retransmitted (fast retransmit).

Most up-to-date operating systems already support a similar optional extension to TCP.

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Flow Control SCTP uses an end-to-end window based flow and congestion

control mechanism similar to the one that is well known from TCP.

The receiver of data may control the rate at which the sender is sending by specifying an octet-based window size (the so-called Receiver Window), and returning this value along with all SACK chunks.

The sender itself keeps a variable known as Congestion Window (CWND) that controls the maximum number of outstanding bytes

Each received data chunk must be acknowledged, and the receiver may wait a certain time before that is done

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Selective Acknowledgement The acknowledgements carry all TSN numbers that have been re

ceived by one side with them. That is, there is a so called Cumulative TSN Ack value, that indi

cates all the data that have successfully been reassembled at the receivers side, and has either already been delivered to the receiving Upper Layer Process, or may readily be delivered upon request.

Moreover, there are so-called Gap Blocks that indicate which segments of data chunks have arrived, with some data chunks missing in between.

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Selective Acknowledgement Should some data chunks have been lost in the course of

transmission, they will either be retransmitted after the transmission timer has expired, or after four SACK chunks have reported gaps with the same data chunk missing. In the latter case, the missing data is retransmitted via the Fast Retransmit mechanism.

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Flow Control for Multihomed Endpoints By default, all transmission is done to a previously selected

address from the set of destination addresses, which is called the Primary Address (or Primary Path).

Retransmissions should be done on different paths, so that if one path is overloaded, retransmissions do not affect this path.

Acknowledgements shall be sent to the transport address from which originated the data.

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Congestion Control According to RFC2960, the congestion control behavior of an S

CTP implementation may have an impact where timely delivery of messages is required.

The congestion control mechanisms for SCTP have been derived from RFC 2581 - TCP Congestion Control, and been adapted for multihoming.

For each destination address (i.e. each possible path), a discrete set of flow and congestion control parameters is kept.

From the point of view of the network, an SCTP association with a number of paths may behave similarly as the same number of TCP connections.

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Slow Start and Congestion Avoidance Similar to TCP, SCTP has two modes, Slow Start and

Congestion Avoidance. The mode is determined by a set of congestion control variables,

which are path specific. For successfully delivered and acknowledged data, the

congestion window variable (CWND) is steadily increased, and once it exceeds a certain boundary (called Slow Start Threshold, SSTRESH), the mode changes from Slow Start to Congestion Avoidance.

In Slow Start, the CWND is increased faster (roughly one MTU per received SACK chunk), and in Congestion Avoidance mode, it is only increased by one MTU per Round Trip Time (RTT) measurement.

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Slow Start and Congestion Avoidance RTO timeout or fast retransmission will trigger retransmission a

nd cause the SSTHRESH to be cut down drastically and reset the CWND.

Fast Retransmission ssthresh = max(cwnd/2, 2*MTU) cwnd = ssthresh

RTO timeout ssthresh = max(cwnd/2, 2*MTU) cwnd = MTU

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SCTP and Multihoming

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An essential property of SCTP is its support of multi-homed nodes, i.e. nodes which can be reached using several IP addresses.

If the according IP networks are configured on physically different paths, associations become tolerant against physical network failures and other problems of that kind.

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Address Management at Association Setup If a client is multi-homed, it informs the server about all its IP

addresses with the INIT chunk's address parameters. The client is only required to know one IP address of the server

although the server provides all its IP addresses to the client in the INIT-ACK chunk.

SCTP is able to handle IPv4 and IPv6 addresses. An SCTP instance regards each IP address of its peer as one

“transmission path” towards this endpoint. If no explicit IP addresses are contained in the INIT or INIT-

ACK chunk, the source IP address of the IP packet which carries the SCTP datagram is used.

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Path Monitoring An SCTP instance monitors all transmission paths to the peer

instance of an association. HEARTBEAT chunks are sent over all paths which are currently

not used for the transmission of data chunks. Each HEARTBEAT chunk has to be acknowledged by a HEARTBEAT-ACK chunk.

Each path is assigned a state: active or inactive. The number of events in which heartbeats were not

acknowledged within a certain time or the number of retransmission events exceeds a certain configurable limit, the peer endpoint is considered unreachable and the association will be terminated.

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Path Selection At the set-up of an SCTP association, one of the IP addresses

from the returned list is selected as initial primary path. Data chunks are transmitted over the primary transmission path by default.

For retransmissions, however, another active path may be selected, if one is available.

The users of SCTP are informed about the status (state and measurements) of a transmission path on request or when a transmission path changes its state. They may then instruct the local SCTP instance to use a new primary path.

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SCTP Streams

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Multiple steams can be used to deliver multiple objects belonging to the same association objects on a webpage, multimedia streams (audio/video/text), files in an

FTP

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Flexible Datagram Delivery Streams are unidirectional logical data flows that the SCTP endp

oints negotiate during association setup. SCTP distinguishes different streams of messages within one SC

TP association. Datagrams can be assigned into different streams within an assoc

iation. Within each stream, SCTP assigns independent Stream Sequence

Numbers (SSN) to the user datagrams. SSNs are used at the receiver to determine the sequence of delivery.

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Flexible Datagram Delivery SCTP performs in-sequence delivery per stream. However, whil

e one stream may be blocked waiting for the next in-sequence user message, delivery from other streams may proceed.

The partial in-sequence delivery mechanism avoids head-of-line blocking between independent streams of datagrams within one association.

With TCP, this could only be achieved by setting-up several connections (one per stream) which would lead to additional cost and overhead.

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SCTP Extensions

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Partial Reliability SCTP (PR-SCTP) PR-SCTP is defined in RFC 3758. It allows an SCTP sender to signal to its peer that it should no lo

nger expect to receive one or more DATA chunks. PR-SCTP consists of two new elements

a single new parameter in the INIT/INIT-ACK exchange that indicates whether the endpoint supports the extension

a single new chunk type, FORWARD TSN, that indicates the receiver should move its cumulative ack point forward

PR-SCTP uses lifetime, which is an SCTP parameter, specifies the life time of the user data, to support "timed reliability" service.

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In base SCTP lifetime is used to avoid efforts to transmit the overdue user messages. The user data will not be sent by SCTP after lifetime expires. Attempted to transmit before lifetime expired, the user data will be (re)tra

nsmitted until the peer receives. In PR-SCTP

The definition of lifetime is extended further. The lifetime of user data is set to be SCTP_LIFETIME_RELIABLE (0xF

FFFFF), it just likes what base SCTP does. The data are delivered as reliable transmission.

The lifetime of user data is set to be zero, it will be sent only once. The lost data will not be retransmitted. The data are delivered as unreliable transmission.

Otherwise, the data are abandoned no matter if the data have been (re)transmitted or not when their lifetime expires. The data are delivered as partial reliability transmission.

If the ack of user data has not been received after lifetime expired, a Forward TSN chunk is sent to receiver to upgrade and advance its cumulative TSN point.

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Advantages of PR-SCTP A single SCTP association can carry both reliable and unreliable

content. That is, instead of sending different types of data using separate protocols, the number of datagrams can be reduced to save network overheads.

PR-data can enjoy the failure detection as the normal reliable SCTP data traffic dose.

PR-SCTP can provide ordered, unreliable data transfer service. PR-SCTP employs the same congestion control for all data traffi

c, which enforces TCP-friendliness.

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Streaming over PR-SCTP MPEG-4 streaming over PR-SCTP PR-SCTP allows to tune the retransmission per packet. The key

point is that more I frames received, better video quality : give a long lifetime to I frame to guarantee the reliable transmission of

them set a shorter lifetime to the P, B frames to limit the retransmission of them

Using PR-SCTP for MPEG-4 video streaming improved video quality and consistency.

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Mobile SCTP (mSCTP) In base SCTP, the IP addresses used are pre-configured and

cannot be changed. However, this feature limits the use of SCTP for mobile hosts. The SCTP extension described in ADDIP makes SCTP a

mobility enabled transport protocol. ADDIP provides SCTP with the ability to:

reconfigure IP address information on an existing association set the remote primary path exchange adaptation layer information during association setup

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ADDIP extension introduced two new chunk types that will be used to transfer the control information reliably. Address Configuration Change Chunk (ASCONF) Address Configuration Acknowledgment (ASCONF-ACK)

There are three major parameters introduced by ADDIP to achieve mobility Add IP Address Delete IP Address Set Primary Address

Mobile SCTP (mSCTP)

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get a new IP, send ASCONF with “Add IP” carrying the new IP

add IP into the association, send ASCONF-ACK

send ASCONF with “Set Primary” to change remote primary

send ASCONF-ACK and change primary path

send ASCONF with “Delete IP” carrying the old IPdelete IP from the association, send ASCONF-ACK

Node A

Node B

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Other STCP Research Topics

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Priority stream scheduling

Researchers are investigating the theoretical and practical implications of adding a priority stream scheduling scheme to SCTP.

Priorities allow the sending end point to give precedence to data specified as critical during periods of increased network delay or decreased throughput.

Priority schemes can help applications adapt to periods of heavy network congestion or poor quality of service.

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Concurrent Multipath Transfer (CMT)

ISP 1

ISP 1

ISP 2

ISP 2

ISP 3

ISP 3

ISP 6

ISP 6

ISP 5

ISP 5

ISP 4

ISP 4

Internet

Existing Paths

With TCPWithcurrent SCTP

With CMT

Path 2

Path 1

Path 3

Not usedone for retransmission

all for transmission

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Transparent SCTP Shim

Migrate existing TCP applications to SCTP transparently Application gains: fault tolerance, SACK support http://www.cis.udel.edu/~bickhart/research.html

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References and Resources

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Implementations www.sctp.org -- a site hosted and maintained by Randy Stewart. www.sctp.de – contains links to all relevant standards, tools and our us

erland-implementation SCTPLIB -- An SCTP Library maintained by Michael Tüxen.

sctplib-1.0.3 is the current stable version. socketapi-1.3.2 is the Socket API Library supports sctplib-1.0.3. sctplib-1.3.1 is the developer's version supports ADDIP extension.

Hypertext Index of current SCTPLIB snapshot Upcoming Linux Kernel Implementation developed by Jon Grimm (IB

M), La Monte Yarroll (Motorola) and many others. The implementation from Randy Stewart has been integrated into the

KAME stack, and additional, more up-to-date patches can be got at www.sctp.org.

Sun was developing a kernel implementation for Solaris.

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RFCs, Drafts and Standards RFC 2960: Stream Control Transmission Protocol. RFC 3257:Stream Control Transmission Protocol Applicability Statem

ent. RFC 3286: An Introduction to the Stream Control Transmission Protoc

ol (SCTP). RFC 3309: Stream Control Transmission Protocol (SCTP) Checksum

Change. RFC 3758: Stream Control Transmission Protocol (SCTP) Partial Reli

ability Extension. RFC 2719 -- Framework Architecture for Signaling Transport, L. Ong

et al. SCTP Extensions for Dynamic Reconfiguration of IP Addresses, R. R.

Stewart, M. A. Ramalho, Q. Xie, M. Tuexen, I. Rytina, P. Conrad The SCTP implementor's guide -- The "Don't Panic!"-Draft for RFC 2

960, R.R. Stewart, L. Ong, et al. Mobile SCTP -- describes how SCTP in combination with the addIP e

xtension can be used for providing a mobility concept

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Papers and Publications T. Dreibholz, A. Jungmaier, M. Tüxen: A New Scheme for IP-based Internet

Mobility,from the Proceedings of the 28th IEEE Conference on Local Computer Networks (LCN 2003), Bonn/Königswinter, October 2003, pp. X-Z. See also the slides from the conference presentation !

A. Jungmaier: A Gentle Introduction to SCTP,a presentation on SCTP given at the 19th Conference of the German CCC, Berlin, December 2002.

A. Jungmaier, E.P Rathgeb, M. Tüxen :On the Use of SCTP in Failover-Scenarios,from the Proceedings of the SCI 2002, Volume X, Mobile/Wireless Computing and Communication Systems II; Orlando, USA; July 2002, pp. 363-368.

A. Jungmaier,M. Schopp, M. Tüxen :Performance Evaluation of the Stream Control Transmission Protocol(English) - From the "Joint IEEE ATM Workshop 2000" (ATM 2000 Conference, Heidelberg, Germany), June 2000.

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Papers and Publications A. Jungmaier, E.P Rathgeb, M. Schopp, M. Tüxen :

SCTP - A multi-link end-to-end protocol for IP-based networks,AEÜ - International Journal of Electronics and Communications, 55 (2001) No.1, pp. 46-54. A. Jungmaier,M. Schopp, M. Tüxen :Das Simple Control Transmission Protocol (German)Veröffentlichung in "Elektrotechnik und Informationstechnik", Heft 06/2000.

Ivan Arias Rodriguez :Stream Control Transmission Protocol - The Design of a New Reliable Transport Protocol for IP Networks

Seok J. Koh, et al., “mSCTP for Soft Handover in Transport Layer”, IEEE Communications Letters, Vol. 8, No. 3, pp. 189 – 191, March 2004.

Hongtao Wang; Yuehui Jin; Wendong Wang, “The performance comparison of PRSCTP, TCP and UDP for MPEG-4 multimedia traffic in mobile network”

Janardhan R. Iyengar, Keyur C. Shah, Paul D. Amer, “Concurrent Multipath Transfer Using SCTP Multihoming”

G. Heinz P. Amer, “Priorities in SCTP Multistreaming”, SCI '04, Orlando, FL, 7/04

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Software Ethereal, the excellent packet tracer that was used throughout most of

our debugging sessions, and has been modified to support SCTP by M. Tüxen.

Dummynet,the excellent network emulation module that comes with FreeBSD.

Tcpdump has become SCTP aware !! It can be downloaded from here. SCTP support was added by Jerry Heinz (Temple University), John Fiore (U of Pennsylvania), and Armando Caro (University of Delaware).

The network simulator (ns-2). An SCTP patch for ns-2 is available here, thanks to Armando Caro and Janardhan Iyengar.

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SYNvictim Flooded!!

SYN Flooding Attack

TCB

TCB

TCB

TCB

TCB

• There is no ACK in response to the SYN-ACK, hence connection remains half-open• Other genuine clients cannot open connections to the victim• The victim is unable to provide service

attackers

128.3.4.5

221.3.5.10

Unavailable, reserved resources

SYN-ACK

ACK

SYN

SYN-ACK

ACK

84

Head-of-Line Blocking in TCPSender Receiver

ACK 1

1

2

3

4

5

ACK 2

ACK 2

ACK 2

Packet 3 is blocking the head of the line.

1

2

Receiver’s App

85

Head-of-line Blocking TCP provides a single data stream. When a segment is lost, subsequent segments must wait

to be processed. Problem for some applications (telephony)

86

Congestionwindow

10

5

15

20

0

Round-trip times

Slowstart

Congestionavoidance

Congestion occurs

Threshold

Slowstart

Congestionavoidance

Timeout

Slow Start & Congestion avoidance

87

Fast retransmitSender Receiver

ACK 1

123

5

6

ACK 2

ACK 2

ACK 2

ACK 2

4

ACK 6

retransmit 3

four consecutive SACKs