Week 8
SCTP and Internet Congestion control
IPv6

Agenda
• Congestion control
• AIMD in TCP
• Explicit Congestion Notification
• SCTP
• Principles
• Differences with TCP
• IPv6

TCP with mild
congestion
Cwnd
Fast retransmit
Threshold
Fast retransmit
Threshold
Slow-start
exponential increase of cwnd
Congestion avoidance
linear increase of cwnd

Severe congestion
Cwnd
Time
Timer expiration
Threshold
Timer expiration
Threshold
Slow-start
exponential increase of cwnd
Congestion avoidance
linear increase of cwnd

AIMD in TCP
# Initialisation
cwnd = MSS;
ssthresh= swin;
# Ack arrival
if tcp.ack > snd.una : # new ack, no congestion
if cwnd < ssthresh :
# slow-start : increase quickly cwnd
# double cwnd every rtt
cwnd = cwnd + MSS
else:
# congestion avoidance : increase slowly cwnd
# increase cwnd by one mss every rtt
cwnd = cwnd+ mss*(mss/cwnd)

AIMD in TCP
else: # duplicate or old ack
if tcp.ack==snd.una: # duplicate acknowledgement
dupacks++
if dupacks==3:
retransmitsegment(snd.una)
ssthresh=max(cwnd/2,2*MSS)
cwnd=ssthresh
else:
dupacks=0
# ack for old segment, ignored
Expiration of the retransmission timer:
send(snd.una) # retransmit first lost segment
sshtresh=max(cwnd/2,2*MSS)
cwnd=MSS

Examples
• Time to transmit 6 TCP segments
without losses
• Time to transmit 6 TCP segments if 3rd
is lost
• Time to transmit 10 TCP segments if 5th
is lost

Simplified model
• Assume all segment losses are periodic
and the every 1/p segment is lost
Cwnd(segments)
W
W/2
0
0 W/2 W 3W/2 Surface 2W time(rtt)
It can be shown that the throughput of a TCP
connection can be approximated by :
Maximum throughput without losses Throughput with
losses/congestion

Tuning TCP @google
• Target
• Minimize time to receive result from
search engine
• HTTP GET in one segment
• HTTP Response in <16 KBytes

Initial retransmission
timer
• What happens if SYN or SYN+ACK is
lost ?

Initial congestion
window
• What is the impact of slow-start on the
time to receive an HTTP response ?

TCP Fast Open
• Can we we reduce the overhead of the
three-way handshake ?
• HTTP/1.1
• Putting data inside SYN and
SYN+ACK

TCP Fast Open
SYN(Src=C,seq=x, HTTP GET)
CONNECT.ind+HTTP GET
CONNECT.req+Data
SYN+ACK(Dest=C,ack=x+1,seq=y, HTTP Resp)
ACK(Src=A,seq=x)
• Is this safe ?
• Risk of denial of service attack

Safe TCP Fast Open
• How to make TCP Fast Open safe in
the presence of attackers ?
• Server needs to ensure that SYN
segment does not come from a
spoofer

Agenda
• Congestion control
• AIMD in TCP
• Explicit Congestion Notification
• SCTP
• Principles
• Differences with TCP
• IPv6

Agenda
• Congestion control
• AIMD in TCP
• Explicit Congestion Notification
• SCTP
• Principles
• Differences with TCP
• IPv6

SCTP
• Segment format

SCTP connection
establishment

INIT Chunk
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 1 | Chunk Flags | Chunk Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiate Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertised Receiver Window Credit (a_rwnd) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Outbound Streams | Number of Inbound Streams |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initial TSN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Optional/Variable-Length Parameters /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

INIT-ACK chunk
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 2 | Chunk Flags | Chunk Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiate Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertised Receiver Window Credit |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Outbound Streams | Number of Inbound Streams |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initial TSN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Optional/Variable-Length Parameters /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

COOKIEs
• COOKIE-ECHO
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 10 |Chunk Flags | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Cookie /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
• COOKIE-ACK
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 11 |Chunk Flags | Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

COOKIE
• Variable length chunk
• Which information should be placed
inside the COOKIE ?

Verification tag
• Why is this tag present in all SCTP
segments ?

Payload chunk
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0 | Reserved|U|B|E| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TSN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Stream Identifier S | Stream Sequence Number n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Protocol Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ User Data (seq n of Stream S) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

SACK chunk
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 3 |Chunk Flags | Chunk Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cumulative TSN Ack |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertised Receiver Window Credit (a_rwnd) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Gap Ack Blocks = N | Number of Duplicate TSNs = X |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Gap Ack Block #1 Start | Gap Ack Block #1 End |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
...
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Gap Ack Block #N Start | Gap Ack Block #N End |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Duplicate TSN 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
...
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Duplicate TSN X |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Dealing with
messages
• How to enable the receiver to
reconstruct the received messages ?
B E Description
============================================================
| 1 0 | First piece of a fragmented user message |
+----------------------------------------------------------+
| 0 0 | Middle piece of a fragmented user message |
+----------------------------------------------------------+
| 0 1 | Last piece of a fragmented user message |
+----------------------------------------------------------+
| 1 1 | Unfragmented message |
============================================================
| Table 1: Fragment Description Flags |
============================================================

Connection release

SHUTDOWN chunks
• SHUTDOWN
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 7 | Chunk Flags | Length = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cumulative TSN Ack |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
• SHUTDOWN ACK
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 8 |Chunk Flags | Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Agenda
• Congestion control
• SCTP
• IPv6
• Addressing architecture
• Packets
• ICMPv6

IPv6 addresses
• Each IPv6 address is encoded in 128 bits
• 3.4 x 10^38 possible addressable devices
• 6.65 x 10^23 addresses per square
meter
• Why 128 bits ?
IPv4
IP version 6

Addressing
architecture • Three types of IPv6 addresses
• Unicast addresses
• An identifier for a single interface. A
packet sent to a unicast address is
delivered to the interface identified by
that address
• Anycast addresses
• An identifier for a set of interfaces.
• Multicast addresses
• An identifier for a set of interfaces. A
packet sent to a multicast address is
delivered to all interfaces identified by
that address.

Textual representation
• Hexadecimal format
• FEDC:BA98:7654:3210:FEDC:BA98:7654:
3210
• 1080:0:0:0:8:800:200C:417A
• Compact hexadecimal format
• Some IPv6 addresses contain lots of zero
• use "::" for one or more groups of 16
zeros.
• 1080:0:0:0:8:800:200C:417A =
1080::8:800:200C:417A
• FF01:0:0:0:0:0:0:101 =
FF01::101

IPv6 unicast addresses
interface ID
128 bits
N bits M bits 128-N-M bits
Usually 64 bits
Based on MAC Address
global routing prefix subnet ID
Can be used to identify the
ISP responsible for this address
A subnet in this ISP or
a customer of this ISP

Agenda
• Congestion control
• SCTP
• IPv6
• Addressing architecture
• Packets
• ICMPv6

The IPv6
packet format
32 bits
Ver Tclass Flow Label
NxtHdr Hop Limit
Payload Length
Source IPv6 address
(128 bits)
Destination IPv6 address
(128 bits)
Version=6
Traffic class
Size of packet
content in bytes
Unclear utilisation
Same as TTL
Used to identify the type
of the next header found
in the packet payload

Sample packets
Payload Length
Payload Length
Sequence number
Acknowledgment number
THL Reserved Flags
• Identification of a TCP connection
Window
• IPv6 src, IPv6 dest, Source and Destination
ports
32 bits
Ver Tclass Flow Label
NxtHdr Hop Limit
Source IPv6 address
(128 bits)
Destination IPv6 address
(128 bits)
Source port Destination port
Length Checksum
UDP
32 bits
Ver Tclass Flow Label
NxtHdr Hop Limit
Source IPv6 address
(128 bits)
Destination IPv6 address
(128 bits)
Source port Destination port
Checksum Urgent pointer
TCP
UDP
TCP

Agenda
• Congestion control
• SCTP
• IPv6
• Addressing architecture
• Packets
• ICMPv6

ICMP
• Internet Control Message Protocol
• Runs on top of IPv6 and provides
various types of services
• tools to aid debugging network
problems
• error reporting
• autoconfiguration of addresses

ping6
#ping6 www.ietf.org
PING6(56=40+8+8 bytes) 2001:6a8:3080:2:3403:bbf4:edae:afc3 -->
2001:1890:123a::1:1e
16 bytes from 2001:1890:123a::1:1e, icmp_seq=0 hlim=49 time=156.905 ms
16 bytes from 2001:1890:123a::1:1e, icmp_seq=1 hlim=49 time=155.618 ms
16 bytes from 2001:1890:123a::1:1e, icmp_seq=2 hlim=49 time=155.808 ms
16 bytes from 2001:1890:123a::1:1e, icmp_seq=3 hlim=49 time=155.325 ms
16 bytes from 2001:1890:123a::1:1e, icmp_seq=4 hlim=49 time=155.493 ms
16 bytes from 2001:1890:123a::1:1e, icmp_seq=5 hlim=49 time=155.801 ms
16 bytes from 2001:1890:123a::1:1e, icmp_seq=6 hlim=49 time=155.660 ms
16 bytes from 2001:1890:123a::1:1e, icmp_seq=7 hlim=49 time=155.869 ms
^C
--- www.ietf.org ping6 statistics ---
8 packets transmitted, 8 packets received, 0.0% packet loss
round-trip min/avg/max/std-dev = 155.325/155.810/156.905/0.447 ms

traceroute6
#traceroute6 www.ietf.org
traceroute6 to www.ietf.org (2001:1890:1112:1::20) from
2001:6a8:3080:2:217:f2ff:fed6:65c0, 30 hops max, 12 byte packets
1 2001:6a8:3080:2::1 13.821 ms 0.301 ms 0.324 ms
2 2001:6a8:3000:8000::1 0.651 ms 0.51 ms 0.495 ms
3 10ge.cr2.bruvil.belnet.net 3.402 ms 3.34 ms 3.33 ms
4 10ge.cr2.brueve.belnet.net 3.668 ms 10ge.cr2.brueve.belnet.net 3.988 ms
10ge.cr2.brueve.belnet.net 3.699 ms
5 belnet.rt1.ams.nl.geant2.net 10.598 ms 7.214 ms 10.082 ms
6 so-7-0-0.rt2.cop.dk.geant2.net 20.19 ms 20.002 ms 20.064 ms
7 kbn-ipv6-b1.ipv6.telia.net 21.078 ms 20.868 ms 20.864 ms
8 s-ipv6-b1-link.ipv6.telia.net 31.312 ms 31.113 ms 31.411 ms
9 s-ipv6-b1-link.ipv6.telia.net 61.986 ms 61.988 ms 61.994 ms
10 2001:1890:61:8909::1 121.716 ms 121.779 ms 121.177 ms
11 2001:1890:61:9117::2 203.709 ms 203.305 ms 203.07 ms
12 mail.ietf.org 204.172 ms 203.755 ms 203.748 ms