Stream Control Transmission Protocol

Stream Control Transmission Protocol

Special thanks to Dr. Paul Amer

Presented by – Viren Mahajan

[email protected] 20, 2007

What we have discussed !

• Motivation• PDU and Chunk format• The ‘a’ word - ASSOCIATION• Association setup & termination• Multihoming

Outline of today’s talk !

• Multistreaming• Framing• Chunk Bundling• Data Transfer• Fragmentation/Reassembly

Multistreaming

file transfer

video

voice

textShall we talk SCTP?

Yahoo! Instant Messenger

TCPmultiple

connections

physical

data link

IP

transportTCP

buffersTCP

buffers

filetransfer text voice video


sender receiver

4 independent connections

SCTPmultistreaming

physicaldata link

IP

transport streambuffers

streambuffers

1 association w/4 streams



sender receiver

Init chunk

Type: 1 Flag: 0 Length

Initiation Tag

Outbound Streams Maximum inbound streams

Optional/Variable length parameters

Advertised receiver window credit

Initial TSN

Init ack chunk

Type: 2 Flag: 0 Length

Initiation Tag

Outbound Streams Maximum inbound streams

Optional/Variable length parameters

Advertised receiver window credit

Initial TSN

Parameter Type: 7 Parameter Length

State Cookie

Multistreaming (cont)

• TCP: 1 stream of data per connection • SCTP: 1 or more streams of data per

association• # of streams negotiated during association

establishment• SCTP partial ordering: ordered delivery

within each stream

delivered to application

Data transfer using TCP

retransmission

receive buffer

Web serverWeb client

loss

sent from application

objects in send buffer

HOL blocking!

persistent, pipelinedTCP connection

stream 1

stream 2

stream 3


The multistreaming advantage

retransmission

receive buffer

Web server Web clientSCTP association

loss

objects in send buffer


Streams

• Streams by definition are ordered

• Unordered data may be sent in a stream (U bit = 1)

• Sequence number is ignored for U = 1

• Unordered messages should be processed first

• Unordered messages may be used for “out-of-band” signaling, as in telnet.

Application work-around to mitigate HOL blocking

• How?– Multiple persistent TCP connections to transfer

independent web objects

• Problems– Aggressive (not TCP friendly)– Possible HOL blocking within one TCP connection– No shared sequence space => Less robust to loss

detection and recovery– Increased load on web server– Increased connection establishment latency during SYN

losses

Message Boundaries

TCP does not preserve message boundaries


TCP connection

Message 3

Message 2

Message 1

bytes 1 - 100

bytes 101 - 200

bytes 201 - 300

bytes 1 – 75

bytes 176 – 230

bytes 231 – 300

bytes 76 – 175

SCTP preserves message boundaries


SCTP association

Message 3

Message 2

Message 1

Message 1

Message 2

Message 3

bytes 1 - 100

bytes 101 - 200

bytes 201 - 300

bytes 1 - 100

bytes 201 - 300

bytes 101 - 200

Message Boundaries

• UDP honors message boundaries– Each app message becomes a datagram

• TCP does not honor message boundaries– App messages become part of a byte stream

• SCTP maintains message boundaries– Each app message is maintained as one or more data chunks– Data flags manage message boundaries

• Simpler coding of applications– no need to do buffering, framing

• Example - DNS– Response size <= 512, UDP is used– If response size > 512, TCP is used

• Application must preface the header with message length

Chunk Bundling

Without bundling

end pt A end pt B

SCTP association

SCTP-PDUs SCTP-PDUs

Application

Application

A-PDUs A-PDUs

SCTP Association

With bundling

end pt A end pt B

SCTP association

SCTP-PDUs SCTP-PDUs

Application

Application

SCTP Association

A-PDUs A-PDUs

Bundling of Chunks in SCTP

• Control chunks bundled before data chunks• Chunk boundary cannot cross SCTP PDU boundary

Source Port Destination Port

Verification Tag

Checksum

Chunk 1

…

Chunk N

Common Header

Bundling

SCTP PDU

Chunk Bundling

• Multiple chunks in one SCTP PDU

• Gain in n/w bandwidth efficiency – reduces ovhd of additional sctp and IP headers

• Path MTU is the constraint

• Control Chunks are always placed first

• Chunk boundaries do not cross SCTP PDU boundaries

• These chunks cannot be bundled– init, init ack, shutdown complete

Data Transfer

SCTP Chunk Format

Type Flags Length

Chunk Data

•Type – Data, Init, SACK, …

• Flags – bit meanings depend on type

• Length – includes type, flags, length, and data/parameters

Data Chunk

Flags = UBE

Data

User supplied Payload Protocol Identifier

Stream Seq Num (SSN)Stream Identifier (SID)

Transmission Sequence Number (TSN)

LengthType = 0x00

0 31

used for ordering, reassembly and retransmission

used for ordering within a stream

identifies the stream to which the data chunk

belongs

used by the application and network monitoring

equipment to understand the type of data being

transmitted

SACK Chunk

Type = 0x03 Flags (none) Length (variable)

Cumulative TSN ack

Advertised receiver window

Number of Gap ACK blocks = N Number of duplicates = X

Gap ACK blk #1 start TSN offset Gap ACK blk #1 end TSN offset

...

Gap ACK blk #N start TSN offset Gap ACK blk #N end TSN offset

Duplicate TSN 1

…

Duplicate TSN X

highest cumulative TSN

no. of Duplicate TSN reports

includednumber of Gap

Ack Blocks included

end offset for a range of

consecutive TSNs received

start offset for a range of consecutive

TSNs received

TSN received more than

once

Data transferTSN: 1

SID: 1 SSN: 1

TSN: 2SID: 2 SSN: 1

TSN: 3SID: 1 SSN: 2

TSN: 4SID: 2 SSN: 2

ACK 4

TSN: 1

SID: 1 SSN: 1

ACK 1

end A

end B

Data transfer

end A

end BTSN: 12

SID: 1 SSN: 2

ACK 12a_rwnd

TSN: 13SID: 2 SSN: 2TSN: 14SID: 1 SSN: 3

TSN: 15SID: 2 SSN: 3 ACK 12

a_rwnd

#Gaps: 1 #Dup: 0

Gap1_start: 2 Gap1_end: 3TSN: 13SID: 1 SSN: 4

TSN: 16SID: 2 SSN: 4


ACK 12a_rwnd

#Gaps: 2 #Dup: 0

Gap1_start: 2 Gap1_end: 3

Gap2_start: 5 Gap2_end: 5



Fragmentation / Reassembly

SCTP fragmentation/reassembly


SCTP association

A-PDU

SCTP-PDUs SCTP-PDUs

A-PDU

Data Chunk

Reserved U B E

Data

User supplied Payload Protocol Identifier

Stream Seq Num (SSN)Stream Identifier (SID)

Transmission Sequence Number (TSN)

LengthType = 0x00

0 31

Fragmentation/Reassembly Flags

U B E Description

* 1 0 (Begin) First Piece of fragmented A-PDU

* 0 0 Middle piece of fragmented A-PDU

* 0 1 (End) Last piece of fragmented A-PDU

* 1 1 Non-fragmented A-PDU

*U set to 1 specifies unordered message

Note: Fragmentation requires sequential TSNs

A fragmentation example

A-PDU

second fragment

E.g. app A-PDU for Stream 2 exceeds Path-MTU

last fragment

First data fragmentSSN=1SID=2TSN=6U=0, B=1,

E=0

Part of Data Chunk Header

U=0, B=0, E=0

TSN=7 SID=2 SSN=1

U=0, B=0, E=1

TSN=8 SID=2 SSN=1

SCTP - User Data Fragmentation

• Large A-PDU fragmented into DATA chunks

• To avoid intermediate routers having to deal with fragmentation

• Constrained by path MTU

• Unique TSN assigned to each DATA chunk

• Same SSN assigned to each DATA chunk

• B/E (Begin/End) bits identify fragments of user message

• U flag same as that in the original message

Re-Assembly of Fragments

• Receiver checks B/E flags of DATA chunk

• User message reassembled using SSN, SID, B/E bits of DATA chunks.

• Number of fragments determined by the TSN of the first and the last fragments

TSN

5TSN6²

TSN2

TSN4

TSN3

TSN5

TSN6²

TSN2

TSN3

TSN4

TSN1¹

TSN1¹

TSN6²

TSN5

TSN4

TSN3

TSN2

TSN1¹

A Large Message Transfer

A2

A1

B2

B1

receive buffer (6)


TSN1¹

TSN3

TSN2


TSN4

TSN5

TSN6²

data

data

data to be sent

1 - B bit set to 12 - E bit set to 1

Path MTU = 512 octets

(2760 octets)

Summary

• Multistreaming– multiple streams avoid HOL blocking

• Framing– message boundaries are preserved

• Chunk Bundling– better bandwidth usage

• Data Transfer– data chunks

• Fragmentation/Reassembly– B/E bits in data chunk

THANKS !

SCTP Vocabulary

• Association

• Chunk

• PMTU – Path Maximum Transfer Unit

• SSN – Stream Sequence Number

• SID – Stream identifier

• TSN - Transmission Sequence Number

• TCB – Transmission Control Block

Documents

Stream Control Transmission Protocol