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Wireless Networks Fall 2007
TCP for Wireless Networks

Outline
 Motivation
 TCP mechanisms
 Indirect TCP
 Snooping TCP
 Mobile TCP
 Fast retransmit/recovery
 Transmission freezing
 Selective retransmission
 Transaction oriented TCP
Adapted from J. Schiller, “Mobile Communications”, Chapter 10

Motivation
 Transport protocols typically designed for
o Fixed end-systems
o Fixed, wired networks
 TCP congestion control
o Packet loss in fixed networks typically due to
(temporary) overload situations
o Routers discard packets as soon as the buffers are full
o TCP recognizes congestion only indirectly via missing
acknowledgements
o Retransmissions unwise, they would only contribute to
the congestion and make it even worse
o Slow-start algorithm as reaction

TCP Slow Start
 Sender calculates a congestion window for a
receiver
 Start with a congestion window size equal to one
segment
 Exponential increase of the congestion window up
to the congestion threshold, then linear increase
 Missing acknowledgement causes the reduction of
the congestion threshold to one half of the current
congestion window
 Congestion window starts again with one segment

TCP Fast Retransmit/Recovery
 TCP sends an acknowledgement only after
receiving a packet
 If a sender receives several acknowledgements
for the same packet, this is due to a gap in
received packets at the receiver
 However, the receiver got all packets up to the
gap and is actually receiving packets
 Therefore, packet loss is not due to congestion,
continue with current congestion window (do not
use slow-start)

Influences of mobility on TCP
 TCP assumes congestion if packets are dropped
o typically wrong in wireless networks, here we often have
packet loss due to transmission errors
o furthermore, mobility itself can cause packet loss, if e.g.
a mobile node roams from one access point (e.g. foreign
agent in Mobile IP) to another while there are still
packets in transit to the wrong access point and
forwarding is not possible
 The performance of an unchanged TCP degrades
severely
o however, TCP cannot be changed fundamentally due to
the large base of installation in the fixed network, TCP
for mobility has to remain compatible
o the basic TCP mechanisms keep the whole Internet
together

Indirect TCP I
 Indirect TCP or I-TCP segments the connection
o no changes to the TCP protocol for hosts connected to the wired Internet,
millions of computers use (variants of) this protocol
o optimized TCP protocol for mobile hosts
o splitting of the TCP connection at, e.g., the foreign agent into 2 TCP
connections, no real end-to-end connection any longer
o hosts in the fixed part of the net do not notice the characteristics of the
wireless part
mobile host
access point
(foreign agent) wired Internet
“wireless” TCP standard TCP

I-TCP socket and state migration
mobile host
access point2
Internet
access point1
socket migration
and state transfer

Indirect TCP II
 Advantages
o no changes in the fixed network necessary, no changes
for the hosts (TCP protocol) necessary, all current
optimizations to TCP still work
o transmission errors on the wireless link do not
propagate into the fixed network
o simple to control, mobile TCP is used only for one hop
between, e.g., a foreign agent and mobile host
o therefore, a very fast retransmission of packets is
possible, the short delay on the mobile hop is known
 Disadvantages
o loss of end-to-end semantics, an acknowledgement to a
sender does not any longer mean that a receiver really
got a packet, foreign agents might crash
o higher latency possible due to buffering of data within
the foreign agent and forwarding to a new foreign agent

Snooping TCP I
 Transparent extension of TCP within the foreign agent
 buffering of packets sent to the mobile host
 lost packets on the wireless link (both directions!) will be
retransmitted immediately by the mobile host or foreign agent,
respectively (so called “local” retransmission)
 the foreign agent therefore “snoops” the packet flow and recognizes
acknowledgements in both directions, it also filters ACKs
 changes of TCP only within the foreign agent (+min. MH change)
„wired“ Internet
buffering of data
end-to-end TCP connection
local retransmission correspondent
host
foreign
agent
mobile
host
snooping of ACKs

Snooping TCP II
 Data transfer to the mobile host
o FA buffers data until it receives ACK of the MH, FA detects
packet loss via duplicated ACKs or time-out
o fast retransmission possible, transparent for the fixed network
 Data transfer from the mobile host
o FA detects packet loss on the wireless link via sequence
numbers, FA answers directly with a NACK to the MH
o MH can now retransmit data with only a very short delay
 Advantages:
o Maintain end-to-end semantics
o No change to correspondent node
o No major state transfer during handover
 Problems
o Snooping TCP does not isolate the wireless link well
o May need change to MH to handle NACKs
o Snooping might be useless depending on encryption schemes

Mobile TCP
 Special handling of lengthy and/or frequent
disconnections
 M-TCP splits as I-TCP does
o unmodified TCP fixed network to supervisory host (SH)
o optimized TCP SH to MH
 Supervisory host
o no caching, no retransmission
o monitors all packets, if disconnection detected
• set sender window size to 0
• sender automatically goes into persistent mode
o old or new SH reopen the window
 Advantages
o maintains semantics, supports disconnection, no buffer
forwarding
 Disadvantages
o loss on wireless link propagated into fixed network
o adapted TCP on wireless link

Fast retransmit/fast recovery
 Change of foreign agent often results in packet loss
o TCP reacts with slow-start although there is no congestion
 Forced fast retransmit
o as soon as the mobile host has registered with a new foreign
agent, the MH sends duplicated acknowledgements on purpose
o this forces the fast retransmit mode at the communication
partners
o additionally, the TCP on the MH is forced to continue sending
with the actual window size and not to go into slow-start after
registration
 Advantage
o simple changes result in significant higher performance
 Disadvantage
o further mix of IP and TCP (to know when there is a new
registration), no transparent approach

Transmission/time-out freezing
 Mobile hosts can be disconnected for a longer time
o no packet exchange possible, e.g., in a tunnel, disconnection
due to overloaded cells or mux. with higher priority traffic
o TCP disconnects after time-out completely
 TCP freezing
o MAC layer is often able to detect interruption in advance
o MAC can inform TCP layer of upcoming loss of connection
o TCP stops sending, but does not assume a congested link
o MAC layer signals again if reconnected
 Advantage
o scheme is independent of data and TCP mechanisms (Ack,SN)
=> works even with IPsec
 Disadvantage
o TCP on mobile host has to be changed, mechanism depends on
MAC layer

Selective retransmission
 TCP acknowledgements are often cumulative
o ACK n acknowledges correct and in-sequence receipt of
packets up to n
o if single packets are missing quite often a whole packet
sequence beginning at the gap has to be retransmitted
(go-back-n), thus wasting bandwidth
 Selective retransmission as one solution
o RFC2018 allows for acknowledgements of single packets,
not only acknowledgements of in-sequence packet
streams without gaps
o sender can now retransmit only the missing packets
 Advantage: much higher efficiency
 Disadvantage
o more complex software in a receiver, more buffer needed
at the receiver

Transaction oriented TCP
 TCP phases
o connection setup, data transmission, connection release
o using 3-way-handshake needs 3 packets for setup and release,
respectively
o thus, even short messages need a minimum of 7 packets!
 Transaction oriented TCP
o RFC1644, T-TCP, describes a TCP version to avoid this
overhead
o connection setup, data transfer and connection release can be
combined
o thus, only 2 or 3 packets are needed
 Advantage
o efficiency
 Disadvantage
o requires changed TCP
o mobility no longer transparent

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